Application Of Iron Based Coagulants Combined With Organic Polymers In Coagulation-Ultrafiltration Hybrid Process

Based on the potential of inorganic-organic coagulants and coagulation-ultrafiltration in water treatment, this article focused on the coagulation performance, coagulation mechanism, and membrane fouling of Fe based coagulants and organic flocculants in coagulation-ultrafiltration hybrid process. The size, strength/recovery factors and fractal dimension of generated floc was studied through online monitoring of coagulation process; their relationship with membrane fouling mechanism was analyzed. The contents and results of this study were as follows:(1) Different mass ratio or dosages of DAM-ECH, PDMDAA and PAM combined use with iron-based coagulants (FeCl3, PFC) in coagulation of HA-Kaolin simulated water. The effects of those polymers on coagulation mechanism of iron salts was studied. Results revealed that proper dose of polymers could reinforce the charge neutralization mechanism so that enhanced the coagulation efficiency of Fe based coagulants on HA-Kaolin (especially hydrophobic organics). When pH of raw water adjusted to 6, coagulation efficiencies of coagulants was enhanced. Different polymers had different effects on PFC coagulation performance, in which PAM and DAM-ECH achieved better DOC removal than PDMDAAC; meanwhile PDMDAAC performed best UV254 removal.(2) Laser light scattering technology was applied to monitor the coagulation dynamic process. The median particle diameter (d0.5) was used as indicator of floc size; floc strength and recovery factor were applied as indicator of floc strength and recovery ability; fractal dimension (Df) was referred as fractal structure indicator. Results showed that organic polymer flocculants enlarged the d0.5 and recovery ability of flocs, and made them looser in structure. Meanwhile, floc became more vulnerble, this was because organic polymer flocculants enhanced the charge neutralization of coagulation process, and thereinto enhanced coagulation efficiencies; the chainlike strucure of organic polymer flocculants made the floc looser in structure. Comparing with PFC/DAM-ECH and PFC/PDMDAAC floc, PFC/PAM floc was looser in structure; meanwhile, PFC/DAM-ECH had better recovery ability than PFC/PAM and PFC/PDMDAAC floc. Weak acidic condition of raw water would contribute to the enlargement of floc size, recovery ability and looser structure.(3) UV254 and DOC were applied as indicators of humic acid (HA) removal. By using different coagulation methods, the effects of coagulation process and coagulation-ultrafiltration process was compared. Results showed that coagulation-ultrafiltration process achieved better HA removal effects than coagulation process. Especially at low coagulant dosages, the ultrafiltration as post treatment process could remarkably increase HA removal rate, and reduce the differences between various coagulation methods. When raw water pH was adjusted from 8.30±0.02 to 6, the HA removal efficiencies were enhanced, which means that weak acidic condition had no negative effects on ultrafiltration.(4) Coagulation effluent of different coagulation methods was channeled into ultrafiltration cell, the change of permeate flux was monitored. Results revealed that comparing with raw water, the permeate flux of coagulation effluent was remarkably higher. For different coagulation effluent, the permeate flux was different. According to SEM image of cake layer, the cake layer formed by floc was much more loose in structure than HA molecules. When raw water pH was adjusted to 6, cake layer structure became more loose, meanwhile the permeate flux was increased. Organic polymer flocculants could remarkably reduce the reduction of permeate flux caused by FC or PFC coagulation effluent. When shearing force was applied, permeate flux decreased as shearing force increased. Same as FC or PFC used alone, the permeate flux increased when raw water pH was adjusted to 6.(5) During ultrafiltration process, membrane was taken out when the structure of cake layer became stable. The membrane fouling resistance was devided by physical cleaning. First, the enternal membrane fouling resistance was devided into loosely attached external fouling (Ref-1) and strongly attached external fouling (Ref-s) according to its immunity to shear force; meanwhile the internal fouling resistance was expressed as revisable and irreversible internal fouling resistance (Rif-r and Rirr) on the basis of backwash treatment. Results revealed that comparing with raw water, the decrease of membrane fouling resistance was mainly because the reduction of external membrane fouling resistance. The reduction of external fouling resistance always followed by the increasing of loosely attached fouling resistance and the decreasing of strongly attached fouling resistance. Meanwhile, internal fouling resistance also reduced because the decrease of low molecular weight organic that may block membrane pore. Comparing with other water samples, PFC/PAM coagulation effluent achieved lowest membrane fouling resistance.(6) PFC with different Fe (III) species distribution respectively combined with DAM-ECH and applied in coagulation-ultrafiltraion process. The coagulation performance, coagulation mechainism and floc properties were studied; meanwhile, their relationship with membrane fouling resistance was also researched. Results revealed that the coagulants with better charge neutralization ability achieved higher HA removal rates(PFC05 and PFCio achieved higher HA removal than PFC15; PFC/DAM-ECH performed better HA removal than PFC). Floc with higher recovery ability achieved lower external membrane fouling resistance, and reduced the membrane fouling resistance. The bridging ability of coagulants (PFC15 and DAM-ECH) contributed to formation of floc with loose structure, so that reduced external fouling resistance. When shearing force was high enough (400 rpm), structure of recovered floc did not remarkably change (floc structure did not become looser). This result leading to observably growth of external fouling resistance; meanwhile, shearing force did not noticeably change the internal fouling resistance, which means that internal fouling resistance was determined mainly by the removal ability of coagulants on low molecular weight organics.