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Rainwater harvesting by Wilo

Rainwater harvesting is becoming an increasingly relevant option here in the UK as the opportunities to include it in new and refurbishment projects grow. Rainwater harvesting put simply, is collecting rainwater which falls onto roofs, storing it and using it as a free resource. Rainwater harvesting is the accumulation and storage of rainwater for reuse before it reaches the aquifer. Currently each person in the UK uses approximately 130-150 litres of water per day. We know that this is not sustainable going forward. We have to find ways to reduce that usage and to provide the water we need. Rainwater harvesting provides an important option.

Environmental considerations are key in rainwater harvesting discussions. Two in particular. We have to manage the demand for what is a finite resource. And we have to learn to deal effectively with surface water run-off. We often get a lot of rainfall in a short period of time and much of it is wasted. It’s a strange fact that in homes over one third of potable water and in offices, over two thirds of potable water is simply flushed away. We spend a lot time and effort ‘polishing’ the water supply and then we flush it down the toilet.
So how can we deal with excess rainwater?

There are three ways - infiltration – a system promoting the passage of surface water through a permeable surface into the ground for example using soakaways; attenuation – which involves a reduction of peak flow and an increased duration of a flow event – for example, storm water storage, linked to the main sewer system; and thirdly there is rainwater harvesting – collecting rainwater from where it falls rather than allowing it to drain away.

There are a number of reasons why rainwater harvesting makes sense. There are very good financial reasons for harvesting water. It offers significant reductions in water costs and also a reduction in capital costs of providing attenuation.

There are serious promotional reasons – rainwater harvesting can enhance the image of the organisation involved and add to its green credentials. And finally there are good ecological reasons – it can help reduce the consumption of potable water and results in a reduction in storm water discharge.

In a typical domestic rainwater harvesting system, rainwater is chanelled via a filter into a storage tank and water is supplied using a pump to utilise the water harvested.

Harvested water can be used for non-potable (non-drinking standard) purposes such as for toilet flushing, for use in washing machines, for irrigation and watering the garden, for car cleaning and for industrial applications. Untreated water is not suitable for use where drinking quality water is required such as for bathing, showering and dish washing.

Using rainwater harvesting can help you to meet the Code for Sustainable Homes. To achieve Code Levels 3 and 4 you will need to reduce mains water use to 105 litres per person per day. To achieve Codes Level 5 and 6, you need to reduce mains water use to 80 litres per person per day.

When designing a rainwater harvesting system, the first steps should include determining the yield to determine the rainfall intensity specific to the location, by seeking the amount of precipitation per year - in litres per square metre. The calculation of rainwater yield per year is the amount of precipitation per year multiplied by the roof area multiplied by the discharge coefficient which may give for example 72,000 litres per year which over 365 days a year gives 200 litres rainwater yield per day.

To ascertain the demand, simply add up the consumption of the different application areas in the development or project – the number of toilet flushes, the washing machine cycles and volume of garden irrigation for example. Experience has shown that a stocking capacity of about 2-3 weeks is optimal for rainwater harvesting.

When selecting your rainwater harvesting system, only high quality products should be used for this key part of the system. Select a rain system with a self-priming multi-stage horizontal high pressure centrifugal pump, complete with pressure and flow controller with integrated low water protection, an integral 11 litre fresh water make up tank and a low water float.

Wilo is one of the leaders in the rainwater harvesting business in Europe where it has been important for far longer than here in the UK. It’s now bringing its expertise to the UK in a big way and already has a large number of successful systems - both domestic and commercial – including the system at the stunning new Belfast Metropolitan College project in the Titanic Quarter in Belfast. For more information, click on to the Wilo website at, and the UK Rainwater Harvesting Association (UKRHA) at

By Gary Wheatley, Training and Technical manager, Wilo UK

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