High nutrient concentrations can cause increased phytoplankton biomass, low dissolved oxygen, and increased harmful algal blooms and toxins, with detrimental effects on species and ecosystems. San Francisco Bay receives high nutrient loads mainly from discharged wastewater, but high turbidity, strong tidal mixing, and abundant filter-feeding clams have kept algal blooms in check. Following the historic algal bloom of 2022, regulators and managers recognize the Bay’s resilience to high nutrient loading is waning and nitrogen concentrations must be managed more proactively.
One potential solution to reducing nutrients discharged in wastewater involves nature-based solutions such as open-water treatment wetlands or horizontal levees. These nature-based options can be an alternative or complement to mechanical nutrient removal strategies. Under the 2019 Nutrient Watershed NPDES Permit, SFEI collaborated with the Bay Area Clean Water Agencies (BACWA) to explore the use of nature-based solutions to reduce nutrients discharged to San Francisco Bay from wastewater treatment plants.
A constraint to developing treatment wetlands is that land in the region is scarce and expensive, increasing the need for collaboration among public agencies capable of sharing land and resources to achieve multiple objectives. NbS can be designed to provide additional benefits beyond nutrient reduction, including wildlife habitat, flood risk management, and recreational opportunities.
In the first phase of work, we used a GIS suitability model to identify potential locations for nature-based solutions at the 37 treatment plants discharging to San Francisco Bay. In the second phase, we collaborated with staff at eight facilities to develop preliminary treatment wetland concepts. In the final stage, we collaborated with HDR to develop early concept designs and cost estimates for three sites (Delta Diablo, Fairfield-Suisun Sewer District, and San Jose-Santa Clara Regional Wastewater Facility).
SFEI also produced two supplemental reports to support the implementation of NbS at wastewater treatment plants: (1) an exploration of the potential to use equalization basins as dual-purpose treatment wetlands (funded by the UC Berkeley Center for Green Chemistry), and (2) a set of case studies on lessons learned from existing treatment wetlands in the San Francisco Bay region. These case studies demonstrate both the promise of nature-based solutions as a long term water quality improvement strategy, as well as some of the pitfalls to avoid in the design process.
