Watershed-based Nutrient Management: Bay Area Update on Treatment and By Other Means Strategies

Problem Recent algal blooms in the Bay (2022 and 2023) have accelerated the need to develop nutrient management strategies across San Francisco Bay (Bay; emphasis on total inorganic nitrogen). This session will discuss the watershed-based collaborative approach that began over a decade ago and how the efforts are informing nutrient management decision-making. Goals and Objectives The session will begin with an update on the regional nutrient watershed permit (third permit), followed by an overview of findings from Regional Watershed Permits (WSPs) 1.0 (R2-2014-0014; HDR, 2018) and 2.0 (R2-2019-0017; HDR, 2023), and the corresponding menu of nutrient management options to inform decision-making. Furthermore, the session will address the importance of regional collaboration in advancing the nutrient management strategies. Introduction The Bay Area Clean Water Agencies (BACWA), working on behalf of the 37 municipal WRRFs discharging to the Bay, has been leading studies for more than 10 years to advance a watershed-based nutrient management strategy. The vision has been to use science to evaluate the health of the Bay ecosystem coupled with strategies for nutrient management at WRRFs (both inside and outside the treatment plant fenceline). Recent algal blooms in August 2022 and 2023 have accelerated the need to develop such a strategy as nutrient limits are looming in 2024 (anticipated adoption by year 2034). Specifically, the focus has been on managing total inorganic nitrogen (TIN). Approach The vision since the initial WSP in 2014 has been to develop a regional nutrient management strategy based on baywide load caps. As such, two separate Regional Watershed Permits (WSPs) were developed that focused on solutions at the treatment plant (WSP 1.0 in 2014) and by other means that focused on recycled water and nature-based solutions (WSP 2.0 in 2019). The results from each WSP menu of nutrient management options are in the process of being used to inform baywide decision making. The menu includes economics for improvements at each WRRF (optimization, sidestream, and upgrades), water recycling, and nature-based solutions. To date, the analyses has been limited to economics. By the end of the 1st quarter 2024, BACWA is including non-economic parameters as a means to effectively compare and contrast both solutions at the WRRFs and beyond (reuse and nature-based solutions (NbS)). Results and Discussion The results from WSP 1.0 are presented in Table 1. Optimization and sidestream concepts collectively are approximately $1B across the bay and it would translate to a combined total of 26 percent TIN reduction. The costs increase exponentially to greater than $10B with either conventional nutrient removal (15 mg N/L) or enhanced nutrient removal (6 mg N/L). Given the relatively high costs associated with WSP 1.0 which would be burdened by ratepayers, BACWA and its members moved forward with WSP 2.0 that considered recycled water and NbS. The overall recycled water findings are presented in Figure 1. BACWA members currently recycle approximately 14 percent of their dry season volumes to recycled water (data not shown). Figure 1 presents that the potential increase in volumes associated with reuse over time. There is the potential to triple reuse volumes over the next 25 years. This increase is predicated on numerous projects advancing beyond master planning, conceptual, and brokering of agreements. As such, confidence levels were applied to each project as illustrated in Figure 1 legend (referred to as confidence levels 1 through 4). The majority of the recycled water projects would divert nutrient loads away from the Bay. However, a significant portion of the TIN loads associated with planned potable reuse projects would end up in the Bay (listed as confidence level 3 and 4 projects). The analysis considers such challenges, as evidenced in a decline in the extent of TIN load reduction after year 2035. The TIN load reductions associated with reuse have the potential to double over the next 25 years compared to a tripling while evaluating water volume. The costs associated with all the listed reuse projects in Figure 1 is approximately $6B. While less expensive than the upgrades at each WRRF (refer to Table 1), this is relatively expensive for nutrient management. However, the other benefits associated with recycled water projects need to be considered while comparing against strategies at each WRRF. The NbS findings are not included in this proposal as it was limited to three WRRFs and it had marginal TIN load reduction. The session will conclude with a comparison (both economic and non-economic) of the nutrient reduction potential for all six nutrient management strategies: i) optimization, ii) sidestream, iii) upgrades (15 mg N/L), iv) upgrades (6 mg N/L), v) reuse projects, and vi) NbS. As previously noted, the non-economic analyses are in process and due the first quarter of 2024 (permit scheduled for April 2024). As such, the economic and non-economic evaluation will be included with the conference proceedings and session. Conclusion and Next Steps This watershed-based approach that considers science along with development of a menu of nutrient management options has the potential to serve as a template for other watersheds managing nutrients (regardless of the nutrient type).