The History of IMHOFF Technology

A Treatment Process for It's Time
By Tom Asmus

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.

This technology was introduced into the United States with its application widespread throughout the 1930's and 1940's. The development of the Imhoff tank was in response to problems associated with solids handling in primary tanks and the use of "regular" septic tanks. These problems included offensive odors resulting from poor sludge digestion.

The Imhoff Tank consists of an upper section known as the sedimentation chamber, and a lower section known as the digestion chamber. (See simplified process diagram below). 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.

The demise of the Imhoff Tank was a result of high construction costs and the inability to meet today's more stringent performance requirements. The combined treatment processes into a single vessel could not compete with the current, more energy-intensive methods used with individual treatment processes. From a process perspective, operational difficulties were often experienced as a result of incomplete sludge digestion that resulted in foaming and excessive scum formation.

Today (2005), many Imhoff Tanks are still in existence, though many have been modified to serve a "second life" as primary clarifiers or sludge holding tanks. The lower portion of the original Imhoff tanks were filled and new sludge removal equipment installed. Imhoff tanks have been superseded in plain sedimentation tanks using mechanical methods for continuously collecting the sludge, which is moved to separate anaerobic sludge digestion tanks. This arrangement permits improved sedimentation results and better temperature control in the digestion process, leading to a more rapid and complete digestion of the sludge.

Today, there is at least one Imhoff Tank operating in the state of Wisconsin, see photo and section drawing. The facility serves an institution and utilizes a trickling filter followed by a secondary clarifier. The facility is well maintained and is treating wastewater well within its permit limits, though the plant is significantly under loaded. Sludge from the Imhoff Tank is hauled via septic tank truck to a nearby sewerage district for treatment.

Imhoff technology served the industry for it's time by improving treatment process performance without adding complexity. During it's hey day, an era that preceded the Clean Water Act and it's associated affect on our industry, the Imhoff tank serves to remind us of simpler times and technologies. That seems to be its legacy. On a more humorous note, Dr. Imhoff's legacy may well be reflected in Imhoff's Law that states "The organization of any bureaucracy is very much like a septic tank -- the REALLY big chunks always rise to the top."


Simplified Process Diagram,

Sectional Drawing and Photo
(click on any image for larger versions)