Effects Of Operational Conditions On The Purification Performance And Membrane Fouling Of Combined Process Of Coagulation And Ultrafiltration

In order to deal with the membrane fouling issue of ultrafiltration, installation of pretreatment units before ultrafiltration membrane has become a common pratice in recent years, and coagulation pretreatment is widely used to relieve this problem. There are two types of coagulation pretreatment process, one is coagulation and sedimentation, the other is coagulation. In this study, the second type was used. Effluent quality and membrane fouling speed are the main topics related to this combined process currently, but little has been reported on the effects of operational conditions on flocs properties and membrane fouling during combined process of coagulation and ultrafiltration. In this study, a pilot-scale experimental system was used. Under different operational conditions of pH of raw water, dose of coagulant and dose of coagulant aids, effluent quality, transmembrane pressure and flocs properties were comparatively analyzed, and the relationship between flocs properties and membrane fouling was investigated. The optimal operational conditions were chosen which can provide some guidance for the wide application of ultrafiltration.1 When the doses of PACl were less than 4~5 mg/L, as the doses increased, the removal rates of combined process for organic matter were remarkablely increased. When the doses of PACl were higher than 4~5 mg/L, the removal rates were stable. Under doses less than 2 mg/L, the flocs with sizes smaller than 300 μm were predominant, which contributed to 79.53% of the total flocs, the fractal dimensions were 1.60±0.05, the TMP increased remarkably, the average increased transmembrane pressure per operation cycle was 0.48 kPa. Under the dose of 5 mg/L, the flocs with sizes among 400~500 μm contributed to more than 51.27% of the total flocs, the fractal dimensions were 1.38±0.07, the TMP increased slowly, the average increased transmembrane pressure per operation cycle was 0.21 kPa.2 When the pH of raw water was 4.5, the combined process had excellent performace of removing organic matter. While the sizes of flocs were relatively small, the fractal dimensions were 1.75±0.15, the rates of TMP increase were relatively fast, the average increased transmembrane pressure per operation cycle was 1.37 kPa, the main pollutants causing membrane fouling were fulvie acid-like and humic acid-like organic matters. When the pH was increased to 7.5, the removal rate of organic matter were decreased slightly, while the flocs in membrane tank had relatively big sizes, the fractal dimensions were 1.37±0.05, the rates of TMP increase were relatively slow, the average increased transmembrane pressure per operation cycle were 0.40 kPa.3 The addition of CPAM improved the removal of organic matter by combined process, but the improvement was not noticeable. When the dose was 0.00 mg/L, the flocs with size among 300~400 μm contributed more than 45.21% of the total flocs, the fractal dimensions were 1.55±0.087, the rates of TMP increase were relatively fast, the average increased transmembrane pressure per increased transmembrane pressure cycle was 0.27 kPa. As the doses increased, the amount of flocs with sizes bigger than 400 μm increased remarkably, the fractal dimensions decreased to 1.34±0.076. When the doses were 0.06 mg/L, the average increased transmembrane pressure per operation cycle was 0.15 kPa, when the doses were higher than 0.08 mg/L, the size of flocs in membrane tank decreased slightly and the TMP increased slightly.