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Energy and Carbon Implications of Rainwater Harvesting and Greywater Recycling (2010)

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This report by the Environment Agency presents the findings of a study that examined the energy and carbon implications of rainwater harvesting and greywater recycling systems for non-potable use. It quantifies lifetime carbon footprints of rainwater harvesting and greywater recycling systems and the contribution to reducing CO2 emissions associated with mains water demand and foul water volumes. The study showed that buildings using harvested rainwater or treated greywater typically increase CO2 emissions. Apart from one system, operational energy and carbon intensities of the systems studied were higher than for mains water.

Executive summary

This report presents the findings of a study into the energy and carbon implications of rainwater harvesting (RWH) and greywater recycling (GWR) systems. The Environment Agency (EA) commissioned the review jointly with the
Energy Saving Trust (EST) and National House Building Council (NHBC) Foundation.

This study quantifies:

  • Lifetime carbon footprints of RWH and GWR systems, consisting of embodiedcarbon and the carbon emitted from operational use; and
  • The contribution of RWH and GWR systems to reducing carbon emissionsassociated with mains water demand and foul water volumes. The key messages from this study are:
    1. 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 –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.
    2. 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.
    3. 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.
    This study focuses on the energy and carbon implications and mains water savings of rainwater and greywater systems that supply water for non-potable use in buildings and are commercially available in the UK. It does not include any other environmental, social or economic costs and benefits assessment.