System Description

A coagulation system has the following functions:

1. Preparation of solutions of coagulants, flocculants, and coagulant aids. Usually employed are alum, ferric chloride, or other inorganic coagulants; cationic polyelec-trolyte flocculants; or anionic or nonionic polyelec-trolyte flocculants.

2. Rapid mixing of coagulants with wastewater.

3. Growth of floc to produce large agglomerates under gentle mixing conditions.

4. Separation of floc from water, usually by settling or flotation.

Figure 7.34.16 shows a complete, fully automated coagu-lation-flocculation system. The example shown involves coagulation aided by an inorganic coagulant followed by the addition of a polyelectrolyte flocculant. This system disperses the polyelectrolyte in the main wastewater line by injecting the solution into the pipeline. At high velocities in the line, rapid mixing is produced. The polyelec-trolyte flocculant is added just before the feed well of the clarifier, which has a slow-moving rake that keeps the settled sludge distributed and induces gentle mixing to enhance floc formation.

The design and nature of coagulation systems are based on the coagulation characteristics of the wastewater and the nature of the coagulant used.

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