Membrane for water reuse: effect of pre-coagulation on fouling and selectivity

Membrane filtration is adequate for producing disinfected clear water suitable for various kinds of applications. However, fouling of membranes is the main limitation. This study has focused on the ability of flocculation to remove from wastewater the organic colloids which play an important role in the fouling phenomena. First, flocculation was optimized for high-efficiency removal of suspended solids and organic material, and then, at a selected ferric chloride dose and pH, was used as a first step before filtration on membranes ranging from UF (50 KDa) to NF. This resulted in an improvement of the filtration flux. Fouling increased when high molecular weight cut-off (MWCO) membranes were used. The fouling mechanism seems to be blocking, by internal clogging and cake formation becoming preponderant with time, pH 5.5 (charge neutralization zone) provided better removal and lower fouling intensity than pH 7.8 (sweep coagulation zone). Ultrafiltration of 4 KDa at acidic pH 5.5 and 150 mg/L ferric chloride could reduce DOC by 70% and UV-254 nm by 60%. The quality of the filtrate was better than that obtained with nanofiltration at basic pH 7.8 with the same dose of flocculant and the fouling was lower with a 4 KDa membrane. Thus coupling of flocculation with a UF membrane might be the best compromise for producing, in very compact units, very clear water for possible reuse in industrial areas.