【作者】 孟凡刚；

【导师】 杨凤林；

【作者基本信息】 大连理工大学 ， 环境工程， 2007， 博士

【摘要】 膜生物反应器（MBR）是将膜分离技术和生物反应器结合而成的一个新型污水处理工艺。它把膜分离过程与生物降解结合起来，以膜分离装置取代普通生物反应器中的二沉池，从而取得高效的固液分离效果。MBR作为一种新型高效的水处理技术，日益受到各国的水处理技术研究者的关注。然而，膜污染是当前限制MBR广泛应用的主要瓶颈，其导致膜通量下降，增加膜组件更换和清洗的频率，从而增加MBR的运行费用。因此，有必要研究膜污染机理，这对于膜污染的有效控制以及MBR的推广应用具有重要意义。本研究采用一体式中空纤维膜生物反应器，对MBR技术中膜污染的关键问题进行了深入研究。主要考察了活性污泥性质和反应器操作条件对膜污染的影响机理，同时，借助分形理论和图像分析仪研究了膜污染的微观机理。本研究包括：（1）采用统计分析方法研究了16种不同性质活性污泥混合液对MBR膜污染的影响机理。结果表明：污泥浓度与膜污染阻力呈指数增加关系；污泥粒径（PSD）、胞外聚合物（EPS）、溶解性有机物（SMP）、上清液胶体颗粒（SS_s）、污泥混合液粘度（μ）、相对疏水性（RH）、Zeta电位均对MBR膜渗透性能有显著的影响作用，其与膜污染阻力的皮尔逊相关系数r_p分别为：-0.730、0.898、0.757、0.810、0.691、0.837、-0.881；同时发现，胞外聚合物是影响活性污泥中溶解性有机物含量（r_p=0.725）、污泥粘度大小（r_p=0.633）、上清液胶体颗粒含量（r_p=0.783）、Zeta电位（r_p=-0.953）及相对疏水性大小（r_p=0.877）的主要因素；在活性污泥性质中污泥浓度、胞外聚合物、污泥粒径是影响膜污染的根本原因，是膜生物反应器膜污染的重要影响因素。经过多元回归，得到了用于预测污泥浓度、污泥粒径及胞外聚合物对膜污染阻力影响的数学模型。该预测模型为：R_f=2.250e^MLSS*9×10-5+0.111EPS-1.99×10^-2PSD-3.201（2）采用短期过滤方式考察了丝状菌浓度或丝状菌生长指数对膜污染的影响机理。本部分研究主要从活性污泥性质和污泥絮体形态学的角度对丝状菌的膜污染机理进行了解释。研究表明：缺少丝状菌的污泥混合液会导致严重的膜孔堵塞污染，而丝状菌的过度繁殖会导致形成厚大、密实的滤饼层；丝状菌的过量繁殖会使得胞外聚合物和相对疏水性增大，污泥絮体Zeta电位下降，从而恶化膜过滤过程；另外，丝状菌的过度繁殖会导致污泥絮体周界分形维数和三维纵横比增大、污泥絮体圆度减小，说明丝状菌的过度繁殖使得污泥絮体变得不规则、松散，而不规则的污泥絮体会沉积并缠绕在膜丝表面，起到固定膜表面污染物的作用；因此，在MBR运行过程中，应采取适当措施控制丝状菌的生长情况以减缓膜污染。（3）采用3个平行运行反应器，系统研究了不同反应器负荷（OLR=0.7-0.8 kgCOD／m³d，OLR=1.1-1.4 kgCOD／m³d，OLR=1.7-2.1 kgCOD／m³d）对膜污染的影响机理。考察了OLR对膜过滤性能、膜污染、污泥性质（污泥活性SOUR、丝状菌浓度、胞外聚合物、污泥粒径、污泥浓度、污泥粘度）的影响，并分析了反应器内污泥混合液的水力学特性。研究发现：随着OLR的增大，反应器COD去除率仅有轻微下降，但污泥活性和污泥混合液溶解氧明显减小；在OLR=1.1-1.4 kgCOD／m³d和OLR=1.7-2.1 kgCOD／m³d的反应器内，发生严重的丝状菌过度繁殖，并导致胞外聚合物和污泥粘度迅速增大，进而加剧膜污染；高的反应器负荷会导致污泥浓度MLSS显著增大；最后，研究发现污泥粘度是决定反应器水力学特性的重要原因，当污泥粘度小于2.0 mPa s时，污泥粘度对反应器气液上升流速基本没有影响，而当污泥粘度大于2.0 mPa s时，气液上升流速急剧下降，其不能有效吹脱膜表面污染物；同时，随着污泥粘度的增大，反应器内污泥混合液的雷诺数迅速下降。（4）从活性污泥性质变化和滤饼层形成两个角度，研究了3个不同曝气强度（150 L／h，400 L／h，800 L／h）对膜污染的影响。研究结果表明：过大或过小的曝气均不利于膜污染的减缓，过小的曝气强度不能有效去除膜表面污染物，而过大的曝气强度会导致污泥絮体破坏，产生大量的胶体粒子和大分子有机物，使得膜通量下降；在大的曝气强度下，胶体粒子和溶解性有机物在膜表面及膜孔的沉积吸附是产生膜污染的根本原因；曝气强度为400 L／h时，在膜面形成了自生动态膜，其起到截留或降解小颗粒物和溶解性有机物、减缓膜污染的作用。（5）考察了不同MBR工艺中污泥混合液的膜过滤性能，并对比研究了污泥混合液的性质。借助共聚焦激光扫描电镜（CLSM）、傅里叶红外光谱仪（FTIR）、X射线荧光光谱仪（XRF）等技术表征了膜污染物的主要成分，并分析了膜表面污染物的形成机制。研究发现：A²／O-MBR中活性污泥的膜污染行为最严重，A／O-MBR和传统MBR（CMBR）的膜污染行为次之，序批式MBR（SMBR）的膜污染最轻；发现丝状菌过度繁殖、结合态EPS、溶解性EPS、芳香性化合物(以SUV₂₅₄表征)等是影响膜污染的重要因素；污泥絮体、多糖和蛋白质在膜表面大量沉积，导致MBR生物污染和有机污染。Ca、Mg、Al、Si、Fe等元素的沉积是造成膜无机污染的主要原因；在膜污染过程中，无机盐的沉淀物、胞外聚合物、微生物等相互结合，沉积并吸附在膜表面，形成粘附性极强、限制膜通量的滤饼层。（6）在膜生物反应器中，活性污泥在膜表面沉积形成滤饼层是造成膜污染的主要因素。采用扫描电镜和自动图像分析技术研究了膜过滤滤饼层的微观结构，并以分形理论和Darcy定律为基础，推导出用于预测膜污染滤饼层渗透性能的数学模型，验证了该渗透模型的可靠性，并利用该渗透模型进行了膜生物反应器中污泥浓度对滤饼层渗透性能影响的研究。结果表明：活性污泥颗粒沉积形成的柔性滤饼层具有明显的多孔结构，并且具有很好的分形特征；膜污染滤饼层的分形维数能够真实反映滤饼层的孔隙率大小；用该渗透模型得到的渗透系数K’可以预测滤饼层渗透性能；滤饼层渗透系数K随污泥浓度的增大而递减，污泥浓度低于10000 mg／L时，K’的变化趋势较小，污泥浓度达到10000mg／L以上时，K’急剧下降；污泥粒径和胞外聚合物是影响滤饼层渗透性能的重要因素，小粒径的污泥颗粒或胞外聚合物容易在滤饼孔隙内沉积，导致滤饼比阻增大，因此，污泥粒径减小或胞外聚合物浓度增大均会使得滤饼层渗透性能严重下降。更多还原

【Abstract】 In recent years, membrane bioreactors （MBRs） have been widely used in wastewater treatment to achieve higher effluent quality, which is often difficult to be effectively met by conventional activated sludge process. The advantages of MBR are a high mixed liquid suspended solids （MLSS） concentration, producing higher rate of removal of biological oxygen demand （BOD） and chemical oxygen demand （COD）, a lower excess sludge production and the treated water can be reused. In addition, the space occupied by MBR systems is greatly reduced due to the absence of settling tanks and the reduction in bioreactor volume made possible by the higher biomass concentration. But a major obstacle for the application of MBRs is the rapid decline of the permeation flux as a result of membrane fouling. The membrane fouling in MBR reduces productivity and increases maintenance and operating costs. Thus, membrane fouling is the crucial problem to be resolved.In this work, fouling mechanisms of submerged MBRs were investigated systematically. The impacts of sludge characteristics and operational conditions on membrane fouling were identified. The micro-mechanism of membrane fouling was also elucidated on the basis of fractal theory and image analysis. This work was performed as follows:（1） In this work, the influence of activated sludge properties such as the MLSS concentration, sludge particle size distribution （PSD）, extracellular polymeric substances （EPS）, soluble microbial products （SMP）, suspended solids in supernatant （SSs）, dynamic viscosity （μ）, relative hydrophobicity （RH）, and zeta potential on membrane fouling was examined. Activated sludge samples taken from different membrane bioreactor processes were used to study their impacts on membrane fouling. The influence of activated sludge properties on membrane permeation was identified using statistical methods. The results showed that MLSS concentration had an exponential relationship with membrane fouling resistance. The sludge particle size （r_p=-0.730） was correlated inversely to the membrane fouling resistance significantly. The total EPS （r_p=0.898） and protein （r_p=0.810） had strong positive effect on membrane fouling resistance, but carbohydrate （r_p=0.626） had a moderate correlation with membrane fouling resistance due to its low amounts. SMP （r_p=0.757）, SSs （r_p=0.810）, dynamic viscosity （r_p=0.691）, RH （r_p=0.837）, and zeta potential （r_p=-0.881） also had significant influence on membrane permeability. However, protein （r_p=0.936）, SMP （r_p=0.725）, SSs （r_p=0.783）, dynamic viscosity （r_p=0.633）, RH （r_p=0.877）, and zeta potential （r_p=-0.953） mainly resulted from the change of EPS concentration. These results suggest that MLSS concentration, PSD and EPS were the predominant factors affecting membrane fouling during membrane filtration of sludge suspension. The membrane fouling resistance can be predicted using a model based on MLSS concentration, PSD and EPS: R_f = 2.250e^MLss*9×10-5＋0.111EPS－1.99×10^-2PSD－3.201（2） The influence of filamentous bacteria on membrane fouling process in membrane bioreactor was analyzed in this study. To illustrate the membrane fouling mechanism of filamentous bacteria, the physical and chemical characteristics of floes such as EPS, zeta potential, RH and floc morphology were investigated systematically. The results showed that the absence of filamentous bacteria in sludge floes led to severe membrane pore blocking, the floes existing excessive filamentous bacteria, however, could cause the formation of a non-porous cake layer on the membrane surface. The over growth of filamentous bacteria could result in much more release of EPS, lower zeta potential, higher hydrophobicity of sludge floes, and caused serous harm to membrane permeation. The results also showed that the boundary fractal dimension （D_p） and three-dimensional aspect ratio （AR） of sludge floes increased with increasing filamentous index （FI）. In addition, the roundness （R_o） of sludge floes decreased with increasing FI. These results indicate that floes of bulking sludge had a very irregular shape, which would do great harm to membrane filtration process. The sludge floes existing a small quantity of filamentous bacteria had a positive effect on membrane permeation. It is an important kind of microbe to be control in the operation of membrane bioreaetors.（3） In this study, three identical MBRs were operated in parallel with sludge retention time （SRT） of 30 days and organic loading rates （OLRs） of 0.7-0.8 kgCOD/m³d, 1.1-1.4 kgCOD/m³d, 1.7-2.1 kgCOD/m³d, respectively, in order to specify the influence mechanism of OLRs on membrane bioreactor. Although OLRs have no direct impact on membrane permeation, OLRs affect sludge characteristics strongly since the MBR system includes living microorganisms and their metabolites. The results showed that COD removal efficiencies were stable though it decreased slightly as OLR increased, but biomass activity and dissolved oxygen （DO） concentration in sludge suspension decreased as OLR increased. The filamentous bacteria grew easily in the bioreaetor with increasing OLR. The EPS concentration and sludge viscosity became much higher as filamentous bacteria excessively grew. The filamentous bacteria could also induce the formation of large and irregular shaped sludge flocs, and worsen membrane permeation. In addition, the MLSS concentration increased significantly as OLR increased. The results also indicated that sludge viscosity was the predominant factor affecting hydrodynamic conditions of MBR systems. The cross flow velocity of the sludge suspension decreased significantly as the sludge viscosity increased larger than 2.0 mPa s. Under the hydrodynamic conditions of low cross flow velocity, the fouling cake layer formed on the membrane surfaces could not be removed effectively. Moreover, the Re of the sludge suspension in the MBRs was decreased significantly with increasing sludge viscosity.（4） Membrane fouling in three parallel MBRs was studied under different aeration intensities （150 L/h, 400 L/h, and 800 L/h） to have a better understanding of membrane fouling mechanism. The impact of aeration on membrane fouling was interpreted from two aspects: evolution of sludge properties and formation mechanism of fouling cake layer. As the membrane fouling in MBRs mainly results from the formation of a fouling cake layer, scanning electron microscope （SEM） and resistance analysis were performed to characterize and quantify the fouling layer. The results showed that small or large aeration intensity had a negative influence on membrane permeability. The smaller aeration intensity could not remove the fouling cake layer effectively. The larger aeration intensity resulted in a severe breakup of sludge flocs, and promoted the release of colloidal and soluble components from the microbial flocs to the bulk solution due to microbial floc breakage, thus caused a rapid loss in membrane permeability. The colloids and solutes had great contribution to membrane foulants as the MBR operated under high aeration intensity. The results suggested that a dynamic membrane would be formed under the aeration of 400 L/h, which had a positive effect on membrane permeation. Aeration had a positive effect on cake layer removal, but pore blocking became severe as aeration intensity increased to 800 L/h.（5） In this study, the fouling behavior of different MBR technologies, including conventional MBR （CMBR）, sequencing batch MBR （SMBR）, A/O-MBR and A²/O-MBR, was investigated on the basis of bacth membrane filtration. The biomass characteristics were analyzed in order to elucidate membrane fouling. The membrane foulants formed on the membrane surface were characterized by such techniques as confocal laser scanning microscopy （CLSM）, Fourier transform infrared （FTIR） spectroscopy, and X-ray fluorescence （XRF）. The formation mechanism of membrane foulants was also analyzed. The results showed that the sludge suspension in A²/O-MBR could result in severe membrane fouling due the higher filamentous concentration and EPS concentration. The sludge suspension in A/O-MBR and CMBR had a moderate membrane fouling tendency. The sludge suspension in SMBR, however, caused a very slight membrane fouling. The filamentous index, bound EPS, soluble EPS and SUV₂₅₄ had strong impacts on membrane fouling. The CLSM and FTIR examination indicated that bacteria clusters, proteins and carbonhydrates were significant contributors to membrane foulants. The XRF suggested that the inorganic foulants resulted from the deposition of Ca, Mg, Si, Al, and Fe. Bridging among bacteria clusters, deposited biopolymers and inorganic materials enhanced the compactness of the fouling layer.（6） The formation of fouling cake layer on the membrane surface is the major problem that hinders the practical application of membrane bioreactor systems. Determination of the cake layer permeability is critical for an accurate analysis and design of membrane filtration. A permeation model, based on fractal theory and Darcy’s law, for evaluating cake layer permeability in microfiltration of sludge suspension was developed. The cake layer permeability was derived and found to be a function of the pore-area fractal dimension and micro-structural parameters. The validity of the model was studied systematically. The permeation model was applied to study the effect of MLSS concentration, PSD and EPS on cake layer permeability in a submerged membrane bioreactor. Results showed that the permeation model was a useful tool to study the micro-mechanism of membrane fouling. There was a close correlation between MLSS concentration and cake layer permeability. There were a slight and a distinct decrease of the cake layer permeation as MLSS increased less and larger than 10,000mg/L, respectively. PSD and EPS were two significant factors affecting cake layer permeability in membrane bioreactor. The decrease of PSD or increase of EPS would result in a sharp decrease of cake layer permeability.更多还原