The Importance of Nonconventional Water Resources under Water Scarcity

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.

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].

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.

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).

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].

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].

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).

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].

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.