Page 124 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

5 Screening

Screening is the retention of particles either by a grid or A means to size a screen is in term of its ‘‘capacity,’’ i.e., the
longitudinal bars with openings smaller than the particles limit of the mass of material fed per unit time per unit area of the
to be removed. Screens have a wide variety of forms and screen. Capacities may be expressed in kilograms of overflow
range from microscreens, with openings as small as 1 mm, to material per hour per square meter. Screens must be rated by data
trash racks. The design of screens is an art that may involve from operation with the particulate matter of interest. Examples
selection of materials, structural calculations, mechanical of numerical values for capacities are 0.045–0.18 metric
2 2
appurtenances, hydraulics, cleaning, conveyance of screening tons=h=m =mm mesh size (0.05 and 0.2 tons=h=ft =mm mesh
2
wastes, disposal of screenings, and provision for maintenance. size) for grizzlies and 0.18–0.73 metric tons=h=m =mm
2
Usually, the design is done by equipment manufacturers who mesh size (0.2 and 0.8 tons=h=ft =mm mesh size) for vibrating
provide a ‘‘package,’’ i.e., a screening ‘‘subsystem’’ that has a screens. Whether the capacity of a screen is exceeded or not
‘‘fit’’ within the treatment train. The role of the designer is depends upon the rate of feed to the unit.
to select the appropriate screening equipment from a manu- Regarding names, a ‘‘grizzly’’ is a grid of parallel metal
facturer. A knowledge of screens is necessary in order to do bars set at an incline and is stationary. A gyrating screen has a
this properly, i.e., work with the manufacturer’s representative significant amplitude and a low frequency. A vibrating screen
and other technical personnel who may in some cases be from has a much higher frequency, e.g., 1800–3000 vibrations per
the manufacturer’s headquarters. minute, and much smaller amplitude than a gyrating screen.
While screening is an essential part of almost any treatment Usual vibrating screen installations are woven or mesh screen,
train, it is not, as a rule, an area that sparks enthusiasm. inclined at a small angle to the horizontal.
The success of a screen is not only in terms of its effectiveness
in removing intended particles but in the support functions. 5.2 TYPES OF SCREENS
The latter include cleaning of the screen; appurtenances for
collection and removal of solids; having materials that are For any screening task a variety of configurations exist.
durable, strong, and noncorrosive; and providing a system One type, for example, is the traveling screen consisting of
that can be maintained easily. All of this is a part of screening wire mesh panels attached to a belt system that operates in a
technology that continues to be developed by proprietary vertical path. The flow of water passes through the screen and
companies. Consequently, the lore of screening technology debris is retained on the upstream side, which is removed by
is found primarily in the catalogs of manufacturers and is a jet of water after the panel rises on the belt to the air portion of
associated with some of the experienced personnel. its travel. Other configurations include bar screens, drum
screens, disk screens, and microscreens (Pankratz, 1988).
Purposes are also varied. Bar screens in wastewater treat-
ment or coarse screens in drinking-water treatment are used
5.1 THEORY OF SCREENING
to protect equipment such as pumps. Trash racks in intakes
Screening is a method of separating particles according to size for drinking-water treatment or industrial cooling water are
alone. The objective of the screen is to accept a ‘‘feed’’ contain- designed to keep out debris that could clog pipes and pumps,
ing a mixture of particles of various sizes and separate it into two cause nuisances, and interfere with treatment. Other screens are
fractions, an ‘‘underflow’’ that is passed through the screen and designed to exclude fish and crustaceans. At the other end of
an ‘‘overflow’’ that is rejected by the screen. An ideal screen the screen spectrum are microscreens, which actually provide a
would sharply separate the feed mixture in such a way that the form of treatment in that the intent is to remove small particles,
smallest particle in the overflow would be just larger than the such as algae and biological flocs.
largest particle in the underflow. Such anideal separationdefines
a cut diameter, d c , which marks the point of separation between
5.2.1 BAR SCREENS
fractions (McCabe and Smith, 1956, 1967, 1976, 1993).
The probability of passage of a particle through a screen Bar Screens are found in the ‘‘headworks’’ of every waste-
depends upon the fraction of the openings relative to the gross water treatment plant and are made of steel bars with openings
surface area, on the ratio of the diameter of the particle to the perhaps 10–30 mm (0.4–1.2 in.). The purpose of a bar screen
width of an opening in the screen, and on the number of is to protect equipment vis-à-vis treatment (albeit treatment
contacts per unit of flow between the particle and screen must occur). The bar screen should exclude large objects and
surface, and on the shear force caused by the fluid velocity rags which could clog intake pipes, flow measuring devices,
relative to a resisting particle. fine screens, or pumps. The openings of the bar screen should

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