反渗透膜的污染分析及其清洗

通过自制的平板膜对沧州炼油厂热电厂水处理车间反渗透设备的膜污染进行了研究。经电镜扫描与ＥＤＸ能谱分析,测出ＲＯ膜膜而Ｃ、Ｏ、Ｃａ和Ｓｉ等元素的分布情况,确定膜面上的主要污染物是胶体的污染物,微生物污染物以及一定量的ＳｉＯ２颗粒,其中胶体污染物的主要成分是Ｓｉ、Ａｌ、Ｆｅ和一定量的有机物。     

Organic fouling of RO membranes: Investigating the correlation of RO and UF fouling resistances for predictive purposes

This study examines the characteristics of deposits from dead-end UF and cross-flow RO filtration of dilute mixtures of sodium alginate (SA) and humic acids (HA), which are representative of common foulants in RO and NF feed waters. The data are interpreted in terms of specific cake resistance, α. The presence of alginates in the mixed fouling layers, even in relatively small percentage, imparts characteristic properties to these deposits; i.e. a gel-like structure which is associated with reduced compressibility and permeability. With SA/HA mixtures, high rejection of organic species and high deposition factors are obtained in UF and RO tests, respectively. The specific resistance of mixed SA and HA deposits tends to increase with increasing SA concentration in solution. The resistances α exhibit fairly strong (power law) dependence on the pressure difference across the cake, ΔΡ_c, for both RO and UF tests. The fairly satisfactory correlation of resistance α versus ΔΡ_c data, obtained from both types of tests (for the mixtures of organic species tested at small salinity) is a significant result of this work. Based on this correlation, an approach is suggested for estimating fouling index I and initial membrane fouling rate of a specific RO process, relying on limited UF tests.     

An assessment of the Silt Density Index based on RO membrane colloidal fouling experiments with iron oxide particles

RO membrane colloidal fouling experiments were performed in the laboratory under well controlled and realistic conditions. Iron oxide was selected as a typical inorganic colloidal foulant, due to its importance, as evidenced from well known manufacturer recommendations on iron concentrations in feed waters and from frequently encountered problems in membrane installations. A range of iron concentrations was identified where a linear relationship existed between flux reduction rate and concentration. The performance of the Silt Density Index (SDI) was tested on the basis of the RO fouling data obtained. The range of iron concentrations where measurable and meaningful SDI values could be obtained was remarkably close to membrane manufacturer recommendations. A notable sensitivity of the SDI was also observed with particles for which retention is negligible. However, on the basis of the RO fouling data obtained, it appears that the SDI is not conservative enough. Furthermore, since the SDI cannot predict fouling rates, it cannot discriminate between different types of membranes.     

Low-pressure RO membrane desalination of agricultural drainage water

Agricultural drainage water is a complex mixture of dissolved and suspended chemical species and may contain a wide variety of microorganisms. The application of membrane systems for desalination of agricultural drainage (AD) water requires careful consideration of feed water quality, suitable membrane selection and operating conditions. In order to evaluate the potential applicability of low-pressure reverse osmosis (RO) to the treatment of AD water, a diagnostic approach to membrane selection and process evaluation was undertaken in support of a pilot field study in the California San Joaquin Valley. Five candidate membranes were evaluated in a diagnostic laboratory membrane system which provided an initial selection based on salt rejection and product water flux performance for model salt solutions of univalent and divalent cations. Biofouling potential of the selected membranes was also evaluated using two standards strains of bacteria. Preliminary pilot plant performance, based on the selected membranes, was encouraging and has provided the basis for long-term pilot plant testing at higher recoveries to assess the impact of fluctuating AD water feed composition.     

Sustainable fuel cell integrated membrane desalination systems

According to the United Nations, between two and seven billion people will face water shortages by the year 2050. Further, it is estimated that the amount of water available per person will shrink by a third during the next two decades. Inadequate supply of good-quality water coupled with higher water demand due to rapid population growth and industrialisation in developing countries are among the major reasons for the worsening water situation. Current shortages of potable water around the world and looming water scarcity especially in the developing countries is the driving force behind the implementation of membrane technologies for seawater and brackish water desalination. Typical energy consumption in seawater reverse osmosis (RO) plants operating at 40–45% product water recovery and with energy recovery from the high pressure reject stream currently is about 3–4 kWh/m³. The near-term goal of the industry is to reduce energy consumption to less than 2 kWh/m³ by using a combination of energy efficient RO pumps, more efficient energy recovery devices, high performance low energy RO membranes, hybrid membrane systems, advanced pretreatment technologies and alternate energy integrated membrane systems. The beneficial aspects of using alternate energy systems such as on-site distributed fuel cell systems integrated with membrane desalination units in remote locations are discussed.     

RO membrane autopsy of Zarzis brackish water desalination plant

Tunisia, located in a semi-arid zone, has limited water resources. The shortage of good water quality in particular at the south regions required a brackish water desalination to supply these regions with potable water. Zarzis desalination plant, working since 1999, has revealed difficulties related to the loss of membrane performances. In the aim to determine the reason of membrane fouling, foulant layer were analysed. The membrane autopsy has achieved by different methods, such as TOC (total organic carbon) measurement, SEM (scanning electron microscopy), AFM (atomic force microscopy), FTIR (fourier transform infrared spectroscopy) analysis, diffraction by X-ray. Results show that the foulant layer composed mostly of SiO₂, clay, organic matter (polysaccharide, protein), CaSiO₃, Fe₃O₄, AlPO₄, and CaSO₄. The characteristics bonds obtained by IR indicate the presence of polysaccharides and proteins that constitute a source for further microbiological growth; this is the problem of biofouling. The metallic elements results of corroded stainless steel and internal coating of cartridges filter that are damaged by the silica particle rejected by sand filters. The scale of CaSO₄ and CaSiO₃ deposition is owing to the bad antiscaling efficiency. The results are presented and discussed in the light of new trends in material.     

Global optimality of RO seawater desalination networks with permeate reprocessing and recycle

In conjunction with the optimality of single-stage reverse osmosis (SRO) system, this work addresses the economic competence of prominent seawater desalination reverse osmosis systems with permeate reprocessing and recycle using differential evolution as global optimization tool. The optimization approach refers to nonlinear programming formulation consisting of mass balances and design specifications as equality constraints and other design specifications as inequality constraints. Further, MATLAB optimization toolbox-based analysis was conducted to compare the obtained solutions with deterministic optimization methods. Among all considered alternatives such as two stages reverse osmosis (TRO) series arrangement with retentate reprocessing and permeate recycle (TRO-TSRR-PR), SRO, TRO series arrangement with permeate bypass and permeate reprocessing and TRO with permeate reprocessing series arrangement processes, TRO-TSRR-RP process configuration has been evaluated to competitive with the SRO process with an optimal cost of 0.9527$/m³ obtained with DE-SQP. While the SRO remains the simplest cost-effective system in terms of topological complexity, the TRO-TSRR-PR process has been evaluated to be 5.75% better than the optimal freshwater production cost of SRO process reported in the literature with an optimal decision variable set values [68.51 bar, 7688, 5881.7 m³/h, 259.8 ppm, 2422, 322.9 m³/h and 1764.9 ppm] for [P₁^F, NM₁, W_RO^RF, C₁^RP, NM₂, W₂^RP, C₂^RP].     

Optimal design of hybrid RO/MSF desalination plants Part II: Results and discussion

This paper presents the results of an optimization study, based on minimum water cost, to explore the feasibility of the hybridization of RO and MSF processes. The study explores the possible improvement of MSF process economics. Nine different scenarios for the production of the same capacity of desalted water are presented and compared from the standpoint of minimum water cost, specific capital cost and water recovery. The process and cost models, formulation of the optimization problem and solution outlines were previously presented in the first part of this study. In this work, results show that RO technology is recommended when building new desalination plants. RO technology becomes preferable at low feed concentrations and for brackish water desalination. Although they come in second position after the RO process, some hybrid plants economically exceed by far the MSF process. Computations gave a water cost of 1.1 $/m³ for the brine recycle MSF process against 0.75 $/m³ for the two-stage RO process. Water cost of the MSF process can be reduced by 17 to 24% through hybridization with RO technology.     

Optimal design of hybrid RO/MSF desalination plants Part III: Sensitivity analysis

This is the last paper in a series of three parts entitled “Optimal design of hybrid RO/MSF desalination plants”. This research is concerned with exploring the feasibility of hybridization of multi-stage flash (MSF) and reverse osmosis (RO) technologies in order to improve the performance characteristics and process economics ofthe conventional MSF process. The research project involved an optimization study where the water cost perunit product is minimized subject to a number of constraints. In the first part, the design and cost models were presented, the optimization problem formulated and solutions for a number of cases were outlined. In the second part, results were presented and discussed. In this paper we discuss the sensitivity of water cost from the alternative plant designs to variations in some cost elements and operating conditions. In general, it is concluded that, for the same desalting capacity, hybrid RO/MSF plants can produce desalted water at a lower cost than brine recycle MSF plants, while hybrid plants are characterized, by lower specific capital costs and higher water recovery fractions. Reduction in steam cost allows MSF to compete more with hybrid RO/MSF plants. This result explains the advantage of coupling MSF plants and steam power plants where the exhaust steam from the back pressure turbine represents a relatively cheaper source of heat for the MSF process. Results showed that the RO technology exceeds all other designs over the whole range of energy, chemicals and membrane costs studied here. However, water cost of the RO process was the most sensitive to variations in membrane and electricity costs compared to other hybrid configurations.     

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.     

Silica and metals removal by pretreatment to prevent fouling of reverse osmosis membranes

Reverse osmosis is being increasingly used for desalination of seawater, brackish water and as well the marginal and polluted wastewaters. The operation of waters bearing substantial amount of silica is only feasible if there is a chemical pretreatment in process to prevent excessive and unbearable fouling by this refractory deposit. In this study, model solutions representing tailings wastewater which usually has high silica content was used in standard jar tests using the Boltac Coagulation and Flocculation simulator to determine the effectiveness of chemical pretreatment (by precipitation and coagulation) for removal of silica and other species such as magnesium, calcium, iron and manganese which affect silica fouling. Two precipitants were tested: lime and soda ash, and caustic soda. Precipitant aids (alum and ferric chloride) were also examined in combination with optimum precipitant doses to observe their differential effects. The optimum precipitant dose of caustic soda was 200 mg/L. Ferric chloride and alum were tested with this optimum caustic soda dose, but did not improve removal significantly, if at all. The optimum precipitant dose of lime and soda ash was 150 mg/L lime and 450 mg/L soda ash. Ferric chloride and alum were tested with this optimum lime and soda dose, but again did not improve removal enough to warrant their use. Neither of the methods were as effective in removal of magnesium and calcium as in removal of other components; lime and soda ash treatment as expected actually increased the calcium content of solutions.     

反渗透膜元件及膜系统的数学模型

介绍了反渗透膜元件性能指标的差异,讨论了建立性能各异膜元件系统运行模型的必要性,给出了单支反渗透膜元件的运行数学模型,分析了并联及串联膜元件的运行数学模型,从而建立了各类膜系统结构的通用数学模型.     

短流程反渗透海水淡化预处理试验研究

针对我国沿海水质的特点,开发了集浸没式超滤及絮凝反应为一体的短流程反渗透海水淡化预处理设备,并进行了中试试验。结果表明,处理后产水水质可达到后续反渗透进水要求,SDI151%,浊度0.1 NTU,并可以在33~40 L/(m~2·h)的高通量下保持6个月稳定运行,可以应用于反渗透海水淡化系统的预处理过程中。     

短流程反渗透海水淡化预处理试验研究

针对我国沿海水质的特点,开发了集浸没式超滤及絮凝反应为一体的短流程反渗透海水淡化预处理设备,并进行了中试试验。结果表明,处理后产水水质可达到后续反渗透进水要求,SDI15<1%,浊度<0.1 NTU,并可以在33⁴0 L/(m40 L/(m²·h)的高通量下保持6个月稳定运行,可以应用于反渗透海水淡化系统的预处理过程中。     

Influence of colloidal fouling and feed water recovery on salt rejection of RO and NF membranes

The influence of colloidal fouling and feed water recovery (or concentration factor, CF) on salt rejection of thin-film composite reverse osmosis (RO) and nanofiltration (NF) membranes was investigated. Fouling experiments were carried out using a laboratory-scale crossflow test unit with continuous permeate disposal to simulate the CF and recovery as commonly observed in full-scale RO/NF systems. For feed waters containing only salt (NaCl), permeate flux declined linearly as CF was increased and salt rejection was nearly constant for both RO and NF membranes. On the other hand, a sharp decrease in permeate flux and significant decline in salt rejection with increasing CF were observed under conditions where colloidal fouling takes place. For both RO and NF membranes, the marked permeate flux decline was attributed to the so-called “cake-enhanced osmotic pressure”. The decline in salt rejection when colloidal fouling predominated was much more substantial for NF than for RO membranes. In all cases, the decline in salt rejection was higher under conditions of more severe colloidal fouling, namely at higher ionic strength and initial permeate flux.     

一种基于膜污染指数的超滤膜污染评价方法

利用浸没式中空纤维膜小试系统,基于膜污染指数（FI）建立超滤膜污染的评价和试验方法。通过试验考察超滤膜污染变化规律,以及运行周期对膜污染指数计算的影响,并确定计算膜污染指数的运行周期;并对膜污染指数评价方法的可重复性进行检验：对化学清洗前后膜污染指数进行比较。试验表明,在过滤初期,滤饼污染和吸附污染并存,所以初期水力不可逆膜污染指数（HIFI）增长较快;中期超滤膜的吸附能力不断减弱,膜污染主要是滤饼污染,所以膜污染较为稳定：后期跨膜压力过大,压缩滤饼层,减小滤饼孔隙率,增加滤饼阻力,所以HIFI有所增加。同时短期的膜污染试验可以用来反映长期的膜污染情况,该超滤膜污染的评价方法重复性好,污染后的超滤膜经过化学清洗后。可以重复利用。     

炼化污水抗污染反渗透膜回用处理中试研究

采用氧化混凝-复合过滤-反渗透组合工艺处理大庆炼化公司外排污水,考察了预处理系统的操作参数和抗污染反渗透膜组件性能的变化规律,为工程设计提供了基本参数。结果表明,预处理出水水质主要受过滤速度和絮凝剂加药量的影响,而膜污染是影响反渗透系统运行的主要因素。系统出水水质符合工业用水软化除盐设计规范。     

RO membrane solute rejection behavior at the initial stage ofcolloidal fouling

This study investigated the rejection of salt and inert organic compounds by reverse osmosis membranes during the initial stage of colloidal fouling. Results of laboratory-scale experiments showed that colloidal fouling caused a marked decrease in flux, salt rejection and rejection of organics with molecular weight (MW) smaller than about 100 g/mol. Removal of neutrally charged organics was mainly by size or steric exclusion. Rejection of xylose, which has MW >100 g/mol, was not affected much by colloidal fouling. The decrease in salt and low MW organic rejections during the initial stage of colloidal fouling was attributed to cake-enhanced concentration polarization, whereby the colloidal cake layer hindered back diffusion of solutes from the membrane surface to the bulk solution, resulting in higher solute concentration gradient across the membrane. At higher channel wall shear rate, the rates of colloidal deposition, flux decline, decrease in salt rejection, and decrease in low MW organic rejection were lower.     

Bench-scale evaluation of seawater desalination by reverse osmosis

In bench-scale tests of seawater reverse osmosis desalination it is important to carefully consider osmotic pressure effects and determine the extent of concentration polarization so that sources of flux variation—whether from fouling, compaction, or osmotic pressure changes—can be properly assessed. Rigorous modeling of concentration polarization is difficult because of the complex geometries and flow regimes in RO modules; typically, concentration polarization must be measured. However, concentration polarization measurement usually requires knowledge of membrane permeability, which can vary from coupon to coupon. In this study a method is presented to determine both the membrane permeability and the concentration polarization regime in a single test. The key to the test is to allow the salt concentration to vary over time in a predictable way and extract parameters from a model fitted to the flux data. The usefulness of this technique is highlighted by evaluating results from several seawater experiments. It was found that specific flux decline in the experiments was caused by changes in osmotic pressure and membrane compaction. RO fouling by seawater organic-matter was not significant for the several seawater samples tested.