Investigation of colloidal biogenic sulfur flocculation: Optimization using response surface analysis

The colloidal properties of biogenic elemental sulfur (S⁰) cause solid–liquid separation problems, such as poor settling and membrane fouling. In this study, the separation of S⁰ from bulk liquids was performed using flocculation. Polyaluminum chloride (PAC), polyacrylamide (PAM) and microbial flocculant (MBF) were compared to investigate their abilities to flocculate S⁰ produced during the treatment of sulfate-containing wastewater. A novel approach with response surface methodology (RSM) was employed to evaluate the effects and interactions of flocculant dose, pH and stirring intensity, on the treatment efficiency in terms of the S⁰ flocculation and the supernatant turbidity removal. The dose optimization results indicated that the S⁰ flocculation efficiency decreased in the following order PAC > MBF > PAM. Optimum S⁰ flocculation conditions were observed at pH 4.73, a stirring speed of 129 r/min and a flocculant dose of 2.42 mg PAC/mg S. During optimum flocculation conditions, the S⁰ flocculation rate reached 97.53%. Confirmation experiments demonstrated that employing PAC for S⁰ flocculation is feasible and RSM is an efficient approach for optimizing the process of S⁰ flocculation. The results provide basic parameters and conditions for recovering sulfur during the treatment of sulfate-laden wastewaters.