Performance of nanofiltration for arsenic removal

Performance of rapid sand filtration inter-chlorination system was compared with nanofiltration (NF) to reduce the arsenic health risk of drinking water. It was found that rapid sand filtration with inter-chlorination is not effective in removing arsenic. If total arsenic concentration in raw water is below 50 μg/L regardless of the turbidity of raw water, arsenic can be removed below WHO guideline value of 10 μg/L by conventional coagulation (polyaluminum chloride dosage is about 1.5 mg Al/L). However, if the raw water arsenic concentration exceeds 50 μg/L, more coagulant dosage or enhanced coagulation is needed. To adopt optimum coagulant dosage for arsenic removal, it needs to monitor raw water arsenic concentration, but it is difficult because arsenic measurement is time consuming. In addition, if raw water contains As(III), it is difficult for rapid sand filtration inter-chlorination system to meet an arsenic maximum contaminant level of 2 μg/L, which would achieve reduction of cancer risk below 10−4. On the other hand, the NF membrane (NaCl rejection 99.6%) could remove over 95% of As(V) under relatively low-applied pressure (<1.1 MPa). Furthermore, more than 75% of As(III) could be removed using this membrane without any chemical additives, while trivalent arsenic could not be removed by rapid sand filtration system without pre-oxidation of As(III) to As(V). Because both As(V) and As(III) removals by NF membranes were not affected by source water composition, it is suggested that NF membrane can be used in any types of waters.