Enhancement effects and pathways of nitrogen removal by plant-based carbon source in integrated vertical flow constructed wetlands

Abstract

Nitrogen removal in integrated vertical flow constructed wetlands (IVCW) is often limited by denitrification processes due to the lack of carbon sources. This study aims to elucidate the enhancement effects and mechanisms of alkali pretreated reeds as plant-based carbon source on the nitrogen removal process of IVCW. The results show near-complete removal of both COD and NH4+-N, with removal efficiencies of circa 98.0%. The alkali pretreated reeds were demonstrated to increase the removal efficiencies of NO3−-N and TN from −15.97% and 53.13% to 95.5% and 93.7%. The plant-based carbon source release organic matter primarily composed of tryptophan-like, fulvic acid-like and humic acid-like substances. Among these substances, the low molecular weight organic matter closely related to the carboxyl functional group showed high bioavailability. 16S rRNA gene sequencing demonstrated the plant-based carbon source to increase the relative abundance of nitrogen removal functional microbial genera up to 24.12%. Consequently, total nitrogen (TN) removal increased notably from 47.77% to 89.10%, primarily driven by a significant increase in denitrification from 0.16 mg/L to 14.11 mg/L. The TN removal was further enhanced by a considerable increase in anaerobic ammonia oxidation from 0.18 mg/L to 1.73 mg/L. The findings could support efficient nitrogen removal in CW via the enhancement of denitrification pathways.