E. coli or Coliforms in Your Water Test? What It Means and What to Do Next
Updated: 2 days ago
Last reviewed: September 2026
Quick answer A positive E. coli result means the water has very likely been contaminated by human or animal faeces, and it isn't safe to drink untreated. Stop drinking it, boil water for drinking, food and teeth, and get the result confirmed by an accredited lab. If you're on mains water, call your water company now. If you're on a private supply, contact your local council's environmental health team. Coliforms without E. coli are a warning sign rather than an emergency — but they still mean something is getting in.
Need to retest or check a fix? After treatment, repairs or heavy rain, a home bacteria screen can flag whether the problem has come back. Confirm any positive with an accredited lab.
E. coli, coliforms and enterococci: what's the difference?
Labs and home tests don't look for every germ that could make you ill. They look for indicator bacteria — organisms that show whether contamination has happened.
Indicator | What it tells you | UK standard at the tap |
E. coli | Faecal contamination from people or animals. The most important result. | 0 per 100 ml |
Enterococci | Also faecal. Survives longer, so it can point to older contamination. | 0 per 100 ml |
Total coliforms | A wider group. Some live in soil and vegetation, so a positive result without E. coli suggests a pathway in (a gap, a crack, a dirty tank), not necessarily faeces. | 0 per 100 ml |
What to do in the next 24 hours
Stop drinking the water untreated. The Drinking Water Inspectorate boil-notice advice is to "boil it before you drink it, use it to brush your teeth, make ice cubes, prepare food, clean feeding equipment or give it to your pets", then let it cool. Or use bottled water. For baby formula, follow the NHS method: boil the water, then "leave the water to cool for no more than 30 minutes, so that it remains at a temperature of at least 70C". The NHS says bottled water can be used "when drinking water is unavailable or has been contaminated". If you use bottled water, North Norfolk council's guidance says: "If bottled water is used ensure it has a sodium concentration of less than 200mg/l."
Tell the right people. On mains water, call your water company today. On a private supply, contact your council's environmental health team. If other homes, tenants, guests or a business use the water, they need to know too.
Get it confirmed. Book an accredited laboratory test, or ask the council. A home screen tells you there's a problem; a lab result is what the council and any treatment installer will need.
Write down what's changed. Heavy rain, flooding, work on the source or tank, a UV lamp that's gone off, livestock nearby — this helps find the cause.
Boiling deals with germs: the DWI says it means "any bacteria or viruses that were in your water are killed or deactivated". It is not a fix for nitrate: North Norfolk council notes that "Boiling the tap water will not reduce nitrate levels."
Mains water or private supply?
On mains water, E. coli at the tap is rare, and the cause is often local — a contaminated tap or sample, or a problem in a storage tank. Your water company has to investigate, so call them rather than acting alone.
On a private supply — a borehole, well or spring — positive bacteria results are far more common, especially after heavy rain. About 1% of people in England and 3% in Wales use a private supply. The council is the regulator: it must risk-assess and sample commercial and shared supplies, and it must monitor a single-home supply if the owner or occupier asks. When E. coli or enterococci are found, councils may serve a notice telling all users about the failure.
Could it be a false positive?
Sometimes. Bacteria tests are very sensitive, and a contaminated sample can fail a clean supply. Common causes:
touching the inside of the bottle or cap,
sampling from a dirty or mixer tap, or through a tap aerator or filter,
sampling from a hose or outside tap,
leaving the sample warm too long before it was tested.
Don't assume it's a false positive. Act on the result, then retest properly.
How to take a clean bacteria sample
Use the sterile bottle supplied. Don't rinse it.
Choose the cold kitchen tap — not a mixer tap if you can avoid it, and not a filtered tap.
Remove the aerator or any attachment, and clean the spout with a disinfectant wipe.
Run the cold water for 2–3 minutes.
Open the bottle without touching the inside of the cap or neck. Fill it to the line, and close it straight away.
Label it, keep it cool, and test or post it the same day.
Why it happens on private supplies
Council guidance lists the usual culprits:
Shallow sources picking up surface water after rain.
Septic tanks, soakaways or leaking waste pipes too close to the source — councils advise more than 50 m.
Muck heaps, slurry or livestock near the source.
Storage tanks with sediment or with missing or ill-fitting lids, which let in dust, insects and rodents.
Treatment that has stopped working — a UV lamp past its life, a blocked pre-filter, or a power cut.
How to fix it
There are two routes, and most supplies need a bit of both:
Protect the source: seal the borehole or well head and casing, fit a proper lid to tanks, divert surface water, move or fence off livestock, and repair nearby drainage.
Treat the water: UV disinfection (with a pre-filter so particles don't shield bacteria) or chlorination. See Private Water Supply Maintenance: Treatment, Servicing and Records.
Then retest. A notice is only lifted after a satisfactory resample — and councils usually charge for resampling.
Reading your E. coli water test result
Result | What it means | What to do |
E. coli present | Faecal contamination likely | Stop drinking untreated, boil, report, confirm with a lab |
Coliforms present, E. coli absent | Something is getting in; faecal source not shown | Boil as a precaution, find the entry point, retest |
Absent / not detected | Nothing detected in this sample | Keep testing regularly, especially after rain or maintenance |
Different results on two tests | Sampling or a changing supply | Retest with a clean sample; confirm with a lab |
For more on reading home test results, see the Result Interpretation Centre.
When to test for bacteria
At least once a year on any private supply (a common council recommendation).
After heavy rain, flooding or a long dry spell.
After work on the source, tank, pipes or treatment.
If the water changes in taste, smell or colour.
Before guests, tenants or a new baby start using the supply.
SustainWater's E. coli Test is a quick screening step. It's designed to flag a problem, not to replace a lab or council test.
Test your water
Keep a bacteria screen on hand for retesting after rain, repairs or treatment changes.
SustainWater home tests are screening tools. If a result is unexpected, or a health, legal or money decision depends on it, confirm it with an accredited laboratory.
Frequently asked questions
Does boiling water kill E. coli? Yes. Bringing water to a full rolling boil kills E. coli. Let it cool and store it covered in the fridge.
Can I shower in water that tested positive for E. coli? Ask your council or water company for advice specific to your supply. Under a boil water notice, the Drinking Water Inspectorate says: "The water is still safe to shower and bathe in, but make sure it does not get into your mouth." Take particular care with young children and anyone likely to swallow the water.
My neighbour uses the same spring. Do they need to know? Yes. A shared supply is regulated by the council, and everyone using it should be told.
How long after heavy rain should I test a private supply? Contamination often shows within a day or two of heavy rain. Testing a few days after a big rainfall event shows how your source responds.
Is coliform bacteria in drinking water dangerous? North Norfolk council's guidance explains that "Coliform bacteria are found widely in the environment", and the standard for drinking water is "0 in 100ml". Finding them means something is getting into the water, so treat it as a warning to find and fix the entry point.
What happens if you drink water with E. coli? It depends on the strain. For Shiga toxin-producing E. coli (STEC), one of the harmful types, NHS inform says "Some people become infected but don’t develop symptoms", while others get symptoms including diarrhoea, stomach pain, vomiting and fever. It warns that "A small number of people, mainly children, with STEC infection go on to develop a serious life-threatening condition called haemolytic uraemic syndrome (HUS)." If anyone in your household feels unwell after drinking the water, contact your GP or call NHS 111.
What level of E. coli in drinking water is safe? The UK standard is zero. North Norfolk council's guidance lists the standard for E. coli as "0 in 100ml", and treats any E. coli found as a "Potential danger to human health". Treat the water as unsafe until the cause is fixed and a follow-up test is clear.
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