Rainwater Harvesting Systems: Design and Regulation

What is Rainwater Harvesting?

Rainwater harvesting (RWH) involves collecting and storing rainwater from roofs, paved surfaces, or other catchment areas for later use. In England and Wales, RWH from roofs and surfaces is generally exempt from the requirement to hold an abstraction licence under the Water Abstraction and Impounding (Exemptions) Regulations 2017.

Regulatory Position

Rainwater harvesting from roofs and hard surfaces is exempt from abstraction licensing because:

• The water is collected before it enters a watercourse or groundwater.
• The volumes involved are typically small.
• The activity does not directly affect the water environment in the same way as abstracting from a river or borehole.

However, if you intend to collect rainwater from a large catchment area (e.g., a reservoir fed by surface runoff), you may need an impoundment licence or an abstraction licence depending on the specific circumstances. Always check with the EA or NRW.

British Standard BS 8515:2009+A1:2013

The principal technical standard for RWH systems in England and Wales is BS 8515:2009+A1:2013, which covers:

• System design and sizing.
• Water quality requirements.
• Installation, commissioning, and maintenance.
• Backflow prevention (critical to prevent contamination of the mains supply).

System Components

Collection Surface

• Roofs: The most common collection surface. Roof material affects water quality (avoid lead, copper, or bitumen-coated surfaces for potable use).
• Paved areas: Can be used but water quality is lower due to contamination from vehicles, animals, and debris.

First Flush Diverter

A first flush diverter discards the initial rainfall (typically 1–2mm) which carries the most contamination from the collection surface. This significantly improves water quality.

Storage Tank

• Underground tanks: Preferred for large systems; maintain cooler temperatures, reducing algal growth.
• Above-ground tanks: Simpler to install but subject to temperature fluctuations.
• Sizing: BS 8515 provides a methodology for sizing the tank based on rainfall data, catchment area, and demand. A typical household system uses a 1,500–7,500 litre tank.

Filtration

• Coarse filter: Removes leaves and large debris at the inlet.
• Fine filter: Removes finer particles; typically 280–380 micron.
• Calmed inlet: Prevents disturbance of settled sediment in the tank.

Pump and Distribution

• Submersible pump: Installed inside the tank; quieter but harder to access for maintenance.
• Suction pump: Installed outside the tank; easier to maintain.
• Header tank: Some systems use a small header tank to provide constant pressure.

Backflow Prevention

Critical: The RWH system must be completely separate from the mains supply, with an air gap or approved backflow prevention device to prevent contamination of the mains. This is a requirement of the Water Supply (Water Fittings) Regulations 1999.

Suitable Uses

Rainwater is suitable for:

• WC flushing (largest domestic use — up to 30% of household water use).
• Garden watering and irrigation.
• Laundry (with appropriate treatment).
• Car washing.
• Industrial processes (with appropriate treatment).

Rainwater is not suitable for drinking, cooking, or bathing without treatment to drinking water standards.

Maintenance

Regular maintenance is essential:

• Clean filters every 3–6 months.
• Inspect and clean the tank annually.
• Check and clean the first flush diverter.
• Test water quality annually if used for any purpose where quality is important.