Mainstream partial nitritation and anammox sustains competent nitrogen removal capability under recurrent seawater intrusions.

Large amounts of saline municipal wastewater are often produced in global coastal areas due to the application of seawater toilet flushing and/or occasional seawater intrusions into municipal wastewater treatment plants (WWTPs). Mainstream anammox has been rated as the best nitrogen removal technology that enables the upgrade of existing WWTPs from energy-intensive to energy-positive. Application of anammox for saline municipal wastewater treatment is yet to be explored. In this study, we investigated the stability and capability of a granular mainstream partial nitritation and anammox (PN/A) process experiencing the recurrent seawater intrusions. Despite fluctuant salinity and ammonium in the wastewater, PN/A achieved stable nitrogen removal of 150.0±8.0 and 78.5±12.1 mgN/L/d on freshwater and saline municipal wastewater, respectively, producing high-quality effluent of <4.5 mgNH4+-N/L and <6.5 mgTN/L. Activity assays revealed that anammox bacteria, ammonium-oxidizing bacteria, and denitrifying bacteria reserved average activities of 402.8, 291.7, and 93.2 mgN/L/d, respectively, over the recurrent seawater intrusions. 16S rRNA gene amplicon sequencing analysis indicated that Nitrosomonas predominating in flocs responded to ammonium oxidation, and anammox bacteria mainly residing at granules shifted from Brocadia (11.3%) to Kuenenia (20.4%) while saline wastewater was supplied. Notably, an unclassified anammox lineage proliferated in flocs and retained with dominance of 8.2%-30.3%, promoting anammox-based nitrogen removal in suspended phase. Nitrospira propagated to 2.3% in flocs at <0.4 mgO2/L and reserved high nitrite oxidation activities of 90.3±18.2 mgN/L/d over the entire mainstream PN/A operation period. Apart from low-oxygen facilitated suppression of Nitrospira, nitrate-to-nitrite reducing bacteria (NRB), i.e., Quisquiliibacterium, Diaphorobacter, and Aridibacter occupying 1.5%-6.8% in flocs and 0.8%-6.1% in granules, were deemed to offset Nitrospira's activity by reducing nitrate to nitrite so as to underpin the metabolism of anammox bacteria and process stability. Our study certified that the mainstream PN/A process exhibited resilience to the recurrent seawater intrusions and was able to sustain competent nitrogen removal comparable to that of the conventional activated sludge process. Knowledge gleaned from this study underscored the vital role of NRB in reinforcing the PN/A process under operation variables, which is currently being overlooked.