Optimization of Semy-Dry anaerobic digestion for a wider diffusion of the Food Waste treatment for energy recovery. Technical and Economic evaluation

Currently, large plants are the most favoured approach for the anaerobic treatment of the organic fraction of municipal solid waste (OFMSW). However, centralised solutions imply certain limitations which prevent large-scale implementation of the anaerobic digestion (AD) of food waste for energy recovery. As a result, we are digesting less than 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, easy-to-operate AD plants is proposed. The purpose of the research is to investigate the better way to apply the Semi-Dry approach to the OFMST treatment, with fermenters that can manage to treat the biowaste as it is (feeding with TS>20-25%), with no dilution or any co-substrate addition needed. A 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. Costs in terms of capital and operating expenditure are estimated, and an economic evaluation is carried on to study the economic sustainability of full-scale installation at different plant sizes.