Distribution in natural abundances of stable isotopes of nitrate and retained sludge in a nitrifying bioreactor: Drastic changes in isotopic signatures

•A nitrifying bioreactor was continuously operated for 61 days.•Drastic changes in stable isotope ratios were observed in the bioreactor.•δ15N and δ18O of NO3− changed spatially with wastewater flow.•Sludge δ13C and δ15N spatially increased from −26‰ to −18‰ and from 5‰ to 30‰•Isotopic signature of sludge and nitrate was related to process performance.