Alkalinity ratios to identify process imbalances in anaerobic digesters treating source-sorted organic fraction of municipal wastes

• A long term anaerobic digestion process of organic fraction of municipal solid wastes was performed and monitored.
• Alkalinity ratios were evaluated as effective indicators of process imbalances in anaerobic digesters treating SS-OFMSW.
• As a cheaper alternative to VFA measurements, limit values of alkalinity ratios were established guaranteeing stability.

Two 5 L anaerobic reactors were used to monitor the mesophilic anaerobic digestion of source sorted organic fraction of municipal solid wastes (SS-OFMSW) focusing the attention on the response of alkalinity ratios. Intermediate/partial alkalinity (IA/PA) ratio can be used as a simple and cheaper alternative to VFAs analysis when digester's stability needs to be assessed in full-scale plants treating these organic wastes. However, lab-scale studies in order to establish a specific limit value of IA/PA referred to SS-OFMSW had not been conducted. In this study, a reference reactor (R1) was operated at low organic loading rates (OLR) and high hydraulic retention times (HRT) during 165 days. Besides, severe disturbances were applied to a second reactor (R2) during 281 days by means of increasing both HRT and OLR in order to assess the digester response under continuous overload conditions. The obtained results show that an IA/PA ratio of below 0.3 is recommended to maintain total VFAs between 2.5 and 3.5 kg m−3 and achieve a stable reactor performance treating SS-OFMSW in a range of total alkalinity (TA) between 13 and 15 kg CaCO3 m−3. These results provide a starting point to develop further works in full-scale digesters, in order to improve the monitoring and process control of full-scale anaerobic reactors treating SS-OFMSW.