Effect of operating conditions in removal of arsenic from water by nanofiltration membrane

The removal of arsenic from synthetic waters and surface water by nanofiltration (NF) membrane was investigated. In synthetic solutions, arsenic rejection experiments included variation of arsenic retentate concentration, transmembrane pressure, crossflow velocity and temperature. Arsenic rejection increased with arsenic retentate concentration. Arsenic was removed 93–99% from synthetic feed waters containing between 100 and 382 µg/L As V, resulting in permeate arsenic concentrations of about 5 µg/L. Under studied conditions, arsenic rejection was independent of transmembrane pressure, crossflow velocity and temperature. In surface water, the mean rejection of As V was 95% while the rejection of sulfate was 97%. The co-occurrence of dissolved inorganics does not significantly influence arsenic rejection. The mean concentration of As in collected permeated was 8 µg/L. The mean rejection of TDS, total hardness and conductivity were 75, 88 and 75% respectively.

Experiments were performed to evaluate the ability of NF-300 membrane module to remove As V from synthetic solution and surface water. In synthetic solutions, the results clearly show that the NF-300 membrane consistently reduced the arsenic concentration in the permeate at values that are below the current and proposed arsenic MCL (50 and 10 µg/L). The rejection of As V by the NF-300 membrane was found to be between 93–99% for retentate arsenic concen- Table 2 Sample analysis in NF test using surface water Parameter Feed Collected permeated Final retentate pH Total dissolved solids a, mg/L Conductivity, µS/cm Total hardnessb, mg/L as CaCO3 Arsenic, µg/L 8.45 8.28 68.1 8.14 240.3 23.4 24.8 c Sulfate , mg/L d Chloride , mg/L a b c = d − SM 2540 C; SM 2340 C; SM 4500 SO4 E; SM 4500 Cl B tration in the range of 100 and 382 µg/L, resulting in permeate arsenic concentrations of about 5 µg/L. Operating under the defined conditions in this test, the rejection of As V was independent of transmembrane pressure, crossflow velocity and temperature. The mass balance shows “loss” of arsenic in the system. The importance of recognizing the possible retention of arsenic within the membrane is that fouling of membranes is a primary cause of high maintenance cost. Application of NF-300 membrane to surface water showed that the co-occurrence of dissolved inorganics do not significantly have influence on the rejection of As V. A concentration factor of 2.66 was applied. The mean rejection of As V was 95% while the rejection of sulfate was 97%. The membrane also reduced the concentrations of TDS and total hardness. Because arsenic in surface waters is typically in the form of As V, low pressure, NF-300 membrane can be used to treat surface waters with unacceptable arsenic concentrations.