| 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. |
