Influence of organic and inorganic flocculants on physical-chemical properties of biomass and membrane-fouling rate

The effects of addition of six types of flocculants (aluminium sulfate, ferric chloride, polyaluminium chloride, polymeric ferric sulfate, Chitosan, polyacrylamide) on mitigation of membrane fouling in membrane bioreactors (MBR) were investigated respectively. The biomasses in various MBRs were characterized by morphological properties (mean floc size (d_p), fractal dimension (df)), physical parameters (surface charge, relative hydrophobicity (RH), dynamic viscosity) and the biochemical components of the mixed liquor (extracellular polymeric substances (EPS), soluble microbial products (SMP)). Statistical methods such as normalization, nondimensionalization and multiple linear regressions were used to identify the dominant membrane-fouling contributors and to simulate membrane-fouling rates. The results demonstrated that addition of flocculants had significant impact on sustainable filtration time and the key factors affecting membrane fouling varied in different flocculants added MBRs. For the organic flocculants added MBRs, membrane-fouling alleviation was mainly due to the decrease in SMP and df as well as the increase in d_p. For the inorganic flocculants added MBRs, the lower fouling rate could be mainly attributed to the decrease in SMP and surface charge as well as the increase in RH. For each type of flocculants, the empirical equations of sustainable filtration time (Γ₄₅) were simulated to predict membrane-fouling rates in different MBRs.     

Modeling cake buildup under TMP-step filtration in a membrane bioreactor: cake compressibility is significant

Fouling is inevitable in membrane bioreactors (MBRs) due to the complex nature of activated sludge, which contains a broad variety of potential foulants. Filter cakes that build up from sludge particles are traditionally highly compressible due to both the deformation of the individual sludge particles and the rearrangement of these particles in the cake. However, this phenomenon has been little examined in studies of fouling mechanisms in MBR systems. This study examines the properties of the cake layer, modeling the cake buildup and specific cake resistance (α), including compressibility, in terms of pressure-dependent α.The changes in fouling resistance during transmembrane pressure (TMP)-step filtration in an MBR setup were simulated using an empirical pressure dependence of the specific cake resistance and a simple mass balance model. The total change in fouling resistance in each TMP step could be divided into an initial rapid change in specific cake resistance due to filter cake compression followed by simple cake buildup. By including cake compression in this simple model, the model fitted the data with high precision. We demonstrated that compressibility should be considered when describing cake fouling in MBRs.     

SMBR在次临界通量下的运行特性

利用“通量阶式递增法”测定了两种膜组件的临界通量,在此基础上考察了次临界通量下的运行特性。试验发现,次临界通量操作下的膜污染过程具有明显的两阶段特征,与第一阶段跨膜压差(TMP)平缓直线上升相对应的膜污染机制主要是膜孔堵塞和凝胶层污染,与第二阶段TMP剧烈直线上升相对应的膜污染机制则是颗粒沉积层污染;先清水冲洗再化学清洗的方式能有效恢复膜的过滤能力,其中清水冲洗能有效去除颗粒沉积层污染,而化学清洗则能有效去除膜孔堵塞和凝胶层污染。     

A comparison of membrane fouling under constant and variable organic loadings in submerge membrane bioreactors

The aim of this study is to compare the effect of constant and variable influent organic loadings on membrane fouling in submerged membrane bioreactors (sMBRs). Two identical lab-scale sMBRs were operated for 162 days at an SRT of 30 days, whereas the influent organic loading was kept constant in one MBR, and varied in another. The microbial characteristics of sludge in terms of MLSS, bound EPS, EPS in the supernatant and particle size distribution were investigated in order to evaluate their respective effect on membrane fouling. During the start-up period, membrane fouling in the MBR fed with variable loadings was more serious than that in the MBR with the constant loading. However, at the stable state, the fouling tendency was clearly reversed with less membrane fouling for variable feed strength. It was shown that the contents of polysaccharides in the supernatant and particle size of the bioflocs were responsible for the observed differences in the fouling tendencies of the two MBRs.     

淹没式膜生物反应器膜面泥饼层的微结构特征

为研究膜生物反应器运行中的膜污染问题,利用扫描电镜（SEM）和原子力显微镜（AFM）分析了膜在污泥混合液及上清液中所形成泥饼层的微结构特征.结果表明,两种类型泥饼层的微结构存在显著差异且与膜污染速度相关,膜面泥饼层微结构的表征技术可以作为膜污染评价的重要手段.在污泥混合液中形成的膜面泥饼层厚度大但表面粗糙度高,存在一定的孔隙性;而在上清液中形成的膜面泥饼层虽然更薄但相对致密,其渗透性差,导致膜污染速度加快.活性污泥混合液及上清液中,膜截留的溶解性微生物代谢产物（SMP）的分子质量分布趋势相近,膜面泥饼层微结构的差异主要与被过滤液中微生物絮体的体积和含量有关.     

Comparison of fouling characteristics in different pore-sized submerged ceramic membrane bioreactors

Membrane fouling, the key disadvantage that inevitably occurs continuously in the membrane bioreactor (MBR), baffles the wide-scale application of MBR. Ceramic membrane, which possesses high chemical and thermal resistance, has seldom been used in MBR to treat municipal wastewater. Four ceramic membranes with the same materials but different pore sizes, ranging from 80 to 300 nm, were studied in parallel using four lab-scale submerged MBRs (i.e., one type of ceramic membrane in one MBR). Total COD and ammonia nitrogen removal efficiencies were observed to be consistently above 94.5 and 98%, respectively, in all submerged ceramic membrane bioreactors. The experimental results showed that fouling was mainly affected by membrane’s microstructure, surface roughness and pore sizes. Ceramic membrane with the roughest surface and biggest pore size (300 nm) had the highest fouling potential with respect to the TMP profile. The 80 nm membrane with a smoother surface and relatively uniform smaller pore openings experienced least membrane fouling with respect to TMP increase. The effects of the molecular weight distribution, particle size distribution and other biomass characteristics such as extracellular polymeric substances, zeta potential and capillary suction time, were also investigated in this study. Results showed that no significant differences of these attributes were observed. These observations indicate that the membrane surface properties are the dominant factors leading to different fouling potential in this study.     

Seasonal variation in membrane fouling in membrane bioreactors (MBRs) treating municipal wastewater

We investigated seasonal variation in membrane fouling in membrane bioreactors (MBRs) treating municipal wastewater regarding the difference between physically reversible and irreversible fouling. Two separate MBRs with different solid retention times (SRTs) operated in parallel for about 200 days including high- and low-temperature periods to evaluate the effect of operating conditions on seasonal variation of membrane fouling. Seasonal variations of both types of membrane fouling (i.e., physically reversible and irreversible fouling) were observed for the MBR with short SRT (13 days). However, in the MBR with long SRT (50 days), there were no significant seasonal variations in both types of membrane fouling. In the MBR with short SRT, the trends in the seasonal variation in the development rates of physically reversible and irreversible fouling were different. Physically reversible fouling was more significant in the low-temperature period, while physically irreversible fouling developed more rapidly in the high-temperature period. The development rates of physically reversible fouling can be related to the concentration of dissolved organic matter in the mixed liquor suspension of MBRs; whereas those of physically irreversible fouling could not be explained by the concentration of dissolved organic matter. The characteristics of dissolved organic matter differed depending on the temperature period, and the trends of dissolved organic matter variation in mixed liquor were similar with those of foulants that caused physically irreversible fouling. The results obtained in this study indicated that seasonal variation in physically reversible and irreversible fouling is related to changes in quantity and quality of organic matter, respectively.     

The influence of aeration intensity on predation and EPS production in membrane bioreactors

Shear, in the form of vigorous aeration, is used to control fouling in membrane bioreactor (MBR) systems. However, shear also influences the physicochemical and biological properties of MBR biomass. The current study examines the relationship between the aeration intensity and extracellular polymeric substance (EPS) production in MBRs. Two identical submerged MBRs were operated in parallel but the aeration rate was three times greater in one of the MBRs. The concentrations of floc-associated and soluble EPS were monitored for the duration of the experiment. Microscopic images and floc-size measurements were also collected regularly. The membrane fouling potential of the biomass was quantified using the flux-step method. Increased aeration did not have a direct effect on soluble or floc-associated EPS production in the microfiltration MBRs. However, aeration intensity had a significant effect on predatory organisms. Large aquatic earthworms, Aeolosoma hemprichi, proliferated under lower shear conditions but were never observed in the high shear reactor. Predation by A. hemprichi resulted in increased floc-associated and soluble EPS production. Thus, the mixing conditions in the low shear MBR indirectly resulted in increased soluble EPS concentrations and higher fouling potential. This research suggests that predation can have a significant impact on the production rates of floc-associated and soluble EPS - key parameters driving membrane fouling in MBRs.     

Recent advances in membrane bioreactors (MBRs): Membrane fouling and membrane material

Membrane bioreactors (MBRs) have been actively employed for municipal and industrial wastewater treatments. So far, membrane fouling and the high cost of membranes are main obstacles for wider application of MBRs. Over the past few years, considerable investigations have been performed to understand MBR fouling in detail and to develop high-flux or low-cost membranes. This review attempted to address the recent and current developments in MBRs on the basis of reported literature in order to provide more detailed information about MBRs. In this paper, the fouling behaviour, fouling factors and fouling control strategies were discussed. Recent developments in membrane materials including low-cost filters, membrane modification and dynamic membranes were also reviewed. Lastly, the future trends in membrane fouling research and membrane material development in the coming years were addressed.     

Further examination of polysaccharides causing membrane fouling in membrane bioreactors (MBRs): Application of lectin affinity chromatography and MALDI-TOF/MS

Membrane fouling remains a major obstacle for wider application of membrane bioreactors (MBRs) to wastewater treatment. Polysaccharides in mixed liquor suspensions in the reactors are thought to be mainly responsible for the evolution of membrane fouling in MBRs. However, details of polysaccharides causing membrane fouling in MBRs are still unknown. In this study, polysaccharides in a mixed liquor suspension of a pilot-scale MBR treating municipal wastewater were fractionated by using lectins, special proteins that bind to specific polysaccharides depending on their properties. Fouling potentials of the fractionated polysaccharides were assessed by bench-scale dead-end filtration tests. It was clearly shown that the degrees of fouling caused by fractionated polysaccharides were significantly different. The amounts of polysaccharides in each fraction could not explain the variations in the fouling, indicating the presence of polysaccharides with high specific fouling potentials. To investigate structures and origins of the polysaccharides with high fouling potentials, matrix-assisted laser desorption/ionization (MALDI)-time of flight (TOF)/mass spectrometry (MS) analysis was applied to the fractionated polysaccharides after partial hydrolysis. Several mass peaks obtained could be assigned to fragments of structures of polysaccharides (i.e., oligosaccharides) reported in a database/literature. This is the first report showing the plausible structures of polysaccharides in MBRs based on MS. A deeper understanding and effective control of membrane fouling in MBRs could be achieved with information obtained by the approach used in this study.     

Changes in characteristics of soluble microbial products in membrane bioreactors associated with different solid retention times: Relation to membrane fouling

A membrane bioreactor (MBR) is a promising wastewater treatment technology, but there is a need for efficient control of membrane fouling, which increases operational and maintenance costs. Soluble microbial products (SMP) have been reported to act as major foulants in the operation of MBRs used for wastewater treatment. In this study, SMP in MBRs operated with different sludge retention times (SRTs) were investigated by means of various analytical techniques and their relations to the evolution of membrane fouling were considered. Bench-scale filtration experiments were carried out in a laboratory with synthetic wastewater to eliminate fluctuations that would occur with the use of real wastewater and that would lead to fluctuations in compositions of SMP. Three identical submerged MBRs were operated for about 50 days under the same conditions except for SRT (17, 51 and 102 days). Accumulation of SMP in the MBRs estimated by conventional analytical methods (i.e., the phenol-sulfuric acid method and the Lowry method) was significant in the cases of short SRTs. However, the degrees of membrane fouling in the MBRs were not directly related to the concentrations of SMP in the reactors estimated by the conventional analytical methods. Non-conventional analytical methods such as excitation-emission matrix (EEM) fluorescence spectroscopy revealed that characteristics of SMP in the three reactors considerably differed depending on SRT. Foulants were extracted from the fouled membranes at the end of the operation and were compared with SMP in each MBR. It was clearly shown that characteristics of the foulants were different depending on SRT, and similarities between SMP and the extracted foulants were recognized in each MBR on the basis of results of EEM measurements. However, such similarities were not found on the basis of results obtained by using the conventional methods for analysis of SMP. The results of this study suggest that the use of conventional methods for analysis of SMP is not appropriate for investigation of membrane fouling in MBRs.     

Alginate block fractions and their effects on membrane fouling

Alginate has been commonly used as a model foulant in studies of membrane organic fouling. As a complex polymer, alginate is composed of two different monomers, namely M ((1 → 4) linked β-D-mannopyranuronic acid) and G ((1 → 4) linked α-L-gulopyranuronic acid) which are randomly arranged into MG-, MM- and GG-blocks. So far, little information is available about fouling propensity of each block in microfiltration. In this study, microfiltration experiments were conducted respectively with MG-, MM- and GG-blocks separated from alginate under defined conditions. Results showed the severest fouling in the filtration of MG-block, and the least flux decline in the filtration of MM-block. The initial pore blocking was found to be responsible for the fouling observed in MG-block filtration, while the cake layer formed on membrane surface during the MM-block filtration could serve as a pre-filter that prevented membrane from further pore blocking. In order to look into fouling mechanisms, the effects of transparent exopolymeric particles (TEP) on membrane fouling were also studied. TEP were found to form through aggregation or cross-link of alginate blocks. As TEP were bigger than original alginate blocks, they could facilitate the formation of cake layer on membrane surface. It was observed that more TEP were produced from MM-blocks than from MG-blocks in solutions. This in turn explained why cake resistance was dominant in the filtration of MM-blocks as compared to MG-blocks. The analysis by the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory further revealed that MM-blocks had lowest cohesive interaction energy among all three alginate blocks, which favoured aggregation of MM-blocks, and ultimately leading to the formation of more TEP. This study provided insights into the roles of different alginate blocks in development of membrane fouling, and suggested that the membrane fouling would be related to molecular structure of alginate.     

复合混凝剂对MBR工艺中污泥性能的影响研究

为了缓解MBR工艺的膜污染,向MBR工艺中投加复合混凝剂,考察了复合混凝剂对污泥性能的影响.结果表明,投加复合混凝剂可降低污泥比阻,明显增大污泥絮体的平均粒径、降低其分形维数,且污泥的可溶性微生物产物(SMP)含量也明显下降,这样有利于增强污泥的沉降性能、降低膜滤饼层的污染、增强膜表面的透水性.     

The effect of calcium on the membrane biofouling in the membrane bioreactor (MBR)

A continuous bench-scale submerged membrane bioreactor (BSMBR) was operated in two modes; low (LC) and optimum calcium (OC) concentrations, to investigate the effect of calcium on membrane biofouling. Both the cake layer and pore blocking resistances were reduced in the operation under OC conditions. In order to find the causes of fouling, molecular weight fractionations of the extracellular polymeric substances (EPS) and soluble microbial products (SMP) were conducted, and the relative hydrophobicity of the sludge, EPS and SMP were also measured. The reduction in the cake layer resistance was assumed to be due to the decrease of filamentous bacteria, and better flocculation caused by the calcium bridges and the increased hydrophobicity of EPS in the operation under OC conditions. It was suggested that the pore blocking resistance was reduced due to the low SMP rejection of the membrane and the fewer hydrophobic reactions of the SMP in the operation under OC conditions. Fourier-transform infrared analysis of the fouled membrane surface supported that proteins and carbohydrates were the main foulants during the BSMBR operation.     

投加颗粒活性炭对膜生物反应器过滤特性的影响

膜污染是制约膜技术应用的重要因素。向膜生物反应器(MBR)中投加颗粒活性炭(GAC),通过分析MBR系统中膜通量、过滤阻力等的变化,考察投加GAC对MBR系统过滤特性的影响。结果表明,运行30d后,未投加和投加GAC的MBR系统的膜通量分别降至初始的31.3%和91.7%;未投加GAC系统的总过滤阻力和极化阻力分别为投加GAC系统的5.8和19.4倍,其污泥的多糖和蛋白质含量为投加GAC系统的近2倍,而其胶体物质和溶解性物质浓度分别为投加GAC系统的3.2和2.2倍。由此表明,投加GAC可大大减缓膜污染,延长膜的过滤周期。     

Effect of temperature shocks on membrane fouling in membrane bioreactors

Temperature is known to influence the biological performance of conventional activated sludge systems. In membrane bioreactors (MBRs), temperature not only affects the bioconversion process but is also shown to have an effect on the membrane performance. Four phenomena are generally reported to explain the higher resistance for membrane filtration found at lower temperatures: (1) increased mixed liquor viscosity, reducing the shear stress generated by coarse bubbles, (2) intensified deflocculation, reducing biomass floc size and releasing EPS into the mixed liquor, (3) lower backtransport velocity and (4) reduced biodegradation of COD. Although the higher resistance at low temperatures has been reported in several papers, the relation with supernatant composition has not been investigated before. In this paper, the composition of the soluble fraction of the mixed liquor is related to membrane performance after exposing the sludge to temperature shocks. Flux step experiments were performed in an experimental system at 7, 15, and 25° Celsius with sludge that was continuously recirculated from a pilot-scale MBR. After correcting the permeate viscosity for temperature, higher membrane fouling rates were obtained for the lower temperature in combination with low fouling reversibility. The soluble fraction of the MBR mixed liquor was analysed for polysaccharides, proteins and submicron particle size distribution. At low temperature, a high polysaccharide concentration was found in the experimental system as compared to the MBR pilot. Upon decreasing the temperature of the mixed liquor, a shift was found in particle size towards smaller particles. These results show that the release of polysaccharides and/or submicron particles from sludge flocs could explain the increased membrane fouling at low temperatures.     

Novel magnetically induced membrane vibration (MMV) for fouling control in membrane bioreactors

Conventional submerged membrane bioreactors (MBRs) rely on the coarse bubbles aeration to generate shear at the liquid-membrane interface to limit membrane fouling. Unfortunately, it is a very energy consuming method, still often resulting in a rapid decrease of membrane permeability and consequently in higher expenses. In this paper, the feasibility of a novel magnetically induced membrane vibration (MMV) system was studied in a lab-scale MBR treating synthetic wastewater. The effects on membrane fouling of applied electrical power of different operation strategies, of membrane flux and of the presence of multiple membranes on one vibrating engine on membrane fouling were investigated. The filtration performance was evaluated by determining the filtration resistance profiles and critical flux. The results showed clear advantages of the vibrating system over conventional MBR processes by ensuring higher fluxes at lower fouling rates. Intermittent vibration was found a promising strategy for both efficient fouling control and significant energy saving. The optimised MMV system is presumed to lead to significant energy and cost reduction in up-scaled MBR operations.     

Correlation of EPS content in activated sludge at different sludge retention times with membrane fouling phenomena

In this study, activated sludge characteristics were studied with regard to membrane fouling in membrane bioreactors (MBRs) for two pilot plants and one full-scale plant treating municipal wastewater. For the full-scale MBR, concentrations of extracellular polymeric substances (EPS) bound to sludge flocs were shown to have seasonal variations from as low as 17mgg(-1) dry matter (DM) in summer up to 51mg(gDM)(-1) in winter, which correlated with an increased occurrence of filamentous bacteria in the colder season. Therefore, it was investigated at pilot-scale MBRs with different sludge retention times (SRTs) whether different EPS contents and corresponding sludge properties influence membrane fouling. Activated sludge from the pilot MBR with low SRT (23d) was found to have worse filterability, settleability and dewaterability. Photometric analysis of EPS extracts as well as LC-OCD measurements showed that it contained significantly higher concentrations of floc-bound EPS than sludge at higher SRT (40d) The formation of fouling layers on the membranes, characterised by SEM-EDX as well as photometric analysis of EPS extracts, was more distinct at lower SRT where concentrations of deposited EPS were 40-fold higher for proteins and 5-fold higher for carbohydrates compared with the membrane at higher SRT. Floc-bound EPS and metals were suggested to play a role in the fouling process at the full-scale MBR and this was confirmed by the pilot-scale study. However, despite the different sludge properties, the permeability of membranes was found to be similar.     

Sub-critical fouling in a membrane bioreactor for municipal wastewater treatment: experimental investigation and mathematical modelling

Fouling is a major limitation for the application of membrane bioreactors (MBRs) in municipal wastewater treatment; the critical flux concept represents a valid tool for process optimisation in planning fouling control strategies. The paper presents the results obtained on a large pilot MBR equipped with a plate-and-frame ultrafiltration membrane. The experimental assessment of flux criticality was carried out by flux-stepping tests showing the positive impact of liquid temperature on the value of the critical threshold. The reliability of short-term tests was then verified over a long period by determining the time of sustainability, t(sust), of six different sub-critical fluxes ranging between 17 and 30Lm(-2)h(-1). An exponential fitting was observed in terms of fouling rate both before and after t(sust), though fouling after t(sust) is likely to be ascribed not only to cake formation. Finally, a new mathematical formulation was proposed according to the local flux approach to model the sub-critical TMP transients. The model involves both bound and free forms of EPS and, once experimentally calibrated, it provided a fair prediction of the TMP jump.     

Influence of shear on the production of extracellular polymeric substances in membrane bioreactors

Shear is used to control fouling in membrane bioreactor (MBR) systems. However, shear also influences the physicochemical and biological properties of MBR biomass. The current study examines the relationship between the level of shear and extracellular polymeric substance (EPS) production in MBRs. Two identical MBRs were operated in parallel where the biomass in one reactor was exposed to seven times greater shear forces. The concentrations of floc-associated and soluble EPS were monitored for the duration of the experiment. The stickiness of extracted floc-associated EPS from each reactor was also characterized using atomic force microscopy. A mathematical model of floc-associated and soluble EPS production was applied to quantitatively describe changes in EPS production with shear. Biomass grown in a high shear environment has lower floc-associated EPS production compared to biomass grown in a lower shear environment. This decrease in floc-associated EPS production also corresponds to a decrease in soluble EPS production, which can be explained by both the lower concentration of floc-associated EPS and the production of stickier floc-associated EPS that is more erosion resistant in the high shear reactor. This research suggests that mechanical stresses can have a significant impact on the production rates of floc-associated and soluble EPS—key parameters governing membrane fouling in MBRs.