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.