Improvements in the reduction coagulation filtration process for hexavalent chromium treatment

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Abstract

Reduction coagulation filtration using ferrous iron (Fe(II)‐RCF), or reduction filtration using stannous tin (Sn(II)‐RF), for the removal of hexavalent chromium (Cr(VI)) from drinking water represent attractive alternatives to ion exchange or reverse osmosis. With a new maximum contaminant level for Cr(VI) anticipated in California, this study sought to expand on previous research by applying Fe(II)‐RCF and Sn(II)‐RF to multiple groundwaters under a variety of reductant dose and reduction time scenarios. Four different granular filter media were also tested for removal of total chromium (Cr(T)) following Cr(VI) reduction to trivalent chromium (Cr(III)). Sodium hypochlorite, instead of aeration, was used to oxidize excess Fe(II) to Fe(III) prior to filtration, with minimal Cr(III) to Cr(VI) reoxidation. Sn(II)‐RF was also able to achieve Cr(T) and Cr(VI) treatment goals without dedicated reduction time prior to filtration. Results of this study were used to develop more up‐to‐date, feasibility‐level, 20‐year annualized cost estimates.

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Regeneration of pilot-scale ion exchange columns for hexavalent chromium removal.

Julie Korak, Richard Huggins, Miguel Arias-Paic (2017)

Due to stricter regulations, some drinking water utilities must implement additional treatment processes to meet potable water standards for hexavalent chromium (Cr(VI)), such as the California limit of 10 μg/L. Strong base anion exchange is effective for Cr(VI) removal, but efficient resin regeneration and waste minimization are important for operational, economic and environmental considerations. This study compared multiple regeneration methods on pilot-scale columns on the basis of regeneration efficiency, waste production and salt usage. A conventional 1-Stage regeneration using 2 N sodium chloride (NaCl) was compared to 1) a 2-Stage process with 0.2 N NaCl followed by 2 N NaCl and 2) a mixed regenerant solution with 2 N NaCl and 0.2 N sodium bicarbonate. All methods eluted similar cumulative amounts of chromium with 2 N NaCl. The 2-Stage process eluted an additional 20-30% of chromium in the 0.2 N fraction, but total resin capacity is unaffected if this fraction is recycled to the ion exchange headworks. The 2-Stage approach selectively eluted bicarbonate and sulfate with 0.2 N NaCl before regeneration using 2 N NaCl. Regeneration approach impacted the elution efficiency of both uranium and vanadium. Regeneration without co-eluting sulfate and bicarbonate led to incomplete uranium elution and potential formation of insoluble uranium hydroxides that could lead to long-term resin fouling, decreased capacity and render the resin a low-level radioactive solid waste. Partial vanadium elution occurred during regeneration due to co-eluting sulfate suppressing vanadium release. Waste production and salt usage were comparable for the 1- and 2-Stage regeneration processes with similar operational setpoints with respect to chromium or nitrate elution.

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Hexavalent chromium removal by reduction with ferrous sulfate, coagulation, and filtration: a pilot-scale study.

Gang Qin, Michael McGuire, Nicole K Blute … (2005)

A flow-through pilot-scale system was tested for removal of Cr(VI) from contaminated groundwater in Glendale, California. The process consisted of the reduction of Cr(VI) to Cr(lll) using ferrous sulfate followed by coagulation and filtration. Results indicated that the technology could reduce influent Cr(VI) concentrations of 100 microg L(-1) to below detectable levels and also remove total Cr (Cr(VI) plus Cr(lll)) to very low concentrations (< 5 microg L(-1)) under optimized conditions. Complete reduction of Cr(VI) to Cr(lll) was accomplished with Fe(ll) doses of 10-50 times the Cr(Vl) concentration even in the presence of significant dissolved oxygen levels. The overall Cr removal efficiency was largely determined by the filterability of Cr(lll) and Fe(lll) precipitates, of which a relatively high filtration pH (7.5-7.6) and high filter loading rate (6 gpm ft(-2)) had negative impacts. The pilot system was able to operate for an extended time period (23-46 h depending on the Fe:Cr mass ratio) before turbidity breakthrough or high head loss. Backwash water was effectively settled with low doses (0.2-1.0 mg L(-1)) of high molecular weight polymer. Backwash solids were found to be nonhazardous bythe toxicity characteristic leaching procedure but hazardous by the California waste extraction test.

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Stannous chloride reduction–filtration for hexavalent and total chromium removal from groundwater

Anthony Kennedy, Rachel Croft, Leah Flint … (2020)

Hexavalent chromium (Cr(VI)) reduction using stannous chloride (SnCl2) has emerged as a possible alternative to chromium treatment technologies such as strong base anion exchange. In an effort to target not only Cr(VI) reduction but, ultimately, total chromium (Cr(T)) removal, SnCl2 addition followed by rapid sand filtration was tested at the pilot scale on a groundwater with a naturally occurring Cr(VI) concentration of 0.090 mg/L. A SnCl2 dose of 1.5 mg/L, followed by filtration, was able to consistently remove Cr(T) to less than 0.010 mg/L following an initial ripening period, with limited head loss for 10 sequential 17‐ to 25‐hr filter runs. Total tin and turbidity removal were similar, decreasing to below 0.050 mg/L and raw water levels, respectively. Analysis of filter sand following backwashes and three different material pipe segments that were exposed to unfiltered water dosed with SnCl2 indicated the accumulation of Cr and Sn on surfaces, which remains a concern.