Effect of Fe (III) species in polyferric chloride on floc properties and membrane fouling in coagulation-ultrafiltration process

Impacts of Fe (III) species on floc properties and membrane fouling in coagulation-ultrafiltration (C-UF) hybrid process were investigated. Fulvic acid (FA)-kaolin synthetized water was used to simulate the actual surface water coagulated by polyferric chloride (PFC) with four different basicity (B) values. Four PFC coagulants with B values of 0.5, 1.0, 1.5 and 2.0 were denoted as PFC₀₅, PFC₁₀, PFC₁₅ and PFC₂₀, respectively. The distribution of Fe (III) species in PFC was determined by a timed complexation spectroscopy method. The results implied that the sum of monomeric and polymeric species (Fe_a and Fe_b) in PFC decreased as B values of PFC increased. PFC with higher Fe_a and Fe_b had better dissolved organic carbon (DOC) removal efficiency and higher zeta potential, which made charge neturalization stronger. Fe_a and Fe_b in PFC were beneficial to form larger and looser flocs. The permeate fluxes of PFC with different B values decreased in the following order: PFC₀₅>PFC₂₀>PFC₁₀=PFC₁₅ and membrane fouling index (MFI) had opposite order. PFC₀₅ displayed a flux advantage due to larger and looser flocs formed by it. In addition, cake resistance (Rc) relative to the total resistance was in the order of PFC₂₀<PFC₀₅<PFC₁₀<PFC₁₅.