Advances and Applications of Hollow Fiber Nanofiltration Membranes: A Review 中空纤维纳滤膜的进展与应用综述
Tim Sewerin ^(1){ }^{\mathbf{1}}, Maria G. Elshof ^(1){ }^{\mathbf{1}} (D), Sonia Matencio ^(2){ }^{\mathbf{2}}, Marcel Boerrigter ^(2){ }^{\mathbf{2}}, Jimmy Yu ^(3)^{\mathbf{3}} and Joris de Grooth ^(1,4,**){ }^{\mathbf{1}, 4, \boldsymbol{*}} Tim Sewerin ^(1){ }^{\mathbf{1}} ，Maria G. Elshof ^(1){ }^{\mathbf{1}} （D），Sonia Matencio ^(2){ }^{\mathbf{2}} ，Marcel Boerrigter ^(2){ }^{\mathbf{2}} ，Jimmy Yu ^(3)^{\mathbf{3}} 和 Joris de Grooth ^(1,4,**){ }^{\mathbf{1}, 4, \boldsymbol{*}}1 NX Filtration, Josink Esweg 44, 7545 PN Enschede, The Netherlands; t.sewerin@nxfiltration.com (T.S.); m.elshof@nxfiltration.com (M.G.E.) 1 NX Filtration，Josink Esweg 44，7545 PN Enschede，荷兰；t.sewerin@nxfiltration.com（T.S.）；m.elshof@nxfiltration.com（M.G.E.）2 LEITAT Technological Center, C/Pallars, 179-185, 08005 Barcelona, Spain; smatencio@leitat.org (S.M.); mboerrigter@leitat.org (M.B.) 2 LEITAT 技术中心，C/Pallars，179-185，08005 巴塞罗那，西班牙；smatencio@leitat.org（S.M.）；mboerrigter@leitat.org（M.B.）3 Pepsi Co., Inc., Global R & D, 350 Columbus Ave, Valhalla, NY 10595, USA; jimmy.yu@pepsico.com 3 百事公司，全球研发部，350 Columbus Ave，Valhalla，NY 10595，美国；jimmy.yu@pepsico.com4 Membrane Science & Technology, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands 4 膜科学与技术，MESA+纳米技术研究所，特温特大学，邮政信箱 217 号，7500 AE 恩斯赫德，荷兰* Correspondence: j.degrooth@utwente.nl * 通讯作者：j.degrooth@utwente.nl
Citation: Sewerin, T.; Elshof, M.G.; Matencio, S.; Boerrigter, M.; Yu, J.; de Grooth, J. Advances and Applications of Hollow Fiber Nanofiltration Membranes: A Review. Membranes 2021, 11, 890. https://doi.org/ 10.3390/membranes11110890 引用：Sewerin, T.; Elshof, M.G.; Matencio, S.; Boerrigter, M.; Yu, J.; de Grooth, J. 空心纤维纳滤膜的进展与应用综述。膜，2021，11，890。https://doi.org/10.3390/membranes11110890
Academic Editors: 学术编辑：
Wolfgang Samhaber and Mohammad Rezaei Wolfgang Samhaber 和 Mohammad Rezaei
Received: 15 October 2021 收到日期：2021 年 10 月 15 日
Accepted: 9 November 2021 接受日期：2021 年 11 月 9 日
Published: 19 November 2021 发表于：2021 年 11 月 19 日
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Hollow fiber nanofiltration (NF) membranes have gained increased attention in recent years, partly driven by the availability of alternatives to polyamide-based dense separation layers. Moreover, the global market for NF has been growing steadily in recent years and is expected to grow even faster. Compared to the traditional spiral-wound configuration, the hollow fiber geometry provides advantages such as low fouling tendencies and effective hydraulic cleaning possibilities. The alternatives to polyamide layers are typically chemically more stable and thus allow operation and cleaning at more extreme conditions. Therefore, these new NF membranes are of interest for use in a variety of applications. In this review, we provide an overview of the applications and emerging opportunities for these membranes. Next to municipal wastewater and drinking water processes, we have put special focus on industrial applications where hollow fiber NF membranes are employed under more strenuous conditions or used to recover specific resources or solutes. 近年来，中空纤维纳滤（NF）膜受到越来越多的关注，部分原因是出现了聚酰胺基致密分离层的替代品。此外，全球纳滤市场近年来稳步增长，预计未来增长速度将更快。与传统的卷式膜组件相比，中空纤维结构具有低污染倾向和有效液压清洗的优势。聚酰胺层的替代品通常化学稳定性更高，因此允许在更极端的条件下运行和清洗。因此，这些新型纳滤膜在多种应用中具有吸引力。在本综述中，我们概述了这些膜的应用及其新兴机遇。除了市政废水和饮用水处理工艺外，我们特别关注了中空纤维纳滤膜在更严苛条件下使用的工业应用，或用于回收特定资源或溶质的场景。
While nanofiltration (NF) membranes have been available since early 1980 [1] only more recently has the attention to these membranes for water treatment processes started growing increasingly, both in academia and in industry. The quality and availability of freshwater sources is becoming a major concern, and this has been included within sustainable development goals (SDGs) by the United Nations (UN), namely SDG 6: Ensure availability and sustainable management of water and sanitation for all. NF membranes offer a semi-selective barrier with selectivities that lie between the dense reverse osmosis (RO) membranes and porous ultrafiltration (UF) membranes. Where RO membranes can retain even the smallest solute to desalinate seawater, UF membranes allow for efficient removal of bacteria, viruses, and total suspended solids (TSS) and NF membranes enable the selective removal of specific solutes or the purification of water at lower energy consumption [1,2]. This semi-permeability of NF also hits the sweet spot within the filtration spectrum for many emerging processes. 虽然纳滤（NF）膜自 20 世纪 80 年代初以来就已问世[1]，但近年来无论在学术界还是工业界，对这些膜在水处理工艺中的关注才开始逐渐增加。淡水资源的质量和可用性正成为一个重大关切，这也被联合国（UN）纳入了可持续发展目标（SDGs）中，即 SDG 6：确保人人获得可持续管理的水和卫生设施。纳滤膜提供了一种半选择性屏障，其选择性介于致密的反渗透（RO）膜和多孔的超滤（UF）膜之间。RO 膜能够截留甚至最小的溶质以实现海水淡化，UF 膜则能高效去除细菌、病毒和总悬浮固体（TSS），而纳滤膜则能够选择性去除特定溶质或以较低能耗净化水[1,2]。纳滤的这种半透性也恰好处于过滤光谱的最佳位置，适用于许多新兴工艺。
NF membranes have been categorized as membranes with effective pore diameters between 1 and 5 nm [3]. This means that these membranes cover the transition from convective flow through distinct physical pores to solution and diffusion in a dense polymer network. Different models have been developed to describe the transport through these membranes [4-7], but their applicability will depend on both the membrane and the solute. The versatility that NF offers also means that there is a wide variation in the possible applications and processes. Several more obvious applications, such as municipal 纳滤膜被归类为有效孔径在 1 到 5 纳米之间的膜[3]。这意味着这些膜涵盖了从通过明显物理孔隙的对流流动到在致密聚合物网络中溶液和扩散的过渡。已经开发了不同的模型来描述通过这些膜的传输[4-7]，但它们的适用性将取决于膜和溶质的性质。纳滤膜所提供的多功能性也意味着其可能的应用和工艺存在很大差异。一些更明显的应用，如市政
wastewater filtration, have received great notice lately [8], but attention on other specific industrial applications is still much more scattered. However, these industrial applications do have the potential of being a main driving force for the growth and acceptance of novel NF membranes. 废水过滤，最近受到了极大关注[8]，但对其他特定工业应用的关注仍然较为分散。然而，这些工业应用确实有潜力成为推动新型纳滤膜增长和被接受的主要动力。
Currently, with an estimated share of 7%7 \%, NF membranes play only a minor part in the total membrane market [9], but this NF market is growing faster compared to conventional membrane technologies (e.g., UF and RO). This is partially attributed to the global increase in demand for potable water, where NF membranes can provide an environmentally friendly and cheaper alternative. Moreover, a substantial part of the projected growth lies in the aforementioned new industrial applications (see Figure 1). 目前，纳滤膜的市场份额估计为 7%7 \% ，在整个膜市场中仅占较小部分[9]，但与传统膜技术（如超滤和反渗透）相比，纳滤市场增长更快。这部分归因于全球对饮用水需求的增加，纳滤膜能够提供一种环保且更经济的替代方案。此外，预计增长的很大一部分来自上述新的工业应用（见图 1）。
Figure 1. Global annual NF market per application in $ million, including the projected growth for 2020-2030 [9]. 图 1. 按应用划分的全球年度纳滤市场规模（单位：百万美元），包括 2020-2030 年的预测增长[9]。
Several different membrane geometries are available for NF membranes. Until recently, predominantly spiral-wound (SW) configurations were commercially available. These types of membranes are comparable to the denser RO modules used for desalination. Typically, the membranes consist of a porous support on which a thin polyamide layer is applied via interfacial polymerization (IP). By using monomers such as piperazine (PIP) [10] a relatively more hydrophilic and open NF membrane is obtained as compared to RO membranes. An advantage of the SW membranes is that the configuration uses the same standard housings, connections, and pressure vessels as RO systems, potentially reducing the total capital expenditures of the total system for the end-user. In addition, due to similarities to the high-pressure RO configuration, these SW NF membranes can also be operated at relatively high pressures [1]. However, the SW configuration necessitates an extensive pre-treatment of the feed, such as UF or sand filtration (SF). The SW configuration cannot handle high levels of suspended solids, as the membranes are prone to clogging and susceptible to (bio)fouling. The latter is especially of concern as the polyamide chemistry of the selective NF layer is not stable in standard oxidizing chemical solutions [11] (preferably based on NaOCl or H_(2)O_(2)\mathrm{H}_{2} \mathrm{O}_{2} ), and thus special biocides need to be applied to remove the biofouling [12]. 纳滤膜有多种不同的膜几何结构可供选择。直到最近，市场上主要供应的是螺旋卷绕（SW）结构的膜。这类膜与用于海水淡化的致密反渗透（RO）模块相似。通常，这些膜由多孔支撑层组成，表面通过界面聚合（IP）工艺涂覆一层薄的聚酰胺层。通过使用如哌嗪（PIP）[10]等单体，相较于 RO 膜，可以获得相对更亲水且孔隙更开放的纳滤膜。SW 膜的一个优势是其结构采用与 RO 系统相同的标准壳体、连接件和压力容器，可能降低终端用户整个系统的总资本支出。此外，由于与高压 RO 结构的相似性，这些 SW 纳滤膜也可以在相对较高的压力下运行[1]。然而，SW 结构需要对进料进行广泛的预处理，如超滤（UF）或砂滤（SF）。SW 结构无法处理高悬浮固体含量的水，因为膜容易堵塞且易受（生物）污染影响。 后者尤其令人关注，因为选择性纳滤层的聚酰胺化学性质在标准氧化化学溶液中不稳定[11]（优选基于次氯酸钠或 H_(2)O_(2)\mathrm{H}_{2} \mathrm{O}_{2} ），因此需要使用特殊的杀菌剂来去除生物污垢[12]。
More recently, hollow fiber (HF) NF membranes have become commercially available. An overview of current manufacturers of HF NF membranes and their products is shown in Table 1. The HF configuration can have a large surface area to volume ratio [13-15], albeit this is dependent on the fiber diameter. HF NF membranes can handle feed streams with much higher levels of suspended solids, matching or even surpassing UF membranes. 近年来，中空纤维（HF）纳滤膜已开始商业化。表 1 展示了当前中空纤维纳滤膜制造商及其产品的概览。中空纤维结构可以具有较大的表面积与体积比[13-15]，尽管这取决于纤维直径。中空纤维纳滤膜能够处理悬浮固体含量更高的进料流，性能可与超滤膜相匹配甚至超越。
In addition, the possibility of hydraulically cleaning the membranes, such as via backwashing [16] and air sparging [17], further enables the filtration of fouling prone feed waters, reducing the necessity of an extensive pre-treatment [18,19]. Nevertheless, biofouling is a potential threat for these membranes and therefore several of the commercially available HF NF membranes have been developed based on selective layers that are chemically stable. This, for instance, allows the use of oxidizing chemical solutions [18,20]. This combination of chemical stability and HF geometry has been a big accelerator for the development of these new membranes, as they enable simple and direct water treatment processes based on NF-like selectivities. In comparison to SW NF membranes, the operating pressures of the commercial HF NF membranes are substantially lower. This stems typically from a specific operational expenditure perspective [2,21], but also, at high operating pressures the fiber integrity is at risk, which can be detrimental to the filtration efficacy [22]. The lower operating pressures can result in a reduced specific energy consumption during operation as compared to SW NF and RO membranes. 此外，通过反冲洗[16]和气体喷射[17]等方式对膜进行液压清洗的可能性，进一步促进了对易结垢进料水的过滤，减少了对广泛预处理的需求[18,19]。然而，生物污垢是这些膜的潜在威胁，因此市售的多款中空纤维纳滤膜是基于化学稳定的选择性层开发的。例如，这允许使用氧化性化学溶液[18,20]。这种化学稳定性与中空纤维结构的结合极大地推动了新型膜的发展，因为它们能够基于类似纳滤的选择性实现简单直接的水处理工艺。与平板式纳滤膜相比，商用中空纤维纳滤膜的运行压力明显较低。这通常是从特定的运营成本角度考虑[2,21]，同时在高运行压力下，纤维的完整性也会受到威胁，可能对过滤效果产生不利影响[22]。 较低的操作压力相比于海水纳滤（SW NF）和反渗透（RO）膜，能够在运行过程中降低单位能耗。
This review paper contains a brief summary of the HF NF membrane preparation methods and then focuses on covering a non-exhaustive overview of different (emerging) applications of HF NF membranes. Special attention is given to the applicability of HF NF membranes in industrial applications, where both existing applications (published in the last 10 years) and emerging processes will be discussed. 本文综述简要总结了中空纤维纳滤膜的制备方法，随后重点介绍了中空纤维纳滤膜的不同（新兴）应用的非详尽概述。特别关注了中空纤维纳滤膜在工业应用中的适用性，讨论了过去十年内已有的应用以及新兴工艺。
2. Preparation Methods 2. 制备方法
The selective layer of HF NF membranes can be prepared via a variety of methods, that can roughly be divided into five different categories: Directly during phase inversion, polymerization, coating, grafting and self-assembly [28]. Phase inversion is one of the key methods to prepare the HF support, but can also be used to create the selective NF layer during fiber spinning [29]. Phase inversion involves the separation of a polymer solution into a polymer-rich and a polymer-lean phase, leading to a porous solid. For HF membranes, the polymer solution is extruded through a double or triple orifice spinneret and phase separation is usually non-solvent induced, where the polymer solution is extruded in a coagulation bath. Co-extrusion via a triple orifice, solvent evaporation or chemical reactions and complexations during phase separation can be employed to yield dense layers directly [30-33]. Alternatively, a post-treatment of the porous fiber is needed to form the selective layer. Interfacial polymerization (IP) is a polymerization method where two reactive monomers react at the interface of two immiscible solvents and form a thin polymeric film [28]. The IP is performed on top of a porous support, which results in a thin-film composite (TFC) NF membrane. Dip-coating is a very simple method to apply a thin film, of which the final properties can be tailored [34]. Grafting can HF 纳滤膜的选择性层可以通过多种方法制备，大致可分为五类：相转化法、聚合、涂覆、接枝和自组装[28]。相转化法是制备 HF 支撑层的关键方法之一，也可用于纤维纺丝过程中形成选择性纳滤层[29]。相转化涉及将聚合物溶液分离成富聚合物相和贫聚合物相，从而形成多孔固体。对于 HF 膜，聚合物溶液通过双孔或三孔喷丝头挤出，且相分离通常是非溶剂诱导的，即聚合物溶液在凝固浴中挤出。通过三孔共挤出、溶剂蒸发或相分离过程中发生的化学反应和络合，可以直接形成致密层[30-33]。或者，需要对多孔纤维进行后处理以形成选择性层。界面聚合（IP）是一种聚合方法，两种反应性单体在两种不混溶溶剂的界面反应，形成薄的聚合物膜[28]。 IP 是在多孔支撑层上进行的，形成薄膜复合（TFC）纳滤膜。浸涂是一种非常简单的涂覆薄膜的方法，其最终性能可以被定制[34]。接枝可以
be used to prepare HF NF membranes: UF membranes can be chemically modified via graft polymerization by different methods, such as plasma, E-beam, UV/photo and gamma\gamma-ray irradiation to result in charged HF NF membranes [35,36]. Finally, a successful method is the preparation of an NF layer via the self-assembly of polycations and polyanions to form polyelectrolyte multilayers (PEM) on a porous support [37], often via a Layer-by-Layer (LbL) type of approach. This method has gained attention recently because of its ease of fabrication, tunability and relatively sustainable preparation method and different methods of applying the PEM have even been developed to obtain controllable thin layers, see Figure 2 [38]. These advantages of the PEM have resulted in the commercialization of these types of membranes. 用于制备中空纤维（HF）纳滤膜：超滤（UF）膜可以通过不同方法的接枝聚合进行化学改性，如等离子体、电子束、紫外/光照射和γ射线照射，从而得到带电的 HF 纳滤膜[35,36]。最后，一种成功的方法是通过多阳离子和多阴离子的自组装，在多孔支撑层上形成聚电解质多层膜（PEM）来制备纳滤层[37]，通常采用逐层（Layer-by-Layer，LbL）方法。该方法因其制造简便、可调性强以及相对可持续的制备方式而受到关注，甚至开发出了多种应用 PEM 的方法以获得可控的薄层，见图 2[38]。PEM 的这些优势促使这类膜实现了商业化。
In the following subsections an overview is provided of both existing and emerging applications for HF NF membranes. For this, a distinction has been made between applications where the aim is to recover or purify freshwater, i.e., where water is the valuable product, and applications where the goal is to recover valuable solutes present in (aqueous) streams. The examples mentioned are meant to show the potential of HF NF membranes; however, the list is not meant to be exhaustive. 在以下小节中，将概述 HF NF 膜的现有和新兴应用。为此，区分了两类应用：一类是旨在回收或净化淡水，即水为有价值的产品；另一类是旨在回收存在于（水性）流体中的有价值溶质。所提及的示例旨在展示 HF NF 膜的潜力；但该列表并非详尽无遗。
Natural organic matter (NOM) is ubiquitous in natural aquatic environments like rivers, lakes and marine systems. However, NOM has a significant impact on drinking water production as it leads to poor water aesthetics (color, taste and odor), forms hazardous disinfection by-products like trihalomethanes by reacting with chlorine, and serves as a carbon and energy source for microbial fouling and regrowth in water distribution systems [39]. In recent decades, increased NOM concentrations have been reported in America, Europe and Asia [40]. Hence, dealing with elevated NOM levels will be a future 天然有机物（NOM）广泛存在于河流、湖泊和海洋等自然水体环境中。然而，NOM 对饮用水生产有显著影响，因为它会导致水质美观性差（颜色、味道和气味），与氯反应生成有害的消毒副产物如三卤甲烷，并且作为微生物污垢和再生长的碳源和能量来源，影响水分配系统[39]。近几十年来，美洲、欧洲和亚洲的 NOM 浓度有所增加[40]。因此，应对升高的 NOM 水平将是未来的一个挑战。
challenge for drinking water systems. NFs can be an effective barrier for NOM due to their typical molecular weight cut-off (MWCO) of 200-1000 Da [41]. Applications for SW membranes have been reviewed extensively elsewhere [42]; however, the implementation of NF is still limited. A main reason for this is that NOM contributes significantly to membrane fouling [39]. This is especially an issue for spiral-wound modules, which are prone to (bio)fouling and have limited cleaning possibilities [12]. HF NF membranes have been named as a promising alternative [20,43], since they offer the advantage to combine the beneficial cleaning conditions of HF UF membranes with the separation efficiency of spiral-wound modules. 饮用水系统面临的挑战。由于其典型的分子量截断范围（MWCO）为 200-1000 Da，纳滤膜（NF）可以有效阻挡天然有机物（NOM）[41]。海水膜的应用已在其他文献中广泛综述[42]；然而，纳滤膜的应用仍然有限。主要原因是天然有机物显著促进了膜污染[39]。这对于螺旋卷绕模块尤为突出，因为它们容易发生（生物）污染且清洗手段有限[12]。中空纤维纳滤膜（HF NF）被认为是一种有前景的替代方案[20,43]，因为它结合了中空纤维超滤膜（HF UF）有利的清洗条件与螺旋卷绕模块的分离效率。
Köhler et al. investigated the performance of commercially available HF NF membranes (HFW 1000, Pentair X-Flow) on NOM removal by running a pilot installation on lake water in Sweden [44]. It was found that a combined process of coagulation and HF NF effectively removed 90%90 \% of the dissolved organic carbon (DOC) and 96%96 \% of the UV absorbance, while less than 20%20 \% of the hardness was retained. Biopolymers and humic substances were almost completely removed. An autopsy of the membrane was performed after 12 months of operation, which demonstrated no significant changes compared to the virgin membrane [20]. In a follow-up pilot study, the same membrane type was evaluated for drinking water production on the same lake water without the prior coagulation [45]. At the optimal process conditions (crossflow velocity of 0.75m*s^(-1),80%0.75 \mathrm{~m} \cdot \mathrm{~s}^{-1}, 80 \% recovery and flux between 12 and 18L*m^(-2)h^(-1)18 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} ), the UV absorbance was reduced by 80%;75%80 \% ; 75 \% of the humic substances (measured molecular weight (MW): 530-660Da530-660 \mathrm{Da} ) were removed as well as 70%70 \% of total organic carbon (TOC) in only one step. However, only 40%40 \% of the low molecular weight acids ( MW=300-400Da\mathrm{MW}=300-400 \mathrm{Da} ) were removed from the water. Lidén et al. tested the commercial HF NF (HFW 1000, Pentair X-Flow) in a pilot installation on NOM removal for three different surface waters from boreal lakes in Sweden. A TOC retention of 88%88 \% and a UV absorbance retention of 93%93 \% were reported [46]. All these results show the potential of HF NF membranes for the treatment of (surface) water where mainly NOM needs to be removed. High removal rates are achieved, which can even be obtained without the addition of coagulants or other pre-treatment steps. Köhler 等人通过在瑞典湖水上运行试点装置，研究了市售 HF NF 膜（HFW 1000，Pentair X-Flow）在去除天然有机物（NOM）方面的性能[44]。研究发现，混凝与 HF NF 的联合工艺有效去除了 90%90 \% 的溶解性有机碳（DOC）和 96%96 \% 的紫外吸收值，而硬度的保留率不到 20%20 \% 。生物聚合物和腐殖物质几乎被完全去除。运行 12 个月后对膜进行了解剖，结果显示与原始膜相比没有显著变化[20]。在后续的试点研究中，同类型膜在未进行预先混凝的情况下，用于同一湖水的饮用水生产进行了评估[45]。在最佳工艺条件下（横流速度为 0.75m*s^(-1),80%0.75 \mathrm{~m} \cdot \mathrm{~s}^{-1}, 80 \% ，回收率和通量介于 12 和 18L*m^(-2)h^(-1)18 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} 之间），紫外吸收值降低了 80%;75%80 \% ; 75 \% ，腐殖物质（测得分子量（MW）： 530-660Da530-660 \mathrm{Da} ）以及总有机碳（TOC）的 70%70 \% 在单一步骤中被去除。然而，水中仅有 40%40 \% 的低分子量酸（ MW=300-400Da\mathrm{MW}=300-400 \mathrm{Da} ）被去除。Lidén 等人 在瑞典北方湖泊的三种不同地表水中，测试了商业化的 HF NF 膜（HFW 1000，Pentair X-Flow）在去除天然有机物（NOM）方面的试点装置。报告的总有机碳（TOC）去除率为 88%88 \% ，紫外吸收去除率为 93%93 \% [46]。所有这些结果表明，HF NF 膜在处理主要需去除 NOM 的（地表）水方面具有潜力。能够实现高去除率，甚至无需添加混凝剂或其他预处理步骤。
Indeed, next to these pilot studies, commercial installations using HF NF for NOM removal are already in operation or under construction. The HFW 1000 membranes from Pentair X-Flow are applied in a full-scale membrane system on surface water in Tasmania, Australia. The water treatment plant provides 60m^(3)*h^(-1)60 \mathrm{~m}^{3} \cdot \mathrm{~h}^{-1} of potable water in a chemical-free one-step process [47]. Currently, a water treatment plant in Indonesia is being upgraded with HF NF dNF80 membranes from NX Filtration to remove NOM from peat water for drinking water production [48]. During pilot study tests, HF NF proved their suitability to achieve drinking water quality in one step, while enabling a chemical-free operation. Initially, a first stage providing a capacity of 180m^(3)*h^(-1)180 \mathrm{~m}^{3} \cdot \mathrm{~h}^{-1} is built, which will be expanded to 1620m^(3)*h^(-1)1620 \mathrm{~m}^{3} \cdot \mathrm{~h}^{-1} in three stages. In an extensive study by Aggarwal, the environmental impact of the HF NF process was compared to more conventional water treatment schemes (based on combinations of, e.g., precipitation, sedimentation processes, rapid sand filtration and UF) [49]. The analysis shows that the HF NF process was the most sustainable alternative, mainly resulting from the low chemical use during operation. 事实上，除了这些试点研究之外，使用 HF NF 去除天然有机物（NOM）的商业装置已经投入运行或正在建设中。Pentair X-Flow 的 HFW 1000 膜被应用于澳大利亚塔斯马尼亚地表水的全规模膜系统中。该水处理厂通过一种无化学品的一步法工艺提供 60m^(3)*h^(-1)60 \mathrm{~m}^{3} \cdot \mathrm{~h}^{-1} 的饮用水[47]。目前，印度尼西亚的一座水处理厂正在升级，采用 NX Filtration 的 HF NF dNF80 膜去除泥炭水中的 NOM，以生产饮用水[48]。在试点研究测试中，HF NF 证明了其在一步法中实现饮用水质量的适用性，同时实现了无化学品操作。最初建成的第一阶段提供 180m^(3)*h^(-1)180 \mathrm{~m}^{3} \cdot \mathrm{~h}^{-1} 的处理能力，计划分三阶段扩展至 1620m^(3)*h^(-1)1620 \mathrm{~m}^{3} \cdot \mathrm{~h}^{-1} 。在 Aggarwal 的一项广泛研究中，HF NF 工艺的环境影响与更传统的水处理方案（基于如沉淀、沉降过程、快速砂滤和超滤等组合）进行了比较[49]。分析显示，HF NF 工艺是最可持续的替代方案，主要得益于运行过程中低化学品使用量。
3.1.2. Water Softening 3.1.2. 软化水处理
Hardness removal, which is often referred to as water softening, is an important step in water treatment. A high level of hardness, which is mainly determined by the presence of Ca^(2+)\mathrm{Ca}^{2+} and Mg^(2+)\mathrm{Mg}^{2+}, can cause scaling problems in plumbing, heat exchangers or boilers. This will decrease the efficiency of heat exchangers leading to increased energy cost and cleaning effort [50]. Therefore, it is beneficial to reduce high hardness levels. Conventional NF membranes are well established for this due to their efficient removal rates for multivalent ions [51]. However, SW NF modules are suspectable to scaling due to the spacer configuration [52]. The HF NF geometry with a smooth inner surface can offer a 去除硬度，通常称为软化水质，是水处理中的一个重要步骤。硬度的高低主要由 Ca^(2+)\mathrm{Ca}^{2+} 和 Mg^(2+)\mathrm{Mg}^{2+} 的存在决定，高硬度会导致管道、热交换器或锅炉结垢问题。这会降低热交换器的效率，导致能源成本和清洁工作量增加[50]。因此，降低高硬度水平是有益的。传统的纳滤（NF）膜因其对多价离子的高效去除率而被广泛应用于此[51]。然而，螺旋卷式纳滤（SW NF）模块由于间隔物的结构，容易发生结垢[52]。具有光滑内表面的中空纤维纳滤（HF NF）结构在结垢问题上可以提供
significant improvement for the scaling issue. It enables the application of diverse cleaning methods as well as mitigating the scaling build-up due to its smooth surface. 显著的改进。其光滑的表面不仅有助于采用多种清洗方法，还能减轻结垢的积累。
Fang et al. developed interfacially polymerized composite HF NF membranes using branched polyethyleneimine (PEI) and trimesoylchloride (TMC) as the monomers on a polyethersulfone (PES) UF membrane support and investigated the performance for water softening [53]. Artificial hard water was created with an overall total dissolved solids (TDS) load of 3000 ppm, mimicking a brackish water source in Florida. At a water flux of 20L*m^(-2)h^(-1)20 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} at 2 bars of feed pressure, the membrane successfully rejected 84.8%84.8 \% and 81.0%81.0 \% of Mg^(2+)\mathrm{Mg}^{2+} and Ca^(2+)\mathrm{Ca}^{2+}, respectively, while Na^(+)\mathrm{Na}^{+}retention was as low as 14.3%14.3 \%. Fang et al. then investigated the performance of a PEI/PIP mixed composite HF NF membrane for hardness removal [54]. The membrane was tested on artificial water with a TDS load of 3000 ppm and an overall hardness of 691mg*L^(-1)691 \mathrm{mg} \cdot \mathrm{L}^{-1} as CaCO_(3)\mathrm{CaCO}_{3}. Hardness removal of over 90%90 \% was reported, whereas Na^(+)\mathrm{Na}^{+}rejection was as low as 18%18 \%. The tests were conducted at 2 bars pressure and a flux of 12.8L*m^(-2)h^(-1)12.8 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} was achieved. Effective hardness removal from artificial waters on a lab-scale has been reported for other HF NF membranes as well [21,55][21,55]. Fang 等人利用支化聚乙烯亚胺（PEI）和三甲酰氯（TMC）作为单体，在聚醚砜（PES）超滤膜支撑体上制备了界面聚合复合中空纤维纳滤膜，并研究了其软化水性能[53]。人工硬水的总溶解固体（TDS）负荷为 3000 ppm，模拟佛罗里达的咸淡水水源。在 2 巴进水压力下，水通量为 20L*m^(-2)h^(-1)20 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} 时，膜成功截留了 Mg^(2+)\mathrm{Mg}^{2+} 和 Ca^(2+)\mathrm{Ca}^{2+} 的 84.8%84.8 \% 和 81.0%81.0 \% ，而 Na^(+)\mathrm{Na}^{+} 的截留率低至 14.3%14.3 \% 。随后，Fang 等人研究了 PEI/PIP 混合复合中空纤维纳滤膜的硬度去除性能[54]。该膜在 TDS 负荷为 3000 ppm、总硬度为 691mg*L^(-1)691 \mathrm{mg} \cdot \mathrm{L}^{-1} 的人工水中进行了测试。报告显示硬度去除率超过 90%90 \% ，而 Na^(+)\mathrm{Na}^{+} 的截留率低至 18%18 \% 。测试在 2 巴压力下进行，达到了 12.8L*m^(-2)h^(-1)12.8 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} 的通量。其他中空纤维纳滤膜在实验室规模上对人工水的有效硬度去除也有报道 [21,55][21,55] 。
Recently, Sengur-Tasdemir et al. investigated various HF NF membranes’ performances during pilot-scale filtration tests of surface water from a lake reservoir in Turkey. The reinforced thin-film composite membrane showed the best results regarding water softening as a hardness removal of 57%57 \% was achieved [56]. A constant flux of 4.76L*m^(-2)h^(-1)4.76 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} was reported over the testing duration of 150 h . 最近，Sengur-Tasdemir 等人研究了多种 HF NF 膜在土耳其一个湖泊水库表层水的中试规模过滤测试中的性能。增强型薄膜复合膜在水软化方面表现最佳，实现了硬度去除为 57%57 \% [56]。测试期间 150 小时内报告了恒定通量为 4.76L*m^(-2)h^(-1)4.76 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} 。
3.2. Municipal Wastewater Treatment 3.2. 城市污水处理
Municipal wastewater treatment focusses on the removal of nutrients and contaminants from wastewater originating from households and businesses. The water streams are traditionally treated by a wastewater treatment plant (WWTP), that consists of several different steps, to produce an effluent that is safe to discard. However, regulations for discharge are becoming stricter for a variety of components and not everything can be sufficiently removed by traditional WWTPs. Membrane filtration offers a possible solution to produce effluent streams that will meet the more stringent disposal limits [57]. 市政污水处理侧重于去除来自家庭和企业的废水中的营养物质和污染物。水流传统上通过污水处理厂（WWTP）进行处理，污水处理厂包含多个不同步骤，以产生安全排放的出水。然而，排放法规对多种成分的限制越来越严格，传统污水处理厂无法充分去除所有物质。膜过滤提供了一种可能的解决方案，以生产符合更严格排放标准的出水流[57]。
The worldwide occurrence of organic micropollutants (OMPs), such as pharmaceuticals, personal care products, hormones, surfactants, industrial chemicals and pesticides, in water bodies and municipal and industrial wastewater streams is creating more and more concern [58]. Due to their ecotoxicological impact and potential risk for human health, further treatment steps are necessary to upgrade WWTPs, which are a main entry source for OMPs in the environment [59,60]. 有机微污染物（OMPs）如药物、个人护理产品、激素、表面活性剂、工业化学品和农药在全球水体以及市政和工业废水流中的存在引发了越来越多的关注[58]。由于其生态毒理学影响及对人类健康的潜在风险，必须对污水处理厂（WWTPs）进行进一步处理升级，污水处理厂是 OMPs 进入环境的主要来源之一[59,60]。
Some of the first results on OMP removal with LbL-based HF NF membranes were published by de Grooth et al. in 2014 [61]. A HF UF support was coated with polyelectrolyte trilayers, consisting of poly(diallyldimethylammonium chloride) (PDADMAC) as polycation, poly N-(3-sulfopropyl)-N-(methacryloxyethyl)- N,N-dimethylammonium betaine (PSBMA) as polyzwitterion and poly(styrenesulfonate) (PSS) as polyanion. Filtration tests on six different OMPs with molecular weights of 215 to 360g*mol^(-1)360 \mathrm{~g} \cdot \mathrm{~mol}^{-1} resulted in high retention for positively and negatively charged OMPs, while relatively low retentions for uncharged OMPs were reported. 关于利用基于层层自组装（LbL）的中空纤维纳滤膜（HF NF）去除 OMPs 的最初研究成果之一由 de Grooth 等人在 2014 年发表[61]。他们在中空纤维超滤（HF UF）支撑膜上涂覆了由三层聚电解质组成的膜层，分别是作为多阳离子的聚（二烯丙基二甲基氯化铵）（PDADMAC）、作为多两性离子的聚 N-(3-磺丙基)-N-(甲基丙烯酰氧乙基)-N,N-二甲基铵甜菜碱（PSBMA）以及作为多阴离子的聚（苯乙烯磺酸盐）（PSS）。对六种分子量在 215 至 360g*mol^(-1)360 \mathrm{~g} \cdot \mathrm{~mol}^{-1} 范围内的不同 OMPs 进行过滤测试，结果显示对带正电和带负电的 OMPs 具有较高的截留率，而对不带电的 OMPs 截留率相对较低。
Using the same HF UF support, Ilyas et al. coated weak polyelectrolyte multilayers (PEMs) using poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) while varying the pH conditions of the coating steps [62]. Coating at a pH of 6.0 resulted in retentions of 60-80%60-80 \% for charged (positively and negatively) as well as neutral OMPs and at the same time allowing a high salt passage. However, the obtained permeability was rather low at 1.8L*m^(-2)h^(-1)1.8 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} bar ^(-1)^{-1}. To further evaluate the membrane’s performance in realistic wastewater conditions, Abtahi et al. tested the same membrane on synthetic wastewater containing four OMPs (diclofenac, naproxen, ibuprofen and 4 n -nonylphenol) in 使用相同的 HF 超滤支撑，Ilyas 等人采用聚(烯丙胺盐酸盐)（PAH）和聚(丙烯酸)（PAA）涂覆了弱聚电解质多层膜（PEMs），同时改变涂覆步骤的 pH 条件[62]。在 pH 为 6.0 时涂覆，带电（正负）以及中性有机微污染物（OMPs）的截留率均为 60-80%60-80 \% ，同时允许高盐通过。然而，在 1.8L*m^(-2)h^(-1)1.8 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} 巴 ^(-1)^{-1} 的压力下获得的渗透率相当低。为了进一步评估膜在实际废水条件下的性能，Abtahi 等人在含有四种 OMPs（双氯芬酸、萘普生、布洛芬和 4-n-壬基酚）的合成废水中测试了相同的膜，浓度具有代表性[63]。
representative concentrations [63]. Measured OMP retentions ranged from 44%44 \% (ibuprofen) to 77%77 \% (diclofenac) and therewith corresponding with previous data, while the reported NaCl retention was as low as 17%17 \%, confirming a potential for OMP removal without producing a saline concentrate. 测得的 OMPs 截留率从 44%44 \% （布洛芬）到 77%77 \% （双氯芬酸）不等，与之前的数据相符，而报告的 NaCl 截留率低至 17%17 \% ，确认了在不产生盐浓缩液的情况下去除 OMPs 的潜力。
In 2019 Abtahi et al. investigated the impact of salt annealing post-treatment of PAH/PAA PEM flat-sheet membranes on OMPs retention [64]. The salt-annealed PEM membranes showed superior rejection rates for the tested compounds diclofenac, naproxen, ibuprofen and 4 n -nonylphenol (52-82% against 43-69%43-69 \% ) in comparison to non-annealed PAH//PAA\mathrm{PAH} / \mathrm{PAA} membranes. At the same time, the ion rejections were moderately low with a NaCl retention of 25%25 \%. Furthermore, the pure water permeability was measured as 11.8L*m^(-2)h^(-1)11.8 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} bar ^(-1)^{-1}, demonstrating no decrease due to salt annealing. Next to that, the applicability of the so-called sacrificial layer approach for fouling removal was investigated by removing and subsequent recoating of the PEMs. The coated multilayers could be entirely removed, or sacrificed, and after recoating a permeability identical to the pristine coated membrane was achieved. In contrast to other studies on sacrificial layers [65], a complete removal of foulants without the necessity to employ physical force was obtained, demonstrating the beneficial effect of the sacrificial layer approach. As municipal wastewater contains a relatively high load of organics, fouling control is an important parameter for stable operation, which makes this approach interesting for applications in wastewater treatment. 2019 年，Abtahi 等人研究了盐退火后处理对 PAH/PAA 聚电解质膜平板膜对有机微污染物（OMPs）截留的影响[64]。盐退火的聚电解质膜对所测试的化合物双氯芬酸、萘普生、布洛芬和 4-正壬基酚的截留率表现出更优异的效果（52-82%对比未退火膜 43-69%43-69 \% ），相比之下，未退火膜 PAH//PAA\mathrm{PAH} / \mathrm{PAA} 的截留率较低。同时，离子截留率适中，NaCl 的截留率为 25%25 \% 。此外，纯水渗透率测量值为 11.8L*m^(-2)h^(-1)11.8 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} bar ^(-1)^{-1} ，表明盐退火处理并未导致渗透率下降。除此之外，还研究了所谓的牺牲层方法在污垢去除中的适用性，通过去除并重新涂覆聚电解质膜实现。涂覆的多层膜可以被完全去除或牺牲，重新涂覆后其渗透率与原始涂覆膜相同。与其他关于牺牲层的研究[65]不同，本研究实现了无需施加物理力即可完全去除污垢，展示了牺牲层方法的有益效果。 由于市政污水含有较高的有机物负荷，控制污染是稳定运行的重要参数，这使得该方法在污水处理应用中具有吸引力。
More recent research on LbL membranes for OMP removal focused on asymmetrical PE layers by combining open and dense PEMs. With this approach, pores are closed, and defects are eliminated by the relatively thick but highly permeable PEM, while high retention is ensured by the thin but dense PEM top layer (Figure 3). Te Brinke et al. achieved a novel type of membrane by coating a porous support with an open PSS/PAH multilayer followed by an ultra-thin top layer of PAA/PAH bilayers with a resulting thickness of 4 nm [66]. Investigation of the salt and OMP removal of this asymmetrical membrane demonstrated an average retention of 98%98 \% for in total nine charged and uncharged OMPs, with MWs ranging from 216 to 624g*mol^(-1)624 \mathrm{~g} \cdot \mathrm{~mol}^{-1}, while the NaCl removal was around 10-15%10-15 \%. These results confirm an outstanding permselectivity regarding salt permeation over OMP permeation combined with a permeability as high as 12.8L*m^(-2)h^(-1)bar^(-1)12.8 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} \mathrm{bar}^{-1}. 最近关于层层自组装（LbL）膜去除有机微污染物（OMP）的研究集中在通过结合开放和致密的聚电解质膜（PEM）形成不对称的 PE 层。通过这种方法，孔隙被相对较厚但高渗透性的 PEM 封闭，缺陷被消除，而由薄而致密的 PEM 顶层确保高截留率（见图 3）。Te Brinke 等人通过在多孔支撑体上涂覆开放的 PSS/PAH 多层膜，随后加上一层厚度仅为 4 纳米的 PAA/PAH 超薄顶层双层膜，制备出一种新型膜[66]。对该不对称膜的盐分和 OMP 去除性能的研究表明，对九种带电和不带电的 OMP 的平均截留率为 98%98 \% ，其分子量范围从 216 到 624g*mol^(-1)624 \mathrm{~g} \cdot \mathrm{~mol}^{-1} ，而 NaCl 的去除率约为 10-15%10-15 \% 。这些结果证实了该膜在盐分透过与 OMP 透过之间表现出卓越的选择透过性，同时具有高达 12.8L*m^(-2)h^(-1)bar^(-1)12.8 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} \mathrm{bar}^{-1} 的渗透率。
As follow-up research, the same research group tested different production techniques and alternative polyelectrolytes to assemble asymmetrical PEMs [67]. A porous HF support was coated with an open PSS/PDADMAC PEM and different processes for creating a dense top layer were investigated. While variation of the ionic strength in the top layer coating did not have significant improvements in comparison to a purely PSS/PDADMAC coated membrane (52% average OMP retention), adding a PSS/PAH PEM top layer at high ionic 作为后续研究，同一研究团队测试了不同的生产技术和替代的聚电解质以组装非对称 PEM 膜[67]。在多孔 HF 支撑体上涂覆了开放的 PSS/PDADMAC PEM，并研究了制造致密顶层的不同工艺。虽然改变顶层涂覆时的离子强度与纯 PSS/PDADMAC 涂覆膜（平均 52% OMP 截留率）相比没有显著改进，但在高离子强度条件下添加 PSS/PAH PEM 顶层...
strength instead led to an average OMP removal as high as 93%93 \% and a pure water permeability of 5.9L*m^(-2)h^(-1)5.9 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} bar ^(-1)^{-1}. The NaCl retention was around 50%50 \%. Moreover, additionally cross-linking the top layer with glutaraldehyde further increased the OMP retention to 98%98 \% on average along with a slight decrease of permeability by 20%20 \%. While the achieved separation is equal to the previously published data at lower permeability [66], the crosslinking is predicted to be more chemical resistant, allowing usage for harsher conditions and cleaning procedures and therewith showing the high potential for OMP removal. 强度反而导致了平均有机微污染物（OMP）去除率高达 93%93 \% ，纯水渗透率为 5.9L*m^(-2)h^(-1)5.9 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} bar ^(-1)^{-1} 。NaCl 的截留率约为 50%50 \% 。此外，使用戊二醛对顶层进行额外交联，进一步将 OMP 的平均截留率提高到 98%98 \% ，同时渗透率略微下降了 20%20 \% 。虽然所实现的分离效果与先前在较低渗透率下发表的数据相当[66]，但预计交联膜具有更强的化学耐受性，允许在更严苛的条件和清洗程序中使用，从而显示出其在 OMP 去除方面的巨大潜力。
Lately, a new type of PEM membrane for OMP removal was produced. In 2020, Li et al. immobilized laccase in between PSS/PAH multilayers and simultaneously on the dense separation layer achieving LbL-based biocatalytic NF membranes [68]. Post immobilization appeared to be the favorable production strategy, resulting in a competitive pure water permeability of 10.9L*m^(-2)h^(-1)10.9 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} bar ^(-1)^{-1} and laccase loading and activity. A bisphenol (BPA) removal of 92.5%92.5 \% was achieved and found to be robust, maintaining the high retention after six cycles within 14 days. 最近，开发出了一种用于 OMP 去除的新型聚电解质膜（PEM）。2020 年，Li 等人在 PSS/PAH 多层膜之间及致密分离层上同时固定了漆酶，制备了基于层层自组装（LbL）的生物催化纳滤膜[68]。后期固定化被认为是更优的生产策略，获得了具有竞争力的纯水渗透率 10.9L*m^(-2)h^(-1)10.9 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} bar ^(-1)^{-1} 以及漆酶的负载量和活性。实现了 92.5%92.5 \% 的双酚 A（BPA）去除率，并且表现出良好的稳定性，在 14 天内经过六个循环后仍保持高截留率。
The high retentions of micropollutants by HF NF membranes are not only limited to LbL PEM membranes. Bolong et al. fabricated PES HF NF membranes blended with charged surface modifying macromolecules [69]. In 2015 Liu et al. showed that thin-film nanocomposite HF membranes could be prepared via dioxane assisted interfacial polymerization [70]. The SAPO-34 nanoparticle incorporated membranes showed pure water permeability of 20.1L*m^(-2)h^(-1)bar^(-1)20.1 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} \mathrm{bar}^{-1} and high rejection ( > 96%>96 \% ) for the micropollutants tris(2-chloroethyl) phosphate, tris(1-chloro-2-propyl) phosphate and tris(1,3-dichloro-2propyl) phosphate. HF 纳滤膜对微污染物的高截留不仅限于 LbL PEM 膜。Bolong 等人制备了掺杂带电表面改性大分子的 PES HF 纳滤膜[69]。2015 年，Liu 等人展示了通过二恶烷辅助界面聚合制备薄膜纳米复合 HF 膜的方法[70]。掺杂 SAPO-34 纳米颗粒的膜表现出纯水渗透率为 20.1L*m^(-2)h^(-1)bar^(-1)20.1 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} \mathrm{bar}^{-1} ，并对微污染物三(2-氯乙基)磷酸酯、三(1-氯-2-丙基)磷酸酯和三(1,3-二氯-2-丙基)磷酸酯具有高截留率( > 96%>96 \% )。
A special type of micropollutant are the so-called “forever chemicals”, compounds that do not naturally break down. Per- and polyfluoroalkyl substances (PFAS) are hydrocarbons in which either all or most hydrogen atoms on the alkyl chain are replaced by fluorine atoms. The most well known examples are perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA). They are used in a wide variety of applications because of their unique physico-chemical properties such as durability and ability to repel both water and oil [71]. Over the last years, concerns have been raised about PFAS, because they were shown to be both toxic and bioaccumulative. Because of this, PFOS and PFOA, among others, have been categorized as persistent organic pollutants (POP) [72]. PFAS are detected in surface, ground and tap water and cause a threat to the secure water. While there are countries that set regulations to limit the use of PFAS, it is still widely used because of a lack of alternatives. Therefore, research focuses on alternative water treatment methods to remove PFAS from water, as the conventional biological water treatment plants do not suffice. 一种特殊类型的微污染物是所谓的“永久化学品”，即那些不会自然分解的化合物。全氟和多氟烷基物质（PFAS）是指烃类化合物，其中烷基链上的全部或大部分氢原子被氟原子取代。最著名的例子是全氟辛烷磺酸（PFOS）和全氟辛酸（PFOA）。由于其独特的物理化学性质，如耐久性以及排斥水和油的能力，它们被广泛应用于各种领域[71]。近年来，人们对 PFAS 的关注不断增加，因为研究表明它们具有毒性且易于生物累积。因此，PFOS 和 PFOA 等被归类为持久性有机污染物（POP）[72]。PFAS 已在地表水、地下水和自来水中被检测到，威胁着水资源的安全。尽管一些国家已制定法规限制 PFAS 的使用，但由于缺乏替代品，其仍被广泛使用。因此，研究重点转向开发替代水处理方法以去除水中的 PFAS，因为传统的生物水处理厂无法有效去除这些物质。
The efficacy of the traditional SW NF membranes for the removal of PFAS was shown on several occasions. Soriano et al. showed the successful use of NF270 membranes for the removal of perfluorohexanoic acid from industrial process waters up to 99.4%99.4 \% [73]. The concentrated retentate stream could be treated separately with an electrochemical cell, leading to a TOC reduction of > 95%>95 \%. In a recent study, Zhao et al. investigated PFOS removal in the presence of humic acids and inorganic ions during NF with a commercial ESNA1-K1 membrane (Hydranautics) [74]. The presence of humic acids, cations and anions improved the PFOS removal up to 99%99 \% retention. 传统海水纳滤（SW NF）膜在去除全氟烷基物质（PFAS）方面的有效性已多次得到验证。Soriano 等人展示了使用 NF270 膜成功去除工业工艺水中的全氟己酸，去除率高达 99.4%99.4 \% [73]。浓缩的截留液流可以通过电化学电池单独处理，实现总有机碳（TOC）降低 > 95%>95 \% 。在最近的一项研究中，Zhao 等人研究了在存在腐殖酸和无机离子的情况下，使用商业 ESNA1-K1 膜（Hydranautics）进行纳滤时对全氟辛烷磺酸（PFOS）的去除效果[74]。腐殖酸、阳离子和阴离子的存在使得 PFOS 的去除率提高至 99%99 \% 。
Wang et al. showed that by using a poly(m-phenylene isophtalamide) HF NF membrane, trace amounts of PFOS could be rejected from an aqueous solution [75]. PFOS rejections of 91.17 to 97.49%97.49 \% were observed, with pH increasing from 3.2 to 9.5 . With an increased concentration of Ca^(2+)\mathrm{Ca}^{2+} in the feed solution, the rejection could be even further enhanced. It is expected that in the removal of PFAS from water sources, NF membranes can play a big role in securing the quality of water and both aquatic and human health. Wang 等人展示了通过使用聚（间苯二甲酰对苯二胺）中空纤维纳滤膜，可以从水溶液中去除痕量的 PFOS [75]。观察到 PFOS 的去除率在 91.17% 到 97.49%97.49 \% 之间，pH 值从 3.2 增加到 9.5。随着进料溶液中 Ca^(2+)\mathrm{Ca}^{2+} 浓度的增加，去除率甚至可以进一步提高。预计在从水源中去除 PFAS 方面，纳滤膜将在保障水质以及水生和人体健康方面发挥重要作用。
3.2.2. Antimicrobial Resistance 3.2.2. 抗菌素耐药性
Antimicrobial resistance (AMR) is an emerging phenomenon as a result of the widespread use of antimicrobial agents. The occurrence can be enhanced via different pathways such as via WWTPs, overuse of antibiotics in aquaculture or animal manure in agriculture. The in- 抗菌素耐药性（AMR）是一种新兴现象，源于抗菌剂的广泛使用。其发生可以通过不同途径增强，例如通过污水处理厂（WWTPs）、水产养殖中过度使用抗生素或农业中使用动物粪便。该现象在-
creased resistance against antibiotics can have a negative effect on both human and animal health. Therefore, it is considered as a global health threat according to the world health organization (WHO), UN and European Union (EU), which needs to be addressed [76]. 对抗生素的抗药性增强可能对人类和动物健康产生负面影响。因此，世界卫生组织（WHO）、联合国和欧盟（EU）将其视为全球健康威胁，亟需加以解决[76]。
AMR is transported by antibiotic resistant bacteria (ARB) and expressed by antibiotic resistant genes (ARGs) [77]. It is important to tackle the AMR issue at the source and ensure that antibiotics are not disposed into wastewater from where they can be discharged into the environment. NF can play a significant role in this by retention of antibiotics from hospital and industrial wastewater streams. The applicability of NF for this is already covered in the previous paragraph on micropollutants. However, AMR intervention consists of more than just micropollutant removal. While the bacterias might be handled with microfiltration (MF)/UF, resistant genes require a much lower cut-off, which open NF can provide [78]. Lan et al. investigated the use of NF for the removal of ARGs in swine wastewater. Removal rates of 5-8log5-8 \log could be obtained for various kinds of ARGs with a polyamide NF membrane. However, these reduction values were calculated for the whole train of the WWTP. A range of membranes, ranging from MF to RO, were studied by Slipko et al. for the purpose of free DNA containing ARG removal [79]. Up to 99.0%99.0 \% of free DNA could be removed by NF membranes. Furthermore, Krzeminski et al. showed that open NF membranes with MWCO < 1kDa<1 \mathrm{kDa} (polyamide TFC) were effective for the complete removal of cell free DNA [76]. The obtained concentrate stream could in turn be treated by UV. Recently, Cristóvão et al. tested the use of Desal 5DK membranes at a domestic wastewater treatment plant [80]. They observed stable permeance, with high rejections for target antibiotics, ARGs and viral genomes. These examples provide promising results for potential application of NF in antibiotic removal. However, the number of publications on the removal of ARGs by NF is still limited and should gain more attention in order to solve the AMR issue. Results show that several NF membranes, with a wide range of cut-off and different geometries, can be employed and the best choice will rely on both the target compounds that need to be removed and the complete water matrix that needs to be treated. 抗生素耐药性（AMR）由抗生素耐药细菌（ARB）传播，并由抗生素耐药基因（ARGs）表达[77]。解决 AMR 问题的关键在于源头控制，确保抗生素不被排入废水中，从而避免其进入环境。纳滤（NF）在这方面可以发挥重要作用，通过截留医院和工业废水中的抗生素。纳滤在这方面的适用性已在前文关于微污染物的段落中讨论过。然而，AMR 干预不仅仅是去除微污染物。虽然细菌可以通过微滤（MF）/超滤（UF）处理，但耐药基因需要更低的截留分子量，开放式纳滤膜可以满足这一需求[78]。Lan 等人研究了纳滤在去除猪舍废水中 ARGs 的应用。使用聚酰胺纳滤膜对多种 ARGs 的去除率可达到 5-8log5-8 \log 。不过，这些去除率是针对整个污水处理厂工艺流程计算的。Slipko 等人研究了从微滤到反渗透（RO）的一系列膜，用于去除含有 ARG 的游离 DNA[79]。纳滤膜可去除高达 99.0%99.0 \% 的游离 DNA。此外，Krzeminski 等人... 研究表明，开孔的 MWCO 为 < 1kDa<1 \mathrm{kDa} 的纳滤膜（聚酰胺 TFC）对细胞游离 DNA 的完全去除效果显著[76]。获得的浓缩液流随后可以通过紫外线处理。最近，Cristóvão 等人在一个生活污水处理厂测试了 Desal 5DK 膜[80]。他们观察到稳定的透水率，并对目标抗生素、抗性基因（ARGs）和病毒基因组表现出高去除率。这些例子为纳滤在抗生素去除中的潜在应用提供了有希望的结果。然而，关于纳滤去除抗性基因的相关文献仍然有限，需引起更多关注以解决抗菌素耐药性（AMR）问题。结果显示，多种纳滤膜具有不同的截留范围和几何形状均可应用，最佳选择将取决于需要去除的目标化合物以及需处理的整体水体基质。
3.2.3. Micro- and Nanoplastics Removal 3.2.3. 微塑料和纳米塑料去除
Roughly ten percent of the total amount of plastic produced eventually ends up in the water environment [81]. Since most plastics are not biodegradable, they accumulate in the aquatic environment and contribute to plastic pollution. The plastic fragments differ in sizes; typically, microplastics are particles smaller than 5 mm and nanoplastics smaller than 1mum1 \mu \mathrm{~m}. These plastics fragments can have a negative impact on the aquatic environment [81,82], e.g., on growth, reproduction and mortality of aquatic animals. Next, potential negative effects to human health are described, e.g., the microplastic polystyrene has shown oxidative stress to human cerebral and epithelial cells [83]. Nanoplastics are considered even more dangerous, because of their small size and large surface area and the fact that they are more difficult to remove in conventional water-treatment processes. In recent years more focus has been put on how to remove these plastics from wastewater. Wastewater treatment plants are able to remove 75%75 \% of microplastics in the pre-, primary and secondary treatment steps. By using a tertiary treatment, the removal can be increased to 98%98 \% [84]. However, plastics smaller than 20 mum20 \mu \mathrm{~m} and nanoplastics are not removed. Therefore, improvements are still needed to also remove these compounds from the water. Malankowska et al. suggested that NF might be an efficient method for this purpose [85]. To the best of our knowledge, no research studies are available yet regarding the effectiveness of NF for this purpose, but it is expected to be evaluated in due time. In research by Trzaskus et al. it was shown that already with MF nanoparticles much smaller than the membrane pore sizes could be retained [86], e.g., 95%95 \% of 92 nm nanoparticles could be separated. Therefore, NF membranes are expected to be useful for the retention of even smaller nanoplastics. 大约百分之十的塑料总产量最终进入水环境[81]。由于大多数塑料不可生物降解，它们在水生环境中积累，导致塑料污染。塑料碎片大小不一；通常，微塑料是指小于 5 毫米的颗粒，纳米塑料则小于 1mum1 \mu \mathrm{~m} 。这些塑料碎片可能对水生环境产生负面影响[81,82]，例如影响水生动物的生长、繁殖和死亡率。接下来，描述了对人类健康的潜在负面影响，例如，微塑料聚苯乙烯已显示对人类脑细胞和上皮细胞产生氧化应激[83]。纳米塑料被认为更危险，因为其体积小、表面积大，且在传统水处理过程中更难去除。近年来，越来越多的关注集中在如何从废水中去除这些塑料。废水处理厂能够在预处理、初级和二级处理步骤中去除 75%75 \% 的微塑料。通过使用三级处理，去除率可提高到 98%98 \% [84]。 然而，小于 20 mum20 \mu \mathrm{~m} 的塑料和纳米塑料无法被去除。因此，仍需改进以去除水中的这些化合物。Malankowska 等人建议，纳滤（NF）可能是一种有效的方法[85]。据我们所知，目前尚无关于纳滤在此方面有效性的研究，但预计未来会有相关评估。Trzaskus 等人的研究表明，即使是微滤（MF），也能截留远小于膜孔径的纳米颗粒[86]，例如，92 纳米的纳米颗粒中有 95%95 \% 被分离。因此，纳滤膜有望用于截留更小的纳米塑料。
3.2.4. Wastewater Reuse 3.2.4. 污水再利用
The aforementioned applications show the promising potential of HF NF membranes for municipal wastewater purification. As the water quality of the permeate will typically not only meet, but exceed the discharge limits, the high-quality treated water can potentially be reused in various applications, e.g., as process or boiler feed water in the industry, for agricultural irrigation, or even as potable water. In contrast to other water sources (e.g., surface water), municipal wastewater is mostly independent from seasonal weather variability and droughts, which enables a shift towards a more circular water economy. In Singapore, five NEWater plants are operated to reclaim pre-treated wastewater for reuse, supplying 40%40 \% of the country’s total water needs [87]. The three-staged NEWater process includes MF/UF, followed by an RO installation and a UV disinfection step (see Figure 4). The achieved water quality allows for use in wafer fabrication plants, where even stricter standards than for drinking water are required. During dry weather, this reclaimed water is also blended to raw water reservoirs. 上述应用展示了 HF NF 膜在市政废水净化方面的广阔潜力。由于渗透水的水质通常不仅能达到排放标准，还能超出标准，高质量的处理水有可能在多种应用中再利用，例如作为工业中的工艺用水或锅炉给水，用于农业灌溉，甚至作为饮用水。与其他水源（如地表水）相比，市政废水大多不受季节性天气变化和干旱的影响，这使得向更循环的水经济转变成为可能。在新加坡，运营着五个 NEWater 厂，回收预处理废水以供再利用，满足该国总用水需求的 40%40 \% [87]。三阶段的 NEWater 工艺包括 MF/UF，随后是 RO 装置和紫外线消毒步骤（见图 4）。所获得的水质允许用于晶圆制造厂，该厂对水质的要求甚至比饮用水更严格。在干旱天气期间，这种回收水也会与原水库水混合使用。
Figure 4. Schematic representation of the NEWater process to recycle treated used water into ultra-clean water [87]. Reprinted with permission from PUB, Singapore’s National Water Agency. 图 4. NEWater 工艺示意图，用于将处理过的废水回收为超净水[87]。经新加坡国家水务局（PUB）许可转载。
Similarly, HF NF could be applied to combine both membrane processes into one step. As has been shown before, HF NF membranes are able to deliver a water quality that allows for drinking water production or reuse in various industries. A wastewater reuse case using HF NF is proposed by the Dutch network organization Energy and Resources Factory. As shown in Figure 5, the WWTP’s effluent enters the HF NF directly after a denitrifying sand filter and provides a level of quality adhering to the Dutch Drinking Water Decree that is suitable for use as process water in most industries [88]. With an additional UV +H_(2)O_(2)+\mathrm{H}_{2} \mathrm{O}_{2} installation as a second disinfection step, total costs (capitalization and operational costs) of 0.50€*m^(-3)0.50 € \cdot \mathrm{~m}^{-3} were reported, which were competitive with local drinking water prices [88]. However, transport was not yet accounted for in the calculation, and it is foreseen that the project requires larger amounts of treated water for true economic feasibility. 同样，HF NF 可以应用于将两种膜工艺合并为一步。如前所示，HF NF 膜能够提供适合饮用水生产或在各行业再利用的水质。荷兰网络组织能源与资源工厂提出了一个使用 HF NF 的废水再利用案例。如图 5 所示，污水处理厂（WWTP）的出水在经过反硝化砂滤后直接进入 HF NF，提供符合荷兰饮用水法令标准的水质，适用于大多数行业的工艺用水[88]。通过增加紫外线（UV）装置作为第二道消毒步骤，报告的总成本（资本和运营成本） 0.50€*m^(-3)0.50 € \cdot \mathrm{~m}^{-3} 具有与当地饮用水价格竞争力[88]。然而，运输成本尚未计入计算中，预计该项目需要更大量的处理水以实现真正的经济可行性。
Figure 5. Schematic illustration of water factory water reuse case. Reprinted from [88] with permission from Energy and Resources Factory. 图 5. 水厂水回用案例示意图。经 Energy and Resources Factory 许可，转载自[88]。
3.3. Industrial Wastewater Treatment 3.3. 工业废水处理
3.3.1. Heavy Metal Removal 3.3.1. 重金属去除
Heavy metals in water are a major concern around the globe due to their persistence and toxicity. Most of them can cause multiple organ damage, even at lower levels of exposure and are classified as either “known” or “probable” human carcinogens [89]. A number of these heavy metals are registered on the Substance Priority List (SPL) of the American Agency for Toxic Substances and Disease Registry, with arsenic, lead and mercury ranked on the first three positions [90]. The SPL indicates substances with significant potential threat to human health due to their known or suspected toxicity. Industrial processes all over the world produce wastewater with substantial concentration of heavy metals, which is then directly or indirectly, via a treatment plant, released to water bodies and drinking water sources. In 2016, over 60%60 \% of the reported heavy metal emissions into water in Europe came from direct or indirect industrial releases, mainly from the chemical manufacturing, non-ferrous metal processing and the energy supply sector [91]. The increasing concentrations of heavy metals in global water bodies observed from 1972 to 2017 [92,93] indicate a worldwide threat for the environment as well as human health and have made the removal of heavy metals a top priority in industrial wastewater treatment processes. 水中的重金属因其持久性和毒性而成为全球关注的重大问题。大多数重金属即使在较低暴露水平下也能引起多器官损伤，并被归类为“已知”或“可能”的人类致癌物[89]。其中许多重金属被列入美国有毒物质和疾病登记局的物质优先清单（SPL），其中砷、铅和汞位列前三位[90]。SPL 指示了因其已知或疑似毒性而对人类健康具有重大潜在威胁的物质。全球各地的工业过程产生含有大量重金属的废水，这些废水随后通过处理厂直接或间接排放到水体和饮用水源中。2016 年，欧洲报告的排入水体的重金属排放中，有超过 60%60 \% 来自直接或间接的工业排放，主要来自化学制造、有色金属加工和能源供应行业[91]。 从 1972 年到 2017 年，全球水体中重金属浓度的不断增加[92,93]表明这对环境及人类健康构成了全球性威胁，因此重金属的去除已成为工业废水处理过程中的首要任务。
The performances of conventional NF membranes regarding heavy metals like arsenic [94], cadmium [95], chromium [96], copper [97,98], lead [99], nickel [100] and zinc [101] have been thoroughly investigated and Abdullah et al. provide a brief overview [102]. However, for many industrial waste streams it is expected that these cannot be directly treated with SW NF due to the high fouling potential of these streams. Several HF NF membranes have proven to be a powerful treatment method since multivalent ions are retained effectively. Gao et al. designed HF NF membranes by adsorption of negatively charged chelating polymers poly(acrylic acid) (PAA) on a positively charged poly(ethyleneimine) (PEI) cross-linked P84 support [103]. The resulting rejections of the PAA adsorbed membrane for lead, copper, nickel, cadmium, zinc, chromium and arsenic were 94%94 \% or higher while a permeability of around 1L*m^(-2)h^(-1)bar^(-1)1 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} \mathrm{bar}^{-1} was achieved. Tests on heavy metal mixtures confirmed an average rejection of 98%98 \%. Additionally, it was shown that the chelating polymers help enhance rejections through adsorption of heavy metal ions. The same research group investigated the performance of thin-film composite HF membranes made of blended sulfonated polysulfone (PS), PES and PEI and further modified with TMC [104]. The NF membrane delivered a pure water permeability of 5.1L*m^(-2)h^(-1)bar^(-1)5.1 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} \mathrm{bar}^{-1} while retaining the heavy metal ions copper, nickel, and zinc more than 90%90 \% in single ion tests as well as in mixtures of those three cations ( > 95%>95 \% ). 关于砷[94]、镉[95]、铬[96]、铜[97,98]、铅[99]、镍[100]和锌[101]等重金属，传统纳滤膜的性能已被彻底研究，Abdullah 等人提供了简要综述[102]。然而，对于许多工业废水流，预计由于这些水流的高污染潜力，无法直接用海水纳滤（SW NF）处理。多种高通量纳滤（HF NF）膜已被证明是一种有效的处理方法，因为多价离子能够被有效截留。Gao 等人通过在带正电的聚乙烯亚胺（PEI）交联 P84 支撑膜上吸附带负电的螯合聚合物聚丙烯酸（PAA）设计了高通量纳滤膜[103]。所制备的 PAA 吸附膜对铅、铜、镍、镉、锌、铬和砷的截留率均达到 94%94 \% 或更高，同时实现了约 1L*m^(-2)h^(-1)bar^(-1)1 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} \mathrm{bar}^{-1} 的渗透率。对重金属混合物的测试确认了平均截留率为 98%98 \% 。此外，研究表明螯合聚合物通过吸附重金属离子，有助于提高截留效果。 同一研究团队研究了由混合磺化聚砜（PS）、聚醚砜（PES）和聚醚酰亚胺（PEI）制成的薄膜复合中空纤维（HF）膜的性能，并进一步用三氯化三甲酰胺（TMC）进行了改性[104]。该纳滤膜在单离子测试以及这三种阳离子混合物测试中，纯水通量为 5.1L*m^(-2)h^(-1)bar^(-1)5.1 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} \mathrm{bar}^{-1} ，同时对铜、镍和锌等重金属离子的截留率均超过 90%90 \% （ > 95%>95 \% ）。
Zhu et al. coated a polybenzimidazole (PBI) and PES bilayer on an HF PES/ polyvinylpyrrolidone support for removal of heavy metal ions (cadmium, chromium, lead) from model wastewater [105]. The rejections of the NF membrane to magnesium and cadmium achieved 98%98 \% and 95%95 \%, respectively. By changing the pH of the solution, the rejections to chromium and lead reached more than 98%98 \% and 93%93 \%, correspondingly. In 2015, 朱等人在中空纤维 PES/聚乙烯吡咯烷酮支撑层上涂覆了聚苯并咪唑（PBI）和聚醚砜（PES）双层膜，用于去除模型废水中的重金属离子（镉、铬、铅）[105]。该纳滤膜对镁和镉的截留率分别达到了 98%98 \% 和 95%95 \% 。通过调节溶液的 pH 值，对铬和铅的截留率分别超过了 98%98 \% 和 93%93 \% 。2015 年，
poly(amidoamine) dendrimer (PAMAM) was grafted on TFC HF NF membranes made of PES [106]. The modification successfully decreased the pore size and improved the pure water permeability, (around 4.0L*m^(-2)h^(-1)4.0 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} bar ^(-1)^{-1} against 3.1L*m^(-2)h^(-1)3.1 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} bar ^(-1)^{-1} ) attributed to an increased hydrophilicity, without affecting the rejection. For the grafted membrane, rejections for arsenic, cadmium, copper and lead reached more than 99%99 \%. In tests with ion mixtures, rejections of more than 98%98 \% for nickel, zinc and chromium were achieved. A pH dependency could be observed, indicating better removal rates at higher pH . 聚酰胺胺树枝状大分子（PAMAM）被接枝到由 PES 制成的 TFC HF NF 膜上[106]。该改性成功减小了孔径并提高了纯水通量（约 4.0L*m^(-2)h^(-1)4.0 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} bar ^(-1)^{-1} 对比 3.1L*m^(-2)h^(-1)3.1 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} bar ^(-1)^{-1} ），这归因于亲水性的增强，同时不影响截留性能。对于接枝膜，砷、镉、铜和铅的截留率均超过了 99%99 \% 。在离子混合物测试中，镍、锌和铬的截留率超过了 98%98 \% 。观察到 pH 依赖性，表明在较高 pH 值下去除率更佳。
Recently, a graphene oxide (GO)/ethylene diamine (EDA) bilayer NF membrane was investigated for heavy metal removal: Zhang et al. coated the GO/EDA network via the LbL approach onto the outside of a modified Torlon ^(®){ }^{\circledR} HF support [107]. The rejections for lead, nickel and zinc were tested as well as the long-term stability. The best results were reported for a 5-bilayer GO/EDA membrane with rejections higher than 95%95 \% for the mentioned heavy metals and a pure water permeability of 4.7L*m^(-2)h^(-1)bar^(-1)4.7 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} \mathrm{bar}^{-1} at a pressure of 3 bar. During a 150 h -long performance test, permeability and rejection (only tested for lead) remained stable with a slight increase in water flux. 最近，研究了一种氧化石墨烯（GO）/乙二胺（EDA）双层纳滤膜用于重金属去除：Zhang 等人通过层层自组装（LbL）方法将 GO/EDA 网络涂覆在改性 Torlon ^(®){ }^{\circledR} 中空纤维（HF）支撑膜的外侧[107]。测试了铅、镍和锌的截留率以及长期稳定性。最佳结果出现在 5 层 GO/EDA 膜，所提及的重金属截留率均高于 95%95 \% ，纯水渗透率在 3 巴压力下为 4.7L*m^(-2)h^(-1)bar^(-1)4.7 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} \mathrm{bar}^{-1} 。在长达 150 小时的性能测试中，渗透率和截留率（仅测试铅）保持稳定，水通量略有增加。
Wei et al. applied thin-film composite HF NF membranes on actual electroplating wastewater for heavy metal removal [108]. Membranes were prepared by interfacial polymerization of PIP and TMC on the HF PS/PES support. At 4 bar operating pressure, the rejection rates reached 96%96 \% for chromium, and 95%95 \% for copper and nickel and ion concentration was more than five times higher in permeate compared to the feed solution. Moreover, the membranes showed a good stability in the low pH (2.31) electroplating wastewater. Wei 等人将薄膜复合中空纤维纳滤膜应用于实际电镀废水中的重金属去除[108]。膜通过在中空纤维 PS/PES 支撑膜上进行 PIP 和 TMC 的界面聚合制备。在 4 巴操作压力下，铬的截留率达到 96%96 \% ，铜和镍的截留率达到 95%95 \% ，且渗透液中的离子浓度比进料溶液高出五倍以上。此外，膜在低 pH 值（2.31）的电镀废水中表现出良好的稳定性。
Despite these several studies on the membrane development and the efficacy of these membranes on synthetic waste streams, little information can be found about the performance of HF NF on real wastewater streams. Such studies are required to assess the true applicability of these membranes for the removal of heavy metals from industrial streams. Upcoming research further needs to include strategies on how to deal with the concentrate streams coming from the NF process. 尽管已有多项关于膜开发及这些膜在合成废水流中的效能的研究，但关于 HF NF 在实际废水流中的性能信息却很少。需要此类研究来评估这些膜在去除工业废水中重金属的实际适用性。未来的研究还需包括如何处理 NF 工艺产生的浓缩水流的策略。
3.3.2. Sulfate Removal 3.3.2. 硫酸盐去除
High loads of sulfates may damage the concrete in the sewage system in a so-called external sulfate attack [109]. Often, a massive formation of gypsum and ettringite formed during this external sulfate attack may cause concrete to crack and scale [110]. Additionally, under anaerobic conditions, sulphate can result in sulfuric acid which further damages the cement of the sewage pipes [110]. As a higher sulfate concentration is related to potentially higher damage [111], reducing the sulfate levels in wastewater can reduce the risk for sulfate attacks. To prevent irreversible damage to the sewage system, in the Netherlands the disposal limit of sulfate is set to 300 ppm [112]. Since NF removes bivalent ions like sulfate effectively, it is a well-suited technology for sulfate removal and has been reported for various applications, among others in mine drainage treatment [113-115], agricultural drainage treatment or the electronic manufacturing industry, where Jin et al. [116] investigated the performance of a commercial thin-film composite NF membrane (DK1812, GE) on a sulfate-rich wastewater from an electronic manufacturing plant. The initial sulfate concentration of 900mg*L^(-1)900 \mathrm{mg} \cdot \mathrm{L}^{-1} could be reduced by more than 98%98 \% to 16mg*L^(-1)16 \mathrm{mg} \cdot \mathrm{L}^{-1}, enabling a direct water reuse as cooling water [116]. Furthermore, a recovery of 83%83 \% was realized proofing the high potential of NF. However, increased scale formation was detected at elevated concentration factors harming the process performance. Here, HF NF could be a suitable alternative because it enables for better scaling control and more effective cleaning methods as these membranes do not contain spacers around which scale growth is known to initiate [52]. 高浓度的硫酸盐可能会损害污水系统中的混凝土，形成所谓的外部硫酸盐侵蚀[109]。通常，在这种外部硫酸盐侵蚀过程中大量生成的石膏和针状硫铝酸盐会导致混凝土开裂和剥落[110]。此外，在厌氧条件下，硫酸盐可转化为硫酸，进一步损害污水管道的水泥[110]。由于较高的硫酸盐浓度与潜在的更大损害相关[111]，降低废水中的硫酸盐含量可以减少硫酸盐侵蚀的风险。为了防止对污水系统造成不可逆转的损害，荷兰将硫酸盐的排放限值设定为 300 ppm[112]。由于纳滤能够有效去除二价离子如硫酸盐，它是一种非常适合的硫酸盐去除技术，并已在多种应用中得到报道，包括矿山排水处理[113-115]、农业排水处理以及电子制造业，其中 Jin 等人[116]研究了商业薄膜复合纳滤膜（DK1812，GE）在电子制造厂含硫酸盐废水中的性能。 初始硫酸盐浓度为 900mg*L^(-1)900 \mathrm{mg} \cdot \mathrm{L}^{-1} ，可被降低超过 98%98 \% 至 16mg*L^(-1)16 \mathrm{mg} \cdot \mathrm{L}^{-1} ，从而实现作为冷却水的直接水回用[116]。此外，实现了 83%83 \% 的回收，证明了纳滤（NF）的高潜力。然而，在较高浓缩因子下检测到结垢增加，影响了工艺性能。在这里，HF 纳滤可能是一个合适的替代方案，因为它能够更好地控制结垢并采用更有效的清洗方法，因为这些膜不含已知结垢起始点的间隔物[52]。
In 2018, Jährig et al. upgraded the commercially available HFW 1000 membrane (Pentair X-Flow) with specific extra coatings to increase the retention capabilities and investigated the performance for sulfate removal in a pilot-scale set-up on well water with elevated sulphate concentration [43]. Pilot operation at a flux of 22.5L*m^(-2)h^(-1)22.5 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} and a recovery of 75%75 \% resulted in sulfate removal of 67%67 \%. 2018 年，Jährig 等人对市售的 HFW 1000 膜（Pentair X-Flow）进行了特定的额外涂层升级，以提高截留能力，并在硫酸盐浓度较高的井水上通过中试规模装置研究了其硫酸盐去除性能[43]。在通量为 22.5L*m^(-2)h^(-1)22.5 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} 、回收率为 75%75 \% 的中试运行中，实现了 67%67 \% 的硫酸盐去除。
HF NF membranes have recently been evaluated for sulfate removal in a German joint research project [117,118]. In the SULEMAN project, commercial HF UF membranes (Multibore ^(®){ }^{\circledR}, DuPont inge GmbH ) were coated via the LbL -approach to create NF separation properties [119] based on PDADMAC and PSS. The novel membranes achieved a MgSO_(4)\mathrm{MgSO}_{4} retention of 90%90 \% and a pure water permeability of 15L*m^(-2)h^(-1)15 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} bar ^(-1)^{-1} was reported. First tests on water with elevated sulfate amounts showed promising results of 80 to 90%90 \% sulfate removal and stable performance. HF NF 膜最近在德国的一个联合研究项目中被评估用于硫酸盐去除[117,118]。在 SULEMAN 项目中，商业 HF UF 膜（Multibore ^(®){ }^{\circledR} ，DuPont inge GmbH）通过层层自组装（LbL）方法涂覆，以基于 PDADMAC 和 PSS 创建 NF 分离性能[119]。新型膜实现了 MgSO_(4)\mathrm{MgSO}_{4} 保留率为 90%90 \% ，纯水通量为 15L*m^(-2)h^(-1)15 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} bar ^(-1)^{-1} 。对含高浓度硫酸盐水的初步测试显示出有希望的结果，硫酸盐去除率为 80 至 90%90 \% ，且性能稳定。
3.3.3. Organics Removal 3.3.3. 有机物去除
Industrial wastewaters usually contain a diverse range of organic compounds in different concentrations. A lot of these substances can have a severe impact on human or environmental health, e.g., being carcinogenic or bioaccumulating in animals. Typical sources of organically polluted wastewaters are, among others, the petrochemical industry, textile industry, tanning industry, pulp and paper industry, rubber and tire production, chemical production, food and beverage industry and the energy supply sector [91,120]. We will show that, due to their robustness and high separation efficiency for small organic pollutants, HF NF has proven to be a successful technology for organic removal from industrial wastewater in single step processes [28,121-125]. 工业废水通常含有多种不同浓度的有机化合物。这些物质中许多可能对人类或环境健康产生严重影响，例如具有致癌性或在动物体内生物累积。有机污染废水的典型来源包括石油化工行业、纺织行业、制革行业、纸浆和造纸行业、橡胶和轮胎生产、化学品生产、食品和饮料行业以及能源供应部门[91,120]。我们将展示，由于其稳健性和对小分子有机污染物的高分离效率，HF NF 已被证明是一种成功的工业废水有机物单步去除技术[28,121-125]。
Textile Industry 纺织行业
One important industrial sector is the textile industry. Since it produces a lot of wastewater loaded with complex mixtures of dyes, a treatment prior discharge is crucial to prevent damage to ecosystems and human health and align with legislations [126,127]. HF NF has been studied intensively in the textile industry and has shown great potential in effectively removing organic dyes from textile wastewater, while retaining low amounts of monovalent salts resulting in beneficial process conditions. 一个重要的工业领域是纺织工业。由于其产生大量含有复杂染料混合物的废水，排放前的处理对于防止生态系统和人类健康受到损害以及符合相关法规至关重要[126,127]。中空纤维纳滤膜（HF NF）在纺织工业中得到了广泛研究，并显示出在有效去除纺织废水中的有机染料方面的巨大潜力，同时保留较少的单价盐，从而带来有利的工艺条件。
Sun et al. fabricated a thin-film composite HF NF membrane by interfacial polymerization of hyperbranched polyethyleneimine and isophthaloyl chloride on a Torlon ^(®){ }^{®} polyamide-imide dual-layer HF substrate [128]. The membrane rejected Safranin O and Orange II effectively ( > 99%>99 \% ), while a flux of 23L*m^(-2)h^(-1)23 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} and 20L*m^(-2)h^(-1)20 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1}, respectively, at a pressure of 5 bars was achieved. Shao et al. developed a thin-film composite HF NF membrane via interfacial polymerization of m -phenylenediamine (MPD) and PIP with TMC on a polyetherimide HF support [129]. The resulting membrane achieved a flux of 17.5L*m^(-2)h^(-1)17.5 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} at 6 bars and showed rejections as high as 90%90 \% and 98%98 \% for Safranin O and Aniline blue, respectively. Sun 等人通过在 Torlon ^(®){ }^{®} 聚酰胺-酰亚胺双层中空纤维基材上采用超支化聚乙烯亚胺和间苯二甲酰氯的界面聚合，制备了一种薄膜复合中空纤维纳滤膜[128]。该膜有效地截留了藏红花素 O 和橙 II（ > 99%>99 \% ），在 5 巴压力下分别实现了 23L*m^(-2)h^(-1)23 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} 和 20L*m^(-2)h^(-1)20 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} 的通量。Shao 等人通过在聚醚酰亚胺中空纤维支撑体上采用间苯二胺（MPD）和 PIP 与三氯甲烷（TMC）的界面聚合，开发了一种薄膜复合中空纤维纳滤膜[129]。所得膜在 6 巴压力下实现了 17.5L*m^(-2)h^(-1)17.5 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} 的通量，并对藏红花素 O 和苯胺蓝分别表现出高达 90%90 \% 和 98%98 \% 的截留率。
Zheng et al. investigated the performance of a submerged thin-film composite HF NF membrane on biologically treated effluent from a textile industry [121]. The selective layer was formed by coating sodium carboxymethyl cellulose onto the outer surface of a polypropylene (PP) support. The submerged HF NF achieved a chemical oxygen demand (COD) reduction rate of 92%92 \% and color removal of 99%99 \% and exhibited a flux of 5L*m^(-2)h^(-1)5 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} at a transmembrane pressure of 0.8 bar and a concentration factor of 4 . Furthermore, the membrane showed low salt retentions enabling the process to be operated at more beneficial process conditions. 郑等人研究了一种浸没式薄膜复合中空纤维纳滤膜在纺织工业生物处理废水中的性能[121]。选择性层通过在聚丙烯（PP）支撑的外表面涂覆羧甲基纤维素钠形成。该浸没式中空纤维纳滤膜在跨膜压为 0.8 巴、浓缩倍数为 4 的条件下，实现了化学需氧量（COD） 92%92 \% 的去除率和色度 99%99 \% 的去除，并表现出 5L*m^(-2)h^(-1)5 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} 的通量。此外，该膜表现出较低的盐分截留率，使得工艺能够在更有利的工艺条件下运行。
Ong et al. performed pilot-scale studies with a polyamide-imide based HF NF membrane on textile wastewater [125]. The wastewater was a complex mixture of textile dyes, suspended solids, mineral oils, electrolytes and surfactants with very high COD (ranging from 3000 to 8000 ppm ). During a 45 day period the membrane module showed excellent COD rejection rates ( > 95%>95 \% ). The initial flux of 3L*m^(-2)h^(-1)3 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} at 5 bars decreased significantly over time but could be regenerated after chemical cleaning several times during testing without influencing the COD rejection. Ong 等人使用基于聚酰胺酰亚胺的中空纤维纳滤膜对纺织废水进行了中试规模研究[125]。该废水是纺织染料、悬浮固体、矿物油、电解质和表面活性剂的复杂混合物，化学需氧量（COD）非常高（范围为 3000 至 8000 ppm）。在 45 天的时间内，膜组件表现出优异的 COD 去除率（ > 95%>95 \% ）。在 5 巴压力下，初始通量为 3L*m^(-2)h^(-1)3 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} ，但随着时间显著下降，但经过多次化学清洗后可以恢复，且不影响 COD 去除效果。
A polysulfone-based HF NF membrane was prepared via interfacial polymerization of MPD and TMC by Mondal et al. [124]. The membrane performance was then investigated by treating textile wastewater from an Indian textile industry. Dye rejection for cibacron yellow, cibacron red, cibacron black and basic blue were 98%98 \% or higher and COD removal Mondal 等人通过 MPD 和 TMC 的界面聚合制备了一种基于聚砜的中空纤维纳滤膜[124]。随后通过处理印度一家纺织工业的纺织废水对膜性能进行了研究。对 Cibacron 黄、Cibacron 红、Cibacron 黑和基本蓝染料的去除率均达到 98%98 \% 或更高，且 COD 去除率
was 75-90%75-90 \%, thereby achieving the local permissible limit. The flux ranged between 4 and 11L*m^(-2)h^(-1)11 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} for various operating conditions. 为 75-90%75-90 \% ，从而达到了当地允许的排放限值。通量在不同操作条件下介于 4 和 11L*m^(-2)h^(-1)11 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} 之间。
Ji et al. fabricated loose HF NF PSF/GO membranes by one-step non-solvent induced phase separation method without post treatment [130]. The membrane was evaluated on simulated textile wastewaters consisting of various congo red dye and NaCl concentrations. It exhibited excellent dye rejection ( > 99%>99 \% ), while NaCl rejection was below 5%5 \%. Moreover, the reported flux was in the range of 74-95L*m^(-2)h^(-1)74-95 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} at operating pressure of 2 bars. Recently, the same research group developed polyvinylidene fluoride (PVDF) HF NF membranes based on a solvent-free process [131]. The unique multilayer structure was constructed by deposition of graphene oxide and polypyrrole on the supporting layer. The membrane showed a high rejection for congo red (99.5%), direct yellow 24 (99.3%), acid orange 10 ( 99.1%99.1 \% ) and rhodamine B ( 94.1%94.1 \% ), whereas NaCl retention was as low as 4%4 \%. Moreover, a flux of 9L*m^(-2)h^(-1)9 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} was achieved at a pressure of 1 bar . 季等人通过一步非溶剂诱导相分离法制备了无后处理的松散中空纤维纳滤 PSF/GO 膜[130]。该膜在含有不同刚果红染料和 NaCl 浓度的模拟纺织废水中进行了评估。其表现出优异的染料截留率（ > 99%>99 \% ），而 NaCl 的截留率低于 5%5 \% 。此外，在 2 巴操作压力下，报告的通量范围为 74-95L*m^(-2)h^(-1)74-95 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} 。最近，同一研究团队基于无溶剂工艺开发了聚偏二氟乙烯（PVDF）中空纤维纳滤膜[131]。通过在支撑层上沉积氧化石墨烯和聚吡咯构建了独特的多层结构。该膜对刚果红（99.5%）、直接黄 24（99.3%）、酸性橙 10（ 99.1%99.1 \% ）和罗丹明 B（ 94.1%94.1 \% ）表现出高截留率，而 NaCl 的截留率低至 4%4 \% 。此外，在 1 巴压力下实现了 9L*m^(-2)h^(-1)9 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} 的通量。
Oil and Gas Industry 石油和天然气工业
Another important industrial sector is the oil and gas industry. In 2019, more than 12 million barrels of oil were produced per day in the US alone [132]. Huge amounts of wastewater are generated during oil extraction, since on average one barrel of oil results in three barrels of so-called produced water [133]. The composition of produced water can vary widely and depends among other things on location, extraction method or type extracted. Produced water typically includes following major groups: Water-soluble low molecular weight organic acids and monocyclic aromatic hydrocarbons, dissolved and dispersed oil, grease, production chemicals (e.g., corrosion inhibitors, biocides) and dissolved minerals [133,134]. Due to the high pollution and amount of generated produced water, sufficient treatment is necessary to meet the environmental regulatory requirements. Several physical, chemical and biological methods or combined technologies are proposed for produced water treatment [134]. Compared to the conventional methods, membrane filtration offers several advantages, including high quality permeate, low footprint, more automation, no need for extraneous chemicals and lower energy demand [135]. NF for produced water treatment is especially interesting as it operates at lower pressure than RO and, compared to UF, it has higher organics rejection [136]. 另一个重要的工业领域是石油和天然气行业。仅在 2019 年，美国每天就生产超过 1200 万桶石油[132]。在石油开采过程中会产生大量废水，平均每开采一桶石油就会产生三桶所谓的产出水[133]。产出水的成分差异很大，取决于地点、开采方法或开采类型等因素。产出水通常包括以下主要成分：水溶性低分子量有机酸和单环芳香烃、溶解和分散的油脂、生产用化学品（如腐蚀抑制剂、生物杀灭剂）以及溶解的矿物质[133,134]。由于产出水的污染程度高且数量庞大，必须进行充分处理以满足环境监管要求。已有多种物理、化学和生物方法或组合技术被提出用于产出水处理[134]。 与传统方法相比，膜过滤具有多项优势，包括产水质量高、占地面积小、自动化程度高、不需要额外化学品且能耗较低[135]。用于处理产出水的纳滤尤其引人关注，因为其运行压力低于反渗透，且与超滤相比，纳滤对有机物的截留率更高[136]。
Xu et al. studied the performance of three commercial NF membranes on produced water from a sandstone aquifer, namely NF-90 (Dow / Filmtec), TFC-S (Koch) and ESNA (Hydranautics). Among those, the NF-90 had the best TOC rejection with 87.6%87.6 \% and was ranked highest regarding the adjusted specific fluxes [137]. Mondal et al. tested the commercially available membranes NF270 (Filmtec) and NF90 (Filmtec) on produced water [138]. It was shown that the TDS concentration was reduced from 2090 ppm to 1780 ppm and 1340 ppm for NF270 and NF90, respectively. Further, TOC could be reduced from 136.4 ppm to 98.1 ppm and 89.7 ppm for NF270 and NF90, respectively. In 2013, Alzahrani et al. studied the performance of a highly hydrophilic NF membrane supplied by AMFOR INC ^(®){ }^{\circledR} on produced water. The membrane achieved a permeate quality that met 96%96 \% of the 74 measured parameters of WHO and United States Environmental Protection Agency quality standards for drinking water and for reuse as indirect potable water [139]. In a second study, they evaluated the toxicity of the obtained NF permeate [140]. While the TOC level was reduced by 48%48 \%, toxicity tests showed that the permeate surprisingly was still toxic which was attributed to unknown substances. Xu 等人研究了三种商业纳滤膜在砂岩含水层产出水中的性能，分别是 NF-90（Dow / Filmtec）、TFC-S（Koch）和 ESNA（Hydranautics）。其中，NF-90 具有最佳的总有机碳（TOC）去除率为 87.6%87.6 \% ，并且在调整后的比通量方面排名最高[137]。Mondal 等人测试了市售的 NF270（Filmtec）和 NF90（Filmtec）膜在产出水中的表现[138]。结果显示，NF270 和 NF90 分别将总溶解固体（TDS）浓度从 2090 ppm 降低到 1780 ppm 和 1340 ppm。此外，TOC 也分别从 136.4 ppm 降至 98.1 ppm 和 89.7 ppm。2013 年，Alzahrani 等人研究了一种由 AMFOR INC ^(®){ }^{\circledR} 提供的高亲水性纳滤膜在产出水中的性能。该膜获得的渗透水质满足了世界卫生组织（WHO）和美国环境保护署（EPA）饮用水及间接饮用水再利用标准中 74 项测量参数中的 96%96 \% 项[139]。在第二项研究中，他们评估了所获得纳滤渗透水的毒性[140]。 虽然 TOC 水平降低了 48%48 \% ，但毒性测试显示渗透液仍然具有毒性，这归因于未知物质。
All these papers reported membrane fouling as a major challenge in produced water treatment via NF. Here, HF NF could be a better alternative to the aforementioned SW membranes, as HFs can be cleaned more effectively (e.g., backwashing) and are less prone to fouling due to the absence of feed spacers, which have a considerable influence on fouling behavior [141]. 所有这些论文都报告了膜污染是通过纳滤处理产出水的主要挑战。在这里，中空纤维纳滤膜可能是上述海水膜的更好替代品，因为中空纤维膜可以更有效地清洗（例如反冲洗），且由于没有进料隔板，其不易发生污染，而进料隔板对污染行为有显著影响[141]。
Liu et al. fabricated a thin-film composite NF membrane via interfacial polymerization on a dual-layer (PES/PVDF) HF substrate [123]. The novel HF membrane has a permeability of 16.6L*m^(-2)h^(-1)16.6 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} and was tested on secondary effluents obtained from a petrochemical 刘等人通过界面聚合在双层（PES/PVDF）中空纤维基材上制备了一种薄膜复合纳滤膜[123]。这种新型中空纤维膜的渗透率为 16.6L*m^(-2)h^(-1)16.6 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} ，并在经过超滤预处理的石化工业厂二级废水上进行了测试。
industry plant pre-treated by UF. A TOC removal of 97%97 \% down to 2 ppm was achieved while the TDS load was reduced by 39.6%39.6 \% with 633 ppm remaining in the permeate, which allowed for direct discharge into the local sewage system or to surface water. 工业厂经过超滤预处理。实现了 97%97 \% 的总有机碳（TOC）去除，降低至 2 ppm，同时总溶解固体（TDS）负荷减少了 39.6%39.6 \% ，渗透液中残留 633 ppm，这使得废水可以直接排放到当地污水系统或地表水中。
Recently, Virga et al. developed a stable HF NF membrane for produced water treatment. By cross-linking their LbL-coated poly(allylamine hydrochloride) (PAH)/PSS multilayers with glutaraldehyde, surfactant stable membranes were achieved [142]. The membrane showed excellent oil removal ( > 98%>98 \% ) for two synthetic produced waters and TOC retention of 96.5%96.5 \% and 83%83 \% for a cationic containing surfactant and an anionic containing surfactant, respectively. More important, flux recovery after cleaning was fully achieved for the cationic solution ( 100%100 \% ) and it was possible to recover 80%80 \% of the initial flux while using the anionic surfactant solution. For both cases high removal rates were achieved and good cleanability was demonstrated, supporting the expectations that these membranes are a promising alternative. 最近，Virga 等人开发了一种用于处理产出水的稳定高通量纳滤膜。通过用戊二醛交联其层层自组装涂覆的聚(烯丙胺盐酸盐)（PAH）/聚苯乙烯磺酸钠（PSS）多层膜，实现了耐表面活性剂的稳定膜[142]。该膜对两种合成产出水表现出优异的去油效果（ > 98%>98 \% ），对含阳离子表面活性剂和含阴离子表面活性剂的水分别表现出 96.5%96.5 \% 和 83%83 \% 的总有机碳（TOC）截留率。更重要的是，阳离子溶液（ 100%100 \% ）清洗后膜通量完全恢复，而使用阴离子表面活性剂溶液时，初始通量的 80%80 \% 得以恢复。两种情况下均实现了高去除率并展示了良好的可清洗性，支持了这些膜作为有前景替代方案的预期。
Other 其他
Wastewater of the soy sauce production process has a high chemical oxygen demand [143]. Before discharging it into the environment it is desirable to remove the color originating from caramel pigments and melanin or melanoidin and to lower the COD. Jang et al. showed the successful treatment of this waste stream by a combination of NE-70 NF membranes and a H_(2)O_(2)//O_(3)\mathrm{H}_{2} \mathrm{O}_{2} / \mathrm{O}_{3} process, resulting in 98.1%98.1 \% color removal and a reduction of 98.2%98.2 \% of COD. 酱油生产过程的废水具有较高的化学需氧量（COD）[143]。在排放到环境之前，最好去除由焦糖色素和黑色素或黑色素类物质引起的颜色，并降低 COD。Jang 等人展示了通过 NE-70 纳滤膜和 H_(2)O_(2)//O_(3)\mathrm{H}_{2} \mathrm{O}_{2} / \mathrm{O}_{3} 工艺相结合成功处理该废水流，达到了 98.1%98.1 \% 的色度去除和 98.2%98.2 \% 的 COD 降低。
3.4. Ultrapure Water Production 3.4. 超纯水生产
In several industrial applications the highest purity of water is needed in the production process. The total global market for ultrapure water was estimated to be USD 5.33 billion in 2016 and is projected to reach USD 10.31 billion by 2025 [144]. In, e.g., semiconductor, pharmaceutical, photovoltaic and microelectronic industries contaminations to the lowest level are still detrimental in the production process. Therefore, several treatment schemes have been developed consisting of multiple different unit operations [145], and membranes often play a crucial role [146] (See Figure 6). A common process for applications in the semi-conductor industry uses both RO and tight UF membranes. The RO removes most of the contaminants, while a tight UF membrane (with an MWCO of around 10 kDa ) serves as a final polishing step to remove the last pathogens and micro- and nanoparticles that end up in the permeate of the RO. Part of these contaminants in the RO permeate can even be a result of leaching out of the membrane modules. In order to assure a clean permeate after UF polishing, HF UF membranes are the preferred option, as they allow the best assurance of a clean permeate side. Typically, these modules are operated in an outside-in configuration. 在多个工业应用中，生产过程中需要最高纯度的水。2016 年，全球超纯水市场总值估计为 53.3 亿美元，预计到 2025 年将达到 103.1 亿美元[144]。例如，在半导体、制药、光伏和微电子行业中，生产过程中对污染物的最低限度控制仍然至关重要。因此，已经开发出多种处理方案，包含多个不同的单元操作[145]，而膜技术通常发挥着关键作用[146]（见图 6）。半导体行业的常见工艺同时使用反渗透（RO）和紧密超滤（UF）膜。反渗透去除大部分污染物，而紧密超滤膜（分子量截留值约为 10 kDa）作为最终的抛光步骤，用于去除反渗透渗透液中残留的病原体以及微米和纳米颗粒。这些反渗透渗透液中的部分污染物甚至可能是膜组件浸出造成的。为了确保超滤抛光后的渗透液清洁，空心纤维超滤膜是首选，因为它们能最好地保证渗透液侧的清洁。 通常，这些模块以外向内的配置方式运行。
Currently, even higher water qualities are being requested [147], which means that, among others, NF membranes can be introduced to substitute the final UF polishing step. Here especially the recent development on HF NF membranes is relevant, to enable clean permeate control just as in the UF membranes. Additionally, NF membranes can be applied when reclaimed water is used as the primary source in the ultrapure water process. The higher organic load of reclaimed water puts a strain on the current treatment schemes [147], but this can be relieved by NF processes. Currently, little attention is given to the application of NF in the ultrapure water processes, but this is expected to change in the future. 目前，对更高水质的需求不断增加[147]，这意味着可以引入纳滤（NF）膜来替代最终的超滤（UF）抛光步骤。其中，最近在中空纤维（HF）纳滤膜方面的发展尤为重要，以实现与超滤膜相同的清洁渗透液控制。此外，当回用水作为超纯水工艺的主要水源时，也可以应用纳滤膜。回用水较高的有机负荷对现有处理方案造成压力[147]，但纳滤工艺可以缓解这一问题。目前，纳滤在超纯水工艺中的应用尚未受到足够关注，但预计未来这一状况将有所改变。
3.5. Food and Beverage Industry 3.5. 食品和饮料行业
In the previous sections, we discussed the (potential) applications of HF NF membranes with the focus on clean water recovery. In the following sections, we shift the focus towards the use of HF NF membranes in different industries for the recovery and reuse of valuable components or solutes. 在前面的章节中，我们讨论了中空纤维纳滤膜在清洁水回收方面的（潜在）应用。接下来的章节中，我们将重点转向中空纤维纳滤膜在不同行业中用于回收和再利用有价值成分或溶质的应用。
Figure 6. Schematic flow of a complex UPW production system for microelectronics manufacturing. A typical hybrid membrane treatment plant is presented which involves a combination of RO-MBDI-UF to provide microelectronics grade UPW. Reprinted from [145], with permission from Elsevier. 图 6. 微电子制造用复杂超纯水（UPW）生产系统的示意流程图。展示了一个典型的混合膜处理厂，结合了反渗透（RO）、膜蒸馏（MBDI）和超滤（UF）技术，以提供微电子级超纯水。转载自[145]，经 Elsevier 许可。
Specifically, the food and beverage industry is an emerging field for the use of NF membranes, and the first developments are starting in the dairy industry [148]. NF membranes can be used as a standalone system, but they are also often applied integrated with other membrane processes [149]. In the following paragraphs various food and beverage industries are covered and a variety of applications of NF membranes are discussed. 具体来说，食品和饮料行业是纳滤（NF）膜应用的新兴领域，乳制品行业的首批开发工作已经开始[148]。纳滤膜可以作为独立系统使用，但它们也常常与其他膜工艺集成应用[149]。在接下来的段落中，将介绍多个食品和饮料行业，并讨论纳滤膜的多种应用。
3.5.1. Dairy Industry 3.5.1. 乳制品行业
The dairy industry is one of the largest food processing industries in the world [150]. Many by-products are formed that can lead to problems of their use or management. Whey is a prime example by-product from the production of cheese [151]. Large volumes are produced, and due to the high organic content whey disposal is difficult. Additionally, it also still contains valuable resources that can potentially be recovered. For example, it can serve as a source of lactose and proteins that in turn can be used not only in the food and dairy industries, but also the pharmaceutical industry [151]. Recovering these compounds also lowers the COD and biological oxygen demand levels in the wastewater, which eases wastewater treatment prior to discharge. Das et al. showed that proteins and lactose could be recovered by using a combination of UF and NF [151]. UF was used to concentrate the proteins in the retentate. The permeate was then led to the NF membrane to concentrate the lactose. 乳制品行业是世界上最大的食品加工行业之一[150]。在生产过程中会产生许多副产品，这些副产品的使用或管理可能会带来问题。乳清是奶酪生产中的一个典型副产品[151]。乳清产量大，由于其有机物含量高，乳清的处理较为困难。此外，乳清中仍含有可回收的宝贵资源。例如，它可以作为乳糖和蛋白质的来源，这些成分不仅可用于食品和乳制品行业，还可用于制药行业[151]。回收这些化合物还能降低废水中的化学需氧量（COD）和生物需氧量，有助于废水排放前的处理。Das 等人展示了通过结合超滤（UF）和纳滤（NF）技术可以回收蛋白质和乳糖[151]。超滤用于浓缩留液中的蛋白质，透过液随后进入纳滤膜以浓缩乳糖。
Next to proteins and lactose as valuable products, whey often also contains minerals at concentrations that are undesirable for applications [152]. Therefore, the concentration of whey is often combined with demineralization. NF membranes take care of partial demineralization and can be combined with an anion-exchange pre-treatment to reach a higher demineralization of whey. 除了蛋白质和乳糖作为有价值的产品外，乳清中通常还含有矿物质，其浓度在某些应用中是不理想的[152]。因此，乳清的浓缩通常与脱矿化相结合。纳滤膜（NF 膜）可以实现部分脱矿化，并且可以与阴离子交换预处理结合，以达到更高程度的乳清脱矿化。
Lactose can be a main ingredient in the production of other valuable products. One of them is galacto-oligosaccharides (GOS), resulting from the trans-galactosylation reaction of lactose. The GOS are of interest because of their prebiotic nature and are increasingly used in infant foods and functional foods. The final product of the synthesis contains, besides the GOS, mono- and disaccharides that reduce the prebiotic function. Therefore, it is key to remove these carbohydrates. In a study by Michelon et al. it was indicated that NP030 NF membranes can be used as a preliminary technique for GOS purification with a recovery of 61%61 \% [153]. 乳糖可以作为生产其他有价值产品的主要原料。其中之一是半乳寡糖（GOS），它是乳糖经转半乳糖基化反应生成的。GOS 因其益生元特性而受到关注，且越来越多地应用于婴儿食品和功能性食品。合成的最终产品除了含有 GOS 外，还含有单糖和双糖，这些糖类会降低益生元功能。因此，去除这些糖类是关键。Michelon 等人的一项研究表明，NP030 纳滤膜可以作为 GOS 纯化的初步技术，回收率为 61%61 \% [153]。
3.5.2. Sugar Industry 3.5.2. 糖业
The production of sugar is one of the most energy-intensive processes in the food industry [154]. In order to reduce the energy consumption membrane processes have been intensively investigated as an alternative to traditional technologies. However, these applications can be challenging, especially due to the viscosities and high osmotic pressures of the solutions [155]. 糖的生产是食品工业中能耗最高的工艺之一[154]。为了降低能耗，膜工艺作为传统技术的替代方案被广泛研究。然而，这些应用具有一定挑战性，特别是由于溶液的粘度和高渗透压[155]。
The removal of colorants from syrups is a key step in the sugar production process, because the final product, sugar, must meet strict standards and the color content should be as low as possible [154]. Usually the removal of non-sucrose compounds is done via liming and carbonation, followed by filtration-operations that are resource intensive and accommodate environmental pollution problems [154]. Gyura et al. showed that NF membranes can be of interest as an alternative method for the use in the separation of colored matter from green syrup [156]. Using a flat-sheet polyamide TFC membrane with an MWCO of 500 Da they proved to be able to decrease the color by 76%76 \%. 从糖浆中去除色素是糖生产过程中的关键步骤，因为最终产品糖必须符合严格标准，且色度应尽可能低[154]。通常，非蔗糖化合物的去除通过加石灰和碳化处理完成，随后进行过滤操作，这些过程资源消耗大且带来环境污染问题[154]。Gyura 等人表明，纳滤（NF）膜作为一种替代方法，在从绿糖浆中分离色素方面具有潜力[156]。他们使用了分子量截留值（MWCO）为 500 Da 的平板聚酰胺复合薄膜（TFC），证明能够降低色度 76%76 \% 。
Another application is the use of NF membranes for the recycling of (anion) exchange resin regeneration effluents. The resins are mainly used to remove high-molecular weight colorants and must be periodically regenerated. This produces a waste brine with high amounts of sodium chloride, organic matter and COD. Salehi et al. investigated the use of polyamide tubular NF membranes for the recovery of usable brine [157]. They showed that brine could be recovered, leading to a reduction in water consumption of 90%90 \% and reduction in salt consumption of 77%77 \% for the regeneration process. Next to this, the colorants were removed for more than 99%99 \%. 另一种应用是使用纳滤膜回收（阴离子）交换树脂再生废液。该树脂主要用于去除高分子量的着色剂，必须定期再生。这会产生含有大量氯化钠、有机物和化学需氧量（COD）的废盐水。Salehi 等人研究了聚酰胺管式纳滤膜用于可用盐水的回收[157]。他们表明，盐水可以被回收，从而使再生过程中的用水量减少了 90%90 \% ，盐的消耗量减少了 77%77 \% 。此外，着色剂的去除率超过了 99%99 \% 。
Oligosaccharides are widely used as ingredients or additives in the food industry, owing to their nutritional and health function. In order to be used, the oligosaccharides should be separated from the monosaccharides present in reaction mixtures. NF membranes can be used for the purpose of purification and separation of oligosaccharides. It is important to note that for these types of processes, microbial growth can be a serious issue. Pruksasri et al. evaluated the use of NP030 NF membranes for the purification of GOS [158]. Microbial growth could be reduced compared to normal operating conditions by purifying at either 5^(@)C5^{\circ} \mathrm{C} or 60^(@)C60^{\circ} \mathrm{C}, where the lower temperature was more advantageous in terms of product yield. A product purity of 85%85 \% could be achieved and oligosaccharide recovery of 82%82 \%. With PEM NF membranes, oligosaccharides could also be separated, as was shown by Shi et al. [159]. The composite NF membranes resulted in 100%100 \% oligosaccharide retention and 63%63 \% glucose rejection, with a selectivity of maltose/glucose of 46 . 低聚糖因其营养和健康功能，被广泛用作食品工业中的成分或添加剂。为了使用，低聚糖应从反应混合物中的单糖中分离出来。纳滤（NF）膜可用于低聚糖的纯化和分离。需要注意的是，对于这类工艺，微生物生长可能是一个严重的问题。Pruksasri 等人评估了使用 NP030 纳滤膜纯化低聚半乳糖（GOS）[158]。通过在 5^(@)C5^{\circ} \mathrm{C} 或 60^(@)C60^{\circ} \mathrm{C} 条件下进行纯化，可以减少微生物生长，其中较低的温度在产品产率方面更具优势。产品纯度可达到 85%85 \% ，低聚糖回收率为 82%82 \% 。Shi 等人也展示了利用聚醚酰胺（PEM）纳滤膜分离低聚糖的可能性[159]。复合纳滤膜实现了 100%100 \% 的低聚糖截留率和 63%63 \% 的葡萄糖排斥率，麦芽糖/葡萄糖的选择性为 46。
In 2016, Malmali et al. investigated the fractionation of sugars by PEM NF membranes [160]. The membranes were prepared by LbL coating of PDADMAC and PSS on a polysulfone support. The resulting membranes had a sucrose to glucose selectivity higher than 11, showing the potential of fractionating di- and monosaccharides with PEM NF membranes. 2016 年，Malmali 等人研究了通过 PEM NF 膜对糖类的分离[160]。这些膜是通过在聚砜支撑体上层层涂覆 PDADMAC 和 PSS 制备的。所得膜的蔗糖与葡萄糖选择性高于 11，显示了利用 PEM NF 膜分离二糖和单糖的潜力。
3.5.3. Beverage Industry 3.5.3. 饮料工业
NF membranes are also gaining increased attention in the beverage industry, especially for the alcoholic beverage industry, in order to control the alcohol content in beverages, but also for juice concentration [149,154]. 纳滤膜在饮料工业中也越来越受到关注，特别是在酒精饮料行业，用于控制饮料中的酒精含量，同时也用于果汁浓缩[149,154]。
Increased interest for NF membranes is observed for the production of low alcohol content wines [149,161]. An important benefit, compared to thermal heat-based processes, to remove alcohol, is that volatile aroma compounds are not lost. A study from Catarino et al. comprises RO and NF membranes (including HF membranes) that were used for the removal of ethanol from a 12vol.%12 \mathrm{vol} . \% red wine [162]. YMHLSP1905 NF membranes from Osmonics and NF99 and NF99HF NF membranes from Alfa Laval showed very promising results, with a good permeability to ethanol, while retaining the aroma compounds. Other tests were performed by first using pervaporation to recover aroma compounds that were later added to the dealcoholized wine, resulting in a better flavor sensation. It must be noted, however, that the final product still contains 8.5vol.%8.5 \mathrm{vol} . \% of alcohol [162]. El Rayess 对于低酒精含量葡萄酒的生产，纳滤膜的兴趣日益增加[149,161]。与基于热处理的工艺相比，去除酒精的一个重要优势是不会损失挥发性香气化合物。Catarino 等人的一项研究涉及反渗透（RO）和纳滤（NF）膜（包括中空纤维膜），用于去除 12vol.%12 \mathrm{vol} . \% 红葡萄酒中的乙醇[162]。Osmonics 的 YMHLSP1905 纳滤膜以及 Alfa Laval 的 NF99 和 NF99HF 纳滤膜表现出非常有前景的结果，对乙醇具有良好的渗透性，同时保留了香气化合物。其他测试首先使用渗透蒸发回收香气化合物，随后将其添加到脱醇葡萄酒中，从而获得更好的风味感受。然而必须指出，最终产品仍含有 8.5vol.%8.5 \mathrm{vol} . \% 的酒精[162]。El Rayess
and Mietton-Peuchot identified the opportunities (e.g., see Figure 7) and advantages of NF and RO in oenology, but highlighted the fouling issues typically observed in these applications [163]. 和 Mietton-Peuchot 识别了纳滤和反渗透在酿酒学中的机遇（例如，见图 7）和优势，但强调了这些应用中通常观察到的污染问题[163]。
Figure 7. Process of wine de-alcoholization by coupling reverse osmosis or nanofiltration and distillation. Reprinted from [163], with permission from Taylor and Francis Ltd., http://www. tandfonline.com (accessed on 14 October 2021). 图 7. 通过反渗透或纳滤与蒸馏耦合的葡萄酒脱醇工艺。转载自[163]，经 Taylor and Francis Ltd.许可，http://www.tandfonline.com（访问日期：2021 年 10 月 14 日）。
An emerging issue in wine-making is the early ripening of grapes, a consequence of global warming, leading to a higher sugar content that results in an alcoholic content that is higher than desired. For this reason, research has focused on controlling the sugar content in grape must. The concept was proven by Salgado et al., who showed the successful reduction in sugar content of red must, using flat-sheet NF membranes [164]. 葡萄酒酿造中一个新出现的问题是葡萄的早熟，这是全球变暖的结果，导致糖分含量升高，从而使酒精含量高于预期。因此，研究重点放在控制葡萄汁中的糖分含量。Salgado 等人通过使用平板式纳滤膜成功降低红葡萄汁中的糖分含量，验证了这一概念[164]。
Juice concentration is a process that helps to reduce the bulk volume and shipping costs of juices. Traditionally it is done by an evaporation process; however, this has some disadvantages such as loss of fragrance, taste and color. Membrane technology provides an alternative technique that allows maintaining flavors and aroma. Warczok et al. indeed showed that apple and pear juices can be concentrated by the use of NF membranes [165]. In other studies, the concept was also successfully applied for the concentration of blackcurrant and strawberry juices [166,167]. Furthermore, NF can also be applied to separate and purify phenolic compounds from pomegranate juice [168]. 果汁浓缩是一种有助于减少果汁体积和运输成本的工艺。传统上，这一过程通过蒸发完成；然而，这种方法存在一些缺点，如香气、口感和颜色的损失。膜技术提供了一种替代方法，能够保持果汁的风味和香气。Warczok 等人确实展示了利用纳滤（NF）膜可以浓缩苹果和梨汁[165]。在其他研究中，这一概念也成功应用于黑加仑和草莓汁的浓缩[166,167]。此外，纳滤还可用于从石榴汁中分离和纯化酚类化合物[168]。
Coffee is another type of beverage for which NF membranes potentially can be of interest. Pan et al. showed that NF-2 SW NF membranes are capable of concentrating coffee extract. In another study, Ong et al. demonstrated the concept of decaffeination with the use of HF NF membranes (MWCO 470) [169]. Early results showed reduction of 25%25 \% in caffeine, with a volume reduction factor of 1.2. 咖啡是另一种可能适用纳滤膜的饮料类型。Pan 等人表明，NF-2 SW 纳滤膜能够浓缩咖啡提取物。在另一项研究中，Ong 等人展示了利用中空纤维纳滤膜（分子量截留值 470）进行咖啡因去除的概念[169]。早期结果显示，在体积缩减因子为 1.2 的情况下，咖啡因含量有所降低。
3.5.4. Other 3.5.4. 其他
There are many more food industries besides the dairy, sugar and beverage industry where NF membranes can (potentially) be successfully applied. One of them is the production of tofu, with whey as a by-product. The tofu whey can be used as fertilizer but is also often discarded. However, this waste contains isoflavones that are a source of antioxidants. A combination of freeze drying and concentration via NF (PVDF SW) was used by Benedetti et al. to show the potential of using this process to recover valuable products from tofu whey [170]. 除了乳制品、糖和饮料行业之外，还有许多其他食品行业中纳滤膜（NF 膜）可以（潜在地）成功应用。其中之一是豆腐的生产，豆腐乳清是其副产品。豆腐乳清可以用作肥料，但也常被丢弃。然而，这种废弃物中含有异黄酮，是抗氧化剂的来源。Benedetti 等人采用冷冻干燥和通过纳滤（PVDF SW）浓缩的组合方法，展示了利用该工艺从豆腐乳清中回收有价值产品的潜力[170]。
Another application is the production of starch, where the NF membranes can serve as an outcome for the recovery of high-value proteins from potato starch wastewater. The potato proteins have a high nutritional value and antioxidant and functional characteristics that can be transferred into valuable food ingredients. Li et al. investigated the potential of 另一个应用是淀粉的生产，纳滤膜可以用于从马铃薯淀粉废水中回收高价值蛋白质。马铃薯蛋白具有高营养价值以及抗氧化和功能特性，这些特性可以转化为有价值的食品成分。Li 等人研究了
HF UF and NF membranes for this purpose [171]. The polyamide TFC HF NF membranes could reject low molecular weight potato proteins for 92.1%92.1 \% combined with a COD rejection of 86.8%86.8 \%. Fouling resulted in flux decline over time, and could be recovered via cleaning to 84.7%84.7 \%. 用于此目的的 HF 超滤和纳滤膜[171]。聚酰胺 TFC HF 纳滤膜能够拒绝低分子量的马铃薯蛋白， 92.1%92.1 \% ，同时化学需氧量（COD）去除率为 86.8%86.8 \% 。膜污染导致通量随时间下降，通过清洗可恢复至 84.7%84.7 \% 。
The above examples highlight the wide range of different food and beverage related applications possible for the use of (HF) NF membranes. While not all examples given here are HF configurations, it can be expected that HF NF will be used for these applications in the future as well, especially because of its benefits compared to SW NF when dealing with higher amounts of suspended solids. With the increasing need for smart use of materials and challenging new questions HF NF membranes can play a role in this. 上述示例突显了（HF）纳滤膜在各种食品和饮料相关应用中的广泛可能性。虽然这里给出的所有示例并非全部是 HF 结构，但可以预期未来 HF 纳滤膜也将用于这些应用，尤其是在处理较高悬浮固体含量时，相较于 SW 纳滤膜具有优势。随着对材料智能利用的需求增加以及新挑战的出现，HF 纳滤膜可以在其中发挥作用。
3.6. Chemical and Petrochemical Industry 3.6. 化工与石化工业
In the chemical and petrochemical industry, separation processes play a paramount role, accounting for 40-70%40-70 \% of the capital and operating costs [172]. For a variety of applications HF NF membranes can be or are already applied. The applications concerning the production of clean wastewater are already discussed in Section 3.3, and here we focus on the recovery and reuse of valuable products from process (waste) streams. 在化工与石化工业中，分离工艺起着至关重要的作用，占据了资本和运营成本的 40-70%40-70 \% [172]。HF 纳滤膜可用于多种应用，且已有应用实例。关于清洁废水生产的应用已在第 3.3 节中讨论，这里我们重点关注从工艺（废）流中回收和再利用有价值产品。
3.6.1. Caustic and Acid Recovery 3.6.1. 碱液和酸液回收
The recovery of acidic and alkaline solutions is an important application in the chemical and petrochemical industry, but also in many other industries such as the food industry. In most cases, they are used for cleaning purposes of apparatuses and production equipment via a so-called cleaning in place (CIP) process that consists of multiple steps and involves both alkaline, often NaOH , and acidic, often HNO_(3)\mathrm{HNO}_{3}, solutions. These solutions are used in large quantities and substantially contribute to the total cost of the process. For example, in the dairy industry, per 1 L of treated milk, 0.2-2.0L0.2-2.0 \mathrm{~L} of acidic and alkaline effluents are produced [173]. By recovering and reusing these products, the process could be operated in a more ecologically and economically efficient manner [174]. 酸性和碱性溶液的回收是化工和石化行业的重要应用，同时也广泛应用于食品工业等许多其他行业。在大多数情况下，这些溶液用于通过所谓的原位清洗（CIP）工艺对设备和生产装置进行清洗，该工艺包括多个步骤，涉及碱性溶液（通常为 NaOH）和酸性溶液（通常为 HNO_(3)\mathrm{HNO}_{3} ）。这些溶液的使用量很大，并且在工艺的总成本中占有重要比例。例如，在乳制品行业，每处理 1 升牛奶，会产生 0.2-2.0L0.2-2.0 \mathrm{~L} 的酸性和碱性废液[173]。通过回收和再利用这些产品，工艺可以以更环保和经济高效的方式运行[174]。
A simple method to recover used cleaning solutions is to store the solution in a tank for a certain time and let the suspended material settle out [175]. The top part of the solution will be partially clarified and can be mixed with fresh cleaning solution to reach the desired properties and used again. However, dissolved and small compounds will not be removed and therefore the final quality of the solution will be relatively low. The quality of the recovered solution can be significantly improved by using an NF process. 一种简单的回收使用过的清洗液的方法是将溶液存放在罐中一段时间，使悬浮物沉降[175]。溶液的上层部分将部分澄清，可以与新鲜清洗液混合以达到所需的性能并再次使用。然而，溶解的和小分子化合物不会被去除，因此最终溶液的质量相对较低。通过使用纳滤（NF）工艺，可以显著提高回收溶液的质量。
Cleaning Solutions 清洗液
Novalic et al. studied the recycling of sodium hydroxide by a tubular NF membrane (MPT-34) in the dairy industry. They showed that it is possible to recycle NaOH and that the quality of the recycled caustic soda is highly dependent on the starting conditions [174]. Santos et al. showed that also in petroleum refineries, the recovery of spent caustic by NF membranes can contribute to the overall performance of an industrial installation [176]. Besides being suitable for the recovery of alkaline solutions, Novalic et al. also showed that recycling acidic cleaning solutions is possible with NF membranes [177]. Here, it is important that the acidic cleaning solution should be applied after a caustic cleaning step (as is done in most cases), because otherwise the high concentrations of salts will lead to high osmotic pressure and the recovery will not be economically feasible. Novalic 等人研究了乳制品行业中通过管式纳滤膜（MPT-34）回收氢氧化钠的过程。他们表明，回收氢氧化钠是可行的，且回收的苛性钠质量高度依赖于初始条件[174]。Santos 等人则表明，在石油炼厂中，通过纳滤膜回收废苛性液也能提升工业装置的整体性能[176]。除了适用于碱性溶液的回收外，Novalic 等人还表明，利用纳滤膜回收酸性清洗液也是可能的[177]。这里需要注意的是，酸性清洗液应在碱性清洗步骤之后使用（大多数情况下都是如此），否则高浓度盐分会导致高渗透压，使回收在经济上不可行。
With time, single-phase detergents are gradually replacing the conventional acids and alkalis [178]. By doing so, the alkaline, acid and disinfection stages are integrated and the lead time and use of chemicals is reduced. However, these single-phase detergents are quite expensive and therefore also here the recovery is of interest. Fernández et al. showed the successful recovery of contaminated single-phase detergent from a CIP system in a yoghurt industrial plant using Koch MPS-34 membranes [178]. Furthermore, Kowalska et al. studied the recovery of acid single-phase detergents by different UF and NF membranes (a.o. tubular polyamide AFC30 and AFC80 NF membranes) [173]. It was shown that the 随着时间的推移，单相洗涤剂逐渐取代了传统的酸和碱[178]。通过这样做，碱性、酸性和消毒阶段得以整合，缩短了前置时间并减少了化学品的使用。然而，这些单相洗涤剂价格相当昂贵，因此回收利用也成为关注的重点。Fernández 等人利用 Koch MPS-34 膜成功回收了酸奶工业厂 CIP 系统中被污染的单相洗涤剂[178]。此外，Kowalska 等人研究了通过不同的超滤和纳滤膜（包括管状聚酰胺 AFC30 和 AFC80 纳滤膜）回收酸性单相洗涤剂[173]。研究表明，
spent acidic single-phase detergent could be recovered. For the AFC30 NF membranes, high protein retention ( 99.8%99.8 \% ) and lactose retention ( 98.7%98.7 \% ) was observed. At the same time the surface tension of the permeate increased, which might have a weakened effect on the detergency properties. 废弃的酸性单相洗涤剂可以被回收。对于 AFC30 纳滤膜，观察到了高蛋白质截留（ 99.8%99.8 \% ）和乳糖截留（ 98.7%98.7 \% ）。同时，渗透液的表面张力增加，这可能会削弱其洗涤性能。
Other Applications 其他应用
Besides cleaning applications, sodium hydroxide is also used for other purposes, such as for the extraction of hemicellulose from wheat bran and barley husks. The hemicellulose is in turn used for the production of food packaging. Arkell et al. showed that the sodium hydroxide could be recovered using an NF membrane and that the pay-back time is expected to be less than two months [179]. 除了清洁应用外，氢氧化钠还用于其他用途，例如从小麦麸皮和大麦壳中提取半纤维素。半纤维素反过来用于生产食品包装。Arkell 等人表明，可以使用纳滤膜回收氢氧化钠，且预计回收期不到两个月[179]。
The recovery of acids is not solely limited to cleaning solutions. There are many other examples of acids that are valuable enough to be recovered. Recent research showed that PEM HF NF membranes (dNF40 NX Filtration) can also be used for the separation of volatile fatty acids (VFA) from anaerobic effluents [180]. The recovered VFAs can be used not only in the chemical industry for the synthesis of, among other things, ketones, esters and alcohols, but also in the textile industry. By comparing four different separation methods (NF, RO, forward osmosis and supported ionic liquid membranes), the study showed that NF provides the highest permeance and allows for adequate selectivity between different VFAs. In another study, the recovery of succinic acid, as a platform chemical, from bioethanol waste was investigated and the potential for flat-sheet NF membranes was demonstrated [181]. In a similar way, carboxylic acids and phenols could be concentrated and recovered from hydrothermal liquefaction wastewater [182]. 酸的回收不仅限于清洗溶液。还有许多其他酸类因其价值较高而值得回收。最新研究表明，PEM HF NF 膜（dNF40 NX Filtration）也可用于从厌氧废水中分离挥发性脂肪酸（VFA）[180]。回收的挥发性脂肪酸不仅可用于化工行业合成酮类、酯类和醇类等，还可应用于纺织行业。通过比较四种不同的分离方法（纳滤、反渗透、正渗透和支持离子液体膜），研究显示纳滤具有最高的通量，并能在不同挥发性脂肪酸之间实现适当的选择性。在另一项研究中，探讨了从生物乙醇废料中回收琥珀酸作为平台化学品的可能性，并展示了平板纳滤膜的潜力[181]。以类似方式，羧酸和酚类也可从水热液化废水中浓缩和回收[182]。
The above examples show that NF membranes can be successful for the recovery and reuse of both alkaline and acidic solutions in several applications. Still, there are only a limited number of acid and alkaline stable NF membranes commercially available that can be used for these applications. Conventional polymeric NF membranes (often polyamides) are not stable, especially under alkaline conditions [183]. Therefore, research is being performed to expand the range of membrane materials suitable for the more demanding conditions. In a recent paper by Elshof et al. it was shown that PEM HF NF membranes can have long-term stability in both extreme acidic and alkaline solutions [184]. For this reason, it is expected that these types of membranes will also be interesting candidates for the above-mentioned applications. 上述例子表明，纳滤膜在多种应用中能够成功回收和再利用碱性和酸性溶液。然而，目前市面上可用于这些应用的耐酸耐碱纳滤膜数量有限。传统的聚合物纳滤膜（通常为聚酰胺膜）在碱性条件下尤其不稳定[183]。因此，研究正在进行，以拓展适用于更苛刻条件的膜材料范围。Elshof 等人在最近的一篇论文中表明，聚电解质多层膜（PEM）中空纤维纳滤膜在极端酸性和碱性溶液中均具有长期稳定性[184]。因此，预计这类膜也将成为上述应用的有趣候选材料。
3.6.2. Metal Recovery 3.6.2. 金属回收
The composition of green energy supply technologies like wind, solar or hydrogen systems is significantly more material intensive than current traditional fossil-fuel-based energy supply systems [185]. As these technologies will very likely be necessary to reach the goals of the Paris Climate Agreement and to ensure a transition to a low carbon economy, the world’s demand of metal resources will rise during the century and pressure on current supplies might increase [186]. Among others, it is expected that the worldwide demand of zinc, lead and copper will exceed available resources by the middle of the 21st century [187]. Therefore, recovering metals from water or wastewater sources will become more and more important to satisfy rising demands. In 2013, Qin et al. investigated the recovery of CuSO_(4),ZnSO_(4),NiCl_(2)\mathrm{CuSO}_{4}, \mathrm{ZnSO}_{4}, \mathrm{NiCl}_{2} and CdCl_(2)\mathrm{CdCl}_{2} from artificial wastewaters by using HF NF membranes [188]. A modified polyacrylonitrile (PAN) UF substrate was coated with bilayers of PEI and PSS, resulting in a positively charged NF membrane. Rejections of 98.0%98.0 \% for CuSO_(4),95.5%\mathrm{CuSO}_{4}, 95.5 \% for ZnSO_(4),95.7%NiCl_(2)\mathrm{ZnSO}_{4}, 95.7 \% \mathrm{NiCl}_{2} and 94.9%94.9 \% for CdCl_(2)\mathrm{CdCl}_{2} were achieved at salt concentrations of 500mg*L^(-1)500 \mathrm{mg} \cdot \mathrm{L}^{-1}, along with permeation fluxes between 19 and 24L*m^(-2)h^(-1)24 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1}. 风能、太阳能或氢能系统等绿色能源供应技术的组成材料强度显著高于当前传统的化石燃料能源供应系统[185]。由于这些技术很可能是实现《巴黎气候协定》目标并确保向低碳经济转型所必需的，全球对金属资源的需求将在本世纪内上升，现有供应压力可能增加[186]。其中，预计到 21 世纪中叶，全球对锌、铅和铜的需求将超过可用资源[187]。因此，从水或废水中回收金属将变得越来越重要，以满足不断增长的需求。2013 年，Qin 等人通过使用 HF NF 膜研究了从人工废水中回收 CuSO_(4),ZnSO_(4),NiCl_(2)\mathrm{CuSO}_{4}, \mathrm{ZnSO}_{4}, \mathrm{NiCl}_{2} 和 CdCl_(2)\mathrm{CdCl}_{2} 的过程[188]。一种改性聚丙烯腈（PAN）超滤基材被涂覆了 PEI 和 PSS 的双层，形成了带正电荷的纳滤膜。在盐浓度为 500mg*L^(-1)500 \mathrm{mg} \cdot \mathrm{L}^{-1} 的条件下，实现了对 98.0%98.0 \% 的 CuSO_(4),95.5%\mathrm{CuSO}_{4}, 95.5 \% 、对 ZnSO_(4),95.7%NiCl_(2)\mathrm{ZnSO}_{4}, 95.7 \% \mathrm{NiCl}_{2} 的 94.9%94.9 \% 和对 CdCl_(2)\mathrm{CdCl}_{2} 的 94.9%94.9 \% 的截留率，同时渗透通量介于 19 和 24L*m^(-2)h^(-1)24 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} 之间。
One metal of high interest is lithium, which is commonly used in Li-ion batteries. Due to their versatile applications in, e.g., plug-in hybrid vehicles, in consumer electronics and in space, military and medical applications, the worldwide lithium consumption for 2025 is expected to be twice as high as it was in 2017 [189]. To keep up with the world’s increasing demand, recovery of lithium from water sources or lithium-containing wastewater streams 一种备受关注的金属是锂，锂通常用于锂离子电池。由于其在插电式混合动力汽车、消费电子产品以及航天、军事和医疗应用中的多样化用途，预计到 2025 年全球锂的消费量将是 2017 年的两倍[189]。为了满足全球日益增长的需求，从水源或含锂废水中回收锂成为必要。
might become more and more attractive in the future. Major challenges for lithium recovery are the low concentration and the separation from many other alkali metals [190]. Here, HF NF membranes can be applied for separation as they can offer an effective barrier for divalent ions. The successful separation of an aqueous MgCl_(2)//LiCl\mathrm{MgCl}_{2} / \mathrm{LiCl} mixture was shown by applying a positively charged polyamide composite HF NF membrane [191]. The membrane was produced by interfacial polymerization of 1,4-Bis(3-aminopropyl) piperazine and TMC on a PAN HF support membrane. Lithium concentration in the permeate was reported to be more than three times the feed and the MgCl_(2)//LiCl\mathrm{MgCl}_{2} / \mathrm{LiCl} ratio changed from 20 in the feed to 7.7 in the permeate. Similarly, Zhang et al. recovered lithium from an artificially created salt lake brine by positively charged three-channel HF NF membranes [192]. For this synthetic solution ( MgCl 21866 ppm and LiCl 134 ppm ), a flux 34L*m^(-2)h^(-1)34 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} was reported. The rejections of MgCl_(2)\mathrm{MgCl}_{2} and LiCl were 95%95 \% and 18%18 \%, respectively, while the MgCl_(2)//LiCl\mathrm{MgCl}_{2} / \mathrm{LiCl} ratio could be reduced from 21.4 to 1.3. However, salt rejection and flux both decreased when a solution of CaCl_(2),MgCl_(2),NaCl\mathrm{CaCl}_{2}, \mathrm{MgCl}_{2}, \mathrm{NaCl} and LiCl was used. MgCl_(2)\mathrm{MgCl}_{2} rejection was lowered to 78%78 \% and LiCl rejection was reported at 15%15 \%, leading to a MgCl_(2)//LiCl\mathrm{MgCl}_{2} / \mathrm{LiCl} ratio of 3.0 in the permeate. However, the generally low concentrations of lithium make these processes not economically attractive, yet. The future demand and availability of lithium will determine its feasibility. 未来可能会变得越来越有吸引力。锂回收的主要挑战是浓度低以及与许多其他碱金属的分离[190]。在这里，HF NF 膜可以用于分离，因为它们能够有效阻挡二价离子。通过应用带正电荷的聚酰胺复合 HF NF 膜，成功实现了水相 MgCl_(2)//LiCl\mathrm{MgCl}_{2} / \mathrm{LiCl} 混合物的分离[191]。该膜是通过在 PAN HF 支撑膜上进行 1,4-双(3-氨丙基)哌嗪和 TMC 的界面聚合制备的。渗透液中的锂浓度被报道为进料的三倍以上，且 MgCl_(2)//LiCl\mathrm{MgCl}_{2} / \mathrm{LiCl} 比率从进料中的 20 降至渗透液中的 7.7。同样，张等人通过带正电荷的三通道 HF NF 膜从人工合成的盐湖卤水中回收锂[192]。对于该合成溶液（MgCl 21866 ppm 和 LiCl 134 ppm），报道了通量 34L*m^(-2)h^(-1)34 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} 。 MgCl_(2)\mathrm{MgCl}_{2} 和 LiCl 的截留率分别为 95%95 \% 和 18%18 \% ，而 MgCl_(2)//LiCl\mathrm{MgCl}_{2} / \mathrm{LiCl} 比率则从 21.4 降低到 1.3。然而，当使用 CaCl_(2),MgCl_(2),NaCl\mathrm{CaCl}_{2}, \mathrm{MgCl}_{2}, \mathrm{NaCl} 和 LiCl 的溶液时，盐的截留率和通量均有所下降。 MgCl_(2)\mathrm{MgCl}_{2} 的截留率降低到 78%78 \% ，LiCl 的截留率报告为 15%15 \% ，导致渗透液中的 MgCl_(2)//LiCl\mathrm{MgCl}_{2} / \mathrm{LiCl} 比率为 3.0。然而，锂的浓度普遍较低，使得这些工艺目前尚不具备经济吸引力。未来锂的需求和供应情况将决定其可行性。
Another metal of growing interest is the relatively scarce and expensive rare earth scandium. Its aluminum alloy forms stronger, heat tolerant and weldable aluminum products, e.g., enabling to apply weldable parts that reduce aircraft weights by 15-20% [193]. Therefore, recovery of scandium from secondary resources is of importance to meet increasing demands. Remmen et al. applied a PEM membrane on a titanium dioxide pigment production waste for scandium recovery [194]. A PES HF UF membrane was coated with bilayers of PDADMAC/PSS and compared against a conventional acid resistant flat-sheet membrane. The LbL membrane showed superior results in regard to scandium retention ( 60%60 \% vs. 50%50 \% ) as well as higher fluxes, with the LbL membrane reaching 27L*m^(-2)h^(-1)27 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1}, whereas the conventional flat-sheet membrane achieved a flux as little as 1L*m^(-2)h^(-1)1 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} under the same process conditions. Additionally, a better selectivity towards major impurities (e.g., iron) was achieved. 另一种日益受到关注的金属是相对稀缺且昂贵的稀土钪。其铝合金能够形成更强、更耐热且可焊接的铝制品，例如，可应用于焊接部件，从而使飞机重量减少 15-20% [193]。因此，从二次资源中回收钪对于满足日益增长的需求具有重要意义。Remmen 等人将质子交换膜（PEM）应用于二氧化钛颜料生产废料中回收钪[194]。一种聚醚砜（PES）中空纤维超滤（HF UF）膜被涂覆了 PDADMAC/PSS 双层膜，并与传统的耐酸平板膜进行了比较。层层自组装（LbL）膜在钪的截留率（ 60%60 \% 对比 50%50 \% ）以及通量方面表现出优越的结果，LbL 膜的通量达到 27L*m^(-2)h^(-1)27 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} ，而传统平板膜在相同工艺条件下的通量仅为 1L*m^(-2)h^(-1)1 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} 。此外，对主要杂质（如铁）的选择性也有所提高。
3.6.3. Phosphorus Recovery 3.6.3. 磷回收
Phosphorus is essential for food supply, mainly because of its use as fertilizer [195]. The element is becoming scarce and therefore phosphorus recovery has become increasingly important. Remmen et al. evaluated the use of PDADMAC/PSS PEM HF NF membranes for phosphorus recovery from sewage sludge ash [196]. For this, they used a 10%10 \% phosphoric acid solution (containing 2g*L^(-1)2 \mathrm{~g} \cdot \mathrm{~L}^{-1} aluminum) as model feed solution. Desired low retentions of phosphoric acid were obtained ( < 10%<10 \% ), such that a purified H_(3)PO_(4)\mathrm{H}_{3} \mathrm{PO}_{4} solution could be achieved in the permeate, with a phosphorus recovery of 75%75 \%. At the same time, impurities such as aluminum were retained > 95%>95 \%. The obtained permeabilities were significantly higher than commercial benchmark membranes (e.g., A-3012 AMS membrane). Later they investigated the stability of LbL based PEM membranes when immersing or filtering 15%H_(3)PO_(4)15 \% \mathrm{H}_{3} \mathrm{PO}_{4} solution [197]. Using PES as support layer, the membranes proved to be very stable after both immersion and filtration. LbL on SPES membranes showed a magnesium retention loss after immersion but were also stable upon filtration. Furthermore, it was observed that magnesium retention decreases with higher ionic strength solutions, but by altering process parameters, magnesium retention values up to 85%85 \% could be reached even at 500mMMg^(2+)500 \mathrm{mM} \mathrm{Mg}^{2+}. Paltrinieri et al. studied the relation between membrane structure properties and phosphorus recovery (see Figure 8) [198]. Three different polycations (PDADMAC, PAH, Modified-PAH) were each combined with PSS as polyanion to form a PEM NF membrane. It was shown that PDADMAC/PSS membranes had the highest permeability and phosphorus recovery, yet lower multivalent metal retention as a result of its loose, less interpenetrated structure. Using PAH modified with guanidium groups, a higher retention was observed and therefore a less-metal contaminated permeate. However, 磷对粮食供应至关重要，主要因为其作为肥料的用途[195]。该元素正变得日益稀缺，因此磷的回收变得越来越重要。Remmen 等人评估了使用 PDADMAC/PSS PEM HF NF 膜从污泥灰中回收磷的效果[196]。为此，他们使用了含有 2g*L^(-1)2 \mathrm{~g} \cdot \mathrm{~L}^{-1} 铝的 10%10 \% 磷酸溶液作为模型进料溶液。获得了所需的低磷酸截留率（ < 10%<10 \% ），从而在渗透液中实现了纯化的 H_(3)PO_(4)\mathrm{H}_{3} \mathrm{PO}_{4} 溶液，磷回收率为 75%75 \% 。同时，铝等杂质被截留 > 95%>95 \% 。所获得的渗透率显著高于商业基准膜（例如 A-3012 AMS 膜）。随后，他们研究了基于 LbL 的 PEM 膜在浸泡或过滤 15%H_(3)PO_(4)15 \% \mathrm{H}_{3} \mathrm{PO}_{4} 溶液时的稳定性[197]。使用 PES 作为支撑层，膜在浸泡和过滤后均表现出非常稳定。LbL 在 SPES 膜上的浸泡后显示出镁截留率下降，但在过滤时也表现出稳定性。 此外，观察到随着溶液离子强度的增加，镁的截留率降低，但通过改变工艺参数，即使在 500mMMg^(2+)500 \mathrm{mM} \mathrm{Mg}^{2+} 时，镁的截留率也可达到 85%85 \% 。Paltrinieri 等人研究了膜结构特性与磷回收之间的关系（见图 8）[198]。三种不同的多阳离子（PDADMAC、PAH、改性 PAH）分别与 PSS 作为多阴离子结合，形成了 PEM NF 膜。结果显示，PDADMAC/PSS 膜具有最高的渗透性和磷回收率，但由于其结构松散、交织较少，导致多价金属的截留率较低。使用带有胍基团修饰的 PAH 时，观察到更高的截留率，因此渗透液中金属污染较少。然而，
the phosphorus recovery was lower. This was attributed to a more dense, compact layer. These examples prove that HF NF membranes can successfully be used for phosphorus recovery and it is expected that these will become an important solution for this purpose. 磷的回收率较低。这归因于其更致密、紧凑的层结构。这些例子证明了 HF NF 膜可以成功用于磷的回收，预计这将成为该目的的重要解决方案。
Organic solvent nanofiltration (OSN) is a promising technology for the purification and recovery of organic solvents [172]. Being able to recover and purify these solvents is an important step to limit the impact on the environment and save costs. Compared to conventional technologies, NF offers advantages such as operating at mild conditions, low footprint and lower energy demand [199]. There are several techno-economic evaluations that show that membrane filtration, either alone or in combination with traditional technologies, can give rise to a reduction in energy consumption, while maintaining the separation efficiency standards. On top of that, membrane filtration offers versatility, while it can be used for organic solvent concentration, purification and solvent exchange [172]. 有机溶剂纳滤（OSN）是一种用于有机溶剂净化和回收的有前景的技术[172]。能够回收和净化这些溶剂是限制环境影响和节约成本的重要步骤。与传统技术相比，纳滤具有操作条件温和、占地面积小和能耗低等优势[199]。多项技术经济评估表明，膜过滤技术，无论是单独使用还是与传统技术结合使用，都能在保持分离效率标准的同时降低能耗。此外，膜过滤具有多功能性，可用于有机溶剂的浓缩、净化和溶剂置换[172]。
Despite the great potential for OSN membranes, their use is still limited. This is mainly because of concerns about the stability of the traditional polymer materials in these challenging conditions and limited fundamental knowledge of transport phenomena in OSN membranes [172]. The commercial OSN membranes used in industry are almost exclusively polymeric SW membranes, because of their ease of fabrication and modification [200]. HF membranes are not in commercial use yet, despite some of the advantages mentioned earlier [201]. Therefore, in recent years, research on OSN HF membranes has addressed these issues and has shown the potential of these membranes for treatment of organic solvents. 尽管有机溶剂纳滤（OSN）膜具有巨大潜力，但其应用仍然有限。这主要是因为传统聚合物材料在这些苛刻条件下的稳定性令人担忧，以及对 OSN 膜中传输现象的基础认识有限[172]。工业中使用的商业 OSN 膜几乎全部是聚合物中空纤维（SW）膜，因为它们易于制造和改性[200]。尽管之前提到了一些优势，中空纤维（HF）膜尚未实现商业化应用[201]。因此，近年来，针对 OSN 中空纤维膜的研究集中解决了这些问题，并展示了这些膜在有机溶剂处理中的潜力。
There are many different materials suggested for the preparation of OSN HF membranes, often with the goal to improve the stability in organic solvents. The most commonly used materials for the preparation of OSN membranes are polyimide, polyacrylonitrile and polybenzimidazole. Goh et al. (2020) fabricated solvent resistant polyimide HF membranes [202]. Polyimide HF substrates were coated with a thin selective polyamide layer via interfacial polymerization. The resulting membranes showed to be stable in acetone and isopropanol and can reject acid fuchsin ( 585 Da ) in acetone for 90%90 \%. Another preparation method involves the formation of a dense layer during phase inversion followed by chemical crosslinking [203,204]. Wang et al. showed the preparation of integrally skinned asymmetric polyimide based HF [204]. It was shown that the swelling degree and solvent stability of HF OSN membranes could be controlled by the shear rate of the spinning solution. The membranes maintained a high rejection for rhodamine B in ethanol after seven days immersion in dimethylformamide (DMF). 有许多不同的材料被建议用于制备 OSN 中空纤维膜，通常目的是提高其在有机溶剂中的稳定性。用于制备 OSN 膜的最常用材料是聚酰亚胺、聚丙烯腈和聚苯并咪唑。Goh 等人（2020）制备了耐溶剂的聚酰亚胺中空纤维膜[202]。聚酰亚胺中空纤维基材通过界面聚合涂覆了一层薄的选择性聚酰胺层。所得膜在丙酮和异丙醇中表现出稳定性，并能在丙酮中截留酸性品红（585 Da） 90%90 \% 。另一种制备方法是在相转化过程中形成致密层，随后进行化学交联[203,204]。Wang 等人展示了整体成皮非对称聚酰亚胺基中空纤维的制备[204]。研究表明，中空纤维 OSN 膜的膨胀度和溶剂稳定性可以通过旋转溶液的剪切速率来控制。该膜在二甲基甲酰胺（DMF）浸泡七天后，仍保持对乙醇中罗丹明 B 的高截留率。
Polyacrylonitrile HF membranes can also be used for OSN applications, as was shown by Tham et al. [205]. They showed that, by chemical crosslinking of PAN HF membranes, NF membranes could be obtained that showed excellent rejection ( > 99.9%>99.9 \% ) of remazol brilliant Blue R with a reasonable ethanol permeance. 聚丙烯腈（PAN）中空纤维膜也可用于有机溶剂纳滤（OSN）应用，正如 Tham 等人所示[205]。他们通过化学交联 PAN 中空纤维膜，制备出了纳滤膜，该膜对重氮蓝 R 表现出优异的截留性能（ > 99.9%>99.9 \% ），且具有合理的乙醇渗透率。
Robust polybenzimidazole (PBI) HF membranes for OSN were prepared by Asadi Tashvigh et al. [206]. Filtration tests with tetracycline/methanol and L- alpha\alpha-lecithin/hexane mixtures showed high rejections and permeances of 3.5 and 7.1L*m^(-2)h^(-1)7.1 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} bar ^(-1)^{-1} for methanol and hexane, respectively, indicating an exciting potential for use in solvent recovery. Other polybenzimidazole membranes were prepared via a green cross-linking method, also showing good prospects for solvent resistant nanofiltration (SRNF) applications [207]. To save time, the crosslinking step can even be applied immediately during the phase inversion step [208]. Asadi Tashvigh 等人制备了用于 OSN 的高强度聚苯并咪唑（PBI）中空纤维膜[206]。用四环素/甲醇和 L- alpha\alpha -卵磷脂/己烷混合物进行的过滤测试显示，对甲醇和己烷的截留率高，渗透压分别为 3.5 和 7.1L*m^(-2)h^(-1)7.1 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} bar ^(-1)^{-1} ，表明其在溶剂回收方面具有极大的潜力。其他聚苯并咪唑膜通过绿色交联方法制备，也显示出在耐溶剂纳滤（SRNF）应用中的良好前景[207]。为了节省时间，交联步骤甚至可以直接在相转化步骤中进行[208]。
Another type of polymer used for preparing OSN hollow fibers is polyaniline (PANi). Loh et al. demonstrated that intrinsically skinned asymmetric PANi hollow fibers are stable in organic solvents, such as DMF and acetone, and could be used for NF in acetone [209]. 另一种用于制备 OSN 中空纤维的聚合物是聚苯胺（PANi）。Loh 等人证明，具有内在表皮结构的不对称聚苯胺中空纤维在有机溶剂如 DMF 和丙酮中稳定，可用于丙酮中的纳滤[209]。
Recent research by Li et al. focused on the improvement of the selectivity of OSN membranes [201]. To this end Torlon ^(®){ }^{\circledR} HF membranes were prepared by tris(2-aminoethyl) amine and 4-sulfocalix[8]arene molecular engineering. The membranes showed excellent stability in DMF and were able to selectively sieve methylene blue over N,N-dimethyl4 -nitroaniline in methanol with a separation factor of 14.5 . This study highlights the potential of these membranes to be carefully engineered, such that sharp molecular sieving capabilities can be obtained. 李等人最近的研究集中在提高 OSN 膜的选择性[201]。为此，通过三(2-氨基乙基)胺和 4-磺基卡立[8]芳烃的分子工程制备了 Torlon ^(®){ }^{\circledR} HF 膜。该膜在 DMF 中表现出优异的稳定性，能够在甲醇中选择性地筛分亚甲蓝，相对于 N,N-二甲基-4-硝基苯胺的分离因子为 14.5。该研究突显了这些膜通过精细设计能够实现尖锐分子筛分能力的潜力。
A completely different application is the use for liquid phase organic synthesis reactions, for example, for solvent exchange. Often, synthesis routes involve a sequence of different reactions. When in the first stage of the synthesis route a solvent is used different to the one used in the second stage, solvents are traditionally exchanged via distillation, but this can also be done via OSN. Livingston et al. showed the proof of principle of this technique with a flat-sheet polyimide membrane for the solvent exchange from toluene to methanol [210]. Similarly, they indicated the potential for homogeneous catalyst recycle. 一个完全不同的应用是用于液相有机合成反应，例如溶剂交换。合成路线通常涉及一系列不同的反应。当合成路线的第一阶段使用的溶剂与第二阶段不同，传统上通过蒸馏进行溶剂交换，但这也可以通过 OSN 实现。Livingston 等人用一张平板聚酰亚胺膜展示了该技术从甲苯到甲醇的溶剂交换的原理验证[210]。同样，他们指出了均相催化剂回收的潜力。
The selective layer cannot only be prepared via interfacial polymerization or chemical crosslinking, the coating of polyelectrolyte multilayers is another successful method. Several researchers have shown the potential of different types of PEM NF membranes for use in SRNF applications [211-214]. For this, both strong and weak polyelectrolytes can be used. For example, Ahmadiannamini et al. showed the preparation of PEM NF membranes with the strong polyelectrolytes, PDADMAC and PSS, that showed retentions up to 99%99 \% for rose bengal in isopropanol (IPA) [215]. On the other hand, Ilyas et al. used the weak polyelectrolytes PAH and PAA, resulting in membranes that are stable long-term in IPA, acetonitrile and DMF [212]. 选择性层不仅可以通过界面聚合或化学交联制备，聚电解质多层膜的涂覆也是另一种成功的方法。多位研究人员展示了不同类型的聚电解质多层纳滤膜在溶剂耐受纳滤（SRNF）应用中的潜力[211-214]。为此，可以使用强聚电解质和弱聚电解质。例如，Ahmadiannamini 等人展示了使用强聚电解质 PDADMAC 和 PSS 制备的聚电解质多层纳滤膜，在异丙醇（IPA）中对玫瑰苯胺的截留率高达 99%99 \% [215]。另一方面，Ilyas 等人使用了弱聚电解质 PAH 和 PAA，制备的膜在 IPA、乙腈和 DMF 中表现出长期稳定性[212]。
The variety of examples shows that NF membranes have an immense potential for use in OSN applications. This includes not only the recovery of solvents and concentration of solvents, but also solvent exchange applications. Nowadays the standard is still focused on mostly SW NF membranes. While for most of the previously mentioned applications HF benefits from a better fouling control, this is typically not an issue in OSN. Therefore, only if HF could become beneficial compared to SW for OSN is it expected that a shift towards HF can be made. An example might be if the specific membrane area could be increased as compared to SW, but first the fiber integrity problems need to be tackled. 各种实例表明，纳滤膜在有机溶剂纳滤（OSN）应用中具有巨大的潜力。这不仅包括溶剂的回收和浓缩，还包括溶剂交换应用。如今，标准仍主要集中在管式纳滤膜（SW NF）上。虽然在之前提到的大多数应用中，中空纤维膜（HF）在防污控制方面具有优势，但这通常不是 OSN 中的问题。因此，只有当中空纤维膜在 OSN 中相较于管式膜具有优势时，才有望实现向中空纤维膜的转变。一个例子可能是如果能够提高单位膜面积的比表面积，但首先需要解决纤维完整性的问题。
3.7. Biorefinery 3.7. 生物炼制
A biorefinery is the processing facility that converts biomass into valuable products or energy that can replace fossil oil refineries. Different biomass materials can be treated, such as wood, straw, starch, sugars, waste and algae, with lignocellulose being the most abundant biomass on earth. The production of valuable compounds in a biorefinery is complex due to the variability of the composition of the biomass. Therefore, separation and purification processes play a critical role in biorefineries as these processes can be 生物炼制厂是将生物质转化为有价值产品或可替代化石油炼厂能源的加工设施。可以处理不同的生物质材料，如木材、稻草、淀粉、糖类、废弃物和藻类，其中木质纤维素是地球上最丰富的生物质。由于生物质成分的多样性，生物炼制厂中有价值化合物的生产过程较为复杂。因此，分离和纯化过程在生物炼制厂中起着关键作用，因为这些过程可能
responsible for up to 50%50 \% of the capital operational costs. Pressure driven membrane filtration processes such as UF and NF have emerged as promising separation technologies in many applications of biorefineries and some examples are presented below. Currently, most of the studies have been carried out on more conventional membrane geometries, attributed to the limited availability of specific HF NF membranes. However, it is foreseen, especially based on the typical high TOC and TSS levels that characterize biorefinery processes, that HF NF membranes will play a significant role here. 占据高达 50%50 \% 的资本运营成本。压力驱动的膜过滤工艺，如超滤（UF）和纳滤（NF），已成为生物炼制厂许多应用中有前景的分离技术，以下列举了一些例子。目前，大多数研究集中在更传统的膜几何形状上，这归因于特定中空纤维纳滤膜的有限供应。然而，特别基于生物炼制过程典型的高总有机碳（TOC）和总悬浮固体（TSS）水平，预计中空纤维纳滤膜将在此发挥重要作用。
Algae are a promising biomass to produce biofuel. Algal biomass harvesting membrane technology is seen as a promising technique, since it provides almost complete retention of biomass and offers a very economical competitiveness when compared to other more energy-consuming methods. Membrane filtration is minimally disruptive to harvested biomass, causes minimal stress to the algal cells and avoids chemical additives (e.g., flocculants or pH adjustments required for flocculation) that may otherwise degrade the quality of harvested biomass or bioproducts [216]. Bhave et al. studied the application of polymeric HF MF (Pall Corp., New York, NY, USA) to dewater microalgae for biofuel production achieving an energy reduction of at least 80%80 \% over traditional methods as centrifugation or froth flotation [217]. Additionally, these membranes also renovated growth media to acceptable levels for recirculation. The major drawback of membrane filtration is the low flux, especially at high biomass concentration. Therefore, several studies have suggested to combine membrane filtration as preliminary separation before centrifugation for microalgae harvesting [218-220]. 藻类是一种有前景的生物质，用于生产生物燃料。藻类生物质收获膜技术被视为一种有前景的技术，因为它几乎能完全保留生物质，并且与其他更耗能的方法相比，具有非常经济的竞争力。膜过滤对收获的生物质干扰最小，对藻类细胞的压力也很小，且避免了可能降低收获生物质或生物产品质量的化学添加剂（如絮凝剂或絮凝所需的 pH 调节）[216]。Bhave 等人研究了聚合物 HF MF 膜（Pall 公司，美国纽约）在微藻脱水用于生物燃料生产中的应用，实现了比传统离心或泡沫浮选方法至少 80%80 \% 的能耗降低[217]。此外，这些膜还将培养基更新至可接受的水平以供循环使用。膜过滤的主要缺点是通量低，尤其是在高生物质浓度下。因此，若干研究建议将膜过滤作为微藻收获前的初步分离步骤，与离心结合使用[218-220]。
Membrane technology can also be applied for the recovery of lignin from lignocellulosic biomass. Lignocellulose is composed mainly of cellulose, hemicellulose and lignin, which must be separated in order to process them into specific products. Cellulose and hemicellulose are biopolymers of sugars and thereby a potential source of fermentable sugars to produce biofuels. Lignin is a complex biopolymer rich in aromatic subunits that can be converted into high value bio-sustainable chemicals such as vanillin, antioxidants or polyamide. Biorefinery of lignocellulose involves pretreatment of biomass, processing and synthesis of biofuel or valuable products and finally purification. Lignocellulose biomass is hardly soluble in common solvents and its economic hydrolysis process into fermentable monosaccharides remains a major challenge. Common pretreatments are acid, alkali, organosolv (aqueous organic solvents like methanol, acetone, ethanol and ethylene glycol) or ionic liquids. 膜技术也可应用于从木质纤维素生物质中回收木质素。木质纤维素主要由纤维素、半纤维素和木质素组成，必须将其分离以便加工成特定产品。纤维素和半纤维素是糖类的生物聚合物，因此是生产生物燃料的可发酵糖的潜在来源。木质素是一种富含芳香基团的复杂生物聚合物，可转化为高价值的生物可持续化学品，如香草醛、抗氧化剂或聚酰胺。木质纤维素的生物炼制包括生物质的预处理、加工和生物燃料或有价值产品的合成，最后进行纯化。木质纤维素生物质在常见溶剂中几乎不溶，其经济高效水解成可发酵单糖的过程仍是一大挑战。常见的预处理方法有酸处理、碱处理、有机溶剂法（如甲醇、丙酮、乙醇和乙二醇等水性有机溶剂）或离子液体法。
Ionic liquids have shown good results, but their reuse is required for the economic viability of the process. Most NF membranes are inefficient since sugars are noncharged, ionic liquids have a low charge density and both have similar molecular weights. Flat NF membranes fabricated by LbL deposition of charged polyelectrolytes (PSS and PAH), with controlled charge density and pore size, have been applied for the recovery of ionic liquid from dilute aqueous solutions containing monomeric sugars [221]. Selectivity above 50 was achieved for 1-butyl-3-methylimidazolium chloride and cellobiose. Lignin concentration from synthetic organosolv liquors by NF has been studied and lignin rejection of 99%99 \% has been achieved with flat thin film composite membranes, consisting of a polyamide selective layer on a commercial polysulfone support (FilmTec NF270 from Dow) [222]. Gomes et al. studied the concentration of synthetic mixture of phenolic compounds by commercial flat-sheet NF membranes (FilmTec NF270 from Dow and MPS-34 from KOCH) [223]. The mixture mimicked the composition of oxidized black liquor, and a significant concentration was achieved with high permeate fluxes and rejections above 90%90 \%. For separating hemicelluloses from process liquors containing sodium hydroxide, Schlesinger et al. showed that NF is preferable over UF for quantitatively retaining hemicellulose at molar masses above 1000g*mol^(-1)1000 \mathrm{~g} \cdot \mathrm{~mol}^{-1} [224]. High-value organic acids can be produced from fermentation of biomass and the primary challenge is the downstream recovery of them. NF has been employed for the recovery of succinic, lactic, butyric, acetic and fumaric acids [225]. Sosa et al. evaluated different NF membranes (Filmtec NF270 from Dow and NF-DK and NF-DL 离子液体已显示出良好的效果，但为了工艺的经济可行性，需要对其进行重复使用。大多数纳滤膜效率较低，因为糖类不带电，离子液体的电荷密度较低，且两者的分子量相近。通过层层自组装（LbL）法沉积带电聚电解质（PSS 和 PAH）制备的平板纳滤膜，具有可控的电荷密度和孔径，已被用于从含有单体糖的稀释水溶液中回收离子液体[221]。对于 1-丁基-3-甲基咪唑氯化物和纤维二糖，选择性超过 50。利用纳滤技术对合成有机溶剂液中的木质素浓缩进行了研究，采用由聚酰胺选择层和商业聚砜支撑层（Dow 公司的 FilmTec NF270）组成的平板薄膜复合膜，实现了木质素的截留率为 99%99 \% [222]。Gomes 等人研究了利用商业平板纳滤膜（Dow 公司的 FilmTec NF270 和 KOCH 公司的 MPS-34）对合成酚类化合物混合物的浓缩[223]。 该混合物模拟了氧化黑液的组成，并在高渗透通量和超过 90%90 \% 的截留率下实现了显著的浓缩。Schlesinger 等人展示了，对于从含氢氧化钠的工艺液中分离半纤维素，纳滤（NF）优于超滤（UF），能够定量截留分子量高于 1000g*mol^(-1)1000 \mathrm{~g} \cdot \mathrm{~mol}^{-1} 的半纤维素[224]。高价值的有机酸可以通过生物质发酵生产，主要挑战在于其下游回收。纳滤已被用于回收琥珀酸、乳酸、丁酸、乙酸和富马酸[225]。Sosa 等人评估了不同的纳滤膜（Dow 的 Filmtec NF270 和 GE Osmotics 的 NF-DK 及 NF-DL
from GE Osmotics), achieving fluxes between 25 and 32L*m^(-2)h^(-1)32 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} and succinate rejections above 90%90 \% and reaching negative rejections for acetate and formate [226]. ），实现了 25 至 32L*m^(-2)h^(-1)32 \mathrm{~L} \cdot \mathrm{~m}^{-2} \mathrm{~h}^{-1} 之间的通量和超过 90%90 \% 的琥珀酸盐截留率，同时对乙酸盐和甲酸盐达到了负截留率[226]。
Another application of membrane technology is the separation of carboxylic acids from biorefinery solutions. Lactic acid is widely used in foods, detergents, pharmaceutical and cosmetic applications and in the chemical industry. Lactic acid is a naturally produced organic acid that can be commonly obtained by either chemical synthesis or carbohydrate fermentation. Lactic acid production by lactose fermentation bears high costs due to the separation steps required for food-grade lactic acid. To reduce the costs, different separation techniques have been studied, such as ion exchange, electrodialysis, distillation and membrane technology. In some studies researchers have shown that NF can be used to separate organic solutes (e.g., glucose and lactic acid) from fermentation broth [227,228]. Figure 9 provides a schematic of an integrated membrane process for the purification of lactic acid. Due to the difference in the molecular weights of glucose ( 180g*mol^(-1)180 \mathrm{~g} \cdot \mathrm{~mol}^{-1} ) and lactic acid ( 90.08g*mol^(-1)90.08 \mathrm{~g} \cdot \mathrm{~mol}^{-1} ), glucose can be effectively separated from lactic acid. The electrical charge of the NF membrane also affects organic molecule separation via Donnan exclusion effects [229]. 膜技术的另一个应用是从生物炼制溶液中分离羧酸。乳酸广泛应用于食品、洗涤剂、医药和化妆品领域以及化工行业。乳酸是一种天然产生的有机酸，通常可以通过化学合成或碳水化合物发酵获得。通过乳糖发酵生产乳酸的成本较高，主要由于获得食品级乳酸所需的分离步骤。为了降低成本，研究了不同的分离技术，如离子交换、电渗析、蒸馏和膜技术。在一些研究中，研究人员表明纳滤（NF）可以用于从发酵液中分离有机溶质（例如葡萄糖和乳酸）[227,228]。图 9 展示了用于乳酸纯化的集成膜工艺示意图。由于葡萄糖（ 180g*mol^(-1)180 \mathrm{~g} \cdot \mathrm{~mol}^{-1} ）和乳酸（ 90.08g*mol^(-1)90.08 \mathrm{~g} \cdot \mathrm{~mol}^{-1} ）的分子量不同，葡萄糖可以有效地从乳酸中分离出来。纳滤膜的电荷也通过 Donnan 排斥效应影响有机分子的分离[229]。
Integrated membrane processes for lactic acid purification 乳酸纯化的集成膜工艺
Figure 9. Integrated membrane processes for lactic acid purification. Reprinted with permission from [229]. Copyright 2017 American Chemical Society. 图 9. 乳酸纯化的集成膜工艺。经许可转载自[229]。版权所有 2017 美国化学学会。
The application of membrane technologies in biorefineries is expected to grow, but still only few studies using hollow fiber nanofiltration can be found. Since these membranes can handle better (bio)fouling-one of the main drawbacks in the application of membrane technologies to biorefineries-they would have the potential to change the economic evaluation of biorefinery downstream processes, making them industrially accessible. 膜技术在生物炼制厂中的应用预计将增长，但目前使用中空纤维纳滤膜的研究仍然较少。由于这些膜能够更好地应对（生物）污染——这是膜技术应用于生物炼制厂的主要缺点之一——它们有潜力改变生物炼制下游工艺的经济评估，使其具备工业可行性。
4. Conclusions and Outlook 4. 结论与展望
NF membranes offer solutions in a wide variety of different processes. The specific selectivity that the NF separation layer provides improves current or enables emerging applications. These include freshwater treatment, to remove hardness and natural organic matter, and municipal or industrial wastewater treatment, to remove harmful contaminants. In addition, NF can be employed to recover solutes or resources from (waste) streams, especially if NF selectivity is tuned or designed to the specific process. In many cases, high fouling rates or conditions of the feed limit the use of the traditional polyamide-based NF membranes. For such processes, a hollow fiber membrane geometry is beneficial and has been studied intensively. 纳滤膜在各种不同的工艺中提供了解决方案。纳滤分离层所具备的特定选择性改善了现有应用或使新兴应用成为可能。这些应用包括淡水处理，用于去除硬度和天然有机物，以及市政或工业废水处理，用于去除有害污染物。此外，纳滤还可用于从（废）流中回收溶质或资源，尤其是在纳滤选择性被调节或设计以适应特定工艺的情况下。在许多情况下，高污染率或进料条件限制了传统聚酰胺基纳滤膜的使用。对于此类工艺，中空纤维膜结构具有优势，并已被广泛研究。
Next to lab-scale tests, many studies have been carried out on larger pilot and demoscales to prove the technical feasibility. This has led to commercial availability of HF NF modules that are now operated in full-scale installations, a testament that this membrane field will grow in the coming years. Given the variety of the streams and solutes, and the 除了实验室规模的测试外，许多研究还在更大规模的中试和示范规模上进行，以证明技术的可行性。这促使中空纤维纳滤模块的商业化，目前这些模块已在全规模装置中运行，这证明了该膜领域在未来几年将持续增长。鉴于流体和溶质的多样性，以及
novelty of the different processes presented here, a better understanding or prediction of performance of NF membranes will be needed to accelerate the acceptance of these new membranes in the market. 鉴于此处介绍的不同工艺的新颖性，需要更好地理解或预测纳滤膜的性能，以加速这些新型膜在市场上的接受度。
Author Contributions: Conceptualization, T.S., M.G.E. and J.d.G.; Investigation, T.S., M.G.E., M.B., S.M. and J.d.G.; Writing-Original Draft Preparation, T.S., M.G.E., M.B., S.M. and J.d.G.; WritingReview & Editing, T.S., M.G.E., S.M., J.Y. and J.d.G.; Administration, M.G.E.; Funding Acquisition, J.Y. and J.d.G. All authors have read and agreed to the published version of the manuscript. 作者贡献：概念构思，T.S.、M.G.E. 和 J.d.G.；调查研究，T.S.、M.G.E.、M.B.、S.M. 和 J.d.G.；原始稿件撰写，T.S.、M.G.E.、M.B.、S.M. 和 J.d.G.；稿件审阅与编辑，T.S.、M.G.E.、S.M.、J.Y. 和 J.d.G.；管理，M.G.E.；资金获取，J.Y. 和 J.d.G.。所有作者均已阅读并同意发表的稿件版本。
Funding: Part of this work has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement 820735 Liberate (www.liberate-project.eu (accessed on 11 October 2021)). 资金支持：本部分工作获得了欧盟地平线 2020 研究与创新计划的资助，资助协议编号为 820735 Liberate（www.liberate-project.eu，访问日期 2021 年 10 月 11 日）。
Institutional Review Board Statement: Not applicable. 机构审查委员会声明：不适用。
Informed Consent Statement: Not applicable. 知情同意声明：不适用。
Data Availability Statement: Data sharing not applicable. No new data were created or analyzed in this study. 数据可用性声明：数据共享不适用。本研究未创建或分析新数据。
Conflicts of Interest: S.M., M.E.B. and J.Y. declare no conflict of interest. T.S., M.G.E. and J.d.G. hold (parttime) positions at NX Filtration, a membrane manufacturer. The views expressed are those of the authors and do not necessarily reflect the position or policy of PepsiCo, Inc. 利益冲突声明：S.M.、M.E.B. 和 J.Y. 声明无利益冲突。T.S.、M.G.E. 和 J.d.G. 在膜制造商 NX Filtration 担任（兼职）职位。文中观点为作者个人观点，不一定反映百事公司（PepsiCo, Inc.）的立场或政策。
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