At last, the UK is squaring up to the problem of organic waste. There’s no doubt that we need to do this – in 2007, landfill sites in the UK released methane totalling 20 million tonnes of CO2 equivalent emissions (CO2e) – the same as driving a car almost half a million times around the equator.
It’s well known that methane is a much more pernicious greenhouse gas than CO2, but it also has the potential to be a valuable source of energy. If not, there are measures to mitigate the impact it has. Currently, the UK sends 10 million tonnes of biodegradable municipal waste to landfill, but steps have already been taken to limit emissions at some sites.
“Historically it’s been the most successful renewable technology in the UK,” says Tim Otley, General Manager of SITA Power, which has 38 landfill gas production projects in the country that currently supply three per cent of Britain’s renewable energy.
In 2007/08, landfill gas was the second largest source of UK renewable energy production at 28 per cent. But landfill legislation means this is going to decrease. The signs are that we are going to have a lot of biodegradable feedstock to process. The question is: what will we choose to do with it? If we look to extract energy, the most commonly touted option is anaerobic digestion (AD), which essentially controls the process that occurs in landfill. AD is already used widely in the UK water industry – 66 per cent of Britain’s sewage sludge is treated this way – and is slowly gaining inroads in solid waste treatment, as governments throw their support behind it.
Although Defra has stated that “at full potential it is thought anaerobic digestion could produce enough electricity to power two million homes”, other estimates put AD’s energy-generating potential at just 350,000 households. However, a recent innovation might improve matters. Anaerobic Membrane Bioreaction (AnMBR) is a modified two-stage AD process. The first stage is a traditional digester, but the second stage more efficiently separates treated water, organic material and the gas that is produced by the process by using submerged membranes to break up the three elements. A series of rectangular, ultrafiltration membranes separate solids from liquid. By applying a slight vacuum to the membrane cartridges, operators can ‘pull’ clean water through the membrane, leaving all bacteria and suspended solids behind.
Of course there are other ways we can get energy (and more) from our organic waste. Pyrolysis has recently been attracting some interest. It’s a process that works by heating biomass in the absence of air. The heating drives off many constituent parts like hydrogen and oil tars, but, crucially, leaves some carbon behind in solid form rather than releasing it as global warming gases.
Not much should necessarily be expected of pyrolysis in terms of its energy potential according to Andrew Godley, Senior Consultant for AEA: “It produces a fuel but you’re not going to get the maximum energy from the fuel or if you do, you don’t then get a material that is carbon free or low carbon. I don’t think it would ever fundamentally change the energy production in the UK.”
Energy isn’t everything, though. Indeed, pyrolysis could fundamentally change how we manage carbon: the main benefit it has over other treatment options is the aforementioned solid carbon or ‘biochar’, which stabilises carbon that has been taken up by growing plants and keeps it out of the atmosphere. This can be significant: when a tonne of agricultural yard waste, for example, is treated through pyrolysis, it sequesters around 550 kilogrammes of C02e.
Pyrolysis is no panacea, but is a useful addition to the arsenal of organic waste treatment options. As Dominic Woolf, Associate Lecturer in Sustainable Development at Swansea University, asserts: “I would make the point that there is no one single process that is the best for all types of biomass for all places for all situations. The pros and cons will very much depend on what your waste stream is and what part of the world you’re in. With very wet waste it may make sense to use anaerobic digestion. If you’ve got poor soils they could really benefit by going down the biochar route.”
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