It is always better to reduce waste before looking for new sources — so prior to considering rainwater harvesting methods, you should first reduce your water wastage as much as possible. However, in our rainy country it seems to make sense to collect rainwater for household use before it goes down the drain. This has a threefold advantage. Firstly, it means that less water is taken from reservoirs and rivers. Secondly, it means that rainwater run-off, causing flash flooding, is slightly reduced, and thirdly, it can be collected in situ in a relatively pure state. Although it is possible to filter rainwater to provide drinking water, it is better to use it for toilet flushing, garden watering and other non-potable uses.
The simplest way to make use of rainwater is to put a water butt on all downpipes, and save the water for use in the garden. This is a very cost-effective solution, as the equipment needed will cost only tens of pounds rather than the thousands you would need to spend on a more complicated system.
Water for toilet flushing needs no treatment, though the rainwater store should be covered to prevent the ingress of sunlight and animals. There are now several manufacturers offering packaged rainwater collection, storage and pump units. At present these are quite expensive (approximately £2,000), and you may be better off just carrying out water efficiency measures.
How much water can be collected? An average house in the UK will have around 100m³ of water per year running off its roof (see calculation, below). As the average household use for toilet flushing is 35-70m³ per year (depending on WC flushing efficiency) it is possible, with enough storage, to save this amount of water — worth perhaps £70-150 per year.
The water from the roof will need to be filtered to prevent dirt, leaves and bird droppings entering and contaminating the flushing system. The best filters (e.g. WISY system at 90% efficiency), operate by surface tension, are self-cleaning, and automatically reject the ‘first flush’ of contaminated rainwater. Between showers they dry out, so contaminant organisms do not survive. Storage tanks should be sized to contain 1m3 of water per 30m² of roof. It is preferable to have the storage tank underground, both for aesthetic reasons and as protection against frost. An automatic pumping system is required to deliver water to the WCs. There will normally have to be a mains ‘back-up’ which can be arranged to come on automatically, and which has to be fitted with approved air gaps to prevent cross-contamination.
A fairly simple calculation can tell you how much rainwater you can harvest, to help you decide if a system (such as above) would be worthwhile: Roof Area (m²) x Annual Rainfall (mm) x System Efficiency (%) x Run-off Coefficient of Roof (a pitched, tiled roof is 0.75) = Annual Collection (l). A tank would normally be sized to store about 5% of this total. For average rainfall figures see www.metoffice.gov.uk.