Reuse, Recovery & Stormwater
Rainwater Harvesting Companies
Rainwater collection, storage, and treatment solutions for industrial, commercial, and community reuse schemes.
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Rainwater Harvesting System Design: Collection Area, Tank Sizing, and Water Quality Standards
Rainwater harvesting (RWH) systems collect precipitation from roof surfaces, filter and store it in dedicated tanks, and distribute it for non-potable uses (WC flushing, irrigation, laundry, industrial process). Rooftop collection efficiency: BS 8515:2009+A1:2013 (UK standard for RWH systems) uses Yf method (yield before spillage) or Yd method (yield after demand) for tank sizing based on local rainfall data (UKCP09 or UKCP18 datasets), roof area, runoff coefficient (concrete/slate tiles 0.75 to 0.90, green roofs 0.15 to 0.40 depending on substrate depth), and demand profile. First-flush diversion: minimum 0.5 to 1.0 L per m2 of roof area is diverted to drain to remove bird droppings, dust, and atmospheric deposition before each rain event; self-cleaning first-flush devices (Wisy Vortex filter, 3P Technik) retain the first flush in a diversion chamber with slow-release drain. Filter requirement: BSEN 12566 equivalent - in-tank floating filter or calmed inlet with 350 to 500 micron mesh filtration.
Tank sizing methodology (BS 8515): the yield-before-spillage (Yb) method calculates daily water balance over a full year of rainfall data: S(t+1) = min(S(t) + Q(t) - D(t), V), where S is tank storage, Q is daily inflow (rainfall times area times runoff coefficient), D is daily demand, and V is tank volume. Optimum tank volume is where annual yield equals approximately 95 percent of annual demand or 95 percent of annual rainfall collected, whichever is limiting. Typical UK residential RWH: roof 80 m2, annual rainfall 650 mm, 2 WCs with 4 occupants (45 L/day toilet demand): tank volume 2,500 to 3,000 L, providing approximately 40 to 50 percent of toilet demand. Commercial systems (large roof, car wash, irrigation): tank volumes 10,000 to 100,000+ L; financial payback 3 to 10 years depending on mains water cost (UK average GBP 1.50 to 2.00 per m3 + sewerage). WRAS (Water Regulations Advisory Scheme) requires backflow prevention (Type AA or AB airgap) between RWH supply and potable mains top-up supply.
Water quality of collected rainwater: pH typically 5.5 to 6.5 (slightly acidic from dissolved CO2 and atmospheric acids); turbidity less than 1 NTU after first-flush and filtration; bacteria (E. coli, coliforms) - variable depending on roof material and bird contamination; Legionella risk in stored rainwater at temperatures 20 to 45 degrees C. UK: WRAS Guidance IGN 9-02-04 requires RWH systems supplying internal non-potable uses to be clearly labelled (WRAS blue-and-green rainwater symbol), colour-coded pipework (buff/beige), and backflow prevention. Australia ARCSA/ANSI 63 and AS/NZS 3500.1 govern RWH design. For potable use of rainwater (permitted in some jurisdictions): disinfection (UV at greater than 40 mJ/cm2 or chlorination to greater than 0.2 mg/L residual) and water quality testing to WHO/national drinking water standards is required; permitted in some Australian states, South Africa, rural areas.
Frequently Asked Questions
How much rainwater can I collect from my roof?
Annual rainwater harvest (m3) = roof plan area (m2) times annual rainfall (mm/1000) times runoff coefficient. Runoff coefficients: concrete/slate tiles 0.80 to 0.90; clay tiles 0.70 to 0.85; corrugated metal 0.85 to 0.95; flat EPDM membrane 0.75 to 0.85; green roof 0.15 to 0.40 (substrate 80 to 200 mm depth). Deduct 10 to 15 percent for first-flush losses. Example: 100 m2 slate roof, 750 mm/year rainfall, runoff 0.85: annual harvest = 100 times 0.75 times 0.85 times 0.85 = approximately 54 m3. For a household using 45 L/day for toilets (4 people): annual toilet demand = 45 times 365 / 1000 = 16.4 m3 - the system provides full toilet supply. Seasonal variation matters: UK summer months (Jun to Aug) typically 45 to 65 mm per month vs autumn/winter 70 to 120 mm; summer irrigation demand coincides with lower rainfall requiring larger tanks for irrigation-dominant systems.
What size rainwater tank do I need?
BS 8515:2009+A1:2013 provides the definitive UK sizing method. Simplified rule of thumb: tank volume (litres) = approximately 5 percent of annual rainfall yield in litres for toilet-only systems (limited demand); for irrigation-dominant systems, tank volume closer to 3 to 4 weeks' irrigation demand. For a residential system (80 to 150 m2 roof, 4 occupants, toilet and laundry demand): recommended tank 2,500 to 5,000 L. Online tools: BS 8515 compliant calculators are available from Rainharvesting Systems, Graf Group, Wisy. Commercial systems: office buildings with 500 m2 roof, car parks, schools benefit from tanks of 20,000 to 100,000 L. Key consideration: most economic tank size is where the marginal cost of additional storage equals the marginal value of additional yield - beyond 70 to 75 percent demand substitution, additional tank volume gives diminishing returns. WRAS requires automatic mains top-up with Type A airgap (300 mm minimum) to prevent backflow contamination of potable supply.
Is rainwater safe to use for toilet flushing?
Yes, for toilet flushing (WC, urinals) and irrigation, collected rainwater is generally safe without treatment if the system is properly designed with first-flush diversion and filtration. UK: BS 8515 and WRAS guidance permit untreated (filtered) rainwater for WC flushing, urinals, external use, and laundry (in dedicated machines). Requirements: first-flush diversion (minimum 0.5 L per m2 roof), 350 to 500 micron filtration at tank inlet, calmed inlet to prevent disturbance of settled sediment, tank overflow, access for cleaning. Legionella risk management: ACOP L8 (HSE) applies to rainwater systems in non-domestic premises; tank temperature should be kept below 20 degrees C (insulate underground tanks) or above 60 degrees C (not practical for RWH); annual risk assessment and tank cleaning (typically every 1 to 2 years). For potable use of rainwater: additional UV disinfection (greater than 40 mJ/cm2) and water quality testing required; not commonly permitted in UK without local authority approval.
What maintenance does a rainwater harvesting system need?
Annual maintenance requirements (BS 8515, WRAS guidance): (1) 6-monthly: inspect and clean first-flush diverter - remove accumulated sediment, check slow-drain function; inspect roof gutters and downpipes for debris, vegetation, and bird fouling; check tank overflow direction and debris screen; (2) Annual: clean calmed inlet filter and floating suction filter (350 to 500 micron mesh) - rinse with clean water; visually inspect inside tank via access hatch - pump out, scrub walls and base if sediment depth exceeds 50 mm; check and test mains top-up solenoid valve and level control; test backflow prevention device (WRAS Type AA airgap float valve); (3) Every 2 to 5 years: replace submersible pump if worn (check flow and pressure at outlets vs commissioning records); (4) Non-domestic systems (ACOP L8 requirement): Legionella risk assessment annually; tank cleaning frequency per risk assessment (typically every 1 to 2 years); records kept for 5 years. Maintenance cost: residential GBP 100 to 200 per year; commercial varies with system size.
A university campus in the East Midlands sought to reduce mains water consumption by 25% across 18 buildings to meet its net-zero water targets, focusing on toilet flushing and irrigation loads that collectively consumed 4,200 m3 per month. Available roof area across academic blocks was 8,400 m2.
A consultant designed a gravity-fed centralised rainwater harvesting system with 80,000 L of below-ground GRP storage, fed from 12 downpipe connections via Wisy WFF280 vortex filters. A twin-pump booster set with frequency control distributed harvested water to all 18 buildings via a dedicated non-potable purple-pipe network. WRAS-approved Type AA airgap top-up ensured regulatory compliance and prevented any cross-connection with the potable supply. BS 8515:2023 sizing methodology was used to maximise annual supply efficiency.
Annual mains consumption fell by 3,850 m3, achieving 91% of the 25% reduction target. Payback period was 8.4 years against a GBP 185,000 installed cost. The system achieved BS 8515 supply efficiency of 72%, confirmed by year-one metering. Zero cross-connection incidents during the DWI site inspection post-commissioning.
Questions to Ask Shortlisted Providers
- 1
What is the available roof catchment area and what is the roof surface material (mineral felt, metal standing seam, GRP, clay tile)?
BS 8515 sizing uses roof area and local rainfall data to calculate annual harvestable yield; roof material affects runoff coefficient (0.75 for flat roofs, 0.9 for metal) and first-flush contamination risk.
- 2
What end uses are specified for harvested rainwater (toilet flushing, irrigation, vehicle wash, cooling tower makeup)?
End-use demand profile determines tank sizing and supply efficiency; toilet flushing is the most common UK application and creates predictable daily demand patterns for BS 8515 modelling.
- 3
Is the building new-build or retrofit, and is a dedicated non-potable distribution network feasible?
Retrofit non-potable pipework requires full identification and separation from the potable supply per WRAS IGN 9-02-04; cross-connection risk in retrofit is significantly higher and must be evidenced to the DWI.
- 4
What is the water hardness, pH, and microbiological quality of local rainfall?
Urban rainfall may carry atmospheric particulates and bird faecal bacteria (E. coli, Cryptosporidium); risk assessment per HSG274 determines whether UV disinfection or filtration is required for non-domestic applications.
- 5
What are the planning authority SuDS requirements and can the RWH tank serve dual purpose as attenuation storage?
Under Schedule 3 of the Flood and Water Management Act 2010, SuDS approval bodies may require attenuation volume; dual-purpose RWH tanks that serve both functions can reduce overall infrastructure cost.
What Drives Cost in This Category
Below-ground polyethylene tanks cost GBP 800 to 3,500 per unit (3,750 to 10,000 L); GRP tanks for large commercial systems (50,000 to 500,000 L) cost GBP 15,000 to 120,000; excavation in poor ground or high groundwater adds 30 to 60% to civil costs.
Dedicated purple-pipe networks for new-build cost GBP 15 to 45 per metre installed; retrofit pipework in occupied buildings costs GBP 60 to 150 per metre due to access constraints and disturbance.
Residential single-pump sets cost GBP 1,200 to 3,500; commercial duty/standby booster sets with frequency control, remote monitoring, and BMS integration cost GBP 8,000 to 35,000.
Annual maintenance for residential systems costs GBP 100 to 200; commercial systems with HSG274 Legionella obligations cost GBP 800 to 3,500 per year depending on tank capacity and risk assessment frequency.
Key Regulations & Standards
UK standard covering system design, sizing (behavioural method or simplified method), component specification, water quality, commissioning, and maintenance; compliance required for systems installed in new-build under Building Regulations Part G.
Require physical separation between rainwater and potable supply via Type AA airgap; non-potable pipework must be clearly identified (purple colour, labels); cross-connection is a criminal offence under Regulation 5.
Non-domestic RWH systems with storage above 300 L and distribution that may generate aerosol require Legionella risk assessment per HSG274; Schematic Risk Assessment (SRA) and written control scheme are required by law under COSHH 2002.
New drainage systems for major developments in Wales (already in force) and England (expected from 2025) must comply with SuDS standards; RWH tanks can be integrated into SuDS attenuation calculations to reduce overall SuDS infrastructure cost.