Composting and anaerobic digestion

An opinion piece published in “The Spinoff” by Kate Walmsley on 23 August 2020 entitled “Why industrial anaerobic digestion is not the answer to food waste” casts doubt on claims that food waste anaerobic digestion(AD) plant will deliver sustainable outcomes for horticultural customers aligned with achieving a more circular economy in New Zealand. The article specifically refers to the Ecogas anaerobic digestion facility in Reporoa.

The article puts the case for composting food wastes to produce biofertiliser, instead of processing the organic waste in anaerobic digestion plant to produce energy and biofertiliser. The article claims that composting provides better environmental outcomes than AD processing due to a better soil conditioning properties of compost versus digestate from AD, lower greenhouse gas emissions and larger societal benefits of composting.

The article misses the point that composting and AD processing of organic waste are two well proven, extensively used and often complimentary technologies used world wide. The choice of processing pathway depends on the composition of the feedstock and the desired products produced in response to the local market demand and opportunities. In some applications with specific feedstocks, composting will be the optimal processing path, while in other applications and feedstock AD processing will be optimal. In general, high energy feedstocks such as food waste are better suited for AD, whereas high fibre feedstocks such as greenwaste are more suited to composting3 . Most importantly, they both keep waste out of landfills and hence substantially reduce greenhouse gas emissions and other environmental risks and impacts associated with that processing practice.

In general terms compost is an organic soil improver and biofertiliser from AD is an organic fertiliser.

The purpose of this response is to provide scientifically proven and verified facts to demonstrate the substantial benefits of using anaerobic digestion for processing of the unavoidable fraction of food and commercial/industrial organic waste that cannot be effectively reused in its raw form alongside the alternative benefits of composting organic waste.

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