Wastewater recycling ‘three times’ more polluting

Wastewater recycling produces more greenhouse gases than traditional water treatment processes but is still worth continuing, research has concluded.

The study found wastewater recycling plants emit around three times more nitrous oxide than traditional water treatments because of the high levels of denitrifying bacteria present.

Despite the production of nitrous oxide, the report concluded that wastewater recycling is still a good idea and proposed that recycled wastewater should be used to supplement drinking water supplies.

University of Cincinnati’s assistant professor of geology and geography, Amy Townsend-Small, who led the research, said: “If wastewater recycling can supplement drinking water resources and not just irrigation water for landscaping, then overall greenhouse gas emissions could be reduced, as could the potential for water scarcity in southern California and beyond.”

Ms Townsend-Small, who worked alongside a team of researchers from the University of California, explained how advanced methods in wastewater treatment are used to supplement fresh water supplies in areas where fresh water is scarce.

She added: “Climate change, caused at least in part by manmade greenhouse gas emissions, is expected to exacerbate freshwater scarcity in many of these areas, including the United States southwest.

“The same microbes that live in agricultural soils also thrive in wastewater treatment and water recycling plants. The same nitrification and denitrification processes are employed in these plants to remove nitrogen from wastewater.”

As part of the research, nitrous oxide emission rates were recorded in recycling plants in southern California to compare the nitrous oxide emission rates and to find out how different types of wastewater treatment affect emissions of the greenhouse gas nitrous oxide.

Nitrous oxide is a by-product of the metabolism of two different types of bacteria known as nitrifying bacteria, which converts forms of reduced nitrogen to oxidized forms of nitrogen, and denitrifying microbes, which converts nitrates into inert nitrogen gas.

The team analysed a conventional wastewater treatment plant, which works to remove organic carbon and return treated wastewater to a river or the ocean, and a wastewater recycling plant, which removes both organic carbon and nitrogen so the treated wastewater can be used for irrigation of landscaping and urban greenspace.

A report of the research is set to appear in the September-October 2011 issue of the Journal of Environmental Quality.

Carys Matthews

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