Water Treatment - The Drinking Water - page 229

its axis. Because of its simplicity of construction, the maintenance require-
ments and operating costs of a drum screen installation are usually lower
than those of a traveling water screen.
bar Screens
Primarily used in wastewater treatment applications, bar screens are also
employed in some water treatment facilities. A bar screen consists of straight
steel bars welded at both ends to two horizontal steel members and is auto-
matically cleaned by one or more power operated rakes. As a rake is operated
up the face of the bar rack, it removes accumulated debris (usually large solid
objects and rags) and elevates in and out of the flow. At the top of the operat-
ing cycle of the rake, the debris is swept from the rake into a debris receptacle
by a wiper mechanism. When installed in a waterway, the bar screen assem-
bly normally is placed at an angle of 60 to 80 degrees from the horizontal.
Passive Screens
Passive intake screens (stationary screening cylinders) have no moving
parts and require no debris handling or debris removal equipment. Passive
intake screens are placed in a surface water body in such a manner so as to
take advantage of natural ambient currents and controlled through-screen
velocities to minimize debris buildup. Usually mounted on a horizontal axis
and oriented parallel to the natural current flow within the water body, cur-
rent flow action works to keep the screen clean. Maximum intake velocity
of about 0.5 foot per second (fps) is typical and works to minimize debris
impingement on the screen surface.
Coagulation
Coagulation, the second step in water purification, is a unit process that has
been used for several years in the treatment of raw water. Basically, coagula-
tion works to settle very fine material of suspended solids.
Note: Chemicals employed for coagulation are expected to be safe for
drinking water when used according to the American Water Works
Association (AWWA) coagulation standards.
Coagulants
Ty p i c a l ly, after screening, raw water is pumped into large settling basins ,
also known as clarifiers or sedimentation tanks . Within the confines of the set-
tling basin, the screened raw water is allowed to sit for some predetermined