Characterization of Exocellular Protein and Its Role in Bioflocculation

The relationship between exocellular biopolymer concentration and cation concentration was examined using laboratory scale activated sludge reactors with bactopeptone as a feed. An increase in the divalent cation concentration in the feed to the reactors was associated with an increase in the bound exocellular protein concentration, and high sodium concentrations resulted in a decrease in the bound protein concentration. The changes in bound biopolymer were explained according to the cation bridging model. Incubation of a laboratory activated sludge with a proteolytic enzyme resulted in deflocculation of the suspension as measured by an increase in the number of particles in the 5–40 μm range, which suggested that the exocellular protein was strongly involved in the aggregation of bacteria into flocs. SDS PAGE results revealed the presence of a single protein in the exocellular biopolymer extract from municipal, industrial, and laboratory activated sludge samples. The molecular weight of the protein was approximately 15 daltons. Amino acid analysis and amino acid sequencing results suggested the protein was a lectinlike protein, and binding site inhibition studies demonstrated that the protein had lectinlike activity.