Reduced influent temperatures can have negative effects on the nitrification in wastewater treatment plants. By three nomogramms, the consequences of a permanent temperature decrease on nitrification safety, aerobic sludge retention time and total nitrogen removal efficiency under steady state conditions are demonstrated. By simulations with a dynamic model, a quantitative relationship between wastewater temperature and ammonium effluent concentration was established. In combination with measured effluent concentrations, this relationship can be used to predict the increase of the ammonium effluent concentration for reduced wastewater temperatures.