Widespread but Overlooked DNRA Process in a Full-Scale Simultaneous Partial Nitrification, Anammox, and Denitrification Plant

Similar to denitrification and anaerobic ammonium oxidation (anammox) processes, dissimilatory nitrate reduction to ammonium (DNRA) is widely distributed in natural ecosystems. However, DNRA in wastewater treatment plants (WWTPs) is usually underestimated or fully ignored. Here, a full-scale simultaneous partial nitrification, anammox, and denitrification (SNAD) WWTP was studied, and the composition of nitrogen metabolism microorganisms in each compartment of the WWTP was analyzed. Ammonia oxidizing bacteria, anammox bacteria, nitrite oxidizing bacteria, denitrifying bacteria, and DNRA organisms coexisted in this full-scale WWTP. The relative abundance of the DNRA functional gene (nrfA) was significantly higher (150–301 reads per million reads (RPMR)) than that of the denitrification functional gene (nosZ, 68–97 RPMR). The SNAD process dealing with high NH4+-N and high organic wastewater provided suitable conditions for the growth of DNRA. However, the unintended selection of DNRA in the SNAD process created an engineering inefficiency of nitrogen removal by utilizing carbon that should be otherwise allocated for denitrification. Our results demonstrate a need to innovate and improve low energy nitrogen removal processes, such as SNAD, to better understand the ecology and function of DNRA organisms in engineered systems.