Anaerobic digestion – a global problem-solver

Anaerobic digestion – a global problem-solver
One of the main selling points of anaerobic digestion (AD) is its versatility. Unlike many other renewable technologies, it doesn’t just generate energy – it’s also helping to treat a variety of issues right around the world…

Improving sanitation
While AD may have proven its environmental and economic worth across a range of European business sectors, in developing countries it is changing the lives of people who have a far more basic need. This is no more evident than in India, where according to the 2011 national census almost half of the country’s 247m households lack access to a toilet, leading to outbreaks of diarrhoea, dysentery and other serious bacterial infections. Based on a two-pit, pour-flush design, household toilets developed by Sulabh International store solid waste that can then be used as a natural fertiliser. The company’s public toilet complexes, meanwhile, are connected to anaerobic digesters that produce biogas for lighting, cooking and generating electricity.

Turning waste into an asset
In Malaysia, the problem lies with a different kind of waste. The country’s palm oil industry uses large quantities of water in mills where oil is extracted from the palm fruits. The palm oil mill effluent, or POME, is usually discharged into the country’s waterways, where it causes significant pollution. It is estimated that for every tonne of crude palm oil produced, up to 7.5 tonnes of water ends up as POME. However, as POME contains a high organic content, it makes an ideal feedstock – rather than polluting the environment, it can be processed through an AD plant, generating renewable energy and biofertiliser. Such is its potential that in Malaysia, all palm oil mills are required to have an on-site biogas plant by 2020 or face closure.

Combating aquatic pollution
In Uganda, it’s a naturally occurring problem in water that’s posing a challenge. Water hyacinth, which has spread from South America and can double its mass in a fortnight, is affecting fresh water bodies and choking the livelihoods of those who rely on it. When repeatedly harvested, the weed can produce ­80 tonnes of dry matter per hectare a year, making it perfect for processing in an AD plant. A pilot biogas project is being established in the Ssese Islands on Lake Victoria, home to 42,000 people. As well as helping to free the lake from the pervasive weed, once digested, the harvested water hyacinth will provide energy to local communities.

Treating domestic food waste
While the USA has more than 2,000 biogas plants, the majority of these are at wastewater treatment and landfill sites. However, as the world’s largest consumer nation, accounting for an astonishing 29 per cent of the global market, the potential for wider-ranging deployment across the business and domestic sectors is huge. Philadelphia’s Mayor, Michael Nutter, has realised this and recently signed a law requiring in-sink food waste disposers to be installed in all of the city’s new residential buildings. The disposers convert food scraps into a slurry that passes through pipes and sewers to water resource recovery facilities, where digesters turn it into biogas. The resulting digestate is being sold as a first-class fertiliser to farmers.

Making soils richer
UK research published by WRAP as part of the DC-Agri project recently confirmed that ‘the increased nutrient supply from organic materials, including digestate and compost, can produce higher crop yields of equal quality to crops grown with bagged fertilisers’. British dairy farmer and cheese-maker Stephen Temple realised the potential of using digestate as a fertiliser to boost crop yields many years ago. His 170 kW digester, built in 2009, processes slurry from the farm’s 100 milking cows, whey from cheese-making, maize silage, energy beet and wholecrop cereals. As well as bringing a financial return, the AD plant produces digestate which provides a nutrient benefit, improves soil structure and reduces the use of manufactured fertiliser, as Stephen explains: “There is a considerable return from the sale of electricity and savings made by not having to buy energy, but the digestate also delivers reduced fertiliser costs, and improves the fertility and value of our farmland.”

 Reducing greenhouse gas emissions
Reducing the amount of synthetic fertilisers used in agriculture will bring significant environmental benefits. However, researchers in Hungary have shown that by capturing methane – a gas that is 21 times more harmful than carbon dioxide – during the digestion process, AD itself also has the ability to slow the effects of climate change. A study of the lifetime emissions of a 600 kW Hungarian AD plant established that the carbon footprint of the complete energy production life cycle was 208,174 kg CO2e. If the country’s fossil-fuel energy facilities produced the same quantity of energy, GHG emissions would be 15 times higher.

Decarbonising transport
Other European countries have already made great strides in reducing their dependence on fossil fuels thanks to AD. Government backing in Sweden has led to buses, trains, lorries and cars all using biomethane, upgraded from biogas, as a vehicle fuel, with filling stations across the country being adapted to meet the demand. Sweden’s ambition is to be the first fossil-free European country, something that the European Parliament wants to see driving change in other Member States.

Improving marine environment
The Environment Committee of the European Parliament said last year that advanced biofuels sourced from seaweeds and other wastes should account for at least 1.25 per cent of energy consumption in transport by 2020. Denmark is already exploring how it can use seaweed – which causes foul odours on public beaches – as an AD feedstock, and then upgrade the biogas for injection into the transport network. The project is still in its early stages, but scientists believe that as well as the obvious benefits of cutting transport emissions, removing the seaweed would also create a better marine environment and reduce nitrogen load to the sea.

Anaerobic digestion is clearly far more than just a renewable energy technology. As these examples demonstrate, its potential to help solve problems in all corners of the globe, for all kinds of businesses and communities, is huge. This is only the beginning…

Processing palm oil mill effluent through an AD plant turns this pollutant into an asset

UK farmer Stephen Temple is reaping the rewards of digestate

Water hyacinth is strangling the livelihoods of Lake Victoria’s communities

All new buildings in Philadelphia are to be fitted with food waste disposers linked to anaerobic digesters

Biomethane is a key transport fuel in Sweden