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

• Distribution of Fe (III) species in polyferric chloride (PFC) was measured.
• Effects of Fe (III) species on floc properties and membrane fouling were studied.
• The relationship between membrane fouling and floc properties was investigated.
• Monomeric and polymeric species in PFC did good to form larger and looser flocs.
• PFC with basicity value of 0.5 exhibited better membrane performance.

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 (Fea and Feb) in PFC decreased as B values of PFC increased. PFC with higher Fea and Feb had better dissolved organic carbon (DOC) removal efficiency and higher zeta potential, which made charge neturalization stronger. Fea and Feb 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.