Sewerage and Sewage Treatment

by Harold E. Babbitt

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.

CHAPTER 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, Inches	Per Cent, Moisture	Rainfall, Inches	Cubic 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:

V₁ = V(100 − P)
(100 − P₁),

in which P = the original percentage of moisture; P₁ = the final percentage of moisture; V = the original volume; V₁ = 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.

Fig. 179.—Sludge drying Beds at Hamilton, Ontario.
Eng. News, Vol. 73, p. 426.

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.

Fig. 180.—Filter Press.

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.

Besco-ter-Meer Sludge Drying Centrifuge at Milwaukee, Wisconsin Courtesy, Barth Engineering and Sanitation Co.

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.

Fig. 181.—Direct-Indirect Sludge Dryer.
Courtesy, the Buckeye Dryer Co.

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.

CHAPTER XXI
AUTOMATIC DOSING DEVICES

283. Types.—Automatic dosing devices are used to apply sewage to contact beds, trickling filters, and intermittent sand filters. These devices can be separated into two classes; those with moving parts and those without moving parts. The latter are better known as air-locked dosing devices. Simple devices without moving parts are less liable to disorders and are nearer “fool-proof” than any device depending on moving parts for its operation.

No one type of moving part device has been used extensively in different sewage treatment plants. Designing engineers have exercised their ingenuity at different plants, resulting in the production of different types.[^[207]] Among the best known forms is the apparatus designed by J. W. Alvord for the intermittent sand filters at Lake Forest, Illinois.[^[208]] In its operation....

A float in the dosing chamber lifts an iron ball in one of a series of wooden columns, and at a certain height the ball rolls through a trough from one column to the next, in its passage striking a catch, which opens an air valve attached to one of ten bell-siphons in the dosing chamber. Each of the siphons discharges on one of the ten sand beds, which are thus dosed in rotation.

Since air-locked dosing devices are in more general use their operation will be explained in greater detail.

284. Operation.—The simplest form of these devices is the automatic siphon used for flush-tanks, the operation of which is described in Art. 61.

In the operation of sand filters, sprinkling filters, or other forms of treatment where there are two or more units to be dosed it is desirable that the dosing of the beds be done alternately. A simple arrangement for two siphons operating alternately is shown in Fig. 182. They operate as follows: with the dosing tank empty at the start water will stand at bb′ in siphon No. 2 and at aa′ in siphon No. 1. As the water enters through the inlet on the left the tank fills. When the water rises sufficiently, air is trapped in the bells, and as the water continues to rise in the tank, surfaces a and b are depressed an equal amount. When b has been depressed to d, a has been depressed to c. Air is released from siphon No. 2 through the short leg, and siphon No. 2 goes into operation. Surface c rises in siphon No. 1 as the tank empties and when the action of Siphon No. 2 is broken by the admission of air when the bottom of the bell is uncovered the water in siphon No. 1 has assumed the position of bb′ and that in No. 2 is at aa′. The conditions of the two siphons are now reversed from that at the beginning of the operation and as the tank refills siphon No. 1 will go into operation. It is to be noted that these siphons are made to alternate by weakening the seal of the next one to discharge and by strengthening the seal of the one which has just discharged.

Fig. 182.—Diagram Showing the Operation of Two Alternating Siphons.

Fig. 183.—Diagram Showing the Operation of Three Alternating Siphons.

285. Three Alternating Siphons.—This principle can be extended to the operation of three alternating siphons as shown in Fig. No. 183. These operate as follows: with the dosing tank empty at the start and water at aa′ in siphons 1 and 2, and at bb′ in siphon No. 3, the dosing tank will be allowed to fill. As the water rises in the tank air is trapped in all the bells and surfaces a and b are depressed. When surface b has been depressed to d, a has been depressed to c. Air is released from siphon No. 3 and this siphon goes into action. Surface c rises in siphons 1 and 2 to the position b, as the dosing tank is emptied. At the same time a small amount of water is passed from siphon No. 3 to the short leg of siphon No. 1, through the small pipes shown, thus filling this leg so that when siphon No. 3 ceases to operate the water in siphons 1 and 3 stands at aa′ and that in No. 2 stands at bb′. Siphon No. 2, having the weaker seal, will be the next to operate. During its operation it will fill siphon No. 3, leaving No. 1 weak. When No. 1 operates it will refill No. 2, leaving No. 3 weak, thus completing a cycle for the three siphons. This principle has not been applied to the operation of more than three alternating siphons and is seldom used on recent installations.

Fig. 184.—Miller Plural Alternating Siphons.
Courtesy, Pacific Flush Tank Co.

286. Four or More Alternating Siphons.—An arrangement for the alternation of four or more siphons is illustrated in Fig. 184. At the commencement of the cycle it will be assumed that all starting wells are filled with water except well No. 1, and that all main and all blow-off traps are filled with water. The following description of the operation of the siphons is taken from the catalog of the Pacific Flush Tank Company:

The liquid in the tank gradually rises and finally overflows into the starting well No. 1 and the starting bell being filled with air, pressure is developed which is transmitted, as shown by the arrows, to the blow-off trap connected with siphon No. 2. When the discharge line is reached, sufficient head is obtained on the starting bell to force the seal in blow-off trap No. 2, thus releasing the air confined in siphon No. 2 and bringing it into full operation.

During the time that siphon No. 2 is operating, siphonic action is developed in the draining siphon connected with starting well No. 2 and as soon as the level in the tank is below the top of the well it is drained down to a point below the bottom of starting well No. 2. It can now be seen that after the first discharge starting well No. 2 is empty, whereas the other three are full.... Therefore when the tank is filled the second time, pressure is developed in starting bell No. 2, which forces the seal of blow-off trap No. 3, thus starting siphon No. 3....

This alternation can be continued for any number of siphons. Other arrangements have been devised for the automatic control of alternating siphons, but these principles of the air-locked devices are fundamental.

287. Timed Siphons.—In the operation of a number of contact beds not only must the dosing of the tanks be alternated, but some method is needed by which the beds shall be automatically emptied after the proper period of standing full. To fulfill this need the principle of the timed siphon must be employed in conjunction with the alternating siphons. Fig. 185 illustrates the operation of the Miller timed siphon. Its operation is as follows: water is admitted to the contact bed and transmitted to the main siphon chamber through the “opening into bed.” Water flows from the main siphon chamber into the timing chamber at a rate determined by the timing valve. The contact bed is held full during this period. As the timing chamber fills with water air is caught in the starting bell and the pressure is increased until the seal in the main blow-off trap is blown and the main siphon is put into operation. As the water level in the main siphon chamber descends, water flows from the timing chamber into the main siphon through the draining siphon and the timing chamber is emptied, ready to commence another cycle.

288. Multiple Alternating and Timed Siphons.[^[209]]—The alternating and timing of a number of beds is more complicated. The arrangement necessary for this is shown in Fig. 186. It will be assumed at the start that all beds are empty and that all feeds are air locked as shown in Section AB except that to bed No. 4 into which sewage is running. As bed No. 4 fills, sewage is transmitted through the opening in the wall into the timed siphon chamber No. 4. When the level of the water in the bed and therefore in this chamber has reached the top of the withdraw siphon leading to the compression dome chamber No. 4, this latter chamber is quickly filled. The air pressure in starting bell No. 4a is transmitted to blow-off trap No. 1a. The seal of this trap is blown, releasing the air lock in feed No. 1 and the flow into bed No. 1 is commenced. At the same time the air pressure in compression dome No. 4 is transmitted to feed No. 4, air locking this feed and stopping the flow into bed No. 4. The alternation of the feed into the different beds is continued in this manner.

Fig. 185.—Miller Timed Siphon.
Courtesy, Pacific Flush Tank Co.

Bed No. 4 is now standing full and No. 1 is filling. When compression dome chamber No. 4 was filled, water started flowing through timing siphon valve No. 4 into timing chamber No. 4 at a rate determined by the amount of the opening of the timing valve. As this chamber fills compression is transmitted to blow-off trap 4b and when sufficiently great this trap is blown and timed siphon No. 4 is put into operation. Bed No. 4 is emptied by it, and compression dome chamber No. 4 is emptied through the withdraw siphon at the same time. This completes a cycle for the filling and emptying of one bed and the method of passing the dose on to another bed has been explained. The principle can be extended to the operation of any number of beds.

Fig. 186.—Plural Timed and Alternating Siphons for Contact Bed Control.
Courtesy, Pacific Flush Tank Co.