The purpose of a primary clarifier is to create a relatively quiescent area where solids with a higher specific gravity than liquid will tend to settle and those with a lower specific gravity will tend to rise. The goal is that readily settleable solids and floating material such as grease can be removed and reduce the load on the subsequent biological treatment units.
After preliminary treatment, the heavier organic material settles and the lighter material floats. The heavier organic material has high volatile solids content (food for bacteria) and is pumped frequently to an aerobic or anaerobic digester. The floating material is skimmed off the top of the primary clarifier and added to the digester feed. If the sludge is not removed frequently from the primary clarifier it has a tendency to go septic due to a lack of dissolved oxygen. If this happens, it causes solids to rise to the surface of the clarifier and it also causes odors.
It is critical to waste sludge from the clarifier in a timely manner to prevent an organic or hydraulic overload in the anaerobic or aerobic digester that the sludge is being pumped to. Organically overloading either type of digester can cause odors, foaming and lid upset on anaerobic digesters.
These types of clarifiers are normally seen in larger facilities where they remove approximately 25 percent to 40 percent of the BOD and 40 percent to 60 percent of the solids in the influent flow at design flow. They are normally used before conventional activated sludge plants or before fixed film reactors such as trickling filters or rotating biological contractors in smaller facilities.
Primary clarifiers, when operated correctly produce an effluent that has a reduced solids and BOD load that should produce an acceptable effluent discharge.