Coagulation - Flocculation

The Coagulation-Flocculation practices are essential pretreatment for water purification process. In the processes of conventional coagulation-flocculation-sedimentation, it is made by adding a coagulant to the source water to create an attraction between the particles in suspension. The mixture is stirred slowly to induce clustering of particles together to form 'flocs'. The water is then moved into the quiet sedimentation tank to settle solids.

The colloidal particles are stable for long periods of time, and have a size from 10^-3 to 10^-6, these are the colloidal silica, clay, hydrated metal oxides, bacteria, proteins, etc., providing the water turbidity and color. Normally can not be removed by sedimentation / flotation as electrostatic forces prevent particles and repulsion by Brownian motion maintains the suspended particles are added.

The coagulation is the destabilization of the colloidal particles electrostatically charged by adding a chemical called coagulant. This can be FeCl3, Fe2(SO4)3, Al2(SO4)3 and Ca(OH)2. Moreover, the coagulation may be aided by synthetic organic polymers called polyelectrolytes high viscosity. There is a need for better rapid mixing turbulence generated by mechanical or hydraulic means to rapidly and uniformly disperse the coagulant in the water.

The solid colloidal particles are carriers of electric charge. The primary sign of this charge can be positive or negative, although most of the colloids and wastewater primary develop a negative charge. This charge can have different origins being the most important ionization of certain functional groups (carboxyl, hydroxyl, sulfate and amino) and the differential adsorption of ions of different charges of dissolution. Said primary charge attracts particles of opposite sign (counterions) having just this primary charge neutralizing ions, therefore, the colloidal dispersion has no net electrical charge.

There is a balance between two forces, electrostatic attraction and dissemination. Electrostatic attraction attracts ions to the surface of the particles, and makes the diffusion of ions go high to low concentration, ions away colloids. The result of these opposing forces results in the formation of a diffuse ion cloud around the particles. This is known as the electrical double layer.

As a result of an excess of cations in the surface of the particle, a difference of electrostatic potential is generated, and maximum at the particle surface and decreasing with increasing distance to it. When the ionic strength is intense, the potential decreases to zero at a shorter distance, thus, in these cases, the repulsive forces are lower.

When two particles approach colloidal interaction between their layers diffuse occurs resulting in a repulsive force (V_r⁰) that prevents the binding of the two particles. These diffused layers interaction forces, attractive or Van der Waals (V_a⁰) responsible for the aggregation of colloids are also generated.

The sum of both forces determines the stability of the colloids, which may have an attractive or repulsive force resulting. At low ionic strengths tend to predominate repulsion. This net repulsion is an activation energy that must be overcome making it smaller or even zero to master the forces of attraction and aggregation occurs.

The Z potential is a measure of the force of repulsion. Colloid in natural water sources, with a pH between 5 to 8, varies between -15 to -30 mV. The greater the charge of the particle in absolute value. As the Z potential decreases can be approximated particles increasing the chance of a collision. The coagulants provide loads of opposite sign to remove that potential.

The coagulation is therefore the process of destabilization of colloidal particles in order to annul or reduce the repulsive forces.