Evaluation of stable isotope ratios (δ15N and δ18O) of nitrate in advanced sewage treatment processes: Isotopic signature in four process types

Abstract

Stable isotope ratios of nitrate are a powerful tool to evaluate aquatic environment stress from treated and untreated sewage. However, there is generally a lack of knowledge on the change in stable isotope ratios within wastewater treatment plants. We investigated nitrogen and oxygen stable isotope ratios (δ¹⁵N and δ¹⁸O) of nitrate in four types of advanced treatment processes operated in parallel; (A) extended aeration activated sludge, (B) anaerobic-anoxic-aerobic (A²O), (C) recycled nitrification-denitrification, and (D) modified Bardenpho. The results exhibited spatial variation of δ¹⁵N and δ¹⁸O for nitrate within the treatment steps. The changes in δ¹⁵N and δ¹⁸O may result from the reactor conditions (aerobic, anoxic, and anaerobic) and the order of these processes. As decreasing nitrate concentration in the anoxic stages, the δ¹⁵N/δ¹⁸O ratio for nitrate increased at a rate of 1.3 to 1.6 coupling with the reduction in the nitrate concentration in the anoxic stages. The δ¹⁵N and δ¹⁸O signatures were attributed to process performance in regard to nitrogen removal. In particular, the modified Bardenpho process has higher nitrogen removal efficiency over other processes, producing effluent with lower nitrate concentration and higher stable isotopes (δ¹⁵N: 23.6 to 25.5‰, δ¹⁸O: 2.8 to 4.5‰). We concluded that the stable isotope signatures mirrored the treatment efficiency and effluent characteristics.

Keywords: Activated sludge; Denitrification; Nitrate; Nitrification; Nitrogen stable isotope ratio (δ(15)N); Oxygen stable isotope ratio (δ(18)O).