Environmental Engineering FOURTH EDITION

Sludge Treatment and Disposal 219

sand ceases. The mechanism of evaporation takes over, and this process is actually
responsible for the conversion of liquid sludge to solid. As the surface of the sludge
dries, deep cracks develop that facilitate evaporation from lower layers in the sludge
mat. In wet areas, sand beds may be enclosed under well-ventilated greenhouses to
promote evaporation and prevent rain from falling into the beds.
For mixed digested sludge the usual design is to allow at least 3 weeks of drymg
time, depending on weather conditions and sludge depth. The addition of chemicals
such as aluminum sulfate often increases the amount of water lost through drainage
during the first few days of treatment, thereby shortening the overall drying time. Some
engineers suggest allowing the sand bed to rest for a month after the sludge has been
removed. This seems to be an effective means of increasing the drainage efficiency
once the sand beds are again flooded.
Raw sludge will not drain well on sand beds and will usually have an obnoxious
odor. Hence raw sludges are seldom dried on beds. Raw secondary sludges tend to
seep through the sand or clog the sand pores so quickly that no effective drainage takes
place. Aerobically digested sludges may be dried on sand, but are more difficult to
dewater than anaerobically digested sludges.
If dewatering by sand beds is impractical, mechanical dewatering techniques must
be used. Wo general processes are used for mechanical dewatering: filtration and
centrifugation.
Sludge filtration is usually done using apressurefifilter or a beltfifilter. The pressure
filter, shown in Fig. 10-10, uses positive pressure to force the water through a filter
cloth. Qpically, pressure filters are built as plateand-frame filters, in which the sludge
solids are captured and compressed between the plates and frames. The plates are then
pulled apart to allow sludge cleanout. The belt filter, shown in Fig. 10-11, uses gravity
drainage and a pressure filter. When the sludge is first introduced onto the moving
belt, water drips through the belt and solids are retained. The belt then moves into the
dewatering zone where the sludge is squeezed between two belts. Belt filters are quite
effective in dewatering many different types of sludges and are widely used by small
wastewater treatment plants.
The effectiveness of a filter in dewatering specific sludge is often measured using
the “specific resistance to filtration” test. The resistance of sludge to filtration can be
stated as

(10.4)

where

r = specific resistance (mkg),

P = vacuum pressure (N/m2),

A = area of the filter (m2),

~r. = filtrate viscosity (N s/m2),

w = cake deposit per volume of filtrate (for dry cakes this may be approximated

b = slope of the filtrate volume vs timehiltrate volume curve.

as the feed solids concentration, kg/m3), and