Low-Dosage Enzymatic Decomposition of OFMSW for Sugar Recovery and Ethanol Production

Enzymatic hydrolysis (EH) of the organic fraction of municipal solid waste (OFMSW) is challenging due to its complexity, inconsistency, and heterogeneous character, requiring industrial hydrolytic enzyme cocktails composed of amylases, cellulases, and hemicelluloses to liquefy and saccharify waste biomass into fermentable sugars. The chemical analysis of OFMSW collected in southern Jutland revealed to have high sugar content, corresponding to 47 g of sugars/100 g DM of OFMSW, of which 15 g is starch. It shows that OFMSW can be an excellent second-generation feedstock for biochemical production if sugars are efficiently liberated. Also, it was discovered that OFMSW has a pH of 4 and a considerable concentration of organic acids, 15 g of acids/100g DM of OFMSW. It shows that organic waste is highly prefermented, which significantly influences its bioprocessing. In this study, the effects of OFMSW compositional characteristics and pretreatment techniques on the enzymatic hydrolysis of OFMSW were investigated. The experimental work involved varying enzymatic dosages and pretreatment methods, including heat pretreatment and anaerobic incubation. The enzymatic hydrolysis of OFMSW showed that a 50% yield of saccharification was achieved by using only 1 FPU of Ctec3/g DM OFMSW and 0.5 U glucoamylase/g DM OFMSW. However, increasing the enzymatic dosages did not result in a significant increase in saccharification. The research revealed that the decomposition of OFMSW by enzymes is limited after reaching about 50% of the sugar recovery yield. Further, ethanol fermentation with Saccharomyces cerevisiae was conducted on sugar-rich hydrolysates, resulting in the desirable conversion of the majority of hexose sugars to ethanol and revealing that product inhibition was not a cause of limitation of OFMSW saccharification. It is suggested that the limitation is due to the high concentration of inhibitors such as organic acids, alcohols, and potentially other organics and inorganics, but on the other hand, these compounds serve as excellent preservatives, achieving contamination control during biological conversion, which is proven by the lack of severe contamination after EH or ethanol fermentation. These findings highlight the need for further investigation of the saccharification process of OFMSW to overcome inhibitory effects and subsequently achieve higher efficiency of the following bioprocess.