What it means
In activated sludge, wastewater is mixed with a population of microorganisms in an aeration basin. The organisms consume organic material, form biological floc, and are then separated in a secondary clarifier. Some settled biomass returns as RAS, while excess solids are wasted.
What operators watch
- Aeration, sludge age, return sludge, and wasting control the biology.
- Clarifier performance is part of the activated sludge process, not a separate afterthought.
- Microscopic observations, settling tests, DO, and effluent quality help diagnose process health.
Common exam traps
- Thinking more air always fixes the process.
- Confusing primary sludge with activated sludge.
- Ignoring wasting and sludge age when troubleshooting.
Field example
A plant with rising effluent TSS may need checks of clarifier blanket depth, sludge settleability, RAS rate, wasting rate, and aeration basin conditions.
