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Rain harvesting

Introduction to rainwater harvesting

We aren’t as rich in water in the UK as popular belief (and our reputation for rain) would have it. According to Environment Agency figures we only have 1,334 cubic metres per person per year. This is not a lot compared with France (3,065), Italy (2,785) and Spain (2,775). South east England is the worst off thanks to high population density and low rainfall. The Thames Valley has just 266m3 of water per person per year.

The average person in the UK uses 150 litres of water every day, and this puts a strain on supply as well as on fish, wetland birds and other wildlife that rely on ponds, rivers and streams.

The water we get through the mains system is cleaned to drinking standard, a process which has significant associated carbon emissions. However, only about half of the water we use needs to be cleaned to this standard. Rainwater can be used for:
  • Toilet flushing
  • Clothes washing machines
  • Watering the garden
  • Washing the car and other outside uses.

How does rainwater harvesting work?

Rainwater harvesting can be as simple as attaching a water butt to the down pipe from your gutters, and using the water you collect to water the garden on wash the car. At the other end of the scale, it can be a pumped system that uses rainwater for toilet flushing and clothes washing, as well as outdoor uses.

The principle is simple. Water from your roof is channelled via your drain pipes to a storage tank. If you want to use it in your home the water will pass through a filter. More sophisticated installations need a control system to monitor water levels and work the pump.

A system that feeds the home will need mains back up that cuts in automatically when the storage tank runs dry. The plumbing from the system must be kept separate from the mains system and labelled. The storage tank can be at ground level, but it is preferable to bury it, both because it looks better, and to protect it against frost.

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Is rainwater harvesting suitable for my home?

Rainwater harvesting systems are most suitable to be fitted when a house is built, or when you are doing significant renovation, as they will need changes to the plumbing system and considerable digging in the garden to bury the tank.

Whether it’s suitable for you will depend on the size of your roof, and how much water you use. It will not be suitable if you have asbestos or other toxic materials on your roof.

On average, half of most households water use can be supplied from rainwater. Most domestic roofs are too small to provide this much water (however big the storage tank), so to find out whether it’s suitable for you it’s worth doing some simple calculations.

First use this simple formula to work out how much you might get per year from a rainwater system:

Roof area (m2) x drainage area x filter efficiency x annual rainfall (mm) = Amount you can collect in a year in litres (divide by 1,000 to get cubic metres)

Drainage varies from 0.9 on a steep pitched roof, to 0.4 on a flat roof with gravel. More detail is available from the Environment Agency.
Filter efficiency is generally estimated at 0.9 Annual rainfall averages are available from the Met Office.

People with a water meter will find it easy to work out what their annual water use is, and divide it by two, and see how it compares with potential yield. For those who don’t, the Consumer Council for Water provides a table of average household water use.

If you want to use most of the water from a rainwater harvesting system for watering your garden, and you live somewhere subject to hose pipe bans, you are better off getting a garden only system. Ones that also serve the house have to be connected to the mains, and so cannot be used during a hosepipe ban.

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What size of rainwater harvesting system do I need?

Generally the tank is sized to hold 18 – 20 days worth of water, or 5 per cent of the amount you can collect in a year, which ever is smaller. To work out the optimum tank size multiply your yield figure (from the above formula) by 0.05.

Maintenance tends to be minimal – usually just rinsing the filter quarterly.

How much will rainwater harvesting system cost?

A good quality domestic system should cost between £2,000 and £3,000 according to the UK Rainwater Harvesting Association. In addition, the cost of running the pump is likely to cost around 5 – 10p a week

For most homes it is more cost effective to reduce your water use than to collect rainwater. This will also reduce your energy use, and your associated carbon emissions. A rainwater harvesting system is unlikely to be cost effective unless you are on a water meter. The Rainwater Harvesting Association estimates that it will take between 10 and 15 years to pay back your investment.

Rainwater harvesting systems may increase your electricity use. For homes with gardens that need a lot of watering water butts or a simple grey water diversion system can save considerable quantities of water, and cost much less than a full rainwater harvesting system.

Top water saving tips include:

  • Getting a low flush toilet or putting a ‘hippo’ (or one litre drinks bottle filled with water) into your cistern
  • Turn off the tap when brushing your teeth
  • Only run your washing machine or dishwasher when there’s a full load, and use the economy setting
  • Mend dripping taps
  • Put the plug in (or use a bowl) when washing hands, veg, or doing the washing up

The Environment Agency has much more detailed tips for saving water in the home and garden. The Centre for Alternative Energy provides some water conservation advice.

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