Sedimentation Basin

Sedimentation is a treatment process in which the velocity of the water is lowered below the suspension velocity and the suspended particles settle out of the water due to gravity.  The process is also known as settling or clarification

Most water treatment plants include sedimentation in their treatment processes.  However, sedimentation may not be necessary in low turbidity water of less than 10 NTU.  In this case, coagulation and flocculation are used to produce pinpoint (very small) floc which is removed from the water in the filters. 

Location in the Treatment Process

The most common form of sedimentation follows coagulation and flocculation and precedes filtration.  This type of sedimentation requires chemical addition (in the coagulation/flocculation step) and removes the resulting floc from the water.  Sedimentation at this stage in the treatment process should remove 90% of the suspended particles from the water, including bacteria.  The purpose of sedimentation here is to decrease the concentration of suspended particles in the water, reducing the load on the filters. 

Sedimentation can also occur as part of the pretreatment process, where it is known as presedimentation.  Presedimentation can also be called plain sedimentation because the process depends merely on gravity and includes no coagulation and flocculation.  Without coagulation/flocculation, plain sedimentation can remove only coarse suspended matter (such as grit) which will settle rapidly out of the water without the addition of chemicals.  This type of sedimentation typically takes place in a reservoir, grit basin, debris dam, or sand trap at the beginning of the treatment process. 

While sedimentation following coagulation/flocculation is meant to remove most of the suspended particles in the water before the water reaches the filters, presedimentation removes most of the sediment in the water during the pretreatment stage.  So presedimentation will reduce the load on the coagulation/flocculation basin and on the sedimentation chamber, as well as reducing the volume of coagulant chemicals required to treat the water.  In addition, presedimentation basins are useful because raw water entering the plant from a reservoir is usually more uniform in quality than water entering the plant without such a holding basin. 

Types of Basins

Three common types of sedimentation basins are shown below:

Rectangular sedimentation basin.
Rectangular basins are the simplest design, allowing water to flow horizontally through a long tank.  This type of basin is usually found in large-scale water treatment plants.  Rectangular basins have a variety of advantages - predictability, cost-effectiveness, and low maintenance.  In addition, rectangular basins are the least likely to short-circuit, especially if the length is at least twice the width.  A disadvantage of rectangular basins is the large amount of land area required.
Double-deck basin.
Double-deck rectangular basins are essentially two rectangular sedimentation basins stacked one atop the other.  This type of basin conserves land area, but has higher operation and maintenance costs than a one-level rectangular basin. 

Square or circular sedimentation basins with horizontal flow are often known as clarifiers.  This type of basin is likely to have short-circuiting problems. 

A fourth type of sedimentation basin is more complex.  Solids-contact clarifiers, also known as upflow solids-contact clarifiers or upflow sludge-blanket clarifiers combine coagulation, flocculation, and sedimentation within a single basin.  Solids-contact clarifiers are often found in packaged plants and in cold climates where sedimentation must occur indoors.  This type of clarifier is also often used in softening operations.