Investigation on Efficiency of Response Surface Methodology (RSM) to Optimize Coagulation Process for Arsenic Removal from Water

Arsenic is a carcinogenic contaminant, and arsenic-contaminated drinking water is the major source of exposure to this hazardous metal. In recent years, the city of Sirjan has faced the problem of water sources pollution to arsenic and due to the existence of conventional water treatment plants in this city, the results of this study can be an important step to reduce and even eliminate this problem. In this study, the response surface methodology used to investigate the effect of pH (7,8,9), turbidity (<1,5,10,15,20 NTU), initial arsenic concentration (50,100,150,200,250 µg/l) and coagulant dose (5,10,15,20,25,30 mg/l) on the residual arsenic concentration after coagulation and flocculation process using ferric chloride. Synthesis of the tested samples was performed using the effluent of the treatment plant. Based on the results, the variables under study follow a quadratic model. The predicted quadratic model for the behavior of the parameters fits the results well. Investigation of the interaction effects of variables showed that the concentration of residual arsenic is greatly affected by pH so that by increasing it in the study area, the concentration of arsenic output also increases. This negative effect of increasing pH can be partially compensated by increasing turbidity. According to the coefficients of factors in this model, linear and square effects of pH, the interaction of pH and turbidity, and the linear effect of coagulant dose had the greatest effect on residual arsenic concentration. Taking into consideration of operation conditions, at pH=8, turbidity=5 NTU, using 20mg/l coagulant, reduced arsenic concentration from 150 µg/l to 3.84 µg/l