Population balance modeling of activated sludge flocculation: Investigating the influence of Extracellular Polymeric Substances (EPS) content and zeta potential on flocculation dynamics

Abstract

Abstract Modeling activated sludge flocculation via particle population balances has been used to investigate the floc size distribution (FSD). The impact of shear rate on the FSD has been stated in previous studies, but the quantitative effect of the Extracellular Polymeric Substance (EPS) content and zeta potential on the aggregation and breakage of activated sludge flocs need to be further investigated. This study defined the balance factor, i.e. the ratio of the breakage rate coefficient to the collision efficiency, to evaluate the combined effect of the EPS content and the zeta potential on the aggregation and breakage of activated sludge, and introduced a binomial breakage function for more probable daughter-particles distributions after the breakage of flocs. The flocculation dynamic parameters were found to be linearly related to the EPS content and the zeta potential. The flocculation balance factor decreased with the increase of EPS content and the decrease of absolute zeta potentials. Smaller flocculation balance factor means better flocculation ability of flocs, leading to larger mean-size flocs. The binomial breakage function parameter values varied from 0.79 to 1.78 for activated sludge flocs with different EPS content, from 0.87 to 1.19 (Al3+ addition) and from 0.96 to 1.93 (Ca2+ addition) for flocs with different zeta potentials, which suggests that the flocs might possess different breakage. Although the increase of EPS content is beneficial to the aggregation of larger flocs, the flocs with high EPS content are susceptible to large-scale fragmentation resulting in more smaller-daughter-particles. Lower absolute values of zeta potentials would promote flocculation, and the activated sludge flocs showed strong stability with Al3+ additions but weak stability with Ca2+ additions.