A harvested-rainwater tank and a greywater recycling unit sit outside the part of the building most people picture when they think about Legionella. They are not the hot water system, they do not feed a drinking tap, and the water in them was never meant to be clean. That combination is exactly why they get waved through.
But non-potable water is still water that is stored, often sits at room temperature or warmer, and is sometimes turned into a fine spray. Two of those three things are precisely what Legionella needs. So the real decision for anyone specifying these systems — designers, sustainability leads chasing a BREEAM water credit, building owners trimming mains demand — is not whether to fit them. It is how they belong in the Legionella risk assessment, as systems in their own right, alongside the potable supply.
The honest way to make that call is to put the three water sources side by side and compare what each one actually does once it is in the building.
What turns non-potable water into a Legionella question
Strip it back and four things decide whether a stored water source is a Legionella concern: how long it stands, what temperature it holds, how much it has to feed bacteria, and whether the end use breaks it into breathable droplets.
Legionella multiplies most readily across roughly the 20–45°C band, sits dormant when genuinely cold below 20°C, and is progressively killed once water is hot [1][2]. Non-potable stores rarely stay cold and are never hot, so they tend to drift into or sit near the growth band, especially in summer. The same conditions that let it grow in tanks, sediment and biofilm in any man-made system apply here, which is why Where Legionella grows is the right mental model to bring to a rainwater or greywater store.
Growth alone does not make people ill, though. Infection needs the bacteria delivered as an aerosol fine enough to reach the lungs [3]. HSE points at systems that store water and can generate a breathable spray as among those most likely to create a foreseeable risk [4]. That is the hinge for non-potable water: a lot of what it is used for does not aerosolise much at all, and a few uses very much do.
Three water sources, side by side
Read the table as a comparison of risk profiles, not a league table. None of these systems is off-limits — they simply demand different controls.
| Factor | Mains / potable | Harvested rainwater | Recycled greywater |
|---|---|---|---|
| Typical storage and temperature | Cold kept below 20°C, hot stored hot and delivered hot; cisterns covered and turned over | Roof runoff held in an underground or plant-room tank at ambient temperature; can stand between rain events, drifting into the growth band [2] | Collected warm from baths, showers and basins — often near body temperature, squarely in the growth band [2] |
| Aerosol-generating end uses | Showers, spray taps and similar fixtures — the familiar potable risks | Mostly WC flushing and drip irrigation (low aerosol); spray irrigation, jet wash-down and water features do spray [4] | Same pattern — WC flushing low, but irrigation, wash-down and any feature spray [4] |
| Contamination / nutrient load | Low — supplied to drinking standard with a disinfectant residual | Moderate — roof debris, bird droppings, pollen and organic matter feed biofilm | High — soap, skin, hair and organic matter; the warmth makes it worse |
| Treatment typically applied | Treated by the supplier; on-site control is temperature and turnover | Coarse inlet filtration, sometimes filtration plus disinfection before use; not to potable standard | Multi-stage: filtration, biological or membrane treatment, then disinfection to a treated non-potable quality |
| Headline Legionella control | Keep hot water hot, cold water cold, and water moving [1] | Assess as a distinct system; restrict aerosolising uses; filter and disinfect; separate from potable | Highest scrutiny: reliable treatment and disinfection, restrict aerosol uses, never cross-connect |
Reading the table: greywater is the one to watch
If you take one thing from the comparison, take this. Recycled greywater is the higher concern, and it is not close.
Rainwater arrives cool and relatively clean, then warms slowly in storage. Greywater arrives already warm from the shower or bath and already loaded with the organic matter Legionella feeds on. Warm plus nutrient-rich is the most hospitable combination in the whole building, which is why greywater systems are designed around active treatment and disinfection rather than storage discipline alone. In my view, a greywater unit with no working disinfection step should be treated as a live hazard, not a maintenance backlog item.
Rainwater is the gentler proposition, but “gentler” is not “ignore”. A tank that sits half-full through a dry fortnight in August is doing a passable impression of a stagnant cistern.
What sprays, and what doesn’t
The good news in non-potable design is that the dominant uses are low-aerosol. Filling a WC cistern, flushing a pan, and sub-surface or drip irrigation move water without misting it. Those are the uses that make rainwater and greywater attractive, and they are also the lowest concern.
The risk concentrates in the spray-producing minority: pop-up sprinkler and spray irrigation, hose and pressure wash-down of yards or vehicles, and any decorative fountain or feature fed from the non-potable supply. A water feature running on harvested rainwater combines an ambient-temperature store with a continuous fine aerosol — the same trap covered in Decorative fountains and water features. The design move is simple to state: keep the spray-producing uses on the most-treated water, away from where people gather, or off the non-potable system entirely.
Keep it separate, and label it
There is one control that is not negotiable on any non-potable system, and it has nothing to do with temperature. Harvested and recycled water must be physically separated from the potable supply, with no cross-connection, and the pipework and outlets must be clearly marked so that nobody — not a tenant, not a contractor a decade later — mistakes it for drinking water or plumbs it into a drinking line. Verify against the UK water fittings regulations for the specific separation and marking requirements that apply to your installation.
This is the failure mode that turns a contained non-potable issue into a building-wide one. Distinct pipework runs, dedicated colours and signage, and backflow protection at any interface are the baseline. Get this wrong and the cleanest temperature regime in the world will not save you.
Treatment helps, but it does not replace the assessment
Most greywater systems and many rainwater systems carry some treatment — filtration, UV, or chemical disinfection — and that is genuinely useful. The general principles for getting water treatment to actually control Legionella are worth reading across from Best practices in water treatment, because a disinfection step only protects you while it is dosing correctly and being checked.
What treatment does not do is take the system out of scope. The monitoring you run, the treatment you specify and how often you sample all follow from a site-specific risk assessment of that particular system, not from a generic schedule [5]. A rainwater system feeding only sub-surface irrigation and a greywater system feeding spray heads near an entrance are different risk pictures and should not inherit the same plan. This is general guidance, not a design for your building; the controls, setpoints and frequencies are decisions for a competent person assessing your actual installation, and nothing here is legal, medical or design advice.
These same systems sit at the heart of low-water, low-carbon design, so it is worth reading this alongside the broader trade-offs in Green buildings and water-saving systems.
What to do next
Pull the drainage and water-reuse drawings for the building, and for every harvested-rainwater or recycled-greywater system, write down two things: where the water is used, and which of those uses produces a spray. Mark each system as a separate entry that needs its own line in the Legionella risk assessment — not a footnote to the potable system.
Then confirm the basics are real and recorded: that separation and labelling are in place, that any treatment or disinfection step is being checked rather than assumed, and that the tanks get inspected. Those checks only prove control if the readings and inspections are logged and visible over time, which is exactly the kind of recurring, easily-forgotten task a digital record keeps from slipping. Start by naming the systems. Most of the risk hides in the ones nobody wrote down.
FAQ
Do rainwater and greywater systems need to be in the Legionella risk assessment?
Yes. Any stored water system that can generate a breathable spray is a foreseeable risk that should be assessed [4], and harvested rainwater and recycled greywater both store water and can feed spray-producing uses. Treat each as a distinct system with its own controls rather than folding it into the potable assessment.
Is greywater more of a Legionella risk than harvested rainwater?
Generally yes. Greywater arrives warm from showers and baths and carries soap, skin and organic matter, so it lands in the growth band already fed [2]. Rainwater arrives cooler and cleaner and warms more slowly. Greywater systems therefore rely on active treatment and disinfection, while rainwater leans more on storage management — though both still need assessing.
Does flushing a WC with non-potable water spread Legionella?
WC flushing produces some aerosol, but it is usually treated as a lower-aerosol use than spray irrigation, wash-down or a water feature [4]. The risk assessment still has to consider it — more so where the water is warm, untreated, or the building houses vulnerable people — but it is the spray-producing uses that warrant the closest attention [3].
Sources
[1] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [2] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [3] CDC, “How Legionella Spreads”. https://www.cdc.gov/legionella/causes/index.html [4] HSE, “Systems most likely to create legionella risk”. https://www.hse.gov.uk/legionnaires/risk-systems.htm [5] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm