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Which of the following is NOT a secondary treatment unit in wastewater treatment ?
aeration tank
trickling filter
aerated lagoon
Imhoff tank
Imhoff tank
An Imhoff tank is a primary treatment unit used for the sedimentation of wastewater and the simultaneous anaerobic digestion of the collected sludge. Secondary treatment units are specifically designed for the biological oxidation of dissolved organic matter using microorganisms, which does not occur in an Imhoff tank.
CPHEEO Manual on Sewerage and Sewage Treatment
An Imhoff tank is a primary treatment unit used for the sedimentation of wastewater and the simultaneous anaerobic digestion of the collected sludge. Secondary treatment units are specifically designed for the biological oxidation of dissolved organic matter using microorganisms, which does not occur in an Imhoff tank.
╬╖=CiтАЛCiтАЛтИТCoтАЛтАЛ├Ч100% тАФ Efficiency of solids removal, where CiтАЛ is influent concentration and CoтАЛ is effluent concentration.
Retention┬аTime=QVтАЛ тАФ Hydraulic Retention Time, where V is volume and Q is flow rate.
The Imhoff tank functions as a two-story sedimentation tank. In the upper compartment, settleable solids are removed by gravity sedimentation. These solids fall through a slot into the lower compartment (digestion chamber), where anaerobic bacteria decompose the organic matter in the sludge.
Primary treatment involves physical processes like sedimentation and flotation.
Secondary treatment involves biological processes (aerobic or anaerobic) to reduce BOD.
Imhoff tanks combine sedimentation and sludge digestion in a single structure, categorizing them strictly as primary/preliminary treatment.
Aeration tanks, trickling filters, and aerated lagoons all utilize microbial degradation of organic matter, defining them as secondary treatment.
Simple operation and maintenance.
Combines clarification and digestion in one unit, saving space.
Does not require external power for operation.
Potential for odor problems due to anaerobic decomposition.
Limited efficiency in BOD removal compared to secondary treatment.
Difficulty in controlling temperature and pH in the digestion chamber.
Small to medium-sized municipal sewage treatment plants.
Isolated rural communities or institutional campuses.
Aeration tank: Uses suspended growth (activated sludge process).
Trickling filter: Uses attached growth process.
Aerated lagoon: Uses mechanical aeration in a pond system.
Imhoff tank: A physical-chemical sedimentation unit with sludge digestion, not biological organic matter oxidation.
D is correct тАФ The Imhoff tank is classified as a primary treatment unit because it focuses on physical sedimentation and anaerobic digestion of solids rather than secondary biological treatment.
Always remember the hierarchy: Preliminary (screening/grit) тЖТ Primary (sedimentation/Imhoff) тЖТ Secondary (biological oxidation) тЖТ Tertiary (polishing/disinfection).
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