The target or operating solids retention time (SRT) depends on treatment goals and water temperature.  Nitrifying bacteria are especially sensitive to water temperatures; growth rates slow as temperature decreases and vice versa. Because nitrifiers are so sensitive to temperature, the right SRT for a facility depends a lot on whether or not nitrification is required to meet permit limits. The curve shown in the figure illustrates how washout SRT changes with temperature for nitrifiers.  Cold temperatures will require longer operating SRTs and vice versa.You as an operator will typically not need to “pick” an SRT.  This is commonly done during design.  All you need to do is know what SRT your WWTP was designed around. What SRT was your WWTP designed with?  Tip - If you have well-defined cold and warm seasons, you might have two SRTs, one for each season.