Technologies and Costs for Removal of Arsenic From Drinking Water

presumably due to changes in electrostatic repulsion at the membrane surface through NOMadsorption. As in the UF pilot study, NF arsenic rejection rate did not seem to be affected by changesin flux or recovery.Single element tests performed on the spiked, finished surface water showed substantial As(V)rejection (>95 percent). As(III) rejection, however, was reduced with an average for all threemembranes of only 40 percent. The authors point out that these results attest to the influence ofdiffusion and electrostatic repulsion on As(III) removal. As(III) is small and can more easily diffusethrough very small NF pores. As(III) is also not as repulsed by surface charge as As(V). CombiningNF with an oxidizing process to convert As(III) to As(V) would probably be the most effective optionfor its removal.TABLE 2-8Arsenic Removal with NF at Pilot-ScaleMembrane MWCO Water Type Charge AsSpeciesAsRejectionSingle ElementAccumem 400High DOC GW ( - ) Total As 80%Low DOC GW ( - ) Total As 60%NF 1 NA Finished SW NANF 2 NA Finished SW NANF 3 NA Finished SW NAV >95%III 52%V >95%III 20%V >95%III 30%ArrayAccumem 400 High DOC GW ( - ) Total As 75% (initial)3-16% (final)NA: Not AvailableThe array test results, as shown in Table 2-8, were somewhat surprising. Arsenic rejectionrate declined over time. Rejection at the beginning of the test was approximately 75 percent butproceeded to decline to 11 percent by day 60. Rejection stayed between 3 percent and 16 percent for2-35