The Imhoff tank is a chamber suitable for the reception and processing of sewage. It may be used for the clarification of sewage by simple sedimentation, along with digestion of the extracted sludge. It consists of an upper chamber in which sedimentation takes place, from which collected solids slide down inclined bottom slopes to an entrance into a lower chamber in which the sludge is collected and digested. The two chambers are otherwise unconnected, with sewage flowing only through the upper sedimentation chamber and no flow of sewage in the lower digestion chamber. The lower chamber requires separate gas vents and pipes for the removal of digested sludge, typically after 6-9 months of digestion. The Imhoff tank is in effect a two-story septic tank and retains the septic tank's simplicity while eliminating many of its drawbacks, which largely result from the mixing of fresh sewage and septic sludge in the same chamber.
Imhoff tanks are being superseded in sewage treatment by plain sedimentation tanks using mechanical methods for continuously collecting the sludge, which is moved to separate digestion tanks. This arrangement permits both improved sedimentation results and better temperature control in the digestion process, leading to a more rapid and complete digestion of the sludge.
This method of sediment removal is also used in some drinking water treatment facilities, in which the tank is often called an Imhoff cone. As in sewage treatment, the collected sludge must be properly disposed of.
