Food and drink producers and hospitality businesses could consider anaerobic digestion of their organic waste. This involves the treatment of organic material in the absence of oxygen (air). The digestion process involves degrading material with microbes but uses a different group of naturally occurring micro-organisms to composting. This process generally has three outputs:
- Digestate - a wetted, solid material that usually requires maturing or composting before it can be used, eg applied to soil.
- Liquid - may be rich in nutrients and can be used as a fertiliser.
- Biogas - can be used to generate electricity or burnt as a renewable fuel. It generally contains around 60 per cent methane, with carbon dioxide the other main component.
An important difference between composting and anaerobic digestion is that biogas is released during the fermentation process. Biogas is suitable for use in some gas engines and is often used in combined heat and power units to generate electricity and heat.
If you intend to set up your own anaerobic digestion system, check whether you need authorisation from the Northern Ireland Environment Agency. If you use a system operated by another organisation, check they are authorised to operate it.
The outputs from anaerobic digestion are still waste unless they have been produced on farms using certain waste types. They are therefore still subject to waste regulation and their use must be registered under a waste exemption or an environmental permit.
To make sure the use of anaerobic digestate isn't regulated by waste legislation you should meet the requirements of the quality protocol for anaerobic digestate.
Digester system design
There are various digester designs, each suited to different circumstances. The design of an anaerobic digestion system provides the optimum conditions in which micro-organisms can flourish and degrade the waste. This requires:
- a balanced and predictable internal environment
- control of the rate of input and output and process conditions such as temperature and acidity
Wet digestion of organic waste
This type of digester has technology that is proven, robust and relatively simple to operate. The digestion of dense materials such as food waste and similar solids/semi-solids requires their dilution in a large volume of water so any solid output will need to be dewatered. Liquid outputs can be recycled within the process to reintroduce the bacteria and reduce water use.
Dry digestion of organic waste
High solids processes are not dry. The waste is processed at 20-40 per cent moisture, generally as a pumpable mass blended to the correct consistency using specialist equipment and injected into the digester. Virtually no liquid make-up is required and the liquid output is modest. This liquid tends to have a relatively high nutrient content and is a useful fertiliser.
Dry digesters can be horizontal and may have a void above the digestion mass, which can act as a buffer for low pressure biogas production and storage. This removes the need for additional gas management and separate storage vessels.
Traditional wastewater anaerobic digestion systems operate at a relatively low temperature of around 35°C. At these temperatures the mass is transformed substantially, coupled with biogas release, but harmful micro-organisms and parasites may not be eliminated. Organic wastes must be subjected to controlled pasteurisation involving temperatures of 55-70°C.