Role of stage-separation in the ubiquitous development of Anaerobic Digestion of Organic Fraction of Municipal Solid Waste: A critical review

Managing the ever increasing load of municipal solid waste (MSW) has been one of the biggest challenges for municipalities all over the world. According to World Bank reports, the global generation of MSW per year is expected to rise to 2.2 billion tons by 2025. Organic fraction of the municipal solid waste (OFMSW) accounts for more than 40% of the generated MSW. Anaerobic digestion (AD) has been a popular technique for OFMSW management due to its potential to generate methane and a semi-stabilized digestate, which can be further processed as a substitute for inorganic fertilizers. Most of the existing full-scale AD treatment plants stabilizing OFMSW are single-stage based. However, with the increase in OFMSW generation-rate phase separation in AD has become more pertinent. Compared to single-stage AD systems, multi-stage AD systems are capable of not only treating an increased load of OFMSW but also producing greater methane and bio-hydrogen. Most recently only a handful of full-scale AD plants in Europe have shifted to two-stage systems. Efficiency in terms of energy recovery has been reported to be up to 94.5% and 86% for multi- and single-stage AD systems, treating organic solid waste, respectively. This can be only enhanced further if the operating conditions optimum for each phase is properly maintained. In three-stage AD systems, pH suitable for hydrolysis (4-5), acidogenesis (5-6.5) and methanogenesis (6.8-7.5) can be easily maintained. Not only that, three-stage AD systems also offer provision for variation in loading-rates and retention-times suitable for individual phases. Furthermore, multi-stage AD systems provide a scope for improved trace-element supplementation, which results in 10-50% performance increase per unit reactor volumes. The present review clearly illustrates the need to switch over to two- and three-stage AD systems for stabilizing OFMSW.