Page 204 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Grit Chambers 159

7.11 Compressor Power for an Aerated Grit Chamber As a note, Rex Chainbelt is a historic company, which
became Rexnord, then Envirex, and later became a division
Given
of Siemens Water Technologies Corp. In 2007, the headworks
A municipal WWTP has flows,
business (bar screens and grit collectors) was sold by Siemens
3
Q(max) ¼ 1.06 m =s (24 mgd), which is the hydraulic
to become WSG & Solutions, Montgomeryville, Pennsylvania
capacity for the plant
(see www.wsgandsolutions.com=). The latter company de-
3
Q(avg) ¼ 0.53 m =s (12 mgd), which was used as the signs, manufactures, and provides a complete product line of
basis for design headworks equipment.
3
Q(min) ¼ 0.26 m =s (6.0 mgd)
GLOSSARY
Two aerated grit chambers are installed, each with
Adiabatic: Without heat transfer; an adiabatic compression
dimensions, w ¼ 4.6 m, D ¼ 4.6 m, L ¼ 7.6 m (15 ft
15 ft 25 ft). Assume both aerated grit chambers are in means that the commensurate equation is used to
operation at the same time, that is, half the incoming estimate power requirement.
plant flow goes to each grit chamber. Aerated grit chamber: Grit chamber with diffused air along
Three positive displacement compressors, each 5.6 kw one side to cause a rotation of the fluid mass.
(7.5 hp), were installed. The air flow capacity for one Blower: For air compression, a centrifugal pump that builds
compressor is Q(air total for one compressor) ¼ 0.050 up significant pressure.
3
m =s NTP, that is, normal temperature and pressure Collection zone: A rectangular inset at the bottom of the grit
(105 scfm) at discharge pressure, p 2 (gage) ¼ 46.2 kPa chamber that collects and stores grit to be scraped or
(6.7 psi). Two compressors are operated at once (one for transported by a screw to a hopper at the head end of
each of the two grit chambers); the third compressor is the grit chamber.
standby and is rotated with the other two. Keep in mind Control section: A particular section following the grit
that the given flows pertain to two parallel grit chambers. chamber used to control the depth velocity relation
Required in the grit chamber; a necessary condition is that for
1. For the conditions given, calculate the power the control section, F > 1.
required for an adiabatic compression, that is, Critical velocity: The velocity at which the Froude number,
P(adiabatic). F,is F > 1.
2. Compare P(adiabatic) with the installed motor Detention time: The mean residence time of a fluid in a given
power and calculate the overall wire-to-compressor- volume; mathematically, u ¼ V=Q.
output efficiency. Diffuser: Device with holes spread out over a plate or mem-
7.12 Volume of an Aerated Grit Chamber—Installed brane through which air flows.
and Calculated Free flow: Refers to flow condition for a Parshall flume in
which the velocity is higher than ‘‘critical,’’ that is,
Given
F > 1.
A municipal WWTP has flows as follows:
Grit: Technically, grit is defined as particles with diameter,
3
Q(max) ¼ 1.06 m =s (24 mgd), which is the hydraulic
d 0.2 mm with SG ¼ 2.65. Any particles that settle
capacity for the plant
readily in a grit chamber, however, could be consid-
3
Q(avg) ¼ 0.53 m =s (12 mgd), which was used as the ered grit.
basis for design Grit chamber: A particular settling basin technology intended
3
Q(min) ¼ 0.26 m =s (6.0 mgd) to settle grit particles and scour organic particles that
may have settled.
Two aerated grit chambers are installed, each with
Header: For the case of a grit chamber, a manifold pipe that
dimensions, w ¼ 4.6 m, D ¼ 4.6 m, L ¼ 7.6 m (15 ft
distributes flow uniformly through diffusers.
15 ft 25 ft). Assume both aerated grit chambers are in
Hydraulic grade line (HGL): Graphical depiction of pres-
operation at the same time, that is, half the incoming
sure profile (usually in feet of water or meters of
plant flow goes to each grit chamber. Q(min for one grit
3
chamber) ¼ 0.13 m =s (3.0 mgd). water) of a pressurized pipeline with associated com-
ponents, for example, a reservoir, a pump, pipeline,
Required bends, valves, and perhaps a terminal reservoir.
Compare the size of the grit chamber as constructed Common in civil engineering.
with empirical guidelines. Organics: With regard to grit chambers, organic matter
entrained in the raw wastewater entering the grit
chamber.
ACKNOWLEDGMENTS
Pneumatic: Refers to a gas characteristic, usually air, for
Permission to use Figures 7.12 and 7.13 was granted by Tom example, pressure, velocity.
Quimby, director of marketing, WSG & Solutions, Montgo- Pneumatic grade line (PGL): Graphical depiction of pres-
meryville, Pennsylvania (www.wsgandsolutions.com). sure profile from air intake, through a compressor,