Wednesday 6 January 2010

Better sewage treatment is the latest thing in clean energy

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.

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.

The second reason is that mixing the sludge moves bacteria away from chunks that they have been digesting and on to "fresher" material that has not had as much bacterial contact. The result is a quicker digestion of the whole. The Fraunhofer pump system, which has already been deployed in 20 sewage plants in Brazil, Germany and Portugal, needs to operate for only a few hours a day, so does not require a large amount of energy.

Sadly, that is not true of the approach used by researchers at the Tema Institute in Linkoping University, Sweden. They are developing a technique that employs ultrasound, rather than pumps, to break up the sludge. This increases methane yields by 13% but, at the moment, the process of generating the ultrasound consumes more energy than it yields.

The consequence of techniques such as these is that an ever-larger proportion of sewage is being used as a raw material for energy generation. Germans already process about 60% of their faeces this way, and the Czechs, Britons and Dutch are close behind (see chart). GENeco reckons the figure in Britain by the end of 2010 will have leapt to 75%—enough, when converted into electricity, to power 350,000 homes. And the latest thinking is to improve yields still further by cutting out the middle man. Faeces are food that has been processed by the human digestive system to extract as much useful energy as possible. An awful lot of waste food, though, never enters anyone's mouth in the first place, and this is an even more promising source of biogas.

In America in particular numerous sewage plants have begun processing undigested food in large quantities over the course of 2009. This is the result of a collaborative policy by the country's Environmental Protection Agency and its Department of Energy, to encourage the recycling of waste food in this way. In Britain, alas, public policy actually discourages such activity. Waste-water facilities there must pasteurise food scraps before they are processed, according to Michael Chesshire, the head of technology at BiogenGreenfinch, a company that modifies sewage digesters to use food scraps. That is a serious waste of brass.

December 31, 2009

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