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]

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:

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.

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.