In April this year, landfill tax will reach £56 per tonne. As a result, it has become imperative to find alternative solutions to waste management, particularly for biodegradable wastes such as food. Anaerobic digestion (AD) could be the answer.
Consider that, each year, the collection of food waste from household rubbish could yield up to four million tonnes of material suitable for AD while supermarket and food industry waste could deliver four times that amount. In addition, farming provides 90 million tonnes of manures and slurries.
This means that, in total each year, the UK produces more than 110 million tonnes of organic waste which AD could convert into gas. If this was made into energy, it could generate 20TWh or 7% of the UK’s anticipated renewable energy by 2020. Achieving this would require an estimated 1,600 farm-based AD plants (500kWe) and 300 larger food waste facilities (1,500kWe). But we currently have just over 30 on-farm and 20 off-farm plants.
So what is holding back the industry? Besides the challenges of securing a supply of feedstock and gaining planning permission, much comes down to cost. Simply transporting material to an AD plant can be expensive, thanks to its widespread distribution and the feedstock’s high water content. To make the economics of AD stack up, plants must be built near the waste source and benefit from having high capacity. This can be costly.
For example, GWE Biogas in Driffield, North Humberside, will use 50,000 tonnes of food waste each year to produce 2,000kW of electricity and the equivalent in heat - enough power for 2,000 homes. The cost of building the plant has been estimated
Most AD plants rely on private finance to get them off the ground, although some banks will lend money to cover installation costs. Government grants are also available, but taking them may leave an AD project ineligible for ongoing support from the feed-in tariff (FIT) and renewable heat incentive (RHI) schemes.
The FIT and RHI schemes support the long-term financial security of renewable energy projects by paying suppliers for each kWh of electricity and heat they produce. These new subsidies are making AD a much more commercially attractive option, and there is increasing interest in using AD as a source of biomethane for transport and injection to the gas grid. Under the right conditions, current payback periods can be between five to eight years for a large plant.
In addition, the Localism Bill has opened up the possibility of local authorities selling their own energy, which potentially allows them to link digesters to household waste processing facilities. This provides an alternative to composting - which is popular with waste managers due to the low equipment costs - but releases methane into the atmosphere and offers no opportunity for energy recovery.
AD could be a better solution for food and farm wastes, as Lucy Hopwood, AD expert at the National Non-Food Crops Centre (NNFCC), explains: “We believe AD is a more efficient option for waste mitigation than its alternatives. It can help local councils to meet their landfill targets and generate renewable energy, while reducing reliance on increasingly expensive fossil fuels. And, of course, the bottom line is that AD can be a valuable source of revenue.”n
Dr Matthew Aylott is science and technology writer at the NNFCC
More on the opportunities for investing in AD and the steps needed to meet regulations can be found at www.biogas-info.co.uk