These factors operate synergistically, and temperature plays roles other than simply affecting the kinetics of a reaction. Neither process has sterilization as its main objective, but it is a definite plus when considering these options. Environmental engineers seldom select heat sterilization except for special situations such as hospital wastes. Various life forms differ widely in their resistance to heat. Most pathogenic organisms thrive only within a very narrow temperature range, with the usual optimum at the body temperature of their host.
Bacterial endospores are seedlike structures that endure through hard times such as desiccation.
While some spores are not particularly resistant to heat, many can survive prolonged exposure to elevated temperature. Once in a while, a strain of organisms may contaminate a laboratory because its spores have abnormally high heat resistance and are not killed by the autoclaving procedures that eliminate its competitors. Batch sterilization uses steam or direct firing to elevate the temperature, and then cooling water stops the process and brings the material back toward room temperature. Both the heat and the cooling water are spent with no opportunity for energy recovery. Large volumes should be passed continuously through heat exchangers for energy economy with the hot, treated fluid heating the cold, incoming feed.