Sedimentation is a
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
than 10 NTU. In this case, coagulation and flocculation are used
produce pinpoint (very small)
floc which is removed from the water in the filters.
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
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 basins are the
simplest design, allowing water to flow
horizontally through a long tank. This type of basin is usually
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
short-circuit, especially if the length is at least twice the
disadvantage of rectangular basins is the large amount of land area
basins are essentially two rectangular
sedimentation basins stacked one atop the other. This type of
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
combine coagulation, flocculation, and sedimentation within a single
basin. Solids-contact clarifiers are often found in packaged
and in cold climates where sedimentation must occur indoors. This
clarifier is also often used in softening operations.