Abstract
Ion exchange (IEX) resins can remove natural organic matter (NOM) from drinking water sources. However, the IEX system produces a waste brine rich of sodium, chloride, NOM and sulfate. The treatment of the waste brine aims to recover a clean solution rich of sodium chloride, that can be reused to regenerate IEX resin. Previous research showed that ceramic nanofiltration partially removes NOM from the waste brine, but sulfate removal requires additional treatment. Sulfate removal by chemical precipitation was previously studied either on brines with low NOM concentrations or water with low concentrations of NOM and salts. The current work focussed on sulfate removal from NOM-rich brines by chemical dosing of (1) BaCl2, resulting in precipitation of barite (BaSO4), and (2) CaCl2, Ca(OH)2 and NaAlO2, resulting in precipitation of calcium sulfate and, subsequently, ettringite (Ca6Al2(SO4)3(OH)12). Additionally, the effect of NOM on SO42− removal was studied. Modelling and batch experiments were conducted with IEX and synthetic brines within the typical ion strength range of 0.1 to 1 M. With doses of 2.2 g of BaCl2 per g of initial sulfate, BaSO4 precipitation removed more than 83 percent of sulfate, resulting in final concentrations below 0.4 g/L even in the presence of NOM. However, NOM inhibited the precipitation of calcium sulfate and, subsequently, ettringite. With doses of 1.3 g of CaCl2, 0.5–0.7 g of Ca(OH)2 and 0.4–0.6 g of NaAlO2 per g of initial sulfate, calcium sulfate and ettringite precipitation removed between 8 and 95 percent of sulfate from NOM-rich brines, resulting in final concentrations between 0.8 and 2 g/L. As a reference, NOM-free brines required doses of 1.3 g of CaCl2, 0.2–0.7 g of Ca(OH)2 and 0.1–0.6 g of NaAlO2 per g of initial sulfate for 89 to 99 percent of sulfate removal, resulting in final concentrations of 0.2 g/L. The inhibition might be attributed to covering of crystal sites by NOM molecules, and to NOM coagulation with aluminium.
Highlights
Anion exchange (IEX) can effectively remove negatively charged natural organic matter (NOM) during drinking water treatment [1]
Sulfate removal with BaSO4 precipitation was above 98 percent for NOM-free brines, and 84 percent for the NOM-rich spent Ion exchange (IEX) brine
This resulted in final concentrations below 0.4 g SO42− /L, even in the presence of NOM
Summary
Anion exchange (IEX) can effectively remove negatively charged natural organic matter (NOM) during drinking water treatment [1]. In IEX processes, resins are reused after cleaning with an electrolyte regenerant solution. In IEX for NOM removal, the regenerant solution is usually NaCl [2,3,4]. The regenerant solution is reused several times before disposal, which increases the concentrations of NOM and anions, like sulfate (SO42− ) [3,4,5]. The composition of spent IEX brines depends on the quality of the water to be treated, the affinity of negatively charged components with the resin, and the specific IEX system opera tion. Discharging waste streams with NOM and salts is often problematic
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