Stoichiometry of coagulation revisited

The roles of particles and natural organic matter (NOM) in determining coagulant (alum) doses in potable water treatment were investigated at two pH conditions (6 and 7). The concentrations of NOM and colloidal silica particles in raw water were systematically varied separately and in combination, and the impacts of these two classes of contaminants on the minimum effective alum doses were investigated using observations of turbidity and dissolved organic carbon (DOC) in laboratory jar tests. At both pHs, coagulant requirements for the removal of these contaminants by sedimentation and filtration were dominated by the DOC concentration in the raw water. The presence of low NOM concentrations (0.75-1.5 mg of C/L) decreased the minimum effective alum dose dramatically for waters low in silica particles, possibly by promoting the precipitation of aluminum hydroxide and/or Al-NOM solids, whose removal would otherwise be limited by low collision opportunities. Strong stoichiometric relationships were observed between DOC and coagulant demand at both pHs regardless of silica particle concentration. Silica contributed to coagulant demand only at very high particle concentrations.