The role of fouling mechanisms in a membrane bioreactor.

The present study has aimed to quantify the role of pore blocking and cake layer in a laboratory scale hollow fibre membrane module in submerged configuration, The membrane reactor (MBR) was fed with raw wastewater, only screened with a 2-mm sieve, collected from the Palermo WWTP. The MBR was characterised by an operating volume of 190 L and equipped with an aeration system located on the bottom of the reactor. The MBR operated for 65 days. The permeate was extracted by imposing a constant flux through the membrane (21 Lh(-1) m(-2)). The results confirm the importance of pore blocking control during start-up. In particular, it provides a rapid irreversible fouling that takes place at the beginning of the filtration process, before the deposition mechanism. Therefore, low suspended solids concentration in the initial phase causes a fast irreversible fouling. This circumstance creates the need for more frequent chemical cleaning after start-up without inoculum. Finally, the results underline that the cake has a mainly reversible feature.     

Characteristics of membrane fouling in submerged membrane bioreactor under sub-critical flux operation.

Recently, the membrane bioreactor (MBR) process has become one of the novel technologies to enhance the performance of biological treatment of wastewater. Membrane bioreactor process uses the membrane unit to replace a sediment tank, and this can greatly enhance treatment performance. However, membrane fouling in MBR restricts its widespread application because it leads to permeate flux decline, making more frequent membrane cleaning and replacement necessary, which then increases operating and maintenance costs. This study investigated the sludge characteristics in membrane fouling under sub-critical flux operation and also assessed the effect of shear stress on membrane fouling. Membrane fouling was slow under sub-critical flux operation. However, as filamentous microbes became dominant in the reactor, membrane fouling increased dramatically due to the increased viscosity and polysaccharides. A close link was found between membrane fouling and the amount of polysaccharides in soluble EPS. The predominant resistance was the cake resistance which could be minimized by increasing the shear stress. However, the resistance of colloids and solutes was not apparently reduced by increasing shear stress. Therefore, smaller particles such as macromolecules (e.g. polysaccharides) may play an important role in membrane fouling under sub-critical flux operation.     

Optimising the operation of a MBR pilot plant by quantitative analysis of the membrane fouling mechanism.

In order to optimize some operational conditions of MBR systems, a MBR pilot plant equipped with a submerged hollow fibre membrane module was employed in this study. The pilot MBR was fed with real municipal wastewater and the filtration flux, backwashing interval, aeration frequency and temperature were varied. A filtration flux below 25 I/m2h is generally recommended, at below this flux, the MBR operated at sub-critical flux conditions, the filter cake was minimized and membrane fouling was mainly attributed to the membrane pore blocking. Moreover, the membrane fouling, at below 25 I/m2h, was more reversible to backwashing; above this value, backwashing became less efficient to clean the membrane. Less frequent backwashing (e.g. 600 s filtration/45 s backwashing) decreased the amount of fouling irreversible to backwashing and its performance was superior to that of frequent backwashing (e.g. 200 s filtration/15 s backwashing). The MBR suffered more fouling at low temperature conditions (e.g. at 13-14 degrees C) than at high temperature conditions (e.g. at 17-18 degrees C). A conceptual model was built up and successfully interpreted this temperature effect.     

Nitrogen removal and sludge reduction in a symbiotic activated sludge system between anaerobic archaea and bacteria.

The possible symbiosis between bacteria and anaerobic archaea was investigated in intermittent aeration (I/A) systems. Archaea solution added to I/A reactor might play an important role in biological activities as well as in improvement of mineralization of organic matter. I/A reactor with archaea solution (I/A-arch) could increase both nitrification and denitrification rate and also reduce the sludge yield remarkably. These results indicate the possibility of the symbiotic activated sludge system with anaerobic archaea by controlling the DO level in the aeration tank. In this study, DO was controlled by intermittent aeration schemes and a successful symbiotic activated sludge system was achieved to reach the following conclusions. 1) SOUR of I/A-arch system was 2.9 mg-O2/g-VSS x min. SOUR and nitrification rate of the sludge from I/A-arch was higher than those from the I/A and A/S reactors. 2) Removal efficiencies of organic matter (TCOD(Cr)) in I/A-arch, I/A and conventional activated sludge (A/S) reactors were 93, 90 and 87%, respectively. 3) Nitrification occurred successfully in each reactor, while denitrification rate was much higher in the I/A-arch reactor. Efficiencies of TN removal in A/I-arch, I/A and A/S reactors were 75, 63 and 33%, respectively. 4) Observed yield coefficients of I/A-arch, I/A and A/S reactors were 0.28, 0.41 and 0.37 g-VSS/g-COD.     

Fouling with aerobic granule membrane bioreactor

Aerobic granulation (AG) and membrane bioreactor (MBR) are two promising, novel environmental biotechnological processes that draw interest of researchers working in the area of biological wastewater treatment. Membrane fouling in the combined aerobic granular membrane bioreactor (AGMBR) process and the conventional MBR process was investigated in this work. The irreversible fouling on hollow-fibre membranes in both reactors were observed with the multiple staining and confocal laser scanning microscope technique. Following physical and chemical washing, the external fouling layers were mostly removed. However, the biofilms built up in the interior surface of membrane remained and contributed to the irreversible fouling resistance. AGMBR retained most cells with granules, thereby reducing their penetration through membrane and thus the chance to form internal fouling layer. The internal biofilm layer was principally composed of live cells embedded in a matrix of proteins and polysaccharides, with that on AGMBR denser and thicker than that on MBR. Prevention of development of internal biofilm is essential to reduce irreversible fouling of AGMBR and MBR membranes.     

Fouling control in submerged membrane bioreactors.

The impact of fouling on the operation of submerged membrane bioreactors (MBRs) is discussed. The discussion proceeds through a review of the impacts of fouling and fouling speciation, with correlations of filterability against candidate foulants being provided. There follows a brief comparison of hydraulic performance of the submerged and sidestream configurations: data is presented to demonstrate the efficacy of coarse bubble aeration in the submerged MBR. The critical flux concept is then discussed and its applicability to MBRs deliberated in view of recent publications demonstrating fouling under sub-critical flux conditions. Finally, the two most commercially important MBR products are briefly reviewed with specific reference to design and operation for fouling amelioration.     

Effects of solid retention time on the performance of submerged anoxic/oxic membrane bioreactor.

In this study, four similar bench-scale submerged Anoxic/Oxic Membrane Bioreactors (MBR) were used simultaneously to investigate the effects of solids retention time (SRT) on organic and nitrogen removal in MBR for treating domestic wastewater. COD removal efficiencies in all reactors were consistently above 94% under steady state conditions. Complete conversion of NH(4+)-N to NO(3-)-N was readily achieved over a feed NH(4+)-N concentration range of 30 to 50 mg/L. It was also observed that SRT did not significantly affect the nitrification in the MBR systems investigated. The average denitrification efficiencies for the 3, 5, 10 and 20 days SRT operations were 43.9, 32.6, 47.5 and 66.5%, respectively. In general, the average effluent nitrogen concentrations, which were mainly nitrate, were about 22.2, 27.6, 21.7 and 13.9 mg/L for the 3, 5, 10 and 20 days SRT systems, respectively. The rate of membrane fouling at 3 days SRT operation was more rapid than that observed at 5 days SRT. No fouling was noted in the 10 days and 20 days SRT systems during the entire period of study.     

磁性悬浮颗粒减缓MBR中膜污染的研究

向MBR中投加磁性悬浮颗粒,通过改善污泥混合液特性来控制膜污染,试验结果表明:最佳吸附剂为四氧化三铁/硅藻土复合颗粒,吸附剂的最佳投加量为0.1 g/L;投加复合颗粒的MBR出水的COD、氨氮和硝态氮均优于参比MBR,说明复合颗粒能改善膜出水水质;投加颗粒的MBR膜表面的EPS各组分均低于参比MBR,说明四氧化三铁/硅藻土复合颗粒能有效的吸附污泥悬浮液中的EPS,加强MBR的通量稳定性,从而延缓膜污染,减少膜清洗次数,降低运行成本。     

Removal of inhibitory phenolic compounds by biological activated carbon coupled membrane bioreactor.

Phenolic compounds cause problems for conventional treatments due to their toxic and inhibitory properties. This work investigated the treatability of phenolic compounds by using two membrane-bioreactor systems, namely: activated sludge coupled with MBR (AS-MBR) and biological granular activated carbon coupled with MBR (BAC-MBR). Initially, the system was fed with phenol (500 mg/L) followed by adding 2,4-dichlorophenol (2,4-DCP). Phenol, 2,4-DCP, TOC and COD removal were higher than 98.99% when the organic load ranged between 1.80 and 5.76 kg/m3.d COD. In addition to MBR system development, removal mechanisms were also investigated. Relatively low values of phenol adsorption of GAC and biomass, and high maximum substrate removal rates obtained from a biokinetic experiment, proved that the removals were mainly due to biodegradation. Analysis of sludge indicated a significant difference in the sludge characteristics of the two reactors. The high EPS content in BAC-MBR led to higher viscosity and poor sludge settling properties. The relationship between sludge properties and EPS components revealed that settleability had no direct correlation with EPS, though it was better correlated to protein/carbohydrate ratios.     

The influence of settling time on the formation of aerobic granules.

Aerobic granular sludge, without the addition of carrier material, has only been reported in one suspended growth system, the Sequencing Batch Reactor (SBR) operated with short fill and settling periods. Recent studies have demonstrated that extracellular polysaccharides increased with the formation of aerobic granules, and that the shear force may stimulate production of these polysaccharides. In the study described herein, two SBRs were operated with the same shear force (air flow rate 275 L h(-1)) and two different settling times (2 and 10 min). Only the reactor with 2 min settling formed completely granular sludge, although granules were present in both reactors. Community analysis using 16S rRNA PCR products and DGGE showed that the communities diverged quickly after reactor start-up. For samples taken at steady-state, the granular population was more stable and less diverse than the flocculent reactor. EPS extraction of samples using cation exchange resin yielded similar values for aerobic granular sludge and previously reported anaerobic granules. While differences in the protein and TOC content between the flocculent and granular reactors increased appreciably as the sludge became more granular, the protein to polysaccharide ratio was relatively constant. The experiment confirmed previous theories that short settling times in SBRs select for granular sludge. The settling time results in granular sludge having a higher EPS protein content and a less diverse but more stable population.     

Quantification of biofilms in a sub-surface flow wetland and their role in nutrient removal.

Subsurface flow wetlands contain gravel or sand substrates through which the wastewater flows vertically or horizontally. The aims of this study were, firstly, to quantify biofilm development associated with different size gravel in sections of a subsurface flow wetland with and without plants, and secondly, to conduct laboratory experiments to examine the role of biofilms in nutrient removal. Techniques to quantify biofilm included: bacterial cell counts, EPS and total protein extraction. Based on comparative gravel sample volume, only EPS was greater on the smaller 5 mm gravel particles. There was no significant difference between biofilm growth in sections with and without plants. Two vertical flow laboratory-scale reactors, one containing fresh wetland gravel, the other containing autoclaved gravel, were constructed to determine nutrient transformations. The autoclaved gravel in the "sterile" reactor rapidly became colonised with biofilm. Both reactors were dosed with two types of influent. Initially the influent contained 7.25 mg/L NO3-N and 0.3 mg/L NH4-N; the biofilm reactor removed most of the ammonium and nitrite but nitrate concentrations were only reduced by 20%. In the "sterile" reactor there was negligible removal of ammonium and nitrite indicating little nitrification, however nitrate was reduced by 72%, possibly due to assimilatory nitrate reduction associated with new biofilm development. When the influent contained 3 mg/L NO3-N and 16 mg/L NH4-N almost 100% removal and transformation of NH4-N occurred in both reactors providing an effluent high in NO3-N. Organic P was reduced but inorganic soluble P increased possibly due to mineralisation.     

Identification of the effect of extracellular polymeric substances on bacterial adhesion to the membrane surface in a membrane bioreactor using Pseudomonas fluorescens.

In a membrane bioreactor (MBR) process containing a variety of bacteria, the bacterial adhesion to the membrane surface, prior to cake formation, causes an increased filtration resistance. In this study, Pseudomonas fluorescens, commonly found in the municipal wastewater treatment process with activated sludge, was used to show the effects of extracellular polymeric substances (EPS) on bacterial adhesion to the membrane surface in the MBR. Of the various roles of EPS in promoting membrane fouling, the adhesion of bacteria to the membrane surface was calculated using the specific cake resistance (alpha, m/kg). Although the amount of EPS binding with bacteria was increased by the addition of Ca2+, there was no significant effect on the bacterial growth. The results of the particle size distribution showed that the addition of Ca2+ increased flocculation, allowing the formation of a complex with the bacteria and EPS. In order to identify the effects of the addition of Ca2+ on the hydrophobicity, the contact angle was also measured. The result showed that the addition of Ca2+ showed no significant differences in the hydrophobicity, even though there was an increase in flocculation. With the bacteria containing a higher EPS concentration, a higher specific cake resistance was observed. From the results of the adhesion experiment, which was conducted with various EPS levels, displayed as the COD and TOC concentration, an increased EPS concentration was shown to promote bacterial adhesion to the membrane surface.     

Comparison of membrane biofouling in nitrification and denitrification for the membrane bioreactor (MBR).

Batch filtration tests were conducted to compare the characteristics of membrane biofouling with regard to nitrification and denitrification. A Modified Fouling Index (MFI) was obtained using a stirred cell tester. The denitrification assays showed higher membrane fouling rates than the nitrification assays. The fouling became worse, not only due to pore blocking resistance, but also from cake layer resistance after denitrification. The Extracellular Polymeric Substances (EPS) concentration and relative hydrophobicity were decreased after denitrification, resulting in floc deterioration. The floc deterioration was assumed to have increased the cake layer resistance in the filtration test. The protein Soluble Microbial Products (SMP) concentration, portion of high molecular weight in carbohydrate SMP and relative hydrophobicity were increased after denitrification, which was assumed to cause membrane pore blocking. The changes in the EPS and SMP characteristics were the main fouling parameters in denitrification.     

Occurrence of EPS in activated sludge from a membrane bioreactor treating municipal wastewater.

EPS are supposed to be among the causes of membrane fouling in membrane bioreactors (MBR). In this work they are measured as total proteins and total polysaccharides. Theoretical and empirical considerations of biomass membrane filtration lead to the conclusion that EPS in the water phase is decisive for the filterability of activated sludge. In this study therefore different ways of separating the water phase from the biomass are investigated, where a simple filtration over a paper filter turned out to be sufficient. Subsequently, a simple batch test set up was used to investigate the influence of substrate conditions on the amount of EPS in the water phase. Dilution of the biomass does not result in changes. Dilution together with substrate addition leads to an increase both in proteins and polysaccharides. Replacement of the water phase leads to no significant changes in protein concentration, but polysaccharide concentration may vary considerably. This phenomenon is more pronounced after replacement of the water phase and substrate addition.     

Impact of dissolved oxygen concentration on membrane filtering resistance and soluble organic matter characteristics in membrane bioreactors

This study investigated the impact of dissolved oxygen (DO) concentration on membrane filtering resistance, soluble organic matter (SOM) and extracellular polymeric substance (EPS) characteristics in a membrane bioreactor (MBR). A laboratory-scale MBR was operated under DO limited (0.2 mg L(-1) DO) and fully aerobic (3.7 and 5.4 mg L(-1) DO) conditions. Membrane filtering resistance was determined for the mixed liquor suspended solids (MLSS) and for resuspended microbial biomass after removing SOM. Regardless of the DO concentration, the cake resistance (Rc) was approximately 95 percent of the total resistance (Rt). The membrane cake resistance was found to decrease significantly after removing the SOM. The total resistance caused by the resuspended biomass was 29 percent of that caused by the MLSS under DO limited conditions, while the total resistance caused by resuspended biomass was 41 to 48 percent of that caused by the MLSS under fully aerobic conditions. Under DO limited conditions, SOM in the MLSS contained a larger amount of high molecular weight compounds, leading to higher cake resistance than under fully aerobic conditions. There was significant variation in the molecular weight fractions of the EPS, with no clear relationship with DO concentration. There was also no distinct relationship between membrane filtering resistance and molecular weight fraction of the EPS.     

Performances of a hybrid adsorption/submerged membrane biological process for toxic waste removal.

This study focuses on a hybrid process, which combines adsorption on powdered activated carbon (PAC), membrane separation using immersed hollow fibers and biological activity. The first part shows that PAC addition in a complex system (containing dissolved molecules and biological particles) can reduce membrane fouling. In that system, DMP removal is function of the activated carbon concentration. Then, respirometric experiments allowed comparison of toxic sensitivity and biological degradation of different bioreactors (membrane bioreactor (MBR), adsorptive membrane bioreactor (PAC-MBR) and classical activated sludge bioreactor (AS)). Results point out that MBR sludge is less sensitive to the toxic than the AS. For high toxic concentration, PAC addition in the MBR decreases rapidly the toxic concentration under the EC50 in the bioreactor, which allows a better biodegradation of the toxic compound. DMP assimilation is completed more rapidly with the PAC-MBR than the MBR.     

Influence of hydraulic retention time on UASB post-treatment with UF membranes

A pilot UASB reactor coupled with an external ultrafiltration (UF) membrane was operated under three different hydraulic retention times (HRT) for domestic wastewater treatment. The aim was to assess the HRT influence on system performance and fouling. The highest concentrations of COD, total solids, extracellular polymeric substances (EPS) and soluble microbial products (SMP) in UASB effluent and permeate were found when the UASB reactor was operated under the lowest HRT studied (4 hours); although the fulfillment of Mexican Standard for wastewater reclamation was not compromised. This fact could be attributed to the higher shear stress forces inside the UASB reactor when it was operated at low HRT, which promoted the release of biopolymeric substances in its effluent. Besides, the fouling propensity in the UASB effluent was worsened with HRT reduction, by increasing the fouling rate and the specific cake resistance. Based on these results, it is recommended to avoid operating the UASB reactor at low HRTs (less than 4 hours) in order to control SMP and EPS fouling potential. The results presented also suggest that HRT reduction has a detrimental effect on performance and fouling.     

Effect of internal recycle rate on the high-strength nitrogen wastewater treatment in the combined UBF/MBR system.

An anaerobic/aerobic system combining an anaerobic upflow-sludge bed filter (UBF) and an aerobic membrane bioreactor (MBR) was operated to enhance organic and nitrogen removal efficiency. The internal recycle rate, which is one of the most important operation factors that affects overall removal efficiency, was varied from 100% to 300% of the influent flow. Under these conditions, the overall removal efficiencies of organic and nitrogen and characteristics of membrane fouling in the combined system treating the synthetic wastewater including high concentration of organics and nutrients were studied. As a result, nitrogen removal efficiency was increased to 67% when the internal recycle rate was 300% of influent flow rate. As the internal recycle ratio increased from 100% to 200%, protein content decreased by 17% and carbohydrate content increased by 12%. However, there was no remarkable difference in total extracellular polymeric substances (EPS) content. At the high recycle rate of 300%, the surface charge of sludge was decreased while hydrophobicity (specific ultraviolet absorbance, SUVA) was increased. The differences in SUVA and surface charge were 11% and 1%, respectively. It is concluded that SUVA and EPS composition were important parameters affecting membrane fouling in the combined system.     

Fouling cake layer in a submerged anaerobic membrane bioreactor treating saline wastewaters: curse or a blessing?

The treatment of inhibitory (saline) wastewaters is known to produce considerable amounts of soluble microbial products (SMPs), and this has been implicated in membrane fouling; the fate of these SMPs was of considerable interest in this work. This study also investigated the contribution of SMPs to membrane fouling of the; (a) cake layer/biofilm layer, (b) the compounds below the biofilm/cake layer and strongly attached to the surface of the membrane, (c) the compounds in the inner pores of the membrane, and (d) the membrane. It was found that the cake/biofilm layer was the main reason for fouling of the membrane. Interestingly, the bacteria attached to the cake/biofilm layer showed higher biodegradation rates compared with the bacteria in suspension. Moreover, the bacteria attached to the cake layer showed higher amounts of attached extracellular polysaccharides (EPS) compared with the bacteria in suspension, possibly due to accumulation of the released EPS from suspended biomass in the cake/biofilm layer. Molecular weight (MW) analysis of the effluent and reactor bulk showed that the cake layer can retain a large fraction of the SMPs in the reactor and prevent them from being released into the effluent. Hence, while cake layers lead to lower fluxes in submerged anaerobic membrane bioreactors (SAMBRS), and hence higher costs, they can improve the quality of the reactor effluent.     

Review on the state of science on membrane bioreactors for municipal wastewater treatment.

Over the past two decades the field of application for membrane bioreactors has broadened towards the municipal wastewater treatment sector. The Global Water Research Coalition (GWRC) determined MBR technology to be of priority for collaborative research and decided to conduct a project with the aim to determine the current state of the science in the field of MBR for municipal wastewater treatment and to develop a phased research strategy represented by priority research projects, like a State of the Science report with regard to MBR technology. This paper describes the state of the science with regard to MBR technology for municipal wastewater treatment by 2007, derived by literature review on recent publications, database analysis and international questionnaires.The research efforts from the past seven years can be characterised by the following prioritised list: (1) membrane fouling, (2) effluent quality, (3) energy consumption (aeration) and (4) cost considerations. The research needs for the near future as identified with the questionnaire are comparable to the main topics of research as identified in the literature review: membrane fouling is still the main problem requiring thorough attention from scientists; effluent quality is a main driver for the application of the technology. There remain some important questions however, with regard to the removal of EDC and micro pollutants. Much of the research activities on MBR are repeated more than once by research groups world-wide. This is only partially caused by a lack of knowledge exchange between researchers. However, lacking information exchange between Europe and the USA on MBR research is identified as a potential bottleneck. Another point of attention is the fast that research results can not easily be translated to other (more practical) situations, since much of the research is carried out at pilot scale.