Alternative and Sustainable Technologies for Freshwater Generation: From Fog Harvesting to Novel Membrane-Based Systems

Abstract

Water scarcity is an escalating global challenge, driven by climate change and population growth. With only 2.5% of Earth’s freshwater readily accessible, there is an urgent need to explore sustainable alternatives. Textile-based fog collectors are advanced tools which have shown great potential and have gained remarkable attention across the world. This review critically evaluates emerging technologies for freshwater generation, including desalination (thermal and reverse osmosis (RO)), fog and dew harvesting, atmospheric water extraction, greywater reuse, and solar desalination systems, e.g., WaterSeer and Desolenator. Key performance metrics, e.g., water yield, energy input, and water collection efficiency, are summarized. For instance, textile-based fog harvesting devices can yield up to 103 mL/min/m², and modern desalination systems offer 40–60% water recovery. This work provides a comparative framework to guide future implementation of water-scarcity solutions, particularly in arid and semi-arid regions.

1. Introduction

Water scarcity is a pressing global crisis that demands immediate attention. Despite advancements in technology and the development of alternative water resources, a comprehensive solution remains elusive for many regions. Innovative approaches can leverage urban economies to create water systems that offer convenience, easy access, sustainability, and affordability, making clean water services more widely available [1,2,3]. Today, over 2.3 billion people lack access to clean, safe drinking water, and every day, more than 18,000 people die due to water-related issues, primarily from unsafe drinking water and poor sanitation [4]. This burden falls disproportionately on poor women and children, who spend hours daily walking long distances to collect water from unsafe sources, e.g., puddles or contaminated rivers. In some regions, they walk for miles along treacherous paths, often carrying up to 40 pounds of water back to their villages while caring for young children, making the task not only physically demanding but perilous [5]. The gravity of the situation is illustrated in Figure 1, which presents a schematic overview of global water resources and highlights the disparity between available water sources and access to safe, clean water.

According to UN Water’s World Water Development Report, global water demand is projected to increase by 40% between 2030 and 2050, exacerbating water scarcity in many regions. Climate change further intensifies this crisis by increasing the frequency and severity of droughts and storms, while rising temperatures elevate water demand in agriculture and livestock [6]. This creates an urgent need for alternative water resources. Technologies, e.g., rainwater harvesting, water reuse, and desalination, are gaining traction as vital solutions to meet global water needs [7,8,9]. These methods provide essential alternatives to relying on a single source of supply, especially in regions heavily impacted by climate variability. Currently, 97.5% of the water on Earth is salty, with only 2.5% being freshwater. This small portion is the source of our drinking water, obtained from natural resources like glaciers, lakes, groundwater, and rivers, as illustrated in Figure 2.

A portion of drinkable water exists in less visible forms, e.g., fog, dew, rain, and water vapor, within the atmosphere. These sources represent viable alternative water resources that can be harnessed without requiring significant energy inputs. For example, technologies for fog and dew harvesting can tap into these resources efficiently [10,11,12,13,14]. Globally, agriculture accounts for approximately 70% of total freshwater withdrawals, followed by industry (19%) and municipal use (11%), according to Food and Agriculture Organization of the United Nations data. In many low-income and arid countries, the share of agriculture can exceed 90%. Water reuse is already being practiced in several regions worldwide. In southern Europe and North America, the majority of reused water supports irrigation-related applications, e.g., landscaping and golf course irrigation [15]. Meanwhile, industrial water reuse is more prevalent in Asia and northern Europe, while urban reuse is being adopted in parts of Asia (Korea and Singapore) for activities that involve lower-quality water [16]. Several studies have aimed to develop eco-friendly and sustainable technologies that identify alternative water sources using renewable energy [17,18,19]. These efforts also include creating artificial water bodies in arid regions or transporting water to remote rural communities. This review provides a comprehensive overview of the potential for drinkable water derived from alternative resources, focusing on areas experiencing acute water scarcity. The collected water from these alternative sources can be utilized for both drinking and irrigation purposes. The primary objective of this review is to highlight advanced technologies capable of generating potable water in regions facing severe water shortages. The final objective is to propose the most suitable techniques, tailored to specific locations and climatic conditions, to effectively meet local water demands.

This review is prepared by systematically screening the recent and relevant literature on alternative freshwater generation technologies. Articles were retrieved primarily from Web of Science, Scopus, and ScienceDirect, with a focus on “fog harvesting”, “dew collection”, “desalination”, “atmospheric water harvesting”, and “textile-based collectors”. The timeframe considered was mainly 2000–2025 to capture both classical and most recent advancements, with priority given to peer-reviewed journal articles, highly cited studies, and reports from international organizations. Publications focusing on experimental data, comparative analyses, and technological innovations were emphasized, while non-peer-reviewed sources were only considered for background or emerging devices, e.g., WaterSeer.

2. Alternative Water Resources

2.1. WaterSeer

WaterSeer device (designed by VICI-Labs from the University of California at Berkeley in cooperation with the humanitarian organization “Peace Corps Association”) pulls moisture from the air with a wind turbine [20]. One conceptual approach to atmospheric water harvesting involves the use of wind-driven condensation chambers like the experimental WaterSeer device. These systems aim to condense water vapor from ambient air using underground cooling chambers. However, scientific evaluations of such technologies remain scarce. In reality, the effectiveness of soil-based cooling is constrained by the fact that subsurface temperatures (~2 m depth) remain close to the average ambient air temperature, meaning condensation is not always favorable. Additionally, as water condenses, it releases latent heat, potentially warming the reservoir and reducing yield over time [21]. Reports on the WaterSeer system’s performance are largely circumstantial, and concerns have been raised online about its scientific credibility. Therefore, while the idea is theoretically interesting, further peer-reviewed research is essential to validate its practical viability and efficiency [22]. The mechanism of this system is to produce cost-effective fresh water from atmospheric air with high efficiency, as shown in Figure 3.

One more similar example of WaterSeer is the airdrop irrigation system, built in Australia (currently under development), which contains a small airstream turbine at the inlet of the setup. This is spun by the airstream and drives air through a vertical duct into a copper condenser pipe positioned under the soil, where the temperature of the air rapidly falls and moisture condenses [23]. But the limited moisture extraction from atmospheric air makes this device less efficient. Even the maintenance of the system is quite costly.

2.2. Desalination

Some published projections warn that by 2050, more than half of the global population could be living in water-stressed areas [24]. With 97.5% of the Earth’s water found in oceans, saturated with salt and undrinkable, finding ways to tap into this vast resource is essential. If a portion of seawater could be filtered and made drinkable, we would have access to an unlimited supply of fresh water. This is where desalination, the process of converting saltwater into freshwater, becomes a key solution. Desalination, in principle, mirrors the natural water cycle, where solar energy heats ocean water, causing it to evaporate and later return as precipitation. Modern desalination technologies are engineered to replicate this process, primarily through two methods: thermal desalination and reverse osmosis. Thermal desalination is one of the oldest methods, which works by boiling seawater and capturing the steam, which is condensed into freshwater. This process is widely used on a large scale in two main forms, i.e., multi-effect distillation (water is boiled in a series of vessels under decreasing pressure to generate steam, which is then condensed into freshwater) and multi-stage flash (seawater is rapidly heated and converted into steam by introducing it into lower-pressure chambers, then condensed to produce fresh water). Multi-effect distillation (MED) operates by evaporating seawater in a series of three or more chambers, each containing heat pipes. In this process, seawater is heated to produce steam, which is then transferred to the heat pipes in the next distillation chamber. As the steam moves through the chambers, it progressively heats the seawater in each subsequent stage, resulting in continuous evaporation. Desalinated water is collected as condensate from the heat pipes inside each chamber, while the concentrated brine (saltwater) exits from the bottom of the distillation system. To maximize efficiency, the steam from the last chamber is reintroduced into the first chamber through a thermocompressor, which compresses the steam, increasing both its pressure and temperature. This recycled steam is then used to further heat the incoming seawater, reducing the overall energy required for the process and making this process more efficient [25,26]. Multi-stage flash (MSF) process distills seawater by flashing a fraction of the water into steam in multiple steps, which is essential for countercurrent heat exchangers. MSF distillation may have up to 30 stages. MSF plants generate about 25% of all desalinated water in the world. Three major parts make up MSF: the heat input section, heat recovery stages, and rejection stages to discharge waste heat to the environment [27]. Heat rejection from the system is removed, and an unevaporated brine recycling system is installed in the place of the heat rejection section. Hence, the cooling seawater stream no longer loses energy, and the blowdown stream loses less energy as well. A demonstration of MED is shown in Figure 4.

RO was introduced in the 1960s and developed further at UCLA (University of California, Los Angeles), and has become the dominant desalination method. This technique forces seawater through a semi-permeable membrane, filtering out salt and impurities to create drinkable water. Desalination methods continue to evolve, offering a potential solution to the growing water scarcity problem. As water stress becomes more widespread, desalination will likely play an increasingly critical role in providing clean, drinkable water to millions around the world [28]. One of the key differences between these two processes (thermal desalination and RO) is that RO does not use heat and does not need to boil anything. Water is highly pressurized and forced through a membrane. The higher pressure forces the water molecule to be separated from the salt. The United Arab Emirates (UAE) and Saudi Arabia produce one-fourth of the desalination water that is currently generated on this planet [29]. Desalination requires a significant amount of energy to separate salts from water. For comparison, the conventional freshwater treatment of surface water or groundwater involving coagulation, sedimentation, filtration, and disinfection typically requires only ~0.2–0.5 kWh/m³, whereas seawater reverse osmosis (SWRO) consumes ~3–6 kWh/m³ and thermal desalination processes; e.g., MSF/MED may exceed 10 kWh/m³, which is up to 25 times higher than conventional treatment. This high energy demand is particularly true for SWRO, MSF, and MED, making cost and sustainability key concerns. San Diego has the largest desalination plant in the world, which has been operating since 2015, producing 50 million gallons of clean water per day [30].

Three-dimensional (3D) printing has recently emerged as a promising technique for fabricating membranes with tailored architectures for water purification and desalination. Both thermal and membrane-based desalination processes stand to benefit from this technology, offering unprecedented efficiency and environmental sustainability through innovative material and design combinations. Unlike conventional phase-inversion or electrospinning methods, additive manufacturing (3D printing) enables precise control over pore geometry, surface roughness, and layer arrangement, thereby allowing membranes to be customized for specific separation needs. This degree of structural control is particularly valuable in optimizing permeability and selectivity trade-offs, which remain a central challenge in membrane technology. Several studies have demonstrated that 3D-printed polymeric membranes can achieve comparable or even superior water flux compared with commercial membranes, while also offering improved antifouling performance due to engineered surface topographies [31,32,33]. In addition, hybrid membranes incorporating nanomaterials, TiO₂, ZnO, graphene oxide, or metallic nanoparticles have been successfully integrated into 3D-printed scaffolds, enhancing mechanical stability and antibacterial activity [34,35,36,37]. Importantly, 3D printing also facilitates rapid prototyping, allowing new membrane designs to be tested within hours rather than weeks, thereby accelerating research and development cycles. With 3D printing, cutting-edge nanostructured materials, e.g., graphene and carbon nanotubes, can be readily integrated into thermal desalination systems, resulting in enhanced solar absorption and improved water evaporation energy density. This allows for the development of advanced solar desalination systems that are more efficient and cost-effective. Furthermore, 3D printing enables the design of highly efficient solar collectors, heat exchangers, and other components of solar thermal desalination systems, inspired by natural systems for optimal performance [38]. Beyond material advancements, 3D printing can also support a sustainable future for water desalination by facilitating the use of renewable energy sources, e.g., wind and solar power in desalination plants. By combining renewable energy with the customization capabilities of 3D printing, desalination systems can be optimized for lower energy consumption and reduced environmental impact.

However, the technology is still at a relatively early stage. Current 3D-printed membranes are often limited to laboratory or pilot-scale demonstrations, with challenges including high material costs, limited printer resolution for nanoscale pore control, and uncertainties about long-term durability in real operating conditions [39]. Moreover, scaling production to the large surface areas required for industrial desalination plants remains a significant barrier. Despite these limitations, the flexibility of additive manufacturing offers unique opportunities to design next-generation membranes that integrate hierarchical pore structures, multi-functional coatings, and even self-cleaning features. As such, while not yet commercially mature, 3D-printed membranes represent a promising frontier in sustainable water treatment technologies. Continued innovation in 3D-printed membranes could lead to significant breakthroughs in efficiency and cost, pushing the boundaries of what is possible in desalination technology [40,41]. Global distribution of desalination, brackish, river, wastewater, greywater, and other water treatment plants is highlighted in Figure 5.

An additional concern is that the outputting of clean desalinated water also generates huge amounts of hyper-salty water, called brine, as a byproduct. Most brine is, in one way or another, emptied back into the ocean [43]. Numerous studies have been published that show that the level of salinity increase will not harm marine life. Continuous climate change on our planet has deformed the balance between resources and the consumption of energy, ultimately affecting reserves of clean water. As a result, desalination is one of the alternatives that can fulfill the requirements of clean water in the near future. Figure 6 provides a comparative overview of countries employing MSF and MED systems at a large scale, highlighting regions such as Saudi Arabia and the UAE where thermal desalination has been widely adopted. This distribution is important to assess the global scalability and practical integration of such technologies.

The water recovery ratio (RR) of a desalination plant, termed volumetric processing efficiency, specifies the fraction of intake water that is transferred to low salinity water for community use. The remaining amount of water is computed as (1-RR), which is the amount of intake water converted into the brine. Thus, a desalination plant operating on 0.4 RR indicates that 40% of water intake is being converted into usable water, and the rest of the 60% of intake water is transferred into brine. The recovery ratio of different desalination means is shown in Figure 7.

A recently published study in Egypt explained that a solar field of 250,000 m² using Parabolic trough collectors with molten salt as the functioning fluid may yield approximately 22,775 m³/day of desalinated water and supply 15.5 megawatts of electricity to the grid under the most favorable circumstances in July, although the system’s minimum production is 18,749 m³/day and 11.9 megawatts of electricity to the grid in January. The average unit water production cost was 0.443 USD/m³ for the MED and 0.417 USD/m³ for the RO plants. The evaluation of water prices and benchmarked desalinated water prices in Egypt shows that the pricey water will be placed in the future, considering the decline in the country’s share of Nile River water [45].

Desalination processes can also be applied to various types of saline water, including seawater, brackish groundwater, and industrial wastewater. Brackish water, moderately saline water typically found in underground aquifers, is easier to treat than seawater due to its lower salt concentration, often making it more energy-efficient to process using RO or electrodialysis (ED).

Table 1. Classification and chemical characteristics of brackish groundwater types [46].

TDS Range (mg/L)	Dominant Ionic Constituents	Classification Notes
~1800	NaHCO3–SO42− (approx. 1/3 of total anions)	Low-salinity brackish water; moderate treatment required
~2500	CaSO4–Na+, Mg2+ (each ~25% of total cations)	Scaling likely; pretreatment recommended
~8400	NaCl	High salinity; similar treatment as seawater RO
~1400	Mixed ions with high silica content	Challenging fouling potential; advanced pre-filtration needed

summarizes typical brackish water types based on total dissolved solids (TDS) and ionic composition, which directly influence pretreatment and membrane selection [46]. Brackish water has more salinity than fresh water but not as much as seawater, which may result from mixing seawater with fresh water by certain human activities. Generally, brackish water contains 1–30 g of salt per liter, so it is not suitable for direct use. The direct use is injurious to human kidneys, while potential damage would be caused to the industrial equipment, ultimately leading to a reduction in productivity caused by inorganic and mineral fouling and scaling. To attain a required quality for conventional and nonconventional purposes, the abundant salinity and other contaminants must be removed. Various elements in brackish water cause scaling, e.g., strontium and barium, which have low solubility that, even in trace concentrations, may precipitate. Recently, the U.S. Geological Survey (USGS) published a report and categorized the brackish groundwater into four groups according to salinity and dominant ionic contents. The treatment of brackish groundwater depends heavily on its chemical composition and salinity level. This classification is essential for selecting appropriate treatment technologies, especially membrane-based desalination like RO [47,48].

Brackish water is low-salinity water, so membrane processes are favorable for the treatment, particularly in regions where electricity rates are quite substantial. Brackish water treatment relies on several membrane-based separation processes, primarily pressure-driven methods such as RO and nanofiltration (NF), as well as electro-driven techniques like ED. Among all the membrane processes, RO has turned into a high-tech process for the desalination of brackish water on a large scale. It has also been recognized for its cost-effective concern. High pressure is exerted on the brackish water forces towards the membrane, and water with less salinity comes out from the other side, as shown in the figure. RO is one of the oldest and most common techniques used for the desalination of brackish water. This technology has been working efficiently by improving the membranes and process optimization. The RO process depends on many factors, e.g., membrane types, operational modes, network structures, energy recovery devices, economical and environmentally friendly management of the concentrate stream, approaches to improve membrane performance, approaches to manage the concentrate stream, and efficiency for brackish water desalination.

A recent study proposed a zero liquid discharge (ZLD) system, which is vastly useful in the treatment and valorization of desalination brine. It was shown that the implementation of this system in industries (food, textile, and leather) that produce brine can improve sustainability. ZLD systems with modern technology should be further studied with the aim of reducing high energy demand and related greenhouse gas discharges and formulating more environmentally friendly ZLD systems [49]. Water purification in RO using the brackish system is shown in Figure 8.

From the results, it was examined that Co-MOF/PANI (cobalt intercalated metal–organic framework/Polyaniline) composite is the best option for arsenic and lead removal. This enables the Co-MOF/PANI to be the finest and most promising adsorbent to eliminate heavy metals from the unhygienic water [50]. PANI is a versatile and widely studied conductive polymer used in various applications such as sensors, batteries, and supercapacitors [51,52,53].

2.3. Desolenator

Desolenator is a patent technology that can convert saltwater and other non-potable water sources into pure, distilled water fit for human consumption. The Desolenator is a solar desalination tool that is capable of producing fresh water, using no power supply other than the sun. This technology is operational on both a small scale and a large scale. For the small-scale installation, the technology offers a smart device to produce about 15 L of fresh water a day [54,55]. This device is durable and easy to maintain, and can provide clean water for a household for a period of up to 20 years. It also ranks as one of the top five technologies for solving water scarcity.

2.4. Fog Harvesting

Fog harvesting depends on environmental and structural factors such as droplet size, mesh surface energy, wind speed, and collector orientation. While several mathematical models have been proposed to estimate water collection efficiency, in this review, we summarize only the key variables influencing system performance. Fog harvesting is a function of wind speed, as moving air is needed to carry fog droplets onto the mesh surface. In conditions with little or no wind, the droplets remain suspended and do not impinge on the collector, resulting in poor water yield. Optimal fog harvesting occurs when horizontal wind speeds range from 2 to 8 m/s, allowing for consistent droplet capture without damaging the structure. Topographically, fog harvesting is most effective in elevated coastal or mountainous areas where fog layers are channeled or pushed by prevailing winds, such as the Andes foothills, the Moroccan coastline, and certain regions of Namibia and Eritrea. Recent years have seen a dramatic increase in fog harvesting as a means of collecting water. Numerous studies have shown that fog plays an important role in supplementing water supply in both arid and semi-arid regions, where conventional water sources are often scarce. Various methods of fog harvesting have been proposed from time to time by researchers, both on a lab scale and a commercial scale [56,57,58]. Many insects and plants in deserts use their body structure to catch the fog from the environment at a specific time. The surface morphological properties of spider silk, cactus, etc., exhibit remarkable water wettability due to their morphological characteristics at the nano and micro levels [59]. While early fog collection systems may have been based on practical observations (condensation on fences or nets), recent advancements in fog harvesting technology have been increasingly inspired by biomimetic designs, such as the Namib beetle’s hydrophilic–hydrophobic shell pattern and cactus spine geometry. Incoming fog from the coastal side is usually captured by a fine mesh made of polyethylene (PE) ribbon-like stripes in a criss-cross structure, as shown in Figure 9. Most of the studies are based on the modification of the design and fog collector element (FCE). FCE is the material used as a mesh or obstacle to construct the fog collector in the course of the fog stream. Recently, a team of researchers developed vertical harps consisting of wire arrays that harvested three times the amount of fog when compared to mesh nets [60]. A recent study showed that multi-layer collectors, whose efficiency is much better than single-layer collectors, could create optimally efficient passive fog collectors by focusing on a geometrical relationship between their elements.

Superhydrophobic FCE favors quick drainage and efficient collection of water droplets, which can help increase collection rates. According to a recent report, surface wettability can enhance moisture harvesting: depending on the conditions, different outcomes have been reported, and it is unclear which type of surface wettability would be best for water collection [61]. Numerous biomimetic surfaces have been developed for the fog harvesting process [62]. In another study, it was described that the super-hydrophobic and super-hydrophilic hierarchical nanocrystal patterns on a biopolymer are more efficient in collecting fog. Fog harvesting is an effective method of recovering water from the moist atmosphere because it harvests water at a maximum efficiency of 103 mL min⁻¹ m⁻² [63].

When a fog stream comes from the coastal side and hits the fog collector, a small fraction of it accumulates on the FCEs. The coalescence of many fog droplets forms a water droplet that falls along the FCE due to the force of gravity. The standard fog collector (SFC) was used in indicant studies to collect the fog water with a surface area of 1 × 1 m². A typical Raschel mesh with a ribbon-like criss-cross structure was utilized as FCE [64]. A large fog collector (LFC) has also been largely used for fog collection. The mechanism of the LFC is identical to that of the SFC, but it is much wider horizontally [65]. Mostly, LFC is 4 m high and 10 m wide. In this article, we will not attempt to explain the physical processes involved in fog droplets, as the literature has already dealt with them. Fog water has been collected using double-layered mesh for the past two decades due to its high water yield. Furthermore, the geometry and structures of the fog collector determine its productivity. The design of the collector enhances the efficiency of the fog collector. The total efficiency of the fog collector is the product of aerodynamic collection efficiency (ɳ_AC), captured efficiency (ɳ_capt), and drainage efficiency (ɳ_drain) [66]. The aerodynamic efficiency (ACE) is the only factor that can enhance the efficiency substantially. However, the capture efficiency has a significant effect on total efficiency. From Equation (3), it can be interpreted that the Stokes number (Stk) is directly related to ɳ_capt, and Stk depends on the diameter of the FCE, which is the only parameter that can enhance efficiency by keeping the value low, up to a minimum level, as described in previous studies [2,67]. The ACE plays an important role in the total efficiency, as it can increase up to a maximum level by capturing the maximum fraction of fog in the collision trajectory passing through the fog collector. By adding multiple layers, the ACE can reach the maximum level, but after five layers, the increase is not significant, so up to 5 layers of the fog collector are reported as optimal. The ACE for the five layers was also described theoretically and pointed out the parameters that are very important to enhance this part of total efficiency. Recently published studies have clearly illustrated that changing the design from the conventional Raschel mesh to a harp collector increased the total efficiency by a factor of 4. Harp-shaped collectors and hydrophilic surfaces have been shown to significantly improve water yield compared to traditional mesh designs.

$${\eta }_{tot}={\eta }_{AC}{\eta }_{capt}{\eta }_{drain}$$

(1)

$${\eta }_{AC}=\underset{\phi }{\underbrace{\left[{\displaystyle \frac{A_{\infty }}{A}}\right]} }\underset{\chi }{\underbrace{\left[(1-{\left(1-s\right)}^N)\right]}}$$

(2)

where A_∞ is the area for the unperturbed incoming fog flow that will filter through the collector of frontal area A. s is the solid fraction, or solidity, of the layer (N layers), which is the probability that a droplet is captured by a layer. Equation (2) is the general form of ACE for the multilayer fog collector; that is, the fraction of droplets in an unperturbed upstream flow of area A that are both filtered by (φ) and incident to (χ), which are the elements of a multi-layer collector [2]. Captured efficiency is defined by the Stokes number, and

$${\eta }_{cap}={\displaystyle \frac{Stk}{Stk+\pi /2 }}$$

(3)

where $Stk= {\displaystyle \frac{2{\rho }_{\omega }{r}^{2}u}{9\mu d}}$ is the Stokes number, ${\rho }_{\omega }$ is the density of the liquid, μ is the dynamic viscosity of air, $u$ is the velocity of the air stream, and d is the diameter of the thread. ${\eta }_{cap}$ increases with increasing Stk, and Stk is inversely proportional to the diameter “d” of the FCE. More precisely, Labbé and coworkers illustrated that the diameter of the thread will be considered with the water film or drops covering it [68].

Raschel mesh has been used as a fog collector in different manufacturing designs, such as knitting patterns, weaving patterns, number of layers, and material characterization (mechanical and chemical) [69]. All the reported studies were found to be less efficient than the harp collectors. Consequently, it was concluded that the design of fog collectors plays an important role in achieving high yield and maximizing the total efficiency of fog collectors.

2.5. Dew Collection

The condensation of atmospheric vapors on a substrate is called the dew. It is a natural radiative cooling process. In meteorology, the process of dew is the same as rain, but in a large amount. Dew is a phenomenon that occurs on the surface, resulting in a non-negligible volume of condensed liquid water. Dew is one of the alternative fresh water sources in the absence of other resources (rain, fog, and groundwater). In hot and dry seasons on coastal sites, the dew droplets that form on materials naturally occur by (radiation-driven) dew water condensation. The meteorological position of the site is very important because it is related to the fraction of liquid droplets of dew in the moisture. Atmospheric vapors (dew) in large amounts could be a smarter move in renewable energy, requiring no external energy and supplying water to communities living in dry arid areas. With the installation of new and smart techniques, alternative water resources on coastal sites act as a substitute for large-scale inland water systems [70].

2.5.1. Dew Point

The dew point is the temperature at which air becomes saturated with moisture, and water vapor begins to condensate. At this point, the relative humidity reaches 100%. A higher dew point indicates more moisture in the air, while a lower dew point reflects drier conditions. The dew point is an important independent parameter in atmospheric water harvesting, as condensation occurs when surface temperatures fall below it. The dew point is directly affected by humidity; higher moisture in the air leads to a higher dew point. It depends on both relative humidity and ambient temperature. When a surface cools below the dew point of the surrounding air, condensation occurs on that surface in the form of liquid droplets, referred to as dew. Therefore, dew is not something that condenses itself, but rather is the result of prior condensation. Dew collection systems typically use radiative cooling surfaces to lower surface temperatures below the ambient dew point, promoting condensation from the air. As the temperature falls below the freezing point, the dew point forms the frost point via deposition instead of condensation. The dew point is affected by two parameters: relative humidity and ambient temperature. Many techniques have been used for dew condensation, such as conical shapes [71], inverted pyramids (similar to cones) [72], a series of ridges, and roof condensation [73,74]. One of the largest dew condensers has been developed by a group of scientists for dew collection or atmospheric moisture for drinking water purposes. It was shown that it can be cost-effective and can generate up to 500 L of water daily. Three different types of systems were developed for large-scale production: (1) condenser-on-roof, (2) condenser-on-ground, and (3) condenser-on-fence. Large areas with low wind (1–4 m/s) are assumed to be favorable places for collection sites. On the other hand, the collection site must not be near poisonous plants or pesticide agricultural areas. Chemically treated or sprayed locations must be avoided. Sites with visible animal waste may be selected for studying contamination risks and assessing water treatment needs. To maintain efficient condensation, the collection surface should be thermally connected to a cooler thermal mass or heat sink. This allows the latent heat released during droplet formation to be conducted away from the surface, helping it remain below the ambient dew point and supporting sustained water collection. Thermally isolating the surface, in contrast, could lead to temperature rise, reducing condensation efficiency.

2.5.2. Dew Collector

Dew collectors are often designed with sloped surfaces to maximize exposure to the night sky and minimize convective heat gain. Several studies have tested inclination angles between 25° and 45°, with around 30° showing favorable results in some climates due to enhanced radiative cooling and water runoff. Materials such as high-emissivity plastic film, corrugated metal, and coated glass have all been used, depending on local conditions and design constraints. The surface is directed toward the sky, but gravity will still pull the water droplets down towards a pipe in which they are collected. The film is made of low-density polyethylene and contains a very low percentage of TiO₂ microparticles with high infrared emissivity to enhance the collection rate. In the previous studies, condensers as planar panels with hydrophilicity at small supersaturation and advanced drop recovery have been practiced [75]. The maximum dew collection of 0.6 L m⁻² per night has been reported [71]. The dew harvesting mechanism has been proposed with different surface morphologies. Generally, nucleation plays an important role in the hydrophilic spots; the size of droplets expands and slides down through the hydrophobic/superhydrophobic background [76]. Dew collection rate relies on the combination of nucleation rate and drop mobility. A recently published article narrated that the superhydrophobic surfaces showed a 40% higher dew collection rate in comparison with plane hydrophobic or hydrophilic surfaces as a result of low contact angle hysteresis that includes the larger surface area for nucleation [77]. Atmospheric water generators are generally categorized into two main types: (i) active systems that use mechanical cooling, such as vapor compression refrigeration to cool air below the dew point, and (ii) passive systems that rely on natural radiative or evaporative cooling processes, often powered by renewable energy. Active systems can operate using various energy sources, including solar panels, wind turbines, or grid electricity, depending on the system design and location.

2.5.3. Water Extraction from Atmospheric Air

Our atmosphere contains a vast reservoir of water vapor, and in hot arid regions such as the Sahara and Arabian deserts, daytime absolute humidity can exceed 7–10 g of water vapor per cubic meter. However, this is not typical for all deserts. Cold deserts, such as Antarctica and the Gobi, exhibit much lower atmospheric moisture content due to their consistently low temperatures. According to the Clausius–Clapeyron relation, which describes the exponential increase in saturation vapor pressure with temperature, warmer air can hold significantly more water vapor than colder air. For example, in the tropics, the surface-specific humidity can reach approximately 20 g/kg, while in polar regions, it may be as low as 1 g/kg. Interestingly, despite high surface temperatures, many desert regions experience low relative humidity due to subsiding air masses (vertical down-flows) that warm adiabatically and inhibit condensation, reducing overall atmospheric moisture near the surface [78]. The methods of fresh water extraction from atmospheric air are presented in

Table 2. Classification of freshwater extraction methods from atmospheric air.

Basis	Categories	Description	References
Moisture Extraction	- Condensing - Adsorptive - Hybrid	Methods based on extracting water via condensation, sorbent materials, or a combination of both.	[79,80,81]
Power Consumption	- Passive - Active	Passive systems use natural cooling; active systems rely on mechanical/electrical input.	[82,83,84]
Power Supply Source	- Grid-powered - Generator - Battery - Renewable energy	Defines how the system is powered: central grid, fuel-powered, battery, or renewable energy like solar or wind.	[85,86,87,88]

.

Atmospheric water harvesting (AWH) technologies are generally categorized into two primary mechanisms: condensation-based systems and sorption-based systems. Condensation-based AWH relies on cooling ambient air below its dew point so that water vapor condenses into liquid form [89]. Several methods are used to achieve this cooling. These include conventional vapor compression refrigeration systems that use a thermal loop with circulating refrigerant, thermoelectric coolers based on the Peltier effect, and passive cooling techniques such as buried ground chambers or radiative cooling surfaces. Novel technologies like the Zero Mass Water device integrate photovoltaic panels and condensation units to generate 2–5 L of drinking water per day [90,91,92]. Other innovations include systems like the Airdrop irrigation device, which uses wind-driven air pushed through underground copper coils to promote cooling and condensation. While these methods are well-suited for regions with moderate to high humidity and energy access, their performance can decline significantly in arid climates due to the high energy cost of cooling dry air. In contrast, sorption-based AWH systems extract water from the air using hygroscopic materials that absorb moisture even at low relative humidity. Common materials include calcium chloride, lithium chloride, silica gel, and advanced metal–organic frameworks (MOFs). Once saturated, these materials are typically heated, often by solar energy, to release the absorbed water as vapor, which is then condensed into liquid. Recent advances in sorption materials have made AWH viable even in regions with relative humidity as low as 10–20%. For instance, MOF-based devices have shown high water uptake and low regeneration temperatures, making them particularly attractive for off-grid water generation [93,94,95,96]. Sorption systems are energy-efficient, scalable, and especially useful in hot, arid climates where condensation-based methods are inefficient.

The morning dew is one of the easiest ways to collect water out in the open fields. A fabric piece would be a t-shirt that can be used for collection purposes. Squeezing this out in any bottle or glass or just holding it and sucking the water out of the cloth will be enough to sustain the body viably. Dew is a phase change that shows two main characteristics. The liquid phase nucleation is adapted by the wetting properties of the substrate and the development is dimensionally inhibited, which provides several unusual properties to the dew.

2.6. Greywater

Greywater is usually wastewater generated in households or office buildings from streams without fecal contamination. The stream of toilets is not included in this wastewater (that is called sewage or black water). Common sources of greywater are sinks, showers, baths, dishwashers, and laundry machines. The greywater contains some pathogens and domestic wastewater that is easier to treat and reuse onsite for toilet flushing or irrigation in mini gardens. However, the use of personal care (non-toxic) and low-sodium soap is recommended to protect vegetation when using greywater for watering the vegetable and fruit plants, especially citric plants. By constructing a wetland, the greywater can be filtered. Wastewater flows into this gravel-filled basin, which is planted with wetland plant species. The wastewater flows underneath the gravel level and is never exposed to the air. The treated water is much cleaner when it flows out through this big plant filter. This system is all about maximizing the contact between the greywater and the plant’s root [97].

Greywater cannot be used as potable water without undergoing thorough treatment and disinfection processes to remove contaminants. The Australian Government has developed guidelines for reusing greywater, including how to reduce the use of cleaning products: use of low- or no-sodium laundry detergents, use soaps and shampoo made with fewer chemicals, use a lint filter, and change it as necessary to ensure proper water flow. A hybrid system (filter unit, phytoremediation, and flocculation processes) could produce high-quality greywater treatment. An economical and feasible solution for greywater treatment involves the use of a combination of primary (a natural filtration unit) and secondary processes (bioreactor system) [98]. The chemical properties of fruits and leaves were not adversely affected by greywater irrigation of olive trees and vegetable crops. The greywater after treatment can be used for irrigation, car washing, watering lawns, and recharging aquifers, but cannot be used for drinking purposes. This is a cost-effective and economical technique to minimize pollutant concentrations and environmental risks. Greyscale filtration is shown in Figure 10.

2.7. Wastewater Treatment

All towns and cities produce wastewater from homes, hotels, factories, and other establishments, which is sent to a sewage treatment plant where this wastewater is treated [99]. It is not safe to release into other water sources, so it undergoes different stages to remove the physical, chemical, and biological contaminants present in it [100]. A pretreatment process called a screening chamber is set to screen the wastewater through grids or vertical bars that can remove large solid substances like metal, cans, papers, and plastic materials. The remaining water is then processed biologically in an aeration chamber, where air blowers bubble air, which helps the aerobic bacteria to grow and feed on organic contaminants such as food waste, feces, and other organisms. The leftover wastewater is treated with chlorine to remove the phosphorus compounds, nitrogen compounds, and bacteria. This chemical process, called chlorination, kills germs. Then, this water is discharged into water bottles [101,102]. The flow chart of this whole mechanism is shown in Figure 11.

2.8. Rain Harvesting

The current consumption rate of water warns that by 2025, two-thirds of the world’s population could face water scarcity. The ecosystem around the world has already been affected by water shortages. In compensation, rainwater could serve as an alternative water resource if harvested and stored with minimal filtration input. Water is also called blue gold because it is a valuable resource that causes wars. There are a lot of methods to collect rainwater. Rainwater harvesting involves either collecting rainwater directly or recharging it to improve groundwater levels. Recharging the groundwater is one of the modern forms of rainwater harvesting. Rainwater benefits our ecosystem in various ways, such as raising the groundwater level, preventing soil erosion and water pollution, and helping to meet water demand and supply in the dry season [103]. A rooftop rainwater harvesting system, which channels rainwater through pipes, is being implemented in different parts of the world. Regardless of what the material of the rooftop is, water can be collected in pits, trenches, or dug wells for long-term use. A gutter linked to the roof must be properly sized to handle runoff during heavy rainfall, so that water does not slide off the roof and spill over the edge. An effective invention in this case is the leaf screen, which is designed to keep leaves and other debris out of the water system. As the water flows down this screen positioned at a steep angle, it passes through another finer screen that filters small particles. However, some contaminants in the water still pass through the leaf screen, requiring additional filtration [104].

A first flush diverter is installed to catch these dirty particles left unfiltered, which drop into a vertical pipe, as shown in Figure 12. While there are various designs for this vertical pipe, this particular design catches the heavier small particles that are denser than water before the cleaner water continues to flow to the tank. But these systems are high maintenance because the emitter or the small hole at the bottom of the pipe tends to become clogged due to the filtering of contaminants; therefore, cleaning of the system keeps it functional. Rainwater is further divided into groundwater and surface water. These are the primary renewable water resources, and their precise use and balanced extractions are critical, given their uneven distribution across locations and seasons. Therefore, simulation modeling tools have been developed to enable equitable distribution of water resources among stakeholders, helping to prevent water shortages, aquifer depletion, and related crises. The main reason for such studies is to simulate the behavior of systems in present and future situations in a vibrant mode. Many models have been developed for optimum water resource allocation, simulation optimization, various conflict resolution, and water allocation [105].

Sustainable groundwater management can be achieved by using groundwater and surface water resources jointly [106]. There are two types of conjunctive systems that have different attitudes toward recharging aquifers: cyclic storage systems and non-cyclic storage systems. Sustainable groundwater management refers to using and exploiting the resource effectively, but without leaving the resource in a state that is unsustainable environmentally, economically, or socially. Efforts to create groundwater sustainability and sustainable water allocation to agriculture were examined by considering cyclic and non-cyclic conjunctive surface and groundwater use strategies in response to climate change. It was found that the sustainability index could be used to assess the reliability, resilience, and vulnerability of water supply plans for the agriculture sector. To maximize the sustainability index in providing water to the agricultural sector, it was designed to minimize deviation from the target groundwater level. By circularly transferring water between groundwater and surface water reservoirs, the surface water reservoir is less likely to spill or lose water, and more water can be stored in the aquifer in wet seasons to be used in dry seasons [107].

The San Antonio Creek integrated model (SACIM) was developed using the USGS coupled surface water and groundwater flow model to simulate the hydrologic system of the SACVW (San Antonio Creek Valley watershed) and present annual and average water budgets for the 1948–2018 water years. In the 1980s, agricultural pumpage accounted for 69% of all groundwater pumpage. Since then, pumpage has increased primarily due to an increase in groundwater evapotranspiration and reduced rates of surface leakage (groundwater discharge to the surface). Furthermore, increased pumping led to a decrease in the inflow into Barka Slough, California, which adversely affected upward flow through the underlying hydrogeologic units. SACIM is a tool that water managers can use to evaluate the effects of changing climatic, hydrologic, and management conditions on the integrated hydrologic system, as well as quantify historical changes [108].

2.9. Humidification–Dehumidification (HDH) Desalination

HDH is a thermal desalination process inspired by the natural hydrological cycle, wherein air is humidified using saline water and then cooled to condense the water vapor, producing freshwater. HDH systems typically consist of a humidifier, a dehumidifier, and a heat source (often solar, geothermal, or waste heat). HDH desalination is particularly suitable for small to medium-scale applications in off-grid or remote regions. It is known for its simplicity, low operating temperatures (60–80 °C), and ability to use low-grade thermal energy [109,110]. HDH systems can achieve water production rates of 1–5 L per hour per square meter, depending on design and climate conditions [111,112,113]. Recent innovations include multi-effect HDH cycles, air-heated loop systems, and integration with solar thermal collectors to improve energy efficiency and water yield [114,115,116]. However, limitations include lower overall recovery ratios compared to RO and scalability challenges. Despite these, HDH remains a promising sustainable solution for decentralized water treatment in arid and semi-arid zones.

Table 3. Comparative summary (advantages and disadvantages) of alternative freshwater generation technologies.

Technology	Water Yield (Approx.)	Energy Source	Scale	Advantages	Disadvantages	References
Desalination (RO, MSF, MED)	40–60% recovery ratio	Electricity/fossil/solar	Large-scale	High yield, established infrastructure	High energy demand, brine disposal issues, expensive infrastructure	[117,118,119]
Desolenator	~15 L/day (small unit)	Solar	Household	Off-grid, sustainable, durable	Small-scale only, relatively slow output, high upfront cost	[120,121,122]
WaterSeer	~11–50 L/day (ideal conditions)	Wind/Geothermal	Individual	Passive, renewable, low maintenance	Limited validated performance, conceptual stage, not field-proven	[123,124]
Fog Harvesting	Up to 103 mL/min/m2	Passive	Community	Eco-friendly, high efficiency (harp design)	Climate dependent, limited to fog-prone areas, moderate yields	[125,126]
Dew Collection	0.6 L/m2/night	Radiative cooling	Household	Low-cost, suitable for arid areas	Low productivity, requires large cooling surfaces, dependent on night conditions	[127,128,129]
Brackish Water RO	Moderate to high (varies)	Electric/solar	Regional	Lower energy demand than seawater RO	Brine disposal issues, moderate energy demand, risk of membrane fouling/scaling	[130,131,132]
Greywater Recycling	50% of household use	Gravity/solar pumps	Household	Reduces freshwater demand, eco-friendly	Requires secondary treatment for safety, public acceptance issues	[133,134,135]
Wastewater Treatment	Up to 90% reclamation	Various	Municipal	Large-scale reuse, supports agriculture	Public acceptance concerns, risk of residual contaminants	[136,137,138]
Rain Harvesting	Variable (site-dependent)	Gravity	Household/community	Enhances groundwater, simple design	Rainfall-dependent, limited storage capacity	[139,140,141]
HDH Desalination	1–5 L/h/m2 (solar-driven)	Solar thermal, waste heat	Small-medium	Low-cost, scalable, ideal for off-grid use	Still pilot-scale, moderate yields, require large surface areas	[142,143,144]

highlights and compares the key technologies discussed, including their water yield, energy source, operational scale, advantages, and relevant literature. This will help the readers better visualize and compare the alternative water technologies presented in this study.

3. Produced Water Treatment

Produced water is a byproduct of oil and gas extraction processes and constitutes one of the largest sources of industrial wastewater globally. It typically contains high concentrations of salts, hydrocarbons, heavy metals, and chemical additives, making its treatment complex but critical for environmental protection and potential reuse [145]. Current treatment methods include physical separation (gravity separators), chemical treatment (coagulation-flocculation), membrane filtration (nanofiltration and reverse osmosis), advanced oxidation processes, and emerging biological treatments [146,147,148]. Recent advances in electrocoagulation and membrane distillation have shown promise in removing both organic and inorganic contaminants with lower energy requirements. Given the sheer volume, estimated at over 250 million barrels per day worldwide, the treatment and reuse of produced water represent a significant opportunity in both water recovery and pollution control, especially in water-stressed regions near oil fields. However, economic feasibility, environmental regulations, and site-specific contaminant profiles remain key factors in selecting appropriate treatment solutions.

4. Water Quality Concerns

The most common type of water that was once or still is used for drinking has been contaminated by wastewater discharges, industrial chemical dumps, and excessive runoff from polluted areas. Filter techniques can effectively remove contaminants in many areas. In certain regions, however, the water supply either cannot be treated to meet potable standards due to a lack of capacity, or the contaminants present cannot be removed by the current technologies. As an example of how a country with contaminated sources has trouble meeting freshwater demands, Bangladesh stands out. With over 30 million people estimated to be exposed to arsenic in concentrations greater than 50 micrograms per liter in aquifers, arsenic contamination has caused epidemic levels of cancer and other diseases. Adding to these problems, much of the country’s surface water is poor for drinking due to high turbidity and salinity. As a result, another source of clean, reliable, and accessible potable water is necessary [149].

A study found that 90% of all infectious diseases are spread through or linked to water issues in developing countries [150]. To overcome this issue, we need to find alternatives to water or implement proper treatment technology to address these issues. The quality of water should be related not only to human beings but also to the water creatures that are living in rivers and lakes. The unnecessary extraction of water from lakes, rivers, and aquifers has disturbed both aquatic ecosystems and the surrounding terrestrial ecosystems that depend on them. Herein, the focus was only on the quality of human drinking water after removing all the hazardous contaminants. Technologies have provided solutions to every problem, contextualized with different costs, impacts, and consequences, so water quality appropriate for health protection and satisfying each user’s needs has been the next challenge.

The microorganisms from the atmosphere and the feces of birds on the rooftop are the most common contaminants in rainwater storage tanks. Thus, it needs the least treatment to make it drinkable water. Rainwater can even be used as a source of freshwater without any additional treatment if the tank and catchment system are maintained properly. Solar disinfection is the most common method to remove possible pathogens from rainwater. Lemon and vinegar, which are inexpensive food preservatives, will usually provide complete disinfection, even under weak weather conditions. Dew and fog act as atmospheric objects, so the chemistry of their formation is a part of the air quality and the heterogeneity in their surroundings. Dew water quality has been found closer to rain and can be used for drinking. The quality of the coming water from the fog collectors meets the World Health Organization (WHO) drinking water standards. In general, the composition of fog water is considered safe to drink. Fog water must always be checked for quality before human use when there is potential for contamination. Similarly, the results showed that the quality of desalinated water and brackish water met the WHO-2017 water quality limits.

5. Water Scarcity and Its Effects

The rapid growth of the population has adversely affected the ecosystem, leading to global warming and stress on natural resources. Freshwater resources are also badly affected by some factors listed below. We will now briefly discuss the aspects that lead to severe water shortages.

5.1. Climate Change

Globally, the pattern of weather and water is expected to change due to the continued release of carbon dioxide and other greenhouse gases into the atmosphere. Some places will experience more droughts, while others will endure more floods. The glaciers and snow peaks are melting and shrinking, which is influencing the freshwater supply to communities downstream. In the future, these changes will reduce the world’s water supply for irrigation, energy production, and ecosystems.

5.2. Pollution

Water pollution is caused by many factors, such as fertilizers deposited in rivers, pesticides, sewage water, untreated human waste, and industrial waste. Many pollutants can leach into underground aquifers, and even groundwater is not safe from pollution. Human waste can pollute water immediately, contaminating it and making it unhygienic to drink or use in the kitchen. As with toxic substances from industrial waste, some of their effects may not be fully recognized for years after they are released into the environment.

5.3. Agriculture

Most of the world’s fresh water is used by agriculture, yet 60% of this is wasted as a result of leaky irrigation structures, inefficient functional processes, and cultivating crops that are highly water-demanding for the conditions in which they are grown. This careless use of water is draining the water reservoirs, such as lakes, rivers, and underground aquifers. The countries that are producing large amounts of food on a daily basis have met or nearly reached their water resource boundaries. Additionally, agriculture can cause substantial freshwater pollution, both through the use of fertilizers and pesticides, which affect humans and other living species [151].

5.4. Population Growth

The world’s population has doubled in the last fifty years. The unstoppable growth of the world’s population and its associated economic progress have led to a severe loss of biodiversity in the world’s water ecosystems. At present, more than 40% of the population resides in river basins that are under water tension, which poses a risk of reaching unsustainable levels. The additional demands for food, shelter, and clothing by these new faces result from the production of commodities and energy and will further stress freshwater resources.

5.5. Disappearing Wetlands

Half of the world’s swamps have been demolished since 1900. As a rich habitat for mammals, birds, fish, and invertebrates, wetlands serve as nurseries for a variety of species and are among the most productive habitats on Earth. Wetlands also maintain the farming of rice, a major ingredient in the diet of half the world’s population. Their ecosystems also provide humans with a variety of ecosystem services, such as flood control, water filtration, storm protection, and recreation.

5.6. Damaged Ecosystems

The insufficiency of water in a particular area indicates that natural landscapes are disappearing. A sea area the size of Lake Michigan has evaporated in just three decades. Increasing pollution and water diversion for irrigation and power generation have made it as salty as the ocean. As the sea has retracted, it has left polluted land. Food shortages and lower life expectancy for local residents have resulted from this ecological catastrophe.

6. Technological Advancement

All the responsible authorities should continue to develop and value advanced water management technologies. As technology has advanced in the water sector, higher-quality water can now be provided cost-effectively and with less environmental impact. Even so, there is always room for improvement to maximize efficiency, reliability, cost-efficiency, and environmental impacts. It would be beneficial to the water sector and the environment if these technologies were more reliable and efficient. Furthermore, even though rainwater harvesting has existed for centuries, technological advances have improved its ability to augment the water supply more effectively. Technological advancements also have a major impact on water supply efficiency in the field of water loss management. Other areas in which technology is advancing include wetland sensors, real-time supervisory control systems, and automatic meter reading. In addition to smart meters, water quality and data analysis are among areas in which continued technological development would contribute to further improvements in water distribution networks. It is important to apply the knowledge effectively as well as with adequate foresight, even though some of this is already available. Regulations must have the capacity to impose high penalties for non-compliance with best practices, while also prioritizing environmental protection and sustainable resource use in decision-making.

Generally, higher water prices tend to match lower usage rates and vice versa (Figure 13). Furthermore, higher prices do not lead to controlled demand for drinking water but to unnecessary uses (European Environment Agency, 2013). Consequently, re-evaluating community water pricing structures may help to manage a country’s water demand.

7. Discussion

This review highlights that a variety of non-conventional water technologies have been developed to mitigate freshwater scarcity, but their performance and feasibility vary significantly depending on environmental, technical, and socio-economic conditions. A key finding is that textile-based fog collectors, particularly those inspired by biomimetic structures, e.g., harp collectors, cactus spines, Namib beetle patterns, stand out as highly energy-efficient and scalable approaches for decentralized water supply. Reported yields of up to 103 mL/min/m² indicate that textiles can provide sustainable freshwater generation without external energy inputs, a critical advantage in arid and semi-arid regions where electricity and infrastructure are often limited. Compared with desalination, which achieves higher volumetric recovery ratios (40–60%) but at the cost of substantial energy consumption and brine disposal challenges, textile-based fog collection represents a more eco-friendly and locally adaptable alternative. When comparing fog harvesting with dew collection, the contrast becomes clear: dew condensers typically yield only 0.6 L/m² per night, and performance is highly dependent on radiative cooling efficiency. Fog harvesting, on the other hand, benefits from wind-driven transport of droplets, enabling higher capture rates. The integration of superhydrophobic and superhydrophilic textile surfaces further enhances drainage and droplet coalescence, confirming direct improvement in water yield. These results underscore the importance of developing optimized textile-based FCEs as a central pathway toward affordable water solutions.

Another significant observation is the trade-off between scalability and sustainability. Large-scale desalination plants, particularly in the Middle East, demonstrate that high-yield solutions are technologically mature and already supply millions of people. However, their dependence on fossil fuels or high-cost renewable energy limits applicability in many developing regions. Conversely, decentralized technologies (Desolenator, WaterSeer, textile-based fog collectors) provide modest water yields but require minimal energy and can be manufactured, maintained, and deployed locally. This indicates that decentralized systems may be more practical for rural and remote communities, whereas large-scale desalination is more appropriate for urbanized coastal regions with robust infrastructure. The comparative analysis in Table 3 reinforces these findings by showing that while desalination dominates in terms of absolute yield, fog harvesting, greywater reuse, and dew collection offer complementary strategies with lower energy input. For instance, greywater recycling can offset up to 50% of domestic water demand, directly reducing pressure on freshwater supplies, even though it cannot serve as potable water. When combined with fog and dew harvesting, such approaches provide an integrated water management strategy that addresses both potable and non-potable needs.

Finally, the review identifies critical research and implementation gaps. Many textile-based fog harvesting systems remain at pilot scale, with limited long-term field data to validate durability and real-world efficiency. Hybrid systems that combine passive fog collection with renewable-powered desalination or greywater reuse remain largely theoretical but represent a promising direction. Additionally, the economic implications of deploying low-cost textile collectors compared with capital-intensive desalination systems warrant further investigation. In summary, our findings suggest that textiles can play a transformative role in advancing atmospheric water harvesting. By integrating material innovations with scalable design, textile-based fog collectors offer one of the most promising, energy-efficient, and environmentally sustainable solutions to address global water scarcity.

Recent advances in artificial intelligence and machine learning (ML) are opening new opportunities to optimize water harvesting and treatment technologies. Currently, ML models are widely used for prediction, classification, and quantification applications [152,153,154,155]. ML models can analyze climatic data (humidity, wind speed, solar radiation) to predict the most suitable times and locations for fog or dew harvesting, thereby maximizing yield. Similarly, ML algorithms have been used to improve desalination processes by predicting membrane fouling, optimizing energy consumption, and enabling predictive maintenance of treatment plants [156,157,158]. In wastewater reuse, ML has been applied for real-time monitoring of contaminants and for automating treatment plant operations to reduce costs and ensure consistent water quality [159,160,161]. These data-driven approaches not only improve operational efficiency but also support adaptive system design by learning from performance feedback. While the integration of ML into water management is still emerging, its potential to enhance sustainability and resilience makes it an important complementary tool for the future of non-conventional water technologies.

8. Summary and Future Direction

This review provides a comparative evaluation of major non-conventional water sourcing technologies, including desalination, fog and dew collection, greywater and wastewater reuse, atmospheric water extraction, and solar-powered devices, i.e., WaterSeer and Desolenator. Key findings are given below:

• The analysis shows that while large-scale desalination plants are technologically mature and capable of delivering high recovery ratios (40–60%), their high energy demand, brine disposal issues, and infrastructure requirements limit their sustainability and accessibility in many regions. In contrast, textile-based fog harvesting emerges as a particularly promising alternative due to its energy efficiency, low cost, and adaptability to local manufacturing and deployment.
• While the technologies vary in maturity and scalability, each provides valuable potential under appropriate environmental, social, and economic conditions. For example, large-scale RO plants are suited to coastal cities with robust infrastructure, while fog harvesters and dew condensers offer passive solutions for remote or arid regions with limited resources. Likewise, household-level systems (WaterSeer and Desolenator) are promising for decentralized, off-grid communities.
• Rather than serving as a prescriptive implementation manual, this article aims to support decision-making by offering a comparative lens through which alternative water technologies can be evaluated based on local needs and constraints. As climate stress intensifies and freshwater demands grow, further research should focus on hybrid solutions, integration with renewable energy systems, and context-specific deployment strategies to improve the sustainability and accessibility of clean water supplies globally.
• Many studies report water yield, energy consumption, or efficiency using different units and environmental conditions, making comparisons difficult across technologies. Technologies such as fog and dew harvesting or passive solar desalination are often evaluated only in lab or pilot-scale settings, with insufficient long-term field studies.
• Combining fog harvesting, atmospheric water generation, and renewable-powered desalination remains largely theoretical; there is a lack of integrated designs tailored to local conditions.

By addressing these gaps, future work can transition these technologies from concept and pilot scale to robust, field-deployable solutions tailored to the evolving global water crisis.