TM 5-814-3/AFM 88-11, Volume III
SLUDGE HANDLING, TREATMENT, AND DISPOSAL
16-1. General considerations.
Sludge, or residual solids, is the end product of wastewater treatment, whether biological or
physical/chemical treatment. Primary sludge is from 3 to 6 percent solids. (Table 11-6 provides more
information regarding sludge solids content.) Treatment objectives are reduction of the sludge and volume,
rendering it suitable for ultimate disposal. Secondary objectives are to utilize the generated gas if anaerobic
digestion is selected as part of the sludge managment strategy. In addition, an attempt should be made to
sell/utilize the sludge as a soil conditioner rather than paying to dispose of it.
16-2. Sludge pumping.
Sludges with less than 10 percent solids can be pumped through force mains. Sludges with solids contents
less than 2 percent have hydraulic characteristics similar to water. For solids contents greater than 2 percent,
however, friction losses are from 1- to 4 times the friction losses for water. Both head losses and friction
increase with decreasing temperature. Velocities must be kept above 2 feet per second. Grease content can
cause serious clogging, and grit will adversely affect flow characteristics as well. Adequate clean-outs and
long sweep turns will be used when designing facilities of these types.
Sludge withdrawal piping
be less than
6 inches in diameter. Minimum diameters for
pump discharge lines are 4 inches for plants less than 0.5 million gallons per day and 8 inches for plants larger
than 1.0 million gallons per day. Short and straight pipe runs are preferred, and sharp bends and high points
are to be avoided. Blank flanges and valves should be provided for flushing purposes.
b. Pumps. Sludge pumps will
be either plunger,
or open-propeller centri-
fugal types. Plunger and progressing-cavity pumps generally should be used for pumping primary sludges;
centrifugal pumps are more suitable for the lighter secondary sludges. Centrifugal and torque-flow pumps
are used for transporting digested sludge in most cases; plunger and progressing-cavity pumps are used when
a suction lift is involved. Plunger pumps are also well suited to sludge elutriation. Standby pumps are required
for primary and secondary sludge pumps as well as for sludge elutriation pumps. The pump information
provided is for guidance only and does not represent design criteria. For more information, refer to Pump
(1) Plunger. The advantages of plunger pumps may be listed as follows:
-- Pulsating action tends to concentrate the sludge in the hoppers ahead of the pumps.
-- They are suitable for suction lifts of up to 10 feet and are self-priming.
-- Low pumping rates can be used with large port openings.
-- Positive delivery is provided unless some object prevents the ball check valves from seating.
-- They have constant but adjustable capacity regardless of large variations in pumping head.
-- Large discharge heads may be provided for.
-- Heavy-solids concentrations may be pumped if the equipment is designed for the load conditions.
Plunger pumps come in simplex, duplex, triplex models with capacities of 40 to 60 gallons per minute per
plunger, and larger models are available. Pump speeds will be between 40 and 50 revolutions per minute, and
the pumps will be designed for a minimum head of 80 feet since grease accumulations in sludge lines cause
a progressive increase in head with use. Capacity is decreased by shortening the stroke of the plunger;
however, the pumps seem to operate more satisfactorily at, or near, full stroke. For this reason, many pumps
will be provided with variable-pitch, vee-belt drives for speed control of capacity.