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Recycling Water Can Increase Carbon Emissions 2012-05-04

4 May 2012, 5:58 PM

Neil Smith, Group Research and Innovation Manager, NHBC, responding to her remarks, commented ¡§While grey water recycling and rainwater harvesting systems can reduce domestic water consumption, they can also lead to increased energy use and carbon emissions.

¡§New homes built to current building regulations already deliver a much better level of water efficiency than existing homes. Therefore, from an overall perspective of saving water in a simple and affordable way, installing water meters in more existing homes would have a far greater impact on reducing overall consumption without these unintended consequences than additional measures in the relatively small number of new homes.¡¨

New homes are currently built to comply with water use targets set out in Part G of the building regulations, which specifies a limit of 125 litres per person per day ¡V and deliver significant water savings well in excess of the UK average consumption of 150 litres per person per day. In addition, water metering is standard in new homes, whereas only about 40%* of existing homes have one fitted.

Although further improvement to the water efficiency of homes can be gained through grey water recycling (the re-use of bath and shower water) and rainwater harvesting ¡V normally for flushing toilets ¡V there are broader environmental issues that need to be taken into account. A study undertaken by the Environment Agency, the Energy Saving Trust and NHBC Foundation in 2010, Energy and carbon Implications of rainwater harvesting and greywater recycling, demonstrated that these systems can increase energy use and carbon dioxide emissions due to the energy used in their manufacture, installation, operation and maintenance.

Key findings of the report were..

Buildings using harvested rainwater or treated greywater typically increase greenhouse gas emissions compared to using mains water, where total cradle to gate embodied and operational carbon are considered. For example over 30 years, where an ¡¥average¡¦ 90m2 house has a RWH system with a polyethylene tank, the total carbon footprint is approximately 1.25 ¡V 2 tonnes of carbon dioxide equivalent (CO2e). This is similar to one year of energy-related emissions from a house built to Code for Sustainable Homes Level 3 energy efficiency standards. The footprints of systems applied to commercial buildings vary widely, but over a 30 year lifespan were found to represent around one month¡¦s operational energy-related emissions in the hotel, office and schools studied.

With one exception, the operational energy and carbon intensities of the systems studied were higher than for mains water by around 40 per cent for a typical rainwater application, and over 100 per cent for most greywater applications. The exception is short retention greywater systems which are around 40 per cent less carbon intensive than mains water supply. The assumed operational intensities of rainwater and greywater systems are based on the limited measured data and information available to this study.

There is scope to improve the efficiency and design of systems to reduce their carbon footprints. Storage tanks account for a large proportion of the embodied carbon footprint of rainwater systems; slightly less so for greywater. Pumps also make up a large proportion of rainwater and greywater embodied carbon and pumping determines net operational carbon. Direct feed rainwater systems have a large operational footprint because both rainwater and mains backup are pumped to end uses via the storage tank. Innovation in these and other areas could reduce carbon footprints. Manufacturers and suppliers should work quickly to reduce the footprints of their systems, and particularly to reduce the energy intensity of pumps and treatment systems.




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