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.