Optimizing functions of coagulants in treatment of wastewater from metalworking fluids: Prediction by RSM method

Background: Coagulation is a treatment procedure for metalworking fluids (MWFs). This study aimed to optimize coagulation using four coagulants and compare the results. Methods: In this research, the outputs of a coagulation procedure in chemical oxygen demand (COD) removal, turbidity, and the release of oil were investigated using four coagulants, ferric chloride (FeCl3), ferric sulfate (Fe2(SO4)3, calcium chloride (CaCl2), and aluminum sulfate (Al2(SO4)3. To optimize the process, a central composite design (CCD) and response surface methodology (RSM) were used. Results: Under optimal conditions for using calcium chloride (coagulant concentration 4.2 g/L and pH 3.71), COD and turbidity removals and oil released were respectively 93%, 96.9%, and 31.8 mL. COD and turbidity removals and oil released using aluminum sulfate as a coagulant under optimal conditions (5.36 g/L, pH 4.64) were 75.7%, 89.9%, and 28.9 mL, respectively. With respect to ferric sulfate under optimal conditions (6.25 g/L, pH 3.4), COD and turbidity removals were 60% and 84.1%, respectively and oil released was 16.1 ml. COD and turbidity removals and the greatest amount of oil released by ferric chloride under optimal conditions (3.16 g/L, pH 3.5) were 54.4%, 84.7%, and 15.9 ml, respectively. Conclusion: Compared with other coagulants in the treatment of MWFs, calcium chloride achieved the highest removal efficiency in eliminating pollution indicators.