Rainwater harvesting (RWH) is a practice of growing importance in the United Kingdom, particularly in the South East of England where there is less water available per person than in many Mediterranean countries. Rainwater harvesting in the UK is both a traditional and reviving technique for collecting water for domestic uses. This water is generally used for non-hygienic purposes like watering gardens, flushing toilets, and washing clothes. There is a growing demand for larger tank systems collecting between 1000-7500 litres of water. The two main uses for harvested rainwater are botanical uses, like gardening for plant irrigation, and domestic uses, like flushing toilets and running washing machines. Rainwater is almost always collected strictly from the roof, then heavily filtered using either a filter attached to the down pipe, a fine basket filter or for more expensive systems like self-cleaning filters placed in an underground tank. UK homes using some form of rainwater harvesting system can reduce their mains water usage by 50% or more, although a 20-30% saving is more common.
Prior to the widespread use of water mains, RWH was a traditional means of getting water in the UK. Even as far back as the 2nd-century AD, archaeological evidence shows that rainwater harvesting was being used by Housesteads Roman Fort in Northumberland as a way to flush the latrines. English castles from the 12th and 13th-century also have notable rainwater harvesting systems, such as Carreg Cennen, Orford, and Warkworth Castle.
In the 19th and the early 20th century, prior to widespread access to water mains, most large middle-class homes got their drinking water from springs and wells, but this water was usually hard which made it unsuitable for washing. Thus, such homes were usually designed to also harvest rainwater to be used in washing. During the interwar period, houses in hard water areas were sometimes built with rainwater storage tanks forming the roof of a scullery. Rainwater was led down to a third tap for washing purposes. Rainwater harvesting declined in popularity as water mains became more widespread through the early 20th century onwards.
In recent years, rainwater harvesting has become more common due to increasing water prices. While rainwater harvesting has been employed in high-profile facilities like the velodrome of the London Olympic Park, the UK's ongoing revival has lagged behind other countries such as Germany (the present world leader in modern rainwater harvesting). At present, only about 400 RWH systems are installed in the UK every year.
Some large retail developments are now incorporating rainwater harvesting even in some of the wetter parts of the UK.
Rainwater harvesting was being encouraged by the government of the UK through the Code for Sustainable Homes. The code ranked homes on a scale of one through six and requires new homes to have a score of at least three. One way to raise the score of a newly designed home is to incorporate a rainwater harvesting system. The code was revoked in 2015.
The Environment Agency has noted that water resources in the UK are under increasing pressure because of the growing population. In addition, the agency has warned that the South East of England is facing more serious water scarcity than anywhere else in England or Wales, such that the per-capita water supply is lower than many Mediterranean countries. The agency encourages a two-pronged approach to both reduce demand and increase supply, such as through the use of rainwater harvesting. However, there is a fundamental mismatch between supply and demand; the areas of the UK suffering water scarcity are in most cases also areas with low rainfall, which means the economics of installing a domestic RWH system are less favourable. The environmental impacts of domestic RWH systems are questioned since the environmental impact of water supply is a very small proportion of the total impact of water use (approximately 4%). For a UK household, the CO2 impact of supplying water to the house is around 100g of CO2 per day, around 1/600th of your total daily impact. However in countries without widespread mains water supplies, or where the environmental impact of mains water is very high, RWH may have more merit.
The installation of rainwater harvesting systems in the UK should be done according to the Water Supply (Water Fittings) Regulations and BS8515, in order to ensure safety. BS8515 also provides details on how to size the storage tank and allows estimation of the potential water savings. If you install a RWH system, you will need to inform your water company.
Rainwater harvesting at large scale may well be appropriate for farms as part of a catchment management strategy to decrease flood risk and diffuse pollution.