Capturing savings

With parts of the UK now technically qualifying as deserts, rainwater harvesting has become a serious consideration among the Government and consumers alike. Gary Wheatley of Wilo UK explains the options.

In recent years, many areas in the UK have been surviving on much lower rainfall totals than they have been used to, or getting their rainfall in much higher quantities, if more infrequently. Because of this, rainwater harvesting is becoming an increasingly relevant option here in the UK, as the opportunities to include it in new build 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, and far from reducing year on year, figures appear to be increasing year on year. This is not sustainable going forward and the UK needs to find ways to reduce that usage and provide the water necessary. Rainwater harvesting offers a vital option to facilitate this.

Holistic considerations of the wider environment are key in discussions around rainwater harvesting. It is essential to both manage the demand for what is a finite resource and to learn to deal effectively with surface water run-off. The UK often gets 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.

Dealing with excess rainwater

There are three ways to deal with excess rainwater: 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, including very sound financial motivations. It offers both significant reductions in water costs and also a reduction in capital costs of providing attenuation. There are also serious promotional reasons. Rainwater harvesting can enhance the image of the organisation involved, be it the Government, a developer, housing association or the owner of the land concerned, adding to a property’s green credentials. Finally, there are good ecological reasons – rainwater harvesting helps reduce the consumption of potable water, resulting in a reduction in storm water discharge.

In a typical domestic rainwater harvesting system, rainwater is channelled 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 toilet flushing, use in washing machines, irrigation, watering the garden, 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.

Choosing a system

When designing a rainwater harvesting system, the first steps should include determining the yield to work out 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. This 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, washing machine cycles and the volume of garden irrigation, for example. Experience has shown that a stocking capacity of about two to three weeks is optimal for rainwater harvesting.

It may seem obvious but it is important to emphasise that when selecting your rainwater harvesting system, only high quality products should be used. 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.

Gary Wheatley is training and technical manager at Wilo UK