Explanation

The word ‘activated sludge system’ is derived from the practice of adding to the incoming sewage of the sludge from a previous batch. This sludge inoculums contains large numbers of metabolizing bacteria, together with yeasts, molds and Protozoa. An especially important ingredient of the sludge are species of Zoogloea bacteria, which from flocculent masses (floc) in the aeration tanks. The activity of these aerobic microorganisms oxidizes much of the effluent’s organic matter into carbon dioxide and water. When the aeration phase is completed, the floc (secondary sludge) is allowed to settle to the bottom just as the primary sludge settle in primary treatment.