Aerobic sludge digestion at cold temperatures

Abstract

Continuous flow, daily fill and draw, and batch aeration digesters were studied on a laboratory scale, to develop low temperature characteristics and design criteria for aerobic digestion of waste-activated sludge. These results were compared against full-scale data, from three independent sources. Raw sludge used in these studies was obtained from a municipal high-rate activated sludge plant. The digestion systems were operated at temperatures of 20, 10, and 5 °C, and at six different sludge ages. Measurements of all parameters studied were made under steady-state conditions.The results show that the effect of low temperature on aerobic digester performance is pronounced. The combined effect of temperature and sludge age is shown to be an important design parameter. Kinetic reaction rates and temperature coefficients were calculated on the basis of total volatile suspended solids. It was shown that batch and continuous feed system reaction rates are not interchangeable. The laboratory results further show that there is little difference between continuous flow and daily fill and draw digestion characteristics, except at 5 °C. Difficulties encountered in making a valid comparison between the laboratory and full-scale results are also discussed.