Imhoff or Emscher Tank - a classic wastewater treatment technology that should still be considered for on-site and low flow treatment
The Imhoff Tank was developed in the early 20th century for treating wastewater. The tank was an improvement over existing septic tank technology and served as a bridge before more advanced systems were introduced. So why talk about Imhoff tanks today? Well for developing countries or small communities without access to significant capital or experienced operators, an Imhoff tank offers a robust, compact unit to pretreat wastewater.
The Imhoff tank is a compact treatment unit usually made of concrete and build in-ground. The Tank consists of a V-shaped settling chamber with a residence time of one to three hours. At the bottom of the V, settled solids drop into a lower anaerobic chamber that starts sludge stabilization. Scum and gas escape from the anaerobic chamber to gas vent channels running parallel to the V-channel (sedimentation chamber). The anaerobic section also has a V-shape for solids accumulation where pumps or screws can eventually remove digested solids for disposal.
As for performance, the Imhoff tank removes between 25 - 50% of inlet COD. With a relatively short residence time in the settling chamber, odors associated with anoxic/anaerobic wastewater are prevented. The anaerobic gases including methane are vented in the side channels with the scum layer helping control organic acid and sulfide odors. As with all anaerobic pretreatment, pathogen removal is low. Normal operations include checking scum & anaerobic chamber sludge levels. Digested sludge should be removed with solids being treated in drying beds or composting - to ensure pathogen removal.
Water passing through the settling chamber, still contains substantial COD/BOD, ammonia, phosphorus, and enteric organisms. Further treatment is required. In traditional Imhoff systems, the discharged water enters facultative lagoons, rock-reed filters, trickling filters, or other low-cost aerobic/oxidation system.
Imhoff tanks with low construction and operational costs are suitable for communities with flows up to 950 cubic meters or 250,000 gallons per day.
Benefits of Imhoff Tanks
Erik Rumbaugh has been involved in biological waste treatment for over 20 years. He has worked with industrial and municipal wastewater facilities to ensure optimal performance of their treatment systems. He is a founder of Aster Bio (www.asterbio.com) specializing in biological waste treatment.
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