Submerged Hollow Fiber Membrane Research Articles

Fouling control of submerged and side-stream membrane bioreactors based on the statistical analysis of mid-term assays

The response surface methodology has been applied to study reversible and irreversible fouling rates caused by anaerobic sludge in membrane bioreactors, with the aim of controlling membrane fouling by adjusting filtration conditions. The challenge of obtaining statistically significant results of long-term fouling by means of mid-term assays has been addressed. The individual and combined effects of the filtration flux, backwashing intensity, gas sparging and crossflow velocity on membrane fouling, were analyzed in two types of membranes: an external tubular membrane and a submerged hollow fiber membrane. In the external membrane, the reversible fouling rate was as low as 0.27 ± 0.10 mbar/min, depending mainly on the filtration flux and gas sparging. However, the principal control parameter of the irreversible fouling rate was the crossflow velocity, reaching 2.12 ± 1.75 1012 m−2 in terms of increase of resistance per cubic meter filtered by square meter of membrane. In the submerged membrane, the irreversible fouling rate was quite lower, 0.78 ± 0.40 mbar/d, despite the reversible fouling rate was higher, 1.26 ± 0.42 mbar/min. In this case, the irreversible fouling depended mainly on the backwashing frequency despite the reversible fouling was more affected by the filtration flux and gas sparging. Hence, the approach used to control the reversible fouling rate does not involve mitigating irreversible fouling on both submerged and external membranes. This study provides a methodological basis for the selection of site-specific operating conditions, under which sustainable operation of membrane bioreactors could be achieved.

Long-Term Continuous Extraction of Medium-Chain Carboxylates by Pertraction With Submerged Hollow-Fiber Membranes.

Medium-chain carboxylic acids (MCCAs), which can be generated from organic waste and agro-industrial side streams through microbial chain elongation, are valuable chemicals with numerous industrial applications. Membrane-based liquid-liquid extraction (pertraction) as a downstream separation process to extract MCCAs has been applied successfully. Here, a novel pertraction system with submerged hollow-fiber membranes in the fermentation bioreactor was applied to increase the MCCA extraction rate and reduce the footprint. The highest average surface-corrected MCCA extraction rate of 655.2 ± 86.4 mmol C m−2 d−1 was obtained, which was higher than any other previous reports, albeit the relatively small surface area removed only 11.6% of the introduced carbon via pertraction. This submerged extraction system was able to continuously extract MCCAs with a high extraction rate for more than 8 months. The average extraction rate of MCCA by internal membrane was 3.0- to 4.7-fold higher than the external pertraction (traditional pertraction) in the same bioreactor. A broth upflow velocity of 7.6 m h−1 was more efficient to extract MCCAs when compared to periodic biogas recirculation operation as a means to prevent membrane fouling. An even higher broth upflow velocity of 40.5 m h−1 resulted in a significant increase in methane production, losing more than 30% of carbon conversion to methane due to a loss of H2, and a subsequent drop in the H2 partial pressure. This resulted in the shift from a microbial community with chain elongators as the key functional group to methanogens, because the drop in H2 partial pressure led to thermodynamic conditions that oxidizes ethanol and carboxylic acids to acetate and H2 with methanogens as the syntrophic partner. Thus, operators of chain elongating systems should monitor the H2 partial pressure when changes in operating conditions are made.

Ammonia capture from human urine to harvest liquid N-P compound fertilizer by a submerged hollow fiber membrane contactor: Performance and fertilizer analysis

In this study, we developed a submerged hollow fiber membrane contactor (HFMC) to recover ammonia from human urine to get compound N-P fertilizers. The ammonia capture performance, water vapor transmembrane performance, ion rejection performance and the liquid fertilizer components using 1–4 mol/L H3PO4 as the stripping solution was comprehensively investigated. Increasing H3PO4 concentration did not significantly affect the ammonia capture performance but the water vapor transfer and fertilizer components. The ammonia mass transfer coefficients were in a range of 1.95×10−6±4.77×10−8 to 2.28×10−6±6.71×10−8 m/s and the ammonia flux fluctuated between 17.80 and 20.80 g/m2·h. The water vapor flux increased with the increase of stripping solution concentration and the time elapsed. The N content (21.29–55.24 g/L) was in the range of the commercial products while the P2O5 content (99.41–281 g/L) was slightly higher, which can be used in the soils or plants with a high demand for phosphorus. The liquid fertilizers were all mixtures of (NH4)2HPO4 and NH4H2PO4, but the distribution ratio slightly changed with the different initial H3PO4 concentration. The economic assessment showed that harvesting liquid N-P fertilizer from human urine using HFMC can make a profit of $7.089/L.

Optimizing the Axial Resistance Profile of Submerged Hollow Fiber Membranes

Submerged hollow fiber membranes (SHFMs) are used for a wide variety of applications. Their applicability and their advantages, however, depend crucially on the prevailing hydrodynamics within single fibers. In this respect, the non-uniform distribution of transmembrane flux is a known problem related to inhomogeneous membrane fouling and disadvantages for cleaning. To address this problem, we propose an approach to homogenize transmembrane flux by varying the local membrane resistance using optimal control methods for the first time in SHFM research. Based on an established model, different scenarios are optimized, namely with different fiber lengths and inner radii. In addition, a double-end setup is explored. It is shown that the optimization goal is reached very well in all tested cases, which underlines the general validity of our strategy. Further uses and extensions of the optimization method are provided, as well as hints for the practical implementation of the suggested measures.

Hysteresis effect on backwashing process in a submerged hollow fiber membrane bioreactor (MBR) applied to membrane fouling mitigation

Hysteresis effect on backwashing in a submerged MBR was investigated with dead-end hollow fiber membranes. The out-of-step changes in TMP and flux is the real hysteresis effect which is common but easily overlooked. Methods of visualization and ultrasonic spectrum analysis were implemented. The results showed that fouling layer is just the culprit of hysteresis effect. Fouling level and fiber length were determined as two key factors that affect hysteresis effect by data and model derivation. Moreover, a hysteresis evaluation index “τbw” is proposed to quantify the result of TMP vs time. The relationship between influence factors and “τbw” is interactive. A linear relationship between fouling level and “τbw” was found as well as an extreme value between fiber length and “τbw”. A lower fouling level (lower backwashing flow) and optimal backwashing duration will be helpful for an effective backwashing no matter for membrane fouling control or energy cost reduce.

Effect of inorganic and organic compounds on the performance of fractional-submerged membrane distillation-crystallizer

A novel approach - fractional-submerged membrane distillation crystallizer (F-SMDC) was evaluated for treating brine. F-SMDC is based on creating concentration gradient (CG) and temperature gradient (TG) in a reactor containing submerged hollow-fiber membrane. This enables water and salt recovery to occur simultaneously in a single reactor. The influence of inorganic and organic compounds present in brine solutions on the development and stability of CG/TG in F-SMDC was evaluated in detail in this study. The results of the study showed that properties of inorganic compounds - molecular weight and electronegativity played a significant role in influencing CG/TG in F-SMDC. A high CG ratio (between 1.51 and 1.83 after crystallization) was observed when using feed solutions with inorganic compounds such as KCl, MgSO4, and Na2SO4. However, only low CG ratio (between 0.94 and 1.46) was achieved in the case of feed solutions containing lower molecular weight compounds, NH4Cl and NaCl. The high CG ratio with KCl resulted in the occurrence of salt crystallization at a faster rate (from VCF 2.4 onwards) compared to the predicted theoretical salt saturation point of VCF 3.0. On the other hands, Na2SO4 showed lower flux decline (12.56% flux decline) compared to MgSO4 (55.93% flux decline) This was attributed to lower cation electronegativity of Na+. The presence of CG in F-SMDC by concentrated inorganic compounds also enhanced organic compounds to gravitate downwards to the bottom of the reactor, potentially mitigating organic deposition on the membrane.

Application of rotational vibration in a submerged hollow fibre membrane system for bioseparation of high concentration yeast suspensions

A novel submerged rotational vibration membrane system was investigated using high concentration yeast suspensions to demonstrate the capability of this system for the separation of high concentration biomass. The critical flux and shear rate for this system can be correlated by the expression, Jcritical=1.2γ̅0.55, for the filtration of 200 g/L yeast suspension. Good fouling control performance as indicated by low and stable TMP at approximately 13 kPa as well as high protein transmission of 84% was maintained over 26 h of filtration using 200 g/L yeast suspension at a constant permeate flux of 15 L/m2 h before a rapid TMP rise was observed mainly due to the increase of feed concentration. It was determined that there is a limiting concentration, defined here as the critical concentration point, below which low and stable TMP operation can be maintained and above which rapid TMP rise was observed. Fractionation analysis of the foulant layer at the end of long term filtration experiments showed that majority of fouling was loose and reversible, which can be easily recovered by simple rinsing. The implementation of intermittent shear enhancement and periodical relaxation were found to be effective in fouling limitation, which extended the critical concentration point. However, applying periodical backwash using the permeate is not advantageous as the permeate contains protein and carbohydrate which can cause internal fouling of the pores.

Volatile fatty acids and biogas recovery using thermophilic anaerobic membrane distillation bioreactor for wastewater reclamation

The effects of bioreactor temperatures and salinities of an anaerobic membrane distillation bioreactor (anMDBR) on the permeation performance and their potential recovery of bioresources were fully examined in this study. To the best of our knowledge, this is the first study of a lab-scale anMDBR process utilizing sub-merged hollow fiber membranes. The hybrid system utilizing both membrane distillation (MD) and anaerobic bioreactors achieved 99.99% inorganic salt rejection regardless the operation temperatures and high initial flux from (2–4 L m−2 h−1) at 45–65 °C. However, after 7-day operation, the flux dropped by 16–50% proportional to the bioreactor temperatures. It was found that the effects of bioreactor temperatures had strong impacts on both the permeation performance and fouling behavior while salinity had insignificant effect. A compact non-porous fouling layer was observed on the membrane surface from the bioreactor operated at 65 °C while only a few depositions was found on the membrane from 45 °C bioreactor. In the present study, the optimal anMDBR temperature was found to be 45 °C, showing a balanced biogas production and membrane permeation performance including less fouling formation. At this bioreactor temperature (45 °C), the biogas yield was 0.14 L/g CODremoval, while maintaining a methane recovery of 42% in the biogas, similar recovery to those at bioreactor temperatures of 55 and 65 °C. The potential recovery of volatile fatty acids made anMDBR a more economically efficient system, in addition to its lower operation cost and smaller footprint compared with most other technologies for on-site wastewater treatment.

New coculture system of Clostridium spp. and Megasphaera hexanoica using submerged hollow-fiber membrane bioreactors for caproic acid production

In this study, a coculture bioprocess was developed with Clostridium strains producing butyric acid and Megasphaera hexanoica producing caproic acid from the butyric acid. The two bacterial strains were each cultivated in two submerged hollow-fiber membrane bioreactors (s-HF/MBRs), separately. Each fermentation broth was filtered through the membrane modules, and the filtered broth was either interchanged on another reactor or obtained sequentially through. Using s-HF/MBRs, the caproic acid concentration increased to 10.08 g L−1, with the fastest productivity of 0.69 g L−1 h−1, which higher than that previously reported.

Theoretical and Computational Analysis on Double-End Submerged Hollow Fibre Membrane Modules

This paper studies the potential increase in permeate output flow rate that submerged hollow fibres can achieve when operating in double-end suction. The flow dynamics of submerged hollow fibre membranes with different combinations of fibre inner diameter, membrane thickness, and membrane permeability were numerically simulated. Fibre features (fibre inner diameter, membrane thickness, and membrane permeability) are then characterised for their effects on the increment in permeate flow rate due to change in configuration. Concurrently, an analytical model of a fibre in double-end suction is modelled. Analysis on the double-end fibre model has indicated that the fibre characteristic ratio, λ, has a direct influence on the relative increase in output flow rate when both ends are open. Parametric investigations on the three fibre features have shown that their effects on relative output increase agree with the co-relations indicated by λ. For fibres with λ less than 4, a proportional relationship between a fibre’s λ value and the percentage increment in permeate flow rate is observed when adopting double-end suction. The fibre characteristic ratio, λ, in addition to characterising flux uniformity, can further be used to consider the effectiveness of applying double-end suction in Submerged Hollow Fibre Membrane Module (SHFMM) systems at the design stage.

The Performance and Fouling Control of Submerged Hollow Fiber (HF) Systems: A Review

The submerged membrane filtration concept is well-established for low-pressure microfiltration (MF) and ultrafiltration (UF) applications in the water industry, and has become a mainstream technology for surface-water treatment, pretreatment prior to reverse osmosis (RO), and membrane bioreactors (MBRs). Compared to submerged flat sheet (FS) membranes, submerged hollow fiber (HF) membranes are more common due to their advantages of higher packing density, the ability to induce movement by mechanisms such as bubbling, and the feasibility of backwashing. In view of the importance of submerged HF processes, this review aims to provide a comprehensive landscape of the current state-of-the-art systems, to serve as a guide for further improvements in submerged HF membranes and their applications. The topics covered include recent developments in submerged hollow fiber membrane systems, the challenges and developments in fouling-control methods, and treatment protocols for membrane permeability recovery. The highlighted research opportunities include optimizing the various means to manipulate the hydrodynamics for fouling mitigation, developing online monitoring devices, and extending the submerged HF concept beyond filtration.

Improved performance of gravity-driven membrane filtration for seawater pretreatment: Implications of membrane module configuration

As a low energy and chemical free process, gravity-driven membrane (GDM) filtration has shown a potential for seawater pretreatment in our previous studies. In this study, a pilot submerged GDM reactor (effective volume of 720 L) was operated over 250 days and the permeate flux stabilized at 18.6 ± 1.4 L/m2h at a hydrostatic pressure of 40 mbar. This flux was higher than those in the lab-scale GDM reactor (16.3 ± 0.2 L/m2h; effective volume of 8.4 L) and in the filtration cell system (2.7 ± 0.6 L/m2h; feed side volume of 0.0046 L) when the same flat sheet membrane was used. Interestingly, when the filtration cell was submerged into the GDM reactor, the flux (17.2 L/m2h) was comparable to the submerged membrane module. Analysis of cake layer morphology and foulant properties indicated that a thicker but more porous cake layer with less accumulation of organic substances (biopolymers and humics) contributed to the improved permeate flux. This phenomenon was possibly associated with longer residence time of organic substances and sufficient space for the growth, predation, and movement of the eukaryotes in the GDM reactor. In addition, the permeate flux of the submerged hollow fibre membrane increased with decreasing packing density. It is thought that the movement of large-sized eukaryotes could be limited when the space between hollow fibres was reduced. In terms of pretreatment, the GDM systems effectively removed turbidity, viable cells, and transparent exopolymer particles from the feed seawater. Importantly, extending the reactor operation time produced a permeate with less assimilable organic carbon and biopolymers. Thus, the superior quality of the GDM permeate has the potential to alleviate subsequent reverse osmosis membrane fouling for seawater treatment.

Control of submerged hollow fiber membrane fouling caused by fine particles in photocatalytic membrane reactors using bubbly flow: Shear stress and particle forces analysis

In current work, bubbly flow was applied to control membrane fouling in photocatalytic membrane reactors (PMRs) under different aeration rate. The fine particles of TiO2 (d0.5 = 1.75 μm) in combination with natural organic matter (NOM) in suspended solutions were employed to conduct membrane fouling experiments in a submerged hollow fiber membrane tank. The results revealed that bubbly flow was an efficient approach to mitigate membrane fouling, leading to a rapidly decreasing fouling rate (dTMP/dt) (0.0908–0.0069 kPa/min) with the aeration increase. Additionally, computational fluid dynamics (CFD) simulations were implemented to calculated shear stress distribution on the spatial point of the membrane surface. Based on the detailed CFD simulation results, it was observed that the mean shear stress (0.505–2.111 Pa) increased rapidly with the increase of air flow rate (0–3.2 L/min). Furthermore, the forces exerted on the fine particles were analyzed in terms of calculated shear stress. It was quantitatively demonstrated that the streaming particles were prone to deposit on the vicinity of the layer and the particles deposited on the layer cannot be removed by hydrodynamics forces. It is expected that this study would be basis for further improvement of hydrodynamic condition design induced by bubbly flow in PMRs.

Control of membrane fouling with the addition of a nanoporous zeolite membrane fouling reducer to the submerged hollow fiber membrane bioreactor

ABSTRACTThe membrane fouling control via the addition of nanoporous zeolite membrane fouling reducer (Z-MFR) to the submerged membrane bioreactor (MBR) was investigated. Using scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) analysis techniques, the characteristics of fouling on a hollow fiber membrane surface were also analyzed. The addition of Z-MFR to the MBR led to the adsorption of foulants and the flocculation of mixed liquor suspended solids (MLSSs), which resulted in substantially enhancing the membrane filterability. The critical flux values obtained from the sewage mixed liquors of 3400 mg L−1 at the effective dosage rate of 0.03 mg Z-MFR mg−1 MLSS was 85 L m−2 h−1 (LMH), which was enhanced by 42%. The transmembrane pressure (TMP) variation under the operating conditions of 30 LMH with 3500 mg MLSS L−1 showed that the addition of Z-MFR extended the time required to reach the critical flux of 0.32 bar by 2.6-fold longer than the control. Thus, due to the hybrid functions of adsorbing foulants and precipitating colloidal substances with the addition of Z-MFR, a decrease in the foulant amount and an improvement of sludge flocculation have been attained simultaneously. As a result, the membrane fouling control was achieved effectively with the addition of the Z-MFR.

Modeling of filtration characteristics during submerged hollow fiber membrane microfiltration of yeast suspension under aeration condition

A modeling of filtration characteristics during the submerged hollow fiber membrane microfiltration of yeast suspension under aeration condition was developed on the basis of hydrodynamic mathematical equations that describe the local flux distribution, particle deposition and gas–liquid two-phase flow. The effects of average operating flux, fiber length, inner diameter and aeration rate on the distributions of local flux and filtration resistance were investigated using the developed model. The model was validated using the literature data. The simulated results showed that the asymmetry of initial local flux distribution was intensified with the increase of aeration rate. The local fluxes along the fiber experienced the self-adjustment and reconstruction due to the changes of filtration resistance as the filtration processed. Furthermore, the local flux distribution along the fiber with a short length and large fiber inner diameter at a high operating flux would tend to become relatively uniform at the initial period of filtration.

Effect of internal pressure and gas/liquid interface area on the CO mass transfer coefficient using hollow fibre membranes as a high mass transfer gas diffusing system for microbial syngas fermentation

This study proposed a submerged hollow fibre membrane bioreactor (HFMBR) system capable of achieving high carbon monoxide (CO) mass transfer for applications in microbial synthesis gas conversion systems. Hydrophobic polyvinylidene fluoride (PVDF) membrane fibres were used to fabricate a membrane module, which was used for pressurising CO in water phase. Pressure through the hollow fibre lumen (P) and membrane surface area per unit working volume of the liquid (AS/VL) were used as controllable parameters to determine gas–liquid volumetric mass transfer coefficient (kLa) values. We found a kLa of 135.72h−1 when P was 93.76kPa and AS/VL was fixed at 27.5m−1. A higher kLa of 155.16h−1 was achieved by increasing AS/VL to 62.5m−1 at a lower P of 37.23kPa. Practicality of HFMBR to support microbial growth and organic product formation was assessed by CO/CO2 fermentation using Eubacterium limosum KIST612.

Direct monitoring of sub-critical flux fouling in a horizontal double-end submerged hollow fiber membrane module using ultrasonic time domain reflectometry

Ultrasonic time-domain reflectometry (UTDR) as a non-invasive and real-time technique was employed to monitor the sub-critical flux fouling in a submerged hollow fiber membrane module (SHFMM) oriented horizontally. A single polyethersulfone hollow fiber membrane was used to filter 5g/L yeast suspension in the fouling experiment. The effect of aeration rate, fiber length and operating flux on the sub-critical flux fouling profiles was investigated. Results showed that the fouling distribution along fiber in the horizontal SHFMM was more uniform than that in the vertical SHFMM under the same conditions. The deposition of foulants gradually migrated from both suction ends to the middle part of fiber in the horizontal SHFMM. Further, a high aeration rate could effectively restrain the deposition of fouling, but could not completely eliminate membrane fouling. Moreover, shortening the length of horizontal SHFMM could not reduce membrane fouling. A low operating flux could effectively prevent from membrane fouling.

Support media impacts on humic acid, cellulose, and kaolin clay in reducing fouling in a submerged hollow fiber membrane system

The impact of support media on a submerged hollow fiber PVDF ultrafiltration membrane system filtering synthetic water with different organic and inorganic matter content was investigated using a factorial design of experiments. Support media was added into a membrane reactor where membrane fouling and water quality parameters were assessed. The addition of support media into the membrane reactor resulted in a 35–50% reduction in membrane fouling with different humic acid and kaolin clay concentrations compared to the control system with no media; however there was no apparent impact of support media in minimizing fouling by cellulose. The support media in the membrane reactor did not affect any of the water quality parameters measured; however, there was a small but measurable difference in the permeate volume and flux recovery (4% flux difference) between the two systems. This favorable difference was attributed to the presence of support media in the membrane reactor.

Ultrasonic visualization of sub-critical flux fouling in the double-end submerged hollow fiber membrane module

Ultrasonic time-domain reflectometry (UTDR) was extensively employed to visualize and analyze quantitatively the effect of aeration rate, operating flux and fiber length on sub-critical flux fouling profiles in a double-end submerged hollow fiber membrane (SHFM) module. Five 10MHz transducers were externally mounted in contact with the outside surface of the double-end SHFM module. A single polyethersulfone hollow fiber membrane was used to filter 5g/L yeast suspension. Results showed that the double-end SHFM module has better filtration performance by comparison with one-end SHFM module. The acoustic measurements revealed that the membrane near the upper suction end of the double-end SHFM module was more easily suffered from fouling than that near the lower suction end under sub-critical flux operation. Furthermore, the progress of foulant deposition gradually migrated from both ends to middle and reached the plateau finally. Moreover, a low operating flux was more effective to reduce membrane fouling. And a short double-end SHFM module was more easily subjected to membrane fouling than a long one at the same operating conditions.

Reverse-flow diafiltration for continuous in situ product recovery

In situ product recovery (ISPR) is a major challenge in downstream processing. ISPR is particularly advantageous for product-inhibited fermentation processes since it removes the product from the vicinity of the producing cell right after it is formed, thereby eliminating the inhibition of the reaction. This work presents a novel ISPR process named reverse-flow diafiltration. This process is characterized by submerged hollow-fiber membranes which are integrated in the bioreactor. The process handles two liquid streams which are exchanged alternatingly over the same membrane in a reverse-flow operation mode: One supplies the nutrient solution, the second one extracts product solution. As a result no net flux over the membrane occurs and cake-layer formation is kept low. Short emptying intervals in-between the two steps are applied, using air or permeate, respectively, for emptying the inside of the hollow-fiber lumen to improve product recovery values. Experiments with yeast suspensions of different concentrations showed that stable filtration performance could be ensured over a period of about one week. At the same time constant product recovery values of up to 100% could be achieved. The reverse-flow diafiltration process is very promising for future ISPR applications towards continuous fermentation processes. It is promising especially for more complex fermentation suspensions such as fungi suspensions, characterized by high viscosity and filamentous organisms, since it enables stable filtration performance and product recovery values over longer time periods than conventional processes.