Reuse of Treated Wastewater to Address Water Scarcity in Viticulture: A Comprehensive Review

Abstract

Water scarcity has become an increasingly critical global issue, affecting various sectors, including industrial, domestic, and particularly agriculture. Agriculture, as the largest consumer of water due to its substantial water requirements for food production, faces significant challenges, which are expected to intensify with the growth of the global population. As a result, many countries have begun to explore innovative solutions to address this pressing problem, one of which is the reuse of wastewater for irrigation purposes. This approach has gained particular attention in viticulture, where water consumption is high, and the need for sustainable practices is paramount. This paper delves into the issue of water scarcity, focusing specifically on the winemaking sector. It reviews several studies investigating the potential of wastewater reuse for irrigating vineyards, highlighting both the promising benefits and the challenges associated with this practice. The findings suggest that using treated wastewater for irrigation in viticulture offers a viable solution to mitigate water shortages, particularly in regions facing severe droughts or limited freshwater resources. However, the successful implementation of this approach requires careful monitoring and management of several factors, including soil quality, plant health, fruit development, and the final wine product. Ensuring the safety and quality of the wine, as well as safeguarding consumer health, necessitates rigorous oversight to prevent any negative impacts from the use of reclaimed water.

1. Introduction

Water is an essential natural resource that plays a pivotal role in the development of countries. A continuous and reliable supply of drinking water is not only necessary for the well-being of individuals but is also a fundamental prerequisite for the establishment and sustainability of permanent communities [1]. Beyond human consumption, water is crucial for maintaining ecosystems, which rely on this resource to support biodiversity and sustain ecological balance [2,3,4]. Its availability and management are integral to both the health of the environment and the progress of human societies.

According to the Food and Agriculture Organization (FAO), the global population is expected to continue growing until 2050, reaching approximately 9 billion people. This population increase will, in turn, create a higher demand for food production [4,5,6], leading to a corresponding rise in water consumption. As highlighted by numerous studies from the FAO, the agricultural sector is the largest global consumer of water resources [7].

In addition to population growth, climate change is exacerbating the challenges we face, primarily due to the rise in greenhouse gas emissions linked to industrial development. This phenomenon is severely impacting the quality and availability of water resources, posing significant risks to human health and food security [8].

In many regions, both human well-being and the integrity of natural ecosystems are being adversely affected by climate change, which has caused substantial disruptions to the global water cycle. Unsustainable water consumption has also led to the depletion of this once-renewable resource, pushing water scarcity to the forefront as a growing concern [9,10,11].

Water scarcity arises when the demand for water exceeds its availability [12], leading to a state of multidimensional human deprivation. This is characterized by the lack of access to safe and affordable water to meet basic needs [12], which can have profound effects on entire regions. The most vulnerable and economically disadvantaged populations are the most likely to suffer from the impacts of scarcity [2].

Several factors contribute to water scarcity, including reduced rainfall and climate change. In addition, population growth, increasing per capita water consumption, and water pollution caused by industrial activities exacerbate the issue [5]. This problem is becoming increasingly evident not only in arid and semi-arid regions but also in areas where rainfall was once abundant [13]. Climate change, which causes severe droughts, combined with population growth, rising demand, and poor water resource management over recent decades, has significantly worsened the situation globally [13,14].

The increased frequency of droughts, coupled with population growth and unsustainable water management, is driving water scarcity to the forefront of global challenges. For viticulture, water scarcity presents both a risk to grapevine health and an opportunity for innovation in water management practices [15,16]. The use of treated wastewater (TWW) for vineyard irrigation has emerged as a potential solution to mitigate water shortages and promote more sustainable practices in regions facing severe water stress [17,18].

Ultimately, addressing water scarcity in viticulture requires a combination of sustainable water use practices, innovative solutions like TWW reuse, and effective water management strategies to ensure that vineyards can continue to thrive in a changing climate while maintaining the ecological balance and supporting long-term food security [15,19].

2. Water Scarcity in Agriculture

Agricultural production, as mentioned earlier, consumes more than 70% of the available potable water [20], while the industrial sector uses approximately 22%, and only 8% is allocated for domestic use. These figures underscore the significant role that agriculture plays in water consumption, contributing substantially to the intensification of water scarcity [2].

Within the agricultural sector, two types of water scarcity are typically identified: green water scarcity and blue water scarcity. Green water scarcity arises when the amount of rainwater is insufficient to meet the water needs of crops, necessitating irrigation to prevent limitations on crop growth and development [12]. On the other hand, blue water scarcity occurs when water is extracted—often without adequate control—from surface or groundwater sources to fulfill irrigation demands for crops facing green water scarcity. This type of scarcity is at the core of the global discourse on water shortages [2,12,21].

The increasing scarcity of water resources presents a critical challenge to sustainable agricultural development worldwide. In the coming years, while there is an expected rise in irrigated areas, available freshwater will be increasingly diverted to meet the growing demands of the domestic and industrial sectors [22].

To address this challenge, it is vital to implement sustainable water management strategies in agriculture. These strategies should include improving irrigation practices, optimizing soil and crop management, reassessing water pricing, and exercising stricter control over water extraction. Moreover, it is crucial to encourage the active participation of farmers in water management, including the adoption of water-saving techniques and the potential use of lower-quality water, such as treated [22,23]. These measures are in line with the four United Nations 2030 Agenda Sustainable Development Goals (SDGs): the 2nd SDG, which focuses on achieving food security, improving nutrition, and promoting sustainable agriculture; the 6th SDG, aimed at improving water quality, increasing efficiency in water use, and reducing water degradation and scarcity globally; the 11th SDG, which seeks to reduce the adverse environmental impact of cities, with special attention to air quality and waste management; and the 12th SDG, which strives to reduce waste generation substantially through prevention, reduction, recycling, and reuse [2,24,25].

Water Scarcity in Viticulture

In recent years, viticulture in the Mediterranean region has faced significant challenges, largely due to the effects of climate change [26,27,28,29]. Once characterized by stable climatic conditions, the region has been experiencing notable shifts, including rising temperatures, irregular rainfall patterns, and more frequent extreme weather events. These changes have had a direct impact on the availability and predictability of water resources, a crucial factor in vineyard management and grape production [30,31,32].

Historically, water scarcity was not a primary concern in Mediterranean viticultural regions. However, prolonged droughts and increasingly erratic precipitation have heightened the risk of water stress in vineyards. This shift has raised concerns among wine producers, as the long-term sustainability of traditional viticultural areas is now uncertain due to unpredictable water availability [29,33,34].

The need for a consistent and reliable water supply is particularly critical in viticulture, as the health of the vines, grape yield, and overall wine quality are closely linked to water availability. Insufficient water can lead to plant stress, reduced productivity, and changes in grape composition, ultimately affecting the sensory characteristics of the wine [15,35,36,37,38]. Additionally, rising global temperatures have increased the frequency and intensity of heatwaves, exacerbating the water demand in vineyards. In many cases, this has led to a decline in natural water reserves, including groundwater and seasonal snowmelt, which have historically supported irrigation [39,40,41,42].

Given these challenges, many winegrowers in the Mediterranean have begun exploring alternative water sources, including the reuse of treated wastewater (TWW) for irrigation. The unpredictability of conventional water supplies has accelerated the search for sustainable water management practices [43,44,45,46]. The use of TWW not only helps mitigate water shortages but also reduces dependence on freshwater sources, which are becoming increasingly scarce due to climate change and over-extraction. While concerns have been raised regarding the impact of wastewater irrigation on grape and wine quality, ongoing studies indicate that, when adequately treated and monitored, TWW can serve as a viable and safe irrigation alternative for vineyards [47,48,49,50,51].

Portugal, a country with a strong viticultural tradition, is particularly vulnerable to climate change, facing risks such as heatwaves, flooding, and prolonged drought periods [52,53,54]. Certain regions, notably the southern and interior areas, are already experiencing significant pressure on water resources, making the implementation of adaptation and mitigation strategies a priority. In response, irrigation has become an essential tool for ensuring vineyard sustainability. However, the effectiveness of irrigation depends on factors such as the quantity and quality of water used, as well as the specific needs of different grape varieties [55,56,57,58].

To optimize water use, many wine producers are adopting precision viticulture techniques, which allow for more efficient water management and contribute to sustainable production. Various studies have examined the effects of water availability and deficit on vine growth and development, highlighting the importance of balanced irrigation strategies [59,60,61]. Water availability plays a key role in vegetative growth, while water deficits can hinder root and shoot development. Severe water stress, particularly during early grape development, can result in smaller berries, altered sugar and acidity levels, and, in extreme cases, delayed ripening or incomplete maturation, ultimately compromising wine quality [59,62,63,64].

In this context, the efficient use of existing water resources and the exploration of alternative sources are essential for ensuring the long-term sustainability of viticulture. Among the potential solutions, treated wastewater reuse stands out as a promising strategy to address water scarcity. Studies conducted in vineyards have demonstrated its viability, reinforcing the need for further research and careful implementation to ensure both environmental sustainability and the preservation of grape quality [43,44,65,66].

3. Wastewater in Viticulture

Water scarcity is a growing global concern, particularly in agricultural regions where irrigation is essential for food production. As freshwater resources become increasingly limited due to climate change, population growth, and over-extraction, alternative water sources are gaining importance [67,68,69]. One such alternative is the use of treated wastewater (TWW) for irrigation, which has the potential to address both water shortages and nutrient management in agricultural systems [1,17,70,71,72].

The increasing use of wastewater for agricultural irrigation in various countries is driven by several factors, primarily the need for a reliable and continuous supply of irrigation water throughout the year [8,73,74]. Additionally, the organic matter and nutrients present in wastewater provide significant value to soils that may be deficient in one or both of these components [75,76,77]. This aligns with the findings of Thebo and collaborators [78], who highlight that studies have shown that using treated wastewater for irrigation is more advantageous than using traditional water sources [79]. This is due to the high nutrient content in wastewater, including nitrogen, phosphorus, and potassium, as well as organic carbon, inorganic micronutrients, and organic matter [80]. These elements contribute to increased crop productivity [81,82] while simultaneously reducing the need for chemical fertilizers, which have negative environmental and economic impacts [83,84,85,86]. Moreover, the application of wastewater for irrigation fosters the circular economy [8].

The reuse of wastewater in viticulture has emerged as an increasingly viable and sustainable solution to address the growing challenges of water scarcity in wine-producing regions. As many areas around the world, particularly those in semi-arid and arid climates, continue to face severe droughts and unpredictable weather patterns, the need for alternative irrigation sources has become more pressing [8,44,87,88,89]. Among these alternatives, treated wastewater (TWW) has garnered significant attention as an effective means of providing a reliable, year-round water supply for vineyards, helping to alleviate the pressure on freshwater resources [47,65,66,90,91].

Treated wastewater offers several advantages for vineyard irrigation, particularly in regions where water scarcity is a growing concern. It provides a consistent and accessible water source throughout the year, which is crucial for maintaining the health of vines and ensuring consistent grape production. The reuse of TWW not only helps mitigate the environmental impact of water extraction from freshwater sources but also supports the sustainable management of water resources, aligning with broader environmental and sustainability goals [17,49,65,92].

A growing body of research has focused on the use of TWW from a variety of sources, including municipal wastewater treatment plants and winery wastewater, for vineyard irrigation [17,47,51,65]. Studies have shown that TWW, when treated to meet specific quality standards, can be safely used for irrigation without compromising soil quality or grapevine health. In some cases, the nutrients present in treated wastewater, such as nitrogen and phosphorus, can even benefit vine growth, reducing the need for additional fertilizers [47,49,51,92,93].

Despite its potential, the use of TWW in viticulture requires careful management to ensure that it does not negatively affect the quality of the final product, namely wine [50,66,87]. Concerns over the presence of salts, heavy metals, and pathogens in untreated wastewater have raised questions about its long-term impact on soil health, vine performance, and wine characteristics [51,94,95]. However, ongoing studies have demonstrated that with proper treatment and monitoring, these risks can be mitigated, and TWW can be used effectively without adversely affecting grape quality [48,62,96,97].

In addition to winery and municipal wastewater, some innovative approaches have explored the use of treated industrial effluents and even greywater for vineyard irrigation [65,98,99]. These sources can offer additional water supplies in regions where traditional water resources are scarce. The diversification of wastewater sources further enhances the reliability of this irrigation method, particularly in areas experiencing severe water stress [14,69,100,101,102,103].

In fact, the reuse of treated wastewater for vineyard irrigation represents a promising and sustainable solution to the growing challenges of water scarcity in viticulture [44,65,72]. As research continues to demonstrate the viability of TWW, particularly when treated and managed properly, it offers a path forward for winegrowers seeking to maintain production levels while reducing their environmental footprint [18,45,104]. The continued exploration and implementation of TWW in vineyards could play a pivotal role in ensuring the resilience of the viticultural sector in the face of ongoing water scarcity and climate change challenges [34,55,105].

In many wine-producing regions facing severe water scarcity, the reuse of treated wastewater (TWW) has emerged as a practical and cost-effective solution for agricultural irrigation, particularly where potable water is not required. Within the European Union, TWW reuse aligns with the objectives of the Water Framework Directive (2000/60/EC), which promotes sustainable water resource management [17,43,44,73,106]. In France, national policies, including the Grenelle Environment Law (Grenelle I), further encourage wastewater reuse as part of broader environmental sustainability efforts [43,44,107,108].

The extent of wastewater treatment required before irrigation depends on local and international water quality standards. In France, the French Agency for Food, Environmental, and Occupational Health & Safety (ANSES) oversees compliance with the national decree of 25 June 2014 (NOR: AFSP1410752A), which establishes purification guidelines [1,8,44,109]. While advanced treatment is necessary to prevent soil contamination, wastewater remains an accessible and nutrient-rich water source for farmers. Therefore, treatment strategies must balance environmental safety with agricultural needs, integrating wastewater reuse into broader resource management approaches to enhance water efficiency and soil productivity [44,110].

TWW irrigation is already practiced in several countries, with studies demonstrating benefits such as improved grape yield and quality. However, potential drawbacks include increased soil salinity, altered vine nutrient absorption, and potential impacts on wine characteristics. While the high salt content of recycled water can influence plant responses, its effects depend on nutrient availability and specific salt composition. Moreover, since most agricultural wastewater irrigation relies on municipal sources with negligible heavy metal content, the accumulation of these elements in vineyard soils typically remains below international safety thresholds [17,111].

Regulatory frameworks play a critical role in ensuring the safe and sustainable reuse of treated wastewater in viticulture. These regulations establish stringent water quality guidelines, defining permissible levels of contaminants such as toxic metals, pathogens, and emerging pollutants [86]. By setting threshold values for hazardous substances, regulatory policies mitigate risks to human health and the environment, preventing the bioaccumulation of harmful elements in soil and crops, which could otherwise compromise food safety and long-term soil fertility [112,113,114].

Enforcement mechanisms, including certification programs, audits, and penalties for non-compliance, help align wastewater reuse practices with public health and environmental protection goals [73,115,116]. By incorporating scientific advancements and technological innovations into policy updates, regulators can refine water reuse guidelines, optimizing both safety and efficiency. Ultimately, a well-structured regulatory framework, supported by rigorous monitoring and enforcement measures, is essential to promoting the responsible use of treated wastewater in viticulture while minimizing potential health and environmental risks [117].

In addition to defining acceptable water quality parameters, regulatory policies emphasize the importance of continuous monitoring and enforcement. Routine water testing, coupled with real-time data collection, allows for early detection of contaminants, ensuring that only treated wastewater meeting established safety criteria is used for irrigation. Furthermore, adherence to best management practices, such as controlled irrigation techniques, periodic soil and crop assessments, and the implementation of risk mitigation strategies, enhances the effectiveness of these regulations [26,28,87,118,119,120].

3.1. Short-Term Studies and Benefits

Mendoza-Espinosa et al. [121] conducted a one-year study in Mexico to evaluate the use of municipal treated wastewater with secondary treatment with chlorine disinfection for vineyard irrigation. The study assessed the grapevines’ response to this irrigation method and compared the biochemical composition of grapes grown to those irrigated with freshwater. The results indicated that grapevines irrigated with municipal TWW exhibited faster growth, extended development periods (approximately 0.75% d⁻¹), and a higher leaf count per shoot compared to those irrigated with freshwater.

These effects were attributed to the total nitrogen (nitrate + ammonia) and phosphorus content in municipal TWW. Nitrate levels varied from undetectable to 18 mg/L, ammonia ranged from 1 to 10 mg/L, and phosphates from 2 to 20 mg/L. Considering the irrigation rate applied throughout the season, TWW supplied 6.10–9.07 kg/ha of ammonia and 13.4–20 kg/ha of phosphate, whereas freshwater irrigation provided only 1.22–1.81 kg/ha of ammonia and 1.2–1.8 kg/ha of phosphate. In contrast, nitrate input was higher with freshwater irrigation (15.85–23.58 kg/ha) compared to TWW (12.19–18.14 kg/ha).

Moreover, the biochemical composition of the grapes, including sugar content and pH, remained unchanged, while productivity increased by 20% per plant, with no reported adverse effects. Hirzel et al. [122] conducted a three-year study in California on the application of winery wastewater, which was enriched with Na⁺ due to the use of NaOH for pH neutralization. The study examined the effects on soil, leaves, grapes, and wine at two key stages—veraison and harvest, focusing on the concentrations of Na⁺, Mg²⁺, K⁺, and Ca²⁺, as well as on the phenolic profile of the grapes and wine.

Even though this TWW was enriched with Na⁺ (134.5 ± 2.7–424.7 ± 9.2 mg/L, compared to 27.2 ± 0.2–35.4 ± 0.7 mg/L in freshwater), the soil did not accumulate excessive amounts of sodium. However, Na⁺ levels were still higher in soil irrigated with TWW (10.7 ± 2.3–72.6 ± 15.8 mg/kg) compared to soil irrigated with freshwater (3.91 ± 0.39–16.1 ± 4.1 mg/kg). Similarly, K⁺, Mg²⁺, and Ca²⁺ concentrations were also elevated in TWW-irrigated soil (0.60 ± 0.16 mg/kg for K⁺, 0.69 ± 0.18–21.9 ± 8.01 mg/kg for Mg²⁺, and 1.44 ± 0.39–9.37 ± 3.09 mg/kg for Ca²⁺) compared to those in soil irrigated with freshwater (0.30 ± 0.06–3.19 ± 1.09 mg/kg, 0.66 ± 0.06–7.32 ± 0.34 mg/kg, and 1.85 ± 0.23–11.7 ± 3.3 mg/kg, respectively).

Plants irrigated with TWW exhibited increased concentrations of Na⁺ and Mg²⁺ at both veraison (145–175% and 20–50%, respectively) and harvest (20–43% and 12–30%, respectively) compared to those irrigated with freshwater, though the levels remained non-toxic. In contrast, K⁺ and Ca²⁺ concentrations were lower in plants grown in TWW-irrigated soils than in those irrigated with freshwater, with reductions observed at veraison (12–87% and <10–86%, respectively) and at harvest (30–66% and <10–80%, respectively).

Grapes from plants irrigated with TWW exhibited higher pH values (3.42 ± 0–3.57 ± 0.04) and lower titratable acidity (TA) (4.52 ± 0.01–4.60 ± 0.15 g/L) compared to those irrigated with freshwater (pH 3.32 ± 0.01–3.44 ± 0.02 and TA 4.11 ± 0.09–1 g/L). A similar trend was observed in the resulting wine, where wines produced from TWW-irrigated grapes showed higher pH values (3.38 ± 0–3.76 ± 0.01) and lower TA (5.66 ± 0.01–6.42 ± 0.12 g/L) compared to wines from freshwater-irrigated grapes (pH 3.32 ± 0.01–3.46 ± 0.05 and TA 6.25 ± 0.03–4 ± 0.39 g/L).

In terms of phenolic compound concentrations, grapes from TWW-irrigated plants had lower levels than those from freshwater-irrigated plants. Additionally, variations in cation concentrations were detected in the wines, which the authors attributed to soil heterogeneity. They concluded that the observed differences throughout the study were influenced not only by the use of TWW but also by soil composition, plant characteristics, and rootstock. Therefore, they emphasized the need for long-term studies to assess potential cation accumulation in the soil and, more critically, in the grapes. They highlighted that soil structure plays a key role, as some soils are more susceptible to cation buildup than others.

Paranychianakis et al. [123] conducted a study of the same duration as Hirzel et al. [122], using municipal wastewater for irrigation and analysing cation concentrations in young grapevines compared to those irrigated with freshwater. Their findings indicated that municipal TWW supplied sufficient levels of P³⁺ (0.28–0.41% d.w.), K⁺ (0.81–1.62% d.w.), Mg²⁺, and Fe³⁺ to meet the grapevines’ nutritional requirements. This reinforces the idea that utilizing TWW can reduce or even eliminate the need for chemical fertilizers. However, the study also highlighted a key limitation: municipal TWW undergoes nitrification–denitrification processes before being used for irrigation, which lowers its nitrogen content. Over the years of the study, N concentrations in TWW-irrigated plants (1.47–2.33% d.w.) were consistently lower than those in plants irrigated with freshwater (1.59–2.46% d.w.), suggesting that additional nitrogen fertilization may be necessary to meet plant demands.

Like Hirzel et al. [122], Paranychianakis et al. [123] emphasized the role of rootstocks in nutrient uptake, noting that certain rootstocks enhance nutrient absorption from both soil and irrigation water. Their findings showed that grapevines grafted onto the 1103P rootstock and irrigated with TWW had lower nitrogen content (1.75–1.80% d.w.) compared to those grafted onto 41B (1.83–1.88% d.w.) and 110R (1.84–1.91% d.w.), which remained stable. Regarding K⁺ concentrations, vines grafted onto 110R (0.91–1.29% d.w.) and 1103P (0.85–1.43% d.w.) exhibited higher levels than those grafted onto 41B (0.68–0.97% d.w.). A similar trend was observed for P³⁺ concentrations, with vines grafted onto 1103P (0.32–0.34% d.w.) and 110R (0.30–0.31% d.w.) presenting higher values than those grafted onto 41B (0.23–0.25% d.w.) when irrigated with TWW [122,123]. These results further underscore the importance of selecting appropriate rootstocks to optimize nutrient uptake and overall plant response to irrigation with treated wastewater.

More recently Abi Saab et al. [48] in Lebanon compared three irrigation methods—freshwater, municipal TWW, and alternating irrigation with both for one year in grapevines for table grapes production. Their findings corroborated previous studies: grapevines irrigated with TWW or alternating methods had higher yields.

An average grape production of 30 kg per vine was recorded under TWW irrigation, while alternating irrigation yielded 29 kg per vine, both surpassing the 25 kg per vine obtained with freshwater irrigation. This further underscores the role of nutrients present in TWW in enhancing productivity. The impact of TWW irrigation was also reflected in the soil’s electrical conductivity, which increased to 1.06 mS/cm compared to 0.36 mS/cm in freshwater-irrigated soil and 0.24 mS/cm in soil subjected to alternating irrigation. This rise in conductivity is attributed to the presence of dissolved salts, such as chlorides, sulfates, and nitrates, introduced through municipal TWW.

Despite these differences, no significant variations were observed in fruit quality, as the pH values remained relatively stable across all irrigation treatments: 3.58 for alternating irrigation, 3.69 for TWW, and 3.72 for freshwater. However, slight differences were noted in total soluble solids (TSS) and sweetness, with freshwater-irrigated vines showing higher values (10 TSS and 10.25 sweetness) compared to those irrigated with TWW (5.88 TSS and 5 sweetness) and alternating irrigation (3.63 TSS and 4.25 sweetness). Conversely, total titratable acidity (TTA) was lower in grapes irrigated with freshwater (2.50 TTA) compared to those irrigated with TWW (8.25 TTA) and the alternating method (8.75 TTA).

In addition to the previously mentioned analyses, Abi Saab et al. [48] extended their study by conducting microbial analyses on the grapes, as they were table grapes intended for direct consumption. This step ensured food safety by confirming that the grapes were free of bacterial contamination. The levels of fecal coliforms, E. coli, S. aureus, and C. perfringens were found to be less than 10 cfu/g, and both Salmonella and L. monocytogenes were absent. The study concluded that long-term tests are necessary to assess the effects of irrigation with treated wastewater on soil, the broader ecosystem, and, most importantly, human health. This conclusion aligns with the findings of other studies that emphasize the need for continuous monitoring and research in this area.

Similarly, to Abi Saab et al. [48], Etchebarne et al. [87] observed positive growth responses in grapevines irrigated with TWW (8461 cm² and 42,621 cm²) compared to vines irrigated with freshwater (7036 cm² and 44,084 cm²) for both grapevine varieties. This reinforces the importance of nutrient availability in TWW as a key factor for grapevine growth and development. The study showed that the doses of nitrogen (21–39 kg N ha⁻¹), phosphorus (0.6–1.1 kg P ha⁻¹), and potassium (15–28 kg K ha⁻¹) provided by TWW were higher than those supplied through conventional fertilization practices on the farm.

For three years, municipal TWW with tertiary treatment (filtration, UV disinfection, and chlorine injection) was applied, and its effects were compared with those of freshwater irrigation. No significant differences were found in the physical-chemical parameters of the grapes, such as pH (3.36 for freshwater and 3.38 for TWW), Brix (25.6° for freshwater and 25.7° for TWW), tartaric acid (7.58 g/L for freshwater and 7.59 g/L for TWW), and malic acid (2.48 g/L for freshwater and 2.39 g/L for TWW). These results were consistent in the wines produced from the grapes.

Microbial analysis confirmed that the grapes were free from contamination, in line with the findings of Abi Saab et al. [48]. Finally, the authors emphasized that the non-significant differences observed throughout the study were attributed to the characteristics of the wine-growing years, rather than the quality of the water used for irrigation. It is well known that wine-growing year conditions, such as temperature, can significantly affect both the physical-chemical parameters and the quality of the grapes and wines, and these factors cannot be controlled.

Petousi et al. [47] observed similar results to those previously mentioned, noting an increase in the concentration of nutrients in young grapevines irrigated with treated wastewater (TWW), including nitrogen (N, 1.99 ± 0.08%), magnesium (Mg, 0.90 ± 0.04%), and calcium (Ca, 4.38 ± 0.06%), compared to vines irrigated with freshwater, which had N (1.97 ± 0.08%), Mg (0.66 ± 0.02%), and Ca (3.95 ± 0.02%). Regarding the mineral concentrations in grapes, N (0.7 ± 0.2 g/L), phosphorus (P, 0.3 ± 0.1 g/L), and potassium (K, 1.6 ± 0.1 g/L) in TWW-irrigated grapes did not differ significantly from those in freshwater-irrigated grapes (N, 0.6 ± 0.2 g/L; P, 0.3 ± 0.1 g/L; K, 1.5 ± 0.1 g/L), nor did physical-chemical parameters such as Brix (20.4° ± 2.0 for TWW and 21.8° ± 1.4 for freshwater) and titratable acidity (TA, 0.6% ± 0.1 for TWW and 0.5% ± 0.1 for freshwater).

However, it is important to note that the use of secondary treated TWW was significant because the irrigated soil contained fecal coliforms (3900–12,000 CFU/g d.w.) and E. coli (10–122 CFU/g d.w.), which can remain in the soil if organic matter levels exceed 8.5 g/kg. In contrast, tertiary treated wastewater did not show such contamination, highlighting the crucial role of advanced wastewater treatment for ensuring the safe agricultural use of TWW.

Although these short-term studies suggest that treated wastewater (TWW) irrigation is viable, regardless of its origin, they all emphasize the critical importance of maintaining high water quality to prevent potential health risks. Concerns are particularly raised regarding the presence of fecal coliforms, E. coli, and heavy metals, which may be absorbed by plants and eventually reach consumers through the consumption of grapes or wine. The studies underscore the need for long-term research to determine whether the positive outcomes observed in the short term are sustained, or whether adjustments to TWW usage may be required to ensure safety and continued effectiveness.

3.2. Long-Term Studies and Considerations

Long-term studies have yielded mixed results. In Algeria, Cherfouh et al. [124] conducted a 14-year study using municipal treated wastewater (TWW) for vineyard irrigation and found no significant changes in soil or plant characteristics. The pH of the soil irrigated with TWW (7.2) was comparable to that of soil irrigated with freshwater (7.0), indicating that irrigation with TWW did not impact soil pH. However, the quantity and availability of nutrients were increased, with soil irrigated with TWW showing superior results compared to soil irrigated with freshwater, particularly for K⁺ (217 mg kg⁻¹ in TWW vs. 47.9 mg kg⁻¹ in FW) and PO₄³⁻ (37.5 mg kg⁻¹ in TWW vs. 18.6 mg kg⁻¹ in FW). In contrast, N levels were lower in soils irrigated with TWW than in those irrigated with freshwater (30.3 mg kg⁻¹ in TWW vs. 32.8 mg kg⁻¹ in FW). This study highlights the benefits of TWW irrigation in improving the agronomic properties of soils but also stresses the importance of continuous monitoring of heavy metal levels in the soil to safeguard environmental health.

Similarly, Howell et al. [49] in South Africa reported that over an 11-year period, municipal treated wastewater (TWW) supplied essential nutrients to grapevines. However, some nutrients exceeded the plants’ needs, such as K⁺ (32 kg/ha), which could potentially lead to negative effects on wine quality if accumulated to toxic levels. Other nutrients, like N (37 kg/ha) and P (5.1 kg/ha), were insufficient to meet the grapevines’ requirements. Additionally, Na⁺ (193 kg/ha) was present in high concentrations relative to other cations like Ca⁺ (74 kg/ha). Over time, this could reduce the soil’s electrical conductivity and interfere with cation circulation between the soil and plant, potentially hindering vegetative growth and suppressing Ca absorption by the vines. On the other hand, Mg (14 kg/ha) levels in the TWW were adequate for the vines. Howell recommended supplementing TWW with fertilizers to correct nutrient imbalances or considering dilution with freshwater to reduce the oversupply of certain nutrients, provided the water analysis supports such measures.

In the case of heavy metal and trace element concentrations, TWW was found to contain low levels, with no expected negative effects on the soil or vines. Howell et al. [49] emphasized a critical point regarding the use of wastewater for irrigating vines and other crops. They highlighted that the composition of wastewater can vary, and thus, it is essential to conduct ongoing analyses of its nutrient content to ensure safety and effectiveness.

A nine-year study by Simhayov et al. [51] in Israel examined various irrigation strategies for table grape production, including the use of TWW combined with fertilizers. Over time, sodium (Na⁺) accumulation in the soil led to vine mortality, especially in vines irrigated with TWW and fertilizer (40% mortality), compared to just 12% mortality in vines irrigated with freshwater. This indicated that the availability of Na⁺ for the plant was higher when fertilizer was added to the TWW irrigation. The problem was further exacerbated by a lack of precipitation, which prevented Na⁺ leaching, and the use of sodium-based compounds in wastewater treatment.

Regarding the concentrations of Mg and Ca in the soil, they were similar across all treatments, suggesting that the high addition of Na⁺ was significantly contributing to the observed trend. Simhayov emphasizes the need for further long-term studies on the use of TWW for irrigation across different soils and crops. These studies should expand the analyses to include factors such as the microbial composition of the soil, which he highlights as a critical point. This would help ensure that the increasing use of TWW for agricultural irrigation does not cause irreversible damage to soil health and, consequently, food security, as noted by Petousi et al. [47].

Similarly, Netzer et al. [125] conducted a six-year study on table grape production, irrigating vines with TWW, TWW combined with fertilizer, and freshwater combined with fertilizer. The study documented sodium accumulation in soil (9.52 meq/L) and vines (4.93 mg/Kg) irrigated with TWW, compared to lower sodium concentrations in the soil (6.15 meq/L) and vines (2.23 mg/Kg) irrigated with freshwater and fertilizer. Despite this, there was no immediate impact on grape yield, as seen in other studies. Over the six years, the average production for vines irrigated with TWW was 29.5 t/ha, while vines irrigated with TWW and fertilizer yielded 29 t/ha. In comparison, vines irrigated with freshwater and fertilizer produced an average of 30.6 t/ha.

However, the authors cautioned that long-term sodium (Na) toxicity could pose risks, emphasizing the need for continuous monitoring. Furthermore, Netzer et al. [125] observed that vines grafted onto a particular rootstock, namely ‘Paulsen’, known for its tolerance to soil salinity, were not significantly affected by TWW irrigation over the six years of the study. This observation aligns with the findings of Hirzel et al. [122] and Paranychianakis et al. [123], highlighting the importance of rootstocks in facilitating or hindering nutrient uptake in such situations.

Weber et al. [126] also conducted an 8-year study on grapevine irrigation using treated municipal wastewater in Napa, California. Over the years, they found that TWW was suitable for irrigating vineyards, adhering to the quality parameters set by the country’s legislation. The electrical conductivity (EC) values were consistently below 0.8 mmhos/cm, well within the acceptable limit of 1.5 mmhos/cm. Based on these findings, the authors concluded that there would be no long-term salinity accumulation in the soil irrigated with TWW.

The nutrients contained in TWWs were beneficial to the vines, making it necessary to reduce nitrogen (N) levels (14 to 21 pounds per acre) compared to the N levels in freshwater (0.7 to 1.0 pounds per acre) in areas where the soils were already rich in N⁺. This surplus of nitrogen in the soil can lead to excessive vegetative growth, causing problems for the plants. As a solution, some vineyards opted to plant cover crops to help absorb the excess nitrogen. This approach demonstrates that wastewater can serve as a viable alternative for addressing water scarcity without requiring dilution or treatment to adjust nutrient levels for crops.

Regarding other nutrients such as phosphorus (P) and potassium (K) present in TWW, they did not harm the vines; rather, they benefited from their application, with concentrations reaching 0.56 pounds/ton for P and 4.94 pounds/ton for K.

Weber et al. [126] also indicated in their study that compounds from hygiene and pharmaceutical products were not analysed. While it is unlikely that these constituents are problematic, they suggest that future research is necessary to determine whether these compounds can accumulate in the vine and subsequently be transported to the grapes and wine. To date, this is the only study that raised concerns about this issue, which is particularly relevant to food safety and human health. Since wastewater is of municipal origin, it may contain components from hygiene products, pharmaceuticals, and even pesticides that could reach consumers through plant absorption.

4. Environmental and Economic Advantages

While short-term studies have generally shown positive results, long-term research has highlighted the complexities of using treated wastewater (TWW) in vineyards. Variations in soil composition, rootstock selection, and the specific characteristics of the wastewater itself mean that irrigation strategies must be adapted to the particular conditions of each vineyard [50,65,66,90,94]. These factors, such as soil permeability and nutrient absorption, can significantly affect how well TWW is integrated into vineyard management. Therefore, further research in this area is essential to developing well-rounded management plans for wastewater reuse, especially in regions where water scarcity is a pressing issue and viticulture plays a central role in the economy and culture [127,128,129,130].

Despite these challenges, the use of treated municipal wastewater for irrigation offers significant environmental and economic benefits [86]. One of the most pressing global concerns is the depletion of freshwater resources, which is worsened by climate change, population growth, and increasing agricultural demands [17,72,95,131]. Reusing TWW helps reduce the pressure on freshwater supplies, ensuring that these limited resources are preserved for essential human consumption and environmental needs. Furthermore, TWW’s nutrient-rich content—containing nitrogen, phosphorus, and potassium—can reduce the need for chemical fertilizers, lowering production costs for winegrowers and minimizing the environmental harm caused by fertilizer runoff into waterways [66,87,94].

Another major benefit of using wastewater for irrigation is its potential to reduce the discharge of untreated or partially treated wastewater into freshwater ecosystems. This often leads to eutrophication, harmful algal blooms, and contamination of water bodies, making them unsuitable for human consumption or ecological health. By repurposing wastewater for vineyard irrigation, these environmental risks are minimized, promoting a more sustainable, circular approach to water use and helping reduce pollution in surrounding water bodies [8,17,45,73,132,133].

However, while TWW presents a viable solution for irrigation, more long-term studies are needed to fully understand its effects on vineyard productivity and soil health. Research should focus on improving wastewater treatment processes to ensure the water is safe and efficient for irrigation [51,65,66,94]. It’s also important to keep track of potential contaminants—such as heavy metals and other harmful substances—to protect the environment, crops, and consumer health. In addition, optimizing irrigation practices based on crop types will help winegrowers choose the best varieties and techniques suited to TWW, maintaining the desired yield and quality of grapes [89,134,135,136].

Another important aspect that needs attention is public perception of wastewater reuse in agriculture. Even with scientific support, concerns about food safety and the potential risks to human health can limit its acceptance. Effective communication and education about the rigorous treatment processes involved in TWW reuse can help alleviate these concerns. Clear regulations on water use rights and safety standards are crucial to building trust among consumers and producers alike, ensuring that wastewater reuse is adopted in a safe and responsible manner [17,73,137,138,139].

In conclusion, using TWW in viticulture is a promising solution to water scarcity, offering both environmental and economic benefits. Yet, to fully realize its potential, ongoing research and public engagement will be necessary to overcome remaining challenges and ensure its safe and effective use in vineyards. By working together, the viticulture industry can pave the way for a more sustainable future.

5. Conclusions and Final Remarks

Water scarcity is one of the most pressing challenges faced by viticulture today, as it threatens the availability of water resources for vineyard irrigation. As climate change intensifies and competition for water increases, vineyard owners need to adopt sustainable strategies to maintain vineyard productivity while preserving water resources. The long-term viability of the wine industry depends on the implementation of innovative solutions that balance agricultural demands with water conservation.

The reuse of treated wastewater (TWW) emerges as a promising solution to mitigate the impact of water scarcity on viticulture. By using treated wastewater for irrigation, vineyards can reduce their reliance on potable water, ensuring a more reliable and sustainable water source. In addition to alleviating water shortages, TWW often contains valuable nutrients such as nitrogen, phosphorus, and potassium, which reduces the need for synthetic fertilizers. This not only lowers production costs but also reduces the environmental footprint of vineyard management by minimizing the use of chemical inputs.

However, the use of treated wastewater in viticulture requires careful monitoring and stringent quality control to prevent negative environmental impacts and safeguard the quality of grapes and wine. While TWW can be rich in essential nutrients, it may also contain harmful substances such as heavy metals, pathogens, or pharmaceutical residues, which can pose risks to soil health, grape quality, and human health. Therefore, it is crucial that treated wastewater besubjected to the highest standards of treatment, with continuous monitoring to ensure that water quality remains within safe limits for agricultural use.

Research on the long-term effects of TWW irrigation on vineyards is still limited, but initial studies show that, when managed properly, this practice can have a positive impact on vine growth and productivity. TWW irrigation has been shown to improve soil fertility and increase vine production in several regions, as the nutrients in the water contribute to the healthy development of the plants. However, there are concerns about the potential accumulation of salts and sodium in the soil over time, which could affect vine health and wine quality. Some studies suggest that using salt-tolerant rootstocks may help mitigate the negative effects of TWW irrigation, making this an important area for future research.

In terms of grape and wine quality, TWW irrigation has shown varied results. While some studies report no significant differences in the physicochemical parameters of grapes (such as pH, sugar content, and acidity) between TWW and potable water irrigation, other research has found slight variations in taste, acidity, or overall quality. It is important to note that factors such as climate, soil type, and vineyard management practices also play a significant role in grape quality, which may explain some of the inconsistencies observed in these studies. Nonetheless, the wine industry must ensure that TWW irrigation does not compromise the sensory characteristics of the wine, as this could impact market acceptance.

Given the potential benefits and challenges of using treated wastewater in viticulture, it is essential that research continues to focus on optimizing irrigation practices, understanding the long-term effects of TWW on soil health, and identifying appropriate rootstocks and grapevine varieties for TWW irrigation. Additionally, more studies are needed to evaluate the impact of TWW reuse on wine quality, including sensory characteristics and the potential presence of contaminants in the final product.

Financial incentives and policy support will be crucial in encouraging the adoption of treated wastewater reuse in viticulture. Governments and agricultural organizations can provide funding, technical assistance, and clear regulatory frameworks to ensure that vineyards can safely and effectively integrate TWW into their irrigation systems. These policies should include guidelines for water quality standards, treatment methods, and contaminant monitoring to protect both the environment and public health.

In conclusion, treated wastewater reuse presents a valuable opportunity for viticulture to address the growing challenge of water scarcity while promoting sustainability. However, its success depends on continued research, careful management, and regulatory oversight to ensure that its application does not compromise grape quality or environmental integrity. By adopting innovative irrigation practices, vineyard owners can help ensure the future of the wine industry in a water-scarce world, ensuring that vineyards thrive while conserving vital water resources.