108 Journal of Membrane and Separation Technology, 2014, 3, 108-118 E-ISSN: 1929-6037/14 © 2014 Lifescience Global RO Membrane to Remove Sulfate: an Inland Brackish Water Desalination Pilot Study Qigang Chang 1,* , Brian R. Bergantine 1 , Robert (Bo) Johnson 2 , Srinivas (Vasu) Veerapaneni 2 , Troy B. Hall 3 , Mark A. Peterson 3 and David Buchholz 3 1 Advanced Engineering and Environmental Services, Inc. (AE2S), USA 2 Black & Veatch, USA 3 City of Fargo, Water Treatment Plant, Fargo, ND, USA Abstract: The City of Fargo completed a Facility Plan of their Water Treatment Plant (WTP) in 2011 to address two main issues: increasing water demands and high sulfate concentrations within a raw water source (Sheyenne River) primarily due to Devils Lake flooding. Reverse Osmosis (RO) was recognized as the most appropriate technology for sulfate reduction, and recommended for use in the WTP expansion. An RO pilot study was performed to evaluate its feasibility for two operational scenarios. RO membranes experienced rapid fouling in the Polishing Scenario, which used RO to further treat filtered water from the existing WTP (pretreatment, lime softening, ozone, and granular filtration). RO membranes exhibited superior performance in the Parallel Scenario, which was a separate treatment process (coagulation/flocculation/sedimentation + microfiltration/ultrafiltration +RO) parallel to the existing WTP. RO membrane autopsies indicated that membrane fouling was organic and biological for the Polishing Scenario while organic and scaling for the Parallel Scenario. Optimization studies were performed in the Parallel Scenario to determine optimal coagulation conditions for pretreatment as well as flux, recovery, and membrane cleaning regimes for both the MF/UF and the RO. Uniquely, an RO membrane selection pilot was conducted for both scenarios to evaluate RO membranes from four different manufacturers. The slight difference surface chemistry among various RO membrane can cause substantial different performance. It was found that one RO membrane could not be cleaned adequately, although it has many successful applications elsewhere. This one year pilot study proved that RO technology is feasible to reduce sulfate concentrations to acceptable levels in the City’s finished water. Keywords: RO membrane, sulfate, fouling, surface water, microfiltration, ultrafiltration. INTRODUCTION The Devils Lake basin was created by the last advance of the continental ice sheets in North Dakota (ND). As one of the largest natural water bodies in ND, Devils Lake has been continuously rising from a water surface elevation of 439.7 meter in 1993 to 443.3 meter on June 27, 2011, as shown in Figure 1. Devils Lake flooding has destroyed hundreds of homes and businesses and inundated thousands of acres of productive farmland. To alleviate the substantial social and economic impact, the State of ND has constructed emergency outlets to discharge Devils Lake water into the Sheyenne River. The west-end emergency outlet was built in 2005 and expanded to 7.1 m 3 /s in 2010. An east-end emergency outlet with a capacity of 9.9 m 3 /s was operational in 2012, for a combined outlet capacity of 17 m 3 /s. In addition, an emergency control structure has been constructed to control a catastrophic overflow from the lake into the Sheyenne River. Operation of the emergency outlets has helped alleviate flooding in the closed basin; however, *Address correspondence to this author at the Advanced Engineering and Environmental Services, Inc. (AE2S); Tel: +1 218-299-5610; Fax: +1 218-299- 5611; E-mail: qigang.chang@ae2s.com, qgchang@gmail.com problems related to water quality have also resulted from the emergency outlets. Devils Lake has high concentrations of dissolved solids (TDS) currently ranging from 1,100 mg/L up to 2,600 mg/L. The elevated TDS concentration is because the lake is a terminal lake, meaning that water will only leave the lake through evaporation, plant uptake, or ground infiltration, and only when the lake elevation is high enough will water overflow into the Sheyenne River. Release of the high TDS water (including high sulfate), causes a degradation of the downstream water quality in the Sheyenne River, which is used as a drinking water source for the City of Fargo (City), the largest city in ND. A United States Geological Survey study predicted that the sulfate concentration in the Sheyenne River could reach 750 mg/L in the lower Sheyenne River at Fargo with the two emergency outlets operating at 17 m 3 /s [1]. The historic average sulfate concentration in the Sheyenne River at Fargo is approximately 160 mg/L, and the secondary standard in drinking water (Secondary Maximum Contaminant Level (SMCL)) is 250 mg/L [2]. This anticipated elevated sulfate concentration causes concerns to municipal water supply systems that rely on the Sheyenne River as major water source, such as the City.