How to design a rainwater harvesting system?

Designing a rainwater harvesting system requires careful consideration of several key factors. A well-designed system maximises water collection, ensures water quality, and provides reliable supply for your intended uses.

Step 1: Determine Your Objectives

Start by defining what you want the system to achieve. Are you aiming to reduce water bills, provide better water for your garden, gain independence from mains supply, or reduce your environmental impact? Your objectives determine system complexity and investment. Also define what you will use the water for — garden irrigation only, or also toilet flushing, laundry, and outdoor cleaning. Each use has different water quality requirements.

Step 2: Calculate Catchment Potential

Measure your roof area in square metres. Multiply by your local annual rainfall in millimetres, then by a collection efficiency factor of 0.75 to 0.8. This gives your annual collection potential in litres. Consider that different roof sections may have different orientations and efficiencies. Include all roof areas that can be connected to the system. Factor in that some rainfall will be too light to generate runoff, and some will be too intense for your gutters to capture fully.

Step 3: Size the Storage Tank

Tank sizing is a balance between collection potential, demand, and budget. Use a water balance calculation comparing daily or weekly water supply versus demand throughout the year. For UK conditions, a tank holding 5 to 15 percent of annual demand is typically appropriate. Larger tanks provide greater drought resilience but cost more and take up more space. Consider modular systems that allow adding storage later.

Step 4: Select Filtration

Choose filtration based on water use. For garden watering: a mesh leaf filter at the tank inlet is sufficient. For outdoor cleaning: add a 25-micron sediment filter. For indoor non-potable use: add 5-micron sediment filtration, activated carbon, and UV disinfection. For drinking water: add reverse osmosis. Always install a first-flush diverter to improve water quality by discarding the most contaminated initial runoff.

Step 5: Plan the Distribution System

Plan how water will be delivered from the tank to each point of use. For garden irrigation, a single outdoor tap supplied by a pump or gravity is usually sufficient. For indoor use, you need a pressurised system with pipework running to toilets, washing machines, and possibly outdoor taps. The distribution system should include isolation valves to allow maintenance without draining the entire system. All non-potable water pipes must be clearly labelled with warning tape or stickers indicating that the water is not for drinking. Pipework should be buried at sufficient depth to prevent freezing in winter. Consider future expansion when laying pipework — installing a spare conduit or leaving space for additional pipes can save significant work later if you decide to extend the system.

Step 6: Address Regulations and Standards

Compliance with regulations is essential, particularly for systems that connect to indoor plumbing. The Water Supply (Water Fittings) Regulations 1999 require backflow prevention to protect the mains water supply. The British Standard BS 8515:2009 provides the code of practice for rainwater harvesting systems and covers design, installation, water quality, and maintenance. Local building regulations may impose additional requirements. Planning permission may be needed for large above-ground tanks, particularly in conservation areas or listed buildings. If you are installing the system yourself, research the requirements thoroughly before starting work. For complex installations, employing a professional installer who is familiar with the regulations is strongly recommended. A properly designed and installed system that complies with all relevant standards will be safe, reliable, and legal.