OVERVIEW

Clarification has more than one application in water treatment. Its usual purpose in a conventional treatment process is to reduce the solids load after coagulation and flocculation. A second application, a process called plain sedimentation, is removal of heavy settleable solids from turbid water sources to lessen the solids load on treatment plant processes. Material presented in this chapter deals primarily with settling flocculated solids.

One way of designing the clarification process is to maximize solids removal by clarification, which generally requires lower clarifier loadings and larger, more costly units. Alternatively, the clarifier may be designed to remove only sufficient solids to provide reasonable filter run times and to ensure filtered water quality. This latter approach optimizes the entire plant and generally leads to smaller, less expensive facilities. Typical loading rates suggested in this chapter or by regulatory guidelines are generally conservatively selected to provide a high-clarity settled water rather than optimization of the clarifier-filter combination.

Clarifiers fall into two basic categories: those used only to remove settleable solids, either by plain sedimentation or after flocculation, and those that combine flocculation and clarification processes into a single unit. The first category includes conventional sedimentation basins (Figure 7.1) and high-rate modifications such as tube or plate settlers and dissolved air flotation (DAF). The second category includes solids contact units such as sludge blanket clarifiers and slurry recirculation clarifiers. Also included in this category is contact clarification in which flocculation and clarification take place in...