Figure 1 Shows the major steps in production of biogas via Anaerobic Digestion. An animation of the process was also created by the Recycle for Wales campaign launched by the Welsh Government, a link to it on youtube is provided. The organic waste, called biowaste, in the figure is first pre-treated with the aim of increasing the biogas yield. The waste is then transported into an AD reactor where AD biochemical processes occur. Biogas is produced during this step which could then be used in a combined heat and power unit to generate both heat and electricity, part of which would be fed back into the system to run the AD plant. An option which GIB did not include in their diagram, is the possibility to upgrade the gas which consequently allows its usage as transportation fuel, or to be fed into gas grid if treated to a certain standard. Some examples of the application of upgraded biogas are described in the case studies section.

 

Another product of the AD process is undigested waste. Digestate could be treated further, ‘post-treatment’ and consequently used as a fertiliser or soil enhancer. The digestate however needs to meet certain criteria, such as BSI PAS110, to be allowed to be used as a fertiliser.

 

It is worth mentioning that in addition to producing biomethane and fertilisers, AD could contribute in various other ways. These include odour reduction, pathogen control, minimising sludge production and the conservation and management of nutrients of Greenhouse Gas (GHG) emission. (Banks et al., 2009)