Rainwater harvesting – frequently asked questions

These faq’s are from Britain but are relevant to the rest of the world too, especially water scarce South Africa. Although one answer below states that rainwater from rain water tanks should not be used for potable purposes; this is not strictly true. Rainwater can be used for drinking as long as it is properly filtered. It also helps to keep gutters clean and to clean out water tanks regularly.

rain water tanks

  • What is Rainwater Harvesting? Rainwater harvesting is a way of saving the rainwater which would normally flow off a roof and down the drain, and using it as piped water to flush toilets and for the garden watering, yard washdown, vehicle and car washing, instead of treated drinking (potable) water.
  • How does rainwater harvesting work? A storage water tank is fitted to your stormwater drain from your roof, and falling rain enters the water tank through a filter which removes leaves and other matter. The water storage tank is usually buried under car or vehicle parks, a garden or under the entrance access or drive, and contains a pump which pumps the rainwater to the building where it is piped to the toilets, and to the outside taps.
  • Is rainwater harvesting suitable for work as well as at home? Yes, in schools, hospitals, offices, commercial premises, rainwater can typically be used for toilets, vehicle washing, yard washdown and watering plant pots/gardens.
  • How much water can a system save? Depending on your normal usage, it can save 30 to 50% of the treated drinking water from the mains in houses and up to and up to 80% of the treated drinking water in a business or commercial building.
  • How much would this save on water bills? Depending on your normal usage, it can save 30 to 50% for the domestic user and 80% for the commercial user of the treated drinking water from the mains. Having metered water is the best way of appreciating the difference.
  • How much rainwater does a system collect? This depends on the area and angle of your roof, and your rainfall. Averages of 100,000 litres per household are commonly quoted, much more for large roofed commercial buildings.
  • What can you use the water for? Filtered, untreated rainwater should only be used for non- drinking or bathing purposes: toilet flushing, gardens and vehicle or yard washdowns.
  • Is it only for new buildings? No, it can be installed in existing buildings, but will cost more, because of the extra plumbing required.
  • Is it only for houses? No, bungalows and commercial premises are also very suitable, the only limitation is the area of the roof to capture rain, compared to the number of users; this puts a limitation on flats and apartments.
  • Where is it installed? The water tanks should be buried under a car or vehicle park, landscaped area, garden, patio or drive, with space left for the round access cover. Most systems are designed so that they can accept cars driving over them if suitably installed.
  • How much does it cost? Domestic systems can cost from about £2500 up to £4000 plus including installation costs, depending on size of tank. Commercial systems can cost a lot more depending on size and requirement, but usually have a much quicker ‘pay back’ period due to the size of roof and high usage.
  • Are there any grants available (UK)? Not for domestic installations yet, although several organisations are petitioning the government for assistance. For commercial installations, there is a tax relief scheme (ECA) for suitable approved equipment on the Water Technology List.
  • So, why should people buy Rainwater Harvesting systems? To save on water bills and show they use a precious resource responsibly to make a difference to our environment.
  • How clean is the water? The rainwater is filtered as it enters the storage tank, to remove particles and other matter. It is kept in the dark and kept oxygenated to discourage algal growth, and properly designed systems are designed with calming inlets, which ensure that any sediment at the bottom of the water tank does not get stirred up. The water is not drinking water fit for humans.
  • Could rainwater get into my drinking supply? Not in a properly designed system, the pipework is entirely separate and should be identified as non-potable.
  • Do I need a big roof area to make it worthwhile? The bigger and flatter the roof area, the more rain will be captured, and the more the rainwater will substitute for treated mains water.
  • What happens when there is no rain? When there is a prolonged spell without rain, the water level in the storage tank will fall to a minimum level. At this level, a float switch will open a valve from the normal mains supply and keep the storage tank topped up, until it is filled by rain again. Normal system design should allow up to a week without rain, given normal usage.
  • Can it be used when there is a hosepipe ban? It is possible to use a hosepipe connected to a rainwater tank, provided that the tank is not connected to a mains water supply. Many people are doing this already by using a water butt or rain barrel.
  • Do I need to have a water meter? Not absolutely necessary although, without a water meter your water provider will make the same blanket charge for all your unmetered water usage. Commercial premises will be metered, and the beneficial effects of rainwater harvesting should be seen very quickly.
  • How is the system maintained? The internal filter should be washed about once a quarter, otherwise maintenance is not needed. The only moving components – pump and float switch have an extremely long life. The system should be designed to overflow a few times a year, to remove floating matter.
  • Who would install a Rainwater Harvesting system? Normally a builder – the skills are the same as installing a septic tank, but a plumber would be needed to complete the internal pipework.
  • Is Rainwater Harvesting a new idea? No, collection re-use of rainwater from roofs can be traced back thousands of years in hot, dry countries around the Mediterranean. In continental Europe, some 100,000 are installed annually; Germany has been using and refining the technology since the early 80s.
  • What regulations govern the installation of a system? No specific regulations yet govern rainwater harvesting as such, although Building Regulations Part H affect siting of the tank and pipe runs, while Part G refers to internal plumbing.

Source: Save The Rain Campaign

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