Application of new generation reverse osmosis membranes for treatment of surface water with high organic content

In order to augment potable water supply requirements, reverse osmosis (RO) membranes are used for demineralization of surface water with high total dissolved solids (TDS) content. When the total organic carbon (TOC) content of the surface water source is high (more than 5 mg/L), the RO membrane is prone to organic fouling causing a decrease in performance and increased chemical cleaning frequency. To address this issue, new generation RO membranes are being introduced by membrane manufacturers. A bench scale study was conducted to determine the treatability of high TOC surface water with organic fouling resistant membranes, to evaluate the efficiency of pretreatment schemes, and to estimate optimum operating conditions and rejection properties for the RO process. The City of West Palm Beach's reservoir water was used as the test water. Pretreatment processes evaluated for reducing TOC content were magnetic ion exchange (MIEX®) and enhanced softening. For the MIEX® process, a bed volume treatment rate of 600 was used. Enhanced softening was performed using 100 mg/L of lime, 25 mg/L of ferric sulfate, and 3.5 mg/L of polymer. Membranes from DowFilmtec and CSM Woongjin Chemicals were evaluated. When the RO membranes were tested without any pretreatment, more than 10 percent decline in normalized specific flux was observed within 24 hours of operation. Reduction of TOC in the raw water using MIEX® as a pretreatment process was substantially higher (60 %) when compared to enhanced softening (30 %). HPSEC analysis revealed that the MIEX process removed smaller size fraction (1000-5000 Da) organics more efficiently than enhanced softening. No substantial reduction in the large size organics (< 100,000 Da) was evident for both pretreatment processes. Although TOC reduction was higher for the MIEX® process, due to the generation of iron oxide fines from virgin resin, the rate of decrease in specific flux of the RO membranes while using MIEX® pretreated water was higher than enhanced softened water. The fouling propensity of RO membranes was found to be lower when microfiltration (MF) was used after the MIEX® process. The rejection of TOC, TDS, alkalinity, and metals were close to (or) below detection limits for the RO membranes. Performance of the RO process was determined to be efficient when a flux of 16 gfd was used. A total feed water recovery of more than 85 % was achievable with the use of antiscalant chemicals to control calcium carbonate precipitation. 2010