Effect of permeate drag force on the development of a biofouling layer in a pressure-driven membrane separation system

New Insights on Biological Activities, Chemical Compositions, and Classifications of Marine Actinomycetes Antifouling Agents

Biofouling is the assemblage of undesirable biological materials and macro-organisms (barnacles, mussels, etc.) on submerged surfaces, which has unfavorable impacts on the economy and maritime environments. Recently, research efforts have focused on isolating natural, eco-friendly antifouling agents to counteract the toxicities of synthetic antifouling agents. Marine actinomycetes produce a multitude of active metabolites, some of which acquire antifouling properties. These antifouling compounds have chemical structures that fall under the terpenoids, polyketides, furanones, and alkaloids chemical groups. These compounds demonstrate eminent antimicrobial vigor associated with antiquorum sensing and antibiofilm potentialities against both Gram-positive and -negative bacteria. They have also constrained larval settlements and the acetylcholinesterase enzyme, suggesting a strong anti-macrofouling activity. Despite their promising in vitro and in vivo biological activities, scaled-up production of natural antifouling agents retrieved from marine actinomycetes remains inapplicable and challenging. This might be attributed to their relatively low yield, the unreliability of in vitro tests, and the need for optimization before scaled-up manufacturing. This review will focus on some of the most recent marine actinomycete-derived antifouling agents, featuring their biological activities and chemical varieties after providing a quick overview of the disadvantages of fouling and commercially available synthetic antifouling agents. It will also offer different prospects of optimizations and analysis to scale up their industrial manufacturing for potential usage as antifouling coatings and antimicrobial and therapeutic agents.

Strategies to Mitigate Biofouling of Nanocomposite Polymer-Based Membranes in Contact with Blood

An extracorporeal blood purification method called continuous renal replacement therapy uses a porous hollow-fiber polymeric membrane that is exposed to prolonged contact with blood. In that condition, like with any other submerged filtration membrane, the hemofilter loses its properties over time and use resulting in a rapid decline in flux. The most significant reason for this loss is the formation of a biofilm. Protein, blood cells and bacterial cells attach to the membrane surface in complex and fluctuating processes. Anticoagulation allows for longer patency of vascular access and a longer lifespan of the membrane. Other preventive measures include the modification of the membrane itself. In this article, we focused on the role of nanoadditives in the mitigation of biofouling. Nanoparticles such as graphene, carbon nanotubes, and silica effectively change surface properties towards more hydrophilic, affect pore size and distribution, decrease protein adsorption and damage bacteria cells. As a result, membranes modified with nanoparticles show better flow parameters, longer lifespan and increased hemocompatibility.

Development of a dynamic model for effective mitigation of membrane fouling through biogas sparging in submerged anaerobic membrane bioreactors (SAnMBRs)

The deterministic mechanistic model derived from the fundamental of the dynamical fouling system was investigated to estimate fouling parameters, with theoretical biogas sparging performance evaluated of a Submerged Anaerobic Membrane Bioreactor treating trade wastewater. The result showed that the sparging effectiveness of EPSc removal was average, 35% higher than the sparging effectiveness of EPSp, with the coefficient of fouling removal characterizing the dynamic time behaviour increasing with the organic loading rate. The dynamic system analysis predicted that the process gain for SAnMBR-1 was more than 30% compared with SAnMBR-2, which supported a widely known theory of fouling dependence of organic loading rate.

An increase in sludge loading rate induces gel fouling in membrane bioreactors treating real sewage

The main objective of this study was to investigate the cause of gel fouling in membrane bioreactors (MBRs) treating real sewage in terms of soluble microbial products (SMPs) and microbial aspects. Two anoxic/oxic-MBRs were operated as the control reactor (S1) and the sludge loading rate increased reactor (S2). The reactors were operated under low-temperature around 11 °C conditions. Membrane permeability substantially decreased in S2, and gel layer biofilm was formed on membrane surface. In contrast, the permeability of S1 gradually decreased and cake layer formed. When gel fouling occurred, the protein and polysaccharide of SMP in S2 were 47 and 23 mg L^-1, which were significantly lower than those recorded in S1 accounted for 118 and 68 mg L^-1, respectively. Furthermore, the total organic carbon concentration of SMPs was 24 mg L^-1, which was lower than the influent in S2, accounted for 62 mg L^-1. Finally, Campylobacteraceae which exists in sewage and uncultured OD1, dominated the gel layer biofilm in S2, unlike the cake layer biofilm in S1. These results indicated that the gel layer biofilm might be composed of influent substances, demonstrating the importance of influent decomposition in MBR for gel fouling mitigation.

Permeation Increases Biofilm Development in Nanofiltration Membranes Operated with Varying Feed Water Phosphorous Concentrations

Nutrient limitation has been proposed as a biofouling control strategy for membrane systems. However, the impact of permeation on biofilm development under phosphorus-limited and enriched conditions is poorly understood. This study analyzed biofilm development in membrane fouling simulators (MFSs) with and without permeation supplied with water varying dosed phosphorus concentrations (0 and 25 μg P·L−1). The MFSs operated under permeation conditions were run at a constant flux of 15.6 L·m2·h−1 for 4.7 days. Feed channel pressure drop, transmembrane pressure, and flux were used as performance indicators. Optical coherence tomography (OCT) images and biomass quantification were used to analyze the developed biofilms. The total phosphorus concentration that accumulated on the membrane and spacer was quantified by using microwave digestion and inductively coupled plasma atomic emission spectroscopy (ICP-OES). Results show that permeation impacts biofilm development depending on nutrient condition with a stronger impact at low P concentration (pressure drop increase: 282%; flux decline: 11%) compared to a higher P condition (pressure drop increase: 206%; flux decline: 2%). The biofilm that developed at 0 μg P·L−1 under permeation conditions resulted in a higher performance decline due to biofilm localization and spread in the MFS. A thicker biofilm developed on the membrane for biofilms grown at 0 μg P·L−1 under permeation conditions, causing a stronger effect on flux decline (11%) compared to non-permeation conditions (5%). The difference in the biofilm thickness on the membrane was attributed to a higher phosphorus concentration in the membrane biofilm under permeation conditions. Permeation has an impact on biofilm development and, therefore, should not be excluded in biofouling studies.

Biocatalytic membranes for combating the challenges of membrane fouling and micropollutants in water purification: A review

Over the last few years, the list of water contaminants has grown tremendously due to many anthropogenic activities. Various conventional technologies are available for water and wastewater treatment. However, micropollutants of emerging concern (MEC) are posing a great threat due to their activity at trace concentration and poor removal efficiency by the conventional treatment processes. Advanced technology like membrane technology can remove MEC to some extent. However, issues like the different chemical properties of MEC, selectivity, and fouling of membranes can affect the removal efficiency. Moreover, the concentrate from the membrane filtration may need further treatment. Enzymatic degradation of pollutants and foulants is one of the green approaches for removing various contaminants from the water as well as mitigating membrane fouling. Biocatalytic membranes (BCMs), in which enzymes are immobilized on membranes, combines the advantages of membrane separation and enzymatic degradation. This review article discussed various commonly used enzymes in BCMs for removing MEC and fouling. The majorly used enzymes were oxidoreductases and hydrolases for removing MEC, antifouling, and self-cleaning ability. The various BCM synthesis processes based on entrapment, crosslinking, and binding have been summarized, along with the effects of the addition of the nanoparticles on the performances of the BCMs. The scale-up, commercial viability, challenges, and future direction for improving BCMs have been discussed and shown bright possibilities for these new generation membranes.

Fabrication of conductive ceramic membranes for electrically assisted fouling control during membrane filtration for wastewater treatment

Membrane technology is widely used in water and wastewater treatment. However, membrane fouling remains one of the biggest challenges for membrane applications. In this study, an electrically assisted technique was developed for the control of fouling on flat-sheet ceramic membranes. The novel conductive membrane was fabricated by coating dopamine and carbon nanotubes (CNTs) onto the surface of an α-alumina membrane support to form a conductive CNT coating. The resulting flat-sheet conductive ceramic membrane (FSCCM) exhibited excellent electric conductivity and stability, which performed well in filtration of the synthetic wastewater containing inorganic matter (kaolin solution) or organic pollutants (oil emulsion). By applying a negative charge on the FSCCM with a DC voltage of 2.0 V, the membrane fouling rate was reduced by approximately 50%. The energy consumption rate for the electrically assisted membrane fouling control was only 22.2 × 10^-3 kWh/m³ in paused-charge mode, with a pause duration of 15 s. A fouling-layer analysis indicted that the imposed electric field greatly reduced the amount of strongly attached foulants on the membrane surface and in the membrane pores. It is believed that the electric field exerted an electrostatic force on the negatively charged pollutants, such as particles and oil droplets, which prevented the foulants from attaching to the membrane surface. This FSCCM-based method provides a clean, effective, and energy-efficient technique for membrane fouling control, thereby enabling high-rate membrane filtration.

Permanent Antimicrobial Poly(vinylidene fluoride) Prepared by Chemical Bonding with Poly(hexamethylene guanidine)

Biofouling is one of the major obstacles in the application of poly(vinylidene fluoride) (PVDF) membrane in water and wastewater treatment. Developing antimicrobial PVDF could kill the attached microbe in the initial stage, thus theoretically inhibiting the formation of biofilm and delaying the occurrence of biofouling. However, the leaching of the antimicrobial component and deterioration of antimicrobial properties remain a concern. In this work, an antimicrobial PVDF (PVDF-g-AGE-PHMG) was developed by chemical bonding PVDF with poly(hexamethylene guanidine hydrochloride) (PHMG). The obtained PVDF-g-AGE-PHMG was blended with pristine PVDF to prepare an antimicrobial PVDF membrane. The results of Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) confirmed that PHMG was successfully grafted into the PVDF membrane. The morphologies, membrane porosity, water contact angles, antimicrobial properties, mechanical properties, and thermostability of the as-prepared membranes were investigated. When the content of PVDF-g-AGE-PHMG reached 10.0 wt %, the inhibition rates of both antimicrobial PVDF membrane against Escherichia coli and Staphylococcus aureus were above 99.99%. Due to the increased hydrophilicity, excellent antimicrobial activity, nonleaching of antimicrobial component, good mechanical properties, and thermostability, the as-prepared PVDF membrane has promising applications in the field of water treatment.

Biodiversity and ecology of microorganisms in high pressure membrane filtration systems

High-pressure membrane filtration (reverse osmosis and nanofiltration) is used to purify different water sources, including wastewater, surface water, groundwater and seawater. A major concern in membrane filtration is the accumulation and growth of micro-organisms and their secreted polymeric substances, leading to reduced membrane performance and membrane biofouling. The fundamental understanding of membrane biofouling is limited despite years of research, as the means of microbial interactions and response to the conditions on the membrane surface are complicated. Here, we discuss studies that investigated the microbial diversity of fouled high-pressure membranes. High-throughput amplicon sequencing of the 16S rRNA gene have shown that Burkholderiales, Pseudomonadales, Rhizobiales, Sphingomonadales and Xanthomonadales frequently obtain a high relative abundance on fouled membranes. The bacterial communities present in the diverse feed water types and in pre-treatment compartments are different from the communities on the membrane, because high-pressure membrane filtration provides a selective environment for certain bacterial groups. The biofilms that form within the pre-treatment compartments do not commonly serve as an inoculum for the subsequent high-pressure membranes. Besides bacteria also fungi are detected in the water treatment compartments. In contrast to bacteria, the fungal community does not change much throughout membrane cleaning. The stable fungal diversity indicates that they are more significant in membrane biofouling than previously thought. By reviewing the biodiversity and ecology of microbes in the whole high pressure membrane filtration water chain, we have been able to identify potentials to improve biofouling control. These include modulation of hydrodynamic conditions, nutrient limitation and the combination of cleaning agents to target the entire membrane microbiome.

One-Step Electrochemically Prepared Graphene/Polyaniline Conductive Filter Membrane for Permeation Enhancement by Fouling Mitigation

In the electrofiltration process, membrane conductivity plays a decisive role in improving the antifouling performance of the membrane. In this paper, combining the preparation of graphene (Gr) with the fabrication of the Gr layer on the surface of a polyaniline (PANI) membrane, a graphene/PANI (Gr/PANI) conductive membrane was prepared creatively by the one-step electrochemical method. The properties of the as-prepared Gr/PANI membrane were studied systematically. By the tests of Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction, and atomic force microscopy, it was confirmed that Gr was successfully produced and was combined with the PANI membrane well. Field scanning electron microscopy with energy-dispersive X-ray analysis further confirmed that the top surface and the upper layer pore walls of the membrane were randomly covered by Gr. The antifouling performance of the prepared membrane was evaluated by studying the permeation flux of the yeast suspension, compared with the ones with no electric field: the total permeation flux at 1 V direct current (dc) increased by 109%; besides, under 1 V dc, the average flux of the Gr/PANI membrane was approximately 1.4 times that of the PANI membrane. This approach may provide a promising strategy for the combination of Gr with conductive polymers to produce separation membranes.

Insignificant Impact of Chemotactic Responses of Pseudomonas aeruginosa on the Bacterial Attachment to Organic Pre-Conditioned RO Membranes

We investigated the impact of conditioning compositions on the way bacteria move and adhere to reverse osmosis (RO) membranes that have been pre-conditioned by organic compounds. We used humic acid (HA), bovine serum albumin (BSA), and sodium alginate (SA) to simulate conditioning layers on the RO membranes. First, we investigated the chemotactic responses of Pseudomonas aeruginosa PAO1 to the organic substances and the impact of changes in physicochemical characteristics of pre-conditioned membranes on bacterial attachment. Second, we observed bacterial attachment under the presence or absence of nutrients or microbial metabolic activity. Results showed that there was no relationship between the chemotactic response of P. aeruginosa PAO1 and the organic substances, and the changes in hydrophobicity, surface free energy, and surface charge resulting from changing the composition of the conditioning layer did not seem to affect bacterial attachment, whereas changing the roughness of the conditioned membrane exponentially did (exponential correlation coefficient, R² = 0.85). We found that the initial bacterial attachment on the membrane surface is influenced by (i) the nutrients in the feed solution and (ii) the microbial metabolic activity, whereas the chemotaxis response has a negligible impact. This study would help to establish a suitable strategy to manage bacterial attachment.

Isolation and characterization of Sphingomonadaceae from fouled membranes

Membrane filtration systems are widely applied for the production of clean drinking water. However, the accumulation of particles on synthetic membranes leads to fouling. Biological fouling (i.e., biofouling) of reverse osmosis and nanofiltration membranes is difficult to control by existing cleaning procedures. Improved strategies are therefore needed. The bacterial diversity on fouled membranes has been studied, especially to identify bacteria with specialized functions and to develop targeted approaches against these microbes. Previous studies have shown that Sphingomonadaceae are initial membrane colonizers that remain dominant while the biofilm develops. Here, we characterized 21 Sphingomonadaceae isolates, obtained from six different fouled membranes, to determine which physiological traits could contribute to colonization of membrane surfaces. Their growth conditions ranged from temperatures between 8 and 42 oC, salinity between 0.0 and 5.0% w/v NaCl, pH from 4 and 10, and all isolates were able to metabolize a wide range of substrates. The results presented here show that Sphingomonadaceae membrane isolates share many features that are uncommon for other members of the Sphingomonadaceae family: all membrane isolates are motile and their tolerance for different temperatures, salt concentrations, and pH is high. Although relative abundance is an indicator of fitness for a whole group, for the Sphingomonadaceae it does not reveal the specific physiological traits that are required for membrane colonization. This study, therefore, adds to more fundamental insights in membrane biofouling.

Low voltage electric potential as a driving force to hinder biofouling in self-supporting carbon nanotube membranes

This study aimed at evaluating the contribution of low voltage electric field, both alternating (AC) and direct (DC) currents, on the prevention of bacterial attachment and cell inactivation to highly electrically conductive self-supporting carbon nanotubes (CNT) membranes at conditions which encourage biofilm formation. A mutant strain of Pseudomonas putida S12 was used a model bacterium and either capacitive or resistive electrical circuits and two flow regimes, flow-through and cross-flow filtration, were studied. Major emphasis was placed on AC due to its ability of repulsing and inactivating bacteria. AC voltage at 1.5 V, 1 kHz frequency and wave pulse above offset (+0.45) with 100Ω external resistance on the ground side prevented almost completely attachment of bacteria (>98.5%) with concomitant high inactivation (95.3 ± 2.5%) in flow-through regime. AC resulted more effective than DC, both in terms of biofouling reduction compared to cathodic DC and in terms of cell inactivation compared to anodic DC. Although similar trends were observed, a net reduced extent of prevention of bacterial attachment and inactivation was observed in filtration as compared to flow-through regime, which is mainly attributed to the permeate drag force, also supported by theoretical calculations in DC in capacitive mode. Electrochemical impedance spectroscopy analysis suggests a pure resistor behavior in resistance mode compared to involvement of redox reactions in capacitance mode, as source for bacteria detachment and inactivation. Although further optimization is required, electrically polarized CNT membranes offer a viable antibiofouling strategy to hinder biofouling and simplify membrane care during filtration.

Electroconductive and electroresponsive membranes for water treatment

In populated, water-scarce regions, seawater and wastewater are considered as potable water resources that require extensive treatment before being suitable for consumption. The separation of water from salt, organic, and inorganic matter is most commonly done through membrane separation processes. Because of permeate flux and concentration polarization, membranes are prone to fouling, resulting in a decline in membrane performance and increased energy demands. As the physical and chemical properties of commercially available membranes (polymeric and ceramic) are relatively static and insensitive to changes in the environment, there is a need for stimuli-reactive membranes with controlled, tunable surface and transport properties to decrease fouling and control membrane properties such as hydrophilicity and permselectivity. In this review, we first describe the application of electricity-conducting and electricity-responsive membranes (ERMs) for fouling mitigation. We discuss their ability to reduce organic, inorganic, and biological fouling by several mechanisms, including control over the membrane’s surface morphology, electrostatic rejection, piezoelectric vibrations, electrochemical reactions, and local pH changes. Next, we examine the use of ERMs for permselectivity modification, which allows for the optimization of rejection and control over ion transport through the application of electrical potentials and the use of electrostatically charged membrane surfaces. In addition, electrochemical reactions coupled with membrane filtration are examined, including electro-oxidation and electro-Fenton reactions, demonstrating the capability of ERMs to electro-oxidize organic contaminates with high efficiency due to high surface area and reduced mass diffusion limitations. When applicable, ERM applications are compared with commercial membranes in terms of energy consumptions. We conclude with a brief discussion regarding the future directions of ERMs and provide examples of several applications such as pore size and selectivity control, electrowettability, and capacitive deionization. To provide the reader with the current state of knowledge, the review focuses on research published in the last 5 years.

Microbial Attachment Inhibition through Low-Voltage Electrochemical Reactions on Electrically Conducting Membranes

Bacterial biofilm formation on membrane surfaces remains a serious challenge in water treatment systems. The impact of low voltages on microbial attachment to electrically conducting ultrafiltration membranes was investigated using a direct observation cross-flow membrane system mounted on a fluorescence microscope. Escherichia coli and microparticle deposition and detachment rates were measured as a function of the applied electrical potential to the membrane surface. Selecting bacteria and particles with low surface charge minimized electrostatic interactions between the bacteria and charged membrane surface. Application of an electrical potential had a significant impact on the detachment of live bacteria in comparison to dead bacteria and particles. Image analysis indicated that when a potential of 1.5 V was applied to the membrane/counter electrode pair, the percent of dead bacteria was 32±2.1 and 67±3.6% when the membrane was used as a cathode or anode, respectively, while at a potential of 1 V, 92±2.4% were alive. The application of low electrical potentials resulted in the production of low (μM) concentrations of hydrogen peroxide (HP) through the electroreduction of oxygen. The electrochemically produced HP reduced microbial cell viability and increased cellular permeability. Exposure to low concentrations of electrochemically produced HP on the membrane surface prevents bacterial attachment, thus ensuring biofilm-free conditions during membrane filtration operations.

Impacts of hydrophilic colanic acid on bacterial attachment to microfiltration membranes and subsequent membrane biofouling

In order to examine the interactions between physicochemical properties of specific extracellular polymeric substances (EPS) and membrane biofouling, we investigated the impacts of hydrophilic colanic acid, as a model extracellular polysaccharide component, on initial bacterial attachment to different microfiltration (MF) membranes and membrane biofouling by using Escherichia coli strains producing different amounts of colanic acid. In a newly designed microtiter plate assay, the bacterial attachment by an E. coli strain RcsF(+), which produces massive amounts of colanic acid, decreased only to a hydrophobic membrane because the colanic acid made cell surfaces more hydrophilic, resulting in low cell attachment to hydrophobic membranes. The bench-scale cross-flow filtration tests followed by filtration resistance measurement revealed that RcsF(+) caused severe irreversible membrane fouling (i.e., pore-clogging), whereas less extracellular polysaccharide-producing strains caused moderate but reversible fouling to all membranes used in this study. Further cross-flow filtration tests indicated that colanic acid liberated in the bulk phase could rapidly penetrate pre-accumulated biomass layers (i.e., biofilms) and then directly clogged membrane pores. These results indicate that colanic acid, a hydrophilic extracellular polysaccharide, and possible polysaccharides with similar characteristics with colanic acid are considered as a major cause of severe irreversible membrane fouling (i.e., pore-clogging) regardless of biofilm formation (dynamic membrane).

Spacer geometry and particle deposition in spiral wound membrane feed channels

Deposition of microspheres mimicking bacterial cells was studied experimentally and with a numerical model in feed spacer membrane channels, as used in spiral wound nanofiltration (NF) and reverse osmosis (RO) membrane systems. In-situ microscopic observations in membrane fouling simulators revealed formation of specific particle deposition patterns for different diamond and ladder feed spacer orientations. A three-dimensional numerical model combining fluid flow with a Lagrangian approach for particle trajectory calculations could describe very well the in-situ observations on particle deposition in flow cells. Feed spacer geometry, positioning and cross-flow velocity sensitively influenced the particle transport and deposition patterns. The deposition patterns were not influenced by permeate production. This combined experimental-modeling approach could be used for feed spacer geometry optimization studies for reduced (bio)fouling.

Bacterial adhesion onto nanofiltration and reverse osmosis membranes: effect of permeate flux

The influence of permeate flux on bacterial adhesion to NF and RO membranes was examined using two model Pseudomonas species, namely Pseudomonas fluorescens and Pseudomonas putida. To better understand the initial biofouling profile during NF/RO processes, deposition experiments were conducted in cross flow under permeate flux varying from 0.5 up to 120 L/(h m(2)), using six NF and RO membranes each having different surface properties. All experiments were performed at a Reynolds number of 579. Complementary adhesion experiments were performed using Pseudomonas cells grown to early-, mid- and late-exponential growth phases to evaluate the effect of bacterial cell surface properties during cell adhesion under permeate flux conditions. Results from this study show that initial bacterial adhesion is strongly dependent on the permeate flux conditions, where increased adhesion was obtained with increased permeate flux, until a maximum of 40% coverage was reached. Membrane surface properties or bacterial growth stages was further found to have little impact on bacterial adhesion to NF and RO membrane surfaces under the conditions tested. These results emphasise the importance of conducting adhesion and biofouling experiments under realistic permeate flux conditions, and raises questions about the efficacy of the methods for the evaluation of antifouling membranes in which bacterial adhesion is commonly assessed under zero-flux or low flux conditions, unrepresentative of full-scale NF/RO processes.

Impact of ZnO embedded feed spacer on biofilm development in membrane systems

The concept of suppressing biofouling formation using an antibacterial feed spacer was investigated in a bench scale-cross flow system mimicking a spiral wound membrane configuration. An antibacterial composite spacer containing zinc oxide-nanoparticles was constructed by modification of a commercial polypropylene feed spacer using sonochemical deposition. The ability of the modified spacers to repress biofilm development on membranes was evaluated in flow-through cells simulating the flow conditions in commercial spiral wound modules. The experiments were performed at laminar flow (Re = 300) with a 200 kDa molecular weight cut off polysulfone ultrafiltration membrane using Pseudomonas putida S-12 as model biofilm bacteria. The modified spacers reduced permeate flux decrease at least by 50% compared to the unmodified spacers (control). The physical properties of the modified spacer and biofilm development were evaluated using high resolution/energy dispersive spectrometry-scanning electron microscopy, atomic force microscopy and confocal laser scanning microscopy imaging (HRSEM, EDS, AFM and CLSM). HRSEM images depicted significantly less bacteria attached to the membranes exposed to the modified spacer, mainly scattered and in a sporadic monolayer structure. AFM analysis indicated the influence of the modification on the spacer surface including a phase change on the upper surface. Dead-live staining assay by CLSM indicated that most of the bacterial cells attached on the membranes exposed to the modified spacer were dead in contrast to a developed biofilm which was predominant in the control samples.

Enhanced biotransformation of fluoranthene by intertidally derived Cunninghamella elegans under biofilm-based and niche-mimicking conditions

The aims of the investigation were to ascertain if surface attachment of Cunninghamella elegans and niche intertidal conditions provided in a bioreactor influenced biotransformation of fluoranthene by C. elegans. A newly designed polymethylmethacrylate (PMMA) conico-cylindrical flask (CCF) holding eight equidistantly spaced rectangular strips mounted radially on a circular disc allowed comparison of fluoranthene biotransformation between CCFs with a hydrophobic surface (PMMA-CCF) and a hydrophilic glass surface (GS-CCF) and a 500-ml Erlenmeyer flask (EF). Fluoranthene biotransformation was higher by 22-fold, biofilm growth was higher by 3-fold, and cytochrome P450 gene expression was higher by 2.1-fold when C. elegans was cultivated with 2% inoculum as biofilm culture in PMMA-CCF compared to planktonic culture in EF. Biotransformation was enhanced by 7-fold with 10% inoculum. The temporal pattern of biofilm progression based on three-channel fluorescence detection by confocal laser scanning microscopy demonstrated well-developed, stable biofilm with greater colocalization of fluoranthene within extracellular polymeric substances and filaments of the biofilm grown on PMMA in contrast to a glass surface. A bioreactor with discs rotating at 2 revolutions per day affording 6-hourly emersion and immersion mimicked the niche intertidal habitat of C. elegans and supported biofilm formation and transformation of fluoranthene. The amount of transformed metabolite was 3.5-fold, biofilm growth was 3-fold, and cytochrome P450 gene expression was 1.9-fold higher in the process mimicking the intertidal conditions than in a submerged process without disc rotation. In the CCF and reactor, where biofilm formation was comparatively greater, higher concentration of exopolysaccharides allowed increased mobilization of fluoranthene within the biofilm with consequential higher gene expression leading to enhanced volumetric productivity.

Microbial adhesion and biofilm formation on microfiltration membranes: a detailed characterization using model organisms with increasing complexity

Since many years, membrane biofouling has been described as the Achilles heel of membrane fouling. In the present study, an ecological assay was performed using model systems with increasing complexity: a monospecies assay using Pseudomonas aeruginosa or Escherichia coli separately, a duospecies assay using both microorganisms, and a multispecies assay using activated sludge with or without spiked P. aeruginosa. The microbial adhesion and biofilm formation were evaluated in terms of bacterial cell densities, species richness, and bacterial community composition on polyvinyldifluoride, polyethylene, and polysulfone membranes. The data show that biofouling formation was strongly influenced by the kind of microorganism, the interactions between the organisms, and the changes in environmental conditions whereas the membrane effect was less important. The findings obtained in this study suggest that more knowledge in species composition and microbial interactions is needed in order to understand the complex biofouling process. This is the first report describing the microbial interactions with a membrane during the biofouling development.

The influence of antiscalants on biofouling of RO membranes in seawater desalination

Antiscalants are surface active polyelectrolyte compounds commonly used in reverse osmosis (RO) desalination processes to avoid membrane scaling. In spite of the significant roles of antiscalants in preventing membrane scaling, they are prone to enhance biofilm growth on RO membranes by either altering membrane surface properties or by serving as nutritional source for microorganisms. In this study, the contribution of antiscalants to membrane biofouling in seawater desalination was investigated. The effects of two commonly used antiscalants, polyphosphonate- and polyacrylate-based, were tested. The effects of RO membrane (DOW-Filmtec SW30 HRLE-400) exposure to antiscalants on its physico-chemical properties were studied, including the consequent effects on initial deposition and growth of the sessile microorganisms on the RO membrane surface. The effects of antiscalants on membrane physico-chemical properties were investigated by filtration of seawater supplemented with the antiscalants through flat-sheet RO membrane and changes in surface zeta potential and hydrophobicity were delineated. Adsorption of antiscalants to polyamide surfaces simulating RO membrane's polyamide layer and their effects on the consequent bacterial adhesion was tested using a quartz crystal microbalance with dissipation monitoring technology (QCM-D) and direct fluorescent microscopy. A significant increase in biofilm formation rate on RO membranes surface was observed in the presence of both types of antiscalants. Polyacrylate-based antiscalant was shown to enhance initial cell attachment as observed with the QCM-D and a parallel plate flow cell, due to rendering the polyamide surface more hydrophobic. Polyphosphonate-based antiscalants also increased biofilm formation rate, most likely by serving as an additional source of phosphorous to the seawater microbial population. A thicker biofilm layer was formed on the RO membrane when the polyacrylate-based antiscalant was used. Following these results, a wise selection of antiscalants for scaling control should take into account their contribution to membrane biofouling propensity.

Effect of antimicrobial residues on early adhesion and biofilm formation by wild-type and benzalkonium chloride-adapted Pseudomonas aeruginosa

Antimicrobial residue deposition can change the physico-chemical properties of bacteria and surfaces and thus promote or impair bacterial adhesion. This study focuses on benzalkonium chloride (BC) deposition on polystyrene (PS) surfaces and the influence of this conditioning film on the physico-chemical properties of PS and on early adhesion and biofilm formation by Pseudomonas aeruginosa wild-type and its laboratory BC-adapted strain. The latter readily acquired the ability to grow in BC, and also exhibited physico-chemical surface changes. The existence of residues on PS surfaces altered their hydrophobicity and favoured adhesion as determined by the free energy and early adhesion characterization. Adapted bacteria revealed a higher ability to adhere to surfaces and to develop biofilms, especially on BC-conditioned surfaces, which thereby could enhance resistance to sanitation attempts. These findings highlight the importance of investigations concerning the antimicrobial deposition effect after cleaning procedures, which may encourage bacterial adhesion, especially of bacteria that have been previously exposed to chemical stresses.

Bacterial attachment to RO membranes surface-modified by concentration-polarization-enhanced graft polymerization

Concentration polarization-enhanced radical graft polymerization, a facile surface modification technique, was examined as an approach to reduce bacterial deposition onto RO membranes and thus contribute to mitigation of biofouling. For this purpose an RO membrane ESPA-1 was surface-grafted with a zwitterionic and negatively and positively charged monomers. The low monomer concentrations and low degrees of grafting employed in modifications moderately reduced flux (by 20-40%) and did not affect salt rejection, yet produced substantial changes in surface chemistry, charge and hydrophilicity. The propensity to bacterial attachment of original and modified membranes was assessed using bacterial deposition tests carried out in a parallel plate flow setup using a fluorescent strain of Pseudomonas fluorescens. Compared to unmodified ESPA-1 the deposition (mass transfer) coefficient was significantly increased for modification with the positively charged monomer. On the other hand, a substantial reduction in bacterial deposition rates was observed for membranes modified with zwitterionic monomer and, still more, with very hydrophilic negatively charged monomers. This trend is well explained by the effects of surface charge (as measured by ζ-potential) and hydrophilicity (contact angle). It also well correlated with force distance measurements by AFM using surrogate spherical probes with a negative surface charge mimicking the bacterial surface. The positively charged surface showed a strong hysteresis with a large adhesion force, which was weaker for unmodified ESPA-1 and still weaker for zwitterionic surface, while negatively charged surface showed a long-range repulsion and negligible hysteresis. These results demonstrate the potential of using the proposed surface- modification approach for varying surface characteristics, charge and hydrophilicity, and thus minimizing bacterial deposition and potentially reducing propensity biofouling.

Effect of conventional chemical treatment on the microbial population in a biofouling layer of reverse osmosis systems

The impact of conventional chemical treatment on initiation and spatiotemporal development of biofilms on reverse osmosis (RO) membranes was investigated in situ using flow cells placed in parallel with the RO system of a full-scale water treatment plant. The flow cells got the same feed (extensively pre-treated fresh surface water) and operational conditions (temperature, pressure and membrane flux) as the full-scale installation. With regular intervals both the full-scale RO membrane modules and the flow cells were cleaned using conventional chemical treatment. For comparison some flow cells were not cleaned. Sampling was done at different time periods of flow cell operation (i.e., 1, 5, 10 and 17 days and 1, 3, 6 and 12 months). The combination of molecular (FISH, DGGE, clone libraries and sequencing) and microscopic (field emission scanning electron, epifluorescence and confocal laser scanning microscopy) techniques made it possible to thoroughly analyze the abundance, composition and 3D architecture of the emerged microbial layers. The results suggest that chemical treatment facilitates initiation and subsequent maturation of biofilm structures on the RO membrane and feed-side spacer surfaces. Biofouling control might be possible only if the cleaning procedures are adapted to effectively remove the (dead) biomass from the RO modules after chemical treatment.