EPA-670/2-74-026
June 1974
THE SWIRL CONCENTRATOR AS A GRIT SEPARATOR DEVICE
By
Richard H. Sullivan
Morris M. Cohn
James E. Ure
Fred Parkinson
American Public Works Association
Chicago, Illinois 60637
Project 11023 GSC (S-802219)
Program Element 1BB034
Project Officer
Richard Field
Storm and Combined Sewer Section (Edison, N.J.)
Advanced Waste Treatment Research Laboratory
National Environmental Research Center
Cincinnati, Ohio 45268
NATIONAL ENVIRONMENTAL RESEARCH CENTER
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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REVIEW NOTICE
The National Environmental Research Center—Cincinnati
has reviewed this report and approved its publication.
Approval does not signify that the contents necessarily
reflect the views and policies of the U.S. Environmental
Protection Agency, nor does mention of trade names or
commercial products constitute endorsement or recommen-
dation for use.
11
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FOREWORD
Man and his environment must be protected from the adverse effects of pesticides,
radiation, noise and other forms of pollution, and the unwise management of solid waste.
Efforts to protect the environment require a focus that recognizes the interplay between
the components of our physical environment-air, water, and land. The National Environ-
mental Research Centers provide this multidisciplinary focus through programs' engaged in
• studies on the effects of environmental contaminants on man and the
biosphere, and
• a search for ways to prevent contamination and to recycle valuable
resources.
As part of these activities, the study described here investigated the applicability
of a swirl concentrator chamber to perform the functions of a grit separation and removal
facility.
A. W. Breidenbach, Ph.D.
Director
National Environmental
Research Center, Cincinnati
111
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ABSTRACT
A study was conducted by the American
Public Works Association to determine the
applicability of a swirl concentrator chamber
to perform the functions of a grit separation
and removal facility. The swirl concentrator
principle was originally developed in Bristol,
England, and subsequently modified and
applied by the APWA to act as a combined
sewer overflow regulator.
The ability of the swirl flow pattern to
effectively remove solids of particular sizes or
specific gravities was noted during the first
study. This hydraulic flow configuration was
developed and adapted to effectively remove
grit from either the underflow from the
combined sewer overflow regulator or from
domestic sanitary sewage.
Hydraulic model studies were used to
develop optimum design configurations. For
an average flow of 0.084 m3 /sec (3 cfs), the
diameter of the unit would be 2.19 m (7.2 ft)
and 1.1 m (3.6 ft) deep. The efficiency of
removing grit particles of 2.65 sg and size
greater than 0.2 mm will be equal to that of
conventional grit removal devices. The unit
has no moving parts. Conventional grit
washers and lifts can be employed.
The complete report on studies carried
out on a swirl grit removal model by the
LaSalle Hydraulic Laboratory Ltd. is included
as an appendix.
The report was submitted in fulfillment
of the agreement between the City of
Lancaster, Pennsylvania, and the American
Public Works Association, under the partial
sponsorship of the Office of Research and
Development, U. S. Environmental Protection
Agency, in conjunction with Research and
Demonstration Project 11023 GSC
(S-802219.)
IV
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CONTENTS
Page
No.
Abstract iv '
Section I Conclusions, Recommendations and Overview 1
Section II The Study 3
Grit Removal: Basic Principles and Practices 3
The APWA Study 4
Section III General Features and Design 7
Basic Criteria of Grit and Organic Constituents of Inflow 7
Flow Rate, and Overflow and Foul Discharge Rates 17
Design Features 21
Other Facility Factors 25
Design 25
Height-Width Relationships 29
Section IV Implementation 31
Section V Glossary of Pertinent Terms ." 33
Section VI Appendices 35
Appendix A Hydraulic Model Study 37
Appendix B The Geiger Grit Chamber, (translated from the German) .... 75
Appendix C Scale Effects on Particle Settlement .91
TABLES
1. Sieve Analysis of Samples from Grit Chambers 10
2. Sieve Analysis of Samples from Aerated Grit Chambers —
New York City Plants 11
3. Sieve Analysis of Samples from Structures Downstream
of Aerated Grit Chamber, New York City Plants 11
4. Typical Grit Gradation 15
5. Specific Gravity, Size and Concentration of Settleable Solids 17
6. Grit in Sanitary Sewage 18
7. Settleable Solids in Swirl Concentrator Used as Combined
Sewer Regulator at Lancaster 20
FIGURES
1. Swirl Concentrator as a Grit Separator, Final Form 2
2. Isometric View, Swirl Concentrator as a Grit Separator 8
3. General Design Dimension 9
4. Gradation Curves of Samples from Grit Chamber 12
5. Gradation Curves of Samples from Aerated Grit Chambers 13
6. Gradation Curves of Samples from Structures Downstream
of Aerated Grit Chambers, New York City Plant 14
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FIGURES (Continued)
7. Gradation Curve of Typical Grit 16
8. Grit Chamber Above Ground with Inclined Screw Conveyor 19
9. Grit Chamber Below Ground with Inclined Screw Conveyor 22
10. Grit Chamber Below Ground with Horizontal Screw Conveyor . . 23
11. Grit Chamber Below Ground with Tubular Conveyor 24
12. Chamber Diameters for 95 Percent Recovery and Hj /Dj =2 26
13. Chamber Diameters for 90 Percent Recovery and HI/D! = 2 27
14. Chamber Diameters for 80 Percent Recovery and H!/D! =2 28
15. Approximate Stage — Discharge Curves Over Weir 30
16. Prototype Unit, Denver, Colorado 32
17. Model Layout 38
18, Typical Grit Gradation 39
19. Particle Settling Velocities for Grit, Pumice and Gilsonite 40
20. Prototype Grit Sizes Simulated by Pumice 41
21. Gradation Curves for Pumice, Gilsonite and Sand Used in Model 42
23. Prototype Grit Sizes Simulated by Sand in Model 43
24. Original Layout , 44
25. Preliminary Layout for Lancaster Study , 45
26. Single Gutter - Small Outlet Scheme 47
27. Three Gutter — Large Hopper Scheme 48
28, Scheme with Hopper at 270° 49
29. Vertical-Sided Hopper at 180° ''. . . 50
30. Hopper at 180° With Three Floor Baffles 51
31. Hopper at 180° with Two Gutters and Three Floor Baffles 52
32. Hopper at 180° with Two Gutters and One Floor Baffle . . . ." I ... 54
33. Solids Separation for Flat Floor Concept 55
34. Conical Floor Principle 57
35. Small Cone, Elbow Flush with Floor | ... 58
36. Small Cone, Elbow 15.2 cm (6 in.) Below Flat Floor 59
37. Final Conical Floor Configuration 60
38. Sketch Layout of Swirl Concentrator Grit Chamber 62
39, Nomenclature for Width-Depth Proving Tests 63
40. Varying Sized Inlets Used in Width-Depth Testing 64
41. Gilsonite Recovery in Final Form ...
3.66 and 2.74m (12 ft & 9 ft) diameters 65
4-2. Gilsonite Recovery in Final Form ;
2.18 and 1.83 m (7.2 ft &6 ft) diameters 66
43, Portions of Prototype Grit Samples
Simulated by Gilsonite, Sand and Pumice 67
44. Portions of Prototype Grit Samples
Represented by Gilsonite Recovered in Tests '. ... 67
45. Portions of Prototype Grit Samples Represented by Gilsonite ;
Recovered in Tests Plus Sand Pumice ; ... 68
46. Grit Recovery for 30.5 cm (1 ft) Inlet with
2.74 and 3.66m (9 ft & 12 ft) Chambers ; ... 69
47. Grit Recovery for 30.5 cm (1 ft) Inlet with i
1.83 and 2.18m (6 ft & 7.2 ft) Chambers i ... 70
48. Grit Recovery for One (Dj = 1 ft = 30.5 cm) Foot Inlet ;
Pipe and Different Sized Chambers 71
49. Grit Recovery for Two (D! = 2 ft = 61 cm) Foot Inlet I
Pipe and Different Sized Chambers ; ... 72
VI
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FIGURES (Continued)
50. Grit Recovery for Three (D1 = 4 ft = 122 cm) Foot Inlet
Pipe and Different Sized Chambers 72
51. Grit Recovery for Four (Dj = 4 ft = 122 cm) Foot Inlet
Pipe and Different Sized Chambers 73
'52. Prototype Discharge Range Covered by Model Tests ' , . 73
53. Schematic View of Flow in a New Type Grit Chamber 79
54. Cause of the Course of the Current in a Model Grit Chamber .82
55. Model Grit Chamber Showing Sediment 84
56. Course of the Current in Model Grit Chamber .' 84
57. Side Views of Course of Current in a Model Grit Chamber 85
58. Grit Chamber for a Smaller City '..... 86
59. Course of Current in an Aerated Grit Chamber 87
60. Double Grit Chamber Layout for a Large City 90
vu
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ACKNOWLEDGEMENTS
The American Public Works Association
is deeply indebted to the following persons
and their organizations for the services they
have rendered to the APWA Research
Foundation in carrying out this study for the
City of Lancaster, Pennsylvania, and the U. S.
Environmental Protection Agency.
CITY OF LANCASTER
Daniel Templeton, Director of Public Works
Arthur Morris, City Engineer
CONSULTANTS
Dr. Morris M. Cohn, Consulting Engineer
Dr. Paul Zielinski, Consulting Engineer
ALEXANDER POTTER ASSOCIATES, CONSULTING ENGINEERS
Morris H. Klegerman
James E. Ure
LA SALLE HYDRAULIC LABORATORY, LTD.
F. E. Parkinson
G. Galiana
U. S. ENVIRONMENTAL PROTECTION AGENCY
Richard Field, Project Officer, Chief, Storm and Combined Sewer Section (Edison, N.J.)
Advanced Waste Treatment Research Laboratory, National Environmental Research Center,
Cincinnati. \
MERIDIAN ENGINEERING, INC.
T. R. Darmody
Vlll
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SECTION I
CONCLUSIONS, RECOMMENDATIONS AND OVERVIEW
CONCLUSIONS
1. Hydraulic model tests indicate that
the swirl concentrator principle can be
utilized to provide the same high degree of
performance in settling and removing grit
particles as conventional devices.
2. The dynamic action of the swirl
concentrator appears to wash the grit and
may result in a minimum of organic materials
settled and entrapped with the inorganic grit
particles.
3. The design of the swirl concentrator as
a grit chamber has been developed for rapid
calculation of the size of the different
elements, allowing ready sizing of units for
various quantities of flow.
4. The small size, high efficiency, and
absence of mechanical equipment in a swirl
grit chamber facility appear to offer
advantages over conventional devices. In
conjunction with a swirl concentrator serving
as a combined sewer overflow regulator, it can1
provide an efficient means of removing the
extremely large concentrations of grit which
can be anticipated, in the foul sewer wastes;
removed from the clarified wastewaters which
are to be discharged to receiving streams or to
overflow treatment or storage facilities.
5. The device could be used to provide.
removal of relatively large quantities of grit-'
and larger organic material from flows which.
may require emergency bypassing or wasting'
at overtaxed sewage treatment plants.
RECOMMENDATIONS
Demonstration facilities should be.
constructed to evaluate the swirl concentrator
in actual field service, for: (1) Removal of grit
from the underflow from the swirl
concentrator as a combined sewer overflow
regulator; (2) grit removal from combined
sewer overflow and storm water; and (3) grit
removal from sanitary sewage, or industrial
wastes as a pre-treatment stage of
conventional wastewater treatment.
OVERVIEW
Previous studies by the American Public
Works Association indicated that the swirl
concentrator principle was very effective at
removing particles of specific grain sizes and
specific gravity combinations from flows. The
City of Lancaster has prepared plans for the
construction of a swirl concentrator as a
combined sewer overflow regulator. The site
requires that flow to the interceptor be
pumped. Project consultants recommended
that the pumps and wet well should be
protected from the anticipated high (up to
13,000 mg/1) concentration of grit in the foul
flow.
The current study was authorized to
determine if, by means of hydraulic modeling
techniques, a design could be evolved for a
grit chamber using the swirl concentrator
technique.
The swirl concentrator principle involves
the development of a flow chamber utilizing
"circular, long-path kinetic energy to produce
separation of solids from liquid and settling of
the particles. The settling is achieved by the
development of optimum hydraulic
conditions to accomplish settling ..• removal of
solids without the use of mechanical
accessories.
A limited number of tests were
conducted to determine the relationship
between the chamber diameter and the height
of the clear water discharge weir above the
floor. Although the usual- parameters of
design will establish a chamber diameter that
will reduce the depth to the bottom of the
conical hopper, variations as desired may "be
made.
Figure 1,. Swirl Concentrator As a Grit
Separator, Final Form, portrays the model in
its final form. The device should be effective
in removing grit from the underflow foul
liquid to the interceptor sewer from the swirl
concentrator combined sewer overflow
regulator; from sanitary sewage, and from
certain industrial wastes.
A complete report of the hydraulic
laboratory studies is attached as Appendix A.
A translation of a German article concerning
the Geiger grit chamber is included as
Appendix B. Although the configuration for
the Geiger device is somewhat similar to the
swirl concentrator, the device is reported to
be efficient only when the flow is relatively
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constant. Experimental testing of the swirl
concentrator has shown that a wide variation
of flow rate can be accommodated without a
loss of efficiency through normal diurnal
variations.
Although the study was performed for
the City of Lancaster, Pennsylvania, with a
specific application defined, all work was
accomplished in a manner which allows ready
application of the resultant design parameters
to conditions which might be found at other
installations and for other purposes.
The Metropolitan Denver Sanitary
District No. 1 constructed a swirl
concentrator/grit separator based upon the
flow requirements of the City of Lancaster.
The unit cost was approximately $4,500,
without necessary valve and grit washing
mechanism — both of which were available to
the District. The co§t and size of the unit is
less than for a conventional unit: Preliminary
evaluation has indicated' satisfactory
operation. Full-scale evaluation is; planned and
will be reported separately. .
Spoilers for
high discharges.
15.3cm;
r-(6")Spoilers
'6.3 em
(2 1/2"
(l 2") 30.5<;m.
(24")
61cm
ELEVATION
section A-A
PLAN
(6")Spo!lers
15.3cm
122cm ,
(48")
Spoilers for
high discharges
Oownplpe
supports
Section B-B
FIGURE 1 SWIRL CONCENTRATOR AS A GRIT SEPARATOR ,
FINAL FORM
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SECTION II
THE STUDY
The ability of a swirl concentrator to
remove solids from a liquid flow field by
means of hydraulic separation, led to the
rational suggestion that an extension of this
principle to other liquid-solids separation
phases of wastewater treatment should be
investigated. Among the proposals for
second-generation study in the report on the
"Swirl Concentrator as a Combined Sewer
Overflow Regulator Facility" —
EPA-R2-72-008. September 1972, was one of
direct relationship to the Lancaster,
Pennsylvania overflow storage-pump
back-partial treatment installation: namely,
the study of the applicability of the swirl
concentrator principle to the problem of
separating grit from combined wastewater
flows and the removal of this inorganic
component of pollution wastes from the
organic portions which could then be diverted
to interceptor-treatment facilities.
The current study, covered by this
Report "The Swirl Concentrator as a Grit
Separator Device" resulted from this
recommendation.
Grit Removal:
Basic Principles and Practices
The degtitting of wastewater is common :
practice. It is one of the conventional
pretreatment stages in sewage and/or
industrial wastes treatment plants. The
removal of inorganic grit is provided to
prevent excessive wear on subsequent
handling operations such as pumping,
comminuting and screening of sewage and
pumping of sludge. Elimination of inert solids
prevents deposition of such material in
settling tanks, sludge hoppers, sludge
digestion chambers, aeration chambers,
pipelines and other locations.
The removal of grit material is normally
carried out by hydraulic classification — a
procedure for separating inorganic and heavier
solids from lighter organic materials contained
in wastewater flows. The principle involves
separation or classification by means of
flow-rate control, thus utilizing the difference
in settling rates, or buoyancy, between the
different specific gravities of these two types
of wastes solids.
Design of sewers is based on the principle
that average sewage solids — organic and
inorganic in character — can be held in
suspension in a so-called self-scouring sewer
line at flow velocities over 0.61 m (2 ft) per
second. Similarly, grit chamber design is based
on the principle that heavier grit will settle at
velocities of flow of 0.3 m (1 ft) per second,
while lighter organics will be held in
suspension under these hydraulic conditions
until they reach settling chambers where flow
velocities are reduced to rates in the general
range of 0.3 m ( 1 ft) per minute more or less.
This, then, is the basic criterion for the
separation of solids-from-solids in grit units,
and the separation of solids-from-liquid in
clarification or settling chambers.
No grit chamber is a perfect solids
classification device. Some grit may pass
through the chamber, regardless of its
configuration and hydraulics, and some
organics may settle and be intermingled with
the inorganic grit. Grit washers or other
auxiliary solids separation facilities are often
used to remove organics.from deposited grit
to make it possible to dispose of innocuous
inorganics by such means as dumping, use as
fill, application of coarse material to sludge
drying beds, dressing of pathways and other
means.
The flow configurations used for grit
removal are varied. They may provide
rectangular channels or various combinations
of flow-through chambers; they may be
circular or square in shape; they may be
equipped with various types of mechanical
collection and removal devices to free
chambers of such deposits. Grit chambers
may be aerated to provide the agitation
needed for washing the deposited solids and
to move depositions to designated points of
concentration and removal.
Grit may be removed from wastewater
flows by mechanical means, such as screens,
but the effectiveness of solids classification is
diluted by such devices because they depend
on solids size, rather than solids gravimetrics
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for removal. It is obvious that gravity
separation or classification is more positive
and more effective. -
The application of the swirl concentrator
• phenomenon to the task of grit removal is
dependent on the ability to provide flow
velocity conditions and internal hydraulic
patterns which will separate heavier, larger
solid particles from lighter, smaller materials
and to allow the two separated classifications
to be collected and removed at separate
points. In this respect, this application is
different from the successful use of the swirl
concentrator as a total solids separator of
combined wastewater flows, as developed in
the research project which led to the finding
that swirl concentration could be applied to
the task of removing grit materials only.
The APWA Study
The proposal by the APWA Research
Foundation to carry out a study of the
application of the swirl concentration
principle to the function of grit removal was
based on the background data outlined above
and on known principles of grit chamber
design, also as outlined above. Continuity of
purpose with the previous study of swirl
applications for combined wastewater
overflow clarification was proposed. The
study's objective was to achieve a workable,
effective grit removal device for the
Lancaster, Pennsylvania regulator-treatment
installation, and a grit separator design which
would have a broader and more universal
application in the treatment of storm,
combined, domestic and industrial
wastewaters.
A realistic appraisal of the proposal led to
the conclusion that a mere substitute for
conventional grit removal facilities by means
of a swirl concentrator should not be the
proper and ultimate goal of the study. Any
application of the swirl principle, as intended
by the proposal, should be judged on its
ability to perform the grit separation and
removal process more efficiently, more
rapidly by flash classification, and therefore
more expeditiously and economically than
other means of performing this function. This
would apply not only to the specific
configuration and dimensions of a swirl
concentrator-separator for the; Lancaster
project but, in addition, to any .necessary
conformations required to make this method
of grit handling amenable to other uses in
other locations.
The proposal provided for initiation of
studies of the action of simulated mixtures of
solids — synthesized to approximate in model
size the admixture of non-organic grit and
non-grit organics that would actually be
contained in the Lancaster four sewer flows,
following handling in the swirl concentrator
overflow regulator previously studied. This
combined sewer regulator device, it was
estimated, could divert as much as a ton of
concentrated solids to the foul sewer out of
the combined sewer overflow within a few
hours. Removal of the grit portion of this
solids loading would protect downstream
pumps against wear and abrasion and benefit
the sewage treatment processes tributary to
the Lancaster interceptor sewer system.
To perform the outlined studies, it was
planned that the basic configurations of the
original regulator overflow swirl concentrator
chamber be examined and such modifications
as would enhance the ability of this hydraulic
facility be made to: (1) separate; grit from
non-grit solids; (2) concentrate and remove
these heavier solids from the cahinber via a
bottom foul sewer connection and/or
mechanical-physical means; and (3) allow the
organic wastes to overflow over a central weir
and discharge assembly into the Lancaster
interceptor sewer and treatment plant.
The particle flow model established for
the overflow concentrator .study is
substantially applicable to the separation of
grit from organic solids. The assumption of
discrete non-interacting particles i would be
valid throughout the body of: the flow
chamber, except near the bottom where
increased solids concentrations may occur and
where these deposits might not continuously
gravitate to the bottom foul sewer outlet.'
Inlet configurations were developed to
provide proper velocities and .directional
injection of solids into the forced swirl
pattern, under flow variations affected by
normal diurnal and abnormal storm
conditions. Other hydraulic factors affecting
model configurations and internal baffles,
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dampeners, and other appurtenant parts of a
workable swirl chamber were studied in the
model having a scale of 1:2 with the ultimate
Lancaster installation. The Froude relations
were particularly attractive at such a large
scale for all the hydraulic parameters.
Swirl concentrator modifications were
made to meet changes in inflow
characteristics from former non-uniform
flows to constant flows that will produce
full-sewer conditions in the inlet line at all
times. Modifications of the bottom shape of
the chamber, the point of grit outlet and the
foul sewer size were also necessary.
No mathematical modeling studies were
proposed for the grit research project. The
basis for solids and liquid behaviors was
adequately determined in the previous studies
to permit their interpretation in connection
with the solids classification patterns
anticipated in the grit swirl hydraulic
investigation.
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SECTION III
GENERAL FEATURES AND DESIGN
The general features of the hydraulic
model are identified in Figure 2, Isometric
View, Swirl Concentrator as a Grit Separator.
(a) Inlet. — The inlet dimension is
normally designed to allow an inlet velocity
of 0.61 m (2 ft) per second. On this basis the
inlet diameter becomes the controlling
dimension for sizing the unit. A set of curves
has been developed to express the relationship
between flow, inlet dimension, chamber
width and depth. The flow is directed
tangentially so that a "long path" pattern,
maximizing solid separation in the chamber,
may be developed.
(b) Covered Inlet. — The Covered inlet is
a square extension of the inlet which is the
straight line extension of the interior wall of
the inlet extending to its point of tangency.
Its location is important, as flow which is
completing its first revolution in the chamber
strikes, and is deflected inwards, forming an
interior water mass which makes a second
revolution in the chamber, thus creating the
"long path" flow pattern.
Without the deflector, the rotational
forces would quickly create a free vortex
within the chamber, destroying the solid
separations efficiency. The height of the
deflector is the height of the inlet port,
insuring a head above the elevation of the •
inlet, a feature which tends to rapidly direct
solids down towards the floor.
(c) Overflow Weir and Weir Plate. — The
diameter of the weir is a function of the
diameter of the chamber, and of the inlet
dimension. Under normal conditions, the weir
diameter is equal to the chamber diameter
minus twice the inlet dimension. The depth,
or vertical distance from the weir to the flat
floor, is normally twice the inlet dimension.
The height, or rise, of the weir plate is
normally 0.25 x the inlet diameter.
The weir plate connects the overflow weir
to a central column, carrying the clear
overflow to the interceptor and primary
treatment. The horizontal leg of the
downshaft should leave the chamber parallel
to the inlet.
(d) Spoilers. — Spoilers are radial flow
guides, vertically mounted on the weir plate,
extending from the center shaft to the edge of
the weir. They are required to break up the
rotational flow of the liquid above the weir
plate, thus increasing the efficiency of the
weir and the downshaft.
The height of the spoilers is the same as
the inlet diameter. This proportionately
large size, as compared to the combined sewer
overflow regulator, is required because of the.
possible large variations in diurnal flow which
may be anticipated.
. (e) Floor.' - The floor of the unit is level
and is in effect a shelf, the width of the inlet.
(f) Central Hopper. - The central hopper
is used to direct the settling grit particles to a
single delivery point -where they may be
removed to a conveyor for washing and
removal from the system.
The hopper is at an angle of 60 degrees to
the floor. If the angle is less than 45 degrees,
particles will build up at the lip. As the angle
is increased, the problem decreases to an
optimum condition at 60 degrees.
The downshaft elbow must be
sufficiently below the floor to prevent
formation of eddy currents. This depth
appears to be the inlet diameter. Structural
supports for the elbow and actual pipe
connections must be designed to prevent rags
from being caught on a protruding bolt head,
flange or strut. The downshaft should exit
parallel to the inlet to assure a minimum
hydraulic interference for settling particles.
Figure 3, General Design Dimensions, lists
the various important dimensions, which are
given as a function of the inlet diameter, Dj.
A supplemental study was made to
determine how the ratio of depth to width, or
diameter, of the swirl chamber might be
varied to allow flexibility of design.
Basic Criteria of Grit and
Organic Constituents of Inflow
Grit Size. — The character of the grit
reaching any plant will depend on many
factors. Chief among these are the nature of
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A Inlet
B Deflector
C Weir and Weir Plate
D Spoiler
E Floor
F Conical Hopper
FIGURE 2 ISOMETRIC VIEW, SWIRL CONCENTRATOR AS A
GRIT SEPARATOR
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NOTE: Inlet and foul outlet
should be parallel for optimum
hydraulic conditions within
conical hopper.
• Downpipc
•I bow
ELEVATION
Section A-A
FIGURE 3 GENERAL DESIGN DIMENSIONS
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the soil, the age and condition of the sewer
pipe and its joints, pipe slope, catch basin and
street cleaning practices, the type of ground
cover in the tributary area, urban street
conditions, and whether the collection system
consists of separate or combined sewers.
Available data on the mechanical analysis
of grit removed from representative
wastewater treatment plants were compared
to establish criteria for grit sizes for this
study.
Data from eight existing plants located in
the United States and Canada are tabulated in
Table 1, Sieve Analyses of Samples from Grit
Chambers. The original data were adjusted to
correspond with the U.S. sieve numbers and
to indicate percent of weight finer than given
sieve sizes. These sieve analyses are shown
graphically in Figure 4, Gradation Curves of
Samples from Grit Chamber in Model. Most
of the grit particles in the samples are* larger
than 0.2 mm. This may be explained by the
fact that most grit chambers are designed to
remove only grit greater than 0.2 mm size. A
notable exception is the sample from Tampa
where 65 percent of the sample is finer than
0.2mm.
, New York City performed an. extensive
study in 1962 and 1963 on the character of
grit collected in aerated grit chambers and in
other plant structures downstream of the grit
chambers.' Samples were taken at several
locations in grit chambers at thre|e plants. The
analyses of samples taken closest and farthest
from the grit chamber inlet at these plants are
given in Table 2, Sieve Analysis of Samples
from Aerated Grit Chamber, N^w York City
Plants, and plotted in Figure 5, Gradation
Curves of Samples from Aerated Grit
Chambers, New York City Plants. Samples
near the inlet have less than 10 percent of the
sample finer than 0.2 mm while ;samples near
the end of the chamber show 15 to 25
percent finer than 0.2 mm. Over 90 percent
of all the samples are finer than 2.0 mm.
Samples were also taken in aeration tanks,
final settling tanks and digestion tanks at
these three plants. Data on the coarsest
Table 1. SIEVE ANALYSIS OF SAMPLES FROM GRIT CHAMBERS
Percentage Finer by Weight
Sieve
Designation
U.S.
mm Sieve No.
6.3 1/2 in.
4.75 4
3.35 6
2.36 8
2.00 10
0.850 20
0.600 30
0.425 40
0.300 50
0.212 70
0.180 80
0.150 100
0.075 200
' (1)
Green Bay
Wis.
I/
96.3
90.9
80.2
70.4
48.3
21.8
3.9
(2)
Kenosha
Wis.
If
88.0
30.0
5.0
(3)
Tampa
Fla.
I/
97.7
40.7
0.5
(4)
St. Paul
Minn.
I/
99.0
95.0-
88.0
80.0
3.0
(5)
St. Paul
Minn.
I/
93.0
80.0
»
-47.0
33.0
0.1
(6) •'" (7)
Winnipeg Winnipeg
Manitoba Manitoba
2/
96.9
83.2
44.3
19.2
4.4
;.. 2/
77.1
:46.3
|:38.9
14.7
6.3
3.5
1.3
(8)
Denver
Colo.
21
94.9
89.2.
75.2
6.7
0.7
Notes:
I/Adapted from data in ASCE Manual No. 36, 1959 edition
2/A11 data adapted from correspondence, 1973
(4) Lower range
(5) Upper range
(6) Inlet end
(7)' Outlet end
10
-------
samples from these structures are shown in
Table 3, Sieve Analysis of Samples from
Structures Downstream of Aerated Grit
Chambers, New York City Plants, and Figure
6, Gradation Curves of Samples from
Structures Downstream of Aerated Grit
Chambers, New York City Plants. The grit at
.these, downstream points is much finer than
Table 2. SIEVE ANALYSES OF SAMPLES FROM AERATED GRIT CHAMBERS
NEW YORK CITY PLANTS
Sieve
Designation -
mm U.S. Sieve
No.
4.75
1.18
0.850
0.600
0.425
0.300
0.212
0.150
0.075
4.
16
20
30
40
50
70
100
200
(9)
Rockaway
99
90
83
74
26
'7.
2
Percentage Finer by Weight
(10) (11) (12)
Rockaway Coney Island Coney Island
100
100
99
98
62
18
4
99
94
86
65
13
5
1
100
99
98
94
45
16
4:
(13)
Jamaica Bay
98
87
80
68
40
15
4
2
r
(14)
Jamaica Bay
100
99
98
96
88
55
26
10
2
Notes: All data adapted from correspondence
(9) 10 feet from inlet, Feb., 1963
(10) 46 feet from inlet, Feb., 1963
(11) Chamber No. 3, May, 1962
(12) Chamber No. 4, May, 1962
(13) Bay No. 1, Dec., 1963
(14) Bay No. 3, Dec., 1963
Table 3. SIEVE ANALYSES OF SAMPLES FROM STRUCTURES DOWNSTREAM
OF AERATED GRIT CHAMBERS
NEW YORK CITY PLANTS
Sieve Designation
mm U.S. Sieve No.
4.75 4
1.18- 16
0.850 20
0.600 30
0.300 50
0.212 70
0.150 100
Percentage Finer by Weight
(15) (16) (17)
Rockaway Coney Island Rockaway
100
96
100 91
99 99 86
68 60 68
29 20 42
8 8 22
Notes: All data adapted from correspondence
(15) Aeration tank, Mar., 1963
(16) Final settling tank, Jan., 1963
(17) Digestion tank
11
-------
100
SIZE OPENINt
INCHES
3/4 Vt 3/1 1/4 4 •
U.S. STANDARD SIEVE NUMBERS
• K> It 1C CO 9O 40 SO TO 100 140 ! 200
O
UJ
>•
m
tn
UJ
30
20
10
30
20
10
20.0
tO.0
C.O 4 O
tO 1.0 0.« O4
GRAIN SIZE IN MM.
o.t
O.I 0.06
FINE
GRAVEL
COARSE I
MEDIUM
FINE
SAND
LEGEND
GREEN BAY ©
KENOSHA (D
TAMPA (D
ST. PAUL (LOWER RANGE) @
ST. PAUL (UPPER RANGE) (§)
WINNIPEG (INLET END) (D
WINNIPEG (OUTLET END) (?)
METRO DENVER (D
FIGURE 4 GRADATION CURVES OF SAMPLES FROM GRIT CHAMBER
12
-------
1
100
90
SO
70
SO
SO
40
30
JSO
10
0
£0
ROC
ROC
cor
cor
JAft
JAF
SIZE ©PENI&J8
INCHES
s/4 t/t V8
1/4
.0 KXO 60
FINE
GRAVEL
4
*•»*
8
&
U.S. ST&NB&R
i W IS !•
^
(T
:>^\
^
s
s
0 S3 EVE NUkligRS
SO SO 40 80 70 IOO 140
^
55
^
•«*
s
&
\
\\
W]
u
\i-~-®
Mi
YJ ^ vU
\VJ
l\'l
iv v\\
M
ll
Iv
^
4.0 S..O I.O 0.6 04 0
GRAIN SIZE IN MM.
i
\\
^±
S O.I
^ewff?? Mgbiuy i FNE
SAND
LEGEND
JKAWAY, N.Y WPCP (10 FEET FROM INLET)
;KAWAY, N.Y WPCP HBFEETFROM INLET)
JEY ISLAND, N.Y. WPCP (CHAMBER NO. 3 )
JEY ISLAND, N.Y. WPCP (CHAMBER NO. 4 )
/IAICA BAY, N.Y. WPCP (BAY!)
rtAICA BAY, N.Y. WPCP (BAY 3)
FIGURE 5 GRADATION CURVES OF SAMPLES FROM A!
GRIT CHAMBERS
£00
0.
®,,,,.
100
90
•0
1-
70 X
&
UJ
80
ffi
SO
ec.
UJ
z
40 U.
30
20
10
0
08
®
fi>
fa •
:RATED
13
-------
,oo!
to
•0
70
•0
SO
40
30
ZQ
10
o
2C
ROCH
CONE
ROCh
SIZE OPENING
INCHES
/4 1/13/11/44
1 1
1
1
«•!
1.0 W.O « 0
FINE
GRAVEL
<
=P
U.S. STANDAR
• 10 It l«
— i-
1 i
"X
•
V
b
so
1
S,
£
«C
>IE
9
-
S
:VE NUMBERS
0 40 SO 70 100 140 200
\
\
S,
>
1
N\
^
1 '
\
(©-
l\\
\\\
1
' '
llr
\\ \
^
\-®
\
1
1
IOO
90
to
K
70 *
&
tit
»
60
>•
CD
90 K
\tt
Z
40 U.
#
90
20
10
0
4 0 t'.O l!o 0 • 04 o'.ft o'.l 0.06
GRAIN SIZE IN MM.
COARSE 1 MEDIUM I FINE
SAND
LEGEND
CAWAY, N.Y. WPCP (AERATION TANK)
:Y ISLAND, N.Y. WPCP (FINAL SETTLING TANK
CAWAY, N.Y. WPCP (DIGESTION TANK)
FIGURE 6 GRADATION CURVES OF SAMPLES FROM STR
DOWNSTREAM OF AERATED GRATE CHAMBE
NEW YORK CITY PLANT
i
;) ©
UCTURES
RS,
14
-------
that captured in the grit chamber, with about
one-third finer than 0.2 mm and all finer than
1.0 mm, except for grit deposited in the
Rockaway digestion tank.
Based on the foregoing, a "typical grit"
for purposes of this study was assumed to
range in size from 0.2 mm to 2.0 mm, with a
gradation corresponding to a straight line on a
mechanical analysis graph.
The assumed gradation is given in Table
4, Typical Grit Gradation, and shown
graphically in Figure 7, Gradation Curve of
Typical Grit.
Table 4
TYPICAL GRIT GRADATION
Size
mm
2.00
0.850
0.425
0.300
0.212
U.S.
Sieve No.
10
20
40
50
70
% Finer
by weight
100
63
31
18
0
Specific gravity of the typical grit is assumed
to be 2.65.
Based on the Unified Soil Classification
System, the typical grit consists of fine sand
with size from 0.212 mm (U.S. Sieve No. 70)
to 0.425 mm (U.S. Sieve No. 40), and
medium sand with size from the latter value
to 2.00 mm (U.S. Sieve No. 10).
Grit Quantity. — A comparison of the
quantity of grit collected in separate and
combined sewer systems, as reported in ASCE
Manual No. 361, was made, based on 11 cities
with combined sewers, and 15 cities with
separate sewers.
This comparison indicated the following
range in the maximum quantities of grit
collected in grit chambers.
Combined Separate
Sewers Sewers
11 Cities IS Cities
Miligrams/liter
Low 32 11
High 290 170
Cubic feet/million gallons
Low 2.7 0.9
High 24.1 14.1
Pounds/million gallons
(assuming 100 Ibs/cf)
Low 270 90
High 2410 1410
It should be noted that two cities, Battle
Creek and Cleveland, in ASCE Manual No. 36,
reported grit quantities in storm periods
which on the above basis would be equivalent
to 2,000 mg/1 and 6,500 mg/1, respectively.
While data are available on the quantity
of grit removed from grit chambers, no
information is available either on total grit
arriving at a plant or on the percent of total
grit recovered in the grit chambers.
Experimental work by Chasick and Burger in
New York City in 19642 indicated that a
cyclone grit separator would remove
practically all sand of 0.1 mm size and
greater. Subsequent work by Neighbor and
Cooper in 19653 indicated that the proper
installation of baffles in aerated grit chambers
could raise the recovery rate of grit of 0.2 mm
size and greater, from 70 to 90 percent. Later
work by Albrecht published in 19674
indicated that with proper design an aerated
grit chamber could remove 95 percent of grit
0.25 mm in size and greater. The last
conclusion is consistent with the removal of
90 percent of grit of 0.2 mm size and larger.
From the foregoing, it appears reasonable
to conclude that the maximum quantities'of
grit 0.2 mm in size and larger will range from
40 to 360 mg/1 in combined sewers and from
20 to 200 mg/1 in separate sewers and that the
grit recovered in the grit chamber will be 90
percent of the foregoing and will range from
36 to 324 mg/1 in a combined sewer system
and from 18 to 180 mg/1 in a separate sewer
system.
In a previous EPA report on the swirl
concentrator5 it was also assumed that the
maximum concentration of grit in combined
sewers would be 360 mg/1.
The summary of grit characteristics used
for this report is given in Table 5, Specific
Gravity, Size arid Concentration of Settleable
Solids.
Organic Constituents. — The character of
settleable solids, excluding grit and floatable
solids, is considered to be the same in sanitary
sewers as in combined sewers except for the
concentration. Based on conclusions reached
in the previous EPA report on the swirl
concentrator5 the quantity and quality of the
settleable solids in both sanitary and
combined sewers are given in Table 5.
Non-settleable and dissolved solids are not
15
-------
SIZE OPENING
INCHES
,ooa
to
•0
70
60
SO
4O
30
20
to
0
M
I/* l/t S/t 1/4 4
1 1
1
1
U.S. STANDARD SIEVE NUMBERS
6 6 K> It 16 20 90 40 90 70 100 140 200
1
.0 KXO 60
FINE
GRAVEL
1
I
V
\
\
s
1
\
\
,
1
\
1
\
\
\
1 I
\
\
.
i
1
1
1
1
1
40 t.O 1.0 0.6 04 0.2 O.I 0.
GRAIN SIZE IN MM.
COARSE 1 MEDIUM 1 FINE
SAND
SIZE
2.
0.
0.
0.
P.
GRADATION
U.S.
% FINER
100
90
•0
" 1
Ul
60
00
50 ^
\H
Z
40 U.
30
20
10
0
36
•i
MM SIEVE BY WEIGHT
00
NO.
IO
100
850 .20 63
425 40 31
300 50 18 i
212 70 O
FIGURE? GRADATION CURVE OF TYPICAL GRIT
16
-------
Table 5. SPECIFIC GRAVITY, SIZE AND CONCENTRATION OF SETTLEABLE SOLIDS
Material
Combined Sanitary and Storm Sewage
(1) Settleable Solids
Excluding Grit
(2) Grit
(3) Floatable Solids
(1) Settleable Solids
Excluding Grit
(2) Grit (see Table 4)
(3) Floatable Solids
Sanitary Sewage
Same as above except for concentration
(1) Settleable Solids Excluding Grit
(2) Grit
(3) Floatable Solids
Specific Concentration
Gravity mg/1
1.05-1.2 200-1550
2.65 ' 40-360
0.9-.998 10-80
Particle Size Distribution
Size (mm)
% by weight
Size (mm)
0.2
10
0.5
10
10
Particle
Size
mm
0.2-5
0.2-2.0
5-25
1.0 2.5
15 25
15 20
5.0
40
25
Concentration
mg/1
200-500
20-200
10- 80
considered herein because it is assumed that
the -swirl concentrator will not affect their
removal.
Flow Rate, and Overflow and
Foul Discharge Rates
The primary purpose of this study was to
design a grit removal device capable of
performing the functions of a conventional
grit chamber, to accommodate the high solid
flow from a combined sewer overflow
regulation with a flow of from 0.028 m3/sec
(1 cfs) to 0.146 m3/sec (5 cfs). This would
conform with the following design criteria:
Peak flow 0.140m3/sec (5 cfs)
Average flow 0.084 m3 /sec (3 cfs)
Minimum flow 0.028 m3 /sec (1 cfs)
When the use of a swirl concentration grit
chamber is considered for sanitary sewage
flow the flow hydrograph will vary from one
location to another but it will generally
follow a diurnal pattern of maximum flows in
the daylight hours and minimum flows in the
late night hours.
Investigations by others have indicated
that the variation in concentration of
suspended matter is greater than the variation
in the flow. The concentration of suspended
matter can be expected to be less than 40
percent of the average at low flows and to be
greater than 150 percent of average during
peak flows. During periods of low flow grit
would be expected to settle out in the sewers
and subsequently to be flushed out when the
flows exceed the average rate. Assuming
periodic flows with a greater concentration of
i grit than the average concentration, it might.
be possible for 70 to 80 percent of the
suspended matter to be discharged during the
period of peak flow. Hence, the grit chamber
must be designed to remove grit at maximum
efficiency during daily peak flow periods.
It is also necessary to operate at
reasonable efficiency during low flow periods.
Grit chambers designed on the basis of
velocity control 'accomplish this by
controlling the velocity through the chamber
between 0.229 m/sec (0.75 fps) and 0.366
17
-------
m/sec (1.2 fps). *Ih addition, removal of grit
may not be achieved during low flow periods
but postponed until peak flow periods when
the higher velocities will keep the organics in
suspension. Aerated grit chambers are
designed to provide a continuous velocity of
flow with a transverse velocity between 0.458
m/sec (1.5 fps) and 0.61 m/sec (2.0 fps) to
keep the organics in suspension.
In the swirl concentrator it is hoped that
the rotational flow pattern induced by the
entrance velocity will be sufficient at all times
to keep the organic matter in suspension
while permitting the settling of the grit. Since
this may not always occur, it is considered
necessary to use a grit washer to remove
organic materials which settle out with the
grit.
Therefore, one of the initial concepts- in
the use of the swirl concentrator as a grit
chamber involved the use of the grit washer
^vith an inclined screw conveyor. This concept
"is shown in Figure 8, Grit Chamber Above
Ground with Inclined Screw Conveyor. The
determination of the grit quantity to be-
removed is given in Table 6, Grit in Sanitary
Sewage. With a grit concentration range of 20
to 200 mg/1 in sanitary sewage, the range of
grit removal for an average daily flow of
0.084 m3sec (3 cfs) would be from 0.002 to
0.0424 m3 /hr (0.1 to 1.2 cuft/hr).
Using data from one manufacturer of this
type of grit washer, this grit quantity would
require a washer with a screw diameter of
22.9 cm (9 in.) with capacity of 0.56 m3/hr
Table 6. GRIT IN SANITARY SEWAGE
Min
m3/sec
1. Sanitary Sewage
flow
2, cfs
3. Grit concentrations mg/1
4. Grit removal mg/1
5. Weight of grit kg/hr
6. Ibs/hr
7. Volume of grit m3/hr
8. cuft/hr
Notes:
Line 4 » 90% x Line 3
Line 6s _3_ x line 4 x 8.345
11.55 24
Max
0.084
3
20
18
5.5
12
0.002
0.1
0.084
3
200
180
55
120
0.024
1.2
(20 cuft/hr) and would require a;wash water
rate of 0.171 m3/rnin (45 gpm). The wash
water in this case would be the quantity of
sewage allowed to flow up the screw casing
from the swirl chamber with the grit. This
would keep the organics in suspension and be
returned to the effluent pipe from the grit
chamber after flowing over _the adjustable
weir.
The secondary purpose of this study was
to determine the feasibility of using the swirl
concentrator for further concentrating the
grit contained in the foul outlet from a swirl
concentrator used as a combined sewer
overflow regulator at Lancaster, Pennsylvania.
The design flow for the regulator in Lancaster
is 4.62 m3 /sec (165 cfs). The flow in the foul
outlet is expected to range from 0.042 m3 /sec
(1.5 cfs) to 0.140 m3/sec (5.0 cfs). Thus, the
influent to the swirl grit chamber will be the
effluent from the foul outlet of the regulator.
The determination of the settleable solids in
the foul outlet from the swirl regulator, based
on the concentrations shown in; Table 5, is
given in Table 7, Settleable Solids in Swirl
Concentrator Used as a Combined Sewer
Overflow Regulator at Lancaster. ,
The derivation of 68 percent in line 7 of
Table 7 is based on data in EPA Report
EPA-R2-72-008. Figure 22 of that report
indicates that the swirl concentrator will
remove 90 percent of grit larger than 0.35
mm. Figure 7 of this grit swirl concentrator'
report indicates that 76 percent of the typical
grit is larger than 0.35 mm. Therefore, the
swirl concentrator will remove 68 percent
(90% x 76) of the total grit entering the storm
water regulator. If the grit chamber is sized on
the basis of the design curves which are based
on removal of 90 percent of grit over the 0.2
mm size, then it is reasonable to (assume that
the chamber will remove 100 percent of the
grit over 0.35 mm in size. Thus, the.grit to be
removed is from 0.28 to 2.52 m3 /hr (10 to 90
cuft/hr). According to da£a 'of one
manufacturer, this will require a screw with
diameter of 50.8 cm (20 in.) with capacity of
3.5 m3/hr (124 cuft/hr), having a wash water
rate of 0.36 m-3'/iriiri (95 gpm). :
The settleable solids, excluding grit, in
the foul outlet flow from the combined sewer
overflow regulator will vary from 12,700 to
29,600 mg/1, as shown'in' Table 7. This is
18
-------
Wash water
Overflow Weir
Grit Washer
Wa sh water
Outlet
Adju stable
Weir
Grit Washer
and Elevator
Sect! on A-A
FIGURE 8 GRIT CHAMBER ABOVE GROUND WITH INCLINED
SCREW CONVEYOR
19
-------
based on the design curves in EPA Report
EPA-R2-72-0085. Figure 22 of that report
indicates that the regulator will remove 90
percent of settleable solids, excluding grit
larger than 1.0 mm. Figure 3 of that report
indicates that 65 percent of settleable solids,
excluding grit, are larger than 1.0 mm. Hence
58 percent (90% x 65) of the settleable solids,
excluding grit, will be removed through the
foul inlet. ...
Table 7. SETTLEABLE SOLIDS IN SWIRL CONCENTRATOR
USED AS COMBINED SEWER REGULATOR AT LANCASTER
Flow
1. Combined flow m3 /sec
2. (cfs)
3. Flow in foul outlet m3 /sec
4. (cfs)
5. Ratio combined to foul outlet flow
Grit
6. Concentration in inflow mg/1
7. Percentage of total grit in foul outlet
8. Concentration in foul outlet mg/1
9. Kilogram/hour
10. (Lbs/hr)
II. m3/hr
12. (cuft/hr)
13. Grit percentage by weight
Settleable solids excluding grit
14. Concentration mg/1
15. Percentage in foul outlet
16. Concentration in foul outlet mg/1
17. Percentage by weight in foul outlet
Total settleable solids
18. Percentage by weight in foul outlet
Min
4.62
(165)
0.042
(1.5)
110
40
68
3000
457
(1010)
0.28
(10)
•0.3
200
58
12,700
1.3
1.6
Notes:
Line 7 = 90% x 76
Line 8 = (line 6 x line 5 x line 7) + 100
Line 10 = (line 8 x 8.345 x line 4 x 0.646) * 24
Line 11 Assuming grit weighs 1640 Kg/m3 (100 Ib/cuft)
Line 13= line 8-^ 10,000
Line 15= 90%x65
Line 16 = (line 14 x line 5 x line 15) * 100
Line 17= line 16-^ 10,000
Line 18 = line 13 + line 17
Max
4.62
(165)
0.14
(5.0)
33
360
68
8100
4100
(9060)
2.52
(90)
0.8
1550
58
29,600
3.0
3.8
20
-------
As shown in Table 7, the inflow to the
grit chamber at Lancaster may have
concentrations of 1.3 to 3.0 percent settleable
solids, excluding grit, 0.3 to 0.8 percent grit,
and 1.6 to 3.8 percent total settleable solids.
Of this material, it is assumed that the grit
chamber will remove all the grit Which may
range from 0.28 to 2.52 m3/hr (1 0 to 90
cuft/hr).
In practice, because of the highly variable
rate of grit discharge from the combined
sewer overflow regulator, it may be desirable
to eliminate the washing of the grit because of
the difficulty in adjusting the wash water flow
rate to the varying concentrations of grit.
Design Features
Grit Removal. — The first concept for
removing the grit involved the use of an
inclined screw and grit washer. If the chamber
is located above ground with an inclined
screw conveyor discharging above ground, the
proportions of the structure would be as
shown in Figure 8. The most suitable material
for construction would be structural steel
shapes and carbon steel plates.
If both the grit chamber and the inclined
screw conveyor are located below ground the
layout may be as shown in Figure 9, Grit
Chamber Below Ground with Inclined Screw
Conveyor. Access must be provided to the
lower part of che screw for lubrication and
maintenance . purposes. The simplest
construction would be to- make the chamber
of steel, as in the previous case, and construct
a concrete rectangular chamber to provide
access, the inclined screw must be lengthened
for the previous case in order to get the
delivery point sufficiently high above ground
level to discharge the grit into a storage
container. From the proportions shown in
Figure 9, the length of the screw conveyqr is
3-1/3 times the diameter. This length may
become excessive for larger diameter chambers
and other methods of removing the grit
may have to be considered, such as a
depressed ramp.
Another method of removing the grit,
which is sometimes used in aerated grit
chambers, is a horizontal screw to convey the
grit to a manhole with a bucket elevator to
lift the grit to the grit washer located above
ground level. This is shown in Figure 10, Grit
Chamber Below Ground with Hprizontal
Screw Conveyor. Access must be provided to
the horizontal screw for lubrication and
maintenance; therefore, it would appear most
feasible to construct the grit chamber of steel
and install it in a concrete box as shown. This
structure is similar to that of an aerated grit
chamber except that the steel grit chamber
supplants the aeration equipment and baffles.
A comparison of dimensions seems in
order. For a design flow of 0.056 m3/sec (2
cfs) and 95 percent efficiency, and an inlet
diameter of 0.305m (1 ft) Figure 14 gives a
D2 of 1.8 meters (6 ft). On Figure 13, the
intersection of 0.056m3/sec (2 cfs) and
0.305m (1 ft) lies between the upper and
lower limits and is therefore satisfactory.
From Figures 10 and 15 it appears the
concrete structure would be about 1.8m (6 ft)
deep and 1.8m (6 ft)-wide and 2.4m (8 ft)
long excluding the bucket elevator chamber.
Aerated grit chambers are usually
designed on the basis of 2 to 3 minutes
detention time at peak design flow. If a 2.4
minute detention time were selected the
chamber would be the same size as indicated
above. Therefore, the concrete structures
would be similar in size for the two types.
Another device that might be used to
elevate the grit is a tubular conveyor. In this
case, the outer structure of the chamber could
be of poured concrete. The tubular conveyor
could be installed in a chase or recess in the
concrete structure. The recess should be
provided with a removable cpver so that all
sections of the conveyor could be reached for
maintenance purposes. The layout of the .
chamber using a tubular conveyor is shown in
Figure 11, Grit Chamber Below Ground with
Tubular Conveyor. Grit dropped into the grit
washer would be relatively dry and it would
be necessary to provide a water supply to the
grit washer.
Another method of removing the grit is
by use of an air lift. This would require the
addition of equipment to provide compressed
air unless such air was also provided for other
purposes at the site. This would require the
installation of two vertical pipes in the
chamber which might interfere with the
rotational flow in the chamber.
Wash Water. — The wash water required
for the grit washer can be supplied from
21
-------
Grit
Chamber
Wash water
Overflow Weir
v- Grit
Wash Water Outlet
Adjustable
Washer
Elevator
Section A-A
FIGURE 9 GRIT CHAMBER BELOW GROUND WITH INCLINED
SCREW CHAMBER
22
-------
Grit Chamber
Grit Washer
Outlet
Water Supply Line
Drive Unit for Bucket
Elevator and
Screw Conveyor
- Qrit Washer
Outlet
Bucket
Elevator
- Screw Conveyor
Seetiona! Elsvalion
FIGURE 10 GRIT CHAMBER BELOW GROUND WITH HORIZONTAL SCREW CONVEYOR
23
-------
Grit Washer
Grit Chamber
Tubular
Conveyor
Wash Water Outlet
Tubular
Conveyor __
Ground
Grit Washer
and Elevator
Grit Can
Sectional Elevation
FIGURE 11 GRIT CHAMBER BELOW GROUND WITH TUBULAR CONVEYOR
24
-------
sewage discharged out of the foul outlet with
the grit. The area of the space above the screw
•should be designed so that the velocity of the
wash water flowing upward will be between
0.042 to 0.070 m/sec (1.5 to 2.5 fps).
Velocities lower than this may permit organic
matter to settle out and velocities 'above this
may produce an upward movement and loss
.of the grit. The adjustable weir must be set so
that the required flow of wash water is
obtained. If the weir is set high so that the
wash water rate is lower than the design rate',
the grit will contain a larger amount of
organic matter. The area of the water surface
upstream of the adjustable weir must be such
that the surface loading of the wash water
rate shall be at least 0.55 m3/min/m2 (13.5
gpm/sq. ft).
Other Facility Factors
Before using the swirl concentrator as a
grit chamber, designers should make a
comparison of the various alternatives. For
large flows the swirl concentrator with a
cone-shaped hopper may require a depth
greater than the more conventional grit
chambers. The presence of high ground water
or bed rock may affect cost estimates
appreciably if the deeper structure is used.
Another major factor is the head available
and the effect of the grit chamber on the
hydraulic flow line of the plant. If a particular
type of grit chamber requires the addition of
pumping facilities it is doubtful if its use can
be justified on an economic basis.
The maintenance of the swirl
concentrator grit chamber should not be
materially different from the maintenance'of
conventional grit chambers. In line with the
usual practice, at least two units — one for
standby - should be constructed so that the.
removal of grit can be continued when one
unit is taken out of service.
The mechanical equipment should be
provided with electrical devices so that the
equipment can be operated either
continuously, or intermittently as regulated
by a time clock, or manually. It is not certain
to what extent organic matter will settle out
in the conical hopper during low flow periods.
For this reason, it may be necessary to
operate the grit washer intermittently at such
times to prevent such accumulations of
organic matter in the hopper.
Design •
The following sequence is recommended
for the design of the swirl concentrator as a
grit separator.
1. Select Design Discharge. The design
engineer must select the design discharge
appropriate to each project based on the
design criteria for the project.
The normal application of the swirl
concentrator as a grit chamber would be its
use in a wastewater treatment plant. In such
an application the grit chamber should be
designed for the maximum design flow.
Another application of the swirl
concentrator considered in connection with
this study was as a grit removal device for the
foul flow from a swirl concentrator used as a
regulator. In that case the design flow for the
grit chamber should be based on the foul flow
discharge from the overflow regulator. It is
proposed to use the grit chamber for this
purpose in Lancaster to remove grit from the
foul flow prior to pumping the foul flow to
the treatment plant. At Lancaster it is also
proposed to regulate the foul flow discharge
from the overflow regulator. A third.
application would be as a combined sewer
overflow or stormwater treatment plant unit
process. Where grit is a problem prior to
syphons or pumping stations within the
collector system, the swirl unit can also be
used. Therefore, the designer must select the
design flow based on these considerations.
2. Select the Operating Efficiency. With
a discharge determined as above, 90 percent
recovery is suggested as an acceptable
operation. However, if there is the possibility
for any future but undefined increase in the
discharge, using 95 percent recovery would
provide some extra capacity.
3. Find the Square Inlet Dimension —
Dt. Having selected the desired recovery rate
and the design discharge, the corresponding
figure in the series of Figures 12., Chamber
Diameters for 95 Percent Recovery and
H!/D! = 2; 13, Chamber Diameters for 90
Percent Recovery and HjDi = 2; and 14,
Chamber Diameters for 80 Percent Recovery
and HJ/D! = 2, would be used. Enter the
25
-------
2 -
mVs
Discharge
m ft
40
IO
a 5
- 30
_ 20
10
167
0.222
278
333
:
10 20 30 4O 50 60 70 80 cf*
FIGURE 12
0.5
CHAMBER DIAMETERS FOR
RECOVERY AND H, /D, = 2
1.0 1.5
Discharge
95 PERCENT
2.0
^5 m'/s
26
-------
m. ft.
8
25
. 20
Q
I
fe
|3
j:
O
15
10
ft
. 40
. 30
S
€>
E
D
JO
E
o
•05
3 -
. 20
10
JL
IO 20 3O 40 5O 60 70 80 cfs
0.5
_L
_L
1.0 1.5
Discharge
J_
2.0
J_
2.5 ms/s
4.
10 12 14 16
20
O.I
0.4
0.5 ms/s
0.2 0.3
Discharge
FTriTUF 1 * CHAMBER DIAMETERS FOR 90 PERCENT
f lUUKfc 13 RECOVERY AND Ht /Dt = 2
27
-------
7
6
l
•±5
1
o
ft
25
20
15
46 8 10 12 14 16 IBcf«20
J I LJ 1 '—I 1
0.1
0.2 0.3
Discharge
0.4
0.5 m»/»
m ft
40
010
!
. 30
20
Jfl.
IO 20 3O 40 50 SO 70 80 cfs
0.5
1.0
1.5
2.0
2.5 m'/s
Discharge
FIGURE 14 CHAMBER DIAMETERS FOR 80 PERCENT .
RECOVERY AND R1/D1 =2
28
-------
figure with the design discharge, and follow
this vertically upward to the broken Dj line
which most nearly corresponds to the supply
sewer diameter. It might be advantageous to
select a larger or smaller Dt to coincide
exactly with the supply sewer size. In the
model tests, the square inlet dimension was
the same as the supply sewer diameter, so
these are the ideal operating conditions for
this unit.
In cases where the square inlet dimension
cannot conveniently be made the same as the
supply sewer, a reducing. or expanding
transition would be necessary. If the supply
sewer is concentrically aligned with the inlet,
the transition should have a length of at least
three times Dj (SDj). Another possibility
would be to have the supply sewer discharge
into an inspection manhole. Leaving the
manhole would be the square inlet
cross section leading into the grit chamber.
The distance from this manhole offtake to the
square inlet discharge in the chamber should
also be a minimum of 3 times Dj (3Dj). This
arrangement could be used to provide the
transition in directions, levels or sizes between
the supply sewer and the square inlet.
4. Find Grit Chamber Diameter —
£>2. The intersection point found in 3 above
defines the chamber diameter, D2 on the
ordinate scale at left. In the considerations for
choosing Dj it might be a valuable aid to
check the D2 size as well. Taking a smaller Dx
means a larger- D2 is necessary; there could
well be an economical or practical optimum
relation between the two dimensions.
5. Select Design Discharge. As stated
previously, another purpose of this study was
to provide criteria for designing a grit
chamber for a daily sanitary sewage flow
varying between 85 and 425 1/s (3 and 15 cfs)
with an inlet pipe diameter of 61 cm (2 ft).
Assume the designer decides to remove
90 percent of the grit over 0.2 mm size with
Inlet Dimension DI
Ratio of Height to Width
Chamber Diameter D2
Chamber Height H!
0.55m (1.8 ft)
0.33
3.3m (10.8 ft)
l.lm (3.6 ft)
discharge at the maximum rate.
Enter Figure 12 with 426 1/s (15 cfs).
Read D2 = 4.88 m (16 ft). Also interpolate
Hi /Da = 0.25. Therefore Ht = 1.22m (4.0 ft).
On Figure 15 the intersection of 0.43m 3/s
(15 cfs) and 61 cm (2 ft) lies between the
upper and lower limits and is therefore
satisfactory. From Figure 15, Approximate
Stage — Discharge Curves Over Weir, the head
on the weir is about 24 cm (0.78 ft).
6. Find Depth-Width Ratio -
•Hi/D2.The same discharge — D/ — D2
intersection point on the pertinent figure in
the Figures 12,13 and 14 series also defines'
the Hj /D2 ratio with respect to the solid lines
on the figure. Interpolation between lines can
be done without extreme care as slight
changes in the ratio are not critical to the
structure's operation. With the ratio
determined, multiplication by the chamber
diameter found in 5 above gives the weir
height.
7. Find Dimensions of Complete Unit
Using D! D2 and H! as found above, go into
Figure 3 to compute dimensions of all
pertinent elements of the structure.
8. Find Water Level in Chamber. With
the unit completely dimensioned, it would
then have to be set with respect to the level of
the incoming sewer. Figure 15 gives the
approximate levels of the water in the
chamber as a function of the flow over the
weir lip.
Height to Width Relationships
To increase the flexibility for design, the
variations of required -chamber diameter and
height or depth were determined for various
inlet dimensions, at various levels of efficiency.
Thus, for design flows of 0.2m3 /sec (7 cfs),
and 95 percent recovery, the following
relationships of design dimensions from
Figure 12 can be used.
0.52m (1.7 ft) 0.48m (1.55 ft) 0.43m (1.4 ft)
0.278 0.222 0.167
3.8m (12.5 ft) 4.3m (14.1 ft) 4.6m (1-5 ft)
1.05m (3.5 ft) 0.95 (3.1 ft) 0.76m (2.5 ft)
29
-------
REFERENCES
1. ASCE, Sewage Treatment Plant
Design, Manual of Practice No. 36, 1959.
2. Chasick, A.H. and Burger, T.B., Using
Graded Sand to Test Grit Removal Apparatus,
Vol. 36, No. 7, July 1964, pp. 884-894.
3. Neighbor, J.B. and Cooper, T.W.,
Design and Operation Criteria for Aerated
Grit Chambers, Water and Sewage Works, Vol.
112, No. 12.
4. Albrecht, A.E., Aerated Grit
Operation Design and Chamber Water and
Sewage Works. Vol. 114, No. 9, Sept. 1967,
pp. 331-335.
5. American Public Works , Association,
The Swirl Concentrator as a Combined Sewer
Overflow Regulator Facility, U.S.
Environmental Protection Agency,
Environmental Protection Technology Series,
EPA-R2-72-008, Sept. 1972. ;
cm
60
50
!30
o
a
o
10
ft.
1.6
• 1.6
1.4
1.2
- 1.0
OB
0.6
0.4
0.2
0
10 20 30 40 50 60 70 80 efs
0.5
1.0 1.5
Discharge
2.0
2.5 mVs
FIGURE 15 APPROXIMATE STAGE - DISCHARGE CURVES OVER WEIR
30
-------
SECTION IV
IMPLEMENTATION
Consideration of the use of a swirl
concentrator as a grit separator device
requires an evaluation of many factors which
include:
1. elevation at which the flow will enter
the facility;
2. the size of the facility based upon
anticipated flow; and
3. an economic comparison with other
types of grit separation devices.
' The operation of the device is predicated
upon the relationship between the inlet
dimension, the chamber diameter, the depth
and the expected operating efficiency. If the
flow is under pressure, then the inlet must be
enlarged to reduce the inlet velocity. The size
of the inlet is the key to determining the
geometric design of the unit.
The rapid increase in depth required with
larger flows suggests that a number of smaller
units operated in parallel would be less costly
to construct and operate as compared to the
costs of units with inlet sizes of three feet or
larger.
The choice of a conventional grit washer
and conveyor 'is predicated upon the
anticipated volume of grit which must be
removed, and the depth of the bottom of the
cone.
As there are no mechanical parts within
the swirl concentrator, maintenance
requirements will be minimal. However, the
grit washer-conveyor system will require
maintenance and points of access in
construction details.
In order to evaluate the efficiency of the
unit, it will be necessary to analyze samples of
the grit collected and of the solids from the
downstream source to determine the size and
amount of grit particles carried through the
unit. In addition, grit from the swirl
concentrator and the grit wash water should
be sampled to learn if an excessive amount of
organic material is being entrained with the
grit. From the survey which was undertaken
by the APWA, it was learned that most
operators interviewed preferred to achieve
high grit removal of small particles, even
though this is generally accompanied by the
removal of an organic loading that may be
equal to the grit loading.
Laboratory studies indicated that a
washing action induced within the swirl
concentrator may minimize organic solid
concentrations in the collected grit.
Cost of Facility. - A prototype unit,
1.83 m (6 ft) in diameter with a 0.305 m (1
ft) inlet has been constructed by the
Metropolitan Sanitary District, of Denver. The
unit is the same size as that required for the
City of Lancaster. The unit was fabricated of
steel and mounted above ground.
Construction cost, exclusive of the grit
conveyor-washer, was approximately $4,500
in 1973.
Figure 16, Prototype Unit, Denver
Colorado, is a photograph of the
demonstration unit constructed by the
Denver Metropolitan Sewage Disposal District
No. 1. '
31
-------
FIGURE 16 PROTOTYPE UNIT, DENVER, COLORADO
32
-------
SECTION V
GLOSSARY OF PERTINENT TERMS
(as applied to this report on the swirl concentrator)
Foul Sewer — The sewer carrying the
mixture of combined sewage and
concentrated settleable solids to the grit
chamber or interceptor sewer from a
combined sewer concentrator overflow
regulator facility.
Grit — Heavier and larger solids which,
because of their size and specific gravity,
settle more readily to the floor of the swirl
concentrator Chamber by the phenomenon of
gravity classification.
Overflow Weir - The structural member
of the swirl concentrator, which is built as a
central circular wall, with a proper form of
overflow edge or crest over which the clarified
wastewater can discharge to the downdraft
outlet leading to predetermined points of
discharge or subsequent holding or treatment
plants.
Regulator — A device or apparatus for
controlling the quantity and quality of
admixtures of sewage and storm water
admitted from a combined sewer collector
sewer into an interceptor sewer or pumping or
treatment facility, thereby determining the
amount and quality of the flows discharged
through an overflow device to receiving
waters, or to combined sewer overflow
retention or treatment facilities.
Scaling — The principle of ascertaining
dimensions and capacities of hydraulic test
units to evaluate the performance of swirl
concentrator chambers, and to up-scale such
sizes to provide actual field design and
construction criteria or parameters.
33
-------
-------
SECTION VI
Page No.
Appendix A, Hydraulic Model Study 37
Appendix B, Grit Chambers for Sewage Treatment Plant
Abridged and Edited from the Archive for Hydraulics 1942;
by H. Geiger, Dr. of Eng.; Translated for the
American Public Works Association by Mrs. Patricia Ure Petersen . . 75
Appendix C, Scale Effects on Particle Settlement, Paul B. Zielinski 91
35
-------
-------
APPENDIX A
REPORT OF HYDRAULIC LABORATORY STUDIES
Previous laboratory work, performed in
developing the swirl concentrator principle,
was directed to its use as a combined sewer
overflow regulator. Studies completed by the
LaSalle Hydraulic Laboratory in February,
1972, made up part of the APWA Report titled
"The Swirl Concentrator-as a Combined Sewer
Overflow Regulator Facility," EPA-R2-72-008,
September 1972.
This earlier work indicated that the
concentrator should be capable of operating
more efficiently on narrower grain-size bands.
The present study was therefore directed to
•separating and removing only the grit from
.the sewage.
The model used in the earlier study was
modified to suit the new parameters.
Additions to and modifications of the study
plan were incorporated into the study
program as test results became available and
were analyzed by the committee of
.consultants.
Principles and Scope of the Study
The swirl concentrator principle, as first
used by Mr. Bernard Smisson in Bristol,
England, and then later modified and
developed by the LaSalle Hydraulic
1 Laboratory, showed great effectiveness as a
solids separator for storm flows in combined
sewer applications. The present investigation
was carried out to apply the knowledge gained
. in these earlier projects to the use of the swirl
concentrator as a grit separation chamber.
The basic swirl concentrator" geometry
resulting from the previous study of the
combined sewer overflow regulator was taken
as the starting point for the present
investigation. Two sets of conditions were
provided to guide the research:
• The first precept was to accept the
bottom foul outlet discharge from the
swirl concentrator overflow regulator at
• Lancaster, Pennsylvania. The discharge was to
be about 42.5 1/s (1.5 cfs) under normal
conditions, and could rise to 141 1/s (5cfs).
The inlet pipe to the grit chamber would be
30.5 cm (1 ft) diameter.
• The second set of conditions was to
investigate a 61 cm (2 ft) inlet pipe diameter,
i assuming a daily sanitary sewage flow varying
between 85 and 425 1/s (3 and 15 cfs)7'The
first case was conceived to comply with the
particular characteristics of the- overall
demonstration project being built at
Lancaster, which would provide a pilot plant
swirl concentrator to serve as a grit chamber.
The second case was to cover the application
.of the swirl grit separation principle to any
municipal treatment plant application,
allowing a general scale-up of the dimensions
over a wide range of different prototype sizes.
Model Description
The main feature of the model was the
separation chamber which took the form of a1
vertical cylinder 91.5 cm (36 in.) in diameter
and 102 cm (40 in.) high, made of 13-mm
(1/2-in.) plexiglas as shown in Figure 17, Model
Layout. Inflow to the chamber was through a
- polyvinyl chloride (PVC) pipe which could
be 7.6, 10.2, 12.7 or 15.2-cm (3, 4, 5 or 6-in.)
diameter, set at a slope of 1:1,000. A
vibrating solids injection system was placed
on this supply pipe, 2.14-m (9 ft) upstream of
the chamber. Water supply to the model
- through the pipe was taken directly from the
: constant level tank in one of the laboratory's
permanent pumping stations.
A flexible 5.1-cm (2-in.) diameter tube.
was placed inside the cylinder, beneath the
floor of the test chamber to pick up the foul
flow. The tube was outletted from the
bottom of the cylinder, .and led to a solids
settling tower fitted with an adjustable level
outlet pipe which could be raised or lowered
at will, to control the discharge drawn off
through the foul outlet.
The clear water outlet came up from the
! base on the centerlirie of the cylinder in the
form of a 15.2-cm (6-in.) diameter PVC pipe.
. Its level could be modified either by adding or
removing sections of the same diameter pipe.
: Outflow from this pipe, which in
operation represents the major part of the
total discharge _ through the structure, was
•delivered to a large settling basin equipped
with a calibrated V-notch weir. The basin
allowed sufficient time for most of the solids
contained in the clearer overflow to settle
37
-------
Foul outflow Foul !»olids
settling tower recovery screen
Chamber cylinder - l/2"(l3mm)
plexiglass- 36"dio. (914mm.)
Clear outflow settling basin
\
A
Calibrated V-notch weir
/
/ '
Clear water overflow
outlet pipe -4 plexiglass (102mm)
•Discharge returned
to pumping »tation
r\
Calibrated V-notch weir
Cleor outflow settling botin
Foul outlet
discharge
control
Butterfly control
valve
Solids hopper
Vibrator
Small water supply
for solids injection
Water supply from
pumping station
Foul Outlet
discharge control
Chamber cylinder - l/2°(l
plexiglass- 36" dia. (914mm)
Foul outflow
settling tower
Butterfly
control valve
Clear water overflow pipe-
4" plexiglass (102mm)
Foul solids
recovery screen
ELEVATION
Section A-A
FIGURE 17 MODEL LAYOUT
38
-------
put. A point gauge on a manometer pot
indicated the level within the basin,
determining the discharge going over the
V-notch weir, hence the clear discharge over
the circular weir in the chamber.
As the model existed at the beginning of
this study, the inlet pipe with its round
cross-section entered on an enlarged
rectangular plexiglas enclosure fixed to the
cylindrical chamber wall in which variations
to the entrance form could be fitted and
tested, as shown in Figure 17.
The floor of the chamber was formed of a
.thin cement mortar crust, supported on a
gravel base which filled the lower portion of
the chamber. This crust could be very easily
broken out and modified either with new
mortar or plasticine. Plasticine was used due
to its speed and simplicity of application.
Solids Simulation
The grit material in sewage to be removed
in the test swirl structure was established, as
shown in Figure 18, Typical Grit Gradation.
The outside grain size limits of 0.2 and 2.0
mm (No. 70 and No. 10 sieve) correspond to
the standard soil mechanics definition of
medium and coarse sand, respectively. The
specific gravity of grit was assumed to be
2.65, and the straight-line grain size
distribution was selected as a representative of
samples taken from existing grit chambers in
sewage treatment plants.
Particle sizes larger than about 1 mm (No.
18 sieve) are known to move along in flowing
water according to equations of the type
propounded by Meyer-Peter and Muller1 or
H.A. Einstein.2 Between 1 mm and 0.2 mm
(No. 18 sieve and No. 70 sieve) the particles
3
6
u
/*
U.S. STANDARD SIEVE NUMBERS
4 6 8 10 16 20 30 40 50 70 IOO 140
1
1
1
4 2
COM8E
v
\
\
\
\
\
\
\
>
V
\
1 1
1 0.6 0.4 0.2 0
Groin size in mm
FME
3MD
S. SIEVE Hi.
IO
20
4O
SO
7O
—SEX. T-
oroettmm
2.000
0.8*0
O.42S
0.300
0.211
%»«R BY WEXSHT
100
63
31
13
O
IUV
9U
BU
7O
6O .e
o>
'5
*n *
SO
>>
.a
An »
40 •
ii_
TLf\ *0
OU 4*
£,U
10
0
FIGURE 1 8 TYPICAL GRIT GRADATION
Source: Alexander Potter and Associates
Memorandum of April 24, 1973
39
-------
3O.O
2O.O
0.01
.05 .04 JO* .0».K> .20 .40 M «0 UO 2.0 4.0
Particl* diam«t«r, mm.
*JO *0 10.0
FIGURE 19 PARTICLE SETTLING VELOCITIES FOR GRIT,
PUMICE AND GILSONITE
are in the transition zone between the above
, case and the Stokes relation. Since the
particles involved in both prototype and
model extended into both ranges, across the
transition zone, the above equations could
not adequately describe the scale relations.
It was therefore necessary to use curves
of particle settling velocities as shown in
Figure 19, Particle Settling Velocities for Grit,
Pumice and Gilsonite. For a given grit size
with S.G. 2.65 in the prototype, the settling
velocity was determined from Figure 19.
Based on Froude's law of similitude, this was
divided by the square root of the scale being
considered to find the required model settling
velocity. Referring to Figure 19 with this
model settling velocity, the model particle
sizes were found for each of the simulating
materials — Gilsonite, pumice and fine sand.
The physical relations used here can be
expressed as follows: '
Model scale = X = Lp / Lm
where Lp and Lm are corresponding
dimensions in the prototype and
the model respectively. \
From Froudes Law, the velocity ;simulation is
given by:
YE-Jb? - > :
Vm Lm
and
The initial tests were conducted where
40
-------
the scale ratio of prototype to model was 4.
For a ratio of 4 the settling velocity in the
prototype should be divided by the square.
root of 4 or 2. From Figure. 19 the settling
velocity of grit of 0.2 mm size is 2.6 cm/sec.
The model settling velocity is then 1.3
cm/sec. Thus in the model the grit of 0.2 mm
size can be simulated by use of 0.12 mm grit
and 0.30 mm pumice or 0.80 Gilsonite.
Figure 20, Prototype Grit Sizes Simulated
by Pumice, shows the prototype grit sizes
simulated by the pumice used on the model.
The range of pumice particle sizes shown are
those which were used on the model as
'indicated by the gradation curves in Figure
21, Gradation Curves for Pumice, Gilsonite
and Sand Used in Model. From this, it can be
seen that the pumice adequately covered all
the required prototype sizes up to, say, scale
4, and only down to 0.4 mm (No. 40 sieve) at
scale 16.
Gilsonite, with a specific gravity of 1.06,
was used to fill in this lower area of finer
particles; its corresponding simulation curves
are shown in Figure 22, Prototype Grit Sizes
Simulated by Gilsonite in Model. The grain
size distribution curve for the material used
on the model is shown in Figure 21.
Sand was used for some of the tests, as a
means of providing a check on the behavior of
the lighter specific gravity particles; i.e.,
•gilsbnite and pumice. The grain size
distribution curve of the sand used is shown
.in Figure 21. The model simulation
characteristics are shown in Figure 23,
Prototype Grit Sizes Simulated by Sand in'
Model. It can be seen that the sand sizes used
on the model covered the prototype grit sizes
fairly well.
The reasons that sand was not used more
extensively in the testing were, first, it moved
more slowly in the model, and secondly,
I 2
6 8 10 12
Model scale - ^
14 16
FIGURE 20 PROTOTYPE GRIT SIZES SIMULATED BY PUMICE
41
-------
U.S. Standard ti«v« numbtrs
34 6 810 16 2030 40 SO 70100140.
6 4
2 I 0.6 0.4 0.2 O.I
, Grain tizt in mm
count I ignuu
±
FIGURE 21 GRADATION CURVES FOR PUMICE, GILSONITE
AND SAND USED IN MODEL
2.0
1.8
1.6
'-4
''2
.0.8
0.6
a.
0.4
0.2
0
12 4 6 8 10 12 . 14 16 18
Modtl seal* - A
FIGURE 22 PROTOTYPE GRIT SIZES SIMULATED BY GILSONITE IN MODEL
42
-------
E
E
I
0)
a
>»
o
*-
o
fc.
a.
i
a
•D
8 10 12 14 16
FIGURE 23 PROTOTYPE GRIT SIZES SIMULATED BY SAND IN MODEL
being so fine, it was •difficult to recover it
conveniently after each test of the existing
model set-up. Both factors would have tended
to lengthen the testing program considerably.
Normal grit concentrations in sewage
were defined as lying between 20 mg/1 and
360 mg/1 (0.00125 and 0.0225 Ibs/cu ft). In
this range, there are few enough particles in
the flow to permit each one to react
individually. Therefore, concentrations used
in the model were kept near the upper limit
of the given range, where the individual
particles still moved with practically no
collision effect with adjacent particles.
An additional particular, concentration
case was considered for the Lancaster Project
where the inflow to the grit chamber will
come from the foul outflow from the Swirl
Concentrator Overflow Regulator. A
suggested concentration for this case in
prototype was 750 to 13,500 mg/1. It was not
possible to reproduce this rate of solids flow
in the model, as it involved the mechanics of
slurry flow in the pipe, and detailed
consideration of this was beyond the scope of
the present study.
However, solids injection rates in the
order of 2000 mg/1 were tested. These
consisted of introducing the solids fast
enough for dunes to form on the pipe invert.
As these concentrated solids masses arrived in
the • chamber, the characteristics of their
behavior could be followed.
The general model testing procedure for
each case consisted of establishing steady-flow
conditions, then injecting into the water
coming through the inlet pipe one full liter of
the solid material under test. The rate of
injection was controllable within the limits set
forth above. After each test; the solid material
43
-------
was collected either in the desired location in
the chamber, on the chamber floor, or in the
stilling basin for the V-notch weir used to
measure the flow over the circular weir in the
chamber. The volume of solids recovered in
the desired location in the chamber floor
(hopper, cone or bottom outlet) was
expressed as a percentage of the original liter
injected, and is referred to as the grit removal
efficiency of the test.
Preliminary Tests with Foul Discharge
At the outset of this test series; the model
configuration was in the form shown in
Figure 24, Original Layout, which was the
layout developed, for the earlier tests of the
Swirl Concentrator as. an Overflow
Regulator. First tests were run using a 1:4
scale ratio, giving a 61-cm (2-ft) diameter inlet.
pipe. Results with this arrangement were not
acceptable over the range of ; discharges,
between 85 and 425 1/s (3 and 15 cfs).
Efficiencies were as low as-75 percent, even
though as much as 10 percent of the inflow
water discharge volume was jdrawn off
through the foul outlet.
Welr-(S') 244cm. dia.
Down pipe
2Qj3 cm
(81* Foul outlet
on floor at 280*
Baffle
368 cm
(12') dia.
61 cm.
(21) dia.
61cm.
FIGURE 24 ORIGINAL LAYOUT
44
-------
Following discussion of these results with
the study's group of consultants, the
configuration shown in Figure 25, Preliminary
Layout for Lancaster Study, was provided in
the model. The Lancaster Project
requirements were undertaken first, so the
30.5-cm (1-ft) diameter inlet pipe was
simulated at scale 1:2.4. Foul discharge
volumes of up to 10 percent of the inflow
were drawn off. Inflows between 42.5 and
141.6 1/s (1.5 and 5 cfs) were tested.
Efficiencies were distinctly higher under
these new configuration conditions, reaching
as high as 95 percent. However, appreciable ;
portions of the deposited grit remained as
accumulations on the chamber floor. The
annular gutter around the central downpipe;
was not particularly advantageous, since'
deposits accumulated at various points ki the:
gutter, and could not be drawn to any single
collection point.
Returning to the use of just a flat floor in.
Weir-(6-8"dia.) 203cm
Annular gutter on floor
(7"X7")I78X 17.8cm
Foul outlet (5"dia.)l2.7cm
at 280* position
Covered baffle entry
A
(7'-2"dia.)
218 cm "
ELEVATION
(A-A)
(I1-;
38.1cm,
n
&
K
l'dia.)
30.5cm
r
J!
Clear
FIGURE 25 PRELIMINARY LAYOUT FOR LANCASTER STUDY
45
-------
the chamber, the layout shown in Figure 26,
• Single Gutter-Small Outlet Scheme, resulted
from a series of cut-and-try development
tests. It showed promise by giving
consistently higher solids recoveries out
through the foul outlet. The foul discharge
volumes were taken as 10, 15 and 20 percent
of the inflow.
It became evident" at' this stage of the
studies that foul discharges as high as these
would be difficult to handle through the
proposed mechanical grit removal equipment.
Also, the bottom outlet at 12.7 cm (6 in.)
diameter was too small to handle such
vojumes. ...
On the other hand, the use of the
floor gutter seemed to be an efficient way of
directing the solids toward the center of the
chamber. As the flow passed over the gutter,
it generated a rolling current with the
longitudinal axis along the gutter. Solid >
particles, moving along on the chamber floor,
fell into this rolling current and were
entrained in it.
If the gutter was placed radially, so the
flow in the chamber passed over it at right
angles, the roller and particles would stay at
about the same relative position out from the-
downpipe. However, when the gutter was,
placed at about a 20° angle from a radial line,
as shown in Figure 26, the rolling current
developed a longitudinal current along the
gutter. All solids particles caught in the rolling
current were" therefore entrained toward the
center of the chamber where they could be
captured by some suitable means.
Flat Floor Concept
Experience gained in the preceding tests:
had provided definition of the particles'
trajectories__in the chamber and along the
floor over the range of discharges being
considered. However, this had been
accomplished with appreciable amounts of
the inflow water discharge being withdrawn
through the foul outlet. - ;
The present series of tests was therefore!
planned to study the characteristics based on
using a flat floor in the chamber, no foul:
discharge, and a large hopper into which the
grit would be directed.
The three gutter arrangement shown in
Figure 27, Three Gutter-Large Hopper
Scheme, was typical of layouts tested
following the findings of the previous tests.
Results were encouraging, although there was
significant turbulence generated in the
hopper. The gutter location at 270° did not
contribute much to the operation, and the
one at the 90° location intercepted the flow
at right angles, so the particles: tended to>
remain out from the hopper, turning in the
roller generated in the gutter as shown in'
Figure 27. j
At this stage, it was decided to adapt the
grit chamber to a definite dimensioned
vertical-sided hopper that would feed a screw
conveyor. The first position tried with this
••30.5 x 45.7-cm (12 x 18-in.) hopper was
270°, as shown in Figure 28, Scheme with
Hopper at 270°. Results were disappointing in
tests either with or without as niuch as 20
percent foul outlet discharge. Although most
of the solids were retained in the chamber,'
heavy deposits remained on the floor as
Shown in Figure 28. This was an unexpected
finding, since previous work had clearly
demonstrated that 270° was the best position;'
for collecting deposited solids.
The hopper was then moved to the 180°
position as shown in Figure 29, Vertical-Sided
Hopper at 180°. The solids deposition pattern
was typical of the discharges of 85! and 141.6
1/s (3 and 5 cfs) tested.'Although this floor
accumulation pattern covered a greater area
than the previous configuration, the deposits
were not so deep, and represented much less.
volume. At this stage it appeared that this was
the best position for the hopper, but that it.
would require guides or vanes of some kind to
carry the solids into it. " .
A series of tests were performed to
evaluate floor baffles or vanes located at.
various positions on the chamber floor. The
arrangement shown in Figure 30, Hopper at
180° with Three Floor Baffles, was the most
efficient in this series. The baffles, 6.5 cm
(2.5 in.) (prototype) high, worked relatively
well in guiding the solids to. the hopper,
but in so doing, they created Additional
turbulence in the flow so that much of the
material was. being thrown back up into
suspension 'continuously. It appeared that
baffles, at least at the 90° and ISO9 positions
where the flow velocities were higher, caused
too much disturbance in the chamber.
; " Figure 31, Hopper at 180? with Two
Gutters, shows .the developed locations of two.
46
-------
I8O°
Tapered floor gutter
.(3"deep) 7.6cm
(5"dio.) Foul outlet at 280°
12.7cm
/ „ ( u
^
^.^^
off le entry ]
1
0°
(7'-2"dia.)
- 218cm '
ELEVATION
(A-A)
(I1 dia.)
30.5cm
Inlet (I1) square 30.5cm
Foul outlet 10, 15 or
20% of total discharge
FIGURE 26 SINGLE GUTTER - SMALL OUTLET SCHEME
47.
-------
Hopper(|'-3"X 2'-6")
38.1X76.3 cm
Gutters(6")wide(3")deep
15.3cm 7.6cm
(7
'-2
218
•
ELEVATION
(A-A1
—tw
^
>"d
en
JO.)
f
:c
s
1
I
^a*
Kl 'dia.)
30.5 cm
FIGURE 27 THREE GUTTER - LARGE HOPPER SCHEME
48
-------
,180s
Vertical sided hopper(l2"x 18")
30.5 X 45.7cm
Tests both with
and without foul
o discharge
270
_^^
0°
±_
(7'-2"dia.)
(XTION
A)
£l<
1
3 ur
r
i
n -
=^=
Ux
•L&
^. Foul d
i .
.
l(l'dia)
3O.5 cm
ischarae — IO. IR nr
20% of total discharge.
JFIGURE 28 SCHEME WITH HOPPER AT 270°
49
-------
180
Vertical sided hopper(12"X 18")
30.5 X 45.7cm
ELEV
(A-
V f
»I8
— «* -v
"
IL
CATION
-A)
T
£ 1
cm
r
J
I
!
i
1(1 'dia.)
30.5 cm
FIGURE 29 VERTICAL-SIDED HOPPER AT 180°
- 50
-------
180
Baffles placed on
chamber floor -(2 1/2"high)
6.3cm
Vertical
sided hopper
(I2"XI8")
30.5 X 45.7cm
(7'-2"dia.)
218 cm.
TT
ELEVATION
(A-A)
r
i
_L
Hl'dia.)
30.5 cm
FIGURE 30 HOPPER AT 180° WITH THREE FLOOR BAFFLES
51
-------
Gutters
I5.3X 7.6cm
(6"X3")
(21")
53.4cm
270'
/
/
T
1
°° 218cm 1
'I
3")
7.6cm
ELEVATION
IA-A)
(7-2" dip
cr
Ti
-u
(I'dia.)
30.5 cm
FIGURE 31 HOPPER AT 180° WITH TWO GUTTERS AND THREE FLOOR BAFFLES
'52
-------
floor" gutters which were very efficient in
directing material to the hopper, even without
any foul outflow. However, the gutter at 180°
was drawing so much water that it caused
high turbulence in the hopper and ejection of
some of the accumulated solids. This material
continued around the central downpipe, and
was deposited on the chamber floor.
Final Flat Floor Configuration
Following a detailed series of tests
intended to reduce to a minimum the deposits
on the chamber floor, the layout shown in
.Figure 32, Hopper at 180° with Two Gutters
and One Floor Baffle, was selected as the
most advanced form of this concept. Also
included in these tests was the procedure of
drawing off a constant discharge of 2.8.1/s
(0.1 cfs) (prototype) through the foul outlet
in the hopper bottom to simulate the flow
through the grit removal screw conveyor.
During these tests, the gutter at 180° was
reduced to a 7.6 x 7.6-cm (3 in. x 3 in.)
cross section from its original 15.2-cm (6-in.)
•width. This effectively reduced the discharge
which it directed into the hopper, inhibiting
the turbulence which had been ejecting too
many solids back into the swirl chamber.
There were, however, still some* solids
escaping from the hopper and migrating
around the central downpipe. The floor baffle
located at the 0° position, in a relatively calm
water zone, served the purpose of redirecting
these particles back into the hopper.
The deposit shown in 'Figure 32 was
typical of tests run at 85 and 141.6 1/s (3 and
5 cfs). The volumes on the floor were very
small; once the approximate pattern shown
was formed, it remained "in equilibrium over a
wide range of the higher discharges. However,
at the 42.5 1/s (1.5 cfs) discharge the deposit
on the floor was substantial. With a discharge
of 56.5 1/s (2 cfs) running during solids
injection, no large deposit was formed.
Having settled on the configuration shown
in Figure 32, as the optimum for the flat floor
concept, detailed tests were carried out to
determine the effects of different overflow
weir levels. Weir heights of 53.3, 61.0, and
68.5 cm (21 and 27 in.) above the chamber
floor were tested for" the three prototype
discharges of 42.5, 85 and 141.6 1/s (1.5, 3
and 5 cfs). The pumice samples smaller than 3
mm (passing No. 6 mesh) shown in'Figures 4
•and 5 were all recovered either on the
chamber floor for 42.5 1/s (1-.5 cfs), or in the
hopper for discharges above 56.5 1/s (2 cfs).
Therefore, a complete series was carried out
using only the Gilsonite, see Figures' 17 and
18, to evaluate the chamber's performance
for the fine end of the grit scale. The results
are shown in Figure 29, Solids Separation for
Flat Floor Concept.
The first point noted was that for the
small discharge of 42.5 1/s (1.5 cfs) the
•deposit on the chamber floor was significant;
as the discharge increased, the deposit no
longer formed, and/or the existing solids
accumulation was swept out. The quantity of
solids recovered in the hopper increased as the
weir height was raised except at 42.5 1/s (1.5
cfs). Similarly, the solids carried out the
overflow increased as the weir level dropped
and the inflow discharge increased.
Remarks
This model configuration operated'very
well in the middle and higher discharge
ranges. It was particularly encouraging to find
that even with the 141.6 1/s .(5 cfs) rate the
chamber still recovered nearly 25 percent of
the finest materials, between 0.1 and 0.4 mm
(No. 140 and No. 40 sieve).
On the other hand, at the lowest
discharge, 42.5 1/s (1.5 cfs), deposits on the
chamber floor always remained. The energy
arriving in the flow at this discharge was not
sufficient to move the material the distance
necessary across the chamber floor to the
gutters or hopper.
At this stage, it was concluded that this
design would be valid if installed on a sanitary
sewage system with a daily flow variation
between the 42;5 and 141.6 1/s (1.5 and 5 cfs)
tested. At the lower discharges, deposits
would be built up in the chamber, but as the
discharge subsequently increased, the deposits
would be swept into the hopper. Another
acceptable mode of operation would be with'
relatively constant discharges, always greater
than 56.5 1/s (2 cfs).
These limitations were considered too
restrictive in view of the goals set for the
project, so a new line- of research was
established as described in the following
sections.
53
-------
Gutter(3"X 3")
7.6 X 7.6 cm
•Gutter
(6"X3")
15.3X7.6 cm
\
Baffle
(2 1/2" high)
6.3 cm
(21")
(53.4cm)
(7'-2"dia.)
218 cm _
"LI
zir
^
ELEVATION ' bs
(A-A)
Kl'dia.)
30.5cm
FIGURE 32 HOPPER AT 180° WITH TWO GUTTERS AND ONE FLOOR BAFFLE
54
-------
o
v>
0>
o
0
100
90
80
70
60
so
40
30
20
10
Discharge, l/s
50 100
-1—I—[I—I—I—jl—I I t' I ' t<
•-*
IN CLEAR
'OVERFLOW
IN FOUL
UNDERFLOW
(HOPPER)
/-61cm __Q-i DEPOSITED ON-
2345
Inflow disch arge - cfs
NOTE :
-Solids represented were: on model, Gilsonite: 1-3 mm
prototype grit, 0,1 -0.26 mm
- Foul outflow through hopper = 0,1 cfs (prototype) = 2.83M
-Chamber layout as on FIG. 16
" "
-21", 24" and 27" indications on graph correspond to
weir heights above chamber floor
FIGURE 33 SOLIDS SEPARATION FOR FLAT FLOOR CONCEPT
• 55
-------
' Conical Floor Concept
Discussions with the project group of
consultants resulted in the proposal to return
to a symmetrical floor arrangement, with a
cone dropping around the central downpipe.-
This approach included the use of an elbow in
the downpipe, at some selected level just
below the chamber floor level, so the clear
flow would be taken off horizontally. At the
same time, it introduced the possibility of
continuing the sloping sides of the cone down
to a single collection point below the
downpipe, on the chamber's central axis, as
shown in Figure 34, Conical Floor Principle.
The use of an annular gutter had been
tried and abandoned in the earliest tests as
shown by Figure 25. The problem at that time f
•was that the central downpipe was considered
as extending down deeper, and that,the grit
had to be concentrated at one point beside.
the downpipe to be drawn off. The grit
showed marked tendencies to deposit in the
annular gutter, but "it was distributed.
irregularly around it, so that it was not
possible to draw it off effectively. However,
in the new concept, the grit could drop
anywhere in the cone and it would migrate
downward under the downpipe where it could
all be picked up mechanically by some form •
of elevator.
At this time it had also been decided that
the chamber at Lancaster must operate
without inlet deposition problems at the>
minimum steady discharge of 42.5 1/s (1.5
cfs). This required the model scale to be 1:2,
giving a prototype 183-cm (6-ft) diameter
chamber for the 30.5-cm (1-ft) diameter inlet
pipe.
The first configuration tried following
this principle is shown in Figure 35, Small
• Cone, Elbow Flush with Floor. The upper
edge of the conical floor basin was 101.5 cm
(40 in.) in diameter, and the downpipe was
considered as having its elbow located with
the top of the horizontal section at the 90°
position just flush with the flat outer chamber'
floor level.
It was immediately evident that the-
downpipe elbow was so high that turbulence
occurred in the chamber. Flow, tended to
build up in the cone, rotating Ground the
vertical section of the downpipe; when it
reached the elbow, it was pushed upward
again, ejecting the solids material into the
upper zone. This resulted in a significant
deposit on the flat floor around the 180°
position.*
The next step was to lower the downpipe
elbow so that it was below the existing cone
bottom on the model; i.e., 15.2 cm (6 in.)
prototype below the flat floor, as shown in
Figure 36, Small Cone, Elbow 6 in. (15.2 cm)
Below Flat Floor. This arrangement removed
all obstruction from the flow circulating in
the cone, and there was no [longer any
re-entrainment of particles that had dropped
onto the bottom. At higher discharges the
deposit on the flat floor at 180° was reduced
to a very small volume which quickly reached
an equilibrium state and did not1 increase in
size. However, for discharges arotind 42.5 1/s
(1.5 cfs) significant deposits remained at the
inlet. •
Observations of the solid particles'
trajectories during the tests with the layout
shown in Figure 36 demonstrated that, once
inside the chamber, the particlejs tended to
work their way to the right of the jet, (i.e.,
toward the center of the chamber). However,
the edge of the cone was too far ;to reach for
these particles with the amount of energy
available in the smallest discharges. This
indicated the need to widen the; cone so its
upper edge extended out to the side of the
square inlet opening.
Final Conical Floor Configuration
Figure 37, Final Conical Floor
.Configuration, shows the basic layout of the
widened conical floor which wasiaccepted as
the most efficient configuration of ..this
principle. The upper'edge of the cone can be
seen extended to a point just outside of the
square baffle-inlet. Preliminary : tests with
various levels and orientations of the
downpipe elbow indicated that it should be
taken out along the 90° line, and should be at,
least 30.5 cm.'O ft) below the flat floor level.
56
-------
A
A
Flat floor level
Grit to screw conveyer
Section A-A
FIGURE 34 CONICAL FLOOR PRINCIPLE
57
-------
180
Conical
floor
;40")IOI.5.em did.
CMIO
Simulated
downpipe
elbow
ELEVATION
Section A-A
s^ Actual model setup
FIGURE 3 5 SMALL CONE, ELBOW FLUSH WITH FLOOR
58
-------
Conical floor
(40") 101.5 cm dia.
Simulated
downpipe
elbow
——Actual model setup
ELEVATION
Section A-A
FIGURE 36 SMALL CONE, ELBOW 15.2 cm (6 in.) BELOW FLAT FLOOR
59
-------
180
Conical floor
(47") 119 cm
*")
m
.5cm
)-^
i
-i rr1 nllx
32cm
Q\_
I
° i
I83cm H
5
\
» -N
! J
A-
P>
V cr«...i ...«
Ml -0") 30.5 cm
1 A«
(I1
Simulated
downpipe elbow
ELEVATION
Section A-A
FIGURE 37 FINAL CONICAL FLOOR CONFIGURATION
60
-------
Figure 38, Sketch Layout of Swirl
Concentrator Grit Chamber, shows a possible
layout for the Lancaster installation, with a
30.5-cm (1-ft) diameter inlet.
Testing to this point had been carried out
over only a limited discharge range, so a
complete detailed series of tests .was
undertaken to accurately define the operating
efficiencies. The tests were planned to include
varying width-depth ratios as well. The model
•modifications are shown in Figure -39,
Nomenclature for Width-Depth Proving Tests.
The same 91.5-cm (36-in.) diameter chamber
was used for all tests, but the 15.3-cm (6-in.)•
supply line was changed respectively to 12.7,
11.1 and 7.6 cm (5,4 and 3 in.), as shown in
Figure 40, Varying Sized Inlets Used in
Width-Depth Tests. To maintain continuity in
reporting, the scale was selected for each so
'that the prototype inlet pipe was always.
considered as being 30.5 cm (1 ft) in
diameter. This procedure resulted, therefore,
in simulating a constant inlet pipe diameter
_and varying chamber diameter.
The same weir assembly was used in all
'tests, so its diameter was considered as
changing along with the chamber diameter.
Three standard prototype discharges were
selected, 28.3, 85 and 141.6 1/s (1,3 and 5
cfs).
The elements which were modified on the
model during this test series are described on
'Figure 39. Following the concept of the inlet
diameter being used as the basic dimension, it
was the first to be given a symbol, Dt in the
test nomenclature. On the plan view, the
value of D! is shown so as to represent the
four different pipe diameters tested. Then on
Section A-A, Dj appears as the height and
width dimension of the square inlet, inside
the chamber. As shown, the four different
Dj 's described above were put on the model,
coming into the chamber tangentially at the
0° position.
The next pertinent dimension defined
was the inside diameter of the chamber, called
D2. This remained constant for all tests.
Finally, the height of the weir crest above the
flat part of the chamber floor was called Ht.
At least three different weir heights were
tested with each inlet pipe diameter.
Proving Test Results and Analysis
In all cases, when pumice and sand were
injected, the recoveries were either 100
percent or .99 percent plus, with only traces
of the finest materials being lost over the
central outlet weir. Therefore, in order to
provide any kind of comparison, Gilsonite
was used as the testing material. Recovery
rates of Gilsonite for the four chamber
diameters simulated are shown in Figures 41
and 42, Gilsonite Recovery in Final Form
(12-Ft and 9-Ft Diameter) and Gilsonite
Recovery in Final Form (7.2-Ft and 6-Ft
Diameter).
To present a comprehensive picture of
the total grit recovery represented by this
data, it was combined with that for the
pumice and sand as well. Reference to Figure
18 shows that the Gilsonite used on the model
simulated the grit sizes in a little more than
the lower half of the graph, leaving the area
above the dm = 3 mm line unrepresented.
Conversely, Figures 20 and 23 show that this
upper area was well covered by the sand and
pumice.
Therefore it was assumed that the
Gilsonite recoveries in the above series of tests
simulated the area below the dm = 3mm on
Figure 22, and that the 100 percent recoveries
for the sand and pumice covered the area
above this. The first step in combining these
two sets of data was to determine the grit size
on the dm = 3 mm line on Figure 22 for the
different model scales. These sizes were taken
to Figure 18, where the percentages of
prototype finer grit were found. The
percentages were plotted on Figure 43,
Portions of Prototype Grit Samples Simulated
by Gilsonite, Sand and Pumice, as indicated
for Gilsonite. The differences between the
Gilsonite line and 100 percent were
determined and plotted simply on Figure 43
representing the portions of the prototype
grit samples covered by the sand and pumice.
Next, the Gilsonite values on Figure 31
for the scales to be used in scaling up to
selected prototype sizes were plotted on
Figure 44, Portions of Prototype Grit Samples
Represented by Gilsonite Recovered in Tests,
above the 100 percent position for Gilsonite
recovery in the tests, and the straight line
61
-------
Spoilers for
high discharges.
15.3cm ;
r(6")Spoilers
Spoilers for
high discharges (12")
30.5 cm.
Adjustable
V weir
(24")
,61cm
(6"/Spoilers
15,3 cm
123cm
(48")
Downpipe
supports
FIGURE 3 8 SKETCH LAYOUT OF SWIRL CONCENTRATOR GRIT CHAMBER
62
-------
L
180°
H,
_j
Simulated
!d^
p Plh
|
,
I
v__
/
nuu
/
/
^-^
,
,
fr=
/so-
Actual m<
setup
ELEVATION
Section A-A
FIGURE 39 NOMENCLATURE FOR WIDTH-DEPTH TESTING
63
-------
7.6cm (3in) Inlet
II.I cm(4in) Inlet
12.7cm (Sin) Inlet
15.3 cm (6 in) Inllet
FIGURE 40 VARYING SIZED INLETS USED IN WIDTH-DEPTH TESTING
64
-------
Scale:
100
90
80
70
^ 60
I 50
S 40
•# 3°
20
10
0
Scale:
100
90
80
70
» 60
2 50
i 4°
K 30
<£ 20
IO
o
4 (I2')Diameter 3.66m
3
-——«»•«.
—
— -=^
=^
"^^
X
N,
.81.3cm,
-1321
x*^^
^(24"
[6t.0cm -
v
^(16")
40.6cm
[H,
J
2 3 4 5 6
Discharge, cfs
(9')Diameter 2.74m
-^
===^^
^v
^>^
^^^
X^
N
61.0cm -
45.7cm
. "
\
76.2cm
V(30")
N>/o,i"\
k, \\Z<* I
Ads")
\(I2")
30.5cm
1 2 3 4 5 cfs
i i i i 1 i i i i i i i , , I
0 50 100 150 |/s
Discharge,
FIGURE 4 1 GILSONITE RECOVERY IN FINAL FORM
3.66 AND 2.74 m (12 ft & 9 ft) DIAMETERS
'Hi
65
-------
Scale
100
a>
0)
cr
Scale:
100
cr
Discharge, cfs
(6')Diameter 1.83m
cfs
l/s
Discharge,
FIGURE 42 GILSONFTE RECOVERY IN FINAL FORM
2.18 AND 1.83 m (7.2 ft & 6 ft) DIAMETERS
66
-------
100
4 ; -6 8 10
Model scale - Pt
12 14
16
FIGURE 43
PORTIONS OF PROTOTYPE GRIT SAMPLES
SIMULATED BY GILSONITE, SAND AND PUMICE
IOO
90 -
16
10 20 30 40 50 60 70
Gilsonite recovered in test - %
80 90 100
FIGURE 44
PORTIONS OF PROTOTYPE GRIT SAMPLES
REPRESENTED BY GILSONITE RECOVERED IN TESTS
67
-------
relations drawn simply to zero. The last step
in constructing the working sheets was to add
the grit percentage for sand and pumice for
each retained scale on Figure 43 to the
varying grit percentages for Gilsonite on
Figure 44. The resulting values were plotted
on Figure 45, Portions of Prototype Grit
Samples Represented by Gilsonite Recovered
in Tests plus Sand and Pumice.
It was then possible to take a Gilsonite
recovery value from Figures 41 and 42, enter
Figure 45 with this, and read off the total grit
represented by that test-on the appropriate
scale line. This procedure was followed to
transpose the data for Gilsonite only on
Figures 41 and 42 to the complete prototype
grit represented by the same tests as shown on
Figure 46, Grit Recovery for 30.5 cm (1 ft)
Inlet with 2.74 and 3.66 m (9 and 12 ft)'
chambers, and Figure 47, Grit Recovery for
30.5 cm (1 ft) Inlet With 1.83 and 2.18 m (6
and 7.2 ft) chambers.
Since it was desired to consider several
different inlet pipe sizes; the laboratory data
as it existed at this stage contained five
variables; discharge, Met dimension, weir
height, recovery rate and chamber diameter.
These could not all be conveniently presented
in a design procedure, so some 'had to be
eliminated. ;
It can be seen on Figures 46 and 47 that
for each inlet pipe diameter tested, the weir
height changes resulted in relatively modest
recovery rate changes. Over most of the
discharge range, a change in weir height equal
to half the Met dimension resulted in a
recovery rate change of less than 3'percent. It
was .therefore decided to select the weir
height of 61 cm (24 in.), corresponding to
twice the inlet dimension, or H! /Dt =2, for
further analyses. i
The choice of the 61 cm (24 in.) high
weir was based first on the fact that it gave
the most consistent results over; the range
tested. Secondly, observations in the model
showed that it still retained a certain degree
100
10
20 30 40 50 60 70
Gilsonite recovered in test - %
80 90 100
FIGURE 45
PORTIONS OF PROTOTYPE GRIT SAMPLES REPRESENTED
BY GILSONITE RECOVERED IN TESTS PLUS SAND PUMICE
68
-------
= 3.66m (12ft.)
8l.3cm(32in.)
?=
61 cm (24in.)
40.6cm (16 in.)
Hi
Scale: 3 D2
100 '
100
Discharge
= 2.74m (9ft.)
76.2 cm (30 in.)
6lcm(24in)
45.7cm(l8in.)
30.5
H,
100
Discharge
FIGURE 46 GRIT RECOVERY FOR 30.5 cm (1 ft) INLET WITH
2.74 and 3.66 m (9 ft & 12 ft) CHAMBERS
69
-------
Scale: 2.4 D2 = 2.18 m (7.2ft)
100
55 90
i
61 cm (24 in)
48.3cm (19in)
36.8 cm (14.4 in)
0
50 100
Discharge
Scale: 2 D2 * 1.83m (6 ft)
100
90
>> 80
a>
o
O)
70
o 60
81.3cm (32in)
.7l.lcm(28in)
1" 6I cm (24 in)
""""-50.8cm (20ir
_L
5 cfs
i—u
I L
i i i
L
50
100
Discharge
FIGURE 47 GRIT RECOVERY FOR 30.5 cm (I ft) INLET WITH
1.83 AND 2.18 m (6 ft & 7.2 ft) CHAMBERS
70
-------
of turbulence in the chamber which was
interpreted as an advantage in separating the
organics from the grit.
Further support for this argument was
found in comparative measurements of
Gilsonite, sand and pumice recoveries for a
given set of conditions. It was found that the
Gilsonite consistently gave lower recovery
rates than the sand and pumice, although for
particular tests they simulated the same
prototype grit sizes. The salient difference
between them was that for a given prototype
grit size, the Gilsonite particle on the model
was much larger than the pumice or sand.
Means available for making this scale
interpretation are a function of the particles'
settling velocities in still water. (See Fig. 19)
However, it appeared that in the presence of
the slightly turbulent flow in the chamber,
the larger Gilsonite particles tended to remain
in suspension longer and to be drawn up over
the weir more easily. This behavior can be
explained by the fact that the larger Gilsonite
particles would have larger Reynolds numbers
than the smaller pumice or sand particles and
therefore reduced drag, since we are dealing
with particle drag coefficients in the
transition zone. Since the reduced drag
coefficient of the Gilsonite resulted in a
smaller force to convect the particle, the
Gilsonite particles in motion tended to behave
more like the larger lighter organic material in
the prototype. • .
The curves for H! =61 cm (24 in.) on
Figures 46 and 47 were then taken for the
30.5 cm (1 ft) inlet and scaled up to three
larger inlet sizes, 61, 91.5, 122 cm (2,3,4 ft)
to give the recovery rates or efficiencies of the
various grit chamber diameters as shown on
Figure 48, Grit Recovery for 30.5 cm (1 ft)
Inlet Pipe and Different Sized Chambers;
Figure 49, Grit Recovery for 61 cm (2 ft)
Inlet Recovery and Different Sized Chamber;
Figure 50, Grit Recovery for 91.5 cm (3 ft)
Inlet Pipe and Different Sized Chambers; and
Figure 51, Grit Recovery for 1.22 m (4 ft)
Inlet Pipe and Different Sized Chambers.
Figure 52, Prototype Discharges - Range'
Covered by Model Tests, shows the discharge
100
I 2 3
Discharge, cfs
i i i i i i i i i
0 20 40 60 80 100 120 140
Discharge, liters/second
•\
FIGURE 48 GRIT RECOVERY FOR ONE (D t = 1 ft = 30.5 cm) FOOT
INLET PIPE AND DIFFERENT SIZED CHAMBERS
71
-------
100
0 100 200 300 400 500 600 700 800
Discharge — liters/second
FIGURE 49 GRIT RECOVERY FOR TWO (D t = 2 ft = 61 cm) FOOT
INLET PIPE AND DIFFERENT SIZED CHAMBERS
100
90
3«
£• 80
e
>
5" 70
S
&
60
50
"
1
1
"--^.
;>
\\
\
\
\
\
\
\
\,
^\|
)2 >(36
•slO.
')
98m
\ ' '^
\D2"<27')
\ 8.23m
\,
\
\
\
D2«(2
6.5
>2 '(1
V 5-!
\
\
.6'N
3m
X,
J1)
Om
S.
-
10 20 30 40 50 6O 7O 80 cfs
Dl.eh.rg.
0.5 1.0 1.5 2.0 2.5
Discharge — m3/*
FIGURE 50 GRIT RECOVERY FOR THREE (Dt = 3 ft = 91.5 cm) FOOT
INLET PIPE AND DIFFERENT SIZED CHAMBERS ,
72
-------
100
20 40 60 80 100 120 140 160 cfs
Discharge, cfs
50
I 2 3
Discharge — m3/s
FIGURE 51 GRIT RECOVERY FOR FOUR (D t = 4 ft = 122 cm) FOOT
INLET PIPE AND DIFFERENT SIZED CHAMBERS
*5*
E
5.0
4.0
3.0
2.0
1.0
•
1 0.5
• 0.4
a
0.3
0.2
0.10
0.05
0.04
0.03
FIGURE 52 PROTOT1
*
200
-150
-ioo
50
40
30
20
10
5
4
; 3
2
• 1
*TE
/
/
/
/
/
/
/
t t
/
/
/
/
<$/
cfiS
50 IOO cm
Square inlet dimension - DI
DISCHARGE-RANGE COVERED BY MODEL TESTS
73
-------
range which the tests covered when scaled up
to these same inlet pipe sizes.
The data presented on Figures 48,49, 50.
and 51 therefore represent the predicted
efficiencies' of the structures as dimensioned.
However, although the figures contain all the
pertinent test results, they are in a form
which is not convenient for design purposes.
The further analysis explained in the
following section attempts to generalize this
information over the complete range of
applicable inlet pipe sizes, presenting it in
such a way as to make it readily usable for
design purposes.
Design Curve Development
As mentioned previously, the weir height
equal to twice the inlet dimension was
Detained to allow reduction of the test data to
a usable form. Then it was necessary to
eliminate another variable to have manageable
design curves. Particular recovery rates were
selected, and the corresponding relations
between discharge, and chamber diameter
were plotted. This data also included the
ratios of weir height to inlet diameter, then
inlet diameter to chamber diameter, meaning
that all of these elements were defined.
The recovery rates selected for design
were 95 percent, 90 percent and 80 percent.
It was felt that 95 percent represented about
optimum operating conditions, whereas 90
percent would be acceptable for higher but
still normal operating discharges. The 80-
percent value was retained as likely the lowest'
recovery that would be desirable for
maximum discharges.
The first step in constructing the design
curves, for example, 95 percent was to pick
off Figures 48, 49, 50 and 51 the discharges
where the 95 percent line cut the various D2
Hnes. Their values were plotted on Figure 12,
Chamber Diameter for 95 percent Recovery
and H! /Di = 2, and a smoothed set of curves
drawn to fit the points. This gave figures for
the even foot sized inlet dimensions, so values
for the half-foot sizes we're interpolated and
drawn individually. The same procedure was
followed for the 90 percent ahd: 80 percent
recovery rates resulting in Figure 13, Chamber
Diameters for 90 percent Recovery and
HI/D! = 2; and Figure 14, Chamber
Diameters for 80 percent Recovery and
Conclusions
1 . A flat floor concept of swirl
concentrator grit chamber was developed but
found inadequate over wider ranges of
discharge. At lower flows, deposits would
likely be a serious problem.
2. A conical floor concept was then
developed to a satisfactory form adaptable
over a wide range of sizes. ;
3. A generalized design procedure has1
been defined for dimensioning the swirl
concentrator as a grit chamber with this;
conical floor concept. |
4. The Lancaster application with the
30.5 cm (1 ft) inlet sewer, discharge range
from 42.5 1/s to 141 1/s (1 to 5 cfs) and
optimum operation for 56.6 1/s (2 cfs) would
require a chamber diameter, D2 , of 1 83 cm (6
ft) as found on Figure 51. A possible layout
for such an installation is shown on Figure 28:
5. The general case using a 61 cm (2 ft)
inlet sewer, and discharge range of 85 to 425
1/s (3 to 1 5 cfs) would need further definition
of the design discharge. Depending on the
shape of the particular daily hydrograph, the •
design discharge could vary, for example,
from 283 to 368 1/s (10 to 13 cfs). From
Figure 37 the chamber diameter, D2, would
'be 3.05 to 4.27 m (10 to 14 ft) respectively.'
REFERENCES
1.
Meyer-Peter, E. and R. Muller, Formulas
for Bed-Load Transport. Proceedings LA.
H.R. Second Meeting Stockholm, 1948.
2. Einstein, H. A., The Bed-Load Function
for Sediment Transportation in Open
Channel Flows. U.S. Department of .
Agriculture, Technical Bulletin No. 1026,
September 1950.
74
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APPENDIX B
GRIT CHAMBER FOR SEWAGE TREATMENT PLANT *
I The Grit Chamber and Its Functions
The purpose of the grit chamber is to
. remove the large amount of insoluble mineral
.matter, which has been mixed with the'
sewage, before the organic putrescible
material has time to settle out. This decreases
the amount of sewage to be treated and also
. decreases the danger of clogging pumps,
while, at the same time, it aids the
putrefaction process.
Grit removal is done chiefly mechanically
in .settling chambers. The grit, which is
generally dragged along the bottom of the
pipes is able to settle out by means of cross-
'sectional expansions at lower velocities. It is
then collected in a lower chamber which is
cleaned out from time to time.
The difficulties in finding a perfect
solution to the complete removal of grit in
sewage through sedimentation are as follows:'
1. In order to separate the mineral
matter from the organic matter in the sewage
different rates of descent are used, but often
the difference is only slight and sometimes
there may be none at all.
2. The organic matter, being sticky,'
often clings to the grit and must be removed •
before the grit settles out.
. 3. Both the amount and composition of
the sewage, from which the grit is to be
'removed, fluctuate with respect to time of
day and amount of precipitation.
4. The usable area of the settling
chamber fluctuates as the grit which has
' already settled out is cleaned out at various
intervals.
5. The flow through the settling chamber
is uneven due to temperature changes in the
sewage and abrupt expansion at the entrance.
Therefore a practical grit chamber must
satisfy the following conditions:
a. The grit must be as clean as possible
and have a kernel size of 0.5 mm to 0.2 mm.
b. The inside of the grit chamber should
be as free of equipment as possible in order to
simplify its construction and maintenance and
to prevent clogging.
c. The grit chamber should be near a low
flat area and take up as little space as possible.
. Increasing its depth increases its cost.
1= Abridged and Edited from the Archive for Hydraulics
1942; by H. Geiger, Dr. of Eng.; Translated for the American
Public Works Association by Mrs. Patricia Ure Petersen.
d. It should be possible to remove the
grit dry during the operation of the chamber.
e. It should be possible to remove it with
simple machinery so that there is no foreign
equipment disturbing the plant's operation.
II The Best Known Grit Chambers.
Even before the turn of the century
settling tanks with sewage running through at
low velocities were used for grit removal.
Fruhling1 was probably the first to point out
the importance of maintaining a sufficiently
constant rate of flow in the settling chamber
so that only the mineral particles remained
and the organic material would be carried on
through the treatment plant. The first
practical structure in accordance with these
principles was the grit chamber in Essen,
Germany built by Imhoff. This chamber had a
long slightly sunken chamber containing a
steplike passage from the inlet channel to the
outlet channel.
A pipe with pores, which were kept.
closed, was built underneath the deepest
section of the floor. Both ends of the grit
chamber had to be closed off when the
settling chamber was full. The water was then;
drained off through the openings in the pipe.:
According to Imhoff, the sectional area of the
grit chamber must be large enough for a
constant average rate of flow of 0.3
meters/second (1 fps).
In order to maintain a constant rate of;
flow in a chamber with a flat bottom and
perpendicular walls the amount of water must
increase exactly linear with the height of the
.water. What cannot be gained by raising the
water level must be gained partly by the use.
of more chambers and partly by the increase
in the width upwards of the grit chamber
channel. Two chambers are necessary in order
to remove the settled sand so that one can be
cleaned while the other is still in use. The
second chamber is also used to control storm
flow which keeps the area of the grit chamber
small.
Correct dimensions for the grit chamber
irj this type of construction creates a difficult
problem to solve. The grit chamber is either
too small when the settling chamber is full or
75
-------
too large when it is empty. These dimensions
are based on the smallest daily flow which is
even smaller at night and during long periods
of drought, especially during the summer.
This creates a heavy sedimentation of organic
material which turns the grit chamber into a
putrefaction chamber with sewage flowing
through it. The collection chamber was finally
separated from the settling chamber since the
use of several chambers did not even out the
disturbing influence of the change in usable
size in the collection chamber. This has the
disadvantage that the sewage no longer flows
through the collection chamber. The
separation effect is now greatly impaired as
the organic materials which stick to the grit
cannot be cleaned off.
Long grit chambers are often difficult to
accommodate and are costly to construct
because of their considerable size. The
equipment used in cleaning them out becomes
more expensive. In shorter, deeper grit
chambers there is the danger of heavy organic
sedimentation during smaller flow periods as
the cleansing action on the bottom decreases
because of the uneven water distribution.
With the above difficulties in mind, grit
chambers were built so large cross-sectionally
that most of the heavy organic particles
settled out with the grit. Separation took
place mechanically using special equipment.
This was done either by blowing air in on the
bottom or by using a circular scraper. This
can be done in the settling chamber. If the
.blowing in of air took place while the
chamber was in operation there was the
danger that the fine grit would be carried
upward on the air stream and finally arrive at
the exit.
Long, extended grit chambers with
separate collecti6n chambers and overly large
(oversized) settling chambers were first
developed by Eddy in America. The grit was
removed from the settling chamber by means
of suction hoses which were led away over the
layers of sand and hung on movable carts.
Suitable, durable suction pumps had to be
used and the rate of depreciation was great.
Grates with bar separations of 10 to 15 mm
were placed in front of the grit chamber to
prevent the pumps from clogging. These bars
were cleaned mechanically.
In order to decrease the amount of space
needed by the grit chamber and tJD facilitate
the cleaning and removing of the sand, it was
obvious that the length of the grit chamber
with oversized settling chambers had to be
limited and therefore it had to be! deepened,
as the danger of heavy organic sedimentation
existed in the long grit chamber too.
The Dorr grit chamber uses scrapers for
later separation of the grit and organic matter.
Sewage is led into a square settling basin with
a flat bottom. Here there is a rotating scraper
with numerous blades and removable arms.
The deposited material is scraped along the
floor toward the outside and here, on one
side, it is caught up by a second scraper
moving back and forth. This then conveys the
material up on a sloping plane. The heavy
organic material which has been deposited is
separated at the same time. Naturally this
procedure, means cumbersome and costly
equipment, which needs a., great deal of
attention as they work continuously in sand
and water. There is also a large consumption
of energy.
The direction of flow in the above grit
chamber is horizontal and is! therefore
perpendicular to the rate of descent, so that
the settling material describes more or less
diagonal paths. Blunk lets the sewage travel
from bottom to top against the direction of
descent. In this way, all the material which
has a rate of descent faster than 'the rate of
ascent of the sewage falls slowly to the floor.
The material with the slower rate o'f descent is
carried on upward. By controlling^he rate of
ascent of the sewage the grit is kept on the
bottom. In putting this idea into practice, the
sewage is next led downward all at .once inside
a round fountain (well) with a funnel-shaped
bottom and then it is forced to rise again
slowly. Individual ductlike partitions of
different heights are built concentrically
arranged. These act as weirs which maintain a
constant rate of ascent that is less than
the rate of descent of the smallest grain of grit
to be collected. The settling grit, still mixed
with heavy organic material, collects on the
tip of the funnel. Here air is pumped in to
remove organic material again ibefore the
clean grit mixed with wastewater is pushed
up by means of a compressed air pump. A
rotary must be installed just under the lowest
water level in order to prevent retention of
16
-------
floating particles in front of the grit chamber.
In this way the grit chamber need only be
measured for the highest amount of sewage
less the amount which flows over the rotary.
There is, however, a danger here that some of
the fine sand will be carried along over the
rotary during greater periods of flow.
HI The Round Grit Chamber
New structures were developed from the
simple square sedimentation tanks with
manual cleaning. These new structures,
although they functioned better, became
more and more complex. This was due to the
desire for a minerally clean sand which should
settle out independently of the change in
amount of sewage and amount of grit already
settled in the collection chamber, and the
desire for a small structure which could be
cleansed mechanically. The author (i.e.,
Geiger) has attempted to develop a grit
chamber which would more than satisfy these
conditions and yet would remain simple in
structure.
1. Hydraulic Basis. It is a known fact
that loose material settles on the inside of
curves during water flow. The cause of this is
not due to a decrease in the rate of flow on
the inside shore, as was often assumed, but
rather just the opposite is' true; that is, by
taking an even cro.ss section, we find that the
velocities are the greatest on the inside of the
curve. An ideal rate of flow in a smooth,
eddy-free current is given by the equation:
v =
= C.
r
This has been proven by Boss, basing his
proof not only on the "Potential Flow
Theory," but also on the Bernouilli Law of
Energy.
The constant C can be found by the
following function:
dr
The following results if we assume a
constant height of the energy line for the
entire current as is stated by:
vf
2g
f =:
H represents the elevation of the energy
contour (line) of the undisturbed parallel
motion and v = C/r.
So that the depth of the water becomes:
t = H -
and with that the entire amount of sewage:
Q
-t
(H-
r2 -2g
^' dr
so that the constant Ccan be determined from
the following:
Q = H • C • In ir
4g
-
R22
2g
+ t0 . = H
with the values for Q, H, R2and Rt already
given.
The equation has unlimited value for
curve currents only at the curve vertex as a
complete circle is not present. We have had
good correlation between actual and
calculated values in models with open channel
curves and flat bottoms except for narrow
strips near the walls. It can be seen from the
above expression for the water depth "t" that
the water surface contour in a curve current
does not form part of a "rotation paraboloid"
curved downward, as is often assumed, but
rather it is curved upward and represents a
part of the surface of a "Rankin individual
eddy." The diagonal slope inward creates a
crosscurrent in a flow subjected to energy
and friction loss. Because of the friction of
the water, particles on the bottom and
especially the loose material, are delayed, so
that the diagonal slope becomes too great for
them here. They are therefore pushed inward
with a greater velocity by the neighboring
particles. In the same way water particles on
the outside wall are constantly being slowed
and therefore forced downward. This action
creates a crosscurrent which runs down the
outside wall along the bottom toward the
inside and eventually along the inside wall up
again. In this way the faster particles present
are able to travel outward, where the flow
disperses upward, and there is no closed
current. It is common to call the entire flow,
consisting of the lengthwise and crosswise
currents, a spiral current.
77
-------
Hinderks had already suggested using
this spiral current for sedimentation plants.
As far as is known, no use has been made of
his suggestion. It is easy to see why the use of
these principles in sedimentation tanks
promises little success. The force of the spiral
current which is to be used to aid the
sedimentation process, ignoring the slight
differences in wall surfaces, is entirely
dependent on the cross slope which is
determined by the equation:
This calls not only for a curve radius as small
as possible but most of all for an adequate
rate of flow. In sedimentation plants there
must be a current as smooth and eddy-free as
is allowed considering the economical size of
the basin, so that the sedimentation process is1
not disturbed.
Besides, the spiral current which is to be
used runs not only on the outside of the curve
from top to bottom and along the floor
toward the inside, but unavoidably it also runs
up the curve again even though somewhat
slower. In sedimentation plants where all
depositable material is to settle out, the
majority of the lighter organic material, with
a rate of descent less than the rate of the
upward current, would of necessity be carried
upward again, and, therefore, not settle out,
providing they do not stick together. It is of
little use therefore, to force the sewage
through such a winding spiral in order to
precipitate all depositable material as
Hinderks had in mind. The effectiveness is'
still less than in normal sedimentation plants.
By comparison, in grit chambers where
adequate rate of flow is required so that the ',
heavier organic material does not settle out
with the mineral particles and where only grit
is to deposited, one can successfully use this
effect of the curve. It would not do to use the
suggested form simply as a grit chamber; that
is, the form as suggested ,T>y Hinderks for a
sedimentation plant, where the collection
chamber is connected by a continuous slit in
the ceiling near the inside wall to the
deposition chamber lying above it As has,
been mentioned, heavy deposits of
organic material are unavoidable in 'those
types of grit chambers with separate
collection chambers with no flow-through. On
the other hand, the rinsing of the grit by the
current again later on must be avoided.
The author, after careful testing of all
possibilities, has arrived at the simplest and
most practical solution. The! collection
chamber is arranged inside a channel curve
(elbow) with a small radius. In this way the
disadvantageous separation of the settling
chamber and collection chamber;is avoided
due to the omission of the inside channel
wall at this point. The basic form, which must
remain simple for practical reasons, was
attained by leading the water to'be treated
into a circular chamber tangentially and after
flowing through a central angle of J80°, flows
out again through a wide opening pin the wall.
The floor is funnel-shaped to aid the spiral
current movement of the grit inw&rd. At the
same time this floor allows for a collection
chamber which is large enough for the grit.
The expected course of the current in
connection with this is diagramed in Figure 53,
Schematic View of Flow in a New Type Grit
Chamber. A plane of separation is created by
the sharp edge at the passageway from the
inlet to the circular settling and collection'
chambers. Otherwise the velocity of one of
the particles streaming around the edge would
approach infinity as has been ! shown by
•Helmholtz. This plane of separation can best
be explained as a crowd of threads of eddies
which periodically become eddies. This
surface separates the actual circular current,
which keeps the same approximate width
from the eddy with perpendicular axis which
is created inside. After the current has
traversed the curve it flows out again over the
outlet notch.
Despite the absence of the inside wall the
current in the curve must run its course to the
boundary layer just the same as a normal
curve current, so that the rule v = C/r and the-
other equations ^already mentioned are also
valid. The water surface rising toward the
outside is curved concavely upward. The
position of the surface of the water in the
vortex must be calculable with sufficient
accuracy after the constant C \ has been
determined. Even the spiral current must
occur undiminished in spite of the!absence of
the inside wall as it is created by the friction
of the water particles on the outside wall and
the floor. Not only is the movement of the
78
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FIGURE 53 SCHEMATIC VIEW OF FLOW IN A NEW TYPE GRIT CHAMBER
loose material in the spiral current aided by
the funnel shaped floor, but, because of the
water depth which is increased, and the
removal of the friction on the -inside wall,
there is an increase in the water velocity
toward the inside.
However, in the region of the eddy, in the
so-called core of the whirlpool, the equation
v = Cr is valid. The water surface here appears
as if in a rotation-paroboloid-shaped container
.which rotates on a perpendicular axis.
Although no current in a rotating
container can develop in radial planes after
inertia has set in, a crosscurrent inward does
develop in connection with an e'ddy due to
the friction of the water particles on the
floor. This is due to the fact that, as in the
curve current, the diagonal slope is too large
for the slowed water particles. For that reason
they are pushed downward toward the center
by faster particles, and there, in the center, an
ascending current develops which enables
these particles to travel outward.
. Since the water content of the eddy must
be assumed approximately constant, a circular
current develops, which, together with the
rotating current, forms a spiral current. The
diagonal slope in connection "with the eddy
decreases to 'the point of zero toward the
.center, contrary to the diagonal slope in the
flow curve which increases toward the center,
so that only a weak crosscurrent develops,
which peters out toward the center.
Rohr had already observed this in water
currents in harbor basins but did not search
for the cause. These investigations also show
how loose material from a current flowing
along a level floor pushes through the surface
79
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of separation into the eddy and is pushed by
it toward the center.
The crosscurrent created by the curve
does not curve upward because of the absence
of the inside channel wall but rather-runs
through the eddy vortex of the plane of
separation, even though a bit delayed. This
course of flow consisting of a curve current
and an eddy is shown schematically in Figure
1. The course continues in the rectified
crosscurrent which forms the eddy here, so
that a spiral current is created over the entire
radius of the circular settling and collection
chambers. This effectively aids the
sedimentation of grit which is pushed
to'gether along the bottom toward the center,
helped along by the slope of the floor. In this
way the grit from the sewage which is near
the floor reaches the region of the eddy
quickly and there it is held in place.
2. Basis for the Experimental Model
a. Description of the Model. In order to
develop a workable grit chamber a model was
built which relied heavily on relationships and
conditions in actual treatment plants. The
model was built as large as possible in the
experimental channel at the Technical
Institute in Karlsruhe. A circular duct was
built onto the channel in order to exclude the
disturbing influence of temperature
differences. Then a finely graded regulating
centrifugal pump was switched on to the duct
and this pump lifted the water from a
collection tank at the end of the channel to a
stagnation basin at the entrance. From there
it flowed on a natural slope through a long
open channel to the grit chamber. The water
surface in the grit chamber could be regulated
in the usual way by the use of removable
locks (blocking apparatus) in the outlet.
There was a sharp-edged measuring weir with
a grate used to calm the water already in place
at the end of the runoff channel. The
measurement of the height of the drop over
the weir and of the height of the water
surface in the inlet channel 'were taken by
means of adjustable, pointed measuring rods.
The amount of water and surface height were
chosen with reference to the actual conditions
in a southern German city:
b. Measurements and Velocities.
According to what was learned from the
model, the following forces determine the
adherence of certain relationships between
the basic measurements for the linear
measurements, times and forces, the force of
inertia, the force of gravity, the force of
friction and the capillary force. If, along with
the force of inertia, more than one of these
forces occur, we are no longer able to obtain
strict mechanical similarity at the same
temperature by using the same fluid in nature
as in the model. In the case at hand, which is
concerned with an outflow in an open
channel with a rough wall, the inside «forces of
friction and the capillary forces vis a vis the
force of inertia and of gravity can be ignored
without hesitation, all the more so as the
experiment occurs during turbulence and the
critical Reynolds number is exceeded by more
than four times. We can expect: therefore a
conformity between the occurrences in the
model and in nature if we use Frouds Law of
Similarity which actually takes into account
the effect of the forces of inertia :and gravity.
A fluid must be used which is similar to
city sewage with regard to physical properties
under study here. As mentioned earlier, only
that sedimentation process where the rate of
descent is decisive can be used for the
separation of mineral and organic particles. In
order to arrive at the correct relationships
(ratios) in the experimental model, the'rate of
descent of the substituted materials must so
suit the model standards, that they are in a
correct ratio to the rates of flow in the model.
The path, which a particle makes in nature
under the simultaneous effect of the rates of
flow and descent, must correspond to the path
in the model, i.e., it must appear; in the same
position on the floor as it is in reality under
given conditions.
According to Fraud's Law of Similarity
the following equation for the relationship
(ratio) of the rate of flow in nature — vn — to
the rate'of flow in the model — vm — is valid
where x = measure (standard) of the model.
Vm =
X0-?
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If we have un as the rate of descent in nature
and um as the rate of descent in the model, it
then follows that:
xo.s
• XO-5
orUm =
Ur
If we take an arbitrary measure for the model,
i.e., x = 10
um =
3.16
For the experimental model with the model
measure x = 10, material must be added to the
liquid which has a rate of descent one-third
that of the rate of descent of the grit. One
realizes, therefore, that it is not only
impractical to work with real sewage in
experimental models, but that it is also
theoretically unsuitable
From experimental work by Blunk, it has
been shown that it is only possible to keep
those mineral particles with a grain size of
over 0.5 mm in the grit chamber because the
smallest rate of descent of those borders on
the largest determined rate of descent of
organic matter.
c. Choice of Suitable Material. In strict
adherence to the laws of similarity, a material
had to be found to be used in place of grit in
the chosen model with the smallest kernel size
of 0.5 mm; 10 = 0.05 mm and a rate of descent
of over 4; 3.16= 1.2 cm/sec, while the rate of
descent of the substitute organic material
could not exceed 1.3 cm/sec. Since the size of
the grain cannot be normally reduced
according to scale in experimental models
because such a fine mud can no longer be
handled in experimental channels, practical
considerations limited the corresponding
reduction in the rate of descent. Lignite slack
was found to be the best suited substitute for
the grit for use in the model and sawdust was
used as a substitute for the organic material.
When the kernel size of the lignite chosen was
over 1 mm the rates of descent of the
materials to be separated bordered on each
other. Wet lignite with a kernel size of 1 to
1.5 mm has a rate of descent of 3 to 6 cm/sec,
whereas normal newly softened sawdust has a
rate of descent of approximately 1 to 3
cm/sec. Moreover, the rate of descent of the
wet lignite grit in the experimental model is
almost exactly the rate of descent of grit with
a kernel size of 1 mm in nature — 9 to 18
cm/sec (taking into consideration the factor
3.16). The experimental model corresponds
to the actual relationships in a grit chamber
which retain grit up to a size of 1 mm,
ignoring the scale reduction of the kernel size.
It could be expected that the experimental
model would show support for practical use.
In fact, existing plants, which shall be
discussed later, have confirmed these
expectations.
. Since the depositable mineral and organic
materials stand in a 1:2.5 ratio in normal city
sewage, one part wet lignite and 2.5 parts wet
sawdust were thoroughly mixed in a special
container and slowly added to the water
through a funnel at the beginning of the inlet
channel. The entire amount of lignite and
sawdust which was added in each experiment
corresponded approximately to the daily load
of depositable material in the sewage in the
treatment plant which was the basis for the
experimental model. Room for the grit
collection was determined for this amount
assuming a daily mechanical cleaning. In order
to shorten the duration of the individual
experiments, the addition of the material had
to take place much more quickly than what
corresponded to the correct measured time
for the model. This was a circumstance which
could only favorably affect the results of the
experiment with respect to reality, as less
time remained to clean out the heaviest
organic matter which had settled out.
d. Determining the Degree of Efficiency.
The efficiency of grit chambers cannot be
determined by one individual percentage
table. The unavoidable dependence on the
changing amount of influent according to
time interval must be taken into account.
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'Even the ratio of deposited grit to added grit
does not give us satisfactory information, but
rather it must be supplemented by the ratio
of the amount of organic material
unintentionally retained to that amount
which was added. A light grit is to be settled
out by means 'of oversized settling chambers
which is all contained in the sewage, if one
accepts a corresponding sedimentation of
organic material at the same time. Without
additional mechanical treatment of the grit it
is only practical, within a narrow area, to have.
100 percent of the mineral material and as
close to 0 percent of the organic material as
possible deposited.
A perfect solution to the problem of grit
sedimentation with a heavy changing rate of
sewage is only possible by means of grit
chambers with oversized settling, chambers
and subsequent mechanical removal of the
heavy organic material which settles out at-
I the same time. Finer grit can be retained in
the grit chamber while the heavier organic
particles must be removed by mechanical
means such as agitators or condensed air
conveyances.
In the experiment the water which
flowed out was led below the measuring weir
over a fine wire sieve which kept back the
remaining loose material. This remaining
amount of wet sawdust and lignite was
determined after every experiment according
to volume, and always set in a ratio to the
constant volume of lignite and wet sawdust
which had been added originally. The rate of
flow according to time interval was changed
from experiment to experiment in close
dependency on the actual conditions of the
smallest drought flow to the largest storm
flow. The percentage, which was applied over
the rate of flow, of the amount of volume of
lignite and sawdust which had been sieved out
in the runoff always gives a pair of lines which
show the effectiveness of the grit chamber.
These lines give information necessary to the
perfection of the grit chamber. There is an
unavoidable inaccuracy inherent when
determining the amount of material strained
according to volume. This must be accepted
as it is not feasible to determine through
weights, for both lignite and sawdust soak up
and emit water quickly and their specific
gravity in the wet state is near 1.
3. Description of an Experimental
Model. As with any new type of building; a
great distance had to be covered from the
initial idea to the finished workable product.
Figure 54, Cause of the Course of the Current
in a Model Grit Chamber, shows the last form
of the model. The water to be cleansed of grit
FIGURE 54 CAUSE OF THE COURSE OF THE CURRENT
IN A MODEL GRIT CHAMBER
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flows tangentially from" the inlet channel
which is divided into a narrow dry-weather
channel and a wider channel for rainy weather
lying above it, into a cylindrical chamber with
a funnel-shaped floor, which serves
simultaneously as a sedimentation chamber
and a collection room. This chamber is
divided the same as the inlet channel, a dry-
weather chamber with a small diameter and a
larger cylinder lying above it to control the
storm flow. After entering the settling
chamber the water must flow around a curve
of more than 180° along the wall before it
can run off through a cut in the outside wall.
As the water to be treated enters the grit
chamber some of the heaviest mineral matter
which is near the floor slides immediately on
the funnel-shaped floor into the collection
chamber and some settles down due to the
slightly reduced rate of flow. Figure 55,
Model Grit Chamber Showing Sediments,
shows this process, looking down onto the
top of the model. In order to see the lignite
which was substituted for the grit, the floors
of the channel and the grit chamber were
painted white. Shortly before the picture was
taken we added a sizable amount of lignite
grit to the entrance of the channel.
The paths of the fine grit which settles as
the water runs through the curve could not be
seen due to their small size at the present rate
of flow. In order to show clearly the effective
flow procedures here, waving flags were
fastened on to show the current on the
. bottom. The course of the surface current was
shown by sprinkling paper shreds on the
surface of the water. Figure 54 shows the
direction of flow as seen from above during
lesser rates of flow according to time
intervals, while Figure 56, Course of the
Current in Model Grit Chamber, shows the
same during greater rates of flow. Figure 57,
Side Views of Course of Current in a Model
Grit Chamber, gives us a side view of the inner
room of the grit chamber.
Along with the current which runs
concentrically on the surface, a powerful
crosscurrent, the so-called spiral current, can
be seen, running along the outside wall
downward and along the bottom inward as it
appears in open-curved channels. Since this
crosscurrent depends on the average velocity,
the flags point.,, for the most part, toward the
center (c.f. Fig. 56 which' has an average
velocity in the inlet channel but a larger flow
as compared with Fig. 54) so that a constant
settling effect is produced. Not only is the grit
being extraordinarily aided in the settling
process by this spiral current, but, once it has
settled, it is also being constantly pushed
toward the center. In this way the sand
particles cross over the plane of separation
between the curve current and the eddy
present inside, are removed from the sewage
which flows onward and are deposited in the
tip of the funnel in the chamber. The
continuous circular flow, which reigns in the
bottom part of the funnel washes away the
lighter particles clinging to the grit: These rise
slowly in the center due to the spiral current,
and are finally torn out into the area of the
curve current by small wandering eddies,
which rythmically dissolve at the tapered
mouth of the inlet and so are carried away
again by the sewage.
Determining the Most Suitable
Shape and Measurements
Separating walls were built in the grit
chamber between the settling room and the
collecting room in the first model. Later on
we tried to increase its effectiveness by adding
conducting walls at the inlet and the outlet.
This only showed us that walls of any kind
produce unwished for and unexpected eddies
and whirlpools which disturb the settling
process. If a separating wall between the
collection chamber and the settling chamber
was built, lighter materials settled out due to
the fact that the cleaning out process. was
stopped. It was further shown that to obtain a
strong circular current it was best to build the
inner chamber of the grit chamber as smooth
and as round as possible.
The position and type of outlet were also
experimented with as an outlet which
functioned as smoothly as possible greatly
influenced the effectiveness of a grit chamber.
The outlet was systematically shifted from a
position which continued the direction of the
inlet channel on the other side further and
further toward the direction of flow. The best
results were reached with an outlet whose
final edge falls with its axis parallel to the
inlet and whose width does not exceed 60°.
In this way an adequate length of bend in the
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FIGURE 55 MODEL GRIT CHAMBER SHOWING SEDIMENT
FIGURE 56 COURSE OF THE CURRENT IN MODEL GRIT CHAMBER
84
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FIGURE 57 SIDE VIEWS OF COURSE OF CURRENT
IN A MODEL GRIT CHAMBER
curve is obtained and there is no opposing
disturbance from outlet and inlet. The
velocity in the runoff may not exceed 0.5
m/sec and its bottom edge should be higher
than the inlet if possible. The outlet is divided
purposely into a narrow part for the diy
weather flow and a symmetrical part lying
above it for the rainy weather flow.
We further ascertained in which way the
effectiveness of the grit chamber depends on
the rate of flow and the size of the
sedimentation chamber. Since the rate of flow
cannot be determined unequivocally, the
average rate in the inlet channel was used as a
point of comparison. It revealed that the rate
of influent should be approximately 0.75
m/sec and should not exceed 1 m. The ratio
between the usable area in the sedimentation
chamber and the amount of water added at
that second proved to be a/useful rule for
'determining the necessary measurements of
the sedimentation chamber. This ratio a = J/Q
is known as theoretical delay time in
•connection with grit chambers which are
flowed through horizontally. In the above
type'of construction a value for a of 25-30 is
recommended when the sedimentation
chamber is riot oversized. In- comparison, a
- has a value of 50 for long grit chambers with
normally sized settling chambers with the
minimum length of 15 meters and the rate of
flow equalling 0.3 m/sec. There is then a
considerable saving in construction costs.
Savings can be increased even more through
compact structure.
The rate of flow and the height of the
water surface were determined with respect to
actual values in a large southern German city.
The main collector there has a slope of
1:2000 with a diameter of 1.8 m, so that
there the ratios are unfavorable as a small
increase in water height corresponds to a
heavy increase in the amount of flow with
this small a slope. For that reason, all ratios
between the rate of flow and the area of the
sedimentation chamber were considered in
the experiment and a higher inlet velocity was
permitted with high rates of flow. These
ratios were much more favorable in reality
because the experiments in the model take
place over such a short period of time that the
current is unable to cleanse out the organic
materials, as it does in reality.
A part of the heaviest organic material
settles out with the sand in the center of the
funnel in dry-weather periods. It can be
85
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FIGURE 58 GRIT CHAMBER FOR A SMALLER CITY
86
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cleaned- out easily in practical applications by
short blasts of air from the middle of the
floor. This creates a gentle rolling and stirring
of the settled material which, along with the
'. spiral current, aids in the separation process
without the danger of the stirred up grit
reaching the outlet of the grit chamber.
A constant, slight over-measurement of
Qhe settling chamber is therefore
recommended if there is a great fluctuation in
the amount of water even in this type of
structure. In this way, post-treatment of the
grit is possible in the simplest way using the
smallest amount of air because of the form of
the settling and collection chambers.
4. Attempts at Practical Applications
a. Surface Current. The various models
were later enlarged for practical applications.
The grit chamber used here was determined as
to size for the minimum amount of sewage as
125 I/sec and the maximum amount of 625
I/sec. It has a diameter of 3.9 meters and is
shown in Figure 58, Grit Chamber for a
Smaller City. Both the surface current and the
rates of flow were shown by means of floating
candles which were interrupted at regular
intervals as shown in Figure 59, Course of
Current in an Aerated Grit Chamber. Even
though the concrete bridge which crosses over
the grit chamber covers a part of the settling
chamber, both the course of the current
through the curve and especially the eddies
which lie inside this current are more clearly
recognizable here than in the model. As was
expected, the current in the curve did not
keep strictly to the width of the inlet-channel
during its course through the settling chamber
but rather it widens ,out toward the inside
FIGURE 59 COURSE OF CURRENT IN AN. AERATED GRIT CHAMBER
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whereby the average velocity decreased
correspondingly. The lines of the course on
the surface push together toward the inside
even on the surface in certain opposition to
the experiments in the model and other
experimental results and also in opposition to
theoretical results. This phenomenon! can be
explained by the influence of the wind which
cannot be entirely eliminated when working
outdoors, but also by the fact that the fairly
heavy floats used were carried inward through
their own weights.
b. Profile of the Water Surface. Along
with the surface current the profile of the
water surface is determined in the curve. The
measurements were taken by means of a
tapered measuring rod and a leveling
. instrument in radial cross sections with angle
intervals of 30° each time. In the crown
(vertex) of the cross section which was the
same as the axis of the concrete path, no
measurements could be taken. The surface
curved upward. The same hydraulic ratios are
as valid here in this type of structure even
though the inside wall is missing, as in a curve
of 180°.
The proof of this can be found in a
comparison with the calculated water surface
line for a channel curve of the same
measurements on the basis of the potential
theory. Next the height of the energy line for
the cross section and the runoff amount for
the inlet sectional area are determined with
reference to the water velocities taken from
Figure 60. The radius of the inside wall of the
settling chamber and a value decreased by
about the width of the inlet channel are
inserted for the radii of the curve. In this way
the constant C can be obtained from the
equation:
Q = H • C • In
as being 420Q/cm/sec.
From the simplified formula:
C =
SflL
to • In Rl
the constant C would be 4250 cm/sec with
the measured average water depth in the
straightaway before the curve equal to 54.6
cm.
c. Properties and Amount of the Settled
Grit. Figures were finally received on the
cleanness and kernel size of the sand in the
three different grit chambers of similar size.
The grit chambers were planned; for a dry
weather flow of 125 to 140 I/sec and a storm
weather flow of 560 to 625 I/sec ;and have a
diameter of 3.9 to 4.5 meters; The grit
deposited was almost completely minerally
clean and also contained a large portion of
fine kerneled grit as is shown by the following
table:
Plant I II III
%. % : %
Mineral Particles 97.0 98.5 f 97.5
Organic Particles 3.0 1.5; 2.5
Kernel size of grit:
Over 4 mm 6.0 2.51 3.5
4-2 mm 15.0 10.5 10.0
2-1 mm 15.5 28.0 ^ 24.0
1-0.5 mm 32.0 42.0; 38.5
0.5-0.2 mm 24.5 14.0 j 20.5
Under 0.2 mm 7,0 3.0; 3.5
5. Description of Practical Application
Grit chambers are generally cleaned out-with
a special air pressure pump which is useful
because of its simple construction and
durability.For this reason the floor of the
chamber is not leveled off but rather built in
the shape of a funnel with the air pressure
pump (compressed air pump) put in its axis.
Since the height (lift) of this pump is only
equal to its immersion depth, the addition of
a corresponding pump basin is most often
necessary. Its diameter should be just a little
larger than the outside diameter of the pump
so that the settling area can be kept as clean
as possible. The settled material is jcleaned in
the following manner before the sand is
finally removed. First the stop valve in the
conveying pipe for the sand-water 'mixture is
closed before the pump is started. Meanwhile,
the compressed air used in operating the
conveying pump gently flows out at the
bottom end of the pump pipe. A small
amount of air is required for only a short
period of time. It has been shown from
experience that a blast of 10 minutes is
sufficient for a complete cleansing before grit
removal. The treated grit and water mixture
flows into an individual settling basin with a
88
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runoff alongside the grit chamber by opening
the stop valve. Here the sand is deposited while
the surplus water is led back into the inlet to
the grit chamber. In this way, traces' of grit'
cannot get into the outlet of the grit chamber.
The grit is then left- to dry and loaded
manually onto carts which carry it away. A
water pipe is led into the pump basin bottom
through which water from the city water
supply line can be led before and during the
operation of the pump. In this way the
removal of the sand-water mixture is made
easier and the material from the grit chamber,
which during long pauses in grit removal, has
thickened on the bottom part of the pump.
The first practical application of the new
type of grit chamber for a smaller city
(Villingen in the Black Forest) is shown in
Figure 58. A grate with bar separation of 40
mm is built into the inlet in order to hold
back large bulky material. This is cleaned
mechanically. It is obvious that this new type
of grit chamber is noteworthy because of its
simplicity and the small amount of space it
needs. It has also been shown here, that the
construction costs for this type of grit
chamber, including the compressed air pump,
are no higher than those for the
two-chambered grit chamber of usual
construction.
The purification plant was started chiefly
without a separated grit chamber in the fall of
1935. The sand (grit) collected, in the settling
basin of the grit chamber had a bright color
and was practically free of organic material.
The kernel size was as small as 0.2 mm and
was completely odorless. Villingen is the
second coldest city in Germany and for that
reason uses a large amount of sand in winter.
For this reason the performance of the grit
chamber is of special interest. The sand which
is collected is reused for spreading on sheet
ice. The servicing of the grit chamber is simple
and in fact is limited to operating the
stop-valve of the sand conveyance. It has also
been shown that there is no danger of
stoppage to the^ compressed air pump. A
rotary can be dispensed with since the sand is
removed during the operation of the grit
chamber.
Double layouts are used for large cities.
Here the inlet pipe is divided and the
chambers lie touchingxeach other. Both the
inlets and outlets are fitted with slide-valves
so that they can be shut off individually. A
common grate can be constructed in front of
the grit chamber.
There are countless solutions with respect
to different physical conditions. Figure 60,
Double Grit Chamber Layout for a Large
City, shows a grit chamber which serves a
population of 750,000, which is the largest
German grit chamber at present (1942). The
grit chambers,.one of which takes the sewage
from the part of the city to the east of the
Rhine River while the other takes the sewage
from the western part, are measured for
different work loads. The ratios here create
great difficulties for the construction of a grit
chamber which is to work perfectly. The
amount of sewage from one area fluctuates
between 1.48 and 6.73 m3/sec (cubic meters)
and for the other area, between 1.26 and 4.8
m3 /sec. The surface heights applied over the
water amount in the inlet pipe at the entrance
to the purification plant result in quite
irregular broken lines due to the effect of very
narrow pipes in the old part of the city from
storm sewers and siphons. Unlike the usual
purification plant, these grit chambers must
operate a greater part of the year with a
strongly fluctuating back-up from the Rhine,
which, can sometimes be as high as 2.9
meters. For that reason a considerable
lengthening of the delay period with a similar
reduction of the inlet velocity must be taken
into account from time to time, especially
with a strong back-up. The heaviest organic
material, which has settled out along with the
sand near the center of the axis of the settling
chamber can be removed by corresponding
blasts of air near the funnel tip before the
sand is removed. If there is a large back-up
and a small rate of influent, which would
seldom occur, an overly large depositation of
organic material can be avoided by constant
blasts of small amounts of air.
In order to prevent a back-up over the
top of the inlet channel from further
worsening the effect of the cleansing in the
settling chamber, this is arched over, which is
easy to do in this type of grit chamber. In this
way a further increase in the ratios between
the content of the settling chamber and rate
of flow at that second is prevented. This gives
better results as the rate and velocity of flow
89
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FIGURE 60 DOUBLE GRIT CHAMBER LAYOUT FOR A LARGE CITY
decrease very quickly with the increase in
back-up from the Rhine. The new type of grit
chamber also allows ihe controlling of
unusual ratios. The grit is conveyed' here, by
means of compressed air pumps. The settling
chambers for the grit-water mixture could
thereby be set at the surface.
Without a doubt this new type of grit
chamber can be used over a wide area, due to
its great simplicity, its outstanding separating
ability, easy operation, and ks minimal
construction and operating costs.
REFERENCES
]. A. Fruhling, Water Removal for Cities,
Handbook of Engineering, 4th Edition,
Volume 4, Leipzig, 1910.
2. A. Hinderks, Spiral Purification Plant for
the Purification of Sewage. The Building
Engineer 11 (1930) Pg. 291.
3. A. Blunk, Measuring Grit Chambers.
Health Engineering 56 (1933) Pg. 478.
4. F. Rohr, Water and Movement of Descent
in River and Ocean Harbours. Munich,
1934. •
5. Dr. P. Boss, Use of the Potential Theory in
the Movement of Water in Curved
Channels of Rivers. The Building
Engineer 15 (1934) Pg. 251.
6. Prof. H. Wittmann and Prof. 'Boss, Water
and Loose Material Movement in Curved
River Stretches. Berlin, 1938.: .
7. Dr. K. Imhoff, Handbook of City Water
Treatment. 8th Edition. Munich and
Berlin, 1939.
90
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APPENDIX C
SCALE EFFECTS ON PARTICLE SETTLEMENT
As discussed in the main portion of the
report, the appropriate modeling parameter to
relate the prototype to model mathematical
relationship is the Froude number, since
gravity effects predominate the flow
performance in this free surface flow case.
This results in the expression
Vm = Vp/Lp/Lm.
If the model to prototype ratio is
Lp/Lm = 2/1, the expression becomes Vm =
Vp/1.414. For example, a 0.2 mm diameter
particle of sand is selected, which has a
specific gravity of 2.65. From Figure 15, the
particle settling velocity for a 0.2 mm
diameter sand particle is 2.6 cm/sec in the
prototype. In the model this corresponds to a
velocity of settlement of 1.84 cm/sec or
0.0603 fps. If other materials are to be used
in the model to simulate a 0.2 mm sand
particle, they must correspond to a
comparable settling velocity of 1.84 cm/sec.
From Figure 15, Appendix A, these particles
would be: sand - specific gravity 2.65,
diameter 0.17 mm; pumice — specific gravity
1.35, diameter 0.34 mm; Gilsonite — specific
gravity 1.06, diameter, 1.0 mm.
When settlement of individual particles
occurs in a flow of water, however, each of
the above particles will settle in accordance
with the principle of particle settling
dynamics, which relates the gravitational and
viscous forces, and therefore follows -the
traditional coefficient of drag and Reynolds
number relationship.
Assume for purposes of this example that
the velocity in the flow field in the model is
0.85 fps. The horizontal motion of a sand
particle 0.17 mm is approximately 0.255 fps,
using an experimentally determined
constant.1
From the same reference, the following
relationship for particle settling holds:
4
o
D
p- -p g =
D 2 vs
PD TT "2" P ~2~ POS 0
this can be reduced to the following equation:
- D - 1 g = PD VS2 pOS0
Substituting values of D = 0.00055 665 ft
(0.017 cm), P! = 2.65 P, and g = 32.2 ft/sec2
(980 cm/sec2) into the left side of the
equation yields the value 0.0394, thus the
above equation is reduced to
0.0394= PD'vs2 POS0
The velocity relative to the particle
horizontally is V - Vp = Vx
where
V = the streamline velocity
Vp = the horizontal velocity of the particle
Vx = the horizontal velocity relative to the
particle
Let V0 equal the actual settling velocity
of the particle; thus the total velocity relative
to the particle as it settles is Vs =VX2 + V02
For this example Vx = 0.85 fps - 0.255 fps -
0.595 fps.
The settling velocity V0 is then obtianed
by selecting a value for V0, calcualting Vs,
Reynolds -No., CD S . For the conditions
described above, V0 = 0.027 fps, which is less
than the settling velocity in still water for this
size particle of 0.0603 fps (1.84 cm/sec).
Similar calculations are performed to
determine the velocity of settlement of the
Gilsonite particle, sp. gr. 1.06, diameter 1.0
mm, which was used in the model to simulate
sand particles 0.2 mm in size. Since this
particle is larger, a different experimental
constant S is used from reference 1. For this
diameter S = 0.5, thus the horizontal velocity
of the particle is 0.5 (.85 fps) or 0.425 fps.
The horizontal component of fluid velocity
relative to the particle is 0.425 fps. Again
utilizing the above equation
Pi
P
g=' -pD Vs2 pOS 0
and specifying values of D = 1 mm (0.00327
ft) g = 32.2 fps2 (980 cm/sec.2) and Pl =
1.06 P, the left hand side of the equation.
becomes 0.0084, or the above equation
reduces to
0.0084= PD Vs2 POS 0
91
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The equation' is solved by selecting values
for V0 and calculating Reynolds No., Cd
(obtained from an experimental plot) and 0.
For the conditions described above, V0 =
0.013 fps, which is considerably different
than the velocity of settlement in still water.
The distance required to settle these
particles can be calculated from the
relationships L = Vp H
V0
where H is the depth of flow
For sand, 0.17 mm diameter, using the
velocities determined above
- 0-455 fps
-
For Gilsonite, 0.2 mm diameter, using the
velocities
T 0.455 fpS TT _ -3 f TT
L = OOT3~fps H~35H
The above calculations illustrate the scale
effects of the model studies. Since the
Gilsonite requires a longer distance to settle,
it can be concluded that if the settlement of
Gilsonite occurs in the model, better settling
performance of grit in the prototype can be
expected. ;
REFERENCES
L Zielinski, P. B., The Vortex Chamber as
a Grit Removal Device for Water
Treatment, Report No. 30, supported by
U.S. Department of the Interior, Office
of Water Resources, at Clemson Uni-
versity, Clemson, South Carolina.
• 92
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-670/2-74-026
2.
3. RECIPIENT'S \CCESSlON«NO.
4. TITLE AND SUBTITLE
5. REPORT DATE
June 1974; Issuing Date
THE SWIRL CONCENTRATOR AS A GRIT SEPARATOR DEVICE
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Richard H. Sullivan, Morris
and Fred Parkinson
i. PERFORK
L Cohn, James E. Ure,
APWA 73-1
9. PERFORMING ORGANIZATION NAME AND ADDRESS
American Public Works Association
1313 East 60th Street
Chicago, Illinois 60637
10. PROGRAM ELEMENT NO.
1BB034;ROAP 21ASY;TASK 221
11. OONTnAB'T/GRANT NO.
11023 GSC (S-802219)
12. SPONSORING AGENCY NAME AND ADDRESS
National Environmental Research Center
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
See EPA-R2-72-008 (PB-214 687)
16. ABSTRACT
A study was conducted by the American Public Works Association to determine the applicability of a swirl concentrator
chamber to perform the functions of a grit separation and removal facility. The swirl concentrator principle was originally
developed in Bristol, England, and subsequently modified and applied by the APWA to act as a combined sewer overflow
'regulator.
The ability of the swirl flow pattern to effectively remove solids of particular sizes or specific gravities was noted during
the first study. This hydraulic flow configuration was developed and adapted to effectively remove grit from either the under-
flow from the combined sewer overflow regulator or from domestic sanitary sewage.
Hydraulic model studies were used to develop optimum design configurations. For an average flow of 0.084 mVsec
(3 cfs)thes diameter of the unit would be 2.19 m (7.2 ft) and 1.1 m (3.6 ft) deep. The efficiency of removing grit particles
of 2.65 sg and size greater than 0.2 mm will be equal to that of conventional grit removal devices. The unit has no moving
parts. Conventional grit washers and lifts can be employed.
The complete report on studies carried out on a swirl grit removal model by the LaSalle Hydraulic Laboratory Ltd. is
included as an appendix.
The report was submitted in fulfillment of the agreement between the City of Lancaster, Pennsylvania, and the American
Public Works Association, under the partial sponsorship of the Office of Research and Development, U.S. Environmental
Protection Agency, in conjunction with Research and Demonstration Project 11023 GSC (S-802219.).
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b. IDENTIFIERS/OPEN ENDED TERMS
c. cos AT I Field/Group
*Grit chambers, Design, Waste treatment, *Grit
removal, Combined sewers, Swirling—separation,
Sewage treatment, *Grit removal-sewage
treatment, Flow rate
*Swirl concentrator,
Combined sewer overflow
regulation, Recovery rate
13B
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
UNCLASSIFIED
101
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
93
U.S. GOVERNMENT PRINTING OFFICE: 1974- 7
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