Content supplied by Rainwater Harvesting

The process of harvesting rainwater has been carried out since Roman times. It is a simple process: rainwater falling on the roof is collected in standard gutters and downpipes, filtered and stored in a tank. When required the water is pumped back into the home.


What can rainwater be used for?

 Rainwater can be used for any non-potable (water that is not of drinking quality) application including:

  • toilets
  • washing machines
  • outside use (gardening and washing the car)

Collecting rainwater at source can also act as a Sustainable Drainage System (SuDS) to prevent flooding. This has become more prevalent as part of planning applications regarding storm water discharge rates.

Type of Tank

Tanks can be composed of a number of materials but most commonly plastic.

If the water is to be used inside the home it is usual to have the tank buried underground. It will be out of sight and not affected by variables of temperature including frost or light.

Shallow dig tanks have made installation far easier and accessible. They can also be installed in areas with a high water table. It is very important that the water is filtered before entering the tank.

Size of the Harvesting Tank

Two important factors for sizing up your tank are the area of the collectable roof and how much rain is likely to fall in that particular part of the country. Our tank size calculator takes into account average rainfall specific to the county.

Other considerations include the number of people who will regularly be in the home, the number of toilets and any other applications the water will be used for. The industry standard is to set the capacity for a 21-day supply, as it is assumed that the tank will be replenished within that time.

Depending on usage, a typical home for a four-person family would require a tank capacity between 3,000 and 7,500 litres.

If combining the benefits of rainwater harvesting with a SuDS option the tank will have extra capacity for storm water.

rainwater harvesting installation

RainActiv is designed to collect the excess and then discharge at a calculated controlled rate.

Type of Mains Back Up

The ideal is to size the harvesting tank so that the toilets will only require rainwater for flushing. However should there be very low rainfall or the garden needed extra water a mains back up would be required. There are two main methods.

Direct Feed: The appliances are directly connected to the harvesting tank. For example when a toilet is flushed a drop in pressure is recognised by the pump and water will flow to it. If back up is required a small amount of mains water will be introduced into the harvesting tank. The system automatically reverts back to rainwater when available.

Gravity Fed: Water is pumped from the underground tank up into a header tank once or twice a day. From the secondary tank water flows to the appliances by gravity. This makes the pump operation more efficient rather than running every time a toilet is flushed. The Rain Director system costs about 1p/person/day to run making it very economical and less wear on the pump. In the back-up mode mains water will be introduced into the header tank.

The pump must be man (or woman) enough for the job. There is no point in having a low wattage pump that cannot draw the water as required. Always look for mains back up systems that are Water Regulation Advisory Service (WRAS) approved. The system will have been rigorously tested to make sure that mains and rainwater cannot mix. WRAS approval is accepted by all water providers as compliant when correctly installed.

Rainwater director system

Rainwater Director System


Keeping leaves or other debris from entering the tank is key to a successful system. This can be achieved with either gutter brushes, down pipe filters but most importantly a filter which flushes any debris into the soak away or drainage system. Look for low maintenance options where the flow of the water helps keep it clean and only needs checking once or twice a year.

Very small particulate may settle on the tank base but water is drawn from a higher level so preventing it being drawn into the system

Correctly sized the tank should overflow once or twice a year. If the system is also being used as a SuDS solution the extra capacity provided for any storm event will drain to the rainwater harvesting level. The importance in both cases is to ensure that there is a sufficient turnover of water in the system. The British Standard advises that the harvesting tank will only require cleaning when ‘routine inspection shows it to be necessary’.

rainwater harvesting tank

How Much Does a Rainwater Harvesting System Cost?

Complete systems including tank, filter, mains back-up controls, pump, pipe, labels and delivery to mainland UK addresses, cost in the region of £1,800-£4,000. This would depend on the tank size and type of mains back-up unit supplied.

Rainwater harvesting installation requires the input from a number of trades and must be in accordance with the manufacturers instructions. This includes groundworkers to site, dig the hole and pipe work to and from the tank. Also a small amount of extra time from the electrician and plumber is required.

As rainwater harvesting is usually installed as part of a larger build project the extra cost of labour varies and would be part of the negotiation.

In most cases installing a system will not yield a short term return but it will future proof your water supply.

RainWater Harvesting Ltd

RainWater Harvesting Ltd has designed the systems in the UK for UK homes. Careful consideration was given to the trade to be as easy as possible to install with push fit fittings and Cat 5 connections. RainWater Harvesting offer training days for installers which covers current legislation and handy tips.

CLICK HERE to visit the Rainwater Harvesting website or call 01733 405 111

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