Exogenous Denitrifying Bioaugmentation for Carbon-Efficient Nitrogen Removal in Low C/N Municipal Wastewater

Low carbon-to-nitrogen (C/N) municipal wastewater often limits heterotrophic denitrification and increases external carbon addition. This study evaluated an exogenous denitrifying bioaugmentation (DEN) strategy for improving nitrogen removal per unit of external carbon added. The screened inoculant reached approximately 2.5 × 109 CFU/mL, and maximum nitrate and nitrite reduction rates of 307.42 and 215.42 g N/(m3 h), respectively, were achieved by the selected isolates. Among the tested carbon sources, the highest denitrification rate was obtained with sodium acetate. Across wastewater samples from six municipal wastewater treatment plants (WWTPs), the apparent C/N requirement was reduced from 2.79–9.41 to 1.92–5.20 by DEN addition, corresponding to a 21–55% decrease in chemical oxygen demand (COD) required per unit total nitrogen (TN) removed. During full-scale validation at a 24,000 m3/d WWTP, effluent TN was maintained at approximately 10 mg/L through startup and adaptive maintenance dosing, while specific denitrification rate (SDNR) was increased by approximately 2.3-fold. Activated-sludge community restructuring was indicated by 16S rRNA sequencing, with an increase in the relative abundance of Pseudomonadota from 18.4% to 27.6% after dosing. Based on external carbon-source expenditure, the specific external carbon-source cost per unit TN removed decreased from 0.036 to 0.011 Chinese Yuan (CNY)/(t water·mg TN), corresponding to a 69.4% reduction in this cost indicator. Overall, DEN bioaugmentation was shown to be a practical engineering tool for carbon-efficient nitrogen removal from low-C/N municipal wastewater.