Sedimentation Tank Hydraulics

Sedimentation Tank Hydraulics

Introduction

A floc blanket is a dense, fluidized bed of particles that forms in the sedimentation tank below the plate settlers. A floc blanket works like a mesh to trap small flocs, thereby reducing effluent turbidity. As flocs are continually resuspended in the blanket, sludge build up on the tank bottom is reduced. The sedimentation needs to be drained and cleaned less frequently, reducing the amount of clean water wasted.

The latest working design of the AguaClara sedimentation tank consists of an inlet manifold with vertical diffusers to channel water into the tank as a line source. A semi-circular half-pipe beneath the diffusers serve as a jet reverser to resuspend flocs. Plate settlers placed above the inlet manifold that catch flocs while allowing water to flow past the settlers into exit launders. The flocs that settle out at the bottom of the sedimentation tank are removed regularly by a sludge drain. Implementing a floc blanket and a floc hopper in the sedimentation tank provide an additional method of filtration and will reduce sludge build up, instead allowing flocs to be wasted at a constant rate from the floc hopper. 

Current and Future 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. For adequate floc resuspension to occur, flocs settling in the sedimentation tank must be directed towards the path of the jet via an incline. We are examining mechanisms for floc resuspension for various bottom geometries by studying the relationships between hydrostatic pressure of solids built up on the sedimentation tank incline and and hydrodynamic pressure of the jet. 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 given a specific 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. 

More Information

Sedimentation Team Research Rationale
Sedimentation Team Research Rationale2
Sedimentation Team Research Report 1