2-15. SLUDGE DISPOSAL
Well-digested sludge withdrawn from the separate sludge digester or Imhoff tank
has a water content of 90 to 96 percent. It is usually granular, has a tar-Iike odor, and
is dark in color. Gray or Iight brown stripes in the sludge are signs of less-digested
material. Total-solids content of well-digested sludge varies from about 4 to 10 percent,
the volatile-solids content (dry basis) is below 55 percent, and the pH is over 7.0.
a. Sludge Drying.
(1) Digested sludge drying takes pIace both by evaporation from the
surface and by drainage through the sand and gravel generally at the treatment plant.
Natural sand areas are sometimes used. Water passing through the bed is returned to
the raw-wastewater flow, where possible, or discharged to other points in the plant or to
a receiving stream if necessary.
(2) Quick and efficient sludge drying depends first on proper functioning of
digesters or Imhoff tanks to produce well-digested sludge. Poorly digested sludge
slows the drying process by forming a heavy, tenacious mat over both sand and sludge
surface. Also, undecomposed grease clogs the sand. Where digestion is incomplete,
the sludge dries poorly and produces objectionable odors. If partly digested sludge or
supernatant is drawn to the bed, hydrated-Iime or chlorinated-Iime suspension may be
used to arrest decomposition by raising the pH. However, when effective quantities of
lime are used, the sludge dries slowly and incompletely, removal is difficult, and the
(3) If the plant has insufficient sludge-bed capacity, the use of coagulants
hastens the drying process. Alum solution is the most effective and economical agent
for treating digested sludge before it is drawn to the sand beds. Alum acts with the
carbonates in the sludge to form carbon dioxide, which keeps the sludge in suspension
while the liquid drains off through the sand.
Wet sludge must not be discharged onto dried or partially dried sludge.
(5) Sludge lines are drained and flushed with a small amount of water or
supernatant after each use to prevent sludge from hardening in them.
b. Sludge Drying Beds.
(1) Underdrained beds ordinarily are level areas of sand supported by
graded gravel layers having open tile drains. Floors are natural earth with dividing and
outside concrete, wood or earth walls 12 to 14 inches high. The beds consist of a
number of adjacent or independent units (see Figure 2-27). Sludge may take 6 weeks
to dry in the summer and 12 weeks in the winter. Glass-covered beds are used if
climatic conditions or possible odor nuisance make them necessary.
(2) Sludge beds must be clean before use. After dried sludge is removed
and before a new batch is added, the sand surface is loosened by Iight raking and