Thermally enhanced solubilization and anaerobic digestion of organic fraction of municipal solid waste

Abstract

Organic fraction of municipal solid waste (OFMSW) is an ideal substrate for biogas production; however, complex chemical structure and being heterogeneous obstruct its biotransformation in anaerobic digestion (AD) process. Thermal pre-treatment of OFMSW has been suggested to enhance the solubilization and improve the anaerobic digestibility of OFMSW. This paper critically and comprehensively reviews the characterization of OFMSW (physical, chemical, bromatological) and enlightens the valuable properties of OFMSW for waste valorization. In following sections, the advantages and limitations of AD of OFMSW are discussed, followed by the application of temperature phased AD, and various thermal pre-treatments, i.e., conventional thermal, microwave, and thermo-chemical for high rate bioenergy transformation. Effects of pre-treatment on COD, proteins, sugars and VS solubilization, and biogas yield are discussed. Formation of recalcitrant during thermal pre-treatment and the effect on anaerobic digestibility are considered. Full scale application, and techno-economic and environmental feasibility of thermal pre-treatment methods are also revealed. This review concluded that thermophilic (55 °C) and temperature phased anaerobic digestion, temperature phased anaerobic digestion, TPAD (55 + 37 °C) processes shows effective and stable performance at low HRTs and high OLRs and achieved higher methane yield than mesophilic digestion. The thermal pre-treatment at a lower temperature (120 °C) improves the net energy yield. However, high-temperature pre-treatment (>150 °C) result in decreased biogas yield and even lower than the non-pre-treated OFMSW, although a high degree of COD solubilization. The OFMSW solubilization in terms of COD, proteins, and sugars cannot accurately reflect thermal/hybrid pre-treatments' potential. Thus, substrate pre-treatment followed by anaerobic digestibility of pretreated substrate together can evaluate the actual effectiveness of thermal pre-treatment of OFMSW.