Optimization of coagulant dose for biopolymer removal: Impact on ultrafiltration fouling and retention of organic micropollutants

• Coagulant dose was optimized for biopolymer removal in natural water matrices.
• A 0.5 mg/L dose was optimal based on diminishing returns for biopolymer removal.
• The 0.5 mg/L dose decreased UF fouling in waters with DOC > 4 mg/L.
• UF fouling rates were similar following coagulation at dosages of 0.5 and 15 mg/L.
• Retention of some micropollutants improved when coagulation was applied prior to UF.

Coagulation as pre-treatment to ultrafiltration (UF) was optimized for the removal of biopolymers, i.e., a primary UF foulant, for three different natural water matrices. The impact of pre-coagulation on membrane reversible and irreversible fouling, as well as the retention of organic micropollutants, was investigated at bench scale. Jar test experiments indicated that the optimum alum dosage for removal of biopolymers, based on a point of diminishing returns analysis, was relatively low (0.5 mg/L as coagulant; 0.05 mg/L as Al3+). This dose was effective at reducing membrane reversible and irreversible fouling (up to 48%) for waters with higher concentrations of organics (>4 mg/L as DOC) over 24 h of permeation and backwash cycles. Biopolymers were identified as contributing to both reversible and irreversible fouling. The retention of organic micropollutants was relatively low for UF alone (<40%) and for coagulation alone (<30%) with higher removals observed in waters with greater concentrations of organic matter. For combined coagulation and UF, retention of some compounds increased by up to 25%. In general, retention was higher for neutral, more hydrophobic compounds (log Kow > 2). This study demonstrates that a low dose of coagulant, optimized for biopolymer removal, may reduce membrane fouling and may provide value added for the retention of some organic micropollutants; however dosages depend on the specific water being treated, as well as treatment-related water quality targets.