AWWA ACE60048

The relationship between raw water characteristics and corresponding minimumeffective alum doses (MEADs) was investigated. To test the effects of raw watercharacteristics on minimum effective chemical conditions for coagulation and, insome cases, subsequent filtration, the concentrations of colloidal particles and naturalorganic matter (NOM) in model raw waters were systematically varied. Jar testswere performed using waters with varying colloidal and NOM concentrations and,for each combination of colloidal and NOM concentrations, the minimum effectivecoagulant dose was observed for the removal of settled and filtered turbidity andDOC. Monodisperse silica particles (129 nm in diameter) and NOM from the GreatDismal Swamp in Virginia were used in preparing the waters to be tested. Theminimum effective alum dose for the coagulation of low silica waters without NOMdecreased as silica concentration increased, whereas it increased proportionally withincreasing silica concentration for high silica waters. At low silica waters, contactopportunity for floc formation is limited by low solid volume and removal isachieved via a sweep flocculation mechanism. Increasing silica concentration withinthis range provides additional floc volume thereby reducing the alum dose required toinduce sweep flocculation. Once the requirement for effective flocculation is met bysufficient silica concentration, the minimum effective alum dose increasesstoichiometrically with increasing silica. Removal of silica in the presence of NOMshowed two distinct results. First, at low silica concentration, the presence of lowNOM (0.75 mg/L) lowered the minimum effective alum dose dramatically, possiblyby promoting the precipitation of Al and/or Al-NOM solids. Jar tests conducted withthe addition of sulfate suggest that the presence of multivalent anionic moieties, suchas those in NOM, accelerate floc formation as aluminum hydroxide precipitate. Thepresence of simple anions (such as chloride) had much less effect. Second, theminimum effective alum dose showed a strong linear stoichiometric relationship withNOM concentration at all silica concentrations, with NOM dominating the alumdemand at low silica concentrations. The minimum effective alum doses for watershigh in both NOM and silica particles were somewhat additive; they increased bothas silica and NOM increased. In summary, NOM controlled the alum demand, fromreducing alum dosages for waters low in both DOC and turbidity to requiringstoichiometric increase at high DOC concentrations. Includes 10 references, tables, figures.

Product Details

Edition:

Vol. - No.

Published:

06/17/2004

Number of Pages:

13

File Size:

1 file , 400 KB