Integrated ion exchange-based system for nitrate and sulfate removal from water of different matrices: Analysis and optimization using response surface methodology and Taguchi experimental design techniques

Abstract Although sequestering nitrate by anion exchange resins (AERs) is a proven and well-established technology, many process problems and challenges still exist, such as interference, nitrate dumping, total dissolved solids (TDS) limitations, brine waste disposal, etc. An engineered-integrated approach was devised and assessed to mitigate such limitations. The overall process optimization was based on screening, analyzing, and optimizing each respective step. A front-end sulfate removal step was suggested, evaluated, which yielded removal efficiencies >95%. Back-end options were also introduced, including reduced brine and brine wastes, recycling possibility, and usage as mixed-fertilizer with a NO3/Cl ratio of 0.65 as confirmed by mass balance calculations. Taguchi robust designs (TRD), Box-Behnken Designs (BBD), and central composite design (CCD) experimental design methods were used, enabling information mining from a minimum number of experiments. A one-factor-at-a-time strategy was applied and the regeneration process was optimized using TRD. Response analysis and regression revealed that acid concentration and volume are the most influential factors. Selectivity order was determined as SO42-->Cl−>HCO3− according to their relative coefficients of the BBD and CCD regression models. Field trials confirmed the robustness and viability of the proposed integrated approach. A process flow diagram was devised. Merits of the proposed system include: a) possible resolution of sulfate competition and nitrate dumping, b) elimination of brine waste problems, c) tolerate higher TDS levels than ever possible for AERs, d) improved economics, e) improved process robustness, f) mitigate the corrosiveness of produced water, and g) fulfilling zero liquid discharge criteria.