Factors affecting biofilm formation and biofouling in membrane distillation of seawater

A review of experimental Assessment Processes of material resistance to marine and freshwater biofouling

Biofouling presents hazards to a variety of freshwater and marine underwater infrastructures and is one of the direct causes of species invasion. These negative impacts provide a unified goal for both industry practitioners and researchers: the development of novel antifouling materials to prevent the adhesion of biofouling. The prohibition of tributyltin (TBT) by the International Maritime Organization (IMO) in 2001 propelled the research and development of new antifouling materials. However, the evaluation process and framework for these materials remain incomplete and unsystematic. This mini-review starts with the classification and principles of new antifouling materials, discussing and summarizing the methods for assessing their biofouling resistance. The paper also compiles the relevant regulations and environmental requirements from different countries necessary for developing new antifouling materials with commercial potential. It concludes by highlighting the current challenges in antifouling material development and future outlooks. Systematic evaluation of newly developed antifouling materials can lead to the emergence of more genuinely applicable solutions, transitioning from merely laboratory products to materials that can be effectively used in real-world applications.

Scientific Investigation of Antifouling Activity from Biological Agents and Distribution of Marine Foulers-Coastal Areas of Tamil Nadu

Biofouling is the result of a biological process that is the accumulation of micro- and macro-organisms on the surfaces of the ship which causes serious environmental problems. The consequence of biofouling includes modifying the hydrodynamic response, affecting heat exchange, can make structures heavier, accelerate or generating corrosion, biodegradation, increasing the fatigue of certain materials, and blocking mechanical functions. It causes severe problems for the objects in the water such as ships and buoys. Also, its impact on shellfish and other aquaculture was sometimes devastating. The main scope of this study is to review the currently available biocides from biological agents for marine submerged foulants and marine foulers that are present around the coastal areas of Tamil Nadu. Biological anti-fouling methods are preferred than that of the chemical and physical anti-fouling methods as it have some toxic effects on the non targeted marine biodiversity. This study focuses on the marine foulers that are present around the coastal areas of Tamil Nadu which will be helpful for the researchers to discover the suitable anti-foulers from a biological source, which will be very useful to protect the marine ecosystem and marine economy. A total of 182 antifouling compounds from marine biological sources were discovered. The marine microbes, Penicillium sp. and Pseudoalteromonas issachenkonii, were reported to possess EC50. The survey results obtained from this study show that Chennai coastal region has a lot of barnacles, and 8 different species were present in Pondicherry region.

Modification of the distribution of humic acid complexations by introducing microbubbles to membrane distillation process for effective membrane fouling alleviation

Membrane fouling caused by inorganic ions and natural organic matters (NOMs) has been a severe issue in membrane distillation. Microbubble aeration (MB) is a promising technology to control membrane fouling. In this study, MB aeration was introduced to alleviate humic acid (HA) composited fouling during the treatment of simulative reverse osmosis concentrate (ROC) by vacuum membrane distillation (VMD). The objective of this work was to explore the HA fouling inhibiting effect by MB aeration and discuss its mechanism from the interfacial point of view. The results showed that VMD was effective for treating ROC, followed by a severe membrane fouling aggravated with the addition of 100 mg/L HA in feed solution, resulting in 45.7% decline of membrane flux. Analysis using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and zeta potential distribution of charged particles proved the coexistence of HA and inorganic cations (especially Ca2+), resulting in more serious membrane fouling. The introduction of MB aeration exhibited excellent alleviating effect on HA-inorganic salt fouling, with the normalized flux increased from 19.7% to 37.0%. The interfacial properties of MBs played an important role, which altered the zeta potential distributions of charged particles in HA solution, indicating that MBs adhere the HA complexations. Furthermore, this mitigating effect was limited at high inorganic cations concentration. Overall, MBs could change the potential characteristics of HA complexes, which also be used for other similar membrane fouling alleviation.

Temporal variations of biofouling assemblages of a coral reef ecosystem during a monsoon period

Coral reefs are highly biodiverse ecosystems, enriched by a range of biofouling species. Temporal variations in biofouling can affect ecosystem stability, but these diverse coral-associated communities remain underexplored in some regions. In the present study, biofouling assemblages of coral reefs in the Chabahar Bay were investigated during a summer monsoon at three deployment periods. In total, 26 taxa were identified with barnacles and polychaetes being the dominant taxa during the whole study. The coverage percentage was driven mostly by the encrusting taxa such as bryozoans and algae while biomass was determined by the dominance of shell-forming taxa. The results of PERMANOVA showed that the effects of the submersion period were significant on the assemblage structure. Biofouling assessment plays a pivotal role in safeguarding the intricate balance and long-term health of coral reef ecosystems. For a comprehensive understanding of biofouling dynamics and interactions with coral-associated species, conducting long-term studies is vital.

Carbon nanomaterials for designing next-generation membranes and their emerging applications

Carbon nanomaterials have enormous applications in various fields, such as adsorption, membrane separation, catalysis, electronics, capacitors, batteries, and medical sciences. Owing to their exceptional properties, such as large specific surface area, carrier mobility, flexibility, electrical conductivity, and optical pellucidity, the family of carbon nanomaterials is considered as one of the most studied group of materials to date. They are abundantly used in membrane science for multiple applications, such as the separation of organics, enantiomeric separation, gas separation, biomolecule separation, heavy metal separation, and wastewater treatment. This study provides an overview of the significant studies on carbon nanomaterial-based membranes and their emerging applications in our membrane research journey. The types of carbon nanomaterials, their utilization in membrane-based separations, and the mechanism involved are summarized in this study. Techniques for the fabrication of different nanocomposite membranes are also highlighted. Lastly, we have provided an overview of the existing issues and future scopes of carbon nanomaterial-based membranes for technological perspectives.

Thermal Effect on Algae, Biofilm and Their Composition Towards Membrane Distillation Unit: A Mini-review

Membrane distillation (MD) has lower operating temperature and potential to recycle waste heat for desalination which catches much attention of the researchers in the recent years. However, the biofouling is still a challenging hurdle to be overcome for such applications. The microbial growth rate, secretion and biofilm formation are sensitive to heat. Membrane distillation is a thermally driven separation, so the increase of temperature in the seawater feed could influence the extent of biofouling on the unit parts. In this review, we present the effect of temperature on algal growth, the range of temperature the microbes, marine algae and planktons able to survive and the changes to those planktons once exceed the critical temperature. Thermal effect on the biofilm, its composition and properties are discussed as well, with association of the biofilm secreting microbes, but the study related to membrane distillation unit seems to be lacking and MD biofouling factors are not fully understood. Characterization of the algae, biofilm and EPS that govern biofouling are discussed. This information not only will help in designing future studies to fill up the knowledge gaps in biofouling of membrane distillation, but also to some extent, assist in pointing out possible fouling factors and predicting the degree of biofouling in the membrane distillation unit.

Progress on Improved Fouling Resistance-Nanofibrous Membrane for Membrane Distillation: A Mini-Review

Nanofibrous membranes for membrane distillation (MD) have demonstrated promising results in treating various water and wastewater streams. Significant progress has been made in recent decades because of the development of sophisticated membrane materials, such as superhydrophobic, omniphobic and Janus membranes. However, fouling and wetting remain crucial issues for long-term operation. This mini-review summarizes ideas as well as their limitations in understanding the fouling in membrane distillation, comprising organic, inorganic and biofouling. This review also provides progress in developing antifouling nanofibrous membranes for membrane distillation and ongoing modifications on nanofiber membranes for improved membrane distillation performance. Lastly, challenges and future ways to develop antifouling nanofiber membranes for MD application have been systematically elaborated. The present mini-review will interest scientists and engineers searching for the progress in MD development and its solutions to the MD fouling issues.

Direct Contact Membrane Distillation for Treatment of Mixed Wastewater of Humic Acid and Reactive Dye: Membrane Flux Decline and Fouling Analysis

The main waste stream from the textile industry is its wastewater with high color, organic matters, and other contaminants. This study aims to investigate the effect of humic acid in mixed wastewater of humic acid and reactive dye on the treatment performance and permeate flux of a direct contact membrane distillation (DCMD) system. In this research, feed temperature and humic acid concentration were the main input parameters for the analysis of DCMD system operation. The fouling resistances significantly increased with higher humic acid concentrations in the mixed wastewater. As compared with the DI water test, 23% of flux decline occurred when the humic concentration in the wastewater was increased up to 20 mg/L. After the DCMD treatment, the 25 ADMI residual color was detected in the permeate when the mixed wastewater contained 20 mg/L humic acid. The mathematical model, based on the Antione equation, was proposed to predict the membrane flux decline of the DCMD system. The reduced pore size of the cake layer by a dimensionless constant β from the Kelvin equation was also considered for the fouling calculation to describe the transport mechanism.

Biofouling control of membrane distillation for seawater desalination: Effect of air-backwash and chemical cleaning on biofouling formation

This study explored the applicability of chemical cleaning and air-backwash to alleviate biofouling on seawater membrane distillation (SWMD). Membrane performance and wettability properties maintained at optimum duration and frequency of the treatments, as indicated by low permeate conductivity throughout the tests. The cleaning of the membrane using 2% NaOH by immersing the membrane for 30 min after 240 min operation removed the biofouling layer, indicated by low permeate conductivity of 370 µScm^-1 after cleaning. However, more frequent membrane cleaning led to membrane damage, more severe wetting, and membrane hydrophobicity reduction. Ten-second air-backwash after 240 min of operation was also effective in controlling the biofouling, particularly when conducted at air pressure of 1 bar. More frequent air-backwash resulted in more aggravated inorganic fouling and accelerated biofouling formation due to the recurring introduction of air, leading to rapid membrane wetting.

Elucidating biofouling over thermal and spatial gradients in seawater membrane distillation in hot climatic conditions

Biofouling is a hurdle of seawater desalination that increases water costs and energy consumption. In membrane distillation (MD), biofouling development is complicated due to the temperature effect that adversely affects microbial growth. Given the high relevance of MD to regions with abundant warm seawater, it is essential to explore the biofouling propensity of microbial communities with higher tolerance to elevated temperature conditions. This study presents a comprehensive analysis of the spatial and temporal biofilm distribution and associated membrane fouling during direct contact MD (DCMD) of the Red Sea water. We found that structure and composition of the biofilm layer played a significant role in the extent of permeate flux decline, and biofilms that built up at 45°C had lower bacterial concentration but higher extracellular polymeric substances (EPS) content as compared to biofilms that formed at 55 °C and 65°C. Pore wetting and bacterial passage to the permeate side were initially observed but slowed down as operating time increased. Intact cells in biofilms dominated over the damaged cells at any tested condition emphasizing the high adaptivity of the Red Sea microbial communities to elevated feed temperatures. A comparison of microbial abundance revealed a difference in bacterial distribution between the feed and biofilm samples. A shift in the biofilm microbial community and colonization of the membrane surface with thermophilic bacteria with the feed temperature increase was observed. The results of this study improve our understanding of biofouling propensity in MD that utilizes temperature-resilient feed waters.

The advent of thermoplasmonic membrane distillation

Freshwater scarcity is a vital societal challenge related to climate change, population pressure, and agricultural and industrial demands. Therefore, sustainable desalination/purification of salty/contaminated water for human uses is particularly relevant. Membrane distillation is an emerging hybrid thermal-membrane technology with the potential to overcome the drawbacks of conventional desalination by a synergic exploitation of the water-energy nexus. Although membrane distillation is considered a green technology, efficient heat management remains a critical concern affecting the cost of the process and hindering its viability at large scale. A multidisciplinary approach that involves materials chemistry, physical chemistry, chemical engineering, and materials and polymer science is required to solve this problem. The combination of solar energy with membrane distillation is considered a potentially feasible low-cost approach for providing high-quality freshwater with a low carbon footprint. In particular, recent discoveries about efficient light-to-heat conversion in nanomaterials have opened unprecedented perspectives for the implementation of sunlight-based renewable energy in membrane distillation. The integration of nanofillers enabling photothermal effects into membranes has been demonstrated to be able to significantly enhance the energy efficiency without impacting on economic costs. Here, we provide a comprehensive overview on the state of the art, the opportunities, open challenges and pitfalls of the emerging field of solar-driven membrane distillation. We also assess the peculiar physicochemical properties and synthesis scalability of photothermal materials, as well as the strategies for their integration into polymeric nanocomposite membranes enabling efficient light-to-heat conversion and freshwater.

Seasonal shift of water quality in China Yangtze River and its impacts on membrane fouling development during the drinking water supply by membrane distillation system

Membrane distillation (MD) technique is increasingly regarded as a promising process for drinking water supply and wastewater treatment owing to its great water purification and usage of renewable energy. Like other membrane separation processes, the membrane fouling issue is widely considered as the main obstacle for real applications of large-scale MD systems. Feedwater characteristics, as the predominant factors for membrane fouling layer formation, mostly determined the membrane fouling trend of MD. Thus the impacts of seasonal shifts of initial feedwater quality on the MD membrane fouling were detailedly researched in this study, and the biofilm development mechanism was especially explored. The bacterial community structure of membrane biofilms was clearly clarified in MD runs of Yangtze River waters that collected in four seasons. The results revealed that the winter run posed a quite sharp flux drop, while a relatively milder flux decline behaviour was seen for other groups despite of the higher bacteria concentration of initial feedwaters. The poorer water quality in winter induced the establishment of a rather thick biofilm on the MD membrane, in which the biofilm-forming bacteria (Gammaproteobacteria and Alphaproteobacteria) and organic matters (EPS) were remarkably observed. Comparatively, a relatively thin biofilm containing abundant live cells and fewer organics finally formed in summer and autumn runs, causing a mitigated flux decline trend. Hence, it can be inferred that the membrane flux decline of MD was likely to be more sensitive to the organic attachment on the membrane in comparison with the bacteria adhesion. Finally, a three-phase pretreatment method was suggested for MD fouling control, including heating course, sterilization course, and filtration course.

Membrane Distillation: Recent Configurations, Membrane Surface Engineering, and Applications

Membrane distillation (MD) is a developing membrane separation technology for water treatment that involves a vapor transport driven by the vapor pressure gradient across the hydrophobic membrane. MD has gained wide attention in the last decade for various separation applications, including the separation of salts, toxic heavy metals, oil, and organic compounds from aqueous solutions. Compared with other conventional separation technologies such as reverse osmosis, nanofiltration, or thermal distillation, MD is very attractive due to mild operating conditions such as low temperature and atmospheric pressure, and 100% theoretical salt rejection. In this review, membrane distillation’s principles, recent MD configurations with their advantages and limitations, membrane materials, fabrication of membranes, and their surface engineering for enhanced hydrophobicity are reviewed. Moreover, different types of membrane fouling and their control methods are discussed. The various applications of standalone MD and hybrid MD configurations reported in the literature are detailed. Furthermore, studies on the MD-based pilot plants installed around the world are covered. The review also highlights challenges in MD performance and future directions.

Evaluation of tire derived rubber (TDR) fixed biofilm reactor (FBR) for remediation of Methylene blue dye from wastewater

The present investigation is focused on development of aerobic biofilm on tire-derived rubber (TDR) media and then evaluation of such system for bioremediation of Methylene blue (MB) dye for 9 weeks. After 9 weeks of operation, the COD, BOD, ammonia and color values have been declined by 89.2%, 98.3%, 99.61% and 99.81%, respectively, While SEM-EDX results showed a variance in weight percent of various elements in TDR without biofilm i.e. raw TDR media, as well as in the 1st and 9th-week samples. Moreover, fine and strong peaks were observed in both the MB simulated wastewater and 9th week TDR samples at 1190, 1300, 1400, 1450, 1500 and 1618 cm^-1 respectively by Raman Spectroscopic analysis. Further, FTIR analysis was performed for the MB simulated wastewater, and absorbance peaks ranging from 1591 to 1363 cm^-1 and 3410 cm^-1 were observed in all the samples with different intensities. To assess the biodeterioration of the TDR media, ATR was performed for the raw, 1st, 2nd and 9th week TDR media samples and in the raw TDR, two important bands, 842 and 2962 cm^-1 were noticed representing -CH = CH and -CH₃. A clear variation of bands and peak intensities were observed in different support media samples. The results indicate that TDR media is a resilient, chemically resistant material and could be employed for the biofilm growth for biological treatment of textile dye wastewater.

Recent Progress in the Membrane Distillation and Impact of Track-Etched Membranes

Membrane distillation (MD) is a rapidly developing field of research and finds applications in desalination of water, purification from nonvolatile substances, and concentration of various solutions. This review presents data from recent studies on the MD process, MD configuration, the type of membranes and membrane hydrophobization. Particular importance has been placed on the methods of hydrophobization and the use of track-etched membranes (TeMs) in the MD process. Hydrophobic TeMs based on poly(ethylene terephthalate) (PET), poly(vinylidene fluoride) (PVDF) and polycarbonate (PC) have been applied in the purification of water from salts and pesticides, as well as in the concentration of low-level liquid radioactive waste (LLLRW). Such membranes are characterized by a narrow pore size distribution, precise values of the number of pores per unit area and narrow thickness. These properties of membranes allow them to be used for more accurate water purification and as model membranes used to test theoretical models (for instance LEP prediction).

A Critical Review of Membrane Wettability in Membrane Distillation from the Perspective of Interfacial Interactions

Hydrophobic membranes used in membrane distillation (MD) systems are often subject to wetting during long-term operation. Thus, it is of great importance to fully understand factors that influence the wettability of hydrophobic membranes and their impact on the overall separation efficiency that can be achieved in MD systems. This Critical Review summarizes both fundamental and applied aspects of membrane wetting with particular emphasis on interfacial interaction between the membrane and solutes in the feed solution. First, the theoretical background of surface wetting, including the relationship between wettability and interfacial interaction, definition and measurement of contact angle, surface tension, surface free energy, adhesion force, and liquid entry pressure, is described. Second, the nature of wettability, membrane wetting mechanisms, influence of membrane properties, feed characteristics and operating conditions on membrane wetting, and evolution of membrane wetting are reviewed in the context of an MD process. Third, specific membrane features that increase resistance to wetting (e.g., superhydrophobic, omniphobic, and Janus membranes) are discussed briefly followed by the comparison of various cleaning approaches to restore membrane hydrophobicity. Finally, challenges with the prevention of membrane wetting are summarized, and future work is proposed to improve the use of MD technology in a variety of applications.

Effect of salt and metal accumulation on performance of membrane distillation system and microbial community succession in membrane biofilms

Membrane distillation (MD) works as a potential technology for the "zero liquid discharge" water treatment owing to its high concentration brine tolerance. The continuous accumulation of salts and metals in the MD system during the "zero liquid discharge" water treatment inevitably posed remarkable impacts on the biofilm formation as well as the MD performance. Hence, the biofouling mechanism of MD was deeply researched in this study with an emphasis on the roles of salt-stress (NaCl) and metal-stress (Zn and Fe) in biofilm development. The membrane flux decline of MD was effectively mitigated by the appearance of NaCl and ZnO, while that was significantly aggravated under the metal-stress of Fe. Considering the serious membrane scaling caused by NaCl crystals, a sharp flux decline was seen for the NaCl group during the later stage of MD operation. Basing on the 16S rDNA and 16S rRNA analysis, heat-stress, salt-stress, and metal-stress all posed certain impacts on the biofouling development in the MD system, and a more remarkable influence was observed for metal-stress. Under the salt-stress from NaCl, a thin biofilm containing high biovolume of dead cells finally formed, in which the bacterial community mainly consisted of halotolerant and thermophile species. Owing to the Zn²⁺-stress and oxidation-stress mechanisms of ZnO, the bacteria in the MD system were largely dead and live bacterial community in biofilms was dominated by some gram-negative species. Under the metal-stress from Fe, a rather thick biofilm containing higher biovolume of live cells clearly developed, in which the prevailing species could secret large amounts of EPS and accumulate metabolites around cells as biological surfactants, inducing aggravated membrane biofouling and high risk of membrane wetting.

Current and future perspectives for controlling Vibrio biofilms in the seafood industry: a comprehensive review

The contamination of seafood with Vibrio species can have severe repercussions in the seafood industry. Vibrio species can form mature biofilms and persist on the surface of several seafoods such as crabs, oysters, mussels, and shrimp, for extended duration. Several conventional approaches have been employed to inhibit the growth of planktonic cells and prevent the formation of Vibrio biofilms. Since Vibrio biofilms are mostly resistant to these control measures, novel alternative methods need to be urgently developed. In this review, we propose environmentally friendly approaches to suppress Vibrio biofilm formation using a hypothesized mechanism of action.

Biodegradation and Absorption Technology for Hydrocarbon-Polluted Water Treatment

Wastewaters polluted with hydrocarbons are an environmental problem that has a significant impact on the natural ecosystem and on human health. Thus, the aim of this research was to develop a bioreactor sorbent technology for treating these polluted waters. A lab-scale plant composed of three 1-L bioreactors with different sorbent materials inside (meltblown polypropylene and granulated cork) was built. Wastewater to be treated was recirculated through each bioreactor for 7 days. Results showed that hydrocarbon retention rates in the three bioreactors ranged between 92.6% and 94.5% of total petroleum hydrocarbons (TPHs) and that after one simple recirculation cycle, no hydrocarbon fractions were detected by gas chromatography/Mass Spectrometry (GC/MS) in the effluent wastewater. In addition, after the wastewater treatment, the sorbent materials were extracted from the bioreactors and deposited in vessels to study the biodegradation of the retained hydrocarbons by the wastewater indigenous microbiota adhered to sorbents during the wastewater treatment. A TPH removal of 41.2% was detected after one month of Pad Sentec™ carrier treatment. Further, the shifts detected in the percentages of some hydrocarbon fractions suggested that biodegradation is at least partially involved in the hydrocarbon removal process. These results proved the efficiency of this technology for the treatment of these hydrocarbon-polluted-waters.

Superhydrophobic nanocarbon-based membrane with antibacterial characteristics

To overcome the biofouling challenge which faces membrane water treatment processed, the novel superhydrophobic carbon nanomaterials impregnated on/powder activated carbon (CNMs/PAC) was utilized to successfully design prepare an antimicrobial membrane. The research was conducted following a systematic statistical design of experiments technique considering various parameters of composite membrane fabrication. The impact of these parameters of composite membrane on Staphylococcus aureus growth was investigated. The bacteria growth was analyzed through spectrophotometer and SEM. The effect of CNMs' hydrophobicity on the bacterial colonies revealed a decrease in the abundance of bacterial colonies and an alteration in structure with increasing the hydrophobicity. The results revealed that the optimum preparative conditions for carbon loading CNMs/PAC was 363.04 mg with a polymer concentration of 22.64 g/100 g, and a casting knife thickness of 133.91 μm. These conditions have resulted in decreasing the number of bacteria colonies to about 7.56 CFU. Our results provided a strong evidence on the antibacterial effect and consequently on the antibiofouling potential of CNMs/PAC in membrane.

Mechanism of biofilm formation on a hydrophobic polytetrafluoroethylene membrane during the purification of surface water using direct contact membrane distillation (DCMD), with especial interest in the feed properties

The impact of feed water quality on biofilm formation during membrane distillation (MD) was investigated in this study, particularly emphasizing the interrelationship between organics, salts, and microbes. Two types of typical natural surface waters in Nanjing, China, were chosen as feed solutions for long-term MD operation, including the Qinhuai River and Xuanwu Lake. The biofilms that developed under different feed water qualities exhibited distinct Foulant compositions and structures, causing different flux decline trends for the MD system. Accordingly, two typical patterns of biofilm formation were suggested for the MD operation of the two different kinds of surface waters in this study. Organics from a primal feed solution and dead bacteria were the key to the establishment of a biofilm on the membrane, and this needs to be effectively removed from the MD system through pre-treatment and process control strategies. Finally, a feasible strategy for MD biofouling control was suggested.

Fouling and wetting in the membrane distillation driven wastewater reclamation process - A review

Fouling and wetting of membranes are significant concerns that can impede the widespread application of the membrane distillation (MD) process during high-salinity wastewater reclamation. Fouling, caused by the accumulation of undesirable materials on the membrane surface and pores, causes a decrease in permeate flux. Membrane wetting, the direct permeation of the feed solution through the membrane pores, results in reduced contaminant rejection and overall process failure. Lately, the application of MD for water recovery from various types of wastewaters has gained increased attention among researchers. In this review, we discuss fouling and wetting phenomena observed during the MD process, along with the effects of various mitigation strategies. In addition, we examine the interactions between contaminants and different types of MD membranes and the influence of different operating conditions on the occurrence of fouling and wetting. We also report on previously investigated feed pre-treatment options before MD, application of integrated MD processes, the performance of fabricated/modified MD membranes, and strategies for MD membrane maintenance during water reclamation. Energy consumption and economic aspects of MD for wastewater recovery is also discussed. Throughout the review, we engage in dialogues highlighting research needs for furthering the development of MD: the incorporation of MD in the overall wastewater treatment and recovery scheme (including selection of appropriate membrane material, suitable pre-treatment or integrated processes, and membrane maintenance strategies) and the application of MD in long-term pilot-scale studies using real wastewater.

Membrane Distillation Biofouling: Impact of Feedwater Temperature on Biofilm Characteristics and Membrane Performance

Membrane distillation (MD) is a temperature driven membrane separation technology that holds great potential for decentralized and sustainable wastewater treatment systems. Yet, similarly to all membrane based systems, microbial fouling (biofouling) might be a critical hurdle for MD wastewater treatment applications. In this study we determined the impact of increasing feedwater temperatures (47 °C, 55 °C, and 65 °C) on biofilm growth and MD performance via dynamic biofouling experiments with Anoxybacillus sp. as a model bacterium. Our results indicated that cell growth was reduced at 47 °C, resulting in moderate distillate water flux decline (30%). Differently, extensive growth of Anoxybacillus sp. at feedwater temperature of 55 °C caused severe distillate water flux decline (78%). Additionally, biofouling induced membrane wetting, which facilitated the passage of bacteria cells and endospores through the membrane structure into the distillate. Although bacterial growth was impaired at feedwater temperatures of 65 °C, excessive production of EPS (compared to bacterial abundance) crippled membrane separation due to severe pore wetting. These results underline the importance of optimized operating conditions and development of antibiofouling and antiwetting membranes for successful implementation of MD in wastewater treatment with high biofouling propensity.

Principles and advancements of air gap membrane distillation

In recent years, membrane distillation (MD) has evidently emerged as one of the promising separation processes, with increasing areas of application including but not limited to desalination, pharmaceutical and textile wastewater purification, food processing, concentration of aqueous solution, breaking azeotropic mixtures, and extraction of volatile organic compounds. Primarily, MD has been categorized on the basis of vapor collection and condensation arrangement methods. Among the various categories, air gap membrane distillation (AGMD), in which an air gap is maintained across the membrane and the cooling plate, turns out to be an important and efficient process. Lately, AGMD has received significant attention of researchers around the world which motivates the present work. This paper aims to review the work done so far concerning the AGMD in order to provide a holistic view that covers the principles and applications of AGMD, effect of process parameters, membrane parameters, mathematical modeling, fouling, temperature and concentration polarization, types of membrane module, energy consumption, recent developments in AGMD process, cost estimation, and heat integration with AGMD. To the best of our knowledge, the present work is the first attempt to exhaustively review the AGMD process.

Wetting phenomena in membrane distillation: Mechanisms, reversal, and prevention

Membrane distillation (MD) is a rapidly emerging water treatment technology; however, membrane pore wetting is a primary barrier to widespread industrial use of MD. The primary causes of membrane wetting are exceedance of liquid entry pressure and membrane fouling. Developments in membrane design and the use of pretreatment have provided significant advancement toward wetting prevention in membrane distillation, but further progress is needed. In this study, a broad review is carried out on wetting incidence in membrane distillation processes. Based on this perspective, the study describes the wetting mechanisms, wetting causes, and wetting detection methods, as well as hydrophobicity measurements of MD membranes. This review discusses current understanding and areas for future investigation on the influence of operating conditions, MD configuration, and membrane non-wettability characteristics on wetting phenomena. Additionally, the review highlights mathematical wetting models and several approaches to wetting control, such as membrane fabrication and modification, as well as techniques for membrane restoration in MD. The literature shows that inorganic scaling and organic fouling are the main causes of membrane wetting. The regeneration of wetting MD membranes is found to be challenging and the obtained results are usually not favorable. Several pretreatment processes are found to inhibit membrane wetting by removing the wetting agents from the feed solution. Various advanced membrane designs are considered to bring membrane surface non-wettability to the states of superhydrophobicity and superomniphobicity; however, these methods commonly demand complex fabrication processes or high-specialized equipment. Recharging air in the feed to maintain protective air layers on the membrane surface has proven to be very effective to prevent wetting, but such techniques are immature and in need of significant research on design, optimization, and pilot-scale studies.

Antifouling membranes for sustainable water purification: strategies and mechanisms

One of the greatest challenges to the sustainability of modern society is an inadequate supply of clean water. Due to its energy-saving and cost-effective features, membrane technology has become an indispensable platform technology for water purification, including seawater and brackish water desalination as well as municipal or industrial wastewater treatment. However, membrane fouling, which arises from the nonspecific interaction between membrane surface and foulants, significantly impedes the efficient application of membrane technology. Preparing antifouling membranes is a fundamental strategy to deal with pervasive fouling problems from a variety of foulants. In recent years, major advancements have been made in membrane preparation techniques and in elucidating the antifouling mechanisms of membrane processes, including ultrafiltration, nanofiltration, reverse osmosis and forward osmosis. This review will first introduce the major foulants and the principal mechanisms of membrane fouling, and then highlight the development, current status and future prospects of antifouling membranes, including antifouling strategies, preparation techniques and practical applications. In particular, the strategies and mechanisms for antifouling membranes, including passive fouling resistance and fouling release, active off-surface and on-surface strategies, will be proposed and discussed extensively.

A review on membrane applications and transport mechanisms in vacuum membrane distillation

The main aim of this article is to provide a state-of-the-art review of the experimental studies on vacuum membrane distillation (VMD) process. An introduction to the history of VMD is carried out along with the other membrane distillation configurations. Recent developments in process, characterization of membrane, module design, transport phenomena, and effect of operating parameters on permeate flux are discussed for VMD in detail. Several heat and mass transfer correlations obtained by various researchers for different VMD modules have been discussed. The impact of membrane fouling with its control in VMD is discussed in detail. In this paper, temperature polarization coefficient and concentration polarization coefficient are elaborated in detail. Integration of VMD with other membrane separation processes/industrial processes have been explained to improve the performance of the system and make it more energy efficient. A critical evaluation of the VMD literature is incorporated throughout this review.

Membrane Distillation Bioreactor (MDBR) - A lower Green-House-Gas (GHG) option for industrial wastewater reclamation

A high-retention membrane bioreactor system, the Membrane Distillation Bioreactor (MDBR) is a wastewater reclamation process which has the potential to tap on waste heat generated in industries to produce high quality product water. There are a few key factors which could make MDBR an attractive advanced treatment option, namely tightening legal requirements due to increasing concerns on the micropollutants in industrial wastewater effluents as well as concerns over the electrical requirement of pressurized advanced treatment processes and greenhouse gas emissions associated with wastewater reclamation. This paper aims to provide a consolidated review on the current state of research for the MDBR system and to evaluate the system as a possible lower Green House Gas (GHG) emission option for wastewater reclamation using the membrane bioreactor-reverse osmosis (MBR-RO) system as a baseline for comparison. The areas for potential applications and possible configurations for MDBR applications are discussed.

Biofouling and microbial communities in membrane distillation and reverse osmosis

Membrane distillation (MD) is an emerging desalination technology that uses low-grade heat to drive water vapor across a microporous hydrophobic membrane. Currently, little is known about the biofilms that grow on MD membranes. In this study, we use estuarine water collected from Long Island Sound in a bench-scale direct contact MD system to investigate the initial stages of biofilm formation. For comparison, we studied biofilm formation in a bench-scale reverse osmosis (RO) system using the same feedwater. These two membrane desalination systems expose the natural microbial community to vastly different environmental conditions: high temperatures with no hydraulic pressure in MD and low temperature with hydraulic pressure in RO. Over the course of 4 days, we observed a steady decline in bacteria concentration (nearly 2 orders of magnitude) in the MD feed reservoir. Even with this drop in planktonic bacteria, significant biofilm formation was observed. Biofilm morphologies on MD and RO membranes were markedly different. MD membrane biofilms were heterogeneous and contained several colonies, while RO membrane biofilms, although thicker, were a homogeneous mat. Phylogenetic analysis using next-generation sequencing of 16S rDNA showed significant shifts in the microbial communities. Bacteria representing the orders Burkholderiales, Rhodobacterales, and Flavobacteriales were most abundant in the MD biofilms. On the basis of the results, we propose two different regimes for microbial community shifts and biofilm development in RO and MD systems.