In-Stream Microbial Denitrification Potential at Wastewater Treatment Plant Discharge Sites

Reactive nitrogen loading from municipal sewage discharge provides point sources of nitrate (NO₃^-) to rivers and streams. Through microbially-mediated denitrification, NO₃^- can be converted to dinitrogen (N₂) and nitrous oxide (N₂O) gases, which are released to the atmosphere. Preliminary observations made throughout summer 2011 near a wastewater treatment plant (WWTP) outfall in the Finger Lakes region of New York indicated that NO₃^- concentrations downstream of the discharge pipe were lower relative to upstream concentrations. This suggested that nitrate processing was occurring more rapidly and completely than predicted by current models and that point “sources” can in some cases be point “sinks”.

Molecular assays and stable isotope analyses were combined with laboratory microcosm experiments and water chemistry analyses to better understand the mechanism of nitrate transformation. Nitrite reductase (nirS and nirK) and nitrous oxide reductase (nosZ) genes were detected in water and sediment samples using qPCR. Denitrifcation genes were present attached to stream sediment, in pipe biofilm, and in WWTP discharge water. A comparison of δ18-O and δ15-N signatures also supported the hypothesis that stream NO₃^- had been processed biotically. Results from microcosm experiments indicated that the NO₃^- transformations occur at the sediment-water interface rather than in the water column. In some instances, quantities of denitrification genes were at higher concentrations attached to sediment downstream of the discharge pipe than upstream of the pipe suggesting that the wastewater discharge may be enriching the downstream sediment and could promote in-stream denitrification.