TM 5-814-3/AFM 88-11, Volume III
the units are serviced either by hand or through auxiliary pipes and valves. As sludge depth increases, scum
accumulation decreases. A deep sludge layer also results in a more dense sludge. This simple system is well
suited for small plants because no mechanical equipment is required. Usually, scum is removed daily if the
freeboard is inadequate. if the tank has more than one compartment, sludge must be resettled by reversing
flow (usually at night) for a short time and allowing "readjustment." Imhoff tanks may be heated and have
mechanical augers to remove densified sludge. Manufacturers produce single and multiple-chambered units
as well as very steep-sided, round tanks.
b. Design criteria. Overflow rates should not exceed 600 gallons per square foot at the average flow rate,
with detention of no less than 3 hours. See figure 6-10 for details of a two-compartment Imhoff tank.