Identifying The Best Coagulant For Simultaneous Water Treatment Objectives: Interactions Of Mononuclear And Polynuclear Aluminum Species With Different Natural Organic Matter Fractions

As some water utilities are facing increasing concentrations of natural organic matter (NOM) in source water, it has been observed that some of this organic content is refractory to conventional coagulation treatment with alum and ferric sulfate. However, prehydrolyzed coagulants containing polymeric and/or cationic aluminum species such as Al-2, Al-3, Al-13, and Al-30 offer many advantages over conventional metal salts, including improved removal of hydrophilic compounds, stronger electrostatic interactions with anionic functional groups, better stability over a wider pH range, etc. Residual NOM after coagulation was characterized using liquid chromatography-organic carbon detection-organic nitrogen detection (LC-OCD-OND), while aluminum species were identified using nuclear magnetic resonance (NMR). Six coagulants having different proportion of Al species were tested on three surface waters, and guidelines for the coagulant selection are proposed for different NOM fractions: humics (HS), building blocks (BB, breakdown products of HS), low molecular weight acids (LMWa), low molecular weight neutral (LMWn) compounds, biopolymers (BP), and dissolved organic nitrogen from the HS fraction (HS-DON). In summary, ferric sulfate outperformed aluminum-based coagulants in acidic conditions (pH < 6) while aluminum chlorohydrate (basicity of 83 %) was repeatedly noted to be the best coagulant in more alkaline conditions (pH > 7). Under more neutral pH (6-7), alum offered a good compromise between DOC and turbidity removals and the coagulant cost. Coagulant performance was also assessed based on typical indicators used in the industry such as turbidity, dissolved organic carbon (DOC), residual aluminum/iron, trihalomethanes/haloacetic acids (THM/HAA) precursors, and alkalinity consumption. Considering all indicators and NOM fractions, this study revealed that PAX 14, a slightly prehydrolyzed coagulant (basicity of 23 %), exhibited the best performance during enhanced coagulation at pH 6.0.