By Justin Mattingly, research manager, WateReuse Association and Research Foundation
New research reveals the value and economics of direct potable reuse (DPR), as well as how to get started.
The prolonged and ongoing drought in California and other regions in the Southwest has made water supplies increasingly scarce, highlighting the need to secure new and sustainable sources of potable water. One such option is to more effectively utilize existing wastewater resources through water reuse, and direct potable reuse (DPR) in particular, to supplement existing water supplies. Through a joint effort between WateReuse, the American Water Works Association (AWWA), and the Water Environment Federation (WEF), the report “Framework for Direct Potable Reuse” was developed by an independent advisory panel administered by the National Water Research Institute (NWRI) to provide information about the value of DPR as a water supply option and what is needed to implement a DPR program. Communities can find numerous advantages — including enhanced water supply reliability, decreased energy usage, greater value from limited natural water supplies, and controlled increases to the cost of water — by considering DPR as part of their water supply portfolios.
In California alone, there is great potential for the expansion of potable water reuse. A 2014 study from WateReuse estimated that by 2020, over 2,300 MGD in treated wastewater will be discharged to surface waters or the ocean. Of this amount, over 1,000 MGD could be used for either indirect potable reuse or DPR. This amount would meet the residential, commercial, and industrial water needs for eight million people, or more than 20 percent of the projected population of California in 2020. There is also significant potential in other regions of the country, including Big Spring, TX, which has a DPR facility currently in operation. The framework document is aimed at making this potential a reality throughout the country by paving the way for a sustainable source of drinking water that is protective of public health and resilient in the face of drought and climate change.
What Is Direct Potable Reuse?
Although not often acknowledged, communities throughout the U.S. currently engage in potable water reuse, where downstream surface waters used as a source of drinking water are subject to upstream wastewater discharges. This is commonly referred to as unplanned or de facto potable reuse. Conversely, planned potable reuse can take the form either of indirect potable reuse, where an environmental buffer such as an aquifer or reservoir is present, or direct potable reuse, where no such environmental buffer is present.
Direct potable reuse can be implemented in two ways:
- Advanced treated water is introduced with or without the use of an engineered storage buffer (ESB) into the raw water supply immediately upstream of a drinking water treatment facility (DWTF). To date, permitted operational DPR facilities in the U.S. involve this form of DPR.
- Finished water is directly introduced — with or without the use of an ESB — into a drinking water supply distribution system, either downstream of a DWTF or within the distribution system. Although a finished water DPR facility has been in operation at Windhoek, Namibia since 1967, the production of finished water is not the focus of the framework document.
A number of communities throughout the country are currently considering implementing a DPR program to supplement their current potable water supply. To ensure that these projects protect public health, decisionmakers need to understand the regulatory and operational components that must be part of a DPR program. While such a program will resemble existing drinking water and wastewater programs, there are some distinct differences and unique characteristics to DPR where further guidance is needed. The framework document sponsored by WateReuse, AWWA, and WEF details these important issues with information on the following topics:
- Public health and regulatory aspects
- Source control programs
- Wastewater treatment
- Advanced water treatment
- Management of advanced treated water in a drinking water system
- Process monitoring
- Residuals management including brine disposal
- Facility operation
- Public outreach
- Future developments.
Paramount among these topics is the implementation of a multi-barrier approach to removing pathogens and chemical constituents. These barriers can include management barriers such as pretreatment policies and proper operations and maintenance procedures, operational barriers such as monitoring and response plans, and the technical barriers of the physical treatment processes. When taken together, these barriers form the foundation of a robust and resilient DPR system.
Mapping The Complete DPR Process
A primary theme of the framework document is that DPR does not include just the treatment processes associated with an advanced treatment facility but also source control programs, traditional wastewater treatment, and the integration of advanced treated water into a drinking water treatment facility. Understanding that the performance of upstream processes can have an effect on downstream processes can go a long way to ensuring that a DPR system is performing properly.
A properly implemented source control program can help eliminate the discharge of constituents into wastewater that can be difficult to treat or impair the final quality of secondary effluent intended for DPR. This can be especially important in communities with large commercial and industrial discharges. There are several important elements to a source control program, including the legal authority to develop source control measures, monitoring discharges within a service area, investigating and maintaining a current inventory of chemical constituents, an effective public outreach plan, and a response plan to be used in case of water quality deviations. Creating a source control program is an important step in DPR because the easiest way to remove chemical constituents from wastewater is to prevent them from entering the wastewater stream in the first place.
Understanding that the performance of upstream processes can have an effect on downstream processes can go a long way to ensuring that a DPR system is performing properly.
Following source control, the next step is the wastewater treatment process. Traditionally, the focus of wastewater treatment has been to produce an effluent suitable for discharge into the environment. However, for DPR there is additional optimization that can be done that has the potential to benefit and increase the efficiency of advanced treatment processes. In the context of DPR, the goal of wastewater treatment should be to provide a consistent and high-quality effluent while recognizing that certain contaminants such as pathogens and constituents of emerging concern may be removed more cost effectively than in advanced water treatment. As an example, the Orange County Sanitation District (OCSD) provides secondary effluent to the Orange County Water District (OCWD) for potable reuse. After completing operational changes to enable OCSD to produce a nitrified effluent, the microfiltration system at OCWD was able to significantly reduce membrane fouling, resulting in cost savings.
The advanced water treatment process is what separates potable reuse from traditional wastewater treatment. This is the process that takes secondary effluent and treats it further to meet potable water quality standards. This process can take various forms, but for DPR the process used at OCWD in its Groundwater Replenishment System (GWRS) has been shown to be effective at producing high-quality product water. This process includes several steps highlighted by microfiltration followed by reverse osmosis (RO) and an advanced oxidation step with UV and hydrogen peroxide. However, for inland communities, there are potential options not requiring RO, including processes with ozonation and biologically active filtration. Indirect potable reuse facilities in Gwinnett County, GA, and Fairfax County, VA, currently operate in this fashion.
Following the production of advanced treated water (ATW), it then must be integrated into the drinking water treatment and distribution system. Typically, ATW will be blended with raw water at the intake of a drinking water treatment facility. However, there are some potential issues that may arise from this process including effects on the coagulation process through reduced alkalinity or turbidity, as well as effects on aesthetics. WateReuse and the Water Research Foundation are currently looking into these issues closely to provide facilities with the best information to seamlessly integrate DPR into their water supply. It is important for all the facilities and agencies involved in each step of this process to work together with the understanding that each step is critical to the successful implementation of DPR.
Economics Of DPR
When comparing DPR to other water supply options, cost is often one of main issues taken into consideration. While precise costs are difficult to generalize, comparing DPR to options like desalination shows DPR often to be the less expensive option, and DPR is also competitive in cost with imported water in California. A 2014 study from the WateReuse Research Foundation estimated the cost of DPR at a range of $820 to $2,000 per acre-foot of water. The range in cost for DPR is based on the cost of treatment, distribution, and brine disposal — if applicable.
Depending on the treatment train used, the cost of treatment can be expected to be relatively uniform. As an example, the GWRS in Orange County has costs of approximately $700 per acre-foot. On the other hand, costs for distribution and brine disposal are largely site-specific and vary based on the distance that water must be conveyed. Siting an advanced treatment facility close to a drinking water facility can dramatically reduce conveyance costs compared to facilities that are located far apart, especially if there is a large change in elevation. The same is true for residuals disposal from facilities utilizing RO. In that case, facilities located on a coastline may have lower costs due to easy access to an ocean outfall.
When determining whether to pursue DPR, economic and social factors should also be considered. Such factors could include energy use and carbon footprint, impact on wastewater discharges and pollution, and the economic benefit of having a local and sustainable water supply. Further detail on the economics of DPR and other water supply options can be found in the framework document, as well as the 2014 report from WateReuse, The Economics and Opportunities of Direct Potable Reuse.
About Framework For Direct Potable Reuse
The report “Framework for Direct Potable Reuse” is available free of charge at www.watereuse.org. The document represents a consensus among the panel, while taking into consideration input from a project advisory committee composed of technical experts in water and wastewater treatment, as well as state and federal regulators. Members of the panel included Panel Chair Dr. George Tchobanoglous of the University of California, Davis; Dr. Joseph Cotruvo of Joseph Cotruvo & Associates; environmental engineering consultant Dr. James Crook; Dr. Ellen McDonald of Alan Plummer Associates; Dr. Adam Olivieri of EOA, Inc.; Andrew Salveson of Carollo Engineers; and Dr. R. Shane Trussell of Trussell Technologies, Inc. The panel was managed by Jeff Mosher of NWRI.
About The Author
Justin Mattingly is a research manager with the WateReuse Association and Research Foundation. WateReuse is internationally recognized as a thought-leader on alternative water supply development. It is the go-to organization for applied research, policy guidance, and educational tools on water reuse, as well as the principal influencer of public opinion, lawmakers, and policymakers on policy and projects related to water reuse.