Choose treatment by the contamination you have: muddy water needs sedimentation first, germs need a ripened slow sand filter followed by disinfection, Cryptosporidium needs the filter because chlorine does not kill it, and dissolved chemicals and heavy metals are not removed by any of these, so the answer for them is another source. Check turbidity before choosing anything, because above about 10 NTU a slow sand filter is unreliable and chlorine and sunlight are both weakened.
Part of Water Treatment at Scale
Treatment selection is the decision of which steps to put in a community water chain, made by matching each known or suspected contaminant to the process that removes or kills it.
Every Treatment Step Has A Class Of Contamination It Cannot Touch
A common error is to pick the treatment that is on hand and assume it covers everything. The table below matches contamination to what works, using this wiki’s pages on each process; figures in brackets name the page.
| Contamination | What works | What does not work |
|---|---|---|
| Mud, silt, clay | Settling, with a coagulant for clay (Settling Basins) | Chlorine or sunlight alone |
| Bacteria | Ripe slow sand filter, then chlorine or sunlight | An unripe, dried-out or overloaded filter |
| Viruses | Chlorine; the filter helps | Filtration alone, which is least reliable for viruses (Sand Filters) |
| Cryptosporidium, Giardia | Slow sand filter, or boiling | Chlorine does not kill Cryptosporidium (Water Treatment Systems) |
| Dissolved minerals, heavy metals, many chemicals | A different source | Sedimentation, sand filtration and chlorination do not reliably remove them |
| Iron (rusty colour) | Aeration, settling, filtering (Hardness and Iron) | Chlorine alone, which only turns it to particles |
| Algae, taste, smell | Settling and filtering; fix the source | Chlorine alone |
What You Need
- A turbidity reading of the raw water (Turbidity Tube)
- A history of the source: what is upstream, what happens after rain
- Test results for bacteria (H2S Test Kit)
- A list of what you can build and sustain, including chlorine supply
Method: Decide In This Order
Step 1: Is the source possibly chemically contaminated? Industrial runoff, mines, old fuel storage or fields sprayed with chemicals upstream. If yes, no treatment in this chain is enough. Choose another source.
Step 2: How turbid is it? If the raw water is cloudy, settle it first. The sand-filter page says a slow sand filter does not reliably remove turbidity above 10 NTU, and the roughing filter or 24-hour settling is the fix (Sand Filters).
Step 3: Add the filter, because it deals with protozoa that chlorine does not.
Step 4: Add disinfection to cover the bacteria and viruses that pass the filter, and to leave a residual in the pipe (Chlorination).
Step 5: Use sunlight only for small volumes of already clear water (UV Disinfection).
Step 6: Check with testing and adjust (System Monitoring).
Worked Example: Three Sources
- A protected spring, clear, no upstream pollution. Turbidity low. The filter is a margin, not a necessity. Chlorine at the tank provides the residual that protects against recontamination in the pipe.
- A stream below a village, muddy after rain. Settling first, then the filter, then chlorine. Without settling, the filter blocks after every storm and the protozoa pass when the operator speeds the flow.
- A well near an old fuel depot with a rainbow film in the bucket. Do not treat it. A film is a sign of petroleum, and the observation table on the community water testing page calls petroleum dangerous. None of the chain’s steps removes dissolved fuel. Choose another source.
What Testing Does And Does Not Tell You
A bacteriological result tells you about one sample at one moment. It cannot see viruses, parasites or chemicals (Community Water Testing). Choose the treatment from the source’s history and from your tests together, never from a single clear result.
Common Mistakes
- Choosing the chain by what is easy. Sunlight alone for a muddy stream; chlorine alone for protozoa.
- Treating a chemical problem with a biological process. Nothing here removes them.
- Skipping the check of the source’s surroundings. Treatment is the second line; the first is keeping contamination out.
Safety
Wrong treatment gives false confidence
Water that looks clear and has been chlorinated can still carry chemicals, protozoa that chlorine does not kill, and viruses that a poorly ripened filter missed. If the source may be chemically contaminated, do not drink it, and do not rely on boiling or chlorine to fix that. Never mix bleach with acids, vinegar or ammonia products when preparing any treatment.
What’s Next
- Learn next: Sedimentation — the first treatment step for most sources
- Combine with: Multi-Barrier Approach — combining the steps you choose