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Published bySteven Gyles Robertson Modified about 1 year ago

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DESIGN OF WASTEWATER TREATMENT PLANT

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Clarifier/Sedimentation Tank Design

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The following design criteria are generally assumed to design a Primary Settling Tank / Sedimentation A) GENERAL No. of Tanks2 or more (usually) Types of tanksCircular or rectangular Removal of Sludge and ScumMechanical (usually) Tank bottom slope mm/m Speed of sludge scraper0.02 – 0.05 rpm Design criteria

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B) DIMENSIONS RangeTypical Rectangular Tank Length (m) Width (m) Depth Circular TankDiameter (m) Depth (m)3-54 Bottom slope, (mm/mm)0.02 – RangeTypical Detention Time, t (hr)1.0 – Flow Through velocity (m/min)0.6 – SLR (m3/m2/hr) at average flow1.2 – Peak Hourly Flow2.0 – WLR (m3/m/d) C) TECHNICAL

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light Clarifiers Types Overflow rate (surface loading rate) Primarily used in WWT to separate solids from liquids in effluent streams. Criteria for sizing clarifier (settling tank) Tank depth at the side wall Detention time Scour velocity

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Clarifiers Definition: The average daily flow rate divided by the surface area of the tank. overflow surface settling rate (m 3 /m 2 d) Average daily flowrate (m 3 /day) Total surface area of the tank (m 2 )

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Depth of tank The water depth at the side wall measuring from the tank bottom to the top of the overflow weir. Exclude the additional depth resulting from slightly sloping bottom that is provided in both circular and rectangular clarifiers. Influent Occupied with sludge Influent Effluent weir H Effluent weir loading (typical= 250 m 3 /m.d) is equal to quantity of WW flowing divided by the total weir length, L w Average daily flowrate (m 3 /day) Total weir length (m)

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Tank volume (m 3 ) Detention time = (day) Average daily flowrate (m 3 /day) Detention time length of time a particle or a unit volume of WW remains in a reactor

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Scour Velocity horizontal velocity through the tank to avoid resuspension of settled particles Where: V H = 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 particles g=acceleration due to gravity (9.81 m/s 2 ) d=diameter of particles f=Darcy-Weisbach friction factor (unitless)

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Example:

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Stabilization pond design

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In series

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Design Parameters

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Facultative Pond

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Design Criteria Surface Organic Loading Rate Depth Detention Time Geometry (L/B ratio)

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Surface Organic Loading Rate Related with algae activity and balance between oxygen production and consumption.

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Depth

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Based on volume, V and the required area, A, the depth (H) of pond = V/A. H = 1.5 to 2.0 m

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Detention time In primary detention time,t usually vary from 15 to 45 days.

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L/B Ratio

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If K = 0.35d -1 θ=1.085 If K = 0.30d -1 θ=1.05

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Example

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Solution

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ANAEROBIC POND DESIGN

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Volumetric Loading Rate

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Detention Time

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Depth

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L/B Ratio

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AEROBIC POND DESIGN Design Criteria Detention time, t = 5 to 10 days Depth, H = 2.5 to 4.0 m Oxygen Requirement Power Requirement

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Oxygen Requirement

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Power Requirement

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Activated sludge design

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