管理提醒： 本帖被 chenshuuu 执行加亮操作(2010-12-27)
Instead of consuming power to treat wastewater, municipalities should extract the energy trapped in its organic molecules, say U.K. researchers. They report that the energy content of household wastewater is significantly greater than previously thought. Wastewater, they report, contains enough energy in each gallon to power a 100-W light bulb for five minutes (Environ. Sci. Technol., DOI: 10.1021/es103058w).
The U.S. treats about 12.5 trillion gal of wastewater a year, a process that consumes about 1.5% of the nation's total electricity. Instead of processing and dumping wastewater, treatment facilities could convert wastewater's organic molecules into fuels such as methane or hydrogen gas, says Liz Heidrich, a graduate student at Newcastle University. They could even generate power directly via microbial fuel cells, which use bacteria that produce electricity as they consume molecules such as acetic acid.
Heidrich wondered how much energy was in the wastewater to recover. In the literature, she found data on solid waste's energy content, but discovered only one wastewater study, which estimated about 6 kJ of energy per L of wastewater. She thought that the calculation underestimated wastewater's total energy, because the researchers may have evaporated important volatiles such as acetic acid when they used an oven to extract organics from the wastewater.
So Heidrich and her colleagues turned to freeze-drying to conserve more of the energy-rich compounds inside the wastewater. The process was excruciatingly slow: Evaporating 2 L of wastewater—the amount needed for a decent-sized dry sample—took over a month. Using a bomb calorimeter to combust the freeze-dried solids, Heidrich found that the wastewater her team had collected from a domestic wastewater treatment plant in northeast England contained 7.6 kJ/L. The same wastewater dried with an oven contained 5.6 kJ/L, about 26% less energy than the freeze-dried sample. Yet, according to the researchers, even freeze-drying may lose volatiles. They estimate that the wastewater's actual energy content probably was closer to 10 kJ/L.
The researchers would like to speed up their technique so that other scientists could use it to measure a water sample's energy. Until now, researchers have relied on chemical oxygen demand, a simple calculation of the amount of organic chemicals in a sample made by measuring the oxygen required to oxidize the compounds. But Heidrich found that such measurements often didn't agree with the energy content that she measured more directly.
David Bagley, an environmental engineer at the University of Wyoming who performed the previous estimate, says the new study is well done and that its figures provide even more incentive to develop technologies to extract energy from wastewater. "We know it's worth the effort," he says. "We should stop messing around and just go get that energy."
Chemical & Engineering News
