Evaluation of iron chemical coagulation and electrocoagulation pretreatment for surface water microfiltration

A bench-scale study was undertaken to evaluate iron chemical coagulation and electrocoagulation pretreatment for dead-end, constant pressure microfiltration of surface water. Fouling of a commercially available PVDF membrane with conventional chemical coagulation pretreatment was found to (1) be insensitive to pH in the range 6.4-8.3, (2) decrease only when ≥10 mg/L Fe3+ were dosed and (3) exacerbate with increasing transmembrane pressure. These results were consistent with creation of larger flocs at high iron dosages that decreased cake specific resistance and formation of compressible cakes that increased hydraulic resistance at higher pressures. A substantial intermediate blocking stage was observed prior to cake formation during raw water microfiltration whereas for coagulated waters, cake filtration was the predominant flux decline mechanism for almost the entire duration of microfiltration. Fouling was not alleviated significantly by iron electrocoagulation in the entire range of experimental conditions investigated since soluble ferrous iron was unexpectedly generated at the anode. Hence, conventional iron chemical coagulation is recommended over iron electrocoagulation for pretreatment of surface waters prior to low-pressure membrane filtration. Chemical- and electrocoagulation pretreatment decreased ultraviolet absorbance to a greater extent than dissolved organic carbon. Hence, disinfection by-product control by coagulation-microfiltration is achieved by decreasing the concentration of natural organic matter precursors as well as their reactivity towards chlorine.