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How does the activated-sludge process treat sewage?
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The activated-sludge process is a secondary treatment method that utilizes microorganisms to remove organic matter from sewage. The process involves several key steps:
- Aeration: Settled sewage is mixed with recirculated, bacteria-rich sludge in an aeration tank. Air is introduced via diffusers or mechanical mixers, providing the oxygen necessary for bacteria to thrive and metabolize dissolved organic materials.12
- Detention: The mixture is agitated for several hours, allowing the microorganisms to absorb or adsorb suspended and organic solids, effectively reducing the biochemical oxygen demand (BOD).12
- Clarification: The mixture flows into a settling tank (secondary clarifier), where the activated sludge settles by gravity. Clear, treated water is skimmed from the surface for disinfection and discharge.12
- Recirculation: A portion of the settled sludge is returned to the aeration tank to maintain a healthy population of acclimated microbes, while the excess is removed for further treatment and disposal.2
Variations, such as high-purity oxygen systems, can accelerate this process by increasing oxygen concentration.2