Co-occurrence and cooperation between comammox and anammox bacteria in a full-scale attached growth municipal wastewater treatment process

Cooperation between comammox and anammox bacteria for nitrogen removal has been recently reported in laboratory-scale systems including synthetic community construct; however, there are no reports of full-scale municipal wastewater treatment systems with such cooperation. Here, we report intrinsic and extant kinetics as well as genome-resolved community characterization of a full-scale integrated fixed film activated sludge (IFAS) system where comammox and anammox bacteria co-occur and appear to drive nitrogen loss. Intrinsic batch kinetic assays indicated that majority of the aerobic ammonia oxidation was driven by comammox bacteria (1.75 ± 0.08 mg-N/g TS-h) in the attached growth phase with minimal contribution by ammonia oxidizing bacteria. Interestingly, a portion of total inorganic nitrogen (∼8%) was consistently lost during these aerobic assays. Aerobic nitrite oxidation assays eliminated the possibility of denitrification as a cause of nitrogen loss, while anaerobic ammonia oxidation assays resulted in rates consistent with anammox stoichiometry. Full-scale experiments at different dissolved oxygen (DO = 2-6 mg/L) set points indicated persistent nitrogen loss that was partly sensitive to DO concentrations. Genome-resolved metagenomics confirmed high abundance (relative abundance 6.53 ± 0.34%) of two Brocadia-like anammox populations while comammox bacteria within the Ca. Nitrospira nitrosa cluster were lower in abundance (0.37% ± 0.03%) and Nitrosomonas-like ammonia oxidizers even lower (0.12% ± 0.02%). Collectively, our study reports for the first time the co-occurrence and co-operation of comammox and anammox bacteria in a full-scale municipal wastewater treatment system. Synopsis Comammox and anammox cooperation resulted in dissolved oxygen concentration dependent nitrogen loss in municipal wastewater treatment system.