Rainwater Harvesting — StormWater Solutions

Rainwater Harvesting is the practice of collecting, storing and reusing rainwater that falls on roofs and hard surfaces. In the UK, harvested rainwater is most commonly used for non-potable applications — WC flushing, irrigation, vehicle washing and industrial process water — reducing demand on mains supply and lowering surface water discharge volumes.

How a rainwater harvesting system works

A modern rainwater harvesting system consists of five core elements working in sequence. Rain falling on the catchment surface (typically the roof) is conveyed via gutters and downpipes through filtration to an underground or aboveground storage tank. A pump set delivers the stored water to point-of-use fittings via a separate supply network, with mains top-up provided when the tank runs dry.

• Catchment surface — usually the roof; non-porous, non-toxic materials yield the best quality.
• Conveyance and filtration — gutters, downpipes, leaf guards and a coarse pre-tank filter (often 0.4–1.0 mm mesh).
• Storage tank — underground concrete, GRP, HDPE or modular cells; tank sizing per BS 8515.
• Distribution — submersible or surface pump with control panel and mains backup.
• Point of use — separate, clearly-marked pipework; signage required at all outlets.

Sizing per BS 8515

BS 8515:2009+A1:2013 is the UK code of practice for rainwater harvesting systems. The standard provides two methods for tank sizing: the simplified approach (smaller of 5% of annual rainfall yield or 5% of annual non-potable demand) and the detailed approach using daily water balance modelling.

Typical UK system sizes range from 1,500 litres for a domestic property (single WC) up to 50,000+ litres for commercial buildings, schools and warehouses where roof areas are large and non-potable demand is constant.

Benefits and incentives

Rainwater harvesting delivers two complementary benefits. It can cut potable water demand by 30–50% in domestic settings and up to 80% in non-domestic buildings. It also functions as part of a SuDS scheme by attenuating surface water discharge — many UK water authorities allow harvesting volume to count towards stormwater attenuation requirements.

BREEAM credits, water bill reductions and reduced surface water drainage charges (where Scottish Water or English water companies apply such charges) provide the commercial case.

Related ViaCon solutions

ViaCon supplies underground storage tanks suitable for rainwater harvesting integration, including ViaCon Storm Solutions and the full stormwater solutions range. Related glossary entries: rainwater tank, rainwater harvesting tank and SuDS.

Frequently asked questions about rainwater harvesting

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