Anaerobic Digestion
The Process
Put into its simplest terms:
Take animal slurry, mix it with other organic matter (e.g. food waste or crops) put into a sealed tank with agitation and heat to around 38°C. The bacteria present will then break down the organic parts releasing bio-gas (approximately 60% methane and 40% Carbon dioxide) This gas can then be used to generate renewable electricity. The resulting ‘soup’ (digestate) is a useful bio-fertiliser with its nutrients in a readily available state.
Some Questions answered:
Does it smell?
No, anaerobic digestion is a sealed process, it is used extensively on municipal sewage plants as a method of odour reduction. It is claimed that it can reduce odour emissions by 80% a benefit to farm neighbours.
Is the energy generated renewable?
Yes, the energy is coming from the materials that have grown, been harvested or eaten the carbon released is just that that was taken out of the atmosphere when they grew, it is basically the suns energy. The net energy exported is eligible for ROCs (Renewable Obligation Certificates). If the heat energy is utilised this should in the future also be eligible for ROC’s.
Will the system Run on slurry alone?
Yes, this is the simplest configuration. The digester will work well as part of a slurry management regime. It will extend your slurry storage and capture the green house gas methane given off as slurry breaks down. If under the NVZ regulations you are having to increase you slurry storage this is an attractive option.
If I run using Farm Slurry alone do I require planning permission?
If you are not bringing any food waste onto the farm it will most likely fall within permitted development you must check with your local planning authority, if you are intending exporting power to the grid you will also need to check that this is falls under permitted development.
If I bring in food waste do I need planning permission?
Yes, If you bring in any food processing wastes you will need planning permission and a waste disposal license or permit. This will require environmental screening, along with approval from the state veterinary service, the environmental agency, your local environmental health and animal health services, we have found these agencies at the local level to be both well informed and supportive (unfortunately at the higher level there is a different story).
Why add other organic matter?
The farm slurry alone does not generate a large amount of energy if your stock regime is good they should be using most of it. However brought in organic wastes e.g., crops, spoiled silage, grass or food processing wastes still have most of their energy available, typically tonne for tonne these can deliver between four and ten times as much energy. It is very dependent on the material. By bringing in other wastes you can create another revenue stream. There is a large environmental benefit, the nutrients and the energy from these wastes are being utilised potentially removing the farms need for expensive chemical fertilisers and the wastes are potentially being diverted from land fill.
Will the process run on food waste alone?
Yes, the advantage of adding the animal slurries are that it provides a constant base load making the process more stable and easier to maintain, it will keep the process running if there is a problem with deliveries, transport or your waste supplier. The slurry provides a continual inoculation of the ‘friendly bacteria’ to keep the process running and healthy. The key is to provide a balanced diet.
Why site digesters on Farm?
The digester acts as part of a farms Slurry management system.
The farm slurry for the base load is generated on farm.
The end product the ‘digestate’ is a valuable fertiliser spread to the farms land.
If you site off farm; The farm manure would have to be transported to an industrial estate and all the ‘digestate’ would have to be transported back to the farm, this represents a five fold increase in tractor or lorry journeys. This double handling significantly reduces the environmental gain making the project economically unviable.
Remember ‘waste miles are as important as food miles.’
What are the limiting factors for the size of the digester?
Availability of animal slurry and other organic inputs.
Land mass available for spreading of the ‘digestate’ the cost of shipping this off the farm generally outweighs the benefits.
Power line capacity. The cost of upgrading the power connection can be prohibitive.
These three factors effectively limit the size of the facility.
What are the benefits of AD?
Increased nutrient availability; between 20 and 25% improved nutrient uptake as compared to undigested slurry i.e., more nutrients into your crops less lost to the environment.
Separated digestate is absorbed quickly leaving little residue on crops reducing rejection by cattle, reducing evaporation, oxidation and run off losses.
Digestion kills most pathogens within the feedstock e.g., TB, F&M etc.,
Digestion kills virtually all weed seeds within the feedstock.
Reduced farm methane emissions.
Reduced farm energy bill and carbon footprint.
Farm
Renewable
Environmental
Energy Ltd
