Layered porous media system for reducing nitrogen and phosphorus runoff in stormwater

Overview

This solution addresses urban stormwater pollution by using a multi-layered porous media system designed to interrupt unsaturated water flow and create a perched water aquifer. When fine-textured material is underlain by coarse-textured material, capillary forces prevent water from draining downward, forming a saturated zone at the interface. This anoxic environment promotes biological denitrification, converting dissolved nitrate into harmless dinitrogen gas that escapes to the atmosphere. The system is enhanced with biochar and plant litter to supply carbon and electrons that stimulate microbial denitrification while biochar also adsorbs phosphorus. The technology is being developed in partnership with Atlanta's Stormwater Management Program to reduce nutrient runoff from ten Atlanta stormwater watersheds, directly supporting the goal of protecting water quality in Atlanta's streams.

Technical specifications

System design:

• Bioretention units with 24-inch depth containing multi-layered porous media
• Pore-size decreases from top to bottom in a specific sequence: green (vegetation layer), silt/clay plus biochar, sand, p-gravel, and coarse gravel
• The texture transition from fine to coarse interrupts unsaturated water flow, creating a perched aquifer at the layer interface
• Anoxic conditions within the perched aquifer drive denitrification, converting nitrate to dinitrogen gas
• Plant litter supplies carbon to fuel microbial denitrification
• Biochar provides electrons and adsorbs phosphorus, addressing both nitrogen and phosphorus runoff simultaneously

Validation approach:

• A planned 6-9 month study measuring nutrient retention and removal performance in-facility
• Installation across ten Atlanta stormwater watersheds in collaboration with the city's Stormwater Management Program
• Public outreach through demonstrations to showcase effectiveness

Technology readiness level

This technology is at an early-to-mid stage of development. The scientific basis draws on established principles of unsaturated water flow and capillary dynamics in porous media, combined with well-documented biological denitrification processes. The research team is preparing to validate the approach through a multi-site field study in partnership with Atlanta's Stormwater Management Program. The upcoming 6-9 month study will measure in-facility nutrient retention and removal performance, providing critical data on real-world effectiveness across diverse urban watershed conditions. Successful validation would position the technology for broader deployment in municipal stormwater management applications.