Environmental Engineering Reference
In-Depth Information
Trash Screens (rakes)
Trash screens or trash rakes are used to remove rough or large debris retained
on a trash rack. They protect pumping equipment and may be used as a pre-
liminary screening device to protect finer screens—drum or traveling water
screens, for example. A trash screen consists of one or more stationary trash
rakes and a screen raking device. Trash rack bar spacings range from 1.5
to 4 inches and are mostly constructed of steel bars. Those constructed of
high-density polyethylene polymers are beginning to replace many of the
older steel bar models—these synthetic screens are lighter and less prone to
microbial growth, corrosion, and ice. Raking mechanisms are available for
use in a variety of intake configurations, including installation on vertical
building and dam walls. Rakes are typically mounted on fixed structures
designed to clean a single trash rack, suspended from an overhead gantry, or
wheel-mounted to traverse the entire width of an intake structure and clean
individual sections of a wide trash rack.
Traveling Water Screens
Traveling water screens (sometimes called
bandscreens
) are placed in a chan-
nel of flowing water to remove floating or suspended debris. These auto-
matically cleaned screening devices protect pumping or other downstream
equipment from debris in surface water intakes. Consisting of a continuous
series of wire mesh panels bolted to basket frames, or trays, and attached
to two matched strands of roller chain, the traveling water screen operates
in a vertical path over a sprocket assembly through the flow. As raw water
passes through the revolving baskets, debris is collected and retained on the
upstream face of the wire mesh panels. The debris-laden baskets are lifted
out of the flow and above the operating flow, where a high-pressure water
spray directed outward removes the impinged debris. This process can be
continuous or intermittent. For intermittent operation, the screen activates
when a specified headloss or time elapsed has occurred. When located on
a river, traveling water screens may be subject to large fluctuations in flow
conditions, debris loading, water depths, and salinity. Depending upon
application, the size of the traveling water screen is determined by consider-
ing such factors as maximum and average flow; maximum, minimum, and
average water levels; wire mesh size; velocity through mesh; basket or chan-
nel width; number of screens; type of service; and/or starting and operating
headloss requirements.
Drum Screens
A drum screen (or cylinder screen) has very few moving parts and is
mounted on a horizontal axis with a series of wire mesh panels attached
or mounted on the periphery of its cylinder. The cylinder slowly rotates on
