Anaerobic Digestion

The Anaerobic Digestion plant is definitely a large-scale enterprise and not for a small farm or single domestic property. Although, if you have access to industrial quantities of animal or human sewage or food waste and a good source of non-food fibre, then this is well worth considering. However, there is a move to introduce viable mini AD plants that will work well for individual Pig and Poultry producers; and for which the gas tank is unlikely to be over 4m in height and easy to hide.

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How might it work for you

For the large scale gas producer your decision will be the start of a large financial investment with commensurate returns.

The modern trend is to avoid landscape problems, and to this end both digesters and gas tanks can be installed below the ground. Another advantage is that this provides a more equable working environment - the microbes like an even temperature. Effectively hidden, this should mean that Planning Permission from the point of view of landscape is not an issue. However, it will be essential to ensure that all external joints can be inspected with ease, as leaks could be disastrous for nearby wells and streams. Where-ever the installation is sited, a proposed schedule for the checking of joints and tanks for leaks could be an additional issue to be considered at the planning stage.

Gas produced from an anaerobic digester, can be used for both heat and electricity. In the latter case the gas is used to produce steam to drive a turbine in the normal manner; the waste water being still quite hot could then be piped to where-ever heat is needed, be it for domestic or glasshouse heating.

The main considerations are:-

  • The volume of 'input fuel' to be produced/brought on site and any anticipated variations due to season etc.
  • Ensuring that the proposed input mix, including fibre content, will provide the optimal output mix of methane and other gases (< 10% namely carbon dioxide, nitrogen, hydrogen, hydrogen sulphide and oxygen) to keep the generator running at its full rated potential.
  • The upfront Capital Costs. Much reduced in the case of mini AD systems.
  • The costs of Planning and Environment Agency issues if not included in the supplier's package.
  • Choosing the best installer.
  • The amount of time that will be required to run the installation on a daily basis once it is fully commissioned.
  • The choice of an energy buying company
  • The cost of connection to the National Grid.
  • The weekly/annual servicing and running costs.

There will be other concerns as the project progresses, but these are the main ones.

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Facts - Energy Produced and Costs of Production

Each situation is unique. However, the following examples of schemes that are up and running will give an indication of how your system might perform.

  • Mini AD systems are expected to be rated at approx. 100kW.
  • Southern UK: A Producer of 1,200t/yr dairy waste has cut their Carbon footprint by > 65%/annum, equivalent to planting 120,000 trees/yr. The associated CHP (Combined Heat and Power) scheme will generate 1,539MW and 1,685MW of heat from the effluent, plus small amounts of de-watered solid digestate.
  • In the case of a dairy or pig farm producing 10,000t of slurry, with a 350kW digester, they would need access to approx. 5,000t of crop, e.g. from 125ha if Maize was used to provide the fibre. However, if it was possible to team up with a vegetable processing unit, then the waste leaf and trimmings from this would provide an adequate replacement for a specifically grown, potential, food crop.

FITs are available on the basis of 11.5p/kWh equivalent for outfits with generating capacities up to 500kW, and 9p/kWh equivalent for outfits producing > 500kW.

Typically an anaerobic digester rated at 556kW/h + Heat + Class A Biosolids (no restrictions on spreading to land) will cost approx. £1.2m. The annual revenue, with ROCs/FiTs = £650,000pa.

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Pros and Cons

Pros

  • Lower Greenhouse Gas emissions than any other type of waste management.
  • Suitable for wet wastes.
  • The whole process is enclosed, so there are no emissions.
  • Can power its own processing.
  • The whole installation can now be sited underground, in which case there will be no landscape issues!

Cons

  • Currently there seems to be a lot of red tape and time involved in getting approval for schemes from government regulators.
  • High initial cost.
  • High Management skills - precise monitoring must be carried out.
  • Accessibility to a good servicing contract.
  • Must have access to land from which to obtain fibre input to the plant.
  • Must have access to land for recycling of the biosolids.
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Variations on the theme

A variety of smaller units are being researched and are at the development stages.

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How and Why it works

Microbes come in assortments in which there are members that are capable of breaking down absolutely anything that humans can throw at them, given time. If one provides this basic assortment with a specific type of waste, then the specific microbes capable of breaking down/digesting this waste, will multiply until they form the dominant forms in the assortment. Those that cannot handle the specific waste simply die off. For this reason it is imperative to keep the contents of the specific waste constant, and the majority of the monitoring that goes on in running the Digester is essential to achieve this constancy. Obviously if the proportions of the mix change, then the dominant microbes will no-longer be effective and it will take many generations in the new mixture before breakdown will be effective once again. Luckily their generation time is measured in minutes rather than decades! So a week will achieve a lot of adaptation.

The gases produced are simply the waste gases produced by the microbes during their lives. When they die, then they too are part of the specific waste. The most important of the waste gases from the point of view of energy production is the hydrocarbon - Methane. This should be at least 50% of the output, and is used to: heat water to produce steam to turn a turbine thus producing electricity; or to heat water for direct heating via a piping network.

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