Formation of density currents in secondary clarifier

Abstract

Density currents exist in most activated sludge secondary clarifiers, but the causes are controversial. The purpose of this study was investigation of the internal flow regimes to identify and evaluate the principal causes of density currents in secondary clarifiers. This study was part of a project focused on controlling the impact of density currents on the performance of activated sludge secondary clarifiers. A 1:25 scale model of a conceptualized prototype rectangular clarifier was used and was operated by the Froude model law. Experiments were conducted in the laboratory with tap water and in the field with activated sludge mixed liquor. Two inlet baffle, intermediate baffle and effluent weir configurations and three sludge draw-off configurations were used in the experiments. In all cases, one-half of the influent flow was removed through the sludge draw-off ports located in the tank bottom. In each experiment, velocity pattern observations and temperature measurements were made throughout the tank. Observed flow patterns were very similar in the laboratory experiments with tap water and the field experiments with mixed liquor. Temperature-driven density currents formed quickly under all inlet baffle and outlet configurations and were extremely stable. Depth of the density currents was inversely related to the difference between temperatures of the influent and tank contents. A temperature difference as small as 0.2°C resulted in formation of density currents. Moreover, the formation of a density current in the tank was independent of the amount of suspended solids in the entering suspension. Temperature appears to be the primary cause of density currents in secondary clarifiers and the type of density current, surface or bottom, will depend on whether the influent is warmer or cooler than the tank contents. In the field, due to the difference between the air and influent mixed liquor temperature, the formation of density currents in secondary clarifiers appears to be inevitable and little affected by the presence or type of inlet or intermediate baffles.