Enhanced Denitrification in Bioretention Using Woodchips as an Organic Carbon Source

Abstract

Bioretention systems fundamentally lack the ability to effectively reduce total nitrogen concentrations from urban stormwater. Column tests were conducted to simulate a bioretention anoxic zone and evaluate the effects of stormwater retention time, limestone addition, and woodchip species, size, and mass percentage on the bioretention denitrification process. Denitrification of an artificial stormwater was demonstrated and appeared to follow first-order kinetics. A 0.8-day average retention time showed the highest nitrate removal percentage of 82.4±0.4%. Willow Oak and Red Maple woodchips resulted in the highest total nitrogen removal efficiencies at 61.9±0.8% and 61.8%, respectively; Wild Cherry, Virginia Pine, and American Beech were less effective. Smaller woodchips and higher woodchip mass percentage corresponded to greater nitrate removal efficiencies, but also higher organic nitrogen leaching. Media containing 4.5% 5-mm Willow Oak woodchips by mass represented optimum conditions. Adding limestone to raise the pH did not increase denitrification rates. Installing a first flush bioretention denitrification zone may represent a way to incorporate the longer retention times needed for denitrification.