The wastewater flows from the grit removal process into two junction boxes and is distributed equally to the primary clarifiers. Primary sedimentation occurs in six open 105-feet diameter primary clarifiers. Four have a side wall depth of 8 feet. Two have a side wall depth of 12 feet. The primary clarifiers are center feed, peripheral overflow type. The primary clarifiers provide quiescent conditions where solids are removed from the liquid stream. The clarified or primary effluent travels from the primary clarifiers through a series of pipes and junction boxes to the trickling filter/solids contact process.
Sludge settling to the bottom of the primary clarifiers is collected by a rotating rake mechanism which carries the sludge to a center hopper. The settled solids are continuously withdrawn at a low concentration by recessed impeller type centrifugal pumps. An in-line grinder upstream of each sludge pump shreds large solids and stringy materials into small fragments to ensure a uniform sludge consistency for downstream processes. Primary sludge is pumped to the dissolved air flotation process (DAFT) for thickening prior to anaerobic digestion.
Each primary clarifier has a scum baffle and rotating scum skimmer which traps floating materials such as grease from the water surface. Skimmings are removed periodically by the same pumps used to remove primary sludge. A 3-way valve on the suction piping of each centrifugal pump alternates on a timed cycle between the clarifier center hopper piping and the skimmings hopper. A below grade scum and sludge pumping station is located between the six primary clarifiers.