Treatment process
Slow sand filtration
The oldest engineered water treatment process still in wide use, and the one that first demonstrated that filtration prevents cholera. Its effectiveness depends on a living biological layer rather than on physical straining alone.
Key facts
- Scale
- Municipal
- Treatment stage
- Filtration
How it works
Water percolates slowly — around 0.1–0.3 m/h — through fine sand. A biologically active layer, the schmutzdecke, develops at the surface and does much of the removal. Cleaning means scraping the surface, after which the filter must mature again.
What it removes — and what it does not
| Row | Removes | Does not remove |
|---|---|---|
| 1 | Bacteria and protozoa to a high degree | Dissolved salts |
| 2 | Turbidity | Nitrate |
| 3 | Some dissolved organic matter, biologically | Most synthetic organic chemicals |
| 4 | Ammonia, by nitrification | Colour, substantially |
Trade-offs
- Very large land requirement for the throughput achieved
- Cannot handle high turbidity without pre-treatment
- Needs weeks to ripen after cleaning
- Minimal chemical and energy input, and very robust once mature
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