Single-phase anaerobic digestion of the organic fraction of municipal solid waste without dilution: Reactor stability and process performance of small, decentralised plants

Currently, centralised plants are the most favoured approach for the anaerobic treatment of the organic fraction of municipal solid waste (OFMSW). However, centralised solutions imply certain environmental impacts, which prevent large-scale implementation of the anaerobic digestion (AD). As a result, we are digesting <5% of organic waste both in Europe and the USA even today. Pursuing the criteria for maximising the balance between profit and impacts, an innovative layout with the ultimate goal of promoting the use of small, decentralised AD plants is proposed. In this study, source-separated OFMSW (SS-OFMSW) was treated in a mesophilic plug flow reactor by applying an atypical combination of conditions such as high SS-OFMSW solid content (214.5 g·kg−1), high organic loading rate (6.2 kg VS·m−3·d−1), and no dilution or co-substrate addition. A suitable and an efficient mixing system is essential to control the process. Accordingly, the process was stable in a single-stage reactor, in the absence of digestate recirculation, obtaining specific gas production of 0.67 m3·kg−1 VS in terms of biogas and 0.41 m3·kg−1 VS in terms of methane. High reactor volume exploitation and small plant construction were feasible, reaching a gas production rate of 4.5 m3·m−3 d−1. The estimated costs in terms of capital and operating expenditure are expected to realize gross economic sustainability of full-scale installation.