Technologies and Costs for Removal of Arsenic From Drinking Water

the remainder of the 80-day period. This was surprising given the fact that the membrane showed higharsenic rejection in single-element tests. Samples taken throughout the array indicate that a speciationchange from As(V) to As(III) was taking place within the filter. Since As(III) is more difficult toremove than As(V), overall arsenic removal dropped. This decrease in rejection over time suggeststhat a negatively charged membrane could not keep high As (V) rejection rates for long durationswithout maintaining arsenic in the As(V) form. Additional long-term testing is needed to verify theseresults for other membranes and situations. If speciation changes are influential for arsenic removal,keeping the membrane surface in an oxidized state may be an option.A NF pilot-scale study to determine arsenic removals with NF membranes was conducted inTarrytown, NY (Malcolm Pirnie 1992). Two NF membranes were tested: (1) NF70 manufactured byDow Chemical Company (FilmTec), and (2) TFCS manufactured by UOP Fluid Systems. The NFmembranes were operated at a flux varying between 17 and 21 gfd and at a recovery of 15 percent.Feed water conductivity varied from 460 to 950 uS, pH ranged from 7.7 to 8.3, and feed water arsenicranged from 0.038 - 0.154 mg/L. A second feed solution was mixed that had approximately twice theTDS and arsenic levels as found in the original test solution to simulate arsenic rejections by the lastelement in an NF membrane system operating at 50 percent recovery. Arsenic rejection was very highwith only one of eight permeate samples from the NF membranes exceeding the detection limit witha level of 0.0025 mg/L, corresponding to 95% rejection.Another study (Chang et al, 1994) revealed that the removal efficiency dropped significantlyduring pilot-scale tests where the process was operated at more realistic recoveries. When themembrane unit was operated at a recovery of 65%, the arsenic removal efficiency dropped to 65%and when the recovery was increased to 90%, the arsenic removal efficiency dropped down to 16%.2.5.7 Reverse OsmosisRO is the oldest membrane technology, traditionally used for the desalination of brackishwater and sea water. RO produces nearly pure water by maintaining a pressure gradient across themembrane greater than the osmotic pressure of the feed water. Osmotic pressure becomes great in ROsystems compared to other membrane processes due to the concentration of salts on the feed side ofthe membrane. The majority of the feed water passes through the membrane, however, the rest isdischarged along with the rejected salts as a concentrated stream. Discharge concentrate can besubstantial, between 10 and 50 percent of the influent flow depending on influent water quality andmembrane properties.2-36