TABLE OF CONTENTS This chapter considers some of the principles that apply to clarification, covering settlement under quiescent conditions, shallow-depth settlement, upward-flow tanks and DAF. It also covers sludge-blanket (or upward-flow solids contact) clarifiers which combine flocculation, in a layer of flocs, with upward-flow clarification.
When silt-laden water is admitted to the still conditions of a sedimentation basin and its velocity falls to near zero, its capacity to transport solids disappears, and suspended solids will begin to settle or rise, depending on whether their density is greater or less than water. Assuming that no forces other than gravity are involved, a particle with a density greater than water will settle. The particle, assuming it to be spherical, will accelerate until it reaches its terminal velocity, given by:
Where:
| D | = diameter |
| ? | 1 = density of sphere |
| ? | = density of water |
| g | = gravitational acceleration |
| V s | = settling velocity |
| C D | = drag coefficient. |
Equation (7.1) is derived from the definition of coefficient of drag, which is the ratio of the actual drag force to the dynamic drag force: [1]
Where:
| F | = actual drag force |
| V | = relative velocity |
| A | = projected area of the moving body. |
Where gravitational forces and viscous forces act on bodies, the relative size of the two forces is defined by the Reynolds number, where:
Where:
| Re | = Reynolds number |
| V | = velocity of the body |
| ? | = dynamic viscosity. |
For Reynolds numbers below 500, flow is predominantly laminar, whereas for...
