Effect Of Fe (Ⅲ) Species In Iron Salts Coagulants On Coagulation-ultrafiltration Hybrid Process

Recently, ultrafiltration (UF) technology was used more and more widely in water treatment. However, membrane fouling hindered UF as an effective as an alternative to conventional treatment for drinking water production. Coagulation was used prior to UF as a pretreatment. In this study, humic acid-kaolin (HA-kaolin) stimulated water and fulvic acid (FA-kaolin) stimulated water were treated by coagulation-ultrafiltration (C-UF) hybrid process. Effects of Fe (III) species in iron salts coagulants on coagulation performance, floc properties including floc size, fractal dimension, floc strength and recovery ability and membrane fouling were studied in this study. Meantime, the relationship between floc properties and membrane fouling was investigated. The main content and results were as follows:1. Impacts of Fe (III) species on floc properties and membrane fouling in C-UF hybrid process were investigated. Four PFC coagulants with basicity values of 0.5,1.0,1.5 and 2.0 were used. The distribution of Fe (III) species in PFC was determined by a timed complexation spectroscopy method. The results indicated 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. Larger and looser flocs resulted in less serious membrane fouling. PFC with B value of 0.5 (denoted PFCos) had highest DOC and UV278 removal efficiency in FA-kaolin synthetic water treatment, and the membrane performance of PFC05 was best.2. The distribution of Fe (III) species in ferric chloride (FeCl3) and PFC with various B values (B= 0.25,0.5,0.8,1.0,1.2,1.5,2.0 and 2.2) was measured according to a timed complexation spectroscopy method. Amount of Feb in PFC with B value of 1.0 (denoted as PFC10) was larger than that in any other coagulants and so was Fec in PFC with B value of 2.2 (denoted as PFC22). Thus, FeCl3, PFC10 and PFC22 were selected for subsequent experiments. Importance of Fe (III) species was investigated in terms of floc properties and membrane fouling during C-UF process. The results showed that the sum of Fea and Feb increased in the order of FeCl3>PFC10> PFC22 and the percentage of Fec was in the reverse order. FeCl3 had better coagulation performance in terms of turbidity, UV254 and DOC than PFC10 and PFC22. The removal efficiencies of turbidity and organic matter by three coagulants increased first and then decreased when pH ranged from 4.0 to 9.0. FeCl3 gained better turbidity and organic matter removal with pH of 6.0-7.0. PFC10 and PFC22 had better turbidity and organic matter with pH 5.0-6.0. FeCl3 produced largest flocs with loosest structure which were, however, weakest with best recoverability. PFC10 took the second place regarding to floe properties. For three coagulants, flocs under acidic condition were larger and looser than these under alkaline condition. However, the flocs were weaker under acidic ranges. The membrane fouling of PFC10 and PFC22 was more serious than FeCl3, which demonstrated that more Fea and Feb could effectively reduce the membrane fouling. 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. In addition, membrane fouling at acidic ranges was less serious than that at alkaline ranges. Membrane fouling was more sensitive to pH variation when iron salt coagulants contained more Fea and Feb.3. Polysilicic acid (pSi) was introduced into PFC and the effect of Fe (III) species and pSi on floc properties and membrane fouling during C-UF process was investigated. The results indicated that the percentages of Fea and Feb in polyferric silicate chloride (PFSiC) were smaller than that in PFC. Turbidity removal for PFSiC was improved compared with PFC while organic removal efficiency for PFSiC was lower than that of PFC. The addition of pSi could improve floc size and floc size increased with increasing Si/Fe ratio. PFSiC with various Si/Fe ratios resulted in more compact and weaker flocs than PFC. For the coagulation-ultrafiltration mode, PFSiC with Si/Fe ratios of 0.07 and 0.10 presented better membrane performance than PFC, which showed that proper pSi introduced reduced membrane fouling. For the coagulation-sedimentation-ultrafiltration mode, PFSiC had better membrane performance than PFC within Si/Fe ratios investigated. Overall, pSi additive was beneficial to membrane fouling reduction.