Page 205 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

160 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

through the pipes, finally to the header pipe and see also ‘‘transition submergence.’’ Determin-
through the diffusers, terminating at the water sur- ation of flow then requires measurement of both
face above the diffusers. The air bubbles emitted H a and H b .
from the diffusers might be considered the terminal Super critical: An hydraulic term that may be defined by
point as opposed to the water surface since the pres- depth or velocity with reference to the ‘‘critical’’
sure in the air bubbles equals the depth of submer- depth or velocity, respectively. If the velocity in a
gence of the diffusers. This description applies to the given channel exceeds ‘‘critical,’’ it is termed ‘‘super
aerated grit chamber application, but may be gener- critical’’; another characteristic of super-critical flow
alized. The graphical depiction would be in terms of is that a ‘‘wave’’ cannot travel upstream. As a given
pressure (absolute pressure would be the clearest, as channel becomes narrower, at some point the depth
opposed to relative pressure), whereas the HGL is in is forced through ‘‘critical.’’ Alternatively, as the
terms of meters (or feet) of water. Normally, the slope of the channel increases, the velocity passes
pressure units would be as kilopascals (kPa) or ‘‘psi’’ through ‘‘critical.’’ The foregoing is a cursory
in U.S. Customary units. The term PGL was sug- description of this topic; more thorough explanations
gested by Professor Robert M. Meroney, Professor are given in most fluid mechanics or hydraulics
of Civil and Environmental Engineering, Colorado texts).
State University during a conversation, c. 2000, Throat: The mid section of a Parshall flume that is used to
when the topic of HGL was brought up and the characterize the flume; for example, a 0.30 m (12 in.)
author questioned him about a corresponding rela- Parshall flume is one that has a throat width of that
tion for air, since the HGL is common in hydraulic dimension.
depictions. Transition-submergence: The condition that occurs when
Pressure: Dimensions are force per unit area. SI units are the tailwater depth, as measured by H b , is high
Newtons per square meter, or pascals. Air pressure at enough such that an additional increase may cause
sea level, p a ¼ 101, 325 Pa ¼ 101.325 kPa (14.7 psi). a ‘‘submerged-flow’’ condition. The criterion for
Conversion to feet of water is obtained from the incipient submergence is that H b =H a ¼ S t . The tail-
water depth is measured by H b .
relation, p ¼ z g w , for example, z(feet of water) ¼
2 2 2 3
p a =g w ¼ (14.7 lb=in 144 in. =ft )=62.4 lb=ft ¼
33.9 ft ¼ 10.34 m.
REFERENCES
Pressure (absolute): Pressure of a fluid with reference to
zero. Usually the local atmospheric pressure would ASCE-WPCF, Sewage treatment plant design, in: ASCE Manual of
be added to a measured gage pressure. A mercury Engineering Practice No. 36 and the WPCF Manual of Prac-
tice No. 8, American Society of Civil Engineers, New York,
barometer with an evacuated tube is an accepted
1959.
standard method of measuring local atmospheric
ASCE-WPCF, Wastewater treatment plant design, in: ASCE Manual
pressure (in mm of mercury). of Engineering Practice No. 36 and the WPCF Manual of
Pressure (relative): Pressure with reference to the local Practice No. 8, American Society of Civil Engineers,
atmosphere, also called ‘‘gage’’ pressure. A Bourdon New York, 1977.
gage is used commonly, in which the pressure on Babbitt, H. E., Sewerage and Sewage Treatment, 5th edn., John
the outside of the elastic coil is atmospheric and the Wiley & Sons, New York, 1940.
Camp, T. R., Grit chamber design, Sewage Works Journal, 14:
inside of the gage contains the fluid in which the
368–381, 1942.
pressure is measured. A manometer or piezometer
Camp, T. R., Sedimentation and the design of settling tanks, Trans-
would also measure relative pressure. actions of the ASCE, III: 895–958, 1946.
Proportional weir: A special weir plate that has narrowing Griffin, F. L., An Introduction to Mathematical Analysis, revised
width toward the top and is designed to give a edition, Houghton-Mifflin Co., New York, 1936.
constant velocity in the horizontal flow rectangular Hirano, R., Pitt, P., Chen, R., and Skelley, E., Grit Overload—
grit chamber. Oceanside Plant in San Francisco, California overcomes grit
accumulation problems, Water Environment Technology,
Separation zone: A hypothetical area near the bottom of an
10(11):55–58, November, 1998.
aerated grit chamber that will capture any grit par-
Londong, J., Beitrag Zur Bemessung Beluufteter Sandfänge Unter
ticle that enters the zone. Besonderer Berücksichtigung der Gleichzeitigen Nutzung Als
Shield’s equation: Empirical mathematical relation that Adsorptionsstufe, 94, Technische Hochschule Aachen, ISSN
relates the mean horizontal velocity for incipient 0342-6068, Aachen, 1987.
scour to the properties of the particles in question. Londong, J., Dimensioning of aerated grit chambers and use as a
Submerged flow: A flow condition for a Parshall flume in highly loaded activated sludge process, Water Science and
Technology, 21:13–22, 1989.
which the tailwater below the flume is increased in
Metcalf, L. and Eddy, H. P., American Sewerage Practice (Volume
depth such that the upstream depth H a is affected.
III, Disposal of Sewage), McGraw-Hill, New York, 1916.
This means that the depth of flow in the Morales, L. and Reinhart, D., Full-scale evaluation of aerated grit
throat of the flume exceeds ‘‘critical-depth.’’ The chambers, Journal of the Water Pollution Control Federation,
submerged-flow condition occurs when H b =H a S t ; 56(4):337–343, April, 1984.