Turbidity is the cloudiness of water caused by suspended particles, and it is the fastest early-warning check for a water supply: in a turbidity tube, a black mark that stays visible beyond 30 centimeters of water means good clarity, 10-30 centimeters is marginal and needs investigating, and under 10 centimeters is bad and means the water must not be distributed without treatment. Clear water is not proven safe, because bacteria, viruses and dissolved chemicals do not cloud it.
Part of Community Water Testing
Turbidity testing is the measurement of how cloudy water is, as a quick indicator of whether a source has changed and whether the water is clear enough for treatment to work.
Cloudy Water Is A Warning And Clear Water Is Only A Partial Reassurance
Turbidity is not a complete safety test, but it is the fastest indicator that something has changed. Particles of clay, silt, plant matter and organisms scatter light and make the water look cloudy. They matter for three reasons:
- Germs can attach to particles and ride through the system on them.
- Particles shield germs from chlorine and from sunlight, and they use up chlorine. Disinfection that works in clear water fails in cloudy water.
- A sudden rise at the source, especially after rain, shows that surface runoff is getting in, which means the source protection is inadequate (Source Protection).
The reverse does not hold. Clear water can still carry bacteria, viruses, parasites and dissolved chemicals, none of which cloud it. A clear reading says the water is suitable for treatment, not that it is safe to drink.
What You Need
- A turbidity tube (Turbidity Tube), or for a rough check, a clear bottle and printed text
- A log to record each reading with the date and weather
Method: Read The Depth At Which The Mark Vanishes
Step 1: Fill the tube slowly with the water sample, looking down from the top at a black cross on white paper beneath it.
Step 2: Note the depth of water at which the cross just disappears.
Step 3: Compare with the carrier’s three bands:
| Depth at which the mark vanishes | Meaning | Action |
|---|---|---|
| Over 30 cm | Good, low turbidity | Continue routine checks |
| 10-30 cm | Marginal | Investigate the cause |
| Under 10 cm | Bad | Do not distribute without treatment |
Step 4: Test at the source, after treatment and at distribution points, and write each reading down.
Worked Example: A Bottle Test And A Tube Reading
Without a tube, fill a clear bottle and hold it over printed text at arm’s length. The SODIS pages use this as the clarity test: if you can read the words through the water, it is clear enough for sunlight treatment; if the text blurs or vanishes, filter or settle the water first (SODIS Method). It is a yes or no check and cannot give a number.
A tube reading is a number. A spring reads 45 cm in dry weather and 25 cm after a storm. The fall from over 30 cm to the 10-30 cm band moves it from good to marginal and tells you that runoff is entering after rain. You would walk the source and fix the cause, not just add more treatment.
Common Mistakes
- Treating a good reading as proof of safety. Germs and chemicals do not show.
- Testing only the source. Treatment steps and taps must be tested too.
- Reading in glare or bad light. Shade the tube, and always use the same lighting.
- Not writing readings down. A trend is invisible without a record.
Safety
Turbidity is not a safety test
Never decide that water is safe to drink from its clarity alone. Clear water from a source that has had a recent flood, a dead animal nearby or an outbreak of diarrhoea in the community should be treated as suspect until treated and tested (Event-Triggered Testing).
What’s Next
- Learn next: Turbidity Tube — building the tester
- Combine with: What Turbidity Means — interpreting the number properly