Treated municipal wastewater often contains nitrogen, which has been linked to algal blooms that can devastate coastal ecosystems. In a recent study, researchers characterized the primary nitrogen-removal pathways in a horizontal levee, an engineered subsurface water-treatment system consisting of a gently sloping strip of land adjacent to storm-control levees. Treated wastewater is discharged through a perforated pipe buried at the top of the slope.
“Historically, water bodies were surrounded by wetlands that functioned as ‘nature’s kidneys’ and would remove many contaminants,” said Aidan Cecchetti, a water resource control engineer with the California State Water Resources Control Board and the study’s first author. “But in places like San Francisco Bay, many of these wetlands have been replaced with urban areas. The system that we studied acts like the historical wetlands by removing these contaminants.”
The study was conducted at a demonstration-scale horizontal levee project adjacent to the Oro Loma Sanitary District wastewater treatment plant in San Lorenzo, California. The system’s layered design includes topsoil layers of fine sediments that support plant growth and lower layers of coarse granular media to promote the flow of wastewater, amended with organic carbon to promote microbial activity.
A variety of methods were used to analyze water, biomass, and soil samples taken over two years. At the X-Ray Fluorescence Microprobe (XFM) Beamline 10.3.2 of the Advanced Light Source, the researchers used microfocused x-ray fluorescence (µXRF) mapping and iron K-edge x-ray absorption near-edge structure (µXANES) spectroscopy to understand the distribution of mineral groups—notably sulfides—in the samples.
The results showed that sulfide-driven denitrification by microbes—a process less sensitive to temperature than other forms of denitrification—became more dominant during winter, when decreased treatment efficiency is often observed in natural treatment systems. Year round, over 95% of the applied nitrogen was removed within the first couple of meters of the subsurface.
As part of regional nutrient-management strategies in San Francisco Bay, horizontal levees are currently being considered and constructed to both improve water quality and mitigate the effects of climate change. With a better understanding of the nitrogen cycle in these systems, designers can maximize nitrogen removal, reducing the likelihood of algal blooms in San Francisco Bay.
A.R, Cecchetti, A.N. Stiegler, E.A. Gonthier, S.R.S. Bandaru, S.C. Fakra, L. Alvarez-Cohen, and D.L. Sedlak, “The Fate of Dissolved Nitrogen in a Horizontal Levee: Seasonal Fluctuations in Nitrate Removal Processes,” Environ. Sci. Technol. 56, 2770 (2022), doi:10.1021/acs.est.1c07512.