Integrating mineral wastes in the anaerobic digestion of OFMSW for improved recovery of renewable energy

Abstract

This thesis investigated the effects of mineral wastes (MW) on laboratory-scale anaerobic reactors treating organic wastes. Different MW resources were used, incineration bottom ash (IBA), fly ash (FA) and boiler ash (BA), taken from a municipal solid waste incineration (MSWI) plants, as well as a cement-based waste (CBW) from construction demolition wastes. The hypothesis was that these MW would provide trace elements (TEs) deficient in the organic fraction of municipal solid waste (OFMSW), and offer moderate alkalinity to prevent reactor acidification of mesophilic anaerobic digestion of the OFMSW. The control and operation of batch biomethane potential (BMP) reactors and continuous stirred tank reactors (CSTRs; single-stage and two-stage reactors), was studied under different feeding regimes, different organic loading rates and hydraulic/solid retention times, in order to determine potential benefits of mineral waste amendments and aqueous trace metal supplements on anaerobic digestion efficiency, methane productivity and process stability. Co-digestion of different solid MW and organic wastes in single-stage CSTR using a liquid-recycled feeding method (LRFM) enhanced process stability (pH of 6.8 – 7.2), increased methane production by 25 - 45%, and yielded 450 – 520 mL CH4/g VS (near to the theoretical maximum) compared to the control. Whereas draw-and-fill feeding method (DFFM) also enhanced digestibility but to a lesser degree. Pre-treatment of the OFMSW with the MW at 37oC improved substrate hydrolysis, and enhanced the performance and stability of the DFFM digestion processes further to values similar to those of LRFM reactors. Amending two-stage CSTRs with aqueous MW extracts provided the reactors with the necessary trace elements deficient in the OFMSW, maintained alkalinity and pH, and hence enhanced hydrogen/methane production and processes stability of both acidogenic and methanogenic reactors. Amendments of IBA, BA and CBW provided trace metals that supported anaerobic digestion processes without adverse effects; however, the metals released from FA provided much lower enhancement of the digestion processes, as some trace metal concentrations were within the toxic range for methanogenic processes. To elucidate and compare the effect and importance of commercial TE supplementation and substrate co-digestion techniques in improving organic waste anaerobic digestion, especially for the single-stage reactors with high organic loading rates, different CSTR feed compositions were studied. Different feedstocks were investigated including synthetic organic waste (SOW), SOW supplemented with TE, SOW supplemented with wheat straw (WS) and ii SOW supplemented with WS and TE. Results showed that high methane yields (450 - 550 mL/g VS), higher microbial numbers and process stability at higher OLRs, were achieved in all reactors having TE supplementation compared to the equivalent reactors without TE supplementation. From analysis of molecular microbial data, the effect that different feeding methods, reaction times and WS co-digestion had on reactor performance was found to be associated directly with microbial community selection and stability. Different feeding regimes altered the microbial communities; Methanoculleus (hydrogenotrophic) and Methanosaeta (acetoclastic) were the most abundant methanogenic genera in the LRFM reactors, and the more metabolically versatile Methanosarcina genus dominated under DFFM. Interestingly, at 25% WS supplementation, the Methanosarcina were found to be acetoclastic (based on indicative coenzyme F420 measurements), but with no WS amendment with highest NH3-N levels the F420 values indicated a predominantly hydrogenotrophic metabolism. These results suggest that, WS co-digestion reduced biological stress on the anaerobic community by reducing the net concentration of ammonia in the feedstock.