Thermodynamic characteristics of nitrifiers reveal the potential NOB inhibition strategies at low temperatures

Nitrification is a highly temperature-sensitive biological process, yet relatively little research has explored thermodynamic characteristics of enzyme-catalyzed nitrification processes in wastewater treatment systems. In this study, a variety of thermodynamic models were employed to evaluate thermodynamic characteristics of four activated sludge samples cultivated under various temperatures (10, 15, and 20°C) and ammonia nitrogen concentration (40 mg/L and 300 mg/L). Results revealed a higher maximum temperature (T_max) for ammonia oxidizing bacteria (AOB) compared to nitrite oxidizing bacteria (NOB), suggesting the robust resistance to high temperatures of AOB. Conversely, the lower minimum temperature (T_min) of NOB indicated its stronger tolerance to low temperatures. Notably, both low temperatures and high-ammonia nitrogen concentrations are conducive to the competition of Nitrotoga over Nitrospira, resulting in Nitrotoga emerging as the dominant NOB within 10∼15°C. Moreover, heat capacity (ΔC_p^ǂ) of enzymes exhibited a similar trend across all samples, i.e. HAO>AMO>NOR, indicating that HAO exhibited the strongest thermodynamic stability, followed by AMO, and NOR has the worst thermodynamic stability. Additionally, the difference in optimal temperature (T_opt) between dominant AOB-Nitrosomonas and dominant NOB-Nitrotoga cultivated at high nitrogen concentration and low temperature could reach up to +8.13°C, enabling selective inactivation of NOR through thermal treatment at T_opt,AOB∼T_opt,NOB. However, despite insignificant differences in T_opt,AOB and T_opt,NOB under low-temperature and low-ammonia nitrogen concentration, the smaller the difference between T_max and T_op (T_max-T_opt) of NOB and smaller D-value of NOR made NOB more susceptible to inhibition than AOB. However, the presence of low-abundance Nitrospira renders thermal treatment alone inadequate for complete NOB inhibition, combing it with other strategies is necessary for effective inhibition. Therefore, the fundamental differences in thermodynamic characteristics of AOB and NOB determine the potential of NOB inhibition strategies at low temperatures.