Flocculation is carried out in enclosures equipped with agitation systems called flocculators. The reactor’s capacity and the energy dissipated will vary depending on the applications concerned, on the fluids to be treated or on the agitation system.
The flocculator receives water that has already been coagulated. The flocculator is characterised by (see coagulation-floculation) its velocity gradient (G), its contact time and the extreme local velocities of both the mobiles and the fluids. These velocities must not apply shear forces to the floc that could cause it to re-disperse.
The tank, agitation system and auxiliary equipment have been designed so ensure that all geometric parameters will:
- prevent the formation of dead spots (area where a deposit is formed on the tank floor, for instance);
- dissipate the energy as effectively as possible throughout the tank (use of peripheral baffles in circular units, for example);
- limit any short circuits between fluid intake and discharge as much as possible.
Finally, once the floc has been formed, it is important that it is not broken up during its transfer from the flocculator to the sedimentation or floatation zone. Depending on the quality of the treated water, appropriate transfer velocities must be applied. For example when clarifying surface water over metal hydroxide floc:
- fragile: v ≤ 0.20 m · s–1;
- firm: v ≤ 0.50 m · s–1.
There are two groups of flocculators:
- those that include a mobile agitator unit (mechanical flocculators);
- offset baffle or static flocculators.