The Environment Agency report claims while rainwater harvesting (RWH) and greywater recycling (GWR) systems may reduce water use, in most cases they result in significantly increased carbon emissions.

It calculates a typical rainwater harvesting system has a carbon intensity some 40% higher than mains water.

Equivalent greywater recycling needs twice the energy used to supply cleans mains water.

It also suggests there is scope to cut the carbon footprint of these systems through use of more efficient technologies.

The report says: “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 users via the storage tank.”

The report looks at embodied energy and energy used over a system lifecycle. Individual systems vary widely but in general running a typical rainwater harvesting system for 30 years used the equivalent energy to that of the rest of the house for one year – or a three per cent increase.

In commercial and municipal buildings such as schools and hospitals the system represented around one month’s operational energy-related emissions – a fraction of a percent of the total over the three decades.

The study’s authors say their work has identified an increase in emissions but they point out the research does not consider the advantages of rainwater harvesting and greywater 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 says.

For a complete picture this should be considered alongside reductions in mains water demand and foul water volumes along with other benefits such as reduced rainwater run-off and increased “resilience” to water shortages from on-site collection and storage.

David Gibbs

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