E. coli in tank water: what a positive coliform result means and what to do next

The lab result arrives with a line that reads "Total Coliform: Positive" or "E. coli: Detected". Many people see the same word, coliform, and assume the two results are equally serious, or dismiss both. The truth lies in between: each result means something different and calls for a specific response. This article explains what E. coli in tank water indicates, how it gets there, and the sensible steps after a result.

What does E. coli in tank water actually mean?

Coliforms are a broad group of bacteria that are tested for not because they necessarily cause illness, but because they are an indicator. The US Environmental Protection Agency (EPA) (external source via the Consumer Safety Center) says total coliforms on their own often do not cause disease, but are a sign that other contaminants may be present or that treatment has failed, whereas the presence of Escherichia coli means the water may have been contaminated with human or animal waste carrying disease-causing organisms.

The WHO guideline tables for drinking-water quality (external source via the Consumer Safety Center) state that E. coli or thermotolerant coliforms must not be detectable in any 100 ml sample of water intended for drinking. A key footnote adds that total coliforms are not an acceptable indicator of the sanitary quality of supplies, particularly in hot regions where many environmental bacteria of no health significance are common.

Are all E. coli strains harmful?

No. Most strains of E. coli live in the gut without harm, and only some cause disease. But their value in a test is that they reveal that faeces has reached the water, and faeces can carry viruses, parasites and other bacteria that a routine test does not look for. So the question is not "is this strain dangerous?" but "where did the contamination enter?"

How does E. coli get into a household tank?

The EPA guidance on positive coliform results (external source via the Consumer Safety Center) directs you to inspect the whole system, including storage tanks, for any place where soil, leaves, insects, animals, sewage or animal waste could get in. In Saudi homes the same entry points come up again and again:

• Cracks in a ground tank: especially if it is close to a septic pit or drain line, or in areas with a high water table.
• A tank opening level with the paving: yard-washing water and dusty rain carrying cat droppings can run in.
• A roof tank lid: a broken or raised lid lets in birds, insects and dust.
• Tanker and hose connections: a hose left on the ground, or a contaminated filling pipe end.
• Accumulated sediment: it gives bacteria a place to shelter and uses up chlorine.

Sometimes the tank is not the source at all, but the tap itself, a household filter, or the way the sample was taken. That is why a single result is read cautiously and the test is repeated from different points to locate the problem.

How do you test a tank for coliforms?

Testing is done in a laboratory, and in its guidelines for testing well water (external source via the Consumer Safety Center) the CDC says that elevated total coliforms suggest other harmful germs may be present, and that a positive result for faecal coliforms or E. coli most likely means faeces has reached the water; it advises using a certified laboratory. These principles apply to a home tank as well.

• Collect the sample in the sterile bottle the laboratory supplies, not in an empty water bottle.
• Draw one sample from a tap connected directly to the tank and, if possible, another from the kitchen tap for comparison.
• Do not disinfect the tank before taking the confirmation sample, or you lose the chance to find the source.
• Ask the laboratory whether it tests total coliforms and E. coli together, and which method it uses.

The details of collecting, storing and transporting samples, and the other indicators worth testing, are in our guide to laboratory testing of tank water.

What should you do if the result is positive?

There is no need to panic, and no room for delay. The steps below run from most urgent to least urgent, and they focus on protecting the family first and then dealing with the cause.

1. 1

   Protect the family straight away

   Stop using tank water for drinking, cooking, infant formula and brushing teeth, and use bottled water. In EPA guidance for utilities, consumers are told to boil water for at least three minutes or use bottled water.

2. 2

   Retest to confirm

   A fresh sample from the tank and from another point of use helps distinguish real contamination from a sampling error.

3. 3

   Inspect the entry points

   The lid, cracks, vents, the area near the septic pit and the overflow pipe. Note what you find and photograph it.

4. 4

   Repair the source

   Close the lid, raise the opening, treat cracks and the internal waterproofing, and move sources of sewage away.

5. 5

   Clean and disinfect the tank

   Remove sediment, then disinfect with a product meant for drinking water and enough contact time. An equipped team should ideally do this, because entering the tank is dangerous.

6. 6

   Retest after disinfection

   Do not go back to drinking until E. coli tests negative, and measure residual chlorine at the tap.

How do you stop coliforms coming back after treatment?

If a positive result returns after a few weeks, it usually means the disinfection treated the effect rather than the cause. Prevention rests on three things: a closed tank that nothing can enter, water that does not stagnate for long, and a residual chlorine level that protects the water in the tank and pipes. To learn the acceptable levels, read our article on permitted chlorine levels in drinking water.

Periodic cleaning through a tank cleaning and disinfection service reduces the sediment bacteria shelter in, and allows the inside walls of a ground tank to be inspected. If cracks or leaks appear in the pipework, plumbing work may be needed to repair the joints before any disinfection. The Azl team can help with tank cleaning and waterproofing repairs where needed. For background on the illnesses involved, see health risks of contaminated water tanks.