Effect of Secondary Aerobic Digestion on Properties of Anaerobic Digested Biosolids

In this study, the performance of sequential anaerobic-aerobic digestion was compared to conventional anaerobic digestion for sludge from the Blue Plains Advanced Wastewater Treatment Plant. Volatile solids removal, polymer conditioning demand for dewatering, and biosolids odor characteristics following anaerobic digestion and sequential anaerobic-aerobic digestion were monitored. Aerobic digesters downstream of anaerobic digesters were found to improve overall process performance. For the sequential anaerobic-aerobic digesters, volatile solids removal was more than 60%. Improvement in the biosolids dewatering properties was also found as evidenced by a decrease in the capillary suction time (CST), polymer conditioner dose requirements. Combined soluble protein and polysaccharides present in the anaerobic digester were reduced by 85% after aerobic digestion. Following sequential anaerobic-aerobic digestion, it was observed that sludges that were digested under thermophilic anaerobic conditions produced approximately 30% less odorants than mesophilic digested biosolids and the addition of an aerobic digestion step reduced odorant production by an additional 40%. A test was conducted to simulate winter storage by exposing the biosolids to a single freeze-thaw cycle. Freeze-thaw treatment of digested biosolids showed that even after anaerobic-aerobic digestion, sludges retain a potential for high and rapid odor production. It appears that following freezethaw treatment, additional proteins are made bioavailable, resulting in the higher odor generation.