Mixing devices for coagulant

Mixing (rapid mixing) must spread the coagulant quickly and uniformly through the whole of the water within 30 seconds to 2 minutes, so that the charge is neutralised before large flocs begin to form. The usual devices are as follows.

1. Hydraulic mixing (no power)

(a) Mixing channel with baffles (baffled channel): the water flows round the vertical baffles or over and under horizontal baffles; the turbulence at each turn mixes the chemical. Velocity 0.3 to 0.45 m/s.

   -->|  |-->|  |-->|  |-->
   ----+  +--+  +--+  +---->
   (over-and-under baffles)

(b) Hydraulic jump (weir) mixing: the coagulant is dropped at a Parshall flume or a weir where a hydraulic jump forms. Simple and cheap, but the head is lost.

        coagulant
            |
  ~~~~~~~~~~v~~\
  water   weir  \
  -->     crest  \___/\/\/~~~~~~~ --> to flocculator
                    jump (turbulence)

(c) Pipe (in-line) mixing / venturi: the coagulant is injected into a pipe at a throat or a bend.

2. Mechanical mixing

(a) Rapid mix tank with a paddle or propeller (turbine): a square or circular tank with a motor-driven impeller; detention 30 to 60 s, velocity gradient $G=300$ to $1000\ \text{s}^{-1}$.

        [Motor]
          |
   ______|______
  |      |      |  <- water in
  |    --+--    |
  |     paddle  |  -> to flocculator
  |_____________|

(b) Flash mixer with deflecting baffle plates: a high-speed impeller, with baffle plates on the tank walls that stop swirling and increase turbulence.

3. Compressed-air mixing

Air bubbles through diffusers in the bottom of the tank cause turbulence.

The mixer is followed by the flocculator (slow mixing), using baffled channels or paddles moving at 0.1 to 0.5 m/s for 20 to 40 min, where the flocs grow.