Effects of sludge properties in a combined process of mesophilic anaerobic-aerobic digestion

Abstract The combined medium-temperature anaerobic digestion and medium-temperature aerobic digestion (MAND/MAD) is a strategy that is suitable for the disposal of conventional activated sludge. The MAND/MAD sludge stabilization process was performed at three retention times (17, 24, and 38 days) of anaerobic digestion (AND-time) with further duration up to 22 days of the MAD process. The retention time of the MAND process had an important role in sludge stabilization. Its extension enhanced sludge stability in the aerobic digestion stage. During a MAD process, values of volatile/total solids ratio (VS/TS) presented the lowering course for the three investigated retention AND-times. The relatively long retention time of MAND leads to a higher level of soluble organic compounds (SCOD) and extracellular polymers (EPS) in the supernatant. Due to the shorter capillary uptake time at the end of digestion in the MAND/MAD system, the sludge with an AND-time of 38 days was extremely susceptible to dewatering compared to the sludge with an AND-time of 17 and 24 days.