Civil Engineering Reference
In-Depth Information
solubilization of objectionable metals. Rivers may carry heavy silt loads that would
overtax or complicate operation of coagulation or softening unit processes. Debris in
either reservoirs or rivers may foul water transport equipment. Pretreatment provisions
may be considered for:
Screening of surface water for removal of general debris
•
Treatment of reservoirs for destratification
•
Treatment of reservoirs for control of algae and plant growths
•
Pretreatment of surface water for control of taste and odors
•
Presedimentation of river waters for removal of suspended silt load
•
Screening
In rivers or lakes where the water transports debris, such as leaves, branches, logs, and
similar objects, it is necessary to provide screening facilities before water is withdrawn
for treatment. When substantial amounts of debris are present, mechanically cleaned
screens should be considered. Most water intakes include protective screens, or bar
racks, that are not mechanically cleaned and remove only the largest pieces of debris.
Screens should be located at the intake structure. Typically, the screenings, whether
manually or mechanically removed, will be returned to the water source in such a way
that they will not return to the screen and accumulate.
Traveling water screens are operated intermittently unless the debris is particularly
heavy. These screens are installed in a channel as shown in Figure 8-10, where they
move in a slot, upward to the surface. Water sprays clean the screens and sluice the
debris to a channel or pipe that carries it back to the river or lake. Screen openings
are normally
1
⁄
8
to
1
⁄
2
inch (3.18 to 12.7 mm) wide.
Protective bar racks are often placed upstream of traveling water screens as barriers
to logs, large fish, and other similar objects that could cause damage or are not suitable
for removal by the screen. These bars may be spaces at 3 to 4 inches (75 to 100 mm),
Fig. 8-10.
Traveling water screen (Courtesy of USFilter / Johnson Screens)
