Flocculation is the process in water treatment where particles are made to combine into floc in preparation for physical removal by sedimentation or filtration. The principle aim of flocculation is to transform destabilized smaller particles into larger aggregates.  The flocculation process of particle agglomeration follows the dispersion and hydrolysis of a coagulant by rapid mixing. The most commonly used coagulants are organic salts that have the most highly charged ions. They are much more effective and in practice A1⁺⁺⁺ and Fe⁺⁺⁺ salts typically are used. During the process, the minute coagulant ions are brought into contact with each other and colloids in the raw water agglomerating progressively into larger and larger floc clusters. In this step the continuous agitation of the coagulated water occurs with decreasing intensity. The opposite-charged coagulant ions and natural colloids come together by absorption and destabilization through contact, and the colloids may be entrapped within the aluminum or iron hydroxide precipitate.

The success of the flocculation process is dependent first on the effectiveness of the coagulation in the raw water. For the flocculation process itself to be successful, the process must provide adequate time for the desired floc size and density to form. The stirring system must be adequate to mix the entire volume of the basin to maximize collision opportunities for the floc and to prevent their deposit on the basin floor.  The flocculation process requires sufficient energy to agitate the water to allow opportunity for floc contact and build-up and at the same time the intensity of the energy must be limited to avoid destruction of the agglomerated floc by shearing forces.

Flocculation is the result of the collision of particles, which is caused by three different mechanisms: (1) Brownian motion of the water mass due to thermal fluid motion; (2) differential settling due to larger particles overtaking and colliding with a slower settling particles; and (3) velocity gradient due to bulk fluid motion. Velocity gradient can be quantitatively described by the concept of Camp and Stein whereby the mean velocity gradient (G-Value) is a function of power input per unit volume or G= (P/uV)^½, where P = total power dissipated, u = absolute viscosity, and V = volume. Typical velocity gradient values will range from 60-70 secs^-1 for the initial or Stage I flocculator mechanism and will decrease (tapered flocculation) to a value of 10-20 secs^-1 in the final stage. The maximum velocity of any component of the flocculator mechanism should not exceed 2 fps at mean operating parameters and should not exceed 3 fps at maximum design values.

Jim Myers & Sons, Inc. can assist in determining the optimum equipment arrangement to produce the proper velocity gradient for your application.