Studies of the control and operation of the aerobic digestion process applied to waste activated sludges at low 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 liquid temperatures of 20, 10, and 5°C, and at six different sludge ages. Measurement of all parameters studied for the continuous feed systems were made under steady-state conditions. Parameters studied were divided into three main groups, namely: (1) Parameters related to aerobic digestion kinetics, such as solids destruction and oxygen uptake rate; (2) Parameters related to aerobic digestion sludge characteristics, such as biochemical oxygen demand, viable bacteria, organic carbon, nitrogen forms, and odour values; (3) Parameters relating to supernatant quality, such as dissolved solids, organic carbon, biochemical oxygen demand, chemical oxygen demand, nitrogen forms, and pH. The results show that the effect of low temperature on aerobic digestion performance was pronounced. The combined effect of sludge age and temperature was shown to be an important design parameter. Kinetic reaction rates and temperature sensitivity coefficients were calculated for the various conditions studied on the basis of volatile suspended solids. It was shown that reaction rates for batch digestion and continuous feed digestion systems were not interchangeable. This is significant, as most pilot plant and bench-scale studies on aerobic digestion are being conducted using batch digestion, the results of which are then being used for design of continuous feed digesters. It appears that, as temperature decreases, the daily fill and draw method of digester operation resulted in significantly higher reaction rates than the continuous feed method of digester operation. At higher temperatures, the two were about equal. Oxygen uptake rate was not considered a reliable indicator of digested sludge stability; instead, mixed liquor BOD₅, was introduced as a possible means of determining such stability. Nitrification and denitrification is shown to be appreciable during aerobic digestion in all systems and at all temperatures studied. Although somewhat tentative, the results show important nitrification and denitrification trends at temperature and pH levels well below optimum values for these processes. The difference between batch and continuous feed digestion is shown in various ways, but probably most clearly through monitoring supernatant characteristics. The continuous feed systems show increased solubilization of organics with increased sludge age, whereas the batch systems do not show any solubilization of organics. Reasonable correlation is demonstrated between the laboratory and full-scale aerobic digestion data.