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.     

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.     

Influence of filtration conditions on membrane fouling and scouring aeration effectiveness in submerged membrane bioreactors to treat municipal wastewater

Membrane fouling and scouring aeration effectiveness were studied using three large pilot-scale submerged membrane bioreactors (MBRs) operated at a series of permeate fluxes, scouring aeration intensities and cyclic aeration frequencies to treat municipal wastewater. The results showed that when operated at the sustainable conditions, the MBRs had a stable reversible fouling resistance. At unsustainable conditions, the reversible fouling resistance increased exponentially as filtration progressed. For each of above two cases, the fouling ratios newly defined by Eqs. (7) and (8) were calculated from the transmembrane pressure increases to compare the relative reversible fouling rates. With the range of sustainable filtration conditions, the fouling ratios at the same reference scouring aeration intensity were found to be proportional to permeate flux. Similarly, the fouling ratios calculated with the same reference permeate flux decreased exponentially with increasing scouring aeration intensity. Moreover, the effects of scouring aeration intensity and permeate flux on the fouling ratios were found to be independent of one another. As a result, an empirical relationship was derived to relate the stable reversible fouling resistance to sustainable permeate fluxes and scouring aeration intensities. Its application was demonstrated by constructing transmembrane pressure contours overlaid with scouring aeration effectiveness contours to aid in the selection of optimal MBR filtration conditions.     

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.     

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.     

Low dose powdered activated carbon addition at high sludge retention times to reduce fouling in membrane bioreactors

The addition of a low concentration of PAC (0.5 g L⁻¹ of sludge, i.e. a dose of 4 mg L⁻¹ of wastewater), in combination with a relatively long SRT (50 days), to improve membrane filtration performance was investigated in two pilot-scale MBRs treating real municipal wastewater. Continuous filterability tests at high flux showed the possibility to run for 18 h at 72 L m⁻² h⁻¹ and 180 h at 50 L m⁻² h⁻¹, while significant fouling occurred without PAC. In addition, measurements of the critical flux showed an increase of 10% for this strategy. Low dosage and high retention time makes it feasible and cost effective. Further advantages with regard to permeate quality and possible micropollutants removal are currently under investigation.     

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.     

Extracellular polymeric substances (EPS) properties and their effects on membrane fouling in a submerged membrane bioreactor

A pilot-scale submerged membrane bioreactor (MBR) for real municipal wastewater treatment was operated for over one year in order to investigate extracellular polymeric substances (EPS) properties and their role in membrane fouling. The components and properties of bound EPS were examined by the evaluation of mean oxidation state (MOS) of organic carbons, Fourier transform infrared (FT-IR) spectroscopy, three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy, and gel filtration chromatography (GFC), etc. Test results showed that MOS of organic carbons in the bound EPS was ranging from −0.14 to −0.51, and major components could be assessed as proteins and carbohydrates. FT-IR analysis confirmed the presence of proteins and carbohydrates in the bound EPS. The organic substances with fluorescence characteristics in the bound EPS were identified as proteins, visible humic acid-like substances and fulvic acid-like substances by EEM technology. GFC analysis demonstrated that EPS had part of higher MW molecules and a broader MW distribution than the influent wastewater. It was also found that a high shear stress imposed on mixed liquor could result in the release of EPS, which would in turn influence membrane fouling in MBRs. Bound EPS solution was observed to have a stronger potential of fouling than mixed liquor. During long-term operation of the MBR, bound EPS demonstrated positive correlations with membrane fouling while temperature was verified as a negative factor affecting EPS concentration. Compared to tightly bound EPS (TB-EPS), loosely bound EPS (LB-EPS) showed more significant correlations with membrane fouling. This critical investigation would contribute towards a better understanding of the behavior, composition and fouling potential of EPS in MBR operation.     

Sludge properties and their effects on membrane fouling in submerged anaerobic membrane bioreactors (SAnMBRs)

Two submerged anaerobic membrane bioreactors (SAnMBRs) (thermophilic vs. mesophilic) were operated for a period of 3.5 months with kraft evaporator condensate at a feed chemical oxygen demand of 10,000 mg/L. The results show that the filtration behavior of the two systems was significantly different. The filtration resistance in the thermophilic SAnMBR was about 5–10 times higher than that of the mesophilic system when operated under similar hydrodynamic conditions. Comparison of sludge properties and cake layer structure from the two systems was made to elucidate major factors governing the different filtration characteristics. There were more soluble microbial products (SMP) and biopolymer clusters (BPC) produced and a larger portion of fine flocs (<15 μm) in the thermophilic SAnMBR. Analysis of bound extracellular polymeric substances (EPS) showed that the thermophilic sludge had a higher protein/polysaccharide ratio in EPS, as compared to that in the mesophilic sludge. A series of analyses, including Fourier transform infrared (FTIR) spectroscopy, energy dispersive X-ray spectroscopy (EDX), confocal laser scanning microscopy (CLSM), scanning electron microscopy (SEM), atomic force microscopy (AFM) and particle size analyzer showed that the cake layer formed in the thermophilic SAnMBR contained higher levels of both organic and inorganic foulants, smaller particle sizes, and especially, a denser and more compact sludge cake structure. These results indicate that floc size, SMP, BPC, bound EPS as well as cake layer structure are the major factors governing membrane fouling in SAnMBR systems.     

Influence of substrate on fouling in anoxic immersed membrane bioreactors

The influence of carbon substrate chemistry on membrane bioreactor (MBR) fouling in anoxic conditions has been evaluated. The use of a weak carboxylic acid (acetic acid) resulted in the production of large open-floc structures (up to 508microm) that were susceptible to breakage. Primary particles (d(10) and d(20) particle sizes, 5.5+/-1.3 and 15.3+/-8.2microm, respectively) and macromolecular soluble microbial products (SMPs) were generated, directly impacting on membrane fouling. The use of a primary alcohol (ethanol), on the other hand, encouraged the growth of flocs similar to activated sludge. These flocs produced low concentrations of primary particles (d(10) and d(20) particle sizes, 120.6+/-36.1 and 185.2+/-62.7microm, respectively) and high-molecular-weight SMP, and the particles had sufficient mechanical integrity to withstand shear. Consequently, the use of ethanol resulted in sufficient suppression of fouling to extend the filtration time by a factor of three. An increase in MLSS concentration did not directly impact upon fouling when operating with ethanol, primarily because of the low concentration of particulate matter produced.     

Novel electrokinetic approach reduces membrane fouling

An innovative submerged membrane electro-bioreactor (SMEBR) was built to reduce membrane fouling through a combination of various electrokinetic processes. The objective of this research was to assess the capability of SMEBR to reduce fouling under different process conditions. At the bench scale level, using synthetic wastewater, membrane fouling of the SMEBR was compared to the fouling of a membrane bioreactor (MBR) in five runs. Different protein concentrations in the influent synthetic wastewater were selected to develop different membrane fouling potentials: high (240 mg/l), low (80 mg/l) and zero protein addition. The MBR and SMEBR were operated at a flux equal to the membrane critical flux in order to create high fouling rate conditions. Membrane fouling rate, expressed as the change in the trans-membrane pressure per day (kPa/d), decreased in the SMEBR 5.8 times (standard deviation (SD) = 2.4) for high protein wastewater, 5.1 times (SD = 2.4) for low protein content, and 1.3 times (SD = 0.7) for zero protein, when compared to the MBR. The supernatant concentrations of the soluble microbial products (SMP) were 195–210, 65–135 and less than 65 mg/l in respective experimental series. Following the bench scale study, membrane fouling was assessed in a pilot scale SMEBR, fed with raw un-clarified municipal wastewater, and operated under real-sewage variable quality conditions. The pilot SMEBR exhibited three times smaller membrane fouling rate than the MBR. It was concluded that electrokinetic processes generated by SMEBR led to a reduction of membrane fouling through: i) removal of soluble microbial products (mainly protein and polysaccharides) and colloidal organic materials; ii) change of the structure and morphology of the suspended solids due their conditioning by DC field.     

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.     

Anaerobic treatment of wastewater with high organic content using a stirred tank reactor coupled with a membrane filtration unit

Using a cross-flow membrane bioreactor, high anaerobic conversion rates of three different types of wastewater with varying organic content were achieved. Loading rates obtained were as follows: 20 g CODL(-1) x d(-1) for artificial wastewater, approximately 8 g CODL(-1) x d(-1) from vegetable processing industry (sauerkraut brine) and 6-8 g CODL(-1) x d(-1) for wastewater from an animal slaughterhouse. At stable conditions, COD-removal rates in all three wastewaters were higher than 90%. Methane yields from the treatment of artificial wastewater, sauerkraut brine, and animal slaughterhouse wastewater were in the range of 0.17-0.30, 0.20-0.34, and 0.12-0.32 L(n) x g(-1) COD(-1) fed, respectively. The complete retention of biomass and suspended solids is a unique feature of this treatment process, which combines a high loading capacity and at the same time, high COD removal rates even for complex wastewater containing high concentrations of particulate matter.     

Engineering of an MBR supernatant fouling layer by fine particles addition: a possible way to control cake compressibility

For membrane bioreactors (MBR) applied to wastewater treatment membrane fouling is still the prevalent issue. The main limiting phenomena related to fouling is a sudden jump of the transmembrane pressure (TMP) often attributed to the collapse of the fouling layer. Among existing techniques to avoid or to delay this collapse, the addition of active particles membrane fouling reducers (polymer, resins, powdered activated carbon (PAC), zeolithe…) showed promising results.Thus the main objective of this work is to determine if fouling can be reduced by inclusion of inert particles (500 nm and inert compared to other fouling reducers) and which is the impact on filtration performances of the structuring of the fouling. Those particles were chosen for their different surface properties and their capability to form well structured layer.Results, obtained at constant pressure in dead end mode, show that the presence of particles changes foulant deposition and induces non-compressible fouling (in the range of 0.5-1 bar) and higher rejection values compared to filtration done on supernatant alone. Indeed dead end filtration tests show that whatever interactions between biofluid and particles, the addition of particles leads to better filtration performances (in terms of rejection, and fouling layer compressibility). Moreover results confirm the important role played by macromolecular compounds, during supernatant filtration, creating highly compressible and reversible fouling.In conclusion, this study done at lab-scale suggests the potential benefit to engineer fouling structure to control or to delay the collapse of the fouling layer. Finally this study offers the opportunities to enlarge the choice of membrane fouling reducers by taking into consideration their ability to form more consistent fouling (i.e. rigid, structured fouling).     

Identification of wastewater sludge characteristics to predict critical flux for membrane bioreactor processes

The effects of sludge characteristics on critical flux were examined using a submerged membrane bioreactor pilot plant operated under different process conditions to treat municipal wastewater. The sludge in the membrane tank was characterized by measuring colloidal particle concentration, extracellular polymeric substances (EPS), mixed liquor suspended solids (MLSS), temperature, time to filter (TTF) and diluted sludge volume index (DSVI). The colloidal particle concentration was represented by the colloidal total organic carbon (TOC), which is the TOC difference between the filtrate passing through a 1.5 microm pore size filter and the permeate collected from pilot ultrafiltration membrane modules with a pore size of 0.04 microm. The results showed that the critical flux measured by the stepwise flux method was almost solely related to the colloidal TOC despite different sludges tested. In contrast, MLSS was shown to have little impact on the critical flux within the range examined. Neither TTF nor DSVI could be used to reliably predict the critical flux. Furthermore, colloidal TOC can be attributed to soluble EPS, but not bound EPS. Therefore, it is suggested that colloidal TOC be used as a new filterability index for MBR processes in wastewater treatment.     

High-concentration food wastewater treatment by an anaerobic membrane bioreactor

The viability of treating high-concentration food wastewater by an anaerobic membrane bioreactor (AMBR) was studied using polyethersulfone (PES) ultrafiltration membranes PES200, PES300, PES500 and PES700 with norminal molecular weight cutoff (MWCO) ranging from 20,000 to 70,000 Da. Hydraulic and solid retention time significantly affected the treatment performance of the AMBR kept at 60 h and 50 days in the study. The four membranes exhibited a similar efficiency in removal of suspended solids, color, chemical oxygen demand (COD) and bacteria. When the volumetric loading rate was below 4.5 kg/m3d, COD removal rate was in the range of 81-94% and the gas yield stabilized at 0.136 m3/kg COD. The effect of membrane properties including MWCO, hydrophobicity and surface morphology on membrane fouling and cleaning was evaluated. The PES200 membranes with the smallest MWCO and smoothest surface exhibited a serious initial flux decline, whereas the PES700 membranes with the largest MWCO and roughest surface were observed related to the highest flux decline and the lowest recoverable flux rate during long-term operation. Membrane autopsy revealed that the significant flux decline was caused by the formation of a thick biofouling layer onto the membrane surfaces.     

Why low powdered activated carbon addition reduces membrane fouling in MBRs

Previous research had demonstrated that powdered activated carbon (PAC), when applied at very low dosages and long SRTs, reduces membrane fouling in membrane bioreactor (MBRs). In this contribution several mechanisms to explain this beneficial effect of PAC were investigated, including enhanced scouring of the membrane surface by PAC particles, adsorption of membrane foulants by PAC and subsequent biodegradation and a positive effect of PAC on the strength of the sludge flocs. It was concluded that the latter mechanism best explains why low dosages of PAC significantly reduce membrane fouling. Cheaper alternatives for PAC may have a similar effect.     

Automatic control systems for submerged membrane bioreactors: a state-of-the-art review

Membrane bioreactor (MBR) technology has become relatively widespread as an advanced treatment for both industrial and municipal wastewater, especially in areas prone to water scarcity. Although operational cost is a key issue in MBRs, currently only a few crucial papers and inventions aimed to optimise and enhance MBR efficiency have been published. The present review summarises the available solutions in the area of automatic control systems and widely explores the advances in automation and control for MBRs. In this review of state of the art, different control systems are evaluated comparatively, distinguishing between control systems used for the filtration process and those used for the biological process of MBRs and describing the challenge faced by integrated control systems. The existing knowledge is classified according to the manipulated variables, the operational mode (open-loop or closed-loop) and the controlled variables used.     

Relationship between types of surface shear stress profiles and membrane fouling

Shear stress has been recognized as an important parameter in controlling particle back-transport from membrane surfaces. However, little is known of the relationship between transient shear conditions induced by air sparging and fouling control near membrane surfaces. In this paper, the different types of surface shear stress profiles that had beneficial effects on minimizing reversible surface fouling were examined. The relationship between different statistical shear parameters (e.g. time-averaged shear, standard deviation of shear and amplitude of shear) and fouling control that have been used by others were examined as well. It was found that the fouling rate for membranes subjected to transient shear conditions was lower than for membranes subjected to constant shear conditions. The magnitude, duration and frequency of the shear conditions were found to have an impact on the fouling rate of membranes. It was also found that although some statistical shear parameters could generally be used to relate shear and fouling, they were inadequate to relate surface shear stress to fouling, for all transient shear conditions examined.     

Baffled membrane bioreactor (BMBR) for efficient nutrient removal from municipal wastewater

Submerged membrane bioreactors (MBRs) are now widely used for various types of wastewater treatment. One drawback of submerged MBRs is the difficulty in removing nitrogen because intensive aeration is usually carried out in the tank and the MBRs must therefore be operated under aerobic conditions. In this study, the feasibility of treating municipal wastewater by a baffled membrane bioreactor (BMBR), particularly in terms of nitrogen removal, was examined. Simultaneous nitrification/denitrification in a single and small reaction tank was possible by inserting baffles into a normal submerged MBR as long as wastewater was fed in the appropriate way. To examine the applicability of the BMBR, pilot-scale experiments were carried out using real municipal wastewater. Although neither external carbon addition nor mixed liquor circulation was carried out in the operation of the BMBR, average removal rates of total organic carbon (TOC), total phosphorus (T-P) and total nitrogen (T-N) reached 85%, 97% and 77%, respectively, with the hydraulic retention time (HRT) of 4.7h. Permeability of the membrane could be maintained at a high level throughout the operation. It was found that denitrification was the limiting step in removal of nitrogen in the BMBR in this study. Various types of monitoring carried out in the BMBR also demonstrated the possibility of further improvements in its performance.