Colloid Formation Resulting from Alum Coagulation of Organic-Laden Sourcewaters
Hardin, William Michael
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This research evaluated natural organic matter (NOM) dissolved-solid phase separation resulting from alum coagulation under the following sourcewater conditions: pH, initial NOM concentration, initial turbidity, and temperature. The solid phase was partitioned into two operationally defined size fractions; colloidal matter was defined as organic carbon (OC) retained by a 30 kilodalton ultrafiltration membrane, and particulate matter was defined as OC retained by a 1μm glass-fiber filter. Coagulation pH had a considerable impact on residual OC colloid formation, signified by more colloids formed as a function of alum dose at pH 6.8 as compared to pH 5.8. Initial NOM concentration strongly influenced the alum dose range over which OC colloid formation occurred and was found to be a proportional relationship. The presence of bentonite clay (used as the initial turbidity source) somewhat affected OC colloid formation by exerting some amount of coagulant demand, signified by decreasing OC colloid formation with increasing initial turbidity. Coagulation temperature had a considerable impact on particulate matter formation, as there was an increase in the dose at which particle formation first occurred at 4 ºC when compared to 25 ºC. Phase separation of OC from dissolved to colloidal matter was very similar at both 4 ºC and 25 ºC. The ability for low doses of polymers to replace a large portion of alum in order to further aggregate colloids during flocculation was unsuccessfully investigated. OC phase separation resulting from alum and iron sulfate coagulation was compared on a molar coagulant metal basis. The amount of residual OC associated with colloidal matter was similar, while the critical coagulant dose at which particulate matter formed was shifted to a much higher dose for iron.
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