The activated sludge process is a process to speed up the rate of decomposition of waste material in water first developed in 1914. It uses air and highly concentrated micro-organisms to degrade organic pollutants, producing a floc containing the oxidized material. The general arrangement of an activated sludge process includes an aeration tank where air or oxygen is injected into the mixed liquor, and a settling tank to let the floc settle out and ready to be separated.
First, air or oxygen is injected into a mixture of micro-organisms and screened, and primarily treated wastewater to form a floc, which removes organic pollutants from the water. After sufficiently treated, this mixture, known as mixed liquor, is discharged into settling tanks and the treated supernatant is run off for further treatments. Part of the settled sludge is returned to the aeration tank to reuse the micro-organisms, which is called return activated sludge. The rest is removed from the process to keep "food to biomass" ratio in balance. The sludge is usually mixed with primary sludge from the primary clarifiers and undergoes further sludge treatment for other applications such as fuel production and composting.
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