An Imhoff tank is a two stage septic where the sludge is digested in a separate tank. This avoids mixing digested sludge with incoming sewage.
Waste that is not decomposed by the anaerobic digestion eventually has to be removed from the septic tank or else the septic tank fills up and wastewater discharges directly to the drainage field. Not only is this bad for the environment, but if the sludge overflows the septic tank into the leach field, it may damage the leach field piping requiring expensive repairs. How often the septic tank has to be pumped out depends on the volume of the tank relative to the input of solids, the amount of indigestable solids and the ambient temperature (as anaerobic digestion occurs more efficiently at higher temperatures).
The tanker truck that pumps out the septic tank sludge is colloquially called a "honey wagon". Users of septic tank systems need to be very careful not to put excessive waste (e.g. through a kitchen food disposal unit) or nonbiodegradable waste through their sewers, thus overloading the system design capacity. Likewise, users of septic systems should limit water usage to the maximum extent practical in order to avoid exceeding the capacity of the leach field to disperse the water portion of the waste.
Erma Bombeck's book The Grass is Always Greener Over the Septic Tank (ISBN 0070064504) and the common expression that is the same as the title is technically incorrect; the grass is greener over the leach field which is better watered and has more nutrients than the surrounding land.