Civil Engineering Reference
In-Depth Information
where:
A
thickening area, ft
2
(m
2
)
t
u
time required to reach desired sludge concentration, sec
h
0
initial height of interface in batch test, ft (m)
Q
flow, ft
3
/ sec (m
3
/s)
Compression
Compression occurs when the subsiding particles accumulate at the
bottom of the sedimentation basin. The weight of the particles is supported by the
compacting mass. Compression (or consolidation of the particles) is a relatively slow
process, as illustrated in Figure 11-2.
Applications of Sedimentation
The two principal applications of sedimentation in water treatment are plain sedimen-
tation and sedimentation of coagulated and flocculated waters. Plain sedimentation is
used to remove solids that are present in surface waters and that settle without chemical
treatment, such as gravel, sand, and silt. It is used as a preliminary process to reduce
the sediment loads in the remainder of the treatment plant; as a result, it is referred to
as
presedimentation.
Sedimentation is also used downstream of the coagulation and
flocculation processes to remove solids that have been rendered more settleable by
these processes. Chemical coagulation may be geared toward removal of turbidity,
color, or hardness.
Presedimentation
The purpose of presedimentation is to reduce the load of sand,
silt, turbidity, bacteria, or other substances being applied to subsequent treatment pro-
cesses, so that those processes may function more efficiently. When the raw water has
exceptionally high concentrations of these substances, good removals are often ob-
tained by plain settling and without the use of chemicals. Waters containing gravel,
sand, silt, or turbidity in excess of 1,000 NTU may require presedimentation.
Presedimentation basins should have hopper bottoms and / or be equipped with con-
tinuous mechanical sludge removal apparatuses especially selected or designed to re-
move heavy silt or sand. Sludge is not removed continuously, and basin design must
allow for sludge accumulation between cleanings. In manually cleaned basins, settled
matter is often allowed to accumulate until it tends to impair the quality of the settled
water, at which time the sludge is flushed out.
Sedimentation basins that are not equipped for mechanical removal of sludge should
have sloping bottoms, so that they can be rapidly drained, allowing most of the sed-
iment to flow out with the water. Because the bulk of the material settles near the inlet
end, the slope should be greatest at this point. In some plants, sluice gates are arranged
to deliver raw water to the sedimentation basin to flush out the sludge; the balance is
generally washed out with a fire hose. At least two basins are needed so one can
remain in service while the other is cleaned.
The time between cleanings varies from a few weeks, in plants that have short
periods of settling and handle very turbid water, to a year or more, where the basin
capacity is large and the water is not very turbid.
6
Because the particles to be removed in presedimentation basins are more readily
settleable than chemical flocs, the detention time may be shorter and the surface over-
