After grit removal, wastewater flows to primary clarifier tanks. Wastewater velocity is slowed once more to 1-2 feet/minute (1 foot /second in grit channel), and detention time is a minimum of 2 hours in the primary clarifier tanks. Removal of floatable material (plastics, wrappers, grease) and settleable material, called sludge or biosolids, is accomplished in the primary tanks. Floatable materials are collected from the surface of the wastewater, deposited in a sump, and pumped to anaerobic digesters. Machinery in the bottom of the clarifier tanks plows the sludge to the center where it is also pumped to anaerobic digesters for treatment.
Approximately 90-95% of the settleable solids, 40-60% of the total suspended solids, and 25-50% of the BOD is removed in the primary clarifiers. An average of 25,000 gallons of 3% solids primary sludge is pumped each day to the anaerobic digesters.