Adsorption combined with ultrafiltration to remove organic matter from seawater

Organic fouling and biofouling are the major severe types of fouling of reverse osmosis (RO) membranes in seawater (SW) desalination. Low pressure membrane filtration such as ultrafiltration (UF) has been developed as a pre-treatment before reverse osmosis. However, UF alone may not be an effective enough pre-treatment because of the existence of low-molecular weight dissolved organic matter in seawater. Therefore, the objective of the present work is to study a hybrid process, powdered activated carbon (PAC) adsorption/UF, with real seawater and to evaluate its performance in terms of organic matter removal and membrane fouling. The effect of different PAC types and concentrations is evaluated. Stream-activated wood-based PAC addition increased marine organic matter removal by up to 70% in some conditions. Moreover, coupling PAC adsorption with UF decreased UF membrane fouling and the fouling occurring during short-term UF was totally reversible. It can be concluded that the hybrid PAC adsorption/UF process performed in crossflow filtration mode is a relevant pre-treatment process before RO desalination, allowing organic matter removal of 75% and showing no flux decline for short-term experiments.     

污水深度回用反渗透膜面微生物污染机理试验研究

微生物污染是最复杂的膜污染形式,是污水深度回用反渗透设备运行过程关注的焦点.通过实验室静态试验和错流过滤动态试验研究了污水回用反渗透膜的微生物污染的机理和特性,结果表明:造成污染的微生物主要为杆菌和丝状菌;在没有微生物源及没有微生物生长和繁殖的必需营养物质的情况下,膜面基本不发生微生物污染;杀菌处理对于防止微生物污染很重要,已经污染的膜面更易于发生微生物污染,做好反渗透设备的停备用保护工作是防止微生物污染的必要手段.     

Reverse osmosis desalination: Water sources, technology, and today's challenges

Reverse osmosis membrane technology has developed over the past 40 years to a 44% share in world desalting production capacity, and an 80% share in the total number of desalination plants installed worldwide. The use of membrane desalination has increased as materials have improved and costs have decreased. Today, reverse osmosis membranes are the leading technology for new desalination installations, and they are applied to a variety of salt water resources using tailored pretreatment and membrane system design. Two distinct branches of reverse osmosis desalination have emerged: seawater reverse osmosis and brackish water reverse osmosis. Differences between the two water sources, including foulants, salinity, waste brine (concentrate) disposal options, and plant location, have created significant differences in process development, implementation, and key technical problems. Pretreatment options are similar for both types of reverse osmosis and depend on the specific components of the water source. Both brackish water and seawater reverse osmosis (RO) will continue to be used worldwide; new technology in energy recovery and renewable energy, as well as innovative plant design, will allow greater use of desalination for inland and rural communities, while providing more affordable water for large coastal cities. A wide variety of research and general information on RO desalination is available; however, a direct comparison of seawater and brackish water RO systems is necessary to highlight similarities and differences in process development. This article brings to light key parameters of an RO process and process modifications due to feed water characteristics.     

Modification of poly(vinylidene fluoride) ultrafiltration membranes with poly(vinyl alcohol) for fouling control in drinking water treatment

A commercial poly(vinylidene fluoride) flat sheet membrane was modified by surface coating with a dilute poly(vinyl alcohol) (PVA) aqueous solution followed by solid-vapor interfacial crosslinking. The resulting PVA layer increased membrane smoothness and hydrophilicity and resulted in comparable pure water permeation between the modified and unmodified membranes. Fouling tests using a 5 mg/L protein solution showed that a short period of coating and crosslinking improved the anti-fouling performance. After 18 h ultrafiltration of a surface water with a TOC of approximately 7 mg C/L, the flux of the modified membrane was twice as high as that of the unmodified membrane. The improved fouling resistance of the modified membrane was related to the membrane physiochemical properties, which were confirmed by pure water permeation, X-ray photoelectron spectroscopy, and contact angle, zeta potential and roughness measurements.     

Impact of microfiltration treatment of secondary wastewater effluent on biofouling of reverse osmosis membranes

The effects of microfiltration (MF) as pretreatment for reverse osmosis (RO) on biofouling of RO membranes were analyzed with secondary wastewater effluents. MF pretreatment reduced permeate flux decline two- to three-fold, while increasing salt rejection. Additionally, the oxygen uptake rate (OUR) in the biofouling layer of the RO membrane was higher for an RO system that received pretreated secondary wastewater effluent compared to a control RO system that received untreated secondary effluent, likely due to the removal of inert particulate/colloidal matter during MF. A higher cell viability in the RO biofilm was observed close to the membrane surface irrespective of pretreatment, which is consistent with the biofilm-enhanced concentration polarization effect. Bacterial 16S rRNA gene clone library analysis revealed dominant biofilm communities of Proteobacteria and Bacteroidetes under all conditions. The Cramer-von Mises test statistic showed that MF pretreatment did not significantly change the bacterial community structure of RO membrane biofilms, though it affected bacterial community structure of non-membrane-associated biofilms (collected from the feed tank wall). The finding that the biofilm community developed on the RO membrane was not influenced by MF pretreatment may imply that RO membranes select for a conserved biofilm community.     

Vacuum membrane distillation of seawater reverse osmosis brines

Seawater desalination by Reverse Osmosis (RO) is an interesting solution for drinking water production. However, because of limitation by the osmotic pressure, a high recovery factor is not attainable. Consequently, large volumes of brines are discharged into the sea and the flow rate produced (permeate) is limited. In this paper, Vacuum Membrane Distillation (VMD) is considered as a complementary process to RO to further concentrate RO brines and increase the global recovery of the process. VMD is an evaporative technology that uses a membrane to support the liquid-vapour interface and enhance the contact area between liquid and vapour in comparison with conventional distillation. This study focuses on VMD for the treatment of RO brines. Simulations were performed to optimise the operating conditions and were completed by bench-scale experiments using actual RO brines and synthetic solutions up to a salt concentration of 300 g L⁻¹. Operating conditions such as a highly permeable membrane, high feed temperature, low permeate pressure and a turbulent fluid regime allowed high permeate fluxes to be obtained even for a very high salt concentration (300 g L⁻¹). For the membrane studied, temperature and concentration polarisation were shown to have little effect on permeate flux. After 6 to 8 h, no organic fouling or biofouling was observed for RO brines. At high salt concentrations, scaling occurred (mainly due to calcium precipitation) but had only a limited impact on the permeate flux (24% decrease for a permeate specific volume of 43L m⁻² for the highest concentration of salt). Calcium carbonate and calcium sulphate precipitated first due to their low solubility and formed mixed crystal deposits on the membrane surface. These phenomena only occurred on the membrane surface and did not totally cover the pores. The crystals were easily removed simply by washing the membrane with water. A global recovery factor of 89% can be obtained by coupling RO and VMD.     

天津反渗透海水淡化示范工程(1000m~3/d)

介绍了天津反渗透海水淡化示范工程(1 000 m3/d)的项目概况、工艺设计、设备配置、运行状况及造水成本分析.结果表明,针对天津海域海水浊度高、污染重的特点,采用初步预处理/双膜法淡化工艺完全能达到设计产水量和产水水质指标,并实现系统的稳定运行.     

Do biological-based strategies hold promise to biofouling control in MBRs?

Biofouling in membrane bioreactors (MBRs) remains a primary challenge for their wider application, despite the growing acceptance of MBRs worldwide. Research studies on membrane fouling are extensive in the literature, with more than 200 publications on MBR fouling in the last 3 years; yet, improvements in practice on biofouling control and management have been remarkably slow. Commonly applied cleaning methods are only partially effective and membrane replacement often becomes frequent. The reason for the slow advancement in successful control of biofouling is largely attributed to the complex interactions of involved biological compounds and the lack of representative-for-practice experimental approaches to evaluate potential effective control strategies. Biofouling is driven by microorganisms and their associated extra-cellular polymeric substances (EPS) and microbial products. Microorganisms and their products convene together to form matrices that are commonly treated as a black box in conventional control approaches. Biological-based antifouling strategies seem to be a promising constituent of an effective integrated control approach since they target the essence of biofouling problems. However, biological-based strategies are in their developmental phase and several questions should be addressed to set a roadmap for translating existing and new information into sustainable and effective control techniques. This paper investigates membrane biofouling in MBRs from the microbiological perspective to evaluate the potential of biological-based strategies in offering viable control alternatives. Limitations of available control methods highlight the importance of an integrated anti-fouling approach including biological strategies. Successful development of these strategies requires detailed characterization of microorganisms and EPS through the proper selection of analytical tools and assembly of results. Existing microbiological/EPS studies reveal a number of implications as well as knowledge gaps, warranting future targeted research. Systematic and representative microbiological studies, complementary utilization of molecular and biofilm characterization tools, standardized experimental methods and validation of successful biological-based antifouling strategies for MBR applications are needed. Specifically, in addition, linking these studies to relevant operational conditions in MBRs is an essential step to ultimately develop a better understanding and more effective and directed control strategy for biofouling.     

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.     

Harmful algae and their potential impacts on desalination operations off southern California

Seawater desalination by reverse osmosis (RO) is a reliable method for augmenting drinking water supplies. In recent years, the number and size of these water projects have increased dramatically. As freshwater resources become limited due to global climate change, rising demand, and exhausted local water supplies, seawater desalination will play an important role in the world's future water supply, reaching far beyond its deep roots in the Middle East. Emerging contaminants have been widely discussed with respect to wastewater and freshwater sources, but also must be considered for seawater desalination facilities to ensure the long-term safety and suitability of this emerging water supply. Harmful algal blooms, frequently referred to as ‘red tides’ due to their vibrant colors, are a concern for desalination plants due to the high biomass of microalgae present in ocean waters during these events, and a variety of substances that some of these algae produce. These compounds range from noxious substances to powerful neurotoxins that constitute significant public health risks if they are not effectively and completely removed by the RO membranes. Algal blooms can cause significant operational issues that result in increased chemical consumption, increased membrane fouling rates, and in extreme cases, a plant to be taken off-line. Early algal bloom detection by desalination facilities is essential so that operational adjustments can be made to ensure that production capacity remains unaffected. This review identifies the toxic substances, their known producers, and our present state of knowledge regarding the causes of toxic episodes, with a special focus on the Southern California Bight.     

反渗透海水淡化脱硼技术研究现状

硼在水中以硼酸的分子形式存在,其浓度过高对农作物和人体会产生不利的影响。由于硼的分子半径小,且不具有电性,因此反渗透海水淡化需要与其他方法相结合才能使硼含量达到饮用水的卫生要求。介绍了目前反渗透海水淡化中的脱硼方法。     

Formation and fate of chlorination by-products in reverse osmosis desalination systems

Chlorination by-products may be formed during pretreatment or posttreatment disinfection in reverse osmosis (RO) desalination systems, potentially posing health, aesthetic and ecological risks. To assess the formation and fate of by-products under different conditions likely to be encountered in desalination systems, trihalomethanes, dihaloacetonitriles, haloacetic acids, and bromophenols were analyzed in water samples from a pilot-scale seawater desalination plant with a chlorine pretreatment system and in benchscale experiments designed to simulate other feed water conditions. In the pilot plant, RO rejection performance as low as 55% was observed for neutral, low-molecular-weight by-products such as chloroform or bromochloroacetonitrile. Benchscale chlorination experiments, conducted on seawater from various locations indicated significant temporal and spatial variability for all by-products, which could not be explained by measured concentrations of organic carbon or bulk parameters such as SUVA₂₅₄. When desalinated water was blended with freshwater, elevated concentrations of bromide in the blended water resulted in dihaloacetonitrile concentrations that were higher than those expected from dilution. In most situations, the concentration of chlorination by-products formed from continuous chlorination of seawater or blending of desalinated water and freshwater will not compromise water quality or pose significant risks to aquatic ecosystems.     

Full-scale simulation of seawater reverse osmosis desalination processes for boron removal: Effect of membrane fouling

The objective of this study is to further develop previously reported mechanistic predictive model that simulates boron removal in full-scale seawater reverse osmosis (RO) desalination processes to take into account the effect of membrane fouling. Decrease of boron removal and reduction in water production rate by membrane fouling due to enhanced concentration polarization were simulated as a decrease in solute mass transfer coefficient in boundary layer on membrane surface. Various design and operating options under fouling condition were examined including single- versus double-pass configurations, different number of RO elements per vessel, use of RO membranes with enhanced boron rejection, and pH adjustment. These options were quantitatively compared by normalizing the performance of the system in terms of E_min, the minimum energy costs per product water. Simulation results suggested that most viable options to enhance boron rejection among those tested in this study include: i) minimizing fouling, ii) exchanging the existing SWRO elements to boron-specific ones, and iii) increasing pH in the second pass. The model developed in this study is expected to help design and optimization of the RO processes to achieve the target boron removal at target water recovery under realistic conditions where membrane fouling occurs during operation.     

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.     

Fouling characteristics of NF and RO operated for removal of dissolved matter from groundwater

A pilot study had been performed for about 6 months in order to investigate the removal efficiency of dissolved matter and its fouling potential during nanofiltration (NF) and reverse osmosis (RO) of local groundwater that was pretreated with an ultrafiltration (UF) membrane system. After pilot plant operation, autopsy tests were performed to identify the characteristics of foulants that were attached to the membrane surface. In the autopsy tests, the flux recovery for each specific cleaning scheme (hydraulic washing, acid cleaning, and alkaline cleaning) was also measured using a dead-end filtration cell unit. The washing solution used in each chemical cleaning was analyzed to identify major components of the foulants, and the membrane surface was observed using the scanning electron microscopy (SEM).Among three kinds of membranes tested, one NF and two RO membranes, the NF and RO1 membranes showed a rapid flux decline after 100 days of operation. Especially, the RO1 membrane showed the more serious flux decline than the NF membrane. The RO2 membrane, with the lowest recovery rate, demonstrated a gradual flux decline. The removal efficiency of dissolved inorganic matter (as conductivity) for each NF, RO1 and RO2 membrane was 76.3%, 88.2% and 95.3%, respectively. The removal of dissolved organic matter (as total organic carbon) was found to be about 80% for both NF and RO membranes used in this study. During the membrane autopsy tests, five sections of the fouled membrane were cut along each NF and RO membrane module from the feed inlet side to the concentrate outlet side, the specific flux for each membrane section was measured before and after each cleaning step. As expected, the degree of fouling was intensified along the membrane surface as the feed flow approached the outlet. Based on the analysis results of wash water used in each cleaning step, the major foulants attached to the membrane surface appeared to be Ca bound with inorganic matter and Si bound with organic matter. Fe seemed to be a great contributor to irreversible fouling. The SEM analysis indicated that the organic matter was forming the first fouling layer close to the membrane and that the inorganic matter was layered top of the organic fouling layer in a tetragonal shape. Any evidence of biofouling was not observed in this study because most of microorganisms had been already removed by the UF pretreatment.     

Impact of flow regime on pressure drop increase and biomass accumulation and morphology in membrane systems

Biomass accumulation and pressure drop development have been studied in membrane fouling simulators at different flow regimes. At linear flow velocities as applied in practice in spiral wound nanofiltration (NF) and reverse osmosis (RO) membranes, voluminous and filamentous biofilm structures developed in the feed spacer channel, causing a significant increase in feed channel pressure drop. Elevated shear by both single phase flow (water) and two phase flow (water with air sparging: bubble flow) caused biofilm filaments and a pressure drop increase. The amount of accumulated biomass was independent of the applied shear, depending on the substrate loading rate (product of substrate concentration and linear flow velocity) only. The biofilm streamers oscillated in the passing water. Bubble flow resulted in a more compact and less filamentous biofilm structure than single phase flow, causing a much lower pressure drop increase. The biofilm grown under low shear conditions was more easy to remove during water flushing compared to a biofilm grown under high shear. To control biofouling, biofilm structure may be adjusted using biofilm morphology engineering combined with biomass removal from membrane elements by periodic reverse flushing using modified feed spacers. Potential long and short term consequences of flow regimes on biofilm development are discussed. Flow regimes manipulate biofilm morphology affecting membrane performance, enabling new approaches to control biofouling.     

Mn oxide coated catalytic membranes for a hybrid ozonation-membrane filtration: Comparison of Ti, Fe and Mn oxide coated membranes for water quality

In this study the performance of catalytic membranes in a hybrid ozonation-ceramic membrane filtration system was investigated. The catalytic membranes were produced by coating commercial ceramic ultrafiltration membranes with manganese or iron oxide nanoparticles using a layer-by-layer self-assembly technique. A commercial membrane with a titanium oxide filtration layer was also evaluated. The performance of the coated and uncoated membranes was evaluated using water from a borderline eutrophic lake. The permeate flux and removal of the organic matter was found to depend on the type of the metal oxide present on the membrane surface. The performance of the manganese oxide coated membrane was superior to that of the other membranes tested, showing the fastest recovery in permeate flux when ozone was applied and the greatest reduction in the total organic carbon (TOC) in the permeate. The removal of trihalomethanes (THMs) and haloacetic acids (HAAs) precursors using the membrane coated 20 times with manganese oxide nanoparticles was significantly better than that for the membranes coated with 30 or 40 times with manganese oxide nanoparticles or 40 times with iron oxide nanoparticles.     

Combined coagulation-disk filtration process as a pretreatment of ultrafiltration and reverse osmosis membrane for wastewater reclamation: an autopsy study of a pilot plant

The effects of the combined coagulation-disk filtration (CC-DF) process on the fouling characteristics and behavior caused by interactions between effluent organic matter (EfOM) and the membrane surfaces of the ultrafiltration (UF) and reverse osmosis (RO) membranes in a pilot plant for municipal wastewater reclamation (MWR) were investigated. The feed water from secondary effluents was treated by the CC-DF process used as a pretreatment for the UF membrane to mitigate fouling formation and the permeate from the CC-DF process was further filtered by two UF membrane units in parallel arrangement and fed into four RO modules in a series connection. The CC-DF process was not sufficient to mitigate biofouling but the UF membrane was effective in mitigating biofouling on the RO membrane surfaces. Fouling of the UF and RO membranes was dominated by hydrophilic fractions of EfOM (e.g., polysaccharide-like and protein-like substances) and inorganic scaling (e.g., aluminum, calcium and silica). The desorbed UF membrane foulants included more aluminum species and hydrophobic fractions than the desorbed RO membrane foulants, which was presumably due to the residual coagulants and aluminum-humic substance complexes. The significant change in the surface chemistry of the RO membrane (a decrease in surface charge and an increase in contact angle of the fouled RO membranes) induced by the accumulation of hydrophilic EfOM onto the negatively charged RO membrane surface intensified the fouling formation of the fouled RO membrane by hydrophobic interaction between the humic substances of EfOM with relatively high hydrophobicity and the fouled RO membranes with decreased surface charge and increased contract angle.     

Development of antifouling reverse osmosis membranes for water treatment: A review

With the rapidly increasing demands on water resources, fresh water shortage has become an important issue affecting the economic and social development in many countries. As one of the main technologies for producing fresh water from saline water and other wastewater sources, reverse osmosis (RO) has been widely used so far. However, a major challenge facing widespread application of RO technology is membrane fouling, which results in reduced production capacity and increased operation costs. Therefore, many researches have been focused on enhancing the RO membrane resistance to fouling. This paper presents a review of developing antifouling RO membranes in recent years, including the selection of new starting monomers, improvement of interfacial polymerization process, surface modification of conventional RO membrane by physical and chemical methods as well as the hybrid organic/inorganic RO membrane. The review of research progress in this article may provide an insight for the development of antifouling RO membranes and extend the applications of RO technology in water treatment in the future.     

Systematic analysis of micromixers to minimize biofouling on reverse osmosis membranes

Micromixers, UV-curable epoxy traces printed on the surface of a reverse osmosis membrane, were tested on a cross-flow system to determine their success at reducing biofouling. Biofouling was quantified by measuring the rate of permeate flux decline and the median bacteria concentration on the surface of the membrane (as determined by fluorescence intensity counts due to nucleic acid stains as measured by hyperspectral imaging). The micromixers do not appear to significantly increase the pressure needed to maintain the same initial permeate flux and salt rejection. Chevrons helped prevent biofouling of the membranes in comparison with blank membranes. The chevron design controlled where the bacteria adhered to the membrane surface. However, blank membranes with spacers had a lower rate of permeate flux decline than the membranes with chevrons despite having greater bacteria concentrations on their surfaces. With better optimization of the micromixer design, the micromixers could be used to control where the bacteria will adhere to the surface and create a more biofouling resistant membrane that will help to drive down the cost of water treatment.