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Activated Sludge Process

Activated sludge consists of sludge particles, teeming with living organisms, produced in either raw or settled waste-water by the growth of organisms (which include bacteria) in aeration tanks where dissolved oxygen is present.

In general, sludge is defined on the basis of the solids content (Figure A-1) but in order to develop a quantitative definition of sludge which will also help with correlating waste forms with disposal options, a simple and measurable characteristic of all three waste forms needs to be identified.

Figure A-1 Simplified distinction between solid waste, sludge, and liquid waste.

Activated sludge processes treat waste streams that contain 1% or less of suspended solids. In this process, flocculated biological growths are continuously circulated in contact with organic wastewater in the presence of oxygen. The process is widely used for industrial wastes and is even more common in municipal treatment plants.

In the process, air or pure oxygen is bubbled through industrial wastewater combined with organisms to develop a biological floc, which reduces the organic content of the wastewater.

The combination of industrial wastewater and biological mass is commonly known as mixed liquor. Once the industrial wastewater has received sufficient treatment, excess mixed liquor is discharged into settling tanks and the treated supernatant liquid is run off to undergo further treatment before discharge. Part of the settled material (sludge) is returned to the head of the aeration system to re-seed the new sewage (or industrial wastewater) entering the tank. Excess sludge which eventually accumulates beyond what is returned is the waste activated sludge which is removed from the treatment process to keep the ratio of biomass to food supplied (wastewater) in balance. The waste activated sludge is stored away from the main treatment process in storage tanks and is further treated by digestion, either under anaerobic or aerobic conditions prior to disposal.