An anaerobic digester receiving food waste collected mainly from domestic kitchens was monitored over a period of 426 days. During this time information was gathered on the waste input material, the biogas production, and the digestate characteristics. A mass balance accounted for over 90% of the material entering the plant leaving as gaseous or digestate products. A comprehensive energy balance for the same period showed that for each tonne of input material the potential recoverable energy was 405 kWh. Biogas production in the digester was stable at 642 m 3 tonne -1 VS added with a methane content of around 62%. The nitrogen in the food waste input was on average 8.9 kg tonne -1 . This led to a high ammonia concentration in the digester which may have been responsible for the accumulation of volatile fatty acids that was also observed.
This is a revised personal version of the text of the final journal article, which is made available for scholarly purposes only, in accordance with the journal's author permissions. The full citation is: Banks, C.J., Chesshire, M. and Stringfellow A. (2008 ABSTRACT Source segregated food waste was collected from domestic properties and its composition determined together with the average weight produced per household, which was 2.91 kg per week. The waste was fed over a trial period lasting 58 weeks to an identical pair of 1.5 m 3 anaerobic digesters, one at a mesophilic (36.5 o C) and the other at a thermophilic temperature (56 o C). The digesters were monitored daily for gas production, solids destruction and regularly for digestate characteristics including alkalinity, pH, volatile fatty acid (VFA) and ammonia concentrations. Both digesters showed high VFA and ammonia concentrations but in the mesophilic digester the pH remained stable at around 7.4, buffered by a high alkalinity of 13,000 mg l -1 ; whereas in the thermophilic digester VFA levels reached 45,000 mg l -1 causing a drop in pH and digester instability. In the mesophilic digester volatile solids (VS) destruction and specific gas yield were favourable, with 67% of the organic solids being converted to biogas at a methane content of 58% giving a biogas yield of 0.63 m 3 kg -1 VS added. Digestion under thermophilic conditions showed potentially better VS destruction at 70% VS and a biogas yield of 0.67 m 3 kg -1 VS added , but the shifts in alkalinity and the high VFA concentrations required a reduced loading to be applied. The maximum beneficial loading that could be achieved in the mesophilic digester was 4.0 kg VS m -3 d -1.
The paper considers the role of anaerobic digestion in promoting good agricultural practice on farms and the contribution this would make to reducing the environmental impacts associated with manure management. There are no regulatory drivers to promote the use of digestion in Europe, and the technology has only been widely adopted where economic drivers and coherent policies have been implemented at a national level. These measures have included direct subsidy on the energy price paid for "green electricity", and exemption of tax when biogas is used as a vehicle fuel. In those countries where financial incentives are not available or where a financial penalty is incurred through the regulatory regime, the uptake of digestion has been poor. Even with subsidies, digestion of animal manures as a single substrate is not common, and countries with successful schemes have achieved this either by permitting the import of wastes onto the farm or offering bonus subsidies for the use of energy crops. Both of these measures improve the energy efficiency of the process by increasing the volumetric methane production, although concerns are expressed that attention could concentrate on energy production at the expense of improving manure management.
The South Shropshire biowaste digester is designed to recycle 5000 t per year of source-segregated household kitchen waste and green garden waste. This waste, collected at the kerbside, is transformed into renewable electricity and biofertiliser. It is one of nine full-scale projects funded by the UK Department for Environment, Food and Rural Affairs (Defra) to demonstrate technologies that enable biodegradable municipal waste to be diverted from landfill. Previous research, over a period of eight years leading to the biowaste digester, concentrated on the anaerobic digestion of food waste, which is able to contribute to the positive reduction in the emission of greenhouse gases in a number of different ways. As a renewable energy technology, anaerobic digestion is unusual in that the overall net reduction in the emission of carbon dioxide is greater than simply the carbon dioxide emissions from the fossil fuels that it displaces; however, apart from its application on sewage works and in landfill sites, the process is not yet widely applied in the UK. The project met the necessary permitting requirements and construction started in June 2005. The first biowaste was processed in March 2006 and the plant is operating at 50% of its design throughput. The first year of operation has shown that the quality of the biowaste is very different from that anticipated, in particular with a low capture of food waste. This issue is being addressed in order that the project can demonstrate the effective role that anaerobic digestion can play in the UK's future waste management strategy.
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