Frequently Asked Questions (FAQs)


What are the benefits of AD?

  • Increased nutrient availability; improved nutrient uptake as compared to undigested slurry i.e. more nutrients into your crops less lost to the environment.
  • Improved land fertility and productivity
  • Reduced reliance on chemical fertilisers
  • Separated digestate is absorbed quickly leaving little residue on crops reducing rejection by cattle, reducing evaporation, oxidation and run
  • Benefits of AD are numerous and successful working digesters have shown AD to be viable even with low energy prices. Utilising the energy in your ‘waste’ organic matter can make a critical difference to your future profitability in times of rising energy prices
  • Feed-in Tariffs help AD become profitable as a standalone enterprise

Is the energy generated renewable?

Yes, the energy is comes 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 the suns energy. The net energy exported is eligible for ROCs (Renewable Obligation Certificates) or FIT’s (Feed In Tariff). If the heat energy is utilised this should in the future also be eligible for RHIs (Renewable Heat Incentive).

Does it smell?

There is some odour but, because anaerobic digestion is a sealed process, odour levels are greatly reduced. It is used extensively on municipal sewage plants as a method of odour reduction and it is claimed that it can reduce odour emissions by 80%; a distinct benefit to farm neighbours.

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 methane (green house gas) emitted to the atmosphere as slurry breaks down. If under the NVZ (nitrogen vulnerable zones) regulations you have to increase you slurry storage this is an attractive option.

Why add other organic matter?

The farm slurry alone does not generate a large amount of energy. Co-digestion, i.e feeding the digester with a combination of feedstocks, increases the nutrient quality of the digestate and produces a higher biogas yield. Imported organic wastes, e.g. crops, spoiled silage, grass or food processing wastes, still contain most of their energy available. Accepting that energy output is feedstock type dependent, tonne for tonne these wastes typically produce between four and ten times as much energy. Importing wastes creates an additional revenue stream and also has a significant environmental benefit; both the nutrients and the energy from these wastes are being fully utilised, instead of being wasted in composting or land fill. Potentially farms utilizing brought in wastes through AD will negate the need to purchase expensive, unsustainable chemical fertilisers.

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, however, it is recommended that you check with your local planning authority. Similarly, if you intend to export power to the grid it is advised that you check that it is covered within permitted development criteria.

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.

Will the process run on food waste alone?

Yes, however 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’ needed 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 the farm.
  • The ‘digestate’ end product is a valuable fertiliser spread to the farms land.
    If you site off farm, e.g. centralized waste site, the farm manure would have to be transported to the site and all the ‘digestate’ would have to be transported back to the farm. This represents a fivefold increase in tractor or lorry journeys; double handling significantly reduces the environmental gain making the project economically unviable as 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 digestate 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 is the difference between a gas engine and an ordinary engine?

A gas engine is basically a diesel engine with spark ignition enabling the engine to run on methane or natural gas. As a gas engine runs hotter and at a lower compression than a diesel, it also requires a spark. Engines that run on natural gas are becoming more common, typically as ‘stand-by’ generators
What is the difference between biogas and natural gas?

Biogas has some significant differences to natural gas. The methane content of biogas (the part that gives the energy) can typically vary between 45% and 75%, whereas the methane content of natural gas remains approximately 97%. The biogas is rarely clean; even with standard gas scrubbing it still contains varying levels of moisture, hydrogen sulphide and other contaminants which love to eat engines, with the contact areas being the cylinder head, pistons and bearings!


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