[Sdpg] Better sewage treatment is the latest thing in clean energy Dec 30thThe Economist

Fri Jan 8 08:00:40 PST 2010

Renewable energy
The seat of power
Dec 30th 2009
From The Economist print edition
Better sewage treatment is the latest thing in clean energy
http://www.economist.com/sciencetechnology/displaystory.cfm?story_id=15172621
Illustration by David Simonds

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.
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.

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