Bacteria are the most common reason a private water supply fails a drinking water test, and E. coli is the one that matters most. Its presence shows that water has been contaminated with faecal matter from people or animals, which means other harmful organisms may be present too. The good news is that bacterial contamination is easy to test for and reliably treated.

At a glance

  • UK limit: 0 E. coli and 0 enterococci per 100 ml; coliform bacteria 0 per 100 ml (indicator value)
  • Most at risk: boreholes, wells and especially springs and surface water
  • Signs: usually none; water can look, smell and taste perfect
  • Fix: sediment filtration followed by UV disinfection

E. coli, coliforms and enterococci: what is the difference?

Laboratories do not test for every germ that could be in water. Instead they look for indicator bacteria, which are easy to detect and tell you whether contamination has happened:

  • E. coli lives in the gut of people and warm-blooded animals. Finding it in water is clear evidence of recent faecal contamination, so the legal limit is zero.
  • Enterococci also come from faeces but survive longer in water, so they can point to contamination that happened some time ago. The limit is zero.
  • Coliform bacteria are a wider group that also live in soil and vegetation. They are not always faecal, but their presence shows the supply is open to outside influence or that treatment is not working properly.

These limits come from the Water Supply (Water Quality) Regulations 2016, and private supplies in England must meet the same standard.

Where do bacteria in water come from?

Mains water is disinfected and kept that way with a small chlorine residual, so bacterial failures at the tap are rare. Private supplies have no such protection. Common routes in are:

  • Septic tanks, cesspits or soakaways too close to the source, or failing.
  • Livestock grazing near a spring or well, and slurry or manure spread on nearby land.
  • Surface water getting in through a cracked or missing well cap, a poorly sealed borehole head or a damaged collection chamber.
  • Heavy rain or flooding washing contamination into shallow sources within hours.
  • Storage tanks without close-fitting lids, which let in insects, birds and small animals.

Health effects

Many E. coli strains are harmless, but some, along with other organisms that travel with faecal contamination such as Campylobacter, Cryptosporidium and norovirus, cause stomach upsets, diarrhoea and vomiting. Households sometimes build up a tolerance to their own supply while visitors fall ill, which is one reason holiday lets and B&Bs on private supplies are regulated more strictly. Babies, older people and anyone with a weakened immune system are most vulnerable.

If a test finds E. coli or enterococci, boil drinking water or use bottled water until the supply has been treated and retested clear.

How to test for bacteria

Bacteria can only be found by a laboratory test. Samples must be taken carefully from the drinking tap and reach the laboratory quickly, because the bacteria in the bottle change over time. Test at least once a year on a private supply, ideally after wet weather, and after any work on the source or treatment. See private water supply testing, or testing for holiday lets and businesses if you supply water to others.

How to remove bacteria from water

UV disinfection is the standard treatment for private supplies. Water flows past an ultraviolet lamp inside a stainless steel chamber, and the UV light damages the DNA of bacteria, viruses and parasites so they cannot reproduce. It adds no chemicals and does not change the taste. Three things make UV work properly:

  1. Pre-filtration. Particles can shade bacteria from the light, so a sediment filter always goes in first.
  2. Treat iron, manganese and hardness first. These coat the quartz sleeve and block the light. See iron and manganese in water.
  3. Annual lamp changes. UV lamps lose output with use even while they still glow, so replace them every 12 months and clean the sleeve.

Browse UV water filtration systems, read our UV FAQs, or see how to purify well water for the full treatment sequence. Always fix the source too: a sealed well cap or relocated soakaway removes the cause rather than just treating it.

Related contaminants

Why Wrekin Water

Wrekin Water is a family run company based in Bettisfield, Shropshire, serving more than 50,000 customers over 15 years and rated Excellent on Trustpilot. We supply, install and service UV systems for private supplies, and in North Shropshire we can fit them for you. Not sure what you need? Ask our team.

Frequently asked questions

No. The UK limit for E. coli in drinking water is zero per 100 ml. Any E. coli shows faecal contamination, so boil drinking water or use bottled water until the supply has been treated and retested clear.

Coliforms are a broad group of bacteria found in soil, vegetation and faeces. E. coli is one member of that group that lives only in the gut of people and warm-blooded animals, so finding it proves faecal contamination. Coliforms alone show the supply is exposed or treatment is not working.

Fit a sediment filter followed by a UV disinfection system, after treating any iron, manganese or hardness that would coat the UV sleeve. Also fix the source, for example by sealing the well cap or moving a nearby soakaway. Retest once treatment is installed.

Yes. Bringing water to a rolling boil kills E. coli and other bacteria, which is why boiling is the emergency advice after a failed test. It is a stop-gap rather than a solution; UV disinfection treats every tap continuously.

At least once a year, ideally after a spell of wet weather, and after any flooding, work on the source or change to your treatment. Supplies used for holiday lets or businesses are also sampled by the local authority.