SEDIMENTATION Outline General Theory Types of sedimentation basin

SEDIMENTATION

Outline • • General Theory Types of sedimentation basin Design example

SEDIMENTATION

SEDIMENTATION • Definition and Application – A physical treatment process used to separate suspended solids (flocs) from water by gravity settling § Preconditions: § Specific gravity of the particles should be larger than the fluid § Applications: § § Pre-sedimentation of river water with high suspended solids Sedimentation after coagulation and flocculation Sedimentation for recovery of filter backwash waste Sedimentation for gravity thickening of sludge

Weir

Concept: Sedimentation Process • Two important terms to understand in sedimentation zone design – The particles (floc) settling velocity, v 5 – The overflow rate, v° or u – the velocity at which the tank is designed to operate

Concept: Sedimentation Process – Overflow rate is the same as water velocity – v° = volume/time = (Depth)(Surface area) = Depth v° surface area (Time) *Surface area) Time v° = / t° = (h)(As) = h = Q As (t° )(As) t° As vs The particles removed is independent of the depth of the sedimentation tank. As long as vs is greater than v° - particles will settle downward and be removed from the bottom of the tank regardless of the depth

General: Sedimentation process • Ideal basins: – The flow through the basin is evenly distributed across the cross section of the basin – The particles are evenly dispersed in water – The settling of the particles is predominant Water surface Inlet Settling Trajectory % Settling Zone Outlet

General: Sedimentation process Type of • • • Non-Ideal Basins Most sedimentation basins perform inefficiently due to: - Eddy currents induced by the inertia of the less incoming fluid inlet - surface currents induced by the wind (Choppy water) - vertical currents induced by the outlet structure -vertical convection currents induced by the temperature differences Note: In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid flows past an obstacle.

Types of Sedimentation • 4 types of sedimentation regimes: – Type I : Discrete settling – Type II : Flocculent settling – Type III : Zone settling – Type IV : Compression settling Type of sedimentation Applications I Settling of silt intake and other headwork before coagulation Eg: sand grit material removal II Settling of softening precipitate (Ca. CO 3) in separate softening basin Eg: sedimentation after coagulation and flocculation III Settling in meddle portion of gravity thickness Eg: Occurs in lime-softening sedimentation, activated sludge sedimentation and sludge thickeners IV Settling near bottom of gravity thickness Eg: Gravity sludge thickener

Geometry of Sedimentation Basin Shapes: - Rectangular, Circular, Square

Geometry of Sedimentation Basin Selection of shape based on: - size of installation - available land site condition - Preferences and experience of operators and ‘or design engineers Advantages of rectangular basin: - Occupy less space by using common walls - Less short circuiting due to near plug flow condition - Less power requirement Disadvantages of rectangular basins: - Restricted in width by collection equipment - Require multiple rows of weirs

Enhancement of sedimentation basin performance • Solids contact – Basic features • A sludge blanket is maintained • Influent solids rise through the blanket • Solids become enmeshed within the blanket • Performance is better • Both circular and rectangular basins are used • Usually used for solids separation in lime softening process or presedimentation of high turbidity raw water

Enhancement of sedimentation basin performance • Inclined surface – Basic features • Using inclined trays, tubes or plates spaced closely • The depth of basin is divided into numerous channels • The depth of fall of particles is reduced • The settling time required is reduced • Flow is laminar • Provide large surface area and low hydraulic loading • Insignificant wind effect

What are the factors that affects sedimentation? • • • Detention time Velocity Surface turbulence Short circuits Temperature Inlet and outlet