31 July 2019
Best in class

Irish organic farming student wins top prize

31 July 2019
HAWL bursaries

Bursaries offered for three-day homoeopathy courses



21 March 2019
In adversity, what are farmers doing to be more resilient?

Opportunities, barriers and constraints in organic techniques helping to improve the sustainability of conventional farming

Anaerobic digestion: farm-scale options & digestate use

Anaerobic digestion continues to invite interest and comment with larger scale systems tending to show greater promise. This session will report on developments in small-scale systems and the role of AD in nutrient cycling.

Laurence Smith (ORC) Chair:

Research results are showing the benefits of adding digested material to crops and the technology of larger digestors is imporoving and proving vilable in Germany and the UK. However, the small scale anaerobic digestors are still limited, due both to technology and policy.

Kurt Moller discussed research results which indicate that digestion of clover/ grass-ley, crop residue and cover crops can increase the crop dry matter and N yields and the N content of wheat grains in organic stockless systems. The use of digested material can also reduce soilborne N20 emissions and reduce the risk of nitrate leaching.

Dr Clare Lukehurst, from Task 37, discussed the vilability of small scale anaerobic digestors (<100kw)  for UK farms and the present limitations by Defra. Current systems available include –

  • Continuously Stirred Tank Reactors
  • Garage reactors
  • Plug flow digesters

The size, style, and cost of these digesters means they are not always viable for the UK. Task 37 will be releasing a report on small scale AD viability in the UK.

Richard Tomlinson from Fre-energy gave some case studies of farm AD. He discussed the differences between crop digesters and waste digesters. He also highlighted the problem and solution to grit and foam in an AD systems. Looking at the economics of AD, Richard mentioned the importance of the Renewable Heat Incentive (RHI), a government led subsidy, in making AD economically vilable on farms.

Key conlcusions

The discussion that followed the presentations brought out the following points and actions:

  • The best practice for ratios of food waste and slurry are 50:50
  • How can we lobby Defra for better policy on AD, looking at the regulation effects in Germany.

Action Points

  • Promotion of Task 37 report when it is published

Speaker presentations and abstracts

Dr Kurt Möller (University of Hohenheim): Effects of AD on nutrient cycles and availability (250KB)

The trend towards specialization in conventional farming led to large agricultural areas in Germany and in Europe lacking livestock. Also stockless organic farming has increased during recent years. In organic farming clover/ grass-ley (CG) provides nitrogen (N) to the whole cropping system via symbiotic N2 fixation and also controls certain weeds. A common practice in organic farming, when ruminants are not present, is to leave the biomass from CG in the field for their residual fertility effect. CG biomass, crop residues (CR) and cover crops (CC) represent a large unexploited energy potential. It could be used by anaerobic digestion to produce biogas. In the presentation, the results of a field experiment carried out by implementing a whole cropping system with a typical crop rotation for such farming systems on the research station Gladbacherhof will be presented. The crop rotation consisted of six crops (two legumes and four non-legume crops). The aim was to evaluate whether the use of N could be improved by processing biomass from CG, CR and CC in a biogas digester and using the effluents as a fertilizer, compared to common practice. Results indicate that digestion of CG, CR and CC can increase the crop dry matter and N yields and the N content of wheat grains in organic stockless systems. Harvesting and digestion of residues and their reallocation after digestion resulted in a better and more even allocation of N within the whole crop rotation, in a higher N input via N2 fixation and lower N losses due to emissions and probably in a higher N availability of digested manures in comparison to the same amounts of undigested biomass. Similar results were obtained also in a similar approach carried out in Sweden. A short presentation of available data on potential effects of anaerobic digestion on soil humus budgets is included.

Dr Clare Lukehurst (Task 37): Small-scale farm anaerobic digestion

The final draft of the International Energy Agency Bioenergy Task 37 ‘Energy from biogas and landfill’ brochure is due for for publication in April 2013. The objectives of the brochure are: to illustrate existing technologies for small-scale plants & possibilities to reduce investment costs; to consider the necessary and favourable framework conditions; and to demonstrate management and operational practices to improve economic viability. A 100- cow dairy herd is used as a basis for the discussion and the examples cited and comparisons made between livestock manure, the additions of crops and food waste as the basis for economic viability. These are illustrated by case studies from Northern Ireland, Brazil and Finland. The brochure also addresses the under valuation of farm scale AD in its contribution to GHG and nitrate emission reduction for which there is no recognition in economic terms or incentives. The authors are preparing a list of companies that can offer small scale applications (<10kWe- 100 kWe) on www.iea-biogas.net as well as a section of case studies also for the web site.

Richard Tomlinson (Calon Wen): On-farm: a farmer’s experience (3.73MB)

Richard will be covering the following areas: obstructions in construction; operations & maintenance; benefits to the farming business; does it pay?