XVII. FILTRATION AND IRRIGATION
=255. Theory.=—The cycle through which the elements forming organic matter pass from life to death and back to life again has been described in Chapter XIII. It has been shown in Chapter XVI that septic action occupies that portion of the cycle in which the combinations of these elements are broken down or reduced to simpler forms and the lower stages of the cycle are reached. The action in the filtration of sewage builds up the compounds again in a more stable form and almost complete oxidation is attained, dependent on the thoroughness of the filtration. In the filtration of sewage only the coarsest particles of suspended matter are removed by mechanical straining. The success of the filtration is dependent on biologic action. The desirable form of life in a filter is the so-called nitrifying bacteria which live in the interstices of the filter bed and feed upon the organic matter in the sewage. Anything which injures the growth of these bacteria injures the action of the filter. In a properly constructed and operated filter, all matter which enters in the influent, leaves with the effluent, but in a different molecular form. A slight amount may be lost by evaporation and gasification but this is more than made up by the nitrogen and oxygen absorbed from the atmosphere. The nitrifying action in sewage filtration is shown by the analysis of sewage passing through a trickling filter, as given in Tables 86 and 87. It is shown by the reduction of the content of organic nitrogen, free ammonia, oxygen consumed, and the increase in nitrites, nitrates, and dissolved oxygen. The reduction of suspended matter is interrupted periodically when the filter “unloads.” The suspended matter in the effluent is then greater than in the influent.
The nitrifying organisms have been isolated and divided into two groups—nitrosomonas, the nitrite formers, and nitrobacter, the nitrate formers. Experiments indicate that the growth of the nitrobacter organisms is dependent on the presence of the nitrosomonas organisms, which are in turn dependent on the presence of the putrefactive compounds resulting from the action of putrefying bacteria. The existence of these organisms is an example of symbiotic action in bacterial growth. The organisms have been found to grow best on rough porous material on which their zoögleal jelly can be easily deposited and affixed. Sewage filters were constructed to provide these ideal conditions before the action of a filter was thoroughly understood.
The action in irrigation is similar to that in filtration. Although more strictly a method of final disposal rather than preliminary treatment, the similarity of the actions which take place, and the grading of sand filtration into broad irrigation with no distinct line of difference has resulted in the inclusion of the discussion of irrigation in the same chapter with filtration.
=256. The Contact Bed.=—A contact bed is a water-tight basin filled with coarse material, such as broken stone, with which sewage and air are alternately placed in contact in such a manner that oxidation of the sewage is effected. A contact bed has some of the features of a sedimentation tank and an oxidizing filter. As such it marks a transitory step from anaërobic to aërobic treatment of sewage. A plan and a section of a contact bed are shown in Fig. 166.
Because of its dependence on biologic action a contact bed must be ripened before a good effluent can be obtained. The ripening or maturing occurs progressively during the first few weeks of operation, the earlier stages being more rapidly developed. The time required to reach such a stage of maturity that a good effluent will be developed will vary between one and six or eight weeks, dependent on the weather and the character of the influent. During the period of maturing the load on the bed should be made light.
The use of contact beds has been extensive where a more stable effluent than could be obtained from tank treatment has been desired, yet the best quality of effluent was not required. The sewage to undergo treatment in a contact bed should be given a preliminary treatment to remove coarse suspended matter. The efficiency of the contact treatment can be increased by passing the sewage through two or three contact beds in series. In double contact treatment the primary beds are filled with coarser material and operate at a more rapid rate than the secondary beds. Double contact gives better results than single contact, but triple contact treatment, though showing excellent results, is hardly worth the extra cost. An advantage which contact treatment has over all other methods of sewage filtration is that the bed can be so operated that the sewage is never exposed to view. As a result the odors from well-operated contact beds are slight or are entirely absent and there should be no trouble from flying insects. Such a method of treatment is favorable to plants located in populous districts and to the fancies of a landscape architect. Another advantage of the contact bed is the small amount of head required for its operation, which may be as low as 4 to 5 feet. This low head consumption by a sewage filter is equaled only by the intermittent sand filter.
The quality of the effluent from some contact beds is shown in Table 85. It is to be noted that nitrification has been carried to a fair degree of completion, and that the reduction of oxygen consumed has been marked. In comparison with the effluent from filters, contact effluent contains a smaller amount of nitrogen as nitrites and nitrates, and suspended solids. Contact effluent is usually clear and odorless, but it is not stable without dilution. The absence of nitrites and nitrates is sometimes advantageous as the effluent will not support vegetable growths dependent on this form of nitrogen. The absence of suspended solids obviates the use of secondary sedimentation basins which are needed with trickling filters. The head of 5 to 8 feet required for contact treatment is low in comparison to the 10 to 15 feet required for trickling filters, but is slightly higher than the head required for intermittent sand filtration. The cost of contact treatment is higher than the cost of trickling filters but is lower than the cost of intermittent sand filtration, as shown in Table 90.
TABLE 85
QUALITY OF EFFLUENTS FROM CONTACT BEDS
Report on Sewage Purification at Columbus, Ohio, by G. A. Johnson, 1905.
──────┬──────┬─────────┬───────┬────────┬───────────────────────────────── Filter│Depth,│ Size of │ Rate, │ Oxygen │ Nitrogen as │ Feet │Material │Million│Consumed│ │ │in Inches│Gallons│ │ │ │ │ per │ │ │ │ │ Acre │ │ │ │ │per Day│ │ ──────┼──────┼─────────┼───────┼────────┼───────┬───────┬────────┬──────── │ │ │ │ │Organic│ Free │Nitrites│Nitrates │ │ │ │ │ │Ammonia│ │ ──────┼──────┼─────────┼───────┼────────┼───────┴───────┴────────┴──────── │ │ │ │ │ Parts per Million │ │ │ │ │ │ │ │ A │ 5│0.25–1.00│ 0.953│ 23│ 3.5│ 8.7│ 0.20│ 1.6 B │ 5│0.25–2.00│ 1.514│ 21│ 4.0│ 8.4│ 0.15│ 1.4 C │ 5│0.25–1.50│ 1.222│ 24│ 3.5│ 10.8│ 0.11│ 0.6 D │ 5│0.50–1.50│ 1.405│ 22│ 3.3│ 9.5│ 0.13│ 0.9 │ │ │ │ │ │ │ │ │ │ │Per Cent Removal of Constituents of Applied Sewage │ │ │ │ │ │ │ │ A │ 5│0.25–1.00│ 0.953│ 48│ 49│ 10│ │ B │ 5│0.25–2.00│ 1.514│ 52│ 40│ 11│ │ C │ 5│0.25–1.50│ 1.222│ 47│ 31│ 12│ │ D │ 5│0.50–1.50│ 1.405│ 46│ 37│ 19│ │ ──────┴──────┴─────────┴───────┴────────┴───────┴───────┴────────┴────────
──────┬──────┬────────────────────┬───────── Filter│Depth,│ Suspended Matter │Dissolved │ Feet │ │ Oxygen │ │ │ │ │ │ │ │ │ │ │ │ ──────┼──────┼─────┬────────┬─────┼───────── │ │Total│Volatile│Fixed│ │ │ │ │ │ ──────┼──────┴─────┴────────┴─────┴───────── │ Parts per Million │ │ │ │ │ A │ 5│ 832│ 94│ 737│ 0.3 B │ 5│ 831│ 85│ 746│ 0.1 C │ 5│ 826│ 92│ 734│ 0.8 D │ 5│ 810│ 91│ 717│ 0.9 │ │ │ │ │ │ Per Cent Removal of Constituents of Applied Sewage │ │ │ │ │ A │ 5│ 73│ 70│ 76│ B │ 5│ 80│ 77│ 83│ C │ 5│ 70│ 70│ 70│ D │ 5│ 67│ 61│ 72│ ──────┴──────┴─────┴────────┴─────┴─────────
The depth of the contact bed is generally made from 4 to 6 feet. The deeper beds are less expensive per unit of volume, to construct, as the cost of the underdrains and the distribution system is reduced in relation to the capacity of the filter. The increased depth reduces the aëration, and the periods of filling and emptying are so increased as to limit the depths to the figures stated. The other dimensions of the bed are controlled by economy and local conditions, as the success of the contact treatment is not affected by the shape of the bed. Contact units are seldom constructed larger than one-half an acre in area, as larger beds require too much time for filling and emptying. A large number of small units is also undesirable because of the increased difficulty of control. In general it is well to build as large units as are compatible with efficient operation, elasticity of plant, and which can be filled within the time allowed at the average rate of sewage flow, or from dosing tanks in which the storage period is not so long as to produce septic conditions.
The interstices in a contact bed will gradually fill up, due to the deposition of solid matter on the contact material, the disintegration of the material, and the presence of organic growths. The period of rest allowed every five or six weeks tends to restore partially some of this lost capacity through the drying of the organic growths. It is occasionally necessary to remove the material from the bed and wash it in order to restore the original capacity. It may be necessary to do this three or four times a year, in an overloaded plant, or as infrequently as once in five or six years in a more lightly loaded bed. The period is also dependent on the character of the contact material and the quality of the influent. This loss of capacity may reduce the voids from an original amount of 40 to 50 per cent of voids to 10 to 15 per cent. If the bed is not overloaded the loss of capacity will not increase beyond these figures.
The rate of filtration depends on the strength of the sewage, the character of the contact material, and the required effluent. It should be determined for any particular plant as the result of a series of tests. For the purposes of estimation and comparison the approximate rate of filtration should be taken at about 94 gallons per cubic yard of filtering material per day on the basis of three complete fillings and emptyings of the tank. This is equivalent to 150,000 gallons per acre foot of depth per day, or for a bed 5 feet deep to a rate of 750,000 gallons per acre per day. The net rate for double or triple filtration is less than these figures, but on each filter the rates are higher.
The material of the contact bed should be hard, rough, and angular. It should be as fine as possible without causing clogging of the bed. Materials in successful use are: crushed trap rock or other hard stone, broken bricks, slag, coal, etc. Soft crumbling materials such as coke are not suitable as the weight of the superimposed material and the movement of the sewage crushes and breaks it into fine particles which accumulate in the lower portion of the filter and clog it. Roughness, porosity, and small size are desirable, as the greater the surface area the more rapid the deposition of material. After a short time, however, the advantages of roughness and porosity are lost, as the sediment soon covers all unevenness alike. The minimum size of the material is limited by the tendency towards clogging. The sizes in successful use vary between ¼ and ¾ of an inch, ½ inch being a common size. The same size of material is used throughout the depth of the bed except that the upper 6 inches may be composed of small white pebbles or other clean material, which does not come in contact with the sewage and which will give an attractive appearance to the plant. In double or triple contact beds 3 or 4–inch material is sometimes used for the primary beds, and ¼-inch material in the final bed.
Sewage may be applied at any point on or below the surface. The sewage is withdrawn from the bottom of the bed. It is undesirable to have too few inlet or outlet openings as the velocity of flow about the openings will be so great as to disturb the deposit on the contact material. The distribution system and the underdrains for the bed at Marion, Ohio, are shown in Fig. 166.
The cycle of operation of a contact bed is divided into four periods. A representative cycle might be: time of filling, one hour; standing full, 2 hours; emptying, one hour; standing empty, 4 hours. The length of these periods is the result of long experience based on many tests and are an average of the conclusions reached. Wide variations from them may be found in different plants, and tests may show successful results with different periods. The combination of these four periods is known as the contact cycle.
The period of filling should be made as short as possible without disturbing the material of the bed nor washing off the accumulated deposits. The sewage should not rise more rapidly than one vertical foot per minute. During the contact or standing full period sedimentation and adsorption of the colloids are occurring on the area of surface exposed to the sewage. This period should be of such length that septic action does not become pronounced, and long enough to permit of thorough sedimentation. The period of emptying should be made as short as possible without disturbing the bed, on the same basis that the period of filling is determined. During the period of standing empty, air is in contact with the sediment deposited in thin layers on the contact material, and the oxidizing activities of the filter are taking place. The filter is given a rest period of one or two days every five or six weeks, in order that it may increase its capacity and its biologic activity.
The control of a contact bed may be either by hand or automatic, the latter being the more common. Hand control requires the constant attention of an operator and results in irregularity of operation, whereas automatic control will require inspection not more than once a day and insures regularity of operation. A number of automatic devices have been invented which give more or less satisfaction. The air-locked automatic siphons, without moving parts, have proven satisfactory and are practically “fool-proof.” The operation of these devices is explained in Chapter XXI.
=257. The Trickling Filter.=—A trickling or sprinkling filter is a bed of coarse, rough, hard material over which sewage is sprayed or otherwise distributed and allowed to trickle slowly through the filter in contact with the atmosphere. A general view of a trickling filter in operation at Baltimore is shown in Fig. 167. The action of the trickling filter is due to oxidation by organisms attached to the material of the filter. The solid organic matter of the sewage deposited on the surface of the material, is worked over and oxidized by the aërobic bacteria, and is discharged in the effluent in a more highly nitrified condition. At times the discharge of suspended matter becomes so great that the filter is said to be unloading. The action differs from that in a contact bed in that there is no period of septic or anaërobic action and the filter never stands full of sewage.
The effluent from a trickling filter is dark, odorless, and is ordinarily non-putrescible. Analyses of typical effluents are given in Tables 86 and 87. The unloading of the filter may occur at any time, but is most likely to occur in the spring or in a warm period following a period of low temperatures. It causes higher suspended matter in the effluent than in the influent and may render the effluent putrescible. The action is marked by the discharge of solid matter which has sloughed off of the filter material and which increases the turbidity of the effluent. Where the diluting water is insufficient to care for the solids so carried in the effluent, they can be removed by a 2–hour period of sedimentation. The effluent may become septic during this time, however. The nitrogen in the effluent is almost entirely in the form of nitrates, and the percentage of saturation with dissolved oxygen is high. The effluent is more highly nitrified than that from a contact bed, and its relative stability is also higher, thus demanding a smaller volume of diluting water.
The principal advantage of a trickling filter over other methods of treatment is its high rate which is from two to four times faster than a contact bed, and about seventy times faster than an intermittent sand filter. The greatest disadvantage is the head of 12 to 15 feet or more necessary for its operation. Sedimentation of the effluent is usually necessary to remove the settleable solids. During the period of secondary sedimentation the quality of the filter effluent may deteriorate in relative stability. In winter the formation of ice on the filter results in an effluent of inferior quality, but as the diluting water can care for such an effluent at this time the condition is not detrimental to the use of the trickling filter. In summer the filters sometimes give off offensive odors that can be noticed at a distance of half a mile, and flying insects may breed in the filter in sufficient quantities to become a nuisance if preventive steps are not taken. The dissemination of odors is especially marked when treating a stale or septic sewage. The treatment of a fresh sewage seldom results in the creation of offensive odors.
TABLE 86
ANALYSIS OF CRUDE SEWAGE, IMHOFF TANK, AND SPRINKLING FILTER EFFLUENTS AT ATLANTA, GEORGIA
(Engineering Record, Vol. 72, p. 4)
─────────┬───────────┬────────────────────────────────────────── │Temperature│ Parts per Million │Fahrenheit │ │ │ │ │ ─────────┼───────────┼─────────────────────────────────┬──────── │ │ Nitrogen as │ Oxygen │ │ │Consumed ─────────┼───────────┼───────┬───────┬────────┬────────┼──────── │ │Organic│ Free │Nitrites│Nitrates│ │ │ │Ammonia│ │ │ ─────────┴───────────┴───────┴───────┴────────┴────────┴────────
Crude Sewage
─────────┬───────────┬───────┬───────┬────────┬────────┬──────── 1913 │ │ │ │ │ │ Maximum │ 77│ 15.6│ 21.8│ 0.1│ 3.0│ 100.0 Minimum │ 61│ 10.4│ 16.5│ 0.1│ 1.4│ 78.3 Average │ 70│ 12.8│ 18.8│ 0.1│ 2.2│ 90.6 1914 (7 │ │ │ │ │ │ months)│ │ │ │ │ │ Maximum │ 74│ 16.0│ 33.4│ │ 2.3│ Minimum │ 60│ 9.5│ 18.1│ │ 1.6│ Average │ 66│ 13.4│ 27.1│ │ 2.0│ ─────────┴───────────┴───────┴───────┴────────┴────────┴────────
Imhoff Effluent
─────────┬───────────┬───────┬───────┬────────┬────────┬──────── 1913 │ │ │ │ │ │ Maximum │ 78│ 13.2│ 21.9│ 0.2│ 3.1│ 68.0 Minimum │ 58│ 6.5│ 16.8│ 0.1│ 1.1│ 53.1 Average │ 68│ 9.0│ 20.0│ 0.2│ 2.1│ 60.1 1914 (7 │ │ │ │ │ │ months)│ │ │ │ │ │ Maximum │ 77│ 10.3│ 30.3│ │ 2.0│ Minimum │ 59│ 4.1│ 18.0│ │ 1.5│ Average │ 65│ 7.7│ 25.9│ │ 1.8│ ─────────┴───────────┴───────┴───────┴────────┴────────┴────────
Sprinkling Filter Effluent
─────────┬───────────┬───────┬───────┬────────┬────────┬──────── 1913 │ │ │ │ │ │ Maximum │ 79│ 5.6│ 14.2│ 0.8│ 11.3│ 32.1 Minimum │ 55│ 2.6│ 6.2│ 0.5│ 5.8│ 23.6 Average │ 66│ 3.8│ 9.9│ 0.7│ 8.2│ 28.2 1914 (7 │ │ │ │ │ │ months)│ │ │ │ │ │ Maximum │ 77│ 8.5│ 20.7│ │ 11.2│ Minimum │ 55│ 4.4│ 8.8│ │ 3.6│ Average │ 63│ 5.7│ 15.2│ │ 7.2│ ─────────┴───────────┴───────┴───────┴────────┴────────┴────────
─────────┬────────────────────┬──────────┬───────── │ Parts per Million │ Per Cent │Relative │ │Saturation│Stability │ │Dissolved │ │ │ Oxygen │ ─────────┼────────────────────┼──────────┼───────── │ Suspended Matter │ │ │ │ │ ─────────┼─────┬────────┬─────┼──────────┼───────── │Total│Volatile│Fixed│ │ │ │ │ │ │ ─────────┴─────┴────────┴─────┴──────────┴─────────
Crude Sewage
─────────┬─────┬────────┬─────┬──────────┬───────── 1913 │ │ │ │ │ Maximum │ 371│ 154│ 163│ 47│ Minimum │ 222│ 98│ 112│ 11│ Average │ 285│ 126│ 138│ 28│ 1914 (7 │ │ │ │ │ months)│ │ │ │ │ Maximum │ 431│ │ │ 48│ Minimum │ 279│ │ │ 12│ Average │ 351│ │ │ 30│ ─────────┴─────┴────────┴─────┴──────────┴─────────
Imhoff Effluent
─────────┬─────┬────────┬─────┬──────────┬───────── 1913 │ │ │ │ │ Maximum │ 90│ 50│ 41│ │ Minimum │ 35│ 42│ 21│ │ Average │ 68│ 46│ 33│ │ 1914 (7 │ │ │ │ │ months)│ │ │ │ │ Maximum │ 73│ │ │ 48│ Minimum │ 49│ │ │ 34│ Average │ 65│ │ │ 43│ ─────────┴─────┴────────┴─────┴──────────┴─────────
Sprinkling Filter Effluent
─────────┬─────┬────────┬─────┬──────────┬───────── 1913 │ │ │ │ │ Maximum │ 60│ 31│ 28│ 76│ 99 Minimum │ 33│ 26│ 28│ 52│ 88 Average │ 49│ 28│ 28│ 64│ 89 1914 (7 │ │ │ │ │ months)│ │ │ │ │ Maximum │ 106│ │ │ 79│ 99 Minimum │ 40│ │ │ 55│ 89 Average │ 62│ │ │ 65│ 95 ─────────┴─────┴────────┴─────┴──────────┴─────────
TABLE 87
EFFICIENCY OF SPRINKLING FILTER CHICAGO, ILLINOIS
Depth of Filter 9 feet. Size of stone 2 in. to 3 in.
────────┬───────────────────────────┬─────────────────────────── Month │ Organic Nitrogen │ Free Ammonia │ │ ────────┼───────────────────────────┼─────────────────────────── │ │ ────────┼─────────┬─────────┬───────┼─────────┬─────────┬─────── │Influent,│Effluent,│ Per │Influent,│Effluent,│ Per │Parts per│Parts per│ Cent │Parts per│Parts per│ Cent │ Million │ Million │Removed│ Million │ Million │Removed ────────┼─────────┼─────────┼───────┼─────────┼─────────┼─────── 1910 │ │ │ │ │ │ October │ 5.1│ 2.8│ 45│ 12.0│ 4.6│ 62 November│ 5.9│ 2.5│ 58│ 12.0│ 5.9│ 51 December│ 4.6│ 3.0│ 35│ 12.0│ 6.9│ 42 │ │ │ │ │ │ 1911 │ │ │ │ │ │ January │ 6.3│ 4.8│ 24│ 11.0│ 7.0│ 36 February│ 9.0│ 4.8│ 47│ 10.0│ 7.2│ 28 March │ 8.3│ 3.5│ 58│ 9.9│ 6.4│ 35 April │ 6.4│ 4.0│ 37│ 8.3│ 3.6│ 69 May │ 7.6│ 5.4│ 29│ 9.2│ 2.4│ 74 June │ 5.9│ 3.2│ 46│ 11.0│ 0.6│ 95 July │ 6.2│ 4.2│ 32│ 11.0│ 1.3│ 88 ────────┴─────────┴─────────┴───────┴─────────┴─────────┴───────
────────┬───────────────────────────┬─────────────────────────── Month │ Oxygen Consumed │ Nitrites │ │ ────────┼───────────────────────────┼─────────────────────────── │ │ ────────┼─────────┬─────────┬───────┼─────────┬─────────┬─────── │Influent,│Effluent,│ Per │Influent,│Effluent,│ Per │Parts per│Parts per│ Cent │Parts per│Parts per│ Cent │ Million │ Million │Removed│ Million │ Million │Removed ────────┼─────────┼─────────┼───────┼─────────┼─────────┼─────── 1910 │ │ │ │ │ │ October │ 30│ 15│ 50│ │ .90│ November│ 35│ 15│ 57│ │ .76│ December│ 39│ 20│ 49│ .07│ .45│ 6.4 │ │ │ │ │ │ 1911 │ │ │ │ │ │ January │ 42│ 20│ 52│ .08│ .15│ 1.9 February│ 46│ 20│ 56│ .09│ .15│ 1.7 March │ 47│ 21│ 56│ .09│ .15│ 1.7 April │ 38│ 21│ 45│ .16│ .21│ 1.3 May │ 33│ 31│ 6│ .08│ .38│ 4.8 June │ 28│ 16│ 43│ .00│ .30│ ∞ July │ 34│ 26│ 24│ .00│ .36│ ∞ ────────┴─────────┴─────────┴───────┴─────────┴─────────┴───────
────────┬───────────────────────────┬─────────────────────────── Month │ Nitrates │ Dissolved Oxygen │ │ ────────┼───────────────────────────┼─────────────────────────── │ │ ────────┼─────────┬─────────┬───────┼─────────┬─────────┬─────── │Influent,│Effluent,│ Per │Influent,│Effluent,│ Per │Parts per│Parts per│ Cent │Parts per│Parts per│ Cent │ Million │ Million │Removed│ Million │ Million │Removed ────────┼─────────┼─────────┼───────┼─────────┼─────────┼─────── 1910 │ │ │ │ │ │ October │ │ 7.8│ │ 0.0│ 8.5│ ∞ November│ │ 5.9│ │ 0.0│ 8.1│ ∞ December│ .15│ 2.6│ 17│ 2.0│ 8.4│ 4.2 │ │ │ │ │ │ 1911 │ │ │ │ │ │ January │ .27│ 2.2│ 8.2│ 3.0│ 7.8│ 2.9 February│ .50│ 2.6│ 5.2│ 2.6│ 8.0│ 3.1 March │ .34│ 3.2│ 9.4│ 2.2│ 6.6│ 3.0 April │ .53│ 4.5│ 8.5│ 2.1│ 7.1│ 3.4 May │ .15│ 7.5│ 4.3│ 0.1│ 7.7│ 77 June │ .16│ 8.3│ 5.2│ 0.0│ 7.6│ ∞ July │ .09│ 7.7│ 8.0│ 0.0│ 6.5│ ∞ ────────┴─────────┴─────────┴───────┴─────────┴─────────┴───────
────────┬───────────┬─────────────────────────────────────────────────────── Month │ Per Cent │ Suspended Matter │Putrescible│ ────────┼───────────┼───────────────────────────┬─────────────────────────── │ │ Total │ Volatile ────────┼───────────┼─────────┬─────────┬───────┼─────────┬─────────┬─────── │ │Influent,│Effluent,│ Per │Influent,│Effluent,│ Per │ │Parts per│Parts per│ Cent │Parts per│Parts per│ Cent │ │ Million │ Million │Removed│ Million │ Million │Removed ────────┼───────────┼─────────┼─────────┼───────┼─────────┼─────────┼─────── 1910 │ │ │ │ │ │ │ October │ 0│ 75│ 40│ 47│ 54│ 25│ 54 November│ 5│ 61│ 16│ 74│ 52│ 15│ 71 December│ 35│ 85│ 40│ 53│ 60│ 26│ 57 │ │ │ │ │ │ │ 1911 │ │ │ │ │ │ │ January │ 38│ 112│ 43│ 63│ 68│ 29│ 57 February│ 29│ 100│ 49│ 51│ 64│ 32│ 50 March │ 28│ 106│ 37│ 65│ 63│ 22│ 65 April │ 9│ 113│ 68│ 40│ 59│ 35│ 41 May │ 6│ 88│ 150│ 1.7│ 54│ 70│ 1.3 June │ 1│ 92│ 77│ 18│ 56│ 36│ 36 July │ 4│ 155│ 130│ 16│ 74│ 61│ 18 ────────┴───────────┴─────────┴─────────┴───────┴─────────┴─────────┴───────
────────┬─────────────────────────── Month │ Suspended Matter │ ────────┼─────────────────────────── │ Fixed ────────┼─────────┬─────────┬─────── │Influent,│Effluent,│ Per │Parts per│Parts per│ Cent │ Million │ Million │Removed ────────┼─────────┼─────────┼─────── 1910 │ │ │ October │ 21│ 15│ 29 November│ 9│ 1│ 89 December│ 25│ 14│ 44 │ │ │ 1911 │ │ │ January │ 44│ 13│ 70 February│ 37│ 17│ 53 March │ 43│ 15│ 65 April │ 54│ 33│ 39 May │ 34│ 80│ 2.4 June │ 36│ 41│ 1.1 July │ 81│ 69│ 15 ────────┴─────────┴─────────┴───────
NOTE.—Italic figures represent increases.
Raw sewage cannot be treated successfully on a trickling filter. Coarse solid particles should be screened and settled out, in order that the distributing devices or the filter may not become clogged. The effluent from an Imhoff tank has proven to be a satisfactory influent for a trickling filter. A septic tank effluent may be so stale as to be detrimental to the biologic action in the filter.
In the operation of a trickling filter the sewage is sprayed or otherwise distributed as evenly as possible in a fine spray or stream, over the top of the filtering material. The sewage then trickles slowly through the filter to the underdrains through which it passes to the final outlet. The distribution of the sewage on the bed is intermittent in order to allow air to enter the filter with the sewage. The cycle of operation should be completed in 5 to 15 minutes, with approximately equal periods of rest and distribution. Cycles of too great length will expose the filter to drying or freezing and will give poorer distribution throughout the filter. Cycles which are too short will operate successfully only with but slight variation in the rate of sewage flow. In some plants it has been found advantageous to allow the filters to rest for one day in 3 to 6 weeks or longer, dependent on the quality of the effluent.
The rate of filtration may be as high as 2,000,000 gallons per acre per day, which is equivalent to 200 gallons per cubic yard of material per day in a bed 6 feet deep. This is more than double the rate permissible in a contact bed. The exact rate to be used for any particular plant should be determined by tests. It is dependent on the quality of the sewage to be treated, on the depth of the bed, the size of the filling material, the weather, and other minor factors.
The filtering material is similar to that used in a contact bed. It should consist of hard, rough, angular material, about 1 to 2 inches in size. Larger sizes will permit more rapid rates of filtration, but will not produce so good an effluent. Smaller sizes will clog too rapidly.
The depth of the filter is limited by the possibility of ventilation and the strength of the filtering material to withstand crushing. The deeper the bed the less the expense of the distribution and collecting system for the same volume of material, and the more rapid the permissible rate of filtration. The depths in use vary between 6 and 10 feet, with 6 to 8 feet as a satisfactory mean. From a biologic standpoint the action of the filter seems to be proportional to the volume of the filtering material and therefore proportional to the depth of the bed, being limited to a minimum depth of about 5 feet, below which sewage may pass through the filter without treatment. The shape and other dimensions of the filter depend on the local conditions and the economy of construction. The filters need not be broken up into units by water-tight dividing walls. One filter can be constructed sufficient for all needs and various portions of it can be isolated as units by the manipulation of valves in the distribution system. Ventilation is provided by the air entrained with the sewage as it falls upon the surface. If the sides of the filter are built of open stone crib work the ventilation will be greatly improved, but it will not be possible to flood the filters to keep down flies, and in cold climates these openings must be covered in winter to prevent freezing. Filters have been constructed without side walls, the filtering material being allowed to assume its natural angle of repose. This has usually been found to be more expensive than the construction of side retaining walls, due to the unused filling material and the extra underdrains required.
The distribution of sewage is ordinarily effected by a system of pipes and spray nozzles as shown in Fig. 168 and 169. Other methods of distribution have been used. At Springfield, Mo.,[160] a moving trough from which the sewage flows continuously is drawn back and forth across the bed by means of a cable. In England circular beds have been constructed and the sewage distributed on them through revolving perforated pipes. At the Great Lakes Naval Training Station[161] the distributing pipes in the plant, now abandoned, were supported above the surface of the filter. The sewage fell from holes in the lower side of these pipes on to brass splash plates 14 inches above the filter. It was deflected horizontally from these plates over the filter surface. Pipes and spray nozzles have been adopted almost universally in the United States. Splash plates, traveling distributors, and other forms of distribution have been used only in exceptional cases. In a distributing system consisting of pipes and nozzles, a network of pipes is laid out somewhat as shown in Fig. 168, in such a manner that the head loss to all points is approximately equal. The number of valves required should be reduced to a minimum. The pipes may be laid out with the main feeders leading from a central point and branches at right angles to them, somewhat on the order of a spider’s web, or they may be laid out on a rectangular or gridiron system. The radial system is advantageous because of the central location of the control house, but it does not always lend itself favorably to the local conditions, and the piping and nozzle location are not so simple. The gridiron system lends itself favorably to the equalization of head losses. The pipes used should be larger than would be demanded by considerations of economy alone, both for the purpose of reduction of head loss and ease in cleaning. No pipe less than 6 inches in diameter should be used, and the average velocity of flow should not exceed one foot per second. Cast-iron, concrete, or vitrified clay pipe may be used, but cast iron is the material commonly used. The system should be arranged for easy flushing and cleaning and the pipes so sloped that the entire system can be drained in case of a shut down in cold weather.
The pipes are placed far enough below the surface of the filling material so that the top of the spraying nozzle is 6 to 12 inches above the surface of the filter. If the pipes are placed near the surface they are accessible for repairs, but are exposed to temperature changes. If the pipes are large their presence near the surface of the filter may seriously affect the distribution of the sewage through the filter. If the distributing pipes are placed near the bottom of the filter they are inaccessible for repairs and the nozzles must be connected to them by means of long riser pipes. The distributing pipes should be supported by columns extending to the foundation of the filter bed, there being a column at every pipe joint with such intermediate supports as may be required. In some plants the pipes have been supported by the filtering material. Although slightly less expensive in first cost the practice of so supporting the pipes is poor, as settling of the material may break the pipe or cause leaks, and if the bed becomes clogged, removal of the material is made more difficult. Valves should be placed in the distributing system in such a manner that different sets of nozzles can be cut out at will, thus resting those portions of the filter and permitting repairs without shutting down the entire filter.
The spacing of the nozzles is fixed by the type and size of the nozzle, the available head, and the rate of filtration. Various types of sprinkler nozzles are shown in Fig. 169 and the discharge rates, head losses, and distances to which sewage is thrown for the Taylor nozzles, are shown in Fig. 170. Nozzles are available which will throw circular, square, or semicircular sprays. In the use of circular sprays there is necessarily some portion of the filter which is underdosed if the nozzles are placed at the corners of squares with the sprays tangent, and there is an overdosing of other portions if the sprays are allowed to overlap so that no portion of the filter is left without a dose. Rectangular sprays will apparently overcome these difficulties, but studies have shown that circular sprays with some overlapping, and the nozzles placed at the apexes of equilateral triangles as shown in Fig. 172 will give as satisfactory distribution as other forms.
The nozzles should be selected to give the best distribution, to consume all of the head available, and to give the proper cycle of operation. The entire head available should be consumed in order that the fewest number of nozzles may be used. An excellent study of the characteristics of various types of nozzles has been published in Bulletin No. 3 of the Engineering Experiment Station at Purdue University, 1920. As a result of the tests on the nozzles shown in Fig. 169, it was determined for all nozzles, except No. 8, that
Q = Ca√(2_gh_);
in which Q = the rate of discharge in cubic feet per second;
C = a coefficient shown in Table 88;
a = the net cross-sectional opening of the nozzle in square feet;
h = the pressure on the nozzle in feet of water.
TABLE 88
COEFFICIENTS OF DISCHARGE FOR SPRINKLER NOZZLES SHOWN IN FIG. 169
──────────────────────┬──────┬──────┬──────┬──────┬──────┬──────┬────── Nozzle Number │ 1 │ 2 │ 3 │ 4 │ 5 │ 6 │ 7 ──────────────────────┼──────┼──────┼──────┼──────┼──────┼──────┼────── Coefficient │ .648 │ .756 │ .696 │ .666 │ .675 │ .598 │ .569 ──────────────────────┴──────┴──────┴──────┴──────┴──────┴──────┴──────
It is evident that if the head on the nozzles is constant and the nozzle throws a circular spray, the intensity of dosing at the circumference will be greater than nearer the center. This difficulty is overcome by so designing the dosing tank from which the sewage is fed that the head on the nozzle and the quantity thrown will vary in such a manner that the distribution over the bed is equalized. Intermittent action is obtained by an automatic siphon which commences to discharge when the tank is full and empties the tank in the period allowed for dosing. Under such conditions the tank should discharge for a longer time at the higher heads than at the lower heads as there is more territory to be covered at the higher heads. The design of the tank to do this with exactness is difficult, and the construction of the necessary curved surfaces is expensive. Where a dosing tank is used for such conditions it has been found satisfactory to construct the tank with plane sides sloping at approximately 45 degrees from the vertical (or horizontal). A tank with curved surfaces is shown in Fig. 171. The dosing siphon is usually placed in the tank as shown in the figure. The head and quantity of discharge through the nozzles can be varied also by maintaining a constant depth in a dosing tank by means of a float feed valve, and varying the head and quantity discharged to the nozzles by a butterfly valve in the main feed line, or by the use of a Taylor undulating valve designed for this purpose. The butterfly valve is opened and closed by a cam so designed and driven at such a rate that the required distribution is obtained. The Taylor undulating valve is opened and closed at a constant rate, the shape of the valve giving the required variations in head and discharge. Other methods of control have been attempted but have not been used extensively.
An example of the design of the nozzle layout and dosing tank for a sprinkling filter follows:
Let it be required to determine the nozzle layout for one acre of sprinkling filters with 5 feet available head on the nozzles.
The selection of the type of nozzle and the size of opening is a matter of judgment and experience. Nozzles with large openings are less liable to clog and fewer nozzles are needed than where small nozzles are used, but the distribution of sewage is not so even as with the use of small nozzles. In this example Taylor circular spray nozzles will be selected. Fig. 170 shows that a Taylor circular spray nozzle will discharge 22.3 g.p.m. under a head of 5 feet, and that the economical nozzle spacing will be 15.3 feet. The least number of nozzles at this spacing required for a bed of one acre in area is found as follows: In Fig. 172, let n equal the number of nozzles in a horizontal row, counting half-spray nozzles as ½, and let m equal the number of rows counting rows of half-spray nozzles as half rows.[162] Then the number of nozzles, N, equals mn, and 15.3_m_ × 13.2_n_ equals 43,560 or mn equals 215.
The next step should be the design of the dosing tank and siphon. It is possible to design a tank which will give equal distribution over equal areas of filter surface. It has been found, however, that the expense of this refinement is unwarranted as there are a number of outside factors which tend to overcome the theoretical design. The effect of wind, unequal spacing, and irregularities in the elevation of the nozzles have a tendency to offset refinements in the design of a dosing tank. It is therefore the general practice to slope the sides of the tank at an angle of about 45 degrees as previously stated. The dosing tank is generally designed to have a capacity which will give a complete cycle of operation once in 15 minutes. In the ordinary design the factors given are the rate of inflow and the given time of filling. In the following example the time of filling will be taken as 10 minutes, the time of emptying as 5 minutes, and the rate of flow as 1,000,000 gallons per day. The capacity of the tank will therefore be (1,000,000)⁄24 x 6 = 7,000 gallons. The diameter of the siphon to be selected can be computed as follows:
Let Q = the capacity of the tank in cubic feet; _q__{1} = the rate of discharge of the siphon in cubic feet per second; _q__{2} = the rate of inflow to the tank in cubic feet per second; q = the rate of emptying the tank in cubic feet per second = (_q__{1} − _q__{2}); A = the cross-sectional area of the free surface of the water in the tank at any instant, in square feet; a = the cross-sectional area of the siphon in square feet; b = the small dimension of the base of the tank in feet; h = the head of water, in feet, on the discharge siphon; _h__{1} = the initial head of water, in feet, on the siphon; _h__{2} = the final head of water in feet, on the siphon; t = the time, in seconds, required to empty the tank,
then dQ = -Adh = _q__{1}dt − _q__{2}dt,
and dt = (dQ)⁄q = − Adh⁄(_q__{1} − _q__{2}),
but _q__{1} = 0.4 A √((2_gh_)),[163]
therefore t = ∫_{_h__{2}}^{_h__{1}} -Adh⁄(0.4_a_√(2_gh_) − _q__{2}),
but A = 4_h_^2 + 4_bh_ + b^2,
therefore t = ∫_{_h__{1}}^{_h__{2}} ((b^2 + 4_bh_ + 4_h_^2)dh)⁄0.4_a_√(2_gh_) − _q__{2}.
The integration of this expression is tedious. Its solution for siphons between 6 inches and 12 inches operating under heads commencing from 3 feet to 6 feet, with a time of emptying of 5 minutes and time of filling of 10 minutes is given in Fig. 173. In the example given the rate of inflow is 1.55 sec. feet and the head is 5 feet. Then from Fig. 173 the size of the siphon to be used is 12 inches. Where a siphon of the size required to empty the tank in the time fixed is not available, combinations of available sizes can sometimes be used.
For example, if the given head is 6 feet, and the rate of inflow is 1.4 sec. feet, it is evident from Fig. 173 that a 6,300–gallon dosing tank and two 8–inch siphons will give the required cycle.
The method used for the design of the setting of Taylor nozzles by the Pacific Flush Tank Co., is less rational but more simple and probably as satisfactory. In this method the steps are as follows:
(1) Divide the maximum daily rate of sewage flow by 1,000 to get the maximum minute inflow.
(2) The number of nozzles required is determined by dividing the preceding figure by 6. Generally a Taylor nozzle with an orifice of ⅞ of an inch will discharge about 20 g.p.m. at the high head and about 8 g.p.m. at the low head, and as the nozzles must have a capacity which will take care of the inflow at the low head, the divisor 6 is used as a factor of safety instead of using 8 as the divisor.
(3) The type of nozzle to be used is selected from experience or as a matter of judgment. Circular-spray nozzles are more generally used.
(4) The spacings are determined from Fig. 170.
(5) The dosing tank of the shape described is then designed. The capacity is such as to give a complete cycle once every 15 minutes. The method of this design is similar to that followed previously.
(6) The dosing siphons are designed so that they will have a capacity at the minimum head of from 40 to 50 per cent in excess of the maximum minute inflow, and the draining depth of the siphon will be limited to a maximum of 5 to 5½ feet. The siphons are all made adjustable with a variation of 6 inches or more on either side of the normal discharge line so that the spraying area and cycle can be varied to secure the best results.
The underdrainage of a trickling filter should consist of some form of false bottom such as the types shown in Fig. 174. Where possible the underdrains should be open at both ends for the purpose of ventilation and flushing. It is desirable that the drains be so arranged that a light can be seen through them in order that clogging can be easily located. The drains should be placed on a slope of approximately 2 in 100 towards a main collector. The length of the drains is limited by their capacity to carry the average dose from the area drained by them. The main collecting conduits must be designed in accordance with the hydraulic principles given in Chapter IV. No valves, or other controlling apparatus, are placed on the underdrains or outlets from the filter.
Covers have been provided in winter for some trickling filters in cold climates. The Taylor sprinkling nozzle has been found to work successfully in extremely cold weather, and it is generally accepted that the covering of filters is unnecessary, if the filter is not to be shut down for any length of time in cold weather.
The operation of devices for automatically controlling the operation of a trickling filter is explained in Chapter XXI.
=258. Intermittent Sand Filter.=—An intermittent sand filter is a specially prepared bed of sand, or other fine grained material, on the surface of which sewage is applied intermittently, and from which the sewage is removed by a system of underdrains. It differs from broad irrigation in the character of the material, the care and preparation of the bed, and the thoroughness of the underdrainage. A distinctive feature of the intermittent sand filter is the quality of the effluent delivered by it. In a properly designed and operated plant the effluent is clear, colorless, odorless, and sparkling. It is completely nitrified, is stable and contains a high percentage of dissolved oxygen. It contains no settleable solids except at widely separated periods when a small quantity may appear in the effluent. The percentage removal of bacteria may be from 98 to 99 per cent. Some analyses of sand filter effluents are given in Table 89. The dissolved solids, the remaining bacteria, and the antecedents of the effluent are the only differences between it and potable water. An effluent from an intermittent sand filter is the most highly purified effluent delivered by any form of sewage treatment. The effluent can be disposed of without dilution, on account of its high stability. The treatment of sewage to so high a degree is seldom required, so that the use of intermittent filters is not common. Other drawbacks to their use are the relatively large area of land necessary and the difficulty of obtaining good filter sand in all localities.
TABLE 89
QUALITY OF EFFLUENTS FROM SAND FILTERS
(Report on Sewage Purification at Columbus, Ohio, by G. A. Johnson, 1905) ────────────┬───────────────────────────────────────────────────────┬─────── Source of │ Parts per Million │Rate of Sample │ │Filtra- │ │ tion │ │Gallons │ │ per │ │ Acre, │ │per Day ────────────┼────────────────────────────────────┬────────┬─────────┼─────── │ Nitrogen as │ Oxygen │ Oxygen │ │ │Consumed│Dissolved│ ────────────┼───────┬──────────┬────────┬────────┼────────┼─────────┼─────── │ Free │Albuminoid│Nitrites│Nitrates│ │ │ │Ammonia│ Ammonia │ │ │ │ │ ────────────┼───────┼──────────┼────────┼────────┼────────┼─────────┼─────── Filter │ 11.0 │ 8.6 │ │ │ 59. │ │ influent │ │ │ │ │ │ │ from grit │ │ │ │ │ │ │ chamber │ │ │ │ │ │ │ Filter │ 1.12 │ 0.88 │ 0.08 │ 11.5 │ 6.9 │ 6.3 │ 0.081 effluent │ │ │ │ │ │ │ Filter │ 0.81 │ 0.88 │ 0.10 │ 12.6 │ 6.5 │ 6.2 │ 0.118 effluent │ │ │ │ │ │ │ ────────────┼───────┼──────────┼────────┼────────┼────────┼─────────┼─────── Filter │ 9.7 │ 5.4 │ │ │ 33. │ │ influent │ │ │ │ │ │ │ from plain│ │ │ │ │ │ │ settling │ │ │ │ │ │ │ tank │ │ │ │ │ │ │ Filter │ 0.62 │ 0.77 │ 0.11 │ 14.9 │ 6.0 │ 8.2 │ 0.139 effluent │ │ │ │ │ │ │ Filter │ 0.99 │ 1.10 │ 0.10 │ 12.6 │ 7.8 │ 6.5 │ 0.274 effluent │ │ │ │ │ │ │ Filter │ 2.61 │ 1.39 │ 0.09 │ 9.0 │ 9.7 │ 3.9 │ 0.357 effluent │ │ │ │ │ │ │ ────────────┼───────┼──────────┼────────┼────────┼────────┼─────────┼─────── Filter │ 10.7 │ 5.6 │ │ │ 38. │ │ influent │ │ │ │ │ │ │ from │ │ │ │ │ │ │ septic │ │ │ │ │ │ │ tank │ │ │ │ │ │ │ Filter │ 1.63 │ 1.16 │ 0.09 │ 11.2 │ 8.0 │ 5.8 │ 0.357 effluent │ │ │ │ │ │ │ ────────────┼───────┼──────────┼────────┼────────┼────────┼─────────┼─────── Filter │ 13.4 │ 4.7 │ │ │ 40. │ │ influent │ │ │ │ │ │ │ from coke │ │ │ │ │ │ │ strainer │ │ │ │ │ │ │ Filter │ 2.24 │ 1.35 │ 1.03 │ 14.6 │ 10.1 │ 6.9 │ 0.372 effluent │ │ │ │ │ │ │ ────────────┼───────┼──────────┼────────┼────────┼────────┼─────────┼─────── Filter │ 8.6 │ 3.6 │ 0.19 │ 1.6 │ 24. │ 0.3 │ influent │ │ │ │ │ │ │ from │ │ │ │ │ │ │ contact │ │ │ │ │ │ │ bed │ │ │ │ │ │ │ Filter │ 2.62 │ 1.35 │ 0.31 │ 8.1 │ 8.3 │ 5.8 │ 0.516 effluent │ │ │ │ │ │ │ Filter │ 2.44 │ 2.41 │ 0.16 │ 9.4 │ 12.5 │ 5.0 │ 0.525 effluent │ │ │ │ │ │ │ Filter │ 3.40 │ 1.15 │ 0.20 │ 10.9 │ 9.7 │ 5.2 │ 0.525 effluent │ │ │ │ │ │ │ ────────────┼───────┼──────────┼────────┼────────┼────────┼─────────┼─────── Filter │ 9.0 │ 4.8 │ 0.42 │ 1.3 │ 27. │ 3.4 │ influent │ │ │ │ │ │ │ from │ │ │ │ │ │ │ sprinkling│ │ │ │ │ │ │ filter │ │ │ │ │ │ │ after │ │ │ │ │ │ │ sedimen- │ │ │ │ │ │ │ tation │ │ │ │ │ │ │ Filter │ 2.95 │ 1.25 │ 0.19 │ 7.0 │ 8.8 │ 3.8 │ 0.675 effluent │ │ │ │ │ │ │ Filter │ 4.77 │ 2.63 │ 0.51 │ 4.6 │ 11.8 │ 2.5 │ 0.749 effluent │ │ │ │ │ │ │ Filter │ 3.47 │ 1.61 │ 0.31 │ 7.2 │ 11.9 │ 3.7 │ 1.129 effluent │ │ │ │ │ │ │ ────────────┴───────┴──────────┴────────┴────────┴────────┴─────────┴───────
The action in an intermittent sand filter is more complete than in other forms of filters because a greater surface is exposed to the passage of sewage by the fine sand particles, and the sewage is in contact with the filtering material a longer time due to the lower rate of filtration and the slow velocity of flow through the filter. It is essential that the sewage be applied to the bed intermittently in order that air shall be entrained in the filter. The period between doses should not be so long that the filter becomes dry.
In the operation of an intermittent sand filter one dose per day is considered an ordinary rate of application, although some plants operate with as many as four doses per day per filter, and others on one dose at long and irregular intervals. It is not always necessary to rest the filter for any length of time unless signs of overloading and clogging are shown. The intermittent dosing action may be obtained by the action of an automatic siphon as is described in Chapter XXI. The sewage is distributed on the beds through a number of openings in the sides of distributing troughs resting on the surface of the filter. The sewage is withdrawn from the bottom of the filter through a system of underdrains, into which it enters after its passage through the bed. There are no control devices on the outlet, as the rate of filtration is controlled by the action of the dosing apparatus and the rate at which sewage is delivered to it. The action of the dosing apparatus should respond quickly to variations in sewage flow. As the doses are applied to a sand filter, a mat of organic matter or bacterial zoöglea is formed on the surface of the bed. The mat is held together by hair, paper, and the tenacity of the materials. It may attain a thickness of ¼ to ½ an inch before it is necessary to remove it. So long as the filter is draining with sufficient rapidity this mat need not be removed, but if the bed shows signs of clogging, the only cleaning that may be necessary will be the rolling up of this dried mat. It is believed that the greater portion of the action in the filter occurs in the upper 5 to 8 inches of the bed, but occasionally the beds become so clogged that it is necessary to remove ¾ of an inch to 2 inches of sand in addition to the surface mat, or to loosen up the surface by shallow plowing or harrowing. The necessity for such treatment may indicate that the filter is being overloaded as a result of which the rate of filtration should be decreased or the preliminary treatment should be improved. The plowing of clogging material into the bed should be avoided as under these conditions the final condition of the bed will be worse than its condition when trouble was first observed.
In winter the surface of the bed should be plowed up into ridges and valleys. The freezing sewage forms a roof of ice which rests on the ridges and the subsequent applications of sewage find their way into the filter through the valleys under the ice. In a properly operated bed the filtering material will last indefinitely without change. If a filter is operated at too high a rate, however, although the quality of the effluent may be satisfactory, it will be necessary at some time to remove the sand and restore the filter.
The rate of filtration depends on the character of the influent, the desired quality of the effluent, and the depth and character of the filtering material. Filters can be found operating at rates of 50,000 gallons per acre per day and others at eight times this rate. For sewage which has had some preliminary treatment, the rate should not exceed 100,000 gallons per acre per day, whereas the rate for raw sewage should be less than this. For rough estimates made without tests of the sewage in question, the rate should not be taken at more than 1,000 persons per acre. If the preliminary treatment of the sewage has been thorough and the material of the sand filter is coarser than ordinary the rate of filtration can be high. For less careful preliminary treatment and fine filtering material the rates must be reduced. The sewage must undergo sufficient preliminary treatment to remove large particles of solid matter which would otherwise clog the dosing apparatus and the filter. This treatment should include grit removal, screening, and some form of tank treatment. Some plants have operated successfully with a stale sewage and no preliminary treatment, as at Brockton, Mass. Septic tank effluent can be treated successfully on an intermittent sand filter, but not so satisfactorily as the effluent from a tank delivering a fresh sewage.
The material of the filter should consist of clean, sharp, quartz or silica sand with an effective size[164] of 0.2 to 0.4 mm., preferably about 0.25 to 0.35 mm., and a uniformity coefficient[165] of 2 to 4. Within the limits mentioned no careful attention need be given to the size of the material. Natural sand found in place has been underdrained and used successfully for sewage treatment. The size of the sand is fixed by the rate of filtration rather than the bacteriological action of the filter. A coarse sand will permit the sewage to pass through the bed too rapidly, and a fine sand will hold it too long or will become clogged. The same size of material should be used throughout the bed, except that a layer of gravel from 6 to 12 inches thick, graded from very small sizes to stones just passing a 2–inch ring should be placed at the bottom to facilitate the drainage of the bed.
The thickness of the sand layer should not be less than 30 inches to insure complete treatment of the sewage. In shallower beds the sewage might trickle through without adequate treatment. Beds are ordinarily made from 30 to 36 inches deep, but when deeper layers of sand are found in place there is no set limit to the depth which may be used. The shape and overall dimensions of the bed should conform to the topography of the site and the rate of filtration adopted. A plan and cross-section of an intermittent sand filter showing the distribution and under drainage systems are given in Fig. 166 and 175.
The distribution system consists of a system of troughs on the surface of the filter, laid out in a branching form, as shown in the figure. The openings in the troughs should be so located that the maximum distance from any point on the bed to the nearest opening should not exceed 20 to 30 feet. If the filters are small enough, troughs need not be used, the sewage being distributed from one corner, or from mid-points on the sides. Where troughs are used they should be supported from the bottom of the filter in order to prevent uneven settling due to the washing of the sand. The openings in the troughs are made adjustable by swinging gates as shown in Fig. 176, or by other means so that after the filter is in operation the intensity of the dose on any portion of the filter can be changed. The troughs may be placed with their bottoms level with the surface of the sand and with sides of sufficient height to give the required gradient to the water surface, or they may be built up above the surface of the filter and given the required slope so that the surface of the flowing water is parallel to the bottom of the trough. In either case a splash plate should be placed at each opening, so that not less than 2 feet of the surface of the sand is protected in all directions from the opening. A stone or concrete slab 2 to 4 inches thick makes a satisfactory splash plate. Either wood or concrete may be used for the construction of the troughs. The former is less durable, but also less expensive in first cost. The capacity of the troughs may be computed by Kutter’s formula with the quantity to be carried equal to the maximum rate of discharge of the feeding siphon, with a reduction in size below each branch or outlet proportional to the amount which will be discharged above this point.
The operation of automatic devices for dosing the bed is explained in Chapter XXI. The dosing tank should have a capacity sufficient to cover the bed to a depth of about 1 to 3 inches at one dose, and the siphon should discharge at a rate of about one second-foot for each 5,000 square feet of filter area. A dose should disappear within 20 minutes to half an hour after it is applied to the filter. With the rate stated and four applications per day to a depth of 1 inch at each dose, the rate per acre per day will be 109,000 gallons.
The filtration of sewage through sand in a manner similar to the rapid sand filtration of water is being attempted at the Great Lakes Naval Training Station. No results of this treatment have been published and the practical success of the method has not been assured.
=259. Cost of Filtration.=—Only comparative figures can be given in stating the costs of filtration, as most data available are based on pre-war conditions, and are therefore unreliable for present conditions. The variations from the figures given may be very large but in general the relative costs have not changed. The figures given in Table 90 are suggestive of the relative costs of the different forms of filtration.
TABLE 90
RELATIVE COSTS OF DIFFERENT METHODS OF SEWAGE TREATMENT
Costs in Dollars per Million Gallons per Day ─────────────────────────┬───────────────┬──────────────┬────────────── Form of Treatment │First Cost[166]│Operation and │ Total │ │ Maintenance │ ─────────────────────────┼───────────────┼──────────────┼────────────── Coarse screens │ │ │ 0.20 Fine screens │ │ │ 3.00 Plain sedimentation │ 7.00│ 3.00│ 10.00 Chemical precipitation │ │ │ 22.00[167] Septic tank │ 7.00│ 1.00│ 8.00 Imhoff tank │ 10.00│ 1.00│ 11.00 Contact bed │ 8.00│ 2.00│ 10.00 Trickling filter │ 4.00│ 2.00│ 6.00 Intermittent sand filter │ 15.00│ 10.00│ 25.00 Activated sludge │ 6.50│ 8.50│ 15.00[168] ─────────────────────────┴───────────────┴──────────────┴──────────────
IRRIGATION
=260. The Process.=—Broad irrigation is the discharge of sewage upon the surface of the ground, from which a part of the sewage evaporates and through which the remainder percolates, ultimately to escape in surface drainage channels. Sewage farming is broad irrigation practiced with the object of raising crops. Broad irrigation can be accomplished successfully without the growing of crops, but it is seldom attempted as some return and sometimes even a profit can be obtained from the crops raised. Broad irrigation and sewage farming differ from intermittent sand filtration in the intensity of the application of the sewage, the method of preparing the area on which the sewage is to be treated, and the care in operation. In broad irrigation and intermittent sand filtration the paramount consideration is successful disposal of the sewage. In sewage farming the paramount consideration is the growing of crops. The growing of crops may be combined with irrigation and filtration, however, but the crop should be sacrificed to the successful disposal of the sewage.
The change which occurs in the characteristics of the sewage due to its filtration through the ground is the same as occurs in aërobic filtration. The effect on the crops is mainly that of an irrigant, as the manurial value of the sewage is small.
=261. Status.=—The disposal of sewage by broad irrigation was practiced in England previous to the development of any of the more intensive biologic methods of treatment. It was considered the only safe and sanitary method for the disposal of sewage, and as a result, areas irrigated by sewage were common throughout England. Crops were grown on these areas as a minor consideration, and sewage farming gained some of its popularity from the apparent success of these disposal areas. The success of sewage farms is due more to generous irrigation in dry years than to fertilization by sewage.
The sewage farms of Paris and Berlin are frequently cited as examples of the successful and remunerative disposal of sewage by farming in connection with broad irrigation. Kinnicutt, Winslow, and Pratt[169] state:
The Berlin Sewage farms offer examples of broad irrigation under better conditions ... of 21,008 acres receiving sewage, 16,657 acres were farmed by the city, 3,956 acres were leased to farmers, and only 395 acres were unproductive. The contributing population at this time was 2,064,000 and the average amount of sewage treated was 77,000,000 gallons, giving a daily rate of treatment of about 3,700 gallons per acre of prepared land. The soil is sandy and of excellent quality. A quarter of the area operated by the authorities is devoted to pasturage, and about a third to the cultivation of cereals, of which winter rye and oats are the most important. Potatoes and beets are grown in considerable amounts and a wide variety of other crops in smaller proportions.... Even fish ponds are made to yield a part of the revenue, and the drains on some of the farms have been successfully stocked with breed trout.
The cost of the Berlin farms to March 31, 1910, was $17,470,000, somewhat more than half being the purchase price of the land. The expenses for this year amounted to $1,300,385 for maintenance, and $741,818 for interest charges. The receipts were $1,240,773 and there was an estimated increase of $122,593 in value of live stock and other property.
The conditions at Berlin are quoted at length to indicate the success which can accompany broad irrigation, and as an example of what is being done abroad, where the rainfall is light and the soil is suitable.
In the United States success in sewage farming has not been marked. This may be due partially to the relative weakness of American sewages, to the cost of labor, to lack of satisfactory irrigation areas, and to inattention to details. An attempt was made to grow crops on the sand filters at Brockton, Mass., but it was finally abandoned as the interests of the crops and the successful treatment of the sewage could not both be satisfied. At Pullman, Illinois,[170] in 1880, there was commenced probably the most extensive attempt at sewage farming in eastern United States. The farm was a failure from the start, because of the clay soil, and it was subsequently abandoned. Sewage farming, mainly as a subsidiary consideration to the filtration of sewage, is practiced in a few cities in the eastern portion of the United States to-day. Among the cities mentioned by Metcalf and Eddy[171] are Danbury, Conn., and Fostoria, Ohio. In the western portion of the United States where water is scarce and the ground is porous, sewage has been used as an irrigant with some success. Such use of sewage cannot be considered as a method of treatment since the prime consideration is the growing of crops. In this process all sewage not used as an irrigant is discharged without treatment into water courses. According to Metcalf and Eddy there were 35 cities in California in 1914 that were operating sewage farms. Among these are Pasadena, Fresno, and Pomona. Other farms, notably the pioneer farm at Cheyenne, Wyo., have been abandoned because of the local nuisance created and the lack of financial success.
=262. Preparation and Operation.=—A porous sandy soil on a good slope and with good underdrainage is most suitable for broad irrigation. Impervious clay or gumbo soils are unsuitable and should not be used. They become clogged at the surface, forming pools of putrefying sewage, or in hot weather form cracks which may permit untreated sewage to escape into the underdrains.
The sewage may be distributed to the irrigated area in any one of five ways which are known as: flooding, surface irrigation, ridge and furrow irrigation, filtration, and subsurface irrigation. In flooding, sewage is applied to a level area surrounded by low dikes. The depth of the dose may be from 1 inch to 2 feet. In surface irrigation the sewage is allowed to overflow from a ditch over the surface of the ground into which it sinks or over which it flows into another ditch placed lower down. This ditch conducts it to a point of disposal or to another area requiring irrigation. Ridge and furrow irrigation consists in plowing a field into ridges and furrows and filling the furrows with sewage while crops are grown on or between the ridges. In filtration the sewage is distributed in any desired fashion on the surface and is collected by a system of underdrains after it has filtered through the soil. In subsurface irrigation the sewage is applied to the land through a system of open-joint pipes laid immediately below the surface, similarly to a system of underdrains. Combinations of and modifications to these methods are sometimes made. Underdrains may be used in connection with any of these forms of distribution.
The preparation of the ground consists in: the construction of ditches or dikes to permit of any of the above described methods of application, grading of the surface to prevent pooling, the laying of underdrains, and the grubbing and clearing of the land. The main carriers may be excavated in open earth or earth lined with an impervious material. The distribution of the sewage from the main carriers to groups of laterals may be controlled by hand-operated stop planks. If the soil has a tendency to become waterlogged it may be relieved by installing underdrains at depths of 3 to 6 feet, and 40 to 100 feet apart. The tile underdrains may discharge into open ditches excavated for the purpose which serve also to drain the land. Drains should be used where the ground water is within 4 feet of the surface, and the open ditches should be cut below the drains to keep the ground water out of them. Four or 6–inch open-joint farm tile may be used for underdrains. The porosity of the soil will be increased by cultivation. Where particular care is taken in the cultivation of the soil so that sewage can be applied at a high rate, broad irrigation merges into the more intensive intermittent filtration through sand.
Before being turned on to the land, sewage should be screened and heavy-settling particles should be removed. The rate of application may be increased as the intensity of the preliminary treatment is increased. The rate at which sewage may be applied is dependent also on the character of the soil, and may vary between 4,000 and 30,000 gallons per acre per day, although higher rates have been used with the effluent from treatment plants and on favorable soil. The sewage should be applied intermittently in doses, the time between doses varying between one day and two or three weeks or more, dependent on the weather and the condition of the soil. The methods of dosing vary as widely as the rates. The dose may be applied continuously for one or two weeks with correspondingly long rests, or it may be applied with frequent intermittency alternated with short rests, interspersed with long rest periods at longer intervals of time. When applying the sewage to the land the rate of application of the dose is about 10,000 to 150,000 gallons per acre per day. The area under irrigation at any one time may be as much as 10 to 15 acres. The rate of the application of the sewage is also dependent on the weather and may vary widely between seasons. It is obvious that a rain-soaked pasture cannot receive a large dose of sewage without danger of undue flooding. One of the principal difficulties with the treatment or disposal of sewage by broad irrigation is that the greatest load of sewage must be cared for in wet seasons when the ground is least able to absorb the additional moisture.
=263. Sanitary Aspects.=—A well-operated sewage farm should cause no offense to the eye or nose, and is not a danger to the public health. In Berlin, a portion of the sewage farms are laid out as city parks. The liquid in the drainage ditches or underdrains may be clear, odorless, and colorless, high in nitrates and non-putrescible. Where the farm has been improperly managed or overdosed the condition may be serious from both esthetic and health considerations. Sewage may be spread out to pollute the atmosphere and to supply breeding places for flying insects which will spread the filth for long distances surrounding the farm. The character of the crop is also a sanitary consideration.
=264. The Crop.=—From a sanitary viewpoint no crops which come in contact with the sewage should be cultivated on a sewage farm. Such products as lettuce, strawberries, asparagus, potatoes, radishes, etc., should not be grown. Grains, fruits, and nuts are grown successfully and as they do not come in contact with the sewage there is no sanitary objection to their cultivation in this manner. Italian rye grass and other forms of hay are grown with the best success as they will stand a large amount of water without injury. The raising of stock is also advisable for sewage farms where hay and grain are cultivated. The stock should be fed with the fodder raised on the irrigated lands and should not be allowed to graze on the crops during the time that they are being irrigated. This is due as much to the danger of injury to the distributing ditches and the formation of bogs by the trampling of the cattle, as to the danger to the health of the cattle.
XVIII. ACTIVATED SLUDGE
=265. The Process.=—In the treatment of sewage by the activated sludge process the sewage enters an aëration tank after it has been screened and grit has been removed. As it enters the aëration tank it is mixed with about 30 per cent of its volume of activated sludge. The sewage passes through the aëration tank in about two to four hours during which time air is blown through it in finely divided bubbles. The effluent from the aëration tank passes to a sedimentation tank where it remains for one-half an hour to an hour to allow the sedimentation of the activated sludge. The supernatant liquid from the sedimentation tank is passed to the point of final disposal. A portion of the sludge removed from the tank is returned to the influent of the aëration tank. The remainder may be sent to any or all of the following: the sludge drying process, the reaëration tanks, or to some point for final disposal. Sections of the activated sludge plant at Houston, Texas, are shown in Fig. 177.
The biological changes in the process occur in the aëration tank. These changes are dependent on the aërobic organisms which are intensively cultivated in the activated sludge. When placed in intimate contact with fresh sewage, brought about by the agitation caused by the rising air, and in the presence of an abundance of oxygen, the organic matter is partially oxidized. The putrefactive stage of the organic cycle is avoided. Colloids and bacteria are partially removed probably by the agitation effected in the presence of activated sludge but the exact action which takes place is not well understood.
=266. Composition.=—Activated sludge is the material obtained by agitating ordinary sewage with air until the sludge has assumed a flocculent appearance, will settle quickly, and contain aërobic and facultative bacteria in such numbers that similar characteristics can be readily imparted to ordinary sewage sludge when agitated with air in the presence of activated sludge. Copeland described activated sludge as follows:[172]
The sludge embodied in sewage and consisting of suspended organic solids, including those of a colloidal nature, when agitated with air for a sufficient period assumes a flocculent appearance very similar to small pieces of sponge. Aërobic and facultative bacteria gather in these flocculi in immense numbers—from 12 to 14 million per c.c.—some having been strained from the sewage and others developed by natural growth. Among the latter are species that have the power to decompose organic matter, especially of an albuminoid or nitrogenous nature, setting the nitrogen free; and others absorbing the nitrogen convert it into nitrites and nitrates. These biological processes require time, air, and favorable environment such as suitable temperature, food supply and sufficient agitation to distribute them throughout all parts of the sewage.
Ardern states that the sludge differs entirely from the usual tank sludge. It is inoffensive and flocculent in character. The percentage of moisture is from 95 to 99 per cent. American experience has generally been that the sludge does not readily separate from its moisture by treatment on fine-grain filters, but the results in England and at Milwaukee, Wisconsin, are in conflict with this general experience. Upon standing 24 hours or more partially dried activated sludge may start to decompose accompanied by the production of offensive odors.
Duckworth states:
The activated sludge at Salford contained three times as much nitrogen, twice as much phosphoric acid and one-half as much fatty matter as ordinary sludge.
TABLE 91
COMPOSITION OF SEWAGE, IMHOFF SLUDGE, AND ACTIVATED SLUDGE AND EFFLUENT AT MILWAUKEE
(W. R. Copeland, Eng. News, Vol. 76, p. 665) ──────┬──────────┬──────────────────────────────────────────────────────────── Period│Source of │ Parts per Million of │ Sample │ Test │ │ ──────┼──────────┼──────┬──────────────────────────────────────┬────────────── │ │ Sus- │ Nitrogen as │Nitrogen │ │pended│ │ Reported as │ │Matter│ │ Ammonia on a │ │ │ │ Basis of │ │ │ │ Sludge Dried │ │ │ │ to 10 Per │ │ │ │ Cent │ │ │ │ Moisture. │ │ │ │ Three │ │ │ │ samples of │ │ │ │ Sludge ──────┼──────────┼──────┼───────┬───────┬────────┬──────┬──────┼────────────── │ │ │ Free │ Albu- │Organic │ Ni- │ Ni- │ │ │ │Ammonia│minoid │Nitrogen│trites│trates│ │ │ │ │Ammonia│ │ │ │ ──────┼──────────┼──────┼───────┼───────┼────────┼──────┼──────┼────┬────┬──── Aug., │Sewage │ 253│ 14.6│ 7.88│ 29│ 0.15│ 0.13│ │ │ 1915│ │ │ │ │ │ │ │ │ │ │Imhoff │ 105│ 16.2│ 6.10│ 27│ 0.19│ 0.13│2.87│3.82│ │ effluent│ │ │ │ │ │ │ │ │ │Activated │ 14│ 3.8│ 3.19│ 6│ 0.29│ 6.00│5.71│4.97│7.04 │ sludge │ │ │ │ │ │ │ │ │ │ effluent│ │ │ │ │ │ │ │ │ ──────┼──────────┼──────┼───────┼───────┼────────┼──────┼──────┼────┼────┼──── Sept.,│Sewage │ 300│ 13.5│ 8.81│ 29│ 0.25│ 0.14│ │ │ 1915│ │ │ │ │ │ │ │ │ │ │Imhoff │ 116│ 15.4│ 7.10│ 27│ 0.12│ 0.09│3.88│ │ │ effluent│ │ │ │ │ │ │ │ │ │Activated │ 8│ 5.7│ 2.22│ 9│ 0.24│ 5.01│8.69│9.00│ │ sludge │ │ │ │ │ │ │ │ │ │ effluent│ │ │ │ │ │ │ │ │ ──────┴──────────┴──────┴───────┴───────┴────────┴──────┴──────┴────┴────┴────
These results have been roughly checked by American experimenters as shown in Table 91.[173] In the recovery of nitrogen from sewage the activated sludge process is the most promising for satisfactory results. In all other processes of sewage treatment the sludge is digested to some extent and nitrogen lost in the gases or in the soluble matter which passes off with the effluent. In the activated sludge process a negligible amount of gasification and liquefaction take place and only a small amount of nitrogen passes off with the effluent as compared with the loss from the Imhoff process as shown in Table 91. The percentage of nitrogen in dried activated sludge is shown in Table 92.
TABLE 92
NITROGEN CONTENT OF DRY ACTIVATED SLUDGE AND SLUDGE FROM OTHER PROCESSES
(G. W. Fuller, Eng. News, Vol. 76, p. 667) ────────────────────────────────────────┬────────────────────────────── Source │ Per Cent Nitrogen ────────────────────────────────────────┼────────────────────────────── Milwaukee (Copeland) │ 4.40 Manchester, England (Ardern) │ 4.60 Salford, England (Melling) │ 3.75 Urbana, Illinois (Bartow) │ 3.5 to 6.4 Armour and Co. (Noble) │ 4.6 Approximate range of all other processes│ 1.0 to 3.0 ────────────────────────────────────────┴────────────────────────────── These figures are expressed in terms of nitrogen and not of ammonia. Nitrogen is only 82 per cent of the ammonia content.
Nitrifying bacteria and other species which have the power of destroying organic matter have been isolated from the sludge. An analysis of the dried sludge at Urbana[174] showed the following results after the weight had been reduced 95.5 per cent by drying: 6.3 per cent nitrogen, 4.00 per cent fat, 1.44 per cent phosphorus, and 75 per cent volatile matter or loss on ignition. Analyses of other domestic sewages have not shown such high contents of these desirable constituents.
The dewatering of activated sludge is a problem which offers serious obstacles to the successful operation of the process. It is its greatest disadvantage. Five to ten times the volume of sludge may be produced by the activated sludge process as by an Imhoff tank, and the activated sludge contains a greater percentage of water. According to Copeland:
The best information now available points to a combination of settling and decantation as a preliminary dewatering process. By this means the water will be cut down from about 99 per cent to 96 per cent. On passing the concentrated residue through a pressure filter the moisture can be cut down to 75 per cent. The press cake can be dewatered in a heat drier to 10 per cent moisture or less.[175]
The quantity of sludge produced at Milwaukee[176] is about 15 cubic yards per million gallons of sewage, the sludge having about 98 per cent moisture. On the basis of 10 per cent moisture it produces ½ ton of dry sludge per million gallons of sewage treated. At Cleveland,[177] 20 cubic yards per million gallons at 97.5 per cent moisture are produced. Methods of drying sludge are discussed in Chapter XX.
Chemical analyses and biological tests indicate that the fertilizing value of the sludge is appreciable. Professor C. B. Lipman states, as the result of a series of tests in which a sludge and a soil were incubated for one month, as follows:[178]
The amounts of nitrates produced in one month’s incubation from the soil’s own nitrogen and from the nitrogen from the sludge mixed with the soil in the ratio of one part of sludge to 100 of soil is, in milligrams of nitrate, as follows: Anaheim soil without sludge 6.0, with sludge 10.0; Davis soil without sludge 4.2, with sludge 14.0; Oakley soil without sludge 2.2, with sludge 4.0.
The effect of the sludge on plant growth is shown in Table 93.[179] The results represent the growth obtained after fifteen weeks from the planting of 30 wheat seeds in each pot.
=267. Advantages and Disadvantages.=—Some of the advantages of the process are: a clear, sparkling, and non-putrescible effluent is obtained; the degree of nitrification is controllable within certain limits; the character of the effluent can be varied to accord with the quantity and character of the diluting water available; more than 90 per cent of the bacteria can be removed; the cost of installation is relatively low; and the sludge has some commercial value.
TABLE 93
FERTILIZING VALUE OF ACTIVATED SLUDGE
(E. Bartow, Journal Am. Water Works Ass’n, Vol. 3, p. 327) ───────────────────────────────────────┬─────────────────────────────── Cultivating Medium │Grams Contained in Experimental │ Pot ───────────────────────────────────────┼───────┬───────┬───────┬─────── │ 1 │ 2 │ 3 │ 4 ───────────────────────────────────────┼───────┼───────┼───────┼─────── White sand │ 19,820│ 19,820│ 19,820│ 19,820 Dolomite │ 60│ 60│ 60│ 60 Bone meal │ 6│ 6│ 6│ 6 Potassium sulphate │ 3│ 3│ 3│ 3 Activated sludge │ 0│ 0│ 20│ 0 Activated sludge extracted with Ligroin│ 0│ 0│ 0│ 20 Dried blood │ 0│ 8.61│ 0│ 0 ───────────────────────────────────────┼───────┼───────┼───────┼─────── Number of heads of wheat │ 14│ 15│ 22│ 23 Number of seeds │ 85│ 189│ 491│ 518 Weight of seeds, grams │ 2.38│ 5.29│ 13.748│ 14.504 Bushels per acre, calculated │ 6.20│ 13.6│ 35.9│ 38.7 Average length of stalk, inches │ 19.40│ 23.0│ 35.4│ 37.1 Weight of straw, grams │ 2.25│ 8.25│ 26.75│ 26.21 Tons per acre, calculated │ 0.18│ 0.68│ 2.23│ 2.18 ───────────────────────────────────────┴───────┴───────┴───────┴───────
Among the disadvantages of the process can be included, uncertainty due to the lack of information concerning the results to be expected under all conditions, high cost of operation under certain conditions, the necessity for constant and skilled attendance, and the difficulty of dewatering the sludge.
=268. Historical.=—The most notable work in the aëration of sewage within recent years was that performed by Black and Phelps for the Metropolitan Sewerage Commission of New York, in 1910,[180] and by Clark and Gage at the Lawrence, Massachusetts, Sewage Experiment Station in 1912 and 1913.[181] The results of these investigations showed that the treatment of sewage by forced aëration might give a satisfactory effluent, but that the time and expense in connection thereto rendered the method impractical.
It remained for Messrs. Ardern and Lockett of Manchester, England, to introduce the process of the aëration of sewage in the presence of activated sludge, as a result of their connection with Dr. Fowler, who attributes his inspiration to his visit to the Lawrence Experiment Station and observing the work of Clark and Gage. Ardern and Lockett commenced their experiments in 1913. Their results were published in the Journal of the Society of Chemical Industry, May 30, 1914, Vol. 33, p. 523. Shortly thereafter experiments were started at the University of Illinois by Dr. Edw. Bartow and Mr. F. W. Mohlmann of the Illinois State Water Survey. At about the same time an experimental plant was started at Milwaukee, by T. C. Hatton, Chief Engineer of the Milwaukee Sewerage Commission. The United States Public Health Service became actively interested in December, 1914, and on February 20, 1915, announced its intention to co-operate with the Baltimore Sewerage Commission in the conduct of experiments. In May, 1915, patent number 1,139,024 was granted to Leslie C. Frank, Sanitary Engineer of the U. S. Public Health Service, covering certain features of the process. Mr. Frank generously donated this patent to the public for the use of municipalities.
The first full sized plant for the treatment of sewage by this method was erected in Milwaukee in December, 1915. This plant had a capacity of 1,600,000 gallons per day. It was used for experimental purposes and is not now in use. The Champaign, Illinois, septic tank, among the first of its kind in the country, was converted into an activated sludge tank on April 13, 1916. The changes, developments, and the results obtained from these and other plants have been reported in the technical press from time to time.
=269. Aëration Tank.=—The sewage on leaving the screen and grit chamber enters the aëration tank, which is usually operated on the continuous-flow principle, although in the early days of experimentation the fill and draw method was practiced. This tank should be rectangular with a depth of about 15 feet and a width of channel not to exceed 6 to 8 feet. Such proportions allow better air and current distribution than larger tanks. The bottom should be level to insure an even distribution of air. The velocity of flow of sewage through the tank is usually in the neighborhood of 5 feet per minute, dependent on the length of the tank and the period of retention. The period of retention is in turn dependent on the desired quality of the effluent. The process is flexible and the quality of the effluent can be changed by changing the period of retention or by changing the rate of application of the air, or both. The period of retention in the aëration tank is usually about 4 hours.
The bottom of the aëration tank is usually made of concrete arranged in ridges and valleys, or small shallow hoppers, at the bottom of which the air-diffusing devices are located, as shown in Fig. 177. The inlet and outlet devices are similar to those in a plain sedimentation tank.
=270. Sedimentation Tank.=—It is evident that as no sedimentation is permitted in the aëration tank, the settleable particles will be discharged in the effluent unless some provision is made for their detention. The effluent from the aëration tank is therefore run through a plain sedimentation tank, usually with a hopper bottom, which has been arranged to permit frequent and easy cleaning. An air lift or a centrifugal sludge pump is satisfactory for this purpose. Another type of sedimentation tank which has been used has a smooth bottom with a slight slope towards the center. A revolving scraper collects the sludge continuously, scraping it towards the center of the tank. Although this arrangement gives better results than the hopper-bottom tank, its expense has usually prevented its installation.[182]
The period of sedimentation in different plants varies from 30 minutes to one hour, although the longer periods usually give the better results. Approximately 65 per cent of the sludge will settle in the first 10 minutes, 80 per cent in the first 30 minutes, and about 5 per cent more in the next half hour.
The effluent from the sedimentation tank is ready for final disposal or if desired, for further treatment by some other method. The sludge, or a portion of it, is pumped back into the influent of the aëration tank, provided the sludge is in a satisfactory state of nitrification. Otherwise it should be pumped to the reaëration tanks. The remainder of the sludge which is not to be used in the process is ready for drying and final disposal.
=271. Reaëration Tank.=—The purpose of the reaëration or sludge aëration tank is to reactivate the sludge which has gone through the aëration tank. During the process of the aëration of the sewage in the aëration tank the activated sludge may lose some of its qualities because of the deficiency of oxygen to maintain aërobic conditions. By blowing air through the sludge in the reaëration tank these properties are returned and the sludge made available to be pumped back into the aëration tank. The reactivation of the sludge obviates the necessity for supplying sufficient air to the entire mass of the sewage to maintain aërobic conditions, and results in an economy in the use of air. The use of mechanical agitators has also been attempted both in the reaëration and the aëration tanks with the expectation of saving in the use of air, but with indifferent success.
It is difficult to say, without experimentation, what the size of the reaëration tank should be, as the necessary amount or reactivation is uncertain. In the experimental plant at Milwaukee, there were eight units of aëration tanks, one sedimentation tank, and two reaëration tanks, all of the same capacity and general design. This represents a ration of about one reaëration tank to four aëration tanks.
=272. Air Distribution.=—Air is applied to the sewage at the bottom of the aëration tank at a pressure in the neighborhood of 5.5 to 6.0 pounds per square inch, dependent on the depth of the sewage, the loss of head through the distributing pipes, and the rate of application. In different experimental plants the pressure has varied from 3 to 30 pounds per square inch. Such pressures are on the line which divides the use of direct blowers for low pressures from turbo and reciprocating pressure machines for pressures above 10 pounds per square inch. Positive-pressure blowers or direct blowers operate on the principle of a centrifugal pump and because of the lighter specific gravity of air they rotate at a very high speed. The Nash Hytor Turbo Blower consists of a rotor with a large number of long teeth slightly bent in the direction of rotation. The rotor, which has a circular circumference, revolves in an elliptical casing. At the commencement of operation the rotor and casing are partially filled with water. The revolution of the rotor throws the water to the outside of the elliptical casing thus forming a partial vacuum between any two teeth as the water is thrown from near the center of the short diameter of the casing to the extremity of the long diameter of the casing. Air is allowed to enter through the inlet port to relieve the vacuum. As the teeth pass from the long diameter to the short diameter of the ellipse, the water again approaches the center of the rotor compressing the air trapped between the teeth and forcing it out under pressure into the exhaust pipe. Among the advantages of this compressor are the washing of the air, cooling, and ease in operation. Reciprocating air compressors operate similarly to direct-acting steam pumps or crank-and-fly-wheel pumps but at much higher speeds, and they require more floor space than either of the other types. Fig. 178 shows the field of serviceability of various types of air compression machinery.
For pressures up to about 10 pounds per square inch the positive blower seems most desirable. It has a low first cost and a relatively high efficiency of about 75 to 80 per cent of the power input. No oil or dirt is added to the air to clog the distributing plates, as in the reciprocating machine. A disadvantage is the difficulty of varying the pressure or quantity of the output of the machine. As the required pressure and volume of air increases the turbo blower becomes more and more desirable within the limits of pressure which are ordinarily used in this process. For small installations the best form of power is probably the electric drive, but when the capacity becomes such as to make turbo blowers advisable they should be driven by directly connected steam turbines.
The quantity of air required varies between 0.5 to 6.0 cubic feet per gallon of sewage, with from 3 to 6 hours of aëration. The quantity of air depends on the degree of treatment required, the strength of the sewage, the depth of the tank, and the period of aëration. The deeper the tank the less the amount of air needed because of the greater travel of the bubble in passing through the sewage, but the higher the pressure at which the air must be delivered. Shallow tanks usually require a longer period of retention. The depth of the tank then has very little to do with economy in the use of air. Hatton states:[183]
The purification of sewage obtained varies decidedly with the volume of air applied. Small volumes applied for 5 or 6 hours do as well as larger volumes applied for 3 or 4 hours, but the time of aëration required to obtain a like effluent does not vary directly with the volume of air applied per unit of time. For instance air applied at a rate of 2 cubic feet per minute purifies the sewage in less time than one cubic foot of air per minute, but will not accomplish an equal degree of purification in half the time.
It has been found that although a low temperature has a deleterious effect on the process, by the use of an additional quantity of air good results can be maintained. The effect of changing the quantity of air and the period of aëration are shown in Table 94 taken from Hatton.
The velocity of the air in the pipes should be about 1,000 feet per minute. There should be relatively few sharp turns in the line, and the distributing mains should be arranged without dead ends. It is desirable to use as little piping as possible and at the same time to make the travel of the sewage long in order to maintain a non-settling velocity and intimate contact with the air. The piping should be accessible and well provided with valves. It should be non-corrodible, particularly on the inside, as flakes of rust will quickly clog the air diffusers. It should drain to one point in order that it can be emptied when flooded, as occasionally happens.
TABLE 94
EFFECT OF VARIOUS RATES AND PERIODS OF APPLICATION OF AIR ON THE RESULTS OBTAINED FROM THE TREATMENT OF SEWAGE BY THE ACTIVATED SLUDGE PROCESS
(Milwaukee Results)
─────────┬──────┬──────┬──────────┬──────── Time of │Cubic │Cubic │Appearance│Per Cent Aëration,│ Feet │ Feet │of Settled│Removal Hours │ Free │ Air │ Liquid │Bacteria │ Air │ per │ │ │ Per │Gallon│ │ │Minute│ of │ │ │ │Sewage│ │ ─────────┼──────┼──────┼──────────┼──────── │ │ │ │ │ │ │ │ ─────────┼──────┼──────┼──────────┼──────── │ │ │ │ │ │ │ │ ─────────┼──────┼──────┼──────────┼──────── 0│ 0│ 0.0│ Turbid │ 0 1│ 160│ 0.67│ Clear │ 52 2│ 160│ 1.32│ Clear │ 81 3│ 160│ 1.98│ Clear │ 92 4│ 160│ 2.64│ Clear │ 94 5│ 160│ 3.31│ Clear │ 98 2.5│ 90│ 1.07│ │ 92 3│ 90│ 1.28│ │ 96 4│ 90│ 1.71│ │ 98 4│ 80│ 1.82│ │ 97.7 4│ 70│ 1.60│ │ 99.6 4│ 46│ 1.67│ │ 88.3 4│ 105│ 1.75│ │ 92.7 3│ 140│ 1.75│ │ 91.2 2.5│ 168│ 1.74│ │ 96.7 │ │ 1.80│ │ 98.1 │ │ 1.53│ │ 99 │ │ 1.12│ │ 91 ─────────┴──────┴──────┴──────────┴────────
─────────┬─────────────────────────────────────────────────────┬────────── Time of │ Parts per Million │Stability, Aëration,│ │ Hours Hours │ │ │ │ │ │ │ │ │ │ ─────────┼─────────────────────────────────┬─────────┬─────────┼────────── │ Nitrogen as │Dissolved│Suspended│ │ │ Oxygen │ Matter │ ─────────┼───────┬────────┬────────┬───────┼─────────┼─────────┼────────── │ Free │Nitrites│Nitrates│Organic│ │ │ │Ammonia│ │ │ │ │ │ ─────────┼───────┼────────┼────────┼───────┼─────────┼─────────┼────────── 0│ 22│ 0.08│ 0.08│ │ 0.00│ │ 000 1│ 17│ 0.00│ 0.04│ │ 0.30│ │ 2 2│ 15│ 0.95│ 0.70│ │ 1.90│ │ 33 3│ 11│ 1.75│ 2.80│ │ 4.30│ │ 120 4│ 7│ 2.20│ 5.60│ │ 5.90│ │ 120 5│ 5│ 2.50│ 8.20│ │ 6.70│ │ 120 2.5│ 11│ 0.05│ 2.00│ │ │ │ 69 3│ 9.9│ 0.12│ 2.9│ │ │ │ 95 4│ 1.8│ 0.14│ 5.2│ │ │ │ 120 4│ 1.95│ 0.08│ 8.5│ │ │ │ 120 4│ 5.79│ 0.14│ 9.0│ │ │ │ 120 4│ 7.90│ 0.02│ 2.0│ │ │ │ 61 4│ 4.86│ 0.36│ 4.9│ │ │ │ 120 3│ 9.39│ 0.60│ 3.0│ │ │ │ 120 2.5│ 11.2│ 0.36│ 1.1│ │ │ │ 84 │ │ │ 8.5│ 4│ │ 11│ 120 │ 5.79│ │ 9.0│ 8│ │ 9│ 120 │ 10.1│ │ 2.3│ 14│ │ 42│ 73 ─────────┴───────┴────────┴────────┴───────┴─────────┴─────────┴──────────
TABLE 95
COMPARATIVE RESULTS FROM THE AËRATION OF SEWAGE IN THE PRESENCE OF ACTIVATED SLUDGE WITH THE USE OF DIFFERENT DISTRIBUTING MEDIA
(T. C. Hatton, Eng. Record, Vol. 73, p. 255) ─────────────┬──────────┬────────┬────────┬────────┬─────────┬───────── Diffusers │Months in │ Pounds │ Air, │Per Cent│Nitrates,│Stability │ 1915 │ per │ Cubic │Bacteria│Parts per│Effluent │ │ Square │Feet per│Removed │ Million │in Hours │ │ Inch │ Gallon │ │ │ ─────────────┼──────────┼────────┼────────┼────────┼─────────┼───────── Filtros plate│June 1 to│ 4.3│ 2.06│ 91│ 3.4│ 78 │ Aug. 15 │ │ │ │ │ Air jet │June 1 to│ 3.5│ 1.94│ 91│ 2.2│ 52 │ Aug. 15 │ │ │ │ │ Filtros plate│Nov. 18 to│ 4.6│ 1.71│ 90│ 0.3│ 113 │ Dec. 7 │ │ │ │ │ Monel metal │Nov. 18 to│ 3.0│ 1.71│ 80│ 0.2│ 63 │ Dec. 7 │ │ │ │ │ ─────────────┴──────────┴────────┴────────┴────────┴─────────┴─────────
It is desirable to diffuse the air in small bubbles as by this means the greatest efficiency seems to be obtained from the amount of air added. A diameter 1/16 to ⅛ of an inch is approximately the maximum limit for the size of an effective bubble. Monel metal cloth, porous wood blocks, open jets, paddles, and other forms of diffusers have been tried, but none have given the satisfaction of the filtros plate. The relative value of different types of diffusers is shown in Table 95 taken from Hatton.[184] The Filtros plates are a proprietary article manufactured by the General Filtration Company of Rochester, N. Y. They are made of a quartz sand firmly cemented together and can be obtained with practically any degree of porosity, size of pore opening or dimension of plate, but they are made in a standard size 12 inches square by 1½ inches thick. The frictional loss through the plate is not very great for the amount of air ordinarily used. The plates are classified in accordance with the volume of air which will pass through them, when dry, per minute when under a pressure of 2 inches of water. These classes run from ½ to 12 cubic feet of air per minute. The type usually specified passes about 2 cubic feet of air per minute. The loss of head through these plates as tested at Milwaukee showed an initial loss of ¾ of a pound and an additional loss of about ¼ of a pound for every cubic foot of air per minute per square foot of surface. It is necessary to screen and wash the air before blowing it through the filtros plate as ordinary air is so filled with dirt as to clog the pores of the diffuser quite rapidly.
The area of filtros plates required in the bottom of the tank is usually expressed in terms of the free surface of the tank or as a ratio thereto. In the Urbana tests the best ratio was found to be less than 1 : 3 and more than 1 : 9. In Milwaukee[185] the ratio adopted is in the neighborhood of 1 : 4 or 1 : 5. At Fort Worth the ratio will be about 1 : 7 and at Chicago it will be 1 : 8. The exact ratio should be determined by experiment and will depend on the construction of the tank and the character of the raw sewage and the desired effluent. It is essential that the filtros plates be placed level and at the same elevation as otherwise the distribution of air will be uneven.
=273. Obtaining Activated Sludge.=—After a plant is once started activated sludge is generated during the process of treatment and with careful management a stock of activated sludge can be kept on hand. When a plant is new, or if shut down for such a length of time that the sludge loses its activation, it is necessary to activate some new sludge. This is done by blowing air continuously through sewage either on the fill and draw method with periodic decantations of the supernatant liquid, or by the continuous-flow process, but more preferably by the latter. Where activated sludge is to be obtained from fresh sewage alone the time required is in the neighborhood of 10 to 14 days, and purification begins at the start. An estimate of the quantity which will be obtained can not be made with accuracy. After the initial quantity of sludge has been obtained activated sludge can be maintained during the process of aëration of the raw sewage, or by means of the reaëration tanks previously described.
The volume of activated sludge present in the aëration tank should be about 25 per cent of the volume of the tank. The volume of the sludge is measured in a somewhat arbitrary manner as the amount by volume which will settle in 30 minutes in an ordinary test tube. It is found that this is almost 90 per cent of the solids settling in 4 to 6 hours.
=274. Cost.=—The available information on the cost of the activated sludge process is meager and unreliable. The factors entering into the cost are: the price of fuel, the size of the plant, the period of sedimentation, the amount of air per gallon of sewage, the air pressure, and the percentage of sludge to be aërated in the mixture. In Milwaukee[186] the cost of construction is estimated at $44,000 per million gallons, and $4.75 per million gallons for operation. At Houston, Texas, the cost is estimated at $24,000 per million gallons, exclusive of the sludge drying plant, which may cost $40,000 per million gallons. At Milwaukee, the cost of pressing the sludge is $4.82 per dry ton and of drying is $3.93 per dry ton. The sludge may be sold at the normal rate of $2.50 per unit of nitrogen. Based on the normal value the evident profit will be $3.75 per ton. The net cost of disposing of Milwaukee sewage is estimated at $9.64 per million gallons of which $4.89 is chargeable to overhead and $4.75 to repairs, operation and renewal. In a comparison of the costs of activated sludge and Imhoff tanks with sprinkling filters,[187] the information given by Eddy has been summarized in Table 96. In comparing the relative areas required for different methods of sewage treatment, activated sludge should be allowed about 15 million gallons per acre per day on the basis of aëration tanks 15 feet deep. This figure represents approximately the gross area of the plants at Milwaukee and at Cleveland.
TABLE 96
COMPARATIVE COSTS OF ACTIVATED SLUDGE, AND OF IMHOFF TANKS FOLLOWED BY SPRINKLING FILTERS
(H. P. Eddy, Eng. Record, Vol. 74, p. 557) ─────────────┬─────────────┬─────────────┬───────────────────────────── Process │ First Cost │Operation per│Total Annual Cost at 4 Per │ per Million │ Million │ Cent with Sinking Fund at │ Gallons, │ Gallons, │ 2.5 Per Cent per │ Dollars │ Dollars │ ─────────────┼─────────────┼─────────────┼──────────────┬────────────── │ │ │ Million │ Capita, │ │ │ Gallons, │ Dollars │ │ │ Dollars │ ─────────────┼─────────────┼─────────────┼──────────────┼────────────── Activated │ 57,100│ 20.00│ 29.85│ 1.09 sludge │ │ │ │ Imhoff tank │ 78,500│ 8.50│ 21.84│ 0.80 and │ │ │ │ sprinkling │ │ │ │ filter │ │ │ │ ─────────────┴─────────────┴─────────────┴──────────────┴──────────────
REFERENCES AND BIBLIOGRAPHY ON ACTIVATED SLUDGE
The following abbreviations will be used: A.S. for Activated Sludge, E.C. for Engineering and Contracting, E.N. for Engineering News, E.R. for Engineering Record, E.N.R. for Engineering News-Record, p. for page, and V. for volume.
No.
1. Cooperation Sought in Conducting A.S. Experiments at Baltimore, by Franks and Hendrick. E.R. V. 71, 1915, pp. 521, 724, and 784. V. 72, 1915, pp. 23, and 640.
2. Sewage Treatment Experiments with Aëration and A.S., by Bartow and Mohlman. E.N. V. 73, 1915, p. 647, and E.R. V. 71, 1915, p. 421.
3. A.S. Experiments at Milwaukee, Wisconsin, by Hatton. E.N. V. 74, 1915, p. 134.
4. A.S. in America, An Editorial Survey, by Baker. E.N. V. 74, 1915, p. 164.
5. Choosing Air Compressors for A.S., by Nordell, E.N. V. 74, 1915, p. 904.
6. A Year of A.S. at Milwaukee, by Fuller. E.N. V. 74, 1915, p. 1146.
7. A.S. Experiments at Urbana. E.N. V. 74, 1915, p. 1097.
8. Experiments on the A.S. Process, by Bartow and Mohlman. E.C. V. 44, 1915, p. 433.
9. Milwaukee’s A.S. Plant, the Pioneer Large Scale Installation, by Hatton. E.R. V. 72, 1915, p. 481 and E.C. V. 44, 1915, p. 322.
10. A.S. Experiments at Milwaukee, by Hatton. Journal American Waterworks Association and Proceedings Illinois Society of Engineers, 1916. Also E.R. V. 73, 1916, p. 255. E.C. V. 45, 1916, p. 104, and E.N. V. 75, 1916, pp. 262 and 306.
11. A.S. Defined. E.N. V. 75, 1916, p. 503, and E.N.R. V. 80, 1918, p. 205.
12. Status of A.S. Sewage Treatment, by Hammond. E.N. V. 75, 1916, p. 798.
13. Trial A.S. Unit at Cleveland, by Pratt. E.N. V. 75, 1916, p. 671.
14. Air Diffuser Experience with A.S. E.N. V. 76, 1916, p. 106.
15. Nitrogen from Sewage Sludge, Plain and Activated, by Copeland, Journal American Chemical Society, Sept. 28, 1916. E.N. V. 76, 1916, p. 665. E.R. V. 74, 1916, p. 444.
16. Tests Show A.S. Process Adapted to Treatment of Stock Yards Wastes. E.R. V. 74, 1916, p. 137.
17. Aëration Suggestions for Disposal of Sludge, by Hammond. Journal American Chemical Society, Sept. 25, 1916. E.R. V. 74, 1916, p. 448.
18. Cost Comparison of Sewage Treatment. Imhoff Tank and Sprinkling Filters vs. A.S., by Eddy. E.R. V. 74, 1916, p. 557.
19. Large A.S. Plant at Milwaukee. E.N. V. 76, 1916, p. 686.
20. A.S. Novelties at Hermosa Beach, Cal. E.N. V. 76, 1916, p. 890.
21. A.S. Experiments at University of Illinois, by Bartow, Mohlman, and Schnellbach. E.N. V. 76, 1916, p. 972.
22. A.S. Results at Cleveland Reviewed, by Pratt and Gascoigne. E.N. V. 76, 1916, pp. 1061 and 1124.
23. Sewage Treatment by Aëration and Activation, by Hammond. Proceedings American Society Municipal Improvements, 1916.
24. A.S., by Bartow and Mohlman, Proceedings Illinois Society of Engineers, 1916.
25. The Latest Method of Sewage Treatment, by Bartow. Journal American Waterworks Association, V. 3, March, 1916, p. 327.
26. Winter Experiences with A.S., by Copeland. Journal American Society of Chemical Engineers, April 21, 1916. E.C. V. 45, 1916, p. 386.
27. A.S. Process Firmly Established, by Hatton. E.R. V. 75, 1917, p. 16.
28. Operate Continuous Flow A.S. Plant, by Bartow, Mohlman, and Schnellbach. E.R. V. 75, 1917, p. 380.
29. Chicago Stock Yards Sewage and A.S., by Lederer. Journal American Society of Chemical Engineers, April 21, 1916. E.C. V. 45, 1916, p. 388.
30. The Patent Situation Concerning A.S. E.C. V. 45, 1916, p. 208.
31. “Sewage Disposal” by Kinnicutt, Winslow, and Pratt, published by John Wiley & Sons. 2d Edition, Chapter 12.
32. A.S. Tests Made by California Cities. E.N.R. V. 79, 1917, p. 1009.
33. Conclusions on the A.S. Process at Milwaukee. Journal American Public Health Association, 1917. E.N.R. V. 79, 1917, p. 840.
34. Dewatering A.S. at Urbana, by Bartow. Journal American Institute of Chemical Engineers, 1917. E.N.R. V. 79, 1917, p. 269.
35. Milwaukee Air Diffusion Studies in A.S. E.N.R. V. 78, 1917, p. 628.
36. A.S. Bibliography (up to May 1, 1917) by J. E. Porter.
37. Air Diffusion in A.S. E.N.R. V. 78, 1917, p. 255.
38. A.S. Plant at Houston, Texas. E.N. V. 77, 1917, p. 236, E.N.R. 83, 1919, p. 1003, and V. 84, 1920, p. 75.
39. A.S. Power Costs, by Requardt. E.N. V. 77, 1917, p. 18.
40. A.S. at San Marcos, Texas, by Elrod. E.N. V. 77, 1917, p. 249.
41. Filtros Plates Made the Best Showing in Air Diffuser Tests. E.N.R. V. 79, 1917, p. 269.
42. Results of Experiments on A.S., by Ardern and Lockett. Journal Society for Chemical Research, V. 33, May 30, 1914, p. 523.
43. Final Plans at Milwaukee. E.N.R. V. 84, 1920, p. 990.
44. A.S. Bibliography, published by General Filtration Co., Rochester, N. Y., 1921.
45. A.S. at Manchester, Eng. by Ardern. Journal Society Chemical Industry, 1921. E.C. V. 55, 1921, p. 310.
46. The Des Plaines River A.S. Plant, by Pearse. E.N.R. V. 88, 1920, p. 1134.
47. Sewage Treatment by the Dorr System, by Eagles. Proceedings, Boston Society of Engineers, 1920. Public Works V. 50, 1920, p. 53.
XIX. ACID PRECIPITATION, LIME AND ELECTRICITY, AND DISINFECTION
=275. The Miles Acid Process.=—The Miles Acid Process for the treatment of sewage was devised and patented by G. W. Miles. It was tried experimentally at the Calf Pasture sewage pumping station, Boston, Mass., 1911 to 1914. In 1916 it was tried experimentally at the Massachusetts Institute of Technology, and it has been tested subsequently at other places, notably at New Haven, Conn., in 1917 and 1918. It is one of the most recent developments in sewage treatment and no extensive experience has been had with it. The process consists in the acidification of sewage with sulphuric or sulphurous acid, as the result of which the suspended matter and grease are precipitated and bacteria are removed. The equipment required for the process consists of devices for the production of sulphur dioxide (SO_{2}), and for feeding niter cake or other forms of acid; subsiding basins; sludge-handling apparatus; sludge driers; grease extractors; grease stills; and tankage driers and grinders.
The first step is the acidification of the sewage. The period of contact with the acid is about 4 hours. Sulphurous acid seems to give better results than sulphuric because of the ease in which it can be manufactured on the spot. It seems also to be more virulent in attacking bacteria than an equal strength of sulphuric acid. In experimental plants the acidulation has been accomplished in different ways such as: by the addition of compressed sulphur dioxide from tanks; by the addition of sulphur dioxide made from burning sulphur; or by the roasting of iron pyrite (FeS_{2}). The acidulation precipitates most of the grease as well as the suspended matter and results in a sludge which gives some promise of commercial value. In referring to the process R. S. Weston states:[188]
(1) It disinfects the sewage by reducing the numbers of bacteria from millions to hundreds per c.c.
(2) If the drying of the sludge and the extraction of the grease can be accomplished economically, it is possible that a large part, if not all, of the cost of the acid treatment may be met by the sale of the grease and fertilizer recovered from the sewage.
(3) The use of so strong a deodorizer and disinfectant as sulphur dioxide would prevent the usual nuisances of treatment works.
(4) The addition of sulphur dioxide to the sewage also avoids any fly nuisance, which is a handicap to the operation of Imhoff tanks and trickling filters.
The amount of acid used varies with the quality of the sewage and the desired character of the effluent. At Bradford, England,[189] 5,500 pounds of sulphuric acid are used per million gallons, producing about 2,340 pounds of grease or 0.43 pound of grease per pound of sulphuric acid. At Boston only 0.215 pound of grease were produced per pound of sulphuric acid. The difference is probably due to the great difference in the amount of grease in the raw sewage. In the East Street sewer at New Haven, Conn.,[190] only 700 pounds of acid are used per million gallons of sewage as the alkalinity is only 50 p.p.m. This amount of acid secures an acidity of 50 p.p.m. whereas in the Boulevard sewer 1,130 pounds of acid had to be added to produce the same result. The results obtained by the experiments conducted by the Massachusetts State Board of Health in 1917 are shown in Table 97. The character of the sludge from the same tests is shown in Table 98. After acidification[191] the sewage contains bisulphites and some free sulphurous acid, with some lime and magnesium soaps which are attacked by the acid liberating the free fatty acids. Part of the bisulphites and sulphurous acid are oxidized to bisulphates and sulphuric acid. It was found as a result of the New Haven[191] experiments that the presence of sulphur dioxide in the effluent caused an abnormal oxygen demand from the diluting water and that this difficulty could be partly overcome by the aëration of the effluent after acidulation and sedimentation, without prohibitory expense. The effluent and sludge are both stable for appreciable periods of time and are suitable for disposal by dilution. The character of the sludge as determined by the New Haven tests[192] is shown in Table 99.
TABLE 97
AVERAGE ANALYSIS OF SEWAGE ENTERING BOSTON HARBOR, BEFORE AND AFTER TREATMENT, JULY 17 TO SEPTEMBER 27, 1917
(Eng. News-Record, Vol. 80, p. 319) ─────────┬───────────────────────────────────────────────┬─────────────── Sample │ Parts per Million │ Bacteria, │ │ Millions ─────────┼─────────────────┬───────────┬────────┬────────┼─────┬───────── │ Ammonia │ Kjeldahl │Chlorine│ Oxygen │ │ │ │ Nitrogen │ │Consumed│ │ ─────────┼─────┬───────────┼─────┬─────┼────────┼────────┼─────┼───────── │Free │Albuminoid │ │ │ │ │ │ ─────────┼─────┼─────┬─────┼─────┼─────┼────────┼────────┼─────┼───────── │Total│Total│Diss.│Total│Diss.│ │ │ 20° │ 37° ─────────┴─────┴─────┴─────┴─────┴─────┴────────┴────────┴─────┴─────────
Paddock’s Island
─────────┬─────┬─────┬─────┬─────┬─────┬────────┬────────┬─────┬───────── Raw │ 14.0│ 3.3│ 1.8│ 6.8│ 3.6│ 134│ 23.1│1.86 │ 4.15 sewage │ │ │ │ │ │ │ │ │ Settled │ 12.2│ 1.6│ 1.1│ 3.5│ 2.2│ │ 15.4│ │ Sewage │ │ │ │ │ │ │ │ │ Acidified│ 20.9│ 5.2│ 3.9│ 10.0│ 7.5│ │ │units│units 91 and │ │ │ │ │ │ │ │ 94 │ settled│ │ │ │ │ │ │ │ │ sewage │ │ │ │ │ │ │ │ │ ─────────┴─────┴─────┴─────┴─────┴─────┴────────┴────────┴─────┴─────────
Deer Island
─────────┬─────┬─────┬─────┬─────┬─────┬────────┬────────┬─────┬───────── Raw │ 23.3│ 8.2│ 4.8│ 16.8│ 8.9│ 3100│ 87.3│2.63 │ 1.50 sewage │ │ │ │ │ │ │ │ │ Settled │ 21.1│ 5.6│ 3.9│ 10.7│ 7.3│ │ 62.2│ │ sewage │ │ │ │ │ │ │ │ │ Acidified│ 20.9│ 5.2│ 3.9│ 10.0│ 7.5│ │ │units│units 85 and │ │ │ │ │ │ │ │ 147 │ settled│ │ │ │ │ │ │ │ │ sewage │ │ │ │ │ │ │ │ │ ─────────┴─────┴─────┴─────┴─────┴─────┴────────┴────────┴─────┴─────────
Calf Pasture
─────────┬─────┬─────┬─────┬─────┬─────┬────────┬────────┬─────┬───────── Raw │ 18.0│ 4.5│ 2.0│ 9.7│ 4.1│ 3254│ 41.2│1.89 │ 0.98 sewage │ │ │ │ │ │ │ │ │ Settled │ 19.1│ 2.3│ 1.4│ 4.9│ 3.3│ │ 25.8│ │ sewage │ │ │ │ │ │ │ │ │ Acidified│ 17.8│ 2.4│ 1.6│ 4.9│ 3.3│ │ │units│units 149 and │ │ │ │ │ │ │ │ 277 │ settled│ │ │ │ │ │ │ │ │ sewage │ │ │ │ │ │ │ │ │ ─────────┴─────┴─────┴─────┴─────┴─────┴────────┴────────┴─────┴─────────
The success of the Miles Acid Process in comparison with other processes is dependent on the commercial value of the sludge produced. The New Haven experiments indicate that 16 to 21 per cent of the grease in the sludge is unsaponifiable and seriously impairs the value of the process.
TABLE 98
AVERAGE AMOUNT OF SLUDGE AND FATS OBTAINED FROM SEWAGE ENTERING BOSTON HARBOR AFTER EIGHTEEN HOURS SEDIMENTATION WITH AND WITHOUT ACIDIFICATION
(Eng. News-Record, Vol. 80, p. 319) ────────────────────┬────────────────┬────────────────┬──────────────── │Paddock’s Island│ Deer Island │ Calf Pasture ────────────────────┼────────────────┼────────────────┼──────────────── │ Sedimentation │ Sedimentation │ Sedimentation ────────────────────┼─────┬──────────┼─────┬──────────┼─────┬────────── │Plain│Acidulated│Plain│Acidulated│Plain│Acidulated ────────────────────┼─────┼──────────┼─────┼──────────┼─────┼────────── Pounds of SO_{2} │ │ 818│ │ 1513│ │ 1189 used per million │ │ │ │ │ │ gallons of sewage │ │ │ │ │ │ treated │ │ │ │ │ │ Dry sludge per │ 782│ 959│ 1709│ 1939│ 1208│ 1427 million gallons │ │ │ │ │ │ Per cent Nitrogen in│ 3.10│ 3.38│ 3.57│ 3.45│ 3.18│ 2.83 sludge │ │ │ │ │ │ Per cent fats in │27.30│ 27.30│24.60│ 19.40│24.30│ 26.30 sludge │ │ │ │ │ │ ────────────────────┴─────┴──────────┴─────┴──────────┴─────┴──────────
TABLE 99
CHARACTER OF MILES ACID SLUDGE AT NEW HAVEN
(Eng. News-Record, Vol. 81, p. 1034) ─────────────────────────┬───────────────────────────────────┬───────── │ East Street Sewer │Boulevard │ │ Sewer ─────────────────────────┼────────┬────────┬────────┬────────┼───────── Length of run in days │ 25│ 24│ 44│ 70│ 29 Total sewage treated, │ 260│ 239.4│ 407.8│ 602.2│ 145.5 thousand gallons │ │ │ │ │ Gallons wet sludge per │ 3750│ 4025│ 3200│ 2600│ 5375 million gallons sewage │ │ │ │ │ Specific gravity │ 1.067│ 1.048│ 1.054│ 1.061│ Per cent moisture │ 86.6│ 88│ 86.3│ 85.7│ 92.5 Pounds of dry sludge per │ 503│ 483│ 439│ 368│ 403 million gallons sewage │ │ │ │ │ Ether extract, per cent │ 23.7│ 24.0│ 29│ 32.6│ 30.9 dry sludge │ │ │ │ │ Ether extract, pounds per│ 119│ 116│ 127│ 120│ 124 million gallons │ │ │ │ │ Volatile matter, per cent│ 47.2│ 51.2│ 57.3│ 63.8│ 78.5 dry sludge │ │ │ │ │ Nitrogen, per cent dry │ 1.6│ 1.6│ 2.4│ 2.0│ 3.0 sludge │ │ │ │ │ ─────────────────────────┴────────┴────────┴────────┴────────┴─────────
The conclusions reached as a result of the New Haven experiments are:[193]
Our experience with New Haven sewage lends no color to the hope that a net financial profit can be obtained by the use of the Miles Acid Process, except with sewage of exceptionally high grease content and low alkalinity. They do, however, suggest that for communities where clarification and disinfection are desirable—where screening would be insufficient and nitrification unnecessary—the process of acid treatment comes fairly into competition with the other processes of tank treatment, and that it is particularly suited to dealing with sewages that contain industrial wastes, and to use in localities where local nuisances must be avoided at all costs and where sludge disposal could be provided for only with difficulty.
The conclusions reached as a result of the Chicago experiments are:[194]
The results on hand indicate that treatment of this sewage with acid results in a somewhat greater retention of fat. An apparent reduction in the oxygen demand over that resulting from plain sedimentation, while remarkable, is probably not real, being simply due to a retardation of decomposition by the sterilization of the bacteria present, the organic matter being left in solution.... However, there appears the added cost of acid treatment and the cost of recovery of the grease, as well as the uncertainty of the price to be received for the grease recovered.
The cost of the treatment is estimated by Dorr to be $18 per million gallons, and the value of the sludge obtained from the Boston sewage as $24 per million gallons, giving a net margin of profit of $6 per million gallons. At New Haven, the total return is estimated at $7.09 per million gallons. Based on the production of sulphur dioxide by burning sulphur (assumed to cost $36 per long ton) and on drying from 85 per cent to 10 per cent moisture with coal assumed to cost $7.50 per ton, it appears that the acid treatment of sewage should be materially cheaper than either the Imhoff treatment or fine screening under the local conditions. A comparison of the cost of the treatment of the East Street and the Boulevard sewage at New Haven and the Calf Pasture sewage in Boston is given in Table 100. The cost of construction was estimated by Dorr and Weston in 1919 as greater than $15,000 per million gallons of sewage per day capacity.
TABLE 100
ESTIMATED COST OF SEWAGE TREATMENT AT NEW HAVEN AND BOSTON BY THREE DIFFERENT PROCESSES
Cost in Dollars per Million Gallons Treated
(Engineering and Contracting, Vol. 51, p. 510) ────────────────┬────────────────────┬────────────────────┬──────────── │ Miles Acid Process │ Imhoff Tank and │Fine Screens │ │ Chlorination │ and │ │ │Chlorination ────────────────┼──────┬─────┬───────┼──────┬─────┬───────┼──────┬───── │ East │Boul-│ Calf │ East │Boul-│ Calf │ East │Boul- │Street│evard│Pasture│Street│evard│Pasture│Street│evard ────────────────┼──────┼─────┼───────┼──────┼─────┼───────┼──────┼───── Tanks and │ 2.47│ 2.47│ 2.47│ 5.28│ 4.44│ │ 4.60│ 4.60 Buildings Int.│ │ │ │ │ │ │ │ and Dep. │ │ │ │ │ │ │ │ Acid treatment │ 6.93│10.74│ 18.65│ │ │ │ │ Drying sludge │ 2.09│ 2.04│ 10.34│ │ │ │ │ Degreasing │ 1.78│ 1.91│ 9.12│ │ │ │ │ sludge │ │ │ │ │ │ │ │ Superintendence │ 1.06│ 2.65│ 1.06│ 0.46│ 1.15│ │ 0.47│ 1.15 Labor on tanks │ 1.00│ 1.00│ 1.00│ 1.20│ 1.50│ │ 1.42│ 2.05 and screens │ │ │ │ │ │ │ │ Disposal of │ │ │ │ 1.00│ 1.00│ │ 0.50│ 0.50 sludge or │ │ │ │ │ │ │ │ screenings │ │ │ │ │ │ │ │ Chlorination │ │ │ │ 4.05│ 4.05│ │ 4.05│ 4.05 Gross cost │ 15.50│20.98│ 42.75│ 11.99│12.14│ │ 11.03│12.35 Revenue │ 6.57│10.66│ 47.59│ │ │ │ │ Net cost │ 8.93│10.32│ 4.84│ 11.99│12.14│ │ 11.03│12.35 ────────────────┴──────┴─────┴───────┴──────┴─────┴───────┴──────┴─────
ELECTROLYTIC TREATMENT
=276. The Process.=—This process has been generally unsuccessful in the treatment of sewage and has grown into disrepute. In the words of the editor of the Engineering News-Record:[195]
Thirty years of experiments and demonstrations with only a few small working plants built and most of them abandoned—such in epitome is the record of the electrolytic process of sewage treatment.
It is probably true that the process has never received a thorough and exhaustive test on a large scale, but the small-scale tests have not been promising of good results. Among the most extensive tests have been those at Elmhurst, Long Island,[196] Decatur, Ill.,[197] and Easton, Pa.[198]
Whatever degree of popularity the method has possessed has been due possibly to the mystery and romance of “electricity” and to the personality of its promoters. The process should, nevertheless, be understood by the engineer in order that it may be explained satisfactorily to the layman interested in its adoption.
In this process, sometimes called the direct-oxidation process, all grit is removed and the sewage is passed through fine screens before entering the electrolytic tank. In the electrolytic tank the sewage passes in thin sheets between electrodes and an electric current is discharged through it. A recent development has been the addition of lime to the sewage at some point in its passage through the electrolytic tank. From the electrolytic tank the sewage flows to a sedimentation tank, where sludge is accumulated, and from which the liquid effluent is finally disposed of.
It is claimed that the action of the electricity electrolyzes the sewage, releasing chlorine, which acts as a powerful disinfectant. The constituents of the sewage are oxidized so that the dissolved oxygen, nitrates, and relative stability are increased and the sludge is rendered non-putrescible. It is said that the addition of lime increases the efficiency of sedimentation and enhances the effect of the electric current. The results obtained by tests at Easton, Pa., are shown in Table 101. It will be observed from this table that the combination of lime and electricity does not have a more beneficial effect than either one of them alone. The amount of sludge produced by the combination is about the same as by chemical precipitation alone, but the character of the sludge produced with electricity is less putrescible. The cost of the treatment as estimated at Elmhurst is shown in Table 102.
As a result of the tests at Decatur, comparing lime alone with lime and electricity together, Dr. Ed. Bartow stated:
The purification by treatment with lime alone was greater than that obtained in several of the individual samples treated with lime and electricity.
TABLE 101
COMPARATIVE RESULTS OBTAINED FROM THE TREATMENT OF SEWAGE BY LIME ALONE, ELECTRICITY ALONE, AND LIME AND ELECTRICITY COMBINED
(Creighton and Franklin, Journal of the Franklin Institute, August, 1919) ───────────────────────┬───────────────┬───────────────┬─────────────── │ Lime and │ Lime Alone │ Electricity │ Electricity │ │ Alone ───────────────────────┼───────┬───────┼───────┬───────┼───────┬─────── │Change,│Change,│Change,│Change,│Change,│Change, │ Parts │ Per │ Parts │ Per │ Parts │ Per │ per │ Cent │ per │ Cent │ per │ Cent │Million│ │Million│ │Million│ ───────────────────────┼───────┼───────┼───────┼───────┼───────┼─────── Chlorine │ +1.2│ +1.9│ +12.3│ +18.2│ +1.6│ +2.2 Nitrites │ +0.014│ +58.3│ -.005│ –10.0│ –0.01│ –20.0 Nitrates │ +0.13│ +23.6│ +.005│ +0.8│ –0.15│ –20.0 Ammonia │ –3.3│ –18.3│ +0.2│ +1.3│ +0.9│ +6.6 Albuminoid ammonia │ –3.6│ –12.1│ –0.4│ –1.7│ –0.5│ –2.3 Oxygen demand │ –13.0│ –20.5│ –7.7│ –8.9│ –6.5│ –10.0 Dissolved oxygen │ +1.78│ +40.9│ –0.93│ –19.1│ +1.61│ +40.1 Total bacteria at 37° │ –343│ –92.7│ –373│ –82.4│ –165│ –37.8 (Thousands) │ │ │ │ │ │ Total bacteria at 20° │ –688│ –92.7│ –1074│ –90.1│ –635│ –70.0 (Thousands) │ │ │ │ │ │ B. Coli (Thousands) │ –77.9│ –99.85│ –96.3│ –92.3│ –45│ –81.8 Oxygen absorbed in 5 │ –3.40│ –81.6│ –1.03│ –21.│ +1.24│ +31 days │ │ │ │ │ │ ───────────────────────┴───────┴───────┴───────┴───────┴───────┴───────
DISINFECTION
=277. Disinfection of Sewage.=—Sewage is disinfected in order to protect public water supplies, shell fish, and bathing beaches; to prevent the spread of disease; to keep down odors, and to delay putrefaction. Disinfection is the treatment of sewage by which the number of bacteria is greatly reduced. Sterilization is the destruction of all bacterial life, including spores. Ordinarily even the most destructive agents do not accomplish complete sterilization. Chlorine and its compounds are practically the only substances used for the disinfection of sewage. The lime used in chemical precipitation, the acid used in the Miles Acid Process, the aëration in the activated sludge process, all serve to disinfect sewage, but are not used primarily for that purpose. Copper sulphate has been used as an algaecide but never on a large scale as a bactericide.[199] Heat has been suggested, but its high cost has prevented its practical application to the disinfection of sewage.
TABLE 102
COST OF ELECTROLYTIC TREATMENT, ELMHURST, LONG ISLAND, AND EASTON, PENNSYLVANIA
────────────────────────────────────────┬───────────────────┬───────── │ │ Three Item │One Million Gallon │ Million │ │ Gallon ────────────────────────────────────────┼─────────┬─────────┼───────── │ unit at │ unit at │ unit at │ Easton, │Elmhurst,│Elmhurst, │ Dollars │ Dollars │ Dollars ────────────────────────────────────────┼─────────┼─────────┼───────── Hydrated lime: │ │ │ Elmhurst, 1300 pounds at $7.90 ton. │ 12.56│ 5.14│ 15.42 Easton, 3720 pounds at $6.75 ton. │ │ │ Electric power electrolysis: │ │ │ Elmhurst, 85 kw-h. at 4 cents │ 4.19│ 3.40│ 9.60 Easton, 6.25 kw-h. at 8.05 cents │ │ │ Electric power, light and agitation: │ │ │ Elmhurst, 60 kw-h. at 4 cents │ 0.50│ 2.40│ 7.20 Easton, 6.25 kw-h at 8.05 cents │ │ │ Heating │ 1.25│ │ Labor and supervision │ 15.00│ 12.50│ 15.00 Maintenance, repairs and supplies │ 1.50│ 1.00│ 3.00 Sludge pressing and removal │ │ 5.11│ 15.33 ────────────────────────────────────────┼─────────┼─────────┼───────── Total │ 35.00│ 29.55│ 65.55 Cost per million gallons │ 35.00│ 29.55│ 21.85 ────────────────────────────────────────┴─────────┴─────────┴─────────
The action which takes place on the addition to sewage of chlorine or its compounds is not well understood. The idea that the bacteria are burned up with “nascent” or freshly born oxygen, has been exploded.[200] Likewise the idea that the toxic properties of chlorine have no effect has not been borne out by experiments. It has been demonstrated, particularly by tests on strong tannery wastes, that the action of chlorine gas is more effective than the application of the same amount of chlorine in the form of hypochlorite. All that we are certain of at present is that the greater the amount of chlorine added under the same conditions, the greater the bactericidal effect.
Chlorine is applied either in the form of a bleaching powder or a gas. In ordinary commercial bleach (calcium hypochlorite) the available chlorine is about 35 to 40 per cent by weight. In order to add one part per million of available chlorine to sewage it is necessary to add about 25 pounds of bleaching powder or 8½ pounds of liquid chlorine per million gallons of sewage. This can be computed as follows:
The molecular weight of calcium hypochlorite is 127.0. This reacts to produce two atoms of available chlorine with a molecular weight of 70.9. If the bleaching powder were pure the available chlorine would therefore represent 70.9 ÷ 127, or 56 per cent of its weight. Then to obtain one pound of chlorine it would be necessary to have 1.79 pounds of pure bleaching powder. Since 1,000,000 gallons of water weigh approximately 8,300,000 pounds, in order to apply one part per million of chlorine to 1,000,000 gallons of sewage it is necessary to apply 1.79 × 8.3 or 14.9 pounds of pure bleaching powder. Commercial bleaching powder is only about 60 per cent calcium hypochlorite. It is therefore necessary to add 14.9 ÷ 0.60 or about 25 pounds of commercial bleach.
Since liquid chlorine is very nearly pure, approximately 8½ pounds of it applied to 1,000,000 gallons of sewage are equivalent to a dose of one part per million.
Commercial bleaching powder is a dry white powder which absorbs moisture slowly, and which loses its strength rapidly when exposed to the air. It is packed in air-tight sheet iron containers, which should be opened under water, or emptied into water immediately on being opened. The strength of the solution should be from ½ to 1 per cent. The rate of the application of the solution to the sewage may be controlled by automatic feed devices, or by hand-controlled devices.
Commercial liquid chlorine is sold in heavy cast steel containers, which hold 100 to 140 pounds of liquid chlorine under a pressure of 54 pounds per square inch at zero degrees C. or 121 pounds per square inch at 20 degrees.
The amount of chlorine used is dependent on the character of the sewage to be treated, the stage of decomposition of the organic matter, the desired degree of disinfection, the period of contact, and the temperature. The amount of chlorine is expressed in parts per million of available chlorine, regardless of the form in which the chlorine is applied. In general about 15 to 20 parts per million of available chlorine with 30 minutes’ contact at a temperature of about 15° C. will effect an apparent removal of 99 per cent of the bacteria from the raw sewage. The effect is only apparent because many of the bacteria encased in the solid matter of the sewage escape the effect of the chlorine, or detection in the bacterial analysis. Stronger and older sewages, higher temperatures, and shorter periods of contact will demand more chlorine to produce the same results. A septic effluent will require more chlorine than a raw sewage because of the greater oxygen demand by the septic sewage. The results of experiments on disinfection made at different testing stations have shown such wide variations in the amount of chlorine necessary, as to demonstrate the necessity for independent studies of any particular sewage which is to be chlorinated. For instance, at Milwaukee approximately 13 p.p.m. of available chlorine applied to an Imhoff tank effluent effected a 99 per cent removal of bacteria, whereas the same result was obtained at Lawrence, Mass., on crude sewage with only 6.6 p.p.m. and at Marion, Ohio, only 9 per cent removal of bacteria was obtained by the addition of 4,815 p.p.m. to crude sewage. The Ohio and Massachusetts reports show irrational variations among themselves. For instance, 6.2 p.p.m. applied to a septic effluent effected 88 per cent removal whereas in another case 7.6 p.p.m. effected only 36 per cent removal. At Lawrence in one case it took 8.6 p.p.m. to remove 99 per cent from a sand filter effluent, but only 6.3 p.p.m. to effect the same result in the effluent from a septic tank. The most consistent results are those found at Milwaukee which show a steadily increasing percentage removal with increasing amounts of chlorine.
Some time after sewage has received its dose of chlorine the number of bacteria may be greater than in the raw sewage. Such bacteria are called aftergrowths. Certain forms of bacteria, particularly the pathogenic or body temperature types, are most susceptible to disinfecting agents. These are killed off and leave the sewage in a condition more favorable to the growth of more resistant forms of bacteria. As the latter are non-pathogenic and are generally aërobic their presence is usually more beneficial than detrimental, as they hasten the action of self-purification.
REFERENCES
The following abbreviations will be used: E.C. for Engineering and Contracting, E.N. for Engineering News, E.R. for Engineering Record, E.N.R. for Engineering News-Record, M.J. for Municipal Journal, p. for page, and V. for volume.
No.
1. Grease and Fertilizer Base for Boston Sewage, by Weston, E.N. V. 75, 1916, p. 913 and Journal American Public Health Association, April, 1916.
2. Getting Grease and Fertilizer from City Sewage, by Allen. E.N. V. 75, 1916, p. 1005.
3. New Haven Tests Five Processes of Sewage Treatment. E.N.R. V. 79, 1917, p. 829.
4. Recovery of Grease and Fertilizer from Sewage Comes to the Front. E.N.R. V. 80, 1916, p. 319.
5. Miles Acid Process may Require Aëration of Effluent, by Mohlman. E.N.R. V. 81, 1918, p. 235.
6. Promising Results with Miles Acid Process in New Haven Tests. E.N.R. V. 81, 1918, p. 1034.
7. Baltimore Experiments on Grease from Sewage. E.N. V. 75, 1916, p. 1155.
8. Report on Industrial Wastes from the Stock Yards and Packingtown in Chicago to the Trustees of the Sanitary District of Chicago, 1914, pp. 187–195.
9. The Separation of Grease from Sewage, by Daniels and Rosenfeld. Cornell Civil Engineer. V. 24, p. 13.
10. The Separation of Grease from Sewage Sludge with Special Reference to Plants and Methods Employed at Bradford and Oldham, England, by Allen. E.C. V. 40, 1913, p. 611.
11. Acid Treatment of Sewage, by Dorr and Weston. Journal Boston Society of Civil Engineers, April, 1919. E.C. V. 51, 1919, p. 510. M.J. V. 46, 1919, p. 365.
12. The Miles Acid Process for Sewage Disposal. Metallurgical and Chemical Engineering, V. 18, p. 591.
13. Miles Acid Treatment of Sewage, by Winslow and Mohlman. Journal American Society Municipal Improvements, Oct., 1918. M.J. V. 45, 1918, pp. 280, 297, and 321.
14. New Electrolytic Sewage Treatment. M.J. V. 37, 1914, p. 556.
15. Electrolytic Sewage Treatment. M.J. V. 47, 1919, p. 131.
16. Electrolytic Treatment of Sewage at Durant, Oklahoma, by Benham. E.N. V. 76, 1916, p. 547. Municipal Engineering, V. 49, 1916, p. 141.
17. Electrolytic Treatment of Sewage at Elmhurst, Long Island, by Travis. Report to the President of the Borough of Queens, Aug. 31, 1914. E.R. V. 70, 1914, pp. 292, 315, and 429. M.J. V. 39, p. 551. Municipal Engineering, V. 47, p. 281.
18. Tests of the Electrolysis of Sewage at Toronto, by Nevitt. E.N. V. 71, 1914, p. 1076.
19. Electrolytic Treatment of Sewage Little Better than Lime Alone, by Bartow. E.R. V. 74, 1916, p. 596.
20. Electrolytic Sewage Treatment Not Yet an Established Process. E.N.R. V. 83, 1919, p. 541.
21. Tests of Electrolytic Sewage Treatment Process at Easton, Pa. Journal of the Franklin Institute, Aug., 1919. E.N.R. V. 83, 1919, p. 569.
22. The Disinfection of Sewage. U. S. Geological Survey, Water Supply Paper, No. 229.
23. Sewage Disinfection in Actual Practice, by Orchard. E.R. V. 70, 1914, p. 164.
24. Water and Sewage Purification in Ohio. Report of the Ohio State Board of Health, 1908, pp. 738–762.
25. Water Purification, by Ellms. Published in 1917 by McGraw-Hill Book Co.
26. Electrolytic Sewage Treatment, A Half Century of Invention and Promotion. E.N.R. V. 86, 1921, p. 25.
XX. SLUDGE
=278. Methods of Disposal.=—Sludge is the deposited suspended matter which accumulates as the result of the sedimentation of sewage. The methods for the disposal of sludge as discussed herein will include the disposal of scum. Scum is a floating mass of sewage solids buoyed up in part by entrained gas or grease, forming a greasy mat which remains on the surface of the sewage.[201] The sludges formed by different methods of sewage treatment are described in the chapter devoted to the particular method. The disposal of sludge is a problem common to all methods of sewage treatment involving the use of sedimentation tanks.
Sludge is disposed of by: dilution, burial, lagooning, burning, filling land, and as a fertilizer or fertilizer base. Certain methods of disposal, such as burning or as a fertilizer, demand that the sludge be dried preparatory to disposal. Sludge is dried on drying beds, in a centrifuge, in a press, in a hot-air dryer, or by acid precipitation.
=279. Lagooning.=—This is a method of sludge disposal in which fresh sludge is run on to previously prepared beds to a depth of 12 to 18 inches or more, and allowed to stand without further attention. The preparation of the lagoons requires leveling the ground, building of embankments, and, if the ground is not porous, the placing of underdrains laid in sand or gravel. At Reading, Pa.,[202] approximately one acre was required for 1,700 cubic yards of wet sludge. The results of lagooning at Philadelphia are given in Table 103.[202]
TABLE 103
RESULTS OF DRYING SLUDGE IN LAGOONS AT PHILADELPHIA
(“Sewage Sludge” by Allen) ─────────────────────┬─────────┬─────────┬─────────┬─────────┬───────── Treatment │ Days │ Depth, │Per Cent,│Rainfall,│ Cubic │ │ Inches │Moisture │ Inches │Yards per │ │ │ │ │ Acre ─────────────────────┼─────────┼─────────┼─────────┼─────────┼───────── Screened │ 0│ 12.20│ 82.8│ 0│ 1600 Screened │ 26│ 7.67│ 57.0│ 0│ 1000 Screened │ 49│ 3.50│ 51.6│ 0.43│ 470 Screened │ 0│ 13.50│ 90.1│ 0│ 1800 Screened │ 62│ 7.00│ 61.0│ 3.14│ 950 Crude │ 0│ 12.00│ 88.7│ 0│ 1600 Crude │ 59│ 4.70│ 62.8│ 2.59│ 640 ─────────────────────┴─────────┴─────────┴─────────┴─────────┴─────────
During the period of standing in the lagoon the moisture drains out and evaporates and the organic matter putrefies, giving off gases and foul odors. In the course of three to six months, biological action ceases and the sludge has become humified and reduced to about 75 per cent moisture. In the utilization of this method of disposal the lagoons must be removed from settled districts and should occupy land of little value for other purposes. The odors created at the lagoons may be intense and offensive. The land so used is rendered unfit for other purposes for many years.
The digestion of sludge in special tanks is a form of lagooning in which an attempt is made to maintain septic action as a result of which a portion of the sludge is gasified or liquefied, leaving less to be cared for by some of the other methods of treatment or disposal. The results obtained by digestion tanks have not been entirely satisfactory. A partial drying and consolidation of the sludge may be effected, however, by the process of decantation, in which the supernatant liquid is run off, followed by further sedimentation, rendering the final product more compact.
=280. Dilution.=—In the disposal of sludge by dilution, as in the disposal of sewage by dilution, there must be sufficient oxygen available in the diluting water to prevent putrefaction, and a swift current to prevent sedimentation. Such conditions exist in localities along the sea coast, and in communities situated near rivers, when the rivers are in flood. In some seacoast towns, for example at London and Glasgow, the sludge is taken out to sea in boats, and dumped. Since it is not necessary to discharge sludge continuously, it can be stored to advantage in the digestion chamber of a tank, until the conditions in the body of diluting water are suitable to receive it.
The amount of diluting water to receive sewage sludge has not been sufficiently well determined to draw reliable general conclusions. A dilution of 1,500 to 2,000 volumes may be considered sufficiently safe to avoid a nuisance provided there is a sufficient velocity to prevent sedimentation. Johnson’s Report on Sewage Purification at Columbus, Ohio (1905), states that a dilution of 1 to 800 is sufficient to avoid a nuisance. The character of the sludge has a marked effect on the proper ratio of dilution, the sludge from septic and sedimentation tanks requiring a greater dilution than that from Imhoff tanks.
=281. Burial.=—Sludge can be disposed of by burial in trenches about 24 inches deep with at least 12 inches of earth cover, without causing a nuisance. The ground used for this purpose should be well drained. This method of disposal is generally used as a makeshift and has not been practiced extensively because of the large amount of land required. Insufficient information is available to generalize on the amount of land required or the time before the land can be used for further sludge burial, or for other purposes. Indications are that the sludge may remain moist and malodorous for years and that the land may be rendered permanently unfit for further sludge burial. Under some conditions the land may be used again for the same or other purposes. For example, Kinnicutt, Winslow and Pratt[203] state that 500 tons of wet sludge can be applied per acre and:
The same land, it is claimed, can be used again after a period of a year and a half to two years, if in two months or so after covering the sludge with earth, the ground is broken up, planted, and, when the crop is removed, again plowed and allowed to remain fallow for about a year.
=282. Drying.=—Before sludge can be disposed of to fill land, by burning, or for use as a fertilizer filler it must be dried to a suitable degree of moisture. The removal of moisture from the sludge decreases its volume and changes its characteristics so that sludge containing 75 per cent moisture has lost all the characteristics of a liquid. It can be moved with a shovel or fork, and can be transported in non-watertight containers. A reduction in moisture from 95 to 90 per cent will cut the volume in half.
The change in volume on the removal of moisture can be represented as:
_V__{1} = V(100 − P)⁄(100 − _P__{1}),
in which P = the original percentage of moisture;
_P__{1} = the final percentage of moisture;
V = the original volume;
_V__{1} = the final volume.
The drying of sludge on coarse sand filter beds is more particularly suited to sludge from Imhoff tanks. This sludge does not decompose during drying, and is sufficiently light and porous in texture to permit of thorough draining. The sludge from plain sedimentation or chemical precipitation tanks is high in moisture, putrescible, and when placed on a filter bed it settles into a heavy, compact, impervious mass which dries slowly. In order to avoid this condition the sludge is run on to the beds as quickly as possible, to a depth of not more than 6 to 10 inches. Lime is sometimes added to the sludge at this time as it aids drying by assisting in the maintenance of the porosity of the sludge, and it is advantageous in keeping down odors and insects.
Sludge filter beds are made up of 12 to 24 inches of coarse sand, well-screened cinders, or other gritty material, underlaid by 6 inches of coarse gravel and 6 or 8–inch open-joint tile underdrains, laid 4 to 10 feet apart on centers, dependent on the porosity of the subsoil. The side walls of the filters are made of planks or of low earth embankments. The sludge filters at Hamilton, Ontario, are shown in Fig. 179.
The size of the bed is dependent mainly upon the characteristics of the sludge. For Imhoff tank sludge which comes from the tank with about 85 per cent moisture, the practice is to allow about 350[204] square feet of filter surface per 1,000 population contributing sludge. For other types of sludge the area varies from 900 to 9,000 square foot per 1,000 population contributing sludge, and only experiments with the sludge in hand can determine the proper allowance. Imhoff recommends 1,080 square feet per 1,000 population for septic tank sludge, and 6,480 square feet for sludge from plain sedimentation tanks.[205] Kinnicutt, Winslow, and Pratt in their book on Sewage Disposal state:
With an average depth of 10 inches per dose of sludge of 87 per cent water content, one square foot of covered (glass) bed should dry to a spadable condition one cubic yard of sludge per year.
The sludge is run on the bed in small quantities at periods from two weeks to a month apart. In favorable weather Imhoff sludge will dry in two weeks or less to approximately 50 to 60 per cent moisture. It is then suitable for use as a filling material on waste land, for burning, or for further drying by heat. Glass roofs, similar to those used on green-houses, have been used to speed the drying process by preventing the moistening of partly dried sludge during rainy weather. In some instances sludge has dried to 10 per cent moisture on such beds. Imhoff sludge can be removed from the drying beds with a manure or hay fork. It has an odor similar to well-fertilized garden soil. It is stable, dark brownish-gray in color, is of light coarse material, and is granular in texture.
Sludge presses are suitable for removing moisture from the bulky wet sludge obtained from plain sedimentation, chemical precipitation, and the activated sludge process. The details of a typical sludge press are shown in Fig. 180. The press shown is made up of a number of corrugated metal plates about 30 inches in diameter with a hole in the center about 8 inches in diameter. The corrugations run vertically except for a distance about 3 inches wide around the outer rim, which is smooth. To this smooth portion is fastened, on each side of the plate, an annular ring about an inch thick and 2 to 3 inches wide, of the same outside diameter as the plate. A circular piece of burlap, canvas, or other heavy cloth is fastened to this ring, covering the plate completely. A hole is cut in the center of the cloth slightly smaller in diameter than the center hole in the plate, and the edges of the cloth on opposite sides of the plate are sewed together. The plates are then pressed tightly together by means of the screw motion at the left end of the machine, thus making a water-tight joint at the outer rim. Sludge is then forced under pressure into the space between the plates, passing through the machine by means of the central hole. The pressure on the sludge may be from 50 to 100 pounds per square inch. This pressure forces the water out of the sludge through the porous cloth from which it escapes to the bottom of the press along the corrugations of the separating plate. After a period of 10 to 30 minutes the pressure is released, the cells are opened, and the moist sludge cake is removed. The liquid pressed from the sludge is highly putrescible and should be returned to the influent of the treatment plant. The pressing of wet greasy sludges is facilitated by the addition of from 8 to 10 pounds of lime per cubic yard of sludge. The cake thus formed is more cohesive and easy to handle. The output of the press depends so much on the character of the sludge that a definite guarantee of capacity is seldom given by the manufacturer.
The simplest form of centrifugal sludge dryer is a machine which consists of a perforated metal bowl lined with porous cloth in which the sludge is placed. Surrounding this bowl is a second water-tight metal bowl so arranged as to intercept the water thrown from the sides of the inner bowl as it revolves. The peripheral velocity of the inner bowl is about 6,000 feet per minute, which makes the effective weight of each particle about 250 times its normal weight when at rest. Very few data are available on the operation of such machines, and their use has not been extensive because of the difficulty of starting and stopping the machine at each filling, and the difficulty of removing the partially dried sludge from the inner basket. The Besco-ter-Meer centrifuge, manufactured by the Barth Engineering and Sanitation Co., can be operated continuously and the difficulties of removing the dried sludge from the machine have been overcome. According to the manufacturers the centrifuge has been operated very successfully in Germany on plain septic tank sludge. A removal of 70 per cent of suspended solids in the raw sludge and a production of 3,600 pounds of sludge per hour, containing 60 to 70 per cent of moisture, can be obtained at less than 900 r.p.m. with a consumption of 15 horse-power. Extensive tests of the machine were made at Milwaukee from October, 1920, to September, 1921, on activated sludge, but results of these tests are not as yet available. Indications are that the centrifuge has acted as a classifier. The coarser particles of sludge have been removed but the finer particles have been continuously returned with the liquid to the sedimentation tank, ultimately filling this tank with fine particles of sludge. An illustration of the unit tested at Milwaukee is shown on this page.
Experiments on the drying of sludge by acid flotation have not progressed sufficiently to allow the installation of a working unit. The method, which has been applied principally to activated sludge, consists in adding a small amount of sulphuric acid to the sludge as it leaves the storage tank. The sludge is coagulated by this action, the coagulated material rising to the surface as a scum containing about 86 per cent moisture. The consistency is such that it can be removed with a shovel. The liquid can be withdrawn continuously from below the scum.
The moisture content of sludge to be used in the manufacture of fertilizer must be reduced to 10 per cent or less. None of the methods of drying described so far can be relied upon for such a product and it becomes necessary to use direct or indirect heat dryers. There are various types of dryers on the market. The details of a Buckeye dryer are shown in Fig. 181. In the operation of this machine moist sludge is fed in at the left end at the point marked “feed.” The hot gases pass from the fire box up and around the cylinder which revolves at about eight r.p.m. The gases are drawn into the inner cylinder through the openings marked A which revolve with the two cylinders. The gases escape from the inner cylinder through the openings to the right and flow towards the left in the outer cylinder. They come in contact with the sludge at this point. The gases then pass off through the fan at the left. The sludge is lifted by the small longitudinal baffles fastened to the outer cylinder, as the drying cylinders revolve. The right end of the cylinder is placed lower than the left so that the drying sludge is lifted and dropped through the cylinder at the same time that it moves slowly toward the right hand end of the cylinder. These dryers require about one pound of fuel for 10 pounds of water evaporated. The odors from the dryer can be suppressed by passing the gases through a dust chamber and washer.
A summary of the results from methods of sludge drying at Milwaukee[206] follows:
Excess sludge produced, 12,100 gallons, having 97.5 per cent moisture, per million gallons of sewage treated.
Sludge cake produced (by presses), 10,083 pounds having 80.3 per cent moisture, per million gallons of sewage treated.
Dried sludge (from heat driers) produced, 2,521 pounds having 10 per cent moisture, per million gallons of sewage treated.
Press will produce 3 pounds of cake per square foot of filter cloth in four and a half hours, or five operations per twenty-four hours.
Dryers will reduce 6,700 pounds of sludge cake at 80 per cent moisture to 10 per cent moisture, and will evaporate 8 pounds of water per pound of combustible.
Thickening devices known as Dorr thickeners, patented and manufactured by the Dorr Co. and originally intended for metallurgical purposes, have been adapted to the thickening of sewage sludge. These thickeners are circular sedimentation tanks, from 8 to 12 feet deep, more or less, and are made in any diameter up to 200 feet or more. An arm, pivoted in the center and extending to the circumference, is provided at the bottom with a number of baffles or squeegees set at an angle with the arm. The arm revolves at from one to fifteen revolutions per hour, and the squeegees, in contact with the bottom of the tank, scrape the deposited sludge towards a central sump, from which it is removed by a pump or by gravity, without interrupting the operation of the thickener. The sludge so thickened may be reduced to 95 or 96 per cent moisture. These devices are ordinarily used only in the activated sludge process in which they have been a pronounced success.