Effect of cake layer structure on colloidal fouling in reverse osmosis membranes

A series of reverse osmosis (RO) membrane filtration experiments was performed systematically in order to investigate the effects of various hydrodynamic and physicochemical operational parameters on a cake layer formation in colloidal and particulate suspensions. Bench-scale fouling experiments with a thin-film composite RO membrane were performed at various combinations of trans-membrane pressure (TMP), cross-flow velocity (CFV), particle size, pH, and ionic strength. In this study, silica particles with two different mean diameters of 0.1 and 3.0 μm were used as model colloids. Membrane filtration experiments with colloidal suspensions under various hydrodynamic operating conditions resulted that more significant permeate flux decline was observed as TMP increased and CFV decreased, which was attributed to the higher accumulative mass of particles on the membrane surface. Results of fouling experiments under various physicochemical operating conditions demonstrated that the rate of flux decline decreased significantly with an increase of the ionic strength as well as particle size, while the flux decline rate did not vary when solution pH changed. The experimentally measured cake layer thickness increased with a decrease in particle size and solution ionic strength. Furthermore, the model estimation of cake layer thickness by using a cake filtration theory based on the hydraulic resistance of membrane and cake layer was performed under various ionic strength conditions. The primary model parameters including accumulated mass and specific cake resistance were calculated from the cake layer resistance. This result indicated that the formation of cake layer could be closely related with solution water chemistry. The model estimated cake layer thickness values were in good agreement with the experimentally measured values.     

Elimination of biological fouling in seawater reverse osmosis desalination plants

The main trouble in the RO method of seawater desalination is biological fouling (bio-fouling) from microorganism growth. In this work, the growth rate of microorganisms in the Red Sea and sterilization by chlorine injection were measured and quantified. Furthermore, actual demonstration of RO pilot tests using cellulose tri-acetate (CTA) RO membranes with chlorine resistance was performed in RO plants where bio-fouling actually occurred. By carrying out direct chlorine sterilization of the RO membrane with an intermittent chlorine injection method, bio-fouling was eliminated. The combination of the CTA membrane and chlorine injection successfully prevented increases of differential pressure and stabilized product water quantity and quality.     

Hydrodynamic factors affecting flux and fouling during reverse osmosis of seawater

In this paper results are presented obtained from investigations on a polyamide reverse osmosis membrane fouled by precipitation of materials which exist in seawater. In these studies the effect of different hydrodynamic factors on fouling of the membrane was investigated. Operating conditions such as temperature, pressure, crossflow velocity and pH, which could play an important role in the fouling processes, were selected for the studies. The results show that increasing temperature, pressure and crossflow velocity enhance fouling of the membrane, but on the other hand, increase permeate flux. Thus, optimization of the parameters was found to be necessary and optimum values of them and pH were determined.     

Chemical cleaning of reverse osmosis membranes fouled by whey

The aim of this research was to establish a rationale for the cleaning of reverse osmosis (RO) membranes fouled by whey. Wheywas processed using a hydrophilic polyamide FT30 RO membrane. The effects ofoperating conditions such as transmembrane pressure, temperature and cross-flow velocity on flux behavior were elucidated before studying the cleaning. A wide variety of cleaning agents including acids, bases, enzymes and complexing agents was used. Resistance removal and flux recovery were used for demonstrating the cleaning efficiency. Hydrochloric acid (0.05 w%) resulted in maximum flux recovery and complete resistance removal. Although sodium hydroxide showed high cleaning efficiency, it may damage the membrane, predominantly at a high pH. Nitric acid and ammonia showed high but not complete resistant removal. Other acids (phosphoric acid and oxalic acid), ammonium chloride, urea and surfactants (SDS, Triton-X100 and CTAB) exhibited moderate effects while EDTA was of low efficiency. The cleaning effectiveness depends on the cleaner concentration. Using sulphuric acid, higher concentration caused lower resistance removal. For HCl the cleaning efficiency increased with the cleaner concentration, passed a maximum and decreased afterwards. Operating conditions such as cleaningtime and temperature affect cleaning efficacy. A longertime and higher temperature provide higher resistance removal. However, the effects are somewhat limited.     

Study of reverse osmosis membranes fouling by inorganic salts and colloidal particles during seawater desalination

Fouling phenomenon is considered among the major reasons that cause significant increase of operating cost of desalination plants equipped with reverse osmosis (RO) membranes. This phenomenon is studied in the present work in the case of RO polyamide aromatic membranes using model seawater containing inorganic salts and colloidal compounds. Different solubility conditions of CaCO₃ and CaSO₄ were applied to study RO performances with and without colloid presence. During experiments, the membrane permeate fluxes were continuously monitored. Moreover, studies of chemical composition, structure, and morphology of the materials deposited on the membrane surface were conducted using energy dispersive microanalysis (EDS) X-ray diffraction and scanning electronic microscopy (SEM). Results show that in conditions of calcium carbonate oversaturation there is a reduction in the permeate flow of 11.2% due to fouling of the membrane by the precipitation of this compound. While in the same conditions of calcium sulphate oversaturation the reduction of the flow is 5%, so we can conclude that in conditions of oversaturation of both salts, calcium carbonate produces a greater fouling of the membrane that in its view causes greater decrease in the flow of permeate. All this based on the results of the test with both salts in oversaturated conditions. Resulting in the formation of calcite and gypsum crystals onto the membranes as XRD analyses stated. Additional presence of colloidal silica in those conditions intensifies strongly the fouling, leading until to 24.1% of permeate flux decrease.     

Investigations of the coupled effect of cake-enhanced osmotic pressure and colloidal fouling in RO using crossflow sampler-modified fouling index ultrafiltration

The aim of this study was to investigate the potential of using the resistivity and deposition rate data from Crossflow Sampler-Modified Fouling Index Ultrafiltration (CFS-MFI_UF) measurements to determine the coupled effects of colloidal fouling and cake enhanced osmotic pressure (CEOP) effect. Cake filtration derived from CFS-MFI_UF was combined with a CEOP model to predict the crossflow RO fouling profile under constant flux filtration. The prediction based on resistivity, I′ from CFS-MFI_UF measurement alone was found to underestimate the RO fouling for high salinity solutions. However, when incorporating the mass information from the CFS-MFI_UF test to account for the CEOP effect, the prediction showed good agreement with the TMP profile of the RO system. The results indicated that the CFS-MFI_UF test which includes the CEOP effect is a very promising technique to provide an estimation of the RO colloidal fouling profile. When the changes of cake thickness and porosity throughout the filtration were considered, the predicted TMP profile based on the model clearly indicated a two-stage of fouling profile which agreed well with the experimental data. Additional studies on the effects of cake thickness and porosity on CEOP highlighted the important influence of cake structure on CEOP.     

Pilot-scale testing of reverse osmosis using conventional treatment and microfiltration

In order to meet the growing needs of its consumers, the Metropolitan Water District of Southern California is looking for ways to desalt water from the relatively hard Colorado River. This study evaluated conventional treatment as the pretreatment step for reverse osmosis (RO) desalting. Other pretreatment options studied include microfiltration (MF) and conventional treatment with ozone and biologically active filters. Each of three pretreatment scenarios produced effluent waters generally considered appropriate for use with RO [median turbidity of less than 0.1 Nephelometric turbidity unit (NTU) and median silt density index of less than 3]. Both microfiltered and ozonated/ biofiltered waters gave steady RO performance over 3 months of testing. However, conventional treatment left the RO system vulnerable to organic and biological fouling. Despite maintaining a 2-2.5 mg/L chloramine residual, pretreatment using conventional treatment required more frequent cleanings than either MF or ozonation/biofiltration (O₃/BF). The good performance for biofiltered water may have resulted from the stabilization of the natural organic matter through the O₃/BF process. Microfiltration, with its superior particle and bacteriological removal characteristics, provided the best RO pretreatment technology.     

中水深度处理后续反渗透系统的化学清洗

通过分析反渗透装置运行参数的变化、保安过滤器的污堵情况以及SDI微孔滤膜的截留物分析结果,判断反渗透膜主要受微生物和有机物污染,同时可能有无机盐垢形成复合污染,并据此确定了杀菌+碱洗+酸洗的清洗工艺。反渗透经化学清洗后进水压力大幅降低,系统出力显著提高,取得了令人满意的清洗效果。     

反渗透膜污染成因与防治

反渗透膜的污染受膜自身性质、水质和操作条件的影响.综述了膜材料、膜结构和膜组件、操作温度、压力、pH值、剪切速率和水质对反渗透膜污染的研究现状,同时指出研制低/抗污染膜和膜组件,优化操作条件.增设水质预处理和有效的膜清洗是反渗透膜技术的发展方向.     

反渗透膜污染成因与防治

反渗透膜的污染受膜自身性质、水质和操作条件的影响。综述了膜材料、膜结构和膜组件、操作温度、压力、pH值、剪切速率和水质对反渗透膜污染的研究现状,同时指出研制低／抗污染膜和膜组件,优化操作条件．增设水质预处理和有效的膜清洗是皈渗透膜技术的发展方向。     

反渗透膜有机污染的研究进展

介绍了反渗透进水中一些主要有机污染物质的污染机理以及操作条件、膜自身特性、溶液化学性质对于污染过程的影响,还综述了预处理技术、清洗方法以及通过膜的表面改性等措施来控制反渗透膜的有机污染,最后指出了目前研究中需关注的一些问题。     

Investigations of silica scaling on reverse osmosis membranes

In reverse osmosis systems with high silica water severe and irreversible membrane scaling can be observed. But suitable test methods to find an appropriate antiscalant agent and the optimal dosage are missing. Usually only the silicate concentration in the feed water is regarded.In this paper the results of laboratory experiments about the influence of Ca²⁺- and Mg²⁺-ions on the behaviour of supersaturated solutions of silica in different test waters are discussed first. After that the new developed membrane-based test method to determine the effectiveness of antiscalants is presented.The applied different methods of analysis enabled the differentiation of three groups of silicates: ‘monomeric’, ‘polymeric’ and ‘filterable’.With the test method the strong impact of silicate scaling was proven, even if only small amounts of scale had been formed. It could be shown, that the ‘polymeric silica’ is mainly responsible for the membrane scaling. The kinetic of the formation of ‘polymeric silica’ is strongly influenced by the cations and the pH-value. It could be demonstrated that the use of a suitable antiscalant makes it possible to operate the plant at significantly higher recovery rates.     

Mathematical modeling of reverse osmosis systems

The present investigation pertains to modeling of seawater desalination system. A simulation model was developed and verified for a small-scale reverse osmosis system. The proposed model combines material balances on the feed tank, membrane module andproduct tank with membrane mass transfer models. Finally a comprehensive simulation model has been developed incorporating the effect of mass transfer inhibition The model is non-linear differential equation representing the feed concentration as a function of operating time and space. The solution of the simultaneous differential equations was obtained using the fourth order Runge-Kutta method, due to self starting and stability. The model was verified using the experimental data from the literature [17,24]. Parameter sensitivity was carried out to select the proper step size. The simulation was run for over 1000 11 enabling a prediction of operational performance at high overall system recoveries.     

Integration of reverse osmosis and membrane crystallization for sodium sulphate recovery

Reverse osmosis and membrane crystallization are evaluated in this work as stand-alone and integrated technologies for the recovery of Na₂SO₄ from aqueous solutions. When SO₂ is removed from flue gases by absorption in an aqueous solution and reacts with NaOH, a reusable product (i.e., Na₂SO₄) of industrial interest can be obtained.For stand-alone reverse osmosis, the effect of the concentration of the feed solution and pressure is studied. For membrane crystallization, the influence of the concentration and flow rate of the feed and osmotic solutions on the process performance has been determined. The characterization of the obtained crystals shows that Na₂SO₄·10H₂O is obtained. From the experimental results, the potential for integration of reverse osmosis and membrane crystallization is simulated. It was concluded that using a reverse osmosis unit prior to the membrane crystallization unit minimizes the total membrane area in comparison with the stand-alone processes.     

某电厂反渗透膜系统的化学清洗

分析了某电厂反渗透系统装置运行参数的变化、反渗透膜污染物的主要组成,确定了清洗方案。通过清洗液浊度、清洗液pH、一二段压差变化等指标,确定清洗终点。该装置化学清洗后进水压力大幅降低,产水脱盐率达到设计要求,产品水的回收率基本达到设计值,取得满意的清洗效果。     

Chemical cleaning of reverse osmosis membranes used for treating wastewater from a rolling mill process

An analysis of fouling material and the effects of chemical cleaning were examined for a reverse osmosis (RO) membrane, which was used for the treatment of wastewater from a rolling mill process in the steel industry. The bulk foulant accumulated in the membrane module consisted mainly of CaSO₄·2H₂O, and the organic contaminants were contained at a very low level. The test pieces obtained from the exhausted RO membrane module (spiral-wound type) were used to examine chemical cleaning with the following solutions: acid and alkaline solutions with EDTA added, 50% methanol, and 10% ethyleneglycol monobutyrate (EGMB). Although a major component of the fouling material was calcium salt, the acid or alkaline solution containing EDTA did not promote the effective recovery of the water flux. On the other hand, cleaning with 50% methanol or 10% EGME solution increased the water flux significantly. The atomic force microscopy images of the membrane surface indicated that relatively large particles accumulated at the surface of the fouled membranes, and the large particles remained even after acid or alkaline cleaning. In the case of EGMB cleaning after alkaline cleaning, large particles did not remain, and uniform and fine particles were observed. The results that calcium salt, a major fouling material, was not removed effectively with the acid and alkaline solution may be due to trace organic materials in the fouling layer that act as a binder for inorganic fouling materials.     

An experimental study of UF membrane fouling by humic acid and sodium alginate solutions: the effect of backwashing on flux recovery

One of the critical issues for the successful application of ultrafitration in water treatment is membrane fouling due to dissolved organic matter, which negatively affects productivity, product quality and process cost. The aim of the present study is to contribute to the understanding of fouling phenomena by organic matter and the efficiency of the backwashing technique, which is applied in practice to restore membrane flux. In this experimental study commercial humic acid and sodium alginate have been used as model substances representative of natural organic matter and extracellular organic matter, respectively. All fouling experiments were carried out in a special single fiber apparatus. An important parameter considered in the study of both model substances is the concentration of calcium ions, which promote their aggregation and influence the rate of flux decline, the reversibility of fouling and rejection. Membrane fouling by humic acid appears to be the outcome of simultaneous action of several fouling mechanisms. Initially, a relatively rapid irreversible fouling takes place due to internal pore adsorption; however, progressively pore blocking becomes important and a fouling layer develops on the membrane. Sodium alginate fouling on the other hand is apparently due to two consecutive mechanisms; i.e. a rapid irreversible fouling due to internal pore constriction, followed by cake development on the membrane surface which becomes the dominant mechanism. Comparing fouling in both cases it can be inferred that even though sodium alginate fouling is more severe than the one caused by humic acids, it is to a large extent reversible by backwashing. On the contrary, fouling caused by humic acid is characterized by greater and increasing with calcium addition irreversibility, which is not remedied by the periodic backwashing. The different fouling propensity of the two types of macromolecules is apparently due to differences of their physical–chemical characteristics.     

Reuse of reverse osmosis membranes in advanced wastewater treatment

In areas where tap water has a high salt content, wastewater is not appropriate for reuse in agriculture, particularly for sensitive crops. One alternative is reduction, via desalination, of the brackish character to the secondary effluent. A filtration stage is also required before desalination. On the other hand, used reverse osmosis membranes can be recycled and used as filters in the advanced treatment stage in order to reduce suspended matter contained in the secondary effluent—one advantage being the environmental recovery of solid waste. Used membranes can be treated with strong chemical oxidants to peel off the active separation layer in order to transform them into microfiltration or ultrafiltration elements. Preliminary tests have been carried out with 8″ elements, aimed at comparing membrane performance before and after the peeling process. An index denoted as peeling effectiveness (high flux, high salt passage) is used for comparison. It was soon observed that potassium permanganate was more effective than others, together with sodium hydroxide. Doses around 1000 mg/L KMnO₄ provided the best results. It was also concluded that membrane cleaning, done with sodium bisulphite prior to peeling, was better when recirculating the cleaning solution around the membrane rather than soaking it. Next steps in the research will test the actual filtration capability of the peeled membranes in actual wastewater.     

Design and optimization of seawater reverse osmosis desalination plants using special simulation software

This paper introduces the Integrated process simulation environment (IPSEpro™) software, and its application to reverse osmosis desalination.The paper introduces the reader to the basic modeling concepts, and how individual process units can be simply simulated with a graphical, flowsheet style interface.The paper then goes on to describe an example simulation for off design of an integrated pump, membrane rack and isobaric energy recovery device, investigating the effects of changing water quality, membrane condition and also, the influence of isobaric energy recovery device overflush.     

Optimizing the coagulant dose to control membrane fouling in combined coagulation/ultrafiltration systems for textile wastewater reclamation

Optimal coagulation conditions need to be re-examined when coagulation is coupled to membrane filtration for wastewater treatment. This work focused on the optimization of coagulant dosing in order to control membrane fouling in ultrafiltration (UF), following coagulation for the reclamation of textile wastewater. The effects of pore size and coagulant types and dosages on flux decline were investigated using a stirred-cell UF unit. The flux was greatly enhanced for the UF membrane when a coagulant was added, whereas for the microfiltration (MF) membrane the flux decreased. This could be attributed to changes in the size of coagulated particles and their interaction with membrane pores. At a low dosage (e.g., 0.0371 mM as Al), the polyaluminum chloride (PACl) coagulant was found to control the flux decline most effectively for low ionic-strength wastewater. The optimal dose minimized the fouling and cake layer resistances, although it was sharp and dependent on influent composition. The cake layer protected the membrane from fouling, but it provided additional resistance to permeation. Analyses of turbidity, particle size, and membrane surface exhibited the characteristics of coagulated particles and their cake structures that are closely associated with flux behavior.