Pilot Testing Of Hexavalent Chromium Removal By Reduction, Coagulation And Microfiltration
- Ying Wu, Nicole Blute, and Leighton Fong
Research on hexavalent chromium Cr(VI) removal has been ongoing for more than a decade at the City of Glendale, California, which has investigated feasible treatment technologies and provided information for a new maximum contaminant level (MCL) expected in California in 2013. Previous demonstration-scale testing of reduction, coagulation and filtration (RCF) showed consistent Cr(VI) removal to below 1 part per billion (ppb), however, total Cr removal to low single-digit ppb levels. Total Cr consists of Cr(VI) and trivalent chromium, Cr(III). Considering Cr(III) can be converted to Cr(VI) by disinfectant in the distribution system as shown in previous testing, the ability to achieve consistent total Cr removal to lower levels was in question. Microfiltration (MF) was substituted in place of granular filtration in the RCF process in this project.
Pilot testing was conducted at Glendale using reduction, coagulation and MF. Two MF pilot units were tested, including pressurized (0.1 micron) and submerged membranes (0.02 micron). The pilot study consisted of several stages to identify the proper reductant dose (i.e. ferrous sulfate) and membrane operational conditions, followed by 30-day continuous runs under steady conditions. Jar testing was also conducted to investigate the impact of a small chlorine dose on conversion of Cr(III) to Cr(VI).
This study demonstrates that an RCF process with MF can consistently achieve a treatment goal of less than 1 ppb total Cr and Cr(VI). A higher iron concentration, 2 mg/L Fe for 15 ppb Cr(VI), improved removals from less than 2 ppb total Cr to less than 1 ppb. The same 2 mg/L Fe dose for an influent concentration of around 90 ppb Cr(VI) was also effective at achieving a total Cr effluent concentration of less than 1 ppb. The higher iron concentration to treat a lower Cr(VI) concentration suggests that a threshold concentration of iron may be beneficial. Detailed results will be presented.
A major finding in this study was the positive bias observed by the ICPMS total chromium method without digestion to remove alkalinity, which is hypothesized to cause an argon-carbon complex with a mass of 52 (like chromium) in the plasma. The positive bias will be discussed in this presentation. This presentation will provide the audience with process details in Cr(VI) removal using reduction, coagulation and MF, utilizing existing coagulation and/or MF processes.
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