Study On Floc Breakage And Re-Growth And Its Effect On Ultrafiltration Membrane Fouling

Coagulation is one of the most important processes in drinking water treatment. Up to now, however, there are still something unclear in the mechanism of coagulation. Generally speaking, coagulation process is consisted of two parts: rapid mixing and slow flocculation. It is inevitable that floc breakage and re-growth will occur due to the asymmetry of liquid field in coagulation reactors. Moreover, at a certain mixing speed, floc breakage and floc growth happen in the same time to obtain a dynamic balance. Therefore, it becomes a new research area to investigate the process and mechanism of floc growth, breakage and re-growth.Charge neutralization and sweep coagulation are main coagulation mechanisms of traditional hydrolyzed coagulant, and sweep coagulation dominates the coagulation mechanism in practical water treatment process (pH 7). Under the condition of sweep coagulation mechanism, formation, breakage and re-growth of flocs were investigated using alum or other coagulants to explore the reversibility of floc breakage. The objective of this study is to better understand how micro-flocs grow to become big flocs, as well as the key factors which determine the size and structure of flocs in the end. The flocs were continuously measured by Photometric Dispersion Analyzer (PDA) and CCD camera. Some research was carried out to explore the intrinsic principle of micro-floc connection and the significant finding had important practical consequences. This dissertation mainly included the following parts.At first, at different coagulant dosages and mechanisms, the aspects influencing floc breakage and re-growth were investigated to obtain more useful information of flocs characteristics. In charge neutralization, there is dramatically reversibility of broken flocs, which is no relationship with the breakage applied shear. While in sweep coagulation, the broken process was dramatically irreversible, and it did not attribute to electrophoretic mobility. In this part, some factors including coagulant species (alum and polymer polyDADMAC), temperature and primary particle size, flocculation time and humic acid concentration, breakage intensity and mixing conditon were also discussed to better understand the process of floc breakage and re-growth. A second low dosage of coagulant, added half way through the floc breakage period caused significantly different effects on the broken flocs for pH 5 and pH 7, even though the initial electrophoretic mobility was close to zero in both cases. At pH 5 a second low alum dosage reduced the re-growth ability of broken flocs, probably because of adsorption of excess cationic species, giving charge reversal and restabilization of broken flocs. By contrast, a second alum dosage at pH 7 greatly enhanced floc re-growth, such that the re-grown flocs could be larger than those before breakage. It is likely that this effect is caused by the adsorption of freshly precipitated hydroxide on the surface of broken flocs, giving improved adhesion. With an extended floc breakage time, the time of addition of the second coagulant dosage had a very large effect. When added shortly after the start of floc breakage, the additional dosage gave only slightly improved floc re-growth, but when added close to the end of the breakage period complete re-growth of broken flocs occurred. The implication is that the beneficial effect of new precipitate is lost after some minutes of exposure to high shear. It seems that the nature of this'surface activation'is the most important factor influencing the re-growth of flocs. Comparing AlCl₃ with PACl₂₅, additional dosage of AlCl₃ significantly improve the re-growth ability of broken floc, and the size of re-grown flocs is the same as or even higher than that before breakage. While for PACl₂₅, it will not change the re-growth ability of broken flocs. The re-growth ability of broken flocs is significantly correlated with the species of Al_a and Al_b, which is distinguished by ferron assay. The most important conclusion was made that Al_a on the surface of flocs was the material which can improve the floc re-growth when sweep coagulation dominated the coagulation mechanism.The effect of surface characteristic of different hydrolyzed product (precipitate) on particles adsorbing was also investigated. Kaolin particles were adsorbed on the surface of but not within the precipitate of AlCl₃. While for PACl₂₅, the precipitate was stable, and the flocs were as same as the one formed in normal coagulation. The surface area of AlCl₃ flocs increased when they were broken, which increased the adsorption ability of kaolin particles. Additional dosage of alum can dramatically improve the adsorption of kaolin particles on the surface of precipitate, and induce the adhesion of precipitate with each other. Hence, if floc sludge is reused, alum should be added to improve the removal of contaminants and floc re-growth.The surface characteristic of floc has an effect on membrane fouling in the coagulation-ultrafiltration hybrid process. Though the average size of flocs after breakage and re-growth was smaller than that without breakage, the fractal dimension of flocs after breakage and re-growth was much lower than that without breakage. Comparing with traditional coagulation process, the development of TMP decreased in breakage and re-growth coagulation process. Flocculation process (about 15 minutes) will not remove any more contaminants than rapid mixing process (about 1 min). The result showed that 1 min is enough for removing particles and organic matter in water and flocculation process may cause higher trans-membrane pressure. Because the flocs formed after only 1 min had lower fractal dimension and looser structure, and there was more pores in the cake layer on the membrane surface and the surface of flocs was more actived. The increase of trans-membrane pressure of KMnO₄ (pre-oxidation) coagulation-ultrafiltration (KCUF) is lower than that without pre-oxidation (CUF), because of the bigger size and lower fractal dimension of flocs. Although assimilable organic carbon (AOC) was increased by permanganate treatment, the AOC of the permeation from KCUF was nearly the same as that from CUF, showing that the cake layer on the surface of KCUF membrane could adsorb small molecules more effectively than that of CUF.