The Imhoff tank obtained its name from its inventor, Dr. Karl Imhoff of Germany. The technology was developed in the Emscher District of Germany and patented in 1906 by Dr. Imhoff. The first plant was put into operation two years later. From the information available, it can be discerned that Imhoff Tanks were an acceptable form of wastewater treatment up until the 1950’s. The main advantage of this type of tank over the septic tank is that sludge is separated from the effluent, which allows for more complete settling and digestion. Documentation from the period states that when operated properly, these systems are capable of removing 30 to 60 percent of the suspended matter, and from 25 to 40 percent of the BOD.
The Imhoff Tank consists of an upper section known as the sedimentation chamber, and a lower section known as the digestion chamber. As a result, Imhoff tanks are very deep and construction costs were high. Forward flow enters the sedimentation chamber. The settling of solids occurs in an upper chamber and digestion of the solids in the lower chamber. The two chambers are separated by a sloping partition that contains narrow slots through which the solids passed into the lower chamber. Solids settle out in the upper sedimentation chamber and gradually flow into the lower digestion chamber. In the digestion chamber, solids accumulate and slowly digest. By design, gas and scum are prevented from entering the sedimentation chamber due to the narrow slots that disallow gas and sludge particles from entering the sedimentation chamber that “stir up” solids as was learned from the septic tank design.
Imhoff tanks are unable to meet today’s treatment standards for either primary clarifier performance or anaerobic digestion. However, there are several qualities worth mentioning:
- It requires a little space, and has a small “foot print”
- It is simple as it has no moving parts, i.e. mixers, collectors
- It requires little operator time as sludge removal is periodic
- At the time, it offered a better solution to solids handling and digestion.
The Imhoff Tank was used in both small and large wastewater treatment facilities during wastewater plant design in the early and mid-1900’s. Imhoff Tanks proved to be better suited for small treatment applications rather than large. Often, the tanks were used in conjunction with a trickling filter.
Photo and references: https://www.wwoa.org/history/treatment/