Temperature affects the activity of the bacteria present in a system. Below certain temperatures, bacteria may slow or reduce activity. Above certain temperatures, bacteria can die and cease nitrification or denitrification or other treatment of wastewater.

Optimal temperatures for bacterial activity range from 25 to 35 degrees Celsius. Aerobic digestion and nitrification stop above 50 degrees Celsius. At 5 degrees Celsius or lower, nitrifying bacteria practically cease functioning.