Prolonged and severe droughts, along with other factors, have made water supplies increasingly scarce in the Southwest and other regions of the United States, as well as elsewhere around the globe.? Because of such water supply constraints, there is a need to more effectively utilize existing water resources, like wastewater, to provide reliable and high-quality potable supplies to our communities.? In response there has been increased interest in water reuse, and direct potable reuse in particular, to help supplement existing water supplies.
Through a joint effort between WateReuse, the American Water Works Association (AWWA), and 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. Individual communities can realize numerous advantages ? including increased water supply reliability, decreased carbon footprints, 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.
Overview of Potable Water Reuse
Potable water reuse involves the use of a community?s wastewater as a source of drinking water with planned and unplanned potable reuse both occurring today. Often identified as de facto potable reuse, unplanned potable reuse occurs when downstream surface waters subject to upstream wastewater discharges are used as a source of drinking water. For planned potable reuse, two forms exist:
- Direct potable reuse, in which highly treated wastewater is introduced immediately after treatment into a community?s water supply system.
- Indirect potable reuse (IPR), in which treated wastewater is introduced into an environmental buffer (e.g., a groundwater aquifer or surface water reservoir, lake, or river) for a period of time before the blended water is introduced into a water supply system.
How Direct Potable Reuse Can Be Implemented
There are two forms of DPR in use today. One involves advanced treated water (ATW) and the other involves finished water.
ATW (see right) 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 projects in the United States 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 (see right). Although a finished water DPR project has been in operation at Windhoek, Namibia since 1967, the production of finished water is not the focus of the Framework document.
Purpose of Framework
A number of communities and utilities throughout the United States are interested in implementing DPR. To ensure that these projects are designed to be reliable source of drinking water that is protective of public health, utility leaders and regulators need to understand the regulatory and operational components that must be part of a DPR program. While such a program will have similarities to 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 man of these important issues with important 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
Of particular concern to utility managers and other high-level decision makers are issues regarding the training and certification of operators and staff at an advanced treatment facility as well as the overall cost and economics of such a facility.
Operational and Management Hurdles for DPR
Current operations at advanced water treatment facilities (AWTFs) utilize processes including microfiltration, reverse osmosis, ozone, and biological filtration in combinations that are not commonly found in typical wastewater and drinking water treatment facilities. Such facilities will need to have qualified operators and staff to ensure the protection of public health. While current training and certification programs for wastewater and drinking water operators contain many elements that will be necessary for operators in an AWTF used for DPR, they are currently insufficient to meet the full needs of such a facility.
The DPR Framework report introduces many of the issues that facility managers will need to confront at the various stages in facility development. The Framework document details the facility operations and maintenance needs through the facility startup and commissioning phases including the initial training of operators, the testing and confirmation of unit processes, and the final commissioning process. In addition, potential challenges during the first year of operations are outlined to help ensure new facilities have the necessary information they need to operate their facility during that crucial time period.
In addition to the DPR Framework report, WateReuse has several other active research projects investigating issues in the operations and maintenance of AWTF facilities. The ongoing project, ?Development of Operation and Maintenance Plan and Training and Certification Framework for Direct Potable Reuse (DPR) Systems? is developing a framework for a potential certification program for DPR system operators. In addition, an upcoming project will develop curriculum and training materials for DPR system operators. All of this is being in coordination with other efforts in California and elsewhere looking at developing certification programs and training materials for DPR system operators. As the adoption of potable reuse and DPR continues to expand, WateReuse plans on being at the forefront of this important issue.
Economics of DPR
A common question regarding DPR is how much it costs compared to other water supply options. Exact costs are difficult to generalize, but when compared to options like desalination DPR compares favorably and is also competitive with other water supply options including 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-ft of water. This total cost is comprised of the cost of treatment, the cost of distribution, and the potential cost of brine disposal.
For the cost of treatment, an excellent model to use is the Groundwater Replenishment System (GWRS) in Orange County, Calif. The GWRS has been operational since 2008 using a treatment train of microfiltration followed by reverse osmosis and UV advanced oxidation with hydrogen peroxide with operating costs of approximately $700 per acre-ft. This value can be used as a rough estimate for the treatment costs of other similar facilities. On the other hand, costs for distribution and brine disposal are site specific based on the distance that water must be conveyed. In general, siting AWTF facilities close to the end use of the advanced treated water (e.g. a DWTF) can dramatically lower costs of distribution. This can be especially true for comparing DPR to IPR where a suitable aquifer or reservoir may be located miles away from an advanced treatment facility. The same is true for brine resulting from reverse osmosis based treatment systems.? Facilities located on a coastline are better suited for brine disposal through an ocean outfall while inland communities may have more costly alternatives.
When determining whether or not to pursue a DPR program, economic and social factors should also be considered alongside the above mentioned financial factors. These factors can include other significant environmental advantages including avoiding issues with seawater intakes for desalination, lower energy costs and associated greenhouse gas emissions compared to other options, decreasing wastewater discharges, and supporting the natural flow of water in some regions by reducing the need for water transfers. Further detail on the economics of DPR and other water supply options can be found 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 document represents a consensus among the Panel, while taking into consideration input from a Project Advisory Committee comprised 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. The report ?Framework for Direct Potable Reuse? is available free of charge at www.watereuse.org.
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.
