Physical chemical processes and environmental impacts associated with home composting

Abstract

This thesis reports on experimental and modelling work carried out in order to make quantitative estimates on the environmental impacts of home composting. The focus of the work was climate relevant gaseous emissions, and developing and utilising a methodology for quantifying them. Experiments using 220L open bottomed home compost bins, alongside purpose built 200L composting reactors with airflow control were performed. A variety of composting conditions were tested, using different compositions of garden and kitchen wastes. The experiments were monitored for headspace gas composition, including CO2, O2, NH3, N2O, CH4 and volatile organic compounds, as well as temperature, humidity, moisture and solids losses and pH.
From the CO2 emission rates calculated from the reactor experiments, theoretical analysis and modelling and airflow pathway tests on home compost bins, it was concluded that molecular diffusion, rather than bulk convective flow, is the dominant gas transfer mechanism from home compost bins. There were no detected emissions of N2O but emissions of NH3 up to 16 g/T feed. Only a few cases of CH4 emission were detected, typically in the first 2-3 days following a feed addition, with the highest single concentration measured at 86 ppm within the headspace.
The total anthropogenic greenhouse gas emissions from home composting were estimated as between 3 and 12 Kg CO2E/Tw with almost 90% coming from the lifecycle of the compost bin. This compares with between 20 and 56 Kg CO2E/Tw from centralised facilities, at least more than double that for home composting. Total anthropogenic CO2-equivalent emissions from home composting in the UK in 2008 were estimated to be in the region of 7 thousand tonnes CO2E.

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ORCID for Ian Williams: orcid.org/0000-0002-0121-1219

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