Enhancing boron rejection in seawater reverse osmosis facilities

Linked to potential health problems and toxicity to crops, boron is present in seawater at concentrations of ranging from 4 to 7 mg/L, and not readily removed by reverse osmosis technology. Commercially available seawater reverse osmosis (SWRO) membranes possess a wide range of rejection characteristics for boron in seawater under ambient temperature and pH, ranging from approximately 50% for low-energy membranes to greater than 90% for the newest high rejection membranes. This level of rejection is typically insufficient to reduce boron concentrations in natural seawater to less than recommended levels. Current World Health Organization (WHO) drinking water concentrations for boron are limited to 0.5-mg/L. Two techniques utilized to mitigate boron concentrations are (1) increasing the dissociation of boric acid by increasing pH prior to SWRO; and, (2) utilizing a second pass reverse osmosis system, potentially coupled with pH adjustment. Utilizing these techniques, the authors tested commercially available SWRO membranes from three different manufacturers utilizing feed water alkalization, coupled with a second pass system. Utilizing feed water alkalization alone, the authors found that all three SWRO membranes were able to produce permeate complying with WHO regulations. Using second pass RO, a boron concentration of less than 0.5 mg/L was achieved for feed pH greater than 6, and less than 0.1-mg/L for pH of 10.