BUOYANT JETS IN HORIZONTAL-FLOW FINAL SETTLING TANKS

摘要

Physical tests at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) at ETH Zurich, Switzerland, show the influence of the inflow jet into a denser sludge bed, near the bottom of a horizontal-flow final settling tank. To gather the development of the buoyant jet, different impact and sludge properties are investigated. Measurements of the total solid fraction and the flow velocity in the settling tank give excellent basic information about the physics of the flow regime. Visual analysis as well as numerical calculations complete the investigation. Turbidity measurements of the effluent finally enable to quantify the influence of the buoyant jet as a filter for the suspended solid fraction. For fast settling sludge with a Sludge Volume Index SVI < 90 ml/g, the buoyant jet develops to a branched flow regime with an inhomogeneous flow distribution. At the sludge blanket the vertical buoyant jet changes to a horizontally dissolved matter jet and discharges suspended solid into the surrounding clear water. With slowly settling sludge, i.e. SVI > 150 ml/g, the jet develops to a fluidized flow regime and stratifies in the sludge bed. In this case the fluidized flow regime acts as a filter for the suspended solid fraction resulting in a reduced turbidity at the effluent of the final settling tank.