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

Organic Fraction of Municipal Solid Waste (OFMSW) is usually stored under variable humidity conditions and long periods before processing them in anaerobic digestion plants. Lately, the fermented OFMSW is mixed with recirculated digestate from the same biogas plants, which is used as methanogenic inoculum. 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, to our knowledge, no studies have been done about the combined effect of these operational parameters on methane production. Therefore, this study focused on determining how humidity conditions during OFMSW storage and the substrate to inoculum ratio (S/I) in the methanisation stage can be adjusted to improve methane production. OFMSW was stored at 35°C and 10, 20, and 28%TS for 15 days. In the second stage, methanisation of previously fermented OFMSW was allowed at different S/I ratios of 0.5, 1.0, and 1.5. Ethanol and acetic acid accounted for 90% of all products of fermentation. The lowest solids concentration reached the highest fermentation degree. Compared to fresh OFMSW (without storing), methane from fermented OFMSW increased 32% and, the times to reach the maximum methane production decreased between 11 and 40%. For fermented OFMSW, 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 improving methane production and reducing reaction times.