Performance of one-stage autotrophic nitrogen removal in a biofilm reactor with low C/N ratio

Abstract

Wastewater with C/N ratios ranging from 1.00 to 0.33 caused by a gradual increase in influent concentration was used to evaluate the performance of the one-stage nitrogen removal process in a biofilm reactor. The system was operated for 197 days under chemical oxygen demand (COD) concentration of 250 mg L−1 and influent concentrations ranging from 250 to 750 mg L−1. The effects of the C/N ratio and dissolved oxygen (DO) on nitrogen removal were evaluated at different influent C/N ratios and DO concentrations, respectively. The microbial composition of the system was examined by scanning electron microscopy and polymerase chain reaction-denaturing gradient gel electrophoresis, and the relative contribution of anaerobic ammonium oxidation (ANAMMOX) to nitrogen removal was assessed by calculating the average rates of ANAMMOX and denitrification in batch experiments. Results showed that the removal efficiencies of total nitrogen (TN), and COD were 74–97%, 75–99% and 64–97%, respectively. The C/N ratio had a significant influence on nitrogen removal efficiency when it was decreased from 1.00 to 0.70, but no significant change was observed when it was reduced from 0.70 to 0.33. DO also correlated with the concentration in the influent, and 3.0 mg L−1 was found to be a suitable concentration for the influent concentration of 450 ± 5 mg L−1. Analysis of microbial composition of the system revealed that biofilm and activated sludge were mainly composed of aerobic ammonium-oxidizing bacteria, anaerobic ammonium-oxidizing bacteria (AnAOB) and denitrifying bacteria. Activity tests suggested that AnAOB played an important role in the one-stage autotrophic nitrogen removal process, contributing to about 52.7% of total TN removal via ANAMMOX.