RAINWATER HARVESTING

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Can I collect and use rainwater?

In the UK, mains water is cleaned to drinking water standards but we don't drink most of it. Energy is used to clean water to a much higher standard than we need for washing, garden watering and toilet flushing. Collecting rainwater and using it for some of these can save both energy and money.

The best way to make use of rainwater in this country is to put a water butt on your down-pipe, and save the water for use in the garden or for car washing. This is very cost-effective, as the equipment costs tens of pounds, rather than the thousands needed for a more complicated system. If you have a big garden, you could even fit two water butts next to each other from the same downpipe. A complete water butt kit will include a diverter pipe to fit to your down-pipe and prevent overflowing, and a stand to allow easy access to the tap.

Reduce, Re-use, Recycle

Remember that just as it is better to reduce waste before recycling what is left, so it is also better to reduce water use before thinking about treating it for re-use. There are many easy ways to reduce water use by a hefty amount, and you can read about these in our Water Conservation in the Home tipsheet. By putting a full, sealed 1 litre drinks bottle in the cistern of an older toilet, you can cut the amount used to flush by one litre each time. A more advanced option is to use a retrofit device replacing the existing flush handle and enabling you to do choose either a full flush or a half flush. CAT Mail Order sell a retrofit flush device and also Tapmagic fittings - which allow a tap to be operated on either full flow or spray flow (for hand washing). See our Water Conservation in the Home tipsheet (available through our pay-per-view page) for many more ideas.

Can I flush my toilet with rainwater?

If your garden is small but you have a large roof area, you may consider a more advanced system to collect water for your toilet or washing machine. A basic filter (such as the 'WISY') can separate water and send it to a collection tank, whilst debris such as leaves drops into the drain. However, such a system will not always be financially or environmentally beneficial - benefits can be outweighed by the environmental costs. Electronic controls, used to monitor water levels and work a pump, have a surprisingly high impact.

A rainwater harvesting system to provide toilet flushing for a 3/4 bedroom house is likely to cost at least £2,000. It may be possible to put in a simpler gravity-fed system (e.g. for a ground-floor toilet) for below £1,000. The payback time can be therefore be very long, and a system may never recover its costs if parts need replacing before savings are realised - especially if your water is not metered.

If you are on a water meter then a rainwater harvesting system might make more financial sense. They are most economic and effective when integrated into new buildings, especially commercial premises or schools with large roofs, high non-potable water use, and metered supply. At CAT, we collect rainwater to flush the frequently used toilets in our visitor centre. Rainwater collection on this scale could also be helpful in urban areas where drainage is poor, as it can help to reduce flash flooding. In rural areas it may actually be more sustainable to look at a composting toilet as a way to reduce water use.

How much water could I collect?

A fairly simple calculation can tell you how much rainwater you can harvest, and help you decide if a system would be worthwhile. You'll need to know the area covered by the roof to be collected from, and the annual rainfall for your area. Average rainfall figures for several UK locations are on the Met Office website.

Roof area (in square metres) x Annual rainfall (in mm) x System efficiency (e.g. Wisy is 90%) x Run-off coefficient of roof (a pitched, tiled roof is 0.75) = Annual collection in litres (divide by 1000 to get a figure for cubic metres).

A tank would normally be sized to store about 5% of this total. The water should enter the storage tank near the bottom - but not right at the bottom as this will disturb the bio-film which collects there. The outlet should be about midway up the tank so as not to collect pollen etc. from the top. You also need to check that your roof is clean and not made of toxic metals.

What about drinking water?

Treating rainwater to a high standard, suitable for bathing or even for drinking, is not something we would recommend. Not only is it very expensive, but also the impact of all the equipment that you would need will outweigh any environmental benefits of reducing mains water use. Small-scale water treatment systems use lots of energy in manufacture and use, and the filters need to be regularly replaced - so creating waste.

If you are not on mains water, collecting and treating rainwater is an option, but in the UK it is usually preferable to use groundwater or even a stream - as this water often needs less treatment. Treating rainwater to a higher level is more suitable in dry countries with scarce supplies (like Australia), or places with a very dirty groundwater supply (such as Germany). In these cases, the extra expense and energy needed can be justified against the alternatives.

Further Information

See the tipsheets mentioned, or get CAT's Water Book, which includes detailed chapters on rainwater and grey water reuse, including the design of garden irrigation systems.

All our Tipsheets are available as pdf files through our pay-per-view system; alternatively, put a couple of stamps per tipsheet in the post - addressed to CAT Information Service, Machynlleth, SY20 9AZ - and we'll send them off to you.

There is more information on rainwater harvesting, including a downloadable report, on the Environment Agency website

You can also contact us with any further questions about the information given here.

Suppliers

CAT Mail Order supplies spray-head 'Tap Magic' attachments (check with them for fitting size), the 'Interflush' retrofit kit to reduce the flush of a siphon toilet and a kit to convert a wheelie bin or other plastic container to a water butt. Visit http://store.cat.org.uk for details.

UK Rainwater Harvesting Association
Tel: 01636 894900; Web: www.ukrha.org
Trade association set up to promote rainwater harvesting. See their website for a list of members.

A good rainwater harvesting system installer should also recommend the use of associated water-saving equipment, such as a low-flush toilet, so this is a good sign to look out for when seeking a suitable system.