Environmental Engineering Reference
In-Depth Information
Screening
Coagulation
Fluoridation
Unit
Processes
Disinfection
Flocculation
Hardness Treatment
Sedimentation
Filtration
Figure 11.1
Typical water treatment unit processes used in treating surface waters.
of these unit processes, which constitute a typical drinking water treatment
system for surface water supplies, in addition to a brief discussion of alterna-
tive approaches.
Screening
Screening (the first important step in treating water containing large solids)
is defined as the process whereby relatively large and suspended debris is
removed from the water before it enters the plant. River water (the source
of water used in our discussion) frequently contains suspended and float-
ing debris varying in size from small rags to logs. Removing these solids is
important, not only because these items have no place in potable water but
also because this river trash may cause damage to downstream equipment
(e.g., clog and damage pumps), may increase chemical requirements, may
impede hydraulic flow in open channels or pipes, or may hinder the treatment
process (Pankratz, 1995). The most important criteria used in the selection of
a particular screening system for water treatment technology are the screen
opening size and flow rate. Other important criteria include costs related to
operation and equipment, plant hydraulics, debris handling requirements,
and operator qualifications and availability. Large surface water treatment
plants may employ a variety of screening devices, such as
trash screens
(or
trash rake),
traveling water screens
,
drum screens
,
bar screens
, or
passive screens
.
We briefly discuss each of these screening devices in the following sections.
