Willem asked for independent info. the following is the first few paragraphs of pp 173-185 in an evauation published on June 10 2011 in such a report - Review of the generation costs and deployment potential of renewable electricity technologies in the UK - covering ALL renewable technologies, by ARUP, for the UK Government, which is available here: http://www.decc.gov.uk/en/content/cms/meeting_energy/renewable_ener/rene....
Electricity generation using anaerobic digestion (AD) technology has been considered for food waste and farm manures. The installed capacity for the treatment of food waste and farm manures in 2010 is about 28MWe.
The available energy from AD is 5,661GWh per year, which is equivalent to 708MWe of installed generation capacity at 2030.
For the low scenario the installed capacity would reach 60.5% of the maximum resource available by 2030.
For the medium scenario, the maximum generation capacity is predicted to be reached by 2030.
For the high scenario the maximum generation capacity is predicted to be reached by 2020.
Anaerobic digestion (AD) is the biological conversion of biodegradable organic material by micro-organisms in the absence of oxygen, which results in a reduction in the quantity of organic material and the production of biogas, consisting of approximately 55-70% methane, 30-45% Carbon Dioxide and approximately 1% nitrogen, with trace elements of hydrogen sulphide. The process also produces a nutrient-rich liquid and solid bio-fertiliser (i.e. digestate). The process is widely employed by the water industry within the UK for the stabilisation of sewage sludge.
In addition to the water industry, there is a growing interest in the digestion of alternative feedstock including food waste, farmyard waste materials and crops grown specifically for AD. However, AD plant development in the UK has been slow compared to some other EU member states (e.g. Austria, Denmark, Germany and Sweden).
There is a number of AD technologies available and their technical complexity and associated capital and operational costs depend on the feedstock to be treated. Typically, food waste AD plants are technically more complex requiring, for example, a greater degree of pre- or post-processing of the feedstock to remove certain contaminants such as plastic packaging, metals, glass etc, and pasteurisation units to meet the strict Animal By-Products Regulations etc. The AD process operates under mesophilic (25-45°C) or thermophilic (50-60°C) conditions.
Furthermore, the process may operate as either a dry digestion process, with material at a dry solids content of greater than 15%, or a wet process, operating below 15% dry solids, more suited to materials such as animal slurry.
Biogas is typically collected and used to heat the digester, to optimise the digestion process, and at larger plants where it is economical, biogas is collected and used for in combined heat and power (CHP) plants. Current advances are also being made in the injecting of biomethane (processed biogas) into the national gas grid and the use of biomethane as biofuel for transport.
Digestion of food waste and of manures may be carried out separately or in a combined treatment process. As stated above, the former can require significant pre-treatment processing, to remove contaminants such as packaging, and may therefore entail a greater capital and operating cost depending on the food waste being processed.
While there are both cost and legislative differences in the treatment of farm and food waste, digestion of these two waste streams is sometimes carried out together.
A small number of the existing anaerobic digestion plants in the UK, both on-farm and off-farm, both on-farm and off-farm, treat a mixture of food waste and manures, and depending on their environmental permits may vary the feedstock to some degree, based on the availability and the increase in income associated with treating food waste from both a gate fee and increased biogas potential.
In addition, to the benefits provided by energy production, the digestion of food waste provides the benefit of a sustainable waste treatment process for the diversion of biodegradable material away from landfill, in line with UK regional waste strategies.
The digestion of farmyard manures improves the fertilising properties and reduces the environmental effects of spreading undigested manure and slurry (Environment Agency, 2010)