XII. MAINTENANCE OF SEWERS
=198. Work Involved.=—The principal effort in maintaining sewers is to keep them clean and unobstructed. A sewerage system, although buried, cannot be forgotten as it will not care for itself, but becoming clogged will force itself on the attention of the community. Besides the cleaning and repairing of sewers and the making of inspections for determining the necessity for this work, ordinances should be prepared and enforced for the purpose of protecting the sewers from abuse. Inspections to determine the amount of the depreciation of sewers with a view towards possible renewal, or to determine the capacity of a sewer in relation to the load imposed upon it are sometimes necessary. The valuation of the sewerage system as an item in the inventory of city property may be assigned to the engineer in charge of sewer maintenance.
The work involved in the inspection and cleaning of sewers in New York City for the year ending May, 1914, included the removal of 22,687 cubic yards of material from catch-basins, and 14,826 catch-basin cleanings. This made an average of two and one-half cleanings per catch-basin per year, or 1½ cubic yards removed at each cleaning. The 6,432 catch-basins were inspected 71,890 times. There were 4,112 cubic yards of material removed from 517 miles of sewers, or about 8 cubic yards per mile. Inspection of 194 miles of brick sewers were made, 4.4 miles were flushed, and 27 miles were cleaned. Inspections of 198 miles of pipe sewers were made, 80 miles were examined more closely, 37 miles were flushed, and 91 miles were cleaned. The field organization for this work consisted of 17 foremen, 8 assistant foremen, 29 laborers, 71 cleaners, 13 mechanics, 7 inspectors of construction, 3 inspectors of sewer connections, 13 horses and wagons, and 28 horses and carts.[105]
=199. Causes of Troubles.=—The complaints most frequently received about sewers are caused by clogging, breakage of pipes, and bad odors. Sewers become clogged by the deposition of sand and other detritus which results in the formation of pools in which organic matter deposits, aggravating the clogged condition of the sewers and causing the odors complained of. Grease is a prolific cause of trouble. It is discharged into the sewer in hot wastes, and becoming cooled, deposits in thick layers which may effectively block the sewer if not removed. It can be prevented from entering the sewers by the installation of grease traps as described in Chapter VI. The periodic cleaning of these traps is as important as their installation.
Tree roots are troublesome, particularly in small pipe sewers in residential districts. Roots of the North Carolina poplar, silver leaf poplar, willow, elm, and other trees will enter the sewer through minute holes and may fill the sewer barrel completely if not cut away in time. Fungus growths occasionally cause trouble in sewers by forming a network of tendrils that catches floating objects and builds a barricade across the sewer. Difficulties from fungus growths are not common, but constant attention must be given to the removal of grit, grease, and roots. Tarry deposits from gas-manufacturing plants are occasionally a cause of trouble, as they cement the detritus already deposited into a tough and gummy mass that clings tenaciously to the sewer.
Broken sewers are caused by excessive superimposed loads, undermining, and progressive deterioration. The changing character of a district may result in a change of street grade, an increase in the weight of traffic, or in the construction of other structures causing loads upon the sewer for which it was not designed. The presence of corrosive acids or gases may cause the deterioration of the material of the sewer.
=200. Inspection.=—The maintenance of a sewerage system is usually placed under the direction of a sewer department. In the organization of the work of this department no regular routine of inspection of all sewers need be followed ordinarily. Attention should be given regularly to those sewers that are known to give trouble, whereas the less troublesome sewers need not be inspected more frequently than once a year, preferably during the winter when labor is easier to obtain.
The routine inspection of sewers too small to enter is made by an examination at the manhole. If the water is running as freely at one manhole as at the next manhole above, it is assumed that the sewer between the manholes is clean and no further inspection need be given unless there is some other reason to suspect clogging between manholes. If the sewage is backed up in a manhole it indicates that there is an obstruction in the sewer below. If the sewage in a manhole is flowing sluggishly and is covered with scum it is an indication of clogging, slow velocity and septic action in the sewer. Sludge banks on the sloping bottom of the manhole or signs of sewage high upon the walls indicate an occasional flooding of the sewer due to inadequate capacity or clogging.
If any of the signs observed indicate that the sewer is clogged, the manhole should be entered and the sewer more carefully inspected. Such inspection may be made with the aid of mirrors as shown in Fig. 140 or with a periscope device as shown in Fig. 141. Sunlight is more brilliant than the electric lamp shown in Fig. 141, but the mirror in the manhole directs the sunlight into the eyes of the observer, dazzling him and preventing a good view of the sides of the sewer. The observers’ eyes can be protected against the direct rays of the electric light, which can be projected against the sides of the pipe by proper shades and reflectors. It is possible with this device to locate house connection, stoppages, breaks of the pipe, and to determine fairly accurately the condition of the sewer without discomfort to the observers.
Sewers that are large enough to enter should be inspected by walking through them where possible. The inspection should be conducted by cleaning off the sewer surface in spots with a small broom, and examining the brick wall for loose bricks, loose cement or cement lost from the joints, open joints, broken bond, eroded invert, and such other items as may cause trouble. An inspection in storm sewers is sometimes of value in detecting the presence of forbidden house connections.
Certain precautions should be taken before entering sewers or manholes. If a distinct odor of gasoline is evident the sewer should be ventilated as well as possible by leaving a number of manhole covers open along the line until the odor of gasoline has disappeared. The strength of gasoline odor above which it is unsafe to enter a sewer is a matter of experience possessed by few. A slight odor of gasoline is evident in many sewers and indicates no special danger. A discussion of the amount of gasoline necessary to create explosive conditions is given in Art. 206. In making observations of the odor it should also be noted whether air is entering or leaving the manhole. The presence of gasoline cannot be detected at a manhole into which air is entering.
As soon as it is considered that the odors from a sewer indicate the absence of an explosive mixture, a lighted lantern or other open flame should be lowered into the manhole to test the presence of oxygen. Carbon monoxide or other asphyxiating gases may accumulate in the sewer, and if present will extinguish the flame. If the flame burns brilliantly the sewer is probably safe to enter, but if conditions are unknown or uncertain, the man entering should wear a life belt attached to a rope and tended by a man at the surface. Asphyxiating or explosive gases are sometimes run into without warning due to their lack of odor, or the presence of stronger odors in the sewer. Breathing masks and electric lamps are precautions against these dangers, the masks being ready for use only when actually needed. More deaths have occurred in sewers due to asphyxiating gases than by explosions, as the average sewer explosion is of insufficient violence to do great damage, although on occasion, extremely violent explosions have occurred. During inspections of sewers there should always be at least one man at the surface to call help in case of accident and the inspecting party should consist of at least two men.
It must not be felt that entering sewers is fraught with great danger, as it is perfectly safe to enter the average sewer. The air is not unpleasant and no discomfort is felt, but conditions are such that unexpected situations may arise for which the man in the sewer should be prepared. It is therefore wise to take certain precautions. These may indicate to the uninitiated, a greater danger than actually exists.
The inspection of sewers should include the inspection of the flush-tanks, control devices, grit chambers, and other appurtenances. A common difficulty found with flush-tanks is that the tank is “drooling,” that is to say the water is trickling out of the siphon as fast as it is entering the tank, and the intermittency of the discharge has ceased. If, when the tank is first inspected the water is about at the level of the top of the bell it is probable that the siphon is drooling. A mark should be made at the elevation of the water surface and the tank inspected again in the course of an hour or more. If the water level is unchanged the siphon is drooling. This may be caused by the clogging of the snift hole or by a rag or other obstacle hanging over the siphon which permits water to pass before the air has been exhausted, or a misplacement of the cap over the siphon, or other difficulty which may be recognized when the principle on which the siphon operates is understood. Occasionally it is discovered that an over zealous water department has shut off the service.
Control devices, such as leaping or overflow weirs, automatic valves, etc., may become clogged and cease to operate satisfactorily. They should be inspected frequently, dependent upon their importance and the frequency with which they have been found to be inoperative. An inspection will reveal the obstacle which should be removed. Floats should be examined for loss of buoyancy or leaks rendering them useless. Grit and screen chambers should be examined for sludge deposits.
Catch-basins on storm sewers are a frequent cause of trouble and need more or less frequent cleaning. Cleanings are more important than inspections for catch-basins for if they are operating properly they are usually in need of cleaning after every storm of any magnitude, and a regular schedule of cleaning should be maintained.
A record should be kept of all inspections made. It should include an account of the inspection, its date, the conditions found, by whom made and the remedies taken to effect repairs.
=201. Repairs.=—Common repairs to sewerage systems consist in replacing street inlets or catch-basin covers broken by traffic; raising or lowering catch-basin or manhole heads to compensate for the sinking of the manhole or the wear of the pavement; replacing of broken pipes, loosened bricks or mortar which has dropped out; and other miscellaneous repairs as the necessity may arise. Connections from private drains are a source of trouble because either the sewer or the drain has broken due to careless work or the settlement of the foundation or the backfill.
=202. Cleaning Sewers.=—Sewers too small to enter are cleaned by thrusting rods or by dragging through them some one of the various instruments available. The common sewer rod shown in Fig. 142 is a hickory stick, or light metal rod, 3 or 4 feet long, on the end of which is a coupling which cannot come undone in the sewer. Sections of the rod are joined in the manhole and pushed down the sewer until the obstruction is reached and dislodged. Occasionally pieces of pipe screwed together are used with success. The end section may be fitted with a special cutting shoe for dislodging obstructions. In extreme cases these rods may be pushed 400 to 500 feet, but are more effective at shorter distances. Obstructions may be dislodged by shoving a fire hose, which is discharging water under high pressure through a small nozzle, down the sewer toward the obstruction. The water pressure stiffens the hose, which, together with the support from the sides of the conduit, make it possible to push the hose in for effective work 100 feet or more from the manhole. A strip of flexible steel about ½ inch thick and 1½ to 2 inches wide is useful for “rodding” a short length of crooked sewer.
Sewers are seldom so clogged that no channel whatever remains. As a sewer becomes more and more clogged, the passage becomes smaller, thereby increasing the velocity of flow of the sewage around the obstruction and maintaining a passageway by erosion. This phenomenon has been taken advantage of in the cleaning of sewers by “pills.” These consist of a series of light hollow balls varying in size. One of the smaller balls is put into the sewer at a manhole. When the ball strikes an obstruction it is caught and jammed against the roof of the sewer. The sewage is backed up and seeks an outlet around the ball, thus clearing a channel and washing the ball along with it. The ball is caught at the next manhole below. A net should be placed for catching the ball and a small dam to prevent the dislodged detritus from passing down into the next length of pipe. The feeding of the balls into the sewer is continued, using larger and larger sizes, until the sewer is clean. This method is particularly useful for the removal of sludge deposits, but it is not effective against roots and grease. The balls should be sufficiently light to float. Hollow metal balls are better than heavier wooden ones.
Plows and other scraping instruments are dragged through pipe sewers to loosen banks of sludge and detritus and to cut roots or dislodge obstructions. One form of plow consists of a scoop[106] similar to a grocer’s sugar scoop, which is pushed or dragged up a sewer against the direction of flow. As fast as the scoop is filled it is drawn back and emptied. The method of dragging this through a sewer is indicated in Fig. 143. At Atlantic City the crew operating the scoop comprises five men, two are at work in each manhole and one on the surface to warn traffic and wait on the men in the manholes. The outfit of tools is contained in a hand-drawn tool box and includes sewer rods, metal scoops for all sizes of sewers, picks, shovels, hatchets, chisels, lanterns, grease and root cutters, etc., and two winches with from 400 to 600 feet of ⅜-inch wire cable.
Another form of plow or drag consists of a set of hooks or teeth hinged to a central bar as shown in Fig. 144. A root cutter and grease scraper in the form of a spiral spring with sharpened edges, and other tools for cleaning sewers are shown in Fig. 145. A turbine sewer cleaner shown in Fig. 146 consists of a set of cutting blades which are revolved by a hydraulic motor of about 3 horse-power under an operating pressure of about 60 pounds per square inch. The turbine is attached to a standard fire hose and is pushed through the sewer by utilizing the stiffness of the hose, or by rods attached to a pushing jack as shown in the figure. This machine was invented and patented by W. A. Stevenson in 1914. Its performance is excellent. The blades revolve at about 600 R.P.M., cutting roots and grease. The revolving blades and the escaping water also serve to loosen and stir up the deposits and the forward helical motion imparted to the water is useful in pushing the material ahead of the machine and in scrubbing the walls of the sewer. In Milwaukee four men with the machine cleaned 319 feet of 12–inch sewer in 16 hours, and in Kansas City 7,801 feet of sewers were cleaned in 14 days.
Sewers large enough to enter may be cleaned by hand. The materials to be removed are shoveled into buckets which are carried or floated to manholes, raised to the surface and dumped. In very large sewers temporary tracks have been laid and small cars pushed to the manhole for the removal of the material. Hydraulic sand ejectors may also be used for the removal of deposits, similar to the steam ejector pump shown in Fig. 97. The water enters the apparatus at high velocity, under a pressure of about 60 pounds per square inch, leaps a gap in the machine from a nozzle to a funnel-shaped guide leading to the discharge pipe. The suction pipe of the machine leads to the chamber in which the leap is made. In leaping this gap the water creates a vacuum that is sufficient to remove the uncemented detritus large enough to pass through the machine, and will lift small stones to a height of 10 to 12 feet. Occasionally barricades of logs, tree branches, rope, leaves, and other obstructions which have piled up against some inward projecting portion of the sewer, must be removed by hand either by cutting with an axe or by pulling them out. Projections from the sides of sewers are objectionable because of their tendency to catch obstacles and form barricades.
Little authentic information on the cost of cleaning sewers is available. A permanent sewer organization is maintained by many cities. The division of their time between repairs, cleaning, and other duties is seldom made a matter of record. From data published in Public Works[107] it is probable that the cost varies from $3 to $15 per cubic yard of material removed. From the information in Vol. II of “American Sewerage Practice” by Metcalf and Eddy the combined cost of cleaning and flushing will vary between $10 and $40 per mile; the expense of either flushing or cleaning alone being about one-half of this.
=203. Flushing Sewers.=—Sewers can sometimes be cleaned or kept clean by flushing. Flushing may be automatic and frequent, or hand flushing may be resorted to at intervals to remove accumulated deposits. Automatic flush-tanks, flushing manholes, a fire hose, a connection to a water main, a temporary fixed dam, a moving dam, and other methods are used in flushing sewers. The design, operation, and results obtained from the use of automatic flush-tanks and flushing manholes are discussed in Chapter VI.
The method in use for cleaning a sewer by thrusting a fire hose down it can also be used for flushing sewers. It is an inexpensive and fairly satisfactory method. There is, however, some danger of displacing the sewer pipe because of the high velocity of the water. An easier and safer but less effective method is to allow water to enter at the manhole and flow down the sewer by gravity. Direct connections to the water mains are sometimes opened for the same purpose.
Sewers are sometimes flushed by the construction of a temporary dam across the sewer, causing the sewage to back up. When the sewer is half to three-quarters full the dam is suddenly removed and the accumulated sewage allowed to rush down the sewer, thus flushing it out. The dam may be made of sand bags, boards fitted to the sewer, or a combination of boards and bags. The expense of equipment for flushing by this method is less than that by any other method, but the results obtained are not always desirable. Below the dam the results compare favorably with those obtained by other methods, but above the dam the stoppage of the flow of the sewage may cause depositions of greater quantities of material than have been flushed out below. A time should be chosen for the application of this method when the sewage is comparatively weak and free from suspended matter. The most convenient place for the construction of a dam is at a manhole in order that the operator may be clear of the rush of sewage when the dam is removed.
Movable dams or scrapers are useful in cleaning sewers of a moderate size, but are of little value in small sewers. The scraper fits loosely against the sides of the sewer and is pushed forward by the pressure of the sewage accumulated behind it. The iron-shod sides of the dam serve to scrape grease and growths attached to the sewer and to stir up sand and sludge deposited on the bottom. The high velocity of the sewage escaping around the sides of the dam aids in cleaning and scrubbing the sewer.
A natural watercourse may be diverted into the sewer if topographical conditions permit, or where sewers discharge into the sea below high tide a gate may be closed during the flood and held closed until the ebb. The rush of sewage on the opening of the gate serves to flush the sewers and stir up the sludge deposited during high tide. Other methods of flushing sewers may be used dependent on the local conditions and the ingenuity of the engineer or foreman in charge.
In some sewers it is not necessary to remove the clogging material from the sewer. It is sufficient to flush and push it along until it is picked up and carried away by higher velocities caused by steeper grades or larger amounts of sewage.
=204. Cleaning Catch-basins.=[108]—Catch-basins have no reason for existence if they are not kept clean. Their purpose is to catch undesirable settling solids and to prevent them from entering the sewers, on the theory that it is cheaper to clean a catch-basin than it is to clean a sewer. If the cleaning of storm sewers below some inlet to which no catch-basin is attached becomes burdensome, the engineer in charge of maintenance should install an adequate catch-basin and keep it clean. Catch-basins are cleaned by hand, suction pumps, and grab buckets. In cleaning by hand the accumulated water and sludge are removed by a bucket or dipper and dumped into a wagon from which the surplus settled water is allowed to run back into the sewer. The grit at the bottom of the catch-basin is removed by shoveling it into buckets which are then hoisted to the surface and emptied.
Suction pumps in use for cleaning catch-basins are of the hydraulic eductor type. The eductor works on the principle of the steam pump shown in Fig. 97, except that water is used instead of steam. The material removed may be discharged into settling basins constructed in the street, or may be discharged directly into wagons.[109] In Chicago a special motor-driven apparatus is used. This consists of a 5–yard body on a 5–ton truck, and a centrifugal pump driven by the truck motor. In use, the truck, about half filled with water, drives up to the catch-basin, the eductor pipe is lowered and water pumped from the truck into the eductor and back into the truck again, together with the contents of the catch-basin. The surplus water drains back into the sewer. The Chicago Bureau of Sewers reports a truck so equipped to have cleaned 1013 catch-basins, removing 1763 cubic yards of material, and running 1380 miles, during the months of August, September and October, 1917. The cost, including all items of depreciation, wages, repairs, etc., was $1,393.89. Orange-peel buckets, about 20 inches in diameter, operated by hand or by the motor of a 3½ to 5–ton truck with a water-tight body, are used for cleaning catch-basins in some cities.
Catch-basins in unpaved streets and on steep sandy slopes should be cleaned after every storm of consequence. Basins which serve to catch only the grit from pavement washings require cleaning about two or three times per year, and from one to three cubic yards of material are removed at each cleaning. The cost of cleaning ordinary catch-basins by hand may vary from $15 to $25, but with the use of eductors or orange-peel buckets the cost is somewhat lower. In Seattle the cost of cleaning large detritus basins by hand is said[110] to vary from $45 to $60. With the use of eductors this cost has been reduced to one-third or one-fifth the cost of cleaning by hand.
=205. Protection of Sewers.=[111]—City ordinances should be wisely drawn and strictly enforced for the protection of sewers against abuse and destruction. The requirements of some city ordinances are given in the following paragraphs.
Washington, D. C.,[112] sewer ordinances provide that:
No person shall make or maintain any connection with any public sewer or appurtenance thereof whereby there may be conveyed into the same any hot, suffocating, corrosive, inflammable or explosive liquid, gas, vapor, substance or material of any kind ... provided that the provisions of this act shall not apply to water from ordinary hot water boilers or residences.
The following extracts from the ordinances of Indianapolis are typical of those from many cities:
2950. No connection shall be made with any public sewer without the written permission of the Committee on Sewers and the Sewerage Engineer.
2953. No person shall be authorized to do the work of making connections until he has furnished a satisfactory certificate that he is qualified for the duties. He shall also file bond for not less than $1,000 that he will indemnify the City from all loss or damage that may result from his work and that he will do the work in conformity to the rules and regulations established by the City Council.
2955. It shall be unlawful for any person to allow premises connected to the sewers or drains to remain without good fixtures so attached as to allow a sufficiency of water to be applied to keep the same unobstructed.
2956. No butcher’s offal or garbage, or dead animals, or obstructions of any kind shall be thrown in any receiving basin or sewer in penalty not greater than $100. Any person injuring, breaking, or removing any portion of any receiving basin, manhole cover, etc., shall be fined not more than $100.
2962. No person shall drain the contents of any cesspool or privy vault into any sewer without the permission of the Common Council.
The Cleveland ordinances are similar and contain the following in addition:
1251. Rule 4. All connections with the main or branch sewers shall be made at the regular connections or junctions built into the same, except by special permit.
Rule 16. No steam pipe, nor the exhaust, nor the blow off from any steam engine shall be connected with any sewer.
Evanston, Illinois, protects its sewers against the additions of grease and other undesirable substances as follows:
1444. It is unlawful for any person to use any sewer or appurtenance to the sewerage system in any manner contrary to the orders of the Commissioner of Public Works.
1446. Wastes from any kitchen sinks, floor drains, or other fixtures likely to contain greasy matter from hotels, certain apartment houses, boarding houses, restaurants, butcher shops, packing houses, lard rendering establishments, bakeries, laundries, cleaning establishments, garages, stables, yard and floor drains, and drains from gravel roofs shall be made through intervening receiving basins constructed as prescribed in par. VIII of this code.
Receiving basins suitable for the work required in the code are illustrated in Chapter VI.
=206. Explosions in Sewers.=—Disastrous explosions in sewers were first recorded about 1886.[113] Up to about 1905 explosions were infrequent and were considered as unavoidable accidents and so rare as to be unworthy of study. For a decade or more after 1905 explosions occurred with increasing violence and frequency causing destruction of property, but by some freakish chance, but little loss of life. A violent and destructive explosion occurred in Pittsburgh on Nov. 25, 1913,[114] and another on March 12, 1916. The property damage amounted to $300,000 to $500,000 on each occasion, but there was no loss of life. Two miles of pavement were ripped up, gas, water, and other sewer pipes were broken, buildings collapsed and the streets were flooded. The streets were rendered unserviceable for long periods during the expensive repairs that were necessary. In recent years the number of explosions in sewers has been smaller, due probably to the gain in knowledge of the causes and intelligent methods of prevention.
The three principal causes of explosions in sewers are: gasoline vapor, illuminating gas, and calcium carbide. It is probable that gasoline vapor is by far the most troublesome. Explosions caused by these gases are not so violent as those caused by dynamite or other high explosives, as the volume of gas and the temperature generated are much less. The violence of sewer explosions may be increased somewhat by the sudden pressures that are put upon them.
Gasoline finds its way into sewers from garages and cleaning establishments. A mixture of 1½ per cent gasoline vapor and air may be explosive. It needs only the stray spark of an electric current, a lighted match, or a cigar thrown into the sewer to cause the explosion. As the result of a series of experiments on 2,706 feet of 8–foot sewer, Burrell and Boyd conclude.[115]
One gallon of gasoline if entirely vaporized produces about 32 cubic feet of vapor at ordinary temperature and pressure. If 1½ per cent be adopted as the low explosive limit of mixtures of gasoline vapor and air, 55 gallons or a barrel of gasoline would produce enough vapor to render explosive the mixture in 1,900 feet of 9 foot sewer provided the gasoline and the air were perfectly mixed. Many different factors, however, govern explosibility, such as: size of the sewer, velocity of the sewage, temperature of the sewer, volatility and rate of inflow of the gasoline. Only under identical conditions of tests would duplicate results be obtained. A large amount of gasoline poured in at one time is less dangerous than the same amount allowed to run in slowly. With a velocity of flow of about 6½ feet per second it was evident that 55 gallons of gasoline poured all at once into a manhole rendered the air explosive only a few minutes (less than 10) at any particular point. With the same amount of gasoline run in at the rate of 5 gallons per minute, an explosive flame would have swept along the sewer if ignited 15 minutes after the gasoline had been dumped. With a slow velocity of flow and a submerged outlet the gasoline vapor being heavier than air accumulated at one point and extremely explosive conditions could result from a small amount of gasoline. Comparatively rich explosive mixtures were found 5 hours after the gasoline had been discharged. High-test gasoline is much more dangerous than the naphtha used in cleaning establishments, yet on account of the large quantity of waste naphtha the sewage from cleaning establishments may be very dangerous.
Illuminating gas is not so dangerous as gasoline vapor as it is lighter than air and it is more likely to escape from the sewer than to accumulate in it. It requires about one part of illuminating gas to seven parts of air to produce an explosive mixture.
Calcium carbide is dangerous because it is self igniting. The heat of the generation of gas is sufficient to ignite the explosive mixture. The gases are highly explosive and cause a relatively powerful explosion. Fortunately large amounts of this material seldom reach a sewer, the gas being generated in garage drains or traps and escaping in the atmosphere.
A hydrocarbon oil used by railroads in preventing the freezing of switches, if allowed to reach the sewers, may cause explosions therein.[116] The oil crystallizes and in this form it is soluble in water. It will thus pass traps and on volatilization will produce explosive mixtures.
Methane, generated by the decomposition of organic matter, is a feebly explosive gas occasionally found in sewers. Its presence may add to the strength of other explosive mixtures.
Sewer explosions may be prevented by the building of proper forms of intercepting basins to prevent the entrance of gasoline and calcium carbide gases, and by ventilation to dilute the explosive mixtures which may be made up in the sewer. There are no practical means to predict when an explosion is about to occur, and after an explosion has occurred it is difficult to determine the cause as all evidence is usually destroyed.
=207. Valuation of Sewers.=—The necessity for the valuation of a sewerage system may arise from the legal provisions in some states limiting the amount of outstanding bonds which may be issued by a municipality to a certain percentage of the present worth of municipal property. The investment in the sewerage system is usually great and forms a large portion of the City’s tangible property. It may be desirable also to determine the depreciation of the sewers with a view towards their renewal.
The most valuable work on the valuation of sewers has been done in New York City[117] by the engineers of the Sewer Department. The committee of engineers appointed to do the work recommended: (1) that the original cost be made the basis of valuation, and that (2), in fixing this cost the cost of pavement should be omitted or at most the cost of a cheap (cobblestone) pavement should be included. Trenches previously excavated in rock were considered as undepreciated assets.
The present worth of sewers depends on many factors aside from the effects of age, such as the care exercised in the original construction, the material used, the kind and quantity of sewage carried, the care taken in maintenance, and finally the injury caused by the careless building of adjoining substructures. During the progress of the inspections the examination of brick sewers, due to their accessibility, yielded better results than the examination of pipe sewers. The routine of the examination of the brick sewers consisted in cleaning off the bricks with a short broom, tapping the brick with a light hammer to determine solidity, and testing the cement joints by scraping with a chisel. In addition, measurements of height and width were taken every 30 feet. The bricks in the invert at and below the flow line were examined for wear.
A study of the reports of these examinations disclosed that the following defects were noticeable:
1. Cement partly out at water line.
2. Cement partly out above water line.
3. Depressed arch and sewer slightly spread.
4. Large open joints.
5. Loose brick.
6. Bond of brick broken.
7. Distorted sides, uneven bottom, joints out of line.
Inspection of pipe sewers from manholes, the pipe being illuminated by floating candles, was found to be unsatisfactory. Reliance was placed on the reports of men experienced in making connections and repairs to the sewers. Early pipe sewers in New York were laid directly on the bottom of the trench. Under these circumstances a small leak at a joint was sufficient to wash the earth away and to drop the pipe, causing serious conditions along the line. No wear or deterioration of pipe sewers were noted, the only defects being cracking of the pipes at the center line due to poor foundation and to defects in the pipe itself.
The depreciation of brick sewers as studied in New York, is shown graphically in Fig. 147. At zero the sewer is in good condition and at 100 it is in such a state of dilapidation as to require instant rebuilding. Repairs are not considered economical in this condition. In the preparation of this diagram each condition on the list above was given a certain number of points, which when added together represented the state of depreciation of the sewer. These sums were plotted as ordinates and the corresponding ages of the sewer were plotted as abscissas. The various points were taken cumulatively, and where the bond of the brickwork was broken (given a value of 72) plus other defects gave a total of 164 the sewer was considered as valueless and not worth repair. The scale of 164 was later reduced to a percentage basis as shown on the right of the figure. Fig. 148 shows a similar diagram for the depreciation of pipe sewers.
It was concluded that the life of a brick sewer in New York is 64 years. Some of the sewers examined were over 200 years old. The total original cost of 483 miles of brick, pipe and wood sewers was figured as $23,880,000 with a present worth of $18,665,000 and an average annual depreciation of 2.2 per cent. In figuring these amounts no account was taken of obsolescence. The deterioration of catch-basins proceeded at about the same rate as for brick sewers.
XIII. COMPOSITION AND PROPERTIES OF SEWAGE
=208. Physical Characteristics.=—Sewage is the spent water supply of a community containing the wastes from domestic, industrial, or commercial use, and such surface and ground water as may enter the sewer.[118] Sewages are classed as: domestic sewage, industrial waste, storm water, surface water, street wash, and ground water. Domestic sewage is the liquid discharged from residences or institutions and contains water closet, laundry, and kitchen wastes. It is sometimes called sanitary sewage. Industrial sewage is the liquid waste resulting from processes employed in industrial establishments. Storm water is that part of the rainfall which runs over the surface of the ground during a storm and for such a short period following a storm as the flow exceeds the normal and ordinary run-off. Surface water is that part of the rainfall which runs over the surface of the ground some time after a storm. Street wash is the liquid flowing on or from the street surface. Ground water is water standing in or flowing through the ground below its surface.
Ordinary fresh sewage is gray in color, somewhat of the appearance of soapy dish water. It contains particles of suspended matter which are visible to the naked eye. If the sewage is fresh the character of some of the suspended matter can be distinguished as: matches, bits of paper, fecal matter, rags, etc. The amount of suspended matter in sewage is small, so small as to have no practical effect on the specific gravity of the liquid nor to necessitate the modification of hydraulic formulas developed for application to the flow of water. The total suspended matter in a normal strong domestic sewage is about 500 parts per 1,000,000. It is represented graphically in Fig. 149. The quantity of organic or volatile suspended matter is about 200 parts per 1,000,000. It is shown graphically in the smaller cube in Fig. 149.
The odor of fresh sewage is faint and not necessarily unpleasant. It has a slightly pungent odor, somewhat like a damp unventilated cellar. Occasionally the odor of gasoline, or some other predominating waste matter may hide all other odors. Stale sewage is black and gives off nauseating odors of hydrogen sulphide and other gases. If the sewage is so stale as to become septic, bubbles of gas will be seen breaking the surface and a black or gray scum may be present. Before the South Branch of the Chicago River was cleaned up and flushed this scum became so thick in places, particularly in that portion of the Stock Yards where the river became known as Bubbly Creek, that it is said that weeds and small bushes sprouted in it, and chickens and small animals ran across its surface.
A physical analysis of sewage should include an observation of its appearance, and a determination of its temperature, turbidity, color, and odor, both hot and cold. The temperature is useful in indicating certain of the antecedents of the sewage, its effect on certain forms of bacterial life, and its effect on the possible content of dissolved gases. Temperatures higher than normal are indicative of the presence of trades wastes discharged while hot into the sewers. A low temperature may indicate the presence of ground water. If the temperature is much over 40° C. bacterial action will be inhibited and the content of dissolved gases will be reduced. Turbidity, color, and odor determinations may be of value in the control of treatment devices, or to indicate the presence of certain trades wastes, which give typical reactions. Since all normal sewages are high in color and turbidity, the relative amounts of these two constituents in two different sewages has little significance regarding the relative strengths of the two sewages or the proper method of treating them. A fresh domestic sewage should have no highly offensive odor. The presence of certain trades wastes can be detected sometimes in fresh sewages, and a stale sewage may sometimes be recognized by its odor.
Sewage is a liability to the community producing it. Although some substances of value can be obtained from sewage[119] the cost of the processes usually exceed the value of the substances obtained. Where it becomes necessary to treat sewage the value of these substances may be helpful in defraying the cost of treatment.
=209. Chemical Composition.=—Sewage is composed of mineral and organic compounds which are either in solution or are suspended in water. In making a standard chemical analysis of sewage only those chemical radicals and elements are determined which are indicative of certain important constituents. Neither a complete qualitative nor quantitative analysis is made. A sewage analysis will not show, therefore, the number of grams of sodium chloride present or any other constituent. A complete standard sanitary chemical analysis will report the constituents as named in the first column of Table 71. The quantities of these materials found in average strong, medium and weak sewages are also shown in this table. These values are not intended as fixed boundaries between sewages of different strengths. They are presented merely as a guide to the interpretation of sewage analyses.
The principal objects of a chemical analysis of sewage are to determine its strength and its state of decomposition. The influents and effluents of a sewage treatment device are analyzed to aid in the control of the device and to gain information concerning the effect of the treatment. Chemical and other analyses, in connection with the desired conditions after disposal, will indicate the extent of treatment which may be required. The standard methods of water and sewage analysis adopted by the American Public Health Association have been generally accepted by sanitarians. These uniform methods make possible comparisons of the results obtained by laboratories working according to these standards.
CHEMICAL ANALYSIS OF SEWAGES
(Parts per million)
From Report on Industrial Wastes from the Stock Yards and Packingtown, Chicago by the Sanitary District of Chicago in 1921, page 231.
──────────┬──────────────────┬───────┬────────┬────────── │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │Waterbury, │ │Boston │Columbus│ Conn., │ Typical Analyses │1905–7 │ 1904–5 │ 1905–6 ──────────┼──────┬──────┬────┼───────┼────────┼────────── │Strong│Medium│Weak│ │ │ ──────────┼──────┼──────┼────┼───────┼────────┼────────── Nitrogen │ │ │ │ │ │ as │ │ │ │ │ │ Organic │ │ │ │ │ │ Nitrogen│ 35│ 20│ 10│ 9.1│ 9.0│ 14.8 Free │ │ │ │ │ │ Ammonia │ 50│ 30│ 15│ 13.9│ 11.0│ 7.8 Nitrites │ 0.10│ 0.05│ 0.0│ 0.0│ 0.09│ 0.14 Nitrates │ 0.40│ 0.20│ 0.1│ 0.20│ 0.20│ 1.52 Oxygen │ │ │ │ │ │ consumed│ 75│ 50│ 30│56[120]│ 51[121]│ 46[120] Oxygen │ │ │ │ │ │ demand │ 300│ 200│ 100│ │ │ Chlorine │ 175│ 100│ 15│ 2300│ 65│ 48 Suspended │ │ │ │ │ │ matter │ 500│ 300│ 150│ 135│ 209│ 165 Volatile│ │ │ │ 91│ 79│ 115 Fixed │ │ │ │ 44│ 130│ 50 Alkalinity│ 200│ 100│ 50│ 125│ 350│ 41 Fats │ 40│ 20│ │ │ 25│ 26 ──────────┴──────┴──────┴────┴───────┴────────┴──────────
──────────┬─────────────┬──────────┬───────────┬─────────── │ │ │ │ Chicago, │ │ │ │ Center │ │ │ Chicago, │ Avenue. │Gloversville,│Worcester,│ 39th St. │Industrial. │ N. Y. │ Mass. │Residential│Day Sewage │ 1908–9 │ 1908 │ 1909–12 │ 1913 ──────────┼─────────────┼──────────┼───────────┼─────────── │ │ │ │ ──────────┼─────────────┼──────────┼───────────┼─────────── Nitrogen │ │ │ │ as │ │ │ │ Organic │ │ │ │ Nitrogen│ 23.0│ │ 7.8│ 79 Free │ │ │ │ Ammonia │ 12.0│ 22.2│ 9.1│ 22 Nitrites │ 0.38│ │ 0.10│ 0.49 Nitrates │ 0.88│ │ 0.33│ 3.04 Oxygen │ │ │ │ consumed│ 95[120]│ 117│ 43│ 268 Oxygen │ │ │ │ demand │ │ │ │ Chlorine │ 158│ 57│ 40│ 1100 Suspended │ │ │ │ matter │ 406│ 258│ 144│ 605 Volatile│ 229│ 166│ 90│ 46 Fixed │ 177│ 92│ 54│ 144 Alkalinity│ 233│ │ 212│ 291 Fats │ 48│ │ 23[122]│ 198[123] ──────────┴─────────────┴──────────┴───────────┴───────────
=210. Significance of Chemical Constituents.=—Organic nitrogen and free ammonia taken together are an index of the organic matter in the sewage. Organic nitrogen includes all of the nitrogen present with the exception of that in the form of ammonia, nitrites, and nitrates. Free ammonia or ammonia nitrogen is the result of bacterial decomposition of organic matter. A fresh cold sewage should be relatively high in organic nitrogen and low in free ammonia. A stale warm sewage should be relatively high in free ammonia and low in organic nitrogen. The sum of the two should be unchanged in the same sewage.
Nitrites (RNO_{2}) and nitrates (RNO_{3})[124] are found in fresh sewages only in concentrations of less than one part per million. In well-oxidized effluents from treatment plants the concentration will probably be much higher. Nitrates contain one more atom of oxygen than nitrites. They represent the most stable form of nitrogenous matter in sewage. Nitrites are not stable and are reduced to ammonias or are oxidized to nitrates. Their presence indicates a process of change. They are not found in large quantities in raw sewage because their formation requires oxygen which must be absorbed from some other source than the sewage. In an ordinary sewer or sluggishly flowing open stream this absorption cannot take place from the atmosphere with sufficient rapidity to supply the necessary oxygen.
Oxygen consumed is an index of the amount of carbonaceous matter readily oxidizable by potassium permanganate. It does not indicate the total quantity of any particular constituent, but it is the most useful index of carbonaceous matter. Carbonaceous matter is usually difficult of treatment and a high oxygen consumed is indicative of a sewage difficult to care for. The amount of oxygen consumed, as expressed in the analysis, is dependent on the amount of oxidizable carbonaceous matter present, the oxidizing agent used, and the time and temperature of contact of the sewage and the oxidizing agent. It is essential therefore that the test be conducted according to some standard method, since the results are of value only as compared with results obtained under similar conditions.
Total solids (residue on evaporation) are an index of the strength of the sewage. They are made up of organic and inorganic substances. The inorganic substances include sand, clay, and oxides of iron and aluminum, which are usually insoluble, and chlorides, carbonates, sulphates and phosphates, which are usually soluble. The insoluble inorganic substances are undesirable in sewage because of their sediment forming properties which result in the clogging of sewers, treatment plants, pumps, and stream beds. The soluble inorganic substances are generally harmless and cause no nuisance, except that the presence of sulphur may permit the formation of hydrogen sulphide, which has a highly offensive odor. The organic substances are: carbohydrates, fats, and soaps, which are carbonaceous and are difficult of removal by biological processes; and the nitrogenous substances such as urea, proteins, amines, and amino acids. The inorganic and organic substances may be either in solution or suspension or in a colloidal condition.
Volatile solids are used as an index of the organic matter present, as it is assumed that the organic matter is more easily volatilized than the inorganic matter. The amount of volatile inorganic matter present is usually so small as to be negligible.
Fixed solids are reported as the difference between the total and volatile solids. They are therefore representative of the amount of inorganic matter present.
Suspended matter is the undissolved portion of the total solids. High volatile suspended matter is an indication of offensive qualities in the nature of putrefying organic matter, whereas fixed suspended matter is indicative of inoffensive inorganic matter. It is difficult to obtain a sample of sewage which will represent the amount of suspended matter in the sewage, since a sample taken from near the surface will contain less inorganic matter and grit than a sample taken near the bottom.
Settling solids are indicative of the sludge forming properties of the sewage and of the probable degree of success of treatment by plain sedimentation. Volatile settling solids indicate the property of the formation of offensive putrefying sludge banks. There is no chemical test which will indicate the scum-forming properties of sewage. Fixed settling solids indicate the presence of inorganic matter, probably gritty material such as sand, clay, iron oxide, etc.
Colloidal matter is material which is too finely divided to be removed by filtration or sedimentation, yet is not held in solution. It can sometimes be removed by violent agitation in the presence of a flocculent precipitate, as in the treatment with activated sludge, or by the flocculent precipitate alone, as in chemical precipitation, or by the acidulation of the sewage so as to precipitate the colloids. Colloidal matter is probably the result of the constant abrasion of finely divided suspended matter while flowing through the sewer or other channel. High colloidal matter may therefore indicate a stale sewage, or the presence of a particular trades waste. Colloids are difficult of removal. For this reason, where sewage is to be treated, turbulence in the tributary channels should be avoided.
Alkalinity may indicate the possibility of success of the biologic treatment of sewage, since bacterial life flourishes better in a slightly alkaline than in a slightly acid sewage. Within the normal limits of the amount of alkalinity in sewage the exact amount has little significance in sewage analyses. Sewages are normally slightly alkaline. An abnormal alkalinity or acidity may indicate the presence of certain trades wastes necessitating special methods of treatment. A method of sewage treatment may be successful without changing the amount of alkalinity in the sewage since the amount of alkalinity is not inherently an objection.
Chlorine, in the form of sodium chloride, is an inorganic substance found in the urine of man and animals. The amount of chlorine above the normal chlorine content of pure waters in the district is used as an index of the strength of the sewage. The chlorine content may be affected by certain trades wastes such as ice-cream factories, meat-salting plants, etc., which will increase the amount of chlorine materially. Since chlorine is an inorganic substance which is in solution it is not affected by biological processes nor sedimentation. Its diminution in a treatment plant or in a flowing stream is indicative of dilution and the reduction of chlorine will be approximately proportional to the amount of dilution.
Fats have a recoverable market value when present in sufficient quantity to be skimmed off the surface of the sewage. Ordinarily fats are an undesirable constituent of sewage as they precipitate on and clog the interstices in filtering material, they form objectionable scum in tanks and streams, and they are acted on very slowly by biological processes of sewage treatment. Although fats are carbonaceous matter they are not indicated by the oxygen consumed test because they are not easily oxidized. They are therefore determined in another manner; by evaporation of the liquid and extracting the fats from the residue by dissolving them in ether.
Relative stability and bio-chemical oxygen demand are the most important tests indicating the putrefying characteristics of sewage. Since stability and putrescibility have opposite meanings the relative stability test is sometimes called the putrescibility test. The relative stability of a sewage is an expression for the amount of oxygen present in terms of the amount required for complete stability.
A relative stability of 75 signifies that the sample examined contains a supply of available oxygen equal to 75 per cent of the amount of oxygen which it requires in order to become perfectly stable. The available oxygen is approximately equivalent to the dissolved oxygen plus the available oxygen of nitrate and nitrite.[125]
TABLE 72
RELATIVE STABILITY NUMBERS
──────────────────────────────────┬────────────────────────────────── Time Required for Decolorization │ at 20° C. Days │ Relative Stability Number ──────────────────────────────────┼────────────────────────────────── 0.5│ 11 1.0│ 21 1.5│ 30 2.0│ 37 2.5│ 44 3.0│ 50 4.0[126]│ 60 5.0│ 68 6.0│ 75 7.0│ 80 8.0│ 84 9.0│ 87 10.0│ 90 11.0│ 92 12.0│ 94 13.0│ 95 14.0│ 96 16.0│ 97 18.0│ 98 20.0│ 90 ──────────────────────────────────┴──────────────────────────────────
The relative stability numbers, given in Table 72, are computed from the expression, S = 100(1 − 0.794_t_) in which S is the stability number and t is the time in days that the sample has been incubated at 20° C. The bio-chemical oxygen demand is more directly an index of the consumption of available oxygen by the biological and chemical changes which take place in the decomposition of sewage or polluted water. As such it is a more valuable, though less easily performed test than the test of relative stability.
The methods for the determination of the relative stability and the bio-chemical oxygen demand are given to show more clearly what these tests represent. The procedure in the relative stability test is to add 0.4 c.c. of a standard solution of methylene blue to 150 c.c. of the sample. The mixture is then allowed to stand in a completely filled and tightly stoppered bottle at 20° C. for 20 days or until the blue fades out due to the exhaustion of the available oxygen. There are three methods in use for the determination of the bio-chemical oxygen demand;[127] the relative stability method, the excess nitrate method, and the excess oxygen method. In the relative stability method the sample to be treated should have a relative stability of at least 50. If it is lower than this the sample should be diluted with water containing oxygen until the relative stability has been raised to or above this point. The oxygen demand in parts per million is then expressed as
O′ = ((1 − P)O)⁄RP,[128]
in which O′ is the oxygen demand, O is the initial oxygen in parts per million (p.p.m.) in the diluting water or sewage, P is the proportion of sewage in the mixture expressed as a ratio, and R is the relative stability of the mixture expressed as a decimal. For the effluents from sewage treatment plants, polluted waters, and similar liquids, the total available oxygen expressed as the sum of the dissolved oxygen, nitrites, and nitrates, divided by the relative stability expressed as a decimal will give the bio-chemical oxygen demand. The excess nitrate method requires the determination of the total oxygen available as dissolved oxygen, nitrites, and nitrates and the addition of a sufficient amount of oxygen in the form of sodium nitrate to prevent the exhaustion of oxygen during a 10–day period of incubation. At the end of the period the total available oxygen is again determined. The difference between the original and the final oxygen content represents the bio-chemical oxygen demand. The excess oxygen test requires the determination of the total available oxygen as before, and the addition of a sufficient amount of oxygen, in the form of dissolved oxygen in the diluting water, to prevent exhaustion of the oxygen in a 10–day period of incubation. The difference between the original and final oxygen content represents the bio-chemical oxygen demand. Theriault concludes as a result of his tests, that the relative stability and excess nitrate methods are open to objections but that the excess oxygen method yields very accurate and consistent results with as little or less labor than is required by other methods.
Dissolved oxygen represents what its name implies, the amount of oxygen (_O__{2}) which is dissolved in the liquid. Normal sewage contains no dissolved oxygen unless it is unusually fresh. It is well, if possible, to treat a sewage before the original dissolved oxygen has been exhausted. Normal pure surface water contains all of the oxygen which it is capable of dissolving, as shown in Table 73. The presence of a smaller amount of oxygen than is shown in this table indicates the presence of organic matter in the process of oxidation, which may be in such quantities as ultimately to reduce the oxygen content to zero. Normal pure ground waters may be deficient in dissolved oxygen because of the absence of available oxygen for solution. The presence of certain oxygen-producing organisms in polluted or otherwise potable surface waters may cause a supersaturation with oxygen however.
The dissolved-oxygen test for polluted water is probably the most significant of all tests. If dissolved oxygen is found in a polluted water it means that putrefactive odors will not occur, since putrefaction cannot begin in the presence of oxygen. It is possible for different strata in a body of water to have different quantities of dissolved oxygen, and putrefaction may be proceeding in the lower strata before the oxygen is exhausted from the upper strata. The oxygen content of a river water will indicate the ability of the river to receive sewage without resulting in a nuisance.
TABLE 73
SOLUBILITY OF OXYGEN IN WATER
Under an atmospheric pressure of 760 mm. of mercury, the atmosphere containing 20.9 per cent of oxygen. ───────────────────────────────────┬─────────────────────────────────── Temperature, degrees C │ Oxygen in parts per million ───────────────────────────────────┼─────────────────────────────────── 0│ 14.62 1│ 14.23 2│ 13.84 3│ 13.48 4│ 13.13 5│ 12.8 6│ 12.48 7│ 12.17 8│ 11.87 9│ 11.59 10│ 11.33 11│ 11.08 12│ 10.83 13│ 10.6 14│ 10.37 15│ 10.15 16│ 9.95 17│ 9.74 18│ 9.54 19│ 9.35 20│ 9.17 21│ 8.99 22│ 8.83 23│ 8.68 24│ 8.53 25│ 8.38 26│ 8.22 27│ 8.07 28│ 7.92 29│ 7.77 30│ 7.63 ───────────────────────────────────┴───────────────────────────────────
=211. Sewage Bacteria.=—A slight knowledge of the nature of bacteria is necessary in order that the biological changes which occur in the treatment of sewage may be understood. Bacteria are living organisms which are so small that it is difficult or impossible to study them either with the eye alone or with the aid of powerful microscopes. They are studied by means of cultures, stains, and certain characteristic phenomena such as the production of a gas, the production of a red colony on litmus lactose agar, etc. Bacteria occur in three forms: spherical, called coccus; cylindrical, called bacillus; and spiral, called spirillum. In size they vary from the largest at about 1⁄10,000 of an inch to sizes so small as to be invisible under the most powerful microscope. An ordinary size is 1⁄25,000 of an inch. The cylindrical or rod bacteria are about four times as long as they are wide. Some bacteria possess the power of motion due to the presence of flagella or hairs which can be moved and cause the cell to progress at a rate as high as 18 cm. per hour, but usually the rate is very much less than this. The composition of the bacterial cell has never been definitely determined.
Bacteria are unicellular plants. They possess no digestive organs and apparently obtain their food by absorption from the surrounding media. Reproduction is by the division of the cell into two approximately equal portions. This reproduction may occur as frequently as once every half hour and if unchecked would quickly mount to unimaginable numbers. The natural cause limiting the growth of bacteria is the generation by the bacterium of certain substances such as the amino acids, which are injurious to cell life. The exhaustion of the food supply is not considered as an important cause of inhibition of multiplication. The products of growth of one species of bacteria may be helpful or harmful to other forms. Where the products are helpful the effect is known as symbiosis, and where harmful the effect is known as antibiosis. In sewage the presence of both aërobic and anaërobic bacteria is usually mutually helpful and the condition is an example of symbiosis. The aërobes, sometimes called obligatory aërobes, are bacteria which demand available oxygen for their growth. The anaërobes, or obligatory anaërobes, can grow only in the absence of oxygen. There are other forms that are known as facultative anaërobes (or aërobes) whose growth is independent of the presence or absence of oxygen.
Spores are formed by some bacteria when they are subjected to an unfavorable environment such as high temperatures, the absence of food, the absence of moisture, etc. Spores are cells in which growth and animation are suspended but the life of the cell is carried on through the unsuitable period, somewhat similar to the condition in a plant seed.
=212. Organic Life in Sewage.=—Living organisms, both plants and animals, exist in sewage. Bacteria are the smallest of these organisms. Others, which can be studied easily under the microscope or can be seen with difficulty by the naked eye but which do not require special cultures for their study, are classed as microscopic organisms or plankton. Organisms which are large enough to be studied without the aid of a microscope or special cultures are classed as macroscopic. The part taken in the biolysis of sewage by macroscopic organisms belonging to the animal kingdom, such as birds, fish, insects, rodents, etc., which feed upon substances in the sewage is so inconsequential as to be of no importance. Both plants and animals are found among the macroscopic organisms.
Organisms in sewage may be either harmful, harmless, or beneficial. From the viewpoint of mankind the harmful organisms are the pathogenic bacteria. Their condition of life in sewage is not normal and in general their existence therein is of short duration. It may be of sufficient length, however, to permit the transmission of disease. The diseases which can be transmitted by sewage are only those that are contracted through the alimentary canal, such as typhoid fever, dysentery, cholera, etc. Diseases are not commonly contracted by contact of sewage with the skin nor by breathing the air of sewers. It is safe to work in and around sewage so long as the sewage is kept out of the mouth, and asphyxiating or toxic gases are avoided.
The beneficial organisms in sewage are those on which dependence is placed for the success of certain methods of treatment. These organisms have not all been isolated or identified.
The total number of bacteria in a sample of sewage has little or no significance. In a normal sewage the number may be between 2,000,000 and 20,000,000 per c.c. and because of the extreme rapidity of multiplication of bacteria a sample showing a count of 1,000,000 per c.c. on the first analysis may show 4 to 5 times as many 3 or 4 hours later. A bacterial analysis of sewage is ordinarily of little or no value, since pathogenic organisms are practically certain to be present, there is no interest in the harmless organisms, and the helpful nitrifying and aërobic bacteria will not grow on ordinary laboratory media. Occasionally the presence of certain bacteria may indicate the presence of certain trades wastes. In general, the total bacterial count, as sometimes reported, represents only the number of bacteria which have grown under the conditions provided. It bears no relation to the total number of bacteria in the sample.
The presence of bacteria in sewage is of great importance however, as practically all methods of treatment depend on bacterial action, and all sewages which do not contain deleterious trades wastes, contain or will support the necessary bacteria for their successful treatment, if properly developed.
=213. Decomposition of Sewage.=—If a glass container be filled with sewage and allowed to stand, open to the air, a black sediment will appear after a short time, a greasy scum may rise to the surface, and offensive odors will be given off. This condition will persist for several weeks, after which the liquid will become clear and odorless. The sewage has been decomposed and is now in a stable condition. The decomposition of sewage is brought about by bacterial action the exact nature of which is uncertain.
It[129] is well established that many of the chemical effects wrought by bacteria, as by other living cells, are due, not to the direct action of the protoplasm, but to the intervention of soluble ferments or enzymes.
Enzymes are soluble ferments produced by the growth of the bacterial cell.
In[130] many cases the enzymes diffuse out from the cell and exert their effort on the ambient substances ... in others the enzyme action occurs within the cell and the products pass out, (for example) ... the alcohol-producing enzymes of the yeast cell act upon sugar within the cell, the resulting alcohol and carbon dioxide being ejected.
Other chemical effects may be brought about by the direct action of the living cells, but this has never been well established.
Metabolism is the life process of living cells by which they absorb their food and convert it into energy and other products. It is the metabolism of bacterial growth that in itself or by the production of enzymes hastens the putrefactive or oxidizing stages of the organic cycles in sewage treatment. Bacteria can assimilate only liquid food since they have no digestive tract through which solid food can enter. The surrounding solids are dissolved by the action of the enzymes, the resulting solution diffusing through the chromatin or outer skin, and being digested throughout the interior cytoplasm.
Bacteria are sometimes classified as parasites and saprophytes. The parasites live only on the growing cells of other plant or animal life. The saprophytes obtain their food only from the life products of living organisms and do not exist at the expense of the organisms themselves. Facultative saprophytes (or parasites) may exist on either living or dead tissue.
The decomposition of sewage may be divided into anaërobic and aërobic stages. These conditions are usually, but not always, distinctly separate. The growth of certain forms of bacteria is concurrent, while the growth of other forms is dependent on the results of the life processes of other bacteria in the early stages of decomposition.
When sewage is very fresh it contains some oxygen. This oxygen is quickly exhausted so that the first important step in the decomposition of sewage is carried on under anaërobic conditions. This is accompanied by the creation of foul odors of organic substances, ammonia, hydrogen sulphide, etc.; other odorless gases such as carbon dioxide, hydrogen, and marsh gas, the latter being inflammable and explosive; and other complicated compounds. An exception to the rule that putrefaction takes place only in the absence of oxygen is the production of other foul-smelling substances by the putrefactive activity of obligatory and facultative aërobes. Hydrogen sulphide may be produced apparently in the presence of oxygen the action which takes place not being thoroughly understood.
The biolysis of sewage is the term applied to the changes through which its organic constituents pass due to the metabolism of bacterial life. Organic matter is composed almost exclusively of the four elements: carbon, oxygen, hydrogen, and nitrogen (COHN) and sometimes in addition sulphur and phosphorus. The organic constituents of sewage can be divided into the proteins, carbohydrates, and fats. The proteins are principally constituents of animal tissue, but they are also found in the seeds of plants. The principal distinguishing characteristic of the proteins is the possession of between 15 and 16 per cent of nitrogen. To this group belong the albumens and casein. The carbohydrates are organic compounds in which the ratio of hydrogen to oxygen is the same as in water, and the number of carbon atoms is 6 or a multiple of 6. To this group belong the sugars, starches and celluloses. The fats are salts formed, together with water, by the combination of the fatty acids with the tri-acid base glycerol. The more common fats are stearin, palmatin, olein, and butyrine. The soaps are mineral salts of the fatty acids formed by replacing the weak base glycerol with some of the stronger alkalies.
The first state in the biolysis of sewage is marked by the rapid disappearance of the available oxygen present in the water mixed with organic matter to form sewage. In this state the urea, ammonia, and other products of digestive or putrefactive decomposition are partially oxidized and in this oxidation the available oxygen present is rapidly consumed, the conditions in the sewage becoming anaërobic. The second state is putrefaction in which the action is under anaërobic conditions. The proteins are broken down to form urea, ammonia, the foul-smelling mercaptans, hydrogen sulphide, etc., and fatty and aromatic acids. The carbohydrates are broken down into their original fatty acid, water, carbon dioxide, hydrogen, methane, and other substances. Cellulose is also broken down but much more slowly. The fats and soaps are affected somewhat similarly to the hydrocarbons and are broken down to form the original acids of their make up together with carbon dioxide, hydrogen, methane, etc. The bacterial action on fats and soaps is much slower than on the proteins, and the active biological agents in the biolysis of the hydrocarbons, fats, and soaps are not so closely confined to anaërobes as in the biolysis of the proteins. The third state in the biolysis of sewage is the oxidation or nitrification of the products of decomposition resulting from the putrefactive state. The products of decomposition are converted to nitrites and nitrates, which are in a stable condition and are available for plant food. It must be understood that the various states may be coexistent but that the conditions of the different states predominate approximately in the order stated. In the biolysis of sewage there is no destruction of matter. The same elements exist in the same amount as at the start of the biolytic action.
=214. The Nitrogen Cycle.=—Nitrogen is an element that is found in all organic compounds. Its presence is necessary to all plant and animal life. The nitrogenous compounds are most readily attacked by bacterial action in sewage treatment. The non-nitrogenous substances such as soaps and fats, and the inorganic compounds are more slowly affected by bacterial action alone. The element nitrogen passes through a course of events from life to death and back to life again that is known as the Nitrogen Cycle. It is typical of the cycles through which all of the organic elements pass.
Upon the death of a plant or animal, decomposition sets in accompanied by the formation of urea which is broken down into ammonia. This is known as the putrefactive stage of the Nitrogen Cycle. The next state is nitrification in which the compounds of ammonia are oxidized to nitrites and nitrates, and are thus prepared for plant food. In the state of plant life the nitrites and nitrates are denitrified so as to be available as a plant or animal food. The highest state of the Nitrogen Cycle is animal life, in which nitrogen is a part of the living animal substance or is charged from protein to urea, ammonia, etc., by the functions of life in the animal. Upon the death of this animal organism the cycle is repeated. The Nitrogen Cycle, like the cycle of Life and Death, is purely an ideal condition as in nature there are many short circuits and back currents which prevent the continuous progression of the cycle. The conception of this cycle is an aid, however, in understanding the processes of sewage treatment.
=215. Plankton and Macroscopic Organisms.=—In general the part played by these organisms in the biolysis of sewage is not sufficiently well understood to aid in the selection of methods of sewage treatment involving their activities. The presence in bodies of water receiving sewage, of certain plankton which are known to exist only when putrefaction is not imminent, indicates that the body of water into which the discharge of sewage is occurring is not being overtaxed. The control of sewage treatment plant effluents so as to avoid the poisoning of fish life or the contamination of shell fish is likewise important. The study of plankton and macroscopic life in the treatment of sewage is an open field for research.
=216. Variations in the Quality of Sewage.=—The quality of sewage varies with the hour of the day and the season of the year. Some of the causes of these variations are: changes in the amount of diluting water due to the inflow of storm water or flushing of the streets or sewers; variations in domestic activities such as the suspension of contributions of organic wastes during the night, Monday’s wash, etc.; characteristics of different industries which discharge different kinds of wastes according to the stage of the manufacturing process, etc. In general night sewage is markedly weaker than day sewage in both domestic and industrial wastes, but in specific cases the varying strength depends entirely upon the characteristics of the district. Some analyses are given in Table 74, which emphasize these points.
TABLE 74
SEWAGE ANALYSES SHOWING HOURLY, DAILY, AND SEASONAL VARIATIONS IN QUALITY ────────────┬────────────┬─────┬────────┬─────────┬──────────┬───────── Place │ Time │ │ │Suspended│ Remarks │Reference │ Nitrogen │Total│Chlorine│ Matter │ │ ────────────┼────────────┼─────┼────────┼─────────┼──────────┼───────── Marion, Ohio│Mid’t-noon, │ │ │ │Industrial│ 1 │ 5–21–06. │ 45│ 53│ 190│ │ │Noon-mid’t │ │ │ │Domestic │ 1 │ 5–21–06. │ 37│ 94│ 133│ │ Westerville,│Day │ │ │ │college │ 1 Ohio │ │ 10.2│ 76│ 118│ town │ │Night │ 2.6│ 74│ 41│ │ 1 Columbus, │1904–1905 │ │ │ │ │ Ohio │ │ │ │ │ │ │Mid’t to 2 │ │ │ │ │ 2 │ a.m. │ 4.6│ 50│ 131│ │ │2 a.m. to 4 │ │ │ │ │ 2 │ a.m. │ 3.0│ 52│ 95│ │ │4 a.m. to 6 │ │ │ │ │ 2 │ a.m. │ 2.3│ 51│ 83│ │ │6 a.m. to 8 │ │ │ │ │ 2 │ a.m. │ 2.7│ 48│ 83│ │ │8 a.m. to 10│ │ │ │ │ 2 │ a.m. │ 16.3│ 66│ 476│ │ │10 a.m. to │ │ │ │ │ 2 │ noon │ 11.4│ 100│ 324│ │ │Noon to 2 │ │ │ │ │ 2 │ p.m. │ 11.3│ 86│ 246│ │ │2 p.m. to 4 │ │ │ │ │ 2 │ p.m. │ 12.3│ 78│ 246│ │ │4 p.m. to 6 │ │ │ │ │ 2 │ p.m. │ 22.0│ 78│ 368│ │ │6 p.m. to 8 │ │ │ │ │ 2 │ p.m. │ 8.2│ 71│ 209│ │ │8 p.m. to 10│ │ │ │ │ 2 │ p.m. │ 7.8│ 80│ 120│ │ │10 p.m. to │ │ │ │ │ 2 │ mid’t │ 6.2│ 56│ 117│ │ │ │ │ │ │ │ Center Ave.,│Mid’t to 3 │ │ │ │ │ 3 Chicago. │ a.m. │ │ │ 123│ │ │4 a.m. to 7 │ │ │ │ │ 3 │ p.m. │ │ │ 316│ │ │8 a.m. to 11│ │ │ │ │ 3 │ p.m. │ │ │ 608│ │ │Noon to 3 │ │ │ │ │ 3 │ p.m. │ │ │ 785│ │ │4 p.m. to 7 │ │ │ │ │ 3 │ p.m. │ │ │ 717│ │ │8 p.m. to 11│ │ │ │ │ 3 │ p.m. │ │ │ 287│ │ │ │ │ │ │ │ Columbus, │Sunday │ │ │ │ │ 2 Ohio │ │ 6.7│ 55│ 858│ │ │Monday │ 9.1│ 66│ 1048│ │ 2 │Tuesday │ 9.4│ 69│ 1024│ │ 2 │Wednesday │ 9.6│ 68│ 1005│ │ 2 │Thursday │ 9.2│ 66│ 990│ │ 2 │Friday │ 9.2│ 67│ 1018│ │ 2 │Saturday │ 9.3│ 67│ 1016│ │ 2 │ │ │ │ │ │ Baltimore, │Aug. 1 to │ │ │ │ │ 4 1907–1908 │ Sept. 1 │ 16.0│ │ 246│ │ │Sept. 4 to │ │ │ │ │ 4 │ Oct. 3 │ 19.0│ │ 190│ │ │Oct. 6 to │ │ │ │ │ 4 │ Nov. 4 │ 20.0│ │ 188│ │ │Nov. 15 to │ │ │ │ │ 4 │ Nov. 29 │ 20.0│ │ 164│ │ │Dec. 3 to │ │ │ │ │ 4 │ Dec. 29 │ 20.0│ │ 123│ │ │Jan. 6 to │ │ │ │ │ 4 │ Jan. 21 │ 19.0│ │ 127│ │ │Feb. 2 to │ │ │ │ │ 4 │ Feb. 26 │ 20.0│ │ 149│ │ │Feb. 29 to │ │ │ │ │ 4 │ Mar. 24 │ 28.0│ │ 274│ │ │Mar. 27 to │ │ │ │ │ 4 │ April 29 │ 25.0│ │ 165│ │ │April 30 to │ │ │ │ │ 4 │ May 26 │ 19.0│ │ 104│ │ │June 8 to │ │ │ │ │ 4 │ July 11 │ 15.0│ │ 88│ │ │July 13 to │ │ │ │ │ 4 │ Aug. 8 │ 9.5│ │ 124│ │ ────────────┴────────────┴─────┴────────┴─────────┴──────────┴─────────
References:
1. 1908 Report of the Ohio State Board of Health.
2. Report on Sewage Purification at Columbus, Ohio, by G. A. Johnson, 1905.
3. Report on Industrial Wastes from the Stock Yards and Packingtown in Chicago, by the Sanitary District of Chicago. 1921.
4. Report of the Baltimore Sewerage Commission, 1911.
=217. Sewage Disposal.=—Previous to the development of the water-carriage method for removing human excreta and other liquid wastes the solid matter was disposed of by burial and the liquid wastes were allowed to seep into the ground or to run away over its surface. Following the development of the water-carriage system, which necessitated the development of sewers, the problem of ultimate disposal was rendered more serious by the concentration of human excreta together with a large volume of water. The unthinking citizen believes the problem of sewage disposal is solved when the toilet is flushed or the bath tub is drained. The problem may more truly be said to commence at this point.
It would appear that the simplest method of disposal of sewage would be to discharge it into the nearest water course. Unfortunately the nature of sewage is such that it may be either highly offensive to the senses or dangerous to health or both, when discharged in this manner. Only the most fortunate communities are favored with a body of water of sufficient size to receive sewage without creating a nuisance.
The problems of sewage disposal are to prevent nuisances causing offense to sight and smell; to prevent the clogging of channels; to protect pumping machinery; to protect public water supplies; to protect fish life; to prevent the contamination of shell fish; to recover valuable constituents of the sewage; to enrich and to irrigate the soil; to safeguard bathing and boating; for other minor purposes; and in some cases to comply with the law. Sewage may be treated to attain one or more of these objects by methods of treatment varying as widely as the objects to be attained.
=218. Methods of Sewage Treatment.=—In studying the subject of sewage treatment it must be borne in mind that it is impossible to destroy any of the elements present. They may be removed from the mixture only by gasification, straining or sedimentation. Their chemical combinations may be so changed, however, as to result in different substances than those introduced to the treatment plant. It is with these chemical changes that the student of sewage treatment is interested.
The methods of sewage treatment can be classified as mechanical, chemical and biological. These classifications are not separated by rigid lines but may overlap in certain treatment devices or methods. Mechanical methods of treatment are exemplified by sedimentation, and screening. Chemical precipitation and sterilization are examples of chemical methods. The biological methods, the most important of all, include dilution, septicization, filtration, sewage farming, activated sludge, etc. If for any reason it is desired to treat sewage by more than one of these methods the procedure should follow as nearly as possible the order of the occurrence of the phenomena in the natural biolysis of sewage. For example, in one treatment plant the sewage would first pass through a grit chamber where the coarse sediment would be removed, then through a screen where the floating matter and coarse suspended matter would be removed, then to a sedimentation basin where some finer suspended matter might settle out, then to a digestive tank where the solid matter deposited would be worked upon by bacterial action and partially liquefied. Simultaneous to the liquefaction of the deposited solid matter the liquid effluent from the digestive tank might proceed to an aërating device to expedite oxidation, then to an aërobic filter, and finally to disposal by dilution.
XIV. DISPOSAL BY DILUTION
=219. Definition.=—Disposal of sewage by dilution is the discharge of raw sewage or the effluent from a treatment plant into a body of water of sufficient size to prevent offense to the senses of sight and smell, and to avoid danger to the public health.
=220. Conditions Required for Success.=—Among the desired conditions for successful disposal by dilution are: adequate currents to prevent sedimentation and to carry the sewage away from all habitations before putrefaction sets in, or sufficient diluting water high in dissolved oxygen to prevent putrefaction; a fresh or non-septic sewage; absence of floating or rapidly settling solids, grease or oil; and absence of back eddies or quiet pools favorable to sedimentation in the stream into which disposal is taking place. The conditions which should be prevented are: offensive odors due to sludge banks, the rise of septic gases, and unsightly floating or suspended matter. In some instances the pollution of the receiving body of water is undesirable and the sewage must be freed from pathogenic organisms and the danger of aftergrowths minimized before disposal. Such conditions are typified at Baltimore, where the sewage is discharged into Back Bay, an arm of Chesapeake Bay. One of the important industries of the state of Maryland is the cultivation of oysters. The pollution of the Bay was therefore so objectionable that careful treatment of the Baltimore sewage has been a necessary preliminary to final disposal by dilution. It is unwise to draw public water supplies, without treatment, from a stream receiving a sewage effluent, no matter how careful or thorough the treatment of the sewage. The treatment of the sewage is a safeguard, and lightens the load on the water purification plant, but under no considerations can it be depended upon to protect the community consuming the diluted effluent.
The sewer outlet should be located well out in the current of the stream, lake, or harbor. Deeply submerged outlets are usually better than an outlet at the surface, as a better mixture of the sewage and water is obtained. The discharge of sewage into a body of water of which the surface level changes, alternately covering and exposing large areas of the bottom is unwise, as the sludge which is deposited during inundation will cause offensive odors when uncovered. Such conditions must be carefully guarded against when selecting a point of disposal in tidal estuaries because of the frequent fluctuations in level.
=221. Self-Purification of Running Streams.=—The self-purification of running streams is due to dilution, sedimentation, and oxidation. The action is physical, chemical, and biological. When putrescible organic matter is discharged into water the offensive character of the organic matter is minimized by dilution. If the dilution is sufficiently great, it alone may be sufficient to prevent all nuisance. The oxidation of the organic matter commences immediately on its discharge into the diluting water due to the growth and activity of nitrifying and other oxidizing organisms and to a slight degree to direct chemical reaction. So long as there is sufficient oxygen present in the water septic conditions will not exist and offensive odors will be absent. When the organic matter is completely nitrified or oxidized there will be no further demand on the oxygen content of the stream and the stream will be said to have purified itself. At the same time that this oxidation is going on some of the organic matter will be settling due to the action of sedimentation. If oxidation is completed before the matter has settled on the bottom the result will be an inoffensive silting up of the river. If oxidation is not complete, however, the result will be offensive putrefying sludge banks which may send their stinks up through the superimposed layers of clean water to pollute the surrounding atmosphere.
The most important condition for the successful self-purification of a stream is an initial quantity of dissolved oxygen to oxidize all of the organic matter contributed to it, or the addition of sufficient oxygen subsequent to the contribution of sewage to complete the oxidation. Oxygen may be added through the dilution received from tributaries, through aëration over falls and rapids, or by quiescent absorption from the atmosphere. The rapidity of self-purification is dependent on the character of the organic matter, the presence of available oxygen, the rate of reaëration, temperature, sedimentation, and the velocity of the current. Sluggish streams are more likely to purify themselves in a shorter distance and rapidly flowing turbulent streams are more likely to purify themselves in a shorter time, other conditions being equal. Although the absorption of oxygen by a stream whose surface is broken is more rapid than through a smooth unbroken surface, the growth of algæ, biological activity, the effect of sunlight, and sedimentation are more potent factors and have a greater effect in sluggish streams than the slightly more rapid absorption of oxygen in a turbulent stream. It is frequently more advantageous to discharge sewage into a swiftly moving stream, however, regardless of the conditions of self-purification, as the undesirable conditions which may result occur far from the point of disposal and may be offensive to no one.
The sewage from a population of about 3,000,000 persons residing in and about Chicago is discharged into the Chicago Drainage Canal. It ultimately reaches tide water through the Des Plaines, the Illinois, and the Mississippi Rivers. The action occurring in these channels is one of the best illustrations known of the self-purification of a stream. In Table 75 are shown the results of analyses of samples taken at various points below the mouth of the Chicago River where the diluting water from Lake Michigan enters, to Grafton, Illinois, at the junction of the Illinois and Mississippi Rivers about 40 miles above St. Louis. The effect of the physical characteristics of the stream on its chemical composition is well illustrated in this table. The rise in the chlorine content between Lake Michigan and the entrance to the Drainage Canal is a measure of the addition of sewage. Since the chlorine is an inorganic substance which is not affected by biologic action, its loss in concentration in the lower reaches of the rivers is due to dilution by tributaries and sedimentation, e.g., between the end of the canal at Lockport and the sampling point at Joliet, the entrance of the Des Plaines River reduces the concentration of chlorine from 124.5 to 41.5 parts per million. The entrance of the Kankakee River at Dresden Heights further reduces the chlorine to 24.5 p.p.m. The increase of albuminoid and ammonia nitrogen accompanied by a decrease in nitrites and nitrates, between the upper end of the canal at Bridgeport and its lower end at Lockport indicates the reducing action proceeding therein. The oxidizing action over the various dams and the effect of dilution with water containing oxygen is shown between miles 34 and 38, at mile 79, and at mile 294. The excellent effect of quiescent sedimentation and aëration in Peoria Lakes is shown between miles 145, 161 and 165.
TABLE 75
ANALYSES OF CHICAGO, DES PLAINES AND ILLINOIS RIVERS
(Parts per million)
────────────┬────────┬────────────────────────────────────────────── Sampling │Distance│January-June, 1900, from “Sewage Disposal,” by Point │in Miles│ Kinnicutt, Winslow and Pratt │ from │ │ Lake │ │Michigan│ ────────────┼────────┼────────┬────────┬──────────┬────────┬──────── │ │Chlorine│ Ammonia│Albuminoid│Nitrates│Nitrates │ │ │Nitrogen│ Nitrogen│ │ │ │ │ │ │ │ ────────────┼────────┼────────┼────────┼──────────┼────────┼──────── Lake │ 0│ 3.0│ 0.03│ 60.13│ 0.002│ 0.008 Michigan │ │ │ │ │ │ │ │ │ │ │ │ Canal, │ 5│ 96.6│ 8.05│ 2.05│ .021│ .074 Bridgeport│ │ │ │ │ │ Canal, │ 34│ 124.5│ 10.90│ 2.07│ .013│ .066 Lockport │ │ │ │ │ │ Joliet │ 38│ 41.5│ 4.22│ 0.83│ .021│ .086 │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ Dresden │ 52│ │ │ │ │ Heights │ │ │ │ │ │ Dresden │ 52│ │ │ │ │ Heights │ │ │ │ │ │ Morris │ 62│ 24.5│ 2.46│ .60│ .075│ .424 │ │ │ │ │ │ Marseilles │ 79│ │ │ │ │ Marseilles │ 79│ │ │ │ │ Ottawa │ 85│ 15.3│ 1.55│ .41│ .197│ .966 La Salle │ 100│ 17.5│ 1.05│ .43│ .109│ .979 Henry │ 129│ 13.3│ .92│ .38│ .102│ .800 Chillicothe │ 145│ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ Averyville │ 161│ 13.5│ .81│ .37│ .004│ 1.150 │ │ │ │ │ │ │ │ │ │ │ │ Wesley │ 165│ 12.0│ .57│ .41│ .083│ 1.03 │ │ │ │ │ │ Pekin │ 175│ 12.3│ .70│ .43│ .060│ .990 Havana │ 205│ 11.2│ .60│ .36│ .065│ .570 Beardstown │ 237│ 10.7│ .69│ .44│ .106│ .685 La Grange │ 249│ │ │ │ │ Kampsville │ 294│ 11.3│ .66│ .44│ .044│ .870 Kampsville │ 294│ │ │ │ │ Grafton │ 325│ 9.8│ .46│ .42│ .031│ 1.06 │ │ │ │ │ │ Grafton │ 325│ │ │ │ │ │ │ │ │ │ │ ────────────┴────────┴────────┴────────┴──────────┴────────┴────────
────────────┬─────────────────────┬─────────── Sampling │ Dissolved Oxygen │ Remarks Point │ │ │ │ │ │ │ │ ────────────┼───────┬──────┬──────┼─────────── │ Jan. │ July │ Nov. │ │30–Feb.│ 8–15 │12–19,│ │2, 1912│ 1912 │ 1912 │ ────────────┼───────┼──────┼──────┼─────────── Lake │ 14.1│ │ 10.8│Typical Michigan │ │ │ │ chemical │ │ │ │ analysis Canal, │ │ │ 6.9│Kedzie Bridgeport│ │ │ │ Avenue Canal, │ 9.9│ │ 1.7│Above dam Lockport │ │ │ │ Joliet │ │ 1.4│ 5.6│Aëration │ │ │ │ over dam. │ │ │ │ Dilution │ │ │ │by Des │ │ │ │ Plaines │ │ │ │ River Dresden │ │ 1.0│ 4.1│Des Plaines Heights │ │ │ │ River Dresden │ │ │ 10.4│Kankakee Heights │ │ │ │ River Morris │ 7.8│ │ 5.7│Illinois │ │ │ │ River Marseilles │ 5.7│ 0.6│ 6.8│Above dam Marseilles │ 8.2│ 4.5│ 9.3│Below dam Ottawa │ 10.0│ │ 8.1│ La Salle │ 5.4│ │ 7.8│ Henry │ │ │ 7.9│ Chillicothe │ 3.4│ 1.5│ 5.9│Above │ │ │ │ Peoria │ │ │ │ Lakes Averyville │ 3.3│ 8.2│ 8.9│Below │ │ │ │ Peoria │ │ │ │ Lakes Wesley │ │ │ 7.1│Below │ │ │ │ Peoria Pekin │ 4.9│ 3.2│ 8.9│ Havana │ 4.8│ │ 8.8│ Beardstown │ 6.5│ │ 9.1│ La Grange │ │ 4.1│ 9.4│Below dam Kampsville │ │ 4.1│ 10.0│Above dam Kampsville │ │ 4.6│ 10.0│Below dam Grafton │ 6.6│ 4.7│ 10.4│Illinois │ │ │ │ River Grafton │ │ 7.3│ 12.0│Mississippi │ │ │ │ River ────────────┴───────┴──────┴──────┴───────────
=222. Self-Purification of Lakes.=—Sewage may be disposed of into lakes with as great success as into running streams if conditions exist which are favorable to self-purification. Lakes and rivers purify themselves from the same causes; oxidation, sedimentation, etc., but in the former the currents are much less pronounced and may be entirely absent. In shallow lakes (20 feet or less in depth) dependence must be placed on horizontal currents and the stirring action of the wind to keep the water in motion in order that the sewage and the diluting water may be mixed. In deeper bodies of water, currents induced by the wind are helpful but entire dependence need not be placed upon them. Vertical currents, and the seasonal turnovers in the spring and fall completely mix the waters of the lake above those layers of water whose temperature never rises higher than 4° C.
In the early winter the cold air cools the surface waters of a lake. The cooling increases the density of the surface water causing it to sink, and allowing the warmer layers below to rise and become cooled. After the temperature of the entire lake has reached 4° C. the vertical currents induced by temperature cease, as continued cooling decreases the density of the surface water maintaining the same layer at the surface. In the spring as the temperature of the surface water rises to 4° C. and above it becomes heavier and drops through the colder water below causing vertical currents. These phenomena are known as the fall and spring turnovers. The former is more pronounced. These turnovers are effective in assisting in the self-purification of lakes.
=223. Dilution in Salt Water.=—The oxygen content in salt water is about 20 per cent less than in fresh water at the same temperature. The greater content of matter in solution in salt water reduces its capacity to absorb many sewage solids. This, together with the chemical reaction between the constituents of the salt water and those of the sewage serve to precipitate some of the sewage solids and to form offensive sludge banks. The evidence of the action which takes place in the absorption of oxygen from the atmosphere by salt water and its effect on dissolved sewage solids is conflicting, but in general fresh water is a better diluting medium than salt water.
Black and Phelps have made valuable studies of the relative rates of absorption of oxygen from the air by fresh and salt water. The results of their experiments are published in a Report to the Board of Estimate and Apportionment of N. Y. City, made March 23, 1911.[131] Concerning these rates they conclude:
Therefore there is no reason to believe that the reaëration of salt water follows any other laws than those we have determined mathematically and experimentally for fresh water. In the absence of fuller information on the effect of increased viscosity upon the diffusion coefficient, it can only be stated that the rate of reaëration of salt water is less than that of fresh water, in proportion to the respective solubilities of oxygen in the two waters, and still less, but to an unknown extent, by reason of the greater viscosity and consequent small value of the diffusion coefficient.
=224. Quantity of Diluting Water Needed.=—In a large majority of the problems of disposal of sewage by dilution it is not necessary to add sufficient diluting water to oxidize completely all organic matter present. Ordinarily it is sufficient to prevent putrefactive conditions until the flow of the stream, lake, or tidal current, has reached some large body of diluting water or where putrefaction is no longer a nuisance. It is never desirable to allow the oxygen content of a stream to be exhausted as putrescible conditions will exist locally before exhaustion is complete. The exact point to which oxygen can be reduced in safety is in some dispute. Black and Phelps have assumed 70 per cent of saturation as the allowable limit; Fuller has placed it at 30 per cent; Kinnicutt, Winslow, and Pratt have placed it at 50 per cent. Since the reaction between the oxygen and the organic matter is quantitative, others have placed the limit in terms of parts per million of oxygen. Wisner,[132] has recommended a minimum of 2.5 p.p.m. as the limit for the sustenance of fish life, which is not far from Fuller’s limit for hot-weather conditions.
Formulas of various types have been devised to express the rate of absorption of oxygen with a given quantity of diluting water which is mixed with a given quantity and quality of sewage. The quantity of sewage is sometimes expressed in terms of the tributary population or in other ways. Knowing the rate at which oxygen is exhausted and the velocity of flow of the stream, the point at which the oxygen will be reduced to the limit allowed is easily determined. The accuracy of none of these formulas has been proven, and their use, without an understanding of the effect of local conditions, may lead to error. They may be used as a check on the bio-chemical oxygen demand determinations, which should be conclusive.
The following formula, based on the work of Black and Phelps, is a guide to the amount of sewage which can be added to a stream without causing a nuisance. It is:
C = (log(O′⁄O))⁄kt,
in which C = per cent of sewage allowed in the water;
O′ = per cent of saturation or the p.p.m. of oxygen in the mixture at the time of dilution;
O = per cent of saturation or the p.p.m. of oxygen in the stream after period of flow to point beyond which no nuisance can be expected;
t = time in hours required for the stream to flow to this point;
k = constant determined by test determinations of the factors in the following expression:
k = (log(_O′__{1}⁄_O__{1}))⁄_C__{1}_t__{1},
in which _O′__{1} = per cent of saturation or the p.p.m. of oxygen in the diluting water before mixing with the sewage;
_O__{1} = per cent of saturation or the p.p.m. of oxygen in an artificial mixture made in the laboratory, after _t__{1} hours of incubation;
_C__{1} = per cent of sewage in the mixture;
_t__{1} = number of hours of incubation of the mixture of sewage and diluting water under laboratory conditions.
In the solution of these formulas it is desired to determine the permissible amount of sewage to discharge into a given quantity of diluting water. This value is expressed by C in the first equation. In solving this equation:
O′ is determined by laboratory tests and should represent the conditions to be expected during various seasons of the year;
O is determined by judgment. It may be 30 per cent or 50 per cent or more as previously explained;
t is determined by float tests or other measurements of the stream flow;
k is determined by laboratory tests in which mixtures of various strengths are incubated for various periods of time. Different values of k will be obtained for different characteristics of the sewage; but for the same sewage the value of k should be unchanged for different periods of incubation.
Rideal devised the formula:[133]
XO = C(M − N)S
in which X = flow of the stream expressed in second-feet;
O = grams of free oxygen in one cubic foot of water;
S = rate of sewage discharge in second-feet;
M = grams of oxygen required to consume the organic matter in one cubic foot of diluted sewage as determined by the permanganate test with 4 hours boiling;
N = grams of oxygen available in the nitrites and nitrates in one cubic foot of diluted sewage;
C = ratio between the amount of oxygen in the stream and that required to prevent putrefaction. Where C is equal to or greater than one, satisfactory conditions have been attained.
In using this formula it is necessary to make analyses of trial mixtures of sewage and water until the correct mixture has been found.
Hazen’s formula is:[134]
D = x⁄S = 4_m_⁄O,
in which D = dilution ratio;
x = volume of water;
S = volume of sewage;
m = result of the oxygen consumed test expressed in p.p.m. after 5 minutes, boiling with potassium permanganate;
O = amount of dissolved oxygen in the diluting water expressed in p.p.m.
For comparison with Rideal’s formula the factor of 7 should be used instead of 4 to allow for the increased time of boiling.
Since the amount of oxygen needed is dependent on the amount of organic matter in the sewage rather than the total volume of the sewage, and since the amount of organic matter is closely proportional to the population, the amount of diluting water has sometimes been expressed in terms of the population. Hering’s recommendation for the quantity of diluting water necessary for Chicago sewage was 3.3 cubic feet of water per second per thousand population. Experience has proven this to be too small. Between a minimum limit of 2 second-feet and a maximum of 8 second-feet of diluting water per thousand population the success of dilution is uncertain. Above this limit success is practically assured and below this limit failure can be expected.
Even with these carefully devised formulas and empirical guides, the factors of reaëration, dilution, sedimentation, temperature, etc., may have so great an effect as to vitiate the conclusions. As shown in Table 75 dilution in winter is far more successful than in summer. The lower temperatures so reduce the activity of the putrefying organisms that consumption of oxygen is greatly retarded.
=225. Governmental Control.=—A comprehensive discussion of the legal principles governing the pollution of inland waters is contained in “A Review of the Laws Forbidding the Pollution of Inland Waters,” by E. B. Goodell, published by the United States Geological Survey in 1905, as Water Supply Paper No. 152.
The disposal of sewage by dilution is subject to statutory limitations in many states. The enforcement of these laws is usually in the hands of the state board of health, which is frequently given discretionary powers to recommend and sometimes to enforce measures for the abatement of an actual or potential nuisance. Such recommendations usually take the form of a specification of certain forms of treatment preliminary to disposal by dilution. No project for the disposal of sewage by dilution should be consummated until the local, state, national, and in the case of boundary waters, international laws have been complied with. The attitude of the courts in different states has not been uniform. Little guidance can be taken from the personal feeling of the persons immediately interested. The opinion of the riparian owner 5 miles down stream may differ materially from the popular will of the voters of a city, and it is likely to receive a more favorable hearing from the court. Statutes and legal precedents are the safest guides.
=226. Preliminary Treatment.=—If the sewage to be disposed of by dilution contains unsightly floating matter, oil, or grease, no amount of oxygen in the diluting water will prevent a nuisance to sight, or the formation of putrefying sludge banks. Under such conditions it will be necessary to introduce screens or sedimentation basins, or both, in order to remove the floating and the settling solids. Biologic tanks, filtration, or other methods of treatment may be necessary for the removal of other undesirable constituents.
=227. Preliminary Investigations.=—Before adopting disposal of sewage by dilution without preliminary treatment, or before considering the proper form of treatment necessary to render disposal by dilution successful, a study should be made of the character of the body of water into which the sewage or effluent is to be discharged. This study should include: measurements of the quantity of water available at all seasons of the year; analyses of the diluting water to determine particularly the available dissolved oxygen; observations of the velocity and direction of currents, and the effect of winds thereon; a study of the effect on public water supplies, bathing beaches, fish life, etc. Good judgment, aided by the proper interpretation of such information should lead to the most desirable location for the sewer outlet. If preliminary treatment is found to be necessary tests should be made to determine the necessary extent and thoroughness of the treatment.
XV. SCREENING AND SEDIMENTATION
=228. Purpose.=—The first step in the treatment of sewage is usually that of coarse screening in order to remove the larger particles of floating or suspended matter. Screens and sedimentation basins are used to prevent the clogging of sewers, channels, and treatment plants; to avoid clogging of and injuries to machinery; to overcome the accumulation of putrefying sludge banks; to minimize the absorption of oxygen in diluting water; and to intercept unsightly floating matter.
By the plain sedimentation of sewage is meant the removal of suspended matter by quiescent subsidence unaffected by septic action or the addition of chemicals or other precipitants. In order to prevent septic action plain sedimentation tanks must be cleaned as frequently as once or twice a week in warm weather but not quite so often in cold weather.
Fine screening may take the place of sedimentation where insufficient space is available for sedimentation tanks, and it is desired to remove only a small portion of the suspended matter. Recent American practice has tended to restrict the field of fine screening to treatment requiring less than 10 per cent removal of suspended matter, thus eliminating screens from the field covered by plain sedimentation tanks. The practice is well expressed by Potter, who states:[135]
Where a high degree of purification is sought, the use of fine screens is of doubtful value. A modern settling tank will give better results and at a less cost for a given degree of purification. A settled liquid is also superior to a screened liquid for subsequent biological treatment in filters.... Again the storing of large quantities of screenings must necessarily be more objectionable than the storing of the digested sludge of a modern settling tank.
=229. Types of Screens.=—The definitions of some types of screens as proposed by the American Public Health Association follow: A bar screen is composed of parallel bars or rods. A mesh screen is composed of a fabric, usually wire. A grating consists of 2 sets of parallel bars in the same plane in sets intersecting at right angles. A band screen consists of an endless perforated band or belt which passes over upper and lower rollers. A perforated plate screen is made of an endless band of perforated plates similar to a band screen. A wing screen has radial vanes uniformly spaced which rotate on a horizontal axis. A disc screen consists of a circular perforated disc with or without a central truncated cone of similar material mounted in the center. The Reinsch Wurl screen is the best known type of disc screen. A cage screen[136] consists of a rectangular box made up of parallel bars with the upstream side of the box or cage omitted. Allen[137] gives the following definitions: A drum screen is a cylinder or cone of perforated plates or wire mesh which rotates on a horizontal axis. A shovel vane screen is similar to a wing screen with semicircular wings and a different method of removing the screenings. Examples of a band screen, a wing screen, a shovel vane screen, a drum screen and a disc screen are shown in Fig. 150. A bar screen is shown in Fig. 151 and a cage screen is shown in Fig. 152.
Screens can be classed as fixed, movable, or moving. Fixed screens are permanently set in position and must be cleaned by rakes or teeth that are pulled between the bars. Movable screens are stationary when in operation, but are lifted from the sewage for the purpose of cleaning. Moving screens are in continuous motion when in operation and are cleaned while in motion. Fixed bar screens may be set either vertical, inclined, or horizontal.
Movable screens with a cage or box at the bottom are sometimes used. The box should be of solid material to prevent the forcing of screenings through it when the screen is being raised for cleaning. A mesh screen should be used only under special circumstances because of the difficulty in cleaning. Screens which must be raised from the sewage for cleaning should be arranged in pairs in order that one may be working when the other is being cleaned. Movable screens are undesirable for small plants because the labor involved in raising and lowering is greater than in cleaning with a rake and the screens are more likely to be neglected. In a large plant rakes operated by hand are too small for cleaning the screens. A fixed screen is sometimes used with moving teeth fastened to endless chains. The teeth pass between the parallel bars and comb out the screenings. If the screen chamber in a small plant is too deep for accessibility a movable cage or box screen may be desirable.
Moving screens are generally of fine mesh or perforated plates. They are kept moving in order to allow continuous cleaning. They are cleaned by brushes or by jets of air, water, or steam.
=230. Sizes of Openings.=—The area or size of the opening of a screen is dependent upon the character of the sewage to be treated and upon the object to be attained.
Large screens, with openings between 1½ inches and 6 inches are used to protect centrifugal pumps, tanks, automatic dosing devices, conduits, and gate valves from large objects such as pieces of timber, dead animals, etc., which are found in sewage. The quantity of material removed is variable, and is usually small.
Medium-size screens with openings from ¼ inch to 1½ inches are used to prepare sewage for passage through reciprocating pumps, complex dosing apparatus, contact beds, and sand filters. The amount of material removed varies from 0.5 to 10 cubic feet per million gallons of sewage treated, dependent on the character of the sewage and the size of the screen. Screenings before drying contain 75 to 90 per cent moisture and weigh 40 to 50 pounds per cubic foot. At times the amount removed may vary widely from the limits stated. Schaetzle and Davis[138] state:
Screenings differ greatly both in amount and character.... The amount varies with the days of the week as well as during the course of the day. It reaches its maximum about noon or shortly before and commences to disappear about midnight, reaching a minimum about 4 or 5 a.m. The material is almost wholly organic and consists of scraps of vegetables or fruit, cloth, hair, wood, paper and lumps of fecal matter. The amount varies so widely that it is impossible to state just what to expect any definite size screen to remove. The amount of water contained is small compared with that in the sludge in sedimentation basins and amounts to from 70 per cent to 80 per cent. On account of its organic origin it is highly putrescible.
Medium-size screens are sometimes placed close together with the bars of the one opposite the openings in the other, thus approaching a fine screen.
Fine screens vary in size of opening from ¼ inch to 50 openings per linear inch or 2,500 per square inch. They are used for removing solids preparatory to disposal by dilution, to protect sprinkling filters, complex dosing apparatus, sand filters, sewage farms, and to prevent the formation of scum in subsequent tank treatment. In general, fine screens will remove from 0.1 to 1 cubic yard of wet material per million gallons of sewage treated. The wet screenings will contain about 75 per cent moisture and will weigh about 60 pounds per cubic foot. The dry weight of the screenings will therefore be about 10 to 400 pounds per million gallons of sewage treated. The effect of the removal of this amount of material is usually not detectable by methods of chemical analysis, the amount of suspended matter before and after screening being found unchanged.
In his conclusions on the discussion of the results to be expected from fine screens, Allen states:[139]
With openings not more than 0.1 inch in size, fine screening should remove at least 30 per cent of the suspended solids and 20 per cent of the suspended organic solids from ordinary domestic sewage, or 0.1 cubic yard of screenings, containing 75 per cent water per thousand population daily.
The effect of the use of different size openings under the same conditions is shown in Fig. 153.[140] Some data covering the amount of material removed by screening are given in Table 76. More extensive data are given in Volume III of “American Sewerage Practice” by Metcalf and Eddy.
TABLE 76
DATA ON SCREENS
(Trans. Am. Society Civil Engineers, Vol. 78, Page 942)
───────┬──────────┬──────────┬────────────────────────── Type of│ Location │ Clear │ Screenings Screen │ │ Opening, │ │ │in Inches │ │ │ │ │ │ │ │ │ │ ───────┼──────────┼──────────┼────────────┬───────────── │ │ │Per Million │ Per 1000 │ │ │ Gallons, │ Population │ │ │y = Cubic │ Daily, │ │ │ Yard │ y = Cubic │ │ │ t = Tons │ Yard │ │ │ │ t = Tons ───────┼──────────┼──────────┼────────────┼───────────── Band │Hamburg │ 0.6 │ 0.34_y_ │ 0.018_y_ │Göttingen │ 0.4 │ 0.35_y_ │ 0.026_y_ │Sutton │0.375[141]│ 0.6_y_ │ │Chicago │ │ 2.4–3.1_t_ │ Wing │Frankfort │ 0.40 │ 0.7_y_ │ 0.040_y_ │Elberfeld │ 0.40 │ 1.15_y_ │ 0.053_y_ │Stralsund │ 0.20 │ │ 0.079_y_ │Wiesbaden │ 0.60 │ 1.1_y_ │ 0.033_y_ Shovel │Strassburg│ 0.10 │ 1.6_y_ │ 0.043_y_ vane │ │ │ │ │Gleiwitz │ 0.12 │ │ 0.192_y_ │Temesvar │ 0.12 │ 0.9–1.7_y_ │0.067–.133_y_ Drum │Bromberg │ 0.08 │ 4.75_t_ │ │Mainz │ Note 6 │ 0.52_y_ │ │Trier │ 0.10 │0.39–0.42_y_│ 0.13_y_ │Osnabruck │ 0.08 │ 3.2–4.0_y_ │ 0.08–.10_y_ Weand │Reading, │ 36[141] │ 1.0_y_ │ │ Pa. │ │ │ │Brockton │ 36[141] │ 1.4_t_ │ Reinsch│Dresden │ 0.08 │ 0.97_t_ │ 0.09_y_ Wurl │ │ │ │ ───────┴──────────┴──────────┴────────────┴─────────────
───────┬────────┬───────────┬─────────┬──────────── Type of│Per Cent│Horse-Power│ Cost of │ Remarks Screen │Moisture│Per Screen │Operation│ │ │ │ Per │ │ │ │ Million │ │ │ │Gallons, │ │ │ │ Dollars │ ───────┼────────┼───────────┼─────────┼──────────── │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ ───────┼────────┼───────────┼─────────┼──────────── Band │ 87 │ 2.5 │ │Note 1 │ │ 2.0 │ │ │ │ │ │ │ 79 │ │ │Stock Yard Wing │ │ 5.0 │ │Note 2 │ 75 │ │ │Note 3 │ │ 4.5 │ │ │ │hand power │ 1.64 │Note 4 Shovel │ 89.3 │ 3.35 │ │Note 5 vane │ │ │ │ │ │ │ 0.90 │ │ 60–70 │ │ small │ Drum │ 40–60 │ │ 2.45 │Experimental │ 75 │ 5.2–6.8 │0.89–3.42│ │ 50–60 │ │ 2.41 │Experimental │ │ 9.00 │ │Note 7 Weand │ 89.5 │ 2.0 │ 1.00± │ │ │ │ │ │ │ │ │ Reinsch│ 84 │ 2.5 │.325–1.76│ Wurl │ │ │ │ ───────┴────────┴───────────┴─────────┴────────────
Notes:—1. After removal of ½ this volume of grit.
2. After removal of 16 per cent by the grit chamber.
3. Including 0.6 cubic yard grit per million gallons.
4. After passing 1.6 inch bar screen.
5. After removal of 0.132 cubic yard grit and coarse screenings per 1000 population.
6. 0.12, 0.04–0.08.
7. Before removal of 0.4 cubic yard grit per million gallons.
=231. Design of Fixed and Movable Screens.=—The determination of the size of the opening is the first step in the design of a sewage screen. This is followed by the computation of the net area of openings in the screen. The final steps are the determination of the overall dimensions of the screen; the size of the bar, wire, or support; and the dimensions of the screen chamber. The net area of openings is fixed by the permissible velocity of flow through the screen and the quantity of sewage to be treated. In determining the velocity of flow the general principle should be followed that the velocity should not be reduced sufficiently to allow sedimentation in the screen chamber. The velocity of grit bearing sewage in passing through coarse screens should not be reduced below 2 or 3 feet per second. If the sewage contains no grit, or the screen is placed below a grit chamber the velocity through a medium or fine screen should be from ½ to 1½ feet per minute. The velocity through the screen in a direction normal to the plane of the screen can be reduced without reducing the horizontal velocity of the sewage by placing the screen in a sloping position.
The final steps are the design of the screen bar and the determination of the dimensions of the screen and of the screen chamber. The size of the bar in a bar screen, or as a support to a wire mesh, is dependent on the unsupported length of the bar. The stresses in the bars are the results of impact and bending, caused by cleaning, and of the load due to the backing up of the sewage when the screen is clogged. Allowance should be made for a head of 2 or 3 feet of sewage against the screen. A generous allowance should be made in addition for the indeterminate stresses due to cleaning. The screen should be supported only at the top and bottom, as intermediate supports in a bar screen are undesirable unless they are so arranged as not to interfere with the teeth of the cleaning devices.
Fixed screens should be placed at an angle between 30° and 60° with the horizontal, with the direction of slope such that the screenings are caught on the upper portion of the screen. A small slope is desirable in order to obtain a low velocity through the screen. The slope is limited since the smaller the slope the longer the bars of the screen and the greater the difficulty of hand cleaning. Small slopes will tend to make the screens self cleaning. As the screen clogs, the increasing head of sewage will push the accumulated screenings up the screen. The use of flat screens in a vertical position is not desirable because of the difficulty of cleaning and the accumulation of material at inaccessible points. If a flat screen is placed in a horizontal position with the flow of sewage downward difficulties are encountered in cleaning and solid matter is forced through the screen as clogging increases. An upward flow through a horizontal screen is undesirable as the material is caught in a position inaccessible for cleaning. Movable screens are more easily handled when placed in a vertical position.
In the construction of small screens, round bars are sometimes used where the unsupported length of the bar is less than 3 or 4 feet. They are not recommended, however, as the efficient area and the amount of material removed by the screen are diminished. Bars which produce openings with the larger end upstream are undesirable as particles become wedged in the screen, and are either forced through or become difficult to remove.[142] Rectangular bars are easily obtained and give satisfactory service except where they are of insufficient strength laterally. For greater lateral thickness a pear-shaped bar is sometimes used, with the thicker side upstream. Fine mesh screens or perforated plates are supported on grids or parallel bars of stronger material designed to take up the heavy stresses on the screen.
The dimensions of the bar may be selected arbitrarily. The length and width of the screen are fixed to give desirable dimensions to the screen chamber and to give the necessary net opening in the screen. The width of the screen chamber and the screen should be the same. The screen chamber should be sufficiently long to prevent swirling and eddying around the screen. If the dimensions thus fixed permit an undesirable, velocity in the screen chamber they should be changed. A sufficient length of screen should be allowed to project above the sewage for the accumulation of screenings. The bars may be carried up and bent over at the top as shown in Fig. 151 to simplify the removal of screenings.
Coarse screens are usually placed above all other portions of a treatment plant. They may be followed by grit chambers or finer screens. Coarse screens are occasionally placed as a protection above medium or fine screens. In sewage containing grit the smaller screens are sometimes placed below the grit chamber. It is desirable to provide some means of diverting the sewage from a screen chamber to allow of repairs to the screen and the cleaning of the chamber. Screen chambers are sometimes designed in duplicate to allow for the cleaning of one while the other is operating.
PLAIN SEDIMENTATION
=232. Theory of Sedimentation.=—Sedimentation takes place in sewage because some particles of suspended matter have a greater specific gravity than that of water. All particles do not settle at the same rate. Since the weights of particles vary as the cubes of their diameters, whereas the surface areas (upon which the action of the water takes place) vary only as the squares of the diameters, the amount of the skin friction on small particles is proportionally greater than that on large particles, because of the relatively greater surface area compared to their weight. As a result the smaller particles settle more slowly. The velocity of sedimentation of large particles has been found to vary about as the diameter and of small particles as the square root of the diameter. The change takes place at a size of about 0.01 mm.
Sedimentation is accomplished by so retarding the velocity of flow of a liquid that the settling particles will be given the opportunity to settle out. The slowing down of the velocity is accomplished by passing the sewage through a chamber of greater cross-sectional area than the conduit from which it came. The time that the sewage is in this chamber is called the period of retention. Although the shape of a basin, the arrangement of the baffles and other details have a marked effect on the results of sedimentation, the controlling factors are the period of retention and the velocity of flow. Another factor affecting the efficiency of the process is the quality of the sewage. Usually the greater the amount of sediment in the sewage the greater the per cent of suspended matter removed. A method for the determination of the proper period of sedimentation has been developed by Hazen in Transactions of the American Society of Civil Engineers, Volume 53, 1904, page 45. The results of his studies are summarized in Fig. 154 which shows the per cent of sediment remaining in a treated water after a certain period of retention. This period of retention is expressed in terms of the hydraulic coefficient[143] of the smallest size particle to be removed. Table 77 shows the hydraulic coefficients of various particles. In Fig. 154 a represents the period of retention and t the time that it would take a particle to fall to the bottom of the basin. The different lines of the diagram represent the results to be expected by various arrangements of settling basins. The meaning of these lines is given in Table 78.
TABLE 77
HYDRAULIC VALUES OF SETTLING PARTICLES IN MILLIMETERS PER SECOND
───────────────────────────────────┬─────────────────────────────────── Diameter in mm. │ Hydraulic Value ───────────────────────────────────┼─────────────────────────────────── 1.00 │ 100 0.80 │ 83 0.60 │ 63 0.50 │ 53 0.40 │ 42 0.30 │ 32 0.20 │ 21 0.15 │ 15 0.10 │ 8 0.08 │ 6 0.06 │ 3.8 0.05 │ 2.9 0.04 │ 2.1 0.03 │ 1.3 0.02 │ 0.62 0.015 │ 0.35 0.010 │ 0.154 0.008 │ 0.098 0.006 │ 0.055 0.005 │ 0.0385 0.004 │ 0.0247 0.003 │ 0.0138 0.002 │ 0.0062 0.0015 │ 0.0035 0.001 │ 0.00154 0.0001 │ 0.0000154 ───────────────────────────────────┴───────────────────────────────────
An example will be given to illustrate the method of using the diagram and tables to determine the size of a sedimentation basin to perform certain required work.
Let it be required to determine the period of retention in a continuously operated sedimentation basin with good baffling, corresponding to two properly baffled sedimentation basins in series. The basins are to remove 60 per cent of the finest particles which are to have a size of .01 mm. The quantity to be treated daily is 3,000,000 gallons.
1st. Entering Table 77, we find that the hydraulic value of the finest particles is .154 mm. per second.
2d. Since we wish to remove 60 per cent of the finest particles, 40 per cent will remain. Since Fig. 154 shows the per cent remaining after the time a⁄t we enter Fig. 154 at 40 per cent on the ordinates and run horizontally until we encounter Line 4 corresponding to good baffling in Table 78. We then run down vertically from this intersection and find that the ratio of a⁄t is 1.0.
Then a equals t, which means that the period of retention should equal the time that it takes a particle 0.01 mm. in diameter to drop from the top to the bottom of the basin. Since this depends on the depth of the basin it is necessary to determine the depth before the other dimensions of the basin can be fixed.
Although this method is seldom used in practice for the final design of a sedimentation basin, it is a guide to judgment and can be used to supplement the data obtained from tests.
TABLE 78
COMPARISON OF DIFFERENT ARRANGEMENTS OF SETTLING BASINS
(From Hazen) ────────────────────────────────────────┬────────────┬───────────────── Description of Basins │Line in Fig.│ Values of │ 154 │ a⁄t. ────────────────────────────────────────┼────────────┼───────────────── │ │ Per Cent of │ │ Matter Removed ────────────────────────────────────────┼────────────┼─────┬─────┬───── │ │ 50 │ 74 │87.5 ────────────────────────────────────────┼────────────┼─────┼─────┼───── Theoretical maximum. Cannot be reached.│ A │ 0.50│ 0.75│0.875 Surface skimming. Rockner Roth system. │ B │ 0.54│ 0.98│ 1.37 Intermittent basins, reckoned on time of│ C │ │ │ service only. │ │ 0.63│ 1.26│ 1.89 Continuous basin. Theoretical limit. │ D │ 0.69│ 1.38│ 2.08 Close approximation to the above. │ 16 │ 0.71│ 1.45│ 2.23 Very well baffled basin. │ 8 │ 0.73│ 1.62│ 2.37 Good baffling. │ 4 │ 0.76│ 1.66│ 2.75 Two basins, tandem. │ 2 │ 0.82│ 2.00│ 3.70 One long basin, well controlled. │ 1.5 │ 0.90│ 2.34│ 4.50 Intermittent basin in service half time.│ E │ 1.26│ 2.50│ 3.80 One basin, continuous. │ 1 │ 1.0│ 3.00│ 7.00 ────────────────────────────────────────┴────────────┴─────┴─────┴─────
The design of sedimentation basins should be based on experimental observations made upon the quantity of sediment removed at certain rates of flow and periods of retention in different types of basins. Hazen’s mathematical analysis is serviceable in making preliminary estimates and in checking the results. The shape of the tank, period of retention and rate of flow producing the most desirable results should be duplicated with the expectation of obtaining similar results or results but slightly modified from those obtained in the tests. This is the most satisfactory method of determining the proper period of retention.
=233. Types of Sedimentation Basins.=—A sedimentation basin is a tank for the removal of suspended matter either by quiescent settlement or by continuous flow at such a velocity and time of retention as to allow deposition of suspended matter.[144] The difference between sedimentation tanks and other forms of tank treatment is that no chemical or biological action is depended on for the successful operation of the tank. Sedimentation tanks may be divided into two classes, grit chambers and plain sedimentation basins.
A grit chamber is a chamber or enlarged channel in which the velocity of flow is so controlled that only heavy solids, such as grit and sand, are deposited while the lighter organic solids are carried forward in suspension. If the velocity of flow is more than about one foot per second, the tank is a grit chamber and below this velocity it is a plain sedimentation basin.
There are six general types of plain sedimentation basins:
1st. Rectangular flat-bottom tanks operated on the continuous-flow principle.
2nd. Rectangular flat-bottom tanks operated on the fill and draw principle.
3rd. Rectangular or circular hopper-bottom tanks operated on the continuous-flow principle, with horizontal flow.
4th. Rectangular or circular hopper-bottom tanks operated on the fill and draw principle, with horizontal flow.
5th. Rectangular or circular hopper-bottom tanks operated on the continuous-flow principle with vertical flow.
6th. Circular hopper-bottom tanks operated on the continuous-flow principle with radial flow.
TABLE 79
CRITICAL VELOCITIES FOR THE TRANSPORTATION OF DEBRIS
Sedimentation will not Occur at Higher Velocities ───────────┬───────────────────────────────────┬────────────────────── Diameter of│Critical Velocity, Feet per Second.│ Size of Screen or Particle in│ │ Number of Meshes per Millimeters│ │ Inch ───────────┼───────────────────────────────────┼────────────────────── │ Specific Gravity │ ───────────┼────────┬────────┬────────┬────────┼────────────────────── │ 1.5 │ 2.0 │ 3.0 │ 5.0 │ ───────────┼────────┼────────┼────────┼────────┼────────────────────── 0.010 │0.13 │0.20 │0.22 │0.28 │ 0.050 │0.23 │0.34 │0.39 │0.50 │More than 200 0.100 │0.30 │0.42 │0.50 │0.65 │More than 150 0.500 │0.55 │0.73 │0.91 │1.15 │More than 28 1.0 │0.71 │0.92 │1.18 │1.50 │More than 14 1.25 │0.77 │1.00 │1.30 │1.60 │ 2.0 │0.92 │1.20 │1.50 │1.90 │More than 10 5.0 │1.30 │1.70 │2.20 │2.60 │More than 4 10 │1.70 │2.20 │2.8 │3.4 │ │ │ │ │ │ Diameter in Millimeters for a Velocity of 1 Foot per Second │ │ │ │ │ │2.5 │1.25 │0.65 │0.32 │ ───────────┴────────┴────────┴────────┴────────┴──────────────────────
=234. Limiting Velocities.=—Sand, clay, bits of metal and other particles of mineral matter will commence to deposit in appreciable quantities when the velocity of flow falls below 3 feet per second. The amount deposited will increase as the velocity decreases. In Table 79 are given the approximate horizontal velocities at which certain size particles of mineral matter will deposit. At a velocity of about one foot per second organic matter will commence to deposit. It will be noticed by interpolation in Table 79,[145] that particles with the same specific gravity as sand (2.6), larger than one mm. in diameter will deposit at a velocity of about one foot per second or less, and that smaller and lighter particles will not deposit at velocity of one foot per second or greater. It will also be noticed that a velocity of one foot per minute is sufficiently slow to permit the deposit of the smallest and lightest particles. For this reason velocities of 1 or 2 or even 3 feet per second have been adopted as the velocities in grit chambers and velocities less than 1 foot per minute in plain sedimentation basins.
=235. Quantity and Character of Grit.=—The amount of material deposited in grit chambers varies approximately between 0.10 and 0.50 cubic yard per million gallons. It is to be noted that grit chambers are used only for combined and storm sewage and for certain industrial wastes. They are unnecessary for ordinary domestic sewage. The material deposited in grit chambers operating with a velocity greater than one foot per second is non-putrescible, inorganic, and inoffensive. It can be used for filling, for making paths and roadways, or as a filtering material for sludge drying beds. An analysis of a typical grit chamber sludge is shown in Table 80.
TABLE 80
ANALYSIS OF GRIT CHAMBER SLUDGE
───────────┬───────────┬───────────┬─────────────────────────────────── Velocity │ Specific │ Per Cent │Calculated to Dry Weight, Per Cent Feet per │ Gravity │ Moisture │ Second │ │ │ ───────────┼───────────┼───────────┼──────────┬──────────┬───────────── │ │ │ Nitrogen │ Fixed │Miscellaneous │ │ │ │ Matter │ ───────────┼───────────┼───────────┼──────────┼──────────┼───────────── 1.0 │ 1.5 │ 45 │ 20 │ 78 │ 2 ───────────┴───────────┴───────────┴──────────┴──────────┴─────────────
=236. Dimensions of Grit Chambers.=—The quantity of sewage to be treated and the amount and character of the settling solids which it contains should be determined by measurement and analysis, and the amount of settling solids to be removed should be determined by a study of the desired conditions of disposal, in order that a grit chamber that will accomplish the desired results may be designed. The period of retention and the velocity of flow are the controlling features in the successful operation of any grit chamber. These should be determined by experiment or as the result of experience. Where neither are available, Hazen’s method can be followed or a decision made based on a study of other grit chambers. In general, the period of retention in grit chambers is from 30 to 90 seconds, and the velocity of flow is about one foot per second.
After having determined the quantity of sewage to be treated, the quantity of grit to be stored between cleanings, the period of retention, the arrangement of the chambers, and the velocity of flow to be used, the overall dimensions of the chambers are computed. The capacity of the chamber is fixed as the sum of the quantity of sewage to be treated during the period of retention and the required storage capacity for grit accumulated between cleanings. The length of the chamber is fixed as the product of the velocity of flow and the period of retention. The cross-sectional area of the portion of the chamber devoted to sedimentation is fixed as the quotient of the quantity of flow of sewage per unit time and the velocity of flow. Only the relation between the width and depth of the portion devoted to sedimentation and the portion devoted to the storage of grit remain to be determined. These should be so designed as to give the greatest economy of construction commensurate with the required results. They will be affected by the local conditions such as topography, available space, difficulties of excavation, etc. Common depths in use lie between 8 and 12 feet, although wide variations can be found. A study of the proportions of existing grit chambers will be of assistance in the design of other basins.
=237. Existing Grit Chambers.=—The details of some typical grit chambers are shown in Figs. 155 and 156. The grit chamber at the foot of 58th Street, in Cleveland, Ohio, is shown in Fig. 155. The special feature of this structure is the shape of the sedimentation basin, the bottom of which is formed by sloping steel plates forming a 6–inch longitudinal slot above the grit storage chamber. Flows between 8,000,000 and 16,000,000 gallons per day are controlled by the outlet weir so that the velocity of flow remains at one foot per second. This is accomplished by increasing the depth of flow in the same ratio as the increase in the rate of flow. The bottoms of the two chambers differ, one having a special hopper for grit and the other a flat bottom. This is due to the method of cleaning the chambers, it being necessary in the one with a flat bottom to shut off the flow when removing the grit while in the one with the hopper bottom it is hoped to remove the grit by the use of sand ejectors without stopping the sewage flow. The details of the chamber at Hamilton, Ontario, are shown in Fig. 156. In studying these drawings the following features should be noted: 1st, the smooth curves in the channel to prevent eddies, undue deposition of organic matter, and difficulties in cleaning; 2nd, the hopper in the upper end of the grit storage chamber and the slope of the bottom of at least 1:20; and 3rd, the simplicity of the inlet and outlet devices which may be either stop planks or cast-iron sluice gates.
The drawings shown are merely representative of some satisfactory types. The number and variety of grit chambers in existence is great. In designing grit chambers consideration must be given to the method of cleaning. They are ordinarily cleaned by such methods as have been described for the cleaning of catch-basins in Chapter XII. Continuous bucket scrapers similar to excavating machines are sometimes used for the cleaning of large grit chambers. The period between cleanings is variable. The design should be such as not to require more frequent cleanings than twice a month under the worst conditions. The fluctuations in quality and quantity of grit will vary the period between cleanings.
=238. Number of Grit Chambers.=—The period of retention in grit chambers is so short and the velocity of flow so near the maximum and minimum limitations that the wide fluctuations in the rate of discharge in storm and combined sewers necessitates the construction of a number of chambers which should be operated in parallel in order to maintain the velocity between the proper limits. Unless arrangements are made permitting the cleaning of grit chambers during operation, more than one grit chamber should be installed in order that when one is being cleaned the others may be in operation. The number of grit chambers must be determined by the desired conditions of operation and the cost of construction. The larger the number of basins the more nearly the flow in any one basin can be maintained constant, but the more expensive the construction. The increase in velocity of flow with increasing quantity is dependent on the outlet arrangements. In a shallow chamber with vertical sides and a standard sharp-crested rectangular weir at the outlet the velocity will vary approximately as the cube root of the rate of flow. Similarly if the outlet is a V notch the velocity will vary as the fifth root of the rate of flow. In all cases the deeper the basin the more nearly the velocity varies directly as the rate of flow. The outlet weir can be arranged as at Cleveland, so that the velocity remains constant for all rates of flow within certain limits. It is seldom that more than three grit chambers are necessary to care for the fluctuations in flow.
=239. Quantity and Characteristics of Sludge from Plain Sedimentation.=—The sludge removed from plain sedimentation basins is slimy, offensive, not easily dried, and is highly putrescible and odoriferous. It contains about 90 per cent moisture and has a specific gravity from 1.01 to 1.05. The amount removed varies between 2 and 5 cubic yards per million gallons of sewage. The percentage of suspended matter removed varies between 20 and 60. The total amount removed and the percentage removal depend on the character of the sewage, the type of basin, and the period of detention.
=240. Dimensions of Sedimentation Basins.=—The dimensions of a sedimentation basin are determined by a method similar to the one given for the determination of the dimensions of a grit chamber in Art. 236. The capacity of the basin is first fixed upon to give the required period of sedimentation and sludge storage capacity. The length of the basin is the product of the velocity and the period of retention. The length, width, and depth of the basin are normally fixed by considerations of economy and the limitations of the local conditions, such as available area, topography, foundations, etc., and examples of good practice. A study of basins in use shows the relation between length and width to vary normally between 2:1 and 4:1. Widths greater than 30 to 50 feet are undesirable because of the danger of cross currents and back eddies which will reduce the efficiency of the sedimentation. Depths used in practice vary too widely to act as guides for any particular design. Theoretically the shallower the basin the better the result. Tanks abroad have been built as shallow as 3 feet and some in this country as deep as 16 feet. The economical dimensions can be determined by trial or by calculus. They will serve as a guide in the adoption of the final dimensions.
The method to be pursued in determining the economical dimensions of any engineering structure are:
I. Express the total cost of the structure in terms of as few variables as possible.
II. Express all of the variables in terms of any one and rewrite the expression for the total cost in terms of this one variable.
III. Equate the first derivative of the expression with regard to this variable to zero and solve for the variable. The result will be the economical value of the variable. The values of the other variables can be computed from the relations already expressed.
For example, let it be desired to determine the dimensions of two continuous-flow sedimentation basins as shown in Fig. 157, in which the period of retention in each is to be 2 hours, the velocity of flow is not to exceed one foot per second, and the sludge accumulated will be 3 cubic yards per million gallons of sewage treated. The quantity of sewage to be treated is 18,000,000 gallons per day. The shortest time between cleanings will be 2 weeks.
The capacity of each basin must be 2/24 of 18,000,000 gallons, or 200,000 cubic feet in order to allow a period of retention of 2 hours. To this volume should be added sufficient capacity to allow for the 2 weeks of sludge storage between cleanings. When a basin is being cleaned the load must be put on the remaining basins. Then if Q represents the rate of accumulation of sludge per day, n represents the number of days between cleanings, m represents the number of basins, and S the sludge capacity of one basin, then
S = (Q(n − 1))⁄m + Q⁄(m − 1)
The sludge storage capacity for the example given will be approximately 11,000 cubic feet.
In expressing the total cost of the basins let
h = the depth in feet. l = the length in feet. b = the width in feet.
The cost of land, floor, etc., per square foot = p dollars. The cost of wall per foot length = qh^2 dollars. The cost of pipes, valves and appurtenances = P dollars.
Then the total cost C = (3_l_ + 4_b_)qh^2 + 2_plb_ + P.
It is now necessary to express the three variables b, l, and h, in terms of one of them. From the relation Q = 2_blh_ it is possible to rewrite the expression for the total cost as:
C = (3_Q_⁄(2_bh_) + 4_b_)qh^2 + (pQ)⁄h + P.
C = (3_l_ + 2_Q_⁄(lh))qh^2 + (pQ)⁄h + P.
Holding h constant and differentiating with regard to b in the first expression and with regard to l in the second expression, equating to zero and solving we get:
b = √((3_Q_)⁄(8_h_)) and l = √((2_Q_)⁄(3_h_)).
The economical relation between b and l is therefore
b = 0.75_l_
regardless of the value of h.
Substituting these values of l and b in the original expression for the total cost, it becomes
C = (3√((2_Q_)⁄(3_h_)) + 4√((3_Q_)⁄(8_h_)))qh^2 + (pQ)⁄h + P.
Differentiating with regard to h, equating to zero, and solving
h = 0.45((pQ^½)⁄q)^⅔.
In the example given if q = 0.2 and p = 1.0 then
h = 11.6 feet, b = 120 feet and l = 160 feet.
Since these are reasonable dimensions and in accord with good practice they should be used, unless other conditions are unsuitable or the velocity of flow is too great. A width of channel of 120 feet as compared to a length of 160 feet is conducive to a poor distribution of velocity across the basin. A ratio of width to length of about 1:4 is desirable. In this case, by the use of three baffles parallel to the length of the basin, thus dividing it into channels 40 feet wide and 11.6 feet deep, the ratio of width to length is changed to 1:4 and the velocity will be increased only to 0.06 foot per second or 3.6 feet per minute, which is a reasonable velocity. It could be reduced by increasing the spacing of the baffles or the depth of the chamber.
Complicated baffling is undesirable. Two or three overflow baffles may be used to permit quiescent sedimentation in the space thus formed, and hanging baffles may be placed before the inlet and outlet to break up surface currents, or to prevent the movement of scum. The hanging baffles should not extend more than 12 to 18 inches below the water surface. The inlet and outlet are sometimes arranged to permit the reversal of flow, and the connecting channels between basins to allow the operation of any number of basins in series or in parallel, although such arrangements are more important in water purification. Sewage should enter and leave at the top of the basin.
Cleaning is facilitated by the location of a central gutter in the bottom of the basin with the slope of the bottom of the basin towards the gutter from 1:25 to 1:80 or steeper. A pipe, 2 inches or larger in diameter, containing water under pressure with connections for hose placed at frequent intervals is a useful adjunct in flushing the sludge from the sedimentation basins. For equal capacity, deep vertical flow tanks are more expensive and difficult to construct than the shallower rectangular type. Deep tanks are advantageous, however, in that sludge can sometimes be removed by gravity or by pumping without stopping the operation of the tank. They will also operate successfully with shorter periods of detention and higher velocities. The upward velocity should not be greater than the velocity of sedimentation of the smallest particle to be removed. The efficiency of sedimentation in them will be increased by the sedimentation of the larger particles which drag some of the smaller particles down with them. The Dortmund tank shown in Fig. 158 is an example of this type.
Ordinarily it is not necessary to roof sedimentation basins as the odors created are not strong, and difficulties with ice are seldom serious.
CHEMICAL PRECIPITATION
=241. The Process.=—Chemical precipitation consists in adding to the sewage such chemicals as will, by reaction with each other and the constituents of the sewage, produce a flocculent precipitate and thus hasten sedimentation. The advantages of this process over plain sedimentation are a more rapid and thorough removal of suspended matter. Its disadvantages include the accumulation of a large amount of sludge, the necessity for skilled attendance, and the expense of chemicals. The process is not in extensive use as the conditions under which the advantages outweigh the disadvantages are unusual. Sewage containing large quantities of substances which will react with a small amount of an added chemical to produce the required precipitate are the most favorable for this method of treatment.
Chemical precipitation accomplishes the same result as plain sedimentation, although the effluent from the chemically precipitated sewage may be of better quality than that from a plain sedimentation basin.
=242. Chemicals.=—Lime is practically the only chemical used for the precipitation of the solid matter in sewage. Commercial lime used for precipitation consists of calcium oxide (CaO), with large quantities of impurities. It should be stored in a dry place and protected from undue exposure to the air to prevent the formation of calcium carbonate (CaCO_{3}), the formation of which is commonly known as air slacking. The active work in the formation of the precipitate is performed by the lime (CaO) or calcium hydroxide (Ca(OH)_{2}). The lime should therefore be purchased on the basis of available CaO, which may be as low as 10 to 15 per cent in some commercial products. The amount of lime necessary depends on the quality of the sewage, the period of retention in the sedimentation basin, the method of application, the required results, and other less easily measured factors. Full scale tests for the amount of lime needed to produce certain results are the most satisfactory. In practice the amount of lime necessary when lime alone is used as a precipitant has been found to be about 15 grains per gallon. This may be markedly different, dependent on the quality of the sewage. For acid sewages, lime alone is not suitable as a precipitant since it is necessary to add sufficient lime to neutralize the sewage before the calcium carbonate will be precipitated.
The use of copperas (FeSO_{4}) together with lime, leads to economy in the use of chemicals as the flocculent precipitate of ferrous hydroxide (Fe(OH)_{2}) is more voluminous than the precipitate of calcium carbonate. This is commonly known as the lime and iron process. The presence of iron in certain trade wastes may reduce the cost of chemical precipitation, as the necessary amount of copperas is reduced. Where 15 grains of lime alone will be needed per gallon of sewage, the total amount of chemicals used will be reduced to 8 to 10 grains per gallon with the use of lime and iron. This combination is less expensive than the use of lime alone, and is even cheaper where the iron is already present in the sewage. Such a condition is well illustrated by the sewage at Worcester, Mass., where the oldest and best known chemical precipitation plant in the United States is located. The amount of lime used at this plant has varied between 6 and 10 grains per gallon of sewage, the normal amount being about 7 grains. No iron is added because of the amount already in solution.
The results of a series of experiments on the chemical precipitation of sewage by Allen Hazen, are given in the 1890 Report of the Massachusetts State Board of Health, on p. 737 of the volume on the Purification of Water and Sewage. Hazen concludes as the result of his experiments: concerning lime,
There is a certain definite amount of lime ... which gives as good or better results than either more or less. This amount is that which exactly suffices to form normal carbonates with all the carbonic acid of the sewage. This amount can be determined in a few minutes by simple titration.
Concerning lime and iron (copperas) he states:
Ordinary house sewage is not sufficiently alkaline to precipitate copperas, and a small amount of lime must be added to obtain good results. The quantity of lime required depends both upon the composition of the sewage and the amount of copperas used, and can be calculated from titration of the sewage. Very imperfect results are obtained from too little lime, and, when too much is used, the excess is wasted, the result being the same as with a smaller quantity.
In precipitation by ferric sulphate and crude alum, the addition of lime was found unnecessary, as ordinary sewage contains enough alkali to decompose these salts. Within reasonable limits the more of these precipitants used the better is the result, but with very large quantities the improvement does not compare with the increased cost.
Using equal values of different precipitants, applied under the most favorable conditions for each, upon the same sewage, the best results were obtained from ferric sulphate. Nearly as good results were obtained from copperas and lime used together, while lime and alum each gave somewhat inferior effluents.... When lime is used there is always so much lime left in solution that it is doubtful if its use would ever be found satisfactory except in case of an acid sewage.
It is quite impossible to obtain effluents by chemical precipitation which will compare in organic purity with those obtained by intermittent filtration through sand.
It is possible to remove from one-half to two-thirds of the organic matter by precipitation ... and it seems probable that ... a result may be obtained which will effectually prevent a public nuisance.
=243. Preparation and Addition of Chemicals.=—Lime is not readily soluble in water. Therefore, it is not best to add the lime as a powder to the sewage, but to form a milk of lime, that is, a supersaturated solution containing from 2,000 to 4,000 grains per gallon, although dry slaked lime has sometimes been applied directly. The solution is prepared in tanks in a quantity sufficient for some part of the day’s run, commonly sufficient to last through one shift of 8 or 10 hours. The lime is prepared by placing the amount necessary to fill one storage tank into a slaking tank containing some cold water. Sufficient water is added to keep the solution just at the boiling point, or steam may be added to make it boil. After slaking, it is run into the milk-of-lime solution tank and sufficient water added to bring to the proper strength. The milk of lime is added in measured quantities, being controlled by a variable head on a fixed orifice or weir, so that it may be varied with the amount of sewage flowing through the plant. The amount of lime to be added is determined by titration with phenolphthalein, experience indicating the color to be obtained when the proper amount of lime has been added.
The use of either copperas or alum has been so rare, for the precipitation of sewage, that a description of the methods of handling these chemicals as a sewage precipitant is not warranted. An excellent description of the methods of handling these chemicals in water purification will be found in “Water Purification” by Ellms.
TABLE 81
RESULTS OF CHEMICAL PRECIPITATION AT WORCESTER, MASSACHUSETTS[146]
──────────────────────────────────────┬──────────┬──────────┬────────── │ 1900 │ 1910 │ 1920 ──────────────────────────────────────┼──────────┼──────────┼────────── Amount of sewage treated, million │ 4,781 │ 5,317 │ 8,893 gallons │ │ │ Amount of sewage chemically treated, │ 3,650 │ 3,574 │ 7,300 million gallons │ │ │ Gallons of wet sludge per million │ 4,450 │ 4,185 │ gallons of sewage treated │ │ │ Per cent of solids in sludge │ 4.42 │ 8.20 │4.64[147] Tons of solids │ 7,294 │ 4,182 │6,431[147] Pounds of lime added per million │ 999[148] │ 762[147] │ 534 gallons of sewage pumped │ │ │ Per cent of organic matter removed: │ │ │ By albuminoid ammonia: │ │ │ Total │52.7[149] │ 58.4 │ 51.9 Suspended │90.0[149] │ 88.7 │ 83.6 By oxygen consumed: │ │ │ Total │62.8[149] │ 61.1 │ 62.5 Suspended │86.6[149] │ 89.7 │ 86.2 ──────────────────────────────────────┴──────────┴──────────┴──────────
=244. Results.=—The results of Hazen’s experiments indicate that a greater amount of suspended matter can be removed in the same time by chemical precipitation than by plain sedimentation. The percentage of removal of suspended matter may be as high as 80 to 90 per cent with a period of retention of 6 to 8 hours and the addition of a proper amount of chemical. That the method is not always a success is shown by the results of some tests at Canton, Ohio.[150] The report states:
... lime treatment removes about 50 per cent of the suspended matter, and in the main about 50 per cent of the organic matter.... These data are instructive as indicating that the addition of lime to the Canton sewage in quantities as previously stated does not materially improve the character of the resulting effluent over and above that which could be produced by plain sedimentation alone.
The plant at Worcester, Mass., is the largest in the United States and information from it is of value. A summary of the results at Worcester for 1900, 1910, and 1920 are shown in Table 81.
XVI. SEPTICIZATION
=245. The Process.=—Septic action is a biological process caused by the activity of obligatory or facultative anaërobes as the result of which certain organic compounds are reduced from higher to lower conditions of oxidation, some of the solid organic substances are rendered soluble, and a quantity of gas is given off. Among these gases are: methane, hydrogen sulphide, and ammonia. The biologic process in the septic tank represents the downward portion of the cycle of life and death, in which complex organic compounds are reduced to a more simple condition available as food for low forms of plant life. The disposal of sewage by septic action, when introduced, promised the solution of all problems in sewage treatment. Septic action is now better understood, and it is known that some of the early claims were unfounded.
The principal advantage of septic action in sewage treatment is the relatively small amount of sludge which must be cared for compared to that produced by a plain sedimentation tank. The sludge from a septic tank may be 25 to 30 per cent and in some cases 40 per cent less in weight, and 75 to 80 per cent less in volume than the sludge from a plain sedimentation tank. The most important results of septic action and the greatest septic activity occur in the deposited organic matter or sludge. The biologic changes due to septic action which occur in the liquid portion of the tank contents are of little or no importance. The installation of a septic tank, although it may fail to prevent the nuisance calling for abatement, has a remarkable psychological effect in stilling complaints. Among other advantages are the comparative inexpensiveness of the tanks and the small amount of attention and skilled attendance required. The tanks need cleaning once in 6 months to a year. If properly designed no other attention is necessary.
The septic tank has fallen into some disrepute because of the better results obtainable by other methods, the occasional discharge of effluents worse than the influent, the occasional discharge of sludge in the effluent caused by too violent septic boiling, and on account of patent litigation. This last difficulty has been overcome as the Cameron patents expired in 1916. Occasionally the odors given off by the septic process are highly objectionable and are carried for a long distance. These odors can be controlled to a large extent by housing the tanks. Over-septicization must be guarded against as an over-septicized effluent is more difficult of further treatment or of disposal than a comparatively fresh, untreated sewage. An over-septicized or stale sewage is indicated by the presence of large quantities of ammonias, either free or albuminoid, frequently accompanied by hydrogen sulphide and other foul-smelling gases. The oxygen demand in an over-septicized sewage is greater than that in a fresh or more carefully treated sewage.
=246. The Septic Tank.=—A septic tank is a horizontal, continuous-flow, one-story sedimentation tank through which sewage is allowed to flow slowly to permit suspended matter to settle to the bottom where it is retained until anaërobic decomposition is established, resulting in the changing of some of the suspended organic matter into liquid and gaseous substances, and a consequent reduction in the quantity of sludge to be disposed of.[151] It is to be noted that a continuous flow is essential to a septic tank. Small tanks containing stagnant household sewage are called cesspools, although sometimes erroneously spoken of as septic tanks.
Septic and sedimentation tanks differ in their method of operation only in the period of storage and the frequency of cleaning. The period of flow in a septic tank is longer and it is cleaned less frequently. The results obtained by the two processes differ widely. A septic tank can be converted into a sedimentation tank, or vice versa, by changing the method of operation, no constructional features requiring alteration. The purpose of the tank is to store the sludge for such a period of time that partial liquefaction of the sludge may take place, and thus minimize the difficulty of sludge disposal. For this reason the sludge storage capacity of a septic tank is sometimes greater than would be necessary for a plain sedimentation tank.
TABLE 82
EFFICIENCIES AND PERFORMANCE OF SEPTIC TANK AT COLUMBUS, OHIO
(Report of Sewage Purification, by G. A. Johnson, Nov. 10, 1905) ──────────────┬────┬─────┬────┬────┬────┬────┬────┬─────┬─────┬───┬────┬──── Month, │Aug.│Sept.│Oct.│Nov.│Dec.│Jan.│Feb.│March│April│May│June│Avg. 1904–1905 │ │ │ │ │ │ │ │ │ │ │ │ ──────────────┼────┼─────┼────┼────┼────┼────┼────┼─────┼─────┼───┼────┼──── Temperature, │ │ │ │ │ │ │ │ │ │ │ │ degrees F. │ │ │ │ │ │ │ │ │ │ │ │ Influent │ 69 │ 70 │ 65 │ 60 │ 54 │ 51 │ 48 │ 50 │ 57 │61 │ 67 │ Effluent │ 69 │ 68 │ 64 │ 59 │ 52 │ 48 │ 45 │ 49 │ 57 │62 │ 68 │ Oxygen │ │ │ │ │ │ │ │ │ │ │ │ consumed, │ │ │ │ │ │ │ │ │ │ │ │ parts per │ │ │ │ │ │ │ │ │ │ │ │ million: │ │ │ │ │ │ │ │ │ │ │ │ Influent │ 49 │ 50 │ 52 │ 47 │ 43 │ 51 │ 44 │ 47 │ 53 │33 │ 40 │ 47 Effluent │ 40 │ 36 │ 40 │ 39 │ 37 │ 35 │ 37 │ 39 │ 50 │34 │ 33 │ 38 Per cent │ 18 │ 28 │ 23 │ 15 │ 16 │ 31 │ 16 │ 17 │ 6 │–3 │ 18 │ 19 removal │ │ │ │ │ │ │ │ │ │ │ │ Organic │ │ │ │ │ │ │ │ │ │ │ │ nitrogen, │ │ │ │ │ │ │ │ │ │ │ │ parts per │ │ │ │ │ │ │ │ │ │ │ │ million: │ │ │ │ │ │ │ │ │ │ │ │ Influent │6.5 │ 8.2 │9.3 │8.4 │8.8 │8.5 │6.7 │ 6.4 │ 7.9 │6.1│6.7 │7.8 Effluent │7.3 │ 5.5 │6.0 │7.4 │8.2 │7.0 │5.4 │ 5.5 │ 5.2 │ │ │ Per cent │–12 │ 32 │ 35 │ 12 │ 7 │ 18 │ 19 │ 14 │ 25 │30 │ 19 │ 19 removal │ │ │ │ │ │ │ │ │ │ │ │ Free ammonia, │ │ │ │ │ │ │ │ │ │ │ │ parts per │ │ │ │ │ │ │ │ │ │ │ │ million: │ │ │ │ │ │ │ │ │ │ │ │ Influent │9.7 │12.2 │12.4│16.3│14.7│10.8│8.3 │ 9.9 │12.3 │6.9│8.3 │11.7 Effluent │10.5│11.5 │12.4│17.2│14.3│11.1│8.9 │10.7 │14.9 │9.0│8.7 │12.1 Per cent │ –8 │ 6 │ 0 │ –6 │ 3 │ –3 │ –7 │ –8 │ –21 │–23│ –5 │ –3 removal │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ Residue on │ │ │ │ │ │ │ │ │ │ │ │ Evaporation,│ │ │ │ │ │ │ │ │ │ │ │ parts per │ │ │ │ │ │ │ │ │ │ │ │ million: │ │ │ │ │ │ │ │ │ │ │ │ Total: │ │ │ │ │ │ │ │ │ │ │ │ Influent │990 │ 952 │993 │961 │989 │949 │890 │ 850 │1067 │912│945 │946 Effluent │935 │ 891 │893 │916 │925 │886 │843 │ 782 │ 895 │800│835 │873 Per cent │ 6 │ 6 │ 10 │ 5 │ 6 │ 6 │ 5 │ 8 │ 16 │12 │ 12 │ 8 removal │ │ │ │ │ │ │ │ │ │ │ │ Volatile: │ │ │ │ │ │ │ │ │ │ │ │ Influent │231 │ 184 │162 │175 │156 │167 │156 │ 168 │ 212 │122│162 │166 Effluent │206 │ 160 │129 │148 │137 │137 │134 │ 137 │ 147 │103│144 │139 Per cent │ 11 │ 13 │ 20 │ 15 │ 12 │ 18 │ 14 │ 18 │ 31 │16 │ 11 │ 16 removal │ │ │ │ │ │ │ │ │ │ │ │ Mineral: │ │ │ │ │ │ │ │ │ │ │ │ Influent │759 │ 768 │831 │786 │833 │782 │734 │ 682 │ 855 │700│783 │780 Effluent │729 │ 731 │764 │768 │788 │749 │709 │ 645 │ 748 │697│691 │734 Per cent │ 4 │ 5 │ 8 │ 2 │ 5 │ 4 │ 3 │ 5 │ 11 │ 1 │ 12 │ 6 removal │ │ │ │ │ │ │ │ │ │ │ │ Cubic yards │ │ │0.10│1.24│1.09│1.17│0.65│0.63 │0.57 │ │1.34│ wet sludge │ │ │ │ │ │ │ │ │ │ │ │ per million │ │ │ │ │ │ │ │ │ │ │ │ gallons: │ │ │ │ │ │ │ │ │ │ │ │ Per cent │ │ │ │ │ │ │ │ │ │ │ │ removal of │ │ │ │ │ │ │ │ │ │ │ │ suspended │ │ │ │ │ │ │ │ │ │ │ │ matter: │ │ │ │ │ │ │ │ │ │ │ │ Total │ 59 │ 54 │ 56 │ 51 │ 42 │ 48 │ 32 │ 47 │ 56 │67 │ 53 │ 50 Volatile │ 60 │ 41 │ 48 │ 52 │ 44 │ 55 │ 47 │ 47 │ 62 │80 │ 15 │ 48 Fixed │ 75 │ 65 │ 60 │ 51 │ 40 │ 38 │ 19 │ 48 │ 53 │64 │ 67 │ 51 Gas evolved, │ │ │ │ │ │ │ │ 29 │ 14 │41 │ 50 │ cubic feet │ │ │ │ │ │ │ │ │ │ │ │ per day: │ │ │ │ │ │ │ │ │ │ │ │ ──────────────┴────┴─────┴────┴────┴────┴────┴────┴─────┴─────┴───┴────┴────
=247. Results of Septic Action.=—The results obtained from the septic tanks at the Columbus Sewage Experiment Station are given in Table 82. The effluent is higher than the influent in free ammonia, but the reduction of other constituents, particularly suspended matter, is marked.
Septic action is sensitive to temperature changes, and to certain constituents of the incoming sewage. Cold weather or an acid influent will inhibit septicization. In winter the liquefaction of sludge may practically cease, whereas in summer liquefaction may exceed deposition. The amount of gas generated is a measure of the relative amount of septic action. The rapid generation of gas in warm weather disturbs the settled sludge and may cause a deterioration of the quality of the effluent because of the presence of decomposed sludge. The results in Table 82 show the effect of cold weather on the process. In warm weather the violent ebullition of gas sometimes causes the discharge of sludge in the effluent, resulting in a liquid more difficult of disposal than the incoming sewage. Since septic action is dependent on the presence of certain forms of bacteria, where these are absent there will be no septic action. Sewage generally contains the forms of bacteria necessary for this action but it has occasionally been found necessary to seed new tanks in order to start septic action.
The sludge from septic tanks is usually black, with a slight odor, though in some cases this odor may be highly offensive. The sludge will flow sluggishly. It can be pumped by centrifugal pumps and it will flow through pipes and channels. It has a moisture content of about 90 per cent and a specific gravity of about 1.03. It is dried with difficulty on open-air drying beds, and it is worthless as a fertilizer. The composition of some septic sludges are shown in Table 83.
=248. Design of Septic Tanks.=—The sedimentation chambers of a septic tank are designed on the same principles as the sedimentation basins described in Art. 240. The velocity of flow should not exceed one foot per minute. The channels should be straight and free from obstructions causing back eddies. The ratio of length to width of channel should be between 2 : 1 to 4 : 1 with a width not exceeding 50 feet, and desirably narrower. The depths used vary between 5 and 10 feet, exclusive of the sludge storage capacity. Hanging baffles should be placed, one before the inlet and the other in front of the outlet, so as to distribute the incoming sewage over the tank, and to prevent scum from passing into the outlet. The baffles should hang about 12 inches below the surface of the sewage. Intermediate baffles are sometimes desirable to prevent the movement of sludge or scum towards the outlet. The placing of baffles must be considered carefully as injudicious baffling may lessen the effectiveness of a tank by so concentrating the currents as to prevent sedimentation or the accumulation of sludge. Baffles should be built of concrete or brick, as wood or metal in contact with septic sewage deteriorates rapidly. In designing the sludge storage chambers it may be assumed that one-half of the organic matter and none of the mineral matter will be liquefied or gasified. The net storage volume allowed is about 2 to 3 cubic yards per million gallons of sewage treated. Variations between 0.1 and 10.0 cubic yards have been recorded, however. If grit is carried in the sewage to be treated, it should be removed by the installation of a grit chamber before the sewage enters the septic tank.
TABLE 83
ANALYSIS OF TANK SLUDGES
──────────┬────────┬────────┬──────────────────────────── Place │Specific│Per Cent│ Per Cent in Terms of Dry │Gravity │Moisture│ Matter │ │ │ │ │ │ │ │ │ ──────────┼────────┼────────┼────────┬─────┬────────┬──── │ │ │Volatile│Fixed│Nitrogen│Fat ──────────┼────────┼────────┼────────┼─────┼────────┼──── Mansfield,│ 1.11│ 80.8│ │ │ │ O. │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ Chicago, │ 1.03│ 90│ 40│ 60│ 1.9│ 7.0 Ill. │ │ │ │ │ │ │ │ │ │ │ │ Columbus, │ 1.09│ 83.3│ 4.4│ 16.7│ 0.25│0.94 O. │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ Atlanta, │ 1.02│ 87.1│ 39.1│ 60.9│ 1.25│6.11 Ga. │ │ │ │ │ │ │ │ │ │ │ │ Baltimore,│ 1.02│ 91.9│ 66.2│ │ 2.45│4.02 Md. │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ Baltimore,│ 1.02│ 92.4│ 62.7│ │ 2.75│ Md. │ │ │ │ │ │ Baltimore,│ │ 79.2│ 73.8│ │ 2.64│9.00 Md. │ │ │ │ │ │ Baltimore,│ │ 92.4│ 58.0│ 3.19│ │ Md. │ │ │ │ │ │ ──────────┴────────┴────────┴────────┴─────┴────────┴────
──────────┬────────┬────────┬─────────┬──────────── Place │ Cubic │ Pounds │ Kind of │ Reference │Yard per│ per │ Sludge │ │Million │Million │ │ │Gallons,│Gallons,│ │ │ Wet │ Dry │ │ ──────────┼────────┼────────┼─────────┼──────────── │ │ │ │ ──────────┼────────┼────────┼─────────┼──────────── Mansfield,│ │ │Septic │1908 Report, O. │ │ │ │ State │ │ │ │ Board of │ │ │ │ Health Chicago, │ 1.0│ 200│Septic │ Ill. │ │ │ │ │ 1.5│ 300│ │ Columbus, │ │ │Septic │G. A. O. │ │ │ │ Johnson │ │ │ │ 1905 │ │ │ │ Report Atlanta, │ │ │Imhoff │Eng. Rec., Ga. │ │ │ │ V. 72, │ │ │ │ 1915, p. 4 Baltimore,│ │ │Digestion│Eng. Md. │ │ │ Tank │ News-Rec., │ │ │ │ V. 87, │ │ │ │ 1921, p. │ │ │ │ 98 Baltimore,│ │ │Imhoff │ do. Md. │ │ │ │ Baltimore,│ │ │Raw │ do. Md. │ │ │ Sludge │ Baltimore,│ │ │Settling │ do. Md. │ │ │ Basin │ ──────────┴────────┴────────┴─────────┴────────────
Two or more tanks should be constructed to allow for the shut down of one for cleaning and to increase the elasticity of the plant. The number of tanks to be used is dependent on the total quantity of sewage and the fluctuations in rate of flow. An average period of retention of about 9 to 10 hours with a minimum period of 6 hours during maximum flow is a fair average to be assumed for design. The period of retention should not exceed about 24 hours, as the sewage may become over-septicized. The sludge storage period should be from 6 to 12 months.
A cover is not necessary to the successful operation of a septic tank. Covers are sometimes used with success, however, in reducing the dissemination of odors from the tank. They are also useful in retaining the heat of the sewage in cold weather and thus aid in promoting bacterial activity. Types of covers vary from a building erected over the tank to a flat slab set close to the surface of the sewage. In the design of a cover, good ventilation should be provided to permit the escape of the gases, and easy access should be provided for cleaning. Tightly covered tanks or tanks with too little ventilation have resulted in serious explosions, as at Saratoga Springs in 1906 and at Florenceville, N. C., in 1915.[152]
The sludge may be removed through drains in the bottom of the tank as described for sedimentation basins, or where such drains are not feasible the sludge and sewage are pumped out. For this purpose a pump may be installed permanently at the tank, or for small tanks portable pumps are sometimes used. Septic tanks should be cleaned as infrequently as possible without permitting the overflow of sludge into the effluent. The less frequent the cleaning the less the amount of sludge removed since digestion is continuous throughout the sludge. It is necessary to clean when the tank becomes so filled with sludge, that the period of retention is materially reduced, or sludge is being carried over into the effluent.
The details of the septic tank at Champaign, Illinois, are shown in Fig. 159. This tank was designed by Prof. A. N. Talbot, and was put in service on Nov. 1, 1897. It was among the first of such tanks to be installed in the United States. The tank shown in Fig. 159 is an example of present day practice in single-story septic tank design.
Small septic tanks for rural homes of 5 to 15 persons, or on a slightly larger scale for country schools and small institutions, are little more than glorified cesspools. Nevertheless much attention has been given to the construction of such tanks by the National Government and by state boards of health.[153] The recommendations of some of these boards have been compiled in Table 84. A typical method for the construction of such tanks, as recommended by the Illinois State Board of Health, is shown in Fig. 160. A subsurface filter, into which the effluent is discharged, is an important adjunct where no adequate stream is available to receive the discharge from the tank.
TABLE 84
CAPACITIES OF SEPTIC TANKS FOR SMALL INSTALLATIONS
──────────────┬─────────────┬─────────┬─────────┬────────────────────── Rule │ Number, │Capacity,│Period of│ Remarks Recommended by│ Persons │ Gallons │Retention│ State Board of│ │ per │ │ Health │ │ Person │ │ ──────────────┼─────────────┼─────────┼─────────┼────────────────────── Wisconsin │ │ 30 │24 hours │ Ohio │ 4 to 10 │ 50 │ │Not less than 560 │ │ │ │ gallons Kentucky │ │ │24 to 48 │Not more than 5 feet │ │ │ hours │ deep Texas │ │ │24 hours │ Illinois │ │ 45 │24 hours │ U.S. Dept. │ │ 40 │24 hours │25 per cent additional Agriculture.│ │ │ │ │ │ │ │capacity for sludge North Carolina│Large Schools│ 15 │ │Not less than 500 │ │ │ │ gallons North Carolina│ 20 pupils │ 25 │ │ North Carolina│Medium School│ 20 │ │ North Carolina│ Homes │25 to 30 │ │ ──────────────┴─────────────┴─────────┴─────────┴──────────────────────
=249. Imhoff Tanks.=—In the discussion of septic tanks it has been brought out that one of the objections to their use is the unloading of sludge into the effluent which occasionally causes a greater amount of suspended matter in the effluent than in the influent. The Imhoff tank is a form of septic tank so arranged that this difficulty is overcome. It combines the advantages of the septic and sedimentation tanks and overcomes some of their disadvantages. An Imhoff tank is a device for the treatment of sewage, consisting of a tank divided into 3 compartments. The upper compartment is called the sedimentation chamber. In it the sedimentation of suspended solids causes them to drop through a slot in the bottom of the chamber to the lower compartment called the digestion chamber. In this chamber the solid matter is humified by an action similar to that in a plain septic tank. The generated gases escape from the digestion chamber to the surface through the third compartment called the transition or scum chamber. Sections of Imhoff tanks are shown in Fig. 161. It is essential to the construction of an Imhoff tank that the slot in the bottom of the sedimentation chamber does not permit the return of gases through the sedimentation chamber, and that there be no flow in the digestion chamber.
The Imhoff tank was invented by Dr. Karl Imhoff, director of the Emscher Sewerage District in Germany. Its design is patented in the United States, the control of the patent being in the hands of the Pacific Flush Tank Co. of Chicago, which collects the royalties which are payable when construction work begins. The fee for a tank serving 100 persons is $10, for 1,000 persons is $80 and for 100,000 persons is $2550. The rate of the royalty reduces in proportion as the number of persons served increases.[154] As designed by Imhoff and used in Germany the tanks were of the radial flow type and quite deep. The depth, as explained by Imhoff, is one of the chief requirements for the successful operation of the tank. As adapted to American practice the tanks are generally of the longitudinal flow type and are not made so deep. An isometric view of a radial flow Imhoff tank is shown in Fig. 162. The sewage enters at the center of the tank near the surface and flows radially outward under the scum ring and over a weir placed near the circumference of the tank. One type of longitudinal flow tank is shown in isometric view in Fig. 163.
=250. Design of Imhoff Tanks.=—The velocity of flow, period of retention, and the quantity of sewage to be treated determine the dimensions of the sedimentation chamber as in other forms of tanks. The velocity of flow should not exceed one foot per minute, with a period of retention of 2 to 3 hours. A greater velocity than one foot per minute results in less efficient sedimentation. A longer period of retention than the approximate limit set may result in a septic or stale effluent, and a shorter period may result in loss of efficiency of sedimentation. The bottom of the sedimentation chamber should slope not less than 1½ vertical to 1 horizontal, in order that deposited material will descend into the sludge digestion chamber. Provision should be made for cleaning these sloping surfaces by placing a walk on the top of the tank from which a squeegee can be handled to push down accumulated deposits. It is desirable to make the material of the sides and bottom of the sedimentation chamber as smooth as possible to assist in preventing the retention of sludge in the sedimentation chamber. Wood, glass, and concrete have been used. The latter is the more common and has been found to be satisfactory. The length of the sedimentation chamber is fixed by the velocity of flow and the period of retention. Tanks are seldom built over 100 feet in length, however, because of the resulting unevenness in the accumulation of sludge. Where longer flows are desired two or more tanks may be operated in series. The width of the chamber is fixed by considerations of economy and convenience. It should not be made so great as to permit cross currents. In general a narrow chamber is desirable. Satisfactory chambers have been constructed at depths between 5 and 15 feet. The depth of the sedimentation chamber and the depth of the digestion chamber each equal about one-half of the total depth of the tank. This should be made as deep as possible up to a limit of 30 to 35 feet, with due consideration of the difficulties of excavation. C. F. Mebus states:[155]
In 9 of the largest representative United States installations, the depth from the flow line to the slot varies from 10 feet 10 inches to 13 feet 6 inches.
Imhoff states, concerning the depth of tanks:
Deep tanks are to be preferred to shallow tanks because in them the decomposition of the sludge is improved. This is so because in the deeper tanks the temperature is maintained more uniformly and because the stirring action of the rising gas bubbles is more intense.
The stirring action of the gas bubbles is desirable as it brings the fresh sludge more quickly under the influence of the active bacterial agents. The greater pressure on the sludge in deep tanks also reduces its moisture content.
Two or more sedimentation chambers are sometimes used over one sludge digestion chamber in order to avoid the depths called for by the sloping sides of a single sedimentation chamber. An objection to multiple-flow chambers is the possibility of interchange of liquid from one chamber to another through the common digestion chamber.
The inlet and outlet devices should be so constructed that the direction of flow in the tank can be reversed in order that the accumulated sludge may be more evenly distributed in the hoppers of the digestion chamber. The sewage should leave the sedimentation chamber over a broad crested weir in order to minimize fluctuations in the level of sewage in the tank. The gases in the digesting sludge are sensitive to slight changes in pressure. A lowering of the level of sewage will release compressed gas and will too violently disturb the sludge in the digestion chamber. Hanging baffles, submerged 12 to 16 inches and projecting 12 inches above the surface of the sewage, should be placed in front of the inlet and outlet, and in long tanks intermediate baffles should be placed to prevent the movement of scum or its escape into the effluent. An Imhoff tank which is operating properly should not have any scum on the surface of the sewage in the sedimentation chamber.
The slot or opening at the bottom of the sedimentation chamber should not be less than 6 inches wide between the lips. Wider slots are preferable, but too wide a slot will involve too much loss of volume in the digestion chamber. One lip of the slot should project at least 3 inches horizontally under the other so as to prevent the return of gases through the sedimentation chamber. A triangular beam may be used as shown in Fig. 161 A. This method of construction is advantageous in increasing the available capacity for sludge storage.
The digestion chamber should be designed to store sludge from 6 to 12 months, the longer storage periods being used for smaller installations. In warm climates a shorter period may be used with success. The amount of sludge that will be accumulated is as uncertain as in other forms of sewage treatment. A widely quoted empirical formula, presented in “Sewage Sludge” by Allen, states:
C = 10.5 PD for combined sewage; C = 5.25 PD for separate sewage,
in which C = the effective capacity of the digestion chamber in cubic feet;
P = the population served, expressed in thousands;
D = the number of days of storage of sludge.
The effective capacity of the chamber is measured as the entire volume of the chamber approximately 18 inches below the lower lip of the slot. The capacity as computed from the above formula is assumed as satisfactory for a deep tank. Frank and Fries[156] recommend the increase of the capacity for shallow tanks to compensate for the decreased hydrostatic pressure. In any event the formula can be no more than a guide to design. No formula can be of equal value to data accumulated from tests on the sewage to be treated. The Illinois State Board of Health requires 3 cubic yards of sludge digestion space per million gallons of sewage treated. Frank and Fries recommend an allowance of 0.007 cubic foot of storage per inhabitant per day for combined sewage and one-half that amount for separate sewage. If this is based on 80 per cent moisture content, the volume for other percentages of moisture can be easily computed. An average figure used in the Emscher District is one cubic foot capacity for each inhabitant for the combined system, and three-fourths of this for the separate system. Metcalf and Eddy[157] recommend the following method for the determination of the sludge storage capacity: (1) From analyses of the sewage or study of the sources ascertain the amount of suspended matter. (2) Assume, or determine by test, the amount which will settle in the period of detention selected, say 60 per cent in 3 hours. (3) Estimate the amount which will be digested in the sludge chamber at about 25 per cent, leaving 75 per cent to be stored. (4) Estimate the percentage moisture in the sludge conservatively, say 85 per cent. The total volume of sludge can then be computed. This method is more rational than the use of empirical formulas, but because of the estimates which must be made its results will probably be of no greater accuracy than those obtained empirically.
The digestion chamber is made in the form of an inverted cone or pyramid with side slopes at most about 2 horizontal to 1 vertical and preferably much steeper without necessitating too great a depth of tank. The purpose of the steep slope is to concentrate the sludge at the bottom of the hopper thus formed. Concrete is ordinarily used as the material of construction as a smooth surface can be obtained by proper workmanship. Where flat slopes have been used, a water pipe perforated at intervals of 6 to 12 inches may be placed at the top of the slopes, and water admitted for a short time to move the sludge when the tank is being cleaned.
A cast-iron pipe, 6 to 8 inches in diameter, is supported in an approximately vertical position with its open lower end supported about 12 inches above the lowest point in the digestion chamber. This is used for the removal of sludge. A straight pipe from the bottom of the tank to a free opening in the atmosphere is desirable in order to allow the cleaning of the pipe or the loosening of sludge at the start, and to prevent the accumulation of gas pockets. The sludge is led off through an approximately horizontal branch so located that from 4 to 6 feet of head are available for the discharge of the sludge. A valve is placed on the horizontal section of the pipe. A sludge pipe is shown in Fig. 162 and 163. Under such conditions, when the sludge valve is opened the sludge should flow freely. The hydraulic slope to insure proper sludge flow should not be less than 12 to 16 per cent. Where it is not possible to remove the sludge by gravity an air lift is the best method of raising it.
The volume of the transition or scum chamber should equal about one-half that of the digestion chamber. The surface area of the scum chamber exposed to the atmosphere should be 25 to 30 per cent of the horizontal projection of the top of the digestion chamber. Some tanks have operated successfully with only 10 per cent, but troubles from foaming can usually be anticipated unless ample area for the escape of gases has been provided.
All portions of the surface of the tank should be made accessible in order that scum and floating objects can be broken up or removed. The gas vents should be made large enough so that access can be gained to the sludge chamber through them when the tank is empty.
Precautions should be taken against the wrecking of the tank by high ground water when the tank is emptied. With an empty tank and high ground water there is a tendency for the tank to float. The flotation of the tank may be prevented by building the tank of massive concrete with a heavy concrete roof, by underdraining the foundation, or by the installation of valves which will open inwards when the ground water is higher than the sewage in the tank. Dependence should not be placed on the attendant to keep the tank full during periods of high ground water.
Roofs are not essential to the successful operation of Imhoff tanks. They are sometimes used, however, as for septic tanks, to assist in controlling the dissemination of odors, to minimize the tendency of the sewage to freeze, and to aid in bacterial activity. In the construction of a roof, ventilation must be provided as well as ready access to the tank for inspection, cleaning, and repairs.
=251. Imhoff Tank Results.=—The Imhoff tank has the advantage over the septic tank that it will not deliver sludge in the effluent, except under unusual conditions. The Imhoff tank serves to digest sludge better than a septic tank and it will deliver a fresher effluent than a plain sedimentation tank. Imhoff sludge is more easily dried and disposed of than the sludge from either a septic or a sedimentation tank. This is because it has been more thoroughly humified and contains only about 80 per cent of moisture. As it comes from the tank it is almost black, flows freely and is filled with small bubbles of gas which expand on the release of pressure from the bottom of the tank, thus giving the sludge a porous, sponge-like consistency which aids in drying. When dry it has an inoffensive odor like garden soil, and it can be used for filling waste land, without further putrefaction. It has not been used successfully as a fertilizer.
Offensive odors are occasionally given off by Imhoff tanks, even when properly operated. They also have a tendency to “boil” or foam. The boiling may be quite violent, forcing scum over the top of the transition chamber and sludge through the slot in the sedimentation chamber, thus injuring the quality of the effluent. The scum on the surface of the transition chamber may become so thick or so solidly frozen as to prevent the escape of gas with the result that sludge may be driven into the sedimentation chamber.
Some chemical analyses of Imhoff tank influents and effluents are given in Table 86 and the analyses of some sludges from Imhoff tanks are given in Table 83. It is to be noted that the nitrites and nitrates are still present in the effluent, whereas they are seldom present in the effluent from septic tanks. The per cent of moisture in the Imhoff sludge is less than that in the septic tank sludge, and its specific gravity is higher. It is heavier and more compact because of the longer time and the greater pressure it has been subjected to in the digestion chamber of the Imhoff tank.
=252. Status of Imhoff Tanks.=—The introduction of the Imhoff tank into the United States, like the introduction of the Burkli-Ziegler Run-Off Formula, and Kutter’s Formula, is to be credited to Dr. Rudolph Hering. He advised Dr. Imhoff to come to the United States to introduce his tank and gave him material aid through recommendations and introductions to engineers. Shortly after its introduction, in 1907, the tank became very popular and installations were made in many cities. This popularity was caused by a growing dissatisfaction with the septic tank, the litigation then progressing over septic patents, the production of inoffensive sludge, and the promising results which had been obtained in Germany. As a result of the extended experience obtained in the use of Imhoff tanks American engineers have learned that, like all other sewage treatment devices introduced up to the present time, the Imhoff tank requires experienced attention for its successful operation. These tanks are now being installed in the place of septic tanks, and they are frequently used in conjunction with sprinkling filters.
=253. Operation of Imhoff Tanks.=—The important feature in the successful operation of an Imhoff tank is the proper control of the sludge and transition chambers. During the ripening process, which may occupy 2 weeks to 3 months after the start of the tank, offensive odors may be given off, the tank may foam violently, and scum may boil over into the sedimentation chamber. This is usually due to an acid condition in the digestion chamber which may possibly be overcome by the addition of lime. A very fresh influent will have a similar effect. Too violent boiling is not likely to occur where the area for the escape of gas has been made large and the gas is not confined. Any accumulation of scum should be broken up and pushed down into the digestion chamber, or removed from the tank. The stream from a fire hose is useful in breaking up scum. The side walls of the sedimentation chamber should be squeegeed as frequently as is necessary to keep them free from sludge, which may be as often as once or twice a week. Material floating on the surface of the sedimentation chamber should be removed from the tank or sunk into the digestion chamber through the gas vents in the transition chamber.
No sludge should be removed, except for the taking of samples, until the tank is well ripened. The ripening of the sludge can be determined by examining a sample and observing its color and odor. An odorless, black, granular, well humified sludge is indicative of a ripened tank. After the tank has ripened, sludge should be removed in small quantities at 2 to 3–week intervals, except in cold or rainy weather. The sludge should be drawn off slowly to insure the removal of the oldest sludge at the bottom of the digestion chamber. After the drawing off of the sludge has ceased the pipe should be flushed with fresh water to prevent its clogging with dried sludge in the interim until the next removal. Under no circumstances should all the sludge in the tank be removed at any time. The removal of some sludge during foaming after ripening may reduce or stop the foaming. The ripening of a tank can be hastened by adding some sludge from a tank already ripened.
Sludge should not be allowed to accumulate within 18 inches of the slot at the bottom of the digestion chamber. The elevation of the surface of the sludge can be located by lowering into the tank, a stoppered, wide-mouthed bottle on the end of a stick. The stopper is pulled out by a string when the bottle is at some known elevation. The bottle is then carefully raised and observed for the presence of sludge. The process is repeated with the bottle at different elevations until the surface of the sludge has been discovered. Another method is to place the suction pipe of a small hand pump at known points, successively increasing in depth, and to pump in each position until one position is found at which sludge appears in the pump. When the sludge in one portion of the digestion chamber has risen higher than in another portion, the direction of flow in the sedimentation chamber should be reversed if possible. In the ordinary routine of operation it is never necessary to shut down an Imhoff tank. Sludge is removed while the tank is operating. The shut down of a tank will be caused by accidents and breaks to the structure or control devices.
=254. Other Tanks.=—The Travis Hydrolytic Tank represents a step in the development from the septic tank to the Imhoff tank. The Doten tank and the Alvord tank are recent developments, and are somewhat similar in operation to the Imhoff tank.
The Travis Hydrolytic Tank when first designed differed from the later design of the Imhoff tank in the slot between the sedimentation chamber and the digestion chamber which was not trapped against the escape of gas from the latter to the former, and in operation a small quantity of fresh sewage was allowed to flow through the digestion chamber. The tank is called a hydrolytic tank because some solids are liquefied in it. The tank is mainly of historic interest as designs similar to it are rarely made to-day. Better results are obtained from the use of the Imhoff tank. Recent developments have altered the original design of the Travis tank so that it is hardly recognizable. The Travis tank at Luton, Eng., is shown in Fig. 164. The detailed description given in the Engineering News in connection with this illustration shows that the governing object of the design is to separate as quickly as possible the sludge deposited by the sewage without septic action being set up. To aid in the collection and settlement of flocculent matter vertical wooden grids or colloiders are used. The suspended matter strikes these and forms a slimy deposit on them that in a short time slips off in pieces large enough to settle readily.
The Doten tank[158] is a single-storied, hopper-bottomed septic tank, views of which are shown in Fig. 165. It was devised by L. S. Doten for army cantonments during the War. Its chief purpose was to avoid the foaming and frothing so common to Imhoff tanks when overdosed with fresh sewage. The first Alvord tank was constructed in Madison, Wis., in 1913.[159] As now constructed the tank consists of three deep, single-story compartments with hopper bottoms. These compartments are arranged side by side in any one unit. Sewage enters at the surface of one of the compartments and is retained here during one-half of the total period of retention. It leaves the first compartment over a weir and passes in a channel over the top of the intermediate compartment to the third or effluent compartment, where it is held for the remainder of the period of detention. Accumulated scum and sludge are drawn off into the intermediate compartment at the will of the operator, this compartment being used for sludge digestion only. Such tanks as the Doten and the Alvord have been used for plants receiving very fresh sewages such as is discharged from military cantonments, in order to assist in the prevention of the foaming to be expected from an Imhoff tank receiving such a fresh influent. The tanks are suitable for small installations, or where excavation to the depth required for an Imhoff tank is not practicable.