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activated-sludge method
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Diagram illustrating sewage treatment using the activated sludge process. Raw sewage passes through screens and grit chambers in primary treatment, then flows to a primary clarifier. The primary effluent enters an aeration tank for secondary treatment, where air is added and activated sludge helps break down waste. The mixture moves to a secondary clarifier, with some sludge returned to the aeration tank and excess sent for disposal. Disinfected secondary effluent is released to surface water or
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activated-sludge method

sewage treatment
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activated-sludge method, sewage-treatment process in which sludge, the accumulated, bacteria-rich deposits of settling tanks and basins, is seeded into incoming waste water and the mixture agitated for several hours in the presence of an ample air supply. Suspended solids and many organic solids are absorbed or adsorbed by the sludge, while organic matter is oxidized by the microorganisms. The amounts of air and sludge used can be varied to control the level of treatment obtained. The sludge is then separated out in a settling tank.