Anaerobic-Aerobic Digestion: Analysis of Process Performance for Secondary and Mixed Sludge

Sequential anaerobic-aerobic digestion has been demonstrated to be an effective alternative to enhance sludge stabilization: the basic motivation behind this approach is related to the different reaction environments provided for biodegradation of the different volatile solids (VS) fractions. In this paper, results of an extensive investigation on waste and mixed sludge from a full scale wastewater treatment plant are reported. Sequential digestion performance was evaluated in terms of VS removal efficiency, biogas production, and dewaterability of the digested sludge. High VS removal efficiencies (47% in the anaerobic phase and 25% in the aerobic one) were obtained for secondary sludge. Even higher performance was observed for mixed sludge with efficiency values of 50 and 45 % for the anaerobic and aerobic stage respectively. The aerobic stage was operated with alternate aeration so providing effective nitrogen removal and a consequent marked reduction of the nitrogen load recycled to the plant. The influence on sludge dewaterability was evaluated by determining the Capillary Suction Time (CST) and a significant positive effect due to the aerobic stage was observed. Biogas production (0.54 and 0.8 Nm3/(kgVSdestroyed) for waste and mixed sludge respectively) was close in both cases to the upper limit of the range of values reported in the specialised literature so demonstrating that the low anaerobic sludge retention time of 15 days is still suitable for efficient energy recovery.