Water reviews, policies and action plans

Reduce mains water use by harvesting rainwater


Rainwater harvesting systems enable you to collect rainwater for use in your business. This reduces the amount of water you need from the mains water supply. To work out if harvesting rainwater could benefit your business, you should consider the following steps.

Quantify water use and costs

Before you install any rainwater harvesting systems, your business should compare the costs of these systems with how much you are paying for water and carefully evaluate any potential cost savings.

You can find out your mains water supply costs by checking your water bills carefully. If you want to quantify how much rainwater you can harvest in a year you should use the following equation:

Annual rainwater yield in cubic metres = P x A x 0.8.

  • P = annual precipitation (in metres)
  • A = collection area (in square metres)
  • 0.8 = typically, you should expect to collect approximately 80 per cent of this water each year, due to small losses in filtering and small rainfalls that do not generate enough runoff.

You can find annual precipitation information on the Met Office website.

Harvesting considerations

Think about the quality of the water you will gain from rainwater harvesting and the quality of water you need to operate your business efficiently. If you need high levels of purity, rainwater may not be suitable.

The reuse of rainwater can improve the overall Building Research Establishment Environmental Assessment Method (BREEAM) performance for a building. BREEAM is an environmental assessment for new and existing buildings, which was developed by the Building Research Establishment and is the leading - and most widely used - environmental assessment method for best practice in sustainable design.

You should invest in a tank large enough to collect water - without it overflowing - at the wettest time of year. There are many businesses that can help you develop bespoke rainwater harvesting systems.