Safe water at the tap depends on two separate things, and most advice about waterborne bacteria only covers one of them. The first is what arrives at the building: whether the water leaving the treatment plant, the well or the tank is free of contamination. The second is what happens inside the building’s own pipes, tanks and heaters once the water is there. Some of the most serious bacteria in household water never come from the supply at all. They grow inside the plumbing, in warm, still water and in the slimy film that lines old pipes.
This guide covers both sides. It explains where waterborne bacteria come from and why the source matters, which organisms are worth knowing about, how to test household water properly, what each treatment method actually removes, and how temperature, flow and the right components stop bacteria growing inside a building. It is written for homeowners who want to understand the risk and for installers and distributors who supply the equipment that controls it.
Two Ways Bacteria Get Into Household Water
The first route is contamination from outside. Disease-causing bacteria such as E. coli and Salmonella live in the gut of people and animals, and they reach water through sewage, septic system leaks, farm runoff, flooding, or a damaged pipe that lets dirty water in. These organisms survive in water for days or weeks, but they generally do not multiply there. The risk comes from how much contamination entered and how recently. Prevention happens at the source and at treatment: protecting wells, disinfecting public supplies, and treating water at home where the supply cannot be trusted.
The second route is growth inside the building. A different group of bacteria, including Legionella, Pseudomonas and some environmental mycobacteria, are natural water organisms that thrive in plumbing. They live in biofilm, the thin slimy layer that forms on the inside of pipes, tanks, shower heads and filters, and they multiply fastest in water that is lukewarm and standing still. A building can receive perfectly safe water and still grow these bacteria to dangerous levels in a hot water tank kept too cool or a pipe that is never used.
The distinction matters because the fixes are different. Boiling water or fitting a drinking water filter does nothing to stop Legionella growing in a hot water cylinder, because that risk is controlled by temperature and flow. A well-run hot water system does nothing to protect against a contaminated well, because that risk is controlled by testing and treatment. Good water safety deals with both.
The Organisms That Matter
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Giardia lamblia
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Escherichia coli (E. coli)
Many microorganisms can be found in water, but a handful account for most of the risk in households. They are not all bacteria. Protozoa such as Giardia and Cryptosporidium cause similar illness and come from the same kind of contamination, and they behave very differently under disinfection, so they belong in any honest list.
| Organism | Type | Usual source | How it causes illness |
|---|---|---|---|
| E. coli | Bacterium | Human or animal faeces | Drinking. Most strains are harmless, but some, such as O157, cause bloody diarrhoea and kidney damage |
| Salmonella | Bacterium | Human or animal faeces | Drinking. Gastroenteritis, and in the case of Salmonella Typhi, typhoid fever |
| Campylobacter | Bacterium | Animal faeces, especially birds and livestock | Drinking. Diarrhoea, cramps and fever |
| Giardia | Protozoan | Human or animal faeces | Drinking. Prolonged diarrhoea, cramps and nausea |
| Cryptosporidium | Protozoan | Human or animal faeces | Drinking. Diarrhoea, and highly resistant to chlorine |
| Legionella | Bacterium | Grows inside building water systems | Breathing in fine droplets, for example from a shower. Causes Legionnaires’ disease, a severe pneumonia |
| Pseudomonas aeruginosa | Bacterium | Grows in biofilm in taps, filters and fittings | Contact and inhalation. Mainly a risk for people who are already seriously ill |
E. coli deserves a special mention because it plays two roles. Most strains are harmless, but because E. coli lives in the gut, finding it in water is a reliable sign of faecal contamination, so it is the standard indicator that water testing looks for. The World Health Organization’s guideline is that E. coli should not be detectable in any 100 mL sample of drinking water. Some strains are dangerous in their own right. Shiga toxin-producing strains such as O157 attack the lining of the large intestine, causing bloody diarrhoea, and in a minority of cases, mostly young children, they lead to haemolytic uraemic syndrome, a serious form of kidney failure. We cover E. coli in detail in E. coli in drinking water.
Legionella is the organism most often missed in household advice, because it is not a drinking problem. It multiplies in water between roughly 20 and 45 °C, especially where water stands still, and people catch it by breathing in tiny droplets from showers, spa baths, some humidifiers and cooling towers. Older people, smokers and people with weak immune systems are most at risk. Because it grows inside the building, a building’s hot water temperatures and how often its outlets are used are the main defences, which we return to below.
Who Is Most at Risk
Anyone can fall ill from contaminated water, but some people suffer much more serious consequences. Babies and young children lose fluid quickly through diarrhoea and can become dangerously dehydrated within a day. Older people are more vulnerable to both dehydration and pneumonia, which is why Legionnaires’ disease is mainly a disease of older adults. People with weakened immune systems, including those having chemotherapy, organ transplant recipients and people with advanced HIV, can develop severe infections from organisms that would barely affect a healthy adult.
That is why the stakes are higher in some buildings than others. Care homes, hospitals and nurseries need stricter water management than an average house, and households that include a vulnerable person have good reason to take extra care, for example by testing a private well more often or keeping the hot water system properly hot. Pregnant women and anyone with a chronic illness should also follow local advice closely during boil-water notices.
The most serious illnesses are those that spread beyond the gut. Bacteria that enter the bloodstream can cause sepsis, and some toxins damage the kidneys. These outcomes are uncommon, but they are the reason any persistent diarrhoea with blood, high fever or signs of dehydration should be treated as a medical matter rather than waited out.
Why Clear Water Can Still Be Unsafe
It is natural to judge water by how it looks, smells and tastes, and those senses do pick up some problems. Brown or orange water usually means rust or sediment. A rotten egg smell often comes from hydrogen sulphide, sometimes produced by bacteria in a water heater. A chlorine smell simply means the supply is disinfected. These are worth investigating, but none of them is a test for harmful bacteria.
Disease-causing bacteria, viruses and protozoa are invisible, have no smell and have no taste. A glass of perfectly clear, fresh-tasting water can carry enough E. coli or Giardia to make someone ill. The reverse is also true: discoloured water is often unpleasant but safe. The only way to know whether water is microbiologically safe is to test it.
How to Test Household Water for Bacteria
If your water comes from a public supply, the water company is responsible for testing it, and in most countries it must meet legal limits and publish the results. That testing stops at the boundary of your property, however. Problems that start inside the building, such as a contaminated storage tank, a backflow from a garden hose, or bacteria growing in rarely used pipes, will not show up in the utility’s results. Testing at your own tap is the only way to check them.
If your water comes from a private well, the responsibility is entirely yours. Public health agencies such as the US Centers for Disease Control and Prevention recommend testing a private well at least once a year for total coliform bacteria, along with nitrates and other basics, and again after flooding, after any work on the well, if the taste or smell changes, or if anyone in the household has unexplained stomach illness. Coliform bacteria are a group that includes E. coli. Their presence shows that surface water or contamination is getting into the supply, even if the particular bacteria found are harmless.
The most reliable method is a test by an accredited laboratory, using the sterile bottle and instructions the laboratory provides. Sample collection matters as much as the test. Remove any aerator from the tap, run the water for a few minutes, avoid touching the inside of the bottle or cap, and deliver the sample within the time the laboratory specifies, usually the same day or within 24 to 30 hours, kept cool. Home test kits for coliform bacteria are available and useful for routine checks between laboratory tests, but a positive result should always be confirmed by a laboratory. Legionella testing is a specialised procedure and is normally done by professionals as part of a building water management programme rather than by householders.
If a test shows E. coli, stop drinking the water untreated straight away and use boiled or bottled water. For a well, the usual next step is to find and fix the route of contamination, shock chlorinate the well, and then retest to confirm the problem has cleared.
Treatment Methods and What Each One Removes
No single method removes everything, and each has limits that matter. The table below compares the common household options by what they do to microorganisms. It says nothing about chemicals such as lead or nitrate, which need different treatment. For help choosing a full system, see our guide to water purification methods.
| Method | Bacteria | Protozoa | Viruses | Main limitation |
|---|---|---|---|---|
| Boiling | Yes | Yes | Yes | Practical only for small amounts. No lasting protection |
| Chlorine disinfection | Yes | Partly. Cryptosporidium is resistant | Yes | Needs correct dose and contact time. Less effective in cloudy water |
| UV sterilisation | Yes | Yes | Yes, at adequate dose | Needs clear water. No residual protection in the pipes. Lamp needs replacing |
| Ceramic filter | Yes | Yes | Limited | Needs cleaning. Can grow bacteria if neglected |
| Ultrafiltration | Yes | Yes | Partly to largely, depending on pore size | Membrane fouls without pre-filtration |
| Reverse osmosis | Yes | Yes | Yes | Wastes some water, slower, needs pre-filtration |
| Sediment pre-filter (for example 40 μm) | No | No | No | Removes particles only. Protects the treatment that follows |
To kill germs by boiling, bring water to a rolling boil for one minute, or three minutes above about 2,000 metres altitude, then let it cool covered. For UV units, look for independent certification such as NSF/ANSI 55 Class A, which corresponds to a dose of 40 mJ/cm², and replace the lamp on schedule even if it still lights, because output fades long before the lamp fails. Membranes and ceramic elements should carry certification for microbiological reduction, not just for taste and odour.
Two points apply to every method. First, the water must be reasonably clear. Particles shield bacteria from UV light and from chlorine, and they clog membranes, which is why point-of-entry systems almost always start with a sediment filter. Second, every filter needs maintenance. A filter cartridge left in place long past its life stops protecting and becomes a surface where biofilm grows, so an old filter can make water worse than no filter at all.
Stopping Bacteria Growing Inside the Building
For bacteria that grow in plumbing, temperature is the main control. Legionella grows between about 20 and 45 °C, is killed more slowly at 50 °C, and dies quickly at 60 °C. Widely used guidance therefore recommends storing hot water at 60 °C or above, keeping circulating hot water at about 50 to 55 °C at the farthest point, and keeping cold water below 20 °C. A water heater turned down to save energy, or a hot water tank that only reaches 45 °C, can turn into a breeding ground.
Storing water that hot creates a different hazard: scalding. At 60 °C, water can cause a full-thickness burn on a child’s skin in about a second. The answer is not to lower the storage temperature but to reduce the temperature at the outlet, using a thermostatic mixing valve or tempering valve that blends hot and cold water to a safe temperature just before the tap or shower. That way the water is hot enough in storage to control bacteria and cool enough at the tap to be safe. Our articles on the tempering valve for the hot water system and why we need thermostatic mixing valves explain how this works.
Flow is the second control. Water that stands still in a pipe warms or cools towards room temperature, loses its disinfectant, and gives biofilm time to grow. Guest bathrooms, holiday homes and outlets in buildings left empty are typical problem spots, as are dead legs, which are lengths of pipe left connected to the system after a fixture was removed. Running rarely used taps and showers for a few minutes each week, removing dead legs, and cleaning and descaling shower heads all reduce the risk.
The third control is keeping contaminated water out. Backflow, where water flows the wrong way back into the supply, can pull dirty water from a garden hose, a boiler circuit or an irrigation system into the drinking water pipes when pressure drops. Check valves and backflow preventers stop this, and many plumbing codes require them on specific connections. Larger buildings such as hotels, hospitals and care homes should have a formal water management programme, for example following ASHRAE Standard 188, which identifies where bacteria could grow in that specific building and how each risk is controlled and checked.
Components in contact with drinking water should also be suitable for it. Valves and fittings made from approved lead-free materials and certified for drinking water, for example to NSF/ANSI 61 in North America or WRAS in the UK, avoid adding contaminants of their own. Our guide to valve materials covers the brass and stainless steel grades involved.
Where a Pre-Filter Fits
A sediment pre-filter is often the first thing installed where water enters a home, and it is worth being clear about what it does and does not do. A 40 μm mesh stops sand, rust flakes, scale and other visible particles. Bacteria are roughly 0.5 to 5 μm across and protozoan cysts only a few micrometres more, so they pass straight through. A pre-filter does not make water microbiologically safe, and it should never be sold or bought as if it did.
What it does is protect everything downstream. It keeps grit out of valves, mixing valves, water heaters, washing machines and taps, where particles wear seals and cause leaks. It also protects the treatment that does remove bacteria. A UV lamp shining through cloudy water leaves bacteria in the shadow of particles, and a membrane clogged with sediment loses flow and fouls far sooner. Keeping water clear in front of those systems is part of what makes them work. We explain the filtration grade in the function of a 40 micron filter in piping systems.
Like any filter, a pre-filter must be cleaned. Particles collected on the mesh hold organic material and give biofilm somewhere to start, so a design that can be opened, rinsed and refitted quickly is more likely to be maintained properly than one that is awkward to service.
Frequently Asked Questions
What are waterborne bacteria?
Bacteria that are carried by water and can cause illness. Some, such as E. coli and Salmonella, come from faecal contamination and survive in water without growing. Others, such as Legionella, grow inside building water systems.
Can you see or smell bacteria in water?
No. Harmful bacteria are invisible and have no smell or taste. Clear, fresh-tasting water can still be contaminated, which is why testing is the only reliable check.
How often should well water be tested for bacteria?
At least once a year for total coliform bacteria, and again after flooding, after work on the well, if the water changes, or if anyone in the household has unexplained stomach illness.
Does boiling water kill bacteria?
Yes. A rolling boil for one minute, or three minutes at high altitude, kills bacteria, viruses and protozoa. It does not remove chemical contaminants and gives no lasting protection once the water cools.
Does a sediment pre-filter remove bacteria?
No. A 40 μm pre-filter removes particles such as sand and rust. Bacteria are far smaller and pass through. It protects the equipment and treatment downstream, such as UV and membrane systems, which do deal with bacteria.
How do you get Legionnaires’ disease from household water?
By breathing in fine droplets of water containing Legionella, typically from a shower or spa. It is not normally caught by drinking. Keeping hot water stored at 60 °C and using outlets regularly are the main defences.
What temperature should a hot water tank be set to?
At least 60 °C in storage to control Legionella, with a thermostatic mixing or tempering valve reducing the temperature at outlets to prevent scalding.
Water Safety Components from Legom
Jiaxing Legom Technology Co., Ltd. manufactures water and hydronic components in Jiaxing, Zhejiang, supplying brands, distributors and wholesalers in more than 90 countries. Our range covers several of the control points described in this article.
At the point of entry, our water pre-filter and first-stage pre-filter use a 40 μm stainless steel mesh to remove rust, sand and larger particles before water reaches valves, appliances and downstream purifiers, while maintaining steady water pressure. A quick-disassembly design with a gear-linkage mechanism lets the mesh be removed, rinsed and refitted without tools, which makes the regular cleaning that keeps a filter safe far more likely to happen. The universal inlet simplifies installation, and the housing is built to withstand pressure surges, as explained in water hammer prevention with modern pre-filters.
For treatment, our water purifier range is designed to follow that pre-filtration stage. On the hot water side, our water mixing valves and thermostatic mixing valves let hot water be stored at a temperature that controls Legionella while being delivered at a safe temperature at the tap. For distributors and brands, our OEM and ODM service covers branding, specification and market documentation.
Reviewed by Maggie Shen, Director at Legom, on November 5, 2026. This article was reviewed for accuracy, including the distinction between faecal contamination and bacterial growth inside building water systems, the characteristics of E. coli, Salmonella, Campylobacter, Giardia, Cryptosporidium, Legionella and Pseudomonas, WHO guidance on E. coli in drinking water, private well testing frequency, sample collection practice, the effectiveness and limits of boiling, chlorination, UV, ceramic, ultrafiltration and reverse osmosis treatment, the particle size range removed by a 40 μm pre-filter, and temperature control of Legionella with outlet tempering to prevent scalding.