Dry thermophilic anaerobic digestion of the organic fraction of municipal solid wastes: Solid retention time optimization

The thermophilic temperature has showed operational advantages in the Anaerobic Digestion process (AD). The increasing of the rate of hydrolysis, which suppose an acceleration of the overall process, and a higher hygienization of the final digestate are one of them. Dealing with OFMSW (Organic Fraction of Municipal Solid Waste) from non-selective collection systems involves additional difficulties due to their heterogeneity. The AD process of OFMSW is conducted usually in the range of 20-35% TS (dry AD). The main objective of this study was to determine the optimum Solids Retention Time (SRT) for the dry thermophilic AD of OFMSW operating in semi-continuous regime. Thus, the following SRTs have been tested: 15, 10, 8, 6, 5, 4 and 3 days. The main results have showed that the process is feasible in SRTs lower that those found in literature. Probably the history of the reactor, joined to the lower organic content in the waste, can be a key factor and the progressive decreasing of SRI is a strategy that permits to maintain stable conditions for lower SRT. The best operating conditions for thermophilic semicontinuous systems ranged in 8-5 days, with a yield of 0.33-0.34 LCH4/gVS(added) and a concentration of Volatile Fatty Acids (VFA) around 100 mg HAc/L. The process becomes unstable for SRTs lower than 4 days, showing a clear decrease in methane productivity (lower than 0.2 LCH4/gVS(added)) and an accumulation of VFA (more than 500 mg HAc/L). In general, SRTs below 4 days were not suitable for single stage of dry AD of OFMSW. (C) 2014 Elsevier B.V. All rights reserved.