Integration and Operational Application of Advanced Membrane Technologies in Military Water Purification Systems

Abstract

Membrane technologies are used in the production of potable water and the treatment of wastewater in the military forces, providing the highest level of contaminant removal at an energy-efficient cost. This review examines the integration and application of membrane technologies, including reverse osmosis, nanofiltration, ultrafiltration, electrodialysis and advanced hybrid systems, in the treatment of wastewater generated at military bases, naval vessels and submarines. Special emphasis is placed on purification technologies for chemically, biologically and radiologically contaminated wastewater, as well as on the recycling and treatment of wastewater streams by mobile systems used in military applications. Given the specific requirements of complex military infrastructures, particularly in terms of energy efficiency, unit self-sufficiency and reduced dependence on logistical supply chains, this work analyses the latest advances in membrane technologies. Innovations such as nanographene membranes, biomimetic membranes, antifouling membrane systems and hybrid configurations of forward osmosis/reverse osmosis and electrodialysis/reverse electrodialysis offer unique potential for implementation in modular and mobile water treatment systems. In addition, the integration and operational use of these advanced technologies serve as a foundation for the development of autonomous military water supply strategies tailored to extreme operational conditions. The continued advancement and optimization of membrane technologies in military contexts is expected to significantly impact operational sustainability while minimizing environmental impact.

1. Introduction

The increasing demand for potable water, driven by rapid industrialization and the growing global population, require advanced treatment methods, particularly in the military sector. Along with increasing demand for drinking water, anthropogenic activity also produces large quantities of harmful and polluted wastewater (WW) [1,2]. Such effluents must be treated by physical, chemical, or biological processes prior to their discharge into the environment. However, conventional methods of water treatment are no longer adequate to meet the growing global demand for water resources [3]. It is therefore necessary to promote the use of advanced separation techniques, such as membrane technologies (MTs), in addition to more responsible use of natural water resources. In recent decades, MTs have become an essential part of systems for WW streams treatment in order to reduce their harmful impact on the environment. In the commercial sector, pressure-driven systems such as reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), microfiltration (MF), as well as electrodialysis (ED), are used for water supply systems, industrial and manufacturing processes and environmental protection to treat WW contaminated with inorganic and organic substances [4,5]. In addition to these human activities, MTs have become suitable for military applications, which include the treatment of potable water in field conditions using mobile container systems, the treatment of water contaminated with chemical, biological and radiological agents (CBR), desalination and treatment systems on ships and submarines and similar applications [6,7]. The military’s operational requirement often involves deployment in extreme field conditions, necessitating the establishment of specialized water treatment and purification standards for both peacekeeping missions and wartime scenarios. These requirements emphasize the need for unit self-sufficiency and enhanced independence from logistical supply chains, increased troop mobility and improved economic and energy efficiency of the water treatment process [8]. To meet these requirements, potential applications of innovations in membrane processes are being explored, such as the use of modularized nanographene or biomimetic membranes, membrane distillation systems, and hybrid configurations like forward osmosis/reverse osmosis (FO/RO) and electrodialysis/reverse electrodialysis (ED/RED). Such systems whether stand alone or in combined synergistic operational modules, offer unique potential for application in stationary and mobile water treatment systems [9,10,11]. The integration and operational application of these advanced technologies are particularly important for the development of autonomous water supply strategies in military operations, tailored to extreme operational conditions [12]. When water is unavailable from public water supply sources, RO water purification units (ROWPU), with a capacity that exceeds 10.0 m³ h⁻¹, are deployed within permanent military areas [13]. NATO has also developed fast deployable alternatives in addition to large-scale systems, like the Lightweight Water Purifier (LWP), a portable device with a 0.470 m³ h⁻¹ capacity designed for use in difficult or isolated environments [14]. Naval platforms use integrated RO, UF, and disinfection systems to recycle over 80% of water and produce up to 1.250 m³ h⁻¹ for crew and operations [15]. Continuous progress and optimization of MTs in a military context have a significant impact on operational sustainability and reduce the environmental footprint. Furthermore, the application of these technologies can contribute to minimizing the environmental impact of military operations, thereby achieving a balance between operational needs and ecological responsibility [16,17]. The aim of this review is to present recent advances in the use and integration of MTs within the military sector. Through an analysis of recent technological advancements in the field of membrane-based water treatment, this review highlights their potential for improving the sustainability of military operations consequently ensuring greater protection of natural resources and the environment.

2. Emerging Technologies Based on Membrane Processes for Water Purification

MTs are among the most effective and ecologically friendly technologies for obtaining potable water (brackish and seawater desalination), treating industrial and municipal WW, purify contaminated water into safe drinking water, and facilitate sustainable energy applications. Technological advances in membrane materials in the second half of the 20th century enabled their widespread use in almost all areas of human activity [18,19,20,21]. Rougher membrane processes, such as MF, are crucial for removing suspended particles and reducing total suspended solids (TSS) levels during WW treatment. While MF primarily retains larger suspended particles, UF retain smaller particles, including bacteria and viruses, thereby enhancing the overall efficiency of the process. RO and NF are highly effective in removing both organic and inorganic pollutants, such as pesticides, heavy metals, and persistent contaminants like pharmaceuticals. MTs play a critical role in water recovery and the reuse of treated water, particularly in industrial and agricultural sectors [22,23]. RO and NF are widely employed in commercial potable water treatment, particularly for the desalination of brackish and seawater, as well as for reducing calcium and magnesium ions that contribute to water hardness [24]. Furthermore, MTs demonstrate high efficiency in water sanitization, proficiently eliminating pathogenic microorganisms and removing residual nitrates, phosphates, pesticides, and other organic pollutants [25]. A review of recent literature and studies highlights the efficiency, cost-effectiveness, and sustainability of membrane separation processes for a wide range of contaminants. Huang et al. had experimentally demonstrated that the closed-circuit reverse osmosis (CCRO) process can remove 90–95% of contaminants of emerging concern (CECs), including dioxane, a wide range of organic compounds, pesticides, antibiotics, analgesics, psychiatric drugs, beta-blockers, narcotics, and flame retardants [26]. Furthermore, Bashir et al. investigated polyelectrolyte multilayer NF membranes, and optimized their selectivity for specific contaminants achieving high removal efficiency [27]. Behroozi et al. investigated the potential of graphene-based membranes for the removal of per- and polyfluoroalkyl substances (PFAS), reporting a high removal efficiency exceeding 90% [28]. Zhang et al. examined thin nanocomposite membranes with covalent organic frameworks (COFs) for the efficient removal of heavy metals from wastewater, achieving rejection of 98.6% for Fe³+, 97.8% for Cu²+, 97.3% for Ni²+, 96.4% for Mn²+, and 95.9% for Zn²+ [29]. A driving force in membrane separation operation might include an electrical, temperature, pressure and concentration gradient (

Table 1. List of membrane processes according to the driving force.

Pressure Difference	Concentration (Activity) Difference	Temperature Difference	Electrical Potential Difference
Microfiltration	Pervaporation	Thermo-osmosis Membrane distillation	Electrodialysis Electro-osmosis Membrane electrolysis
Ultrafiltration	Gas separation
Nanofiltration	Vapor permeation
Reverse osmosis	Dialysis
Forward osmosis	Diffusion dialysis
Pressure retarded osmosis	Carrier-mediated transport

) [30].

In these processes, specific chemical substances are retained by the membrane (retentate), while others (preferably water) are selectively transmitted through it (permeate). The specificity of each membrane process is determined by membrane characteristics, such as pore size and the shape and surface properties (porosity, hydrophobicity), as well as the configuration, dimensions, and geometric properties of the membrane [22,30]. Due to these factors, two main types of membrane separation processes are employed in WW treatment and water recycling for potable use: isothermal and non-isothermal processes. Isothermal membrane processes, such as pervaporation and membrane gas separation, are driven by concentration gradients. MF, UF, NF, and RO are pressure-driven membrane processes. ED and electrophoresis (EF) are electrically driven processes. In contrast, non-isothermal processes are thermally driven and include technologies such as membrane distillation (MD). These membrane-based separation processes play a crucial role in water treatment technologies by enabling efficient phase separation and selective removal of contaminants [31,32].

Basic Principles of the Most Industrially Significant Membrane Processes and Separation Mechanisms

RO is the most well-known and highly efficient membrane operation that has been very well established in the industry for decades. On an industrial scale, it is mainly used for the desalination of seawater and brackish water into potable water. RO is also particularly important for the production of high-purity water, which is of great importance for pharmaceutical processes in vaccine and pharmaceutical production and in the chemical industry for chemical process syntheses of new compounds. It is also used in households and small facilities for the purification of drinking water from excess salts, chlorides, heavy metals, pesticides, nitrates, and others, as well as for the recycling and treatment WW [33]. The applications of this process are numerous and include the concentration, purification and selective separation of valuable components of a mixture from waste effluent [22]. The separation of RO membranes is based on three mechanisms: size exclusion, charge exclusion (with a minor contribution) and membrane-solution interactions, which depend on the compatibility of the solute physico-chemical characteristics and the chemistry of the RO membrane. Recent studies have shown that the permeability of water through RO membranes is governed by pore flow rather than diffusion [34]. The pressures required for the RO process range from 20 to 80 bar to overcome the osmotic pressure of the solution (feed) [34,35,36]. On the other hand, components that are intended to be separated from a homogeneous or heterogeneous mixture of substances, such as high-value compounds, pharmaceuticals, proteins and similar substances, as well as pollutants such as hydrocarbons, pesticides and inorganic heavy metal-based compounds, are rejected by the membrane. Despite RO has many advantages, including high efficiency in removing contaminants and the absence of additional chemicals that impact the environment, it also has disadvantages, such as its limited use with highly loaded WW due to intensive membrane fouling. To overcome these limitations, the RO process is most often integrated with other processes in hybrid membrane systems and advanced, improved versions of membranes with greater fouling resistance are used [37]. In military applications, during crisis situations or military threats, RO is a key process for water recycling and purification as well as for CBR decontamination, with high removal rates of synthetic chemical warfare agents, organic molecules, radionuclides, and toxic industrial compounds as well as heavy metals. The process, in both military and civilian sectors, is most frequently combined with other membrane processes such as UF and NF, ED, and adsorption on activated carbon/zeolite [38]. In addition to the RO process, UF and NF have also been successfully used for decades to purify and treat WW. They are similar to RO in terms of process characteristics, but differ significantly in the size of the particles that can be separated from the system, i.e., the membrane pore size and the operating pressure [39]. The UF process is most commonly used as pre-treatment for RO or NF due to their larger pore size (0.01–0.1 μm), which retains over 98% of larger suspended solids, colloidal particles, viruses, and bacteria. The operation is based on classic mechanical filtration: larger particles are retained while water containing smaller dissolved molecules, such as salt ions, passes through the porous membrane under relatively low pressure of 1–5 bar. The military application of UF focuses on the treatment and sanitization of biologically contaminated wastewater containing coliform or pathogenic microorganisms, as well as larger organisms [40,41]. On the other hand, the porous structure of composite NF membranes consists of smaller pores, of less than 2 nm, and in the membrane separation process, in addition to size exclusion, electrostatic repulsion and interactions between ions occur due to the membrane’s surface charge [42]. Therefore, NF successfully rejects most organic compounds and polyvalent ions such as Ca²⁺, Mg²⁺, SO₄²⁻ and heavy metals with a higher valence numbers, while monovalent ions dissolved in the aqueous medium usually pass through the NF membranes to a greater extent at applied pressure of up to 20 bar. In addition to its widespread use in industry for the concentration, demineralization, water softening, and recycling of WWr streams, NF is used in the military sector to remove larger organic compounds such as organophosphorus warfare agents, radiological contaminants, and heavy metals, and to achieve removal of more than 99.9% of biological warfare agents [43,44]. In order to effectively optimize separation processes with multifunctional application and reduced energy consumption, membrane technologies integrate different membrane processes with each other or in combination with other physico-chemical or biological separation processes to form so-called hybrid membrane technologies (HBT). In conventional wastewater treatment, primary, secondary and tertiary WW treatment are very often carried out separately, which requires higher capital and investment costs, larger facilities to carry out such an extensive process and increased energy consumption. The use of hybrid integrated systems reduces the need for multiple treatment stages, increases flexibility and adaptability to various pollutants and process conditions, and makes it possible to meet multiple requirements within a single process module (purification, concentration, separation, complexation, etc.) [45,46]. In the context of sustainable management of water resources, under strict legal standards of environmental protection legislation, the hybrid FO/RO membrane process represents an innovative, sustainable and practical solution. Such a concept is particularly suitable for the treatment and concentration of industrial wastewater streams, but can also be used successfully used for seawater desalination and production of ultra-pure water. Due to the difference in concentration between draw solution (DS) (the higher concentrated solution) and feed solution (FS) (the lower concentrated solution), FO takes place as a natural osmosis process through a semi-permeable membrane. An ideal DS should have a high transmembrane osmotic pressure gradient, at the same time negligible salt permeability from DS to FS, so that it can extract water molecules from FS [47]. In the second step, the RO process is applied under the influence of high external pressure to remove the remaining salt concentration from the DS. The result of the process is clean drinking water. The concentrated FS can now be used as a DS in the next step. Using FO as a pretreatment prior to the RO process reduces membrane contamination, fouling, and downtime of the RO membrane systems, which increases the membrane recovery rate and minimizes chemical consumption [48,49,50]. A visual overview of the main types of pressure membrane processes and the specific contaminants they remove, categorized by membrane pore size, can be found in Figure 1.

The membrane operation that uses electrically charged membranes (anion/cation exchange membranes) and an electric potential to separate ionic species from neutral species in aqueous solution is ED. This process is most commonly used in the desalination of sea and brackish water, as well as for the production of drinking water in rural areas of the world. With the development of new ion-exchange membranes that have improved thermal, mechanical and chemical properties, high selectivity and lower electrical resistance, ED has begun to be widely applied in biotechnology, wastewater treatment and various sectors of industry such as the chemical and pharmaceutical sectors. Between the cathode and the anode there is a series of ion exchange membranes (multi-chamber electro dialyzer). A stream of diluted or concentrated water flows out of the chambers. Cations move through selective cation exchange membranes to the cathode when an electric current flows through the system, and anions move through anion exchange membranes to the anode [51,52]. To maintain a positive energy balance of the ED process, it is integrated with the complementary process of reverse electrodialysis (RED). In RED, the migration of cations and anions moves through selective membranes towards the cathode and anode, where an electrochemical potential is generated and electrical energy is obtained. The migration of ions in the solution is driven by the natural process of establishing equilibrium concentrations. The combination of ED/RED is very suitable for military field operations for the conversion of contaminated or saline water into potable water and for the generation of autonomous energy systems and green energy production [53,54] The separation process of MD combines membrane technology with traditional distillation principles. The operation is based on a temperature difference across a hydrophobic, microporous membrane that only allows water vapour to pass through. All solid particles and non-volatile materials remain on the feed side (retentate), while the vapour moves towards the cooler side and condenses there. Since MD operates at much lower temperatures than conventional thermal distillation, it is more energy efficient and consumes less energy overall. However, as with other membrane-based systems, there are still operational issues with membrane fouling and long-term durability of its hydrophobic surface. MD is suitable for the recovery and concentration of volatile chemicals in chemical and pharmaceutical operations, and treating various wastewater streams. By improving other separation processes, it can also be used as a post-treatment step to further purify effluents and reduce the volume of the waste. Important dissolved molecules that might otherwise be lost can also be concentrated and recovered via MD [55,56]. On the other hand, membrane bioreactors (MBR) combine biodegradation, typically achieved by a diverse microbial community in the activated sludge, with membrane filtration processes such as MF or UF. In addition to their ability to capture suspended solids, dissolved inorganic particles and heavy metals, these systems have demonstrated high effectiveness in removing organic contaminants from wastewater [57]. By integrating MD/MBR into a hybrid system, significant potential is created for the treatment and purification of water. The MBR system removes the majority of organic substances and solid particles that could cause membrane fouling during subsequent filtration, while the MD system serves to concentrate and reduce the volume of waste after treatment [58]. For military purposes, MD is used in mobile units for desalination and water purification, but also for the treatment of water contaminated with synthetic chemicals in areas affected by chemical weapons or toxic industrial waste [59]. MBR, on the other hand, is used for biological water treatment in military bases and field hospitals, where high-quality effluent is obtained which can be used for hygienic purposes such as human showering or for other technical purposes [60]. A key component of any membrane process is the membrane, a physical barrier that has the ability to selectively pass or retain substances from the feed solutions. The chemical nature of membranes can be organic, inorganic or hybrid. The two most important characteristics that affect the cost-effectiveness of a separation process are permeability and selectivity, which is referred to as perm selectivity. Relying on more intricate, multi-stage systems is frequently necessary to achieve the required degree of separation when membrane selectivity is low. However, it is important to emphasize that permeability is also crucial—highly selective membranes often have lower flux. Therefore, developing an effective and useful membrane process involves achieving the correct balance between selectivity and permeability. In other words, if the membrane selectivity is low, a more complex membrane configuration and more complex multi-stage processes are required. The number of membrane modules (the smallest practical unit) and the surface area of membranes in them, are determined by the permeability factor, which also correlates with the productivity of the membrane. The specific requirements of the process, such as resistance to extreme process conditions (e.g., high concentrations of pollutants or salts), selectivity and product purity requirements, availability of energy sources and economic viability, influence the choice of membrane separation process and membrane material [61,62]. The types of inorganic, polymeric and hybrid membranes with a range of practical applications and basic characteristics are presented in

Table 2. Inorganic, polymeric, and hybrid membrane types with a variety of practical applications and fundamental properties [63,64,65,66].

Type of Membranes		Spectrum of Use	Advantages	Disadvantages
Inorganic membranes	Al2O3, TiO2, ZrO2 ceramic membranes, Al-Si oxide membranes, titanium/stainless steel porous metal membranes, carbon based molecular sieves, silicate, borosilicate and porous glass membranes	used in various sectors of the chemical industry for water filtration and gas separation, in analytical sensors, pH electrodes etc.	excellent mechanical, thermal and chemical stability, variable pore size, work under challenging process conditions, separation mechanism: molecular sieving, capillary condensation surface diffusion, Knudsen diffusion	membranes have extremely fragile pores, preparation process is expensive, difficulties in scale-up implementations
Organic (polymeric) membranes	polyamide (PA) membranes	used in NF and RO processes, for seawater desalination—thin film composite membrane (TFC), for industrial wastewater treatment—spirally wound membranes (SWM)	low production cost, good mechanical stability, easy for upscaling, realtively easy preparation, separation mechanism: size exclusion, charge exclusion and membrane-solution interactions	plasticization, depending on the nature of the polymer relatively low thermal and chemical stability, not controllable pore size, trade-off between permeability and selectivity, need for regular cleaning procedure (intensive pore fouling)
	polyvinylidene fluoride (PVDF) membranes	used in UF i MBR processes, used in municipal and industrial plants for wastewater treatment etc.
	polyurethane (PU) membranes	utilised as biosensors for the detection of ions and for the controlled release of drugs in medicine etc.
	membranes based on cellulose derivatives	used in ED i RO, used as membranes for desalination of seawater (1. generation)
Hybrid and mixed matrix membranes	graphene-modified polymer membranes (graphene-polymethyl methacrylate PMMA nanolaminate)	used in biosensors, electronic and optical devices, for the removal of organic and inorganic pollutants, as well as for water purification and desalination etc.	reduced plasticization, enhanced thermal and mechanical stability, low energy consumption, separation mechanism: combined inorganic and polymeric membrane principle	at high fraction of filler within the polymer matrix, fragility of the system, thermal and chemical stabilities depend on the polymeric matrix
	zeolite nanoparticles MFI/polycrystalline membrane MOF ZIF-8	used in biomedicine for drug delivery, catalytic reactions, and in the separation of petroleum fractions etc.
	biomimetric and bioinspired membranes	application in various sectors, for water purification, biosensor development, and industrial processes etc.

.

3. Application of Membrane Technologies in Military Wastewater Treatment

Access to clean water, both for drinking and for other purposes required by the military, is an essential tactical, operational and strategic requirement for the execution of missions and the fulfilment of assigned tasks. Regardless of whether troops are stationed in permanent military bases or camps or deployed in the area of operation in mobile, expeditionary camps, ships or submarines, the implementation of water purification and quality improvement systems within the armed forces is necessary to maintain combat readiness. Water cleanliness prevents the transmission of infectious diseases through water and eliminates the toxic effects of pollutants on the health of soldiers [67,68]. Furthermore, extreme and physically demanding missions require continuous rehydration to maintain operational capability and physical fitness of individual members of the armed forces. Today, all integrated water recycling and reuse systems must comply with military-standards and documents that define the required quality of treated effluent (NATO Standard “AMedP-4.9. Requirements for water quality during operations”,WHO “Guidelines for Drinking Water Quality”, and national regulatory documents for water purity standards and environmental protection). There are defined key requirements that water purification systems within the military sector must fulfil. The first requirement is adaptability to the military system. This refers to the ability to purify and regenerate water from a variety of environmental sources, including contaminated wastewater and water from lakes, rivers and oceans. The term contamination includes CBR pollution with warfare agents, synthetic chemicals, biological pathogens and their toxic by-products, and radionuclides. The second requirement is the operational speed of the separation process with both quantitative and qualitative characteristics. Within the military sector, it is often necessary to purify large volumes of water quickly and efficiently, with capacities of more than 5.0 m³ h⁻¹ for troop requirements, or more depending on demand. The third important requirement for treatment systems is modularity, robustness and durability, especially with regard to mobile and portable systems. Such equipment must be relatively easy to transport and lightweight so that it can be deployed by mobile units along the route of movement within the operational area. In addition, the devices must be resistant to harsh field and environmental conditions—i.e., resistant to vibration and shock, mechanical impact, high and low temperatures, high humidity, sand, dust and similar factors [13,69,70,71,72]. Figure 2 highlights the areas within the military system where water treatment, purification and desalination devices are used, as well as the most commonly applied membrane technologies in practical use.

3.1. Water Recycling and Reuse in Stationary Military Bases and Mobile Units

Large systems and plant for the treatment and improvement of drinking water quality, WW treatment, and desalination are installed in the stationary military bases and camps of NATO troops. By combining different membrane processes (FO/RO, UF/RO, MD/RO, ED/RO, etc.), many contemporary armed forces around the world have modernized and standardized their water treatment systems in the last decade to increase their capacity and reduce energy and financial costs. Depending on the degree of contamination and the required purity of the water, three purification stages are used. Media filters, bag filters, activated carbon filters and sedimentation tanks are among the filtration and sedimentation processes used in the initial stage, known as pre-treatment. The main treatment of the water stream is represented by the second stage, which could include a combination of membrane or hybrid technologies or a single process such as RO, MD or UF. To replace the vital minerals lost during the RO process, the final, third stage of treatment involves necessary UV sterilization, chlorination, pH correction and remineralization [71,72,73,74,75]. RO water purification units (ROWPU), General Dynamics, Taunton, MA, USA, with a capacity of more than 10.0 m³ h⁻¹ of water, are installed within fixed military facilities when water is not available from public water supply sources. ROWPU systems are enhanced with other purification techniques such as UF and adsorption on activated carbon, all of which are carried out in multi-stage processes based on a desired level of purity of the final product. Processes like UF and NF, electro deionization systems (EDI), advanced oxidation processes (AOP), UV sterilization and ozonation may further enhance purification in healthcare facilities where ultra-pure water is essential for operating procedures. It is crucial that the energy efficiency of the purification process matches the demand for clean, potable water. For instance, air-delivering bottled drinking water to troops stationed in desert, or mountain and highland regions during peacekeeping missions in Iraq and Afghanistan in the early 2000s was neither practical nor sustainable from a financial perspective. In order to supply the clean water to friendly units, the technical units were ordered to start digging wells. Mobile purification devices were subsequently installed at these sites to remove dust and sand. The price of the water made the initial investment worthwhile, and the risk of airstrikes on coalition forces was eliminated [14,76]. RODI Systems’ CemPureTM ceramic membrane bioreactor (CMBR) RODI Systems Corporation, Aztec, NM, USA, is another advanced stationary-modular vessel technology that can be applied and utilized in the military system. The CMBR employs continuously aerated, submerged flat sheet ceramic membranes to minimize fouling and is designed for the treatment of wastewater streams at bases. In a bioreactor, the system uses UF membrane separation (pore size 0.1–0.5 μm) along with biological treatment to separate inorganic materials and break down organic contaminants [76,77,78]. In addition to large stationary and modular systems, NATO forces are developing and implementing rapidly deployable water purification systems. A small portable military water purification system known as the Lightweight Water Purifier (LWP), MECO Incorporated, LA, USA was developed to provide military forces deployed to difficult or isolated locations access to pure drinking water, with a flow rate of approximately 0.470 m³ h⁻¹. Its rapid implementation and user-friendliness enable it to effectively purify water from a variety of sources, such as lakes, rivers, and even polluted supplies, ensuring consistent availability to potable water in the field. This system uses a two-stage membrane process: a spiral-wound RO membrane is used to remove dissolved components from the water stream, while UF is used to remove larger suspended particles, sediment, and bacteria. Seawater, contaminated natural water, and water contaminated with CBR chemicals may all be effectively purified using the LWP. The LWP is extremely durable, can withstand both high and low temperatures, and is resistant to vibration during transportation. The use of mobile LWP systems during operations in Iraq led to a significant reduction in financial and logistical costs. The average price of water purification by LWP was 0.0185 US$ L⁻¹, compared to 1.32 US$ L⁻¹ for supplied bottled water [75,79]. The Croatian Armed Forces operate the ROWPU TWPS 1500 GPH (Reverse Osmosis Water Purification Unit) in accordance with the NATO capability target MILENG-WAT-TM (Military Engineering Water Purification Team). This system consists of RO membranes assembled in hermetically sealed spiral modules and is resistant to CACs (controlled atmosphere covers). Its main goal is to support the armed forces and the general public in reducing the effects of natural disasters and chemical spills into aquatic ecosystems. Furthermore, the ROWPU system used by the Croatian Armed forces enables military operations in the field to utilize a range of water sources to provide safe drinking water. Using a water source with a maximum of 1000 ppm of TDS, this system generates 5.680 m³ h⁻¹ of water. The production capacity drops to 4.543 m³ h⁻¹ if the feed has more than 35,000 ppm TDS, which is typical for seawater. The ROWPU can be implemented in mobile systems placed on trucks or trailers and transported by rail or road, or it can be installed at a stationary site. The incoming raw water passes through a multi-stage pretreatment system designed to remove suspended solids, prevent scale formation, and protect the RO membranes from chemical damage. At the intake point, screens and multilayer sedimentation filters are installed to eliminate fine sand and silt (50–100 µm). In cases of turbid surface water (ponds, lakes, rivers), the ROWPU system includes a flocculant dosing module that neutralizes the charge of colloidal particles. The integrated ROWPU system also features a bisulfite dosing module that acts as a dechlorination agent, reducing chlorine to chloride, thereby effectively protecting polyamide membranes prior to MF/UF filtration. The separation process continues with microfiltration and ultrafiltration, which remove turbidity and most bacteria larger than 0.1 µm, with continuous monitoring of transmembrane pressure and implementation of backwash procedures. When treating wastewater generated during decontamination processes, it is necessary to remove free chlorine prior to RO treatment. If the water source contains high concentrations of specific organic contaminants, a granular activated carbon (GAC) adsorption process is conducted before RO treatment. This comprehensive pretreatment process ensures the production of filtered, chemically conditioned, and microbiologically controlled water (Figure 3) [80,81].

In the second phase, the treated water from the separator is fed into the RO module, the system pressure is increased and based on the principles of RO, separation of pollutants or CBR agents is carried out. Semipermeable membranes retain dissolved salts and contaminants such as heavy metals, organic molecules, microbiological agents, and toxins. The permeate contains purified water and a small percentage of residual salts. In the case of thin-film composite (TFC) polyamide membranes, typical salt rejection rates range from 95% to 99%, meeting the standards for potable water (<500 mg L⁻¹ TDS). The ROWPU system features a self-flushing membrane design activated after the purification process. The system utilizes a portion of the clean permeate to rinse the RO modules at a lower pressure, effectively removing concentrated particles and residual salt crystals from the membrane surfaces. The output RO permeate is of very high purity and can be discharged into the environment as such. However, if the treated water is intended for use as drinking water, post-treatment is essential to meet legal standards for quality and microbiological safety. Water disinfection is carried out either by chlorination, with a residual chlorine concentration of 1–2 mg L⁻¹ (in accordance with military standard TB MED 577), or by UV sterilization using ultraviolet lamps. In addition to these processes, remineralization is performed since RO permeate contains less than 10 mg L⁻¹ of minerals. This is achieved by filtering the water through a calcite or dolomite filter, which restores alkalinity and hardness to approximately 60 mg L⁻¹ and adjusts the pH to a range of 7–8 [81,82,83]. CBR agents are removed by multi-stage filtration, adsorption and RO, although these methods are not effective for all types of agents. RO effectively rejects chemical agents (warfare agents) through TFC-PA membranes. However, certain low molecular weight organic compounds can still pass through the membrane and be found in the permeate stream. Additionally, RO is effective in removing radionuclides from contaminated water, depending on their type and concentration. Further separation by adsorption or ion exchange is required, as the radionuclides Cs-137 and I-131 dissolved in the water could potentially pass through the membrane [84,85]. In terms of biological agents, the UF system efficiently eliminates 99% of the polluted feed water’s bacteria, fungi, viruses and biological pathogens. To ensure effective disinfection of toxic bio-agents, additional treatments such as UV disinfection or chlorination must be carried out after the osmosis process [86,87].

Furthermore, Croatia military forces supported the Haitian population following the devastating earthquake by participating in the UN MINUSTAH mission between 2010 and 2011. To support the local residents and provide them with drinking water, UN member states deployed mobile RO/UF equipment to purify water from nearby rivers and wells. In addition, more than 1500 mobile water supply units and community water tanks were installed in the reception centers, particularly following the first outbreak of cholera cases in October 2010. The Tactical Water Purification System (TWPS) devices, Aqua-Chem, Knoxville, TN, USA, have proven to be extremely efficient in producing field water during NATO deployments and international military exercises. For example, the water supply logistics units used field-deployable mobile equipment to produce sufficient drinking water to supply all deployed military personnel during the NATO military exercise Saber Strike 2018 [88].

3.2. Water Recycling and Reuse in Military Naval Vessels and Submarines

Conventional methods for the purification and desalination of brackish and sea water for drinking included a classical thermal distillation unit. Multi-stage flash distillation (MSF) was used on large warships, such as aircraft carriers, and submarines that had exceptional energy sources [89]. In this process, the water is heated at high temperatures and then fed into a low-pressure chamber where rapid evaporation takes place. After condensation of the water vapor, purified drinking water would be collected. Diesel generators or small nuclear reactors on submarines were required to carry out such processes, whereas on smaller ships and submarines that did not have such high energy sources, vacuum distillation was used, which required lower operating temperatures and pressures as well as lower energy consumption [90]. At the beginning of the 21st century, thermal desalination processes in naval ports, ships and submarines were replaced by more energy-efficient and modern membrane processes, of which RO is the most widely used. With the introduction of RO systems, the desalination process became more efficient, economically viable, and more flexible in rapid response situations. In addition to lower maintenance costs for the system and membranes, high process efficiency and fuel savings, RO devices are significantly smaller and more compact, offering a better space-saving solution for limited ship and submarine environments [91,92]. Modern RO devices installed in NATO submarines (SUBRO) are upgraded with high-pressure pumps that produce minimal noise and must be able to operate under high external pressure. They are also upgraded with an energy recovery mechanism that reduces the load on the generators. Notably, legacy energy recovery system designs predating 2000 were capable of reducing generator load by approximately 40%; however, modern iterations achieve comparable efficiency with enhanced operational stability, reduced maintenance requirements, and integration with smart grid protocols [93]. Research within the military sector continues to advance the energy efficiency of desalination technologies. Under the Expeditionary Unit Water Purification (EUWP) initiative, prototype shipboard reverse osmosis (RO) desalination systems have been developed, achieving a 65% reduction in energy consumption, 40% lower mass and volume, and 75% reduced maintenance demands compared to legacy systems. Furthermore, these systems demonstrated double the water recovery efficiency: while conventional systems typically yield a 20% recovery rate (e.g., 0.757 m³ of permeate from 3.785 m³ of seawater), the enhanced module achieves approximately 40% recovery from the same feed volume [94,95]. In submarines, RO is often combined with UF methods and ozonation/chlorination to recycle and recirculate used water through the system at rates exceeding 80%. The FlexOsmosis project demonstrated the positive impact and improved safety of naval operations when FO was integrated into submarines alongside RO. In the event of an attack and hull breach, or in emergency situations where the RO system fails, FO would take over the role of desalination and wastewater treatment without additional generated power (it operates solely on natural osmotic pressure) [96]. On ships, RO and UF systems are integrated for obtaining drinking water from seawater and for ballast water management. These advanced systems can produce up to 1.250 m³ h⁻¹ for crew purposes and for operational propulsion processes. In addition to the aforementioned membrane technologies RO and UF, other methods for wastewater stream treatment, such as MBR systems, are also integrated into naval military bases. All of these systems are now digitalized and controlled by autonomous, intelligent servers incorporating artificial intelligence (AI). In recent years, advanced, highly optimized shipboard RO desalination units, MECO MN-100, MECO Incorporated, Mandeville, LA, USA, have been developed for the needs of the naval forces and as a part of the Future Naval Capability (FNC) program. These units have a capacity to desalinate seawater to potable and ultrapure water for special purposes of more than exceeding 15.417 m³ h⁻¹. The improved membrane systems are resistant to chlorine, temperature fluctuations and seawater turbidity and have an automatic cleaning and backwash function for sediment removal. The resistance to chlorine was attained by substituting traditional thin-film composite (TFC) membranes with a polyamide layer, which is highly susceptible to chlorine, with modified polymeric membranes featuring aromatic functional groups that exhibit chlorine resistance, or by employing specialized coatings such as TiO₂ or nanoparticles integrated within the membrane’s active layer [15]. The units are constructed from titanium and high-alloy steel, are lightweight and corrosion resistant and fireproof. They are fully automatic and more than 75% energy efficient than existing systems [97].

3.3. Treatment of Chemically, Biologically and Radiologically (CBR) Contaminated Water

Water contaminated with chemical warfare agents, radionuclides, or biological pathogenic agents poses a major challenge for treatment and purification. Such types of water can be generated either through intentional or accidental releases (ROTA—Release Other Than Attack) into environmental components or after the implementation of decontamination procedures for personnel, equipment or vehicles. MTs within the military system, apart from being used for providing drinking water and municipal purification, have been investigated as practical methods for treatment in the case of CBRN contamination [98]. Chemical warfare agents, such as nerve agents sarin, soman, or VX, are structurally classified as organophosphorus compounds and can be partially removed from aqueous solutions through nanofiltration (NF) for larger molecules (e.g., VX) or reverse osmosis (RO). However, removal efficiency varies depending on the physicochemical properties of the agents, such as ionization, polarity, and molecular weight. Therefore, complete decontamination necessitates the application of combined methods [99]. For instance, blood agents, due to their low molecular weight and polarity, can be separated via RO membranes at pH 9 through ionic repulsion, but further removal requires electrodialysis (ED) with ion-selective membranes or activated carbon adsorption. Blister agents (e.g., lewisite) exhibit pronounced hydrophobicity and are selectively rejected by RO membranes. However, their viscosity and structural stability lead to rapid membrane fouling, necessitating frequent cleaning and the implementation of hybrid systems (e.g., combining RO with enzymatic degradation or AOP). In addition to agents defined under the Chemical Weapons Convention (CWC), military systems also analyze the risks posed by toxic industrial chemicals (TICs) as improvised chemical weapons. While NF and RO effectively remove many of these substances, complex compounds with variable solubility or reactivity demand a combination of membrane technology with chemical oxidation (e.g., Fenton reaction) or biological remediation [100,101,102,103]. Technologies like UF, NF, RO, and ED are implemented in military applications for eliminating biologically harmful bacteria and the toxic by-products of their metabolism. Bacteria including Salmonella typhi, Legionella sp., Escherichia coli, coliform bacteria, as well as viruses, are effectively removed by UF. Larger exotoxins, such as botulinum toxin, can also be effectively removed using this method, while smaller endotoxins are harder to effectively retain by UF. More efficient membranes for removing these endotoxins are membranes with pores smaller than 1 nm, i.e., TFC-PA NF and RO membranes. These membranes can reduce the concentration of toxins by 99% via adjusting to high pH values due to hydrophobic interactions and electrostatic repulsion. The combined NF and RO approach can be replaced by EDF for further elimination. MBR are used at large stationary military sites for biological treatment, biodegradation of organic material, and harmful microorganism suppression [104,105,106]. The most commonly applied MTs in practice are RO, NF and ED for the treatment of radionuclide-rich water. Radionuclides like Cs137 and I131 are extensively rejected by membranes due to their ionic form and size, with a removal factor of 99% [107]. Effluents contaminated with CBR are not commonly encountered in practice, but in order to ensure effective treatment of this type of contamination, multi-stage treatment using membrane separation processes in combination with certain chemical processes is required. The process of thorough, multi-stage treatment of CBR-contaminated water is shown in Figure 4. Within the pre-treatment stage, coarse filters and sedimentation units are used to remove larger particles from the wastewater stream. UF is applied for the removal of pathogenic microorganisms and larger toxins, while NF removes larger organic compounds and heavy metal-containing compounds. For radionuclides, low molecular weight endotoxins and chemical warfare agents, RO offers a highly effective separation technique. As a final treatment step, adsorption on activated carbon or zeolite can be used to remove smaller hydrophilic molecules and unpleasant odors, along with chlorination or UV sterilization to ensure additional disinfection [108,109].

The resulting treated effluent obtained by such multi-stage processes would meet strict standards for water quality. The NATO standard „AMedP-4.9 Requirements for water quality during operations“ prescribes the appropriate standards that all member states must meet with regard to the quality of water in terms of physical, chemical, microbiological and radiological limits for specific pollutants, biological agents and their by-products, and radionuclides. Aforementioned NATO’s minimum standards for contaminants, pathogenic microorganisms, and radionuclide exposure in emergency situations are presented in

Table 3. Minimum standards for contaminants, pathogenic microorganisms, and radionuclide exposure in emergency situations (for water consumption up to 7 days).

Constituents	Unit	Min. Emergency Standards	Potential Health Effect
Phisical
colour	CU/cobalt-platinum method	50	Risk of dehydration due to reduced water consumption caused by decreased palatability; symptoms of dehydration include weariness apathy, impaired co-ordination, delirium, heat stroke.
turbidity	NTU	1	Risk of dehydration due to reduced water consumption caused by decreased palatability. Mostly gastro-intestinal effects due to presence of pathogenic microorganisms, caused by decreased disinfection efficiency.
conductivity	µS cm−1	1500	Risk of dehydration due to reduced water consumption caused by decreased palatability.
pH	-	5–9.5	More corrosive activity on lower pH and decreased disinfection efficiency at higher pH.
Microbiological
Escherichia coli	CFU 100 mL−1	0	Mostly gastro-intestinal effects due to presence of pathogenic microorganisms. Symptoms: dehydration, abdominal cramps, diarrhea, vomiting, bloating, high fever, HUS syndrome, etc.
coliform bacteria	CFU 100 mL−1	0	Gastrointestinal infections, diarrhea, vomiting, abdominal pain, urinary tract infections, hemolytic syndrome caused by Shiga toxin, etc.
Chemical
cyanide	mg L−1	6	Headache, breathlessness, weakness, palpitation, nausea, vomiting, giddiness, tremor, rapid heartbeat, dizziness, confusion, anxiety, agitation, cardiac arrhythmias, seizures, stupor, coma.
arsenic	mg L−1	0.3	Facial swelling, vomiting, loss of appetite, abdominal pain, diarrhoea, shock, muscle cramps, headache, chill, cardiac abnormalities, anaemia, decreased white blood cell count, enlargement of liver, delayed effects including sensory and motor peripheral polyneuropathies.
sulphate	mg L−1	300	Laxative effect that can lead to symptoms of dehydration including weariness apathy, impaired co-ordination, delirium, heat stroke.
inorganic mercuric compounds	mg L−1	0.003	Mercury compounds mainly have health effects on the kidney and the central nervous system.
lewisite (arsenic fraction)	mg L−1	0.080	Nausea, vomiting, diarrhoea, abdominal pain, intense thirst, weakness, hypotension, hypothermia.
sulphur mustard	mg L−1	0.140	Nausea, vomiting of blood, diarrhoea, abdominal pain, fever, headache, cardiac arrhythmias, dizziness, malaise, loss of appetite, lethargy, convulsion, leukopenia, anemia, immumosuppression.
nerve agents	mg L−1	0.012	Nausea, vomiting, diarrhea, abdominal cramps, headache, giddiness, dizziness, excessive salivation, tearing, miosis, blurred or dim vision, difficult breathing, cardiac arrhythmias, loss of muscle coordination, muscle twitching, random jerking movements, convulsions, coma.
T-2 toxins	mg L−1	0.026	Nausea, vomiting, diarrhea, generalised, burning erythema, mental confusion.
Radiological
alpha, Pu239	activity limit, Bq L−1	28,500	Nausea, vomiting, diarrhea The standard of each type of radiation correspondents with an exposure of 250 mSv.
beta, St90		255,000
gamma, I131		300,000

[69].

Until few years ago, the military system lacked a complete and integrated mobile treatment and purification unit for CBR-contaminated water generated during decontamination. In addition to the military-relevant contaminants, these waters are also heavily contaminated by chlorine-based decontamination agents (e.g., calcium hypochlorite, sodium hypochlorite, etc.) and the by-products of the hydrolysis of additional agents. This type of water was not purified at its point of origin; but was collected in specialized reservoirs and transported for further treatment, which entailed additional costs and a potential risk of leakage. The aforementioned ROWPU system is designed to remove wide range of contaminants, including CBR agents. However, a prototype field device called Decontamination effluent treatment system (DETS) was constructed few years ago. This device treats the contaminated water generated after decontamination within the actual operational area and at the point of origin with a capacity of over 2.1 m³ h⁻¹. The operating principle of the DETS is a multi-stage treatment, in which the first stage involves sedimentation in a settling tank, along with coarse filtration with a sand filter to remove large suspended solids from the wastewater. In the second stage, granulated activated carbon is used to adsorb surfactants and organic compounds such as oils and petroleum derivatives. The third stage consists of a six-stage RO process to remove organic pollutants, dissolved salts, radionuclides, and endotoxins. Retentate is recirculated back into the system, thereby enhancing the reduction of waste by-product. Through this treatment process, more than 80% of the initially used decontamination water is recirculated, reducing the overall demand for water resources. It also effectively reduces turbidity, water hardness and total chlorine by 100%, surfactant concentration by 98.7%, total organic carbon by 98.0%, and organophosphate insecticides malathion and cesium radionuclides by 100%. The aforementioned system is already being upgraded to a PFAS Effluent treatment system by replacing the granular activated carbon with ion exchange resins, to enable the removal of PFAS. These substances are widely used in the military sector in fire suppression liquids and foams, as well as for improving the binding properties of rocket propellants (viton) and stability of missile systems (teflon). PFAS compounds are also components of cleaning agents and maintenance products for military equipment. Their resistance and persistence to environmental conditions and degradation make them extremely hazardous if released into environment. Through biomagnification, they accumulate through the food chain and readily bioaccumulates in soil, sediments, and water. In living organisms they cause various physiological disorders and are highly carcinogenic. For this reason, the membrane systems for the separation of CBR agents and toxic industrial chemicals need to be upgraded with processes for the removal of these specific pollutants. Additionally, hybrid membranes were also tested as part of the upgraded RO system. To further improve the removal of toxic agents, graphene oxide and chitosan membranes have been developed [110,111]. The integrated hybrid container system from German manufacturer Kärcher Futuretech, WTC 8000/15000 UF/RO C, is an advanced dual-purpose water purification and treatment system adapted to conditions of potential CBR contamination. This system uses UF and RO for obtaining drinking water with a capacity of 8.0 m³ h⁻¹ and freshwater treatment up to 15.0 m³ h⁻¹. It is widely used in NATO field exercises to ensure potable water supply in CBR-contaminated environments, as well as for decontamination of personnel, vehicles, and equipment, and for recirculation and recycling of the resulting contaminated water. The WTC membranes are robust and durable, resistant to salinity, turbidity, corrosive effects of chemical warfare agents, and fouling caused by biological agents and radionuclides. In addition to military applications, it is also used in peacekeeping operations under the direction of the United Nations to supply of drinking water to the affected civilian population [112].

4. Innovations and Future Trends in Membrane Technologies

MTs in industrial applications of obtaining drinking water by desalination of sea water and purification of drinking water, as well as in the treatment of CBR-contaminated water are being technologically improved and upgraded on a daily basis. An innovative approach to solving problems such as membrane fouling, reduction of economic process costs, higher efficiency, improved robustness, operational autonomy and modularity has enabled a more comprehensive approach and wider application in human activity sectors [113,114]. Within the military system, on-site water availability and self-sufficiency, as well as independence from logistical supply, are key elements for the successful fulfillment of assigned unit missions. Real challenges identified after the long-term implementation of MTs in the military system are related to real-time detection. For greater system autonomy and monitoring of the separation process quality, innovative sensors for continuous water monitoring are being developed and integrated into the membrane filters. These IoT (Internet of Things) sensors are then integrated with detection systems of stationary or mobile units in the field or within submarines and ships [115]. An additional challenge is presented by the customized modular approach and optimization of the membranes used for different types of water: seawater and brackish water, municipal wastewater, hydrocarbon-contaminated water, CBR-contaminated water, etc. The modern approach to improving the selectivity and energy efficiency of membranes is closely related to the development of nanohybrid, graphene and graphene oxide membranes. Since carbon is present in nearly every component of the environment, the supply of raw materials is limitless [116]. With a surface area of 2700 m² g⁻¹, graphene is also incredibly light, chemically inert, brittle and flexible, and about 200 times stronger than steel and 1000 times more electrically conductive than copper. By layering several two-dimensional nanosheets on top of each other, graphene oxide membranes have a unique mechanical strength. Significant separation efficiency, e.g., in desalination processes (over 99%), is achieved by modifying the exact distance between the layers of graphene oxide sheets depending on the pH value of the solution, graphene oxide sheet size, or the intercalating agent. Due to the minimal material consumption, the application of graphene oxide membranes represents an economical and sustainable approach [117,118,119]. In membrane-based separation processes for biological agents from wastewater streams, graphene oxide coatings enable antimicrobial activity and the inhibition of over 99.9% of microorganisms [120]. Recent research conducted by a team of scientists from the Institute of Technology, Republic of China, includes the use of superhydrophilic photothermal membranes composed of graphene-modified titanium dioxide nanotubes (Gr-TiO₂) for application in interfacial solar-driven water evaporation (ISDWE) processes. This module utilizes the solar photothermal effects and unlimited seawater resources for the production of fresh, potable water. Such technology provides an innovative complementary process to the standard RO system used by naval forces, offering a more environmentally friendly and cost-effective version of seawater desalination, as well as a system for wastewater recycling [121]. Other nanomaterials are also used for water disinfection, such as silver nanoparticles and carbon nanotubes (CNTs) immobilized on membranes, which cause radical damage to a wide spectrum of gram-positive and gram-negative bacterial cells as well as coliform bacteria. Additionally, specialized nanofilters are being developed from poly (ε-caprolactone)—based polyurethane nanofiber mats containing Ag nanoparticles for effective water disinfection following physico-chemical and biological treatment [122]. Membranes can also be functionalized with specific enzymes that catalyze the degradation of targeted pollutants. Bioremediation of organophosphate compounds such as the pesticides TEPP, parathion, malathion and CWA nerve agents sarin, cyclosarin, soman, tabun, VX and DFP can be carried out using decontamination enzymes incorporated on the membrane surface. Enzymes that have been shown in scientific research to be particularly effective in degrading OPs include: organophosphate hydrolase (OpdA) isolated from Agrobacterium radiobacter, phosphotriesterase (PTE) isolated from Brevundimonas diminuta, Flavobacterium sp., and Deinococcus radiodurans, diisopropylfluorophosphatase (DFPase) isolated from Loligo vulgaris, paraoxonase (PON1), a component of human liver, and organophosphate acid anhydrolase (OPAA) isolated from several Alteromonas bacterial strains (Alteromonas undina and Alteromonas haloplankts) [123,124]. Enzyme-immobilised biocatalytic membrane reactors (BMRs) are used to degrade chemical warfare agents based on synthetic organophosphates. The hyperthermophilic archaeon Sulfolobus solfataricus produced the thermostable mutant enzyme phosphotriesterase (SsoPox), that was then covalently bound to polymeric MF membranes made of polyethersulfone (NSG-PES) and PVDF in a continuous BMR. The herbicide paraoxon is hydrolyzed by this immobilization process. By preventing the disintegration of the neurotransmitter acetylcholine in neuronal synapses by the enzyme acetylcholinesterase (AChE), paraoxon functions similarly to conventional nerve agents. This causes overstimulation of the receptors at the nerve endings, which has very negative effects on the nervous system and the structure of the brain. Several studies have shown that the paraoxon conversion rate in the continuous BMR reached 90% at each passage through the biocatalytic membrane. Additionally, the immobilized enzyme demonstrated higher stability of up to 10 months compared to the free enzyme in the system [125,126]. In recent studies, advanced porous membrane materials such as porous organic polymers (POP), porous aromatic frameworks (PAF), covalent organic frameworks (COF), and metal-organic frameworks (MOF) have shown promising performance in the treatment of radionuclide contaminated water. In contrast to conventional materials such as activated carbon, clays, resins and carbon nanotubes, which have low selectivity, limited adsorption coefficients and low kinetics, among others, the new materials have proven to be highly effective due to their selectivity, high specific surface area, broad pore structure spectrum, and exceptional stability [127]. For the removal of radioactive elements from water, RO membranes with immobilized crown ethers (15C5 and 18C6), obtained by interfacial polymerization, have shown promising results in research. MPD@15C5-TMC and MPD@18C6-TMC membranes demonstrated excellent pure water permeability and pronounced rejection of Cs137, Sr90, and Co60, confirming the high potential for the practical application of RO-modified membranes in the treatment of radionuclide-rich water [128]. Progress in membrane processes also relates to the use of self-healing antifouling membrane materials impregnated with hydrogel coatings to extend their operational lifespan. Zwitterionic membrane materials are a special type that contains both cationic and anionic groups with an overall neutral charge. Zwitterionic hydrogel has the ability of rapid self-healing based on the principle of electrostatic attraction between cationic and anionic functional groups forming pairs, thereby improving hydrogel longevity, reducing operational costs, and increasing sustainability in the use of new membranes. The self-healing efficiency of formed cracks can reach up to 90% [129]. The Science and Technology Organization (STO) within NATO framework actively leads antifouling projects using nanocoatings (graphene, graphene oxide, silver) and explores their application in the protection and enhancement of existing membrane systems, desalination equipment, underwater electronic devices, surfaces on ships and submarines, and similar. Core innovations refer to the aforementioned zwitterionic polymer coatings, coatings with biomimetic microstructure (microstructures of lotus flower, shark skin, reed leaf, beetle, etc.), self-assembling coatings made of chitosan and dialdehyde starch, conductive antifouling coatings, and photocatalytic coatings [130,131]. Modernization of MTs with respect to fouling issues and costly maintenance can be addressed by using spiral-wound module (SWM) in RO processes, according to recent studies. This configuration enables compact operating modules in which the use of spacers between the membranes, promotes fluid mixing, resulting in a reduction in concentration polarization (CP) and an increase in permeate flux [132]. In the field of automation, optimization, and process control within the military system, intelligent sensors are integrated for continuous monitoring of separation efficiency and the quality of feed, permeate and retentate streams. Supervisory control and data acquisition (SCADA) has been used for many years for real-time management of water treatment processes, data acquisition and remote monitoring via parameters such as salinity, turbidity, pH, free chlorine and similar. This system has proven to be ideal for field operations and mobile bases as it is fully automated and does not require any particular specialization from operators and crew. Based on IoT sensor data, Programmable logic controller (PLC) systems automatically adjust the process (chemical dosing, system pressure generated by pumps, etc.) depending on process conditions. The system is monitored by operators from the SCADA control center and is connected to a web interface, allowing remote access from any smart device. Experion HS SCADA is used for the comprehensive control and monitoring of seawater desalination, potable water distribution and wastewater treatment on Nimitz and Gerald R. Ford class aircraft carriers In addition to these processes, SCADA also manages other protection systems on the ship and performs real-time diagnostics. In practice, such automated systems, which operate 24/7, have demonstrated exceptional performance in terms of efficiency, safety and resilience. Additionally, ABB Freelance/800xA SCADA is used for the control and automation of desalination and water drainage processes as part of the KFOR operation in Kosovo [133,134,135].

5. Conclusions

One major issue affecting the entire globe nowadays involves the shortage of drinkable water, which necessitates the implementation of contemporary technological solutions for treating existing sources like wastewater and seawater. In this regard, MTs are vital for ensuring a dependable and superior water supply. Among them, RO is the most widely used method for desalination and treating challenging water compositions. MTs are frequently employed in military applications, including as desalination and water purification on ships and submarines, producing drinking water in the field with mobile units, and treating water tainted with chemical, biological, and radioactive contaminants. While stationary bases also use MBR, UF, and MF, mobile systems are usually based on RO, NF, and ED. A multi-stage strategy is required in demanding situations like treating water contaminated with biological pathogens or chemical warfare agents, where MTs serve as the main component of integrated systems when combined with physico-chemical treatment techniques. Future advancements in military MTs will concentrate on improving robustness and sustainability, decreasing reliance on logistical supply networks, and boosting operational autonomy and energy efficiency. Graphene-based membranes, membranes functionalized with enzymes, biocatalytic membrane reactors, self-healing antifouling materials, nanotechnology, IoT-enabled sensors, and real-time automation and monitoring of membrane processes are examples of innovative approaches.