Can biochar application improve nitrogen removal in constructed wetlands for treating anaerobically-digested swine wastewater?

In this study, aerated constructed wetlands (CWs) modified with biochar were used to treat anaerobically-digested swine wastewater. Nitrogen removal performance and mechanisms were comparatively investigated by exploring the dynamic transformation of dissolved organic matter (DOM) in water. It was demonstrated that removal efficiencies of chemical oxygen demand (COD) and ammonium nitrogen (NH₄⁺-N) were 63.06–77.18% and 87.19–96.54% in aerated CWs with and without biochar application under various influent strengths. However, total nitrogen (TN) removal could not be enhanced in aerated CWs with biochar application (30.92–40.12%) compared that in conventional aerated CWs (40.83–48.70%), which might be related to insufficient carbon sources and lower pH values in systems. More average N₂O emission fluxes (185.43–443.33 μg m⁻² h⁻¹) were detected in aerated CWs with biochar application at higher influent strengths. Humic-like substances were identified as major components of DOM in water by using fluorescence excitation-emission-matrix combined with parallel factor analysis (EEM-PARAFAC), which could result in insufficient carbon source for denitrification due to the refractory characteristics. The results suggested that the high-temperature biochar may be inappropriate to effectively increase anaerobically-digested swine wastewater treatment performance in CWs under intermittent aeration operations.