Sedimentation

Where are we? Unit processes* designed to remove _________________________ remove __________ ___________ inactivate ____________ *Unit process: a process that is used in similar ways in many different applications Unit Processes Designed to Remove Particulate Matter Screening Coagulation/flocculation Sedimentation Filtration Particles and pathogens dissolved chemicals pathogens Empirical design Theories developed later

Sedimentation the oldest form of water treatment uses gravity to separate particles from water often follows coagulation and flocculation

Sedimentation: Effect of the particle concentration Dilute suspensions Particles act independently Concentrated suspensions Particle-particle interactions are significant Particles may collide and stick together (form flocs) Particle flocs may settle more quickly At very high concentrations particle-particle forces may prevent further consolidation

Types of Settling Four types of sedimentation: Discrete settling Flocculant settling Hindered settling Compression

Examples of Settling Types Discrete Flocculant Hindered

Type I settling During settling, a particle will accelerate until the drag force, FD, equals the impelling force, F1 Identify forces CD (drag force) :depend on whether the flow regime around the particle is laminar or turbulent or function of NRe

Drag Coefficient on a Sphere Stokes Law Tambo, N. and Y. Watanabe (1979). "Physical characteristics of flocs--I. The floc density function and aluminium floc." Water Research 13(5): 409-419. turbulent boundary laminar turbulent

NRe < 1 : laminar flow; CD = 24/NRe Stokes equation 1< NRe < 104 : transition flow; NRe > 104 : turbulent flow; CD=0.4 Newton equation

Sedimentation Rate vp = particle settling velocity (m/s or ft/s) p = particle density (kg/m3 or lbm/ft3)  = fluid density (kg/m3 or lbm/ft3) d = particle diameter (m or ft) g = gravitational acceleration (9.81 m/s2 or 32.2 ft/s2) μ = dynamic viscosity (kg/m·s or lbm/ft·s)

Applications Stoke’s Law can be used to determine the surface area of a settling tank Set the critical velocity equal to the settling velocity of the smallest particle The overflow rate is equal to the flow rate into the tank divided by the surface area Setting the overflow rate equal to the critical settling velocity allows time to capture smallest particles of interest

Applications OFR = over flow rate (m/s or ft/s ) vc = critical settling velocity (m/s or ft/s) Q = the flow rate into the basin (m3/s or ft3/s) A = the surface area of the basin (m2 or ft2)

t＝V/Q＝(L×W×H)/Q for rectangular settling basin Circular basin