Installing rainwater harvesting systems can provide a variety of benefits, financial as well as reducing the risk of flooding in some areas.
However, it may not be suitable for every installation, and needs some careful thought and assessment beforehand before committing to any expenditure.
Rainwater harvesting is very simple, and has been practiced for centuries. Rainwater is collected from a clean surface (i.e. a roof), filtered, stored and reused. It is a system used by millions of people around the world who don’t have access to a mains water supply. In Germany, harvesting rainwater is widely practised with approximately 60,000 systems installed a year.
In the UK, rainwater harvesting is not so straightforward, and depends upon the location and end use. The financial benefits depend upon the cost of the mains water which is relatively cheap compared to parts of continental Europe.
To establish whether a rainwater harvesting system will benefit your residence or business follow the basic steps set out below:
1. Quantify the amount of water you currently use
– simply done by checking recent water bills
2. Quantify the maximum water you can harvest in a year
– based upon rainfall in your area, the area at the installation (house or business) over which it can be collected.
Annual rainwater yield (Y) in m3 = P x A x 0.8
Where P = annual precipitation (in metres) and A = collection area (m2)
0.8 = typically you should expect to collect approximately 80% of this water each year, due to small losses in filtering and small rainfalls that do not generate enough run-off.
3. Quantify the cost
Check water bills to assess current cost, and possible saving
Water quality is an important consideration depending upon the reuse application. Water for general domestic use such as WC flushing or irrigation of gardens, can be of a lower quality than that required for any application relating to food processing etc.
The table below illustrates the amount of water you could save, based upon the collection area.