Design of slow mixer or flocculator for WTP

In this session you will learn how to design a slow mixer and it's components
Coagulation and flocculation
The term coagulation and flocculation are often used to describe the process of removal of turbidity caused by fine suspension, colloids and organic colour.
Coagulation describes the effect produced by addition of coagulant, resulting in particle destabilization
Flocculation is the second stage of formation of settlable flocs achieved by gentle and prolonged mixing.
In modern terminology , this combination of rapid mixing and slow mixing is designated by a single term ‘ flocculation’.
After this the flocs are separated by the process of sedimentation or clarification.
After thoroughly mixing of coagulant and raw water process followed is relatively slow and gentle steering to permit built up and agglomeration.
From the mixing basin the water is taken to a flocculation tank called flocculator where it is given a slow steering motion.
Rectangular tank is fitted with pedals operated by electric motor can best serve this purpose.
The paddles usually rotated at a speed of about 2 to 5 RPM. 
Design Parameters
Detention time (td)….Time for which the water is kept in the tank
Temporal mean velocity gradient(G)…. Rate of change of velocity per unit distance normal to a section (m per sec./m)or (per sec…s-1)
Impeller speed…. Speed at which the propeller type impeller rotates.
Power requirement…. Power consumed for the operation of slow mixer.
Peripheral velocity at the tip of the blade…. Velocity of blade at the tip while rotating with given RPM)
Design Parameters
Detention time (td)….Time for which the water is kept in the tank
Temporal mean velocity gradient(G)…. Rate of change of velocity per unit distance normal to a section (m per sec./m)or (per sec…s-1)
Impeller speed…. Speed at which the propeller type impeller rotates.
Power requirement…. Power consumed for the operation of slow mixer.
Peripheral velocity at the tip of the blade…. Velocity of blade at the tip while rotating with given RPM)
Detention time (td)…. 10-40 min. (normally 30 min)
Detention time (td) = (Volume of tank/Flow)=(V/Q)
Temporal mean velocity gradient(G)…. 10-75/sec. or more
Temporal mean velocity gradient(G)= √ (P/ μV)
μ.... Absolute viscosity (N-sec/m2)
Impeller speed…. 2-3 rpm
Tangential velocity at the tip of the blade…. 0.2-0.6 m/sec
Power requirement(P)…. 10-36 Kilo-watts/MLD.
Power requirement(P)=0.5*CD * ρ * Ap *vr 3
CD … Coefficient of drag…. 1.8
ρ … Density of water … 1000kg/m3
Ap … Area of paddle (m2)
vr …. Relative velocity …. (0.75 * Tangential velocity )
Factor….. G.td = 104-105


Tangential velocity =(Impeller speed* Impeller periphery/60)
Depth of tank…. 3-4.5m
Velocity of flow…. 0.2-0.8m/sec (normally 0.3-0.4 m/sec)
Area of paddle…. 10-25% of the cross-sectional area of the tank.