Biological indicators are signs that living things are growing in or entering a water supply: green tint or slime, reddish slime, tiny worms or larvae, and a rotten-egg smell. They are clues, not proof, because harmful bacteria, viruses and parasites cannot be seen, so the one test aimed at the main danger, faecal contamination, is a bacteria test such as the hydrogen sulfide test, and even that is only evidence about one sample at one moment.
Part of Community Water Testing
Biological indicators are the visible, smellable and testable signs that point to living organisms in a water supply, from algae and larvae to the faecal bacteria whose presence signals that human or animal waste has reached the water.
Look For Growth, Then Test For Germs
Two kinds of indicator exist, and they answer different questions.
Signs you can observe tell you that something is living in the water or tank:
| Observation | What it usually means |
|---|---|
| Green tint or slime | Algae growing, usually where light reaches the water |
| Tiny worms or larvae | Mosquito breeding, or biofilm; gaps in screens or covers |
| Rotten-egg smell | Hydrogen sulfide from anaerobic bacteria: serious, flush and investigate |
| Oily film | Petroleum contamination or natural minerals: investigate, as petroleum is dangerous |
These come from the observation table on the community water testing page (Community Water Testing). Each is treated in detail in Visual Bio Indicators and Smell and Taste.
Indicator bacteria are the second kind. The germs that cause disease are many and hard to test for, so testing looks for a stand-in: bacteria that live in the gut and are found only where faeces have been. If they are found, faeces reached the water and any gut pathogen could have come with them. The hydrogen sulfide (H2S) test turns black when such bacteria are present (Coliform Detection).
What You Need
- Your eyes and nose, used daily at the source, tanks and taps
- A log of what you see and when
- An H2S test kit for the monthly check and after events (H2S Test Kit)
Method: Observe Daily, Test On Schedule
Step 1: Look at the source, the tank and a tap each day. Note colour, film, slime, larvae and smell.
Step 2: Write down anything unusual, however small. Slow change is the early warning.
Step 3: Run the H2S test monthly at the source, tank outlet and two or three taps, and immediately after rain, repairs or illness.
Step 4: Act on what you find: clean the tank and block light for algae, repair screens for larvae, flush and investigate a rotten-egg smell (Responding to Failures).
Worked Example: Two Signs, Two Different Meanings
A tank shows green slime on the walls and mosquito larvae under the lid. That is growth: light is getting in and the screen has a gap. It is a maintenance fault and a mosquito problem, and not by itself proof of faecal contamination. The operator cleans the tank, closes the gap, and runs an H2S test to check.
At the same time a household reports a rotten-egg smell from one tap, and its water is cloudy. This is a different case: the smell points to hydrogen sulfide from anaerobic bacteria, and the cloudiness to sediment. Together they call for flushing the branch, testing, and walking the pipe for a cause. Clear water in one place and growth in another are not evidence about each other.
What A Test Result Does And Does Not License
A negative H2S test says the sample showed no sign of faecal bacteria in that vial after 24-48 hours. It does not say the water is safe. It does not detect viruses, parasites or chemicals, bacteriological tests can miss low levels, a sample is only as good as how it was collected, and a safe result the day before an outbreak is normal. A positive result is a reason to act at once. A negative result is a reason to keep the routine, not to relax it.
Common Mistakes
- Treating clean-looking water as safe. Germs are invisible.
- Treating slime as the same as faecal contamination. It is a different problem.
- Ignoring a smell because it “always smells a bit”. Record and investigate it.
- Declaring victory on one negative test.
Safety
Observation is not a safety test
Do not taste water to find out whether it is safe. Water that smells of rotten eggs may carry hydrogen sulfide, a gas that is poisonous in closed spaces and that dulls the sense of smell at high levels, so do not enter a tank or a pit that smells of it. Do not test an atmosphere with a flame.
What’s Next
- Learn next: Coliform Detection — the bacteria test and how to read it
- Combine with: Visual Bio Indicators — reading algae, biofilm and aquatic life