Energy Optimization of Aerobic Digestion of MBR Sludge

Microaerophilic sludge digestion is better suited to excess MBR sludge digestion because the long SRTs already in place during operation of MBRs. A comparison of the design parameters of aerobic and micro-aerophilic sludge digestion of MBRs is presented. When using option 4 of 503 rule for reduction of vector attraction, SOUR less than 1.5 mg O2 / g TS d, the criteria is met consistently within two days of digestion for all conditions tested in this project. For initial SOUR rates of 3 mg O2/g TS h the required digestion time is only 5 days. This is in contrast to over 35 days required for obtaining 38% reduction in VS. No significant differences were observed in the rate of SOUR reduction or the time required for achieving the target SOUR when digesting MBR sludge under aerobic or anoxic conditions. No significant effect of the initial TSS concentration was observed either. This effect is consistent with theory. It is advantageous to control the ORP of the digestion process to a level over – 200mV when using anoxic digestion. ORP values lower than this would induce significant accumulation of ammonia in the system. Ammonia has a strong effect on the determination of SOUR. Concentrations as low as 1 mg/L induce saturation of the kinetics of oxygen uptake creating a confounding factor in the SOUR determinations. It is recommended to aerate the sludge for a short aeration period of 30 minutes in order to remove trace ammonia levels and to obtain consistent results for comparison with aerobic digestion. The aerobic process exhibited higher pathogen reduction rate than the microaerophilic process. It is likely that pathogen reduction criteria might control the overall sizing of the digester if Class B sludge is desired. Combination of SOUR and pathogen reduction criteria could be optimized for different disposal scenarios.