Look at water and its containers every day for three kinds of living growth: algae, which tint water green and coat tank walls with slime where light gets in; biofilm, a slimy skin in pipes and tanks that sheds into the water and shelters germs from chlorine; and animal life such as larvae and tiny worms, which show gaps in covers and screens. These signs tell you what to repair and clean, but they do not show whether harmful germs are present, and blue-green scum on open water should be avoided as a source because it can carry toxins that boiling does not remove.
Part of Community Water Testing
Visual biological indicators are the algae, slime and small animals that can be seen in a water source, tank or pipe, read as evidence of light, nutrients, warmth and openings that let living things in.
Growth Shows Where Light, Nutrients And Gaps Are, Not Whether The Water Is Safe
The community water testing page lists what to look for and what to do:
| Observation | Possible cause | Action |
|---|---|---|
| Green tint or slime | Algae growth | Clean the tank, block light, increase circulation |
| Tiny worms or larvae | Mosquito breeding, or biofilm | Clean the tank, repair screens, check for openings |
Algae need light and nutrients. In a tank, light enters through a cracked lid, a clear pipe or an open hatch, and nutrients arrive in dust, leaf litter or with the water. A tank in the dark does not grow algae. The fix is to cover it, not just to clean it.
Biofilm is the thin slimy layer of microbes that coats the inside of every wet surface. In pipes and tanks it sheds in pieces, which show as a rise in cloudiness or as flecks, and it shelters organisms from chlorine, so a system with heavy biofilm keeps a low residual. Flushing the system monthly with the lowest valve open for 5 minutes, as the community water system page directs, removes some. This is a different thing from the useful living layer on top of a slow sand filter (Biofilm Maturation).
Larvae and small animals. Mosquito larvae hang at the surface and wriggle when disturbed; they mean an opening large enough for adults to lay eggs. Red worms or larvae in the sediment of a tank suggest a lot of organic matter settling at the bottom. In a stream, the presence of mayfly, stonefly and caddisfly larvae generally suggests cleaner, well-oxygenated water, and only pollution-tolerant animals suggests the reverse. I am recalling that from general ecology; it describes the stream’s condition and is not a safety test for drinking it.
What You Need
- A bright light to look into tanks, and a clean white container to hold a sample against
- A tank inspection routine with a list of what to look at: lid, vents, screens, overflow, walls
- Tools for cleaning: a brush, a bucket, clean water, a mop
- A log and, if you can, a simple sketch of the tank with notes (Test Log)
Method: Look, Record, Fix The Cause
Step 1: Open the hatch carefully and look, keeping dirt and hands out of the water.
Step 2: Note: water colour, film, slime, flecks, larvae, anything on the surface or floor.
Step 3: Find how it got there. Light gap? Torn screen? Open overflow? A ditch draining into the source?
Step 4: Clean and repair, covering the tank so that no light enters and no insect can. Rinse with clean water, and flush the system.
Step 5: Test afterwards with the bacteria test and the residual, since cleaning stirs things up and opens the tank to dirt.
Reading A Tank Inspection
A tank shows a green tint at the top and slime on the walls above the water line, no larvae, and a fine screen intact on the vent. The tint and slime say light is entering: the lid has a crack or the access hatch is not opaque. No larvae and an intact screen mean insects are excluded. The action is to seal the light gap and clean the walls, not to dose the water.
A second tank has no algae but a layer of silt on the floor and larvae under the lid. The silt means the outlet is set too low or the tank has not been desilted; the larvae show a gap in the cover. The action is to desilt, move the outlet above the floor, and fix the cover.
A third source, an open pond, has a blue-green scum. Do not use it for drinking. The scum may be cyanobacteria, which can release toxins into the water, and, as I recall, boiling does not destroy those toxins, and the treatment chain in this section is not built to remove them. Take water from another source.
Common Mistakes
- Cleaning a tank and leaving the light gap. The algae come back.
- Treating slime as a cleaning job only. It tells you about the cover.
- Reading a living stream as safe water. Healthy animals do not mean it is free of germs.
- Stirring sediment during cleaning and then going straight back into service. Flush and test first.
Safety
Cleaning a tank means entering a confined space, and cleaning chemicals can make gas
A tank can hold air low in oxygen or carrying hydrogen sulfide or carbon dioxide. Nobody enters without ventilation, a harness on a line held by a person outside, and a rule that the person outside never goes in after a collapsed worker. Do not test the air with a flame. If you use bleach to scrub walls, never mix it with acids, vinegar, rust or toilet cleaners, or with ammonia products: they release chlorine or chloramine gas inside the tank, which is where people have been hurt cleaning.
What’s Next
- Learn next: Smell and Taste — the other early signs a supply is changing
- Combine with: Event-Triggered Testing — what to test after you find growth