Enhanced nitrogen retention and transformation in biochar-amended bioretention media

Overview

Stormwater bioretention systems are widely used to remove pollutants from urban runoff, but conventional media often struggle to achieve consistent nitrogen removal, particularly at low temperatures and short hydraulic residence times. This research demonstrates that amending bioretention media with wood-derived biochar pyrolyzed at elevated temperatures (above 550°C) substantially improves nitrogen retention and transformation.

Field and laboratory results show total nitrogen, ammonium, and nitrate removal rates exceeding 70% with biochar amendment. For example, in cold-weather field trials at 12°C, control media removed 0% nitrate while biochar-amended media removed 30%. Biochar amendment also increased hydraulic residence times from 0.7 to 3.8 hours and improved both infiltration and water retention, supporting healthier plant growth in bioretention installations.

Technical specifications

Key features and mechanisms:

• High-temperature wood-derived biochar (pyrolyzed above 550°C) serves as the active amendment
• Increased media porosity enhances water movement and storage capacity
• Enhanced sorption of ammonium (NH4) through high cation exchange capacity
• Sorption of organic nitrogen, the largest dissolved nitrogen fraction in many stormwater sources, currently under final validation
• Improved denitrification driven by enhanced reducing conditions, longer residence times, and electron storage and transfer on biochar surfaces
• Measured electron storage capacity of biochar, a property uniquely characterized by the research team
• Compatibility with traditional bioretention media formulations meeting North Carolina and Delaware specifications

Performance highlights:

• Greater than 60% nitrogen removal efficiency across tested conditions
• Greater than 70% removal rates for total nitrogen, ammonium, and nitrate in optimized scenarios
• Hydraulic residence time increases of up to five-fold depending on configuration
• Validated in both cool-season (12°C) and warm-season (23°C) field conditions and equivalent laboratory stormwater simulations

Technology readiness level

The technology has been validated through pilot-scale field experiments and controlled laboratory studies across multiple temperature regimes and media formulations. Biochar's ability to enhance ammonium sorption and denitrification has been confirmed, and its impact on hydraulic performance and plant growth has been demonstrated. Remaining validation work focuses on confirming biochar's capacity to sorb organic nitrogen and quantifying nitrogen removal under short hydraulic residence times typical of shallow bioretention installations with high infiltration rates (1–2 GPM/ft²). This final stage of testing will complete the performance picture for practical deployment scenarios.