Technologies and Costs for Removal of Arsenic From Drinking Water

4.4.10 Reverse OsmosisReverse osmosis (RO) membranes will remove much smaller ions typically associated withTDS (AWWARF, 1998). The recovery rate for RO membranes is dependent on the source waterquality but typically falls between 30 and 85 percent. The reject stream from this process containselevated levels of arsenic and other contaminants. While reverse osmosis is an effective removaltechnology, it is unlikely to be selected solely for arsenic removal because other options are more costeffective for arsenic removal and do not reject a large volume of water. RO may be cost effectiveoptions if removal of other contaminants is needed and water quantity is not a concern.The disposal technologies were presented in the November 1999 Technology and CostDocument. They were: direct discharge, indirect discharge, and chemical precipitation/non-hazardouslandfill disposal. Since the recovery rate is typically less than 85% percent, reverse osmosis rejectstreams can be characterized as high volume with lower arsenic concentrations compared to the brinestreams from anion exchange and activated alumina. Arsenic concentrations should not exceed the TCregulatory level since the maximum concentration effect is less than 6 to 1.Selection of Handling and Disposal OptionsDirect discharge of RO residuals to a receiving surface water is one possible disposalalternative. The large volume of the waste stream makes this alternative unlikely except for smallsystems (which produce a small waste volume) or systems located near the coast.Indirect discharge to the POTW is also an option. Similar to anion exchange, the existingTBLLs would likely need to be modified because the background arsenic will change. The water lossbetween the drinking water plant and the POTW will be the critical factor in determining the arsenicincrease to the POTW. The volume of the reject stream compared to the total volume at the POTWwill also be a critical factor in determining if the POTW would accept this waste stream. Systemsshould check with the POTW before selecting this option.Chemical precipitation of reverse osmosis streams is likely to be cost prohibitive. Reverseosmosis is already a fairly expensive treatment technology compared to other options and chemicalprecipitation of the large volume waste stream would significantly increase those costs. If directdischarge and indirect discharge are unavailable, systems should consider another treatmenttechnology.Costs are not presented for reverse osmosis waste disposal because it was not used to developnational costs and other options are more cost effective and produce much smaller waste streams.4-21