SedimentationSedimentation
University Curriculum Development for Decentralized Wastewater TUniversity Curriculum Development for Decentralized Wastewater TreatmentreatmentFundamental Concepts for Environmental ProcessesFundamental Concepts for Environmental Processes
Kenimer et al., 13 August 2004Kenimer et al., 13 August 2004FINALFINAL
AcknowledgementAcknowledgement
This work was supported [in part] by the National This work was supported [in part] by the National Decentralized Water Resources Capacity Development Project Decentralized Water Resources Capacity Development Project with funding provided by the U.S. Environmental Protection with funding provided by the U.S. Environmental Protection
Agency through a Cooperative Agreement (EPA No. Agency through a Cooperative Agreement (EPA No. CR827881CR827881--0101--0) with Washington University in St. Louis.0) with Washington University in St. Louis. The The
results have not been reviewed by EPA or Washington results have not been reviewed by EPA or Washington University in St. Louis. The views expressed in this University in St. Louis. The views expressed in this
presentation are solely those of NCSU, and University of presentation are solely those of NCSU, and University of Arkansas and EPA and Washington University in St. Louis do Arkansas and EPA and Washington University in St. Louis do not endorse any products or commercial services mentioned in not endorse any products or commercial services mentioned in
the presentation.the presentation.
SedimentationSedimentation
SedimentationSedimentation is the gravitational is the gravitational accumulation of solids at the bottom of a fluid accumulation of solids at the bottom of a fluid (air or water)(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 is It occurs when there is a relatively low solids a relatively low solids concentrationconcentration
Types of SedimentationTypes of Sedimentation
In In flocculant settlingflocculant settling, , individual particles individual particles stick together into stick together into clumps called flocsclumps called flocsThis occurs when This occurs when there is a greater solids there is a greater solids concentration and concentration and chemical or biological chemical or biological reactions alter particle reactions alter particle surfaces to enhance surfaces to enhance attachmentattachment
Types of SedimentationTypes of Sedimentation
In In hindered settlinghindered settling, , particle concentration particle concentration is great enough to is 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 settlingCompression settlingoccurs when occurs when particles settle by particles 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 work Assumes no fluid mixing, so usually will not work for gassesfor 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 The overflow rate is equal to the flow rate into the tank divided by the surface areatank 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 smallest settling velocity allows time to capture smallest particles of interestparticles 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))
