Effect of Secondary Aerobic Digestion on Properties of Anaerobic Digested Biosolids

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ABSTRACT
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.

INTRODUCTION
Engineers are constantly searching for ways to increase volatile solids destruction in anaerobically digested biosolids. An increase in volatile solids destruction results in a decrease in dry solids mass, and often an increase in overall dry cake solids content. These changes can thus reducing biosolids management costs. Anaerobically digested biosolids often have poor dewatering properties and they may also generate odors, especially if they are dewatered using high solid centrifuge (Novak, et al., 2006). Poor dewatering and offensive odors are a cause for concern for wastewater treatment utilities due to the additional costs of conditioning biosolids, solids disposal and public relations problems. Therefore, methods to improve dewatering and reduce odors are also desired.

The release of extra-cellular polymer or biopolymer, primarily soluble proteins and polysaccharides, during digestion is considered to be responsible for increasing polymer demand during dewatering (Novak et al., 2003). Research by Novak and Higgins (1997b), and Novak et al. (2003) suggest that different type and quantity of biopolymer are released by anaerobic and aerobic digestion. Subsequent to aerobic digestion, soluble extra-cellular polysaccharides are the predominant biopolymers, while after anaerobic digestion, both soluble polysaccharides and protein accumulates. Thus anaerobically digested biosolids can have a substantially greater polymer demand than aerobically digested biosolids. The post-aerobic treatment of anaerobically digested biosolids could potentially remove some of the accumulated protein and reduce polymer demand.

Park et al. (2006) found that proteins released during anaerobic digestion of various sludge was strongly correlated with VS removal. They also observed that the total VS reduction by either anaerobic-aerobic digestion or aerobic-anaerobic digestion is the same; only the fraction of VS removed during the separate anaerobic or aerobic stage changes. This suggests that there is some portion of VS that can be degraded only by aerobic digestion and some portion degraded only under anaerobic conditions. Subramanian (2005) found improved cake solids and reduced polymer demand for sequential anaerobic-aerobic digested biosolids due to the reduction of bound water and low concentration of proteins and polysaccharide.

The literature suggests that both anaerobic and aerobic digestion processes degrade different portions of sludge and a small fraction of solids can be digested under both digestion processes. Therefore, by utilizing combined anaerobic-aerobic digestion, higher solids destruction and lower odors generation might be possible. The main objective of this study was to determine the effect of sequential anaerobic-aerobic digestion volatile solids removal, biosolids dewatering and odorant production. This study also investigated the influence of simulated winter storage (a single freeze-thaw cycle) on odorant production from digested biosolids.

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