Decline in microbial activity does not necessarily indicate an end to biodegradation in MSW-biowaste: a case study

The aim of this study was to examine whether a decline in microbial activity (i.e., CO2 output) during the biodegradation of municipal solid waste (MSW) indicated an end to biodegradation and the appearance of a stable, final product. As the organic fraction of MSW was biodegraded in an 8 clamp Cambridge Batch System composter, CO2 output declined by 50% and moisture content at the end of the process was <20%. Levels of biodegradable material remaining in the product were determined by the dynamic respiration index (DRI) method but despite 151 days in the composting system biodegradable material was still present at levels (24130 mg O2/kg dry matter (DM)) which exceeded draft EU biowaste directive (2001) guidelines (10,000 mg O2/kg DM). Further laboratory based incubations demonstrated that microbial activity and hence biodegradation of organic material could be restarted if moisture levels were adjusted suggesting that dehydration limited microbial activity. Low levels of microbial activity alone did not therefore indicate and end to biodegradation, biodegradable material was not exhausted and the final product was not stable which has serious implications for its end-use.