Achieving stable advanced nitrogen removal from mainstream municipal wastewater using integrated strategies in partial nitrification anammox (PNA) granular hybrid system

Partial nitrification anammox (PNA) is a promising technology for energy saving and nitrogen removal. The bottlenecks limiting the widespread application of PNA in mainstream municipal wastewater treatment include the difficulty in stably suppressing nitrite-oxidizing bacteria (NOB) and failure to meet discharge standards for total nitrogen (TN) due to high nitrate concentration in the effluent. To this end, this study aims to stably suppress NOB using integrated strategies (including the inoculated activated sludge pretreatment with FNA, low DO, residual ammonia nitrogen, and floc sludge discharge) while also investigating the feasibility and performance of coupling PNA with endogenous partial denitrification/anammox (EPDA) in a PNA granular hybrid system. During the 300 days of operation, the integrated strategies effectively stabilized the suppression of NOB, decreasing its abundance from 1.22 % to 0.55 %, and the final activity was reduced to 0.36 ± 0.12 mg-N/L/h. Meanwhile, the relative abundance of AnAOB as the dominant microbial community increased from 9.16 % to 24 %, with its activity reaching 2.84 ± 0.16 mg-N/L/h, limiting the supply of nitrite and ensuring the stable suppression of NOB. Typical cycle tests showed that EPDA and PNA were successfully coupled during the aeration stage with a nitrogen removal contribution rate of 80.55 %, ultimately achieving the average effluent TN and PO3- 4-P were 4.22 mg/L and 0.09 mg/L, respectively. The metagenomic and metatranscriptomic sequencing results revealed that anammox bacteria (AnAOB) coexisted with ammonia-oxidizing bacteria (AOB), NOB, and endogenous denitrifying bacteria (EDB), and AnAOB played a primary role in the nitrogen removal process of mainstream PNA. This study brings mainstream PNA closer to widespread application and provides significant technical support for its engineering implementation.