Study says water saving technologies increase carbon emissions

The research, compiled by the Environment Agency and supported by the NHBC Foundation and the Energy Saving Trust, is especially relevant since the Code for Sustainable Homes and Part G of the Building Regulations call for these water saving technologies.

Need to improve technology
The study suggests that it is possible to cut the carbon footprint of these systems by developing more efficient technologies and it urges suppliers and manufacturers to look at ways to reduce the carbon footprint of their systems.

"Storage tanks account for a large proportion of the embodied carbon footprint of rainwater systems; slightly less so for greywater," the report said.

"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 back-up are pumped to end users via the storage tank."

Dynamics of study
With the assistance of RWH and GWR systems suppliers, the water saving systems were tested in different buildings including homes, flats, a hotel, an office building and a school. The carbon footprints were calculated over 15, 30 and 60 year system lifetimes.

Four scenarios were used to explore the effects of future changes: changing water demand; mains water leakage and carbon intensity; emissions factors for UK grid electricity; and annual rainfall. 

The analysis of these scenarios found that only grid decarbonisation had a major impact on the carbon footprints of the systems studied.

The report analysed embodied energy and energy used over a system lifecycle. Over 30 years, a typical RWH system used the equivalent energy to that of the rest of the house for one year.

Benefit of water saving systems
Researchers said their work has identified an increase in carbon emissions as a result of these water saving systems, but they pointed out the research does not consider the advantages of RWH and GWR recovery.

"This study identifies a 'carbon gap’: a building with RWH and/or GWR systems has an increase in carbon emissions and so a larger carbon footprint," the report said.

 "However, for a complete picture this should be considered alongside reductions in mains water demand and foul water volumes, and other benefits such as reduced rainwater run-off, and increased 'resilience’ to water shortages from on-site collection and storage. The value of water demand reductions and the wider benefits of rainwater and greywater systems was outside the scope of this study."