Seasonal variations in the reductions of total coliform, fecal coliform, fecal streptococci, and enterovirus densities that occur during conventional aerobic sludge digestion in cold climates were characterized. Also, the potential to improve reductions in the densities of these four groups of enteric microorganisms in cold climates by simple modifications that increase process temperature by reducing heat losses was demonstrated. To obtain this data, two 32 cu m aerobic digesters located at a small municipal wastewater treatment plant were operated continuously over a period of twenty months. One digester was a conventional digester while the other was designed to minimize heat losses, and thus, facilities autoheating. When the results obtained during 11 separate periods of steady-state operation at mean mixed liquor temperatures ranging from 8 to 40 C and at residence times of 10, 15, and 20 days were combined for analysis, it was evident that significant reductions in the densities of the four groups of enteric microorganisms was dependent both residence time and temperature. Using the Arrhenius equation, it was possible to describe mathematically the temperature dependence of the rate of log10 reduction in density of each of these four groups of enteric microorganisms. The four mathematical relationships developed provide a rational basis to determine residence times necessary to achieve desired levels of indicator organism and enterovirus reductions during aerobic sludge digestion at mixed liquor temps. ranging from 8 to 40 C. (Copyright (c) Pergamon Press 1990.)