In previous editions of The Moorlander, we have been looking at the future of green energy in Britain. So far we have covered: wind, solar and tidal power. Now, in our last piece in this series, we turn our attention anaerobic digestion and the production of biogas.
What is anaerobic digestion?
The term anaerobic digestion (AD) refers to a special treatment applied to organic materials. It can be applied to a range of natural biodegradable materials, including food waste, slurry, sewage sludge and manure.
This material, known as biomass, is naturally broken down until it emits a new gas – known as biogas.
Biogas is a methane-rich gas, comprising of around 60% methane and 40% carbon dioxide. This gas can then be used to generate energy.
AD is recognised by the government, Defra, the Welsh Assembly, the Scottish Parliament, Friends of the Earth and the National Farmers Union as one of the best methods for food waste recycling and dealing with farm waste and sewage sludge.
The word anaerobic actually means ‘in the absence of oxygen’. The biogas naturally created in the sealed tanks is used as a fuel in a combined heat and power (CHP) unit to generate renewable energy, ie: electricity and heat.
What’s left from the process is a nutrient-rich biofertiliser which is pasteurised to kill any pathogens and then stored in large covered tanks ready to be applied twice a year on farmland in place of fossil fuel-derived fertilisers.
Every tonne of food waste recycled by anaerobic digestion as an alternative to landfill prevents between 0.5 and 1.0 tonne of CO2 entering the atmosphere, one of the many benefits of anaerobic digestion.
How does it work?
The process takes place inside an anaerobic digester; a large, sealed tank which is void of oxygen.
The biomass is heated to around the temperature of blood, when it will react with the naturally occurring microorganisms and bacteria.
It goes through four stages: hydrolysis, acidogenesis, acetogenesis and methanogenesis. The end result is that the biogas is emitted, and a material called digestate is left behind. Both the gas and the digestate material can be
re-used, therefore making it a very effective way to recycle waste materials.
What are the benefits?
AD provides the opportunity to turn waste products into clean energy. It can be utilised by any industry that produces food or sewage waste, including agricultural, sewage and food processing and there are different sized systems available dependent on the amount of waste produced.
The methane-rich biogas that is generated can be used as a source of renewable energy to power electricity generators and provide heat.
It can even be altered further and upgraded to filter out much of the carbon dioxide – the end result is biomethane, which can then be used as vehicle fuel or to provide gas.
Plus, the digestate can be used as fertiliser that is suitable for organic farming systems. By utilising anaerobic digestion, the amount of waste you send to landfill can be dramatically decreased.
This in turn helps to reduce emissions of harmful greenhouse gases, because biodegradable material that is simply sent to landfill will emit a large amount of methane and carbon dioxide into the atmosphere if it is simply left to rot.
Anaerobic digestion in the UK
There are now 579 operational anaerobic digestion plants in the UK, including 88 biomethane-to-grid plants, and a further 331 anaerobic digestion projects under development.
According to a new study recently published by Britain’s largest gas distribution network, renewable gas could heat up to 15 million homes in the UK every year by 2050.Titled The Bioenergy Review, the report was commissioned by Cadent to estimate how much of the UK’s energy demand could be met by renewable gas.
The study concludes that bioenergy in general, and renewable gas in particular, can make a significant contribution to meeting 2050 climate change targets, especially when supporting decarbonisation of the heat and transport sectors.
Like with all of the renewable energy options discussed so far in this series, the production of biogas through anaerobic digestion has its detractors. At this point, I’ll pass over to Dr Phillip Bratby to tell you more.
Anaerobic digestion (AD) is a process in which microorganisms break down biodegradable organic material in the absence of oxygen and produce methane and carbon dioxide.
The process is carried out in a sealed tank. Small-scale on-farm anaerobic digestion has been used successfully for many years. Farmers fed an AD tank with manures, slurries and waste crops and the resultant methane could be burnt to heat the farmhouse and farm buildings. The waste product, called digestate, is a fertiliser which could be spread on the land instead of the manure or slurry.
However, problems started to arise ten years ago after the government interfered and decided that, since AD was a source of renewable energy (electricity and heat), it should be heavily subsidised to help meet EU renewable energy targets. Subsidies were set at three to four times the wholesale price of electricity – another scheme increasing energy prices and fuel poverty. This was yet another ill-thought out renewable energy scheme, the unintended consequences of which soon became apparent.
Developers quickly realised that there was big money to be made from the massive subsidies. Instead of small-scale on-farm AD plants, the developers realised that big-scale, centralised AD plants were necessary. Also, because there is little energy in manures and slurries (the stomachs of farm animals would not be doing their job properly if they did not extract as much energy as possible from their food), they realised that it would be more profitable not bothering with feeding the crops to animals, but to put the crops straight into the AD plants and bypass food production. Thus AD plants changed the farms from food producers to energy producers.
Generally speaking, the methane produced by the AD plants would be burnt in a combined heat and power unit (a generator) to produce electricity that would be fed into the local electricity network. Sometimes, if a gas grid was available, the methane would be cleaned up and injected into the grid.
However, even in crops the energy content is low and so huge quantities of crops are necessary. Each AD plant requires several square miles of farmland to be turned over from food production to producing the crops.
The main crops that are used are grass, maize, beet and wheat. We all know that maize is a totally unsuitable crop to grow in Devon’s wet and heavy soils. It is harvested in late autumn and results in soil compaction and soil degradation. The digestate that is produced is, in theory, stored in sealed tanks or pits and, when the ground conditions are suitable, is spread back on the land on which the crops have been grown. Since the introduction of the AD subsidies we have seen the development in Devon of many centralised AD plants on farms. They are big industrial complexes, usually consisting of several big domed tanks, storage sheds, silage clamps, generators and other industrial equipment. They are totally out of character in the countryside. The AD plants use minimal amounts of manures and slurries but source crops from far and wide.
It is evident that the amount of diesel fuel used to cultivate, harvest and transport the crops to the AD plants and to transport the digestate and spread it back on the fields is enormous. In many cases, the energy content of the diesel fuel exceeds the usable energy produced by the AD plant. But that does not matter to the developers as long as they get the subsidies and it does not seem to matter to the government as long as renewable energy targets are met.
The impact of operation of these centralised AD plants on the people living locally has been unacceptable. The operators of the plants minimise their costs of transporting the crops and digestate by using enormous vehicles.
These vehicles include HGVs as well as tractors and trailers/tankers. The large vehicles are unsuitable for many of the lanes between field and AD plant and result in damage to the lanes, verges and banks. Normal users of the lanes: car drivers, walkers, cyclists and horse-riders no longer feel it is safe to use the lanes. Many local people feel intimidated by the large vehicles. In some cases, the damage to lanes has been so great that the lanes have been closed to normal users and apparently the lanes have been abandoned by Devon Highways.
It has also become apparent that a lot of the digestate has an awful smell and people who live near pits in which it is stored before spreading have suffered ill health. Residents who live near fields on which the digestate is spread suffer from the smells.
CPRE Devon believes that farmland in Devon should, wherever possible, be used for the production of high-quality meat and dairy products, something for which it is famous. Fortunately, the vast majority of Devon farmers agree with this and are dedicated to the production of top-quality food to put on our tables.
Dr Phillip Bratby, energy spokesman for the Devon branch of The Campaign to Protect Rural England (CPRE Devon)