Fundamental Concepts: Fundamental Concepts: SedimentationSedimentation

Ann KenimerAnn KenimerTexas A & M UniversityTexas A & M University

University Curriculum Development for University Curriculum Development for Decentralized Wastewater Decentralized Wastewater

ManagementManagement

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CitationCitation

Kenimer, Ann L., J. Kenimer, Ann L., J. VilleneuveVilleneuve and S. and S. SheldenShelden. . 2005. Fundamental Concepts: Sedimentation 2005. Fundamental Concepts: Sedimentation --Power Point Presentation. Power Point Presentation. inin (M.A. Gross and (M.A. Gross and N.E. Deal, eds.) University Curriculum N.E. Deal, eds.) University Curriculum Development for Decentralized Wastewater Development for Decentralized Wastewater Management. National Decentralized Water Management. National Decentralized Water Resources Capacity Development Project. Resources Capacity Development Project. University of Arkansas, Fayetteville, AR.University of Arkansas, Fayetteville, AR.

SedimentationSedimentation

SedimentationSedimentation is the gravitational is the gravitational accumulation of solids at the bottom of a accumulation of solids at the bottom of a fluid (air or water)fluid (air or water)

Types of SettlingTypes of Settling

Four types of sedimentation:Four types of sedimentation:Discrete settlingDiscrete settlingFlocculant settlingFlocculant settlingHindered settlingHindered settlingCompressionCompression

Examples of Settling TypesExamples of Settling Types

DiscreteDiscrete FlocculantFlocculant HinderedHindered

Types of SedimentationTypes of Sedimentation

In In discrete settlingdiscrete settlingindividual particles individual particles settle independentlysettle independentlyIt occurs when there It occurs when there is a relatively low is a relatively low solids concentrationsolids concentration

Types of SedimentationTypes of SedimentationIn In flocculant flocculant settlingsettling, individual , individual particles stick particles stick together into together into clumps called flocsclumps called flocsThis occurs when This occurs when there is a greater there is a greater solids concentration solids concentration and chemical or and chemical or biological reactions biological reactions alter particle alter particle surfaces to enhance surfaces to enhance attachmentattachment

Types of SedimentationTypes of Sedimentation

In In hindered settlinghindered settling, , particle particle concentration is concentration is great enough to great enough to inhibit water inhibit water movementmovementWater must move in Water must move in spaces between spaces between particlesparticles

Types of SedimentationTypes of Sedimentation

Compression Compression settlingsettling occurs occurs when particles when particles settle by settle by compressing the compressing the mass belowmass below

Sedimentation RateSedimentation Rate

Stoke’s LawStoke’s LawUsed for spherical particlesUsed for spherical particlesAssumes no fluid mixing, so usually will not Assumes no fluid mixing, so usually will not work for gasseswork for gasses

µ

ρ−ρ=

18)( wp gd

v2

p

Sedimentation RateSedimentation Rate

vd g

p

2

=−( )ρ ρ

µp w

18vvpp = particle settling velocity (= particle settling velocity (mm//ss or or ftft//ss))ρρpp = particle density (= particle density (kgkg//mm33 or or lblb//ftft33))ρρww = fluid density (= fluid density (kgkg//mm33))d = particle diameter (m or ft)d = particle diameter (m or ft)g = gravitational acceleration (9.81 g = gravitational acceleration (9.81 mm//ss22 or 32.2 or 32.2 ftft//ss22))µµ = dynamic viscosity (Ns/m= dynamic viscosity (Ns/m22 or lbs/ftor lbs/ft22))

ApplicationsApplications

Stoke’s Law can be used to determine the Stoke’s Law can be used to determine the surface area of a settling tanksurface area of a settling tank

Set the critical velocity equal to the settling Set the critical velocity equal to the settling velocity of the smallest particlevelocity of the smallest particleThe overflow rate is equal to the flow rate into The overflow rate is equal to the flow rate into the tank divided by the surface areathe tank divided by the surface areaSetting the overflow rate equal to the critical Setting the overflow rate equal to the critical settling velocity allows time to capture settling velocity allows time to capture smallest particles of interestsmallest particles of interest

ApplicationsApplications

OFR vQAc= =

OFR = over flow rate (OFR = over flow rate (mm//ss or or ftft//ss ))vvcc = critical settling velocity (= critical settling velocity (mm//ss or or ftft//ss))Q = the flow rate into the basin (Q = the flow rate into the basin (m3m3//ss or cfs)or cfs)A = the surface area of the basin (mA = the surface area of the basin (m22 or ftor ft22))