Effects of Intensity and Mixing Speed on the Efficiency of Coagulation and Flocculation Processes and Improving Effluent Quality from Yazd SBR Wastewater Treatment Plant

Coagulation and flocculation techniques are appropriate methods for improving the quality of efflunets from wastewater treatment plants that help achieve higher standards of water reuse. In this study, PAC was used as the coagulant in the SBR treatment system of Yazd Wastewater Treatment Plant and the effects of mixing intensity during rapid and slow mixing steps were investigated on the improved efficiency of coagulation and flocculation processes. Meanwhile, constant and declining mixing speeds during the flocculation step were compared. For the purposes of this study, the Jar test unit was employed as a batch reactor. Results showed that from among the three mixer speeds of 100, 150, and 200 rpm in the rapid mixing tank, the 150 rpm mixing speed yielded higher removal efficiencies for turbidity, BOD, COD, and TSS. In addition, comparison of the constant speeds (20, 30, and 40 rpm) and the declining speeds (40 to 10 rpm) in the slow mixing stage showed that the declining speed yielded higher turbidity, BOD, COD, and TSS removal efficiencies than each of the three fixed speeds. Moreover, the lowest volume of sludge produced was 34 ml/L at a slow mixing speed of 30 rpm from among the constant speeds used in the slow mixing tank. Finally, it was found that the declining speed flocculation led to an improved sludge production of 26 mg/L.