A Review of Membrane Fouling in MBRs: Characteristics and Role of Sludge Cake Formed on Membrane Surfaces

Abstract

Membrane bioreactor (MBR) has been deemed to be a promising technology for wastewater treatment and reclamation; however, the MBR filtration performance inevitably decreases with filtration time attributed to the deposition of soluble and particulate materials onto and into the membrane under the interactions between activated sludge components and the membrane. Cake layer formation on membrane surfaces has been a major challenge in the operation of MBRs under supra-critical flux operation, and/or caused by uneven distribution of aeration intensities, etc.; however, it was argued that a thin cake layer might improve filtration operation by some researchers. This paper provides a critical review on the formation mechanisms, properties, the role of sludge cake in membrane filtration, and the corresponding strategies of controlling cake fouling in MBRs. Drawbacks and benefits of the formation of sludge cake were also discussed in order to better understand the characteristics and role of sludge cake formation in MBRs.     

Effect of Sludge Retention Time on Membrane Bio‐Fouling Intensity in a Submerged Membrane Bioreactor

Abstract

The submerged membrane bioreactor (sMBR) is being increasingly applied for municipal and industrial wastewater treatment. This paper examines the role of sludge retention time (SRT), an important operating parameter of the MBR as it affects the biological characteristics of the sludge and therefore influences membrane fouling. Well controlled runs were carried out at short SRT (10days) and moderate SRT (30days) in a 30 L submerged MBR equipped with KUBOTA flat‐sheet membranes. At steady operation, the mixed liquor suspended solids (MLSS) stabilized at approximately 5–6 g/L and 8.5–10 g/L for SRT 10 and 30days respectively. The DGGE profiles suggested a shift in the dominant bacterial community with the prolonged SRT. The soluble microbial products (SMP) were 9.3 mg/L and 5.4 mg/L at the SRTs of 10days and 30days respectively. The total amount of extracellular polymeric substance (EPS) extracted from the floc and the supernatant was approximately constant at the two SRTs under the same organic loading rate. However, the polysaccharide concentration in the supernatant was about 100% higher for the SRT of 10days than that for 30days. The viscosity of the biomass increased with the prolonged SRT, while the estimated average air induced water velocity was similar for the two SRTs. The results of flux stepping tests showed that the membrane fouling rate (dTMP/dt) at SRT 10days was always higher than that at 30days at each flux step. Similarly, long term experimental runs at a constant flux of 20 L/m²·h clearly showed more severe membrane fouling for the SRT of 10days than that at 30days. This implies that fouling is more influenced by the concentration of SMP and Polysaccharides than the MLSS.     

膜生物反应器污泥混合液对膜污染影响研究进展

膜生物反应器(membrane bioreactor,MBR)作为一种高效的污水处理及回用工艺,近年来备受关注,然而MBR的膜污染问题严重制约了该工艺进一步快速的商业化推广.本文综述了污泥混合液各组分对MBR膜分离的影响,包括溶解性微生物代谢产物(soluble microbial product,SMP)、胞外聚合物(extracellular polymeric substances,EPS)、污泥浓度(mixed liquor suspended solids,MLSS)、颗粒粒径及无机物对膜污染影响,并对该领域未来的研究方向进行了论述.     

Modeling of Particle Fouling and Membrane Blocking in Submerged Membrane Filtration

Abstract

The models of particle fouling and membrane blocking in a submerged membrane filtration are developed in this study. The effects of operating conditions, such as aeration intensity (air flow rate) and filtration pressure, on the filtration flux, membrane blocking, and cake formation are discussed thoroughly. The experimental results show that the filtration resistances due to cake formation and membrane blocking play significant roles in determining the overall filtration resistance, but the latter one is more dominant. An increase in aeration intensity leads the filtration flux to increase due to the reduction of cake formation on the membrane surface. However, a higher filtration pressure causes more severe membrane internal blocking and then to lower filtration flux. The cake properties and the filtration resistance due to membrane blocking are analyzed and can be regressed to empirical functions of filtration pressure. A force balance model for particle deposition on the membrane surface is also derived. In order to estimate the shear stress acting on the membrane surface, the diameter, shape, and rising velocity of air bubbles are analyzed based on hydrodynamics. Once the model coefficients are obtained, the pseudo‐steady filtration flux under various conditions can be estimated by the proposed model and the basic filtration equation. The calculated results agree fairly well with the available experimental data.     

Flux-Dependent Fouling Phenomena in Membrane Bioreactors under Different Food to Microorganisms (F/M) Ratios

This paper investigated the effect of food to microorganisms (F/M) ratios in the MBRs on membrane fouling propensities at fluxes of 10, 20, and 30 L/m² hr (LMH). The high F/M-MBR had different biomass properties, more soluble extracellular polymeric substances (EPS), and faster fouling rate compared to the low F/M-MBR. However, the fouling mechanisms at the three fluxes were dissimilar. At a low flux, the microbial flocs dominant cake layers facilitated catching the soluble EPS to increase resistance. At a high flux, the great accumulation of soluble EPS (especially soluble polysaccharides) to form gel-like cake layers predominantly induced membrane fouling. Supplemental materials are available for this article. Go to the publisher's online edition of Separation Science and Technology to view the free supplemental file.     

不同污泥对印染废水中残余染料的吸附特性

研究了四种不同污泥(活性污泥、厌氧污泥、干活性污泥、干厌氧污泥)对染料阳离子嫩黄X-4GL的吸附,并考察了胞外聚合物(EPS)在此过程中所起的作用.结果表明四种污泥对阳离子嫩黄的吸附在90min内基本达到平衡,干活性污泥的吸附性能最好,干厌氧污泥的吸附性能最差,污泥浓度为250 mg/L时,其吸附量分别为133和50 ...     

Effect of Substrate Composition (C/N/P ratio) on Microbial Community and Membrane Fouling Tendency of Biomass in Membrane Bioreactors

In this study, biomass characteristics including bacterial community, extracellular polymeric substances (EPS) production, and membrane fouling propensity were examined when the membrane bioreactors (MBRs) were fed with different substrates (i.e., different C/N/P ratios). Denaturing gradient gel electrophoresis (DGGE) analysis revealed that significant shifts of bacterial communities happened when increasing nitrogen or phosphorus loading in the MBRs, which followed in an almost similar way. At steady state, the biomass from the low C/N- and C/P-MBRs had comparable concentrations and produced similar EPS levels as those in the control MBR. However, the median particle size increased when the MBRs fed with low C/N- or C/P- substrate, possibly associated with the filamentous bacteria propagating in the MBRs. Increasing nitrogen or phosphorus loading 1-fold could not induce more serious membrane fouling compared to the control MBR.     

聚乙烯醇(PVA)对模拟胞外聚合物(EPS)在错流超滤中膜污染的影响

研究了在模拟胞外聚合物(EPS)溶液中添加聚乙烯醇(PVA)对错流超滤膜过滤过程的影响. 对有无添加PVA的模拟EPS溶液分别用错流平板膜进行超滤实验,用Hermia修正模型对实验数据进行拟合,分析验证膜污染机理. 结果表明,无论是否添加PVA,模拟溶液的通量-时间实验数据对滤饼堵塞模型的拟合度都最高,为0.891~0.994;添加PVA的模拟EPS溶液形成的吸附阻力和膜污染阻力均比无添加的溶液高约0.5倍,滤饼比阻比无添加的溶液高约4倍,分别为2.29′1014和9.57′1014 m-1;模拟EPS溶液添加PVA后,通量对操作压力的敏感度增加,对膜面流速的敏感度却降低.     

微生物代谢产物对膜生物反应器膜污染的影响

针对膜生物反应器（MBR）在运行过程中溶解性微生物代谢产物（SMP）及胞外聚合物（EPS）对膜污染进行研究。实验过程中对MBR内的污泥混合液进行了定期膜阻力监测。结果表明,SMP和EPS对膜过滤阻力有负面的影响。SMP中相对分子质量分布（Mw）在3～10 kDa对膜内部阻力影响显著,SMP中Mw＞10 kDa的大分子有机物及EPS浓度对膜外部阻力影响明显。通过傅里叶转换红外光谱（FTIR）检测膜表面污染物表明,EPS主要由多聚糖、蛋白质和腐殖酸组成,而污染层中的SMP主要是多聚糖和腐殖酸。     

Fenton试剂对剩余污泥脱水性能的改善

研究了利用Fenton试剂调理城市污水处理厂剩余污泥,通过测定污泥毛细吸水时间(CST)、污泥比阻(SRF)和泥饼含水率来表征污泥脱水性能的变化,分别考察了污泥初始pH、H2O2投加量、H2O2/Fe2+、反应温度和反应时间对污泥调理效果的影响。综合考虑,确定Fenton调理污泥的最佳条件为:pH=3、抽滤脱水和离心脱水H2O2最佳投加量分别为3 g/L和9 g/L、H2O2/Fe2(+质量比)最佳范围为8～12、反应温度50℃、反应时间60 min。对污泥离心上清液中胞外聚合物(EPS)含量的研究表明,Fenton调理后污泥上清液中蛋白质和多糖含量有大幅升高,说明Fenton试剂能有效氧化破解EPS,从而提高污泥的絮凝性,改善污泥的脱水性能。Fenton氧化后污泥颗粒粒径变小,比表面积增大。     

臭氧-活性炭技术对膜生物反应器膜污染减缓研究

本研究目的是探讨臭氧-活性炭技术对膜生物反应器（membrane bioreactor,MBR）膜污染减缓的影响。通过短期批式实验表明,粉末活性炭（power activated carbon,PAC）可强化臭氧的氧化效果,臭氧投加量超过0.25mg/(gSS)将恶化污泥混合液可滤性;对滤出液残余臭氧浓度检测表明,PAC的加入有利于维持本体溶液臭氧浓度。臭氧-活性炭技术引入MBR系统有助于膜污染的减缓,反应器内微生物活性受到一定的抑制作用,但对MBR出水水质影响较小;臭氧-活性炭减小了反应器内溶解性微生物产物（soluble microbial products,SMP）中的蛋白质及多聚糖含量,显著降低了污泥絮体中松散的胞外聚合物（loosely bound EPS,LB）及胞外聚合物（extracellular polymeric substances,EPS）中蛋白质浓度,以上结果表明应用臭氧-活性炭技术来延缓MBR膜污染是可行的。     

温度对MEC阳极膜形成及胞外聚合物的影响

温度是影响微生物活性和优势菌种以及反应器性能的重要参数之一。实验采用单池微生物电解池（MECs），以厌氧活性污泥为接种物，葡萄糖为碳源，在不同温度条件下培养运行微生物电解池阳极生物膜。实验结果表明：在25℃、30℃、35℃和45℃的培养温度下最大电流密度分别达到2.04A/m²、7.75A/m²、12.27A/m²和2.5A/m²。阳极生物膜电化学活性和氢气产率变化趋势与MECs电流密度相类似，均表现出在一定温度范围内升高温度有利于阳极膜电化学活性与阳极膜氢气产率。进一步分析阳极膜生物量与胞外聚合物（EPS）成分结果表明：一定温度范围内升高温度有利于阳极膜生物量的提高，大量的阳极附着细菌可以产生更高的电流密度与EPS含量，阳极膜EPS中蛋白质成分明显高于多糖，并且蛋白质含量随产电密度增高而增大。傅里叶变换红外光谱（FTIR）分析证实了生物膜EPS中存在蛋白质和碳水化合物。     

Effect of Hydraulic Retention Time on Sludge Properties,Cake Layer Structure,and Membrane Fouling in a Thermophilic Submerged Aerobic Membrane Bioreactor

Sludge properties, cake layer structure, and membrane fouling in a membrane bioreactor were studied under various hydraulic retention times (HRT). A decrease in HRT resulted in an increase in extracellular polymeric substance production in bulk sludge and changes in cake layer structure from gel layer to one or two cake layers. Particle size distribution in cake sludge changed with respect to HRT. An evolution in cake layer thickness and porosity was observed with trans-membrane pressure (TMP) jump. The change in cake layer structure might bear more responsibility for the TMP jump than the quantity of cake layer.     

Influence of Dissolved Organic Carbon and Suspension Viscosity on Membrane Fouling in Submerged Membrane Bioreactor

Abstract

This paper deals with the membrane fouling in membrane bioreactor (MBR). Based on the experimental data obtained in the MBR pilot plant study, the influence of F/M ratio on the irreversible and reversible fouling was discussed in the wide range of MLSS concentration. In the case of lower MLSS concentration (2,000–3,000 mg/L), irreversible fouling rate of membrane increased with increasing F/M ratio because of the accumulation of DOC in the mixed liquor. It seems that soluble microbial products with the similar size of the membrane pore will be most responsible for the irreversible fouling. In the case of higher MLSS concentration (8,000–12,000 mg/L), reversible fouling rate of membrane increased with increasing F/M ratio because of the increased suspension viscosity caused by the increased activated sludge size or volume even in the same MLSS concentration.     

EGSB处理中药废水过程中厌氧颗粒污泥特性变化

通过分析厌氧颗粒污泥粒径分布、机械强度、二价金属含量、胞外聚合物含量等,探讨了EGSB反应器处理中药废水过程中颗粒污泥特性变化。结果表明,在中药废水COD浓度为2000～5000 mg·L^-1、HRT为12 h、T为30℃的条件下,颗粒污泥粒径分布在500～1000 μm之间,完整系数（integrity coefficient,IC）小于20,颗粒污泥EPS总量、蛋白含量、多糖含量分别为85.59、63.67和21.92 mg·（g VSS）^-1,此时颗粒污泥絮凝性良好,机械强度高。当HRT减少为6 h时,IC为30.03,蛋白与多糖的比值增大到6.86,但多糖含量仅为18.11 mg·（g VSS）^-1;而当T降低为20℃时,颗粒污泥粒径分布在250～750 μm之间,IC增大到32.11,Ca²⁺、Mg²⁺、Mn²⁺含量减少 为20.78、4.79和0.94 mg·L^-1,颗粒污泥EPS显著降低,其总量、蛋白含量、多糖含量仅为69.04、58.87和10.17 mg·（g VSS）^-1,在此运行条件下,出现了颗粒污泥的解体、流失,同时出水水质变差。     

胞外聚合物对膜污染的影响及对策

膜生物反应器(MBR)中膜污染问题直接影响了工艺的稳定性和经济性。通过分析膜污染机理发现,胞外聚合物(EPS)在膜污染中起重要作用。从EPS的性质、组成及影响因素等方面进行了归纳总结,并对膜污染控制提出了相应的对策。     

Effect of magnetic field on surface properties of Bacillus cereus CrA and its Extracellular Polymeric Substances (EPS)

The effect of magnetic field on surface properties of Bacillus cereus CrA and its EPS was studied by zeta potential and FT-IR spectrum. The weak magnetic fields of 6.0–10.0 mT promoted the predominant strain B. cereus CrA to form EPS. The weak magnetic field of 2.4–10.0 mT could improve the bio-adsorption efficiency of anions by decreasing the negative charge of the intact strain, while the magnetic field of 17.4 mT could increase the negative charge of the strain’s EPS by 20% and be responsible for the bio-adsorption of cations. Magnetic field could affect the band intensity and the slight shifting of peaks and the fluctuation of the bacterial absorption capability depended on the magnetic density. The magnetic field energy could affect the hydroxyl group and carboxyl group by forming hydrogen bonding, which were responsible for the adsorption ability of the strain and its EPS.     

无排泥条件下膜生物反应器污泥混合液可滤性分析

膜生物反应器（MBR）在无排泥条件下运行100 d,定期对溶解性微生物产物（SMP）、SMP分子量分布、胞外聚合物（EPS）中的蛋白质和多糖进行监测,应用修正的污染指数（MFI）考察污泥混合液可滤性的变化。实验表明：长时间无排泥运行模式下将导致污泥混合液可滤性的恶化;污泥混合液上清液中分子量（M_w）大于10 kDa 的SMP浓度对污泥混合液的可滤性产生强烈的负面影响;污泥浓度（MLSS）与混合液可滤性之间关系复杂,MLSS对污泥可滤性的影响存在一个临界值;EPS中的蛋白质60 d后发现可被微生物迅速降解,多糖类物质对污泥混合液可滤性有较强的负面影响。     

典型金属赋存形态对MBR膜污染影响研究进展

膜污染是膜生物反应器(MBR)进一步发展的最大障碍,污泥混合液性状是影响膜污染的关键因素.本文综述了金属离子在MBR混合液中的赋存形态,并分别对金属离子3种不同赋存形态对MBR污泥混合液及膜分离过程的影响进行了探讨.文中揭示了同价离子或异价离子共存对混合液及膜分离的影响,对共存体系中金属离子间的协同或拮抗行为也进行了讨论,并对利用高价金属离子减缓膜污染领域未来的研究趋势进行了展望.