Some aspects of silica polymerization and fouling and its pretreatment by sodium aluminate, lime and soda ash

In desalination of brackish water, silica is one of the major foulants that forms on the reverse osmosis membranes and limits the water recovery. In addition, it is a very adherent scale and once it forms, it is very difficult to clean and cleaning may damage the membrane. There are also complicating factors affecting silica fouling, such as presence of cations (e.g. Ca, Mg, etc), that usually promote silica polymerization. Pretreatment is used as a measure to reduce silica levels in the feed and hence mitigate silica fouling. This paper examines the effect of temperature, cations on silica polymerization as well as silica removal by sodium aluminate, lime and soda ash in laboratory tests using field waters.     

核电站海水淡化系统反渗透膜的在线清洗

反渗透膜的污染是反渗透系统应用中最大的问题之一。及时的清洗可有效地恢复反渗透膜的性能、延长其使用寿命。本文以某核电站海水反渗透项目为例,对海水反渗透膜的在线清洗技术进行了介绍。实践表明,定期的物理清洗和适当的化学清洗可以保证海水反渗透装置的良好运行。     

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.     

Cellulose membranes for reverse osmosis part II. Improving RO membranes prepared from non-woody cellulose

Experiments were carried out to minimize the stages of preparing reverse osmosis (RO) desalination membranes at economical cost and to improve the transport properties of RO membranes prepared from non-wood fibrous materials (cotton linters and bagasse pulp) to approach those prepared from imported viscose pulp and purchased cellulose acetate (see Part I). Further study was carried out on examining the effects of long-term testing and subsequent washing of membranes by a special mixture. Finally the quality of the product water reverse osmosis was examined by comparing its water analysis with standard specification for drinking and domestic water.     

Scaling salts removal in RO concentrates by inorganic induced method

Scaling on heat transfer surface and reverse osmosis （RO） membrane surface is one of the main problems in desalination processes. To mitigate scales, anti-scalants are often used. For RO system, membrane concentrates contain high amounts of dissolved salts and anti-scalants. Commonly, concentrates are discharged directly. In fact, concentrates are high quality water due to the strict pre-treatment of RO feed water.     

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.     

地下水反渗透除盐预处理工艺的优化

地下水基本没有悬浮物,污染指数SDI低,比较适宜做反渗透的水源,但硬度高,容易引起反渗透膜结垢,故地下水反渗透除盐,预处理的重点是防止膜表面结垢。软化法预处理操作繁琐,设备复杂,讨论了在地下水中加入阻垢剂防止反渗透膜表面结垢的方法,结果表明此法有效可行。     

反渗透膜的污染及其化学清洗

反渗透膜的污染是反渗透系统应用中最大的问题之一,及时的化学清洗可有效地恢复反渗透膜的性能、延长其使用寿命。对反渗透膜的污染及其化学清洗方法进行了综述,主要内容包括反渗透膜污染物的种类,污染物形成的原因,以及清洗时机的选取,清洗药品的选择,影响清洗效果的主要因素、清洗系统的构成,化学清洗的方法和清洗效果的评价指标。     

反渗透海水淡化的预处理技术研究进展

本文分析了国内外关于反渗透海水淡化预处理技术的应用研究现状，并对预处理技术的发展趋势做了展望。     

Chemical control of colloidal fouling of reverse osmosis systems

Fouling of membranes by colloidal organic and inorganic particles continues to be documented as the most common and challenging obstacle in attaining stable continuous operation of reverse osmosis (RO) and ultrafiltration (UF) systems. Much current research is being conducted on physical parameters to mitigate such fouling. The focus has been on membrane synthesis and element design; microfiltration and ultrafiltration pretreatment; electromagnetic devices; correlation with physical factors such as Silt Density Index, zeta potential and critical flux; technique of direct observation of fouling process through a membrane; and classification of macromolecular organics for correlation with fouling characteristics. We report initial successes with chemical control of colloidal fouling. Through screening with a large number of observable coagulations of natural colloids, we have developed a group of proprietary anticoagulants and dispersants that would, at less than 10 ppm dosage to the RO feedwater, control various classes of colloidal foulants. Case studies of the control of humic matter, elemental sulfur and colloidal silicate in problematic RO systems that became stabilized are briefly presented. We conclude that a great need and potential exists in economically controlling the myriads of fouling interactions of colloidal particles during concentration within the brine channels of RO membrane elements. Low dosages of antifoulants can in many cases obviate the need for installation and maintenance of pretreatment unit or operations designed to remove such colloidal foulants from the process stream.     

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。     

KDF过滤介质在反渗透预处理中的应用

介绍了一种反渗透预处理的新方法,阐述了应用KDF过波介质控制结垢形成,减少悬浮固体,减少氧化剂,控制细菌和藻类,减少重金属和减少硫化氢的机理,并给出了应用实例。     

Forward-osmosis strategy and chemical cleaning of seawater desalination reverse osmosis membranes

In order to settle the membrane fouling of reverse osmosis membranes in seawater desalination process, this study reported a novel strategy based on forward-osmosis process and discussed the effects of different factors like different cleaning combination among reverse osmosis product, simulated reverse osmosis concentrate and simulated seawater, as well as cleaning time on the membrane permeate flux and salt rejection. For irreversible fouling, the effects of different chemical cleaning agents, immersion time and concentration were also investigated in this study. The results exhibited that the cleaning combination between diluted water and simulated reverse osmosis concentrate possessed the best cleaning performance in the process of forward-osmosis cleaning. Such approach also enhanced normalized flux from 9.48 L/(m²·h·MPa) to 13.6 L/(m²·h·MPa) and enhanced NaCl rejection from 80.59% to 92.80%. Furthermore, the normalized flux was enhanced from 9.48 L/(m²·h·MPa) to 14.3 L/(m²·h·MPa) and NaCl rejection was also enhanced from 80.59% to 96.27% after soaking in 2%(mass) citric acid solution for 2h, soaking with 1%(mass) ethylenediamine tetra-acetic acid tetrasodium salt and 0.3%(mass) sodium tripolyphosphate solution for 1.5 h. According to the result of SEM images and AFM images, the forward-osmosis cleaning strategy could not cause the damage of selective layer of membrane surface and caused the drop of inorganic and organic fouling on the membrane surface. Hence, cleaning fouled RO membranes by such approach could prolong the chemical cleaning cycle and reduce the amount of chemical cleaning agent, which has certain industrial application perspectives.     

正渗透策略和化学清洗海水淡化反渗透膜

为解决海水淡化过程中反渗透膜的污染问题,研究了基于正渗透策略的反渗透产水、模拟反渗透浓水、模拟海水不同的组合清洗和清洗时间对膜通量和截留率的影响。针对不可逆污染,研究了不同化学清洗药剂、浸泡时间、浓度对膜通量和截留率的影响。结果表明,正渗透策略清洗方式中,淡水/模拟反渗透浓水的组合清洗方式效果最佳,其归一化通量从9.48 L/(m²·h·MPa)提升至13.6 L/(m²·h·MPa),截留率从80.59%提升至92.80%。此外,经质量分数为2%的柠檬酸溶液浸泡2 h后,再使用质量分数为1%的乙二胺四乙酸四钠盐和0.3%的三聚磷酸钠溶液浸泡1.5 h,其归一化通量从9.48 L/(m²·h·MPa)提升至14.3 L/(m²·h·MPa),截留率从80.59%提升至96.27%。从SEM和AFM图可以看出,正渗透清洗策略并未对膜表面选择层造成损坏,且可以清洗膜表面的有机污染物和无机污染物,因此,应用这种方法对污染的反渗透膜进行清洗,可延长化学清洗周期,减少化学清洗剂用量,具有一定的工业应用前景。     

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.     

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.     

反渗透海水淡化系统中膜组件的清洗策略

反渗透海水淡化法中膜组件的定期清洗是防治膜污染的主要措施之一.膜清洗可将膜表面的污染物去除,达到恢复透水量的目的.在保证产水量的情况下,选择恰当的清洗策略和清洗时机可使清洗次数最合理,清洗费用最低.以规划期内总操作费用最小为目标,提出了新的污染模型,给出了清洗的判据,将清洗策略问题表达为一个混合整数非线性规划（MINLP）.对一个实例进行计算,给出了该例中的反渗透系统在5年规划期内的最优清洗策略和清洗时机.结果证明这种数学规划方法对于寿命为3～5年的膜组件的清洗维护是有效的.     

全流程卷式反渗透海水淡化系统操作优化

在对反渗透海水淡化系统流程和实际应用分析的基础上,提出了一种旨在降低总体操作费用的全流程反渗透海水淡化系统优化方法。首先根据系统变参数特点并充分利用蓄水池的缓冲能力,建立了反渗透过程机理模型、蓄水池动态过程模型以及变参数方程模型,实现了整个流程的方程描述。然后根据工艺流程和操作过程费用组成情况建立了总的操作费用模型,得到了全流程单位产水费用指标。在此基础上建立了以总体操作费用最低为目标、以开放方程描述的各模型方程为约束、以设备和产品质量限制为边界的优化命题,采用联立求解技术将该微分代数方程组成的优化（DAOP）问题转化为NLP问题后进行求解。最后对某海水淡化系统进行了实例研究。优化求解结果不仅表明本优化方法可以大幅降低实际操作费用,而且通过求解还可得到各种变参数条件下最优操作费用组成,以及实现费用最低的最优操作压力和流量变化曲线。本研究对优化海水淡化系统操作、降低总体操作费用具有重要意义。     

内外压式超滤组件在反渗透预处理中的应用比较

分别选取一种内压式和一种外压式中空纤维膜超滤组件,从超滤膜材质、运行参数和出水水质等方面对这2种组件在海水淡化反渗透预处理中的应用做了比较。结果表明:由于超滤膜的材料不同,使得2种组件运行工艺有较大差异:内压式组件在跨膜压差和膜通量方面要好于外压式组件,而2种组件出水水质在同一水平上。在进水浊度小于20NTU时,内压式膜组件更适合作为反渗透的预处理工艺。