ENVIRONMENTAL PROTECTION TECHNOLOGY SERIES
September 1973
The Dual Functioning Swirl
Combined Sewer Overflow
Regulator/Concentrator
National Environmental Research Center
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
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EPA 670/2-73-059
September 1973
THE DUAL FUNCTIONING SWIRL COMBINED SEWER
OVERFLOW REGULATOR/CONCENTRATOR
by
Richard Field, Chief
Storm & Combined Sewer Section
Advanced Waste Treatment Research Laboratory
National Environmental Research Center - Cinn.
Edison, New .Jersey
U.S. Environmental Protection Agency
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ABSTRACT
A hydraulic laboratory pilot project was run in conjunction with
mathematical modeling to refine and demonstrate the swirl flow regu-
lator/solids-liquid separator. The device, of simple annular shape
construction, requires no moving parts. It provides a dual function,
regulating flow by a central circular weir while simultaneously treat-
ing combined wastewater by a 'swirl' action which imparts liquid-
solids separation. The low-flow concentrate is diverted via a bottom
orifice to the sanitary sewerage system for subsequent treatment at the
municipal works, and the relatively clear liquid overflows the weir into
a central downshaft and receives further treatment or is discharged to
the stream. The device is capable of functioning efficiently over a
wide range of combined sewer overflow rates, and can effectively separ-
ate suspended matter at a small fraction of the detention time required
for conventional sedimentation or flotation. For these reasons, serious
thought is being given to the use of swirl units in series and in paral-
lel solely as wet-weather (and domestic sewage) treatment plant systems.
11
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CONTENTS
Page
Abstract ii
Figures iv
Tables vi
Introduction 1
Background 1
Combined Sewer Overflow Problems 3
"Swirl" Not a "Vortex" 4
General Description of Swirl Device 6
Modeling . 9
Hydraulic Model Description 9
Elements of the Swirl 15
The Prototype at Lancaster, Pennsylvania 23
Overall Description 23
Design Flow and Dimensions 26
Structural Layout 26
Solids Separation Efficiencies 29
Model Particle Materials and Sizes 29
Removals Expected 32
Design Rationale 34
Hydraulics 34
Head Considerations 34
Sizing 35
/>
Special Design Features 40
Potential Applications and Research Needs ' 43
Universalization - 43
Potential Uses ' 43
Conclusions 45
References 48
111
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FIGURES
No. Page
1. "Vortex" Device Originally Developed by Smisson, 2
Bristol, England.
2. Comparison of Particle Flow Mathematical Model Results 2
with Test Data
3. Unimpeded Free-Surface Vortex Action, Pilot Model, 5
LaSalle, P.Q.
4. Crude Deflector Plate Inducing Gentle Swirling Action, 7
Pilot Model, LaSalle, P.Q.
5. Overhead View of Swirl Flow Regulator/Solids-Liquid 8
Separator in Operation, Pilot Model, LaSalle, ,P.Q.
6. Dry Swirl Device with Floor Gutters Visible, Pilot Model, 8
LaSalle, P.Q.
7. Elevation View of the Swirl in Operation with Solids 10
being Separated Towards the Floor, Pilot Model,
LaSalle, P.Q.
8. Overall Hydraulic Model Layout, LaSalle, P.Q. 10
9. Particle Settling Velocities as a Function of Size and 14
Specific Gravity
10. Isometric View of Swirl Regulator/Concentrator 17
11. Floatables Emerging Under Weir Plate from Vortex 20
Cylinder, Pilot Model, LaSalle, P.Q.
12. Floatables Trapped Under Weir after Test Operation, Pilot 20
Model, LaSalle, P.O.
13. Empty 3-foot Chamber Laboratory Model with Floor 22
Gutters Visible, Pilot Model, LaSalle, P.Q.
14. Preliminary Flow Diagram, Combined Sewer Overflow 24
Control/Treatment System, Lancaster, Pa.
15. Installation Site, Lancaster, Pa. 25
16. Preliminary Drawing - Elevation View of System, 27
Lancaster, Pa.
IV
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17. Preliminary Drawing - Plan View of System, Lancaster, Pa. 27
18. Top View of Weir Assembly, Onondaga County, Syracuse, 28
New York
19. Elevation View of Outside Tank into Which Weir Assembly 28
will be Set, Onondaga County, Syracuse, N. Y.
20. Model Simulation of Prototype Solids - Organic '' 31
21. Model Simulation of Prototype Solids - Grit 31
22. Percent Gilson.ite Recuperation vs. Discharge Rate for 33
Grit and Organics
23. Head Above Weir Crest vs. Discharge Rate 33
24. Hydraulic Head Requirements 36
25. Design Overflow Rate vs. Chamber Diameter 37
26. Swirl Plan and Elevation Views - Below Roof 38
27. Swirl Plan and Elevation Views - Floor Area 38
28. Separation Efficiency Curve 41
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TABLES
No. Page
I. Specific Gravity, Size and Concentration
of Settleable Solids ' 29
II. Swirl Chamber Dimensions 39
VI
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"THE DUAL FUNCTIONING SWIRL COMBINED SEWER
OVERFLOW REGULATOR/CONCENTRATOR"
Introduction
Background
An intensive study(1,2) to develop and design a new type of com-
bined sewer overflow regulator device was fostered by and piloted under
the general supervision of the U.S. Environmental Protection Agency's
(EPA's) Storm and Combined Sewer Technology Branch, Edison Water Quality
Research Laboratory of the National Environmental Research Center -
Cincinnati, Edison, New Jersey. The intent was to optimize the initial
configuration of a circular combined sewer overflow regulator device
referred to as a "vortex" which was originally developed by Smisson
(2-6) and installed in Bristol, England in 1964. (Figure 1)
An outstanding team effort carried out the successful model develop-
ment and optimization endeavor under the technical direction of Mr.
Richard H. Sullivan of the American Public Works Association, Research
Foundation. Hydraulic modeling to determine swirl concentrator configura-
tions, flow patterns, and solids removal efficiency was performed by the
LaSalle Hydraulic Laboratory, Ltd. of LaSalle, Quebec Province. Mathe-
matical modeling by the General Electric Company, Re-entry and Environ-
mental Systems Division was prepared in conjunction with actual hydraulic
model results to determine a design basis. As indicated by Figure 2
good correlation was found between the' two model studies. Mr. Bernard
S. Smisson, responsible for the development of the forerunner vortex
device, along with Dr. Morris M. Cohn, Mr. J. Peter Coombes, and Alexander
Potter Associates, provided hydraulic model test planning, technical
design requirement guidance and translation of findings into practicable
application, and project reviews.
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Figure 1
"Vortex" Device Originally Developed by Smisson,
Bristol, England.
COMPARISON OF PARTICLE FLOW
MATHEMATICAL MODEL RESULTS WITH TEST DATA
OJ (FT/SEC)
10 - lo'.O
(CM/SEC)
PROTOTYPE SCALE SETTLING VELOCITY
Figure 2
Comparison of Particle Flow Mathematical Model Results
with Test Data
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Combined Sewer Overflow Problems
The basic difficulty with combined sewers involves their built-in
inefficiencies, which are their overflow points (2). Untreated overflows
from combined sewers, have proved to be a substantial water pollution
source during both wet and dry weather periods. In total nation-wide,
there are roughly 15,000 to 18,000 combined sewer overflow points(7).
It has been estimated in a study(7) for the EPA that on a national level
the expenditure for combined sewer overflow pollution abatement would be
30 billion dollars (at today's cost).
In considering wet and dry-weather water pollution abatement, first
attention must be directed to control of the existing combined sewerage
system and replacement or stricter maintenance of faulty regulators.
Consulting and municipal engineers will agree with the findings(2,4,5,7)
that regulator mechanical failures and blockages persist at the overflow
or diversion points resulting in unnecessary bypassing especially a
problem during dry-weather periods. Malfunctioning overflow structures,
both of the static and dynamic varieties, are major contributors to the
overall water pollution problem.
The American practice of designing regulators for just flow rate
control or diversion for dividing the quantity of combined wastewaters
to the treatment plant, and the overflow to receiving waters must be
given new consideration. Sewer system management which emphasizes the
"dual function" of combined sewer overflow regulator facilities to im-
prove overflow quality by concentrating sewage solids to the sanitary
interceptor, as well as conventionally diverting excess storm flow to
the outfall, will pay significant dividends(1,2,4,5). A new phrase,
the "two Q's" representing both the quantitative and qualitative aspects
of overflow regulation has been coined(l,4,5). "Regulators and their
appurtenant facilities should be recognized as devices which have the
dual responsibility of controling both quantity and quality of pverflow
to receiving waters, in the interest of more effective pollution con-
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trol"(l) .
"Swirl" Not a "Vortex"
The circular chamber concept which was evolved in England in order
to obtain adequate weir length for overflows without the space require-
ment and expense of constructing a long lateral weir. As a bonus, it
was found that this device could concentrate and divert as much as 70
percent of the combined sewage settleable solids along with 30 percent
of the flow volume to the treatment works.
The concept of solids removal by rotationally induced forces causing
inertial separation other than vertical gravity sedimentation, in rela-
tively small tankage, lies behind the "vortex" principle utilized at
Bristol, England. However, this investigation, working with the rela-
tively larger flow diversions of 30 percent as compared to 2-3 percent
in American practice, showed that a completely free-surface vortex con-
dition must be avoided. Ackers(8,9) thought he might improve separation
by developing a true' free-surface vortex and instead concluded that this
approach was not feasible. Without a deflector, unimpeded free vortex
action as illustrated by Figure 3 is too violent, allowing a significant
solids portion to overflow and is not nearly the optimal liquid-solids
separation flow field.
Initially in the study(1), a forced vortex or "swirl" action was
artifically induced, and the free vortex eliminated, by the manual in-
sertion of a simple flow deflector which prevented flow completing its
first revolution from merging with the inlet 'flow. A condition of
rotating motion was established whereby the sewage was caused to follow
an even longer-spiral path around the circular chamber. Rotary motion
at the surface was later further impeded by a vertical baffle (spoiler)
arrangement perpendicular to the flow. Some rotational movement remained,
but in the fprm of a gentle swirl, so that liquid entering the chamber
from the inlet pipe is slowed down and diffused with very little turbu-
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Figure 3
Unimpeded Free-Surface Vortex Action, Pilot
Model, LaSalle, P.Q.
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lence. The particles entering the basin are thereby induced to spread
more easily over the full cross section of the stream tube and settle
more rapidly. Solids are entrained along the bottom, around the chamber,
and are concentrated at the dry-weather outlet.
Figure 4 illustrates a crude deflector plate inducing a gentle
swirling action which encourages a greater inertial separation of solids
to take affect. As will be described later, investigations resulted in
an optimized .device capable of greater separation efficiencies (than the
English vortex unit) with a concentrate to the interceptor of only 2% as
compared to the 30% (for the British device).
General Description of Swirl Device
The swirl flow regulator/solids-liquid separator is of simple
annular-shaped construction and requires no moving parts. An overhead
view of the final form of the device in operation is shown in Figure 5.
It provides a dual function—regulating flow by a central circular weir-
spillway while simultaneously treating combined wastewater by swirl
action which imparts liquid-solids separation. Dry-weather flows are
diverted through a cunette-like channel in the floor of the chamber
into a bottom orifice located near the central standpipe which outlets
to the intercepting sewer for subsequent treatment at the municipal
plant. Figure 6 depicts another overhead view with the top weir plate
arrangement removed and the device dry so that the floor gutters are
clearly visible. During higher flow, storm conditions, the low-volume
concentrate is diverted via the same, bottom orifice leading to the
interceptor, and the excess, relatively clear, high-volume supernatant
overflows the center circular weir into a downshaft for storage and/or
treatment or discharge to the stream. This device is capable of func-
tioning efficiently over a wide range of combined sewer overflow rates
having the ability to separate settleable light weight organic matter
and floatable solids at a small fraction of the detention time required
for primary separation. Figure 7 illustrates an elevation view of the
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Figure 4
Crude Deflector Plate Inducing Gentle Swirling Action,
Pilot Model, LaSalle, P.Q.
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Figure 5
Overhead View of Swirl Flow Regulator/Solids-Liquid
Separator in Operation, Pilot Model, LaSalle, P.Q.
Figure 6
Dry Swirl Device with Floor Gutters Visible,
Pilot Model, LaSalle, P.Q.
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swirl in operation with solids being separated towards the floor.
Modeling
A prototype chamber was modeled in hydraulic and mathematical
studies and specific calculations were performed for both the laboratory
model and a proposed prototype unit to be installed at Lancaster, Pen-
nsylvania. Over and above the specificity of the mathematical inves-
tigations of Lancaster conditions, the results are applicable to a broad
range of chamber sizes, flow rates and particle characteristics.
Hydraulic Model Description
The swirl concentrator model took the form of a vertical cylinder 36
inches in diameter and 40 inches high, made of 1/2-inch plexiglass. The
overall model layout is shown in Figure 8 The inlet was a six-inch diam-
eter polyvinyl chloride (PVC) pipe. A vibrating solids injection system
was placed on this supply pipe. A movable one-inch diameter tygon tube
was placed inside the cylinder, beneath the floor of the test chambers
to pick up the concentrated flow. The tube was led out the bottom of
the cylinder, and its free end could be raised or lowered to control the
discharge drawn off through the concentrate outlet.
The overflow water outlet came up from the base, on the center-
line of the cylinder in the form of a six-inch-diameter PVC pipe. Its
level and diameter could be changed easily either by adding or removing
elements of the same diameter pipe, or with adaptors to provide either
larger or smaller diameter downshafts. Similarly, different diameters
or configurations of weir could be adjusted and held-in place on top of
the shaft by a s-imple threaded bras's rod coming up the center of the
shaft.
Outflow from this pipe, which in operation represents the major
part of the total discharge through the structure, entered a large sett-
ling basin equipped with a calibrated V-notch weir. The basin allowed suf-
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Figure 7
Elevation View of the Swirl in Operation with Solids
being Separated Towards the Floor, Pilot Model,
LaSalle, P.Q.
SWIRL CONCENTRATOR MODEL LAYOUT
SOLIDS HOPPER
fOUL OUTLET PIPE 1
TVCON FLEXIBLE TUBING
DISCHARGES RETURNED
10 PUMPING STATION
' FOUL OUTFLOW !
. SETTLING BASIN
CHAMBER CYLINDER - 1/2
PLEXIGLASS 36" 0
VIBRATOR! /
SMALL WATER SUPPLY
FOR SOLIDS INJECTION
P.VC
s' _ SUPPLY PII'E
WATER SUPPLY FROM
PUMPING STATION
CLEAR WATER OVERFLOW OUTLET PIPE
4" PLEXIGLASS
' CLEAB OUTFLOW SETTLING BASIN
Figure 8
Overall Hydraulic Model Layout,
LaSalle, P.Q.
] )
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ficient time for most of the solids contained in the clarified overflow
to settle out for measurement. The clear discharge over the circular
weir in the chamber was determined with the V-notch weir. Underflow
and concentrate solids were measured in a like manner by a settling
basin-V-notch weir arrangement. Removal effiencies were determined by
expressing the total volume of solids separated as a percentage of an
original full liter introduced.
Another form of comparison in the optimization process was measure-
ment of rotational velocity. Study of these velocities in the form of
detailed contour lines served as an indication of. any tendencies to ap-
proach ranges found to have reduced removal efficiency.
The swirl chamber floor was moldable to enable easy changes of the
bottom shape and its gutters. The entire model structure was designed
for the flexibility of a trial and error procedure to optimize its over-
all configuration.
Normal scaling laws were used to establish the geometry of the
hydraulic model and, in turn, of the mathematical model used in verify-
ing the hydraulic findings. A ratio of 1:12 was used for converting the
3 foot diameter chamber laboratory model to an actual 36 foot diameter
prototype size for the full-scale application in Lancaster, Pennsylvania.
By using these scaling laws, the results calculated for a few special
cases can be extended to other flow rates, chamber sizes, particle diam-
eters and specific gravities provided that geometric similarity is main-
tained. Scaling, therefore, greatly reduces the amount of computation to
be performed and extends the usefulness of both the mathematical and phy-
sical model results.
11
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The Froude number
V
where: V = reference velocity (inlet velocity)
g = acceleration due to gravity
s = reference length (chamber diameter)
is used as a scaling parameter between the model and prototype swirl
concentrators because gravitational forces are critical. For a fixed
size relationship (s , ,/s ,_ . ) , the flow rate in the model must
model prototype
then be adjusted so that
V
V
model
prototype
model
'prototype
Flow rate is proportional to Vs . Therefore the laboratory model
was operated at a flow rate of
- Q
(Vs2)
m
(Vs2)
Qp /s_\5/2
C
For example, to represent the prototype swirl at 100 cfs for a 1:12
size relationship, the scale model flow rate,-
m
100
(12)
5/2
= 0.20037 cfs
12
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At this flow rate, the same Froude number in the model and prototype is
maintained, and the fluid motion and the balance between the gravitional
and inertial forces will be identical in both concentrators. However,
the foul sewer or concentrate flow fraction must be the same in both
cases.
The equations of motion also show that the flow velocities at any
point in a given concentrator are proportional to the flow rate. At
very high flow rates, however, the equations are no longer applicable,
due to the increasing importance of non-axisymmetric effects. The pro-
portionality between local velocities and flow rate is only valid below
about 250 cfs.
The analysis of the particle flow equations shows that it is not
possible to reproduce the three-way balance between inertial, gravita-
tional, and drag forces on a model to prototype basis. However, repre-
sentation of the full-scale particle flow in the laboratory is possible
by preserving only the balance between gravity and drag forces as in-
ertial forces are negligible on particles. To achieve this balance it
is only necessary to scale the particle settling velocities according
to the Froude number, as for the liquid velocities. The separation
efficiency of the concentrator will be the same for all combinations of
particle size and specific gravity which give the same settling velocity
according to Stoke's equation:
V = gd2 (f>S~PW)
S
18.
where: V = settling velocity
s
d = particle diameter
/i = water viscosity
pw = density of water
ps = density of solids
g = acceleration due to gravity
This equation is represented by the family of curves on Figure 9.
13
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100-
10-
a
10
10
10
10
-3
-2
10 (inches)
10
'-2 '-1
10 (centimeters) 10
PARTICLE DIAMETER
Figure 9
Particle Settling Rates
14
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Suppose it is desired to represent in the scale model the behavior
of 01. -in. (0.254mm) particles with a specific gravity of 1.05 moving
in the 36-ft. chamber. These particles have a settling rate of 0.146
fps (see Figure 9) . They can be represented in the 3-ft. laboratory
concentrator by particles with settling velocity Yv ^ scaled by the
Froude number to equal 0.042 fps:
\s)
m = \spj^ = (0.146) _3 = 0.0420 fps
36
This' scaled settling velocity can be achieved with 0.034-in. particles
with a specific gravity 1.05, or 0.080-in. particles with a specific
gravity of 1.01, or any other combination of diameter and specific
gravity yielding the same settling velocity. The movement and separa-
tion efficiency of these scaled particles in the laboratory-scale con-
centrator will closely duplicate the movement and separation efficiency
of the full size particles in the full size concentrator.
-" * In a similar fashion, once the separation efficiency for particles
with a settling velocity of 0.0420 fps is measured in the laboratory,
the same efficiency applies to all particles with a settling rate of
0.146 fps in the 36-ft. -diameter concentrator. The same measurement
can also be applied to other concentrator sizes (say 20 ft.) by scaling
the flow rate and settling velocity according to the Froude number.'
Elements of the Swirl
The swirl- flow regulator/concentrator will be subjected to widely
varying flow rates and suspended solids concentrations, characteristic
of combined sewer networks. For an essentially static device to perform
efficiently, under such conditions, special attention must be given to
the various pertinent elements within the chamber as learned from the
15
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modeling study.
Figure 10—Isometric View of Swirl Regulator/Concentrator, identi-
fies by small letters the various special features which will be discussed.
(a) Inlet Ramp - The inlet ramp should be designed to introduce
the incoming flow at the bottom of the chamber, while preventing problem-
atical surcharges on the collector sewer immediately upstream. Introduc-
ing the inflow at the chamber floor will allow the solids to enter at as
low a position as possible. The ramp slope chosen in the hydraulic model
was 1:2 but greater treatment efficiency can be expected as this slope
is decreased, reducing inflow turbulence. Local conditions may govern
slope selection as drastic modifications to the combined sewer upstream
of the chamber may be necessary to reduce the slope, and the affected
section of the collector sewer may become seriously surcharged during
overflow periods.
The floor of the inlet ramp should be V-shaped to the center, pro-
viding self-cleansing velocity during small storm-flow events and for
the dry-weather flow. It is essential that this ramp and its entry port
introduce the flow tangentially so that the "long path" maximizing solids
separation in the chamber may be developed.
(b) Flow Deflector - The flow deflector is a vertical free-standing
wall which is a straight line extension of the interior wall of the
entrance ramp, extending to its point of tangency. Its location is
important so as to direct flow which is completing its first revolu-
tion in the chamber, to strike, and be deflected inwards forming an in-
terior water mass which makes a second revolution in the chamber, thus
creating the ".long path".
Under the energy conditions normally produced by combined sewer
flows, rotational forces in the chamber would quickly form a vortex of
relatively low separating efficiency if the flow deflector were not used.
16
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toui
sewer
overflow
a
b
c
d
e
f
g
h
Legend
Inlci Ramp
Flow Deflector
Scum Ring
Overflow Weir and Weir Plate
Spoilers
Floatables Trap
Foul Sewer Outlet
Floor Gutters
Figure 10
Isometric View of Swirl Regulator/Concentrator
17
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The height of the deflector is equivalent to the height of the
inlet port, thus insuring a head above the wall slightly greater than
the weir height during overflow events. This head passes over the
flow deflector after one revolution in the chamber and acts as a damper
on inflow, thus forming incoming solids nearer to the floor and result-
ing in clearer supernatant at the overflow.
(c) Scum Ring - The purpose of the scum ring is to prevent float-
ing solids from overflowing. It should extend a minimum of six inches
below the level of the overflow weir crest, and vertically above the
weir crest. Its diameter is such that its edge is located vertically
above the flow deflector, thus further establishing a boundary between
the outer and inner flow masses. During overflow events, and because
of the great difference in volume of liquid overflowing and discharging
to the interceptor, the velocities of the outer flow mass are much
greater than those of the inner flow mass, allowing solids in the inner
zone a greater opportunity to settle.
For large diameter scum rings-weir-configurations, the upward
overflow velocity component will be large. Any particles entrained
in this flow will be readily swept out with the overflow. As a scum
ring diameter is decreased with constant weir diameter the cross sec-
tion area between the scum ring and weir is decreased and the upward
velocity is increased.
(d) Overflow Weir and Weir Plate - The optimum diameter of the
overflow weir is not totally dependent on the design overflow. The
diameter must be such that an underflow beneath the scum ring will not
be created that would allow floating solids to be lost to overflow.
Experiments in the hydraulic laboratory indicated that the relation
between the weir diameter and the scum ring diameter should be 5:6.
The weir plate is a horizontal circular plane that connects the
overflow weir to a central downshaft, carrying the overflow liquid
18
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to discharge. Its underside acts as a storage cap for floating solids
directed beneath the weir plate through the floatables trap. The ver-
tical element of the weir is extended below the weir plate to retain
and store floatables. The weir skirt should be extended a minimum of
eighteen inches below the weir plate, but not lower than the top of
the flow deflector.
(e) Spoilers - These are radial flow guides, vertically mounted
on the weir plate extending from the center shaft to the scum ring.
They are required to reduce rotational energy of the liquid above the
weir plate and between the scum ring and weir, thus increasing the
overflow capacity of the downshaft, and improving the separation ef-
ficiency. Four to eight spoilers should be installed. These spoilers
should extend in height from the weir plate to a position, approxi-
mately six inches above the crest of the emergency weir, thus assuring
efficient and controlled operation of the swirl concentrator well
beyond the design flow and preventing formation of a free-surface
vortex under all loading conditions.
(f) Floatables Trap - This trap is a surface flow deflector which
extends across the outer rotating flow mass and directs floating mater-
ial into a channel crossing the weir plate to a vertical vortex cylinder
located near the wall of the overflow downshaft. Floating material is
drawn down beneath the weir plate by the vortex and dispersed under the
plate around the downshaft. The trap and its deflector are located at
the point of least surface velocity in the outer liquid mass. Locating
the device in other positions resulted in floating materials which were
collecting at the mouth of the channel being swept under the deflector
and scum ring, and then over the weir to overflow. The depth of the
deflector should coincide with that of the scum ring. If lower, eddy
currents under the deflector will sweep floatables to overflow.
The next two Figures, 11 and 12 show the handling of floatables
by the hydraulic model. Figure 11 illustrates floatables emerging under
19
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Figure 11
Floatables Emerging Under Weir Plate from Vortex
Cylinder, Pilot Model, LaSalle, P.Q.
Figure 12
Floatables Trapped Under Weir after Test Operation,
Pilot Model, LaSalle, P.Q.
20
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the weir plate from the vortex cylinder. Figure l2 depicts floatables
(polythene and alathon) trapped on the underside of the weir plate after
a test operation at 103 cfs.
(g) Foul Sewer Outlet - The foul outlet is the exit orifice designed
to direct peak0dry-weather flow and separated combined sewage solids in
the form of a concentrated slurry, to the interceptor. It has been
positioned at the point of maximum settlement of solids and is shaped
to create a vortex for effective draw down of the surface in dry-weather
flow thus improving the efficiency and reducing the clogging problems of
a horizontal orifice. Its down draft velocities minimize deposited
solids in the vicinity and floatable materials on the surface of the
sewage to a depth of one foot.
During the hydraulic investigation, it was determined that the
optimum location of the floatables trap and the foul sewer outlet were
similar in plan view. Consequently, they have been located in vertical
alignment so that these important elements of the swirl concentrator
can be readily inspected from above the device.
(h) Floor Gutters - The primary floor gutter is the peak dry-
weather flow channel connecting the inlet ramp to the foul sewer out-
let to avoid dry-weather solids deposition. Its location has been
chosen to eliminate shoaling of settled solids during wet-weather
operation. A secondary gutter follows the wall of the overflow down-
shaft and aids the primary gutter in the minimization of deposits.
Although rectangular shaped gutters were used in the laboratory model,
a semi-circular section should prove more efficient in minimizing
shoaling of solids. Figure 13 shows the empty 3 foot chamber labora-
tory model with the floor gutters 'clearly visible.
(i) Floor Shape - Under design flow conditions, flat floors per-
formed very.well. However, at low flow conditions and reduced chamber
velocities, sedimentation and local shoaling can become a problem.
21
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Figure 13
Empty 3-foot Chamber Laboratory Model with Floor
Gutters visible, Pilot Model, LaSalle, P.Q.
22
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Therefore, the floor should be sloped a minimum slope of 1/4-in. per
foot toward the center to permit the chamber to be flushed out. To
facilitate flushing out the chamber, a ring water main should be
installed around the outer perimeter wall with radial jets to flush the
floor clean following combined sewer overflow events. For greatest
efficiency, this flushing action should be activated by level control
sensors timed to operate as the water level, on draining, reaches the
floor level at the exterior chamber wall. This is discussed in more
detail under the section "Special Design Features".
The Prototype at Lancaster, Pennsylvania
Overall Description
This study was performed under EPA's ongoing demonstration grant
(No. 11023 GSC) to the City of Lancaster, Pennsylvania entitled "Demon-
stration of an Underground Storage Silo - Vortex (Swirl) Regulator/
Solids Separator System for Control of Combined Sewer Overflow". In-
vestigations were specifically aimed at optimizing the design of a
full-scale unit to be installed at the Lancaster demonstration site.
A flow diagram of the proposed Lancaster installation is presented in
Figure 14. Figure 15 is a photograph of the installation site. The
"two Q" dual purpose ability of the swirl device will be demonstrated
at full scale. Aside from the swirl's function as the incoming flow
regulator, it will concentrate a solids slurry to the interceptor and
sanitary sewage treatment plant. Evaluations will be made of the swirl's
ability to minimize solids loading to the storage silo, and to act as a
treatment device by itself.
Other full-scale modes of combined sewer overflow control and treat-
ment to be assessed at Lancaster after initial overflow storage in the
silo are: pump back to the interceptor during low-flow periods; and
microstraining and disinfecting prior to discharge into the Conestoga
River.
23
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| EXIST 60" 'Combined
SEWER
Swirl
Degritter
EXIST '10 SAN -SEW£R
EXIST 6" SAN -SEWER
-EXIST. OUTFALL 60
Underground
SILO TANK
(100* Deep
50' Diam.;
NEW. INTERCEPTOR.
" ~42"
I
NEW
WET
WELL
I I
h — TREAT,V!EN'
PLANT
CONESTOGA
Figure 14
Preliminary Flow Diagram, Combined Sewer Overflow Control/Treatment System, Lancaster, Pa.
-------
Figure 15
Installation Site, Lancaster, Pa.
25
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Design Flow and Dimensions
The prototype design flow selected for this study was 165 cfs which
represents a 5-year frequency storm flow for the 130 acre drainage area.
A 3-year storm was estimated to produce 103 cfs. It was decided to
evaluate model solids removal efficiencies (in the ranges of grit, set-
tleable suspended solids, and floatables) not only at 165 cfs, but at
the 15, 50, and 103 cfs flow levels also. Foul outlet flow was kept
constant at 3 cfs, the peak dry-weather flow which is approximately 2
percent of the design flow. The device was hydraulically designed to
allow 450 cfs, the peak upstream sewer capacity, without flooding. It
was desired to obtain 85 percent of the maximum synthesized settleable
solids removal at peak design flow of 165 cfs. On this basis it was
found that for the intermediate frequency flow of 103 cfs, optimum set-
tleable solids removal would be provided.
Structural Layout
The recommended•primary dimensions of the swirl unit to be installed
at the Lancaster site are: a 36 foot diameter chamber, a 20 foot diam-
eter overflow weir with a 1.5 foot weir skirt, a 24 foot diameter scum
ring, and a 9 foot vertical distance between the chamber floor and top
of the weir. Figures 16 and 17 contain preliminary drawings of the
elevation and plan views, respectively for the Lancaster installation.
The cost estimated for the prototype Lancaster installation is
$100,000 which is equivalent to $700 per acre; 1972 figures apply.
This estimate includes a roof, foul sewer out-let control gate, and a
wash-down system.
Figures 18 and 19 depict a 12.5-ft. diameter chamber recently shop-
fabricated out of carbon steel for installation in Onondaga County,
Syracuse, New York. The cost of this prototype including installation,
appurtenances, and pumping is approximately $30,000 and will be demon-
strated under the sponsorship of EPA.
26
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ICUM RINO .VOITIX FOUL
=fc=HmL i i~m i i i i
Figure 16
Preliminary Drawing - Elevation View of System, Lancaster, Pa.
A1ION 2S6'-0"
Figure 17
Preliminary Drawing - Plan View of System, Lancaster, Pa.
27
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Figure 18
Top View of Weir Assembly
Onondaga County, Syracuse, New York
Figure 19
Elevation View of Outside Tank into which Weir Assembly
will be Set, Onondaga County, Syracuse, New York
28
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Solids Separation Efficiencies
Model Particle Materials and Sizes
Since it was not possible to use actual combined wastes in the
scaled-down hydraulic model investigations, it was necessary to repro-
duce ranges of particle sizes and specific gravities with synthetic
materials. It was not possible to reproduce the entire spectrum of
size and specific gravity, nor was this essential to the accuracy of
the model studies because combined sewer flows vary markedly in com-
position due to geographical and climatic conditions.
After' intensive reviews of recorded analytical data(3,9-13) for
representative flows from various systems, and consideration of all of
the factors outlined above, an acceptable "range" of particle sizes and
specific gravities was chosen for the studies. It was necessary to make
a basic assumption of the firm analytical data to be used. Table I con-
tains the desired characteristics of grit, settleable solids, and float-
able solids to be simulated in the hydraulic laboratory. The character-
ization on the table includes specific gravity, size, and concentration.
TABLE .1
Specific Gravity, Size and Concentration of Settleable Solids
Particle Size Distributed
Specific Concentration Particle (upper line - size mm)
Material Gravity (mg/1) Size (mm) (lower line - % by weight)
Settleable 1.2 ' 200-1,500 0.2-5 " .2 .5 1.0 2.5 5.0
Solids ex- 10 ..10 15 . 25 40
eluding grit
Grit 2.65 20-360 0.2-2 .2 .5 1.0 1.5 2.0
10 10 15 25 40
Floatable 0.9-.998 10-80 5-25 5 10 15 20 25
10 10 20 20 40
29
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The material most used in the hydraulic testing program was gil-
sonite, a natural hydrocarbon with a specific gravity of 1.06 which had
a grain size between 1 and 3 mm. Following the Stokes relation at a
scale of 1:12 - laboratory test unit to full-sized prototype - this
material reproduces grit (with a specific gravity of 2.65) between 0.36
and 1.0 mm and settleable suspended solids (with a specific gravity of
•1.2) between 1 and 3 mm. Figures 20 and 21 illustrate the relation-
ships between simulated and actual particles on a size vs. cumulative
weight (as a percentage of total weight) basis, for grit and organic
settleable solids, respectively.
The grit range leaves a small part of the fines unrepresented, as
well as a wide part of the coarser particles. The coarser end of the
scale was assumed to be covered, since any larger particles would
obviously settle out if those represented in the chosen material had
settled. The fines at the lower end of the scale in turn were simu-
lated less often with Petrothene, a compounded plastic with grain sizes
between 2 and 4 mm and a specific gravity of 1.01. This also covered
prototype settleable solids in the neighborhood of 0.2 mm.
Similar reasoning was utilized in establishing particle character-
istics to simulate settleable suspended solids - the larger particles
were considered to have been removed if the gilsonite settled. Also
at times ground gilsonite having a mean particle diameter of 0.3 mm
(45-mesh) was used to approximate the finer organic settleables of 0.2
mm effective diameter.
The rates of settleable solids injection- used in the hydraulic
pilot unit correspond to the 50-1,550 mg/1 range in -the prototype flows.
Tests for the removal of floatables were carried out using uniformly
sized polythene particles of 4 mm diameter, with a specific gravity of
0.92; and Alathon, another plastic compound with particle size of 3 mm
diameter and specific gravity of 0.96. Injection rates for this mater-
30
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a
2 SO
ORGANIC SETTLEABLE SOLIDS SG = 1.2
1 i '»
h
2.0
HI
Ib)
*
3.0 40
PARTICLE SIZE mm
PARTICLE SIZE mm
MODEL MATERIALS l«l GILSONITE 1-3 mm SG = 1 .06
AND SIZE (H GILSONITE ON 30 MESH SG - 1.06
SIMULATIONS AT Id GUSONITE ON 45 MESH SG ' 106
PROTOTYPE SCALE (d) PETROTHENE 2-4 mm SG = 1.01
MODEL SIMULATION OF PHOTOTYPE SOLIDS
Figure 20
Model Simulation of Prototype Solids - Organic
PROTOTYPE GRIT SG = 2.65
AND SIZE
SIMULATIONS AT
PROTOTYPE SCALE
la) GUSONITE 1-3 mm SG I 06
(b) PETROTHENE 2-4 mm SG = 1.01
MODEL SIMULATION OF PROTOTYPE SOLIDS
Figure 21
Model Simulation of Prototype Solids - Grit
31
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ial varied from 30 to 150 mg/1, at prototype scale.
Removals Expected
Predicted efficiencies based on model testing at 165 cfs are:
For Floatables: with a specific gravity range 0.9 to 0.96, having
particles sizes between 5 and 50 mm, the chamber should remove between
65 and 80 percent.
For Grit: with a specific gravity 2.65, having particles larger
than 0.3 mm, removal should be 90 to 100 percent. For smaller particles
there would be a reduction of efficiency, so that at 0.2 mm it would be
about 75 percent, and at 0.1 mm, probably less than 40 percent.
For Settleable Solids: with a specific gravity of 1.2, having
particles larger than 1 mm, the efficiency should be between 80 and 100
percent. As shown on Figure 20 this fraction represents 65 percent of
the total amount of settleable solids in the design solids concentra-
tion. For the finer particles, removal efficiencies decrease so that
for 0.5 mm particles it would be about 30 percent and for 0.3 mm, prob-
ably less than 20 percent.
The discharge vs. efficiency curves for removal of the larger
organic and grit particles greater than 1 mm and 0.36 mm, respectively
are shown on Figure 22.
Spot checks were carried out on separation efficiency by using the
large gilsonite. The separating flow characteristics in the chamber
remained remark-ably steady up to a'bout 250 cfs in each case, then they
seemed to break up. Note that the separation efficiency curve on Fig-
ure 22 begins to decelerate more rapidly after 250 cfs. Figure 23
further indicates a breaking off of the established flow field as the
rate of head build up above the weir increases markedly for discharges
greater than 250 cfs.
32
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* *•
£ no
I H
GILSONITE RECUPERATION EFFICIENCY
24' Weir Alone
20' Weir «llh 24' scum ring
300 400
Clear Overflow Discharge cfs
Figure 22
Percent Gilsonite Recuperation vs. Discharye Rate for
Grit and Oryanics
STAGE DISCHARGE CURVES
24' Weir Alone
100 150 ' ZOO 250 300
Cteor Overflow Discharge - cfs.
Figure 23
Head Above Weir Crest vs. Discharge Rate
33
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To get a more complete perspective of efficiencies for the entire
representative solids concentration range chosen for the model tests,
and as depicted by Figure 21 gilsonite removal varies from 65 percent
at design flow or 165 cfs, to 87 percent at 103 cfs, up to 97 percent
at 50 cfs. It is emphasized that these removals are accomplished at
chamber retention times in the order of 5 to 15 seconds.
Design Rationale
Hydraulics
Three flow quantities must be considered in the design: (1) the
peak dry-weather flow; (2) the design flow, i.e., the flow for which
the optimum desired treatment is established and (3) the maximum flow
likely to occur through the chamber.
As the cost of the facility and the hydraulic head loss for dry-
weather flows increase with the flow rate, to provide optimum solids
removal, choice of the design flow and degree of settleable solids
removal is very important. The wisest choice of design flow rate can
only be made after analyses of a history of "pollutographs" in the form
of dependent curves representing mass emissions of specific pollutants.
These analyses may be based on either real-time values or model predic-
tions resulting from rainfall records.
Here it is important to realize that self -leansing efficiency
is improved at smaller chamber diameters because of the tendency of
the solids to shoal at low rotational velocities.
Head Considerations
The vertical distance between the hydraulic grade lines in the
combined sewer and interceptor must be great enough to permit installa-
tion of the regulator. There must be sufficient hydraulic head avail-
34
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able to allow dry-weather flows to pass through the facility and remain
in the channel.
The total head required for wet-weather operation is shown in
Figure 24. The total available head should be the differential eleva-
tion between the highest point in the combined sewer system that can
be tolerated before flooding or undesirable surcharging occurs, and
the level in the interceptor.
If sufficient head is not available to operate the foul sewer dis-
charge by gravity, an economic evaluation would be necessary to deter-
mine the value of either pumping the foul sewer outflow continuously,
or pumping the foul flow during storm conditions and bypassing the swirl
concentrator during dry-weather conditions.
Sizing
From the design discharge, Q,, selected, the diameter of the chamber
D , may be determined from the curve on Figure 25 which represents the
equation of equivalent model and prototype Froude numbers, that is,
°2 = 3 0 / Qd \2/5
• I . The chamber diameters are 29.5 and 22.5 feet for
103 cfs and 50 cfs design storm discharges, respectively.
The other dimensions of the chamber will have the same ratio to
the diameter as those in the model. On this basis the dimensions for
design discharges of 50, 103 and 165 cfs are .shown in Table II. The
location of the various dimensions are shown in Figures 26 an<3 27.
35
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Maximum elevation-
of flooding in
collector sewer
Overflow weir tsule)
Overflow \veir 'central I
Collector sc\\vr
invert —
Foul outlel
Interceptor
sewer inlet
height of How
over weir
chamber depth
losses due to outlet,
jliite, connecting
sewer ;nul :'!-. >w
through chamber
hydniulic
head required
Figure 24
HYDRAULIC HEAD REQUIREMENTS
36
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60 —i
50 —
3 40
to Tab.H to I'iiul lirst Dj, then calculate the
dimensions of the other chamber elements.
Figure 25
DESIGN OVERFLOW RATE VS CHAMBER DIAMETER
37
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Shear
golf
ELEVATION C-C SCALI „ . r o"
Figure 26
Swirl Plan and Elevation Views - Below Roof
SWIRL PLAN ELEVATION - FLOOR AREA
TO IHIEBCEP
OUTLET
PIPI
FLOW DEFLECTOR
HEVATIOH A-A
Figure 27
Swirl Plan and Elevation Views - Floor Area
38
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TABLE II
Swirl Chamber Dimensions
(all dimensions in feet)
Design Storm Discharge - cfs
Diameter of Chamber
Diameter of Overflow and
Diameter of Inlet
Diameter of Circular Scum Ring
Diameter of Circular Weir
Radius of Inlet Gutter (0-90°)
Radius of Inlet Gutter (90-180°)
Radius of Secondary Gutter (90-270°)
= D2 (Fig.25)
= Dl = 1/6 D,
= D3 = 4 D]
= D4 = 3 1/3 D]
= Rx = 2 1/3 D3
= R2 - 1 1/2 D]
= 5/8 Dn
Radius of Secondary Gutter (0-90°)
= R4 = 1 1/8
Radius of Secondary Gutter (270-360°) = R = 3 2/3
Difference in Radius Between
Secondary and Circular Weir
Offset Distance for Determining
Gutter Radii
Distance Between Floor and Top of
Circular Weir
Depth Invert to Bottom of Chamber
Height of Circular Weir
Height of Scum Ring
= bjL = 1/3
= b2 = 1/6
= d1 = 1 1/2
= d2 = 5/6
= hx = 1/2
= h = 1/3
50 100 165
22.5 29.5 36.0
3.75 4.92 6.00
15.00 19.68 24.00
12.50 16.40 20.00
8.75 11.48 14.00
5.62 7.38 9.00
2.34 3.08 3.75
4.22 5.54 6.75
13.75 18.04 22.00
1.25
0.62
1.64
0.82
2.00
1.00
5.62 7.38 9.00
3.12 4.10 5.00
1.87 2.46T 3.00
1..25 1.64 2.00
39
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The percent of solids diverted to the foul sewer can be obtained
from Figure 28 for any given multiple of design discharge. Thus, at
design flow the concentrate through the bottom outlet will contain 90
percent of grit larger than 0.35 mm and 90 percent of settleable solids
larger than 1.0 mm. Smaller percentages of finer materials would also
pass through the foul outlet. Importantly, this curve shows good re-
moval efficiencies are maintained throughout an extremely wide range of
overflow rates.
Special Design Features
Roof: A roof is considered desirable for safety and aesthetic
reasons.
Inspection Walk: A walk should be provided around the chamber
periphery and located to allow easy inspection and maintenance of the
weir and scum plate.
Automatic Flushing: In order to clear floatables from the under
side of the horizontal weir plate it is recommended that a circumfer-
ential (4 in. diameter) water pipe be installed below the plate adjacent
to the inner side of the skirt. Eight 3/4-in. pipe nozzles should be
aimed upward at the bottom of the plate. When the sewage level falls
below some point in the chamber under the plate a pump should automat-
ically apply 80 gpm of water at 40 psi.
Another (4-in.) pipe should be anchored to the chamber wall at
approximately weir level for cleaning the chamber bottom. Sixteen
3/4-in. nozzles pointed straight downward are recommended which would
automatically spray water when the' sewage level in the chamber falls
below, the bottom floor.
Positive Control Gate: At low flow rates, discharge through
the foul outlet pipe may occur as gravity flow while at higher flows
40
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I
•o
_o
I
g
o
o
o
1/5
u
100
90
80
70,
60.
50-
40
30
20,
10 ,
0
Curve valid t'oi both
-- (Jrit larccr than 0.35mm
Settleable Solids larger than 1.0mm
Qa
Hydrograph Peak Discharge
2Qd
NO'I (•:
1. Sohifs rc^ovt'rcj ;irc only 'tlnisi.' kirm'i' lli.ui tin.1 ^l/^^s shown nf filler material wnuld he reniovud,
hut are not defined by this eurxe.
2. Percentages of solids recoveries calculated as the volume of solids
t.iKen out through the foul outlet, with respect to the total volume
of solids entering the chamlier during the complete storm hydro-
graph period.
Figure 28
SEPARATION EFFICIENCY CURVE
41
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discharge will occur under varying hydraulic head. It is difficult to
size the pipe to act as a "throttle" pipe to pass a specific flow equiv-
alent to dry-weather flow. Therefore, it is recommended that a sluice
gate or other flow control devices be installed on the pipe to regulate
dynamic flow to the interceptor. The use of a gate will permit adjust-
ment of the opening and the discharge rate. Furthermore, it will allow
the use of a larger size pipe with less chance of clogging and, if
clogging occurs at the gate, the gate can be opened to clear out the
debris.
If the necessity to limit the variation in flow of the foul sewage
to a minimum is determined critical, then an automatic motorized gate
should be used. Such gates could be controlled by either the level or
flow rate in the downstream sewerage system of the stage in the swirl
chamber. Remote sensing of interceptor and/or sewage treatment plant
flow coupled with remote positive control of the gate affords maximum
utilization of these downstream facilities.
To limit clogging potential, the minimum diameter of foul outlet
line should be 8 in., but preferably 12 in.
Side Overflow Weir: In many cases a side overflow weir should be
provided on the periphery of the chamber to take part of the flow when .
the flow exceeds an undesirable level above the design flow. This
would help to achieve the desired removal of suspended solids. The
side weir would tend to alleviate upsetting flowfields; and also in-
crease the hydraulic system capacity.
Enlarged Inlet: In order to maintain low-inflow" velocities, in
the range of 3 tc 5 fps, for minimized turbulence, enlarged inlet pipe
sections may be useful.
Grit and.. Solids Removal: The downstream sewer system and treat-
ment works must provide capacity to handle the increase in grit and
42
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settleable solids which will be captured from the combined sewer over-
flow. This could easily amount to more than a ton of solids from one
device in a very short period of time. Additional grit removal and
sludge processing equipment may be necessary on the foul sewer prior
to the interceptor. Should the concentrated flow be pumped, sumps and
pumps should be designed to handle the anticipated high solids content;
and the use of hydrocyclones for degritting should be considered.
Potential Applications and Research Needs
Universalization
A further swirl regulator principle requiring development must now
be emphasized,, that is, universalization of the device. By enabling
interchange of primary dimensions, such as, chamber diameter for height,
engineers will have the flexibility of designing a future swirl instal-
lation under restrictive structural and hydraulic head limitations im-
posed at their particular installation site. The Storm and Combined
Sewer Technology Program of EPA is now working towards this goal.
Potential Uses
The swirl principle has many potential applications. It may be
employed anywhere it is desirable to keep solid particles out of liquid
flows. In the field of water pollution control this could relate to
the degritting of sanitary and combined sewage, straight urban storm
runoff, and silt-laden runoff from eroded land areas; to the primary
separation of domestic wastewaters, combined sewer overflows, storm-
water, and industrial wastewaters; to- the sludge thickening of sanitary
stormwater, and various industrial processing concentrates; and to the
final clarification process. In potable water purification practices,
it may be feasible to apply a form of the swirl for chemical mixing,
coagulation, and clarification of raw water. Other uses including in-
dustrial processing and pollution control may prove to be realistic.
43
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Each of the above applications in the sanitary engineering spectrum
may involve less arduous conditions of operation than the combined sewer
regulator application. Both the hydraulic laboratory and the mathematical
model investigation have indicated that greater efficiency of solids
separation may be experienced if the device operates under steady flow
conditions, and if a narrower range of solids size and specific gravity
is to be removed.
Similarly, better efficiencies may be achieved with two half-size
chambers as opposed to one full-size unit. With two units operating in
parallel, one chamber could be used for all flows lower than 103 cfs...
(at the site of the proposed prototype regulator where 165 cfs was the
design flow)...and the second would be required if the storm flow exceeded
that value. This would provide better separation at both higher and lower
flow rates.
The possibility also exists of operating units in series to improve
solids removal by breaking a wide range of particle characteristics into
narrower grain size/specific gravity bands.
The EPA has recently awarded a supplemental grant to the City of
Lancaster, Pennsylvania for the development of a swirl degritter for
treatment of the swirl regulator/separator concentrate before entering
the interceptor system.
The Storm and Combined Sewer Program of EPA is also actively pur-
suing a project which includes optimization of a swirl concentrator as
a primary separator for combined sewer overflow, sanitary sewage, storm-
water runoff, and erosion runoff along with universalization-of regulator.
More effective removals should result at the more confined diameter and
specific gravity ranges as compared to the broad ranges of this regula-
tor study. Also chamber detention times will be in the order of 5 to 15
min., whereas those times in the swirl regulator were in the range of 10
to 20 seconds.
44
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Conclusions
The dual functioning swirl unit is the first regulator device of
its kind in this country offering the basic advantage of controlling
the "two Q's", that is, quantity and quality of combined sewer over-
flows, simultaneously. It is a practical and simple facility which can
effectively reduce significant portions of grit, settleable solids, and
floatables over a wide range of varying overflow rates.
The swirl principle employes an innovative approach to the clar-
ification and concentration of solid-liquid mixtures which does not
require moving or mechanical parts and their associated power require-
ments. Studies have confirmed that the kinetic energy produced by
swirl flow action can be harnessed to accelerate the solids separation
process. Deposited solids are self-cleansed by its own flow patterns.
This is in contrast with standard grit and sedimentation facilities
which require some form of collection and removal mechanisms to perform
this function. Density and thermal current short circuiting can be over-
come by the swirl action.
Conventional static and dynamic regulators have a known history of
chronic failures due to clogged orifices and malfunctioning moving
parts. The absense of moving parts overcomes the mechanical breakdowns
problem and the need for standby equipment. Corrosion of metallic parts
could be avoided by construction of a swirl chamber with relatively
inert materials such as, concrete, stainless steel, or plastic.
Relative detention times are extraordinarily short, being only
seconds. It is further envisioned that swirl concentrators can be
constructed to take the place of primary settling tanks which could
have as little as 1/8 of the two hour retention time of these conven-
tional units. Tankage requirements and costs would be greatly reduced
by substituting swirl units for primary separation in future construc-
tion.
-------
Although the study was performed as part of an EPA demonstration
grant for the City of Lancaster, Pennsylvania, with design and develop-
mental criteria defined by a specific site for installation, all work
was accomplished in a manner which readily allows translation of results
to many conditions which exist at other locations and possibly for
other types of flow treatment purposes. The device is simple to design -
a procedure has been established as part of the final report(1) for the
study. The report can be considered a "cookbook" manual for rapid design
of the swirl facility at various rates of flow and site requirements.
Before using the swirl concentrator as a combined sewer overflow
regulator for a' given application, the following must be evaluated:
1. Hydraulic head differential between the collector and inter-
ceptor sewers taking maximum advantage of the head available in the
collector sewer to allow in-system storage;
2. Hydraulic capacity of collector sewer;
3. Design flow;
4. Dry-weather flow and capacity of interceptor sewer; and
5. Amount of character of settleable solids.
Small changes in the design of the concentrator and its appurtenant
elements may produce wide variations in its operation efficiency. In
this regard, particular care must be taken during design and construc-
tion to avoid irregularities or intrusions in the walls, floors, and
elements of the device.
Solids separation efficiencies noted in this paper relate to
specific gravities, sizes, and concentrations selected for the model
studies. Such conditions of size and specific gravity may riot reflect
local conditions'. An examination of the mathematical modeling design
methods in Appendix 2 of the more complete final report(1) will indicate
necessary adjustments for greater removal efficiency of specific particle
types. If, for example, grit is a problem in a particular design area,
scaling down of concentrator dimensions established by the hydraulic
46
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design should be considered.
We are at the stage now where we feel confident with what can be
done with the device. The swirl flow regulator/solids separator will
be very useful to communities as a tool for combatting the combined
sewer overflow problem. In addition, as a primary treatment device
for domestic wastewaters it should allow facilities to be constructed
and operated more efficiently and at less cost.
As combined sewer systems are upgraded and improved regulators
constructed to reduce the pollutional impact of overflows on receiv-
ing waters, the swirl concentrator must be considered.
47
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References
1. "The Swirl Concentrator As A Combined Sewer Overflow Regulator
Facility", U.S. Environmental Protection Agency, EPA-R2-72-008
(September 1972).
2. Field, R. , and Struzeski, Jr., E.J., "Management and Control of
Combined Sewer Overflows", Jour. Water Poll. Control Fed., 44, 7,
1393 (July 1972).
3. Smisson, B., "Design Construction, and Performances of Vortex
Overflows", Proceedings, Symposium on Storm Sewage Overflows, May
4, 1967, Inst. Civil Eng. (1967).
4. "Combined Sewer Regulation and Management - A Manual of Practice",
U.S. Environmental Protection Agency, 11022 DMU 08/70 (August 1970).
5. "Combined Sewer Regulator Overflow Facilities", U.S. Environmental
Protection Agency, 11022 DMU 07/70 (July 1970).
6. "Technical Committee on Storm Overflows and the Disposal of Storm
Sewage - Final Report", Ministry of Housing and Local Government,
Her Majesty's Stationery Office, London, Eng. (1970).
7. "Problems of Combined Sewer Facilities and Overflows - 1967", U.S.
Environmental Protection Agency, 11020 12/67 (WP-20-11) (December
1967).
8. Ackers, P., and Crump, E.S., "The Vortex Drop", Proc. Inst. Civil
Eng. (G.B.)", 16, 433 (1966).
48
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9. Ackers, P., et al., "Laboratory Studies of Storm Overflows with
Unsteady Plow", Proceedings, Symposium on Storm Sewage Overflows
Inst. Civil Eng., May 4, 1967.
10. Prus-Chacinski, T.M., and Wielgorski, J.W., "Secondary Motions
Applied to Storm Sewage Overflows".
11. Fair, G.M., and Geyer, J.C., "Water Supply and Waste-Water Disposal",
John Wiley and Sons, Inc., New York, pp. 563, 609, 613 (1954).
12. "Stream Pollution and Abatement from Combined Sewer Overflows,
Bucyrus, Ohio", U.S. Environmental Protection Agency, 11024 FKN
11/69 (November 1969) .
13. "Rotary Vibratory Fine Screening of Combined Sewer Overflows", U.S.
Environmental Protection Agency, 11023 FDD 03/70 (March 1970).
49
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