Are rainwater and greywater systems a legionella risk?
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Key points
- Harvested rainwater and greywater are stored, unchlorinated water at ambient temperature, often sitting squarely in the legionella growth range with organic nutrients included.
- The exposure route decides the risk: spray uses (sprinklers, hosepipes, pressure washers) can carry legionella to the lungs; drip irrigation and WC flushing create far less respirable aerosol.
- Greywater arrives warm and nutrient-rich, so it is a more demanding medium than rainwater and needs shorter storage times.
- Reuse pipework must be identified as non-potable and never cross-connected to the drinking supply; backflow protection is not optional.
- A reuse system is a water system like any other: it goes in the risk assessment, gets maintained, and its tasks get recorded.
Rainwater harvesting collects roof runoff into a tank for reuse, usually for toilet flushing and garden irrigation. Greywater systems do the same with the waste from baths, showers, basins and laundry. Both are increasingly common in UK buildings, and both create the same basic legionella problem: a volume of unchlorinated water, stored at ambient temperature, carrying organic material, connected to outlets in your building. Whether that problem is small or serious depends almost entirely on one question: does the water ever become spray?
Why stored rainwater supports growth
Rain does not arrive sterile, and the roof adds whatever was sitting on it: dust, bird droppings, leaf debris. Once collected, the water sits in a tank with no chlorine residual, at whatever temperature the season provides. In summer, a tank in a warm plant room or in the sun sits comfortably inside the 20°C to 45°C band where legionella grows, with a supply of organic nutrients thrown in. The parallel with a neglected cold water storage tank is exact, except the reuse tank rarely gets the same inspection attention because nobody drinks from it.
Greywater tightens the screw. It arrives already warm from baths and showers, and it carries soap, skin cells and organic matter, so it is a richer medium that spoils faster. Greywater systems are therefore designed around short storage times, and a greywater tank run like a rainwater tank, holding water for weeks, is a warm nutrient broth.
The exposure route is the risk
Legionella causes harm when contaminated water is inhaled as a fine aerosol, so the use decides the risk level. A drip irrigation line or a toilet cistern produces very little respirable spray. A garden sprinkler, a hosepipe with a spray gun, or a pressure washer fed from the harvest tank atomises the water thoroughly, and anyone downwind breathes it. The same tank of water is a minor maintenance issue on the first list and a genuine exposure route on the second.
This leads to the practical hierarchy. Lowest risk: sub-surface or drip irrigation, and WC flushing with a well-maintained tank. Higher risk: any spray use, where the water should either be avoided or the system designed and maintained to a standard that matches, including treatment where appropriate. Vehicle washing with a hose sits awkwardly in between and deserves a thought about who is standing in the mist. Design decisions of that kind belong with a competent installer and should be recorded in the risk assessment, not improvised at the tap.
A harvest tank is a minor chore until the day someone connects a sprinkler to it. The water has not changed; the route into someone's lungs has. Assess the use, not just the tank.
Cross-connection: the other way reuse systems bite
The second risk has nothing to do with aerosols. A reuse system operating anywhere near the potable plumbing creates the possibility of a cross-connection, and a fault or an unwise plumbing alteration can push non-potable water back into the drinking supply. The Water Fittings Regulations require appropriate backflow protection for exactly this scenario, and harvested water counts as a contamination risk to be kept out of the mains. Pipework carrying reused water should be clearly identified as non-potable, outlets labelled, and any top-up connection from the mains arranged through a proper air gap. A cross-connection is how a garden feature becomes a whole-building contamination event.
Managing the system
The management regime is the familiar one, applied to a tank nobody drinks from:
- Match storage to demand so water turns over rather than standing for weeks, and keep greywater storage especially short.
- Keep the tank shaded, screened and filtered so debris and sunlight stay out and the water stays as cool as it can.
- Clean on a planned basis: filters, gutters, first-flush diverters and the tank itself, with the same discipline as a cold water tank inspection.
- Flush little-used draw-off points and avoid spray uses unless the system was designed for them.
- Put it in the paperwork: the reuse system goes in the risk assessment, its tasks in the written scheme, and its records in the log book. Our risk assessment builder can structure that assessment alongside the rest of the water system.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Where this fits the guidance
Nothing in ACOP L8 or HSG274 exempts a water system because its water is not for drinking. The duty is to assess and control the risk from any water system on the premises that can create an exposure, and a harvest or greywater system qualifies the moment it stores water or makes spray. Assessed honestly, used for the purposes it was designed for, and maintained on schedule, a reuse system is a manageable part of the water safety picture. Installed, plumbed in and forgotten, it is an unmonitored warm tank with your name on it.
Free legionella risk assessment template
A structured Word document following the five-step approach in ACOP L8. Covers risk identification, written scheme, monitoring, and records. If it isn't written down, you can't evidence it.
Follows ACoP L8 and HSG274 Part 2. Free. No spam.
Frequently asked questions
Can you get legionella from rainwater?
Rainwater itself is not sterile, and once it is collected it becomes stored environmental water: it sits at ambient temperature, carries organic debris washed off the roof, and has no chlorine residual. Stored warm for long enough it can support legionella growth like any other unchlorinated stored water. The risk becomes real when that water is turned into spray, through irrigation sprinklers, hosepipes or pressure washers, because inhalation of contaminated aerosol is the route of exposure. Sub-surface or drip uses present a much lower risk than spray uses.
Is greywater a legionella risk?
Greywater, the waste from baths, showers, basins and laundry, arrives warm and carries organic material such as soap and skin cells, so it is a richer growth medium than rainwater. Stored greywater in a warm tank is well inside the temperature band legionella favours. The controls mirror the risk: keep storage times short, avoid using greywater in ways that create spray, maintain the tank and filters, and keep the system completely separate from the drinking-water supply.
Can harvested rainwater be used for flushing toilets?
Toilet flushing is the most common use for harvested rainwater in UK buildings and is generally the lowest-risk use, because a cistern flush creates little respirable spray compared with a shower or sprinkler. The tank still needs maintenance: filters cleaned, debris removed, and storage kept turning over. The pipework must be clearly identified as non-potable and must never be cross-connected to the drinking-water supply, because backflow from a rainwater system into the mains is a contamination event for the whole building.
Do you need to treat harvested rainwater?
It depends entirely on the use. Water for sub-surface garden irrigation needs little beyond filtration to remove debris; water for any use that creates an aerosol, or for indoor uses, needs more thought and often disinfection. The design decision should be made with a competent installer and recorded in the risk assessment, because a reuse system is a water system like any other and the duty holder is responsible for assessing the risk it presents.
Does a rainwater harvesting tank need legionella controls?
Yes, in proportion to how the water is used. The tank is stored, unchlorinated, ambient-temperature water, so the familiar principles apply: keep storage volumes matched to demand so water turns over, keep the tank shaded and screened from debris, clean it on a planned basis, and flush the distribution if outlets are little used. Where the water feeds anything that sprays, the control standard rises sharply. The system belongs in the legionella risk assessment alongside the cold water tank and the calorifier.
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