Treatment process
Rapid gravity filtration
Sand or multimedia filtration after clarification, and the barrier that matters most against chlorine-resistant pathogens such as Cryptosporidium.
Key facts
- Scale
- Municipal
- Treatment stage
- Filtration
How it works
Water passes down through a bed of graded sand, often over anthracite, at rates around 5–15 m/h. Particles are captured within the bed depth rather than only at the surface. Beds are backwashed periodically.
What it removes — and what it does not
| Row | Removes | Does not remove |
|---|---|---|
| 1 | Residual floc and turbidity | Dissolved substances of any kind |
| 2 | Cryptosporidium and Giardia, when operated to a strict turbidity target | Viruses reliably, without disinfection |
| 3 | Precipitated iron and manganese | Nitrate, chloride, sodium |
| 4 | Algae | — |
Trade-offs
- Performance degrades immediately after backwash, so the first filtrate is often run to waste
- Requires continuous turbidity monitoring per filter
- Turbidity breakthrough is the mechanism behind most large Cryptosporidium outbreaks
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