3 Design criteriaThe following design criteria are generally assumed to design a Primary Settling Tank / SedimentationA) GENERALNo. of Tanks2 or more (usually)Types of tanksCircular or rectangularRemoval of Sludge and ScumMechanical (usually)Tank bottom slopemm/mSpeed of sludge scraper0.02 – 0.05 rpm
4 B) DIMENSIONS C) TECHNICAL Range Typical Rectangular Tank Length (m) 15-10030Width (m)3-3010Depth2.5-54Circular TankDiameter (m)3-60Depth (m)3-5Bottom slope, (mm/mm)0.02 – 0.050.03B) DIMENSIONSRangeTypicalDetention Time, t (hr)1.0 – 4.02.0Flow Through velocity (m/min)0.6 – 3.60.9SLR (m3/m2/hr) at average flow1.2 – 2.51.6Peak Hourly Flow2.0 – 5.04.2WLR (m3/m/d)250C) TECHNICAL
5 Primarily used in WWT to separate solids from liquids in effluent streams. TypeslightClarifiersCriteria for sizing clarifier (settling tank)Tank depth at the side wallOverflow rate (surface loading rate)Detention timeScour velocity
8 Criteria For Sizing Clarifier (Settling Tank) -Overflow Rate (Surface Settling Rate)-Definition: The average daily flow rate divided by the surface area of the tank.ClarifiersAverage daily flowrate(m3/day)overflowsurface settling rate(m3/m2d)Total surface area of the tank(m2)
9 Criteria For Sizing Clarifier (Settling Tank) Average daily flowrate -Depth Of Tank-The water depth at the side wall measuring from the tank bottom to the top of the overflow weir.Depth of tankEffluent weirExclude the additional depth resulting from slightly sloping bottom that is provided in both circular and rectangular clarifiers.InfluentInfluentHOccupiedwith sludgeEffluent weir loading (typical= 250 m3/m.d) is equal to quantity of WW flowing divided by the total weir length, LwAverage daily flowrate(m3/day)Total weir length(m)
10 Criteria For Sizing Clarifier (Settling Tank) Average daily flowrate -Detention Time-length of time a particle or a unit volume of WW remains in a reactorDetention timeTank volume (m3)Detention time =(day)Average daily flowrate(m3/day)
11 Criteria for sizing clarifier (settling tank) -Scour velocityhorizontal velocity through the tank to avoid resuspension of settled particlesScour VelocityWhere:VH = horizontal velocity that will just produce scour (m/s)k = cohesion constant that depends on type of material being scoured (unitless)s=specific gravity of particlesg=acceleration due to gravity (9.81 m/s2)d=diameter of particlesf=Darcy-Weisbach friction factor (unitless)
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