Wastewater reuse

Wastewater reuse is often used synonymously with the terms wastewater recycling and wastewater reclamation and can be defined as the use of reclaimed water for beneficial purposes, such as agricultural, landscape and field irrigation, industrial processes, toilet flushing, fire protection, and replenishing of surface water and groundwater (the latter referred to as groundwater recharge).^[1] Water/wastewater reuse is integral to sustainable water management because it allows water to remain in the environment and be preserved for future uses while meeting the water requirements of the present.^[2] Water and wastewater reuse is a long-established practice used for irrigation especially in arid countries. The World Health Organization ^[3] has recognized the principal driving forces for global wastewater reuse as the (i) increasing water scarcity and stress, (ii) increasing populations and related food security issues, (iii) increasing environmental pollution from improper wastewater disposal, and (iv) the increasing recognition of the resource value of wastewater, excreta and greywater.^[4] Water is a limiting resource, and the pressure exerted on surface and groundwater resources should be reduced or at best maintained, rather than increased, as the human population and industrial development increase. Water recycling and reuse is thus of increasing importance, not only in arid regions but also in cities and contaminated environments.^[5] Already, groundwater aquifers used by over half of the world population are being over-drafted.^[6] As a result, it is no longer advisable to use water once and dispose of it; it is important to identify ways to reuse water. Reuse will continue to increase as the world’s population becomes increasingly urbanized and concentrated near coastlines, where local freshwater supplies are limited or are available only with large capital expenditure.^[7][8] Large quantities of freshwater can be saved by wastewater reuse and recycling, lowering costs, reducing environmental pollution and improving carbon footprint; since reuse is a sustainable and cost-effective alternative water supply.^[9]

Historical information on wastewater reuse (known facts on international level)

Wastewater reuse is an ancient practice, which has been applied since the dawn of human history, and is being contemporarily practiced in many parts of the world, which suffer from drought conditions.^[10][11] Reuse of untreated municipal wastewater has been practiced for many centuries with the objective of diverting human waste outside of urban settlements. Likewise, land application of domestic wastewater is an old and common practice, which has gone through different stages of development. This has led to better understanding of process and treatment technology and the eventual development of water quality standards.^[12] Domestic wastewater was used for irrigation by prehistoric civilizations (e.g. Mesopotamian, Indus valley, and Minoan) since the Bronze Age (ca. 3200-1100 BC).^[13] Thereafter, wastewater was used for disposal, irrigation, and fertilization purposes by Hellenic civilizations and later by Romans in areas surrounding cities (e.g. Athens and Rome).^[14] Moreover, in China, use of human excreta for fertilizing agricultural crops has been practiced since ancient time.^[15][16] In the more recent history, the “sewage farms” (i.e. wastewater application to the land for disposal and agricultural use) were operated in Bunzlau (Silesia) in 1531, in Edinburgh (Scotland) in 1650, in Paris (France) in 1868, in Berlin (Germany) in 1876 and in different parts of the USA since 1871, where wastewater was used for beneficial crop production.^[17][18] In the following centuries (16th and 18th centuries) in many rapidly growing countries/cities of Europe (e.g. Germany, France) and the United States, “sewage farms” were increasingly seen as a solution for the disposal of large volumes of the wastewater, some of which are still in operation today.^[19] Irrigation with sewage and other wastewater effluents has a long history also in China and India;^[20] while also a large “sewage farm” was established in Melbourne, Australia in 1897.^[21] The use of the land treatment systems continued into the nineteenth/twentieth century in central Europe, USA, and other locations all over the world, but not without causing serious public health concerns and negative environmental impacts. During 1840s and 1850s, this practice resulted in disastrous spread of waterborne diseases like cholera and typhoid.^[22] However, when the water supply links with these diseases became clear, engineering solutions were implemented that include the development of alternative water sources using reservoirs and aqueduct systems, relocation of water intakes, and water and wastewater treatment systems.^[23]

Wastewater reuse types

The main reclaimed water applications in the world are shown below:^[24][25][26]

De facto wastewater reuse

Also, commonly referred as “unplanned water reuse”, this refer to a situation where reuse of treated wastewater is, in fact, practiced but is not officially recognized.^[27] Common examples occur where a wastewater treatment plant from one city discharges effluents to a river which is subsequently used as a drinking water supply for another city downstream.

Example of de facto wastewater reuse

The Trinity River in Texas is a representative example of an effluent-dominated surface water system where de facto potable water reuse occurs. The section of the river south of Dallas/Fort Worth consists almost entirely of wastewater effluent under base flow conditions. In response to concerns about nutrients, the wastewater treatment plants in Dallas/Fort Worth that collectively discharge about 2 million m³ per day of effluent employ nutrient removal processes. Little dilution of the effluent-dominated waters occurs as the water travels from Dallas/Fort Worth to Lake Livingston, which is one of the main drinking water reservoirs for Houston. Once the water reaches Lake Livingston, it is subjected to conventional drinking water treatment prior to delivery to consumers in Houston.^[28]

Urban reuse

Unrestricted: The use of reclaimed water for non-potable applications in municipal settings, where public access is not restricted. Restricted: The use of reclaimed water for non-potable applications in municipal settings, where public access is controlled or restricted by physical or institutional barriers, such as fencing, advisory signage, or temporal access restriction.^[29]

Agricultural reuse

Food crops to be eaten raw: crops which are intended for human consumption to be eaten raw or unprocessed. Processed food crops: crops which are intended for human consumption not to be eaten raw but after treatment process (i.e. cooked, industrially processed). Non-food crops: crops which are not intended for human consumption (e.g. pastures, forage, fiber, ornamental, seed, forest and turf crops).^[30]

Environmental reuse

The use of reclaimed water to create, enhance, sustain, or augment water bodies including wetlands, aquatic habitats, or stream flow.^[31]

Industrial reuse

The use of reclaimed water to recharge aquifers that are not used as a potable water source.^[32]

Potable reuse

Two categories of potable wastewater reuse have been proposed, ‘direct’ and ‘indirect’ potable reuse applications, depending on whether the reclaimed wastewater is used directly or mixed with other sources.^[33][34][35]

Direct Potable Reuse (DPR)

In other words, DPR is the introduction of reclaimed water derived from urban wastewater after extensive treatment and monitoring to assure that strict water quality requirements are met at all times, directly into a municipal water supply system.^{[36][37][38][39]} While this practice is rarely used, it has gained some traction in recent years ^[40](USEPA, 2004).

Examples of Direct Potable Reuse (DPR)

A representative example of DPR is the case of Windhoek (Namibia, New Goreangab Water Reclamation Plant (NGWRP)) (1969), where treated wastewater has been blended with drinking water for more than 40 years. It is based on the multiple treatment barriers concept (i.e. pre-ozonation, enhanced coagulation/dissolved air flotation/rapid sand filtration, and subsequent ozone, biological activated carbon/granular activated carbon, ultrafiltration (UF), chlorination) to reduce associated risks and improve the water quality.^[41][42] Since the year 1968 the capital of Namibia, Windhoek, has used reclaimed wastewater as one of their drinking water sources,^[43] which nowadays represent about 14% of the city’s drinking water production.^[44] In 2001, the New Goreangab Reclamation Plant (NGWRP) was built by the City of Windhoek and it started to deliver drinking water in 2002 (about 21,000 m³ of water per day).^[45][46]

In July 2014, the city of Wichita Falls, Texas (USA), became one of the first in the United States to use treated wastewater directly in its drinking water supply (production of 45,000-60,000 m³ per day). Treated wastewater is disinfected and pumped to the Cypress Water Treatment Plant where it goes through clarification, microfiltration (MF), reverse osmosis (RO), and ultraviolet light disinfection before being released into a holding lagoon where it is blended with lake water (50:50). The blended water goes through a seven-step conventional surface water treatment.^[47]

In Beaufort West, South Africa’s a direct wastewater reclamation plant (WRP) for the production of drinking water was constructed in the end of 2010, as a result of acute water scarcity (production of 2,300 m³ per day).^[48][49] The process configuration based on multi-barrier concept and includes the following treatment processes: sand filtration, UF, two-stage RO, and permeate disinfected by ultraviolet light (UV) (Burgess et al., 2015). Another example of DPR is the reuse plant constructed and operated in the town Hermanus (Overberg) in South Africa, where now 2,500 m³ per day of effluent reused, with a future plan to increase the capacity to 5,000 m³ per day. The treatment processes applied include UF pre-treatment, RO desalination, as well as advanced oxidation and carbon filtration. The product from the reuse plant is fed directly into the drinking water reticulation system.^[50]

While there are currently no full-scale DPR schemes operating in Australia, the Australian Antarctic Division is investigating the option of installing a potable reuse scheme at its Davis research base in Antarctica. To enhance the quality of the marine discharge from the Davis WWTP, a number of different, proven technologies have been selected to be used in the future, such as ozonation, UV disinfection, chlorine, as well as UF, activated carbon filtration and RO.^[51]

Indirect Potable Reuse (IPR)

IPR occurs through the augmentation of drinking water supplies with urban wastewater treated to a level suitable for IPR followed by an environmental buffer (e.g. rivers, dams, aquifers, etc.) that precedes drinking water treatment. In this case, urban wastewater passes through a series of treatment steps that encompasses membrane filtration and separation processes (e.g. MF, UF and RO), followed by an advanced chemical oxidation process (e.g. UV, UV+H₂O₂, ozone).^{[52][53][54][55]}

Examples of Indirect Potable Reuse (IPR)

IPR or even unplanned potable use of reclaimed wastewater exists in many countries, where the latter is discharged into groundwater to hold back saline intrusion in coastal aquifers. IPR has generally included some type of environmental buffer, but conditions in certain areas have created an urgent need for more direct alternatives.^[56] A well-known and frequently quoted success story on water reuse (IPR) is Singapore (Singapore). At the end of 2002, the programme - successfully branded as NEWater - had garnered a 98 per cent acceptance rate, with 82% of respondents indicating that they would drink the reused water directly, another 16% only when mixed with reservoir water.^[57] MF/RO and UV are used for the wastewater treatment by the PUB-Singapore's National Water Agency.^[58] The produced NEWater after stabilization (addition of alkaline chemicals) is in compliance with the WHO requirements and can be piped off to its wide range of applications (e.g. reuse in industry, discharge to a drinking water reservoir).^[59] NEWater now makes up around 30% of Singapore’s total use, by 2060 Singapore’s National Water Agency plans to triple the current NEWater capacity as to meet 50% of Singapore’s future water demand.^[60][61] Today, the water is mostly used by industry, including high-tech industries requiring ultra-pure water (Growing Blue, 2011), and only a very small percentage of NEWater is blended with reservoir water used to produce drinking water.^[62]

Orange County is located in Southern California, USA, and houses a classic example in IPR.^[63] A large-scale artificial groundwater recharge scheme exists in the area, providing a much-needed freshwater barrier to intruding seawater.^[64] Part of the injected water consists of recycled water, starting as of 1976 with Water Factory 21, which used RO and high lime to clean the water (production capacity of 19,000 m³ per day).^[65][66] This plant was de-commissioned in 2004 and has since made place for a new project with a higher capacity (265,000 m³ per day with an ultimate capacity of 492,000 m³ per day), under the name of Groundwater Replenishment System.^[67][68] This newer scheme uses the newer technological combination of RO, MF, and ultraviolet light with hydrogen peroxide.^[69][70] Plans are also underway to further increase the capacity of the system,^[71] which already provides up to 20% of the water used by the country.^[72]

In the USA, San Diego, California is the leading state implementing IPR. MF, RO and UV/H₂O₂ are employed prior to groundwater replenishment with the treated effluents (CDPH, 2013). In San Diego, the effort to increase the share of recycled water was rekindled with an extensive study in 2006.^[73] MF provides substantial removal of the dissolved effluent organic matter (dEfOM), while dEfOM reduction down to 0.5 mg/L (in terms of TOC) is achieved through RO application. The chemical oxidation treatment (UV/H₂O₂) following the membrane steps, results in the mitigation of N-nitrosodimethylamine (NDMA), as well as in the improvement of the effluent quality with respect to its organic content.^[74]

The City of El Paso’s (Texas, USA) water sources include groundwater aquifers and surface water from the Rio Grande. In order to increase groundwater levels, the El Paso Water Utilities injects advanced treated reclaimed water into the aquifer. The advanced treatment facilities use two-stage powdered activated carbon (PAC), addition of lime, two-stage recarbonation, sand filtration, ozonation, granular activated carbon (GAC), and chlorination for purifying the water.^[75] The Hueco Bolson Recharge Project, which initially began in 1985, currently recharges 1,700 acre-feet per year of reclaimed water at 10 injection wells and 800 acre-feet per year at an infiltration basin for groundwater recharge.^[76]

In Colorado, USA, the Colorado River Municipal Water District implemented a project to capture treated municipal effluent from the City of Big Spring, and provide additional advanced treatment prior to blending into their raw surface water delivery system (2012). Advanced treatment of the municipal effluent consisted of MF, RO, and ultraviolet oxidation, producing very high quality water, which is blended with surface water from Lake E.V. Spence for distribution to their member and customer cities (production of 6,700 m³ per day).^[77]

The IPR project in Wulpen, Belgium (2002), discharges recycled water to an unconfined dune aquifer. Initially the recycled water comprised 90% RO permeate and 10% MF permeate (approx. 6,000,000 m³ per year). However, it was observed that some herbicides were present in the recycled water at levels below drinking water standards due to detection of herbicides in the MF permeate. As a result, since May 2004, only the RO permeate after UV disinfection is injected into the aquifer with addition of sodium hydroxide to adjust the pH.^[78][79]

In South Africa, the town Garden Route, George faced water shortages and had decided on an IPR strategy (2009/2010), where final effluents from its Outeniqua WWTP are treated to a very high quality through UF and disinfection prior to being returned to the main storage facility, the Garden Route Dam, where they are combined with current raw water supplies. This initiative augments the existing supply by 10,000 m³ per day, approximately one third of the drinking water demand. The process configuration includes the following treatment processes: drum screen, UF, and chlorine disinfection. Provision has been made for powdered activated carbon (PAC) addition at George WTW, if required as an additional operational barrier.^[80]

IPR has been considered for regional communities in Goulburn, NSW, the Australian Capital Territory (ACT) and Toowoomba, Queensland (2008). The Western Corridor Recycled Water Scheme in South East Queensland was designed and built to produce drinking quality water suitable for release into the Wivenhoe Dam, Brisbane's principal water storage. The advanced WWTP incorporated MF and RO followed by an advanced oxidation system using UV-light and hydrogen peroxide to remove specific disinfection by-products and non-specific low molecular weight organics. The project had a production capacity of 232,000 m³ per day and over 200 km of interconnecting and product water delivery pipelines.^[81][82]

In Perth in Western Australia, the Western Australia Water Corporation operated a three-year demonstration project investigating the feasibility of reclaiming water from the Beenyup WWTP using MF, RO and UV disinfection prior to injection into the Leederville aquifer (production of 5,000 m³ per day). The demonstration concluded in 2012, and in 2013 the Western Australian Government agreed to a full-scale groundwater recharge scheme, which commenced construction in 2014. When complete, the full-scale facility will provide 14,000,000 m³ per annum to the aquifers supplying Perth's drinking water, with the option to expand to 28,000,000 m³ per annum in the future.^[83][84]

Advantages of wastewater reuse

Water/wastewater reuse, as an alternative water source, can provide significant economic, social and environmental benefits, which are key motivators for implementing such reuse programmes. Specifically, in agriculture, irrigation with wastewater may contribute to improve production yields, reduce the ecological footprint and promote socioeconomic benefits.^[85] These benefits include:^{[86][87][88][89][90]}

• Increased water availability
• Integrated and sustainable use of water resources
• Drinking water substitution - keep drinking water for drinking and reclaimed water for non-drinking use (i.e. industry, cleaning, irrigation, domestic uses, toilet flushing, etc.)
• Reduced over-abstraction of surface and groundwater
• Reduced energy consumption associated with production, treatment, and distribution of water compared to using deep groundwater resources, water importation or desalination
• Reduced nutrient loads to receiving waters (i.e. rivers, canals and other surface water resources)
• Reduced manufacturing costs of using high quality reclaimed water
• Increased agricultural production (i.e. crop yields)
• Reduced application of fertilizers (i.e. conservation of nutrients, reducing the need for artificial fertilizer (e.g. soil nutrition by the nutrients existing in the treated effluents))
• Enhanced environmental protection by restoration of streams, wetlands and ponds
• Increased employment and local economy (e.g. tourism, agriculture).

Health and environmental risks of water reuse

As stated in the 2002 Hyderabad Declaration, on Wastewater Use in Agriculture, "without proper management, wastewater use poses serious risks to human health and the environment".^[91][92] The main potential risks that are associated with reclaimed wastewater reuse for irrigation purposes, when the treatment is not adequate are the following:^{[93][94][95][96][97][98][99]}

1. contamination of the food chain with microcontaminants, pathogens (i.e. bacteria, viruses, protozoa, helminths), or antibiotic resistance determinants;
2. soil salinization and accumulation of various unknown constituents that might adversely affect agricultural production;
3. distribution of the indigenous soil microbial communities;
4. alteration of the physicochemical and microbiological properties of the soil and contribution to the accumulation of chemical/biological contaminants (e.g. heavy metals, chemicals (i.e. boron, nitrogen, phosphorus, chloride, sodium, pesticides/herbicides), natural chemicals (i.e. hormones), contaminants of emerging concern (CECs) (i.e. pharmaceuticals and their metabolites, personal care products, household chemicals and food additives and their transformation products), etc.) in it and subsequent uptake by plants and crops;
5. excessive growth of algae and vegetation in canals carrying wastewater (i.e. eutrophication);
6. groundwater quality degradation by the various reclaimed water contaminants, migrating and accumulating in the soil and aquifers.

Guidelines/policies on wastewater reuse

The recommendations on wastewater reuse established by the State of California, the World Health Organisation (WHO) (i.e. "Guidelines for the safe use of wastewater, excreta and greywater" (2006)) and the US Environmental Protection Agency (USEPA) (i.e. "Guidelines for Water Reuse. Environmental Protection Agency (EPA)" (2012)) ^{[100][101][102]} and by Australia, constitute the background of most of the legal guidelines proposed in countries such as the United States, Portugal, Spain, Italy, Cyprus, France, Australia, Israel, Jordan, Mexico, South Africa, Tunisia, and China. Compliance with these regulatory frameworks requires the analysis of the treated wastewater prior to its reuse.^[103]

Water reuse guidelines developed by international organisations

• World Health Organization (WHO): “Guidelines for the safe use of wastewater, excreta and greywater” (2006).
• United Nations Environment Programme (UNEP): “Guidelines for municipal wastewater reuse in the Mediterranean region” (2005).
• United Nations Water Decade Programme on Capacity Development (UNW-DPC): Proceedings on the UNWater project “Safe use of wastewater in agriculture” (2013).
• Food and Agriculture Organization (FAO): “Water quality for agriculture” (1994).
• Canada: “Canadian guidelines for domestic reclaimed water for use in toilet and urinal flushing” (2010).
• China: China National Reclaimed Water Quality Standard; China National Standard GB/T 18920-2002, GB/T 19923-2005, GB/T 18921-2002, GB 20922-2007 and GB/T 19772-2005.
• Israel: Ministry of Health regulation (2005).
• Japan: National Institute for Land and Infrastructure Management: Report of the Microbial Water Quality Project on Treated Sewage and Reclaimed Wastewater (2008).
• Jordan: Jordanian technical base n. 893/2006 Jordan water reuse management Plan (policy).
• Mexico: Mexican Standard NOM-001-ECOL-1996 governing wastewater reuse in Agriculture.
• South Africa Policies: The latest revision of the Water Services Act of 1997 relating to grey-water and treated effluent (Department of Water Affairs and Forestry, 2001).
• Tunisia: Standard for the use of treated wastewater in agriculture (NT 106-109 of 1989) and list of crops that can be irrigated with treated wastewater (Ministry of Agriculture, 1994).
• USA National: United States Environmental Protection Agency (USEPA) “Guidelines for water reuse” (2012).
• Australia National level Guidelines: Government of Australia (the Natural Resource Management Ministerial Council, the Environment Protection and Heritage Council, and the Australian Health Ministers Conference (NRMMC-EPHC-AHMC)): Guidelines for water recycling: managing health and environmental risks” Phase 1, 2006.^[104]

Water reuse regulations in Europe

The health and environmental safety conditions under which wastewater may be reused are not specifically regulated at the European Union (EU) level. There are no guidelines or regulations at EU level on water quality for water reuse purposes. In the Water Framework Directive (WFD) (2000/60/EC), reuse of water is mentioned as one of the possible measures to achieve the Directive’s quality goals, however this remains a relatively vague recommendation rather than a requirement: Part B of Annex VI refers to reuse as one of the “supplementary measures which Member States within each river basin district may choose to adopt as part of the programme of measures required under Article 11(4)”.^[105] Besides that, Article 12 of the Urban Wastewater Treatment Directive (91/271/EEC) concerning the reuse of treated wastewater states that “treated wastewater shall be reused whenever appropriate”, is not specific enough to promote water reuse and it leaves too much room for interpretation as to what can be considered as an “appropriate” situation to reuse treated wastewater. Despite the lack of common water reuse criteria at the EU level, several Member States (MS) have issued their own legislative frameworks, regulations, or guidelines for different water reuse applications (e.g. Cyprus, France, Greece, Italy, and Spain). However, after an evaluation carried out by the European Commission (EC) on the water reuse standards of several member states it was concluded that they differ in their approach. There are important divergences among the different standards regarding the permitted uses, the parameters to be monitored, and the limit values allowed. This lack of harmonization among water reuse standards might create some trade barriers for agricultural goods irrigated with reclaimed water. Once on the common market, the level of safety in the producing member states may be not considered as sufficient by the importing countries.^[106] The most representative standards on wastewater reuse from EU MS are the following:^[107]

• Cyprus: Law 106 (I) 2002 Water and Soil pollution control and associated regulations (KDP 772/2003, KDP 269/2005) (Issuing Institutions: Ministry of Agriculture, Natural resources and Environment, Water Development Department).
• France: Jorf num.0153, 4 July 2014. Order of 2014, related to the use of water from treated urban wastewater for irrigation of crops and green areas (Issuing Institutions: Ministry of Public Health, Ministry of Agriculture, Food and Fisheries, Ministry of Ecology, Energy and Sustainability).
• Greece: CMD No 145116. Measures, limits and procedures for reuse of treated wastewater (Issuing Institutions: Ministry of Environment, Energy and Climate Change).
• Italy: DM 185/2003. Technical measures for reuse of wastewater (Issuing Institutions: Ministry of Environment, Ministry of Agriculture, Ministry of Public Health).
• Portugal: NP 4434 2005. Reuse of reclaimed urban water for irrigation (Issuing Institutions: Portuguese Institute for Quality).
• Spain: RD 1620/2007. The legal framework for the reuse of treated wastewater (Issuing Institutions: Ministry of Environment, Ministry of Agriculture, Food and Fisheries, Ministry of Health).

Challenges

• The identification of the CECs, their transformation products originating both during treatment and while being in the environment through biotic and abiotic processes, their potential uptake by crops during wastewater reuse for irrigation, and the effects that these contaminants may induce in the environment ^[108]
• The analytical methods to identify and quantify such contaminants in complex matrices like wastewater; the development of bioassays that can be applied to assess the effects of such contaminants and of treated wastewater as complimentary or alternative methods to the chemical methodologies, in order to evaluate the potential of the treated flows to cause harm to the human and environmental health.^[109]
• The evolution and release of antibiotic resistance.^[110]
• The identification of technologies that are able to remove CECs from wastewater, and the identification of means and solutions to overcome these problems, and promote safe reuse practices further (Sanz and Gawlik, 2014;.^[111]
• Full-scale implementation and operation of water reuse schemes still face regulatory, economic, social and institutional challenges.^[112]
• Economic viability of water reuse schemes.^[113]
• Cost effectiveness of water quality monitoring and identification of contaminants.^[114]
• Full cost recovery from water reuse schemes - lack of financial water pricing systems comparable to already subsidized conventional treatment plants.^[115]
• Legislative requirements: Amendments of license conditions.^[116][117]
• Each location and situation has different advantages and challenges to be evaluated for making the best decisions for implementation.^[118]
• Wastewater reuse for DPR still meets a lot of resistance due to social and cultural aspects, perceptions about the anticipated health risks.^[119][120]
• In general, people are not comfortable with the idea of wastewater reuse, especially for potable purposes.^[121]

References