Sewage For Energy, Too

: Image by karenwithak via Flickr

ON several quiet streets in Sheffield, a northern English city an hour from here, are street lamps that look like ordinary gas lamps, but do not burn ordinary gas. . Thus, they are both relics of the past, when gas lamps lighted our streets, and of the future, when excrement and wastewater will again be seen as a resource, not a waste.

“Wastewater” has always been recognized to have some value. In 1860, as waterborne sewer systems were becoming the norm, an alderman named Mechi told Farmer’s Magazine that “if the money value of our sewers could be shown to the British farmer in bright and glittering heaps of sovereigns, he would gasp at the enormous wealth, and make great efforts to obtain the treasure.” Mechi was talking about the fertilizing nutrients in human “waste,” which he thouInstead, their light comes from gas released from the sewers that run beneath themght were needlessly ruined by mixing excrement with water, but he might also have been talking about its wasted energy potential.

Sludge, the solids that remain after sewage has been cleaned into effluent, has a high B.T.U. content (a measurement of fuel’s energy); it burns efficiently and well. Other aspects of wastewater treatment can also reap energy: anaerobic digestion (whereby bacteria munch on the organic contents) produces methane, which with turbines can become combined heat or power. Microbial fuel cells can use bacteria to get electricity from sewage, while gasification, a high-temperature process, can reap fuel-ready gas from sludge.

When it comes to harnessing energy from wastewater treatment, it sounds as if we are spoiled for choice. Then you look at the numbers. Of the 16,000 wastewater treatment plants in the United States, about 1,000 process enough gallons (five million daily) to be able to generate cost-effective energy using anaerobic digestion. Yet only 544 use anaerobic digestion, and only 106 of those do anything more with the gas produced than to flare it.

If those 544 treatment plants generated energy from their sewage, the E.P.A. concluded in a 2007 report, they could provide 340 megawatts of electricity (enough to power 340,000 homes), and offset 2.3 million tons of carbon dioxide that would be produced through traditional electricity generation. In the effort to reduce greenhouse gases, the E.P.A. said, this would be equivalent to planting 640,000 acres of forest or taking some 430,000 cars off the road.