Animals produce huge amounts of methane, but it is hardly practical to collect it so the gas is released into the atmosphere – where, incidentally, it has a far more potent greenhouse effect than CO2. If only, sages reasoned, the digestion process could be made an out-of-body experience and the resulting gas collected for use.

Well in fact it can, and that is the principle behind anaerobic digestion (AD). Although the first AD was built by a leper colony in Bombay as long ago as 1859, the concept has, for the most part, remained the preserve of specialist sectors like agriculture, sewage treatment and waste management.

ADs have taken time to join the renewable energy mainstream alongside technologies such as wind and solar. Today, however, biogas fuels are coming into sharper focus as a potential contributor to the future energy mix. The drive for renewable energy coincides with pressures to dispose usefully of a whole range of organic wastes – food waste, animal by-products, farm waste, sewage, green garden waste etc – and without recourse to landfill. ADs can help meet both energy and waste management needs at the same time.

Using ADs to treat biodegradable waste results in a biogas that can be used to generate electricity and heat.

Environmentalists, not surprisingly, are keen on the concept. NGO Friends of the Earth, for instance, point out that producing renewable energy in this way helps tackle climate change instead of aggravating it through landfill and incineration. They argue that treating 5.5 million tonnes of food waste per year in ADs could generate between 477 and 761 GWh of electricity, enough to meet the needs of 164,000 European households.

The UK alone produces 10m tonnes of food waste per year and currently only 0.4% of this goes to AD. Friends of the Earth estimate that at least 0.36% of UK electricity could be generated by AD of food and other household wastes, much more if waste from non-household sources such as shops and restaurants could be included as well.

Compared with composting – which is organic breakdown in the presence of oxygen – digesting the 5.5 m tonnes anaerobically would save 0.22 m–0.35 m tonnes of CO2 equivalent, assuming that the biogas produced is used for electricity generation. Using AD to treat sewage waste provides energy while emitting 16% less carbon than conventional sewage treatment.

And when used with mixed municipal waste, AD preceded by mechanical sorting provides a solution known as mechanical biological treatment (MBT). Recyclable and non-digestible materials are first separated from the waste mechanically, the remainder then being available for anaerobic digestion. The final AD digestate residue is biologically inactive so that it can go to landfill without releasing further methane.

The gas generated by an AD is actually a mixture of methane and CO2, in a ratio of about 60/40 to 70/30. A scrubbing process can be used to remove the CO2, which has industrial and food uses, leaving a marketable, clean burning fuel that resembles natural gas. Burning this biomethane in a combined heat and power (CHP) plant produces effective renewable-only heating and electricity.

AD-CHP plants are well suited to distributed generation schemes, with local siting of small plants reducing fuel transportation and its associated pollution. Alternatively, the fuel can be burned in stationary engines or in vehicles.

Where is AD taking place?

AD usage is popular in parts of Europe, and around the world:

• In Denmark, for instance, AD plants operated by farm cooperatives provide district heating for local communities as well as electricity;
• Thousands of ADs in Austria and Germany digest manure, food waste and energy crops, producing biogas that is then used for electricity generation;
• In Sweden, biogas plants produce fuel for fleets of town buses;
• Extremely large AD plants exist in the USA;
• What is claimed to be the world's largest, however, is found in Germany at Penkun, where a 15 hectare plant feeds up to 20 MW of electricity to the national grid, enough to supply 40,000 households. This facility, with its multiple 50 m diameter digestate tanks and other plant, is more reminiscent of a refinery than the conventional idea of an AD (see image).

What is the process for AD?

Anaerobic digesters range from small, simple systems used in homes and smallholdings in developing countries through farm-scale digesters to sophisticated industrial-scale units found in developed nations. Designs vary according to the feedstocks and bio-outputs intended, as well as the planned scale of operation. Because the AD process is biological, careful seeding and control of micro-organisms is a prerequisite for high efficiency:

• The process stream typically starts with shredding, giving digester microbes more surface area to work on. Shredded material then enters the digester itself. This comprises one or more sealed airtight chambers that have been seeded with appropriate organisms.
• Acidogenic bacteria first break the waste down into simpler molecules along with ammonia, CO2 and hydrogen sulphide (H2S) by-products. Acetogenic bacteria digest the simple molecules to produce CO2, hydrogen and acetic acid.
• In a third bacterial action, methanogens act on the acid to produce methane, CO2 and water.
• Digestion rates are optimised by keeping the temperature at 30–60 degress c, and acidity to 5.5–8.5 pH. The system can be self-powered by the exothermic digestion process itself, though usually with additional heat provided by burning some of the biogas.

Gaseous emissions are low because the process is enclosed. Although some nitrogen oxides are produced when biogas is burned, AD-CHP emissions are generally lower than for other forms of waste disposal.

Health risks are low provided the wastes are separated at source so that no contaminants can enter the system. Plant maintenance can be an issue, particularly where metals are used, due to the warm acids and other corrosives that are part of the process. Use of stainless steel, high-grade plastics, composites and other corrosion-resistant materials secures durability, albeit at some cost.

Advances in AD – minimising waste

In some countries (Britain is one example), AD exploitation lags behind that in other European countries – though the sector is not negligible.

One problems is that AD often has to be installed on a substantial scale before it can be ‘economic' in the conventional sense of the word. To address this and other issues, increasing professionalism is characterising the evolution of AD.

Organic Power Ltd, a British company that was formed in 1997. The company was convinced that a major inhibitor to the AD's development were technical problems arising from waste handling – before and after digestion – and from traditional cylindrical tank designs.

Working with (and adapting) ideas and patents originated by Maltin Pollution Control Systems over the previous three decades, led to the present Maltin system, marketed as a “21st Century approach to AD”.

Features include advanced feedstock preparation, input of the minimum amount of energy needed to optimise efficiency, and ability to process a range of different feedstocks in a multi-tank digestion system.

Naturally occurring bacteria are used to digest diverse organic materials, including most wastes and energy crops, to yield high quality methane, food-grade CO2 and clean fertiliser.

The company's philosophy is that final waste is the result of poor or incomplete processing, and its technology controls the system to avoid this happening: “AD is the only solution that ‘ticks all the boxes', being carbon neutral; providing total energy recovery; eliminating waste; offering 100% water recovery; enabling heavy metals to be recovered from the digestate; and yielding clean fertiliser as well as high-quality biogas and CO2”. So producing biomethane from liquid organic waste in this way is better for the environment, asserts the company, than any other energy source include hydrogen, wind power, biogas made from dry manure, biogas made from crops such as rapeseed and sunflowers, fossil natural gas, conventional diesel and petrol, and bioethanol made from sugar beet.

Since 2003 Organic Power has been part of a European consortium of 11 partners in 6 European countries, which has been investigating how AD can best be used to provide a versatile, low cost, carbon-neutral fuel in an environmentally sound way within a sustainable agricultural framework (for more information see http://www.cropgen.org/).

And with a particular focus on vehicle fuels, the company has worked with Daimler Chrysler in the development of Mercedes EcoVito gas-powered vehicles, and is part of the EU-funded Market Development for Gas Driven Cars (MaDeGasCar) project, running from September 2007 to January 2010. This is aimed at developing the market and infrastructure for gas driven vehicles.

What are the best feedstocks for AD?

Another AD and biogas specialist Greenfinch Ltd points out that where the primary aim is to maximise biogas production, cattle and pig slurry may not be the best feedstock, because this is what is left when animals have digested readily-digestible elements of food, and produced their own biogas. Better, says the company, to use food waste or add some wet energy crops such as grass silage, whole crop cereals, or maize. Operators of farm digesters can consider importing food waste from other local sources. Meeting the onerous requirements of food waste legislation inevitably increases capital and operating costs, but AD operators can offset these by charging gate fees to those disposing of waste.

Greenfinch has been a key contributor to a project aimed at providing a large-scale biowaste digester in the UK. The industrially-sized AD processes up to 125 tonnes per week of kitchen waste, garden waste and cardboard collected form 19,000 households in South Shropshire.

Located in Ludlow, the South Shropshire District Council facility generates biogas that is fed to a CHP unit. Around 150 kW of electrical output is fed to the national grid. The unit also produces biofertiliser used in local agriculture to improve soil quality. Public cooperation in separating out kitchen and garden wastes and putting them into the appropriate bins for collection ensures that the right mix goes into the digester.

The AD project, funded by Advantage West Midlands (the local rural development authority) and the Department for Environment Food and Rural Affairs (DEFRA), includes a visitor centre used to demonstrate to other local authorities how they might divert biodegradable waste from landfill.

Despite having the capacity to treat some 5,000 tonnes per year of waste, the unit is seen as a pilot-scale plant in preparation for a potential facility about three times its size. The existing plant comprises a waste reception area, buffer storage area, pasteurisation plant, AD unit, digestate processing unit, digestate storage unit as well as shredders, pumps mixers, the CHP unit, instrumentation and controls.

Meanwhile, a new trend has seen supermarket branches send their food wastes and other biodegradable products to AD plants. One example, again in the UK, is Waitrose, which sends waste to a plant run by Biogen in Bedford. The gas generated is used to produce power. Waitrose recycling waste manager Arthur Sayer says the signs so far are positive, and he believes that the AD will prove to be a “sustainable way of eliminating the need to send waste food to landfill”.

And in the south west of the UK, what is claimed to be Britain's first centralised AD facility, at Holsworthy in North Devon, produces enough electricity for some 3,600 homes, whilst also providing heat for a health centre, hospital, swimming pool, school and council offices.

The company responsible, Andigestion Ltd, says that feedstocks include local farm slurry plus waste from industrial bakers, food processors, abattoirs, cheese producers, biodiesel manufacturers and councils. Two 8,000 m3 digesters can process 80,000 m3 per year of organic material, generate up to 2.7 MW of electricity – some 800–1,000 MWh per month. Around 90% of the electricity generated is exported to the national grid, the other 10% is used to power the plant itself.

Down on the farm

Farmers around the world are showing interest in ADs as a result of higher energy and fertiliser prices, the growing costs of complying with waste disposal legislation in many countries and the continuing need to diversify in order to maintain farm incomes.

Sentiment has previously been depressed by poor experience with earlier digesters that proved insufficiently robust, the uncertain value of renewable electricity, the regulatory environment and difficulties with connection to the electricity grid in some places.

Now, however, the potential of biogas as a fuel for machinery and vehicles is being better appreciated, while that for generating heat for polytunnels and animals in rearing units is not lost on farmers either.

AD-CHPs can help rural communities to become self-sufficient in energy. Towns should benefit too, as waste disposal technology maintains its shift away from landfill towards more sustainable options. Over and above the economic drivers, AD operators gain from the fact that greenhouse gases are substantially reduced and AD can make a positive contribution to the environment.