The impact of ferrous ion reduction of chlorite ion on drinking water process performance

The use of chlorine dioxide (ClO2) as a primary disinfectant and pre-oxidant in drinking water treatment is being explored as an alternative to chlorine for reducing disinfection by-product formation and to assure compliance with United States Environmental Protection Agency's Stage 1 Disinfection/ Disinfection By-Products Rule. However, the ClO2 by-product chlorite ion (ClO2-) is also regulated by the same regulation. Ferrous iron (Fe(II)) has been shown to effectively reduce chlorite ion to chloride ion (Cl-) and this study was conducted to evaluate the impact on overall treatment process performance due to the ferric hydroxide solids that form from the reaction. Ferrous iron application was explored at three different points in a pilot-scale water treatment system: pre-rapid mix. pre-settling and pre-filter. Chlorite ion concentrations were effectively reduced from 2mg/L to less than 0.3mg/L using an Fe(II) dose of approximately 6mg/L for all trials. Fe(II) addition at the rapid mix caused no adverse effects and, in fact, allowed for reduction of the alum dose due to the newly formed ferric hydroxide acting as a supplemental coagulant. An increase of 241 and 247% of total suspended solids influent to the filter process was observed when Fe(II) was applied at the pre-settling and pre-filter locations. Pilot-scale filter runs during these trials were less than 2 h and never obtained true steady state conditions. Jar testing was performed to better understand the nature of the ferric hydroxide solids that are formed when Fe(II) was oxidized to Fe(III) and to explore the effectiveness of Fe(II) addition at intermediate stages in the flocculation process.