water Review Worldwide Regulations and Guidelines for Agricultural Water Reuse: A Critical Review Farshid Shoushtarian and Masoud Negahban-Azar * Department of Environmental Science and Technology, University of Maryland, College Park, MD 20740, USA; farshid@umd.edu * Correspondence: mnazar@umd.edu; Tel.: +1-301-405-1188 Received: 18 February 2020; Accepted: 25 March 2020; Published: 29 March 2020 ���������� ������� Abstract: Water reuse is gaining momentum as a beneficial practice to address the water crisis, especially in the agricultural sector as the largest water consumer worldwide. With recent advancements in wastewater treatment technologies, it is possible to produce almost any water quality. However, the main human and environmental concerns are still to determine what constituents must be removed and to what extent. The main objectives of this study were to compile, evaluate, and compare the current agricultural water reuse regulations and guidelines worldwide, and identify the gaps. In total, 70 regulations and guidelines, including Environmental Protection Agency (EPA), International Organization for Standardization (ISO), Food and Agriculture Organization of the United Nations (FAO), World Health Organization (WHO), the United States (state by state), European Commission, Canada (all provinces), Australia, Mexico, Iran, Egypt, Tunisia, Jordan, Palestine, Oman, China, Kuwait, Israel, Saudi Arabia, France, Cyprus, Spain, Greece, Portugal, and Italy were investigated in this study. These regulations and guidelines were examined to compile a comprehensive database, including all of the water quality monitoring parameters, and necessary treatment processes. In summary, results showed that the regulations and guidelines are mainly human-health centered, insufficient regarding some of the potentially dangerous pollutants such as emerging constituents, and with large discrepancies when compared with each other. In addition, some of the important water quality parameters such as some of the pathogens, heavy metals, and salinity are only included in a small group of regulations and guidelines investigated in this study. Finally, specific treatment processes have been only mentioned in some of the regulations and guidelines, and with high levels of discrepancy. Keywords: water reuse; agriculture; irrigation; regulation; guideline; standard; recycled water 1. Introduction Climate change, industrialization, high rate of urbanization, and population growth are among the main reasons that have made many countries, especially in the arid and semi-arid areas, suffer from the water crisis [1]. For instance, water scarcity in Australia has caused population losses in north-eastern, south-eastern, and western rural areas. These areas have experienced further unemployment, lack of success in local businesses, and downtrend in irrigation [2]. Countries in the Middle East, Central Asia, and some parts of Southeast Asia have been struggling on water-related issues. It is anticipated that these struggles may result in conflicts over shared water resources in these regions [3]. Considering the adverse consequences of the water crisis, countries around the world have been trying to increasingly cope with this problem by implementing sustainable water management plans and looking for alternative water supply sources [1]. Water conservation, water reuse, and desalination of seawater and brackish groundwater are among those strategies that have been tried to address the water crisis [1]. Water 2020, 12, 971; doi:10.3390/w12040971 www.mdpi.com/journal/water http://www.mdpi.com/journal/water http://www.mdpi.com https://orcid.org/0000-0001-6485-1881 http://dx.doi.org/10.3390/w12040971 http://www.mdpi.com/journal/water https://www.mdpi.com/2073-4441/12/4/971?type=check_update&version=2 Water 2020, 12, 971 2 of 58 In recent years, more and more countries are considering water reuse as an alternative water supply to supplement the freshwater sources [1,4,5]. Water reuse decreases the pressure on the freshwater resources, reduces the pollution that is being discharged to water bodies, and can be a reliable source compared to other water resources that are directly dependent on rainfall [1]. Due to these advantages and along with the recent developments in wastewater treatment technologies, scientists reported that the worldwide volume of recycled water in the 2010–2015 period was increased from 33.7 (million m3/d) to 54.5 (million m3/d) [6]. 1.1. Water Reuse History Water reuse has been practiced by humans for a very long time, of course, sometimes not in an appropriate way. Ancient civilizations during the Bronze age, 3200–1100 BC, used their domestic wastewater to irrigate their crops [7]. Ancient Greeks conveyed their domestic wastewater to a storage chamber using a sewer system in public latrines [8]. Moreover, Greeks and Romans used wastewater in agricultural irrigation, preparing fertilizer for crops and orchards [9]. During early modern history (1550–1700), direct use of wastewater in agriculture was being applied in Germany, Scotland, and England [8]. Beginning in the 19th century, irrigation with wastewater gained more popularity in some European and U.S. cities such as Paris, London, and Boston [8]. About the same time, the first wastewater irrigation in agriculture happened in Australia [9,10]. However, conveying and discharging the untreated wastewater in urban fields caused waterborne disease epidemics, such as cholera and typhoid fever outbreaks [8]. Unsafe application of wastewater in urban and agricultural areas, industrialization, and urbanization resulted in unhealthy situations for the societies in the 19th century [8]. To address the existing problems, some helpful efforts were as follows: (1) establishing the Great Britain’s Public Health Act, (2) holding a lot of sanitary conferences on sanitation and demography, (3) the constitution of International Office of Public Hygiene, and (4) constructing the underground sewage systems [8]. Generally, the application of recycled water can be divided into seven categories including urban reuse, agricultural reuse, impoundments, environmental reuse, industrial reuse, groundwater recharge/non-potable reuse, and potable reuse [11]. Of note is water reuse applications are different in various countries and depend on several factors such as levels of treatment, the conditions of water resources, environmental status, and public willingness [11]. Agricultural water reuse, by far, is the most dominant application of water reuse in the world [1]. In total, 91% of the recycled water in this section is allocated for crops and pastures irrigation, including the growing of fruit, tree nut, vegetables, cotton, and grain farming [1]. The residual 9% is dedicated to the cleaning of piggeries, and drinking water for stock and dairy [1]. Agricultural water reuse has multiple advantages such as reducing pressure on fresh water sources [8,12], nutrients management and recovery [13,14], and higher reliability due to constant yield [12,15]. However, wastewater needs to be adequately treated to be used for agricultural irrigation, especially for food crop irrigation due to potential health risks [16]. Other major limiting factors in agricultural water reuse include technical feasibility (e.g., treatment technologies and management), economic factors (e.g., water distribution cost), social factors (e.g., social acceptance and consumer response), and regulatory considerations (e.g., lack of regulations or guidelines) [17,18]. Of note is while the focus of this study was agricultural water reuse, there might be some other challenges in the future related to water reuse in general (such as developing methods of coupling advanced wastewater treatment with seawater desalination facilities; developing efficient methods of risk assessment for water reuse practices; establishing regulations and guidelines which ensure promoting and regulating water reuse practices) [19]. A list of benefits and constraints of water reuse in agriculture is provided in Table 1. It should be noted that not every water reuse project will result in all of these benefits immediately, nor will face all of these challenges at the same time [4]. Water 2020, 12, 971 3 of 58 Table 1. Benefits and challenges of agricultural water reuse (adapted from [4] and modified). Benefits Challenges Sustainable development: Increasing food production [20]. Improving aquatic life/fish production [21]. Sustainable development of dry regions [20]. Technical issues: Operation/maintenace reliability [22]. Increasing water system complexity [23]. Proper design of treatment processes [24]. Water reuse infrastructure resilience [25]. Available knowledge/expertise/experience [26]. Water conservation: Closing water cycle [27]. More efficient water use [28]. Saving high-quality water [29]. Social concerns: Unequal development. Social acceptance [30,31]. Consumer response/crops marketability [32]. Conflicts between different stakeholders. Socioeconomic/cropping patterns change [33]. Water supply: Reliable/secure/drought-proof water source [1]. Alternative/efficient/independent water supply [1]. Future challenges: Developing methods of coupling advanced wastewater treatment with seawater desalination facilities [19]. Developing efficient methods of risk assessment [19]. Establishing regulations and guidelines which ensure promoting and regulating water reuse practices [19]. Health benefits: Improving public health [34]. Improving health/environmental justice [1,4]. Health concerns: Microbial/chemical polluion [35]. The health of farmers/workers/consumers [35]. Inadvertent exposure/unreliable operation [35]. Environmental benefits: Linking rural-urban areas [36]. Reducing pollutants discharge [34]. Avoiding groundwater pollution [34]. Avoiding new water supply impacts [37]. Effective use of wastewater nutrients [34]. Improving recreational value of waterways [38]. Alternative to wastewater permits restrictions [39]. Environmental concerns: Polluting soils [34]. Endangering wildlife [40]. Polluting water bodies [41]. Greenhouse gas emissions [42]. Negative effects on crops/food [43]. Legal benefits: Policy awareness [44]. Compatible with treatment regulations [45]. Legal issues: Water rights. Lack of reuse regulations/guidelines [46]. Economic benefits: Avoiding development cost [47]. Increasing land/property value [48]. Increasing tourism activities in dry regions [49]. Additional revenue from recycled water sale [50]. Secondary revenue for costumers/industries [50]. Reducing/eliminating commercial fertilizers [51]. Lowering water treatment costs for downstream [34]. Economic challenges: Water pricing. Demand variations. Vulnerability to market change [50]. Difficult revenue and cost recovery [50]. Large storage capacity requirement [52]. Cost of water reuse infrastructure/operation and maintenance [50]. Need for well-adapted economic approach [50]. 1.2. Current Status of Water Reuse FAO (Food and Agriculture Organization of the United Nations) has estimated that 3.928 × 1012 m3 of freshwater was withdrawn from existing water sources in the world in 2010. In total, 11% of the total water withdrawal in the world was municipal water demand, of which 3% was consumed and 8% was discharged as municipal wastewater [53]. There was 2.75× 106 million m2 of land consisting of irrigated agriculture worldwide, of which about 15% (4 × 105 million m2) could be irrigated by the municipal wastewater [53]. Moreover, 32% of the world wide water withdrawal was discharged as agricultural wastewater and drainage [53]. The majority of wastewater that is recycled in agriculture is municipal wastewater, but these results show the need to change the focus of water reuse policies and plans from municipal wastewater management to sustainable management of municipal and agricultural (drainage and return flow) wastewater [1]. Furthermore, approximately, 5 × 104 to 2 × 105 million m2 Water 2020, 12, 971 4 of 58 of the irrigated land is irrigated by raw and diluted wastewater, with the largest portion being in China [53]. This just includes 2–7% of the world’s total irrigated area. Accordingly, there is a great potential for implementing planned and safe water reuse in agriculture. While irrigation with recycled water is recognized as an alternative source to reduce the pressure on fresh water sources, the ultimate goal is safe implementation of water reuse practices [1]. One of the basic necessities for safe application of recycled water is to make sure that it has the desired quality and poses no harm to human health and the environment [16]. Started by the state of California in 1918, countries around the world alongside international organizations (e.g., World Health Organization (WHO) and FAO) have started to establish their water reuse regulations and guidelines to ensure safe water reuse practices [7]. In general, countries and organizations have taken different approaches to establish regulations and guidelines [54]. For instance, some countries like Canada, Australia, and many states in the U.S. have issued more restrictive regulations, while others have chosen to take less restrictive approaches to develop water reuse regulations and guidelines. Of note is that there is no federal regulation or guideline for agricultural water reuse in the U.S. [11]. It is up to the states to establish their own regulations or guidelines [11]. When compared in more details, it becomes apparent that current agricultural water reuse regulations and guidelines vary significantly [1]. For instance, some of the regulations and guidelines do not consider some of the biological and microbial quality parameters, and some others do not consider some of the physico-chemical parameters. Furthermore, even in regulations and guidelines that do consider the same parameters, the threshold levels for those parameters vary significantly. As water reuse in agriculture is becoming popular as a beneficial approach to address the water scarcity, the disparity in regulations and guidelines may become a source of problems, at both regional and global levels [54]. At the regional level, the absence of unified or at least relatively comparable water reuse regulations and guidelines may result in uncertainty among stakeholders (e.g., farmers, consumers, and policy-makers), thereby slowing down the promotion of water reuse in agriculture. The U.S. is a good example in that respect. In the U.S. there are 42 and 28 states that have regulation or guideline for nonfood crop/processed food crop, and food crop irrigation, respectively [11]. Eight states do not have any type of regulation or guideline for agricultural water reuse [11]. When looked at in more details, the water quality parameters and the threshold levels in those regulations and guidelines are different. This not only may create uncertainty among stakeholders, but also may increase the risk of public acceptance, thereby slowing down the process of implementing agricultural water reuse [1]. In addition, agriculture has a global market and agricultural commodities are being imported/exported all around the world. As a result, the difference in regulations and guidelines between the countries of origin and the end-use countries may pose major obstacles in food safety, market acceptability, and import/export relationships. To date, there are very few studies which investigate and compare the existing agricultural water reuse regulations and guidelines in the U.S. and worldwide. The main objectives of our research were to compile and compare the existing agricultural water reuse regulations and guidelines around the world, and to identify the gaps in those regulations and guidelines. To achieve this goal, the most up to date regulations and guidelines that were issued by national and international organizations (e.g., Environmental Protection Agency (EPA), International Organization for Standardization (ISO), FAO, WHO, European Commission), and by pioneering countries in water reuse (e.g., U.S., Canada, Mexico, Iran, Egypt, Tunisia, Jordan, Israel, Oman, China, Kuwait, Saudi Arabia, Australia, France, Greece, Portugal, Cyprus, Spain, and Italy) were obtained and investigated in this study. In addition, the water quality criteria in those regulations and standards were compared, and the major differences between those criteria were identified. Results from this study identify the discrepancies in the current regulations and guidelines. They also highlight the challenging areas that need to be addressed to promote the agricultural water reuse with respect to the existing regulations and guidelines. Water 2020, 12, 971 5 of 58 2. Methodology In order to compile a complete worldwide agricultural water reuse regulations and guidelines database, Google Scholar search engine was used as the first step of this study. In this step, key words including “water reuse”, “water reclamation”, “water recycling”, “wastewater reuse”, “wastewater recycling”, “recycled water”, “reclaimed water”, “agriculture”, “regulation”, “guideline”, “standard”, and “criteria” were used. Peer reviewed journal articles related to agricultural water reuse regulations and guidelines were compiled and reviewed. In the second step, based on the results obtained from the first step, study cases were identified (e.g., countries, international organizations, and state agencies that have issued/established agricultural water reuse regulations or guidelines). In the third step, the official website of the organizations (e.g., state agencies, ministries, governmental institutes, etc.) were investigated. Moreover, official representatives at organizations/agencies were contacted if needed to make sure that the obtained regulations and guidelines were the latest version. In total, 70 agricultural water reuse regulations and guidelines were gathered for this study. All of these regulations and guidelines were thoroughly analyzed and compared in this study. 2.1. Definitions and Terminologies 2.1.1. Technical Definitions and Terminologies The use of treated wastewater for beneficial purposes is generally called water reuse [5]. However, there are different terminologies that have been used in various water reuse regulations and guidelines such as water reuse, water recycling, water purification, reclaimed water, recycled water, reused water, repurified water, NEWater, and more. To clarify, in this manuscript, water reuse refers to treatment or processing of wastewater and then the application of the treated wastewater in agriculture. In addition, recycled or reclaimed water refer to the treated wastewater that is used for different applications. Of note is that “recycled” and “reclaimed” water have been used in this manuscript interchangeably. 2.1.2. Legal Definitions and Terminologies Similar to scientific and technical terminologies, different legal terminologies have been used for water reuse regulations. While the main focus of this manuscript was technical and scientific aspects of agricultural water reuse, it is helpful to clarify these legal terminologies, which are commonly used in the reference documents (Table 2). Table 2. The definition of standard, criteria, guideline, and regulation [5]. Term Definition Comments Standard A rule, principle, or measure established by an authority. Standards are usually quite rigid, official, or quasi-legal. As standards may be written using safety factors, they can be potentially unfair, inequitable, or ignoring scientific knowledge. Standards typically include qualitative restrictions in terms of numerical limits. Criteria As the basis for standards, criteria are developed based on available data and scientific opinion. It is common that technical and economic feasibility are not considered in the process of developing criteria. Effective criteria have the potential to be evaluated quantitatively through suitable analytical procedures. Criteria include qualitative restrictions (these restrictions can be numerical limits and narrative statements). Guideline Best practices that are used prior to development of standards or regulations. Usually, guidelines are voluntary, advisory, and non-enforceable. These guidelines can be used in water reuse permits to become enforceable requirements. Regulation When a state legislature or a water pollution control agency officially adopt a standard, criteria or guideline. Enforceable and mandatory by governmental agencies, water reuse regulations include treatment requirements, cross connection controls, signage, and setback distances. Act Passed by Congress, state legislatures or Parliament, depending on each country’s type of government, acts set out the broad/policy principles. Water 2020, 12, 971 6 of 58 3. Results and Discussion In total, 70 regulations, guidelines, standards, criteria, and acts were obtained and included in this study (Table 3). The State of California in the U.S. was the first to issue a specific regulation for agricultural water reuse in 1918. After 48 years, the next regulation document was issued by the state of Iowa in the U.S. in 1966, followed by Mexico’s standard in 1971. WHO is the first international organization that issued a guideline for agricultural water reuse in 1973. As illustrated in Table 3, among the 70 investigated documents, there were 30 regulations, 29 guidelines, six standards, four criteria, and one act. It was found that most of these regulations and guidelines were issued after 1973, and the majority of them were issued after 1998. Starting from the 1970s and 1980s, international organizations including WHO, FAO, and the World Bank tried to effectively notify countries and organizations around the world the importance of safe water reuse practices, resulting in the propagation of establishing water reuse regulations and guidelines [1]. Table 3. Agricultural water reuse regulations or guidelines included in this study. # Year 1 Country (State) Current Edition Type 1 1918 US (California) Title 22: California Water Recycling Criteria [55], Water Code-division 7–article 7 [56]. Regulation 2 1966 US (Iowa) 567 IAC Chapter 62: Effluent and Pretreatment Standards: Other Effluent Limits or Prohibitions [57]. Regulation 3 1971 Mexico Standard NOM-001-ECOL-1996 [58,59]. Standard 4 1973 WHO 2 WHO guideline for the safe use of wastewater, excreta and greywater-volume II—wastewater use in agriculture [60]. Guideline 5 1975 US (Alabama) Alabama Environmental Regulations and Laws-division 6-volume 3—reclaimed water reuse program [61]. Guideline 6 1976 US (South Carolina) Regulation 61-9, Water Pollution Control Permits [62]. Regulation 7 1977 Italy National Inter ministry Committee for the Protection of Waters from Pollution [63]. Regulation 8 1980 EPA 2 Guidelines for water reuse [11]. Guideline 9 1981 US (Arizona) Arizona administrative code, title 18, chapters 9 and 11 [64]. Regulation 10 1985 US (Delaware) Regulations governing the design, installation and operation of on-site wastewater treatment and disposal systems [65]. Regulation 11 US (Wisconsin) Chapter NR 206—land disposal of municipal and domestic wastewaters [66]. Regulation 12 1987 FAO 2 Wastewater quality guidelines for agricultural use [67] Guideline 13 1989 US (North Dakota) Chapter 33-16-01—North Dakota pollutant discharge elimination system [68]. Guideline 14 Tunisia Tunisian standards NT 106-03 [69,70]. Standard 15 1990 US (Oregon) Department of environmental quality-Chapter 340-Division 53—Graywater reuse and disposal systems [71]. Regulation 16 1991 US (Florida) Reuse of reclaimed water and land application [72]. Regulation 17 France Water reuse criteria for agricultural and landscape irrigation in France [73]. Criteria 18 US (South Dakota) Recommended design criteria manual-wastewater collection and treatment facilities [74]. Guideline 19 1992 US (Washington) Chapter 90.46 RCW [75]. Guideline 20 1993 Oman Ministerial decision no. 145 of 1993 issuing the regulations on waste water reuse and discharge [76]. Regulation 21 1995 US (Illinois) Title 35: environmental protection–Subtitle c: water pollution-Chapter ii: environmental protection agency—Part 372 Illinois design standards for slow rate land application of treated wastewater [77]. Regulation 22 US (Montana) DEQ 2—design standards for wastewater facilities [78]. Regulation 23 1996 CA (Atlantic Canada) Atlantic Canada wastewater guidelines manual [79]. Guideline 24 1997 US (Texas) Chapter 210-use of reclaimed water [80]. Regulation 25 1998 US (Indiana) Article 6.1—land application of bio solid, industrial waste product, and pollutant-bearing water [81]. Regulation Water 2020, 12, 971 7 of 58 Table 3. Cont. # Year 1 Country (State) Current Edition Type 26 1999 AU (Australian Capital Territory) ACT—wastewater reuse for irrigation [82]. Guideline 27 CA (British Columbia) Chapter 10—use of reclaimed water [83]. Regulation 28 Israel Israeli guideline for wastewater reuse [84–86] Guideline 29 2000 CA (Alberta) Guidelines for municipal wastewater irrigation [87]. Guideline/Act 30 US (Colorado) Regulation 84: reclaimed water control regulation [88]. Regulation 31 Greece [89,90] Criteria 32 Saudi Arabia [91] Regulation 33 2001 Kuwait Standards of the Kuwait environment public authority (KEPA) [92]. Standard 34 2002 China GB20922-2007 [93]. Standard 35 US (Hawaii) Volume 1: recycled water facilities [94]. Guideline 36 Jordan Jordanian standard (JS: 893/2002) [95]. Standard 37 US (Maryland) Guidelines for use of class iv reclaimed water [96]. Guideline 38 AU (Tasmania) Environmental guidelines for the use of recycled water in Tasmania [82] Guideline 39 2003 AU (New South Wales) The guidelines for sewerage systems: use of reclaimed water (ARMCANZ-ANZECC-NHMRC 2000) [82]. Guideline 40 Palestine [97] Regulation 41 AU (Victoria) The guidelines for environmental management: use of reclaimed water, guidelines for environmental management: dual pipe water recycling schemes—health and environmental risk management [82]. Guideline 42 2004 CA (Saskatchewan) Treated municipal wastewater irrigation guidelines-EPB 235 [98]. Guideline 43 2005 Cyprus Cyprus regulation K.D.269/2005 [11]. Regulation 44 Egypt [99] Regulation 45 US (New Jersey) Reclaimed water for beneficial reuse [100] Guideline 46 Spain Spanish regulations for water reuse-royal decree 1620/2007 of 7 December [101]. Regulation 47 2006 AU (AGWR) The Australian guidelines for water recycling: augmentation of drinking water supplies [102]. Guideline 48 Portugal Portuguese standard NP 4434 [103]. Criteria 49 2007 US (Ohio) 3745-42-13 Land application systems [104]. Guideline 50 2008 US (Idaho) Rules for the reclamation and reuse of municipal and industrial wastewater [105]. Regulation 51 AU (Queensland) The water quality guidelines for recycled water schemes (DNRW 2008c) [82]. Guideline 52 US (Virginia) Chapter 740. Water reclamation and reuse regulation [106]. Regulation 53 2009 US (Massachusetts) 314 CMR 20: Reclaimed water permit program and standards [107]. Regulation 54 AU (Western Australia) Guidelines for the use of recycled water in western Australia (WA DoH 2009) [82]. Guideline 55 2010 Iran Criteria for using recycled water (In Farsi) [108]. Criteria 56 ISO 2 Guidelines for treated wastewater use for irrigation projects [109]. Standard 57 US (Minnesota) Municipal wastewater reuse [110]. Guideline 58 2011 US (Kansas) Kansas EPA 503 land application of septage–updated [111]. Guideline 59 US (North Carolina) Subchapter 02U—reclaimed water [112]. Regulation 60 2012 US (Georgia) Guidelines for slow-rate land treatment of wastewater [113]. Guideline 61 US (Pennsylvania) Reuse of treated wastewater guidance manual 385-2188-002 [114]. Guideline 62 US (Rhode Island) Guidance for wastewater reuse projects [115]. Guideline 63 US (Wyoming) Department of environmental quality, water quality, chapter 21: reuse of treated water [116]. Regulation 64 2013 US (New Mexico) Title 20, chapter 7, part 3 [117]. Guideline 65 US (Utah) Title R317. Environmental quality, water quality [118]. Regulation 66 2014 AU (Northern Territory) Guidelines for wastewater works design approval of recycled water systems [82]. Guideline 67 2015 US (Oklahoma) Title 252. chapter 656. Water pollution control facility construction standards [119]. Regulation 68 2016 US (Nevada) Use of reclaimed water [120]. Regulation 69 2017 European Commission 2 Minimum quality requirements for water reuse in agricultural irrigation and aquifer recharge [121]. Guideline 70 US (Nebraska) Title 119, chapter 12 [122]. Regulation 1 The dates indicate when the documents were established/issued for the first time. 2 Organizations. Water 2020, 12, 971 8 of 58 3.1. Reference Regulations and Guidelines In this section, the pioneer agricultural water reuse regulations and guidelines which have been the source of inspiration and adoption for many of other states, countries, and organizations are discussed. They include WHO, EPA, FAO, Australian Guideline for Water Recycling (AGWR) guidelines, ISO standard, California, and European Commission regulations. 3.1.1. World Health Organization (WHO) guideline WHO issued three guidelines for water reuse in 1973, 1989, and 2006. The first document was published in 1973 entitled “Reuse of effluents: methods of wastewater treatment and health safeguards”, which became one of the main references for other international standards. The main goals of this document were to protect the public health and to guide the safe application of wastewater and excreta in agriculture and aquaculture. However, the document had minimal health risk approach and lacked epidemiological studies [8]. Later, the WHO updated its prior guideline in 1989 by implementing a complete epidemiological studies analysis. In this version, entitled “Health guidelines for the use of wastewater in agriculture and aquaculture”, WHO focused on the microbiological quality of the recycled water for irrigation. Additionally, risk assessment and necessary information to determine the societies’ tolerable risks were included. This guideline lacked to give any information about surveillance guidelines [8]. WHO’s final guideline was published in 2006, entitled “Safe use of wastewater, excreta, and greywater”, to contribute to forming governmental guidelines, standards, and regulations relating to wastewater management for each country regarding its specific situation. There are significant improvements regarding risk assessment in this guideline including microbiological analysis, based on the information gathered from present pathogens, and health risk management, estimations were made based on person per year (PPY) and disability-adjusted life year (DALY) [8]. This risk-based guideline is mainly focused at microbial health risks, but it also contains recommended maximum tolerable soil concentrations for various organic and inorganic pollutants which are assessed by QMRA (Quantitative Microbial Risk Assessment) and epidemiological evidence. The DALYs are used in this guideline in order to compare the results of a disease from one exposure pathway to another pathway. WHO indicated the determination of DALYs as follows: “DALYs are calculated by adding the years of life lost to premature death to the years lived with a disability”, accounting for acute and chronic health effects [60]. A water-borne disease burden of 10−6 DALYs per person per year is determined as the tolerable risk by WHO [60]. Critics claim that this is not the most appropriate value to use, especially in low-income countries [123]. In this guideline (volume 2, section 4.5), it is mentioned that the appropriate value is less than 10−4 or 10−5 DALY (loss) per person per year which Mara et al. supports less than 10−4 DALY to be used for water reuse in agriculture [123]. Moreover, this guideline defines two exposure scenarios for agricultural irrigation including unrestricted irrigation and restricted irrigation, suggesting the required log pathogen reductions for each of them (Table S1). Regarding physico-chemical quality of the water, WHO refers to the FAO’s requirements for irrigation practices. Of note is that the WHO guideline requires no specific type of treatment for the restrictions mentioned above. 3.1.2. FAO Guideline FAO issued two guidelines for water reuse in 1987 and 1999. In the latest version, FAO divides the application of recycled water in agriculture into three categories, including (A) Irrigation of crops likely to be eaten uncooked, sports fields, and public parks, (B) Irrigation of cereal crops, industrial crops, fodder crops, pasture, and trees, and (C) localized irrigation of crops in category B if exposure of workers and the public does not occur [67]. Moreover, FAO drafts some requirements for interpretation of water quality for irrigation (Table S2) including three degrees of restriction including severe, slight to moderate, and none, on use of the recycled water based on its quality. Additionally, threshold levels of trace elements for crop production are introduced by FAO in its last guideline in 1999 [67]. Water 2020, 12, 971 9 of 58 In terms of microbial parameters, FAO follows a less restrictive approach, similar to WHO, considering epidemiological evidence. FAO recommends stabilization ponds, category A and B, and at least primary sedimentation, category C. In unrestricted category, A, FAO recommends stricter limitations for fruit trees as Fecal Coliforms <200/100 mL. For physico-chemical parameters, FAO guideline has been the leading guideline to which the standards, criteria, guidelines, and regulations of other organizations, countries, and states agencies have referred. 3.1.3. Environmental Protection Agency (EPA) Guideline EPA developed four guidelines for water reuse in 1980, 1992, 2004, and 2012. The first guideline was issued as a technical research report in 1980. Later in 1992, EPA updated the first version by including toxicity in crops that were irrigated with wastewater [8]. This version was provided for project planners and state regulatory officials in order to develop water reuse systems in different states. EPA included two new scopes in its updated guideline in 2004 consisting of “indirect potable reuse” and “industrial reuse”. New treatment and disinfection technologies, concerning pathogens and emerging chemicals, information about economics, research actions, funding alternatives, and data sources were also elaborated in the 2004 document [8]. The EPA along with the United States Agency for International Development (USAID) issued an updated version of the 2004 EPA guideline in 2012 (Table S3). The ultimate goal of this guideline was to make the water reuse process easy to implement based on global databases. In addition, EPA and USAID included the progresses made in the technologies of wastewater treatment, regional variations of water reuse, best management practices (BMPs) in communities’ involvement, case studies of water reuse around the world, and development of safe and sustainable water reuse. In this guideline, EPA suggests some requirements for each of the water reuse categories mentioned in the guideline [8]. Regarding agricultural water reuse, EPA divides agricultural water reuse into two categories including water reuse for food crops and water reuse for processed food crops/nonfood crops irrigation. This guideline also provides some suggestions for the required treatments, recycled water quality, recycled water monitoring, setback distances, and chemical constituents’ limits [11]. Secondary treatment, filtration, and disinfection are the required treatments for food crops and secondary treatment and disinfection are the required treatments for processed food crops/nonfood crops which were the most common treatments in existing regulations and guidelines. Furthermore, EPA used a very high-demanding approach for its microbial requirements, resulting in being a restrictive guideline in terms of microbial water quality. Moreover, EPA recommends FAO’s water quality criteria for irrigation. 3.1.4. Australian Guideline for Water Recycling (AGWR) As a national guideline, supply and use of recycled water has been regulated through Australian Guideline for Water Recycling: Managing Health and Environmental Risks (AGWR). This guideline was issued by the Australia’s Environment Protection and Heritage Council and the Natural Resource Management Ministerial Council in 2006 to address water crisis, as a result of widespread droughts and population growth in Australia [124]. In order to manage the risks to human and the environment, the guideline focuses on two situations, namely, the effluent of a centralized wastewater treatment plant and recycled water from greywater recycling. The guideline helps to identify major health risks and recommends preventive practices to lower those risks to an acceptable level [124]. Regarding human health, AGWR focuses on microbial risks which are addressed using DALYs. The tolerable risks in AGWR, like WHO 2006 guideline, is 10−6 DALYs per person per year. Reference pathogens, including Campylobacter for bacteria, rotavirus and adenovirus for viruses, and Cryptosporidium parvum for protozoa and helminths, are used for risk identification [124]. Additionally, two categories of intended and unintended use are included in the exposure consideration. Moreover, maximum risk, risk with no preventive practices, residual risk, and remaining risk with the presence of preventive practices, are considered for risk characterization in AGWR. For environmental risks, instead of DALYs and health-based targets, environmental values which are related to the Water 2020, 12, 971 10 of 58 impacts on specific endpoints in the environment are used (e.g., native tree species and specific grasses). Eighteen environmental hazards were identified by AGWR, including Boron, Cadmium, Chlorine disinfection residuals, hydraulic loading (water), Nitrogen, Phosphorus, salinity, Chloride and Sodium, Ammonia, Aluminum, Arsenic, Copper, Lead, Mercury, Nickel, and Surfactants [124]. 3.1.5. California’s Regulation As was mentioned before, California’s regulation (the first version) was the first water reuse regulation worldwide issued in 1918 by the California Department of Health Services. As a pioneer in water reuse regulations, California’s regulation has been the basis for many other state agencies as well as other countries and international organizations. This regulation has been considered a very comprehensive and restrictive regulation as it covers a wide range of water quality parameters and other requirements in terms of type of crops and irrigation types (Table S4). Similar to EPA guidelines, California’s regulation requires a high level of disinfection along with total coliform inactivation of <2.2 (total coliform/100 mL). Although this regulation is one of the most developed regulations in terms of water quality monitoring, treatment train design, and operation, it lacks any requirement for irrigation rates or storage requirements [4]. Since its first edition establishment, California’s agricultural water reuse regulation has been continuously studied and revised. The terms “reclaimed water” and “water reuse” in earlier versions of the regulation have been changed to “recycled water” and “water recycling” in more recent versions, respectively. Additionally, oxidized wastewater which is undisinfected secondary treated wastewater was chosen as the requirement for industrial crops [4]. Moreover, turbidity requirements for high-level recycled water uses were added. Of note is that this regulation requires no other physico-chemical water quality parameter. 3.1.6. International Organization for Standardization (ISO) Standard The first ISO standard for water reuse was issued in 2010 based on a request from Israel for water reuse in agriculture, titled PC 253 [109]. The next ISO standard for water reuse was proposed by Japan to be established along with Israel and China, titled TC 282, in 2015. WHO guideline (2006), Australian national water reuse regulations (2006), Israeli regulations for agricultural irrigation (1978, 1999, and 2005), and California Code of Regulations (Title 22, division 4, chapter 3, water recycling criteria (2000)) were the references in order to establish the ISO standard [109]. ISO standard consists of three sections: (1) Treated wastewater use for irrigation, (2) Treated wastewater use in urban area, and (3) Risk and performance evaluation of water reuse systems. In the first section, ISO introduces 5 categories of water quality for water reuse applications for irrigation, A: Very high quality treated wastewater, B: High quality treated wastewater, C: Good quality treated wastewater, D: Medium quality treated wastewater, and E: Extensively treated wastewater [109]. As required treatments, combinations of secondary treatment, filtration, and disinfection are used in this guideline, depending on the water quality [109]. Although disinfection is needed for A, B, and C categories, there was no requirements for residual chlorine in this guideline. The microbial approach in this standard is close to the restrictive approach but it also includes the intestinal nematodes. For a higher quality water, A and B, the low concentrations of thermo-tolerant coliforms are considered adequate to make sure the water is suitable for unrestricted and restricted food crops irrigation. Irrigation of nonfood, industrial, and seeded crops, C, D and E, are regulated by thermo-tolerant coliforms and intestinal nematode restrictions. For physico-chemical qualities, ISO includes biological oxygen demand (BOD5) and total suspended solids (TSS) restrictions, while turbidity is used only for category A recycled water [109]. 3.1.7. European Commission Regulation A proposal has been put forward by European Commission in order to stablish a European regulation for agricultural water reuse, since May 2018 [125]. The proposal goals are to encourage the Water 2020, 12, 971 11 of 58 application of recycled water and to help address the water crisis in Europe [125]. As an EU-wide project, it has been estimated that the project can decrease the water stress in Europe by 5% through increasing the application of recycled water from 1.7 billion m3 to 6.6 billion m3, annually [125]. The references used to establish the proposal included a commission impact assessment for the 2012 Blueprint communication [126], a study on guidelines, needs, and barriers related to water reuse [127], a 2017 report on minimum quality requirements for wastewater reuse [128], a 2017 hydro-economic analysis [129], a 2013 report on wastewater reuse in the EU [130], a 2015 report on optimizing water reuse in the EU [131], a 2016 report on EU-level instruments on water reuse [132], and a 2017 report on the patterns of unplanned water reuse [133]. The proposal requires the operators of water reuse practices to comply with minimum recycled water quality requirements summarized in Table S5. Moreover, the proposal requires the operators to establish a risk management plan to ensure addressing the potential additional dangers [125]. The Committee on Environment, Public Health, and Food Safety (ENVI) is the responsible committee for this proposal in the European Parliament. 3.2. Recycled Water Quality Standards In most cases, agricultural water reuse regulations and guidelines include three categories of water quality, treatment processes, and irrigation technologies [46]. Recycled water quality can be categorized into three groups including human-health parameters, agronomic parameters, and physico-chemical parameters, each of which consists of many specific water quality parameters [4] (Figure 1). Water 2020, 12, 971 13 of 74 physico-chemical parameters, each of which consists of many specific water quality parameters [4] (Figure 1). Figure 1. Agricultural water quality parameters. 3.2.1. Human-Health Parameters Human-health parameters are of prominent importance in safe agricultural water reuse practices. The health of farmers, workers, consumers, and people who live in the close vicinity of farms have to be considered for safe agricultural water reuse practices. This issue has been addressed mainly by including microbial and chemical water quality parameters related to human health. 3.2.1.1. Pathogens The presence of pathogens is the main health concern when recycled water is used for irrigation. Scientists and experts have concluded that it is not practical to monitor the existence of all of the pathogens in recycled water. Therefore, the indicator organism concept has been used to monitor the pathogens in a more practical manner [4]. There are many waterborne pathogens and their microbiological indicators, which have been included in the regulations and guidelines. In general, there are two major approaches to microbial water quality including “no fecal indicator bacteria” and “no real risk of infection”. No Fecal Indicator Bacteria in the Water: In this approach, the assumption is that it is not viable to monitor all of the pathogenic microorganisms [134]. Therefore, coliforms are considered as substitute parameters. Fecal coliforms are the most common bacteria of thermo-tolerant coliforms. Additionally, E. coli is the most common fecal indicator bacterium used by different organizations and countries. Under this approach, no detectable fecal indicator is required using fecal indicator as the microbial indicator. The advantage of this approach is that there is no need to monitor all of the pathogenic microorganisms. The Recycled water quality Human-health partameters Pathogens Chemicals Agronomic parameters Salinity Toxic ions Sodium adsorption ratio (SAR) Trace elements pH Bicarbonate and Carbonate Nutrients Free Chlorine Physico-chemical parameters Turbidity TSS/SS and TDS BOD5, CBOD5, and COD Figure 1. Agricultural water quality parameters. 3.2.1. Human-Health Parameters Human-health parameters are of prominent importance in safe agricultural water reuse practices. The health of farmers, workers, consumers, and people who live in the close vicinity of farms have to be considered for safe agricultural water reuse practices. This issue has been addressed mainly by including microbial and chemical water quality parameters related to human health. Water 2020, 12, 971 12 of 58 Pathogens The presence of pathogens is the main health concern when recycled water is used for irrigation. Scientists and experts have concluded that it is not practical to monitor the existence of all of the pathogens in recycled water. Therefore, the indicator organism concept has been used to monitor the pathogens in a more practical manner [4]. There are many waterborne pathogens and their microbiological indicators, which have been included in the regulations and guidelines. In general, there are two major approaches to microbial water quality including “no fecal indicator bacteria” and “no real risk of infection”. No Fecal Indicator Bacteria in the Water: In this approach, the assumption is that it is not viable to monitor all of the pathogenic microorganisms [134]. Therefore, coliforms are considered as substitute parameters. Fecal coliforms are the most common bacteria of thermo-tolerant coliforms. Additionally, E. coli is the most common fecal indicator bacterium used by different organizations and countries. Under this approach, no detectable fecal indicator is required using fecal indicator as the microbial indicator. The advantage of this approach is that there is no need to monitor all of the pathogenic microorganisms. The disadvantage of this approach is that it is so strict and costly even though there is no need to monitor all of pathogenic microorganisms [134]. Shuval et al. argued that using this approach would increase cost per case of disease averted [135]. They estimated that the cost of using no detectable fecal coliform/100 mL was near US $330 million more than using 1000 fecal coliform/100 mL per each case of an infectious disease (i.e., hepatitis A) prevented [135]. As the level of endemic enteric diseases in developed counties are low, this higher cost may be justified. However, other countries with high levels of endemic enteric diseases which usually are transmitted through low levels of sanitation and hygiene may not justify this higher cost [134]. Of note is in most cases, the state of California’s regulation has been widely used as the benchmark regulation under this approach by other organizations and agencies. In this study, we call regulations and guidelines which used this approach “restrictive”. From the Epidemiological Point of View; no Real Risk of Infection: According to this approach, epidemiological evidence must be used to issue any microbial quality requirement [134]. The advantage of using this method is that the risk assessment process is done by studying the infection between exposed people to the recycled water. In this method, people exposed to different recycled water qualities would be studied to determine what level of recycled water quality results in no more excess infection cases in the study population. On the other hand, this approach is only valid for the specific time and place that the risk assessment has been conducted. Therefore, to use the results in regulations and guidelines, they must be extrapolated, which requires making some assumptions about the changes to the variables, making it less precise. Additionally, conducting epidemiological studies are not always easy, especially in developing countries. For example, there are critics about insufficiency of these studies and existence of some groups of people who have been immunized to many enteric infections. In addition, there may be a lack of health risk assessment methodologies which were used before for these types of studies [134]. The other disadvantage of this method is that epidemiological studies do not consider the secondary transmission [134]. Of note is in most cases, the WHO guideline has been considered as the benchmark guideline under this approach by other organizations and agencies. In this study, we call regulations and guidelines which used this approach “less restrictive”. Among the regulations and guidelines that were evaluated for this study, there were 49 documents that were considered restrictive, and 15 documents that were less restrictive (Tables 4 and 5). The microbial quality parameters in restrictive agricultural water reuse regulations and guidelines were Fecal Coliforms (25 documents), E. coli (21 documents), Total Coliforms (7 documents), Intestinal Nematodes (6 documents), Thermo-tolerant Coliforms (5 documents), Enterococci (2 documents), Somatic Coliphages (2 documents), Clostridium Perfringens (1 document), and F-RNA Bacteriophages (1 document). In general, the threshold limits for food crops irrigation and unrestricted public access categories have lower limits compared to processed food crops/non-food crops irrigation and Water 2020, 12, 971 13 of 58 restricted public access categories. When compared, regulations and guidelines established by different organizations and agencies sometimes had different threshold levels for the same parameters (Table 4). The comparison also showed a considerable level of discrepancy among the restrictive regulations and guidelines with respect to microbial water quality (Table 4). It was apparent that restrictive regulations or guidelines have been adopted by developed countries in the world due to their costs and high-tech requirements. It was estimated that restrictive regulations and guidelines cost an additional $3–30 million per prevented enteric disease [135]. Table 4. Restrictive agricultural water reuse regulations and guidelines. Reuse Categories Required Microbial Quality (cfu/100 mL) (Monitoring) EPA (2012) Food crops Fecal coliforms (daily): 0 (median of last 7 days), 14 (max) Processed food crops/non-food crops Fecal coliforms (daily): 200 (median of last 7 days), 800 (max) ISO (2015) A: very high-quality treated wastewater; unrestricted urban irrigation and agricultural irrigation of food crops consumed raw Thermo-tolerant coliforms: 10, 100 (max) B: high quality treated wastewater; restricted urban irrigation and agricultural irrigation of processed food crops Thermo-tolerant coliforms: 200, 1000 (max) C: good quality treated wastewater; agricultural irrigation of non-food crops Thermo-tolerant coliforms: 1000, 10,000 (max) Intestinal nematodes: 1 Egg/L (average) D: medium quality treated wastewater; restricted irrigation of industrial and seeded crops Intestinal nematodes: 1 Egg/L (average), 5 Egg/L (max) E: extensively treated wastewater; restricted irrigation of industrial and seeded crops Intestinal nematodes: 1 Egg/L (average), 5 Egg/L (max) British Columbia Restricted Fecal coliform (weekly): 200 Unrestricted Fecal coliform (daily): 2.2 Alabama E. Coli (daily): 18 (median of the last 7 results), 34 (max) Atlantic Canada Restricted E. Coli (2/month): 200 (only golf courses and parks) Unrestricted E. Coli (2/month): 2 (only golf courses and parks) Saskatchewan Food crops Fecal Coliform or E. Coli (1/Week): 2.2 (Median), 23 (Max) Non-food crops Fecal Coliform or E. Coli (1/Month): 1000 Arizona Food crops Fecal Coliform (Daily): 0 (4 of the last 7 daily samples), 23 (Max) Processed food crops/non-food crops Fecal Coliform (Daily): 1000 (4 of the last 7 daily samples), 4000 (Max) California Food crops Total Coliform Bacteria (Daily): 2.2 (Last 7 Days), 23 (One sample in any 30-day period), 240 (Max) Colorado Processed food crops/non-food crops E. Coli: 126 (Monthly geometric mean), 235 (Max) Delaware All types Fecal Coliform (2/Month): 20 Florida Food crops Fecal Coliforms: 0 (75% of samples), 25 (Max) Processed food crops/non-food crops Fecal Coliforms: 200 (Average), 800 (Max) Georgia Processed food crops/non-food crops Fecal Coliform (Daily): 23 (Monthly geometric mean), 46 (Weekly geometric mean), 100 (Max) Hawaii Food crops R-1: Fecal Coliform (Daily): 2.2 (Last 7 days), 23 (More than 1 sample in any 30-day period), 200 (Max) R-2: Fecal Coliform (Daily): 23 (Last 7 Days), 200 (More than one sample in any 30-day period) Water 2020, 12, 971 14 of 58 Table 4. Cont. Reuse Categories Required Microbial Quality (cfu/100 mL) (Monitoring) Idaho Food crops B: Total Coliform (Daily): 2.2 (Median), 23 (Max) C: Total Coliform (Weekly): 23 (Median), 230 (Max) Processed food crops/non-food crops C: Total Coliform (Weekly): 23 (Median), 230 (Max) D: Total Coliform (Monthly): 230 (Median), 2300 (Max) Indiana Food crops Fecal Coliform (Daily): 0 (Median Value), 14 (Max) Processed food crops/non-food crops Fecal Coliform (Daily): 200 (Median Value), 800 (Max) Kansas Restricted E. Coli (2/Month): 160 Unrestricted E. Coli (2/Month): 20 Maryland Class I (restricted access) Fecal Coliform: 200 (Monthly geometric mean) Class II (restricted access) Fecal Coliform: 3 (Monthly geometric mean) Class III1 (restricted access) Fecal Coliform: 2.2 (Monthly geometric mean) Massachusetts A: food crops, unrestricted Fecal Coliform: 0 (Median, continuous 7-day sampling), 14 (Max) B: pasture for milking animals, unprocessed food crops (no contact with the edible part of crop), restricted Fecal Coliform: 14 (Median, continuous 7-day sampling), 100 (Max) C: orchard and vineyard (no contact with the edible part of crop), processed food crops Fecal Coliform: 200 (Median) Minnesota Food crops Total Coliform: 2.2 Processed food crops/non-food crops Fecal Coliform: 200 Montana All types Total Coliforms (Weekly): 2.2 (Last 7 days), 23 (Max) Nevada Processed food crops/non-food crops Fecal Coliform: 200 (30-day geometric mean), 400 (Max) New Jersey Food crops Fecal Coliform: 2.2 (7-day median), 14 (Max) Processed food crops/non-food crops Fecal Coliform: 200 (Monthly geometric mean), 400 (Weekly geometric mean) North Carolina All types E. Coli or Fecal Coliform: 14 (Monthly geometric mean), 25 (Max) North Dakota Processed food crops/non-food crops E. Coli (Weekly): 126 (Max) Ohio Processed food crops/non-food crops Fecal Coliform (3/Week): 1000 E. Coli (3/Week): 126 Oklahoma Processed food crops/non-food crops Fecal Coliform (3/Week): 200 (Monthly geometric mean), 400 (Max) Oregon Food crops Total Coliform: 2.2 (Last 7 days), 23 (Max) Processed food crops/non-food crops Total Coliform: 23 (Last 7 days), 240 (Any 2 consecutive samples) Pennsylvania Food crops Fecal Coliform (2/week): 2.2 (Monthly average), 23 (Max) Processed food crops/non-food crops Fecal Coliform (Weekly): 200 (Monthly average), 800 (Max) Rhode Island Processed food crops/non-food crops Fecal Coliform: 23 Texas Food crops Fecal Coliform or E. Coli (2/Week): 20 (30-day geometric mean), 75 (Max) Enterococci (2/Week): 4 (30-day geometric mean), 9 (Max) Processed food crops/non-food crops Fecal Coliform or E. Coli (Weekly): 200 (30-day geometric mean), 800 (Max) Enterococci (Weekly): 35 (30-day geometric mean), 89 (Max) Utah Food crops E. Coli: 0 (Daily grab samples), 9 (Max) Processed food crops/non-food crops E. Coli: 126 (Weekly median), 500 (Max) Water 2020, 12, 971 15 of 58 Table 4. Cont. Reuse Categories Required Microbial Quality (cfu/100 mL) (Monitoring) Virginia Food crops Fecal Coliform: 14 (Monthly geometric mean), Cat 2 > 49/100 mL, E. Coli: 11 (Monthly geometric mean), Cat > 35/100 mL Enterococci: 11 (Monthly geometric mean), Cat > 24/100 mL Processed food crops/non-food crops Fecal Coliform: 200 (Monthly geometric mean), Cat > 800/100 mL E. Coli: 126 (Monthly geometric mean), Cat > 235/100 mL Enterococci: 35 (Monthly geometric mean), Cat > 104/100 mL Washington Food crops Total Coliform (Daily): 2.2 (Median of last 7 days), 23 (Max) Processed food crops/non-food crops Total Coliform (Daily): 23 (Median of last 7 days), 240 (Max) Cyprus Agglomerations > 2000 p.e. 3 E. Coli (1/15 Days): 5 Intestinal Nematodes: 0 Agglomerations < 2000 p.e. 3 all crops Fecal Coliforms: 5 (80% of samples per month (Min. number of samples = 5)), 15 (Max) Intestinal Nematodes: 0 Agglomerations < 2000 p.e. 3 unlimited access and vegetables eaten cooked (potatoes, beetroots, colocasia) Fecal Coliforms: 50 (80% of samples per month (Min. number of samples = 5)), 100 (Max) Intestinal Nematodes: 0 Agglomerations < 2000 p.e. 3 limited access and crops for human consumption Fecal Coliforms: 1000 (80% of samples per month (Min. number of samples = 5)), 5,000 (Max) Intestinal Nematodes: 0 Agglomerations < 2000 p.e. 3 fodder crops Fecal Coliforms: 1000 (80% of samples per month (Min. number of samples = 5)), 5,000 (Max) Intestinal Nematodes: 0 Italy NS E. Coli: 10 Greece Restricted irrigation, fodder and industrial crops, pastures, trees (except fruit trees), provided that fruits are not in contact with the soil, seed crops, and crops whose products are processed before consumption. Sprinkler irrigation is not allowed E. Coli (Weekly): 200 (Median) Unrestricted irrigation: all crops including all irrigation methods E. Coli (4/Week): 5 (80% of samples), 50 (95% of samples) European Commission A: E. Coli (Weekly): 10 (90% of The Samples) Intestinal Nematodes (2/Month): 1 Egg/L B: E. Coli (Weekly): 100 Intestinal Nematodes (2/Month): 1 Egg/L C: E. Coli (2/Month): 1000 Intestinal Nematodes (2/Month): 1 Egg/L D: E. Coli (2/Month): 10,000 Intestinal Nematodes (2/Month): 1 Egg/L Israel NS Fecal Coliforms: 10 Jordan A: cooked vegetables, parks, playgrounds roadsides in the city E. Coli or Fecal Coliform: 100 Intestinal Nematodes: 1 Egg/L B: fruit trees, landscaped roadsides of highways E. Coli or Fecal Coliform: 1000 C: industrial crops, forest trees NS D: cut flowers E. Coli or Fecal Coliform: 1.1 Kuwait NS Total Coliforms: 400 Fecal Coliforms: 20 Saudi Arabia Restricted Thermo-Tolerant Coliform: 1000 Intestinal Nematodes: 1 Unrestricted Thermo-Tolerant Coliform: 2.2 Intestinal Nematodes: 1 Act (Australia) Pasture and fodder for grazing animals (except pigs) Thermo-Tolerant Coliforms (Weekly): 1000 (Median) Silviculture, turf, and non-food crops Thermo-Tolerant Coliforms (Monthly): 10,000 (Median) Food crops in direct contact with water e.g., sprays Thermo-Tolerant Coliforms (Weekly): 10 (Median) Food crops not in direct contact with water (e.g., flood or furrow) or which will be sold to consumers cooked or processed Thermo-Tolerant Coliforms (Weekly): 1000 (Median) Water 2020, 12, 971 16 of 58 Table 4. Cont. Reuse Categories Required Microbial Quality (cfu/100 mL) (Monitoring) NSW (Australia) Food production, raw human food crops in direct contact with effluent e.g., via sprays, irrigation of salad vegetables Thermo-Tolerant Coliforms (Weekly): 10 (Median) Intestinal Nematodes: 1 Egg/L Food production, raw human food crops not in direct contact with effluent (edible product separated from contact with effluent, e.g., use of trickle irrigation) or crops sold to consumers cooked or processed. Thermo-Tolerant Coliforms (Weekly): 1000 (Median) Food production, pasture and fodder (for grazing animals except pigs and dairy animals, i.e., cattle, sheep, and goats) Thermo-Tolerant Coliforms (Weekly): 1000 (Median) Food production, pasture, and fodder for dairy animals (with withholding period). Thermo-Tolerant Coliforms (Weekly): 1000 (Median) Food production, pasture, and fodder for dairy animals (without withholding period). Drinking water (all stock except pigs). Wash-down water for dairies Thermo-Tolerant Coliforms (Weekly): 100 (Median) Non-food crops, silviculture, turf and cotton, etc. Thermo-Tolerant Coliforms (Weekly): 10,000 (Median) NT (Australia) A+: (high level of human contact) commercial food crops consumed raw or unprocessed (e.g., salad crops) E. Coli (Weekly): 1 B: (medium level human contact) commercial food crops E. Coli (Weekly): 100 C: (low level of human contact) commercial food crops E. Coli (Weekly): 1000 D: (very low level of human contact) non-food crops (trees, turf, woodlots, flowers) E. Coli (Annually): 10,000 QLD (Australia) (Minimally processed food crops) a+: Clostridium Perfringens (Weekly): 1 (95%) E. Coli (Weekly): 1 (95%) F-RNA Bacteriophages (Weekly): 1 (95%) Somatic Coliphages: 1 (95%) (Minimally processed food crops) a: E. Coli (Weekly): 10 (95%) (Minimally processed food crops) b: E. Coli (Weekly): 100 (95%) (Minimally processed food crops) c: E. Coli (Weekly): 1000 (95%) (Minimally processed food crops) d: E. Coli (Weekly): 10,000 (95%) TAS (Australia) A: direct contact of reclaimed water with crops consumed raw Thermo-Tolerant Coliforms (Daily): 10 (Median) B: crops for human consumption Thermo-Tolerant Coliforms (Weekly): 1000 (Median) C: non-human food chain Thermo-Tolerant Coliforms (Weekly): 10,000 (Median) VIC (Australia) A: commercial food crops consumed raw or unprocessed E. Coli: 1 B: dairy cattle grazing E. Coli: 100 C: human food crops/processed, grazing, fodder for livestock E. Coli: 1000 D: non-food crops including instant turf, woodlots, flowers E. Coli: 10,000 WA (Australia) (High level of human contact) commercial food crops consumed raw or unprocessed (e.g., salad crops) E. Coli (Weekly): 1 Coliphages (Weekly): 1 Clostridia (Weekly): 1 (Low level of human contact) non-edible crops E. Coli (Weekly): 1000 D: (extra low level of human contact) non-food crops (subsurface reticulation) E. Coli (6 monthly): 10,000 AGWR (Australia) Commercial food crops consumed raw or unprocessed E. Coli: 1 Commercial food crops E. Coli: 100 Commercial food crops E. Coli: 1000 Non-food crops- trees, turf, woodlots, flowers E. Coli: 10,000 1 Irrigation of Class III effluent on fruit and vegetables not commercially processed, including crops eaten raw, is prohibited. Irrigation of Class III effluent on bare soil is prohibited except for providing adequate moisture for seed germination in the seeding area. The irrigation area shall be planted with healthy vegetation cover. Irrigation on high water table or saturated soils which cause persistent surface runoff and ponding is prohibited. 2 Corrective active threshold. 3 Population equivalents. Water 2020, 12, 971 17 of 58 Table 5. Less restrictive agricultural water reuse regulations and guidelines. Reuse Categories Required Microbial Quality (cfu/100 mL) (Monitoring) FAO A: irrigation of crops likely to be eaten uncooked, sports field, public parks Fecal Coliforms: 1000 (Geometric mean) Fecal Coliforms: 200 (In case of fruit trees, geometric mean) Intestinal Nematodes: 1 Egg/L (Arithmetic mean) B: irrigation of cereal crops, industrial crops, fodder crops, pasture and trees Intestinal Nematodes: 1 Egg/L (Arithmetic mean) C: localized irrigation of crops in category B if exposure of workers and the public does not occur NS WHO Restricted E. Coli: 10,000 (Labor), 100,000 (Highly mechanized) Intestinal Nematodes: 1 Egg/L Unrestricted (Drip irrigated) E. Coli: 1000 (Low-growing), 100,000 (High-growing) Intestinal Nematodes: 1 Egg/L Unrestricted E. Coli: 1000 (Root crops), 10,000 (Leaf crops) Intestinal Nematodes: 1 Egg/L Alberta Restricted Total Coliform (Weekly or daily): 1000 (Geometric mean) Fecal Coliform (Weekly or daily): 200 (Geometric mean) Unrestricted Total Coliform (Weekly or daily): 1000 (Geometric mean) Fecal Coliform (Weekly or daily): 200 (Geometric mean) Nebraska Unrestricted Fecal Coliform: 200 (30-day geometric mean), 400 (No more than 10% samples) South Dakota Food Crops Total Coliform: 200 (Geometric mean) Wyoming Food Crops Fecal Coliform: 200 Processed food crops/non-food crops Fecal Coliform: 1000 Mexico Restricted Fecal Coliforms: 2,000 (Daily averages), 1000 (Monthly average) Unrestricted Fecal Coliforms: 2,000 (Daily average), 1000 (Monthly average) France A: unrestricted irrigation of all crops including these accessed by the public Enterococci (Weekly): ≥ 4 Logs E. Coli (Weekly): 250 B: all crops except those consumed raw or green areas with public access Enterococci (1/15 days): ≥ 3 Logs E. Coli (1/15 days): 10,000 C: other ornamental crops, shrubs, cereals; horticultural crops drip irrigated, forests with controlled access Enterococci (Monthly): ≥ 2 Logs E. Coli (Monthly): 100,000 D: forests with no access Enterococci: ≥ 2 Logs Spain 2.1 E. Coli (Weekly): 100 Intestinal Nematodes: 1 Egg/10L 2.2: quality 2.2 (A) irrigation of crops for human consumption using application methods that do not prevent direct contact of reclaimed water with edible parts of the plants, which are not eaten raw but after an industrial treatment process. (B) irrigation of pasture land for milk- or meat-producing animals. (C) aquaculture. E. Coli (Weekly): 1000 Intestinal Nematodes: 1 Egg/10L 2.3: (A) localized irrigation of tree crops whereby reclaimed water is not allowed to come into contact with fruit for human consumption. (B) irrigation of ornamental flowers, nurseries and greenhouses whereby reclaimed water does not come into contact with the crops. (C) irrigation of industrial non-food crops, nurseries, silo fodder, cereals and oilseeds. E. Coli (Weekly): 10,000 Intestinal Nematodes: 1 Egg/10L Water 2020, 12, 971 18 of 58 Table 5. Cont. Reuse Categories Required Microbial Quality (cfu/100 mL) (Monitoring) Iran A: irrigation of crops likely to be eaten uncooked, sports field, public parks Fecal Coliforms: 1000 (Geometric mean) Intestinal Nematodes: 1 (Arithmetic mean) B: irrigation of cereal crops, industrial crops, fodder crops, pasture and trees Intestinal Nematodes: 1 (Arithmetic mean) C: localized irrigation of crops in category b if exposure of workers and the public does not occur NS Egypt A: plants and trees grown for greenery at touristic villages and hotels and inside residential areas at the new cities Fecal Coliforms: 1000 B: fodder/feed crops, trees producing fruits with epicarp trees used for green belts around cities and afforestation of highways or roads nursery plants roses and cut flowers fiber crops mulberry for the production of silk Fecal Coliforms: 5000 C: industrial oil cropswood trees NS China Fiber crops Fecal Coliforms: 40,000 Intestinal Nematodes: 2 Dry field corn oil crops Fecal Coliforms: 40,000 Intestinal Nematodes: 2 Paddy field grain Fecal Coliforms: 20,000 Intestinal Nematodes: 2 Vegetable Fecal Coliforms: 20,000 Intestinal Nematodes: 2 Palestine A: High quality Fecal Coliforms (1 sample/2 days): 200 B: Good quality Fecal Coliforms (1 sample/2 days): 1000 C: Medium quality Fecal Coliforms (1 sample/2 days): 1000 D: Low quality Fecal Coliform (1 sample/2 days): 1000 Portugal A: vegetables consumed raw Fecal Coliforms: 100 B: public parks, and gardens, sport lawns, forests with public access Fecal Coliforms: 200 C: vegetables to be cooked, forage crops, vineyards, orchards Fecal Coliforms: 1000 D: cereals (except rice), vegetables for industrial process, crops for textile industry, crops for oil extraction, forest and lawns in places of restricted or controlled public access Fecal Coliforms: 10,000 Oman A: vegetables likely to be eaten raw, fruit likely to be eaten raw and within 2 weeks of any irrigation Fecal Coliform: 200 Intestinal Nematodes: 1 Egg/L B: vegetables to be cooked or processed, fruit if no irrigation within 2 weeks of cropping, fodder, cereal, seed crops, pasture no public access Fecal Coliform: 1000 Intestinal Nematodes: 1 Egg/L Fifteen documents were gathered under less restrictive regulations and guidelines (Table 5). The indicator parameters in these regulations and guidelines included Fecal Coliforms (11 documents), Intestinal Nematodes (6 documents), E. coli (3 documents), Total Coliforms (2 documents), and Enterococci (1 document). Similar to restrictive regulations and guidelines, the threshold limits for food crops irrigation and unrestricted public access categories had lower values than processed food crops/non-food crops and restricted public access categories. In addition, threshold levels for same parameters are sometimes very different, when regulations and guidelines are compared with each other (Table 5). BY comparing the required microbial quality thresholds, one can simply notice large discrepancies among the existing regulations and guidelines. Of note is that none of the less restrictive regulations and guidelines used thermo-tolerant Coliforms in their documents. To have a better idea of these Water 2020, 12, 971 19 of 58 thresholds, common pathogen indicators were analyzed using descriptive statistical analysis (Table 6). Fecal Coliform was used more than the other indicators in the regulations and guidelines (Table 6). E. Coli thresholds had the largest range, 100,000, among the indicators. Additionally, the most frequent threshold of Fecal Coliform, E. Coli, Total Coliform, Thermo-Tolerant Coliform, Intestinal Nematodes, Enterococci, Coliphages, Clostridia, and F-RNA Bacteriophages were 200, 1000, 23, 1000, 1, 35, 1, 1, and 1, respectively (Table 6). Table 6. Descriptive statistical analysis of common pathogen indicators included in agricultural water reuse regulations and guidelines. Microbial Indicator (cfu/100 mL) Number of Documents Total Number of Indications Mean Standard Error Median Mode Minimum Maximum Fecal Coliform 36 100 1810.62 627.08 200 200 0 1 40,000 2 E. Coli 24 69 6017.74 2465.17 126 1000 0 3 100,000 4 Total Coliform 9 22 284.18 113.92 23 23 2.2 5 2300 6 Thermo-tolerant Coliform 5 20 2417.61 875.33 1000 1000 2.2 7 10,000 8 Intestinal nematodes (Egg/L) 12 33 0.97 0.09 1 1 0 9 2 10 Enterococci 3 10 11 30.5 12.92 23 35 4 12 89 13 Coliphages 2 2 1 0 1 1 1 14 1 Clostridia 2 2 1 0 1 1 1 15 1 F-RNA Bacteriophages 1 1 1 0 1 1 1 16 1 1 EPA, Arizona, Florida, Indiana, and Massachusetts. 2 China. 3 Utah. 4 WHO and France. 5 California, Minnesota, Montana, Oregon, and Washington. 6 Idaho. 7 Saudi Arabia. 8 ISO, ACT, NSW, and TAS. 9 Cyprus. 10 China. 11 Four of the Enterococci thresholds, issued by France, are in terms of log reduction, excluded from the statistical analysis. 12 Texas. 13 Texas. 14 QLD and WA. 15 QLD and WA. 16 QLD. One of the main human health concerns related to water reuse is intestinal parasitic infections [136]. Verbyla et al. [137] showed that the consumption of lettuce irrigated with river water, contaminated with fecal contamination, resulted in an estimated median health burden that represented 37% of Bolivia’s overall diarrheal disease burden. However, irrigation with filtered riverbank resulted in an estimated health burden that was only 1.1% of this overall diarrheal disease burden. Median concentrations of different contaminants in the river water were as follows: 3.2 × 108 adenovirus copies/L, 6.4 × 107 pepper mild mottle virus copies/L, 1.8 × 107 E. Coli cfu/L, 1.4 × 107 human-specific HF183 Bacteroides copies/L, 3.6 × 106 human rotavirus group A copies/L, 1.1 × 105 Coliphage pfu/L, 530 Giardia cysts/L, and 4.0 Cryptosporidium oocysts/L [137]. The following contaminants were detected in the filtered riverbank (lower median concentrations): 1.1 × 105 adenovirus copies/L, 7.7 × 104 pepper mild mottle virus copies/L, 3.0 × 103 E. coli cfu/L, 4.5 × 101 human-specific HF183 Bacteroides copies/L, 5.9 × 102 Coliphage pfu/L, 2.0 Giardia cysts/L, and 0.04 Cryptosporidium oocysts/L [137]. Based on Table 5 and the study by Verbyla et al. [137] it is apparent that the microbial parameters and their thresholds in the existing regulations and guidelines are not adequate to make sure agricultural water reuse practices are safe for human health. For example, E. Coli threshold set by WHO and France is 100,000 cfu/100mL, however other studies showed that irrigation with a water with E. Coli concentration of 18,000 cfu/mL caused an estimated median health burden that represented 37% of Bolivia’s overall diarrheal disease burden [137]. It has also been reported that the greatest health risk in developing countries is the high concentration of Nematode eggs (>1 Egg/L) when water reuse is practiced using spray irrigation technology, especially in case of vegetables eaten raw by children [4]. Among the regulations and guidelines that were evaluated in this study, only 12 documents have included intestinal nematodes including ISO, Cyprus, E.U., Saudi Arabia, Tunisia, and NSW, from the restrictive group, and FAO, WHO, Spain, Iran, China, and Oman from the less restrictive group (Tables 4 and 5). Chemicals Another human health concern related to recycled water use is potential contamination of crops and groundwater by chemical constituents that may be present in water. These chemicals may include heavy metals, pharmaceuticals, personal care products, and compounds which exert Water 2020, 12, 971 20 of 58 endocrine disruption properties such as hormones or other chemicals including PCBs, Octilphenol, Nonilphenol, etc. [4]. These hazardous chemicals are of great concern for human health especially in heavily polluted industrial wastewater [4]. On the other hand, there is still a huge data gap in terms of characterization and treatment of these chemicals which their concentrations are very low in concerning waters. Only a few agricultural water reuse regulations and guidelines included the chemical constituents in their documents (Table 7). Despite the potential negative consequences of these chemical constituents, only 17 regulations and/or guidelines included some of these chemical parameters (Table 7). Among the studied regulations and guidelines, Italy, China, Oman, and AGWR have included the highest number of chemical constituents in their documents (32, 22, 21, and 20 chemical parameters respectively, Table 7). Table 7. Chemicals and trace elements thresholds in agricultural water reuse regulations and guidelines (numbers in parentheses show the threshold level of chemical constituents and trace elements). Chemical/Trace Element Number of Documents that Included this Parameter Range (mg/L) Regulation/Guideline (Thresholds as mg/L) Cadmium (Cd) 17 0.0001–0.2 EPA (0.01), FAO (0.01), WHO (0.01), British Columbia (0.05), Atlantic Canada (0.005), Cyprus (0.2), Italy (0.005), Greece (0.01), Israel (0.01), Jordan (0.01), Kuwait (0.01), Oman (0.01), Saudi Arabia (0.01), Tunisia (0.1), China (0.01), ACT (0.01), AGWR (0.0001–0.005) Chromium (Cr) 17 0.001–0.15 EPA (0.1), FAO (0.1), WHO (0.1), British Columbia (hexavalent: 0.008), Atlantic Canada (hexavalent:0.008, trivalent:0.005), Cyprus (0.1), Italy (0.1), Greece (0.1), Israel (0.1), Jordan (0.1), Kuwait (0.15), Oman (0.05), Saudi Arabia (0.1), Tunisia (0.1), China (0.1), ACT (0.1), AGWR (0.001–0.021) Nickel (Ni) 17 0.002–0.2 EPA (0.2), FAO (0.2), WHO (0.2), British Columbia (0.2), Atlantic Canada (0.2), Cyprus (0.2), Italy (0.2), Greece (0.02), Israel (0.2), Jordan (0.2), Kuwait (0.2), Oman (0.1), Saudi Arabia (0.2), Tunisia (0.2), China (0.1), ACT (0.2), AGWR (0.002–0.02) Iron (Fe) 16 0.3–4.7 EPA (5), FAO (5), WHO (5), British Columbia (5), Atlantic Canada (5), Italy (2), Greece (3), Israel (2), Jordan (5), Kuwait (5), Oman (food crops:1, non-food crops:5), Saudi Arabia (2), Tunisia (0.5), China (1.5), ACT (1), AGWR (0.03–4.725) Arsenic (As) 16 0.004–0.1 EPA (0.1), FAO (0.1), WHO (0.1), British Columbia (0.1), Atlantic Canada (0.1), Italy (0.02), Greece (0.1), Israel (0.1), Jordan (0.1), Kuwait (0.1), Oman (0.1), Saudi Arabia (0.1), Tunisia (0.1), China (0.05), ACT (0.1), AGWR (0.004) Copper (Cu) 16 0.002–1 EPA (0.2), FAO (0.2), WHO (0.2), Atlantic Canada (0.2–1), Cyprus (0.1), Italy (1), Greece (0.2), Israel (0.2), Jordan (0.2), Kuwait (0.2), Oman (food crops:0.05, non-food crops:0.1), Saudi Arabia (0.4), Tunisia (0.5), China (1), ACT (0.2), AGWR (0.002–0.091) Lead (Pb) 16 0.001–5 EPA (5), FAO (5), British Columbia (0.2), Atlantic Canada (0.2), Cyprus (0.15), Italy (0.1), Greece (0.1), Israel (0.1), Jordan (0.2), Kuwait (0.5), Oman (food crops:0.1, non-food crops:0.2), Saudi Arabia (0.1), Tunisia (1), China (0.2), ACT (0.2), AGWR (0.001–0.02) Cobalt (Co) 15 0.004–1 EPA (0.05), FAO (0.05), WHO (0.05), British Columbia (0.05), Atlantic Canada (0.05), Italy (0.05), Greece (0.05), Israel (0.05), Kuwait (0.2), Oman (0.05), Saudi Arabia (0.05), Tunisia (0.1), China (1), ACT (0.05), AGWR (0.0004–0.0013) Water 2020, 12, 971 21 of 58 Table 7. Cont. Chemical/Trace Element Number of Documents that Included this Parameter Range (mg/L) Regulation/Guideline (Thresholds as mg/L) Zinc (Zn) 15 0.5–5 EPA (2), FAO (2), WHO (2), Atlantic Canada (1–5), Cyprus (1), Italy (0.5), Greece (2), Israel (2), Kuwait (2), Oman (5), Saudi Arabia (2), Tunisia (5), China (2), ACT (2), AGWR (0.049–0.11) Aluminum (Al) 14 0.011–5 EPA (5), FAO (5), WHO (5), British Columbia (5), Atlantic Canada (5), Italy (1), Greece (5), Israel (5), Jordan (5), Kuwait (5), Oman (5), Saudi Arabia (5), ACT (5), AGWR (0.011–0.665) Manganese (Mn) 14 0.019–0.5 EPA (0.2), FAO (0.2), WHO (0.2), British Columbia (0.2), Atlantic Canada (0.2), Italy (0.2), Greece (0.2), Israel (0.2), Kuwait (0.2), Oman (food crops:0.1, non-food crops:0.5), Saudi Arabia (0.2), China (0.3), ACT (0.2), AGWR (0.019–0.069) Beryllium (Be) 13 0.002–2 EPA (0.1), FAO (0.1), WHO (0.1), British Columbia (0.1), Atlantic Canada (0.1), Italy (10), Greece (0.1), Israel (0.1), Kuwait (2), Oman (food crops:0.1, non-food crops:0.3), Saudi Arabia (0.1), China (0.002), ACT (0.1) Selenium (Se) 12 0.02–0.05 EPA (0.02), FAO (0.02), WHO (0.02), Atlantic Canada (0.02–0.05), Italy (0.01), Greece (0.02), Israel (0.02), Oman (0.02), Saudi Arabia (0.02), Tunisia (0.05), China (0.02), ACT (0.02) Lithium (Li) 11 0.07–2.5 EPA (2.5), FAO (2.5), WHO (2.5), British Columbia (2.5), Atlantic Canada (2.5), Greece (2.5), Israel (2.5), Jordan (2, citrus:0.075), Oman (0.07), Saudi Arabia (0.07), ACT (2.5) Molybdenum (Mo) 11 0.001–0.05 EPA (0.01), FAO (0.01), WHO (0.01), Atlantic Canada (0.01–0.05), Greece (0.1), Israel (0.01), Oman (food crops: 0.01, non-food crops: 0.05), Saudi Arabia (0.01), China (0.5), ACT (0.01), AGWR (0.001–0.021) Vanadium (V) 11 0.1 EPA (0.1), FAO (0.1), WHO (0.1), British Columbia (0.1), Atlantic Canada (0.1), Italy (0.1), Greece (0.1), Israel (0.1), Oman (0.1), Saudi Arabia (0.1), China (0.1) Mercury (Hg) 11 0.0001–0.2 Cyprus (0.005), Italy (0.001), Greece (0.002), Israel (0.002), Jordan (0.02), Kuwait (0.002), Oman (0.001), Saudi Arabia (0.001), Tunisia (0.001), China (0.001), AGWR (0.0001–0.002) Total phenol 5 0.0005–1 Italy (0.1), Kuwait (1), Oman (food crops:0.001, non-food crops:0.002), Saudi Arabia (0.002), AGWR (0.0005–0.007) Copernicum (Cn) 3 0.05–0.1 Italy (0.05), Oman (food crops:0.05, non-food crops:0.1), Saudi Arabia (0.05) Silver (Ag) 3 0.0001–0.5 Oman (0.01), Saudi Arabia (0.5), AGWR (0.0001–0.005) Magnesium (Mg) 3 0.5–150 Oman (150), Tunisia (0.5), AGWR (6–40) Uranium (U) 2 0.01 British Columbia (0.01), Atlantic Canada (0.01) Benzene 2 0.01–2.5 Italy (0.01), China (2.5) Cyanide (Cn) 2 0.001–0.5 China (0.5), AGWR (0.001) Calcium (Ca) 1 10–74.00 AGWR (10–74) Tin (Sn) 1 3 Italy (3) Titanium (Ti) 1 0.001 Italy (0.001) Pentachlorophenol 1 0.003 Italy (0.003) Total aldehydes 1 0.5 Italy (0.5) Tetrachloroethylene 1 0.01 Italy (0.01) Total Chlorinated solvents 1 0.04 Italy (0.04) Total trihalomethanes 1 0.03 Italy (0.03) Total aromatic solvents 1 0.001 Italy (0.001) Benzo(a)pyrene 1 0.00001 Italy (10−5) Total organic Nitrogen solvents 1 0.01 Italy (0.01) Total surfactants 1 0.2–0.5 Italy (0.5), AGWR (anionic:0.2) Chlorinated biocides 1 0.0001 Italy (0.0001) Water 2020, 12, 971 22 of 58 Table 7. Cont. Chemical/Trace Element Number of Documents that Included this Parameter Range (mg/L) Regulation/Guideline (Thresholds as mg/L) Phosphorated pesticides 1 0.00001 Italy (0.00001) Other pesticides 1 0.05 Italy (0.05) Volatile Phenol 1 1 China (1) Linear alkynate sulfunic 1 5 China (5) Trichloracetic aldehyde 1 0.5 China (0.5) Acrolein 1 0.5 China (0.5) Methanol 1 1 China (1) Barium (Ba) 1 0.001–0.0375 AGWR (0.001–0.0375) 3.2.2. Agronomic Parameters Agronomic parameters are of prominent importance in safe agricultural water reuse practices. Crops quality and yield, soil productivity, and ecological health have to be considered in safe agricultural water reuse practices. pH As the indicator of water acidity and alkalinity, pH is one of the water quality parameters that can be easily measured, and can be an indicator of the presence of toxic ions [4,67]. Although the normal pH range for safe irrigation is 6.5–8.4 [67], different pH ranges are used in the agricultural water reuse regulations and guidelines (Table 8 and Figure 2). Recycled water outside the normal pH range might result in nutritional imbalance, which may alter the crops growth and health, and facilitate the corrosion in pipelines, sprinklers, and control valves [4,138–141]. Lower pH makes heavy metals move easier in the soil, contaminating crops, and water bodies [142]. Out of 70 agricultural water reuse regulations and guidelines studied in this research, 34 documents included pH as one of their requirements (Table 8). The most common ranges are 6–9 and 6.5–8.5 (Figure 2). Table 8. pH ranges in the agricultural water reuse regulations and guidelines. 6.0–9.0 6.5–8.5 6.5–8.0 6.0–8.5 5.0–10.0 6.0–9.5 5.5–8.5 5.5–8.0 6.2–9.8 EPA Maryland FAO Alabama Mexico Italy China TAS (AU) AGWR (AU) British Columbia Massachusetts Alberta Saudi Arabia Georgia Cyprus Indiana Iran Iowa Israel Nevada Kuwait Ohio Tunisia Rhode Island Act (AU) Utah NSW (AU) Virginia NT (AU) Oman WA (AU) VIC (AU) Salinity It has been reported that salinity is one of the most important recycled water quality parameters for agricultural water reuse practices. This is due to the fact that high concentration of dissolved salts increases the soil water pressure, requiring more energy from plants to take up water from soil and also resulting in specific ion toxicity [4,142–145]. The salinity of irrigation water often determines the salinity in the soil. Total dissolved solids (TDS (mg/L)) or electric conductivity (EC (dS/m)) are often used as indicators of salinity. While each of these parameters are important individually, there is an approximate correlation between TDS and EC, as shown by Equation (1) [4]. Water 2020, 12, 971 23 of 58 Water 2020, 12, 971 31 of 74 agricultural water reuse regulations and guidelines studied in this research, 34 documents included pH as one of their requirements (Table 8). The most common ranges are 6–9 and 6.5–8.5 (Figure 2). Table 8. pH ranges in the agricultural water reuse regulations and guidelines. 6.0–9.0 6.5–8.5 6.5–8.0 6.0–8.5 5.0–10.0 6.0–9.5 5.5–8.5 5.5–8.0 6.2–9.8 EPA Maryland FAO Alabama Mexico Italy China TAS (AU) AGWR (AU) British Columbia Massachusetts Alberta Saudi Arabia Georgia Cyprus Indiana Iran Iowa Israel Nevada Kuwait Ohio Tunisia Rhode Island Act (AU) Utah NSW (AU) Virginia NT (AU) Oman WA (AU) VIC (AU) Figure 2. Required pH ranges in agricultural water reuse regulations and guidelines. 3.2.2.2. Salinity It has been reported that salinity is one of the most important recycled water quality parameters for agricultural water reuse practices. This is due to the fact that high concentration of dissolved salts increases the soil water pressure, requiring more energy from plants to take up water from soil and also resulting in specific ion toxicity [4,142–145]. The salinity of irrigation water often determines the salinity in the soil. Total dissolved solids (TDS (mg/L)) or electric conductivity (EC (dS/m)) are often 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 EPA, British Columbia, Georgia, Indiana,… Maryland, Massachusetts, Cyprus, Iran,… FAO, Alberta Alabama, Saudi Arabia Mexico Italy China TAS (AU) AGWR (AU) pH Re gu la tio ns a nd g ui de lin es Figure 2. Required pH ranges in agricultural water reuse regulations and guidelines. Three categories for EC and TDS thresholds are included in FAO’s guideline based on the water use restriction as none, slight to moderate, and severe negative impact (Table 9 and Figure 3). Only 13 and 10 regulations and guidelines (out of 70) have included EC and TDS in their agricultural water reuse requirements, respectively (Table 9). FAO, Saskatchewan, Iran, Jordan, Oman, ACT, and AGWR regulations and guidelines have included both of EC and TDS thresholds in their documents. In addition, Kuwait, Saudi Arabia, and China have only included TDS in their regulations and guidelines. Of note is that none of the U.S. states included EC and TDS thresholds in their agricultural water reuse regulations and guidelines. TDS (mg L ) = EC (dS m ) × 640 (1) Table 9. Salinity (electric conductivity (EC) and total dissolved solids (TDS)) thresholds in agricultural water reuse regulations and guidelines. Regulation/Guideline. EC (dS/m) TDS (mg/L) FAO and Saskatchewan None: <0.7 None: <450 Slight to moderate: 0.7–3.0 Slit to moderate: 450–2000 Severe: >3 Severe: >2000 Alberta and Atlantic Canada Unrestricted: <1.0 NS 1 Restricted: 1.0–2.5 NS Unacceptable: >2.5 NS Oman Restricted (public access): 2.7 Restricted (public access): 2000 Unrestricted (public access): 2.0 Unrestricted (public access): 1500 China NS Saline-alkali land: 2000 NS Non-saline-alkali land: 1000 Cyprus 2.2 NS Italy 3 NS Iran 0.7 450 Israel 1.4 NS Jordan 2.34 1500 Tunisia 7 NS ACT 0.8 500 AGWR 0.2–2.9 145–1,224 Kuwait NS 1500 Saudi Arabia NS Restricted irrigation: 2000 1 Not specified. Water 2020, 12, 971 24 of 58 Water 2020, 12, 971 33 of 74 Figure 3. Salinity (electric conductivity (EC) and total dissolved solids (TDS)) thresholds in agricultural water reuse regulations and guidelines. 3.2.2.3. Sodium Adsorption Ratio (SAR) Sodium is one of the important ions in irrigation water which has to be regulated for agricultural practices. Its presence in the exchangeable form in soil causes harmful effects on the physical and 0 1 2 3 4 5 6 7 8 Ir an A C T A lb er ta -u nr es tr ite d A tla nt ic C an ad a Is ra el O m an -u nr es tr ic te d C yp ru s Jo rd an A lb er ta -r es tr ic te d O m an -r es tr ic te d A G W R Ita ly Tu ni si a EC (d S/ m ) Regulations and guidelines EC (dS/m) FAO-none FAO-slight to severe 0 500 1000 1500 2000 2500 Ir an A C T C hi na -n on sa lin e- al ka li la nd A G W R O m an -u nr es tr ic te d Jo rd an K uw ai t O m an -r es tr ic te d C hi na -s al in e- al ka li la nd Sa ud i A ra bi a- re st ri ct ed ir ri ga tio n TD S (m g/ l) Regulations and guidelines TDS (mg/l) FAO-non FAO-slight to moderate Figure 3. Salinity (electric conductivity (EC) and total dissolved solids (TDS)) thresholds in agricultural water reuse regulations and guidelines. Sodium Adsorption Ratio (SAR) Sodium is one of the important ions in irrigation water which has to be regulated for agricultural practices. Its presence in the exchangeable form in soil causes harmful effects on the physical and Water 2020, 12, 971 25 of 58 chemical properties of the soil. Excessive amounts of Sodium results in particle dispersion and reduction of water and air infiltration into the soil [4,143–146]. The most common Sodium indicator which has been used in literature was the sodium adsorption ratio (SAR) index, calculated by Equation (2). SAR = Na+√ 0.5 ( Ca2+ + Mg2+ ) (2) In this equation, SAR is the amount of Sodium adsorption ration, Na, Ca, and Mg are the concentrations of Sodium, Calcium, and Magnesium in me/L, respectively. There are no states in the U.S. with SAR threshold in their regulations or guidelines. In Canada, provinces of Alberta, Atlantic Canada, and Saskatchewan have included SAR thresholds for restricted and unrestricted agricultural water reuse practices (Table 10 and Figure 4). In Iran’s guideline, the SAR threshold is set as 3 when the EC < 0.7 dS/m, which is its required EC threshold. Additionally, Iran includes other SAR thresholds when EC > 0.7 dS/m. Moreover, the highest SAR thresholds we issued by Italy and Oman. In total, 7 out of 70 regulations and guidelines investigated in this study have included SAR in their requirements. Table 10. The SAR thresholds in the agricultural water reuse regulations and guidelines. Organizations/Countries/States SAR Organizations/Countries/States SAR Alberta 4–9, restricted use when EC > 1.0 dS/m Italy 10 <4, unrestricted use Iran <3, EC < 0.7 3–6, EC > 1.2 6–12, EC > 1.9 12–20, EC > 2.9 20–40, EC > 5 Atlantic Canada 4–9, restricted use Israel 5 <4, unrestricted use Oman 10 Saskatchewan <3, no restriction 3–9, slight to moderate restriction >9, severe restriction ACT 6 AGWR 3–12.2Water 2020, 12, 971 35 of 74 Figure 4. Required sodium adsorption ratio (SAR) ranges in agricultural water reuse regulations and guidelines. 3.2.2.4. Ions: Chloride, Sodium, and Boron Resulting in crops growth and yield reduction, morphology changes, and death, the presence of toxic ions can be detrimental to crops if their concentrations are more than the desired levels [147]. Despite this potential negative impact, these ions are beneficial at relatively low concentrations. Among these ions, Sodium (Na), Chloride (Cl−), and Boron (B) are of great significance. The crop is affected by these ions which can be either direct by interference with the metabolic processes or indirect by influencing other nutrients [4]. Roots and leaves are the main parts of crops by which Sodium and Chloride can be absorbed. Usually, when the ion is absorbed by leaves, it increases the rate of absorption which results in toxic ion accumulation and can be the primary toxicity source [148]. Due to extensive use of perborate as a bleaching agent, residential wastewater often contains considerable amounts of Boron. While 1 mg/L of Boron is essential for crop growth, if its concentration reaches 2 mg/L or more, most of the crops will suffer from Boron toxicity [4]. The highest and lowest thresholds for Boron were issued by Atlantic Canada and Israel as 6.5 and 0.4 mg/L, respectively. Even though Boron concentrations of more than 2 mg/L can result in toxic crops, Tunisia and Atlantic Canada have set their Boron thresholds as 3 and 6.5 mg/L, respectively. Excess of Chloride can result in acute physiological dysfunctions. A salty taste is another result of more than desired amounts of Chloride, which affects crops market negatively [149]. The lowest chloride thresholds have been issued by Iran and Saudi Arabia, 100 mg/L, and the highest one has been issued by Tunisia, 2000 mg/L (Table 11). Additionally, among the U.S. states, only Delaware has included Chloride in its regulation. Extra Sodium increases osmotic stress and can kill crop cells [150]. For Sodium, just 6 documents out of 70 investigated documents have included Sodium among their water quality parameters. The lowest thresholds were issued by FAO, and Iran as 69 and 70 mg/L, respectively, and highest were issued by AGWR as 312 mg/L, and by Oman for non-food crops as 300 mg/L. Of note is none of the U.S. states have included Sodium thresholds in their regulations and guidelines. 0 5 10 15 20 25 30 35 40 Saskatchewan (slight to moderate restriction) saskatchewan (no restriction) Italy Iran (EC < 0.7 dS/m) Iran (EC > 1.9 dS/m) Iran (EC > 5 dS/m) Atlantic Canada (restricted) Alberta (restricted) AGWR SAR Re gu la tio ns a nd g ui de lin es Figure 4. Required sodium adsorption ratio (SAR) ranges in agricultural water reuse regulations and guidelines. Ions: Chloride, Sodium, and Boron Resulting in crops growth and yield reduction, morphology changes, and death, the presence of toxic ions can be detrimental to crops if their concentrations are more than the desired levels [147]. Despite this potential negative impact, these ions are beneficial at relatively low concentrations. Water 2020, 12, 971 26 of 58 Among these ions, Sodium (Na), Chloride (Cl−), and Boron (B) are of great significance. The crop is affected by these ions which can be either direct by interference with the metabolic processes or indirect by influencing other nutrients [4]. Roots and leaves are the main parts of crops by which Sodium and Chloride can be absorbed. Usually, when the ion is absorbed by leaves, it increases the rate of absorption which results in toxic ion accumulation and can be the primary toxicity source [148]. Due to extensive use of perborate as a bleaching agent, residential wastewater often contains considerable amounts of Boron. While 1 mg/L of Boron is essential for crop growth, if its concentration reaches 2 mg/L or more, most of the crops will suffer from Boron toxicity [4]. The highest and lowest thresholds for Boron were issued by Atlantic Canada and Israel as 6.5 and 0.4 mg/L, respectively. Even though Boron concentrations of more than 2 mg/L can result in toxic crops, Tunisia and Atlantic Canada have set their Boron thresholds as 3 and 6.5 mg/L, respectively. Excess of Chloride can result in acute physiological dysfunctions. A salty taste is another result of more than desired amounts of Chloride, which affects crops market negatively [149]. The lowest chloride thresholds have been issued by Iran and Saudi Arabia, 100 mg/L, and the highest one has been issued by Tunisia, 2000 mg/L (Table 11). Additionally, among the U.S. states, only Delaware has included Chloride in its regulation. Extra Sodium increases osmotic stress and can kill crop cells [150]. For Sodium, just 6 documents out of 70 investigated documents have included Sodium among their water quality parameters. The lowest thresholds were issued by FAO, and Iran as 69 and 70 mg/L, respectively, and highest were issued by AGWR as 312 mg/L, and by Oman for non-food crops as 300 mg/L. Of note is none of the U.S. states have included Sodium thresholds in their regulations and guidelines. Table 11. Toxic ions (Chloride, Sodium, and Boron) thresholds in agricultural water reuse regulations and guidelines. Organization/Country/State Chloride (Cl−), mg/L Sodium (Na+), mg/L Boron (B), mg/L EPA 0.75 FAO surface irrigation: <142 (unrestricted use) 142 < Cl < 355 (restricted use) sprinkler irrigation: <3 m3/L (unrestricted use) 3 < m3/L (restricted use) surface irrigation: <3 SAR (unrestricted use) 3 < Na < 9 SAR (restricted use) sprinkler irrigation: <69 (unrestricted use) 69 < (restricted use) <0.7 (unrestricted) 0.7 < B < 3 mg/L (restricted) Atlantic Canada 0.5–6.5 Delaware 250 Cyprus 300 1 Italy 250 1 Greece 2 Iran 100 70 0.7 Israel 250 150 0.4 Kuwait 2 Oman 650 (food crops) 200 (food crops) 0.5 (food crops) 650 (non-food crops) 300 (non-food crops) 1 (non-food crops) Saudi Arabia 100 0.5 Tunisia 2000 3 China 350 1 AGWR 340 62 < Na < 312 0.009–0.480 Trace Elements As mentioned before, trace elements (such as lead, cadmium, mercury, etc.) exist in low concentrations in wastewater but are hardly included in routine irrigation water analysis. Industrial and urban wastewater may contain considerable amount of trace elements and may result in accumulating of these compounds in soil and plants, reducing crop growth and polluting groundwater [4,151,152]. Trace elements accumulation in soils depends on their chemical form, consisting exchangeable, sorbed, organic-bound, carbonate, and sulfide. The uptake of these elements by plants mostly depend on rhizosphere environment, plant root system features, and soil characteristics. The soil pH has the most Water 2020, 12, 971 27 of 58 effect on the plant’s uptake. Toxicities caused by trace elements have been mostly reported in acidic soils [4]. The threshold levels indicated by agricultural water reuse regulations and guidelines were issued using limited research by different agencies which have made these thresholds relatively restrictive (Table 7). Therefore, if the suggested threshold is not met, it does not mean that it will always result in phytotoxicity. Trace elements thresholds were issued to help practice a safe and sustainable agriculture, but as these elements are used extensively nowadays, these thresholds need to be updated regularly. Additionally, more regulations and guidelines should include trace elements. Currently, at the best case, only 17 out of 70 investigated documents include trace elements. Of note is the discrepancies among the trace elements thresholds ranges in the documents. Cadmium (0.0001–0.2 mg/L), Arsenic (0.004–0.1 mg/L), Lead (0.001–0.5 mg/L), Copper (0.002–1 mg/L), and Mercury (0.0001–0.2) are some of the most frequent trace elements in current regulations and guidelines with 17, 16, 16, 16, and 11 number of mentions, respectively (Table 7). Bicarbonate and Carbonate Bicarbonate and Carbonate are among important ions for plant health. High concentrations of these ions in the recycled water for irrigation can cause different consequences [153–156]. Irrigation of crops by recycled water which has high concentrations of Bicarbonate and Carbonate, using overhead sprinklers, leaves white lime deposits on the crops leaves during hot irrigation days. These white deposits not only reduce the crops sells due to undesirab