Assessing Nitrogen Loss by Coupled Nitrification-Denitrification in an Intermittently Drained Floating Treatment Wetland Mesocosm Experiment

Floating treatment wetlands (FTWs) offer a promising approach for removing nitrogen (N) pollution from small to medium-sized water bodies, comparing favorably to traditional land-based constructed wetland remediation practices due to reduced construction and infrastructure requirements. There is a growing consensus that the main mechanism for N removal in FTWs is microbially mediated rather than plant-mediated, so recent focus has been in creating FTWs that stimulate microbial populations involved in N transformations that eventually lead to N loss by denitrification. In this study, a new FTW technology is proposed, called the intermittently drained floating treatment wetland (IDFTW), seeking to mimic the conditions of a riparian wetland environment, which has been shown to be particularly efficient at removing N due to nitrification-denitrification coupling. The IDFTW intermittently fills and drains a bed of light expanded clay aggregate (LECA) via a solar-powered water pump and a passive bell siphon, which leads to alternating high-oxygen and low-oxygen conditions that are theorized to stimulate this coupling. In this 14-week mesocosm experiment, two 340L tanks were spiked with 10 mg/L NH4+ and NO3- and monitored for several water chemistry parameters including pH, DO, and temperature across different components of the IDFTW. Additionally, ON content was measured, striving for a mass-balance approach in accounting for all N inputs and outputs in the system to reach a final value of total N loss. The experimental tank, which included the IDFTW bed showed a 98.2% decrease in NO3- and 100% decrease in NH4+ compared to the respective 24.1% and 95.7% decrease in the control tank. Further analysis shows that these decreases in NO3- and NH4+ were not accounted for by increases in ON. The NO3-, NH4-, and NO2-, concentrations over time show recognizable nitrification curves and the overall loss of nitrogen over time suggests denitrification is the mechanism for N removal.