leveled with a slight slope away from the point where the wet sludge enters. Sludge
chunks, weeds, and other debris are removed. When the sand layer decreases to 4
inches or less because of sand being removed with the dried sludge, clean coarse sand
is added. Improper cleaning and preparation of the beds between sludge doses may
clog sand surfaces and retard drying. Clogged sand surfaces may be remedied by
removing the top 1/4 to 1 inch of sand. At the inlet, sludge must be prevented from
falling directly on the sand surface by an adequate splash plate of concrete, brick,
masonry, or wood so the surface is not appreciably disturbed.
(3) Filling the bed to excessive depths may clog the bed and lengthen
drying time since water must then be lost almost entirely by evaporation. Sand depth
varies from 6 to 18 inches, but the optimum drying depth is generally between 8 and 12
inches, depending on solids content. The dried cake should be about 3 to 4 inches
thick under normal drying conditions. If the sludge application is comparatively thin, it
dries quickly; but the thinner cake requires more labor to remove a unit column than
thicker applications. A greater percentage of sand is removed with thin cakes.
However, if bed area is limited, digested sludge must be drawn more frequently and
applied at a minimum depth so it can dry more quickly.
c. Sludge Removal and Use.
(1) Removal from drying beds. Unless large drying areas are available,
dried sludge is removed from beds as soon as it can be handled and piled where it is
accessible for grinding, hauling, or both. Dried sludge is ready to handle when it can be
picked up with a fork without excessive sand adhering to the underside. Moisture
content of this sludge usually ranges from 55 to 70 percent.
(2) Use as soil builder. Only well-digested sludge, wet or dried, is used on
the post. Sludge is particularly suitable for growing vegetation cultivated for dust and
erosion control or for lawns, flower beds and shrubbery. The principal value of dried
sludge as a soil builder is the humus content, which averages from 25 to 35 percent.
Sludge also has Iimited power as a fertilizer. On established lawns, pulverized sludge is
spread uniformly but not so thick as to blanket the grass and cause blanching. On
areas to be seeded, the sludge is spread uniformly and worked into the soil. Sludge
must not be applied to crops that are to be eaten raw. The frequent presence of
hookworm eggs in sludge may cause infection where climate and soil favor continued
hookworm activity. In most areas, health authorities prohibit the use of dried sludge as
fertilizer unless it receives further processing for sterilization. The sludge is also
checked for the presence of heavy metals. The standards need to be met for the sludge
to be used.
(3) Use as fill. Dry sludge may be put on dumps or in low areas. The
sanitary landfill and land surface applications are respectively, disposal and utilization
methods most suitable for military installations. Trestles or other devices for dumping
must be provided since the sludge does not support heavy hauling equipment.