Start-up of a High-Solid Pilot-Digester Treating Low Quality Mechanically-Separated Small Particle Fraction of OFMSW

A pilot reactor layout (0.300 m3) was developed to accommodate the high-solid mesophilic anaerobic digestion (AD) of the fine particle fraction of the mechanically sorted OFMSW. Digestion of the low-quality substrate was also performed in a laboratory reactor (0.013 m3) to enable a comparison to be made with the pilot unit. The substrate contained a sufficient concentration of lipids and carbohydrates to produce a significant amount of methane during digestion, maintaining also a low risk of acidification for the AD system. Stability in methane generation was attained soon after process initiation for both digesters due to a prompt acclimatization of methanogenic bacteria to the substrate and to the dry AD conditions. Slight fluctuations in methane release appearing over the entire start-up process were associated to the feeding and the quantity of soluble organics inserting the reactors on a daily basis. For the conventional reactor, a layer of unreacted substrate formed on the AD medium surface limited the exploitation of all the available substrate biodegradable organics. A similar effect could not be seen in the pilot configuration where the formation of a surface solid layer was prohibited, leading to a greater consumption of soluble organics contained in OFMSW and eventually to an elevated methane generation of 0.144 m3 kgVS−1 compared to the 0.134 m3 kgVS−1 produced in the laboratory reactor.