Influence of membrane properties on physically reversible and irreversible fouling in membrane bioreactors

Nano-clay modified membranes: A promising green strategy for microalgal antifouling filtration

Membrane fouling is a major challenge which limits the sustainable application of membrane filtration-based microalgal harvesting at industrial level. Membrane fouling leads to increased operational and maintenance costs and represents a major obstacle to microalgal downstream processing. Nano-clays are promising naturally occurring nanoparticles in membrane fabrication due to their low-cost, facile preparation, and their superior properties in terms of surface hydrophilicity, mechanical stability, and resistance against chemicals. The membrane surface modification using nano-clays is a sustainable promising approach to improve membranes mechanical properties and their fouling resistance. However, the positive effects of nano-clay particles on membrane fouling are often limited by aggregation and poor adhesion to the base polymeric matrix. This review surveys the recent efforts to achieve anti-fouling behavior using membrane surface modification with nano-clay fillers. Further, strategies to achieve a better incorporation of nano-clay in the polymer matrix of the membrane are summarised, and the factors that govern the membrane fouling, stability, adhesion, agglomeration and leaching are discussed in depth.

Understanding and Designing a High-Performance Ultrafiltration Membrane Using Machine Learning

Ultrafiltration (UF) as one of the mainstream membrane-based technologies has been widely used in water and wastewater treatment. Increasing demand for clean and safe water requires the rational design of UF membranes with antifouling potential, while maintaining high water permeability and removal efficiency. This work employed a machine learning (ML) method to establish and understand the correlation of five membrane performance indices as well as three major performance-determining membrane properties with membrane fabrication conditions. The loading of additives, specifically nanomaterials (A_wt %), at loading amounts of >1.0 wt % was found to be the most significant feature affecting all of the membrane performance indices. The polymer content (P_wt %), molecular weight of the pore maker (M_Da), and pore maker content (M_wt %) also made considerable contributions to predicting membrane performance. Notably, M_Da was more important than M_wt % for predicting membrane performance. The feature analysis of ML models in terms of membrane properties (i.e., mean pore size, overall porosity, and contact angle) provided an unequivocal explanation of the effects of fabrication conditions on membrane performance. Our approach can provide practical aid in guiding the design of fit-for-purpose separation membranes through data-driven virtual experiments.

Insights on fouling development and characteristics during different fouling stages between a novel vibrating MBR and an air-sparging MBR for domestic wastewater treatment

Membrane fouling remains a major hindrance to a prevalent application of membrane bioreactor (MBR) for wastewater treatment. Vibrating membrane technology has recently attracted increasing attention in energy-efficient fouling control in MBR compared to air sparging. However, little is known about its fundamental fouling control mechanism and whether the vibrating MBR (VMBR) is a highly effective strategy to control fouling constitutions and fouling sources compared to the conventional air-sparging MBR (ASMBR). This study operated two parallel MBRs with vibrating or air-sparging membrane modules for long-term (215 d) real domestic wastewater treatment. Effects of air sparging and vibration rates on fouling control, fouling development and fouling sources across three fouling stages were comprehensively evaluated. Results showed that the VMBR achieved 70% lower fouling rates compared to the ASMBR due to a remarkable retardation in each fouling stage by membrane vibration. The VMBR significantly reduced over 62.7% of colloid_CL and SMP_CL within the cake layer (CL) to simultaneously alleviate the reversible and irreversible fouling compared to the ASMBR. The comparatively lower dissolved organic matter (DOM) and biopolymer contents in the cake layer of the VMBR resulted in a slower TMP rise. The main DOMs in the foulants of both MBRs were found in the following order: aromatic protein > soluble microbial by-products > other organics. EPS_ML from mixed liquor (ML) contributed more DOMs to form membrane foulant than the SMP_ML in both MBRs. Aromatic proteins and soluble microbial products in the EPS_ML were markedly reduced in the VMBR but increased in the ASMBR in high-shear phase, demonstrating higher effectiveness in fouling control by membrane vibration. This study provided insights into understanding fouling control, fouling development characteristics and fouling mechanisms between the VMBR and ASMBR, which might guide the researchers and engineers to apply novel vibrating MBRs to better control membrane fouling for holistic wastewater treatment in full scale.

Polyvinylidene Fluoride Membrane Via Vapour Induced Phase Separation for Oil/Water Emulsion Filtration

Membrane-based technology is an attractive option for the treatment of oily wastewater because of its high oil removal efficiency, small footprint and operational simplicity. However, filtration performance is highly restricted by membrane fouling, especially when treating oil/water emulsion as a result of strong interaction between oil droplets and the hydrophobic property of the membrane. This study explores the fabrication of polyvinylidene fluoride (PVDF)-based membrane via the vapour induced phase separation (VIPS) method while incorporating polyvinyl pyrrolidone (PVP) as a hydrophilic additive to encounter membrane fouling issues and improve membrane filterability. The resulting membranes were characterized and tested for oil/water emulsion filtration to evaluate their hydraulic, rejection and anti-fouling properties. Results show that the changes in membrane morphology and structure from typical macrovoids with finger-like substructure to cellular structure and larger membrane pore size were observed by the prolonged exposure time from 0 to 30 min through the VIPS method. The enhanced clean water permeability is attributed to the addition of PVP-LiCl in the dope solution that enlarges the mean flow pore size from 0.210 ± 0.1 to 7.709 ± 3.5 µm. The best performing membrane was the VIPS membrane with an exposure time of 5 min (M-5), showing oil/water emulsion permeability of 187 Lm-2 h-1 bar-1 and oil rejection of 91.3% as well as an elevation of 84% of clean water permeability compared to pristine PVDF developed using a typical non-solvent induced phase separation (NIPS) method. Despite the relatively high total fouling, M-5 was able to maintain its high permeability by water flushing as a simple operation for membrane fouling control. The performance was achieved thanks to combination of the large mean flow pore size and hydrophilic property from residual PVP in the membarne matrix. Overall, the results demonstrate the potential of the optimum VIPS method in the presence of PVP and LiCl additives for oil/water emulsion treatment.

Recent Advances in the Rejection of Endocrine-Disrupting Compounds from Water Using Membrane and Membrane Bioreactor Technologies: A Review

Water is a critical resource necessary for life to be sustained, and its availability should be secured, appropriated, and easily obtainable. The continual detection of endocrine-disrupting chemicals (EDCs) (ng/L or µg/L) in water and wastewater has attracted critical concerns among the regulatory authorities and general public, due to its associated public health, ecological risks, and a threat to global water quality. Presently, there is a lack of stringent discharge standards regulating the emerging multiclass contaminants to obviate its possible undesirable impacts. The conventional treatment processes have reportedly ineffectual in eliminating the persistent EDCs pollutants, necessitating the researchers to develop alternative treatment methods. Occurrences of the EDCs and the attributed effects on humans and the environment are adequately reviewed. It indicated that comprehensive information on the recent advances in the rejection of EDCs via a novel membrane and membrane bioreactor (MBR) treatment techniques are still lacking. This paper critically studies and reports on recent advances in the membrane and MBR treatment methods for removing EDCs, fouling challenges, and its mitigation strategies. The removal mechanisms and the operating factors influencing the EDCs remediation were also examined. Membranes and MBR approaches have proven successful and viable to eliminate various EDCs contaminants.

Development of Hydrophilic PVDF Membrane Using Vapour Induced Phase Separation Method for Produced Water Treatment

During the production of oil and gas, a large amount of oily wastewater is generated, which would pollute the environment if discharged without proper treatment. As one of the most promising treatment options, membrane material used for oily wastewater treatment should possess desirable properties of high hydraulic performance combined with high membrane fouling resistance. This project employs the vapor induced phase separation (VIPS) technique to develop a hydrophilic polyvinylidene fluoride (PVDF) membrane with polyethylene glycol (PEG) as an additive for produced water treatment. Results show that thanks to its slow nonsolvent intake, the VIPS method hinders additive leaching during the cast film immersion. The results also reveal that the exposure of the film to the open air before immersion greatly influences the structure of the developed membranes. By extending the exposure time from 0 to 30 min, the membrane morphology change from typical asymmetric with large macrovoids to the macrovoid-free porous symmetric membrane with a granular structure, which corresponds to 35% increment of steady-state permeability to 189 L·m⁻²h⁻¹bar⁻¹, while maintaining >90% of oil rejection. It was also found that more PEG content resides in the membrane matrix when the exposure time is extended, contributes to the elevation of surface hydrophilicity, which improves the membrane antifouling properties. Overall results demonstrate the potential of VIPS method for the fabrication of hydrophilic PVDF membrane by helping to preserve hydrophilic additive in the membrane matrices.

A review on the applications of ultrasonic technology in membrane bioreactors

Membrane bioreactors (MBRs) have received increasing attention in the field of wastewater treatment in recent years. However, membrane fouling is the main problem of MBRs, limiting their widespread and large applications. Membrane cleaning methods can be mainly classified into four types including chemical, physical, physico-chemical and biological clean the fouled membrane. In recent years, ultrasonication has been reported as a promising cleaning technique for the membranes fouled in MBRs. Ultrasonic irradiation can clean the fouled membrane by creating important physical phenomena including microjets, microstreams and shock waves. Moreover, the ultrasonic method can be combined with other cleaning methods e.g. chemical cleaning and backwashing in order to improve the cleaning efficiency. It should be noted that the application of ultrasonic in the MBR system is not limited to the cleaning of membrane. The pretreatment of the wastewater by ultrasonic irradiation or ultrasound coupled with other methods, e.g. ozonation, prior to MBR system, can decrease the organic loading of the wastewater and subsequently postpone the fouling of the membrane. This paper critically reviews the recent advances in the applications of ultrasound in MBR systems. Emerging issues associated with application of on-line ultrasound and also hybrid on-line ultrasound for controlling the membrane fouling in MBR systems are critically reviewed. Moreover, application of the ultrasound in ex-situ form for cleaning the fouled membranes and pretreatment of wastewater prior to the MBR system is discussed.

New insight into the fouling behavior of hydrophobic and hydrophilic polypropylene membranes in integrated membrane bioreactors

To investigate the effect of hydrophobic and hydrophilic polypropylene hollow fiber membranes (PPHFMs) applied in membrane bioreactors (MBR), the fouling behaviors of membrane surfaces and pores have been tested. The structural and morphological features on the membrane surface were characterized using attenuated total reflection-Fourier transform infrared spectroscopy, field emission scanning electron microscopy, atomic force microscope, energy dispersive X-ray spectroscopy and laser granularity distribution analysis. The results showed that significantly more polysaccharide, protein and inorganic ingredients were accumulated in the original membrane compared to the hydrophilic membrane. Furthermore, it was found that the pore size influenced the particle distribution and accumulation, such that smaller pore size membranes tended to contain fewer pollutants and a narrow size distribution. Under a constant flux of 11.5 L/m² h, the transmembrane pressure (TMP) varied narrowly between 38 and 53 KPa. Alongside this, a relatively hydrophilic membrane (PP-g-AA) showed the characteristics of lower TMP in comparison to hydrophobic membranes (PP). Indeed, the flux recovery was 30% higher than those of the original PPHFM. This investigation broadens our understanding of membrane modifying and fouling behavior in integrated MBRs.

Numerical simulation of filtration performance in submerged membrane bioreactors: effect of particle packed structure

It is widely known that the accumulation of solid matter forming a cake layer on the membrane surface is one of the major limitations of the filtration performance in submerged membrane bioreactors (SMBR). This study is focused on the influence of the cake porosity of different particle microscopic packed structures on the filtration performance of hollow fiber systems. An integrated model based on the finite element method to simulate numerically the flow in an SMBR is presented. The model coupled the Navier-Stokes and Darcy Brinkman equations to simulate a complete filtration run. The cake growth took into consideration not only the deposition with local filtration velocity but also the effect of aeration scouring. A novel solution of mesh deformation was adopted to investigate transient cake growth along the fiber. Comparisons between simulations and experiments are in good agreement. The results show that a higher porosity particle packed structure causes non-uniform filtration and cake thickness but also higher permeate flux. Meanwhile, the proportion of cake resistance to total resistance increases with the decrease of porosity.

Enhancing the permeation and fouling resistance of PVDF microfiltration membranes by constructing an auto-soak surface

A new kind of hydrophilic polyvinylidene fluoride microfiltration membrane with an auto-soak surface was fabricated by plasma treatment and interfacial crosslinking to improve the separation efficiency and fouling resistance in water treatment.

Membrane biofouling in a wastewater nitrification reactor: Microbial succession from autotrophic colonization to heterotrophic domination

Membrane biofouling is a complex process that involves bacterial adhesion, extracellular polymeric substances (EPS) excretion and utilization, and species interactions. To obtain a better understanding of the microbial ecology of biofouling process, this study conducted rigorous, time-course analyses on the structure, EPS and microbial composition of the fouling layer developed on ultrafiltration membranes in a nitrification bioreactor. During a 14-day fouling event, three phases were determined according to the flux decline and microbial succession patterns. In Phase I (0-2 days), small sludge flocs in the bulk liquid were selectively attached on membrane surfaces, leading to the formation of similar EPS and microbial community composition as the early biofilms. Dominant populations in small flocs, e.g., Nitrosomonas, Nitrobacter, and Acinetobacter spp., were also the major initial colonizers on membranes. In Phase II (2-4 d), fouling layer structure, EPS composition, and bacterial community went through significant changes. Initial colonizers were replaced by fast-growing and metabolically versatile heterotrophs (e.g., unclassified Sphingobacteria). The declining EPS polysaccharide to protein (PS:PN) ratios could be correlated well with the increase in microbial community diversity. In Phase III (5-14 d), heterotrophs comprised over 90% of the community, whereas biofilm structure and EPS composition remained relatively stable. In all phases, AOB and NOB were constantly found within the top 40% of the fouling layer, with the maximum concentrations around 15% from the top. The overall microbial succession pattern from autotrophic colonization to heterotrophic domination implied that MBR biofouling could be alleviated by forming larger bacterial flocs in bioreactor suspension (reducing autotrophic colonization), and by designing more specific cleaning procedures targeting dominant heterotrophs during typical filtration cycles.