COMBINING RO WITH ION EXCHANGE USING RO CONCENTRATE AND ALTERNATIVE REGENERATION BRINE SOURCES TO TREAT MULTIPLE FRESH AND BRACKISH GROUNDWATER SUPPLIES

摘要

Lee County Utilities owns and operates the Green Meadows lime softening facility that treats ablend of surficial aquifer groundwater and intermediate Sandstone aquifer groundwater, bothhard, freshwater sources. The facility’s aging treatment infrastructure is nearing the end of itsuseful life and needs to be replaced to maintain reliability and to continue meeting regulations.Furthermore, due to the increased demand on fresh water aquifers throughout Florida and theresultant concern for cumulative negative impacts on water quantity, quality, and on wetlands,Lee County will use an alternative Upper Floridan Aquifer (UFA) brackish groundwater sourcefor a planned expansion. An initial process evaluation identified NF/RO as the most costeffective and reliable process to treat a blend of the existing fresh and new UFA brackishgroundwater.The surficial supply represents a treatment challenge for NF/RO because the water has highhardness, iron, color and organic material with seasonal spikes in SDI and turbidity as well aslow levels of hydrogen sulfide. In contrast, the intermediate Sandstone wells produce a higherquality fresh water with low turbidity, SDI, chloride, color, and organics, but with high hardnessand hydrogen sulfide. The deep UFA brackish water is characterized by low levels of organicsand low SDI and turbidity, but high levels of hardness, sulfide, and TDS. This combination offresh surficial, fresh intermediate, and brackish deep source wells presents an interestingtreatment process optimization challenge for a new expanded NF/RO facility.A pilot-scale treatability study was conducted to determine the optimal blending arrangement forthe three supplies and their subsequent treatment by NF/RO. The goal of the testing was toidentify treatment options that minimized the level of pretreatment and effectively managedNF/RO membrane fouling while allowing for maximum flexibility in the utilization of eachsource. Initial testing indicated that blending the iron-containing surficial source with the sulfidecontainingSandstone aquifer or UFA water was not feasible prior to a membrane process. Thestudy then evaluated separate RO/NF treatment of the surficial water and blended freshSandstone Aquifer and brackish UFA water.Additional testing was conducted to evaluate the use of a two-step ion exchange (IX) process(strong acid sodium-form cation followed by strong base chloride-form anion) as an alternative2012 ? American Water Works Association Membrane Technology Conference Proceedings. All Rights Reserved.treatment of the freshwater surficial source to remove iron, hardness, and color prior to blendingwith RO permeate from treatment of the other water sources. Additionally, some of thefreshwater Sandstone aquifer would be blended with the RO permeate to improve finished waterstability.The operational costs associated with IX systems are generally high due to the large amount ofsalt required to regenerate the resin, particularly for the cation resin where the elevated ironlevels in the surficial aquifer water require frequent regeneration. Therefore the operational costcan be significantly reduced if an alternative regenerant (to purchased sodium chloride dry salt)can be found. Testing was conducted to evaluate the use of alternative salt sources to replace orsupplement the use of purchased sodium chloride salt. These sources included RO concentratewith approximately 2,000 mg/L sodium produced as a byproduct of desalination of the UFAwater and recovery of Lower Floridan Aquifer water with seawater salt concentrations.LCU selected IX for surficial aquifer water treatment over membrane treatment to improvereliability and to reduce lifecycle cost of the expanded facility. The proposed overall treatmentfacility will soften and remove organic material from the existing surficial aquifer wells. Thefacility will treat blended UFA and Sandstone aquifer well water with RO and bypass blendSandstone aquifer water to increase treated water hardness and stabilize the finished water. Theproposed facility would have a firm finished water capacity of 14 mgd with a maximumtreatment capacity of 16 mgd.This paper will outline the treatment goals for the upgraded water treatment plant, summarize thesource water treatment evaluation, and describe the pilot-scale evaluation of the two-step IXprocess, including a comparison of performance using different concentrations of purchased saltand RO concentrate as well as different manufacturer’s resins. The paper will also include asummary of the lifecycle cost comparison for surficial water treatment using NF/RO versus twostepIX based on different regeneration approaches that led to the final process decision.