5.6 Rapid mix Rapid mixers should provide sufficient agitation to disperse the coagulant in raw water. Rapid mixing units can be classified according to the method of agitation mechanical or static. A mechanically agitated rapid mixer utilizes a mechanical mixer with an impeller or propeller to create turbulence in the mixing chamber. Examples of impellers and propellers used in water treatment are shown in fig (5.3). Fig (5.4) shows the mechanical radial flow.

Straight blade turbine Disc turbine

Normally, the ratio between turbine diameter and the width of mixing tank range from 1/2 to 1/3, also the ratio between turbine diameter and the distance between the radial impeller and the bottom of the mixing tank equal to 1. D=1/2 W

For static rapid mixing, arrangements are used such as channels or chambers with baffles producing turbulent flow conditions, overflow weirs, and hydraulic jumps (fig 5.5, 5.6, 5.7). Fig (5.4) Mechanical rapid mixer

Fig (5.5) Baffled channel for rapid mixing (not very effective)

Fig (5.6) Overflow weir

Fig (5.7) Hydraulic jump

G = velocity gradient, s-1 Traditionally, in water treatment plants, the degree of agitation in a mixing unit is measured by velocity gradient. The value of velocity gradient is given by:   Where G = velocity gradient, s-1 P = power input, Watt. V = volume of water in mixing tank, m3.µ = dynamic viscosity, Pa.s. Some design criteria for rapid mix shown below: G: 700 <G <1000 s-1 D.T: 60 to 120 s GT: 30000 to 60000 (D.T = GT / G)