Jan 222011
Structural formula of the methane molecule, CH...
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The seat of power

WHERE there’s muck, there’s brass—or so the old saying has it. The cynical may suggest this refers to the question of who gets what, but thoughtful readers may be forgiven for wondering, while they are recovering from the excesses of Christmas in the smallest room in the house, what exactly happens when they flush the toilet.

The answer is encouraging. Less and less waste, these days, is actually allowed to go to waste. Instead, it is used to generate biogas, a methane-rich mixture that can be employed for heating and for the generation of electricity. Moreover, in an age concerned with the efficient use of energy, technological improvements are squeezing human fecal matter to release every last drop of the stuff. Making biogas means doing artificially to faeces what would happen to them naturally if they were simply dumped into the environment or allowed to degrade in the open air at a traditional sewage farm—namely, arranging for them to be chewed up by bacteria. Capturing the resulting methane has a double benefit. As well as yielding energy, it also prevents what is a potent greenhouse gas from being released into the atmosphere.

Tanked up

Several groups are testing ways of making the process by which faeces are digested into methane more efficient. GENeco, a subsidiary of Wessex Water, a British utility company, uses heat. Instead of running at body temperature, the firm’s process first stews the excrement at 40°C for several days. It then transfers the fermenting liquid to a tank that is five degrees cooler.

This two-tank system produces more methane than conventional methods because different strains of bacteria, which chew up different components of faeces, work better at different temperatures. The result of giving diverse groups of bugs a chance to operate in their ideal environments is, according to Mohammed Saddiq, GENeco’s boss, about 30% more methane from a given amount of excrement.

In Germany a team at the Fraunhofer Institute in Stuttgart, led by Walter Trösch, is using a different approach. Dr Trösch has reduced the amount of time it takes to digest sewage from two weeks to one, by employing a pumped mixing system. This works faster than traditional methods for two reasons. The first is that stirring the sludge causes methane to bubble to the surface faster. From the bacterial point of view, methane is just as much of a waste product as faeces are from the human viewpoint. Encouraging this poison to escape allows the bacteria to survive longer and thus produce yet more methane.

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