Secondary clarifiers are constructed and operated like the primary clarifiers except that they follow a biological treatment process such as a trickling filter or activated sludge. The type of biological treatment they follow dictates how they are operated.

Secondary clarifiers following trickling filters or RBCs are used to settle biological solids that have separated from the media.

Secondary clarifiers following activated sludge processes help settle and clarify the effluent while also providing a concentrated source of return sludge for process control.

Secondary clarifiers may be round or rectangular, and may be designed for natural gravity settling or enhanced chemically aided settling.

Key factors in secondary clarifier operation include the following concepts82:

– Retention   time,   hours= ( volume,    gal )   ( 24   hrs/day ) flow,    gallons   per   day ( inc   RAS ) https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780203734216/c7e56d1a-db1f-4fcf-8803-cf49bae89672/content/eq16.tif"/>

– Surface   loading   rate,   gal/day sq/ft = flow,    gallons   per   day surface   area,   sq/ft ( w/o   RAS ) https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780203734216/c7e56d1a-db1f-4fcf-8803-cf49bae89672/content/eq17.tif"/>

– Surface   loading   rate,   lbs/day sq/ft = solids   into   clarifier,   lbs/day surface   area,   sq/ft ( inc   RAS ) https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780203734216/c7e56d1a-db1f-4fcf-8803-cf49bae89672/content/eq18.tif"/>

– Weir   overflow   rate,   gal/day lineal   ft = flow,    gallons   per   day weit   length,   lineal   ft ( w/o   RAS ) https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780203734216/c7e56d1a-db1f-4fcf-8803-cf49bae89672/content/eq19.tif"/>