TM 5-814-3/AFM 88-11, Volume III
Imhoff tanks provide removal of settleable solids and the anaerobic digestion of these solids in the same unit.
They are two-level structures (fig 6-9) which allow the solids to settle out in the upper level. The settled
solids then fall through slots into the lower level where they undergo digestion. The gas produced during
digestion escapes through the vent areas along the sides of the upper level. A more detailed discussion of
Imhoff tanks is included in paragraph 6-6. A two-compartment Imhoff tank is illustrated in figure 6-10. The
upper level will be designed for a surface overflow rate of 600 gallons per day per square foot and a retention
period of 3 hours at the average daily flow rate. The bottom of the lower digestion zone has sides which are
sloped 1.4 vertical to 1.0 horizontal. The slot, which allows the solids to flow from the upper level to the
lower level, is a 6-inch opening. An Imhoff tank can be designed so that a single digestion compartment can
receive settled solids from multiple settling compartments. The digestion compartment should be designed
to provide storage for 6 months* accumulation of sludge.
Table 11-4 represents typical characteristics of domestic sewage sludge.