Floc Kinetics in Dual-coagulation for the Treatment of High-concentration Surfactant-kaolin Wastewater

Large amounts of surfactants pose a threat to water environment and dual-coagulation has been proven to be effective in the pretreatment of surfactant-polluted wastewater. However, the influence of water quality and hydraulic parameters on floc formation and its mechanisms are still unclear, especially under high-concentration condition. In this study, polymeric aluminum chloride (PAC) and polyacrylamide (PAM) were optimized to treat surfactant-kaolin wastewater. Influence of chemical dosage, initial pH and shear force on floc properties was investigated. In addition, mechanisms for soil particle coagulation in the matrix of simulated washing solution were also discussed. Results showed that adsorption of surfactants or kaolin particles onto aluminum-based hydroxides through entrapment and sweeping effect was the dominating mechanism at the pH of raw water. At pH 5, floc size and growth rate were relatively larger. When pH was between 6 and 9, the effect of initial pH on floc formation was not significant. At the lower PAC dosage, flocs formed in PAC + PAM dual-coagulation could be partly recovered and the size of regenerative flocs was larger than that of PAC. With the increase of PAM dosage, the time for achieving the balance between floc formation and breakage was shortened. Charge neutralization and adsorption bridging of PAM played a significant role in the dual-coagulation. Therefore PAM can be used as an efficient coagulant aid for PAC to promote floc properties. Results of this study would be beneficial for the application of dual-coagulation in the high-concentration wastewater treatment.