RainWater Harvesting
Guide to Using / Recycling Rain Water
Rainwater harvesting comes in two forms; basic or complex.
(Also see this related article on Agricultural rainwater harvesting)
A typical basic system is the common garden butt, fed from a gutter / drainpipe and stored for use in the garden – possibly distributed with the help of a small pump.
The more complex techniques involve draining rainwater from a surface (typically a roof) and feeding it into a sunken tank.
The water is then pumped into your home for uses such as flushing toilets, feeding washing / dishwashing machines, car washing, garden irrigation and so on.
There's a full guide below. But, first, here are some basic points.
Drinking Harvested Rain Water
Drinking harvested rainwater is not approved of in the UK.
This is mainly because bacteria is almost guaranteed to be in harvested rainwater. A simple image of birds sitting on your roof should help explain this...
While it is certainly possible to fully filter this rainwater using Ultra Violet systems, the concern is that whereas UV filtration is used happily on “non mains” supplied water from boreholes or well water, these sources tend to be fairly clean to start with.
The Ultra Violet filtration of this water is used as an extra precaution - “just in case” there are any bugs in the water.
However, rainwater that's come off a roof is practically guaranteed to have bugs.
The fear is that the Ultra Violet filter systems won't be maintained properly and, to be blunt, that people will die.
What You Can Do about it
To counter this you simply need to make sure you have:
a) a good quality Ultra violet water filter system - make sure it is a top quality brand sold by a known and reputable business (hint hint).
b) That you change the bulb every year on time
c) That you change the pre-filters regularly.
All of this is easily done. it just needs to actually be done - or you'll probably regret it.
Other Issues with Drinking Rain Water
If that hasn't put you off then, as we said, filtering rain water certainly is possible.
However, you will be stymied by having to have a totally separate water system to your mains water supply.
This is because strict building regulations mean you cannot have a “bi-valve” type of arrangement - whereby you can opt between using mains supplied water vs your own harvested rain water.
(This is mainly due to concerns that polluted water might wash back into the mains water supply).
So you would need to build a separate plumbing system inside your home into which you pump your harvested rain water in from the tank.
This is often impractical and certainly expensive.
Better News: Rain Water Harvesting for Non-drinking Uses
However harvesting rain for non drinking uses is a different matter.
This is becoming an increasingly popular way to supply the aforementioned loos, washing machines and so on.
Benefits of Non-drinking Rainwater Harvesting
There is a perhaps surprising extra benefit that concerns planning permission.
It is said that adding a rain water harvester to your plans for a building extension or a new build will dramatically increase your chances of success with your local planning department.
The reasons for this are a mixture of environmental / conservation AND - a crucial keyword - “attenuation”.
Attenuation is the storage and slow release of water.
Attenuating water has benefits such as preventing flood water from rushing into drains (which can cause damage to the mains water system).
It also minimises the risk of feast / famine that comes from reliance, in some areas, on rainwater for public supplies.
Will it Save You Money?
The economic benefits of rain water harvesting are long term.
It's useful to compare them to solar power systems: your initial outlay will only be repaid over many years. But meanwhile you will be less likely to run out of water for your garden etc in the event of droughts.
Plus you'll be helping the environment.
Read more about UV Filters here
Full UK Guide to Harvesting Rainwater at Home
A plain-English but thorough guide to collecting, storing, filtering and using rainwater on domestic properties in the United Kingdom - including how your roof covering changes both how much you collect and how clean it is, with filtration recommendations for each roof type.
1. What rainwater harvesting is, and why bother in the UK
Rainwater harvesting (RWH) simply means catching the rain that lands on your roof, storing it, and using it instead of mains water for jobs that don't need drinking-quality water. It is not a new idea, but it has become more relevant in the UK for three reasons:
- Water stress is rising. Despite our reputation for rain, much of England - especially the South and East - is officially "water-stressed." Dry springs and summers are becoming more common, and mains demand keeps climbing.
- Bills. If your home is on a water meter, every litre of rainwater you use is a litre you don't pay for (twice - most bills charge for water in and waste water out).
- Flooding and drainage. Slowing down the rain that leaves your property eases pressure on drains and sewers. In new developments, rainwater harvesting is now treated as a top-priority way of managing surface water.
A realistic expectation: a well-designed domestic system can supply roughly 40–50% of a typical household's total water use - because toilet flushing, washing machines and outdoor use together make up around half of what an average home gets through, and all of those can run on rainwater.
The golden rule: harvested rainwater is non-potable (not for drinking) unless it is treated to a full drinking-water standard, which is expensive, tightly regulated, and a serious undertaking. Sections 1–12 of this guide assume non-potable use. If you want to treat rainwater to genuine drinking quality, read Part Two (Sections 13–18), which covers the treatment engineering and how to get regulatory approval.
2. What you can (and can't) use it for
Suitable uses (no drinking-quality treatment needed): - Flushing toilets (the single biggest indoor saving) - Washing machines / laundry - Watering the garden, greenhouse, allotment - Washing cars, patios, windows, bikes - Filling ponds and water features - General outdoor and indoor cleaning
Not suitable without full treatment and testing: - Drinking, cooking, making ice, or brushing teeth - Filling baths or showers you'll bathe in - Dishwashers used for eating utensils (borderline - many people avoid it) - Anything a vulnerable person (baby, elderly, immunocompromised) might ingest
3. How much water can you actually collect?
The amount of rain you can capture depends on four things: roof size, local rainfall, roof material, and filter efficiency.
The simple formula
Litres per year = Roof plan area (m²) × Annual rainfall (mm) × Runoff coefficient × Filter efficiency
A handy fact: 1 mm of rain falling on 1 m² of roof = 1 litre of water.
- Roof plan area is the footprint the roof covers when seen from above (not the sloped surface area). A 10 m × 7 m footprint = 70 m².
- Annual rainfall varies enormously across the UK - see below.
- Runoff coefficient is how much of the rain actually runs off rather than soaking in or evaporating (this is where roof material matters - Section 6).
- Filter efficiency is typically around 0.9 (good filters waste about 10% flushing dirt away).
UK rainfall varies hugely
| Region (rough guide) | Typical annual rainfall |
|---|---|
| East Anglia / South-East England (driest) | 550–650 mm |
| Midlands / Central England | 700–850 mm |
| South-West / North-West England | 1,000–1,400 mm |
| Wales, Cumbria, Scottish Highlands (wettest) | 1,500–3,000 mm+ |
Always check your local figure - the Met Office publishes averages by area, and it can vary within a single county.
A worked example
A semi-detached house with a 70 m² collecting roof footprint, a clean tiled roof (runoff ~0.85), and standard filtering (0.9):
- Drier East (700 mm): 70 × 700 × 0.85 × 0.9 ≈ 37,000 litres/year
- Wetter West (1,200 mm): 70 × 1,200 × 0.85 × 0.9 ≈ 64,000 litres/year
For comparison, a family of four flushing toilets and running a washing machine might use roughly 70,000 litres/year of non-potable water - so even the drier example covers around half of that demand.
4. The main types of system
Systems range from a £30 barrel to a £7,000 buried installation. Pick the level that matches your goals.
a) Water butt (entry level)
A barrel connected to a downpipe with a diverter. Gravity-fed tap at the bottom, feeds a watering can or hose. - Best for: garden use only. - Pros: cheap (£30–350), no plumbing, no electricity, install yourself in an afternoon. - Cons: small (100–350 L typically), runs dry in a dry spell, no filtration to speak of, outdoor use only.
b) Direct-pumped ("pressurised") system
A larger tank (often underground or in a garage/cellar) with a pump that sends filtered rainwater directly to toilets, the washing machine and outside taps on demand. - Best for: whole-house non-potable supply. - Pros: high capacity, feeds indoor appliances, compact controls. - Cons: relies on the pump/electricity; needs a mains "top-up" arrangement for dry periods.
c) Gravity-fed (header tank) system
A pump lifts rainwater to a header tank in the loft; it then flows down to appliances by gravity. - Pros: the pump runs less often (longer life), and you still have stored water in the loft if the power fails. - Cons: needs loft space and structural capacity for a filled tank (water is heavy - ~1 kg per litre).
d) Underground / below-ground system
A large tank (typically 1,500–7,500 L for homes) buried in the garden, with filtration, a submersible pump, calmed inlet and overflow. - Best for: serious water savings, new builds, and homes wanting maximum offset. - Pros: big storage, out of sight, water stays cool and dark (discourages algae). - Cons: most expensive, needs excavation, best fitted during building work or landscaping.
5. The parts of a proper system (and what each does)
Working from roof to tap:
- Gutters and downpipes - collect and channel the water. Keep them clear.
- Leaf guards / gutter mesh - stop leaves and debris entering.
- First-flush diverter - discards the first few litres of each downpour, which carry the most dust, bird droppings and pollutants washed off the roof. One of the most important quality features.
- Pre-tank filter - usually a self-cleaning vortex or basket filter (mesh around 0.3–0.8 mm) that removes grit and organic matter before storage.
- Calmed inlet - feeds water into the tank gently so it doesn't stir up settled sediment on the bottom.
- Storage tank - dark, cool, with an overflow (ideally to a soakaway) and access for cleaning.
- Floating suction filter - draws water from just below the surface, where it's cleanest (the dirtiest water sinks, and a surface skin of oils stays on top).
- Pump - pressurises the supply.
- Mains top-up - automatically adds a little mains water when the tank runs low, via an air gap so the two supplies can never physically connect (see regulations).
- Point-of-use filtration/disinfection - fine filters, carbon and/or Ultra Violet systems where higher quality is needed (Section 7).
6. Roofing materials: how your roof changes the result
This is the part most guides skip, and it matters twice over. Your roof covering affects:
- Quantity - via the runoff coefficient (smooth, non-absorbent roofs shed more water; rough, porous or planted roofs hold onto it).
- Quality - via what the material sheds or leaches into the water (dust, metals, organic matter, granules, biological growth).
Research consistently finds that metal (coated steel), concrete tile, clay/ceramic tile and "cool" roofs give the cleanest harvested water, while asphalt/bitumen shingle and green (living) roofs shed the most dissolved organic matter and need the most treatment. Importantly, every roof type benefits from first-flush diversion and filtration - no roof gives water that's clean straight off the tiles.
Runoff coefficients by roof type (how much water you actually get)
| Roof type | Typical runoff coefficient | Effect on quantity |
|---|---|---|
| Slate (pitched) | 0.85–0.95 | Excellent - sheds almost everything |
| Glazed clay / ceramic tile | 0.85–0.90 | Excellent |
| Concrete tile (pitched) | 0.80–0.90 | Very good |
| Coated / profiled metal (pitched) | 0.90–0.95 | Excellent - smoothest of all |
| Polycarbonate / glass (conservatory) | 0.90–0.95 | Excellent (but usually small area) |
| Flat roof - asphalt / mineral felt | 0.50–0.80 | Moderate (pooling and absorption) |
| Gravel-ballasted flat roof | 0.50–0.70 | Moderate |
| Green / living roof | 0.30–0.50 | Poor - retains most of the rain |
| Thatch | very low | Unsuitable - absorbs water |
Material-by-material breakdown (quantity and quality)
Natural slate - Quality: Excellent. Slate is smooth, hard and chemically inert, so it leaches virtually nothing. Among the best surfaces for clean runoff. - Watch out for: Moss and lichen in the damp UK climate; lead flashing or lead valleys on older roofs, which can add lead to the water - a genuine contamination concern.
Clay / ceramic tile (fired) - Quality: Excellent. Once fired, clay is inert and among the top performers in water-quality studies. - Watch out for: Slightly rougher, textured surfaces can harbour moss and algae; unglazed tiles are more porous than glazed.
Concrete tile (very common in the UK) - Quality: Very good, and one of the best performers in independent testing once weathered in. - Watch out for: When new, concrete sheds fine cement dust and can leach a little lime, which raises the water's pH and causes cloudiness (turbidity) and white "efflorescence" for the first few months. A larger first-flush and a settling-in period fix this. Porous surfaces can grow moss.
Metal roofs - behaviour depends heavily on the metal: - Coated/painted steel (e.g. plastisol-coated, Galvalume-type, "Colorbond"-style): Excellent quality - smooth, high runoff, and studies show lower bacterial contamination than other roofs. - Bare galvanised (zinc-coated) steel, zinc, or copper: These leach metal into the water - zinc from galvanised/zinc roofs, copper from copper roofs. Fine for toilets, but zinc can harm sensitive plants at higher concentrations and copper can stain. Leaching is worst when the roof is new. - Aluminium: Generally inert and clean. - Lead (roofs or flashing): Toxic - never use lead-contact runoff for anything anyone might ingest, and treat/divert it even for garden use.
Asphalt / bitumen shingle and mineral felt (garages, sheds, flat roofs, extensions) - Quality: Lower. These roofs leach dissolved organic carbon and hydrocarbons and shed mineral granules, giving higher turbidity, colour and organic content, sometimes with an odour or taste. Newer roofs leach more. - Quantity: On flat felt roofs, pooling and slight absorption lower the runoff coefficient. - Verdict: Usable for garden/toilet with proper filtration, but needs more treatment than tile, slate or metal.
Fibre-cement and asbestos-cement sheeting (older corrugated roofs) - Quality: Older (pre-2000) corrugated sheets may contain asbestos, which can shed fibres, and such roofs have been linked to heavy-metal leaching (lead, zinc, mercury). Modern fibre-cement is asbestos-free but can shed fine cement particles when new. - Verdict: If you suspect asbestos cement, do not harvest from it and do not disturb it - seek professional advice. Best avoided as a collection surface.
Green / living roofs - Quality: Water comes off brown and organic-rich - high in dissolved organic matter, tannins, nutrients and sediment. - Quantity: Poor for harvesting - a living roof deliberately retains 40–70% of rainfall. - Verdict: Great for slowing runoff and biodiversity, but a poor harvesting surface. Usable for irrigation with generous filtration; not recommended for indoor use without serious treatment.
Wood shingles / shakes - Quality: Often treated with preservatives and prone to leaching tannins and organics; moss-prone. Not ideal.
Thatch - Verdict: Unsuitable - it soaks up water (tiny yield) and adds heavy organic contamination.
Polycarbonate / glass (conservatory and greenhouse roofs) - Quality: Excellent - smooth, inert, clean runoff. - Quantity: High runoff, but usually a small collecting area.
A note on solar panels: Roof-mounted PV panels shed clean water and generally don't harm quality; they can slightly channel or concentrate flow. Just keep the surrounding roof and gutters clear of the extra debris that can gather around mountings.
7. Filtration solutions - by roof type
First, understand the standard "treatment train." You add stages depending on how clean the roof runoff is and how demanding the end use is.
The filtration ladder (least to most treatment)
- Gutter guards / mesh - keep out leaves and large debris.
- First-flush diverter - throw away the dirty first few litres of each storm.
- Vortex or basket pre-filter - remove grit and fine organic matter before the tank (≈0.3–0.8 mm mesh).
- Calmed inlet + floating suction - keep settled sediment undisturbed and draw the cleanest water.
- Fine sediment cartridge - 20 → 5 → 1 micron, for clearer water at the point of use.
- Activated carbon - removes dissolved organics, colour, taste, odour and some chemicals. Essential for "dirty" roof types.
- UV disinfection - kills bacteria and viruses; used when water goes to indoor appliances or where hygiene matters.
- Full potable train (not recommended DIY): multi-barrier sediment + carbon + UV (sometimes reverse osmosis) plus regular laboratory testing - only route to drinking quality, and legally/practically a professional job.
Recommended filtration by roof type
| Roof type | Recommended filtration |
|---|---|
| Slate | Standard train: gutter mesh → first-flush → vortex/basket pre-filter → calmed inlet + floating suction → 5–25 µm cartridge for non-potable. Add UV if feeding indoor appliances. Divert or treat any lead-flashing runoff separately. |
| Clay / ceramic tile | Same standard train as slate. Very reliable; carbon + UV only if you want near-mains clarity indoors. |
| Concrete tile | Standard train plus a larger first-flush and a settling-in period when new (to handle lime dust and initial cloudiness). Clean the pre-filter more often in the first few months. |
| Coated / painted steel, aluminium | Standard train - one of the cleanest sources. UV optional for indoor use. |
| Bare galvanised / zinc / copper | Standard train plus metal-reduction media (activated carbon, or specialist media such as KDF; ion-exchange if levels are high). Avoid this water on metal-sensitive plants. UV if used indoors. |
| Lead roof or lead flashing present | Do not use for anything ingestible. For garden use only: divert lead-contact runoff if possible, or use lead-reducing media (e.g. activated alumina / a certified lead cartridge) and have the water tested. |
| Asphalt / bitumen shingle & mineral felt | Enhanced train: larger first-flush → sediment filter (for granules) → activated carbon (essential) for hydrocarbons/organics/colour → UV if used indoors. Not recommended for potable. |
| Fibre-cement / asbestos-cement | Best avoided. If modern (asbestos-free) and unavoidable: sediment + activated carbon, and test. If asbestos is suspected: do not harvest - seek professional advice; don't disturb the roof. |
| Green / living roof | Coarse pre-filter for sediment → generous first-flush → sediment cartridge → activated carbon for colour/tannins/organics → UV if indoor. Expect discoloured water; best kept to irrigation. Extra settlement/storage helps. |
| Wood shingle / shake | Sediment → activated carbon (for tannins/preservatives) → UV. Non-potable only. |
| Thatch | Not recommended for harvesting. |
| Polycarbonate / glass | Standard train; very clean source, minimal treatment needed. |
Three points that apply to every roof: - First-flush diversion is the cheapest, highest-value quality improvement you can make - fit it whatever your roof. - The longer the dry spell before rain, the dirtier the first flush (dust, droppings and pollen build up), so size your first-flush diverter generously. - Carbon and UV are the two "upgrade" stages that turn marginal water (dirty roofs, or any indoor use) into consistently good non-potable supply.
8. The rules: staying legal and safe in the UK
Rainwater harvesting is legal and encouraged, but non-potable plumbing is regulated to protect the public drinking-water supply. The overriding concern is preventing any cross-connection between rainwater and mains water.
Key points to know:
- Water Supply (Water Fittings) Regulations 1999 (in Scotland, the Water Byelaws 2014) govern how any non-mains water system connects to plumbing. You must notify your water company before installing certain systems, and the work should follow the regulations.
- Backflow prevention is mandatory. Any mains top-up must be delivered through a physical air gap (Type AA/AB) so rainwater can never be sucked or pushed back into the mains. A plumbed cross-connection is illegal and dangerous.
- Pipework must be clearly identified. Non-potable pipes and outlets must be distinctly marked (following the relevant identification standard) and taps labelled, so no one mistakes rainwater for drinking water - now or during future work. Outside taps supplying rainwater should carry a "not drinking water" label.
- Code of practice: The current British/European code of practice for rainwater harvesting is BS EN 16941-1 (the 2024 edition is current; it replaced the older BS 8515:2009). It covers design, sizing, installation and maintenance. Following it is the recognised way to get a system right, and matters especially for new builds.
- Building Regulations: Part G (water efficiency) and Part H (drainage) are relevant, particularly for new construction and extensions.
- Sustainable drainage (SuDS): For new developments, national standards now rank rainwater harvesting as a high-priority method of managing surface water - so it increasingly features in planning.
Practical takeaway for a homeowner: a simple garden water butt needs no notification. A plumbed-in system feeding toilets or the washing machine should be installed by a competent installer who understands the Water Fittings Regulations, notifies your water company where required, and builds in the air gap and pipe labelling.
9. Costs, savings and funding
Rough costs
- Water butt: £30–350
- Above-ground pumped kit: several hundred pounds upwards
- Full underground system (supply + tank + install): typically £2,000–£7,000+
Savings
- Only metered households save money on bills (unmetered homes save water, not cash).
- A metered home offsetting toilets, laundry and outdoor use might save on the order of £150–£220 a year, though this depends heavily on tariff, household size and rainfall.
- Payback: a water butt pays back quickly; a full underground system can take 20 years or more on bill savings alone. Many people install for water security, resilience and environmental reasons as much as pure economics.
Funding
- There is no general national grant for domestic rainwater harvesting in the UK.
- Some water companies run occasional schemes offering free or subsidised water butts (often tied to flood-resilience projects) - worth checking with yours.
- Surface-water drainage rebates: if your system's overflow goes to a soakaway rather than the public sewer, you may qualify for an ongoing reduction in sewerage charges (often £20–£60+ a year) - ask your water company.
- Grants do exist for farms and businesses (agricultural and commercial schemes, capital allowances for qualifying water-efficient equipment), but these generally don't apply to ordinary homes.
10. Looking after your system (maintenance)
A neglected system produces poor water; a maintained one runs for decades.
- Every few weeks / after storms: clear gutter guards and leaf traps.
- Every 1–3 months: rinse the pre-filter / vortex filter (more often in autumn leaf-fall, and while a new concrete roof is settling in).
- Every 6–12 months: check and empty the first-flush diverter; inspect the calmed inlet and overflow.
- Annually: check the pump, the mains top-up air gap, and replace sediment/carbon cartridges as specified.
- UV lamps: replace roughly yearly (they lose output long before they stop glowing).
- Tank: inspect for sludge every 1–2 years; de-sludge as needed. Keep it dark to discourage algae.
- Water quality: frequent lab testing isn't necessary for non-potable use, but during maintenance look for unusual colour, smell or cloudiness and act on it.
11. Common mistakes to avoid
- Skipping the first-flush diverter - the cheapest big win, often left out.
- Cross-connecting to the mains - illegal, unsafe, and easy to do by accident without an air gap and labelled pipes.
- Harvesting from the wrong roof - bitumen felt, asbestos cement, thatch or a green roof without matching the filtration to the material.
- Undersizing or oversizing the tank - too small and it overflows constantly; too big and it stagnates. Aim to store roughly 2–3 weeks of demand, and follow BS EN 16941-1 sizing.
- Letting the tank see daylight - light plus warmth breeds algae.
- Forgetting the overflow - it must go somewhere safe (ideally a soakaway), sized for a downpour.
- Assuming it's drinkable - it isn't, without full treatment and testing.
12. Quick decision summary
- Just want to water the garden cheaply? A water butt (with a diverter and mesh) is all you need.
- Want to cut mains use meaningfully (toilets + laundry)? A pumped system with proper filtration, professionally installed to the Water Fittings Regulations.
- Building or renovating? Fit an underground system now - it's far cheaper during works, and rainwater harvesting is favoured in modern drainage standards.
- Best roofs for clean, plentiful water: slate, clay/ceramic tile, concrete tile (once weathered), coated steel - all with a standard filter train.
- Roofs needing extra treatment: galvanised/copper metal (metal reduction), bitumen/felt and green roofs (carbon filtration), anything with lead (divert/treat), and asbestos cement (avoid entirely).
- For any indoor use: add carbon and UV, label your pipes, and keep the mains top-up on an air gap.
PART TWO - Treating Rainwater to Drinking-Water (Potable) Standard
Read this first. Turning rainwater into safe drinking water is entirely possible and is done across the UK, but it is a different order of undertaking from a garden butt. Rainwater collects bird and animal droppings off the roof, so the headline hazard is faecal pathogens - bacteria like E. coli, and especially the protozoan parasite Cryptosporidium, which causes serious gastrointestinal illness and shrugs off ordinary chlorine. The whole philosophy of potable treatment is redundancy: several independent barriers, so that if one fails, others still protect you. This is a professional design-and-test job, not a DIY weekend project. For most UK homes with a mains connection it isn't worth it - mains water is cheap, safe and reliable. Potable rainwater makes real sense mainly for off-grid, remote or mains-less properties.
13. What has to be removed, and what removes it
Rainwater is soft and low in dissolved minerals, but it is not clean. To be "wholesome" (the legal word for water safe to drink) it must meet the UK drinking-water quality standards for microbes, chemistry and appearance. Here is what you're dealing with and the barrier that handles each:
| Contaminant | Where it comes from | What removes/inactivates it |
|---|---|---|
| Sediment, silt, turbidity (cloudiness) | roof surface, atmosphere, tank | Sediment/depth filters, settling, first-flush |
| Bacteria (E. coli, coliforms, Legionella, Pseudomonas) | bird/animal droppings, biofilm in tank | UV, ultrafiltration, chlorination, ozone |
| Protozoa (Cryptosporidium, Giardia) | animal faeces | Ultrafiltration, RO, sub-1-micron absolute filtration, UV (inactivates Crypto) |
| Viruses | faecal contamination | Reverse osmosis, tight ultrafiltration, UV (adequate dose), chlorination |
| Heavy metals (lead, zinc, copper) | roof, lead flashing, metal roofs | Reverse osmosis, ion exchange, specialist media (e.g. activated alumina for lead) |
| Dissolved organics, pesticides, hydrocarbons | atmosphere, bitumen roofs | Activated carbon (granular or block) |
| Nitrate / nitrite | atmosphere, nearby agriculture | Reverse osmosis, ion exchange |
| Low pH / softness / corrosivity | rainwater is naturally acidic & soft | pH correction / remineralisation (calcite/calcium carbonate contactor) |
| Chlorine taste (if you chlorinate) | the disinfection step itself | Activated carbon (as a final "polish") |
Two things to internalise: no single device does all of this, and the barriers must be in the right order - because, for example, UV can only disinfect water that is already clear, and reverse-osmosis membranes clog if fed dirty water.
14. The specific treatment stages - how each works
Working in the order water should pass through them:
1. Roof-to-tank pre-treatment (your first barrier). Everything from Part One still applies and matters more for potable: gutter guards, a generously sized first-flush diverter, a vortex/basket pre-filter, a calmed inlet, and a floating suction filter drawing the cleanest water from just below the surface. Keeping gross contamination out of the tank makes every downstream stage work better and last longer.
2. Graduated sediment filtration. A series of cartridge filters of decreasing pore size - typically 20 micron → 5 micron → 1 micron. These remove particulates, protect the membranes and UV downstream, and start reducing turbidity. A 1-micron absolute (not "nominal") rated filter is the point at which you begin physically excluding Cryptosporidium cysts.
3. Activated carbon (granular activated carbon, GAC, or a carbon block). Adsorbs dissolved organic carbon, pesticides, hydrocarbons, colour, taste and odour, and removes any chlorine. This both improves palatability and protects/assists the UV stage (organics and colour absorb UV light). A tight carbon block (0.5–1 µm) also adds another physical barrier against cysts.
4. Membrane filtration - choose your barrier:
- Ultrafiltration (UF) - hollow-fibre membranes with pores around 0.01–0.02 micron. This physically strains out bacteria, protozoa (including Cryptosporidium and Giardia) and most viruses, while leaving the water's minerals intact. It wastes very little water and has become the practical "workhorse" barrier for potable rainwater. Lower running cost than RO and no remineralisation needed.
- Reverse osmosis (RO) - an even finer membrane (~0.0001 micron) that removes virtually everything, including dissolved solids, heavy metals, nitrate and viruses. The trade-offs: it wastes water (a reject stream goes to drain), it is slower and pricier, and it strips out minerals to produce aggressive, acidic water that must be remineralised afterwards. Use RO where you have a dissolved-contaminant problem (heavy metals, high nitrate) or want maximum belt-and-braces.
Many good domestic systems use UF for microbes plus targeted media (e.g. a lead-reducing cartridge) rather than full RO, precisely to avoid RO's water waste and demineralisation.
5. Disinfection - the final microbial kill:
- Ultraviolet (UV) is the standard final disinfection for private and rainwater supplies. It inactivates bacteria, viruses and protozoa - including Cryptosporidium, which is exactly why it's preferred over chlorine for rainwater. It needs clear water (hence sediment + carbon first) and the correct dose (a validated unit delivering at least ~40 mJ/cm², sized to your flow rate). The lamp is replaced roughly annually and the quartz sleeve kept clean; a good unit has a UV-intensity monitor and alarm.
- Chlorination leaves a residual that keeps stored and distributed water protected, but it does not reliably kill Cryptosporidium and creates a taste and by-products (removed by a final carbon polish). Used mainly where water is stored/distributed and needs ongoing protection.
- Ozone is powerful but complex and rare in homes.
6. pH correction / remineralisation. Rainwater is naturally soft and slightly acidic, which is corrosive - it can attack copper pipes and leach metals into your finished water. A calcite (calcium carbonate) neutraliser or remineralisation cartridge raises the pH and hardness to a stable, non-aggressive, better-tasting level. This is essential after RO and usually worthwhile for raw rainwater too. Aim to land comfortably inside the legal pH window (6.5–9.5) with water that is non-corrosive.
7. Monitoring. Turbidity or UV-transmittance sensing, a UV-intensity alarm that cuts the supply if disinfection fails, and - above all - regular laboratory testing (Section 16).
15. How the barriers combine - a realistic domestic potable train
A typical, well-designed order looks like this:
Roof pre-treatment → storage tank → pump → 20 µm sediment → 5 µm sediment → 1 µm absolute → activated carbon → ultrafiltration (or reverse osmosis) → remineralisation / pH correction → UV disinfection → to the tap
Why this order: - Coarse-to-fine sediment first, so later stages aren't overwhelmed. - Carbon before the membrane, to strip organics (and any chlorine, which damages RO membranes). - Membrane before UV, so the UV sees clear, low-organic water. - Remineralise after RO (RO water is aggressive) and before final UV polish. - UV last, as the final kill immediately before use.
If you use RO instead of UF, you'd place remineralisation after the RO membrane; UV still comes at or near the end. Some designs add a small treated-water storage with a chlorine residual plus a point-of-use UV at the kitchen tap for extra assurance - that layering is the multi-barrier principle in action.
Rules of thumb that keep people safe: - Never rely on one barrier. Aim for at least: physical removal (fine filtration/membrane) plus disinfection (UV) plus the roof/first-flush pre-treatment. - Size to your flow rate. UV and membranes only work if water passes through them slowly enough; undersizing silently lets pathogens through. - "Absolute" beats "nominal" ratings for anything you're relying on to exclude cysts. - Fail-safe the disinfection. A UV unit feeding drinking water should shut the supply if its lamp fails or intensity drops.
16. Testing - proving the water is actually safe
Treatment you don't verify is just hope. Test against the UK drinking-water parameters, using an accredited laboratory (UKAS / ISO 17025), ideally with an approved sampler:
- Microbiology (the priority): E. coli and enterococci must be 0 per 100 ml; also coliforms; and Cryptosporidium where risk indicates.
- Core chemistry & appearance: pH, turbidity, colour, conductivity, taste/odour, ammonium.
- Metals & nutrients: lead, copper, nickel, iron, manganese, nitrate, nitrite.
Test at commissioning (before anyone drinks it), then periodically - microbiology at least a few times a year (more often at first), and a fuller chemical suite annually. For any regulated supply (see below) the local authority sets the sampling schedule. Keep records of every result and every filter/lamp change.
17. Getting regulatory approval - the UK process
The moment you use rainwater for drinking, cooking, food preparation or other domestic consumption, it legally becomes a private water supply, and a different regulatory regime applies. The whole system - roof, tank, treatment, pipework and fittings - is in scope, not just the tap.
Who regulates what: - Your local authority (Environmental Health / private water supplies team) is the regulator you deal with. In Scotland this is the local council; the principle is the same UK-wide. - The Drinking Water Inspectorate (DWI) sets standards, advises local authorities, and oversees quality nationally - but you don't apply to the DWI; you work through your council. - Ofwat and your former water company don't regulate a private supply's quality (though the water company still cares about any mains cross-connection).
The governing rules: - Private Water Supplies (England) Regulations 2016 (as amended 2018) - and the equivalents: Wales 2017, Scotland 2006, and separate Northern Ireland regulations. These require the water to be wholesome (meet drinking-water standards) and give the local authority duties and powers. - Water Supply (Water Fittings) Regulations 1999 (Scotland: Water Byelaws) - apply to any mains connection: mandatory backflow prevention via an air gap, and no cross-connection, ever. - Materials and fittings in contact with the water must be suitable and non-contaminating - in practice, WRAS-approved (or equivalently certified) products. - Building Regulations Part G (and Part H for drainage) for the installation. - Follow BS EN 16941-1:2024 for the harvesting side, plus DWI guidance and recognised good practice for treatment.
A step-by-step route to compliance:
- Confirm your category early. Single owner-occupied dwelling, or shared/rented/commercial/food premises? It changes your obligations (below).
- Contact your local authority's private water supplies team before you install. Discuss the design; they are the people who will assess and, if applicable, monitor it.
- Risk assessment. For supplies serving more than a single private dwelling, or any commercial or food premises, the local authority must carry out a risk assessment and regular monitoring (you pay statutory fees). For a single dwelling occupied by its owner, the authority is not obliged to monitor - but the water still has to be wholesome, you can (and should) request testing, and the authority retains powers to require improvements if there's a risk.
- Register / notify. There is no blanket legal duty to register a single-dwelling supply, but the DWI strongly recommends notifying your local authority, and it is effectively required for shared, rented, commercial or food-business supplies. If you ever sell the property, a private supply must be disclosed.
- Design and build to standard. Multi-barrier treatment producing wholesome water; WRAS-approved materials; Water Fittings Regulations compliance (air gap on any mains backup, labelled pipework); Part G.
- Commission and test through a UKAS-accredited lab before anyone drinks it.
- Operate, maintain and re-test on the authority's schedule (or at least annually for microbiology if unregulated), keeping full records. The authority can serve notice requiring improvements if standards aren't met.
In short: the "approval" isn't a single certificate you buy - it's satisfying your local authority (under the Private Water Supplies Regulations) that the supply is wholesome, safe and well-maintained, backed by accredited test results, compliant fittings and proper backflow protection.
18. Costs and honest expectations for potable
- A potable-grade treatment train typically adds £1,500–£5,000+ on top of the harvesting/storage system, depending on whether you use UF or RO and how much redundancy you build in.
- Ongoing costs matter more than for non-potable: sediment and carbon cartridges (every 6–12 months), UV lamp (yearly), membrane servicing, remineralisation media, plus laboratory testing fees and any local-authority monitoring charges.
- Reliability is on you. A neglected filter or a failed UV lamp on a garden system is a nuisance; on a drinking system it's a health risk. Build in monitoring and stick to the maintenance schedule religiously.
- Verdict: worthwhile mainly where mains water isn't available (remote or off-grid properties). Where mains exists, most households are better served using rainwater for non-potable duties and drinking the mains supply.
PART THREE - Two Worked Potable Treatment Trains
These are illustrative reference designs, not off-the-shelf shopping lists - final specification (flow rates, UV dose, membrane sizing) must be done by a competent water-treatment engineer for your actual water and household. Component sizes below assume a typical family home with a peak simultaneous flow of roughly 12–20 litres/minute. Any component in contact with the finished water should be WRAS-approved / meet BS 6920 so it doesn't contaminate the water it's meant to clean.
Choose your train by your water chemistry, not just preference:
- Train A (Ultrafiltration) is the sensible default when the main risk is microbial and your raw water chemistry is reasonable (no serious heavy-metal or nitrate problem). It keeps the water's minerals and wastes almost nothing.
- Train B (Reverse Osmosis) is for problem water - lead from flashing, zinc/copper from a metal roof, high nitrate - or where you want the most complete removal and accept water waste and remineralisation.
19. Train A - Ultrafiltration-based (recommended default)
Flow schematic:
ROOF
└─ gutter guards ─ first-flush diverter ─ vortex pre-filter
└─ STORAGE TANK (calmed inlet · overflow to soakaway · floating suction)
└─ pump + pressure vessel
└─ 20 µm sediment ─ 5 µm sediment ─ 1 µm ABSOLUTE
└─ activated carbon block (0.5–1 µm)
└─ ULTRAFILTRATION (~0.02 µm hollow-fibre)
└─ remineralisation / pH correction (calcite)
└─ UV DISINFECTION (≥40 mJ/cm², fail-safe)
└─ to potable taps
Indicative component list:
| # | Stage | Indicative component type | What it does |
|---|---|---|---|
| 1 | Roof pre-treatment | Gutter mesh; first-flush diverter (sized per BS EN 16941-1); self-cleaning vortex/basket pre-filter (~0.3–0.65 mm) | Keeps leaves, grit, droppings and the dirty first flush out of the tank |
| 2 | Storage | Cool/dark tank with calmed inlet, overflow to soakaway, floating suction filter with fine screen | Lets sediment settle; draws the cleanest water from just below the surface |
| 3 | Pressurisation | Submersible or booster pump + pressure vessel (~2–3 bar) | Delivers steady pressure to the treatment stages |
| 4 | Sediment (coarse→fine) | 10"/20" "Big Blue" housings: 20 µm → 5 µm → 1 µm absolute cartridges | Removes particulates; the 1 µm absolute stage begins excluding Cryptosporidium cysts |
| 5 | Carbon | Activated carbon block, 0.5–1 µm | Removes organics, pesticides, colour, taste/odour; protects/assists the UV |
| 6 | Membrane | Ultrafiltration hollow-fibre module, ~0.01–0.02 µm, ideally back-flushable | Physically strains out bacteria, protozoa and most viruses - keeps minerals |
| 7 | Conditioning | Calcite (calcium carbonate) remineraliser (± a little magnesium media) | Raises pH/hardness so soft, acidic rainwater isn't corrosive; improves taste |
| 8 | Disinfection | UV reactor delivering ≥40 mJ/cm² at rated flow (e.g. to NSF/ANSI 55 Class A), with intensity monitor + solenoid shut-off on fault | Final kill of bacteria, viruses and Cryptosporidium; fails safe |
| 9 | Monitoring | Turbidity / UV-transmittance sensing; UV-intensity alarm; pressure gauges each side of the filters | Warns of cloudiness, disinfection failure or a blocked filter |
Why this order: coarse-to-fine sediment first so nothing downstream is overwhelmed; carbon before the membrane; remineralise before the final UV; UV last, immediately before use. Ultrafiltration here is the physical microbial barrier and UV is the independent second barrier - that redundancy is the point.
20. Train B - Reverse-osmosis-based (for problem water / maximum assurance)
Because RO is slow and wastes water, you don't run your whole house through it. The practical design is: treat the whole potable supply for microbes and sediment centrally, then add a point-of-use RO unit at the kitchen tap for drinking and cooking, where dissolved-contaminant removal actually matters.
Flow schematic:
ROOF ─ (pre-treatment & storage exactly as Train A)
└─ pump + pressure vessel
└─ 20 µm ─ 5 µm ─ 1 µm ABSOLUTE ─ carbon block
└─ UV DISINFECTION (whole-supply microbial barrier)
├─ to bathroom / utility potable taps
└─ UNDER-SINK RO UNIT (kitchen drinking/cooking tap):
sediment + carbon pre-filters
└─ RO MEMBRANE (~0.0001 µm) ── reject water → drain
└─ remineralisation cartridge
└─ post-carbon polish (± point-of-use UV)
└─ dedicated drinking tap
Indicative component list (additions/changes vs Train A):
| # | Stage | Indicative component type | What it does |
|---|---|---|---|
| 1–5 | Roof → carbon | Same as Train A (pre-treatment, storage, pump, 20/5/1 µm sediment, carbon block) | Removes solids and organics; protects everything downstream |
| 6 | Whole-supply disinfection | UV reactor (≥40 mJ/cm², fail-safe) serving the general potable network | Microbial safety for all potable outlets, including any that bypass the RO |
| 7 | Point-of-use RO | Under-sink RO membrane ~0.0001 µm with its own sediment + carbon pre-filters and a small pressurised product tank; reject stream to drain | Removes dissolved solids, heavy metals, nitrate and viruses from drinking/cooking water |
| 8 | Post-conditioning | Remineralisation cartridge (essential after RO) + post-carbon polish | Restores minerals/pH to aggressive, flat RO water; final taste polish |
| 9 | Optional final barrier | Small point-of-use UV at the drinking tap | Belt-and-braces disinfection right before the glass |
| 10 | Monitoring | TDS meter on RO output; UV-intensity alarm | TDS creep signals a tiring RO membrane; alarm signals disinfection failure |
Why this layout: RO strips everything (which is why the remineraliser is not optional), but it's wasteful (typically 2–4 litres to drain per litre produced, better on efficient units) and slow - so it's reserved for the tap where dissolved contaminants matter, while the rest of the potable supply relies on filtration + UV.
21. Ultrafiltration vs Reverse Osmosis - quick comparison
| Factor | Ultrafiltration (Train A) | Reverse Osmosis (Train B) |
|---|---|---|
| Removes bacteria & protozoa | Yes (physical) | Yes |
| Removes viruses | Most | Yes |
| Removes dissolved metals / nitrate | No | Yes |
| Keeps beneficial minerals | Yes | No - must remineralise |
| Water wasted | Almost none | ~2–4 L per 1 L produced |
| Speed / flow | Fast (whole-house) | Slow (point-of-use + storage) |
| Running cost | Lower | Higher |
| Best when | Chemistry is fine, microbial risk is the concern | Heavy metals / nitrate present, or maximum assurance wanted |
Many real installations are Train A with a targeted extra cartridge (e.g. a lead-reducing media stage) rather than full RO - you only take on RO's waste and demineralisation if your water chemistry genuinely needs it.
22. Maintenance calendar (both trains)
Potable treatment is safety-critical - a lapsed filter or dead UV lamp is a health risk, not just a nuisance. Keep a written log of every check and change.
| Interval | Task |
|---|---|
| Weekly / after heavy storms | Glance at pressure gauges and the UV-intensity display; clear gutter guards and leaf traps |
| Monthly | Rinse the vortex/pre-tank filter; confirm UV intensity is in range and the alarm/shut-off is armed; check pump pressure |
| Every 3 months | Empty/inspect the first-flush diverter; clean the floating-suction screen; check the pressure drop across the sediment filters (rising = time to change) |
| Every 6 months | Replace sediment cartridges (20/5/1 µm) - or sooner on pressure drop; replace the carbon block; check/top up remineralisation media; back-flush the UF module |
| Every 12 months | Replace the UV lamp (it stops disinfecting long before it stops glowing) and clean the quartz sleeve; service/integrity-check the UF module; RO: replace RO pre/post filters; full accredited lab water test (microbiology + chemistry) |
| Every 2–3 years | Replace the UF membrane (life ~2–5 yr) or RO membrane (life ~2–5 yr, sooner if TDS climbs); de-sludge and inspect the storage tank |
| Event-driven (do immediately) | After any UV fault or lamp failure: don't drink until restored and re-tested. After roof/gutter work: extra first-flush and a check. After any positive microbiological test: stop drinking, investigate, disinfect/shock the system, and re-test until clear |
Consumables to keep as spares: a set of sediment cartridges, a carbon block, a spare UV lamp (and quartz sleeve), and remineralisation media. Running out mid-fault is what tempts people to keep drinking untreated water.
23. Commissioning - before anyone drinks it
- Install and pressure-test the full train; confirm no cross-connection to mains and that any mains backup is on an air gap.
- Flush new carbon and remineralisation media to waste until the water runs clear and stable.
- Energise and verify the UV: correct lamp, clean sleeve, intensity in range, and the fail-safe shut-off actually stops flow when you simulate a fault.
- Run a full accredited (UKAS/ISO 17025) laboratory test against UK drinking-water standards - microbiology (E. coli/enterococci must be 0/100 ml) and the chemical suite (pH, turbidity, metals, nitrate).
- Only once results confirm the water is wholesome, and your local authority is satisfied under the Private Water Supplies Regulations, put it into use for drinking.
- Set calendar reminders for every maintenance interval above, and file the commissioning results as your baseline.
This guide is for general information about domestic rainwater harvesting in the UK, including both non-potable use and treatment to potable (drinking) standard. It is not a substitute for professional design, installation and testing, or for advice from a plumber competent in the Water Supply (Water Fittings) Regulations. Do not use harvested rainwater for drinking, cooking or bathing unless it has been treated by a properly designed multi-barrier system, verified by accredited laboratory testing, and cleared with your local authority under the Private Water Supplies Regulations. Rainwater carries faecal pathogens, including Cryptosporidium; treat potable systems as safety-critical. Always confirm current requirements with your local authority (environmental health / private water supplies team), the Drinking Water Inspectorate, your water company and building control before installing a plumbed-in or potable system.
