The Weekly Geek: anaerobic digestion

Posted by bex — 20 February 2008 at 12:59pm - Comments

Ken Livingstone wants it for London, Hilary Benn is giving money to it and Adam and Debbie are bringing it to Ambridge. After a couple of millennia in the sidelines, anaerobic digestion has finally hit the big time (well, The Archers, anyway) - which is why we've chosen it for this second edition of the Weekly Geek.

Every year, we bury thousands of tonnes of waste food in landfill sites around the UK. We produce almost one and a half million tonnes of sewage a year (don't do the maths - it's disturbing), which is mostly spread on land, incinerated or buried as landfill. And we produce enormous amounts of agricultural waste on our farms. All of this waste breaks down to release greenhouse gases as it decomposes.

In all, about half of our total landfill comes from biodegradable waste, where it becomes part of the problem that contributes to climate change. Instead of sending it to landfill, anaerobic digestion allows us to convert this waste into ‘biogas', making it part of the solution.

Anaerobic digestion can help us to replace fossil fuels, reduce methane emissions from landfill sites and increase the efficiency of our energy system. As well as helping us to fight climate change, it can solve many of our waste management problems, reduce freshwater pollution from organic wastes, increase fuel security and reduce our dependence on chemical fertilisers.

This is animation from EfficienCity outlines how it works:

The organic matter used can be pretty much any biodegradable material: food waste from households, markets, shops, restaurants, caterers, breweries, distilleries, industrial kitchens and companies that process food and drink; abattoir waste; agricultural waste like manure, slurry, straw, feathers and crop residues; industrial waste and residues from, say, pharmaceutical processes or paper manufacturing; and sewage sludge.

After being collected and stored in a sealed reception hall, shredded by a huge industrial shredding machine, this matter is pasteurised in a tank at about 70degC for an hour to eradicate any lurking nasties like salmonella or E. Coli before being pumped into the main digester.

In the sealed digestion tank, micro-organisms break the matter down (digestion) in the absence of oxygen (anaerobic). By releasing enzymes, bacteria convert the matter into fatty acids, hydrogen and acetic acid, eventually producing methane and carbon dioxide.

The composition of the matter determines how long it is digested for and at what temperature; liquid wastes are usually broken down for 15-30 days, heated to 30-35 oC whereas solid wastes usually take 12-14 days heated to 55 oC.

While the biogas produced is mostly methane (60-odd per cent) and carbon dioxide (about 40 per cent), it still contains some trace gases. So, before the gas is burned, these trace gases are removed. The end result, biogas, can either be burned as it is, or can be turned into ‘biomethane' by removing the carbon dioxide.

After being compressed, the biogas or biomethane is ready to be used. Obviously, the best place to do this and make the most out of the energy is to burn it in a combined heat and power plant - the most efficient way possible to burn a fuel - where it generates both electricity and heat:

On top of producing a renewable, very low carbon fuel, anaerobic digestion produces a useful by-product: a high nutrient solid ‘digestate' which can be used instead of normal compost and inorganic fertiliser. So, it can help stop climate change, solve our waste problems and produce biofertiliser. Not a bad result for a load of old muck. It's also far quicker to build than most other kinds of energy plants (in around two and a half years).

So why aren't we using it?

Well, we are - to a degree. But while a few places have built anaerobic digesters to supply district energy schemes or the national grid, as a country, we've been left way behind our European neighbours. For a change.

According to Defra, the UK has fewer than 20 agricultural anaerobic digesters producing electricity. Compare this to Germany, which already has over 3000 and where anaerobic digestion is the fastest growing renewable technology (digesters are being built at a rate of about 1000 every year). Germany has also taken the first steps towards providing a ‘biogas feed in law' which will allow anaerobic digesters to feed biogas into the natural gas network - exactly the kind of bold regulatory measures the UK needs to emulate if our renewables industry is to be allowed to thrive.

While Hilary Benn's £10m funding for showcasing commercial scale anaerobic digestion is all very nice, the technology has already been showcased around the world; now we need to actually start building it on a large scale.

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