The treatment plant has three circular tanks that accept the raw sewage from the grit chambers. The sewage is held in these tanks for periods of four to six hours to allow the solids to settle by gravity.
Settled solids are collected by scrapers at the bottom of the tank and pushed toward the center of the tank. Here the sludge falls into a pit where the primary sludge is collected and pumped to a gravity thickener.
This process removes an estimated 50% to 60% of the suspended solids that comes from the grit chambers. The settling process also reduces the organic content by approximately 30%. The organics are called BOD (Biochemical Oxygen Demand) because oxygen is needed for the bacteria to consume this waste product. The bacteria process is called activated sludge.
The Primary Clarifiers are part of the original plant construction in 1975, and are slated for upgrade in 2015.