Increased salinity improves the thermotolerance of mesophilic nitrification.

Nitrification is a well-studied and established process to treat ammonia in wastewater. Although thermophilic nitrification could avoid cooling costs for the treatment of warm wastewaters, applications above 40 °C remain a significant challenge. This study tested the effect of salinity on the thermotolerance of mesophilic nitrifying sludge (34 °C). In batch tests, 5 g NaCl L(-1) increased the activity of aerobic ammonia-oxidizing bacteria (AerAOB) by 20-21 % at 40 and 45 °C. For nitrite-oxidizing bacteria (NOB), the activity remained unaltered at 40 °C, yet decreased by 83 % at 45 °C. In a subsequent long-term continuous reactor test, temperature was increased from 34 to 40, 42.5, 45, 47.5 and 50 °C. The AerAOB activity showed 65 and 37 % higher immediate resilience in the salt reactor (7.5 g NaCl L(-1)) for the first two temperature transitions and lost activity from 45 °C onwards. NOB activity, in contrast to the batch tests, was 37 and 21 % more resilient in the salt reactor for the first two transitions, while no difference was observed for the third temperature transition. The control reactor lost NOB activity at 47.5 °C, while the salt reactor only lost activity at 50 °C. Overall, this study demonstrates salt amendment as a tool for a more efficient temperature transition for mesophilic sludge (34 °C) and eventually higher nitrification temperatures.