Characteristics of N2O generation within the internal micro-environment of activated sludge flocs under different dissolved oxygen concentrations

Abstract

Nitrous oxide (N2O) is a strong greenhouse gas that is produced in significant quantities through biological nitrogen removal processes in wastewater treatment plants; however, N2O generation within the internal micro-environment of activated sludge flocs (ASFs) is poorly understood. In this study, microelectrodes and molecular techniques were employed to investigate the concentrations of N2O and other chemicals and the composition and distribution of microbes within ASFs, respectively. The results showed that N2O generation was correlated with the ASF micro-environment, and was significantly influenced by the dissolved oxygen (DO) concentration of the bulk wastewater. Equal N2O, DO, NH4+–N, and NO3−–N concentrations were found in small flocs (<100 μm). By contrast, higher N2O generation rates and lower DO, NH4+–N, and NO3−–N concentrations were detected in the center of large flocs (>200 μm) compared with those at their surfaces. Microbial structures of varying particle sizes were distinct and depended on the micro-environmental characteristics.