A flash mixer, also known as a rapid mixer, is a mechanical device used in water treatment and other industrial processes to rapidly and uniformly disperse chemicals into a liquid. It's typically the first stage after adding chemicals, designed to ensure thorough mixing before the liquid enters a flocculation basin or other treatment stages.

1. Prof. Mrs. D.A. Latthe
Flash Mixer

2.  The flash (or rapid) mixing and coagulation stages consist in
adding chemicals to raw water to foster coagulation while
attracting particles that doesn’t settle or cannot be filtered.
 Coagulants, such as ferric chloride (FeCl3), PAC (poly
aluminium chloride Al2Cl(OH)5), aluminium sulphate
(Al2(SO4)3), attract these particles, so it will be easier to
agglomerate them during flocculation.

3. Flash Mixer

4. Impeller type Flash Mixer

5.  The duration of the flash mixing and coagulation stages
determines the quality of the whole process.
 For example, a short retention time won’t allow the
homogeneous distribution of the chemicals within the basin,
making the reaction unsuccessful.
 Therefore, the residence and contact time must be calculated
accurately, considering the main parameters, such as the
basin’s volume and the water’s flow rate.

6.  vertical agitators are efficient during the flash mixing and
coagulation stages as they quickly and thoroughly agitate the
chemicals in raw water before releasing them into the
flocculation chamber.

7. Types of Flash Mixers:
1. Mechanical Flash Mixer
* Uses an impeller or turbine rotating at high speed.
* Provides high energy input for rapid mixing.
2. Hydraulic Flash Mixer
* Mixing achieved by hydraulic turbulence (e.g., through a
baffled channel, jet mixing, or hydraulic jump).
* Economical as it avoids mechanical equipment.

8. Purpose
*To disperse the coagulant quickly and
uniformly in water.
*To avoid localized high concentration of
chemicals.
*To initiate destabilization of colloidal particles
for floc formation

9. Design Considerations
 Retention time:
 30 to 60 seconds (very short).
 Velocity gradient (G-value):700 – 1000 s ¹.
⁻
 Power requirement: 1–2 hp per million liters per day (MLD)
of flow.
 Tank volume:Small (since detention time is very low).

10. Process Flow
1. Raw water enters the flash mixer.
2. Coagulant is dosed just before or at the entry point.
3. Rapid mixing disperses the chemical uniformly.
4.Water then flows to the **Flocculator**, where slow mixing
promotes floc formation.

11. ThankYou!