Physical Unit Process:
Common physical unit operations include among other processes screening, flow equalization, sedimentation, clarification and aeration.
A screen with openings of uniform size is used to remove large solids such as cloth, which may damage process equipment, reduce the effectiveness of the ETP or contaminate waterways.
There are several different steps in the textile dyeing process and therefore wastewater quality and quantity varies over time. ETPs are usually designed to treat wastewater that has a more or less constant flow and a quality that only fluctuates within a narrow range. The equalization tank overcomes this by collecting and storing the waste, allowing it to mix and become a regular quality before it is pumped to the treatment units at a constant rate. To determine the required volume of equalization tank the hourly variation of flow needs to be determined.
Sedimentation and Filtration
The flock formed in flocculation (see chemical unit processes for a description of flocculation) are large enough to be removed by gravitational settling, also known as sedimentation. This is achieved in a tank referred to as the sedimentation tank, settling tank or clarifier. Sedimentation is also used to remove grit and suspended solids, to reduce clarified effluent, and to thicken the sludge produced in biological treatment. Flocculation and sedimentation should remove most of the suspended solids and a portion of the BOD.
Aeration is required in biological treatment processes to provide oxygen to the microorganisms that breakdown the organic waste (this is described in more detail in the biological treatment section). 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 into the tank through diffusers.