Figure 20. Example of an in-line static mixer for polyelectolyte additions.

Once the negative charges of the suspended solids are neutralized, flocculation begins. Flocculation can be thought of as the second step of the coagulation process. Charge reduction increases the occurrence of particle-particle collisions, promoting particle agglomeration. Portions of the polymer molecules not absorbed protrude for some distance into the solution and are available to react with adjacent particles, promoting flocculation. Bridging of neutralized particles can also occur when two or more turbidity particles with a polymer chain attached come together. It is important to remember that during this step, when particles are colliding and forming larger aggregates, mixing energy should be great enough to cause particle collisions but not so great as to break up these aggregates as they are formed. In some cases flocculation aids are employed to promote faster and better flocculation. These flocculation aids are normally high molecular weight anionic polymers. Flocculation aids are normally necessary for primary coagulants and water sources that form very small particles upon coagulation. A good example of this is water that is low in turbidity but high in color (colloidal suspension).

A final are we should discuss is color removal. This is perhaps the most difficult impurity to remove from waters. In surface waters color is associated with dissolved or colloidal suspensions of decayed vegetation and other colloidal suspensions. The composition of this material is largely tannins and lignins, the components that hold together the cellulose cells in vegetation. In addition to their undesirable appearance in drinking water, these organics can cause serious problems in downstream water purification processes. For examples:

1. Expensive dernineralizer resins can be irreversibly fouled by these materials.

2. Some of these organics have chelated trace metals, such as iron and manganese within their structure, which can cause serious deposition problems in a cooling system.

There are many ways of optimizing color removal in a clarifier. The three most common methods are:

• Prechlorination (before the clarifier) significantly improves the removal of organics as well as reducing the coagulant demand.

• The proper selection of polymers for coagulation has a significant impact on organic removal.

• Color removal is affected by pH. Generally, organics are less soluble at low pH.

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