Shortening sludge retention time promotes the competitive advantage of heterotrophic nitrification and aerobic denitrification bacteria in the halophilic aerobic granular sludge treating saline wastewater.

Heterotrophic nitrification-aerobic denitrification (HN-AD) microorganisms enable simultaneous removal of inorganic nitrogen and organic pollutants in a single reactor. Sludge retention time (SRT) is a critical operational parameter shaping microbial community dynamics and functional competition. However, the ecological mechanisms underlying the succession of HN-AD bacteria under different SRT regimes remain poorly understood. This study evaluates how the HN-AD bacteria compete with autotrophic ammonium oxidizing bacteria under different SRTs in a halophilic aerobic granular sludge (HAGS) system. When the SRT increased from 10 to 20 days, the total inorganic nitrogen removal efficiency decreased from 100 % to 51.3 %, with the proportion of heterotrophic bacterial biomass reduced from 80.1 % to 47.7 %. Paracoccus was the main HN-AD bacteria, and its relative abundance decreased from 65.8 % to 55.5 %. A significant reduction of napA gene was also found in this study. Results showed that short SRT conditions ensure that HN-AD bacteria an advantage in niche competition, assisting the maintenance of HN-AD function during long-term operation. By unraveling the SRT-dependent dominance of HN-AD bacteria in HAGS, this work offers a cost-effective approach to improve nitrogen and organic matter removal efficiency in saline wastewater, which holds significant potential for scaling up in industrial saline wastewater treatment.