Insights into Global Water Reuse Opportunities

Abstract

The growing population, intensified anthropogenic pressures and climate variability have increased the demands on available water resources, and water reuse has become a high priority, particularly in areas of the world suffering from water stress. The main objectives of this review paper are to consider and identify the potential opportunities and challenges in the implementation of water reuse schemes worldwide by considering and analyzing different fields of interest in water reuse, the current and future global drivers of water reuse policies, the existing advances in treatment and reuse technologies promising elimination of environmental footprint and human health risk, an analysis of the trends in potable and non-potable reuse, and the development of quality criteria and issues related to transition circular economy. Moreover, the major knowledge gaps in critical issues on different domains of water reuse schemes are discussed. For this study, a thorough analysis of the current literature was conducted, using research and review articles, technical reports, specific national (and EU) proposals, guidance documents, and legislative initiatives and actions, as well as any validly disseminated findings by scientists around the world in the wider scientific area of (alternative) water resources, water supply, water management, sustainable development, and protection of public health. Water reuse practices are expected to increase in the future, mainly in developed countries and climate-vulnerable areas of the planet. Current advances in wastewater treatment and water reuse technologies can provide the opportunity for the foul exploitation of alternative water resources, increasing the potential of potable and non-potable water reuse systems worldwide, relying on pollutant/contaminant elimination, and improving economic and energy performances. Moreover, paradigmatic and technological switches based on an improved understanding of the relationships between the water cycle and the Water–Energy–Food (WEF) Nexus will increase the perspective of water reuse schemes. The benefits of the recovery of nutrients through sewage wastewater treatment are also highlighted, arising from reduced costs associated with their sheer removal and the supplement of fertilizers to the WEF Nexus. On the other hand, reduced nutrient removal may promote agricultural or landscape reuse practices, contributing to less energy consumption and reducing GHGs emissions. Regarding the management of water use schemes, a holistic approach (integrated management) is proposed, incorporating regulatory actions, actions increasing public awareness, interconnection among actors/stakeholders, and efficient control and monitoring. The establishment of quality criteria is paramount to preventing undesirable impacts on humans and the environment. The study considers the “one water” concept, which means equal water quality criteria independent of the origin of water, and instead differentiates among different types of water reuse as a means to facilitate implementation and management of potable and non-potable water reuse. Finally, it highlights the need to understand the impacts of water reuse systems on ecosystem services (ESs) and the consequences of achieving the global sustainable development goals (SDGs).

1. Introduction

The growing population, intensified anthropogenic activity, and changing climate have dramatically increased the pressures on available water resources [1,2,3]. Water reuse has become a high-priority, emerging issue around the world, particularly in areas suffering from water scarcity [4,5,6]. Vulnerability to weather extremes, the frequency and intensity of droughts, and their environmental and economic damages have drastically increased over the past thirty years [7]. Recent droughts illustrate the extent of the issue: summer 2022 is considered Europe’s worst dry spell in 500 years, with a combination of record-high temperatures and low rainfall that caused rivers to dry, crops to fail, and food insecurity and prices to increase [8,9,10]. The American West experienced its most severe drought conditions in 1200 years: Lake Mead and Lake Powell, feeding water to large areas in the western US, reached record low levels [11]. In China, sections of the Yangtze (China’s longest river) reached the lowest level since 1865 [12]. The drought had severe impacts on agriculture: in Italy, the Po River experienced its worst water crisis in approximately 70 years, disappearing completely in some areas [13]; consequently, local rice and maize harvests significantly dropped [14]. Similar situations occurred across Europe, increasing fears of a food security crisis. The drought also exacerbated the critical energy situation developed as a consequence of the Ukrainian war: reduced hydroelectric supply (in Norway, hydropower reservoir levels, which produce about 90% of the country’s electricity, dropped to a 25-year low), lack of sufficient flow for the cooling of nuclear plants, and river navigation severely affected industrial production, energy generation, and prices [15]. At the same time, the prolonged overexploitation and degradation of surface and ground waters lead to a decrease in base flows in rivers, a reduction in subsurface waters, and saltwater intrusion in coastal aquifers [16,17].

The awareness of users about water resources and environmental protection is encouraging more efficient and sustainable use of conventional water resources. Water reuse is compatible with the circular economy concept and constitutes an important strategy to alleviate the adverse impacts of excessive pressures on water resources [18,19]. The reuse of treated wastewater from Wastewater Treatment Plants (WWTPs) has emerged as an important option to alleviate water scarcity situations: the United Nations defines wastewater as an untapped available water source; furthermore, according to UNESCO, improved wastewater treatment and an increase in reuse, as indicated in Sustainable Development Goals Target 6.3 (Clean Water and Sanitation), would support the global transition to a Circular Economy [20]. Remarkably, water reuse can affect water and non-water-related sustainable development goals (SDGs) [21,22,23], such as SDG 2 “Zero Hunger”, SDG 7 “Affordable and Clean Energy”, SDG 12 “Responsible consumption and production”, and SDG 13 “Climate Action”. The reuse of treated wastewater has thus become a priority across European Union (EU) Member States [24,25] to reduce competition among freshwater users and mitigate the adverse impact on natural water bodies that are under overexploitation and degradation, as mentioned above. The spreading of water reuse in the EU was recorded by a survey in 2017 that identified 787 schemes and main sectors of water reuse across 16 countries [26].

Although Europe is experiencing increasing water scarcity, it still shows a small percentage of global reuse applications, lagging behind the Americas and Asia in the implementation of these solutions. Half of the European countries are facing water stress, as reported by Ungureanu et al. [5] (Figure 1), who divided European member states into the following four categories according to the water stress index: low (<10%), moderate (10–20%), medium (20–40%), and high (40–80%). It is estimated that 10% of the European area and 14% of the population were affected by water scarcity. The available data showed a total installed capacity for water reuse in the EU of 1 billion m³/y [27], compared to a global water reuse just for irrigation of 7.1 billion m³/yr. Reuse schemes are more common in coastal areas, where freshwater resources are limited and often adversely affected by droughts as well as by over-abstraction due to intensive tourism and agricultural activities. The identified schemes in the EU cover both non-potable and indirect potable uses across a wide spectrum of possible purposes, from agricultural irrigation to landscape irrigation, including golf courses, industrial uses, and the augmentation of the water supply. Agricultural reuse remains the most common application in Europe (39% of identified schemes), followed by industrial reuse (15%), mostly located in northern Europe and reuse for recreational purposes (11%). At the global level, industrial use, followed by agriculture and landscape irrigation, is the largest reuse sector [26].

Industrial water consumption accounts for about 20% of global water use and is expected to increase by a factor of five from the year 2000 to 2050 [28]. In the USA, the largest industrial use of reclaimed and reused wastewater effluents is for cooling, especially for power plants, but it also finds application as process water at metal works, chemical plants, petroleum refineries, pulp and paper industry, sand and gravel washing, laundries, and dust suppression operations. High-technology reclamation and reuse processes were also introduced in manufacturing (e.g., semiconductors), requiring high-quality treatment to meet the high-purity process water specifications [28]. Agriculture is the largest consumer of freshwater, accounting for more than 80% of total water use worldwide [29]. Considering growing food demand and the uneven spatial and temporal distribution of available water resources, the use of alternative water supplies appears to be a reasonable option to close the gaps between supply and demand [5,30]. When natural water reserve availability declines, reclaimed water constitutes one of the most reliable sources due to its relatively constant production, which is relatively unaffected by punctual climatic conditions. Already, agricultural uses make up more than 32% of global wastewater reuses, and landscape uses make up an additional 20% [20]. Often, the reuse of reclaimed wastewater might be the best quality source available for irrigation and fertilization (“fertigation”), representing a free source of fertilizers; irrigation and fertilization (“fertigation”) with reused effluents can simultaneously provide water and nutrients to crops and urban green and, as such, are potentially ideal applications of reclaimed water [5,31]. A techno-economic evaluation of agricultural fertigation sustainability showed that significant mineral fertilizer savings could be achieved on selected crops that could be easily supplied with most of the required N through WWTP effluent fertigation [32]. A 100-year-long, high-resolution simulation of monthly N and P loads to global river networks showed that the proportion of the inorganic forms of total N and P inputs originating from fertilized agricultural lands rose from 30 to 43% and from 56 to 65%, respectively, during the 20th century [33]. Soil loss from agricultural lands, delivering large amounts of bound P to surface waters, has become a significant P source and is likely to increase long-term accumulation in lakes and reservoirs [34]. Fertigation could thus be a mitigating factor for nutrient runoff losses [35,36].

The most recent EU directive on the “Minimum Requirements Applicable to Reclaimed Water Intended for Agricultural Irrigation” (EU, 2020/741) [25] scheduled for adoption by all EU member states defines water quality classes for irrigation. The US and several other countries have also adopted water reuse legislation. This is addressed in Section 4.5. Despite this, discussion is still ongoing on which target contaminants should be included in the standards for safe wastewater reclamation and reuse since many pollutants, such as antibiotics, antibiotic resistance genes (ARGs) [37,38,39,40,41], Endocrine Disrupting Compounds (EDCs) [42,43], Pharmaceuticals and Personal Care Products (PPCPs) [44,45], and Contaminants of Emerging Concern (CECs) [38,46], are still not included even in the most restrictive drinking water and wastewater treatment regulations. These substances, although present in very low concentrations, could threaten human and ecosystem health and alter the biochemical processes occurring in nature [45,47,48,49,50,51,52] and must be considered when applying current technologies (or even developing new advances in treatment technology) and identifying water reuse opportunities. Therefore, the formerly mentioned technologies for the effective treatment of wastewater effluents are analyzed and discussed in the present review.

The management of water reuse schemes is still a great challenge, arising mainly from certain deficiencies in different domains of governance and the involvement of numerous actors/stakeholders [53]. This review highlights the application of a holistic approach in the management of water reuse schemes, based on an integrated and flexible management framework, covering local and national social-economic, political, environmental, and climate specialties and involving important actors/stakeholders (e.g., authorities, decision makers, the private sector, and end-product consumers) [6,23,54,55]; Recent work has discussed the integrated water management model and the most important elements that should be included in it [6]. The main goal of this is to build a supportive framework of actions that will help in considering, identifying, and prioritizing the issues, challenges, and opportunities in the domain of water reuse and in achieving specific objectives, such as the establishment and application of quality criteria for the different reuse types either for effluent or receptors, ensuring the effective involvement and collaboration of all actors/stakeholders by applying an efficient control and monitoring program, based on the evaluation of the potential impacts and protection of the environment, natural resources, and humans’ health and prosperity. From this point of view, the consideration and assessment of the potential impacts on ecosystem services (ESs) [56,57,58] and the consequences of achieving the established Sustainable Development Goals (SDGs) [59,60] are also necessary; however, little work has been done so far in the domain of water reuse.

Considering the above issues, deficiencies, and lack of knowledge in specific domains of water reuse, the main objectives of the review paper, depicting the needs and significance of the study, are:

(a)

To consider and identify the potential opportunities and challenges in the implementation of water reuse schemes worldwide by considering and analyzing different fields of interest in water reuse, such as the current and future global drivers of water reuse policies, the existing advances in treatment and reuse technologies promising the elimination of environmental footprints and human health risk, an analysis of the trends in potable and non-potable reuse, and the development of quality criteria and issues related to the transition to a circular economy.

(b)

To reveal the major knowledge gaps in critical issues in different domains of water reuse schemes in the context of improving their performance in terms of benefits gained and minimizing the potential impacts on natural resources, biodiversity, ecosystems, and human health.

For the study, a thorough consideration of the current bibliography dealing with the above issues was conducted, considering research and review articles, technical reports, specific national (and EU) proposals, guidance documents, and legislative initiatives and actions. In addition, any valid disseminated findings by scientists around the world in the wider scientific areas of (alternative) water resources, water supply, water management, sustainable development, and the protection of public health were considered.

2. Evolution of Water Recycling and Reuse

In practice, all the water on the planet is eventually reused. Ionian philosophers recognized that all freshwater on the planet has been naturally recycled since the end of archaic times. Anaximander (ca 610–547 BC), dissertating on meteorological phenomena, identified the main hydrological processes and, broadly speaking, the essence of the water cycle. He reported that: “Rains are generated from evaporation (atmis) that is sent up from the earth toward under the sun” (Hippolytus, Ref. I6, 1-7-D.559W.10). Thereafter, Aristotle (384–322 BC) observed that regarding water phase changes and the role of energy, “The sun causes the moisture to rise; this is similar to what happens when water is heated by fire” (Meteorologica, II.2, 355a 15). He also recognized water mass conservation on a global scale by reporting that: “Even if the same amount of water does not come back every year and in a given place, in a certain period all quantity that has been abstracted is returned” (Meteorologica, II.2, 355a 26) [19,61].

Indirect (unplanned) water reuse is hence a natural phenomenon. Furthermore, throughout the ages, human civilizations have paid close attention to communal sanitation and wastewater management, which has included reuse since the dawn of urbanization. The related knowledge has accumulated throughout humankind’s history, and land application of wastewater, an ancient common practice, underwent subsequent development stages in time with increasing knowledge of the processes, treatment technologies, and evolution of regulations/guidelines [19,62]. Since prehistoric civilizations (e.g., Mesopotamian, Indus Valley, Minoan) and thereafter in Classical, Hellenistic, and Roman times, land application has been used in peri-urban areas for disposal, irrigation, and fertilization purposes [63].

For many centuries after the Roman period and the Middle Ages (ca 330–1650 AD), there had been no substantial improvement in the methods of waste removal. The lack of proper sanitation increased the effects of epidemics in crowded medieval towns. However, in Asian countries (e.g., China, India, and Vietnam) various types of drainage techniques were developed in temples for wastewater disposal on agricultural soils [64]. In recent history, documentation of wastewater application to the land (i.e., ‘sewage farms’) for beneficial crop production has existed in Bunzlaw (Poland) since 1531 and in Edinburgh (Scotland) since 1650 [19,65]. In the following years, in many cities in Europe and the United States, ‘sewage farms’ were increasingly seen as an ideal solution for the disposal of large volumes of generated wastewater. Paris is a good example, with the sewage farms established at Gennevilliers in 1872 eventually handling wastewater from the entire city. In the early 1900s, sewage farms had reached their maximum extent, with established ‘sewage farms’ in Gennevilliers (900 ha), Achères Plain (1400 ha), Pierrelaye (2010 ha), and Triel (950 ha), fed with raw wastewater by pumping stations in Paris [62]. A big ‘sewage farm’ was built in 1897 in Melbourne, Australia [66].

Land treatment systems existed throughout the 20th century in central Europe, the USA, and other areas, but not without generating serious public health concerns and limiting environmental impacts. By the late 19th Century, these systems, due to drawbacks such as areal footprint, operational problems, and inability to achieve modern hygiene requirements, were not easily accepted any longer.

As a reaction to the exacerbation of sanitary conditions due to heavy industrialization and urbanization, modern sewerage systems were developed in the mid-19th century. The first modern water reuse projects were started in California in the early 20th century. These projects were the main drivers for practices for local water scarcity and the benefits to crop yield. Based on these, the State of California recognized its environmental and economic benefits and issued the very first regulations governing water recycling and reuse in agriculture to limit its potential health risk [67].

Today, projects for treated wastewater effluent reclaim and reuse are significantly increasing in several areas of the planet. The main destinations of treated effluents are irrigation (agricultural and landscape), aquifer recharge (including seawater intrusion barriers), industrial uses (including cooling and process water), and other urban uses. High-income countries treat about 70% of the municipal and industrial wastewater they generate; the ratio drops to 38% in upper-middle-income countries, 28% in lower-middle-income countries, and 8% in low-income countries [68]. Estimates indicate that over 80% of all globally generated wastewater is still discharged without treatment [69,70]; the release of untreated discharges remains common practice, especially in developing countries, due to a lack of infrastructure, technical and institutional capacity, and financing.

International organizations estimated more than 25% of the average annual increase in reuse water volumes in the USA, China, Japan, Spain, Israel, and Australia [71]. In Spain, the total volume of treated effluent reuse is expected to reach 1000 Mm³/y in 2025 from 500 Mm³/y in 2015 [72]. In Israel, more than 90% of treated wastewater effluents are reused, mainly in agriculture. In Singapore, NEWater already meets up to 40% of the nation’s current water needs [23], which is expected to increase to 55% by 2060 [73]. Wastewater, whose value had not been fully appreciated until recently by modern society, is increasingly recognized as a potential water source for potable and non-potable uses, with associated social, environmental, and economic benefits. In addition to the more common reuse applications, high-grade reuse for both indirect potable reuse (IPR) and direct potable reuse (DPR) has been implemented in the last 60 years. An overview of selected IPR/DPR projects, made possible by very efficient wastewater treatment and reuse processes, is shown in

Table 1. Overview of selected IPR and DPR projects (adapted from [23]).

Location	Capacity (m3/d)	Description
Wulpen, Belgium	7200	Treated water is injected into the aquifer before being reused indirectly as a potable water source.
Windhoek, Namibia	21,000	Wastewater effluent is blended with conventionally treated surface water for DPR.
Essex, UK	40,000	Treated water is returned upstream to the river for IPR.
Orange County, California, USA	379,000	Treated water is injected into an aquifer before its reuse in IPR. Capacity to be increased to 492,000 m3/d.
Singapore	462,000	Treated wastewater is used for DPR and IPR.
Australia	4100	Wastewater effluent is treated by MF/RO and advanced oxidation is injected into groundwater and then is reused.

[23].

3. Critical Drivers of Water Reuse Policies

Notwithstanding the usefulness and unavoidability of appropriate water reuse policies, several critical issues in the domains of technology, policy, economy, and societal acceptance should be preliminarily identified and addressed. A thorough understanding of the role of critical factors and drivers behind the need for “new” sources is needed. In this section, a discussion of the most important factors concerning the adoption and implementation of water reuse projects in critical areas of the planet is carried out. The discussion emphasizes human (e.g., population growth, urbanization, and migration), environmental (e.g., stresses for the quality of water resources), climatic, regulatory, and technological factors.

3.1. Increasing Anthropic Water Demand and Its Environmental Impacts

It has been estimated that the world population will increase in the next 30 years from 8 to 10 billion, a trend that mostly reflects growth concentrated in developing countries; at the same time, a corresponding increase in urbanization is expected [74]. The UN projects a 70% increase, from 4 to 6.3 billion, in urban population from 2015 to 2050 [75]. Urban population will develop unequally; by 2030, 60% of urban dwellers will live in coastal regions; and the largest predicted increase in urbanization will occur in the poorest areas of the planet, South Asia and Sub-Saharan Africa, accounting for extensive cropland losses. According to different forecast scenarios, global urban land cover will increase by up to 110% by year 2030 and 210% by 2050, respectively, compared to the year 2000, with land uses of nearly 1.3 and 1.9 million km², respectively [76]. Such concentration will inevitably cause drastic and irreversible land-use changes in peri-urban areas, further increasing the pressure on rural environments and highly productive farmlands, as well as on the quality and availability of natural resources [77,78,79,80]. Other related implications of urbanization include changes in the water–energy–food nexus [81,82], climate impacts [83], the modification of local hydrology and floods [84,85,86], biogeochemical cycles, losses in biodiversity [86,87], and ecosystem functioning and services [86,88,89,90]. According to the USDA, water quality and stream integrity can be negatively affected by as little as 5 percent of the land surface imperviousness increase from urban expansion [91].

This situation creates enormous challenges in terms of the governance and management of resources and infrastructure. China is a typical example of a country with a growing population, urbanization, and industrialization already facing significant challenges in the sectors of economy and environment, as well as in the management and supply of water; estimates showed an increase of over 10% in water consumption between 2000 and 2014, i.e., from 550 to 610 billion m³/y, respectively [92]. Additional requirements of 201 billion m³/y are expected by 2030 [93]. On the other hand, China was characterized until recently by a low wastewater treatment rate (about 69%), discharging large quantities of raw wastewater to the environment, up to 69.5 billion m³ in 2013 [92].

A large number of cities in the USA [94], the EU (EC 2011), and the Middle East [95] are also experiencing severe water supply issues. In Europe, Cyprus suffers from the highest water stress due to a combination of limited supply, urban spread, and population growth, which is exacerbated by intensive tourist fluxes [96]. Such pressure on water resources could in turn exacerbate competition and conflicts among water users, particularly among agricultural, urban, and industrial sectors.

Water resources in many regions are at high risk regarding both capacity and quality status due to anthropogenic activities. Aquatic ecosystem deterioration caused by overexploitation and over-abstraction, chemical, and microbiological contamination is evident. In water-stressed areas, effluent discharges can represent up to 100% of a water body stream’s flow; for example, in the South Platte River (Colorado, USA), WWTPs discharges can account for more than 69% of the total flow [97]; in Texas, WWTP effluents account for 70 to 100% of the base flow in many water courses [98].

The runoff from urban and agricultural lands can significantly contribute to nutrient transfer to rivers and water basins, impacting environmental quality, biodiversity, ecosystems’ sustainability, human welfare, and health [99]. Aquatic ecosystems in peri-urban areas are more vulnerable to high levels of nutrients due to the contribution of municipal and industrial WWTPs. In the US, at a national scale, more than 50% of the overall nutrient content in waters has been attributed to discharges from urban WWTPs [100]. A typical example is the Reedy River watershed, where it is estimated that the elimination of WWTP effluent could reduce nitrogen and phosphorus loadings by 53 and 49%, respectively [101]. At a global scale, estimates indicate sewage-related contributions of 10% of the total N and P inputs to surface freshwaters; in highly urbanized areas, these loadings could constitute up to 48–53% N and 49–81% P total inland contributions [102].

Common problems in water-stressed areas are groundwater overexploitation and quality degradation [103]. In China, the overexploitation and degradation of groundwater contributed up to 91% of the water supply in some regions [104]. Similarly, in Crete (Greece), groundwater depletion from agricultural abstractions (about 80% of the island’s water supply, relying 92% on subsurface water) is reported [53]. Finally, an example of surface water deterioration is Lake Haromaya in Ethiopia, which became dry due to an inefficient use of water and over-pumping [105]. In many areas, groundwater is the primary source available to compensate for the surface water deficit. A recent study quantified the impacts of human activities on aquifer systems in the USA, Germany, and Iran, highlighting deficits between water consumption and the natural, renewable water supply [103].

Water reuse policies could selectively address the most critical issues arising from anthropic modifications to water resource dynamics, based on local analysis of the possible impacts.

3.2. Climate Variability

Despite uncertainties regarding current and future climate variability impacts, arising mainly from the uncertainty of applied forecasting methodologies [106], and even from debates on the very nature of “climate change” [107,108], the fact remains that climate is one of the critical drivers of natural resources and ecosystems’ services on our planet [109,110]. The potential climate variability, which is still largely unpredictable, such as long-term shifts in rainfall and temperature patterns, flood intensity and frequency, and/or an increase in drought phenomena [84,111,112,113], could induce disasters of unexpected proportion in urban human habitat and infrastructure, affect ecosystems and water resources [114,115,116,117], and impair the security and economic prosperity of society [118,119].

In the food sector, climate variability may cause drastic changes in crop production and food security [8,9], as recently observed in 2022. An increased risk could be expected for human health, particularly in developing countries, due to water-, air-, food-, and vector-borne diseases, mainly as a consequence of changes in nutrients/pollutants/contaminants distribution patterns [120,121,122,123,124,125]. Threats to terrestrial and marine biodiversity, impacting ecosystem functioning and derived services, could also be expected [126,127]. Climate variability is of critical importance in high water-stressed regions [128,129]; a recent study covering 192 countries suggested that Africa is the most vulnerable continent regarding climate impact due to low economic, governance, and social adaptation readiness [115]. Poverty, unsuitable infrastructure, and lack of strong institutions make adaptation and resilience to climate challenges increasingly difficult.

Water reuse could be a resilience-building strategy against the extremization of water resource availability induced by climate variability; in times of drought, reused water could compensate for diminishing supplies from traditional sources [1,4,130]. In addition, water reuse could save energy for long-range water transfer and unnecessary excessive treatment since non-potable reuse will have lower quality requirements than potable uses, avoiding the associated CO₂ and N₂O emissions (the latter in the case of elimination of N removal for fertigation application) [131,132,133,134]. Another field of the positive effect of water reuse on the energy budget associated with the GHGs emissions is its potential for energy recovery during wastewater treatment, referred to as potential, thermal, or even chemical bond energy. These different energy sources can be obtained through the use of domains of technology, such as turbines/water wheels (potential energy), heat exchange (thermal), and biogas production/steam generation (chemical energy) [131,135]. There also technological advances in the of control of GHG emissions during wastewater treatment, such as the sludge-based biochar production, the application of constructed wetlands (CWs), the application of microbial electrochemical processes, and the cultivation of microalgae [136], which could improve the overall climate footprint of reuse schemes.

3.3. Emerging Contaminants and Public Health Issues

Conventional WWTPs are not designed to remove all anthropogenic contaminants; hence, effluents may still contain pathogens, antibiotics, antibiotic resistance genes (ARGs) [37,38,39,40,41], and CECs originated from non- or incompletely metabolized substances in household and industrial discharges [38,46]. These could threaten human and ecosystem health and alter biochemical processes occurring in nature [48,49,50,51]. It has been estimated that in Germany, up to 16,000 tons/year of pharmaceuticals are discarded; 60–80% of them are either flushed down toilets or disposed of with household waste, possibly finding their way to sewers, treatment plants, and water bodies [137]. These compounds may be partly eliminated or, more often, further transformed into metabolic byproducts within WWTPs. Even though high removal rates of specific compounds can be occasionally observed in the aqueous phase, detailed studies often reveal the presence of transformation byproducts in the effluent. Although advances in analytical technology make it possible to detect these contaminants at the nanogram-per-liter level, their detection is still laborious as typical screening approaches are not yet suitable for comprehensive online identification [138].

Effluents may still contain large numbers of bacteria (about 10⁹–10¹² CFU/d-P.E.), and at least 10⁷–10¹⁰ of them could have some kind of acquired antibiotic resistance [139,140]. Gene transfer during biological treatment processes could spread antibiotic-resistant genes (ARGs) that may subsequently propagate in aquatic ecosystems [139]. Advances in molecular biology techniques have improved the ability to detect and identify antibiotics and ARGs in wastewater [41]. Antibiotic resistance was recognized as the most critical public health issue for the 21st century by the World Health Organization (WHO) [37]; hence, water reuse applications must account for this possible presence.

Very recently, during the 2020 COVID-19 epidemic, numerous studies have investigated the relationship between wastewater and the diffusion of this specific virus. Traces of viral SARS-CoV-2 RNA could be detected in sewer systems, allowing public health authorities to determine or even anticipate the trend of infection rates in urban areas; however, there is no evidence that anyone became infected with COVID due to direct exposure to treated or untreated wastewater [141]. The risk posed to human health and the environment due to the presence of the virus in sewage is minimal since its persistence is negligible as it gets mostly destroyed at ambient temperature [142]. Furthermore, viruses and bacteria can be easily removed by physical means (e.g., membrane filtration, now a common wastewater treatment process) from effluents. The typical size of the waterborne microorganisms of concern varies; it can be seen that most bacteria are removed by effluent microfiltration, and most viruses by ultrafiltration, which are shown in

Table 2. Size range of waterborne microorganism types of concern.

Microorganism Type	Size Range [µm]	Removed by
Helminths	10–100	Microfiltration (pore size 0.1 µm)
Protozoa	1–60
Bacteria	0.2–10
Viruses	0.01–0.3	Ultrafiltration (pore size 0.01 µm)

.

Endocrine-disrupting chemicals (EDCs) (e.g., nonylphenol, bisphenol, and triclosan) are defined as “exogenous substances or mixtures that alter the endocrine system functions, and consequently cause adverse health effects in an intact organism or its progeny”. These and other pharmaceuticals have been identified in domestic effluents, sludge/biosolids, industrial, livestock, and other discharges [42,43] and can be transferred to surface water bodies, posing significant risks to aquatic organisms and humans. Wastewater irrigation may accumulate EDCs in soils at toxic concentrations, threatening microbiota and crops [47].

WWTPs are one of the main sources of microplastics’ (<5 mm) environmental diffusion [143]. Microplastics are not removed by conventional wastewater treatment processes and may be introduced into the food chain by discharges to water bodies and/or irrigation causing direct effects on small organisms. They can also impair the environment and human health indirectly; microplastics absorb organic pollutants, heavy metals, and bacterial contaminants dyes [144,145,146,147]; hence, effluent discharges pose potential threats to biodiversity, ecosystems, and humans [148,149,150,151].

Proper attention and consideration of these issues are necessary. Since the presence of such contaminants at micro-concentrations is generally unavoidable in source and wastewater, care must be taken to avoid their environmental diffusion in discharge and reuse water equally. Although there are no set discharge limits for most of these micropollutants, some regulations exist. One of them, the EU Water Directive 2008/105/EC, lists 33 priority substances/groups of substances, for which environmental quality standards (EQS) were defined based on available data on the acute and chronic effects on the aquatic environment and human health. Regarding EDCs, the first list for monitoring beta-estradiol and nonylphenol in drinking water was released in January 2022 [152]. Moreover, recently, the EU introduced a proposal to monitor microplastics in WWTPs [153]. Work is in progress on the determination of admissible limits for new contaminants; however, their number and possible metabolites make this task quite arduous; specific applicable criteria for these substances via the application of newly proposed wastewater treatment measures should be applied in 2040 [153].

4. Wastewater Reuse Opportunities: Existing Technologies and Future Challenges

The increase in WWTP effluent reuse potential is necessarily based on the adoption of sustainable, upgraded processes [37,38,154,155,156,157,158]. An analysis of the characterization and needs of water reuse quality can lead to a “fit-for-purpose” design of wastewater treatment facilities to fulfill reuse with optimized energy inputs and environmental and health protection [159]. WWTP facilities, given the advances in treatment technology, can generate effluents that can properly find applications in several uses, such as demanding agriculture [160], replenishment of overexploited aquifers [161], solving seawater intrusion problems [162], or even satisfying drinking water requirements, especially in areas with acute water shortage problems [163].

Technological advances in decentralized treatment facilities can help promote local reuse, and the development of specific on-site facilities can promote water reuse at the site of generation or use, such as farms, sports facilities, or industries [164]. Decentralization can be an important step towards the implementation of new development paradigms for water systems, encouraging local reuse and perhaps involving the recovery of specific materials/nutrients or even energy [165]. Hybrid systems, such as constructed wetlands and the use of plants for phytoremediation, may increase wastewater treatment and reclamation potential [166].

4.1. Upgrading Wastewater Treatment Technologies for Reuse (Removal of EDCs, CECs, Antibiotics, ARGs, and Other Microcontaminants)

Processes to improve WWTPs pollutants’ removal capacity from nutrients and organic substances to EDCs, CECs, antibiotics, ARGs, and other microcontaminants to prevent their environmental diffusion are constantly under study. Membrane technology is among the most significant technologies over the last 30 years. Membranes comprise a thin layer of polymers/ceramics or another material to achieve the physical separation of pollutants/contaminants. This is achieved by pushing a liquid stream through the membrane under pressure; a clear permeation is obtained, and solids are retained in a retentate (or concentrate); ion-size constituents (0.001 µm or less) can be removed by reverse osmosis (RO), achieving up to 95–99% of removals (e.g., dissolved organics, inorganics, and microorganisms). This technology produce high-quality drinking water. Ultrafiltration (UF) can remove fine particles smaller than 0.01 µm; microfiltration (MF) operates at particle sizes an order of magnitude larger. Membrane technology can be applied as a purely physical process for drinking water and treated effluents (tertiary polishing, physical removal of pathogens, desalination of sea/brackish water), or in conjunction with biotreatment. In the latter case, the process is commonly identified as Membrane Bioreactors (MBR), which are used for high-grade wastewater treatment. High-grade membrane filtration (NF and RO) is often applied in wastewater reuse-oriented treatments (

Table 3. Specific characteristics of membrane techniques for water treatment and desalination (adapted from [167]).

Membrane Type	Particle Capture Size	Typical Contaminants Removed	Typical Operation Pressure Ranges	Key Applications
Microfiltration	0.1–10 μm	Suspended solids, bacteria, and protozoa	0.1–2 bar	Water treatment plants, pretreatment in desalination plants, and the preparation of sterile water for industries, such as pharmaceuticals, etc.
Ultrafiltration	ca 0.003–0.1 μm	Colloids, proteins, polysaccharides, most bacteria, and viruses (partially)	1–5 bar (cross-flow) 0.2–0.3 bar (dead end)	Drinking water treatment, the pretreatment process in desalination, and membrane bioreactors
Nanofiltration	ca 0.001 μm	Viruses, natural organic matter, and multivalent ions (including hardness in water)	5–20 bar	Treatment of fresh, process and wastewater
Reverse osmosis	ca 0.001 μm	Almost all impurities, including monovalent ions	10–100 bar	Treatment of fresh, process and wastewater and the desalination of seawater

).

The elimination of many contaminants remains a challenge since many current water treatment techniques, including membrane filtration, concentrate some contaminants in biological sludge without degrading them completely [168]. Even in RO, some compounds (e.g., nitrosamines) can escape, raising possible health concerns.

Advanced oxidation processes (AOPs) (i.e., UV, chlorination and ozonation, sequential chlorine disinfection, solar photocatalysis, photocatalytic ozonation, and advanced oxidation process) are based on the in situ generation of strong oxidants, achieving high-levels of disinfection (UV/H₂O₂ or Fenton oxidation Fe²⁺/H₂O₂) [168]. In addition, advanced reduction processes (ARPs) combine methods such as UV, ultrasound, and microwave with reducing agents (e.g., sulfite, ferrous iron, sulfide and dithionite) to form reactive reducing radicals. However, complete mineralization, either by AOPs or ARPs, is still a challenge attributed to low energy consumption [169]. In the particular subclass of Advanced Oxidation-Reduction Processes (AORPs), AOP oxidants (•HO, •H, H₂O₂, H₃O⁺) and reducing compounds (•H, e⁻_aq), released by the high energy of radiolytic processes [168], can achieve the complete mineralization of almost all anthropogenic pollutants, apart from some forms of PFAS (polyfluoroalkyl substances, also dubbed “forever contaminants”, due to their extremely stable molecular structure) and can inactivate antibiotic-resistant organisms through DNA damage.

In recent times, wastewater stream segregation has been advocated by researchers as a means to curtail urban water consumption, increase local reuse, and increase resource recovery [170]. By “taking the water out of wastewater”, i.e., separating domestic greywater from blackwater at the source, the lightly polluted greywater could be treated on the spot to provide readily reusable supplies for non-potable uses. Treatment technologies for greywater reuse have been discussed in the recent literature [171,172,173].

4.2. Wastewater Treatment and Water–Energy–Food (WEF) Nexus

Energy is a key factor in WWTP sustainability [174]. WWTPs are generally not energetically self-sufficient; high treatment efficiency is usually connected with increased energy use and greenhouse gas (GHG) emissions [175]. The typical energy requirements for conventional and non-conventional treatment processes are shown in

Table 4. Energy use of non-conventional wastewater treatment technology (Data from OCWD ¹).

Technology/Water Source	Energy Required (kWh/m3)		C Footprint 2 (kgCO2eq/103m3)
	Range	Typical 1
Secondary treatment without nutrient removal	0.28–0.37	0.33	0.17
Tertiary treatment with nutrient removal by effluent filtration	0.42–0.52	0.49	0.25
Advanced water treatment facility	0.86–1.10	0.95	0.48
Brackish water desalination	0.82–1.64	1.55	0.77
Ocean desalination	2.52–3.90	3.17	1.59
Inter-basin transfer of water, California State Water Project	2.09–2.62	2.43	1.22
Inter-basin transfer of water, Colorado River	1.62–1.95	1.62	0.81
Conventional water treatment	0.08–0.11	0.10	0.05
Membrane-based water treatment	0.26–0.40	0.33	0.17

¹ Orange County Water District project. ² Based on a plant capacity of 37,854 m³/d.

.

The amount of kWh required to reduce the concentration of a pollutant by one order of magnitude (90%) in 1 m³ of solution defines the electrical energy per order (EE/O) [176]; the latter represents best the energy efficiency of a process and can be used to characterize and compare different technologies regardless of solutions, location, and actual energy prices.

WWTPs effluents contain significant amounts of nutrients; at the global scale, total N and P embedded in wastewater have been estimated at 16.6 × 10⁶ and 3.0 × 10⁶ t/y, respectively [177]. These concentrations may significantly vary at the local scale, depending on the sewage sources, treatment technology, and dilution effects on influents due to climatic or other reasons [92]. Nutrients can be reused in agriculture [32,178]; in China, reused N and P have been estimated at 2.05 × 10⁵ and 2.92 × 10⁴ t/y, respectively, and reused organics, as organic C, have been estimated at 2.12 × 10⁶ t/y [92]. In terms of the percentage of total fertilizer use, estimates at the EU scale indicate that wastewater-recovered P or N could meet up to 14% of the fertilization requirements [179]. This is consistent with estimates at a global scale, suggesting a potential 13.4% demand offset by recovered nutrients from wastewater effluents [177].

The recovery of nutrients through sewage wastewater treatment is highly compatible with the promotion of sustainable practices, gaining concomitant economic benefits arising from the reduced costs associated with their sheer removal and the supplement of fertilizers to the WEF Nexus. Normally, nutrient recovery is carried out from residual biosolids generated by WWTPs in the form of mineral precipitates (e.g., struvite, calcium phosphates, and others) [180]; however, recovery can also occur from the liquid stream processing by appropriate exploitation of existing technologies [181]. In the future, an increase in N and P recovery from WWTPs is expected due to improved biological P-removal techniques and beneficial synergies during P and energy recovery in anaerobic digesters [182,183].

Nowadays, WWTPs are mostly required to remove nitrogen and phosphorous from effluents due to possible eutrophication concerns. This, however, requires large amounts of energy; nitrification alone could demand around 50% of the total energy consumption of an aerobic wastewater treatment facility, or between 11.7–12.5 kWh/kg N removed. On the other hand, the energy-intensive Haber–Bosch process, from which ammonia is produced as industrial fertilizer, requires 10–12.5 kWh/kg N fixed, using 1–2% of the global energy consumption [184]. The full process cycle to first remove, and then generate ammonium can thus require up to 25 kWh/kg N. By calibrating (reducing) nutrient removal according to reuse applications (e.g., agricultural or landscape fertigation), reused effluents may not only constitute a greater added value for such use but also contribute to the energy consumption of WWTPs and the reduction in related GHG emissions [32].

4.3. On-Site Reuse of Wastewater Produced by Agro-Industries

Agro-industries, including slaughterhouses; fish farms; pig farms; and dairy, livestock, and vegetable farms/processing, produce large amounts of wastewater [185,186]. Due to its content (rich in organic materials and nutrients), this type of wastewater has excellent potential to be used in agricultural lands and/or reused on-site, saving significant amounts of freshwater and protecting resources from deterioration. Areas that could be benefited are those with intensified agro-industrial and agricultural production and/or that are prone to climate variability and water scarcity phenomena, such as those in the Mediterranean basin [187,188]. There are estimates of a profit of about 10,000 m³ha⁻¹ of water reused for irrigation purposes over 1.5 years, saving about 6000 m³ha⁻¹ of groundwater annually [187]. In regards to cost, in a case in Italy, the overall cost of the tertiary treatment was estimated at 0.61 EUR/m³, higher than that required for municipal wastewater reclamation (0.35 EUR/m³) [188,189]. However, reuse must be done with great caution to avoid the contamination and pollution of resources and prevent human exposure, a risk that mainly arises from the presence of organic/inorganic pollutants, pathogens, enteric bacteria, viruses, and other emerging pollutants [40,190]. Technological advances in small treatment units should fulfil this task as well as in developing on-site treatment plants that can promote reuse at the site of their production (industrial processing or close agricultural fields) and/or in assembly and transporting to other users, such as farmers or industries, such as biogas production units [191,192].

4.4. Rainwater Harvesting

Rainwater harvesting has been practiced since prehistoric times (e.g., Minoan and Indus Valley civilizations) [193]. Water scarcity in some of the world’s dry regions is not unavoidably caused by low precipitation levels, but it could be increased by the absence of proper methods for the collection of excess rainwater that is thus lost to the environment. Ancient populations in settlements in many parts of the Middle East were able to create a near-permanent water supply in places where nature was not kind enough to provide springs or surface waters. Nabateans, the inhabitants of Petra, excelled in managing rainwater through the use of reservoirs more than 2500 years ago. Underground cisterns in Umayyad desert castles in Jordan desert reveal similar activities during the Islamic era [194]. Many ancient examples of effective water harvesting systems have survived in various areas, and these lessons from old civilizations can be starting points for new solutions today. The collection of rainwater is still achieved today in similar ways, such as in situ harvesting, cisterns/tanks, check dams, percolation tanks, and others.

Sustainable stormwater management and harvesting belong to blue–green infrastructure (BGI). Sustainable stormwater management allows source control of the quality and quantity of the runoff near the source. Retention tanks could be used to reduce overflows to water bodies and supply water to local urban areas [195]. Rainwater harvesting is not just an effective measure to mitigate floods and pollutant migration in urban environments but also provides an alternative water source that could increase the reliability and sustainability of urban water supplies. Treatment may be required for some higher reuses; a review on stormwater treatment technologies for reuse was recently presented [196]. Overall, rain harvesting is considered a cost-efficient and low-health and environmental-risk technology [197,198,199], which has allowed for its expansion in many developing and developed countries [193]. However, there are still challenges to be addressed in several different domains, such as technology, economy, policy, and the protection of human health [193,198,200,201,202,203].

4.5. Trends in Potable and Non-Potable Reuse

WWTPs generate large amounts of effluent that can be reused for domestic, industrial, and agricultural uses, complementing available freshwater resources. Technological advances in wastewater treatment have allowed the promotion of high-quality uses with many examples all over the world [159]. On the national scale, Israel and Singapore are among the countries with the highest percentages of water reuse, reaching up to 24.1 and 30% of the total water supply, respectively [204]. In the EU, potable reuse represents only 2.3% of the total tertiary-treated water, while irrigation (%), industry (%), and non-potable urban applications represent 52, 19.3, and 8.3%, respectively [205]. In China, reused WWTP effluents have been estimated at 3.76 billion m³ per year [92].

Although the percentage of potable water reuse applications worldwide is still limited, there is an increasing trend in this area, which is approaching 15% of the water requirements in Chennai (India) and 25% in Windhoek (Namibia) [158,179,206]. Numerous potable reuse projects are also ongoing in Singapore, Australia, and the US. In terms of potable reuse, either DPR or IPR is possible. DPR is intended as the introduction of highly treated, reclaimed water into a distribution system or water supply reservoirs downstream of a purification plant; hence, water is not subject to additional purification processes, save perhaps for final disinfection. IPR refers to the water supply to reservoirs and/or aquifers where water is buffered and mixed. IPR applications still represent the majority of potable reuse cases, with 20 recorded sites worldwide, including Singapore, Orange County (CA, USA), Gwinnett County (GA, USA), and Southeast Queensland (Australia). In addition, DPR sites so far include El Paso and Big Spring (TX, USA), Windhoek (Namibia), and Beaufort West (South Africa) [207].

The main constraint that still need to be addressed is the limited public acceptance rather than technological issues; the latest advances in treatment technology promise potable water from non-conventional water resources [208,209]. Low public acceptance is a critical issue behind the low diffusion of potable water reuse and is related to the cultural, socio-technical, demographic, and contextual characteristics of the areas [208]. The establishment of specific quality criteria for the effluents, regulations, collaborations among stakeholders, and efficient dissemination actions have been proposed to increase potable water reuse [54].

Membrane filtration is the main advanced technology for potable reuse purposes, capable of removing pollutants and pathogens from wastewater [210]; adsorption and AOPs are also available technologies [211,212].

Concerns have arisen regarding the detection of extremely low concentrations of contaminants and pathogens (e.g., enteric virus, Cryptosporidium, or specific chemical compounds) (those that are regulated or even unregulated by legislation) [19,23,213]. Online water monitoring technologies are applied nowadays for the real-time detection of pollutants in water supplies [214].

Finally, besides technical challenges, the economic and energy requirements for potable uses should also be considered; recent studies show that there are lower energy demands (less than 1 kWh/m³) for the potable reuse of wastewater compared to seawater desalination [132,210]. From an economic viewpoint, the long-term (48 years) analysis of the Windhoek facility operation showed that the cost of sewage reclaimed for safe, direct potable reuse is currently at 0.72 EUR/m³ [215]. By comparison, California Public Utilities Commission estimated the cost of raw freshwater supply in the western US ranging between 0.02–1.18 USD/m^3, with an average value of 0.64 USD/m³ [216], which not dissimilar from the former. The economic cost of DPR for different treatment technologies is presented in

Table 5. The economic cost of direct potable reuse (DPR), dependent on technology and water source (data from OCWD ¹).

Technology/Water Source	Cost Required (USD/m3)
	Treatment	Residual Management	Concentrate Management	Conveyance and Blending Facilities
Advanced water treatment facility with RO	0.55–0.73	0.01–0.04	0.06–0.63	0.08–0.81
Advanced water treatment facility without RO	0.32–0.57	0.01–0.04	Not applicable	0.08–0.81
Bragish groundwater desalination (inland)	0.32–0.65	0.01–0.02	0.06–0.63	0.08–0.81
Sea water desalination	1.58–2.84	0.02–0.08	0.06–0.16	0.32–2.43
Retail cost of treated imported surface water	0.32–1.05	0.32–1.05	Not applicable	0.08–0.49
Water use efficiency, conservation, and use of restrictions	0.36–0.77			Not applicable

¹ Orange County Water District project.

.

4.6. Water Reuse and Circular Economy Concept

The circular economy is based on a new paradigm concerning the relationships between markets, customers, and natural resources, to promote sustainable and resource-efficient practices and enable a, economy to grow while at the same time minimizing the number of primary resources extracted. The transition to the circular economy will encourage more efficient water use, enhancing society’s ability to reconcile the imbalance between supply and demand.

According to the results of an Organisation for Economic Co-operation and Development (OECD) Survey on the circular economy in cities and regions, 66% of the surveyed circular economy initiatives identified water and sanitation as key sectors for circular economy implementation, followed closely by the waste sector (78%). Closing the loop in water management and use could offer a range of environmental, economic, and social benefits; reuse can improve the status of the environment by substituting abstractions, thus alleviating anthropic pressure, and reducing the pressure of WWTP discharges on sensitive areas. From an economic point of view, reuse will extend the useful water life cycle and increase resource productivity and the dependence on primary resources, including water itself, nutrients (nitrogen and phosphorus), and energy. From the social perspective, reuse’s public acceptance is generally not straightforward nor granted, but it is more common when water scarcity is already affecting the public so that it is easier to perceive it as a solution and a possibility for improving development and public health than as problem.

The reuse of treated wastewater could transform the linear model that ends up with large disposals to the environment into a circular system, decoupling consumption from adverse impacts on natural resources [217]. Because of the current and predicted shortages, the business case for a transition to a circular water economy is compelling, considering the environmental and social-economic impacts [217]. Winning over public opinion has been recognized as a key challenge for the implementation of reuse schemes; however, in times of climatic stress and increasing environmental consciousness, the acceptability of these approaches is increasing.

The lack of proper infrastructure could constitute a serious impairment to water reuse; for example, in Gulf Cooperation Council (GCC) countries (Bahrain, Kuwait, Oman, Qatar, Saudi Arabia, and the UAE), in recent years, high-capacity WWTPs with advanced treatment technologies have produced high-quality water. The reuse of this water depends on several factors, such as local demands, infrastructure, and policy; some policymakers are still reluctant to use it. This may suggest a holistic approach to governance, involving institutional and organizational aspects to achieve a successful transition to the circular economy.

Cities are places where the inflows and outflows of materials and resources are generated in connection with the surrounding areas; for the water sector, the proper functional area should at least be the size of the hydrological basin, often not corresponding to the administrative boundaries of cities or regions. This mismatch complicates the efficient management of water resources across the multiple institutions in charge and can also add complexity when it comes to recycling resources and organic residuals in the proximity of their generation site. Site-specific solutions are therefore required to counter these mismatches, promoting cooperation between cities and their surroundings. In the EU, water management plans for the river basin districts (RBDs) of member states could be the basis for building integrated water reuse plans.

Generalized water reuse still faces several barriers from public perception to pricing, safety, and regulatory issues; in comparison to conventional sources, DPR is often approached carefully by policymakers considering higher water quality standards [23]. Despite its inevitably higher level of initial contamination, reused water may, in the end, provide greater public health protection than many conventional water sources, although the costs involved could be increased by the additional required protection. If regenerated water is used for productive uses, such as golf courses, agricultural irrigation, or industrial uses, projects should be financially viable as regular business endeavors. Artificially low water pricing for certain uses, such as drinking or irrigation water, which in most countries are subsidized, makes the reuse of water uncompetitive.

4.7. Criteria for Water Reuse; Developing Criteria Based on the “One Water” Concept?

The treatment and reuse of wastewater effluents have passed through continuous stages over the centuries, keeping up-to-date with scientific knowledge on technical, environmental, and public health issues. These processes are characterized by the constant development of a regulatory framework addressing critical water-reuse-related issues, providing guidance, and establishing quality criteria and standards.

Such development has occurred gradually and unequally among different countries. Generally, developed countries have addressed these issues early; in the State of California (USA), the first regulations for agricultural reuse were applied in 1918 [218]. The United States Environmental Protection Agency (US-EPA) developed nationwide regulations and criteria for water reuse, which were then adopted as references by many other countries, since 2004 [141,219]. The World Health Organization (WHO) has its own set of guidelines concerning effluent reuse in agriculture, addressing the known possible risks associated with the reuse of water for edible crops and approaches to reduce those risks [220]. The United Nations Food and Agriculture Organization (FAO) also developed its guidelines [221]. In the US, 27 States regulate reuse for unprocessed food crops, 43 for non-food or processed food crops, 40 for urban, 17 for the environment, 31 for industries, and 9 states regulate for potable indirect reuse [28,159]. In European and Mediterranean countries, detailed legislation exists, while countries such as Germany or the United Kingdom do not regulate reuse as it is seldom practiced due to higher availability. Spain has set limits on pathogens (fecal coliforms and nematodes) based on the 1985 Water Law [222]. South American countries (Brazil, Costa Rica, Chile, Mexico, and others) have developed regulations for water reuse based on mainly microbial content and metals, freely allowing the use of beneficial organic matter and nutrients on land [222]. In the Middle East, 11 out of 22 Arab States have adopted legislation concerning the reuse of treated wastewater [20]. In Africa, with several countries facing a persistent water crisis, many have lax or non-existent regulations, and wastewater reuse practice occurs mostly uncontrolled; some countries’ (e.g., Tunisia) guidelines include physicochemical and biological parameters and heavy metals. Some countries generally adopt international organizations’ (e.g., FAO, WHO) guidelines. A thorough survey of the available worldwide regulations/guidelines for agricultural water reuse was recently conducted by Mainardis et al. [32]; as the related legislative framework is in continuous evolution, a highly inhomogeneous situation was highlighted.

Among the legislative initiatives adopted in conjunction with EU policies on water, the recent directive on “Minimum Requirements Applicable to Reclaimed Water Intended for Agricultural Irrigation” (EU, 2020/741) defines water quality classes for irrigation purposes. Other regulation initiatives that could boost increased water reuse include the “Strategic approach to pharmaceuticals” [223], the “Strategy for a sustainable and toxic-free environment” [224], the “Zero Pollution for Air, Water and Soil” [225], and the “Proposal to introduce the monitoring of microplastics in WWTPs” [153]. In the proposed guidelines on water reuse [24], emerging pollutants are of concern. The framework for endocrine disruptors’ identification, detection, and establishment of criteria was set by [226]; however, so far, there are no established criteria for these substances, except for recently introduced limits for two EDCs (beta-estradiol and nonylphenol) in drinking water [152].

The European Commission has released a proposal on new regulations and measures for urban water treatment, with a deadline of 2040 [153]. This will include, among others, new standards for decentralized facilities, integrated water management plans for large agglomerations (>10.000 p.e.), stricter limits for nutrient releases for facilities of more than 10,000 p.e., new limits for micropollutants, monitoring and tracing at the source of non-domestic pollution, key parameter monitoring, and energy neutrality for wastewater facilities (for >10,000 p.e.). So far, the proposal does not include standards for IPR, DPR or other non-agricultural uses.

The “one water” concept: The study by [55] suggests the establishment of quality criteria independently of the water source, as opposed to the current situation where criteria are largely based on the water source’s origin, in agreement with the previous study [227]. By contrast, it is necessary to develop different criteria for each different type of reuse (e.g., potable, irrigation, and industrial), which may help in further promoting this practice. This implies a supportive legislative framework and the undertaking of actions aimed at improving public acceptance through better water management and cooperation among stakeholders. Setting incentives [53] and highlighting the benefits resulting from water reclamation should be part of the efforts to promote reuse.

5. Analysis of Major Knowledge Gaps and Research Needs

Despite the progress made in water reuse in recent years, some major knowledge gaps need to be considered, as follows:

(a)

Possible interactions of agricultural reuse with soils, plants, and crops;

(b)

Fate of organic microcontaminants in receiving environmental media and targets;

(c)

Epidemiological risk of antibiotic-resistant bacteria and/or resistance genes released in the environment with treated effluents;

(d)

Issues concerning climate change and/or variability;

(e)

Strategies to overcome barriers to water reuse;

(f)

Links among reuse schemes, ecosystems’ services, and SDGs.

5.1. Interactions of Agricultural Reuse with Soils, Plants, and Crops

The non-potable (e.g., agriculture) reuse of WWTP effluents has shown great potential in areas of intense water scarcity as an option for overcoming low water resource availability and degradation. The challenge consists of the identification, detection, and elimination of specific contaminants (either regulated or currently unregulated) present at extremely low concentrations that can, however, have critical effects [23,213]. Reuse in agriculture requires advanced knowledge of the mechanisms and pathways behind contaminants’ transfer to the environment (soil and water resources), crops, and the food chain. Besides the threat to humans, potential impacts on soil (bio)chemical characteristics, and crops and their interaction should be evaluated and addressed, including the effect on critical soil biochemical processes (e.g., N turnover processes) [160]. Practices that improve water use efficiency (WUE) and nutrient use efficiency (NUE) are necessary to reduce losses to the environment [228,229,230]. Crop selection criteria could need more adjustments compared to traditional practices.

Deeper knowledge of these issues would allow the introduction of the necessary adjustments to reuse techniques.

5.2. Fate of Organic Microcontaminants in Receiving Environments and Targets

Treated wastewater is not free of microcontaminants; residual organics (recalcitrant or degradation byproduct compounds, EDCs, DBPs, etc.) [231] remain even after conventional treatment. A recent study reviewed specific persistent contaminant groups, such as pharmaceuticals, restricted chemicals, perfluoroalkyl and polyfluoroalkyl substances (PFAS), disinfection by-products, and solvents in drinking water. A total of 333 different chemicals were identified, of which 246 were found in in drinking water [232]. In particular, PFAS, anthropogenic hydrocarbons in which hydrogen atoms in the alkyl chain are replaced by fluorine, were detected at hundreds of locations worldwide. Exposure to these chemicals was linked to harmful health effects in humans, affecting the liver, kidneys, blood, and immune system; furthermore, they bio-accumulate, with the result that even small amounts can build up to damaging levels over years of exposure. Industrial and municipal WWTPs have been identified as the main recipients of PFASs as well as significant pathways for their environmental diffusion [233]. Limited options are currently available for removing PFAS from water solutions; reverse osmosis, which can passively trap PFAS, and destruction technologies, based on ARP processes, have been proposed, however, they are not yet available for full-scale application and could lead to the formation of smaller-chain fluorinated molecules. Both are quite expensive and may generate concentrated pools of contaminants that may further increase the risk of environmental dispersion.

5.3. Epidemiological Risk of Antibiotic-Resistant Bacteria and/or Resistance Genes

According to the European Centre for Disease Prevention and Control, infections caused by ARBs could be responsible for about 25,000 deaths in Europe and about that much in the USA annually; in addition, additional healthcare costs and productivity losses due to their presence are estimated to reach EUR 1.5 billion [234]. In 2002, an estimated 100 to 200 million kg of antibiotics were consumed worldwide [235]; most of these are excreted from the human body after use, ending up at the municipal treatment plant. Generally, their removal is achieved via chemical treatment with AOPs or bio-adsorption onto particulates and subsequent physical separation from effluents. Given the small molecular size of antibiotics, even ultrafiltration membranes could not be able to provide their complete removal from municipal effluents, and even more advanced processes (e.g., RO) may not achieve full removal [236]. Studies suggest that the presence of ARG/ARBs may even increase during conventional wastewater treatment [139], leading to their possible diffusion through disposal and reuse applications.

Antibiotic resistance genes can be present within resistant bacteria or extracellularly, as naked DNA; their fate and persistence in the environment depend on the host’s ability to proliferate and survive after their release. Although it was observed that ARBs were not able to survive in soil environments, the persistence of viable resistant cells or DNA is of concern as mobile genetic elements could be transferred to indigenous microbial populations [237]. The unanswered questions on this topic include whether crops could be able to take up antibiotic residues or ARGs from manure or effluents and whether ARB/ARGs could be detrimental to human health after the consumption of contaminated crops.

AOPs have been tested for antibiotic molecule degradation; a survey in the recent literature showed that AOPs can significantly reduce ARBs but not eliminate ARGs [238]. The potential hazard of ARB/ARG could be strongly reduced by the application of high-energy AOPs, such as plasma or electron beam technology [239].

5.4. Issues Concerning Climate Change and/or Variability

Climate variability affects water availability, requirements, and consumption; however, there is still high uncertainty regarding the magnitude of such impacts in the future due to biases, assumptions, and the inherent uncertainty of climatic and hydrological model projections and simulated scenario accuracy [240]. In light of such uncertainties, water reuse projects should focus on building resilience into existing systems; this can be achieved by diversifying water sources and decentralizing the water supply. The existing literature highlights the importance of distributed treatment and reuse systems for enhancing the sustainability and resiliency of water infrastructure. The techno-economic impact assessment of distributed wastewater treatment and reuse, given all these uncertainties, can be conducted using innovative modeling frameworks to evaluate the cost–benefit tradeoff of distributed reuse strategies based on the existing infrastructure and alternative system configurations and provide decision-making scenarios for the design of an improved system architecture [241].

5.5. Overcoming Barriers to Water Reuse

Among the most important barriers in the development and implementation of wastewater reuse schemes, deficiencies in governance, incomplete legal and regulatory frameworks, limited public awareness, and inadequate collaboration among actors/stakeholders can be cited. These mainly arise from national- and/or local-scale social, economic, political, and environmental constrains [23]. The situation is made more complex considering the dependence on decisions from a plethora of actors, political interference, and the limited public acceptance of water reuse due to the perceived potential adverse impacts on human health and the environment. Therefore, despite exiting studies and knowledge, further investigation into the factors controlling water reuse, spanning different productive sectors and different levels of administration, is needed. This knowledge could include novel management methods for the implementation and monitoring of water reuse schemes, methodologies, and practices to increase public acceptance and strengthen links among different actors, incorporating them into the planning of reuse projects [23,54,205]. It is also crucial to improve the methods that could involve citizens in proactive water reuse habits [54,55,242].

The domain of potable reuse is the most challenging and needs further research investigations regarding the ways of increasing public acceptance, criteria establishment, economy, and energy management of non-potable reuse schemes; an interesting option, that needs also further investigation is the development and establishment of criteria for water quality independently of the origin of water [55], introduced previously as the “one water” concept [227], as presented above. Furthermore, an increase in the number of case studies on non-potable reuse would help in achieving the continuous validation and improvement of water quality criteria as well as in updating the recommendations for users [54,55].

5.6. Reuse Schemes, Ecosystem Services, and Sustainable Development Goals (SDGs)

The ecosystem service (ES) concept includes the goods (services) provided as a result of the continuing functions of natural ecosystems [243,244]. This includes benefits from social, economic (circular economy), cultural, environmental, climate, and human health perspectives, from a variety of beneficial functions, such as freshwater availability, land productivity, raw natural materials, C sequestration, nutrient cycling, wastewater self-purification, and aesthetic value, which are dependent on ecosystems’ characteristics [245,246]. Studies dealing with the effect of various drivers (e.g., land use changes, climate change adaptation practices, intensive use of resources, and pollution by organic substances) on potential changes in the quality of natural resources and related available ecosystem services have been published [243,247,248,249,250]; however, little information is available specifically regarding the consequences of water reuse on ecosystem services. In the domain of agriculture, it is essential to decipher the short- and long-term consequences of reuse practices on the changes in related ESs. From this perspective, it is also important to provide links to the potential consequences of the achievement of the planned SDGs. A conceptualization of such a general model is presented in Figure 2. Future research should move from theory to practical solutions, focusing on efficient collaboration among actors/stakeholders, the identification of the pathways/processes/mechanisms by which reuse practices can influence ES, and their quantification [251]. ES quantification is still a critical domain of such research through updated methodologies [252,253] and could facilitate the implementation, management, and monitoring of water reuse schemes consistent with the objectives of the SDGs.

6. Conclusions

Based on the main objectives of the review paper, the main conclusions can be extracted:

• The growing world population, climate variability and/or change, and water resource degradation by overexploitation are the major drivers of increased water demands worldwide, stressing the need for alternative good quality water supplies. As a result of this global water crisis, an increase in water reuse practices is expected, mainly in developed countries and climate-vulnerable areas of the planet.
• The current advances in wastewater treatment and water reuse technologies provide the opportunity for the foul exploitation of alternative water resources, increasing the potential of potable and non-potable water reuse systems, relying on pollutant/contaminant elimination, and improving economic performances (arising from various benefits); reclaimed water derived from wastewater is now a very promising water resource capable of satisfying even high-quality drinking water requirements with a decreasing production cost.
• Paradigmatic and technological switches based on an improved understanding of the relationships between the water cycle and the Water–Energy–Food Nexus will increase the perspective of water reuse.
• The recovery of nutrients through sewage wastewater treatment is a highly sustainable practice promising economic benefits arising from the reduced costs associated with their sheer removal and the supplement of fertilizers to the WEF Nexus. In the future, an increase in N and P recovery from WWTPs is expected due to improved biological P-removal techniques and beneficial synergies during P and energy recovery in anaerobic digesters. On the other hand, reduced nutrient removal based on water reuse applications (e.g., agricultural or landscape fertigation) may constitute a greater added value and contribute to less energy consumption and a reduction in related GHG emissions.
• The circular economy and water reuse are perfectly compatible concepts that target more efficient water use by users, ensuring parallel environmental and economic benefits.
• Revisions in the management of water and water reuse schemes should adopt a more holistic approach (integrated management), incorporating regulatory actions and actions that will increase public awareness and interconnectivity among actors/stakeholders and research institutions/universities, and applying sophisticated control and monitoring of water reuse schemes.
• The adoption of quality criteria for effluents is mandatory to prevent undesirable impacts on humans and the environment. The consideration of the “one water” concept, which means equal criteria independent of the origin of water, and instead differentiates among different types of water reuse, along with the decreasing treatment cost, could facilitate the management and implementation of potable and non-potable water reuse schemes.
• We need improved knowledge in specific fields of water reuse, such as the optimization of treatment systems, the interaction and impacts of reclaimed water on the quality of natural resources and biodiversity, the fate of microcontaminants in the soil–water–plant–atmosphere–humans matrix, an epidemiological risk assessment, and issues concerning the expected changes due to climate variability and/or change.
• Finally, this review highlights the importance of understanding the potential impacts of water reuse systems on ecosystem services and the related consequences in achieving the global Sustainable Development Goals (SDGs).