Wednesday, 6 July 2016

Anaerobic Digesters - good or bad?

Nocton Fen AD

My research into these type of facility started with the Nocton Dairy campaign. I have previously reported on the construction of the anaerobic digester on Nocton Fen - which is now owned by Beeswax Farming Ltd:
This particular digester was commissioned in 2011. It has a 12.900 m3 reactor capacity with a 2 MW gas engine application, with a potential annual production of 16 MW of electricity and heat.

I have since discovered two notices related to this digester that were served by the Enforcement Agency:
  1. Beeswax Farming (Rainbow) Ltd - Nocton Digester LN4 2AY - EPR Reg 36 - Comply with conditions and remedy pollution [Case Ref: 19727 - 29/04/2014]
  2. Beeswax Farming (Rainbow) Ltd - Nocton Digester LN4 2AY - EPR 07 Reg 38 (1)(b) [Case Ref: 19727 - 30/05/2014]

What are the benefits of anaerobic digesters?

Anaerobic digestion was initially designed to provide an opportunity to generate 100 per cent renewable energy from biodegradable waste. The organic material is broken down producing biogas for both heat and power... and this can be converted into a healthy financial return.

The income involves a government Feed in Tariff (FiT) and Renewable Heat Incentive (RHI), which can provide an attractive 20-year income for renewable energy generation. Other incentives are also available, such as grants to assess feasibility of on-farm anaerobic digestion and business planning.

Furthermore, the nutrient rich digestate that is a by-product of the anaerobic digestion process can save money by replacing expensive chemical fertilisers. But all this also comes at a cost.

What are the costs of anaerobic digesters?

There is a large capital investment of building the facility in the first place.

The material that is used in anaerobic digestion is called feedstock. It was always intended that land unsuitable for the production of food crops was the sustainable option for the supply of biomass for anaerobic digester plants. However, good agricultural land is increasingly being used to grow crops purely for energy production.

The diversion of land use from human food supply to energy use is very much a contentious issue and it is important to be mindful of future unintended consequences should there be significant areas of productive agricultural land grown for energy production.

Finally, there is the ongoing specialist skill required to operate these anaerobic digestion plants effectively, efficiently and safely.

What is involved?

Basically, what goes into an anaerobic digester determines what comes out, so a careful choice of feedstock is essential. There are three potential types of feedstock that can go into an anaerobic digester:
  1. Food waste - Industrial, commercial and domestic.
  2. Agricultural residues - These include animal wastes such as cattle and pig slurry, manures, feed waste and bedding.
  3. Energy/Arable crops - Purpose grown crops cultivated for their high calorific value. The key crops used in the UK are maize silage, grass silage and whole crop cereals.
I seem to recall Nocton Fen anaerobic digester is only licensed to process energy and arable crop feedstock. As such, securing a reliable feedstock supply is fundamental to profitable anaerobic digestion.

The anaerobic digestion process is a treatment that composts feedstock in the absence of oxygen, producing biogas that can then be used to generate electricity and heat. As well as biogas, the digester produces a solid and liquid residue called digestate, which is used as an organic soil conditioner to fertilise the land. This can achieve increased crop yields by improving the soil with careful application. However, because digestate is derived from an anaerobic digestion process (devoid of oxygen), it contains only anaerobic microorganisms.

Agriculture normally requires aerobic conditions, and actually anaerobic conditions can lead to severe problems. For example worms don't like anaerobic conditions and require aerobic conditions in order to thrive, no different from plants, or humans. Scientific literature is very clear that anaerobic processes result in outputs that can be highly toxic, and not at all beneficial to plants.

Fortunately, as long as the anaerobic digester is managed properly, the health risk from the solid and liquid residue from an anaerobic digester plant should be low. This will ensure no chemical contaminants are entering the system. It is crucial to analyse the chemical content of the digestate produced, select the best manner of application and to calculate the correct irrigation dose and frequency, maintaining records diligently for inspection when required.

Digestate is a rich source of nutrients for plants; and assuming it is not full of contaminants, it should serve the land well.

Anaerobic Digestion Explained - in more detail.

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