Assessing the Sustainability of Spent Mushroom Compost Disposal Options
|Spent mushroom compost (SMC) is the organic waste product of intensive mushroom farming. Disposal by spreading on fields sometimes results in environmental damage, necessitating alternative options. SMC disposal by land application, landfilling, and incineration for energy generation were assessed for sustainability using a three-pillar (environmental, economic, and social) approach.
A life cycle assessment (LCA) compared the environmental impacts on climate change, freshwater eutrophication, human toxicity, and freshwater aquatic eco-toxicity (FAET). Land application had the least global warming potential (GWP), greatest impact on eutrophication and FAET, and moderate human toxicity impacts. Landfilling had the greatest short-term GWP due to methane emissions, but impacts decreased as timescale increased. It had high impacts on eutrophication and human toxicity, but negligible impacts on FAET. Energy generation had the greatest long-term GWP because of CO2 emissions, but negligible or positive impacts on eutrophication, human toxicity, and FAET because of offset conventional energy generation. Economically, landfilling is prohibitively expensive because of already high and rapidly increasing landfill fees in the UK. Costs for 64 tonnes weekly for 15 years exceed £2 million net present value (NPV). In contrast, land application costs less than £400,000 NPV. Energy generation is profitable, earning revenues of £9000 NPV, however at significant capital expense of £250,000. Energy generation is very favourable socially (and politically) as it has the least impact on local populations and is a favoured technology by the government. Land application has societal impacts that, if adequately considered, would make it acceptable. Landfilling is socially unacceptable and strongly discouraged by the government.
Landfilling is a poor disposal option from all perspectives, but the question of whether land application or energy generation should be pursued depends on context. If cheap or free land spreading is possible, and local conditions amenable, land application is the best option for mushroom farms. Under all other contexts, energy generation is optimal, especially since it provides a secure disposal route, revenue generation, and has minimal (sometimes positive) overall environmental impact.