Mass and energy balance for a rotating-drum composting plant

The Bioganix in-vessel composting system at Leominster, UK, received 33 515 t of waste from January 2006 to December 2007, of which 9200 t was source-segregated biodegradable municipal waste (BMW). The process operated with a nominal in-vessel retention time of 4 days. Mass balances indicated 17·1% reduction (excluding reject materials). The process was estimated to consume 186 kWh/t of waste processed as electrical energy for static plant and a further 9·6 kWh as diesel for mobile plant. Taking into account transportation and application of the compost, the estimated consumption was ~560 kWh/t of waste processed, calculated as primary energy (including electrical conversion). 96·7% of this was for processing; transportation consumed 1·3% and application 2·0%. The mixed waste compost had a high nitrogen content but, for a typical source-segregated biodegradable municipal waste, the energy potentially offset from nitrogen fixation is likely to be considerably less than that used in processing.