Technologies and Costs for Removal of Arsenic From Drinking Water

AWWARF (1998) also performed UF pilot-scale tests. Single element pilot tests wereperformed on two groundwaters, one with a DOC level of 11 mg/L and one with a DOC level of 1mg/L, and a spiked, finished surface water. Arsenic removal results from these tests are shown inTable 2-6.TABLE 2-6Arsenic Removal by UF at Pilot-ScaleMembrane MWCO Water Type As Species As RejectionDesal GM2540F 8,000 High DOC GW Total As 70%Low DOC GW Total As 30%Desal GM2540F 8,000Finished SWV 47%III 10%As seen in Table 2-6, arsenic removal varied with DOC levels, being much higher in the highDOC groundwater (70%) than in the low DOC groundwater (30%). The authors postulated that thisdifference was due to a reduction in electrostatic forces caused by adsorption of NOM to themembrane surface. Adsorption of NOM would reduce the surface charge of the membrane and would,in effect, increase the repulsion towards negatively charged arsenic compounds. Increases in theapparent size of the arsenic molecules through “bridging” with humic substances was ruled out sincea concurrent increase in UV 254 removal was not seen. In contrast to DOC levels, changes in flux andrecovery did not seem to impact the arsenic rejection rate. As shown in Table 2-6, testing on thefinished surface water showed fairly effective removal of As(V), but unimpressive As(III) removal.Considering the MWCO, however, these removals were expected.2.5.6 NanofiltrationNanofiltration membranes are capable of removing significant portions of the dissolvedarsenic compounds in natural waters due to their small pore size. NF will primarily remove divalentions (e.g., Ca, Mg), but not monovalent salts (e.g., Na, Cl). Through size exclusion, NF can removeboth dissolved As(V) and As(III). This makes NF a reliable arsenic removal process for groundwaterwhich contains up to 90% dissolved arsenic (AWWARF, 1998). The small pore size, however,makes NF membranes more prone to fouling than UF or MF membranes. The application of NF for2-33