A novel ammonia—carbon dioxide forward (direct) osmosis desalination process

A novel forward (direct) osmosis (FO) desalination process is presented. The process uses an ammonium bicarbonate draw solution to extract water from a saline feed water across a semi-permeable polymeric membrane. Very large osmotic pressures generated by the highly soluble ammonium bicarbonate draw solution yield high water fluxes and can result in very high feed water recoveries. Upon moderate heating, ammonium bicarbonate decomposes into ammonia and carbon dioxide gases that can be separated and recycled as draw solutes, leaving the fresh product water. Experiments with a laboratory-scale FO unit utilizing a flat sheet cellulose tri-acetate membrane demonstrated high product water flux and relatively high salt rejection. The results further revealed that reverse osmosis (RO) membranes are not suitable for the FO process because of relatively low product water fluxes attributed to severe internal concentration polarization in the porous support and fabric layers of the RO membrane.     

正向渗透膜分离技术及其应用综述

正向渗透是一项新型的利用半透膜两侧溶液渗透压差作为驱动力的膜分离技术。文章介绍了正向渗透膜分离技术的原理和影响因素,对其在各个领域(包括海水淡化、废水处理、橙汁浓缩、水袋)的研究进展进行了综述。现有的研究表明,可用于正向渗透工艺的膜不同于常规的反渗透膜,需要从膜结构开发适合的膜组件;采用NH3和CO2制备提取液是目前研究中具有应用前途的方式之一,具有产水率高且易于分离浓缩的优点。     

优化聚酰胺分离层制备高选择透过性反渗透膜

反渗透（RO）膜分离技术由于具有高效、低耗和产水水质高等优点，已成为现阶段解决水资源短缺的有效手段。进一步提高RO膜的选择透过性能有利于降低产水成本和提高产水质量，因此制备高选择透过性能的RO膜一直是膜领域研究的重点。从优化界面聚合工艺、优化基膜及开发新型制膜工艺三方面对近年来改善RO膜选择透过性能的研究进行了综述。通过优化界面聚合工艺和开发新型制膜工艺可以直接改变分离层的结构和性质，通过调节基膜的孔径、孔隙率及亲疏水性可以影响分离层的结构，从而改善RO膜的性能。最后对制备高选择透过性能的反渗透膜的研究方向与发展前景进行了总结与展望。     

Distillation vs. membrane filtration: overview of process evolutions in seawater desalination

The worldwide need for fresh water requires more and more plants for the treatment of non-conventional water sources. During the last decades, seawater has become an important source of fresh water in many arid regions. The traditional desalination processes [reverse osmosis (RO), multi stage flash (MSF), multi effect distillation (MED), electrodialysis (ED)] have evoluated to reliable and established processes; current research focuses on process improvements in view of a lower cost and a more environmentally friendly operation. This paper provides an overview of recent process improvements in seawater desalination using RO, MSF, MED and ED. Important topics that are discussed include the use of alternative energy sources (wind energy, solar energy, nuclear energy) for RO or distillation processes, and the impact of the different desalination process on the environment; the implementation of hybrid processes in seawater desalination; pretreatment of desalination plants by pressure driven membrane processes (microfiltration, ultrafiltration and nanofiltration) compared to chemical pretreatment; new materials to prevent corrosion in distillation processes; and the prevention of fouling in reverse osmosis units. These improvements contribute to the cost effectiveness of the desalination process, and ensure a sustainable production of drinking water on long terms in regions with limited reserves of fresh water.     

反渗透膜的研究进展与展望

反渗透具有低能耗、高效率等突出优点,是目前应用最为广泛的分离技术之一。反渗透膜的性能是影响反渗透过程效率的决定因素,反渗透膜的研制一直是国内外膜领域的研究热点。特别是近年来,石墨烯、碳纳米管等新型材料展现出优异的水传递行为,成为新型反渗透膜材料的研究热点。本论文回顾了反渗透膜的研制发展历程,介绍了不同单体通过界面聚合反应成膜的研究进展,综述了国内外新型混合基质膜和无机分子筛反渗透膜材料及其成膜研究,最后提出了新型反渗透膜的研究方向。     

反渗透系统的污染及清洗

反渗透作为水处理领域的一种高新除盐技术应用日益广泛。如何防止膜元件的污染及污染后的清洗是当前反渗透技术应用中的一个难点。本文结合长铝公司 40 0m3 /h反渗透系统的运行和清洗经验 ,就该问题进行一些探讨     

海水反渗透淡化技术的分析与探讨

反渗透海水淡化技术是一种高效、节能、先进的液体分离技术.论述了目前国内外海水反渗透淡化技术的应用现状,着重介绍了反渗透膜材料及特点、膜污染及清洗、典型的海水反渗透淡化流程,探讨了反渗透海水淡化技术目前存在的问题及未来发展趋势.     

Study of the structure and transport of asymmetric polyamide membranes for reverse osmosis using the electron spin resonance (ESR) method

The electron spin resonance technique (ESR) was used to study the structure and transport of asymmetric aromatic polyamide membranes. TEMPO (2,2,6,6-tetramethyl-1-piperridinyloxy free radical) was used as a spin probe that was brought into the membrane either by (a) immersion ofthe membranes in aqueous TEMPO solutions, (b) reverse osmosis (RO) experiments with feed solutions involving TEMPO or (c) blending TEMPO in casting solutions. The membranes were further tested for the separation of sodium chloride and TEMPO from water by RO. It was concluded that aromatic polyamide membranes contain water channels in the polymer matrix like cellulose acetate membranes. The presence of such water channels allows aromatic polyamide membranes to be used as RO membranes. The diffusion of organic solutes through the water channels seems much slower in aromatic polyamide membranes than in cellular acetate membranes, which probably causes a higher separation of organic solutes by aromatic polyamide membranes than cellulose acetate membranes. A comparison was made with other RO membranes (cellulose acetate, CA) and ultrafiltration membranes (polyethersulphone, PES). It was observed that the ESR technique can be used to study the structure of OF and RO membranes. The presence of water channels in the polymer matrix seems indispensable for the RO membrane.     

正渗透技术及其应用

正渗透技术作为一种新型的膜分离技术，因其能耗低、污染少等特点已经引起了广泛的关注。目前正渗透己经在海水淡化、污水净化、食品处理、药物释放和能源等领域得到了迅速的发展。综述了正渗透的基本原理、技术特点及其应用现状，并展望了该领域的未来前景。     

正渗透原理及分离传质过程浅析

正渗透是一种新兴的膜分离技术,因其低能耗、抗污染、对污染物截留能力广等的潜力,在脱盐、废水处理、农业和电力等领域的应用前景备受瞩目。本文介绍了正渗透概念和原理,通过正渗透传递过程的现象学模型,对浓差极差极化与质量传递的关系作出分析,提出了强化正渗透传质过程的一些建议。     

Reverse osmosis performance of organosilica membranes and comparison with the pervaporation and gas permeation properties

Hybrid organosilica membranes were successfully prepared using bis(triethoxysilyl)ethane (BTESE) and applied to reverse osmosis (RO) desalination. The organosilica membrane calcined at 300^°C almost completely rejected salts and neutral solutes with low‐molecular‐weight. Increasing the operating pressure led to an increase in water flux and salt rejection, while the flux and rejection decreased as salt concentration increased. The water permeation mechanism differed from the viscous flow mechanism. Observed activation energies for permeation were larger for membranes with a smaller pore size, and were considerably larger than the activation energy for water viscosity. The organosilica membranes exhibited exceptional hydrothermal stability in temperature cycles up to 90^°C. The applicability of the generalized solution‐diffusion (SD) model to RO and pervaporation (PV) desalination processes were examined, and the quantitative differences in water permeance were accurately predicted by the application of generalized transport equations. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1298–1307, 2013     

新型准对称无机膜的正渗透去除Cd2+的效能

正向渗透(forward osmosis, FO)是一种以溶液渗透压差为驱动力的新型膜技术。课题组在先前研究中使用微界面溶胶凝胶法制备了一种全新的准对称结构无机薄膜(QSTFI膜), 与传统的有机聚合FO膜相比具有更大的优势。本文考察了QSTFI膜分离去除水中重金属Cd²⁺的效能, 讨论了Cd²⁺浓度、提取液浓度以及膜表面带电性对Cd²⁺去除的影响机制。采用扫描电子显微镜(SEM)表征了QSTFI膜的微观形貌, 使用能量色散光谱(EDS)、傅里叶变换红外光谱(FTIR)表征了膜的化学组成, 并使用原子力显微镜(AFM)表征膜表面带电特性。结果表明, QSTFI膜表面带负电荷, 能够与液相主体中的Cd²⁺通过静电引力形成双电层结构, 双电层的Debye厚度越大越有利于膜对Cd²⁺的截留。FO实验测试中原液Cd²⁺浓度为10 mg·L^-1的条件下, QSTFI膜对Cd²⁺截留率超过99%, 水通量最大值可达到69 L·m^-2·h^-1(提取液为2.0 mol·L^-1 NaCl)。本研究为拓展FO技术在含重金属废水处理的潜在应用前景提供了理论依据和指导。     

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。     

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.     

Characteristics and potential applications of a novel forward osmosis hollow fiber membrane

There has been a resurgence of interest in forward osmosis (FO) as a potential means of desalination, dewatering and in pressure retarded osmosis, which Sidney Loeb was advocating over 3 decades ago. This paper describes the characteristics and potential applications of a newly developed FO hollow fiber membrane, which was fabricated by interfacial polymerization on the inner surface of a polyethersulfone (PES) hollow fiber. This FO membrane presents excellent intrinsic separation properties, with a water flux of 42.6 L/m² h using 0.5 M NaCl as the draw solution and DI water as the feed with the active layer facing the draw solution orientation at 23 °C. The corresponding ratio of salt flux to water flux was only 0.094 g/L, which is superior to all other FO membranes reported in the open literature. To evaluate different application scenarios, various NaCl solutions (500 ppm (8.6 mM), 1 wt.% (0.17 M) and 3.5 wt.% (0.59 M)) were used as the feed water to test the performance of the FO membrane. The membrane can achieve a water flux of 12.4 L/m² h with 3.5 wt.% NaCl solution as the feed and 2 M NaCl as the draw solution, suggesting it has good potential for seawater desalination.     

Study on Potassium Permanganate Chemical Treatment of Discarded Reverse Osmosis Membranes Aiming their Reuse

Membrane-based industrial desalination/demineralization plants generate a considerable amount of discarded membranes. This work studies the effects of the chemical treatment with potassium permanganate solutions in combination with chemical cleaning of three years old composite polyamide/polysulfone reverse osmosis membranes (8″ modules) in order to make them reusable in other applications than reverse osmosis and to increase their life cycles. The performance of membranes with 60 cm² was evaluated in terms of water permeate flux and salt rejection, two properties that can be easily measured in industrial systems. Potassium permanganate treatment degraded the selective layer, improving the water permeate flux of at least 15% from its initial value at the expense of decreased salt rejection. The formation of a manganese oxide layer was detected which reduced subsequent oxidative treatment efficacy. Citric acid was used as a cleaning agent after the oxidation steps, removing part of the manganese oxide layer; the result was an enhanced oxidative process, which increased the permeate flux even more and decreased the loss of salt rejection. Furthermore, to verify the stability of the treatment, the membrane was submitted to a long-term oxidizing experiment.     

Poly(vinyl) alcohol coating of the support layer of reverse osmosis membranes to enhance performance in forward osmosis

Membrane hydrophilicity influences the transport of water through the membrane in osmotically driven separations such as forward osmosis. In this paper, we coated the polysulfone support layer of two types of commercially available reverse osmosis membranes (brackish water and seawater) with hydrophilic polyvinyl alcohol (PVA). The aim of this was to increase the support layer hydrophilicity and, correspondingly, the rate of water transport through the membrane. Previous work with polydopamine coatings of the polysulfone support of reverse osmosis membranes has yielded promising results. In this work, we explore more readily available materials. Specifically, we studied the effects of two different PVA crosslinking agents – maleic acid and glutaraldehyde – on the resultant membrane properties and osmotic performance. For seawater membranes we found that PVA crosslinked to a limited degree with maleic acid creates a significant improvement in water flux in RO and FO systems, as compared to membranes with PVA crosslinked by glutaraldehyde. However, brackish water membranes did not have comparably significant changes in membrane performance. We conclude that the smaller pores of the brackish water membrane become clogged, and this effect is magnified by the lack of fractional free volume available within PVA that is highly crosslinked with glutaraldehyde.     

Nitrate removal with reverse osmosis in a rural area in South Africa

The nitrate-nitrogen concentration (>6 mg/l) and the salinity (>1000 mg/l TDS) of many borehole waters in rural areas in South Africa are too high for human consumption. Therefore, an urgent need for water denitrification and water desalination exists in these areas. Reverse osmosis (RO), electrodialysis (ED), ion-exchange (IX) and certain biological technologies can be very effectively applied for water denitrification. Each of these technologies, however, has its own advantages and disadvantages. Reverse osmosis technology, however, has been selected for this study because the technology is well known in South Africa and because it can be very effectively applied for water desalination. The objectives of this study were: (a) to transfer RO technology through process demonstration performance for water denitrification and water desalination to people living in rural areas; (b) to build capacity regarding the operation and maintenance of an RO application in a rural area; (c) to produce a preliminary operational and maintenance manual for the operation of an RO unit in a rural environment; (d) to train local operators to operate and maintain an RO plant in a rural environment; (e) to evaluate stock watering as brine disposal option; and (f) to determine the preliminary economics ofthe process. The following conclusions were drawn. It was demonstratedthatthe RO process could be very effectively applied for water denitrification and water desalination in a rural area. Nitrate-nitrogen was reduced from 42.5 mg/l in the RO feed to only 0.9 mg/l in the RO product water. The TDS of the RO feed was reduced from 1292 mg/l to 24 mg/l in the RO permeate. Therefore, an excellent quality water could be produced for potable purposes. The RO brine at approximately 50% water recovery should be suitable for stock watering if the conditions for stock watering are met in terms of nitrate-nitrogen concentration, TDS and other constituent concentrations. The capital cost for an approximately 50 m³/d output RO plant is approximately US $29,900. Preliminary cost estimates have shown that the operational cost for water denitrification is approximately US $0.50/m³. This cost, however, should be significantly reduced by optimisation of the chemical dosing and by blending borehole water with RO product water.     

Effect of Stream Mixing on RO Energy Cost Minimization

Various mixing operations between the feed, retentate and permeate streams are studied in this work to determine their effectiveness in decreasing the specific energy consumption (SEC) of single-stage (single-pass), two-pass and two-stage reverse osmosis (RO) processes operated at the limit of the thermodynamic restriction. The results show that in a single-stage RO process, partial retentate recycling to the feed stream does not change the SEC, while partial permeate recycling to the feed stream increases the SEC if targeting the same overall water recovery. Energy optimization of two-pass membrane desalination, with second-pass retentate recycling to the first-pass feed stream and operated at the limit imposed by the thermodynamic restriction, revealed the existence of a critical water recovery. When desalting is accomplished at recoveries above the critical water recovery, two-pass desalination with recycling is always less efficient than single-pass desalination. When desalting is accomplished at recoveries below the critical water recovery, an operational sub-domain exists in which the SEC for a two-pass process with recycling can be lower than for a single-pass counterpart, when the latter is not operated at its globally optimal state. For the two-stage RO process, diverting part of the raw feed to the second stage, in order to dilute the feed to the second-stage RO, does not decrease the minimal achievable SEC of a two-stage RO process. The various mixing approaches, while may provide certain operational or system design advantages (e.g., with respect to achieving target salt rejection for certain solutes or flux balancing), do not provide an advantage from an energy usage perspective.     

New chlorine‐resistant polyamide reverse osmosis membrane with hollow fiber configuration

New asymmetric hollow fiber reverse osmosis (RO) membrane was developed from a new chlorine‐resistant copolyamide [4T‐PIP(30)] with a piperazine moiety by a conventional phase‐separation method. The new 4T‐PIP(30) hollow fiber membrane has the same low‐pressure RO performance as cellulose triacetate hollow fiber membrane (FR = 205 L/m² day, Rj = 99.6%) and superior chlorine resistance as well as pH resistance to conventional aramid RO membranes. Structural analysis and viscoelastic study revealed that the new hollow fiber consisted of a top skin, dense layer, and microporous layer, and that it began to decrease its elasticity at 80°C in water, which is possibly related to its good and stable RO performance around room temperature. Several kinds of RO modules were made from the new hollow fiber membranes, for which RO performances were stable for 2 years in chlorinated feed water desalination (the free residual chlorine ranged from 0.l to 1.1 mg/L). © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 517–527, 2001