Reverse Osmosis Pretreatment Research Articles

Graphene oxide functionalized polyethersulfone membrane to reduce organic fouling

Graphene-oxide can be used to modify a polymeric membrane surface to render the surface less susceptible to organic fouling. In this study, a three-step method is developed to covalently bind graphene oxide (GO) nanosheets onto a polyethersulfone (PES) support to create an ultrafiltration membrane that is resistant to organic fouling. First, amine functional groups are grafted onto a PES surface by UV-induced graft co-polymerization of allylamine monomer. Next, trimesoyl chloride (TMC, 0.2wt% in hexane) and GO nanosheets (2, 4 and 6wt% in anhydrous acetonitrile) are attached onto the polyallylamine modified membrane surface by spin coating, allowing covalent binding to take place between TMC and the amine functional group, and also between TMC and the hydroxyl or carboxylic functional groups of GO. Covalent bonding was confirmed by the emerging ester or anhydride peaks by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). The GO membrane morphology was observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The molecular weight cut offs (MWCO) of the GO (2, 4 and 6wt%) modified membranes were determined to be 380, 140, and 75kDa respectively. The membrane with 4wt% GO (PES-GO-4) was chosen for further study because it exhibited higher pure water flux and rejection than the other GO formulations. The PES-GO-4 membrane exhibited 2.6 times greater flux recovery than an unmodified PES-UF membrane of comparable MWCO. This method is a simple, effective way to create a robust, high flux UF membrane that is resistant to organic fouling. Such a membrane could be applied in water or wastewater treatment applications, such as pretreatment for reverse osmosis.

Use of innovative inge® Multibore® ultrafiltration membranes for the treatment of challenging seawater

Ultrafiltration (UF) has established itself as one of the key technologies in the water treatment industry over the last decade, providing superior filtrate water quality regardless of fluctuations in the feedwater quality and protecting downstream treatment steps. UF has demonstrated its advantages as seawater reverse osmosis pre-treatment for desalination application improving reverse osmosis membranes performances while extending their service life. In-depth knowledge of polymeric chemistry is mandatory for developing, manufacturing and operating new membranes. On-site evaluation of membrane performance on real water is also essential. Seawater properties can vary significantly depending on location and region. This makes it very important to pilot in order to understand the challenges and how to address them. This paper presents results obtained at different sites operating on seawater under particularly difficult conditions using inge® Multibore® membranes. The study describes membrane behavior when operated during algae blooms, during a monsoon period and during extreme low water temperatures (0°C). It presents process adjustments realized to optimize the overall performance. The study shows that system optimization yields stable and long-term operation on challenging seawater without pre-treatment upstream of the UF membranes.

Novel ultrafiltration membranes from low-fouling copolymers for RO pretreatment applications

For a consistent and reliable ultrafiltration (UF) membrane operation, the membrane flux level has always been determined conservatively so that fouling rate is controllable, and therefore, chemical cleaning requirements are minimized. A better solution is to prevent the fouling to happen in the first place by modifying the surface properties of the membrane materials. This paper presents novel approaches to improve the fouling-propensity performance of existing UF membranes based on polyethersulfone. More hydrophilic membranes and anti-adhesive membranes were developed by using various novel copolymers as base membrane materials or as an additive in the spinning process. Incorporating these copolymers, improved Multibore® fibers were produced and validated in multiple pilot trials for various applications, namely surface and wastewater purification and seawater reverse osmosis pretreatment for desalination. The paper demonstrates that, for each application, a reduced fouling propensity performance was achieved, allowing the fibers to be operated at a substantially higher flux of up to 30%. This study underscores the importance of chemistry in obtaining low-fouling and high-performance UF membranes by modifying their surface properties.

State-of-the-art of R&D on seawater desalination technology

Fresh water deficit has been an increasingly critical global environmental issue, and seawater desalination is considered as the most effective and promising solution, which promotes the development of different desalination technologies. In order to provide a comprehensive knowledge about the development of desalination technologies, state-of-the-art of research and development on the application of desalination technologies is reviewed in this paper. At first, the overview of desalination technologies is briefly introduced, including the definition of desalination, the evaluation index of water quality and the resulting classification of water resources, the overview of desalination industry, the usages of freshwater produced by desalination, and the sources of raw water. Besides, conventional desalination methods are categorized into physical method and chemical method according to whether there is new substance generated in the desalination process. In the following section conventional desalination methods are reviewed from the perspectives of basic processes and principles, the performances and technical characteristics. However, the shortcomings of conventional desalination technologies mainly lie in high energy consumption and complex treatment process, such as the decrease of heat transfer coefficient caused by the scaling and fouling generated on the solid heat transfer walls for multi-stage flash (MSF) and multiple effect distillation (MED), and rigorous pre-treatment and low recovery factor for reverse osmosis (RO). With the aims of overcoming these disadvantages and thus decreasing the energy consumption, different improvements have been put forward and new desalination methods have been developed. Therefore, the development of desalination technology is then exhaustively analyzed and forecasted in the following four aspects: improvement for the existing technology, combination of different technologies, desalination technology in combination with new energy, and development of new desalination technologies utilizing unemployed physical phenomena. The improvements mainly focus on the shortcomings of MSF, MED and RO, which are the current major large-scale industrial desalination technologies. The combination of different technologies can integrate their respective advantages to improve the desalination performance and decrease the energy consumption and the cost. The limitations of the decrease of traditional fossil energy and its increasing cost can be broken through the combination with solar, wind, geothermal, ocean and nuclear energies, which is also regarded as a potential solution for climate change. The newly developed desalination technologies have the advantages of lower energy consumption and cost, but most of which are still in lab-scale and need further improvement. Due to the important role in the assessment of the desalination efficient, special attention is paid to the analysis of energy consumption of each desalination technology.

Insight in to the Initial Stages of Silica Scaling Employing a Scanning Electron and Atomic Force Microscopy Approach

The performance of reverse osmosis (RO) desalination can be limited by membrane scaling. Of particular concern is silica scale, which once deposited on the membrane is extremely difficult to remove. In this work, the deposition of silica-rich nanoparticles was considered. A novel in situ sample preparation method was developed for a microscopy investigation into the deposition and adhesion of the silica-rich nanoparticles. The method involves placing a clean silica wafer in agitated brine to collect particles to simulate initial stages of scaling. The ‘scaled’ surfaces were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Model brines, with varying nanoparticle, cation, and organic composition and concentration were tested, as well as reject brine from a full-scale operational water treatment facility. Microscopy revealed that silica-rich nanoparticles were deposited from all waters, with smaller nanoparticles more readily attaching to the wafer compared to larger ones. The presence of organics increased nanoparticle adhesion whereas divalent cations (Ca2+ and Mg2+) decreased nanoparticle adhesion. These results have implications for the evaluation, selection and operation of RO pre-treatment processes and chemical dosing strategies, particularly the requirement for weak acid cation ion exchange (WAC-IX) and anti-scalant chemicals, respectively.

Experimental study of pretreatment of brackish reverse osmosis concentrate streams using seeded precipitative softening process for improvement of water recovery

This study investigates the pretreatment of brackish water reverse osmosis concentrate streams as a feed to secondary reverse osmosis (RO) process aiming at reuse and enhancement of the potable water recovery from the concentrate streams and reduction in its volume. Seeded crystallization process was tested as a pretreatment method. The results obtained indicated that the seeded crystallization process has the potential to accomplish a 96.76% reduction of calcium ion concentration and a reduction of Langelier saturation index from 1.6 to 0.05. The optimum conditions for treating concentrates continuously at flow rate 1 L/min are: pH 10.6; seeds concentration = 3 g/L; seeds size 100 μm, residence time = 1 min. The treated concentrate can be fed to a secondary RO process with low risk of membrane fouling.

A new flat sheet membrane bioreactor hybrid system for advanced treatment of effluent, reverse osmosis pretreatment and fouling mitigation

This paper introduces a new hybrid electro membrane bioreactor (HEMBR) for reverse osmosis (RO) pretreatment and advanced treatment of effluent by simultaneously integrating electrical coagulation (EC) with a membrane bioreactor (MBR) and its performance was compared with conventional MBR. Experimental results and their statistical analysis showed removal efficiency for suspended solids (SS) of almost 100% for both reactors. HEMBR removal of chemical oxygen demand (COD) improved by 4% and membrane fouling was alleviated according to transmembrane pressure (TMP). The average silt density index (SDI) of HEMBR permeate samples was slightly better indicating less RO membrane fouling. Moreover, based on the SVI comparison of two reactor biomass samples, HEMBR showed better settling characteristics which improved the dewaterability and filterability of the sludge. Analysis the change of membrane surfaces and the cake layer formed over them through field emission scanning electron microscopy (FESEM) and X-ray fluorescence spectrometer (XRF) were also discussed.

Extracellular polymeric substances removal of dual-layer (PES/PVDF) hollow fiber UF membrane comprising multi-walled carbon nanotubes for preventing RO biofouling

The reverse osmosis (RO) pretreatment of the petrochemical refinery wastewater has been investigated using the dual-layer (PES/PVDF) hollow fiber nanocomposite membranes, which were fabricated comprising MWCNTs in both layers via the two-step TIPS/NIPS process. The dual-layer hollow fiber nanocomposite membrane had the reduced fouling propensity, the improved compaction resistance and the higher filtration performance than the dual-layer hollow fiber (PES/PVDF) membrane. The low concentration of bacteria in the permeate caused RO membrane biofouling due to the rapid growth in the environment of extracellular polymeric substances (EPS). In order to inhibit the RO membrane biofouling, EPSs were removed by using the hollow fiber nanocomposite UF membrane, including polysaccharides, biopolymers and proteins. The rejection of biopolymers (MWCO=33kDa) was higher than that of polysaccharides (MWCO=40kDa). With respect to protein removal, it was found that the bovine serum albumin and rejection of 90% could be achieved, which was strongly dependent on pH. The EPS removal achieved up to 98.2% with the optimized carboxylated MWCNTs loading in the (NcPES-0.2%)/(NcPVDF-0.5%) hollow fiber nanocomposite membrane. The permeate treated by the hollow fiber nanocomposite membrane had effectively inhibited the RO membrane biofouling by Escherichia coli, as a result of the lower concentration of turbidity and EPS. The resultant hollow fiber nanocomposite ultrafiltration membrane shows the feasibility of removing EPS coupled with a low rejection of total dissolved salts, which yields a superior pretreatment method to reduce biofouling of the subsequent purification by reverse osmosis.

Direct water reclamation from sewage using ceramic tight ultra- and nanofiltration

Sewage is a nutrient rich reliable water source that is rather consistent in quality, volume and temperature, and is available in large amounts in urban areas. Decentralised reclamation of water including its constituents from municipal sewage, further referred to as sewer mining, is a concept in which municipal sewage is considered a resource instead of a waste stream.In this research, water reclamation in the sewer mining concept was studied using ceramic tight ultra- (UF) and ceramic nanofiltration (NF). In our current approach, ceramic membrane filtration is proposed as pre-treatment for reverse osmosis (RO) to produce demineralised water for industries from municipal sewage. The objectives of this research are to study (i) the membrane performance, (ii) the organic matter and ion rejection, and (iii) the biofouling potential of RO using permeate water from the ceramic filtration.The application of ceramic tight UF and ceramic NF for direct treatment of domestic sewage has been demonstrated in this study. The cross flow ceramic tight UF and NF fed with filtered sewage, can be operated for 1–4days without any cleaning required. The membrane performance remained high with chemical cleaning with NaClO (0.1%) and HCl (0.1molL−1) solutions. On average about 81% of organic matter was rejected by both ceramic tight UF and NF membranes. Finally, the pressure drop increase in the MFS fed with ceramic NF permeate was low during an operation of 14days. These results were comparable with the increase in pressure drop of an MFS fed with Dutch drinking water.

Treatment of reverse osmosis concentrate by electrolysis and MBR process

In this study, the reverse osmosis (RO) concentrate treatment technology, which combines the electrolysis process with the MBR process, has been suggested to treat the RO concentrate generated in the water recycle process, which applied the RO membrane. The electrolysis process constructed in this study is to reduce the MBR process load by removing the non-degradable matters and the nitrogen compound, which are the weakness of biological treatment. When treating the RO concentrate through the electrolysis, the disinfection by-products—like residual chlorine, THMs and so forth—are generated. Since this reaction of the by-products affects the microbes, PAC was injected within the MBR reactor to desalinate the treated water. As the MBR process shows considerably high treatment efficiency for the organic matters and can increase the concentration of the activated sludge within reactor, it is effective to treat the nitrogen and phosphorus, which cause the eutrophication of water, but since there is a limit in treating the RO concentrate with this single process, such a hybrid process was suggested. The comparative experiment was performed depending on whether the electrolysis-applying pretreatment of RO concentrate has been applied or not, and whether the PAC has been injected within MBR or not. As a result, the DOC, T-N and T-P removal efficiency of the suggested process was each approximately 98.0, 40.0 and 45.6%, respectively. The experiment results showed that the suggested process has great potential in RO concentrate treatment.

Biofilm formation on RO membranes: the impact of seawater pretreatment

Membrane biofouling is an acute problem that interferes with filtration and pressure-driven desalination processes. In this study, ultrafiltration (UF) and membrane bioreactor (MBR) system were examined for their potential use in the removal of organic matter from seawater as pretreatment for reverse osmosis (RO) desalination. The study showed that MBR treatment equipped with UF decreases total organic carbon, polysaccharides, and biofouling potential of RO membrane in comparison to feed seawater after UF treatment alone. Bacteria in the feed water and in the MBR system were characterized. The most abundant heterotrophic bacteria nourished from organic substances present in the MBR system belonging to the Alphaproteobacteria and Gammaproteobacteria classes increased from ~40% in seawater to ~60% in the MBR. These results indicate that pretreatment using a seawater MBR system can improve RO feed water quality and reduce the biofouling potential of RO membranes.

Gravity-driven membrane filtration as pretreatment for seawater reverse osmosis: Linking biofouling layer morphology with flux stabilization

In this study gravity-driven membrane (GDM) ultrafiltration is investigated for the pretreatment of seawater before reverse osmosis (RO). The impacts of temperature (21 ± 1 and 29 ± 1 °C) and hydrostatic pressure (40 and 100 mbar) on dynamic flux development and biofouling layer structure were studied. The data suggested pore constriction fouling was predominant at the early stage of filtration, during which the hydrostatic pressure and temperature had negligible effects on permeate flux. With extended filtration time, cake layer fouling played a major role, during which higher hydrostatic pressure and temperature improved permeate flux. The permeate flux stabilized in a range of 3.6 L/m2 h (21 ± 1 °C, 40 mbar) to 7.3 L/m2 h (29 ± 1 °C, 100 mbar) after slight fluctuations and remained constant for the duration of the experiments (almost 3 months). An increase in biofouling layer thickness and a variable biofouling layer structure were observed over time by optical coherence tomography and confocal laser scanning microscopy. The presence of eukaryotic organisms in the biofouling layer was observed by light microscopy and the microbial community structure of the biofouling layer was analyzed by sequences of 16S rRNA genes. The magnitude of permeate flux was associated with the combined effect of the biofouling layer thickness and structure. Changes in the biofouling layer structure were attributed to (1) the movement and predation behaviour of the eukaryotic organisms which increased the heterogeneous nature of the biofouling layer; (2) the bacterial debris generated by eukaryotic predation activity which reduced porosity; (3) significant shifts of the dominant bacterial species over time that may have influenced the biofouling layer structure. As expected, most of the particles and colloids in the feed seawater were removed by the GDM process, which led to a lower RO fouling potential. However, the dissolved organic carbon in the permeate was not be reduced, possibly because some microbial species (e.g. algae) could convert CO2 into organic substances. To further improve the removal efficiency of the organic carbon, combining carrier biofilm processes with a submerged GDM filtration system is proposed.

Desalination for irrigation use: chemical pre-treatment study of Reverse osmosis plant

Desalinated brackish water is becoming an important water source for agricultural irrigation. In Brackish water desalination, pretreatment of reverse osmosis is the key step in designing the plants to avoid membrane fouling and scaling. It is enormously important to carry out a study designed at ensuring the optimization of the pretreatment system for brackish desalination plant in order to optimize the quality of the water fed through the reverse osmosis membranes, to guarantee the highest performance and to minimize the number of shutdowns for chemical cleaning. In this paper, performance evaluation carried out for a brackish water reverse osmosis plant for agricultural application, located in Dokkala Region in Morocco.

Advanced treatment of textile dyeing secondary effluent using magnetic anion exchange resin and its effect on organic fouling in subsequent RO membrane

Strict regulations are forcing dyeing factory to upgrade existing waste treatment system. In this study, advanced treatment of dyeing secondary effluent by magnetic anion exchange resin (NDMP) was investigated and compared with ultrafiltration (UF); NDMP as a pre-treatment of reverse osmosis (RO) was also studied. NDMP resin (20mL/L) gave higher removal of dissolved organic carbon (DOC) (83.9%) and colority (94.9%) than UF with a cut-off of 10kDa (only 48.6% and 44.1%, respectively), showing that NDMP treatment was effective to meet the stringent discharge limit of DOC and colority. Besides, NDMP resin (20mL/L) as a pretreatment of RO increased the permeate flux by 12.5% and reduced irreversible membrane fouling by 6.6%, but UF pretreatment did not mitigate RO membrane fouling. The results of excitation-emission matrix fluorescence spectra and resin fractions showed that NDMP had more efficient removal than UF for transphilic acid and hydrophilic fraction, such as protein-like organic matters and soluble microbial products, which contributed to a significant proportion of RO membrane fouling. In sum, NDMP resin treatment not only gave effective removal of DOC and colority of dyeing secondary effluent, but exhibited some improvement for RO membrane flux and irreversible fouling.

One-year operational experience with ultrafiltration as pretreatment of seawater reverse osmosis desalination system (Maspalomas-I Plant)

The selection of a proper pretreatment for reverse osmosis (RO) systems is essential to produce a high-quality feed water in order to ensure a more sustainable and reliable operation. Traditionally, seawater RO installations have been operated with conventional pretreatments based on single- or two-staged media filtration, and occasionally preceded by coagulation/flocculation processes. In recent years, however, hollow fiber ultrafiltration (UF) technology has increasingly gained acceptance as a viable pretreatment alternative for seawater desalination, due to advantages such as greater capability to cope with fluctuations and high solid loads in raw waters (typical of open intakes), smaller footprint, higher and more consistent filtrate water quality, and higher environmental sustainability, ultimately demonstrating lower total water cost in the long-term operation. In this article, design and operational aspects of the country’s largest integrated UF and RO system to treat seawater in a remote island of Spain will be provided. Special emphasis will be given on the benefits that the UF technology has brought to the operator compared to the conventional pretreatment it replaced.

Suitability of cationic flocculants for RO membranes performance improvement

As it has already been published and demonstrated, one of the main causes of membranes failures is related to a poor plant pretreatment. Data from more than 600 autopsies demonstrated that both biofilm and colloidal matter are the most common components on reverse osmosis (RO) membrane fouling; therefore, main lack on membrane pretreatment is related to disinfection and small size of the particles. The main deficiency in membrane pretreatment is related to the dosing of chemicals such as biocides, and flocculants, which are the only tool for membrane pretreatment improvement. Although on previous papers, cationic flocculants effectiveness was already proved, there is still a reluctance to use these kinds of products on RO pretreatments. Genesys Membrane Products laboratories has broad experience on the optimization of cationic flocculants use which proves the effectiveness of these products for RO pretreatment with no risk for the membranes. This paper will demonstrate the compatibility of some cationic flocculant with membranes, as well as the compatibility of novel formulations including biocides in the flocculant formulation. On this paper, the authors will review data from laboratory tests, case studies, and compatibility tests carried out through different analytical techniques (SEM-EDX, ATR/FTIR, etc.) for the different flocculants developed by Genesys International Limited and will try to demonstrate the safe use of these products for the best performance of RO systems.

Using modelling approach to validate a bench scale forward osmosis pre-treatment process for desalination

Forward osmosis (FO) has recently attracted growing attention in wastewater, brackish groundwater and seawater desalination, and power generation. This study evaluates the potential of using a batch laboratory-scale FO system as a pre-treatment for the reverse osmosis (RO) process. FO is a low pressure-driven process that offers many advantages compared to the conventional pre-treatment for RO especially for brackish water with high potential of scaling and fouling. FO can help to reduce the RO process cost by avoiding RO membrane fouling and achieving higher water recovery. An experimental modelling has been employed to describe the FO process taking into account water flux, water recovery and final draw solution. Based on this experimental modelling, the energy consumption for RO has been estimated. It has been found that the treatment time for the FO process and the initial draw solute concentration are important parameters that have an interrelated effect on FO and RO efficiency. The optimal conditions for this FO pre-treatment process are determined by modelling and are experimentally validated by using real brackish water as feed in a bench-scale FO system.

Long-term effect on membrane fouling in a new membrane bioreactor as a pretreatment to seawater desalination

Submerged membrane adsorption bio-reactors (SMABR) were investigated as a new pretreatment for seawater reverse osmosis (SWRO) desalination. They were tested with different doses of powder activated carbon (PAC) on-site for a long-term. The biofouling on the membrane was assessed in terms of DNA (cells) and polysaccharide distribution. MBR without PAC addition resulted in severe fouling on membrane. When PAC is added in the MBR, PAC could reduce the organic fouling. Hence the biofilm formation on membrane was reduced without any membrane damage. PAC also helped to remove low molecular weight (LMW) organics responsible for biofouling of RO membrane. A linear correlation between assimilable organic carbon (AOC) and LMW organics was observed. A small amount of PAC (2.4–8.0g of PAC/m3 of seawater) was sufficient to reduce biofouling. It indicated that SMABR is an environmentally-friendly biological pretreatment to reduce biofouling for SWRO.

Comparison of sand and membrane filtration as non-chemical pre-treatment strategies for pesticide removal with nanofiltration/low pressure reverse osmosis membranes

Pilot plant investigations of sand and membrane filtration (microfiltration (MF)/ultrafiltration (UF)/nanofiltration (NF)/low pressure reverse osmosis (LPRO)) have been performed to treat groundwater polluted with pesticides. The results show that simple treatment, with use of aeration and sand filtration or MF/UF membranes, does not remove pesticides. However, by reducing the content of key foulants, the techniques can be used as a pre-treatment for nanofiltration and low pressure reverse osmosis that has proved to be capable of removing pesticides. It was found that a lower fouling potential could be obtained by using the membranes, but that sand filter was better at removing manganese and dissolved organic matter. The results indicate that combining aeration, sand filtration and membrane techniques might be a good option for pesticide removal without any addition of chemicals and minimized membrane maintenance.

Dynamic shift in community structures of biofilm-forming bacteria by the pre-treatment systems of seawater reverse osmosis processes

In this study, the bacterial compositions of raw seawater and permeates from the conventional pre-treatment system and the membrane-based microfiltration system were examined during the seasons of autumn, winter and spring. The biofilm-forming potential was calculated based on the relative abundances of the putative biofilm-forming bacteria for the two seawater reverse osmosis pre-treatment systems. The cloning and sequencing analysis of the 16S rRNA gene reveals the dynamic shift in bacterial community structure to the genera capable of biofilm-formation through the pre-treatments. The highly diverse bacterial community structure of the raw seawater and the permeates hinders the statistical analysis for generalizing the bacterial components according to the sources or the seasons. This study proposes that the operational regimes of pre-treatment systems should be reconsidered based on the target bacterial groups of key biofilm formers.