Anaerobic digestion - renewable heat, electricity, waste disposal and fertiliser production!

When people think of renewable energy they think mostly of wind power and photovoltaics. Any discussion of renewable electricity policy tends to refer to these and criticise them, and by implication all renewable energy, because they are unpredictable and variable and need backup.

There is an astonishing ignorance even at high government level over the potential of other kinds of renewable power generation. So I want to redress this balance with occasional posts looking at different technologies. Recent posts have referred to marine current turbines, for example.

This post is about the unsexily named anaerobic digestion. Mostly it's about small, farm-scale versions, and I hope to get around to talking about larger scale ones soon.

However, it's worth mentioning right up front that larger plants are able to produce gas for the mains and for vehicles running on gas.

A chief worry if we don't rely on nuclear power, is where will all the power come from to decarbonise transport? Well, here's one answer.

A new income stream - and more - for farmers

A survey last December found that 80% of farmers in the UK wanted to have solar photovoltaics on their roofs within the next three years - and yet the fact is, that in terms of the carbon saving and other benefits anaerobic digestion (AD) provides better value for money than solar PV.

For example, farmer Clive Pugh (above) at Bank Farm, Mellington, near Churchstoke, Wales, put in his first AD plant 20 years ago. He now has a state-of-the-art, three chamber unit that provides all of the farm's own energy needs, and that for two homes and the farm dairy, as well as generating an income of up to £10,000 a month from supplying the National Grid - without the new FiTs subsidy, because he was an 'early adopter' and so the scheme is excluded from it.

“We initially went for an anaerobic set-up because we needed a new slurry store and it was something we had been looking into for some years,” says Mr Pugh.

“It revolves around using the slurry from our 140-cow dairy herd. In order to keep the gas production fairly constant throughout the year, we also use poultry manure, silage effluent, waste silage, discarded milk and whatever other green waste we can get hold of.”

While 10 cows are needed to produce 1kw of energy, in fertiliser value terms 1,000 gallons of separated liquid will provide around 30 units of nitrogen, 40 units of potash and 12 units of phosphate.

“The quality of our grass is certainly most noticeable these days, and our need for phosphate and potash is now nil. We also only need top-up units of nitrogen depending on the type of crops being grown,” says Mr Pugh.

How does it work?

In a typical plant, vats ferment farm slurry and crop waste (and can also process food waste) in the absence of oxygen to produce gas which can be used to generate heat and power.



The facility would normally be owned and operated by the farmer/farm business, but might sometimes be part of a co-operative venture. They often would not be approved to accept animal by-products at this scale.

The biogas produced in AD is a mixture of methane (65%) and carbon dioxide (35%) which can be used to generate heat through a boiler, or heat and power through a combined heat and power (CHP) system. In addition, following further processing, biogas is also a suitable fuel source for vehicles.

Hot water may be used on site, for example to heat polytunnels or greenhouses for market gardening. Some farms use AD to power a generator for the digester and pasteurisation. Other benefits include:

• it avoids landfilling of organic wastes;
• the biogas can be burnt as a fuel;
• there is a reduction in the use of fossil fuels, offsetting carbon dioxide emissions;
• it is a predictable and reliable source of electricity and energy, unlike wind power and PV;
• the digestate products return nutrients to the land, reducing dependence on inorganic fertilisers;
• there are economic benefits from reduced fuel and fertiliser use, as well as the subsidy;
• farms can become more self-sufficient, with socio-economic opportunities, e.g., gate fees can be charged for waste taken in and electricity, biogas, fertiliser and soil conditioner can be sold;
• odour is reduced by around 80% compared to farm slurry;
• methane (a greenhouse gas) emissions are reduced;
• a range of organic waste materials can be processed - the highest gas yields come from the co-digestion of fatty (food processing wastes), liquid wastes (animal slurries) and green wastes;
• the amount of farm slurry sprayed onto farmland - and of run-off and pollution of waterways - is reduced;
• harmful bacteria and viruses are destroyed, reducing the spread of harmful disease causing pathogens.

The energy generating potential is determined by the size of the digester and waste feedstock composition.

A typical farm installation might be up to 0.5MW. A small farm using farm waste can produce enough heat to warm the digester and meet domestic heating requirements. If electricity is generated through CHP of 10kWe capacity, enough electrical energy could be generated to supply up to 13 homes.

A brand new installation can cost anything from £150,000 for a fairly basic liquid-only unit to more than £375,000 for an all-embracing 120 kw producing version.

Better payback than PV

This high initial cost is why the technology needs support at this stage. Without support, simple economic payback is approximately 20 years.

Factoring in savings made in waste disposal, according to the Carbon Trust, mean that payback times for installations tend to be under 5 years.

Compare this to solar PV in much of the UK, which is 40 to 60 years without subsidy.

Larger plants

A range of AD scales exists, from single on-farm digesters through to large centralised anaerobic digesters (CAD) collecting waste from a larger surrounding area.

These CADs will usually accept animal by-product wastes for digestion. The gas produced at this scale can also be used for other purposes, for example to power vehicles or be injected into the National Grid.

AD at this scale is economically viable and requires little support. Most plants operate as co-digestion plants with slurries, in additional to wastes from the food, brewing and other industries.

This recent post contains other examples.

This website is a useful source of further information, although slightly out of date.

In Germany there are more than 3,000 on-farm anaerobic digesters - in the UK perhaps around 50.