Study of an innovative anaerobic (A)/oxic (O)/anaerobic (A) bioreactor based on denitrification–anammox technology treating low C/N municipal sewage

• Study the A/O/A bioreactor based on den–anammox treating low C/N municipal sewage.
• Study the performance of den–anammox system in low nitrogen concentration.
• Build nitrogen removal model of the A/O/A bioreactor.
• Study nitrogen removal contribution of every approach by mass-balance theory.

In this study, an integrated anaerobic hydrolysis (A)/nitrification (O)/denitrification–anammox (den–anammox) (A) fixed-bed baffled bioreactor was constructed to treat low C/N (COD/NH4+–N) municipal sewage. With the influent COD/NH4+–N of 1.38–5.01, the average TN and COD removal percentages of bioreactor arrived to 89.9% and 91.5% under the room temperature (25 °C). By the innovative design (partial nitrification, den–anammox technology and no sludge returning), the A/O/A bioreactor can theoretically save at least 54.2% of energy consumption (calculated by aeration quantity) compared with traditional nitrification–denitrification process. Under the COD/NO3-–N of 0.94–1.69, the collaborative denitrification of denitrifying bacteria and anammox bacteria was successfully achieved in the Den–Anammox Pool (DAP) of bioreactor. The maximum NH4+–N and TN removal percentages of DAP arrived to 97.6% and 93.8%, respectively. The nitrogen removal contribution of den–anammox process was above 70% by mass-balance theory. DGGE (denaturing gradient gel electrophoresis) and gene sequencing results showed that the microbial community was diverse and the dominant bacteria of DAP changed from single anammox bacterium group into one group containing anammox bacterium and denitrifying bacterium by 175-day domestication. According to FISH (fluorescence in situ hybridization) results, the percentage of anammox bacterium declined from 89% to 58% in DAP.