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.     

Synergistic behavior between silica and alginate: Novel approach for removing silica scale from RO membranes

The formation of mineral scale deposits on membranes is a pervasive and expensive problem for the water treatment industry. A series of experiments run on a laboratory-scale reverse osmosis membrane system examined the fouling of membranes when the feed water was spiked with organic and inorganic foulants. Alginic acid was used as the organic foulant and silica was used as the inorganic foulant. Studies involving interactions of these two foulants have not previously been reported in literature. Experiments were run with each foulant individually to characterize fouling at different velocities and pressures. Experiments were then run using both foulants together to characterize the synergistic effects on membrane fouling. One set of experiments with both foulants demonstrated that alginic acid inhibits silica fouling on reverse osmosis membranes. Further experiments indicated that alginic acid added after silica fouling had already occurred was able to remove silica scale from the membrane and restore permeate flux.     

Economic benefits of membrane technology vs. evaporator

European Union food ingredients production operations face operating cost challenges. Ingredient production steps frequently incorporate product recovery steps employing centrifugation, rotary vacuum filtration, evaporation and spray drying demanding high energy inputs.Product and process waste streams contain small molecular weight components that can be concentrated, desalted or, in some applications, fractionated with membrane filtration technology.Substantial capital and operating cost benefits together with selective separations can be derived by use of low energy consumption membrane technology as a complete stand alone process step or in conjunction with evaporation enabling product recovery and water for reuse.Reverse osmosis membrane technology can offer greater than 75% reduction in operating cost when compared with 5 multi-effect evaporation with thermal vapour re-compression process. Mechanical vapour recompression evaporators offer the lowest operating cost, similar to membrane technology, but with a considerably higher capital investment.Many companies which employ evaporation as a standard de-watering step in the manufacture of products such as dairy food ingredients, sugars, sweeteners, beverages, organic acids, renewable source bio-fuels, etc., can potentially reduce their carbon foot print with membrane technology.     

Pilot study for reclamation of secondary treated sewage effluent

A pilot study for reclamation of secondary treated sewage effluent in Singapore was conducted using a MF/RO system with the capacity of 20 m³/d. A 0.1 μm MF membrane from Asahi and RE-4040-FL RO membrane from Saehan were used in this study. The pilot plant consists of six spiral-wound RO elements. The RO train was configured in single stage. The pilot plant was designed with automatic control system and it was operated continuously (24 h) during the study. Trial runs on various flux rates of the RO membrane at different operating pressures were conducted over 3 months. The pilot results showed that the optimal operation flux rate of the RO membrane ranged from 10 to 15 gal/f²/d (GFD) for this application. The normalized flux after CIP was 97% of the initial one. At a flux rate of 10 GFD and water recovery of 50%, the average operating pressure of 57 psi was noted corresponding to a high normalized flux of 38 L m⁻² h⁻¹ MPa⁻¹ at 25°C. Rejections of the RO membrane in terms of conductivity, TOC, ammonium and nitrate were higher than 96%,97%,90% and 85%, respectively. It was concluded that the RO permeate quality in terms of conductivity, turbidity, TOC, ammonium, nitrate, hardness, total bacteria and total coliform matched the quality of high-grade water (NEWater) for use in the electronics industry.     

Flow Parameter Profiles in the Crossflow of a Two-Component Fluid through Semipermeable Membranes

Abstract

The analytical solution for spiral-wound module performance is a useful tool for developing explicit equations for the local values of variables like effective pressure, water flux, salt wall concentration, and velocities for seawater feed solutions. Depending on the operating conditions, knowledge of the local values of these variables could be useful to predict possible areas on the membrane surface where scale formation or fouling is likely to occur. Reasonable values for all variables have been found by using the developed equations at any point in the permeate and the feed side of the membrane. Although the method has been applied for spiral wound reverse osmosis membranes, it is believed that the same method could be used in similar hydrodynamic situations where flow through porous media is taking place.     

A general approach to modelling membrane modules

Mathematical models of hollow-fibre and spiral-wound membrane modules are presented in this paper. The models are developed from rigorous mass, momentum and energy balances and can be used to describe a generic membrane separation. This is in contrast to most existing models which are typically process specific and are only valid within a limited operating range. The generality of the new approach is demonstrated by application to gas separation, pervaporation, and reverse osmosis case studies. Simulation results for these systems show excellent agreement with experimental data.     

硅垢防垢剂ITSA合成及性能研究

由于三元复合驱中碱的引入,导致采出液在采出系统中形成大量硅垢,而硅垢一旦形成便难以去除。自共聚物防垢剂开发以来,针对硅垢的防垢剂研究报道较少,因此硅垢共聚物防垢剂一直是油田处理药剂研究的热点。作者以衣康酸(IA)、三乙醇胺(TEA)、烯丙基磺酸钠(SAS)、丙烯酰胺(AM)为单体,过硫酸铵为引发剂,合成了一种新型四元共聚物硅垢防垢剂,简称ITSA。通过实验确定合成共聚物防垢剂的最佳工艺条件为:聚合温度65℃,聚合时间3h,m(引发剂)∶m(单体)=8%,单体配比n(IA)∶n(TEA)∶n(SAS)∶n(AM)=1.0∶1.8∶0.2∶1.5。在此合成条件下对硅垢的防垢率达到72.11%。说明该四元共聚物防垢剂对硅垢有较好的防垢效果。防垢剂的作用机理主要是晶格畸变和分散作用。     

The effect of the silica content of silica fume on its ability to control alkali-silica reaction

The use of silica fume (SF) has been instrumental in the development and utilization of high-strength and high-performance concrete. In the interests of economics, questions have been raised regarding the possible use and effectiveness of “lower grade” SFs with SiO₂ contents less than 85%. Such materials do not meet current CSA and ASTM standards for SF. In this study, the performance of two SFs from the same U.S. plant but with different silica contents (68% and 88% SiO₂) were compared by examining the effect of the materials on the expansion due to alkali-silica reaction (ASR) and the composition of the pore solution. The mixtures tested with these procedures included 0%, 4%, 8%, and 12% SF replacement by mass of cement. Results show that the SF with lower than standard silica contents cannot control ASR at the levels of replacement examined in this program.     

pH and Particle Structure Effects on Silica Removal by Coagulation

Coagulation is presented as an efficient alternative to reduce the silica content in effluents from recovered‐paper mills that are intended to be recycled by a final reverse‐osmosis (RO) step. Coagulation pretreatment by several polyaluminum chlorides (PACls) or FeCl₃ was optimized prior to the RO process. PACls with low alumina content and high basicity achieved almost a 100 % removal of silica at pH 10.5. A good reduction of the silica content was attained without regulating the pH by adding one of these PACls. Silica removal was related to the structure of the produced clots in which cylindrical particles produced higher silica removal. All coagulants removed more than 50 % of the chemical oxygen demand (COD).     

Considerations for selection of seawater filtration pretreatment system

Two types of pretreatment systems are typically used to protect seawater reverse osmosis membranes from fouling: conventional granular media filtration and membrane filtration. While granular medial filtration is still a dominating seawater pretreatment technology, microfiltration and ultrafiltration membrane systems for seawater pretreatment have evolved rapidly over the past decade and may offer cost and performance benefits for the site specific conditions and challenges of a given seawater desalination project. This article presents a critical review of widely used conventional granular and membrane pretreatment technologies and addresses key factors, and issues which would need to be taken under consideration when selecting a seawater pretreatment system.     

Characterization of uncertainties in the operation and economics of the proposed seawater desalination plant in the Gaza Strip

In the Gaza Strip, the available freshwater sources are severely polluted and overused. Desalination of seawater through reverse osmosis (RO) has become the most realistic option to meet a rapidly growing water demand. It is estimated that the Gaza Strip will need to develop a seawater desalination capacity of about 120,000 m³/d by the year 2008, and an additional 30,000 m³/d by the year 2016 in order to maintain a fresh water balance in the coastal aquifer and to fulfill the water demand for different uses in a sustainable manner. Cost and reliability of a large RO facility are still subject to much uncertainty. The cost of seawater desalination by RO systems varies with facility size and lifetime, financing conditions, intake type and pre-treatment requirements, power requirements, recovery rate, chemicals cost, spare parts cost, and membrane replacement cost. The permeate salinity is a function of feed water temperature, recovery rate, and permeate flux. The quantity of water produced depends mainly on plant size, recovery rate, and operating load factor. Many of these parameters are subject to a great deal of uncertainty. The objective of this work is to develop a probabilistic model for the simulation of seawater reverse osmosis processes using a Bayesian belief network (BBN) approach. This model represents a new application of probabilistic modeling tools to a large-scale complex system. The model is used to: (1) characterize the different uncertainties involved in the RO process; (2) optimize the RO process reliability and cost; and (3) study how uncertainty in unit capital cost, unit operation and maintenance (O&M) cost, and permeate quality is related to different input variables. The model utilizes information from journal articles, books, expert opinions, and technical reports related to the study area, and can be used to support operators and decision makers in the design of RO systems and formulation of operational policies. The structure of the model is not specific to the Gaza Strip and can be easily populated with data from any large-scale RO plant in any part of the world.     

以硅灰、白炭黑、硅溶胶为硅源合成碳化硅晶须的研究

以硅灰、白炭黑、硅溶胶为硅源,炭黑为碳源,采用碳热还原法合成碳化硅晶须,通过XRD及SEM对合成产物的物相及形貌进行分析,探讨了合成温度(分别为1 400、1 450、1 500、1 550℃)、硅源、n(C)∶n(SiO2)对合成碳化硅晶须的影响.结果表明:n(C)∶n(SiO2)为2.4～3.6,合成温度为1 500 ℃,保温3 h时,硅溶胶与炭黑反应没有生成碳化硅晶须,硅灰、白炭黑与炭黑反应均生成碳化硅晶须;以硅灰为硅源合成碳化硅晶须的质量及数量明显优于以白发黑为硅源合成碳化硅晶须;合成碳化硅晶须的最佳n(C)∶n(SiO2)为3.3.     

Forecasting the economic costs of desalination technology

Government policy, in the form of grants and contracts for desalination technology, has had a major impact on steadily declining costs of desalination. The process, reverse osmosis (RO), exhibits economies of scale, which increases its feasibility as a water treatment technology for large populations. Ultrafiltration, an RO pre-treatment, also shows economies of scale. The real economic costs of desalination technology can be forecast using an ARIMA model. If these costs fall below those of conventional water treatment processes, RO and ultrafiltration become competitive with conventional water treatment technology. Our ARIMA forecasts are validated by using independent plant level cost data.     

The liquid phase removal of no and H2S pollutants using membrane separation of the homogeneous catalyst

Using the liquid phase reaction 2NO + H₂S + 2O₂ 2HNO₃ +S in the presence of heteropolyanion catalyst PW₁₁Fe¹¹(H₂O)O^5– ₃₉, the potentialities of the membrane method in separation of a homogeneous catalyst from a soluble reaction product have been demonstrated. The use of membranes for this purpose is expected to give a further impact for the search of industrial processes based on homogeneous catalysis.     

甘油三酸脂对硅溶胶的分散作用

通过对硅溶胶泥浆的流变性实验确定了硅溶胶泥浆的流变模型，验证并分析了加入少量经过低温氧化过的甘油三酸脂（ＧＴＯ）不改变硅溶胶泥浆的流变模型，但能有效地改善其流变性能。因此，ＧＴＯ可作为硅溶胶结合浇注料的高效减水剂。     

Discussion of silica speciation, fouling, control and maximum reduction

In the production of ultrapure water for the power and microelectronics industries, multiple pass reverse osmosis (RO) process is commonly the major step in the reduction of dissolved and suspended matter before polishing by ion exchange and other methods to attain the high purity requirements. With the diverse location of power plants and microelectronic manufacturing facilities around the world, silica concentrations in source waters can range between 1 and 60 ppm (mg/L) to even 300 ppm in some volcanic regions. High pressure steam generators and fine microelectronic structures now require water containing less than 1 ppb (ug/L) concentrations of silica. In designing purification processes, silica has presented issues not only as formidable challenges in many locations as RO membrane foulants, as well as a contaminant requiring efficient removal. Analyses of RO membrane foulants and correlation with water chemistry in the course of trouble-shooting numerous RO processes continues to offer us opportunities to understand silica chemistry, the patterns of silica fouling and methods by which we can chemically control the RO process. Such understanding is applicable to the operation of ion-exchange resin beds as well. In this paper we review the speciation of silica in feedwaters, and chemical approaches to controlling fouling and maximizing silica reduction. Silica and silicates are addressed in the three categories of reactive soluble, non-reactive soluble (colloidal, not filterable) and non-reactive insoluble (particulate, filterable) forms. A brief review of geochemistry, the chemical and biochemical dissolution and deposition of silica and silicates in nature is provided for insights and understanding of natural processes that can be applied to the task of process design and control in silica removal from water.     

硅溶胶结合浇注料的流变性能研究

对高纯硅溶胶电熔莫来石细粉泥浆的流变性能研究表明：这种泥浆是存在触变结构的高浓度胶溶悬浮体系，在受较低剪切力作用时产生的是粘滞流动，总的表现符合Ｂｉｎｇｈａｍ流变方程。研究还表明：少量超细粉的加入不改变泥浆的流变模型，但能有效地改善其流变性能，不同的超细粉影响力不同，且这种改善作用有一最佳加入量（活性ＳｉＯ２超细粉影响力大于活性Ａｌ２Ｏ３微粉，其最佳加入量分别为２％和１０％），超过这一加入量将导致流变常数上升，使其流变性恶化。     

Hydration,porosity and water dynamics in the polyamide layer of reverse osmosis membranes: A molecular dynamics study

This study employs molecular dynamics to gain insight into the mechanisms underlying the formation, hydration and functioning of aromatic polyamide membrane used for water purification by reverse osmosis. A polyamide membrane structure was successfully generated with a chemical composition and degree of crosslinking that closely match recent experimental data. The density of dry polymer was found to be slightly lower than in previous studies but closer to recent experimental measurements. The volume expansion and mass gain upon hydration show marked variations due to the presence of a significant fraction of permanent voids, revealed by dynamically averaged water concentration maps. The mass hydration and porosity were also larger than previously obtained, but water self-diffusivity was similar to previous simulations, presumably, since the larger porosity was offset by the higher cross-linking. Using a radial distribution function of water within polyamide, strong next-neighbor correlations and random long-distance water–water correlations could be differentiated, which could be assigned to small network and large aggregate pores. Unfortunately, this dual porosity picture could not be linked to the experimentally found bimodal distribution of the pKa values of the COOH groups, since the distribution of the local water content around COOH was found to be unimodal.