Examination of three theories for mechanisms of cation-induced bioflocculation

Research from different studies has been used to support three different theories pertaining to the role of cations in bioflocculation. These theories are the alginate theory, Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, and divalent cation bridging (DCB) theory. The objectives of this research were to examine the role of cations in bioflocculation to determine which theory, if any, best describes cation induced bioflocculation. Experiments were performed using laboratory scale activated sludge systems with bactopeptone as a feed. The feed was supplemented with either calcium, magnesium, or sodium at increasing concentrations. Floc properties were analyzed in each reactor during steady state periods. The addition of calcium or magnesium to the feed individually resulted in improvements in SVI, CST, SRF, cake solids and floc strength and each of these divalent cations produced similar results. The addition of sodium to the feed resulted in a deterioration in floc properties relative to a control reactor. Analysis of these results suggest that the DCB theory best explains the role of cations. The discrepancies between different studies were examined and are thought to be a result of different experimental procedures in different studies and in particular the use of short-term batch tests versus continuous flow reactor studies. In addition, the implications of DCB theory suggests that activated sludge systems should attempt to lower the ratio of monovalent to divalent cations to improve floc properties and treatment performance.