Sewage Treatment Plant

Physical Unit Processes

Physical Unit Processes

Some of the common physical unit operations include among processes such as screening, flow equalization, sedimentation, clarification and aeration.


Screening process includes the use of a screen with uniform sized openings to remove larger solid particles thatcan damage the equipment, reduce the effectiveness of the ETP or pollute the waterways.

Flow Equalization

Equalization tanks are used to collect and store the wastes that allows to mix and become a regular quality before it is pumped to thetreatment units at a constant rate. ETPs are designed to treat wastewater that normally has a constantflow and a quality that fluctuates only within a narrow range. The equalization tank overcomes this. The hourly variation of flow needs to be determined so that the required volume of the tank is determined.


Sedimentation is used to remove grit and suspended solids, to produce clarified effluent, and to thicken the sludge produced inbiological treatment. A sedimentation tank (settling tank or clarifier) is used for this process. Flocculation and sedimentation should remove most of the suspended solids and a portion of the BOD.


This involves aerating the wastewater. The water is aerated (or shaken up) to release dissolved gases such as hydrogen sulfide. Aeration also replenishes oxygen, which is consumed by decaying organic matter found in wastewater. Bacteria are also used at this stage to consume nutrients and organic materials. Aeration at times can remove up to 90 percent of all solids and organic material. Two main methods are used for this, either mechanical agitation of the water so that air from the atmosphere enters the water, or by introducing air intothe tank through diffusers.

A) Chemical Unit Processes
B) Biological Unit Processes

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