Down to Earth: An illustration of life cycle inventory good practice with reference to the production of soil conditioning compost

The need to minimise the environmental impacts of production and consumption are increasingly being recognised by governments, industry and the general public. Biodegradable waste composting is able to make a contribution to this need, not only in reducing the biodegradable waste being landfilled, but also lessening the horticultural industry's and gardeners’ reliance on peat based compost, whilst the application of compost to land has the potential to enhance soil and biomass carbon sequestration.As a result of the consequent interest in the potential environmental benefits accruing from composting, there have been a number of studies attempting to measure the gains from composting the biodegradable fraction of commercial and household waste streams. However, a review of such studies reveal that there is much scope for improvement in defining the product process and in data accuracy, as well as the technical framework employed to construct the life cycle inventory stage. These shortcomings can invalidate life cycle impact assessments, and at worst lead to invalid management and operational decisions based on their findings.This study addresses the inventory process deficiencies, and issues of data reliability and completeness, contained in much of the published compost production life cycle assessment literature. It provides a comprehensive cradle to grave inventory of the soil conditioning compost produced by a social enterprise company in Manchester, England. Based on the specifics of the system model, it provides the basis for valid comparisons of the environmental benefits of producing soil-conditioning compost from biodegradable municipal waste with other composting and waste management options. Overall energy use has been calculated as 1034 MJ/tonne of compost, equivalent to 633 MJ/tonne for processing the wet feedstock, with the processing inventory stage accounting for 57%.