Impact of Biochar on Nitrogen Removal and Nitrous Oxide Emission in Aerated Vertical Flow Constructed Wetland

Abstract

In an intermittent aerated vertical flow constructed wetland, the dissolved oxygen (DO) distribution tends to be inhomogeneous because of poor diffusivity resulting in the production and emission of nitrous oxide (N2O). As a multifunctional environmental material with numerous porosities and a large specific area, biochar has been recently applied to enhance pollutant removal and reduce greenhouse gas emissions in traditional wetland systems. Using the conventional aerated vertical flow constructed wetlands (CW) as the comparison, biochar-amended wetland microcosms (SW) were built in greenhouses to investigate the influence of biochar on nitrogen removal and N2O emissions. The results showed that DO concentration in the aeration stage increased by 0.42 mg·L-1 in SW. Furthermore, SW achieved higher removal efficiencies for NH4+-N (99.5%) and total nitrogen (TN; 95.0%) than CW. Similar removal rates of chemical oxygen demand (COD), close to 94%, were observed in CW and SW, indicating that no significant effects resulted from adding biochar (P>0.05). Additionally, N2O emission fluxes of CW and SW were 0.92 mg·(m2·h)-1 and 0.27 mg·(m2·h)-1, respectively. The N2O cumulative emission in SW was 6.58 mg·m-2, which was significantly lower than that in CW (22.03 mg·m-2). Biochar addition could be an effective strategy to promote nitrogen removal and reduce N2O emissions.