CH₄ and N₂O from mechanically turned windrow and vermicomposting systems following in-vessel pre-treatment

Hobson, A. M.; Frederickson, J. and Dise, N. B. (2005). CH₄ and N₂O from mechanically turned windrow and vermicomposting systems following in-vessel pre-treatment. Waste Management, 25(4) pp. 345–352.

DOI: https://doi.org/10.1016/j.wasman.2005.02.015

Abstract

Methane (CH₄) and nitrous oxide (N₂O) are included in the six greenhouse gases listed in the Kyoto protocol that require emission reduction. To meet reduced emission targets, governments need to first quantify their contribution to global warming. Composting has been identified as an important source of CH₄ and N₂O. With increasing divergence of biodegradable waste from landfill into the composting sector, it is important to quantify emissions of CH₄ and N₂O from all forms of composting and from all stages. This study focuses on the final phase of a two stage composting process and compares the generation and emission of CH₄ and N₂O associated with two differing composting methods: mechanically turned windrow and vermicomposting. The first stage was in-vessel pre-treatment. Source-segregated household waste was first pre-composted for seven days using an in-vessel system. The second stage of composting involved forming half of the pre-composted material into a windrow and applying half to vermicomposting beds. The duration of this stage was 85 days and CH₄ and N₂O emissions were monitored throughout for both systems. Waste samples were regularly subjected to respirometry analysis and both processes were found to be equally effective at stabilising the organic matter content. The mechanically turned windrow system was characterised by emissions of CH₄ and to a much lesser extent N₂O. However, the vermicomposting system emitted significant fluxes of N2O and only trace amounts of CH₄. In-vessel pre-treatment removed considerable amounts of available C and N prior to the second stage of composting. This had the effect of reducing emissions of CH₄ and N₂O from the second stage compared to emissions from fresh waste found in other studies. The characteristics of each of the two composting processes are discussed in detail. Very different mechanisms for emission of CH₄ and N₂O are proposed for each system. For the windrow system, development of anaerobic zones were thought to be responsible for CH₄ release. High N₂O emission rates from vermicomposting were ascribed to strongly nitrifying conditions in the processing beds combined with the presence of de-nitrifying bacteria within the worm gut.

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