Rainwater Harvesting FAQs

  1. 9000530K-Wall-Tank-GreenWhat Happens during a drought?

    A full rainwater harvesting system will automatically take water from the mains when rain is scarce.

  2. Is the water clean?

    Rainwater tanks have filters to remove debris and particles, so the water is clean.

  3. How much water can a system harvest?

    This depends on variables such as the catchment area and how much it rains where you are based. On average, a domestic system will cut mains reliance by about 50%, rising to around 80% for commercial applications.

  4. Why buy a system?

    Rainwater harvesting has significant environmental benefits, cutting mains demand and lowering storm water run-off. Controlling your storm-water run-off is becoming more important in the eyes of Planning departments.

  5. Can rainwater harvesting systems be retrofitted?

    The systems are best when built into the design from the beginning, but they can be retrofitted to existing buildings.

  6. Is there a danger of Legionella?

    Legionella cannot cultivate in the dark, cool, and oxygenated conditions of underground tanks.

  7. How big are the systems?

    System sizes vary considerably, and depend on the harvesting capabilities of your roof as well as the levels of use of non-potable water usage in the building.

  8. What maintenance do rainwater harvesting systems need?

    This varies from one system to the next, but is always minimal. Typically, you’ll need to wash off the filter with a garden hose for five minutes once per quarter.

  9. How long will a system last?

    The buried parts of the system will last indefinitely, and the control system, pump, and filter all have long lives and are easily replaced if necessary.

  10. How long has rainwater harvesting been in use?

    The Romans created the first rainwater harvesting systems, but those used today are based on mid-1980s’ designs.

  11. Is the modern technology proven?

    Yes – Systems fitted in Germany in the 1980s are still in use, and one benefit of the UK being behind the rest of Europe is that any issues with the systems have been worked out through all of the work done in Germany.

  12. How often will the rainwater harvesting system need to be topped up from the mains supply?

    Systems with good balances of supply and demand should only need to be topped up when it hasn’t rained for a while. Severn Trent monitored a domestic system and found it rarely needed topping up and was usually around 50% full – ideally balancing having enough water to use and enough space to hold future rainfall.

  13. What uses does the water have?

    Harvested rainwater is suitable for all non-potable purposes from watering plants, washing cars, flushing toilets, and running washing machines. In some cases, further treatment can make the water safe to drink.

  14. How much do pumps cost to run?

    A pump will normally use less than 1.0kWh of electricity to provide 1 cubic metre of water (1,000 litres). Pumping costs are around 10p per week for a 3-bed house using rainwater for WCs, gardening, and the washing machine.

  15. How much do rainwater harvesting systems cost?

    Smaller water butts used for gardening or washing cars will be significantly cheaper, but larger domestic systems cost around £1500 to £2000, depending on size. Commercial systems are more expensive, but will save more due to the greater roof areas.

  16. Is rainwater potable?

    In the UK, rainwater usually isn’t harvested to be drunk. However, some manufacturers can offer solutions to bring water to potable quality with sufficient maintenance.

  17. Are there any rainwater harvesting grants or tax allowances available?

    At the moment, there are none. However, rainwater harvesting systems do qualify for 100% capital allowance in commercial applications.


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