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The partial nitritation (PN)–anammox (PN/A) process offers a sustainable alternative to nitrogen management in wastewater treatment, addressing the high costs and increasing the low eco-friendliness associated with traditional nitrification/denitrification processes. Stable partial nitritation (PN) is critical for effective PN/A operation, and this study specifically focused on the need to suppress nitrite-oxidizing bacteria (NOB) to facilitate the enrichment of ammonia-oxidizing bacteria (AOB). Utilizing two sequencing batch reactors (SBRs), PN1 and PN2 with different free ammonia (FA) concentrations, this study aimed to evaluate the NOB suppression strategy while enriching AOB. The PN2 reactor, which operated with a higher initial FA concentration (50 mg/L), successfully maintained high nitritation activity, with 96.1% ammonium removal efficiency (ARE) and 95.1% nitrite accumulation efficiency (NAE) at reduced influent NH4+-N concentrations (50 mg NH4+-N/L, FA 10 mg/L). In contrast, PN1 showed inadequate NOB suppression due to lower FA concentrations (10 mg/L). These results suggest that initiating the nitritation process with higher FA concentrations can effectively suppress NOB, enhancing the stability and efficiency of PN/A processes in mainstream applications.

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.