Water Online

November 2016

Water Innovations gives Water and Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.

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requirement for pH depression for operation (Clifford 1990). Once pretreated, the water passes through pressure vessels containing SBA-IX resin where the Cr(VI) and other anions are exchanged for chloride. Following the ion exchange step, the treated water is typically disinfected, and if needed, pH adjustment and/or other stabilization may be performed prior to sending the water to the distribution system. When the exchange sites are filled with contaminants, the resin is said to be exhausted and requires regeneration (Brandhuber et al. 2004). Regeneration is accomplished by using a 1.5 to 12 percent sodium chloride (NaCl) solution to impart a concentration gradient to replace the contaminant anions on the resin with chloride. Multiple bed volumes (BVs) of the regenerant are typically used to restore the exchange capacity (Siegel and Clifford 1988). Brine Management Management of brine often limits the applicability SBA-IX for drinking water treatment. Brine management options typically include the following: • Discharge to a sewer or septic system • Waste volume reduction using drying beds • Trucking to an off-site approved disposal location • Ocean discharge through a coastal pipeline • Deep well injection • Advanced treatment and disposal Waste brine quantity and quality characteristics (e.g., salinity, metals, and radionuclides) and geographical location can affect the feasibility and costs of these disposal options. Proximity and access to offshore disposal options, such as a brine line to the ocean, can also be significant factors in determining the burden of brine disposal. Without the ability to dispose of the brine in municipal sewers or via a brine line, the regenerant brine requires off-site disposal. This can be complicated, as it is likely to be designated as hazardous waste due to elevated concentrations of hexavalent chromium and other co-occurring contaminants. Alternatively, the regenerant brine can be treated to remove the Cr(VI) rendering it nonhazardous. Siegel and Clifford (1988) conducted bench-scale experiments with different reductants to evaluate their ability to reduce Cr(VI) to Cr(III) and determine the optimal conditions for precipitation of Cr(OH) 3(s) . The results showed that acidic sulfite, ferrous sulfate, and hydrazine are all capable of reducing Cr(VI) to Cr(III); however, ferrous sulfate was the only reductant that did not require pH adjustment for the reduction reaction to proceed and did not require additional chemical feed to achieve precipitation. In laboratory studies, Cr(VI) recovery with sodium chloride regenerations of SBA-IX was always demonstrated to be less than 100 percent, which was attributed to Cr(VI) reduction to trivalent chromium with subsequent precipitation of a greenish solid (chromium hydroxide) (Clifford 1990). Regenerant brine optimization has also been investigated for Cr(VI) treatment during pilot-scale testing conducted in Glendale, CA (McGuire et al. 2006). In that case, a regenerant brine with a sodium chloride concentration of 6 percent was found to be insufficient to fully regenerate SBA-IX, and the BV to breakthrough during treatment declined from 1,900 BVs with fresh regenerant to less than 500 BVs after the first recycle pass. Further treatment capacity reduction after subsequent regeneration cycles was also noted. Increasing the sodium chloride concentration from 12 to 26 percent improved performance; however, the Cr(VI) exchange capacity continued to diminish after subsequent cycles. In this instance, the diminished capacity was attributed to sulfate accumulation in the brine. Soquel Creek Research Results Research conducted with the Soquel Creek Water District's San Andreas well proved that SBA-IX can be effective for Cr(VI) treatment. At bench scale, commercially available SBA-IX resins were able to achieve 15,000 to 30,000 BVs of treatment prior to an 8 µg/L treatment threshold. SBA-IX operating in this fashion is extremely efficient (greater than 99.97 percent water-efficient), especially when compared to its use for nitrate removal where resins are typically regenerated after only 500 to 1,500 BVs. While some diminished capacity was observed, performance generally appeared to stabilize after the initial loading cycles. Regeneration of the resins at bench scale showed that the Cr(VI) could be recovered from the resin with mass balances showing 76 to 106 percent recovery of the Cr(VI), with the variability likely due to sampling and analytical limitations. At pilot scale, the exceptional loading capacity of the initial loading cycle was replicated, but without the subsequent diminished capacity experienced at bench scale. This reduced capacity observed at bench scale may be the result of irreversible fouling of the resin that occurred when the feedwater became contaminated with organic material. Regardless, there was no discernible performance decrease of the SBA-IX resin after five loading and regeneration cycles. The feasibility of direct brine reuse was also proven at pilot scale. Regenerant brine was used eight times consecutively at pilot scale with each of the reuses yielding loading cycles of at least 15,000 BVs prior to 8 µg/L Cr(VI) breakthrough. A caveat to this is that there is extended leakage of total chromium, which is illustrated in 26 wateronline.com n Water Innovations INORGANICCONTAMINANTS Pre-Filter Raw Water Strong Base Anion Exchange Contactors Treated Water Treatment Residuals Brine Treatment Waste Brine Waste Solids Brine Tank Brine Recycle Brine Pump Figure 1. SBA-IX process schematic

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