36 ENERGY IN BUILDINGS & INDUSTRY

waTER maNaGEmENTEIBI 11&12.2010

Rainwater harvesting is a popular sustainable option. It is financially accessible and the concept is easy to

understand – after all it is what our great, great grandparents used to do until mains water came along. And even in the UK with its reputation for a wet climate, people realise that mains water can be in short supply. Using rainwater when non-drinking quality water is not needed (outdoor use, WCs and washing machines) can save 50 per cent on mains water use and it makes sense for people to use water off their own roof.

Official pressure is on to reduce mains water consumption. With new Building Regulation Part G that came into force this April and the targets set by the Code for Sustainable Homes since 2007, reducing water consumption in new construction has become an obligatory requirement.

A report by the Environment Agency1 published in August suggested that treating and transporting mains water to the home has a less negative overall environmental impact than installing a rainwater harvesting system. However, many of the criteria used were based on older systems and now great improvements have been made in reducing the carbon footprint, in particular with regard to pumps and tanks.

Two distribution methodsRainwater is taken from the main storage tank to where it is needed from a choice of two distribution methods. With “direct feed”, a pressure sensitive pump in the storage tank maintains pressure on the rainwater pipes typically to toilets, washing machines and for outdoor use. The pump will be activated whenever water is drawn. Furthermore, the pump will try to maintain pressure in the pipe network even when no water is drawn and will, therefore, activate itself from time to time. These start ups obviously use power and this continual activity shortens the life of the pump.

In another type of direct feed

Marcus Bicknell argues that rainwater harvesting should be encouraged despite criticism in a recent report on the carbon footprint of such systems

It makes sense to harvest rain

ENQUIRY No. 136www.rainwaterharvesting.co.ukwww.raindirector.co.uk

Using rainwater on household appliances can save up to 50 per cent of mains water use

Marcus Bicknell is marketing and technical partner of RainWaterHarvesting.co.uk

system, the pressure-sensitive pump is in a console located in the house, sucking from the tank and delivering water to the appliances. Also in the console is a mini header tank, which provides mains water backup. However, pumps that suck are less efficient and again the pump is activated whenever water is drawn.

With an indirect gravity feed system, instead of supplying directly, the pressure-sensitive pump in the main storage tank supplies a plain header tank in the roof space and keeps it full using a classic ball cock and float. If the rain runs out, a lower ball cock and float admits mains into the header tank across an A-B gap. The advantage of this is that water is assured should there be a power cut. However, the pump works whenever water is drawn.

Where progress has really been made in reducing power use by the pump is in the development of a special header tank equipped with electric float sensors so that the pump is activated to refill the tank only when it is completely empty rather than every time water is drawn. In this way, about 5 times less electricity is used.

Use solar powerEven better are systems already in existence which use solar power to reduce electricity use to zero such as RainWaterHarvesting.co.uk’s Solar Rain Director.

Storage tanks made of polyethelyne (PE) tend to be used now rather than GP and concrete tanks. Furthermore many PE tanks do not need to be concreted in. Some tanks are now delivered in halves and assembled on site. This reduces transport requirements.

The Environment Agency Report also says: “There are often trade-offs and conflicts that need to be considered in sustainable design…. Rainwater systems should be considered both in terms of water management and energy and carbon implications.” Surely the trade-off in terms of water management is positive against the energy and carbon implications. Another recent study2 calculates that a standard rainwater harvesting system (without the advantages of a smart header tank or solar power), has lifetime emissions (30 years) equivalent to a 3.5 hour flight or 2050 miles of driving. These are not significant over 30 years, and already will be considerably reduced with newer systems.

These emissions are surely justified in terms of an overall saving in water use. Water is ultimately our most precious resource. Official pressure currently encourages rainwater harvesting but it places more emphasis on using water efficient appliances to reduce water consumption. The problem with this is that it can create a dramatic change in lifestyle by requiring consumers, for example, to have slow-flow taps, small baths, short showers. Encouraging consumers to use water-efficient appliances in conjunction with rainwater harvesting will make using less mains water a more attractive proposition. z

References(1) Environment Agency Science

Report 090018 “Energy and Carbon Implications of Rainwater Harvesting & Greywater Recycling”

(2)Rainwater Harvesting: Environmentally Beneficial for the UK?

CM Way*, DB Martinson**, SE Heslop*, RS Cooke***

* Civil Engineering Department, University of Bristol

** Civil Engineering Department, University of Portsmouth

*** Buro Happold, Camden Mill, Bath.

A Solar Rain Director control panel