Evaluation of waste activated sludge as a coagulant aid for the treatment of industrial wastewater containing mixed surfactants

Abstract

Wastewater generated by the industry manufacturing detergents and various kinds of consumer products normally contains very high contents of mixed surfactants, organic matters expressed as chemical oxygen demand (COD), and phosphates that must be treated prior to discharge to the aquatic environment. In this study, jar-test experiments were conducted to evaluate the waste activated sludge (WAS) as a coagulation aid in the coagulation-flocculation process with ferric chloride or aluminum sulfate as a coagulant for the treatment of wastewater collected from the aforementioned industry. The WAS was selected because of its adsorption capability of anionic surfactants and its availability from the wastage stream of biological wastewater treatment process. The effective dosages of both coagulants with and without the WAS additions were determined in this study. Without the WAS addition, the concentrations of 800 mg/L aluminum sulfate at the optimum pH value of 8 and 2208 mg/L ferric chloride at the optimum pH value of 12 were the optimum chemical dosages. It appears that aluminum sulfate was more effective than ferric chloride based on the chemical dosage and removal efficiency. The turbidity, suspended particles, anionic surfactants, COD, and phosphates removal efficiencies of aluminum sulfate and ferric chloride under the optimum dosage were 95.6, 88.2, 78.4, 73.5, 47.3% and 98.8, 92.0, 72.7, 67.5, 53.1%, respectively. The addition of 200 mg/L WAS was sufficient to reduce the optimum dosages of both chemicals by 200 mg/L. The cationic surfactant existing in the wastewater worked as a flocculating agent leading to the flocculation of waste activated sludge resulting in the enmeshment of the suspended particles and colloids on which the COD, anionic surfactants, and phosphates were adsorbed. However, the substances removal efficiencies of ferric chloride and aluminum sulfate were slightly enhanced and reduced, respectively. It is possibly explained that the settling time is insufficient to settle the aluminum hydroxide floc when it is compared to the ferric hydroxide floc because the iron floc is normally heavier than the alum floc.