Performance and microbial adaptation of a continuous granular sludge-type nitritation process facing the decreasing temperature condition until 5 ℃

The nitritation process, a critical prerequisite for energy-efficient anammox-based nitrogen removal, often encounters operational instability in low-temperature treatment. The feasibility of nitritation at an ultralow temperature of 5.0 ℃ using a continuous-flow aerobic granular sludge (CAGS) reactor, thereby challenging conventional temperature limitations. The reactor was operated under a fixed ammonium load (250.0 mg/L) and hydraulic retention time (12.0 h), maintaining robust performance during a stepwise temperature reduction from 25.0 to 5.0 ℃. At the finally ultralow temperature of 5.0 ℃, stable nitritation was achieved, with the average nitrite production rate of 0.29 kg/m³/d and nitrite production efficiency of 59.77 %. As the temperature decreased, the specific activity of ammonium-oxidizing bacteria (AOB) was partially inhibited, and the granular sludge exhibited a tendency to evolve into smaller sizes with a gradual reduction in calcium content, while still functioning as a temperature buffer. Genus Nitrosomonas was identified as the sole AOB genus at 5.0 ℃. Throughout the study, Nitrite-oxidizing bacteria were effectively suppressed. Moreover, certain denitrifying bacteria appeared less susceptible to the temperature condition, potentially resulting in nitrate production. The relative abundance of amo genes remained stable, ensuring the stability of nitritation at 5.0 ℃. These findings redefine the operational boundaries of nitritation processes and provide a mechanistic framework for implementing nitrogen removal technologies in cold-climate regions without the need for energy-intensive heating.