Sequential urban stormwater treatment using bioretention and photocatalytic advanced oxidation

Overview

Urban stormwater runoff carries a complex mixture of nutrients, organic contaminants, and emerging pollutants that conventional treatment systems struggle to remove efficiently. This proposed solution combines bioretention-based heterotrophic denitrification with solar light-enabled advanced oxidation processes and pump-and-treat technology to deliver a more efficient and stable treatment approach over extended operating periods.

The core innovation is a metal-free graphitic carbon nitride (g-C3N4) photocatalyst suspended in a self-reliant photochemical reactor. By integrating biological nutrient removal with photocatalytic advanced oxidation, the system targets nitrate, phosphate, oxalate, and trace organic contaminants in a single sequential workflow.

Technical specifications

Key features:

• Metal-free photocatalyst: Surface-engineered graphitic carbon nitride nanosheets (g-C3N4) that operate under visible (solar) light, avoiding secondary pollution associated with metal-based catalysts
• Selective nitrate reduction: Reduces nitrate preferentially to N2 gas, avoiding formation of ammonium (NH4+), which is a common drawback of existing catalytic technologies
• Biocompatible formulation: Suitable for integration with biological treatment stages such as heterotrophic denitrification
• Sequential treatment train: Bioretention (biological denitrification) followed by advanced oxidation in a photochemical reactor, supported by pump-and-treat infrastructure
• Custom photochemical reactor: Designed for batch operation at 10 L laboratory scale, with optimization of pH, influent load, nitrogen concentration, hydraulic retention time, and catalyst dosage
• Ion chromatography monitoring: Quantification of NO3-, PO4-, C2O42-, NO2-, and NH4+ throughout treatment using suppressed ion chromatography

How it works:

Stormwater first passes through a bioretention system where heterotrophic denitrifying bacteria convert nitrate to nitrogen gas under controlled conditions. The effluent is then routed to a photochemical reactor containing suspended g-C3N4 photocatalyst, which under solar irradiation drives advanced oxidation of residual organic contaminants and further transformation of nutrients. Operational parameters are tuned to maximize removal efficiency while minimizing unwanted byproducts.

Technology readiness level

This proposal is at an early research stage. The team has prior published work demonstrating mineralization of the hormone EE2 in real hospital effluents using the same catalyst family, which supports feasibility of the photocatalytic component. However, no proof-of-concept data currently exists for the combined bioretention plus photocatalytic sequential system.

Future validation plan:

• Synthesis and surface engineering of the g-C3N4 photocatalyst
• Batch nitrate removal experiments in a custom 10 L photochemical reactor at laboratory scale
• Optimization of operational conditions including pH, influent load, nitrogen concentration, hydraulic retention time, and catalyst concentration using simulated runoff
• Transition to real stormwater samples after laboratory optimization
• Monitoring of NO3-, PO4-, C2O42-, NO2-, and NH4+ concentrations via ion chromatography

The technology readiness level is currently low (TRL 2-3), with laboratory validation of individual components underway and integrated system demonstration planned as the next milestone.