Ques 2) consider the following information for a rectangular primary clarifier a

Question

Ques 2)

consider the following information for a rectangular primary clarifier

a) determine the required surface area(m^2),tank volume (m^3) and detention time (hours) under both average flow conditions and peak flow conditions

b) calculate and compare the scour velocity (m/s) and peak flow horizontal velocity (m/s)

Based on these two values,do you expect the particles to re-suspend during the clarification process.

number of clarifiers = 2

Qaverage=20,000m^3/d

Qpeak=50,000 m^3/d

channel width= 6m

sidewater depth = 4m

overflow rate at average flow =40m^3/m^2.d

density of water =1000kg/m^3

beta=0.05

diameter of particle =200 micrometer

g=9.81m/s^2

darcy-weisbach friction factor (f)=0.03

Explanation / Answer

Calculate the required surface area

A = Q/OR

= (20,000 m3 /d) / (40 m3 / m2 . d)

= 500 m2

Determine tank length

L = A/W

= 500 m2 / 2* 6 m = 41.7 m

However for the sake of convenience, the surface dimensions will be rounded off to 6 m by 42 m

Tank Volume = 4 m * 2(42m * 6m) = 2016 m2

Overflow rate = Q/A = (20,000 m3 /d) / 2(6m * 42 m)

                         = 39.7 m3 / m2.d

Detention time = Vol/Q

                             = (2016 m3) (24h/d) / (20,000 m3 /d)

                             = 2.42 h

Determine the detention time and overflow rate at peak flow

Overflow rate = Q/A = (50,000 m3/d) / 2(6m * 42 m)

                                      = 99.2 m3 / m2.d

Detention time = Vol / Q

= (2016 m3) (24h/d) / (50,000 m3 /d)

                             = 0.97 h

Scour Velocity

Cohesion constant (k) = 0.05

Specific gravity (s) = 1.00

Acceleration due to gravity (g) = 9.81 m/s2

Diameter of particle (d) = 200 * 10-6 m

Darcy Weisbach friction factor (f) = 0.03

VH = 8k (s-1)gd

      --------------                                            ………………………………(1)

              f

= (8)(0.03)*(0.25)*(9.81)*(200*10-6)

   ----------------------------------------------

                               0.03

Compare the scour velocity with the peak flow horizontal velocity

V = Q/Ax

= ((50,000 m3/d) / 2 * (6m *4 m)) ((1/ (24 h/d) * (3600 s/h))

= 0.012 m/s

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