Use of the nitrogen/ oxygen isotope fractionation technique for tracing nitrogen removal from activated sludge in municipal wastewater treatment plants

The dual isotope fractionation technique of nitrogen and oxygen has been successfully applied to analyze microbially-driven nitrogen transformations and partitioning in aquatic ecosystems, but not in municipal water treatment facilities. Nitrogen removal efficiency in wastewater treatment plants using activated sludge is critical and difficult to monitor. Here, we tested the nitrogen and oxygen isotope fractionation technique for capacity to trace nitrogen removal from municipal wastewater. Long-term monitoring of Anaerobic/anoxic/aerobic (A2/O) process municipal wastewater treatment plants revealed that nitrogen removal performance gradually improved with increasing seasonal temperatures. The δ15N value of activated sludge increased from 5.18‰ (January) to 6.37‰ (June). An increase in temperature promoted the increase in NO3--N from 1.23 mg/L (January) to 6.06~7.02 mg/L (June to August) in the aerobic reaction. The δ15NNO3 decreased from 17.82~37.37‰ to 4.09~17.68‰, and variability in utilization of light and heavy isotopes gradually decreased. The slope of the ratio δ15NNO3:δ18ONO3 gradually decreased from 21.58±12.15 (January) to 0.57±0.51 (August). The dynamic response of δ15N values in sludge and isotopes in water revealed the migration transformation of nitrogen and realized the differential analysis to denitrification performance with seasonality. It also provided a technical basis for long-term stable nitrogen removal system.