Characterization of nitrogen transformation and microbial interactions of floc sludge and biofilms in single-stage gas-lift partial nitritation/anammox reactor with hollow cylindrical carriers.

Floc sludge and biofilms are common biomass types used in the partial nitritation/anaerobic ammonium oxidation (anammox) (PN/A) process to treat ammonium wastewater; however, the interactions between these coexisting biomass types for stable nitrogen removal have not yet been well characterized. This study employed single-stage PN/A reactor with hollow cylindrical polypropylene carriers to investigate nitrogen transformation and microbial interactions in floc sludge and biofilms during long-term stable nitrogen removal. Floc sludge primarily drove ammonia oxidation, demonstrating 11.6-fold higher efficiency than biofilms, whereas biofilms mediated the anammox reaction, 3.6-fold more efficiently than floc sludge. Floc sludge and biofilms contributed 18.4 % and 65.9 % to nitrogen removal, respectively. Ammonia-oxidizing bacteria dominated the floc sludge (Nitrosomonas) and cooperated with anammox biofilms (Candidatus Kuenenia) to enable stable nitrogen removal. These findings deepen understanding of nitrogen transformation and microbial interactions across different biomass types and facilitate the optimization of PN/A efficiency through flexible control strategies.