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Environmental Impact of Composting E-mail
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Sunday, 12 August 2007
Environmental Impact of Composting

Performance and Environmental Impact of a Combined In-Vessel Composting and Vermicomposting System

Frederickson J., Howell G. and Hobson A. M

Integrated Waste Systems research group, The Open University, Walton Hall, Technology Faculty, Milton Keynes, MK7 6AA

There is a growing interest in the use of combined composting systems designed to comply with legislation and/or to achieve specific technical objectives, such as producing high specification composts. However, there is little published data on the effectiveness of combined systems, especially relating to vermicomposting systems. In England, the use of commercial-scale vermicomposting to process and add-value to kitchen wastes is prohibited without first subjecting the waste to thermophilic composting, under enclosed conditions. Hence, a combined system is required if vermicomposting is to be utilised. The aim of this study was to investigate the performance and environmental impact of a combined system of composting comprising in-vessel thermophilic composting and vermicomposting, compared with a system comprising in-vessel thermophilic composting and windrow composting. Source segregated household waste was first subjected to in-vessel composting for 14 days to comply with the Animal By-Products Regulations (2003). After this, the partially stabilised waste was matured for 92 days using either a large-scale vermicomposting system or a windrow composting system. The maturation phase was replicated five times. The mature windrow compost and vermicompost were then subjected to bioassay evaluation using the PAS100 (2005) method. During all stages of processing, physico-chemical characteristics of the waste were recorded, stabilisation rates were measured using respirometry and greenhouse gas emissions were monitored.

During the initial stage of in-vessel composting, the waste lost 12% of its carbon content and 8% of its nitrogen content. The in-vessel composting system generated CH4 and N2O. During the 92 day maturation process, respirometric analysis of the windrow compost and vermicompost was undertaken. This showed the two processes to be equally effective at maturing the waste. In terms of greenhouse gas emissions, the vermicomposting process emitted N2O mainly while the windrow composting process emitted CH4. At the end of maturation, the physico-chemical characteristics of each compost were compared. The nitrate (NO3) content for the vermicompost was 43% greater than for the windrow compost while the mass of material in the smallest particle size fraction (<10 mm) was significantly higher for the vermicompost. Also, the electrical conductivity (EC) of the vermicompost was approximately 50% less than the windrow compost. When subjected to the PAS100 bioassay test, the vermicompost, compared with the windrow compost, produced significantly increased yields for one out of the three plant types tested.

It may be concluded that the combination of thermophilic in-vessel composting and vermicomposting produced a horticultural grade compost with enhanced physico-chemical qualities compared with the combined windrow system. However, there was no evidence to suggest that the vermicomposting system increased compost maturation time compared with the windrow system. Equally, the enhanced characteristics found for the vermicompost only resulted in increased yield for one plant type out of three tested. The study demonstrated that all composting and vermicomposting systems investigated had the potential to generate greenhouse gases (CH4 or N2O) and the implications arising from this are discussed in the paper.


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