Why small scale anaerobic digestion faces an exciting future

Written by: RWW | Published:

Richard Gueterbock, marketing director of Clearfleau, examines the relevance of small scale anaerobic digestion plants in terms of Britain’s renewable energy sector and explains why installing on-site installations treating bio-degradable residues where they are created will make better use of unwanted resources.

Government agencies are taking greater interest in opportunities for smaller AD plants to treat bio-degradable materials where they are generated - not only farm residues, but also on food processing sites, with more efficient handling of residues and generation of renewable energy. Also as the policy framework changes, more innovative solutions are needed especially in rural areas.

In January 2014, Scotland introduced a ban on food residues being sent to landfill. By 2016 all biodegradable wastes will be excluded (including sewer discharge of liquid production residues).

Installation of on-site AD plants for food and drink processors is part of the solution.

Should the British government look at imposing a similar ban for the rest of the UK?

Although Britain has lagged behind other European countries in adopting AD, and initial drivers for deployment here came from larger waste management companies - smaller scale AD should have an exciting future. Concerns with centralised plants can include issues such as planning, cost, footprint, process efficiency and process risk.

But the main issue is disposal of digestate (also involving a transport cost), as AD only removes degradable solids (reducing feedstock volume by just 5%).

Very large plants also require a sufficient land area to manage the volume of digestate and prevent nutrient overloading in soils.

Farm scale AD

The agricultural sector should see significant deployment of farm scale AD plants in the next decade. Digestate should be returned to land and it makes sense to site plants on or close to farms. AD can reduce odour emissions from livestock slurries and manures, plus nitrate and phosphate run-off from storage and spreading of undigested manures. But the digester must fit the farm.

Britain’s farm manures have a methane potential of 10bn kWh of thermal energy per year. But, if not digested, estimated GHG potential is 3m tonnes CO2 equivalent.

Hence, AD should be more widely adopted on livestock farms - with farm scale plants. Germany recognises the value of AD for on-farm pollution control and nutrient recycling, with enhanced incentive payments for smaller on-farm slurry based digesters.

Britain should do the same. Also smaller German farm AD plants can take in food waste, and this should be encouraged in Britain.

More landfill exclusions

More household and catering food residues must be excluded from landfill. Larger centralised AD plants may offer a viable solution for bio-degradable wastes from urban conurbations, but rural communities require local solutions not urban style plants. Local plants have been built (e.g. at Llangadog in Wales and Warminster in Wiltshire) but many more are required across the UK.

By allowing the addition of local food waste, the commercial viability of slurry-based digesters would be enhanced. Treating these materials in community AD plants, based on farms, will reduce transport costs.

However, regulations inhibit the use of such energy-rich materials in farm-scale AD.

One solution is pasteurisation at a centralised facility and delivery of high energy ‘soup’ to on-farm digesters. The hub and point of digestion (PoD) system conceived by Professor Charles Banks of Southampton University, has generated some interest but needs government support. On-site digestion of industrial production residues reduces the carbon footprint of the site and can provide renewable energy for production processes. Extracting energy from these bio-degradable materials will reduce treatment and disposal costs.

Small scale AD can provide flexible, lower-risk, robust solutions for organics treatment, not just on farms, but for factories and in communities. On-site installations treating bio-degradable residues where they are created make better use of unwanted resources.

This has the potential to become a dynamic part of the AD industry and one that does not rely on imported technology and it should be better supported by regulators and policy makers.

Fact File: Clearfleau

Clearfleau is a British company that designs and builds small on-site plants that treat bio-degradable production residues and which seeks to drive innovation in the anaerobic digestion sector. Its anaerobic digestion process converts liquid feedstocks into energy, to replace fossil fuels used in the factory, with enhanced treatment efficiency.

- www.clearfleau.com

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