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

Abstract

This study determined the spatial and temporal changes in natural abundance of stable isotopes (δ13C, δ15N, and δ18O) with regard to nitrate (NO3−) and retained sludge in a nitrifying bioreactor. The bioreactor was continuously fed with synthetic wastewater including ammonium for 61 days at 20 °C. After the start-up period of the bioreactor, the NO3− concentration in the effluent gradually increased. The stable isotopes (δ15N and δ18O) of NO3− in the effluent also increased in a phase of incomplete nitrification. The profile experiments showed that the concentration and stable isotopes of NO3− changed simultaneously along the wastewater flow in the bioreactor. The stable isotope analysis revealed that nitrification efficiency seems to be strongly related to the δ15N of NO3−. Moreover, the δ13C and δ15N of the retained sludge drastically changed along the reactor length, from −26‰ to −18‰ and from 5‰ to 30‰, respectively, after 61 days of operation. The isotopic composition of the retained sludge might be affected by the isotope ratios (δ15N and δ18O) of NO3− in the bioreactor. Therefore, the isotope signatures of the retained sludge seem to closely reflect process performance such as nitrification efficiency throughout the operational period. Our findings suggest that the spatial distribution of the isotopic composition of the retained sludge can be used to detect process occurrence within the bioreactor over time.