Aerobic digesters in small package-type wastewater treatment plants have been studied to provide better operational control at temperatures lower than 20°C. Investigations at treatment plants in British Columbia, Canada have indicated that the SRT must be increased as operating temperatures decrease to ensure acceptable volatile solids reductions (Koers and Mavinic, 1977; Mavinic and Koers, 1979). At temperatures between 5 and 20°C, the system should operate at 250 to 300 degree-days (product of SRT in days and operating temperature in °C) to maintain an acceptable level of volatile solids reduction. Heated air or wastewater can be used to decrease detention time and to prevent freezing of digesters.
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