Environment Hub

Anaerobic Digestion

Pig manures can be digested in an oxygen-free environment to produce biogas – a process called Anaerobic Digestion (AD). Biogas contains up to 60% methane which can be used to fuel vehicles, fuel static engines to generate electricity, or have the remaining 40% carbon dioxide removed to inject into a nearby gas main, or into bottles.

Also, manure produces ammonia which can disrupt the bacteria that carry out the digestion process in standard systems. This can be alleviated by using co-digestates such as wholecrop maize or green/food wastes which increase the relative carbon to nitrogen content (C:N ratio) to create optimum digestive conditions. However, many pig farmers do not have land on which to grow supplementary crops (or borrow against), or have access to wastes.

BPEX have produced a factsheet about anaerobic digestion and the feasibility of standard systems on pig units:

Anaerobic Digestion Factsheet

For further information on anaerobic digestion, biogas and digestate visit the The Official Information Portal on Anaerobic Digestion set up the NNFCC www.biogas-info.co.uk

The issue for UK producers is that most of the standard systems on the market were designed in Germany where AD is highly subsidised, so they are expensive to purchase in the UK.

Anaerobic Digestion Technology for Biomass Projects - Commercial Assessment Short Form Report - Renewables East, June 2007

Straight slurry digestion
Some manufacturers have developed technologies that reduce the nitrogen content of manure during pre-treatment but these are expensive.

Lower cost solutions compartmentalise the stages of digestion using horizontal tanks. The liquid digestate is infused with bubbles to cause it to flow past a series of bulkheads within the tanks. Each compartment is a microenvironment for the different bacteria and processes involved in digestion. The bacteria/processes that can cope with high nitrogen levels are therefore able to flourish in their own niche.

Straight FYM digestion
FYM can be digested on its own, but the C:N ratio is low because the carbon is bound in the woody lignin of the straw. Pre-treatment includes heating and removal of the liquor which is then digested in a covered tank using the residual heat. The straw residue can be composted.

If FYM is used to supplement other feedstocks, it needs thorough maceration to break up the straw before entering the digester. This requires modifications to most systems on the market.

Plug Flow installations are based on horizontal digesters. These digesters are long cylindrical or rectangular tanks, usually built partially below ground level. Pre-mixed material is added to the tank at one end and pushes older material to the opposite end. As a result the material flows though the tank in a ‘plug’. The average retention time of substrates in plug flow is 15 to 20 days.

The retention time can be controlled very precisely so that the maximum amount of gas can be obtained. The dry-matter content of the substrate can be increased to 25 % or even 35% through this process depending on whether the substrate is augered or tumbled through the tank.

Batch Dry digestion
FYM can also be fermented in garage style all-in all-out systems, but care must be taken that the unit does not leak gases.

Anaerobic Digestion in Agriculture: Workshop on Increasing the Uptake of Anaerobic Digestion (Exeter University, 3-4 September 2007)

On 3-4 September 2007, Defra held a workshop on working with stakeholders to facilitate a faster uptake of anaerobic digestion in England at Exeter University. This was organised in collaboration with the Renewable Energy Association (REA), the Waste and Resources Action Programme (WRAP), the Institute of Grassland and Environmental Research (IGER) and the Country Land and Business Association (CLA). The aim was to help identify what now needs to be done to achieve our goal of stimulating markets for anaerobic digestion and its products, and to address the administrative and technical challenges, which may hamper their development.

A Factsheet produced by TASK37 provides links to relevant information and articles; this can be found on the Biomass Energy Centre website.

Renewables East has produced guidance booklets, giving background information on AD technology, a comparison of equipment suppliers and their offer, case studies and financial data:

Reaseheath Agricultural College are offering free open days at their recently opened Anaerobic Digestion plant. This will give interested farmers and landowners the opportunity to look around an AD plant and a discussion of the plants results and the practical lessons that have been learnt so far.

Tour date are:

13 September 2011, 4 October 2011, 1 November 2011, 6 December 2011, 10 January 2012, 7 February 2012,

6 march 2012,

Admission - Free

Time - 10 am - 12pm

Contact hub@reaseheath.ac.uk to register your interest