Reverse Osmosis

Pretreatment Options for Removing Arsenic and Iron before RO Treatment

By Jacob M. White, P.E., and Jesus Leal, P.E.


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Located in Brownsville, Texas, the Southmost Regional Water Authority (SRWA) regional desalination plant uses state-of-the-art reverse osmosis (RO) technology to treat previously untapped and unusable brackish groundwater resources as an alternative water supply. Serving 5 individual water suppliers (Brownsville Public Utility Board, Valley Municipal Utility District No. 2, Los Fresnos, Indian Lake, and the Port of Brownsville), the facility was originally designed to treat 6 million gallons per day (mgd) of RO permeate with 1.5 mgd of blended water for a total treated water design capacity of 7.5 mgd. During the first several years of operation, two water quality issues emerged. First, a change in the drinking water standard for arsenic concentrations was lowered by the U.S. Environmental Protection Agency (EPA) from 50 parts per billion (ppb) to 10 ppb with a compliance date of January 23, 2006. Second, in October 2005 complaints were received of colored water that was caused by iron in the distribution system. The 20 wells associated with the SRWA facility show arsenic levels ranging from 0.0140 milligrams per liter (mg/L) to 0.035 mg/L. The combined groundwater feed to the RO process ranged between 0.025 mg/L (25 ppb) and 0.029 mg/L (29 ppb) and an average iron content of about 0.6 mg/L. Monitoring conducted at points throughout the treatment process indicated that the RO membranes were providing removal of arsenic, but not enough to comply with the new standard.

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