Balancing long-term enriched partial denitrifying bacteria and anammox bacteria for carbon-neutral mainstream nitrogen removal

Anaerobic ammonium oxidation (anammox)-based process has become a method for achieving carbon-neutral wastewater treatment. However, in mainstream wastewater with a low-strength ammonium, obtaining partial nitritation (PN) for the anammox process can be challenging and often result in NO₃^– accumulation. The recently proposed partial denitrification (PD), which reduces NO₃^– back to NO₂^–, can provide NO₂^– for anammox. For a successful PD-anammox (PD/A), it is crucial to efficiently cultivate PD bacteria (PDNB) and maintain a balance between the activities of PDNB and anammox bacteria (AnAOB). In this study, an efficient PDNB enrichment was cultivated for a long period of 400 days in a sequential batch reactor (SBR) by feeding it with acetate (300 mg COD/L) and nitrate (100 mg NO₃^–-N/L) at an exchange ratio of 50 %. The nitrite accumulation efficiency (NAE) gradually increased to >90 %, with Thauera phenylacetica identified as the key species for achieving high NAE. When PDNB was applied with AnAOB to remove 50 mg NH₄⁺-N/L and 50 mg NO₃^–-N/L, different total nitrogen (TN) removal efficiencies were observed depending on the mixing ratio of PDNB and AnAOB (1:5–1:20). The most rapid and complete TN removal was achieved at the mixing ratio of 1:15. At lower mixing ratios of 1:5 and 1:10, PDNB activity exceeded that of AnAOB, resulting in incomplete TN removal. Conversely, at a higher inoculation ratio of 1:20, AnAOB activity surpassed of PDNB, leading to delayed T-N removal. These results indicate that maintaining a balance between NO₂^– accumulation by PDNB and NO₂^– consumption by AnAOB is essential for successful PD/A process operation. The findings of this study can be utilized as fundamental data for the operational strategy of an anammox-based process to achieve a carbon-neutral wastewater treatment.