Seasonal Impacts of Nonpotable Reuse on DPR Feasibility

APPLICABILITY
California's new direct potable reuse (DPR) regulations allow agencies to accurately consider DPR implementation costs and challenges. The City of Napa (City) and the Napa Sanitation District (NapaSan) conducted an evaluation that illustrates key considerations for utilities interested in DPR, and sheds light on strategies that can be used to incorporate DPR into a system that has an existing nonpotable reuse (NPR) program with seasonal demands. Innovative approaches for augmenting wastewater flows with stormwater were evaluated for augmenting DPR and NPR supplies when NPR demands are high, taking advantage of 'one water' principles.

The Napa evaluation compared treatment alternatives for DPR including both raw water augmentation (RWA) and treated water augmentation (TWA). Regulatory requirements for operations and monitoring, impacts on staffing, and project costs were also assessed. More specifically this study evaluated challenges related to discharge permit compliance, seasonal DPR impacts and limitations, supplemental dry weather urban runoff to increase available water for reuse, interagency collaboration and visioning for DPR, and RO concentrate disposal.

NapaSan has a long history of water recycling. In winter when non-potable demand is lower, excess water is discharged to the Napa River. As a result, both agencies were interested in exploring how excess water could be used for DPR supply. However, costs of intermittent DPR treatment operation were expected to be high. Therefore, an assessment of collecting and conveying dry weather urban runoff stormwater to NapaSan to supplement the seasonal supply fluctuations expected at the DPR facility was performed.

While portions of the Napa feasibility study are site-specific, the outcome highlights the broader implications of what elements are necessary and important to consider in the early stages of a DPR project. Attendees will gain an understanding of the overall challenges that arise with DPR project implementation and potential solutions.

DEMONSTRATED RESULTS AND OUTCOMES
Methodology and Results
Sizing the DPR Facility
To size the treatment facility, the study assessed seasonally-available flow at NapaSan after meeting NPR demands. Figure 1 illustrates historical existing river discharge and recycled water flows at NapaSan, and seasonal precipitation as an indicator of the potential for capturing and reusing stormwater runoff.

Dry weather urban runoff and wet weather stormwater flows can be used to augment supplies and increase the percent utilization of the DPR facility. The evaluation considered three different DPR facility sizes. It was determined that the capture and use of large wet weather flows would require cost prohibitive equalization basins in the collection system. However, targeted connections to divert known sources of dry weather urban runoff and small wet weather flows (subject to wastewater collection system availability) could prove useful in increasing the overall supply to the NPR and DPR facility while dampening the temporal variability. Table 1 shows the percent utilization with and without the addition of dry weather urban runoff.

Infrastructure Considerations
For Napa, both TWA and RWA were considered. With TWA, purified water is discharged directly into the potable distribution system (Figure 2). With RWA, purified water is sent to the water treatment plant prior to potable distribution (Figure 3). RWA will require more pipelines than TWA including the pipe used to send water from the DPR facility to the water treatment plant along with the disposal pipelines needed at the DPR facility.

Staffing and Agency Discussions
Potable reuse requires collaboration between water and wastewater agencies. To understand how a future DPR project could be governed, interviews and a workshop were conducted with both agencies. Three governance structures (Contract, Joint Powers Agreement, and Merger) were reviewed and evaluated. Both agencies identified project components of concern (cultural differences, underperformance, and water delivery) and compared these against the proposed governance structures. These discussions allowed both agencies to initiate and work through potential issues that could arise in coordinating a DPR project.


Conclusions/Outcomes
The Napa study illustrates the key elements that should be considered in a DPR feasibility study, with strategies to identify and address potential implementation challenges. The Napa DPR feasibility study takes a holistic approach to considering the project viability and corresponding regulatory, operational, and structural components that make up these projects. By integrating dry weather urban runoff, utilities may be able to effectively augment recycled water supplies and optimize NPR and DPR systems.

RELEVANCE TO AUDIENCE
Utilities and agencies interested in DPR will gain an understanding of the project elements that should be considered at the initial stage of a DPR project, and strategies for augmenting recycled water supplies with stormwater. The presentation will highlight potential challenges and solutions that are applicable beyond the Napa project. DPR projects are in the initial stages of analysis, design, and implementation across California, Texas, Colorado, and Utah. Consequently, as the need for recycled water grows, it will be beneficial to learn from others.