The roles and interactions of cations, proteins, and polysaccharides in the settling and dewatering of activated sludge

The roles of cations and exocellular biopolymer on the settling and dewatering of activated sludge was investigated. Both laboratory and full-scale activated sludge systems were examined. The results of the study showed the settling and dewatering properties of the activated sludge were dependent on the calcium, magnesium, potassium and sodium concentrations added to the feed. A minimum of 0.72 meq/L each of calcium and magnesium in the feed was necessary for acceptable settling and dewatering. Two types of microbial cultures were observed, one required both calcium and magnesium in the feed while the other required either calcium or magnesium, but not both, for optimization of settling and dewatering. Sodium addition to the feed improved the settling of activated sludge when the ratio of sodium to calcium plus magnesium was equal to approximately one on a meq/L basis. When this ratio was greater than two, the settling and dewatering properties deteriorated, but the deterioration could be reversed by increasing the calcium and magnesium concentration in the feed. In general, the data supported the cation bridging model for bioflocculation, and the cations act to bind protein to the biofloc structure.

Results from the full-scale activated sludge plants correlated well with results from laboratory activated sludge systems and demonstrated the cation content in these systems had a direct impact on the settling and dewatering properties. Field trials in which divalent cations were added to activated sludge systems resulted in dramatic improvements in the settling properties of these systems. Characterization of the exocellular protein extracted from laboratory, industrial and municipal activated sludges revealed the presence of a single protein, which appears to be a lectin. The molecular weight of the protein measured by SDS PAGE was approximately 16,000 Daltons with similar amino acid composition as microbial lectins. Also, amino acid sequencing analysis indicated the N-terminal sequence of the protein was consistent with those of microbial lectins. In addition, the activated sludge cultures exhibited lectin activity as demonstrated by binding site inhibition experiments. A model of bioflocculation that includes the role of protein was proposed.