In advanced wastewater treatment plants that achieve high levels of nitrogen (N) removal,
up to one-third of the N in effluent is organic, herein referred to as effluent organic N (EON).
While we know that inorganic N is highly labile, it is unclear what fraction of EON is
bioavailable. In this study, we demonstrate the utility of a method that can be used to
examine the reactivity of EON in natural receiving waters to better understand both the
ecosystem response and the potential bioavailability of EON. The technique is suitable for
analyzing polar organic matter in natural waters; electrospray ionization coupled with
Fourier transform mass spectrometry. Bioassays were performed on samples collected at
the end of the biological process from two wastewater treatment plants achieving
advanced N removal. The samples were concentrated, and then added to natural water
samples collected from the oligohaline James River, a major tributary of the Chesapeake
Bay. Our results demonstrate that while the lignin-like fraction of the effluent dissolved
organic matter (some of which contains N) was conserved, a large portion of aliphatic and
aromatic compounds containing N was removed (79e100%) during incubations, while
other compounds were produced. Furthermore, the two effluents exhibited differences in
the degree of degradation and type of degradation, which can be related both to the various
processes employed in the two WWTPs and the dramatic differences in the type of influent
they received. These findings suggest that EON is highly reactive in the natural environment
and that simple assays examining net consumption or production of bulk dissolved
organic N pools are inadequate for assessing the bioavailability of EON.