Sedimentation Tank Hydraulics

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Introduction

A floc blanket is a dense, fluidized bed of particles that helps to reduce effluent turbidity by trapping small flocs and reduces clean water waste through less frequent draining of the sedimentation tank. Floc resuspension is necessary for floc blanket formation so that flocs are recirculated through the tank instead of settling on the tank bottom as sludge.

The current design of sedimentation tanks in Agua Clara plants consists of plate settlers placed above the inlet manifold that catch flocs formed in the flocculator while allowing water to flow past the settlers into exit launders. The flocs then settle out at the bottom of the sedimentation tank and are removed regularly by a sludge drain. Because of non-uniform flow, 1) flocs accumulate unevenly in the tank which causes difficulty in removal, 2) dead zones are created in the tank that lower the efficiency of the system.

Current Research

We wish to determine optimal bottom geometry of a sedimentation tank for floc blanket formation. Floc resuspension is necessary for floc blanket formation so that flocs are recirculated through the tank instead of settling on the tank bottom as sludge. We are examining mechanisms for floc resuspension of various bottom geometries by studying the relationships between hydrostatic and hydrodynamic pressure. We are also looking at parameters such as floc blanket formation time, clarified effluent turbidity, and tube settler turbidity for various bottom geometries, which will help us to determine the optimal bottom geometry to meet a given design objective.

Our team is also working on designing a floc weir for the sedimentation tank, which will waste flocs at a constant rate from the floc hopper and serve to keep the floc blanket at a constant height.