Denitrification performance enhanced by optimizing the sites of external carbon addition in a full-scale anoxic tank.

Abstract

Low influent carbon-to-nitrogen (C/N) ratios often limit denitrification in municipal wastewater treatment systems. This study evaluated denitrification performance in a full-scale anaerobic-anoxic-oxic (AAO) process equipped with a 6 m-deep anoxic tank containing spherical fixed carriers. Sludge flocs and carrier-attached biofilms were sampled at depths of 1 m, 3 m and 5 m along the horizontal flow path. Denitrification kinetics were quantified using batch tests, and microbial community structures were analyzed by 16 S rRNA gene sequencing. Sludge flocs exhibited the highest denitrification rates at 1 m, whereas biofilms performed optimally at 3 m. Along the horizontal direction, sludge flocs near the influent and external carbon dosing site showed enhanced denitrification, while biofilms downstream of the propeller demonstrated improved denitrification. Elevated dissolved oxygen (DO) introduced by internal reflow reduced the effective utilization of the external carbon source. Nitrosomonas was more abundant in sludge flocs, whereas Thauera dominated denitrifying community and peaked at 3 m in biofilms. Based on the spatial distribution of denitrification kinetics and microbial communities, the conventional "pre and top" carbon dosing strategy was re-evaluated, and an optimized "post and top" dosing strategy was proposed. This strategy reduced chemical oxygen demand (COD) consumption per unit of total nitrogen (TN) removed by 16%, providing a practical approach to enhance denitrification efficiency and external carbon utilization in full-scale anoxic tanks.

Keywords

Identifiers

DOI

41636831

PubMed

41636831

Journal

Bioprocess and biosystems engineering

Year

2026