AWWA MTC64631

The effects of coagulant residuals on nanofiltration (NF) fouling were investigated. Experiments were carried out with a pilot microfiltration(MF)-NF plant and a laboratory-scale MF-NF setup. The pilot influent was drawn from a sedimentation basin after poly-aluminum chloride (PACl) coagulation. In the pilot, MF (PVDF membrane nominal pore size, 0.1 µm) permeates were transferred to the NF modules (nominal NaCl rejection 55%; polyamide), which was operated with constant flux (0.6 m/day) and water recovery (95%) by adjusting the feed pressure. In the laboratory-scale experiment, groundwater was pretreated with PACl, alum, or ferric chloride in-line coagulation and MF; then the MF permeate was pumped to an NF cross-flow cell with 0.6-m/d flux and 10% raw water recovery. The trans-membrane pressure of the NF in the pilot doubled in the 6 months of operation, although the mean silt density index of the NF feed water was 2.25, suggesting an absence of severe fouling by particulate matter. The aluminum concentration in the NF influent was not high (average 30 µg/L), but analysis of membrane foulants revealed excessive accumulation of aluminum (46 mg/m²) as well as silicate (30 mg/m²), suggesting that aluminum residuals probably caused the membrane fouling by forming alumino-silicates or aluminum hydroxide. In the laboratory-scale experiments, the permeability of NF declined when the feed water contained residual aluminum at 18 µg/L or more, but not when the aluminum concentration was lower than about 9 µg/L. Thermodynamic calculations and elemental analyses of the foulants recovered from the membranes indicated that the majority of inorganic foulants were compounds composed of aluminum, silicate, and possibly potassium. Includes 4 references, tables, figures.