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.
“There are two main ways to do it. We either burn it in a ?high-temperature, low-emissions flare, which will convert the methane to CO2 but the energy will be lost. Or we utilise the inherent energy potential, the calorific value of the methane, and convert that into electrical 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. A recent Defra study reports that 78 per cent of people now support separate food waste collections. 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 – compacting the organic waste and placing it in digesters for bacteria to break it down – again producing methane for capture. 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.
In 2008, Welsh Environment Minister Jane Davidson indicated the Welsh Assembly Government’s enthusiasm: “Anaerobic digestion… is the most carbon efficient way of managing [food waste]. Not only does this have the advantage of producing renewable energy but it also produces a high-quality soil improver and fertiliser.” Last year, Defra followed suit, issuing a paper on shared goals for stakeholders relating to AD and the Scottish Government has likewise come out in favour of 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.
Graham Brown, President for ADI systems (a Canadian firm that has developed an AnMBR technique), explains the benefits: “This technology makes a very reliable biological system because anaerobic bacteria cannot leave the system as they cannot pass through the membrane. This means the anaerobic system cannot fail due to a loss of bacteria, which is the most common reason for failure of biological treatment systems. The AnMBR is much more efficient than a traditional anaerobic digester, producing more biogas and cleaner effluent than a conventional approach.”
Of course, although AD gets the lion’s share of attention, 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 best with dry organic content – less than 50 per cent water content is ideal – and 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. The parts that are driven off can be collected for use, usually as syngas.
Not much should necessarily be expected of pyrolysis in terms of its energy potential, though, 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. The process has so much potential, in fact, that it was identified as one of the ‘Breakthroughs for the 21st century’ that could transform the UK into a sustainable society by the Sustainable Development Commission. As a soil additive, biochar also increases fertility – limiting the need for fossil-fuel derived fertilisers – and improves water retention capability, reduces nitrogen leaching into the water table and neutralises acid soil.
Dominic Woolf, Associate Lecturer in Sustainable Development at Swansea University, has been conducting research into its potential. He agrees with Godley that a balance must be struck between ?energy generation and carbon sequestration: “If you maximise ?the amount of char you get you’ll get about a third amount of the energy. Basically, there’s a trade-off you get in the process. If you want lots of energy, you won’t get much char; if you want lots of char, you won’t get much energy.”
To date, AD has received a lot of investment, but if payments are offered for the sequestering of carbon as biochar, then pyrolysis could become attractive. Woolf, for one, is convinced that pyrolysis is viable on a large scale. “The big question in my mind is whether we as a society are prepared to pay to do that.
Stern was making predictions that we really have to spend something in the order of two per cent GDP to have any hope of fighting climate change. And, within that ballpark, it’s a reasonably cost-effective way of doing so.”
Pyrolysis is no panacea, but is a useful addition to the arsenal of organic waste treatment options. As Woolf 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.”