Leveraging coagulation mechanisms to reduce fouling and increase natural organic matter removal during coagulation/flocculation-ultrafiltration treatment

Despite widespread application of coagulation/flocculation prior to ultrafiltration (UF), a lack of knowledge remains regarding the appropriate selection of conditions to reduce fouling and/or improve the removal of natural organic matter (NOM). The present study investigates the impact of conditions (alum dosage, pH) associated with specific coagulation mechanisms along with two different coagulation/flocculation configurations, on UF performance in terms of fouling and NOM removal. In-line coagulation/flocculation configurations were provided at bench-scale under continuous-flow conditions, which enabled quantification of hydraulically reversible/irreversible fouling over multiple permeation cycles. When compared to raw water without the addition of coagulant, utilization of coagulation/flocculation generally increased hydraulically irreversible fouling resistance. Conditions that promote adsorption destabilization (2.5 mg/L alum, pH 5.5) substantially reduced or eliminated the hydraulically irreversible accumulation of NOM, likely by minimizing interactions between certain NOM components (e.g. biopolymers and humic substances) and the membrane surface. When considering conditions that promote combined (8 mg/L alum, pH 7.5) and sweep (15 mg/L alum, pH 7) mechanisms, inclusion of flocculation increased TOC removal while reducing total/hydraulically irreversible fouling resistance (when compared to coagulation alone), despite greater amounts of NOM being retained by the membrane. The reduced fouling resistance was likely due to the formation of larger floc which formed a more permeable foulant layer. Results from the present study suggest that if only fouling control is required, conditions that promote adsorption destabilization are optimal, whereas if NOM removal is required, conditions that promote sweep are optimal. Inclusion of flocculation provided an optimal trade-off between fouling reduction and NOM removal for both adsorption destabilization and sweep, which should be considered during the design of coagulation/flocculation-UF systems to reduce operating costs despite potentially higher construction costs when compared to coagulation alone.