Date of Graduation

5-2014

Document Type

Thesis

Degree Name

Master of Science in Civil Engineering (MSCE)

Degree Level

Graduate

Department

Civil Engineering

Advisor/Mentor

Julian Fairey

Committee Member

Ashley Pifer

Second Committee Member

Wen Zhang

Keywords

Chlorite Reduction, Coagulant, Drinking Water, Ferrous

Abstract

In November 2012, the Beaver Water District (BWD) switched primary oxidants from free chlorine to chlorine dioxide (ClO2) to curb formation of trihalomethanes, a regulated group of organic disinfection byproducts (DBPs). Because ClO2 is reduced to chlorite (ClO2-), a regulated inorganic DBP, BWD incorporated ferrous salt into their coagulation regime to quench chlorite to chloride. In this study, two ferrous salts - ferrous chloride (FC) and ferrous sulfate (FS) - were compared as secondary coagulants in combination with alum, the primary coagulant. Ferrous salts were dosed stoichiometrically to reduce chlorite and alum was dosed at percentages (0-100%) relative to a typical alum dose at the BWD, 30 mg/L as alum. Settled water turbidity, dissolved organic carbon (DOC), chloride to sulfate mass ratio (CSMR), solids settleability, and water filterability were assessed in laboratory scale bench tests and pilot plant studies as a function of percent alum dose reduction (ADR). Results from bench scale jar tests and pilot plant study showed that settled water turbidity and DOC were not compromised up to ~40% ADR (18 mg/L as alum) and were indifferent to the type of ferrous salt added. As expected, CSMR, a corrosion surrogate, increased with increasing ADR (or decreasing sulfate addition) and was higher for the FC-treated waters compared to the FS-treated waters. However, CSMR remained below the suggested threshold of 0.6 for ADR values up to 60%, indicating both ferrous salts were viable options for chlorite quenching at BWD. Notably, the headloss rate during filtration in the pilot plant study was lower for the FS-treated waters at all values of ADR tested, and thus is the recommended secondary coagulant for the BWD.

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