Coagulation and Flocculation-I



Design a conventional vertical-shaft rapid mix tank unit for uniformly dispersing coagulant in 10 MLD of settled raw water as per design parameters given below:


Detention time (t): 20 60 s

Ratio of tank height (H) to diameter (D): (1:1 to 1:3)

Ratio of impeller diameter (DI) to tank diameter (D): (0.2:1 to 0.4:1)

Velocity gradient (G): >300 /s

Gt: 10000 20000

Tank diameter (D): < 3 m

Paddle tip speed (vp): 1.75 2.0 m/s

Velocity of paddle relative to water (v): 0.75 x paddle tip speed

Paddle area (Ap)/Tank section area (AT): 10:100 20:100

Coefficient of drag on impeller blade (CD): 1.8

Maximum length of each impeller blade (L): 0.25 x impeller diameter

Maximum width of impeller blade (B): 0.20 x impeller diameter

Impeller height from bottom (HB): 1.0 x impeller diameter

Kinematic viscosity : 1.003 x 10-6 m2/s

Dynamic viscosity of water : 1.002 x 10-3 N.s/m2


Determine tank dimensions (provide a freeboard of 0.5 m), impeller diameter, paddle dimensions, number of paddles, clearance of the impeller from tank bottom, paddle rotation speed and power input requirement.



Let the detention time (t) be 40 s.

Therefore, volume of tank (V) =

Let the tank diameter (D) be 2 m

Tank cross-sectional area (Acs) = 3.14 m2

Tank height (H) = 1.47 m, provide freeboard of 0.5 m

Total height (HT)= 1.97 m, say 2 m.

Tank height (H) to tank diameter (D) ratio =

(within the range of 0.331.0, hence okay)

Let velocity gradient, G = 400 s-1

Therefore, G.t = (400).(40) = 16000 (within the limit of 10000-20000, hence okay)

Let the paddle tip speed (vp) be 1.8 m/s

Hence, velocity of paddle relative to water (v) = 0.75.(1.8) = 1.35 m/s


, or,

or, Ap = 0.335 m2

Tank sectional area = (D).(H) = (1.47).(2) = 2.94 m2

, which is around 0.15, hence okay.

Let the impeller diameter be 0.8 m, i.e., , which is okay

Choose length of each impeller blade (L) as 0.20m, i.e., (okay)

Choose breadth of each impeller blade (B) as 0.15m, i.e., (okay)

Area of each blade = (L.B) = (0.2).(0.15) = 0.03 m2

Therefore, number of blades to be provided = , say 12

Clearance of the paddles from the tank bottom = 0.8 m

Paddle rotation speed (w, radians/s) = radians/s,

i.e., revolutions per minutes

Power requirement is given by, , or,

or, P = 742 Watts, i.e., provide 1 KW motor for driving the impeller at 45 rpm.