Potable Reuse of Treated Wastewater: An Emerging Strategy to Protect Water Security
Water reuse: an ancient phenomenon naturally built into the water cycle
Water availability is governed by the hydrologic cycle, also known as the water cycle, which is a natural cycle that many learn about as early as elementary science classes. The hydrologic cycle has worked in geologic time scales going back to the origin of life on Planet Earth. As implied, water has been naturally circulating through different stocks like the atmosphere, snowpack, rivers, lakes, oceans, soil moisture, and groundwater. Managing the movement and quality of water by humans through dam construction, diversions and transfers, and water and wastewater treatment for agricultural, urban, and industrial uses has affected the flow, extent, and speed with which water moves between different stocks. But all the water that exists in the Earth’s biosphere (or regions occupied by living organisms) is all the water there has ever been. The ongoing nature of the hydrologic cycle points to a de facto reuse of wastewater (water that has been used before at some point in the past) within the water cycle, albeit unintentionally. The inevitable natural recycling and reuse of water is the basic premise for the science-based implementation of this strategy to increase water security.
Potable reuse of treated wastewater: why, what, and how?
As humans tighten their grip over water resources that sustain different forms of life and socio-economic and ecological functions, these resources are under increasing pressure worldwide. The declining reliability of water supply systems has motivated many water utilities in the U.S. and around the world to think of recycling and reuse of valuable fresh water for different purposes, including drinking. Potable reuse of treated wastewater is gaining traction as an adaptation strategy to mitigate water shortages in water-scarce regions that face prolonged droughts, especially in a warming world.
Besides unintended wastewater reuse within the hydrological cycle, potable reuse of treated wastewater can happen either indirectly or directly. Indirect potable reuse involves blending highly treated wastewater with raw water in an environmental buffer such as river, lake, or groundwater to add “a touch of nature” before re-treating the water to drinking water standard for delivery to end users. Direct potable reuse of treated wastewater means that highly treated and monitored wastewater is directly injected into delivery pipelines without going through an environmental buffer and retreatment. However, blending with other treated water may occur within the distribution infrastructure.
From a technical standpoint, potable reuse of treated wastewater is implemented through engineered treatment systems that are designed to include multiple treatment processes to minimize acute and chronic health risks such as infection and cancer (Mirchi et al., 2019). Potable reuse treatment systems employ several advanced processes to purify treated effluent from conventional wastewater treatment plants. Design engineers can choose from microfiltration, ultrafiltration, reverse osmosis, and ultraviolet irradiation, membrane filtration, ozonation, and activated carbon filtration for the potable reuse of treated wastewater. The multiple barrier approach is essential to provide safe and reliable drinking water using wastewater and increase water supply. Because of the importance of protecting public health, potable reuse systems are designed to have redundancy of treatment processes and be robust and resilient. In other words, these engineered systems use more treatment units in parallel or series than the minimum required, use various treatment processes to remove diverse contaminants, and include measures that ensure unsafe water will not reach the public in case of a unit malfunction or system failure. Economic cost and energy requirements of treatment, residual management, concentrate management in the case of RO systems, and conveyance and blending must be considered to select suitable treatment processes for potable reuse based on the environmental setting.
Applications around the world: Two prominent cases in the U.S.
There is growing interest in both indirect and direct potable reuse of treated wastewater. In some cases, like the International Space Station, direct potable reuse has been implemented for a long time. Potable reuse of treated wastewater has been implemented in numerous countries, including Australia, Namibia, Singapore, Belgium, and the U.S. Singapore is a great example where large-scale indirect potable reuse operations have been successfully implemented along with effective public relations campaigns to meet about 40% of the country's water demand. Likewise, direct potable reuse provides drinking water to about 350,000 residents in the City of Windhoek, Namibia. In the U.S., potable reuse of treated wastewater is also among popular supply-side water management options in populous water-scarce and semi-arid/arid metropolitan areas, like the Southwestern U.S., with facilities in operation or under planning. Two notable cases of plans to use potable reuse of treated wastewater are projects in San Diego, California, and El Paso, Texas (Mirchi et al., 2019).
The first example is Pure Water San Diego, which is planned to start operation in 2035. The project will add 30 million gallons per day (mgd) to Miramar Reservoir before entering the city’s water distribution network. San Diego’s plan to use treated wastewater for drinking purposes originated in the late 1970s, with different pilot projects helping to analyze the techno-economic feasibility. To minimize risks to public health, water quality must meet stringent state and federal regulations. The evolution of Pure Water San Diego has involved fierce public relations battles backed by rigorous scientific evidence about the quality of the purified water to overcome bad publicity due to the use of the phrase “toilet-to-tap” by opponents of the plan. Comprehensive outreach efforts were needed in the last nearly three decades to overcome the polarization and build public support for the indirect potable reuse of treated wastewater. The outreach team tapped into the potential of community leaders and stakeholder groups to inform residents about the plans. These efforts have been proven effective, as indicated by increasing public support for the indirect potable reuse of treated wastewater in San Diego.
A notable example of direct potable reuse is the Advanced Water Purification Facility in El Paso, Texas, which is planned to address potential drinking water challenges by supplying up to 10 mgd in a desert metropolitan environment. The area is vulnerable to surface water shortages because of the snowpack decline in headwater catchments in Colorado and the dropping of groundwater tables due to overdraft to compensate for the surface water deficit. The plan is backed by rigorous pilot research and third-party expert review to ensure the reliability and safety of the produced drinking water. In the case of El Paso, too, public information activities have been an integral part of the overarching plan to add direct potable reuse to the water supply portfolio. However, public engagement has been less widespread relative to the San Diego case. There is an opportunity to expand outreach efforts to ensure the public is informed about direct potable reuse as a water supply option to form a science-based public perception and pre-empt potential backlash due to waves of panic as was experienced in the past in San Diego.
Takeaways: lessons for potable reuse of treated wastewater in Iran
The inevitable growth in the number of plans to reuse treated wastewater for drinking purposes in water-scarce metropolitan areas worldwide is out of necessity. The necessity arises because of the compounding effects of demand growth, climate-related risks to renewable water supply, declining groundwater, and running out of less contentious water supply alternatives due to cost and environmental concerns. Advancements in the science and engineering design based on the multi-barrier concept make potable reuse technologically feasible. However, this emerging strategy to protect water security has not been widely adopted in Iran despite the egregious urgency of the country’s state of water bankruptcy. The lack of transparent, well-thought-out plans to tap into treated wastewater to augment the reliability of the water supply is a missed opportunity. It is timely and critical to rethink the country’s continued implementation of the hydraulic mission to increase water supply by building more dams and inter-basin water transfer projects. Iranians can no longer afford to use water as a disposable good, and allowing wastewater discharge into water bodies pose public health and environmental quality risks where, in fact, it can be used as a valuable resource to reduce the need for a new fresh water supply.
Implementing potable reuse of treated wastewater is a sensitive sociotechnical decision that requires several important considerations. A critical public concern is safety and “yuck” factor associated with using treated wastewater, which is a formidable psychological challenge to overcome in the absence of public trust in authorities and institutions, and without the engagement of independent experts to endorse the technology based on rigorous scientific investigations. Counterintuitively, using treated wastewater will be more expensive than using readily available freshwater, despite the users’ expectation to pay less for treated wastewater as they perceive this type of water to have lower quality. The presented examples from the U.S. highlight the importance and nuances of meaningful, all-encompassing public engagement to support techno-economically feasible design alternatives to implement potable reuse. The social dimension of potable reuse concerns not only surveying public attitudes but also governance, institutions, and politics. The cultural and religious contexts in Iran add a compounding factor that must be considered in engaging opinion leaders and stakeholder groups for effective public information sharing to ensure people from different walks of life and religious backgrounds are informed and engaged. Careful assessment of downstream effects of recycling and reuse of treated wastewater is also needed to minimize adverse impacts in downstream areas that rely on upstream return flows. Given the potential of potable reuse of treated wastewater to increase water security, the water management discourse in Iran will benefit from serious consideration and analysis of the widespread implementation of this strategy.
________________ Ali Mirchi is an Assistant Professor of Water Resources Engineering at the Department of Biosystems and Agricultural Engineering at Oklahoma State University, U.S. He applies systems modeling and analysis techniques, including system dynamics simulation, hydro-economic optimization, and watershed hydrologic modeling, to advance understanding of coupled human-natural systems at different scales. His research focuses on water resources planning and management to derive policy insights that promote water sustainability in the face of population growth, competing demands, and climatic extremes.