Coagulation Followed By Settling

Coagulation systems used in combination with settling are available in two basic design configurations: conventional systems and sludge-blanket systems. In conventional system, the rapid-mix step is completed before the water enters the large settler, where the flocculation and clarification steps are completed. In a sludge-blanket-type unit, the coagulant mixing, flocculation, and settling steps all take place in a single unit.

Figure 7.34.15 shows a conventional system where rapid mixing of the cationic inorganic coagulant is done in the comminutor (a mix tank can also be used), and the water is then fed to the primary clarifier. The anionic polyelectrolyte flocculant is injected into the wastewater line before the clarifier unit. A moving scraper collects the settled floc and removes it to the sludge outlet. Recycling part of the floc usually improves clarification efficiency.

The flocculation step can also occur in a separate floc-culation tank with slow-moving, horizontal paddles located upstream to the conventional settler.

Sludge-blanket units combine coagulation, settling, and upward filtration in one compact unit (see Figure 7.34.17). In this design, influent water is mixed with coagulation chemicals as it is fed to the inner chamber. Floc formation begins in this chamber. The formed floc slurry then moves to the outer chamber where the water rises through a bed of previously formed floc which functions as a filter bed, effectively retaining the floc.

The velocity of the water decreases as it moves up the outer chamber because the cross-sectional area increases. This decreased velocity assists in the separation. The clear effluent water flows out through a weir at the top while draining after blowoff removes the sludge collected at the bottom. This type of unit has a small space requirement.

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