Greenhouse gas emissions from vertical and horizontal subsurface flow constructed wetlands in tropical climate

Abstract

In this study, CH4 and N2O fluxes from HSSF and VF constructed wetland operated in tropical climate and the effect of different plant harvesting intervals on CH4 and N2O fluxes from VF constructed wetlands were compared. Results show that the average CH4 fluxe was higher in the HSSF and the average N2O flux was higher in the VF system. Average CH4 fluxes of 7.13 and 4.05 mg CH4-C.m-2.h-1 and average N2O fluxes of 0.13 and 0.20 mg N2O-N.m-2.h-1 were measured from the HSSF and the VF constructed wetland, respectively. CH4 and N2O fluxes in the wet period were lower than the dry period for both systems. CH4 and N2O fluxes dropped during a rainfall event. Inflow TOC and TN were the dominant factors regulating the fluxes. The average TOC and TN concentrations in the influent were 80.2 and 55.9 mg/L, respectively. The study on the effect of plant on VF systems with average influent TOC and TN of 74.5 and 40.0 mg/L, respectively shows that TN removal, effluent DO concentrations, N2O flux and the number of nitrifying and denitrifying bacteria in the system increased but TOC removal and CH4 flux decreased with the presence of plants. Harvesting plants after reaching the final height improved DO concentrations, organic nitrogen and NH3 removal and lower the effluent NO3 concentrations and increases N2O flux. The results suggest that harvesting plants after reaching the final height would increase microbiological processes involving in nitrogen transformation which results in the improvement of nitrogen removal efficiencies but would also increase N2O flux.