As a new scheme opens to supply 200 Oxfordshire households with gas made from what they recently flushed from their own toilets, a number of water and gas companies around the UK are now beginning to look at sewage in a completely different light.
The idea isn’t particularly new, of course. Camel dung has been burnt as a fuel for centuries, farm manures have been used to make gas for decades and human waste has been helping to provide the power for wastewater treatment plants long before anyone managed to make it clean enough to use in a domestic hob. It may not be quite the right topic to discuss over the dinner table, but there’s no escaping the fact that sewage and manure do make surprisingly good sources of energy.
Time to find out about the power of poo!
Energy Content
It’s been estimated that the world’s farm animals produce a staggering 2 billion tonnes of manure each year – and that’s after it’s been dried out! Each tonne of this dry dung has an energy content of between 12 and 18 gigajoules, which is roughly half as much as coal and calculations show that if only half of it – 1 billion tonnes – was to be harvested and used, it would be the equivalent of some 300 million tonnes of oil. That’s getting on for 10 per cent of the world’s overall oil demand.
While burning camel dung may work well enough outdoors in the Sahara, setting fire to dried human sewage isn’t such a practical proposition for a kitchen in Surrey and in any case, it would be just too gross to contemplate! Convenience, however, is only part of the appeal of turning human sewage into gas; weight for weight, the energy content is nearly double.
Turning Poo into Power
The key to harvesting the power of poo lies in a process called anaerobic digestion (AD for short) and the real stars of it are a special group of microorganisms which have the unique ability to turn sewage – and a whole range of other organic material too – into methane. It’s a bit like composting, except that the bugs responsible for making AD happen don’t live in air, so the vessel in which the sewage solids are digested has to be air tight, to avoid oxygen poisoning them – and besides, methane and air makes for a bit of an explosive mix!
Although there are a few different ways of achieving AD, and the operating details vary as a result, the general process involves filling the digester with a fairly thick slurry of sewage, heating it, to keep it at the sort of warm temperature that the microbes like and keeping it mixed. The actual digestion process has a number of stages and different kinds of bugs are responsible for each, with one group starting off by breaking down the original sewage to make the right ‘food’ for the next type, and so on until the whole thing is finally over. The methane-rich gas, which also contains carbon dioxide and a number of impurities, is collected and piped away, and at the end, the material left behind – called digestate for obvious reasons – is removed and sometimes gets used as a fertiliser.
Making Use of Biogas
Normally the biogas itself is burnt in on-site generators, providing electricity for the treatment plant, and the waste heat produced during generation goes to keep the digesters warm, helping the whole process to continue. Assuming the Didcot trial – which uses a cleaned and more purified type of gas – is as successful as everyone thinks it will, however, we may soon see more homes benefiting from this infinitely renewable energy source. If you really can’t wait until then to slash those fuel bills, there are even plans available to help you build your very own personal sewage-fuelled digester – always assuming that the whole idea appeals in the first place, of course. That said, it’s probably not for everyone!
