Chemistry parameter
Turbidity
The cloudiness of water caused by suspended particles. Turbidity is not itself a health parameter, but it is the most important operational indicator in drinking water treatment because particles shelter micro-organisms from disinfection.
Key facts
- Unit
- NTU
- Metric
- Turbidity
What it measures
Light scattered by suspended particles at 90 degrees to an incident beam. It counts nothing directly; it responds to particle number, size and shape.
How it is measured
By nephelometer, reported in nephelometric turbidity units. Continuous online monitoring on each filter is standard practice at modern works.
How to read a value
| Value (NTU) | What it indicates |
|---|---|
| Below 0.1 | Excellent filtered water. The operational target for works treating Cryptosporidium-risk sources. |
| 0.1 – 0.5 | Good treated water. |
| 0.5 – 1 | Acceptable but indicates reduced filter performance. |
| Above 4 | Visible cloudiness; the regulatory limit at the consumer tap in several jurisdictions. |
Bands are conventional interpretation aids, not regulatory limits. Where a legal standard exists it is stated on the relevant standards page.
What causes it
- Clay and silt washed in after rainfall
- Algae in reservoirs
- Iron and manganese precipitates
- Disturbed sediment in distribution mains
Why it matters
- Filter performance is judged by turbidity because it responds within seconds, whereas a microbiological result takes at least a day
- A turbidity spike after rainfall is the classic signature of a Cryptosporidium risk event
- Discoloured water at the tap with normal turbidity at the works points to the distribution network rather than to treatment
Related intelligence
Sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- National Institute of Standards and Technology — NIST Chemistry WebBook — thermophysical properties of water. Open · US Government work — public domain