Effects of a temporary increase in OLR and a simultaneous decrease in HRT on dry anaerobic digestion of OFMSW

The effects of the temporary increase of organic loading rate (OLR) combined with the simultaneous decrease of hydraulic retention time (HRT) on the stability of a pilot scale dry anaerobic digester were investigated. The separately collected organic fraction of municipal solid waste in mesophilic conditions (T = 40°C) was treated. The objective of this study was to verify whether it is possible to feed the digester for short periods, about three consecutive weeks, with higher OLRs and lower HRTs than those considered optimal without generating process failure or long-term instability. Starting from stable operation at a daily OLR of 10.0 kg of total volatile solids (TVS) for digester volume and an HRT of 23 d, the reactor was fed with an OLR of 10.8, 11.7 and 12.5 kg TVS m^-3 d^-1 corresponding to an HRT of 21, 19 and 18 d, respectively. It was observed that after using an OLR of 10.8 and 11.7 kg TVS m^-3 d^-1 for 3 weeks with satisfying results, it was possible to restore stable operating conditions at an OLR of 10.0 kg TVS m^-3 d^-1 in a short time. Otherwise, after using an OLR of 12.5 kg TVS m^-3 d^-1 the anaerobic digestion was deeply unbalanced and quickly failed. In this latter case, however, it was possible to fully recover and restore the stable conditions of the process within two months.