Date of Award:

2016

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Civil and Environmental Engineering

Advisor/Chair:

Laurie S. McNeill

Abstract

The USEPA Maximum Contaminant Level (MCL) for Total Chromium in drinking water is 100 μg/L. Total chromium includes both trivalent chromium (Cr(III), a trace nutrient) and hexavalent chromium (Cr(VI), a probable human carcinogen). The State of California set a Cr(VI)-specific MCL of 10 μg/L in 2014, and USEPA is considering a new federal MCL for Cr(VI). This would have a significant impact on drinking water systems across the US, with estimated annual cost of compliance between $0.6 to 5.1 billion per year.

While Cr(VI) is the species of concern for health effects, water utilities must also consider Cr(III) since it can be oxidized to Cr(VI) by various chemicals. This oxidation has been documented for commonly used disinfectants. However, past studies were conducted with higher levels of chromium (e.g., 20 to 500 μg/L) and it is unknown if the reactions proceed at the same rate and extent at the lower concentrations relevant to most water treatment plants (< 10 μg/L).

This project, funded by the Water Research Foundation, systematically evaluated the extent of oxidation of Cr(III) by drinking water oxidants under conditions relevant to drinking water utilities. Five oxidants (chlorine, monochloramine, chlorine dioxide, potassium permanganate, and ozone) were tested. Two doses were used for each chemical with their respective reaction times reflecting the typical application of the chemical in treatment. Three different water qualities were evaluated, each at pH 5.5, 7, and 9, and at two different temperatures (5 and 16 °C).

Chlorine consistently oxidized an average of 80% of the available Cr(III), with the majority of the oxidation happening within the first 7 hours. Monochloramine did not significantly oxidize Cr(III) at any of the conditions tested. Chlorine dioxide was an effective oxidant at pH 7, with complete oxidation occurring in 6 hours, but was less effective at pH 5.5 and 9. Potassium permanganate achieved complete oxidation in 4 hours at each pH, with pH 7 experiencing the fastest oxidation. Ozone oxidized all available Cr(III) within minutes at all pH values. Quantifying the Cr(III) oxidation as a result of using these oxidants provides understanding of potential Cr(VI) addition into drinking water.

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