Shanks. Waste Solutions

Anaerobic digestion is the natural breakdown or organic material in the absence of air.  Anaerobic digestion (AD) provides an effective method for turning residues from livestock farming, organic waste and food processing industries into:

• Biogas(rich in methane) which can be used to generate heat and/or electricity
• Fibre which can be used as a nutrient-rich soil conditioner, and
• Liquor which can be used as liquid fertiliser

The Science of AD

The digestion process takes place in a warmed, sealed airless container (the digester) which creates the ideal conditions for the bacteria to ferment the organic material in oxygen-free conditions.  The digestion tank needs to be warmed and mixed thoroughly to create the ideal conditions for the bacteria to convert organic matter into biogas (a mixture of carbon dioxide, methane and small amounts of other gases).

There are two types of AD process:

• Mesophilic digestion:  The digester is heated to 30 - 35˚C and the feedstock remains in the digester typically for 15-30 days.  Mesophilic digestion tends to be more robust and tolerant than the thermophilic process, but gas production is less, larger digestion tanks are required and sanitisation, if required, is a separate process stage.
• Thermophilic digestion:  The digester is heated to 55˚C and the residence time is typically 12-14 days.  Thermophilic digestion systems offer higher methane production, faster throughput, better pathogen and virus ‘kill’, but require more expensive technology, greater energy input and a higher degree of operation and monitoring.

During this process 30-60% of the digestible solids are converted into biogas.  This gas must be burned, and can be used to generate heat or electricity of both.  It can be burned in a conventional gas boiler and used as heat for nearby buildings, and to heat the digester.  It can be used to power associated machinery or vehicles.  Alternatively, it can be burned in a gas engine to generate electricity.  If generating electricity, it is usual to use a more efficient combined heat and power (CHP) system, where heat can be removed in the first instance to maintain the digester temperature, and any surplus energy can be used for other purposes.  A larger scale CHP plant can supply larger housing or industrial developments, or supply electricity to the grid.

As fresh feedstock is added to the system, digestate is pumped from the digester to a storage tank.  Biogas continues to be produced in the storage tank; collection and combustion may be an economic and safety requirement.  The residual digestate can be stored and then applied to the land at an appropriate time without further treatment, or it can be separated to produce fibre and liquor.  The fibre can be used as a soil conditioner or composted prior to use or sale.  The liquor contains a range of nutrients and can be used as a liquid fertiliser which can be sold or used on-site as part of a crop nutrient management plan.

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