PREDICTING THE DIGESTIBILITY OF WASTE ACTIVATED SLUDGES USING CATION ANALYSIS

Laboratory digestion studies revealed that different cations in the feed sludge play a unique role in determining the digestibility of sludge under anaerobic and aerobic conditions. Batch anaerobic and aerobic digestion were carried out for 30 days at room temperature using waste activated sludge (WAS) collected from various wastewater treatment plants. Changes in solution and floc cations, solution biopolymer (protein and polysaccharide) and sludge dewatering characteristics were monitored before and after digestion. The anaerobic digestion data showed that protein increased significantly in solution phase and there were strong positive correlations between soluble protein, NH4+ production, % VS reduction and iron in WAS. These observations indicate that the release of Fe-bound protein from floc to solution and its degradation is strongly associated with VS destruction in anaerobic digestion. In aerobic digestion, polysaccharide was accumulated in solution with substantial increases in Ca2+ and Mg2+. Good correlations between divalent cation release and the production of inorganic nitrogen were obtained. This implies that divalent cation-bound biopolymer, thought to be lectin-protein, would be the primary organic fraction that is easily degraded under aerobic conditions. These data support the recently suggested floc concept that flocs consist of two types of organic matter with different cation bindings and each type is degraded under distinct digestion environments. The study further found other activated sludge characteristics such as effluent biopolymer and conditioning and dewatering properties of WAS were strongly influenced by iron and aluminum in floc. The more trivalent cations the sludge contained, the more favorable was the effluent and sludge characteristics. These results suggest that cations play a pivotal role in activated sludge and digestion process and better understanding of floc structure along with cation analysis would bring improvement in several biological processes in wastewater treatment field.