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CHAPTER 7

CLARIFICATION

John F. Willis
CDM
Cambridge, Massachusetts

Clarification has more than one application in water treatment. Its usual purpose in a con-
ventional 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 clar-
ification, 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 opti-
mizes the entire plant and generally leads to smaller, less expensive facilities. Typical
loading rates suggested in this chapter or by regulatory guidelines are generally con-
servatively 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 sed-
imentation 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 cate-
gory is contact clarification in which flocculation and clarification take place in a coarse
granular media bed.

CONVENTIONAL CLARIFICATION DESIGN

Most sedimentation basins used in water treatment are the horizontal-flow type in rectan-
gular, square, or circular design. Both long, rectangular basins and circular basins are com-
monly used; the choice is based on local conditions, economics, and personal preference.
Camp (1946) states that long, rectangular basins exhibit more stable flow characteristics
and therefore better sedimentation performance than very large square basins or circular
tanks. Basins were originally designed to store sludge for several months and were peri-

7.1