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Review

The Importance of Nonconventional Water Resources under Water Scarcity

by
Andreas N. Angelakis
1,2,
George Tchobanoglous
3,
Andrea G. Capodaglio
4 and
Vasileios A. Tzanakakis
5,*
1
School of History and Culture, Hubei University, Wuhan 430061, China
2
HAO-Demeter, Agricultural Research Institution of Crete, 71300 Iraklion, Greece
3
Department of Civil and Environmental Engineering, University of California at Davis, Davis, CA 95616, USA
4
Department of Civil Engineering & Architecture, University of Pavia, Via Adolfo Ferrata, 3, 27100 Pavia, PV, Italy
5
Department of Agriculture, School of Agricultural Science, Hellenic Mediterranean University, 71410 Iraklion, Greece
*
Author to whom correspondence should be addressed.
Water 2024, 16(7), 1015; https://doi.org/10.3390/w16071015
Submission received: 29 February 2024 / Revised: 25 March 2024 / Accepted: 29 March 2024 / Published: 31 March 2024
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Abstract

:
According to FAO, water scarcity is now affecting all five continents and is expected to intensify in the coming years as the water demands of the growing population increase and the impacts of climate variability become more pronounced. The existing unevenness of water resource availability and insufficient investment in relevant infrastructure have forced the water sector to recognize the importance of nonconventional water resources (NWR) in planning for a sustainable water future. The purpose of this review is to highlight the available and potentially available NWR and to discuss the future application of these water sources.

1. Prolegomena

Waters that are not originally suitable for domestic and/or agricultural uses without prior extensive treatment are considered nonconventional water resources (NWR) or unconventional water resources (UWR). Nonconventional water resources include those generated as a by-product of specialized processes (e.g., desalination or treatment of already used water), needing suitable pre-use treatment, or special technology/processes to be collected or accessed. For example, seawater, saline, and brackish surface and ground waters contain levels of dissolved salts and other constituents (e.g., microbiological pollution) that must be removed; wastewater from industrial or municipal origin may also contain, after treatment, a variety of pathogens and potentially harmful chemical constituents, which render these sources unsuitable for such reuses. Atmospheric water (vapor) can be harvested with special techniques and reused.
With the global population and economic growth and the subsequent intensified anthropogenic activities in many sectors of human activity (e.g., intensified agriculture), many of the existing available water supply sources have become stressed and unpredictable. Furthermore, changes in climate patterns, which have put further stresses on water resources due to extreme weather events such as drought phenomena, increased temperatures, and floods, affect not only humans but also crops, soils, and biodiversity, impacting the overall performance of ecosystems, their provided services, and climate feedback [1,2]. Thus, the water sector has begun to recognize the importance of NWR in planning for a sustainable future [2,3,4].
Current global water demand for anthropic uses and consumption is estimated at up to 4600 km3/yr [5]; global freshwater (conventional water resources) withdrawal is estimated at approximately 3756 km3/yr, with an additional estimated 255 km3/yr NWR used worldwide [6]. It is, however, difficult to estimate precisely the extent of all NWR contributions as there is no exact record of their actual or potential usage. A recent study [3] has summarized from the literature the volumes of NWR, reporting 1.35 billion km3 for seawater and 27 million km3 for Antarctic ice sources, with 2000 km3/year by icebergs breaking off. Moreover, 25 million km3 are reported for fossil water, whereas wastewater/greywater and atmospheric water have been estimated at 380 km3/yr and 13,000 km3, respectively.
Despite the promising possibilities of NWR, it is important to recognize that it currently has limited applicability worldwide due to specific constraints arising from social, economic, climatic, and geographical characteristics of areas and an incomplete understanding of the potential benefits of available NWR [3]. In what follows, current and potential NWR are identified, and each source of water is examined further from a historical and ongoing perspective. For this purpose, a thorough review of the literature was carried out, covering different areas of interest, such as the current status of NWR, technological advances, and current issues and challenges in different domains.

2. Nonconventional Water Resources

Based on a review of the current literature and ongoing practice, the principal forms of NWR are identified in Table 1, along with important nonconventional applications. Currently, the principal NWRs are reclaimed wastewater and seawater. In many parts of the world, NWR represents an important alternative or, at least, a complementary one to locally meet water needs.
In Australia, water recycling is defined as the use of water originating from the treatment of wastewater effluents to provide safe water for beneficial purposes. According to the US EPA, reclaimed water originates from municipal wastewater after treatment to meet quality criteria [2]. In the EU, water reuse includes the use of water generated from wastewater and achieves after-treatment minimum quality standards dependent on the use of water. In addition, monitoring and risk management assessments are included. The main focus is to protect natural resources and reduce competition among users, particularly in climate-vulnerable Member States. In this framework, several EU releases cover a wide range of substances or groups of substances, such as pathogens, antibiotics and antibiotic resistance genes (ARGs), microplastics, pharmaceuticals, and other organics [2]. As discussed below, each of the water sources has been classified as currently or potentially available.

2.1. Currently Available Nonconventional Water Resources

All of the sources of water listed in Table 1 could be classified as currently available NWR; although their use is still not mainstream worldwide, they have been used as water supply sources for both domestic and agricultural uses for some time in water-stressed locations. For example, wastewater after conventional and advanced water treatment has been utilized for indirect and direct potable reuse (IPR and DPR). The desalination of seawater is used throughout the world for the production of drinking water.
It should be noted that the spatial distribution of the currently available NWR on Earth at present time is worldwide, almost everywhere, e.g., in the Gulf Cooperation Council countries, such as Bahrain, Kuwait, Saudi Arabia, the Sultanate of Oman, Qatar, and the United Arab Emirates [7].

2.2. Potential Nonconventional Water Resources

The sources of water listed in Table 1, classified as potentially available NWR, have been used locally but have not been used widely as water supply sources for both domestic and agricultural uses. They are NWRs that need more research and development before they can be utilized fully.

2.3. Treated Wastewater

Wastewater treatment and reuse have a long history; knowledge has been accumulated during the history of humankind [8]. Land application of human waste is also an old practice, known since prehistoric times, that has undergone several development stages from ancient to contemporary times [9]. In Greece, during the Minoan period (ca 3200–1100 BC) and thereafter during the Classical and Hellenistic periods (ca 480–31 BC), untreated wastewater was applied to agricultural lands for irrigation and fertilization [8]. Progress in wastewater reclamation and reuse began in the middle of the last century, mainly due to population growth and the need for more agricultural production [10,11].
One of the first modern comprehensive evaluations of the “economics and technical status of water reclamation from sewage and industrial wastes” was published in 1951 [12]. This report was one of the first to provide detailed cost comparisons of fresh and reclaimed water. Two additional important studies were published in 1955: one a comprehensive review of water reclamation and the other an analysis of sewage spreading [10,11].
Today, wastewater is considered any source of water that has been contaminated by human activities (e.g., domestic, industrial, agricultural, commercial, and any other). It can include greywater, defined as wastewater generated from non-toilet-related domestic uses resulting from in-house source segregation [13], i.e., any return water from land irrigation not involved in growing crops [14]. Agricultural drainage water can be a source for further irrigation when collected; however, as it first passes through soil and drainage networks, it accumulates salts, fertilizers, and agricultural chemicals (e.g., pesticides), so it might need treatment before reuse [15]. In water-scarce regions, treated greywater can reduce water stress as an alternative NWR for non-potable uses, pending treatment to remove contaminants [16,17,18]. Also, stormwater and other urban runoff ending up in municipal, industrial, or other sewerage systems should be considered [19].
Treated wastewater is one of the most important NWRs [3,20,21]). Depending on the type of treatment, it may be employed in agricultural and urban landscape irrigation (fertigation if it still contains suitable quantities of nutrients) [22] and industries [23]. It has great potential in water-scarce regions, especially in semi-arid and arid regions [24,25].
Globally, the main use of treated wastewater is agricultural irrigation and, to a lesser extent, urban and industrial uses and washing. While often advertised by the public and especially farmers, as they are directly impacted by the costs and consequences of these projects [26,27,28]. Recently, due to freshwater scarcity issues and more reliable treatment technologies [29], wastewater reuse acceptance has improved.
In some developed countries, treated wastewater is widely used for various purposes. In the European Union (EU), especially in southern countries, it is mainly reused for irrigation, especially in Cyprus and Malta. Worldwide, the pioneer country in water reuse is Israel, where about 90% of treated wastewater is reused for irrigation. The EU water reuse regulations, in compliance with the EU Urban Wastewater Treatment Directive [30,31], focus on the reuse in agriculture of urban wastewater treated and further reclaimed by a set of national requirements. Those requirements apply to reclaimed water destined to be used for agricultural irrigation and depend on the crop and the irrigation method [32].
Currently, one of the most important developments in the field of water reuse is the reuse of treated wastewater effluent to produce potable water. Following advanced water treatment, two types of potable reuse (PR) applications are used: (a) indirect potable reuse (IPR) employing an environment barrier (i.e., groundwater or surface water augmentation) and (b) direct potable reuse (DPR) (distribution after treatment without any environmental barrier). These two PR applications are illustrated in Figure 1. It should be noted that the oldest continuously operational DPR facility in the world is in Windhoek, Namibia, which has been in operation since 1968. Interest in potable reuse has increased steadily over the past 15 years. In December 2023, the State of California adopted regulations for DPR. Currently, the largest IPR facility is the Orange County Water District, a groundwater replenishment system in Southern California that has a capacity of 130 MGD. The purified water is used for groundwater augmentation (see Figure 1). An important advantage of using treated effluent for PR is its continued availability, subject to conservation measures, considerably lower energy requirement relative to seawater desalination, and economic benefits as compared with seawater desalination. For most large cities, agricultural reuse is not a viable reuse option for treated effluent because of the costs associated with the transport and storage of the treated effluent to locations where it can be used for agricultural irrigation. For these and the reasons cited previously, it Is anticipated that In the future, the PR of treated effluent will be an important part of the water portfolio of most large cities.
The development of new and Improved wastewater treatment technologies has progressed rapidly in the past few years. For example, carbon-based advanced treatment (CBAT) is being promoted to provide safe and reliable augmentation of drinking water supplies [33,34]. Treatment technologies, such as reverse osmosis or CBAT, are used in medical centers where the use of high-quality water is required, such as, e.g., in hemodialysis. It is a clear indication of the high quality of the results achieved with ongoing technological progress.

2.4. Sea, Saline, and Brackish Water

To utilize sea, saline, and brackish water, salts and other specific TDS and other constituents must be removed. The World Health Organization (WHO) set criteria for drinking water quality, with a limit of 300 ppm for total dissolved solids (TDS). However, the limit of 500 ppm has been applied by some authorities [35,36,37]. Desalination is the process used to eliminate salts from these NWRs to produce water suitable for human consumption and industrial or agricultural uses, but is often perceived as an environmentally damaging and expensive alternative and is affordable only for affluent countries due to high energy and technological requirements [38].
The desalination practice has a long history. It appears that Minoans in prehistoric times (ca 3200–1100 BC) had implemented water desalination (e.g., distillation) by boiling seawater and capturing the vapor as freshwater separated from the salts [39]. The ability to obtain freshwater from seawater was of fundamental importance in the development of the Minoan Civilization and its Thalassocracy (sea power) during the Bronze Age. Minoan vessels were able to travel great distances without the need to stop and obtain fresh water. The ability to travel all over the Mediterranean Sea allowed the Minoans to contribute to the sustainability of the region through the sharing of their scientific development [40]. It is also interesting to note that during historical times, the Greek philosopher Aristotle (384–322 BC) recognized that seawater could become freshwater through the exchange of energy [39]. In the Roman and Hellenistic eras, major developments were made in hydraulics, transport, and storage of drinking water, especially in enclosed cisterns. The technologies involved in the use of seawater as a source of fresh water in the mid-1950s were described in an article by [41].
Water 16 01015 g001
Figure 1. Schematic diagram illustrating various forms of PR for advanced treated water where satellite wastewater and advanced water treatment facilities are employed. IPR is depicted in the upper left as groundwater and surface water augmentation. DPR is depicted in the upper center as raw drinking water augmentation. Although a satellite plant is shown, the advanced water treatment facility could be located adjacent to or at some distance from the wastewater treatment facility (adapted from [42]).
Figure 1. Schematic diagram illustrating various forms of PR for advanced treated water where satellite wastewater and advanced water treatment facilities are employed. IPR is depicted in the upper left as groundwater and surface water augmentation. DPR is depicted in the upper center as raw drinking water augmentation. Although a satellite plant is shown, the advanced water treatment facility could be located adjacent to or at some distance from the wastewater treatment facility (adapted from [42]).
Water 16 01015 g001
Several desalination techniques exist today that may be classified into conventional and nonconventional methods, such as distillation, nanofiltration, electrodialysis, and reverse osmosis (Figure 2). The latter is the most widely used desalination technique in the world.
A major advantage of desalinating seawater is that the source of supply is essentially endless, and the impacts of climate or weather conditions are minimal. Lastly, by combining desalination technologies with renewable energy, the cost of desalinated water has been reduced. However, it does have some disadvantages, such as impacts on marine ecosystems in the case the sea water has an increased concentration of salts.
Desalination technologies are developing rapidly to support the sustainability of water resources. However, issues related to energy consumption by these technologies require more attention to increase their applicability [39]. Globally, estimates on the capacity of desalination technologies include over 100 million m3/d for seawater and brackish water (Figure 3).

2.5. Harvested Rainwater

The term “rainwater harvesting” refers to a variety of rain runoff collection and storage systems used to increase water availability for other purposes, such as groundwater recharge, irrigation, or even domestic use, mostly in arid and semi-arid areas (e.g., regions of the Mediterranean basin) [44,45,46,47,48]. For these areas, the use of rainwater harvesting to supply drinking water in urban areas has a long history, dating from the late Neolithic and early Bronze periods [49]. Mesopotamia (e.g., today Iraq and Jordan) and Minoan Crete, Greece, are known for their water supply systems [47,50,51]. In Minoan Phaistos palace, for example, cisterns were used to collect rainfall water while care was taken to protect water from contamination (e.g., cleaning of roofs or using sandy filters before water flowed into the cistern) (Figure 4a,b) [52,53]. The Ancient Greeks had their settlements away from wet areas, e.g., lakes and rivers [54,55]. Rainwater harvesting has also been practiced in other areas of the world, such as India or China, since the 3rd millennium BC [52]. Rainwater harvesting has also been widely used in the developing world [56].
In general, rain harvesting was mainly used in areas under water scarcity, e.g., in arid and semi-arid regions [50,57,58,59]. The collected water, from rooftops and non-rooftop areas, can be used in settlements and cultivated areas [60,61]. Besides water supply, it can protect areas from flooding due to extreme precipitation events [62,63]. Rain harvesting, since it is a low-cost and low-risk technology [64,65,66], has great potential for both developed and developing countries [67]. It should be noticed that harvested stormwater and other runoff in urban drainage areas are included.
In rural and regional areas where rainwater from roofs is harvested and stored in water tanks for potable use, the stored water should be tested regularly for safety and health concerns. For example, in Australia, in the region of Cadia, more than 800,000 homes are not served by a public water supply. After prolonged dry periods, heavy rainfall events can contaminate the stored water, even with the diversion of the first runoff, because of the extent of rooftop accumulation of hazardous metals in dust emissions from the operation of the Cadia mine [68]. Therefore, treatment of the harvested water should be considered dependent on reuse [69]. Also, several other issues still exist in the domains of technology and economy [2,67].

2.6. Recharged Groundwater

Groundwater recharge is defined as the practice of augmenting groundwater aquifers through human intervention [70]. This technology, known since Hellenistic times, however, had limited application. During the industrialization of Europe in the 19th century, technology developed progressively to supply water to the growing population [71]. It was developed through the evolution of different methods [3,72]. Specifically, two principal methods of groundwater recharge exist: recharge through surface application in spreading basins, also known as infiltration basins (see Figure 5a), and recharge through subsurface injection wells (Figure 5b) [73]. Although surface spreading of wastewater has been used for years, one of the first truly scientific studies of the process involved in sewage spreading was conducted by the Sanitary Engineering Research Laboratory at the University of California, Berkeley, CA, USA [10]. One of the principal findings of this study was that the rate of infiltration was governed by the quality of the applied wastewater.
Groundwater recharge combines different reuse perspectives, such as wastewater reuse, mitigation of seawater intrusion, and stormwater runoff. The latter includes the use of rainwater to recharge groundwater aquifers, a strategy that is suitable for managing floods in urban areas [74]. The potential locations for artificial recharge are dependent on several factors regulated by terrestrial and climate characteristics, such as rainfall, drainage density, lineament density, slope, soil type and permeability, land use/land cover, geology, and geomorphology [3,75]. One of the first uses of the term “the indirect cycle of water reuse” was in an article published in 1969 [76]. Several novel approaches, such as the use of artificial intelligence (e.g., machine learning algorithms and models) or GIS-based technologies, have been developed for identifying suitable artificial groundwater recharge locations [77,78] and evaluating their efficiency [79]. Such techniques facilitate managers and policymakers in establishing and managing proper groundwater recharge. In any case, all groundwater artificial recharge projects should be designed and considered regarding their technological validity and environmental and economic viability [80,81].

2.7. Agricultural Drainage Water

Agricultural drainage water is considered excess water, which is removed from natural, artificial, and sub-surface irrigated agricultural areas. Drainage water is considered a NWR [3]. It can be reused to irrigate crops; however, before its use, it should be considered for the potential existence of harmful substances in it; it may contain leached nutrients, salts, agrochemicals, or other pollutants dependent on the use of the land and the quality of the applied water, from where drainage water is captured [82,83,84]. Therefore, technologically cost-effective strategies for agricultural drainage water management, such as control drainage systems, better timing for fertilization, subsoil irrigation practices, and treatment technologies, are needed before reuse [85,86]. A recent study has proposed a model-based assessment method for agricultural subsurface drainage strategies to achieve sustainable crop production and environmental protection [87].

2.8. Cloud-Seeded Water

Cloud seeding, a new technology, was developed in 1946 by Vincent Joseph Schaefer (4 July 1906–25 July 1993). Schaefer conducted the first true cloud seeding experiments by aircraft. He dropped 6 pounds of crushed dry ice into a cloud in the Adirondack Mountains of New York. The technology, by dispersing agents/substances into clouds, can change their characteristics, affecting precipitation. It should be noted that the water obtained from cloud seeding is generally considered to be of high quality. However, the technologies in cloud seeding projects are characterized by high cost, requiring in parallel experienced personnel [3]. Moreover, the chemicals used in this technology (e.g., silver iodine) can harm the environment, plant species, and humans. In areas with a lack of infrastructure, cloud seeding may increase the risk for the population due to potential extreme weather conditions, such as flooding, caused by the application of cloud technology [88]. Due to its disadvantages, novel approaches, such as AgI-loaded silica aerogels, and optimization models have been developing [89,90].
Israel applied cloud seeding in the 1950s in the northern parts of the country. They used airplanes and ground stations to emit silver iodide [91]. However, since 2021, the projects have been stopped for several reasons, including the variability of results, high cost, and the development of other technologies (e.g., desalination of seawater).

2.9. Dew and Fog Water

Water harvesting has emerged as a viable solution to address the water shortage. Besides rain harvesting, fog and dew waters are also an option, even though water collected from the atmosphere is at low levels dependent on the local conditions. In the first, the capture of water is from fog, whereas in the second, water originates from the condensation of vapor on surfaces with a temperature below the dew point [92,93,94]. Previous experimentation in semi-arid and arid regions (e.g., Syria) highlights the potential of dew and fog water as complementary sources to the existing freshwater supply [95].
Dew occurs in high frequency in many different places and climates and therefore lately has received attention as an alternative source of water [96]. In general, dew is a meteorological phenomenon commonly occurring on a global scale [97]. Also, it is a potable water resource since it originates from atmospheric moisture that is altered into liquid water [98]. There is some evidence of its use in arid and semi-arid regions [99,100]. Several recent investigations have focused on dew water collection [101,102,103].
Fog water, suspended water droplets (non-rainfall), and moisture near the earth’s atmosphere are available in fog-prone areas. Passive collection of fog water has been applied in different locations of the world, depending on the geographical and climate locations [3]. The world’s increasing water demand, especially potable fresh water, has led to the use of NWRs such as rain and fog water collection. As mentioned previously, although rainwater collection is relatively old, simple practice is often an erratic source of water.
Fog water harvesting, as a source of potable water, could be a sustainable practice for providing drinking water for human consumption without any energy consumption and at a low cost. It is a good quality of fresh water and vital for water harvesting within Integrated Water Resources Management (IWRM) [104,105,106].

2.10. Fossil Water

Fossil water, also called “paleowater”, is a freshwater resource that was formed through millennia since the Holocene era (40,000 years ago) in an underground aquifer that is undisturbed and can be found on every continent. These aquifers are often geologically sealed by impermeable rock formations at both their lower and upper limits [107]. Undisturbed aquifers of fossil water have been discovered and exploited since the 19th century, thanks to the development of mechanically motorized pumps. Fossil water is a highly valuable but finite and non-renewable water resource that can provide a significant supply source in areas under severe water scarcity, often for agricultural irrigation; however, because these reservoirs are naturally isolated, any further water recharge is impossible or extremely reduced.
In the USA, one such source is the Ogallala Aquifer, an underground sediment formation spanning under eight western states from the Canadian border to the north to the Gulf of Mexico in the south (Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming). It contains the nation’s largest underground reserves of fresh water, currently supplies 30% of all irrigation water in the US, and supports 20% of the nation’s wheat, corn, cotton, and cattle production. Since large-scale irrigation in the western states began in the 1950s, the average water level decline across the aquifer was over 5.5 m, according to the last 2017 published USGS data, with highs of about 15 m in Texas and over 9 m in Kansas [108], but in some isolated areas, they have dropped even more than 30 m since 2001. In 2023 alone, levels in some wells dropped over 3 m due to increased pumping during the severe 2022 drought. To reduce water level drawdown and extend the aquifer’s life, measures are being implemented locally to substitute nonrenewable fossil water with other NWR (e.g., effluents from WWTPs for golf courses/crop irrigation) and aquifer recharge with treated wastewater.
A recent study, via a model, has quantified and predicted water deficits and groundwater depletion in fossil aquifer systems in North Africa and the Arabian Peninsula [109]. In that model, different climatic and socio-economic scenarios from 2016 until 2050 were considered, projecting severe water deficits for North Africa, particularly in Egypt and Libya, and the depletion of North African fossil aquifers up to 15% of their exploitable water capacity. Foul depletion of fossil aquifers was projected for a period of 200–350 years. Regarding the countries of the Arabian Peninsula, more severe deficits were projected, leading to the full depletion of the exploitable fossil aquifer systems and groundwater resources in a period of up to 90 years [109]. The authors also highlighted anthropogenic rather than climatic drivers of the situation, projecting subsequent social-economic impacts, particularly for economically weak countries.

2.11. Iceberg Water

Iceberg water is one of the purest NWRs in the world [110] and does not require any treatment process to increase its quality. Floating icebergs will eventually melt in the ocean and may cause damage to offshore structures and ships; therefore, the iceberg transferring (towing) could provide a suitable NWR by supplying melted ice water to locations in need of water (e.g., arid and semi-arid areas under water scarcity) [111]. Major issues, however, concern the difficulty of their transportation over open seaways and dispatching the resources to landlocked areas [3]. Potential environmental impacts are also a critical challenge to exploiting icebergs as a water resource [112].

3. Challenges for NWR

The use of NWR has numerous potential benefits, especially for the areas subject to water scarcity and climate vulnerability (e.g., Mediterranean and MENA countries), such as preservation or even improvement of water resources and problematic lands, support of crop production, and increasing the economic perspectives of the area [3,113], mentioned in the present study. However, technological, environmental/hygienic, socioeconomic, and policy issues are barriers that have significantly reduced the use of NWR in the past.
Currently, one of the most critical challenges that needs to be addressed is the high cost of the infrastructure required for the implementation of NWR, such as inland saline and brackish water desalinization, fossil water extraction, and cloud seeding technologies, as well as iceberg exploitation due to increased transportation costs, which have high demands in energy and in some cases require consumption and dispersion of chemicals, threatening in parallel the environmental and atmospheric quality and increasing further the economic cost. Other issues and challenges include mainly the efficiency and viability of technologies such as dew/fog and rain harvesting, due to uneven meteorological conditions and climate variability that constitute severe constraints in arid and semi-arid areas and influence the commercial exploitation of these technologies. The connection between the use of NWR and energy and the critical goal of applying policies/technologies/practices according to the high-efficiency concept, based mostly on lower water and energy consumption, especially in arid and semi-arid regions, is also an issue [114].
The potential threats to the environment and increased risk to human health are other issues and challenges associated mostly with the use of treated municipal and industrial wastewater, agricultural drainages, and rainwater harvesting, which, despite the available conventional treatment technologies, can increase the risk of spreading dangerous pollutants and contaminants (e.g., antibiotics, toxic elements, etc.) and threaten the quality of natural resources, biodiversity, plant species/crops, and humans [1,115]. The latter is a critical issue as there are still severe knowledge and legislation/policy gaps regarding the transfer/spreading mechanisms (within the soil-water-crop-atmosphere matrix) and protection measures for potential pollutants/contaminants contained in specific types of NWR [2,116]. The potential impacts of the polar seafloor habitats referring to iceberg water exploitation are also unknown [3].
Concerning policy and legislation, the main problems that remain are the deficiencies and failures in the establishment of valid quality criteria and a legislative framework around the use of NWR, a fact that creates uncertainties in the value and perspective of the NWR. For example, a recent study for the MENA region reported that one of the main problems regarding the use of NWR is the inability of policymakers to provide a valid adaptation to the general water reuse agenda based on the country’s economic context [113]. It has been proposed that the use of NWR be promoted by governments as an integral component of a national development strategic plan [113]. Other barriers, according to the latter study, are the limited knowledge exchange from regional and international experiences, the lack of coherence across the stakeholders involved in the use of NWR, and the uncertainties regarding the economic profit for the users. A recent study dealing with global water reuse proposed a connection among actors/stakeholders, and beyond that, the assessment of impacts on ecosystem services by the applied water reuse practices as a tool to assess and improve the efficiency of the reuse systems. In addition, consideration of the reuse schemes in terms of their compliance with the established global sustainable development goals (SDGs) was proposed [2]. From this perspective, the use of NWR should be assessed on the basis of its impacts on ecosystem services and contribution to the SDGs.
Overall, more sophisticated, economic, environmental, and climate-friendly technologies and strategies/policies are needed, adjusted to site-specific conditions, to improve substantially the efficiency, validity, and viability, and therefore the public and economic acceptability, of the use of NWR to protect the environment and life.

4. Epilogue and Recommendations

In this review, NWR has been identified and described. NWR can be categorized as (a) currently available and (b) potentially available. Within the currently available category, all of the NWRs are not at the same level of development. The two most important and widely used NWRs are wastewater and high-salinity sources, including sea, saline, and brackish waters, due primarily to their easy availability and access. The implementation of the currently available NWR at a regional and local scale is influenced by technical aspects, socio-economic and climatic conditions, and policy issues. While the potentially available NWR is of interest, their application is limited currently. It is believed that the question of ownership will limit the utilization of iceberg water. Regardless of whether NWR is available or has potential, future work on NWR should focus on the development of more effective extraction and energy-efficient methods and technologies. Moreover, improved documentation of the potential and perspective of the NWR in the context of a viable economy, environmental protection, and life protection could highlight and enhance the utilization of NWR as a viable and sustainable practice, not only for water supply but also in particular in vulnerable water-short areas of the planet. Also, policy and legislation are pillars in the promotion, acceptability, and use of NWR by farmers and stakeholders and therefore should be thoroughly considered and applied in the country’s economic context.
This paper is dedicated to World Water Day. The principal objective of this review is to highlight the sources and importance of NWR and the need to begin planning for their future utilization in developing a sustainable water portfolio.

Author Contributions

Conceptualization, A.N.A. has the original idea and was written as the original draft. G.T. contributed to all sections and reviewed them. A.G.C. and V.A.T. also contributed to sections, reviewed, and edited the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflicts of interest.

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Water 16 01015 g002
Figure 2. Typical reverse osmosis (RO) membrane installation (a) Detail of spiral wound thin film composite RO membrane; (b) Schematic of pressure vessel, which can contain up to 8 RO membranes modules with a length of 1 m, and (c) One bank of a large RO installation at the Orange County Water District Groundwater Replenishment System. The additional RO pressure vessel shown above the stack of six RO vessels is used to compare the performance of new RO membranes to the existing RO membranes.
Figure 2. Typical reverse osmosis (RO) membrane installation (a) Detail of spiral wound thin film composite RO membrane; (b) Schematic of pressure vessel, which can contain up to 8 RO membranes modules with a length of 1 m, and (c) One bank of a large RO installation at the Orange County Water District Groundwater Replenishment System. The additional RO pressure vessel shown above the stack of six RO vessels is used to compare the performance of new RO membranes to the existing RO membranes.
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Figure 3. Variations of desalination across the world since 1960 [43].
Figure 3. Variations of desalination across the world since 1960 [43].
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Figure 4. Water supply system in Phaestos palace: (a) an open yard used to collect rainwater, which was diverted to a series of small cisterns, and (b) a special cistern with a coarse sandy filter (photographs by A. N. Angelakis).
Figure 4. Water supply system in Phaestos palace: (a) an open yard used to collect rainwater, which was diverted to a series of small cisterns, and (b) a special cistern with a coarse sandy filter (photographs by A. N. Angelakis).
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Figure 5. Methods of groundwater recharge: (a) surface spreading basins and (b) subsurface injection wells, both used by Orange County Water District for groundwater replenishment. The water that is spread or injected is treated wastewater effluent, which has undergone advanced water treatment, including reverse osmosis and advanced oxidation. The water is also stabilized before injection (photographs by G. Tchobanoglous).
Figure 5. Methods of groundwater recharge: (a) surface spreading basins and (b) subsurface injection wells, both used by Orange County Water District for groundwater replenishment. The water that is spread or injected is treated wastewater effluent, which has undergone advanced water treatment, including reverse osmosis and advanced oxidation. The water is also stabilized before injection (photographs by G. Tchobanoglous).
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Table 1. Important Applications of Nonconventional Water Resources.
Table 1. Important Applications of Nonconventional Water Resources.
Water ResourceImportant Nonconventional Applications
Currently available nonconventional water resources
Treated wastewaterAgricultural and landscape irrigation, toilet flushing, industrial applications, drinking water, high-purity industrial uses, hydroponics, etc.
Sea, saline, and brackish waterDrinking water; industrial applications; irrigation.
Harvested rainwaterDrinking water with or without treatment; landscape irrigation.
Recharged groundwaterDrinking water with or without treatment; agricultural use; industrial applications with treatment.
Potentially available nonconventional water resources
Agricultural drainage water
Cloud-seeded water		Secondary irrigation.
If captured drinking water with treatment; irrigation.
Dew and Fog waterDrinking water; landscape irrigation.
Fossil waterIrrigation; drinking water with treatment.
Iceberg towed waterDrinking water; irrigation
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MDPI and ACS Style

Angelakis, A.N.; Tchobanoglous, G.; Capodaglio, A.G.; Tzanakakis, V.A. The Importance of Nonconventional Water Resources under Water Scarcity. Water 2024, 16, 1015. https://doi.org/10.3390/w16071015

AMA Style

Angelakis AN, Tchobanoglous G, Capodaglio AG, Tzanakakis VA. The Importance of Nonconventional Water Resources under Water Scarcity. Water. 2024; 16(7):1015. https://doi.org/10.3390/w16071015

Chicago/Turabian Style

Angelakis, Andreas N., George Tchobanoglous, Andrea G. Capodaglio, and Vasileios A. Tzanakakis. 2024. "The Importance of Nonconventional Water Resources under Water Scarcity" Water 16, no. 7: 1015. https://doi.org/10.3390/w16071015

APA Style

Angelakis, A. N., Tchobanoglous, G., Capodaglio, A. G., & Tzanakakis, V. A. (2024). The Importance of Nonconventional Water Resources under Water Scarcity. Water, 16(7), 1015. https://doi.org/10.3390/w16071015

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