Dynamic variation in nitrogen removal of constructed wetlands modified by biochar for treating secondary livestock effluent under varying oxygen supplying conditions.

Vertical flow constructed wetlands are facing the problem of low removal efficiency when treating high-load wastewater. This study explored the effect of oxygen supplement (1.2, 1.0, 0.8 L min-1 for 6 h; 1.0 L min-1 for 4 h and 2 h) on enhancing nitrogen removal in vertical flow constructed wetlands modified by adding biochar for treating secondary livestock effluent. The results indicated that biochar addition could enhance contaminant removal due to its adsorption of contaminants under the condition of no aeration. Additionally, the aeration addition of 1.0 L min-1 for 2 h per day in biochar-based constructed wetlands achieved the high efficiencies for chemical oxygen demand (95%) and total nitrogen (73%) removal. However, total nitrogen removal efficiency in biochar-based constructed wetlands declined under excessive dissolved oxygen conditions which might restrain the denitrification process. Average nitrous oxide emission fluxes in biochar-based constructed wetlands (575-1877 μg m-2 h-1) were lower than those in non-biochar constructed wetlands (745-2298 μg m-2 h-1). In addition, the variation of N2O emission under different aeration conditions was similar with the maximum value at the aeration condition of 1.2 L min-1 for 6 h d-1. These results could be useful for improving the sustainable design and operation of constructed wetlands for high-load wastewater treatments.