【作者】 刘婷；

【导师】 陈忠林；

【作者基本信息】 哈尔滨工业大学 ， 市政工程， 2011， 博士

【摘要】 近年来,超滤膜在水处理中的应用价值得到了广泛认可。而超滤膜在净水过程中,自身会不可避免地受到污染。如何在保证水质的前提下降低膜污染,是膜技术领域研究的重点和难点。预处理技术具有降低膜污染的潜能,本研究中选取了具有较强实用性、可操作性和成本优势的三种预处理技术——混凝预处理、吸附预处理和氧化预处理,选择常用试剂为代表,研究了采用不同预处理技术及不同运行方式时,浸没式超滤膜的运行效能,重点考察了三种预处理技术对膜污染的影响及其机理。采用硫酸铝为混凝剂,在中性pH值条件下,研究发现预混凝-超滤系统在常规最佳混凝剂投量下对水中UV₂₅₄和溶解性有机碳（DOC）的去除率分别达到82%和67%。混凝预处理还能够有效降低膜污染,在恒定膜通量10 L/（m²h）条件下,系统运行至280小时后,无混凝预处理的超滤系统跨膜压差比有混凝预处理的超滤系统高出13.5kPa。原水中含有少量颗粒物（浊度8¹0NTU）并不会影响有机物的去除效果和膜污染程度,混凝剂的投量由有机物的浓度决定。尽管省略絮凝过程的微絮凝-超滤系统缩短了工艺流程,不影响膜的出水水质,但在10L/（m²h）的膜通量下运行250小时后,微絮凝-超滤系统的跨膜压差比常规混凝-超滤系统高出11kPa。原因是微絮凝过程产生了体积百分比约占50%的尺寸在20μm以下的微絮体,这部分絮体更容易造成膜污染。尽管减量投药可减少药剂的投加和絮体污泥的生成,对UV₂₅₄的去除率仅降低了8%,但对DOC的去除率降低了20%。当运行300小时后,减量投药混凝-超滤系统的跨膜压差比常规最优投量混凝-超滤系统高出10kPa。原因是此时絮凝速率降低,絮凝阶段末期絮体的平均尺寸降低了约25%,絮体分形维数从常规最优投量下的2.69增加到2.79,均对降低膜污染不利。将混凝剂在混合和絮凝阶段二次投加的新型投药方式明显降低了膜污染,当运行330小时后,二次投药混凝-超滤系统的跨膜压差比常规单次投药混凝-超滤系统低了16kPa。原因是二次投药下平衡时絮体平均尺寸提高了近25%,分形维数从单次投药的2.68降低至2.51,同时膜的内部污染程度明显减轻。这可能由于新生成的混凝剂水解产物具有不同的表面特性,使最终形成的絮体过滤性能得到改善。研究发现粉末活性炭预吸附对提高超滤系统去除有机污染物的效能作用显著,去除率随着粉末活性炭投量的增加而显著提高,尤其对于超滤膜几乎无法截留的中小分子有机物去除能力很强,50mg/L粉末活性炭的投量对乙草胺和异丙甲草胺两种目标污染物的去除率分别达到了85%和90%左右。粉末活性炭在低投量下对膜污染的影响不明显,而在高投量下会加剧膜污染,10 L/（m²h）的膜通量下,运行200小时后,系统跨膜压差比低投量时提高了约10kPa。水中腐殖酸会将炭颗粒粘结在膜表面,并进入膜表面沉积的活性炭层孔隙中将其堵塞,二者的协同作用导致了复合膜污染。在膜池前的混合池中投加粉末活性炭与直接投加到膜池中的膜污染速率无明显差别。运行约300小时后,在膜池中连续投加比间歇投加跨膜压差低了7kPa,这是由于间歇投加造成粉末活性炭分布不均匀和膜表面更严重的炭颗粒沉积。超滤膜表面预涂粉末活性炭层后对膜污染速率的影响并不显著,说明了预涂层技术是否能够降低膜污染与膜材料的性质有关。臭氧预氧化不仅能够提高超滤系统对有机污染物的去除能力,还可以有效降低膜污染。浸没式超滤膜持续运行一段时间后,膜反应器内还存在微生物的降解作用。研究发现臭氧预氧化使整个系统对COD_Mn、DOC和UV₂₅₄的去除率分别提高了17%、13%和30%左右。运行55天后,在10 L/（m²h）的膜通量下,无臭氧预氧化-超滤系统的跨膜压差比有臭氧预氧化-超滤系统高出12.5 kPa。三维荧光光谱技术的研究结果显示,臭氧预氧化使进水中类腐殖酸物质含量降低了30⁴0%,使类蛋白质物质含量降低了60⁷0%,对应出水中两类物质也有相应含量的减少。臭氧预氧化显著降低了混合液胞外聚合物中类蛋白质物质的含量,此类物质与膜表面滤饼层阻力密切相关。臭氧预氧化能够使外部膜污染物中类蛋白质物质的含量明显降低,以及类腐殖酸物质的结构发生变化;同时,臭氧预氧化造成了内部膜污染物中类蛋白质与类腐殖酸物质的含量均大幅度降低,但结构变化很小。在此基础上,采用松花江水为原水,进一步证明了臭氧预氧化技术对提高超滤系统运行效能的积极作用。更多还原

【Abstract】 The significance of ultrafiltration （UF） technology applied in water treatment has been gradually recognized in recent years. However, when UF membrane is employed in the water purification process, it will be inevitably fouled. On the premise of water quality ensurance, how to mitigate the membrane fouling effectively is a technical difficulty in membrane technologies. Pretreatment methods are considered to be effective ways to reduce membrane fouling. In this study, three feasible, practical and inexpensive pretreatment methods were selected: coagulation pretreatment, adsorption pretreatment and oxidation pretreatment. In addition, commonly used reagents were selected as representatives. The process performance of a submerged polyvinyl chloride ultrafiltration membrane was investigated at different pretreatment strategies and different operating conditions, with focus on membrane fouling degree.Using aluminum sulfate as coagulant at neutral pH, the removal efficiencies in terms of UV absorbance at 254 nm (UV₂₅₄) and dissolved organic carbon （DOC） were achieved 82% and 67% respectively by coagulation-UF system at optimal coagulant dosage. Moreover, coagulation pretreatment could alleviate membrane fouling effectively. In the same constant flow of 10 L/（m²h）, trans-membrane pressure （TMP） of UF system without pre-coagulation was 13.5 kPa more than that of UF system with pre-coagulation after 280 h operation period. It was found that a certain amount of particulate matter （turbidity of 8¹0NTU） had little influence on organic matter removal and TMP increase rate of UF system, and coagulant dosage was determined by the concentration of organic matter.Although a shorter treatment duration and similar permeate quality can be achieved by a micro-flocculation with UF system, it caused a TMP gap of 11 kPa, that is more serious membrane fouling, after 250 h operation period in the constant same flow of 10 L/（m²h） compared to a conventional coagulation process with flocculation. It was mainly because that micro-flocs （smaller than 20μm） took approximately 50% volume percent, which resulted in a denser foulant layer and more severe fouling. Although under-dosing coagulation saved coagulant and produced less floc sludge, 8% and 20% lower removal efficiencies of UV₂₅₄ and DOC were achieved compared with the case of optimal dosage of conventional coagulation. After 300 h operation period, TMP of under-dosing system was 10 kPa higher than that of optimal dosage system. This was mainly due to a decreased flocculation rate resulted from insufficient coagulant dosage. The average size of flocs decreased about 25% and fractal dimension of flocs increased from 2.69 to 2.79 at the end of flocculation stage, which was unfavourable for membrane fouling control. This study found that two-stage coagulant addition exhibited obvious advantages to mitigate membrane fouling. The TMP of two-stage coagulant addition system was 16 kPa higher than that of conventional one-stage addition system after 330 h operation period. It was found that the average size of flocs was increased nearly 25% and fractal dimension of flocs decreased from 2.68 to 2.51. Internal fouling in membrane pores was also reduced by two-stage addition strategy. This was attributed to freshly formed precipitates in the second coagulant dosage which could improve the floc properties to produce preferable flocs for filtration performance.Powdered activated carbon （PAC） was used as adsorbent to study the adsorption pretreatment on ultrafiltration performance. It was found that pre-adsorption of PAC could significantly reduce concentration of organic matter in permeate. Removal effeciency increased with the increasing of PAC dosage, especially for the target micro-pollutants with small molecule which could be hardly removed by UF membrane. The removal efficiencies of two target pollutants, i.e. acetochlor and metolachlor, were achieved 85% and 90% at PAC dosage of 50 mg/L.However, at a low dosage, TMP increase rate seemed to have little change compared to that without PAC addition. While at a high dosage, membrane fouling could be actually aggravated. At higher PAC dosages, humic acid may help to connect carbon particulates to the membrane surface, and carbon layer which deposited on the membrane surface could be also filled with humic acid. The joint effect of humic acid and powdered carbon together could form a denser foulant layer which was adverse to fouling control. The results of addition mode indicated that there was no significant difference in TMP increase rate between adding into mixing tank and directly into membrane reactor. It was showed that TMP of continuous dosing system was 7 kPa lower than that of intermittent dosing after 300 h operation period, which may be due to uneven distribution of PAC in membrane reactor and more PAC deposition on membrane surface in the intermittent dosing mode. The PVC membrane after PAC pre-coating was investigated. The results exhibited that the effect of pre-coated carbon was not significant, which indicated that the effect of pre-coating was related with the property of membrane material.Oxidation process may be beneficial to both organic matter removal and membrane fouling reduction. There was accumulation of microorganism in a submerged UF membrane system as the filtration process continued. It was found that removal efficiencies of UF system in terms of COD_Mn, DOC and UV₂₅₄ were increased 17%, 13% and 30% respectively by pre-ozonation. After 55 d operation period, TMP of UF system without pre-ozonation was 12.5 kPa higher than that of UF system with pre-ozonation.Three-dimensional fluorescence spectroscopy was utilized to analyze the mechanism of membrane fouling control by pre-ozonation. The results showed that about 30⁴0% of humic-like substances and 60⁷0% of protein-like substances could be decreased by pre-ozonation. Pre-ozonation induced a significant reduction of protein-like substances in extracellular polymeric substance, which closely related with membrane fouling. Both external and internal fouling could be effectively mitigated by the pre-ozonation. The content decrease of protein-like substances and structural changes of humic-like substances were observed in external foulants from EEM fluorescence spectra due to pre-ozonation. At the same time, ozonation pretreatment resulted in remarkable reduction of both protein- and humic-like substances in internal foulants but relatively little structural changes. On the basis of above, the effectiveness of ozone pre-oxidation technology to improve UF performance was further confirmed with Songhua River as feed water.更多还原