混凝絮体破碎再絮凝机理研究及对超滤膜污染的影响
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摘要
混凝是给水处理中的核心单元,而混凝机理一直还存在着较多的争议。一般来讲,混凝包括两个阶段:投加混凝剂后的快速混合阶段,以及絮体逐渐形成增大的慢速絮凝阶段。由于混凝构筑物中各处流场的速度梯度分布不均匀,导致了絮体在构筑物中发生破碎,而破碎后的絮体也会发生再絮凝。此外,在一定的搅拌强度下,絮体的破碎和絮凝过程同时存在,最终会达到一个稳定的动态平衡。因此,混凝工艺中絮体的絮凝、破碎以及再絮凝过程,逐渐成为了近年来研究混凝机理的热点领域。
常规水解混凝剂的混凝机理主要是电中和作用和网捕卷扫作用,而在实际给水处理中(pH值为7左右),网捕卷扫作用占主导机理。本文主要针对网捕卷扫作用为主的混凝过程中,通过研究絮体破碎再絮凝过程,来探讨水中的微小絮体依靠什么作用、以怎样的方式粘结在一起,以及什么因素决定了最终平衡状态下絮体的尺寸和形态。研究中主要采用了透光率脉动检测仪(PDA-2000)和CCD摄像技术对絮体进行监测和分析,探讨了高岭土和腐殖酸絮体的絮凝、破碎和再絮凝过程,从以下几个方面逐步揭示了微小絮体粘结聚集的过程和规律,该研究结果具有工程应用的价值和潜力。
在不同混凝剂投量、不同混凝机理作用下,研究了絮体破碎再絮凝过程的主要影响因素,考察了不同影响因素下絮体的变化特征。研究发现:在以高岭土为悬浮颗粒物的原水中,电中和条件下硫酸铝絮体破碎后能够完全恢复,并且与破碎强度无关;而在网捕卷扫条件下,破碎后的絮体只能部分程度得到恢复,而破碎前和再絮凝后的絮体电泳迁移率(或zeta电位)并不会发生变化。此外,考察了不同混凝剂种类(包括铝盐和高分子电解质)、温度和初始颗粒大小、絮凝时间和腐殖酸浓度、破碎强度以及搅拌条件对絮体破碎再絮凝过程的影响。
针对网捕卷扫条件的普遍性及该机理条件下絮体难以完全恢复的情况,考察了再次投加少量硫酸铝混凝剂对破碎絮体再絮凝的影响。在不同混凝机理主导下,通过等电点(电泳迁移率接近于零)确定了硫酸铝的最佳投量。研究发现:当pH值为5的电中和条件下,再次投加少量硫酸铝会导致颗粒再稳,反而降低了破碎絮体的再絮凝能力;而当pH值为7的网捕卷扫条件下,再次投加少量硫酸铝能够显著提高絮体的再絮凝能力,使再絮凝后的絮体尺寸完全恢复到破碎前的水平。在网捕卷扫条件下,为了进一步研究再次投加硫酸铝的作用机理,将一分钟破碎时间延长至十分钟,发现了在长时间破碎的初期再次投加硫酸铝时,再絮凝能力提高不明显;而在长时间破碎快结束时再次投加硫酸铝,破碎絮体能够完全恢复。这说明硫酸铝新析出物的效力,会在其暴露于高剪切速率下的几分钟后消失。由此证明,在絮体破碎再絮凝过程中,是絮体的表面活性,而非絮体的带电性质(表现为电泳迁移率)决定了絮体的恢复能力。通过比较再次投加氯化铝(AlCl_3)和聚合氯化铝(PACl_(25))后发现,AlCl_3能够促进破碎絮体的完全恢复,而PACl_(25)并不能提高破碎絮体的再絮凝能力。利用试铁灵试剂(Ferron)研究后发现,很可能是由于AlCl_3析出物表面存在单体水解产物(Al_a),使微絮体之间能够相互粘结聚集;而对于PACl_(25),其多核水解产物(Al_b)转化成了其它稳定的物质,再次投加并不能促进破碎絮体的再絮凝。上述结果得出了本论文的核心观点:在网捕卷扫条件下,絮凝及再絮凝过程取决于絮体表面的Al_a。
研究了AlCl_3和PACl_(25)水解产物的表面特性对水中颗粒物高岭土的吸附作用。pH值为7时,投加AlCl_3后无定形氢氧化铝析出,由于表面Al_a的存在使析出物粘结聚集而逐渐长大。随后加入的高岭土颗粒主要吸附在了AlCl_3析出物的表面,而析出物内部的颗粒很少。然而,PACl_(25)的水解产物在水中较稳定,并不会发生粘结聚集,随着高岭土颗粒的投加,由于颗粒失稳而相互吸引,聚集形成了较大的絮体,因此絮体各部分(包括内部)颗粒均匀分布。由此可知,AlCl_3析出物具有粘结能力,而PACl_(25)则不具有这样的能力。这个研究结果可用于混凝絮体(污泥)的回流再利用。将絮体破碎后可增大其比表面积,可以促进对颗粒物的吸附,然后通过再次投加少量硫酸铝,不仅可以进一步吸附颗粒物,而且能够将破碎后的絮体粘结,最终形成较大的絮体。
研究了絮体的特性对混凝-超滤组合工艺中膜污染的影响。虽然絮体破碎再絮凝后的平均尺寸比常规混凝的絮体要小,但其分形维数比常规混凝的絮体要低,并且小絮体数目减少,从而减缓了膜污染的速度。另外,研究还发现慢速搅拌的絮凝过程并没有增加对水中高岭土和腐殖酸等物质的去除,而一分钟的快速混合足以完成对污染物的去除作用。快速混合后形成的絮体其分形维数较低,结构较松散,因此吸附在膜表面上形成的滤饼层孔隙度较高,同时絮体还具有较高的表面活性,对小颗粒的吸附能力较强,从而降低了跨膜压差的增长速度。研究还发现了高锰酸钾(KMnO_4)预氧化助凝时,硫酸铝絮体破碎再絮凝后的分形维数比单独投加硫酸铝的絮体要低,因此能够起到缓解膜污染的作用。
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 AlCl_3 with PACl_(25), additional dosage of AlCl_3 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 PACl_(25), 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 AlCl_3. While for PACl_(25), the precipitate was stable, and the flocs were as same as the one formed in normal coagulation. The surface area of AlCl_3 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 KMnO_4 (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.
常规水解混凝剂的混凝机理主要是电中和作用和网捕卷扫作用,而在实际给水处理中(pH值为7左右),网捕卷扫作用占主导机理。本文主要针对网捕卷扫作用为主的混凝过程中,通过研究絮体破碎再絮凝过程,来探讨水中的微小絮体依靠什么作用、以怎样的方式粘结在一起,以及什么因素决定了最终平衡状态下絮体的尺寸和形态。研究中主要采用了透光率脉动检测仪(PDA-2000)和CCD摄像技术对絮体进行监测和分析,探讨了高岭土和腐殖酸絮体的絮凝、破碎和再絮凝过程,从以下几个方面逐步揭示了微小絮体粘结聚集的过程和规律,该研究结果具有工程应用的价值和潜力。
在不同混凝剂投量、不同混凝机理作用下,研究了絮体破碎再絮凝过程的主要影响因素,考察了不同影响因素下絮体的变化特征。研究发现:在以高岭土为悬浮颗粒物的原水中,电中和条件下硫酸铝絮体破碎后能够完全恢复,并且与破碎强度无关;而在网捕卷扫条件下,破碎后的絮体只能部分程度得到恢复,而破碎前和再絮凝后的絮体电泳迁移率(或zeta电位)并不会发生变化。此外,考察了不同混凝剂种类(包括铝盐和高分子电解质)、温度和初始颗粒大小、絮凝时间和腐殖酸浓度、破碎强度以及搅拌条件对絮体破碎再絮凝过程的影响。
针对网捕卷扫条件的普遍性及该机理条件下絮体难以完全恢复的情况,考察了再次投加少量硫酸铝混凝剂对破碎絮体再絮凝的影响。在不同混凝机理主导下,通过等电点(电泳迁移率接近于零)确定了硫酸铝的最佳投量。研究发现:当pH值为5的电中和条件下,再次投加少量硫酸铝会导致颗粒再稳,反而降低了破碎絮体的再絮凝能力;而当pH值为7的网捕卷扫条件下,再次投加少量硫酸铝能够显著提高絮体的再絮凝能力,使再絮凝后的絮体尺寸完全恢复到破碎前的水平。在网捕卷扫条件下,为了进一步研究再次投加硫酸铝的作用机理,将一分钟破碎时间延长至十分钟,发现了在长时间破碎的初期再次投加硫酸铝时,再絮凝能力提高不明显;而在长时间破碎快结束时再次投加硫酸铝,破碎絮体能够完全恢复。这说明硫酸铝新析出物的效力,会在其暴露于高剪切速率下的几分钟后消失。由此证明,在絮体破碎再絮凝过程中,是絮体的表面活性,而非絮体的带电性质(表现为电泳迁移率)决定了絮体的恢复能力。通过比较再次投加氯化铝(AlCl_3)和聚合氯化铝(PACl_(25))后发现,AlCl_3能够促进破碎絮体的完全恢复,而PACl_(25)并不能提高破碎絮体的再絮凝能力。利用试铁灵试剂(Ferron)研究后发现,很可能是由于AlCl_3析出物表面存在单体水解产物(Al_a),使微絮体之间能够相互粘结聚集;而对于PACl_(25),其多核水解产物(Al_b)转化成了其它稳定的物质,再次投加并不能促进破碎絮体的再絮凝。上述结果得出了本论文的核心观点:在网捕卷扫条件下,絮凝及再絮凝过程取决于絮体表面的Al_a。
研究了AlCl_3和PACl_(25)水解产物的表面特性对水中颗粒物高岭土的吸附作用。pH值为7时,投加AlCl_3后无定形氢氧化铝析出,由于表面Al_a的存在使析出物粘结聚集而逐渐长大。随后加入的高岭土颗粒主要吸附在了AlCl_3析出物的表面,而析出物内部的颗粒很少。然而,PACl_(25)的水解产物在水中较稳定,并不会发生粘结聚集,随着高岭土颗粒的投加,由于颗粒失稳而相互吸引,聚集形成了较大的絮体,因此絮体各部分(包括内部)颗粒均匀分布。由此可知,AlCl_3析出物具有粘结能力,而PACl_(25)则不具有这样的能力。这个研究结果可用于混凝絮体(污泥)的回流再利用。将絮体破碎后可增大其比表面积,可以促进对颗粒物的吸附,然后通过再次投加少量硫酸铝,不仅可以进一步吸附颗粒物,而且能够将破碎后的絮体粘结,最终形成较大的絮体。
研究了絮体的特性对混凝-超滤组合工艺中膜污染的影响。虽然絮体破碎再絮凝后的平均尺寸比常规混凝的絮体要小,但其分形维数比常规混凝的絮体要低,并且小絮体数目减少,从而减缓了膜污染的速度。另外,研究还发现慢速搅拌的絮凝过程并没有增加对水中高岭土和腐殖酸等物质的去除,而一分钟的快速混合足以完成对污染物的去除作用。快速混合后形成的絮体其分形维数较低,结构较松散,因此吸附在膜表面上形成的滤饼层孔隙度较高,同时絮体还具有较高的表面活性,对小颗粒的吸附能力较强,从而降低了跨膜压差的增长速度。研究还发现了高锰酸钾(KMnO_4)预氧化助凝时,硫酸铝絮体破碎再絮凝后的分形维数比单独投加硫酸铝的絮体要低,因此能够起到缓解膜污染的作用。
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 AlCl_3 with PACl_(25), additional dosage of AlCl_3 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 PACl_(25), 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 AlCl_3. While for PACl_(25), the precipitate was stable, and the flocs were as same as the one formed in normal coagulation. The surface area of AlCl_3 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 KMnO_4 (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.
引文
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