Bygrave Lodge anaerobic digestion (AD) plant at Weston Bygrave, UK [Image Source: Peter O'Connor, Flickr]
Anaerobic digestion (AD) involves a number of processes in which microorganisms break down biodegradable material in the absence of oxygen in order to either manage waste and/or generate bioenergy. In nature, it is the main source of methane, as Italian scientist Alessandro Volta discovered in 1776.
Nowadays, AD is used in the water treatment industry to treat sewage sludge and also by the waste industry to convert landfill gas into energy. However, this latter application is not currently being used to its full potential, particularly given the amount of food wasted by producers and consumers every year. For this reason, there is huge potential for the generation of biogas as a form of renewable energy. This can either be used as fuel, for example in combined heat and power plants or be converted to biomethane, which can in turn be used as a replacement for natural gas. The remaining digestate can also be used productively – as fertilizer.
In recent years, the recycling of waste combined with new technology has meant that capital costs for AD plants have fallen, with the result that a number of governments around the world are now looking at AD seriously, including the UK, Germany and Denmark.
In the UK, the AD sector, outside of the water treatment industry has grown by 622 percent, according to Charlotte Morton, Chief Executive of the Anaerobic Digestion and Bioresources Association (ADBA), writing in Biocycle Magazine last year. Over the same period, energy generated from sewage gas has increased by a quarter. Around 100 new AD plants opened in the UK last year, across all sectors, including water treatment. This means that the AD industry now has over 456 MW of electricity and biomethane capacity from 396 plants across the country, more or less equivalent to the power produced by one of the UK’s nuclear power plants at Wyfla, which is in the process of being decommissioned. ADBA believes the potential for growth in AD is huge. It could be generating in excess of 80 terrawatt hours (TWh) of green gas, equivalent to about 30 percent of the UK domestic gas demand, as well as generating 35,000 green jobs and contributing about £3 billion ($4.6 billion) to the UK economy. Nutrient rich biofertilizer from the industry could be worth around £200 million ($309 million) to UK farmers while AD could also help to reduce UK greenhouse gas emissions by 2 percent.
Recycling of food waste alone through AD could produce 9.3 TWh per year by 2025. Currently, over 91 food waste AD plants are now in operation, with many more due to be developed. These plants typically receive food waste from food and drink manufacturers, food processing companies, hospitality and food service outlets such as pubs, cafes, restaurants and hotels, local authorities (via household food waste collections), schools, colleges and universities, hospitals, supermarkets and retail stores.
UK Energy Secretary Amber Rudd MP formally opens Nestle's AD plant in Newcastle [Image Source: Department of Energy and Climate Change (DECC), Flickr]
Biomethane plants can produce clean fuel for vehicles and there are now around 30 biomethane plants in operation across the UK, with the number of UK gas refueling points supplying biomethane doubling year on year for the past three years to 11 locations in total. In addition to significant demand from vehicle fleet operators, the European Commission (EC) last year produced a new regulatory framework that could require EC Member States to develop publicly accessible refueling points every 400 kilometers (250 miles) by the end of 2025.
AD plants on farms are increasing in number, with 139 plants in operation last year. Thankfully, farmers are increasingly recognizing the benefits of the technology as a means of processing manures and slurries in addition to generating renewable energy.
However, as with other renewable energy sectors, growth in AD will only come if there is a stable policy environment, including provision of incentives, in order to maintain certainty and confidence for investors. With regard to vehicle fuel, at present, the UK still has much to do before it will be able to meet the Renewable Energy Directive (RED) 10 percent renewable transport fuel 2020 target. Continuing debate around Indirect Land Use Change (ILUC) and the use of crops for biofuel in general is hampering efforts to reach this target.
The Roe family, who live near Bridgwater, Somerset, have developed a successful business from turning to AD. They farm 158 hectares of land in south west England, the family having been in farming for more than 100 years, focusing largely on arable crops, particularly cereals. When the potato market declined in the late 1990’s, they switched to storing food products, but now, faced with rising electricity costs, they have looked at anaerobic digestion (AD) as a means of covering their energy requirements.
Electricity previously accounted for a quarter of their outlay, but in 2007 those costs looked as if they were about to rise by 72 percent above the previous two years. The Roe’s responded to this by developing one of the largest AD operations in the country which is now generating the equivalent of 9,300 kilowatts of electricity and biogas, based largely on home-grown maize. In 2012, they increased the amount of electricity exported to the grid to 2 MW and in 2013 a combined heat and power unit increased that capacity to 3.8 MW. In May 2015, a gas-to-grid plant started producing power which has enabled them to generate 3.2 MW of electricity and 1,200 cubic metres of gas, using 60,000 – 65,000 tonnes of food waste per year. A number of local farmers have agreed to supply bioenergy crops such as maize and beet.
A maize sileage digester near Neumunster, Germany [Image Source: Wikipedia Commons]
The use of beet helps to reduce waste, whereas maize can lead to a fibrous mixed waste generated by the mixing of scraps of plastic food packaging with non-digestible material from the maize. This is very difficult to recycle and is usually sent to landfill. Furthermore, beet can generate 30 tonnes of biogas per acre, compared to only 16-18 tonnes per acre for maize. Energy beet varieties such as KWS Gerty and Linova can generate some of the highest biomass yields per acre, 25 percent to 45 percent more than a fodder beet variety. They can also produce more gas per kilogram of root. Lab tests at Cannington have shown that a typical fodder beet variety yields 112 cubic meters of gas per tonne compared to 160 cubic meters per tonne for energy beet.
Some UK politicians are now starting to push for expansion of the industry, by helping to raise awareness of the benefits of AD, while ADBA itself has called on the Chancellor to allocate £25 million to help boost research in biogas, digestate and bioproducts. Meanwhile, recent discoveries at universities in the UK and in Australia have found that fungi in the digestive tracts of cattle could help to make AD and biogas production more efficient.
With the right incentives from government, this sector could have a vibrant future ahead of it.
Written by Robin Whitlock