Page 5: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Chromium

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Treatment Technology

Because chromium has not been identified as a problem in many water supplies, there have been few studies to determine the effectiveness of water treatment processes in removing chromium. Most of the information is based on laboratory or pilot-plant studies.^{Footnote 28}

Of the two common valence states of chromium, chromium(III) is the less toxic and the easier to remove from water supplies. Alum coagulation, iron coagulation (using ferric sulphate), and lime softening have all been found to be capable of removing at least 90 percent of an initial chromium(III) concentration of 0.15 mg/L. The effectiveness of iron coagulation was independent of pH in the range of 6.5 to 9.3. Alum coagulation was most effective between pH 7.5 and 8.5 and removed only about 80 percent of the chromium at pH 9.2. The optimum range for lime softening was above pH 10.5; the amount of chromium removed decreased to about 70 to 80 percent at pH 9.5. At an initial chromium concentration of 10 mg/L, all three processes were capable of reducing the chromium concentration by 98 percent.^{Footnote 29}

None of these three methods is very effective in removing hexavalent chromium. The best results were found using iron coagulation with ferric sulphate, which removed 35 percent of the hexavalent chromium at pH 5.5. Neither alum coagulation nor lime softening removed more than 10 percent.^{Footnote 29}

The method of choice for removing hexavalent chromium is iron coagulation using ferrous sulphate, which reduces the chromium to the trivalent state. Studies on river water containing 0.15 mg/L of chromium(VI) showed that ferrous sulphate was capable of removing 98 percent of the chromium in the pH range 6.5 to 9.3. For initial chromium(VI) concentrations above about 0.2 mg/L, it was found necessary to adjust the pH several minutes after coagulation to allow time for the chromium(VI) to be reduced to chromium(III).^{Footnote 29}

Chlorination of water before treatment tends to oxidize any trivalent chromium to the hexavalent state and so reduce the effectiveness of water treatments aimed at chromium(III). Chlorine doses of 2 mg/L with contact times of up to 6 hours decreased the effectiveness of alum coagulation by only about 10 percent, but much larger decreases were found with longer contact times.^{Footnote 29} Chlorination of water after treatment would be expected to convert any residual trivalent chromium to the hexavalent state. It is not clear, however, if passage of water through iron pipes would tend to reverse the process.

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