Secondary Treatment begins with the activated sludge process. The aeration tanks are where bacteria are grown and cultured. These bacteria use oxygen and feed on the remaining suspended and dissolved organic matter from the primary clarifiers.
Air is introduced into the liquid/biological mixture (mixed liquor), by large mechanical mixers (think of a home batter electric mixer) to assure enough air is in the wastewater for the bacteria to thrive. To ensure enough bacteria are available to consume this waste product, sludge must be returned to the tank from the secondary clarifiers (returned activated sludge).
The activated sludge will constantly increase in quantity as it eats more organic material in the wastewater. When there are too many bacteria it becomes necessary to remove the excess quantity from the system. The excess microbiological life removed is termed waste activated sludge and is pumped to tanks called sludge thickeners. All of the return and activated sludge is collected in the bottom of the secondary clarifiers.
The secondary clarifiers are another set of circular tanks where the bacteria from the activated sludge tanks are settled out by gravity. Manchester has three clarifiers where the mixed liquor suspended solids (MLSS) is separated from the clean overflow. The MLSS settles to the bottom of the clarifier, is collected by a series of spiral blades and sludge intake ports along the bottom of the tank abd the center column and is either wasted to the thickener(s) or returned back to aeration tanks to maintain the biological population.
The size of each of the three (3) secondary clarifiers is 145 feet in diameter x 12 feet deep. This provides a volume of 198,060 cubic feet which is 1,481,489 gallons.