In many communities around the world, the growth of populations and economies are causing demand for freshwater to increase at an alarming rate. Without a sound and sustainable strategy for integrated water resource management (IWRM), demand in these areas can quickly expand to exceed available supply. One integrated approach that is gaining acceptance is to consider municipal wastewater as a vital resource for appropriate applications, including agricultural and other irrigation, industrial and domestic uses. This practice is called water reclamation and reuse and is an example of an Environmentally Sound Technology because it protects the environment, results in less pollution, utilizes resources in a more sustainable manner, allows its waste and products to be recycled, and handles residual wastes in a more acceptable manner than the technologies for which it substitutes (UNEP and GEC, 2004).
The terms reclamation and reuse are often used in various contexts to mean different things. This article will adopt conceptual definitions from the textbook Water Reuse: Issues, Technologies, and Applications. Water reclamation is the treatment or processing of wastewater to make it reusable with definable treatment reliability and meeting appropriate water quality criteria; water reuse is the use of treated wastewater (or reclaimed water) for a beneficial purpose. It is also important to mention that, in common parlance, the term reclaimed water is used interchangeably with the often more culturally-acceptable term recycled water (Asano et al., 2007).
Though this article will focus on applications of water reuse that directly affect drinking water supplies, it is important to note that agricultural use accounts for the majority of freshwater consumption worldwide. Therefore, augmentation of agricultural irrigation with reclaimed water could potentially yield the greatest benefits to global water resources. In fact, reclaimed water is used to supplement agricultural irrigation in almost all arid areas of the world (US EPA, 2004). The WHO published an updated set of guidelines in 2006 (WHO, 2006) that were intended to serve as a framework for the development of national and international standards and regulations for managing the health risks associated with the use of reclaimed water in agriculture. These guidelines should be consulted when developing approaches for agricultural reuse.
A number of sustainable and safe approaches to meeting increasing water demand with municipal wastewater have been identified (Asano et al., 2007). These general approaches include:
- Substituting reclaimed water for applications that do not require potable water
- Augmenting existing water sources and providing an additional source of water supply to assist in meeting both present and future water needs
- Protecting aquatic ecosystems by decreasing the diversion of freshwater, as well as reducing the quantity of nutrients and other toxic contaminants entering waterways
- Postponing and reducing the need for water control structures
- Complying with environmental regulations by better managing water consumption and wastewater discharges
Typical wastewater treatment schemes incorporate multiple levels of physical, biological, and chemical treatment in order to ensure that water discharged to the environment does not pose a significant risk of adverse environmental or health impacts. Treated wastewater is usually discharged to surface water and that surface water is often used by a water source for a water utility downstream. Thus, many water systems reuse wastewater inadvertently. Though such unintentional reuse (also called “unplanned”, “incidental” or “natural” reuse) occurs often, it is rarely acknowledged (US EPA, 2004). Drawing attention to unintentional reuse could possibly reduce public resistance to wastewater reuse (see the section about barriers below).
Water reclamation and reuse approaches utilize the same treatment technologies as conventional wastewater treatment, including secondary clarifiers, filtration basins of various designs, membranes, and disinfection basins. Further reading regarding the applicability of such technologies to water reclamation and reuse is available (Asano et al., 2007). Though it is likely that each and every water reclamation treatment scheme will require some degree of customization, a great deal of work has been done to define appropriate applications for water that has been treated by primary (such as sedimentation), secondary (such as biological oxidation and disinfection), and tertiary (such as chemical coagulation, filtration and disinfection) conventional wastewater treatment processes. The most comprehensive set of guidelines that recommend treatment processes for specific uses, reclaimed water quality limits, monitoring frequencies, and other controls for various water reuse applications have been developed by the United States Environmental Protection Agency (US EPA, 2004). These guidelines are a valuable resource for water resource managers who are planning water reclamation and reuse programs. In general, uses that correspond to increasing levels of human exposure require water that has received higher levels of treatment. Though not comprehensive, Figure 1 depicts a number of such uses that have been suggested by the US EPA. The UNEP report on water and wastewater reuse (UNEP and GEC, 2004) and the Water Reuse: Issues, Technologies, and Applications textbook (Asano et al., 2007) also contain a great deal of information regarding treatment considerations for appropriate uses of reclaimed water.
Direct potable reuse is very rarely recommended, regardless of the level of treatment reclaimed water has received. There are two technical reasons for this: (1) even when it is technically feasible to remove all known contaminants from wastewater, unknown contaminants may be present; and (2) in the case of an undetected failure in the treatment process, major risks to health are likely. These two issues still pose enough of a potential threat to health to render direct potable reuse impractical in most contexts. However, in most settings the main reason for not pursuing direct reuse is public opposition (see the section about barriers below). In fact, the Windhoek plant in Namibia is the only case where drinking water supplies have
been directly augmented by reclaimed water on a long-term basis (Asano et al., 2007).
Traditionally, it has even been uncommon for drinking water reservoirs to be augmented with reclaimed water. However this practice, known as indirect potable reuse, has increased in popularity over the last decade and has been successfully implemented in a number of cases around the world. For potable reuse, treatment requirements generally go beyond conventional tertiary treatment steps. For example, the direct potable reuse plant in Namibia and the indirect potable reuse plants in Singapore (NEWater) and in Orange County, California (Water Factory 21) all incorporate advanced drinking water treatment technologies into water reclamation schemes, such as dissolved air flotation, membrane filtration, reverse osmosis, and UV irradiation (Asano et al., 2007). It is still generally believed that nonpotable reuse can conserve water resources to the same extent as potable reuse while avoiding most of the public health risks (Okun, 2000).
A vast amount of information on water reclamation and reuse is available in peer-reviewed literature. A number of notable textbooks (Asano et al., 2007; Jimenez and Asano, 2008), guideline documents (UNEP and GEC, 2004; US EPA, 2004), and comprehensive reviews (Okun, 2000; Anderson, 2003) have been developed in an attempt to gather and analyze this information. Water resource managers should consult these resources for guidance on water reuse regulations and guidelines, public health risks, appropriate water reuse technologies and treatment systems, applications for reclaimed water, and appropriate steps for planning and implementing water reuse approaches.
It is important to mention that most of the wastewater used in developing countries for agricultural irrigation is done so without adequate treatment (Asano et al., 2007). This often results in high burdens of enteric disease when these crops are consumed raw or undercooked. Such diseases diminish economic productivity and confine people to poverty. However, when implemented appropriately, water reclamation and reuse can contribute to social and economic development by reducing environmental pollution and enteric disease burden and increasing household water availability and crop production. The WHO Guidelines for the Safe Use of Wastewater, Excreta and Grey Water, Volume 2: Wastewater Use in Agriculture were developed in order to provide guidance on the safe use of reclaimed water. According to these guidelines, treatment options that can enable safe use of wastewater in resource-poor settings without modern centralized wastewater treatment include: waste stabilization ponds, wastewater storage reservoirs, and constructed wetlands. WHO provides basic guidance on design factors, retention time and climatic conditions to achieve adequate pathogen reduction (US EPA, 2004).
Among a number of other predictions made by the Intergovernmental Panel on Climate Change, it is anticipated that climate change will lead to increased periods of drought, reduced freshwater stores, and sea level rise (IPCC, 2007). Such changes can have drastic impacts on both the quantity and quality of the world’s water resources. However, water reclamation and reuse approaches can and have been shown to be effective for adapting water resource management in the face of such stressors. Most importantly, water reclamation and reuse contributes to climate change adaptation by allowing water resources to be diversified and conserved. Using reclaimed water for applications that do not require potable water can result in greatly decreased depletion of protected water sources and prolong their useful lifespan. In addition, reclaimed water can be applied to permeable land surfaces or directly injected into the ground for the purpose of recharging groundwater aquifers and preventing saline intrusion in coastal areas. A successful example of this is the Montebello Forebay Ground Water Recharge Project, where for over 40 years recycled water has been applied to the Rio Hondo spreading grounds to recharge a potable ground water aquifer in south-central Los Angeles County in California (US EPA, 1998).
The water and nutrients that can be recovered from wastewater are simply too valuable to waste in areas where resources are limited. For this reason, it is very common for farmers in developing countries to supplement their crop irrigation supplies with wastewater. In fact, except for a handful of cases where applications such as natural filtration systems for water reclamation (Takizawa, 2001), sewage reclamation for industrial uses (Kurian and Visvanathan, 2001), or direct potable reusei have been implemented, almost all water reclamation and reuse in developing countries is dedicated to agricultural irrigation. Not only does this practice increase the volume of water available for crops and utilize the nutrients in wastewater in a beneficial way, it also contributes to greater quality of human life by increasing household water availability.
In general, the most economically viable applications for water reuse are those that replace potable water with reclaimed water for use in irrigation, environmental restoration, cleaning, toilet flushing, and industrial uses (US EPA, 2004). These applications of reclaimed water contribute directly to conservation of water resources and pollution reduction.
The financial requirements for implementing water reclamation and reuse programs will vary significantly based on the type of application that is planned for the reclaimed water. Therefore, water resource managers must fully understand the costs associated with developing and managing the particular water supply, wastewater management system, and proposed water reuse system in order to compare the costs and benefits of implementing water reclamation and reuse programs with that of maintaining traditional water and wastewater management approaches. An economic analysis should be conducted in order to weigh the cost of maintaining traditional approaches and of possibly needing to develop additional water sources versus the cost of retrofitting existing and constructing new infrastructure for reuse applications. Such analyses should also consider the number of financial benefits associated with water reclamation and reuse approaches, such as reduced treatment costs and the recovery of valuable nutrients from wastewater.
A particular type of economic analysis known as life cycle cost (LCC) analysis has been used to evaluate conditions under which water reclamation and reuse programs would be cost effective (UNEP and GEC, 2004). This approach, which has been described extensively in a report by UNEP’s Division of Technology, Industry and Economics, considers the cost of the reclamation and reuse program over its entire lifespan, including design, production, installation, operations, maintenance, repair, and disposal (UNEP, 1996). An example of a case when the LCC approach has been used for such an application is in Tokyo, Japan, where options for wastewater reuse were compared with a conventional freshwater and sewage treatment option in a number of office buildings (Yamagata et al., 2003). The analysis found that if the reclaimed water volume was more than 100 m3 per day, the wastewater reuse option was more cost effective when compared to the conventional freshwater and sewage treatment option. Such analyses can be very helpful in determining the economic feasibility of water reclamation and reuse programs.
A number of key capacity building elements that are required to ensure the quality of decision-making and managerial performance in the planning and implementation of water reclamation and reuse programs have been previously identified by UNEP (UNEP, 2002). Mentioned briefly here, these requirements are described in detail along with examples in the UNEP Water and Wastewater Reuse report (UNEP and GEC, 2004).
- Human resources: Implementation of water reclamation and reuse approaches requires the strengthening of local water and wastewater personnel’s technical and managerial ability to evaluate limitations of current practice, potential benefits and requirements of wastewater reuse as well as the fostering of their capability to implement new programs.
- Policy and regulatory framework: It will be necessary for policies and legal frameworks that facilitate safe and appropriate reclamation and reuse programs to either be created or aligned in order to ensure the protection of human health and the environment.
- Institutions: National, regional, and local institutions will likely need to be supported in their efforts to identify ways in which they can improve effectiveness in regulating and managing water reclamation and reuse programs.
- Financing: Financing opportunities and services for water reclamation and reuse initiatives will need to be expanded in order to facilitate such initiatives. It is also likely that the capability of utilities and potential users to understand and access these services will need to be improved.
- Participation: Since public perception often determines the success or failure of water reclamation and reuse initiatives (Po et al., 2003) civil society will need to be educated about the benefits of water reclamation and reuse as well as encouraged to participate in the decision-making process and implementation of such programs.
The institutions that are most likely to be involved in water reclamation and reuse projects are those responsible for water supply, wastewater management, water resources management, environmental protection, public health and agriculture (US EPA, 2004). Because of the complexity inherent in an initiative that attempts to coordinate so many institutions at the local, regional and national levels, it may be necessary to reorganize administrative duties into a general group that coordinates reclamation and reuse projects. In addition, it would be a clear conflict of interest for either the water supplier or the wastewater manager to function as a regulatory agency that has oversight and enforcement responsibility over all the partners involved in water reuse. Therefore, it may also be necessary to task an independent agency, such as the entity charged with protection of public health or the environment, with such a role. A number of developing countries such as Tunisia, Morocco, and Egypt have successfully made such institutional changes in order to facilitate water reclamation and reuse programs (US EPA, 2004).
There are a number of socio-political barriers that often limit successful implementation of water reclamation and reuse programs. In many cases, public opposition to the use of reclaimed water for any application to which humans might be exposed (especially for potable reuse) can hinder progress. The NEWater initiative in Singapore provides a strong example of how expansive public education campaigns and appropriate marketing can be used to positively influence public opinion towards water reclamation and reuse (Pahl-Wostl et al., 2005). Lack of communication and collaboration between stakeholders is also another significant socio-political barrier to water reclamation and reuse programs. The first step in the design and implementation of water reclamation and reuse initiatives should be to identify these institutional gaps and to forge the necessary links among agencies (US EPA, 2004).
Technical barriers can hinder successful implementation of water reclamation and reuse programs as well. Physical issues with transporting reclaimed water in distribution systems, such as corrosion of pipes, blockage of pipes and strainers, and biofilm formation in reservoir tanks due to reduction of residual chlorine in reclaimed water, are often cited as major concerns. In addition, implementation of reclamation and reuse programs often requires the retrofitting and construction of new and dual distribution systems as well as the development of new technologies for decentralized and satellite wastewater treatment. This can lead to prohibitively high costs that effectively limit the implementation of such programs. In depth discussions of dual distribution system and decentralized wastewater treatment can be found elsewhere (Asano et al., 2007; US EPA, 2004; Okun, 2000). The issue of unknown contaminants continues to be a barrier to the implementation of potable reuse. A series of publications issued by the Water Science and Technology Board of the National Research Council (NRC, 1994; 1996; 1998) reported a general consensus that technology exists for rendering almost any wastewater safe for drinking by current standards, but that the uncertainties regarding trace organics and emerging contaminants impose risks, suggesting that potable reuse should be an option of last resort (US EPA, 2004).
The planning and management approaches of 65 international nonpotable water reuse projects were documented in a 2001 survey conducted by the Water Environment Research Foundation (WERF) (Mantovani et al., 2001). The survey, which covered agricultural, urban, and industrial water reuse projects in both industrialized and developing countries in arid and semi-arid areas around the globe, showed that operational performance, sound institutional arrangements, conservative cost and sales estimates, and good project communication are the basis for success in reclamation and reuse projects. Likewise, the survey also showed that institutional obstacles, inadequate valuation of economic benefits, and lack of public information can delay water reclamation and reuse projects or cause them to fail (US EPA, 2004).
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