Ultralow-Greenhouse Gas Urban Stormwater Denitrification through Engineered Bioretention

Impervious surfaces, covering approximately 60% of urban land, generate stormwater runoff that contributes roughly 6% of riverine nitrogen. Existing bioretention systems face a denitrification-methane trade-off due to an imbalance between labile organic carbon and nitrate availability following nitrate depletion during stormwater retention. A 240-day monitoring campaign quantified this trade-off, revealing methane emissions of 1993.7 mg CO2-eq·m-2·h-1 at 98.5% denitrification efficiency. Metabolic analyses identified formate as a key intermediate linking nitrate depletion to methanogenesis. Temporal coupling between formate accumulation and hydrogenotrophic methane production generated characteristic oxidation-reduction potential (ORP) and pH signatures, providing a mechanistic basis for adaptive process control. Combined microbial, isotopic, and redox evidence suggests immediate drainage disrupts reducing conditions, redirecting electron flux from methane production and limiting formate utilization. Based on these findings, we developed Stormwater Runoff Adaptive Denitrification (SRAD), an ORP/pH-triggered immediate drainage strategy that maintained 97.7% denitrification efficiency and reduced methane flux by 96.3% over 90 days. Simulations across Beijing's approximately 2336 km2 urban area indicate SRAD could annually reduce 4.2 × 104 t nitrogen and 7.2 × 105 t CO2-eq.