Efficiency of a Coagulation/Flocculation Process Combined to Membrane Filtration for the Treatment of Heavily Polluted Soap Industry Wastewater

The mismanagement of various industrial wastes especially highly polluted wastewater produced by soap manufacturing causes serious environmental problems and ultimately impaired human health. Therefore, constant efforts are made to alleviate the pollution of this type of effluents with the aim of reducing their environmental impact, on one hand, and considering the possibility of reuse, on the other. Within this framework, this present investigation focuses on the treatment of soap wastewater using coagulation/flocculation (CF) and membrane filtration processes. CF experiments were performed using jar test protocol. Response surface methodology (RSM) via Box–Behnken design (BBD) was employed for the optimization process. Seven variables were involved: coagulant concentration (X1) [0.5–2.5 g/L], flocculant dosage (X2) [20–80 mg/L], initial pH (X3) [5–13], mixing speed of coagulation (X4) [100–200 rpm], contact time for coagulation (X5) [3–7 min], mixing speed of flocculation (X6) [20–40 rpm] and contact time for flocculation [20–40 min]. The manipulated variables were varied on three levels (+ 1, 0 and − 1). The maximum removal efficiencies (99% turbidity and 27% COD) were recorded under initial pH of 5.3, at 2.17 g/L Al2(SO4)3 concentration, 55.32 mg/L dose of flocculent, mixing speed of about 141.42 rpm for coagulation and 28.92 rpm for flocculation, contact time of about 4.78 min for coagulation and 33.55 min for flocculation. Further, the membrane filtration process was carried out under dead-end filtration (DEF) mode in order to select the most appropriate membrane pore size allowing to obtain the best quality of treated water. Different flat microfiltration (MF) (5, 1.2, 0.45 and 0.22 µm) and ultrafiltration (UF) membranes (5, 10, 30 and 100 kDa) have been used. Based on the results found, CF combined to UF process seems to be more efficient and showing more satisfactory results comparing to CF/MF. Indeed, maximum turbidity and COD removal rates of 100% and 88%, respectively, were achieved using UF membrane with molecular weight cutoff (MWCO) of 5 kDa. Therefore, the proposed hybrid process CF/UF demonstrated a significant improvement in the quality of the treated water, with high potential for application in soap wastewater treatment.