Floc properties and membrane fouling of different monomer and polymer Fe coagulants in coagulation-ultrafiltration process: The role of Fe (III) species

Importance of Fe (III) species in ferric chloride (FeCl₃), polyferric chloride with basicity values (B, OH^-/Fe³⁺ molar ratio) of 1.0 and 2.2 (denoted as PFC₁₀ and PFC₂₂) was investigated in terms of floc properties and membrane fouling during coagulation-ultrafitration process. The Fe (III) species distribution in these three coagulants was measured by a timed complexation spectroscopy method. Floc characteristics by FeCl₃, PFC₁₀ and PFC₂₂ were studied using a laser diffraction particle sizing device. Moreover, SEM images and resistance analysis were conducted to explore mechanism of membrane fouling. The results indicated that the monomeric and polymeric species (Fe_a and Fe_b) increased as follows: FeCl₃>PFC₁₀>PFC₂₂. FeCl₃ had better coagulation performance in terms of turbidity, UV₂₅₄ and DOC than PFC₁₀ and PFC₂₂. Meanwhile, FeCl₃ produced largest flocs with loosest structure which were, however, weakest with best recoverability. PFC₁₀ took the second place regarding to floc properties. The membrane fouling of PFC₁₀ and PFC₂₂ was more serious than FeCl₃. SEM images and resistance analysis showed that cake layer resistance relative to the total resistance produced by FeCl₃ was lowest due to the larger and looser flocs with high re-growth ability. Thus, more Fe_a and Fe_b could effectively reduce the membrane fouling.