Page 138 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Screening 93

is recommended to select a suitable fabric and to deter- GLOSSARY
mine HLR and operating conditions, such as rotation
Bar screen: A screen that has steel bars typically 25 < opening
velocity and backwash velocity (in terms of pressure)
< 50 mm (1–2 in.) able to withstand the impact of
and flow. Enumerate the variables and uncertainties and
large objects. Bar screens are an integral part of the
generate scenarios that provide a reasonable exploration
headworks of every municipal WWTP. See also
that relates to a design.
comminutor.
5.11 Microscreen Appurtenances
Coanda effect: A phenomenon in which a high-velocity flow
Size the appurtenances to a microscreen design, such as
tends to adhere to a surface—observed by Henri-
motor power, backwash pump power, flow capacity for
Marie Coanda in 1910.
backwash, weir design for effluent flow, etc. [This will
Comminutor: A screen with blades that cut material that has
be handled most easily if manufacturer’s catalogs are
attached, e.g., stringy matter. The comminutor is
available.]
located usually after the bar screen and prior to
pumping (if required). In the 1990s, the trend was
ACKNOWLEDGMENTS to omit comminutors and use, instead, a bar screen
with 5 < opening < 12 mm (1=4–1=2 in.).
The comminutor rating curves, Figure 5.5, were from a catalog
Disk screen: A screen that is disk shaped and that rotates
(c. 1969) of Lyco Equipment, Williamsport, Pennsylvania. In
with material collected being removed from the disk
trying to trace the lineage of the company since 1969, in order
area that leaves the intake flow.
to seek permission to use the figure, Ranvir Singh, director of
Drum screen: A screen with fabric that covers a drum struc-
research and development for Lyco during the period
ture that rotates on an axis, with partial submergence;
1967–1969, and presently with the Office of Surface Mining,
water flow is from inside to outside. As the drum
Denver, Colorado, confirmed that the company had gone out of
area leaves the water, the material collected is
business about 1971. This meant, of course, that the question
removed by water jet to be carried away in a flume
was moot regarding permission. Singh was coauthor of the
structure.
Lyco catalog and stated that the catalog data were generated
Fabric: Material that covers the screen structure. The term is
from their own laboratory testing or by contracts with univer-
applicable to fine screens and other screens that have
sity laboratories.
a mesh material used to exclude particles.
The section on wedge-wire hydro-screens was added after
Intake crib: A screen structure at the bottom of a lake
lectures and demonstrations of their performance by Robert
intended to exclude fish and whatever other larger
K. Weir, presently a consulting water engineer in Denver,
material might occur in the vicinity of the intake.
Colorado (he retired from the Denver Water Department in
Manually cleaned bar screen: A bar screen in which the
1999 as the deputy director of operations and maintenance).
debris that accumulates is removed manually, usu-
Weir also designed, fabricated, and installed the screen at
Empire, Colorado, which provided a field demonstration of ally by a pitchfork-like tool.
Microscreen: A screen with opening 1 < opening < 60 mm.
how well the screens have worked to eliminate debris from
Such screens came into use about the 1950s with
the slow sand filters both during summer and in winter with
the intent to exclude filter-clogging algae that were
ice and freezing conditions prevalent. The Town of Empire
a particular problem for eutrophic lake water
also provided hospitality to visit its plant for drinking-water
sources. In the 1980s, a slot type of microscreen
treatment on numerous occasions.
was used in wastewater treatment, eliminating pri-
The photograph of mechanically cleaned bar-screen, Figure
mary settling.
5.1, was taken by Uma Wirutskulshai, a former graduate student
Trash rack: A large opening bar screen, e.g., 25 < opening <
atColoradoState University,(1996); (currently(2010) adoctoral
75 mm (1–3 in.), intended to exclude large debris
student at the Asian Institute of Technology (AIT), Bangkok,
such as branches as found in streams.
Thailand). In addition, Steve Comstock, formerly supervisor,
Wedge-wire: A screen with V-shaped screen wire with flat
Fort Collins WWTP, granted permission to use the photograph.
portion parallel to the flow or offset at a slight angle
Comstock hosted my class on numerous occasions during which
to the main flow velocity vector. Opening is about
photographs were often taken with his permission.
1mm.
The late Paul Grundeman, Marcy Gulch WWTP,
Centennial Water and Sanitation District, Highlands Ranch,
Colorado, provided hospitality to visit the plant to obtain REFERENCES
several photographs, including the drum screen installation.
Dr. John Hendrick, general manager of the district gave per- Anon, Catalog of Water Treatment Equipment, Lyco Engineering,
Inc., Williamsport, PA, c. 1969.
mission to use the photographs taken at the plant for this book.
Burns and Roe, Inc., Process Design Manual for Suspended Solids
Permission to use the data for construction of Figure 5.9 was
Removal, U.S. Environmental Protection Agency, Washing-
given by Michael Quick, Siemens Envirex Division, Wauke- ton, DC, October 1971.
sha, Wisconsin Quick noted that the manufacturing of micro- Envirex, Rex Products, Envirex–A Rexnord Company, Waukesha,
screens was discontinued by Envirex in the early 1990s. WI, 1982.