Treatment process
Activated sludge
The dominant secondary wastewater treatment process worldwide. A suspended community of micro-organisms is aerated so that it consumes the organic pollution in sewage, and is then settled out and partly returned.
Key facts
- Scale
- Municipal
- Treatment stage
- Secondary wastewater treatment
How it works
Settled sewage enters an aeration tank containing suspended microbial flocs. Aeration supplies oxygen for the organisms to oxidise organic matter. The mixture settles in a final clarifier; most sludge is returned to maintain the population and the surplus is removed for treatment.
What it removes — and what it does not
| Row | Removes | Does not remove |
|---|---|---|
| 1 | Biochemical oxygen demand, typically over 90% | Most pharmaceuticals and micropollutants, substantially |
| 2 | Suspended solids | PFAS |
| 3 | Ammonia, by nitrification, where sludge age is long enough | Nitrate, unless an anoxic denitrification zone is included |
| 4 | Nitrogen and phosphorus, in configurations designed for it | Pathogens to drinking water standard |
Trade-offs
- Aeration is typically the single largest electricity consumer at a treatment works, and often the largest on a municipality’s entire estate
- Produces sludge requiring treatment and disposal
- Sensitive to toxic shock loads and to hydraulic surges during storms
Related intelligence
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Sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- NSF International / American National Standards Institute — NSF/ANSI drinking water treatment unit standards. Public but restricted · Publisher terms — public access, reuse not clearly granted
- American Water Works Association — AWWA standards, water audits and utility benchmarking. Public but restricted · Publisher terms — public access, reuse not clearly granted