EFFECTS OF SOLIDS CONCENTRATION AND SUBSTRATE TO INOCULUM RATIO ON METHANE PRODUCTION FROM FERMENTED ORGANIC MUNICIPAL SOLID WASTE

The organic fraction of municipal solid waste (OFMSW) is usually stored under variable humidity conditions and extended periods before processing in anaerobic digestion plants. Digestate from previous batches is used as inoculum and mixed with previously stored (fermented) OFMSW for methane production under different substrate (OFMSW) to inoculum (digestate) ratios. Although both the moisture content during the storage of OFMSW and the inoculum concentration in the feed mixture to the anaerobic reactors are determining factors for the process, no studies were found on how these combined parameters affect methane production. This research is focused on determining how humidity affects OFMSW during storage (fermentation) and how the substrate to inoculum ratio affects the methanisation of the previously stored OFMSW. OFMSW with solids concentrations of 10, 20, and 28% was stored/fermented at 35 degrees C for 15 days. The methanisation of the previously fermented OFMSW was allowed at S/I ratios of 0.5, 1.0, and 1.5. The fermentation results show that ethanol and acetic acid accounted for 90% of all products (metabolites). The lowest solids concentration allowed the highest fermentation degree with the highest VFA and alcohols production. Compared to fresh OFMSW, methane from fermented OFMSW increased 32%, and the time required to reach the maximum methane production decreased between 11 and 40%. For fermented OFMSW, the S/I ratio of 1.0 is the best condition to produce methane. ANOVA shows that, independently of solid concentration during storage, the S/I ratio is the main parameter to consider for methane production and reducing reaction times.