AD is the process where plant and animal material (biomass) is converted into useful products by micro-organisms in the absence of air. Biomass is put inside sealed tanks and naturally occurring micro-organisms digest it, releasing methane that can be used to provide clean renewable energy. The material left over at the end of the process is rich in nutrients and can be used as fertiliser. The process reduces emissions of methane and produces a product that provides more nitrogen to crops than slurry.

Outputs

Biogas is a mixture of 60% methane, 40% carbon dioxide and can be combusted to provide heat, electricity or both. Alternatively, the biogas can be cleaned up and the pure methane injected into the mains gas grid or used as a road fuel.

Digestate comprises of left over indigestible material and dead micro-organisms. It contains valuable plant nutrients like nitrogen and potassium and can be used as a fertiliser and soil conditioner.

What can AD process?

Almost any biomass can be processed by AD including food waste, energy crops, crop residues, slurry and manure. Cereals and rape meal can be used as AD feedstocks, giving high biogas yields, but are generally expensive. Dedicated energy crops with high biomass such as maize, can be grown specifically for anaerobic digestion.

What can’t AD process?

Woody biomass cannot be used in AD because the micro-organisms can’t breakdown the lignin, the compound that gives wood its strength.

Yield

The yield of biogas from a particular feedstock will vary according to:

  • Dry matter content. Food wastes in particular will vary greatly.
  • The energy left in the feedstock. If ithasundergone prolonged storage it may already have begun to break down.
  • Length of time in the digester .
  • The type of AD plant and the conditions in the digester.
  • The purity of the feedstock.

The table below shows data on the potential biogas yields of the feedstock commonly used in anaerobic digestion (from a report by the Andersons Centre).

Feedstock

Dry Matter %

Biogas Yield M3/tonne 

Cattle slurry 10 15-25
Pig slurry 8 15-25
Poultry 20 30-100
Maize silage 33 200-220
Grass silage 28 160-200
Maize grain 80 560
Whole wheat crop 33 185
Wheat grain 85 610
Crude glycerine 80 580-1000
Rape meal 90 620
Fats up to 100 up to 1200

Permissions

  • Planning permission is generally required, apart from small scale digesters using only on-farm waste, these may only require an Agricultural Notification.
  • Depending on the size of the digester, either an environmental permit or exemption from the Environment Agency will be required.
  • If you wish to spread the digestate onto land, it will either have to be PAS110 verified (it is then a product and not a waste) or a permit/exemption will be required.
  • Animal By-Products permits may be required depending on the feedstock.

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