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-------
Using the D2/Di abscissa chosen for the standard design above,
move vertically to trie intersection with the bold standard design line
to locate the working point. Constant operating conditions for the
specific design then lie on the geometry modification curve passing
through the working point. Moving down to the right corresponds to
increasing the chamber diameter or width and lowering the weir height
or chamber depth. Moving up to the left reduces the chamber diameter
and increases the weir height.
Any choice of D D or H,/D, relationship can then be made, and the
corresponding values found. It will then be necessary to redimension
the other elements of the structure, based on the general design
details in Figure 7.
6. Foul Discharge: The unit was designed to operate on the assump-
tion that 3DWF would be handled through the foul outlet for discharge to
the wastewater treatment facility. Greater Solids removal efficiency
can be obtained with larger discharges as explained in 9 below.
7. Find Dimensions for the Whole Structure; Having made decisions on
acceptable D and D values, these can be applied to Figure 7 to
determine the necessary dimensions for all the features of the entire
swirl chamber.
8. Geometry Modifications: The above steps have provided the
geometric configurations to meet the design hydraulic conditions.
However, at this stage other considerations such as available space,
depth or head, or economic factors, might make it desirable to modify
the general proportions of the chamber. The same operating conditions
can be obtained if the geometry is modified according to Figures 8 to
11. This procedure assumes that the inlet dimension DI is retained from
the above procedures, and that the chamber diameter and weir height
would be modified.
9. Foul Discharge Modification: There may be a reason to allow a
greater foul discharge than 3DWF, If the diameter of the foul outlet
is increased from 30 cm (1 ft) to 91 cm (3 ft) there will be a marginal
drop in efficiency, perhaps 3 percent. Figures 26 to 28 reflect the
changes in this unit's efficiency as the percentage of flow to the foul
sewer is increased to 50 percent of the total flow for D9/D ratios
of 4.5 to 6.0.
These curves may be used to evaluate efficiency of separation at
various stages of the unit's operation on either the rising or falling
stage of the storm hydrograph. Laboratory data is not available for
D /D ratios above 6.
It must be realized that a major portion of the increased
efficiency at higher foul sewer flows is due to the increased flow split,
and not to the swirl concentration treatment received.
30
-------
IOC!-
-TO
S
o so
£
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02 03 04 OS
10 IS 2 .34 .5 6niV«
2 3 4 5
Discharge
10
20 eli
Figure 19 Settleable Solids Percent Recovery vs Discharge for Inlet Diameter of
30.5 cm (1 ft)
IOC
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20
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I i , i . i i i i I
« S « • lOm'/i
i i 1 I I
2 S 4 9 « T I 10 IS tO IS 30 40 ell
Discharge
Figure 20 Settleable Solids Percent Recovery vs Discharge for Inlet Diameter of
45.8 cm (1.5 ft)
31
-------
I I 1
100
D,/D, 4.5 5.25 6 7.2 9 12
H,:D2 = 0.25
O.I O.I5 02 0.3 040.5
i i i i 1 i I I
1.0 15 2.0
1 I I
45 10 20 30 40 50 KX)cf»
Discharge
Figure 21 Settleable Solids Percent Recovery vs Discharge for Inlet Diameter of
61.5 cm (2 ft)
IOO
90
O 80
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3 .4 .5 6 7 8 10 15 20 30
| | ii i I
10 20 30 40 50 IOO
Discharge
200 cf>
Figure 22 Settleable Solids Percent Recovery vs Discharge for Inlet Diameter of
91.5 cm (3 ft)
32
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100
90
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E?
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£?
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Da/D, 4.5 5.25 6 7.2 9 12
I
456 8 10
15 20
3.0 m'/s
I I
IOO 200 300 400 500 800 IOOO cfs
Discharge
Figure 25 Settleable Solids Percent Recovery vs Discharge for Inlet Diameter of
183.0 cm (6 ft)
Design Example
Table 1 illustrates the design procedure. Item 1 is the design
discharge. Item 2 is the design settleable solids recovery efficiency.
Item 3 is the possible inlet diameters selected from Figure 9. Item 4
is the ratio of D2/Di from Figure 9. Item 5 is the computed chamber
diameter D~. Item 6 is the actual recovery efficiency obtained from
Figures 23 to 25. If the recovery efficiency is below 90 percent a greater
diameter (l>2^ must be selected from Figures 13 to 18 or 19 to 23 to conform
with 90 percent recovery. The revised chamber diameter is shown in Item 7.
The final design chamber diameter is given in Item 8. The ratio of chamber
height of weir (H^) to width ^2) in Item 9 is equal to 0.25.
The inlet velocity is shown in Item 10. It is evident that where
there is a choice of inlet sizes, the largest inlet size will result in
the lowest inlet velocity, and the smallest and most economical structure.
Hence, the designer should select the largest inlet size shown on the design
34
-------
100-
90-
80-
70-
60-
0>
o
£40n
(T
30-
20-
10-
-r
2
—T
4
T
5
NOTE :
I I T I 1
50 100 150
Inf low Discharge - cfs
200
250
Percentages shown on curves denote portion of inflow
drawn through foul outlet.
Figure 26 Efficiency of Separation of Higher Foul Sewer Discharges, D2/D, = 6
35
-------
100-
0
100 150
Inf low Discharge - cfs
200
NOTE:
Percentages shown on curves denote portion of inflow
drawn through foul outlet.
250
Figure 27 Efficiency of Separation of Higher Foul Sewer Discharges, D^D, = 5.25
36
-------
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Table 1
Design Procedures for Swirl Combined Sewer Overflow Regulator/Separator
Design Discharge nv/s (cfs)
1. Design Discharge
2. Operating efficiency
3. Inlet D (Fig. 9)
4. D2/D, (Fig. 9)
5. Diameter D2
6. Recovery (Fig. 23)
(Fig. 24)
(Fig. 25)
7. Revised D2 (Fig. 23)
(Fig. 24)
8. Design diameter D2
9. Depth H,(0.25 D2)
10. Inlet Velocity
11. Final D2/D,
12. Design modification
(H, = D,)
Revised H,/D, Fig. 12
Revised D2/D, Fig. 12
Revised D2
13. Maximum discharge
(2 x design discharge)
14. D,(Fig. 7)
15. Length of Weir
16. Maximum discharge/length
17. Maximum head on weir
(Fig. 6)
Note NA — not available
%
m
(ft)
—
m
(ft)
%
m
(ft)
m
(ft)
m
(ft)
m
(ft)
cm/s
(fps)
—
m
(ft)
—
_
m
(ft)
m-/s
(cfs)
m
(ft)
m
(ft)
l/s/m
(cfs/ft)
m
(ft)
1.42
(50)
90
1.2
(4)
5.8
7.0
(23)
90
7.0
(23)
1.8
(5.8)
94
(3.1)
5.8
1.2
(4)
1.0
7.7
8.2
(27)
2.84
(100)
4.57
(15)
14.36
(47.1)
19.78
(2.12)
0.27
(0.9)
90
1.8
(6)
4.4
7.9
(26)
NA
2.83
(100)
90
1.5
(5)
6.0
9.1
(30)
85
—
—
—
9.8
(32)
9.8
(32)
2.4
(6.4)
122
(4-0)
6.4
1.5
(5)
1.0
7.6
11.6
(38)
5.66
(200)
6.43
(21.1)
20.21
(66.3)
28.01
(3.02)
0.43
(1.4)
90
1.2
(4)
9.0
11.0
(36)
88
11.3
(37)
11.3
(37)
2.8
(9.2)
189
(6.2)
9.2
1.2
(4)
1.0
11.5
14.0
(46)
5.66
(200)
7.77
(25.5)
24.48
(80.3)
23.12
(2.49)
0.33
(1-1)
90
1.8
(6)
6.4
11.6
(38)
—
90
11.6
(38)
2.9
(9.5)
140
(4.6)
6.4
1.8
(6)
1.0
7.6
13.7
(45)
9.34
(310)
7.62
(25)
23.93
(78.5)
39.03
(3.95)
0.58
(1.9)
4.67
(165)
90
1.5
(5)
8.3
12.5
(41)
63
14.6
(48)
14.6
(48)
3.6
(12)
201
(6-6)
9.6
1.5
(5)
1.0
1.2
18.3
(60)
9.34
(310)
10.16
(33.3)
31.88
(104.6)
29.30
(2.96)
0.43
(1.4)
90
I.2
(4)
11.6
14.0
(46)
78
14.9
(49)
—
—
14.9
(49)
3.7
(12.2)
314
(10.3)
12.2
1.4
(4)
1.0
NA
9.34
(310)
8.30
(27.2)
26.1
(85.5)
35.79
(3.63)
0.52
(1-7)
38
-------
figures as being suitable for the design discharge with the hydraulic head
available and the hydraulic constraint of the inlet sewer.
The foregoing design is based on a ratio of chamber diameter to depth
of 4:1.
The ratio of D~ to Hi can be modified by use of the geometry
modification curves in Figure 12.
Assume it is desirable to reduce the depth to its minimum value.
Determine the final ratio of D7/D, as shown in Item 11 of Table 1.
Then, with the use of Figure 12, proceed as shown in Item 12.
Enter ^2^l in Figure 12,
extend line vertically to standard design line, to working point.
Move down parallel to modification curves to horizontal line where
ratio of H, /D-^ is 1.0.
Then proceed down vertically to obtain revised ratio of f>2/^\-
The resultant depth (H^) is equal to the inlet dimension (D-^)
and
the chamber diameter (02) is larger than the diameter selected in
Item 7 of the standard design.
Obviously other ratios of HI/DI could be selected to obtain other size
chambers for comparison purposes.
The maximum discharge, Item 13, without a peripheral side weir is twice
the design flow. The diameter of the weir, D^, is Item 14, with the
corresponding length of weir, Item 15. The maximum discharge per unit length
over the weir, Item 16, is computed, and from Figure 6 the maximum head over
the weir, Item 17, is determined.
A D_/D-| ratio of 6 was developed (1) as the "standard design."
Design flexibility has been extended to cover the range of 4.5 to 12 for
D2/D1' However, in the absence of major constraints, the "standard design"
is considered preferable.
Table 2 lists the areas and volumes for the structures shown in
Table 1 for 2.832 cu m/sec (100 cfs) and 4.673 cu ml sec (165 cfs). For the
standard design it is obvious that the largest inlet size results in the
minimum area and volume. The areas and volumes of the modified design with
minimum depth are compared with the areas and volumes of the smallest chamber
in standard design. For the two sizes shown the design with minimum depth
compared to the smallest standard design show an increase in area of 41
percent and a decrease in volume of 11 to 12 percent. This table indicates
that for any given situation the designer has several choices and must weigh
the advantages of each before reaching a final decision.
39
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Table 2
Comparison of Variation in Area and Volume Between Standard Design
and Design With Minimum Depth
m
(ft)
Design Discharge 3ls (cfs)
2.832 4.673
(100) (165)
1.5 ~ 1.2 1.8 1.5
(5) (4) (6) (5)
m2
(sf)
m3
(cf)
%
%
74
(800)
181
(6,400)
0
0
97
(1,040)
280
(9,900)
+ 30
+ 55
105
(1,130)
305
(10,700)
0
0
166
(1,810)
598
(10,400)
+ 60
+ 63
m2
(sf)
m3
(cf)
%
%
105
(1,130)
158
(5,650)
+ 41
-12
154
(1,660)
185
(6,640)
+ 101
+ 4
147
(1,590)
265
(9,540)
+ 41
-11
262
(2,830)
393
(14,150)
+ 150
+ 32
1. Design discharge
2. Inlet D
Standard design
3. Area
4. Volume
5. Area change from smallest
6. Volume change from smallest
Modified design minimum H,
7. Area
8. Volume
9. Area change from smallest standard
10. Volume change from smallest standard
Note Area and volume are based on dimensions given in Table 1
Hydraulic Head Requirements
There must be sufficient hydraulic head available to allow dry-weather
flows to pass through the facility and remain in the channel. The total head
required for operation is shown in Figure 29.
Determination of the maximum elevation in the collector sewer that can
be utilized for in-system storage and the differential between the maximum
elevation in the collector and the elevation of the interceptor sewer is
the total available head.
The head required will vary directly with flow and the outlet losses in
the foul sewer.
If sufficient head is not available to operate the foul sewer discharge
by gravity, an economic evaluation would be necessary to determine the value
of either pumping the foul sewer outflow continuously, pumping the foul
flow during storm conditions or bypassing the swirl concentrator during dry-
weather conditions.
Sit e Requiremen ts
The location of the swirl regulator/separator is dependent upon the
elevation of the combined sewer and the location of the interceptor sewer.
In some instances it may be feasible to construct the facility underground
in the public right-of-way.
40
-------
Maximum elevation
of flooding in
collector sewer
height of flow
over weir
Overflow weir (side)
Overflow weir (central)
Collector sewer
inlet
Foul outlet
Interceptor
sewer inlet
chamber depth
losses due to outlet
gate, connecting
sewer, and flow
through chamber
Required difference
in elevation between
collector and
interceptor sewer
Figure 29 Head Requirements with Free Flowing Interceptor
The site should minimize construction of transition sewers from the
collector and the clear overflow discharge to receiving waters. The foul
discharge, due to its relatively small diameter, is not usually a critical
cost consideration.
If the facility is to be housed in a building, only normal side yard
restrictions of the adjacent property will be required. If the unit is to be
left open, there should be about 7,5 m (25 ft) clearance. This distance,
if the site is fenced, should eliminate most problems of vandals throwing
items into the unit. Aesthetics dictate that proper screening of an open
facility be provided.
Construction Considerations
The primary element is the circular chamber which normally would be
constructed of reinforced concrete. However, it is not necessary to make the
interior wall surface a perfect circle and the use of 61 cm (2 ft) wide
prefabricated steel forms is considered permissible. The chamber could also
be constructed with Gunite or steel. The interior features could be
constructed of steel, plastic or fiberglass. However, the flow deflector
should be constructed of steel due to abrasion from coarse solids.
It is suggested that the floor have a slope of one to fifty from the
wall toward the center. Steeper slopes may reduce separation efficiency.
41
-------
The layout of the gutter is extremely critical for elimination of
deposits on the floor. The foul outlet should be located at the 320 degree
position. The floor should have a circular depression around the outlet
sewer with a diameter of about three times the diameter of the outlet sewer
and a depth of the gutter. A semi-circular shape for the gutter is
considered preferable for moving solids in low flow periods. The gutter
should have sufficient capacity for the peak dry-weather flow.
The size of the outlet sewer will be governed to a large extent by the
required size of the flow control device on the outlet pipe.
The inlet to the chamber must be aligned so as to introduce the storm
flow or combined sewer overflow tangentially to the outer periphery of the
chamber. An important element is the "flow deflector," a wall extending from
the entrance of the inlet sewer to the zero degree position of the chamber.
The top of this wall is the same level as the bottom of the weir skirt and is
not connected thereto. Flow entering the chamber is directed toward the
outside of the chamber by this deflector. Stormwater rotating in the chamber
passes over the deflector wall and tends to cause the entering solids to be
directed downward in the chamber.
It is important that the inlet sewer enter the chamber with its invert
at the same elevation as the chamber bottom. A minimum grade into the unit
is desirable, as long as self-cleansing low flow velocities are maintained.
This criteria results in more rapid settling of solids to the bottom. If it
is possible to surcharge the inlet sewer, then the chamber can be raised the
amount of the surcharge and the drop in the inlet transition decreased
accordingly.
It is suggested that the "clear water" downshaft and the weir be
constructed of steel. Concern must be given to the support of the weir in
order that it may be kept level. Temperature changes may tend to distort
the elevation of the weir and the shape of the scum ring, if the weir and
ring are supported from the top. The latter is not considered to be of maior
importance.
The downshaft supports a horizontal circular plate. The outer edge of
the plate has a vertical plate welded to it which forms a weir above and a
skirt below the plate. So-called "spoilers" are vertical radial plates
located on the circular plate to prevent vortex action in the downshaft. At
least four to eight evenly spaced spoilers should be used extending from the
edge of the downshaft to the scum ring. To prevent floatables from flowing
over the weir, a scum plate is set away from the weir with the lower edge of
the scum plate 15 cm (6 in.) below the weir crest. This scum plate can be
supported by the spoilers or by separate brackets.
Studies have indicated that there is less collection of debris on
broad-crested weirs than on sharp-crested weirs. Therefore it is suggested
the weir be semi-circular in shape .
The floatable deflector consists of a steel plate extending from the
outer wall of the chamber to the scum ring and having the same height as
42
-------
the scum ring. From the scum ring two plates form a passage 30 cm (1 ft)
wide through the weir. From the weir two plates resting on the horizontal
weir plate form a passage to a location near the center. At this location
a cylinder is provided through the horizontal weir plate. Vortex action
at this point carries the floatables to the underside of the circular
plate. The floatable deflector should be constructed as shown in Figure 30a.
The vortex cylinder through the circular horizontal weir plate should
be located directly above the foul sewer outlet, Figure 30b.
Details of the gutter layout are shown in Figure 30c. The radii should
be adjusted as required to provide a smooth transition in the gutter.
Structure Features
Plans and sections through a typical chamber are shown on Figure 31,
The provision of a roof for the chamber is not necessary for functional
reasons but is considered desirable for safety and aesthetic considerations.
Several openings are required in the roof. A manhole 60 to 75 cm
(2 to 2.5 ft) diameter should be placed directly over the vortex cylinder
for the floatables. This will permit rodding of the cylinder in case of
clogging. Since the cylinder is located directly over the foul sewer
outlet this manhole will also permit rodding of the outlet pipe. A large
sidewalk door should be provided to permit removal of large floating objects.
The size of the door should be related to some extent by the size of the
inlet sewer and the possible size of floating objects. Although a poured
roof is shown, a precast unit could also be used.
Three types of entrance stairs are shown in Figure 6.1.3 of the
Combined Sewer Overflow Regulator Manual of Practice (15). The preferred
access is the use of a 38 degree stairway with 20 cm (7.75 in.) risers and
25 cm (10 in.) treads surmounted with a superstructure with exterior
dimensions of 4 m by 1.5 m by 2.4 m (13 x 5 x 8 ft) high. Minimum openings
of 60 cm (2 ft) square should also be provided in the sluice gate manhole
and the overflow manhole.
An inspection walk should be provided around the periphery of the
chamber with a minimum width of 61 cm (2 ft). The walk should be located so
that the weir and scum plate can be cleared of debris if required. A pipe
handrail 76 cm (42 in.) high should be provided on the walk and stairs.
After each storm the chamber should be inspected. It may be necessary
following storms to flush down the bottom of the chamber to prevent
subsequent nuisance odors. Floatables collected under the horizontal plate
may have a tendency to remain attached to the plate. Floatables may be
subjected to heads of up to 1.4 m (5 ft) and this may cause them to adhere
to the horizontal plate. Therefore, it may be desirable to remove the
materials by flushing after each storm. In cities with many regulators,
several days may elapse after a storm before each regulator can be inspected.
Hence, automatic cleansing of the chamber bottom and horizontal plate is
desirable.
43
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Floatables deflector
Floatables Trap
0.5ft
Section A-A
Note: 1ft = 30cm
DETAILS OF SPECIAL STRUCTURES - FLOATABLE TRAPS D2/D, = 6
Figure 30a Details of Special Structures
44
-------
Gutter and outlet 2.75 m (9 ft) deep
FOUL OUTLET
30 cm (1 ft)
91 cm (3 ft)
DETAILS OF OPEN VORTEX FOUL OUTLET D2/D, = 6
Figure 30b Details of Special Structures
If water used for this purpose comes from a potable supply there
should be no physical connection between the supply and the flushing system.
A more feasible source of flushing water may be either the nearby receiving
waters to which the chamber discharges or the stormflow that passes through
the chamber. The use of receiving water requires the construction of a sump
and pumps. The use of stormflow requires the construction of a reservoir
adjacent to the chamber to store the stormflow during the storm so that it
can be used after the storm is ended.
One suggested method of using stormflow for flushing the chamber is
shown in Figure 31a. This comprises a 1.2 m (4 ft) square chamber,
2.7 m (9 ft) deep, adjacent to the sluice gate chamber. The capacity is
about 3,800 1 (1,000 gal). Stormflow enters the manhole through a
30 cm (1 ft)-square opening in the chamber wall set with top of opening level
with the circular weir crest. The opening is covered with 1.2 cm (0.5 in.)
mesh to prevent solids from entering. The velocity parallel to the chamber
wall should keep the screen from clogging. A shear gate is installed in the
common wall between the two chambers so that the stormflow chamber can be
emptied into the sluice gate chamber after each storm. A vertical wet-pit
non-clog pump is used to pump the stormflow into the flushing lines.
45
-------
Note: Both gutters
45 cm (1.5 ft) by 22.5 cm (0.75 ft)
Secondary gutter goes
down on 1:4 slope to
bottom of gutter below
chamber floor.
90°
GUTTER OUTLET
Figure 30c Details of Special Structures
A 10 cm (4 in.)-diameter pipe is installed on the underside of the horizontal
plate adjacent to the skirt. This pipe has eight 1.3 cm (0.75 in)-nozzles
aimed upward at the bottom of the plate. When the water level in the chamber
has fallen to some point below the plate, the pump will operate for 5
minutes, discharging 300 1/min (80 gpm) at 40 psi.
For flushing the bottom of the chamber another 10 cm (4 in.)-diameter
pipe is attached to the chamber wall above maximum flow level with sixteen
13 cm (0.75 in.)-nozzles pointed straight downward. When the water level in
the chamber has fallen to below the chamber bottom the pump will again
operate for about 5 minutes.
46
-------
Sluice gate stand
or Hydro-Brake®
To interceptor
Flushing water
pump motor
Sidewalk door over
floatable deflector
j~"| | Superstruct
'
— Inlet sewer
Walk
60 cm (2 ft)
Elevation D-D
Figure 31 a Plan and Elevation
47
-------
Sheargate
Wall opening
Walk with
railing
*~ Horizontal
plate
pipes
Elevation C-C
Figure 31 b Plan and Elevation
48
-------
To interceptor
Outlet pipe
Flow deflector
A
Flushing pipe
•i
tm
, 1 r- Weir
CM
x"
1 1
14
/
/
Downshaft — /
V
X
r Spoiler J^,
•ph o
L^^^^^^i fcScum
^^^^^| plate
J 5 HI
d"*-
/ .
^- Flow deflector
ci
Elevation A-A
PLAN ELEVATION — FLOOR AREA
Figure 31 c Plan and Elevation
49
-------
The use of an epoxy covering of internal surfaces is being tested.
Preliminary results indicate that with this coating, the unit is essentially
self-cleansing.
Hyd rau1i c Comp i1at ion
Most combined sewer overflow regulators are designed for use in
connection with existing combined sewers and either existing or proposed
intercepting sewers. The vertical distance between the hydraulic grade lines
in the combined sewer and interceptor must be great enough to permit
installation of the regulator. It may be necessary to run through the
hydraulic computations at any specific location in order to determine if the
swirl concentrator can be used. Table 3 indicates the nature of the
computations required to illustrate the factors that should be considered.
In the following computation the "foul sewer" is the outlet pipe from
the chamber to the sluice gate manhole and the "branch interceptor" is the
sewer from the sluice gate manhole to the interceptor.
As stated previously, some type of control should be provided on
the foul sewer where it leaves the chamber. In the following computations
the control is assumed to be a manually operated sluice gate. This type of
control will result in the greatest variation in flow to the interceptor
between dry-and wet-weather periods. One way to decrease the amount of the
variation is to design the branch interceptor to flow full under peak dry-
weather conditions. Increasing the length of the branch interceptor will
also help to decrease the variation. Under these conditions when wet-weather
flows occur, the flows will surcharge the sewer and the hydraulic grade line
will rise and limit the discharge capacity.
If the variation in flow is too great, then a tipping gate or motor- or
cylinder-operated gate should be used instead of the manually operated gate.
The hydraulic gradient and energy lines for peak dry-weather flow
should be computed starting at the interceptor and proceeding upstream
through the sluice gate manhole to the chamber. The quantity diverted to the
interceptor during storm periods is determined in a similar manner by trial
and error method assuming various discharges.
In the initial computation, the hydraulic computations should start at
the water surface in the interceptor at peak dry-weather flow. In subsequent
trials it may be necessary to raise the branch interceptor at its junction
with the main interceptor which will result in flow at critical depth at the
end of the branch interceptor. In this case it may be necessary to compute
the backwater curve for the flow in the branch interceptor to determine the
depth of flow at the upstream end. Figures 32 and 33 present the results of
the design computations for flow in the foul outlet.
50
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Table 3
Design Example, Swirl Combined Sewer Overflow Regulator/Separator
Design Example
Sample Computations
L —
A
D
V
d
Q
b
9
C
W.S.
H.G.L
E.L
n =
C —
d,
Interceptor
D =
Combined Sewer
D
Peak dry weather flow =
Design flow = 165 cfs
Maximum flow = 300 cfs
Length
Cross-sectional area
Diameter
Velocity
Depth of flow
Discharge
Width of opening
Acceleration of gravity
Coefficient
Water Surface
Hydraulic grade line
Energy line
0.013 (Manning)
Slope
Depth of swirl concentrator
3 ft; invert el. = 10 ft; W.S. = 12.4 ft
6 ft; invert el. = 19.14 ft; S = 0.0005
3 cfs
Note Conversion factors -
1 ft = 0 305 meters -
Interceptor
Assume
Branch Interceptor
L
D
V (full)
d/D
- U S customary to metric
-1 cuft/sec = 28 32 I/sec
Invert
10.00
H.G.L
12.40
E.L.
100 ft, Q = 3 cfs
1.0ft, S = 0.007
3.8 fps
0.8
V (0.8 full) = (1.14) (3.8) = 4.3
V2/2g = 0.28 ft
Set downstream end so flow line is
same as interceptor
Invert 12.40 — 0.8
Exit loss =
Upstream end
Rise
12.40
11.60
0.28; 12.4 + 0.28
12.68
(100) (0.007) = 0.70
11.60 + 0.70
12.40 + 0.70
12.68 + 0.70
12.30
13.10
13.38
51
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Table 3 (continued)
Sluice Gate Manhole
Entrance loss (0.5) V2/2g = 0.14
13.38 + 0.14
Assume loss of velocity head in manhole
Sluice gate
Use 12 inch by 12 inch gate
Assume opening 0.67 ft high
V = 3/0.67 = 4.5 fps
V2/2g = 0.31 ft
Exit loss = 0.31
13.52 + 0.31
Contraction loss at gate
0.3V2/2g = 0.09
13.83+ 0.09
Set gate invert at manhole invert
Use 1.0 ft square conduit
Top conduit 13.30
V = 3/1 = 3 fps
V2/2g = 0.14 ft
13.92 - 0.14
Outlet Pipe
D = 1.0 L = 20 A = 0.785
Start pipe 1 ft upstream of gate
V = 3/0.785 = 3.8 fps
V2/2g = 0.22 ft
Enlargement loss = (0.25) (0.22)
= 0.06
= 13.92 + 0.06
= 13.98 - 0.28
= 20 ft S = 0.007
Invert H.G.L E.L.
12.30
13.52
13.52
13.83
13.92
12.30
13.78
12.30
= (20) (0.007) = 0.14
E.L
H.G.L.
L
Rise
Upper end 12.30 + 0.14
13.70 + 0.14
13.98 + 0.14
Use 90° C.I. bend
Length invert to bell 1.85 ft
Top of bell 12.44 + 1.85 = 14.29
Bend loss 0.25V2/2g = 0.06
E.L. = 14.12 + 0.06
H.G.L. = E.L.
H.G.L. is below top of bell at 14.29
Chamber Bottom
Gutter invert
Make gutter 0.75 ft deep
Chamber invert at center
14.29 + 0.75
Use transverse slope of 1/4 in. per ft
Rise = (15) (1/4) = 3 3/4 in.
= 0.31 ft
13.98
12.44
13.70
13.84
14.12
14.18
14.18
14.29
15.04
52
-------
Table 3 (continued)
Chamber invert at wall
15.04 + 0.31
Invert
15.35
H.G.L.
E.L.
Gutter
Try one-half 18-in. pipe
Length from end of ramp to foul
outlet = 64 ft (from Fig. 7)
Total fall = (12) (1/4) = 3 in.
= 0.25 ft
S = 0.25/64 = 0.004
Q = 6.5 cfs (full pipe)
V = 3.7 fps (full pipe)
One-half pipe
Q = (0.5) (6.5) = 3.2cfs>3.0
V = (1.0) (3.7) = 3.7 fps OK
Chamber
For design flow of 165 cfs
d = 9.0 (Table 1)
Weir crest 15.35 + 9.00
Weir diameter = 20 ft
Weir length = 62.8 ft
Weir discharge per ft
165/62.8 = 2.6
Weir head = 1.2 (Fig. 6)
H.G.L. for 165 cfs
24.35 + 1.2
Set emergency weir 28 ft long at
elevation 25.55
Determine W.S. for maximum flow
of 300 cfs
By trial and error
Weir
24.35
25.55
Circular weir
Emergency weir
Foul outlet
H
2.0
0.8
Q
248
45
3±
296
Water surface 24.35 + 2.0
This is at 180° position
Assume same at 0° position
At 0° position area between
deflector and wall equals
(6) (9 + 2.0) = 66 sq ft
V = 300/66 = 4.6 fps
V2/2g = 0.33 ft
At 0° position
26.35
24.35
26.35
26.68
53
-------
Table 3 (continued)
A = 28.3 sq ft
Inlet Pipe
D = 6ft
V = 10.6
V2/2g = 1.74
Enlargement Loss
(0.25) (1.74 - 0.33) = 0.35
Required E.L.
Required H.G.L.
Required invert so pipe is not
surcharged 25.29 - 6.0
Required vertical distance from
W.S. in interceptor to invert of
inlet sewer 19.29 - 12.40 = 6.89 ft
Determine flow to interceptor when
maximum flow is 300 cfs and W.S.
in chamber is 26.35
Assume 8.6 cfs
Interceptor
Assume W.S. as before
Branch Interceptor
D = 1.0; V = 11.0; V2/2g = 1.88
S = 0.06
Exit loss 1.88
Rise = (100) (0.06) = 6.00
Manhole
Entrance loss 0.5 V2/2g = 0.94
Sluice gate (from before)
A = 0.67; V = 12.9; V2/2g = 2.58
Exit loss 2.58
Contraction loss (0.3) (2.53) = 0.77
Outlet Pipe
L = 20 S = 0.06
Rise = (20) (0.06) = 1.20
Bend loss (0.25) (1.88) = 0.47
H.G.L. for 8.6 cfs
Actual H.G.L.
Invert H.G.L.
Invert
E.L
27.03
25.29
19.29
12.40
26.24
26.35
14.28
20.28
21.22
23.80
24.57
25.77
26.24
Therefore discharge through foul
outlet will be about 8.6 cfs when
maximum flow of 300 cfs occurs.
54
-------
COSTS
Typical dimensions for three sizes of the swirl regulator/separator are
given in Table 4. Basic dimensions are taken from Table 1. Cost estimates
are based on these dimensions and the construction details shown in
Figure 31.
Table 4
Swirl Regulator/Separator Dimensions
Design discharge m3/s 1.42 2.83 4.67
(cfs) (50) (100) (165)
Diameter of chamber D2 m 7.0 9.8 11.6
(ft) (23) (32) (38)
Diameter of inlet D, m 1.2 1.5 1.8
(ft) (4) (5) (6)
Height-invert to roof floor to weir H1 m 1.8 2.4 2.9
(ft) (5.8) (8.0) (9.5)
Head on weir Fig. 6 m 0.27 0.40 0.55
for 150% design flow (ft) (0.9) (1.3) (1.8)
Clearance to walk m 0.30 0.30 0.30
(ft) (1.0) (1.0) (1.0)
Headroom m 2.44 2.44 2.44
(ft) (8.0) (8.0) (8.0)
Total height from invert of m 4.8 5.5 6.2
chamber to underside of roof (ft) (15.7) (18.3) (20.3)
D3 = 5/9 D2 m 3.9 5.4 6.4
(ft) (12.8) (17.8) (21.1)
Weir length m 12.2 17.0 20.1
(ft) (40) (53) (66)
Discharge per unit length m3/s 0.18 0.25 0.34
at 150% design flow (cfs) (1.8) (2.8) (3.7)
Note: obtain other dimensions from Fig. 7
Assumptions for Estimating
The height of the structure from the floor of the chamber to the
underside of the roof is based on the following criteria:
Assumption 1: The clearance between the top of the walk and the water
surface is 0.31 m (1.0 ft) when the discharge is 150 percent of design
discharge and the foul outlet is not functioning. The head on the weir
is determined from Figure 6.
Assumption 2: The headroom above the walk is 2.44 m (8.0 ft).
If the underside of the roof is assumed to be at ground level and the
inlet sewer approaching the chamber is assumed to have 2.44 m (8 ft) cover,
55
-------
then the crown of the sewer would be at the level of the walk or 0.3 m (l ft)
above high water level. Hence, the inlet sewer would not be subject to
surcharge. The estimates are based on these assumptions. If a "modified"
design is used with smaller H-^ and a larger D£ , the resultant costs may be
more or less than those shown.
Additional assumptions for estimation purposes are as follows:
A. The walls are 0.30 m (1.0 ft) thick.
B. The roof is of poured concrete, about 0.25 m (0.83 ft) thick, with
two beams 0.92 m (3.0 ft) by 0.46 m (1.5 ft).
C. The bottom concrete slab is 0.61 m (2.0 ft) thick.
D. The concrete walk is 1.22 m (4.0 ft) wide and supported on concrete
beams.
E. The superstructure is 3.96 m (13.0 ft) long by 1.52 m (5.0 ft) wide
by 2.44 m (8.0 ft) high.
The miscellaneous cost is taken as 25 percent and is intended to
include stairs, handrails, scum baffle, circular weir, flushing water system
and pipes, a manual sluice gate and manhole, electrical work, ventilating
work, and doors.
Contingent and engineering costs are taken as 25 percent of the
foregoing.
Constr u c tion Cos t s
The costs of the three selected sizes of swirl regulators, designed
for 90 percent removal of grit, are shown in Table 5. These costs are for a
unit requiring sheet piling, poured concrete walls, and a roof slab.
Alternate Approach
A 1976 report for USEPA (23) approached the cost from the standpoint of
only the facility without site improvements and interconnecting piping.
Rather, only unit material costs were developed. Based upon a H1/D2 ratio of
0.25, chamber diameters of 4.4 m (12 ft) to 17.6 m (48 ft) were developed.
Costs were then related to the surface area. Table 6 presents the results in
tabular form.
The cost of the swirl regulator/separator includes the basic
chamber, which does not include roof, pumping stations, flow measurement
or basin dewatering facilities. These items, if applicable, must
be added to derive a total estimated project cost. The chamber dewatering
facility is normally incorporated in the sludge (or concentrate) pumping
station or concentrate discharge facilities. Costs for raw wastewater
pumping stations, sludge (or concentrate) pumping stations, and flow measure-
ment facilities are presented later in this section.
56
-------
r
Foul Outlet
L = 20ft D = 1.0ft
Chamber Invert 15.04
Gutter Invert 14.23
Energy line 14.20
12.40
12.44
Note Conversion factor
1 ft = 0 305 m
10.00 —i
Figure 32 Hydraulic Profile 85 I/sec (3 cfs)
V Energy line , Weir Crest 24.35
\ /"" r
^r^-—^sLL
W S i — — - """^
TA -
•
•£
i
"u-^A
V/ /-/ 19.29
\~
^\
<^^
v^
?w___
j
^
I
/
a
«
c
0
CT^
u
v>
«'
I
1
i
- 26.35 H.G.L. for 300 CFS
- 26.24 Energy line for 8.6 cfs in Foul Outlet
7~---,
Energy line •
foul outlet
* ' • — vi
L = 20ft D = 1.0 It
r-1504 15.35\
i \
&//////////,
i— 24.57
/ ,- 23.80 |
-X *
4T
. '
V
N
«
«
O
«
_o
1
S
Di
^ Energy line «
Branch Interceptor
L = 100 ft D = 1.0 It
•— 21.22
\
v^ — Energy line
^^
\
\ 14.28 -
>H^
'/y/k///////////.^
I— 12.44
•• 12.30 ~7
11.60^
c
t
, 12.40
1 —
Note Conversion factor
1 ft = 0.305 m
1 cfs = 28 32 I/sec
Figure 33 Hydraulic Profile 245 I/sec (8.6 cfs)
57
-------
Table 5
Construction Cost — Swirl Regulator/Separator
Capacity 1.42 m 3/s (50 cfs)
Item
Sheet piling
Excavation
Reinforced concrete
Concrete block walls
Roof
Outlet pipes
Downshaft and plate
Subtotal
Miscellaneous costs
Bypass sewer
Subtotal
Contingent and engineering costs 35%
Total * * *
Capacity 2.83 m3/s (100cfs)
Item
Sheet piling
Excavation
Reinforced concrete
Concrete block walls
Roof
Outlet pipes
Downshaft and plate
Subtotal
Miscellaneous costs
Bypass sewer
Subtotal
Contingent and engineering costs 35%
Total
Quantity
200m2
(2,160sf)
460m3
(600 cy)
98m3
(128 cy)
27m2
(290 sf)
6 m2
(65 sf)
25%
Amount
$ 25,920
10,800
48,000
3,480
910
1,940
3,000
94,030
23,500
15,000
132,530
46,400
$178,930
Quantity
290m2
(3,120sf)
900m3
(1,180cy)
156m3
(204 cy)
27m2
(290 sf)
6m2
(65 sf)
25%
Amount
$ 37,440
16,200
76,500
3,480
910
4,500
4,500
135,430
33,860
29,400
198,690
69,540
$268,230
58
-------
Table 5 (continued)
Capacity 4.67 m3/s (165 cfs)
Item
Street piling
Excavation
Reinforced concrete
Concrete block walls
Roof
Outlet pipes
Downshaft and plate
Subtotal
Miscellaneous costs
Bypass sewer
Subtotal
Contingent and engineering costs 35%
Total
EN R 3140
Quantity
375m2
(4,030 sf)
1,360 m3
(1,780cy)
216m3
(282 cy)
27 m2
(290 sf)
6m2
(65 sf)
25%
Amount
$ 48,360
32,040
105,750
3,480
910
6,000
7,500
204,040
51,010
42,000
297,050
103,950
$401,000
Table 6
Cost Estimate Unit Prices, Swirl Combined Sewer Overflow Regulator/Separator D^D, = 0.25
90% efficiency
Cost Component
Sq. meters
Sq. feet
Manufactured equipment
Concrete
Steel
Labor
Metal pipe and valves —
Concrete pipe —
Housing —
Electrical and instrumentation 3,000
Miscellaneous items 3,100
Contingency 3,560
Total estimated cost 27,315
EN R 2205 (June 1975)
Surface Area (square feet)
10.5
113
10,500
600
2,480
4,075
23.6
254
16,500
1,340
4,885
9,765
42.0
452
22,500
2,220
8,305
17,770
65.7
707
28,500
3,500
13,250
29,650
94.6
1018
34,500
5,120
20,580
45,355
128.7
1385
40,500
8,470
30,425
68,560
168.2
1810
48,000
12,970
42,530
95,100
3,200
5,350
6,160
47,200
3,400
8,130
9,350
71,675
3,600
11,780
13,540
103,820
3,800
16,400
18,860
144,615
4,000
22,790
26,210
200,955
4,200
30,420
34,980
268,200
59
-------
OPERATION AND MAINTENANCE
The operation and maintenance requirements for swirl
regulators/separators has been assumed to be constant for all sizes of
units.
Cleaning of the unit may be done with automatic washdown facilities.
Many utilities perform routine visits to unattended stations to assure the
facility is operable when needed and for a check to assure the facility has
not been vandalized. It is assumed that the unit will be visited every other
week (26 times per year) and the inspection visit will require two hours,
including travel time. Such costs are common to all types of facilities and
are not unique to the swirl.
With automatic washdown facilities only one or two special site visits
to remove large objects can be anticipated. If manual hosing is required
after each storm event, four to eight hours of labor per event for cleanup
should suffice, particularly if the interior of the facility is covered with
epoxy to minimize adherence of solids to the walls. Absence of square
corners and the presence of the gutters greatly facilitates washdown.
If a bar screen is used, frequency of cleaning will be dependent upon
the normal amount of large floatables in the combined sewer. As four to
five centimeter (1.75 to 2 in.) openings are sufficient, it is likely that the
screen will need to be cleaned only after each storm event. Such cleaning
and handling of the solids should take less than one hour per storm event.
PROTOTYPE INSTALLATIONS
Results from three demonstration units are available which have tended
to validate the laboratory testing. Problems of sampling the combined sewer
flow and the clear and foul sewer flow have proved to be very difficult.
Merely characterizing the settling velocity of the typical solids in order
to determine the theoretical solids removal efficiency as set by the assumed
solids concentration used in. the laboratory has proved very difficult.
Applicable data from three installations will be categorized under
construction cost, operating experience, and pollution removal efficiency.
The units which have been constructed include those by
Onondaga County (Syracuse), New York; Lancaster, Pennsylvania; and
Boston, Massachusetts. The unit in Syracuse is unique in that it was
constructed even though there was not sufficient hydraulic head
available to allow gravity flow during dry-weather flow without maintaining
the swirl unit almost full. A pump was placed on the foul line. Costs and
problems associated with the pumping have not been included in this text as
they are, in effect, independent of the unit's operation. The Boston
facility is a part of a large demonstration project to test the swirl
and helical bend regulator/separator. Comparison tests are to be run
on combined sewer overflows and stortnwater. However, only generalized
operating experience is available at this time.
60
-------
The Lancaster unit is a full-scale unit which has been in operation for a
year. Sampling problems to date do not allow definitive conclusions to be
drawn as to the actual solids removal efficiency.
A small pilot facility has also been tested at San Francisco,
California. The swirl was tested on dry-weather sanitary flow. The
unit, as predicted, was not suitable as the size and concentration of
solids was not in the operating range of the unit.
Table 7 gives information on the three prototype units.
Table 7
Comparison of Prototype Units
Item Syracuse Lancaster Boston
D2/D, 0.25 0.25 0.25
Design efficiency 90% 90% 90%
D2 3.7m (12.3 ft) 8.8 m (24 ft) 3.2 m (10.5 ft)
Design flow 23.4 cu m/min (8.9 mgd) 68 cu m/min (26 mgd) 10.2 cu m/min (3.9 mgd)
Dry-weather flow 1.3-2 cu m/min (0.5-0.75 mgd) 4.7 cu m/min (1.8 mgd) Not applicable
Cons true t i on Cost
Available data from Syracuse and Lancaster are presented in Table 8
and 9. The Boston unit was prefabricated from an available storage tank and
labor and fabrication costs are not comparable to a regular construction
contract.
Table 8
Construction Cost — Syracuse. NY
Current Portion Attributable
Reported Costs Equivalent Cost* To Swirl Unit
Site work $ 18,700 $ 30,600 —
Piping 19,700 32,300 -
Swirl chamber 19,700 32,300 $32,300
Electrical 4,100 6,720 6,720
Miscellaneous 3,500 5,740 2,870
Totals $65,700 $107,660 $41,890
*ENR 3140
61
-------
Table 9
Construction Cost — Lancaster, PA
Concrete $ 69,000
Excavation 14,000
Miscellaneous metal 28,000
Roof 5,000
Valves and gates 8,000
Paint 4,000
Ventilation 3,000
Total estimated cost $131,000
Estimated cost at-
tributable to swirl unit $ 88,000
ENR 3140
Operating Experience
The experience of all three agencies with the operation of the swirl
unit has been very good. Generally, operational problems which have become
apparent have been due to design deficiencies. Structural members in the
flow field induce motions which impede concentration of solids. Among the
other problems which have been found, two demand careful attention.
Floatables - Where large branches or other debris can be found in the
flow, the possibility exists of blocking the floatable inlet. This has
been reported to have happened at least twice in Syracuse. Lancaster
has placed a bar screen ahead of the swirl at a diversion chamber. If
the floatables trap is blocked, floatables escape to the receiving
waters. Thus, for full protection, the bar screens are desirable, even
though they add an element of maintenance of up to one hour per storm
event.
Shoaling of Solids - The transition in the inlet structure must be
designed with care to prevent shoaling of solids. At Lancaster, the
flow path through the inlet diversion-screening chamber is offset.
Shoaled material has built up in the backwater. This could be
prevented by having the flow channel continue in a straight line.
Lancaster reports that the unit is self-cleansing with the automatic
washdown facilities and that no additional cleaning has been necessary in
16 months of operation. Hangup of floatable debris can be almost completely
eliminated by the application of an epoxy coating as was done in Boston.
The use of the HydroBrake^R) as discussed in Section XI appears to be
satisfactory in Lancaster and Boston as a means of controlling the foul sewer
flow.
62
-------
Treatment Effectiveness
Monitoring for treatment evaluation has been performed at the Syracuse
facility. Efficiency has been calculated on the basis of actual performance
without comparison to theoretical values based upon the laboratory work.
Suspended Solids - Relatively good SS removal efficiencies were
determined over the entire storm flow range at the Syracuse prototype
as shown in Table 10. Total mass loading and concentration removal
efficiencies ranged from 33 to 82 percent and 18 to 55 percent,
respectively, with flowrates from 0.54 cu m/min (0.2 mgd) to
20.5 cu m/min (7.6 mgd). Figures 34 and 35 illustrate the total SS
mass removals with respect to time and storm flowrate. The shaded
areas between curves indicate a trend of higher removals at storm
onset when concentrations are generally higher, and again near the
end of the storm when flowrates drop.
Table 10
Suspended Solids Removal, Syracuse, NY
Swirl Concentrator
Average SS Mass Loading
per storm, mg/l kg
Storm # Inf. Eff. Removal" Inf. Eff. Removalb
2-1974
3-1974
7-1974
10-1974
14-1974
1-1975
2-1975
61975
12-1975
14-1975
15-1975
535
182
110
230
159
374
342
342
291
121
115
345
141
90
164
123
167
202
259
232
81
55
36
23
18
29
23
55
41
24
20
33
52
374
69
93
256
99
103
463
112
250
83
117
179
34
61
134
57
24
167
62
168
48
21
52
51
34
48
42
77
64
45
33
42
82
Conventional Regulator
Mass Loading
kg
Inf. Under- Removal
374
69
93
256
99
103
463
112
250
83
117
flow
101
33
20
49
26
66
170
31
48
14
72
(%)
27
48
22
19
26
64
37
28
19
17
62
a — For the conventional regulator removal calculation it is assumed that the SS concentration
of the foul underflow equals the SS concentration of the inflow.
b — Data reflecting negative SS removals at tail end of storms not included.
Source EPA Technology Transfer Capsule Report (EPA-625/2-77-012)
Figure 36 further reveals the trend of greater SS mass loading
reduction as the SS influent concentrations increase. Suspended solids
influent concentrations greater than 250 mg/l generally resulted in
removals of better than 50 percent of the total mass loading to the swirl.
63
-------
0* w
0
0.076 -
0.752-
0.229 -
0.305
•c
00
s
J
I
3
w
o
n
STORM #1 3/24/75
TOTAL SUSPENDED SOLIDS
o MASS LOADING (INFLUENT)
£ MASS LOADING (EFFLUENT)
-FLOW
77:00 72.-.00 73:00 74:00 75:00 76:00 77:00 78:00
TIME, hrs
Source EPA Technology Transfer Capsule Report (EPA-625/2-77-012)
Figure 34 Suspended Solids Removal, Syracuse, NY, Storm #1
64
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tu
1200-
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7000-
900-
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700-
600-
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300-
200-
700-
0
O STORM #6 6/5/75
6 STORM #2 4/3/75
X STORM #7 3/24/75
70
20 30
40 50 60
MASS REMOVAL, %
70
90 700
Source EPA Technology Transfer Capsule Report (EPA-625/2-77-012)
Figure 36 Suspended Solids Influent Concentration vs Percent Mass Loading
Removal, Syracuse, NY
Care must be taken in evaluating swirl solids treatability since under
dry-weather flow conditions, all regulators are designed to divert the entire
flow volume and associated solids to the intercepting sewer until a prede-
termined overflow rate is reached. This diversion to the interceptor con-
tinues at a maximum throughout the storm. However, the swirl has the added
advantage of concentrating solids as well as conventionally diverting flow
during overflow events. This concentrating effect is evidenced by removal
efficiencies in terms of SS concentrations varying from 18 to 55 percent
(Table 10) as previously stated whereas conventional regulators are
assumed not to concentrate solids at all (zero percent removal based
upon concentration) (Table 10, footnote a).
If the swirl regulator was replaced by a conventional flow regulator,
the net mass loading reductions (attributable to the SS conventionally going
to the intercepted underflow) would have ranged from 17 to 64 percent
(Table 10) as compared to a more effective range of 33 to 82 percent
(Table 10) for the swirl. This may be a better way to compare the effec-
66
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tiveness of the swirl to conventional combined sewer overflow regulators
since conventional devices will remove the solids associated with the flow
diverted for treatment.
For low-flow storms approaching the maximum dry-weather capacity of
the interceptor, the advantages of swirl concentration are reduced as would
be expected based on the physical principle of mass balance involved. In
other words, as the ratios of "inflow to foul outlet underflow" or "weir
overflow ro foul outlet underflow" decrease, the SS removal advantage from
swirl concentrating also decreases. This is because the intercepted hydrau-
lic loading to underflow becomes more significant in the net mass loading
calculation of the hypothetical conventional regulator. This phenomenon is
exemplified by the SS total (of the swirl) compared to SS net (of the con-
ventional regulator) mass loading removals of Storm No. 1-1975 (Table 10).
where the hydraulic loadings to the swirl were low, approaching dry-weather
conditions.
Many outfalls are designed to pass 20, 100 and even 1,000 times average
dry-weather flow as opposed to the Syracuse facility which, at best, passes
only 10 times average dry-weather flow. For these cases, the swirl concen-
trating effect will exhibit distinct advantages over conventional regulators
for SS removal.
BODr Removal
Prototype analyses indicated BOD^ removals of 50 to 82 percent for mass
loading, and 29 to 79 percent in terms of concentration (Table 11^.
Figures 37 and 38 indicate the trend for BOD,- total mass loadings removal
for the swirl prototype. Figure 39 indicates higher removals at higher
BOD^ influent concentrations.
Removal of BODr could not be modeled in the laboratory. Association of
the BOD,- load with the various sizes of settleable solids will dictate removal
loads. Such relationships are also important for other pollutants such as
phophorus, ammonia, and heavy metals.
Table 11
BOD5 Removal, Syracuse, NY
Average BOD5
Mass Loading, kg per storm, mg/l
Storm - Influent Effluent Rem. (%) Influent Effluent Rem. (%)
7-1974 26,545 4,644 82 314 65 79
1-1975 3,565 1,040 71 165 112 32
2-1975 12,329 6,164 50 99 70 29
Source EPA Technology Transfer Capsule Report (EPA-625/2-77-012)
The floatables trap has been reported to work well and there has been
almost no observable floatables passing over the weir. Adequate testing has
not yet been reported for removal of oils and floating debris as sampling is
all but impossible in the influent pipe.
67
-------
< t
o.
0.076.
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STORM #7 3/24/75
o MASS LOADING (INFLUENT)
MASS LOADING (EFFLUENT)
— FLOW
r-—*
77:00 72:00 73:00 74:00 75:00 76:00 77:00 78:00 79:00
TIME, hn
Source EPA Technology Transfer Capsule Report (EPA-625/2-77-012)
Figure 37 BOD5 Removals, Syracuse, NY, Storm # 1
68
-------
6£'0—
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69
-------
500 -
O STORM *2 4/3/75
X STORM ft 3/4/75
STORM *7 6/21/74
0 20 40 60 80
MASS REMOVAL, %
Source EPA Technology Transfer Capsule Report (EPA-625/2-77-012)
Figure 39 Swirl Regulator BOD5 Influent Concentration vs Percent Mass Loading
Removal, Syracuse, NY
70
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Section III
HELICAL BEND COMBINED SEWER OVERFLOW REGULATOR/SEPARATOR
DESCRIPTION
The helical bend combined sewer overflow regulator/separator consists
of an enlarged section of the sewer which acts as a solids and floatables
trap prior to diversion of the overflow to additional treatment or receiving
waters. The device requires considerable space to construct. However,
right-of-way requirements may be minimal as the bulk of the device may at
times be constructed from the overflow point back along the sewer. Operation
and maintenance are minimized as there are no mechanical or moving parts
within the device. The channel is curved to develop helical secondary
motions within the flow. The helical motion effectively captures particles
which have a greater settling velocity than the upward velocity of the
helical motion. Relatively high velocities are achieved as the chamber
empties to treatment at the end of the storm event which will remove deposited
solids. Thus, the helical bend separator is unique in that most of the
removed solids are released at the end of the storm event.
The helical bend combined sewer overflow separator was developed in
Great Britain by T. M. Prus-Chacinski, and a full scale test was made in
Nantwich, G.B., following laboratory studies. The design developed for
USEPA was based upon English experience as well as hydraulic and mathematical
model studies. A demonstration unit has been installed (September, 1979) in
Boston, Massachusetts, where it will be tested on both combined sewage and
separate stormwater flows.
Figure 40 is an isometric view of the separator. The transition section,
which is 15 times the diameter of the inlet combined sewer in length, is
covered to allow development of improved flow lines as the width of the chamber
is expanded to three times the inlet diameter. Figure 41 shows the transition
section in a prototype unit. A straight section of five times the inlet
diameter allows the development of a less turbulent flow field and the
development of the shape of the bottom cross section. A sixty degree bend
with a radius of sixteen inlet diameters uses the outside edge of the curve
as an overflow weir. The weir is baffled to trap floatables. At the end of
the curved section is a wall with an outlet sized to allow dry-weather flow
to flow unimpeded to treatment. The outlet may require a mechanical flow
regulating device. The regulator has been designed to allow maximum flows
consistent with downstream hydraulic capacity to continue to treatment.
71
-------
INLET
CHANNEL FOR
OVERFLOW
WEIR
/
OUTLET TO
STREAM
TRANSITION SECTION
15D
STRAIGHT
SECTION
5D
HELICAL
BEND 60C
-v R = 16D
NOTES:
1. Scum baffle is not shown.
2. Dry-weather flow shown in channel
OUTLET TO PLANT
Figure 40 Isometric View of Helical Bend Combined Sewer Overflow Regulator/Separator
Available data indicates that the helical bend separator can be as
efficient as the swirl separator/regulator. The helical bend separator
compared to the swirl separator should have less head loss, may require
less acquisition of additional right-of-way, and allows the solids to be
delivered to treatment over a relatively short period of time at the end of
the storm event. However, the cost of the device may be up to fifty percent
more than an equivalent swirl separator/regulator and almost three times more
than a swirl unit designed to remove 80 to 90 percent of the solids.
DESIGN GUIDELINES
The decision as to the design flow should be based upon the same type
of considerations as presented in the design flow information of Section I.
Figures 42 to 46 are required for designing the helical separator.
72
-------
Figure 41 Transition Section, Boston
> 90
O
Design Flowrate Ratio
Figure 42 Grit Recovery vs Design
Flowrate Ratio
0 5Q[>
OQ I 5 QO
Design Flowrate Ratio
Note: Organic material as defined on Figure 86
Figure 43 Settleable Recovery vs
Design Flowrate Ratio
73
-------
Note' For Profile C-C Location
See Figure 47
Figure 44 Transition Profile
Hydraulic grade line
Water surface
Velocin head
Velocity Head
2D
Profile with Level Invert
H\draulic Grade line
Profile \\ ith Level Top
Figure 45 Effect of Transition Slope
74
-------
20 30 40 50 IOO
I i i i>i
200 300 400500
i 1 i i i i 1 I II
100%-
cfs
0.5
2345
Design Flowrate
10
15 20 25 m3/s
Figure 46 Design Flowrate vs Inlet Diameter
The first step is to determine the design flow rate. As mentioned
in Section I, design flows should be less than the system capacity.
Figure 42 indicates that the grit recovery will decrease to 97 percent with
a flow equal to 1.5 times the design flow rate and to 93 percent with a
flow equal to 2 times the design flow rate. Hence, the efficiency of grit
removal is not greatly affected by flows up to twice the design flow rate.
75
-------
Similarly, Figure 43 indicates the decrease in recovery of organic
matter with increase in flow. Thus, for 1.5 times design flow rate, the
efficiency decreases to about 87 percent; and for 2 times design flow rate,
the efficiency decreases to about 75 percent.
From the foregoing, it would appear that a considerable increase in
flow above the design flowrate can occur without greatly affecting the
operating efficiency of the helical separator.
Transition Slope
As shown in Figure 44, the recommended transition has a length of
15 D and a height of D at the inlet and 2 D at the outlet.
The invert should have some slope. To prevent any surcharge at the
inlet, the top of the transition should be kept level, and the invert should
be either the slope of the inlet or the slope that will satisfy the hydraulic
slope S in the Manning equation, whichever is greater.
The resultant hydraulic conditions either with the invert level or the
top level is shown in Figure 45. The transition with the level top has the
following advantages: 1) The sewer is not surcharged upstream of the tran-
sition except for loss of head in the transition, which may be minor; and
2) the slope will increase the velocity through the helical separator as the
storm flow subsides which may aid in flushing deposits out of the helical
section. The chief disadvantage of providing too great a slope in the tran-
sition is that the outlet pipe to the stream from the helical separator may
be lowered so much that the extension of the existing sewer cannot be utilized
for this purpose. Therefore, each situation must be evaluated before select-
ing the slope. Again as a minimum, the transition should have the same slope
as the incoming sewer.
Transition Length
The transition length as given in Figure 45 is 15 D. The value of D
is selected from Figure 46. Assume the designer selects a D of 1.83 m
(6 ft) from Figure 46 as appropriate for the design flow rate. Then the
recommended transition length is 27.4 m (90 ft). However, assume the existing
sewer has a diameter of 1.52 m (5 ft) rather than 1.83 m (6 ft). The problem
is how to effect the connection from the existing sewer to the transition.
The most logical way would be to extend the transition to meet the existing
sewer while reducing the area at the same rate as occurs in the transition.
The area of the transition at the entrance would be 0.785 D squared and at the
exit 4.70 D squared. These areas are equivalent to squares with a side of
0.885 D at the entrance and of 2.16 D at the exit. Accordingly, the slope of
the side of the transition would be equal to 1.28 D divided by 30 D or 0.0426.
This slope has an angle of 2 degrees 26 minutes. Thus, if the diameter of the
existing sewer is 0.30 m (1 ft) smaller than the selected D, the transition
should be extended by 3.5 m (11.6 ft).
It would also seem logical to reduce the length of the transition by a
similar process if the area of the existing sewer is larger than the area of
the transition inlet selected from the design charts.
76
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Transition Inlet Size
All dimensions of the helical separator are related to D, the diameter
of the transition inlet. After determining the design flow rate, the designer
should select the inlet diameter, D, from Figure 46 which shows the simple
scaled-up values according to the Froude Law, for the design discharge, 0~,
as well as 1.5 QD and 2 QQ. These have been computed covering the likely
range of applicable flood discharges and pipe sizes that will be encountered
in any prototype installations.
Seldom will the value of D be that of a standard pipe size. Hence, the
designer should select the nearest D corresponding to a standard pipe size.
If the indicated D falls between two pipe sizes, the larger D will give a
separator with greater efficiency than the smaller D. For instance, if the
design flow rate is 2.83 cu m/sec (100 cfs), the indicated D will be 1.45 m
(4.75 ft). The designer can select a D of 1.37 m (4.5 ft), equivalent to a
design flow rate of 2.40 cu m/sec (85 cfs). If the latter capacity is
chosen, the design flow rate will be 18 percent larger than the separator
capacity. From Figure 42, the grit removal efficiency will be reduced to
99 percent of the total grit load. From Figure 43 the settleable organic
removal will be reduced to about 96 percent.
If, in the example given above, the existing sewer should have a dia-
meter equal to one of the possible D selections, then it would be logical to
select the D which matches the existing sewer size. Otherwise, the transi-
tion should be extended as discussed previously.
The overall length of the helical separator is approximately 37 D
including the transition and straight sections, as shown in Figure 47. If
a D of 1.37 m (4.5 ft), is selected, the length will be 50.9 m (167 ft) where-
as if a D of 1.52 m (5 ft) is selected, the length will be 56.4 m (185 ft).
A third possibility, assuming the existing sewer is 1.37 m (4.5 ft) and the
D indicated by the chart is 1.4 m (4.75 ft), would be to base all dimensions
on the indicated D and to extend the transition according to the method ex-
plained previously. In this case, the transition would be extended an amount
equal to one-half the difference in diameters, divided by 0.0426 or 0.91 m
(3 ft). The overall length in this case would be 37 times D plus 0.91 m
(3 ft), or 54.5 m (179 ft).
Thus, the designer is faced with the choice of three lengths—either
50.9 m (167 ft), 56.4 m (185 ft) or 54.5 m (179 ft). Obviously the largest
helical separator will provide the most efficient operation.
The design calculations in this section have been based upon the need
to provide regulation of the combined sewer overflow. If treatment is the
primary objective, the design should be modified to further enhance solids
separation.
77
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D/6
to
Section A-A
Scum Board
Weil
Typical Section B-B
RECOMMENDED CROSS SECTIONS
PLAN LAYOUT 1
Inlet Diameter '
-D-
Figure 47 Plan Layout and Cross Sections
78
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1.2
1.0
0.9
0.8
d- 0.7
0)
S 0.6
0}
o
.a
< 0.5
"8
-------
Velocities in Transition
As a matter of interest the velocities at the inlet and outlet ends of
the transition were computed for five values of D, from Figure 48. The D
values selected were 0.91, 1.22, 1.52, 1.83 and 2.13 m (3,4,5,6, and 7 ft).
The results are shown in Table 12. These data indicate an outlet velocity
from the transition ranging from 0.22 m/sec (0.71 fps) to 0.36 m/sec
(1.17 fps). This compares with the usual criteria of velocities between
0.23 m/sec (0.75 fps) and 0.38 m/sec (1.25 fps) in a rectangular grit channel
with velocity control. In general, the outlet velocities are about one-sixth
the inlet velocities. All velocities are based on the sections flowing full.
Channel Slope
The channel should be given enough slope to maintain a self-cleansing
velocity of 0.61 m/sec (2.0 fps) with DWF (average dry-weather flow).
Table 12
Velocities in Transitions
Area
Velocity
Inlet
Diameter
0.91 m
(3ft)
1.22m
(4ft)
1.52m
(5ft)
1.83m
(6ft)
2.13m
(7ft)
Design
Discharge Inlet
0.85 m3/s 0.65 m2
(30 cfs) (7.0 sf)
1.84 m3/s 1.17 m2
(65 cfs) (12.6sf)
3.11 m3/s 1.82m2
(110 cfs) (19.6sf)
4.96 m3/s 2.63 m2
(175 cfs) (28.3 sf)
7.65 m3/s 3.58 m2
(270 cfs) (38.5 sf)
Outlet
3.93 m2
(42.3 sf)
6.98 m2
(75.2 sf)
10.9 m2
(117sf)
15.7 m2
(169 sf)
21.4 m2
(230 sf)
Inlet
1.31 m/s
(4.3 fps)
1.58 m/s
(5.2 fps)
1.71 m/s
(5.6 fps)
1.89 m/s
(6.2 fps)
2.13 m/s
(7.0 fps)
Outlet
0.22 m/s
(0.71 fps)
0.26 m/s
(0.86 fps)
0.29 m/s
(0.94 fps)
0.30 m/s
(1.0 fps)
0.36 m/s
(1.17 fps)
The following example is based upon what would be a maximum ratio of
dry-weather flow to design flow. Generally the ratio will be less.
Assume the following:
D
Design flow rate =
DWF
Peak DWF
0.91 m (3.0 ft)
0.85 cu m/sec (30 cfs)
1 percent of design flow rate
0.008 cu m/sec (0.3 cfs)
0.025 cu m/sec (0.9 cfs)
From a chart showing hydraulic properties of circular sections when the
flow rate is one percent of the full section, the depth is seven percent of
the full depth and the velocity is 31 percent of the velocity of the full sec-
tion when flowing full at a velocity of 0.61 m/sec (2.0 fps), divided by 0.31,
or 1.98 m/sec (6.5 fps). From a nomograph of flow for Manning n = 0.013, the
required slope of the channel for a diameter of 0.91 m (3.0 ft) is 0.48
percent. If the peak dry-weather flow is 3 DWF the following data prevail:
80
-------
When slope is 0.48%
Q = 0.025 cu m/sec (0.90 cfs)
d = 0.11 m (0.36 ft)
v = 0.58 m/sec (2.0 fps)
The foregoing assumes a circular section in the channel when flow is
1 percent and 3 percent of design flow rate. From a visual comparison of a
large-scale section of channel with a circular section it is evident that
flow conditions in a circular section will prevail for the depths of flow
considered above. The foregoing indicates that peak dry-weather flows should
cause no deposition in the channel.
Weir Discharge
Previous research on side overflow weirs indicates that with a rela-
tively high weir, as proposed in the helical separator, the usual weir dis-
charge equations provide a reasonable basis of design. The usual equation is
as follows:
Q = CLH3/2
where
Q = flow rate in cu m/sec (cfs)
C = coefficient
L = length of weir in m (ft)
H = head on weir in m (ft)
The coefficient C varies depending on whether the weir is sharp crested
or broad crested and depending on the head and width of the weir.
Experience in Great Britain, where side overflow weirs have been used
more widely than in the United States, favors the use of a weir with a semi-
circular shape. This shape seems preferable for the helical separator.
The coefficient of a broad crested weir varies with the width of crest
and head on the weir. For the widths and heads likely to occur in the heli-
cal separator the value of C (for U.S. customary units) may range from 2.8 to
3.3. The use of a C value of 3.0 (for U.S. customary units) for design pur-
poses is suggested. An example follows:
Design flow rate = 0.85 cu m/sec (30 cfs)
D = 0.91 m (3.0 ft)
L (weir length) = 18.83 D
<= 17.2 m (56.5 ft)
Assume flow to plant = 0.028 cu m/sec (1.0 cfs)
Q (over weir) = 0.82 cu m/sec (29.0 cfs)
In U.S. Customary ,,/„
Units Q = CLH
If C = 3.0 then H = 0.095 m (0.32 ft)
81
-------
The weir height is 1-5/6 D from Figure 47. Therefore:
Weir Height = 1.68 m (5.50 ft)
Head on weir = 0.095 m (0.32 ft)
Water Depth = 1.77 m (5.81 ft)
However, to meet the laboratory demonstrated requirements that the tran-
sition be flowing full, the water depth should be 2D or 1.83 m (6.0 ft).
Therefore, in this case the weir height should be a minimum of 1.73 m (5.68 ft)
so that the transition outlet is flowing full when design flow rate occurs.
Outlet Control
Various methods of controlling the flow from combined sewer overflow
regulators are discussed in an EPA report (13). This report indicates that
close control of the outlet flow requires the use of a sluice gate controlled
by a float and actuated by either water power or an electric motor. On
smaller structures where the use of such devices is not justified, one method
of control is by use of a manually-operated gate. A HydroBrake'R / would also
be effective for all structure sizes. The intent is to only operate such
gates to clear them of debris or to change the opening size.
The use of such gates may result in considerable variation in the flow
diverted to the treatment plant. This may not be serious when this flow is
only a small percentage of the total tributary to the plant. To indicate the
possible range in flow, the following example is based on the use of a
manually-operated gate on the outlet to the treatment plant. The mininum size
gate used should be 0.20 m (0.67 ft) square but a gate with a. minimum size of
0.30 m (1.0 ft) square is preferable.
Legend
A = Cross-sectional Area
D = Diameter
V = Velocity
d = Depth of flow
Q = Flow rate
b = Width of opening
C = Coefficient - 0.7
DWF = Average dry-weather flow
g = 9.81 m/sec2 (32.2 ft/sec )
Pertinent Data
DWF = 0.008 cu m/sec (0.30 cfs)
Peak DWF = 0.025 cu m/sec (0.90 cfs)
82
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Try sluice gate 0.30 m (1.0 ft) square
Assume opening 0.10 m (0.33 ft) high
Then A = 0.03 sq m (0.33 sf)
Determine depth upstream of gate when:
Q = 0.025 cu m/sec (0.90 cfs)
"
Q = C A
0.025 cu m/sec (0.9 cfs)
= 0.7 x 0.031 sq m (0.33 sf) x 4.43 (8.03) x\HT
= 0.21 m (0.69 ft) on center line of orifice
Depth of flow is H, plus one-half height of orifice, or 0.26 m (0.86 ft)
This is much greater than the normal depth of flow at the peak dry-weather
flow of 0.11 m (0.36 ft) computed previously. Therefore, the velocity will
be much less than the 0.58 m/sec (2.9 fps) computed previously and may cause
deposition of grit at peak dry-weather flow.
Determine flow to the treatment plant when the water level is at weir
crest.
Depth of flow in chamber is 1.83 m (6.0 ft)
Head on center of orifice is 1.77 m (5.83 ft)
Q = C A ^2P
= 0.7 x 0.03 sq m (0.33 sf) x 4.43 (8.03) 1.33 m (2.41 ft)
= 0.127 cu m/sec (4.46 cfs) = 15 DWF
Hence, the flow to the plant will exceed 15 DWF during periods of de-
sign discharge if there is no further restriction to flow downstream of the
sluice gate. One way to restrict the flow is to design a sewer between the
sluice gate manhole and the interceptor in such a way that it will convey the
peak dry-weather flow without surcharge but will become surcharged when the
flow exceeds the peak dry-weather flow. This procedure is described and
illustrated by an example in an EPA report (13) .
Principles of sewer design must be maintained in the design of the out-
let. The size of the pipe must be sufficient to allow blockages to be re-
moved readily from a convenient point of access. The slope of the line must
be sufficient to maintain self-cleansing velocities through the unit and the
outlet pipe. To be self-cleansing, velocities through the unit should exceed
0.6 m (2 ft) per second at low flow in order that solids will not be retained.
Spillway Channel
The side channel in the helical section which conveys the overflow from
the weir to the outlet sewer leading to the stream should be designed for the
maximum flow expected to pass through the separator. The maximum flow will
83
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depend on the storm frequency for which the combined sewer is designed, as
well as on the extent to which the combined sewer can be surcharged by storm
flows greater than the design flow. It should also be assumed that the pipe
outlet to the treatment plant is not in use either by design or by accident.
On this basis it is possible for the maximum flow to exceed the design flow
rate by 50 to 100 percent.
As an example, assume that the design flow rate is 0.85 cu m/sec
(30 cfs) and the maximum flow is 1.27 cu m/sec (45 cfs). The side channel
can be designed as a lateral spillway channel with the weir discharge spilling
into it throughout its length. To aid in self-cleaning, it is desirable to
set the downstream end of the channel above the invert of the outlet pipe
and to provide a slope in the channel so that at low depths of flow the ve-
locity will exceed 0.31 m/sec (1 fps).
The channel should be designed large enough so that the upstream water
surface will not cause submergence of the weir.
The general equation for determing the depth of flow in a lateral spill-
way channel is the following:
h0 =J^c£ + (hl -A ii)2 -2 n_
If hl 3 3
where
h = upstream water depth
hc = critical depth
h-^ = downstream water depth when flow is submerged
i = slope of channel
1 = length of channel
and 2
where
b = width of channel
The factors in the for.egoing equation are depicted in Figure 49.
Actually, only 17.83/18.83 or 95 percent of the maximum flow discharges
directly into the spillway channel. The balance is discharged by the foul
sewer. In the following example, however, it is assumed that all the maximum
flow is conveyed by the channel.
Assume the following data:
Design flow rate = 0 85 cu m/sec (30 cfs)
Q (side channel) = 1.27 cu m/sec (45 cfs)
Outlet pipe diameter = 0.91 m (3.0 ft)
Weir height = 1.83 m (6.0 ft)
Then:
Outlet velocity 2 = 1.95 m/sec (6.4 fps)
Entrance loss = -| = Q.29 m (0.96 ft)
Elevation invert outlet pipe = 0.00 m (0.00 ft)
Elevation weir = 1.83 m (6.00 ft)
84
-------
Elevation water at entrance
Length of channel = length of
weir, less outlet diameter
1.21 m (3.96 ft)
16.3 m (53.5 ft)
Initial computation indicated that a channel 1.83 m (6.0 ft) deep and
0.31 m (1.0 ft) wide would cause submergence of the weir. For maintenance
purposes a minimum width of channel of 0.61 m (2 ft) is considered desirable.
Preliminary computation with zero slope and the downstream end of the
channel at elevation 0 indicated a water depth at the upstream end of the
channel of 1.49 m (4.9 ft).
Note D= 1 Mnsition inlet did
Figure -ssumes 'h^t outlet
sewer has same diameter
Figure 49 Spillway Channel Profile
The effect of submergence on broad crested weirs is surprisingly small.
If necessary the fall in the water surface over the weir can be limited to
50 percent of the head on weir without affecting the discharge over the weir.
As computed previously the head on the weir is 0.095 m (0.32 ft) and little
elevation can be gained by assuming a submerged weir. Therefore, design can
be based on no submergence of weir. It is also desirable to locate the down-
stream end of the discharge channel above the outlet pipe invert to prevent
deposition in the channel. Therefore the downstream end of the channel was
set at elevation 0.30 m (1 ft) and the channel slope set at 0.005. The re-
sultant freeboard of 0.18 m (0.6 ft) indicated that the channel could have
been raised an additional 0.18 m (0.67 ft). The final data are as follows:
Elevation downstream invert of channel 0.3 m (1.0 ft)
Rise in channel 0.09 m (0.3 ft)
Elevation upstream invert of channel 0.40 m (1.3 ft)
85
-------
h0 1.25 m (4.1 ft)
Elevation upstream water surface 1.65 m (5.4 ft)
Elevation weir 1.83 m (6.0 ft)
Freeboard 0.18 m (0.6 ft)
Design Example
The design calculations for a helical bend combined sewer overflow
regulator/separator are illustrated in Table 13.
Table 13
Design Example, Helical Bend Combined Sewer Overflow Regulator/Separator
Sample Computations
Note: Conversion factors
1 ft = 0.305 m
1 cfs = 28.32 I/sec
D = Diameter inlet pipe
D = Diameter outlet orifice
S = Slope
1 = Diameter outlet pipe
n = Manning roughness coefficient
v = velocity, max flow
v
1 = velocity, design flow
2 = velocity, peak DUF
D = velocity, discharge pipe
V
3 = peak velocity, discharge pipe
0 =
D = design flow
2 = max flow
Q3 = peak dry-weather flow (3 DWF)
4 = flow, discharge pipe
1 = depth of design flow in pipe
2 = depth of peak DWF
D = depth of flow in discharge pipe
DWF = dry-weather flow
g =32 ft/sec/sec
A = area
K = Rehbock K
L = unit length of weir
1 = length of throttle pipe
H = head on weir
C = coefficient for orifice discharge
86
-------
Straight Pipe
Assume pipe design so that at DWF the velocity is about 2 fps
D = 3.0 ft S 0.44% n = 0.013
Q2 = 45 cfs v = 6.5 fps
For Q^ (design) = 30 cfs
% 30
Q2 ^
d
1 = 0.6 from standard charts
D
d, = 0.6 x 3.0 = 1.8 ft
For
For
v = 1.07 from standard charts
vl = 1.07 x 6.5 = 7.0 fps
Q (DWF) = 0.3 cfs
_D = 0^1 = 0.007
45
= 0.06
= 0.29
v2
Q3 (Peak DWF) = 0.9 cfs
Q3 0.9
_£ = =0.02
d, = 0.06 x 3 = 0.18 ft
vx = 0.29 x 6.5 = 1.9 fps
OK, almost 2.0 fps
d2 = 0.1 x 3 =0.3 ft
v= 0.4x6.5 =2.6 fps
Straight Pipe
D = 3 ft
Q3 = 0.9 cfs
Q = 0.3 cfs
87
-------
0.3 ft
— Q3 = 0.9 cfs
At 3 DWF the area will only be slightly larger in the separator than in 3 ft
diameter pipes. Assume velocity the same
vr. - 2.6 fps
Exit Pipe — Assume 1.0 ft diameter (D-, )
- 0.42 ft
S
QD
0.44%
2.4 cfs
°'9 = 6.37
= 3.0 fps
0.9 cfs
= 0.42
_D = 0.92
v3
dD = 0.42 ft
v = 0.92 x 3
= 2.8 fps
Lower invert of outlet pipe 0.12 ft below invert of separator so as not to
raise water surface
Determine outlet design
when
so that
Q
D
= 30 cfs
= 0.9 cfs
Weir Length Angle = 60
D = 3 ft
Weir Radius = 16 D + 2.5 D + D/3
= (16) (3) + (2.5) (3) + 1
= 56.5, ft
Weir Length = ^?_ f ? TT R l =
360 I /
= 59 ft '
Head on Weir
Q per ft
= |^ = 0.51 cfs
88
-------
Use Rehbock K
Q
H 3/2
H
Depth of Water
1-5/6 D
Head on weir
Total water depth
Assume short tube exit
A
Q,
ac
r~-
wi
Outlet Design
KLH 3/2
0.15
0.28 ft
3.41 (1) H 3/2 = o.51
II (3) = 5.50 ft
6
0.28
5.78 ft
1.0 ft
7rD2 = o.785 sf
-ZT
CA \J 2gH
(0.7) (0.785) (8.03)
10.1 cfs
x
0.5
0.12
Determine orifice area for
Q3
0.9
A
A
D.
= 7TD2
.03 \J5.28
0.07
0.09 = 0.3 ft
Orifice should be greater than 0.67 ft. If orifice is made this size so that
only 0.9 cfs or 3 DWF would pass when unit is full, then it is apparent that
the unit would fill up whenever 3 DWF occurs which might be the peak daily flow.
To prevent deposition of solids on the separator floor and to prevent cleaning
the separator in dry weather periods, the separator and outlet should pass up
to 3 DWF without raising levels in separator to weir levels.
89
-------
Try throttle pipe on outlet
00
Assume free fall
Outlet Design
Use 8-in. pipe as minimum
Use S = 0.4% as minimum
For n = 0.013
Q = 0.75 cfs
V
= 0.08 ft
For
QDWF
Ql
Q2
0.3 cfs
0.3
0.75
= 0.4
If discharge ratio is 0.4, then depth is 44% and velocity 94%
D0 = 0.44 x 0.67 = 0.3 ft
VD = 0.94 x 2.2 = 2.1 fps
Assume
For
LI = 100 ft H = 5.78 - 0.67 = 5.11 ft
Q3DWF = 0.9 cfs
V3 = 2.5 fps
S = 0.5%
90
-------
Entrance & exit loss 1.5 x 0.1
Slope hydraulic gradient 100 x 0.5%
Actual pipe slope
Water surface above top pipe
Water surface in tank
Depth water in tank
Determine Q (max) when separator is full
Assume entrance and exit loss = 1 . 2 f t
Slope H.G =
Q
4 (max)
100
cfs
- 1.2) 1QQ + Q_
v = 7-2
0.15
0.50
0.65
0 . 40
0.25
0.67
0.92 ft
= 0.8
2g
Thus with 8 in. throttle pipe 100 ft long the maximum Q will be 1.6 cfs or
about 5 times DWF .
Try other lengths of pipe
L ft 5% HG
200
300
400
600
2.45
1.9
1.45
1.2
Q4(max) cfs
1.8
1.6
1.4
1.3
5.4
4.7
4.0
3.8
V2
2g~
0.5
0.4
0.3
0.2
Q4(DWF)
6
5.3
4.7
4.3
A length of 400 ft should be the maximum for an 8 in. sewer. Therefore, it
is obvious that if the discharge is to be limited to 3 DWF some type mechani
cal device should be used to close the outlet opening as the water level
rises in the separator.
Determine depth of water in separator with throttle pipe 400 ft long, when
Q^= 0.9 cfs
Required S = 0.5% (See previous page)
400 x 0.5% = 2.0 ft
Slope sewer 400 x 0.4%
Entrance & exit loss 1.5
Head on top of pipe
Diameter pipe
Depth of water in tank
V2
2g
2.
1.
0.
0.
0.
0.
0
6
4
15
55
67
ft
ft
ft
1 . 22 ft
TYPICAL DIMENSIONS
In order to develop cost estimates, three design discharges were chosen
and helical bend separators sized for each. Table 14 presents typical
dimensions for flows of 1.42, 2.83, and 4.67 cu m/ sec (50,100, and 165 cfs).
91
-------
Table 14
Helical Bend Combined Sewer Overflow Regulator/Separator Dimensions
Design discharge
Inlet diameter
Transition length
Straight section — length
Radius
Width
Minimum wall height
Channel to top weir
Height end of transition
Scum baffle height
Distance from weir to
bottom of baffle
Weir height
Distance wall to weir (Max)
(Min.)
m3/s
(cfs)
D-m
(ft)
15 D-m
(ft)
5D-m
(ft)
16D-m
(ft)
3D-m
(ft)
2.5D-m
(ft)
1 5/6 D-m
(ft)
2D-m
(ft)
D/3-m
(ft)
D/12-m
(ft)
D/3-m
(ft)
D/3-m
(ft)
D/6-m
(ft)
1.42
(50)
1.07
(3.5)
16.0
(52.5)
5.33
(17.5)
17.1
(56.0
3.20
(10.5)
2.67
(8.75)
1.95
(6.4)
2.13
(7.0)
0.36
(1.2)
0.09
(0.3)
0.36
(1.2)
0.36
(1.2)
0.18
(0-6)
2.83
(100)
1.52
(5.0)
22.9
(75.0)
7.62
(25.0)
24.4
(80.0)
4.57
(15.0)
3.81
(12.5)
2.77
(9.1)
3.05
(10.0)
0.52
(1-7)
0.13
(0.4)
0.52
(1.7)
0.52
(1.7)
0.26
(0.85)
4.67
(165)
1.83
(6.0)
27.4
(90.0)
9.14
(30.0)
29.3
(96.0)
5.49
(18.0)
4.57
(15.0)
3.35
(11-0)
3.66
(12.0)
0.61
(2.0)
0.15
(0.5)
0.61
(2.0)
0.61
(2.0)
0.30
(1.0)
SITE REQUIREMENTS
The location and depth of the combined sewer will determine the area
required for its installation. The depth of the sewer may suggest that an
underground chamber is appropriate. If this is the case or if adjacent land
is expensive, it may be desirable to construct a chamber for the separator
along the existing right-of-way of the sewer. Figure 50 shows the site
requirements assuming that 2 m (6 ft) is allowed for construction clearance
and the thickness of the structure. Figure 51 shows the site requirements
for a swirl unit.
The size of the buffer or protective zone required around the uncovered
helical separator will depend to a large extent on the environment of the
neighborhood. In any locality, a buffer zone at least 15.2 m (50 ft) wide
would be desirable. Therefore, the site requirements given herein are based
on a 15.2 m (50 ft) buffer zone around all open or above ground parts of the
facility. Because the transition is below the surface, no buffer zone is re-
quired for that part of the structure; however, it is assumed that all of the
transition section is located on the site.
92
-------
11.8D +30.5m (100 ft)
Notes:
1 If D <0 76 (2.5 ft) the length is 16 9 D + 30 5 m (100 ft)
2 Or 15 2 m (50 ft) whichever is greater
3 D = Diameter of transition inlet
D, +31.1m (102ft)
o
5
1. Da = Inside diameter of swirl concentrator
Figure 50 Site Requirements, Helical Bend
Combined Sewer Overflow
Regulator/Separator
HYDRAULIC HEAD LOSSES
Figure 51 Site Requiements, Swirl Concentrator
Combined Sewer Overflow
Regulator/Separator
The available head at a specific site may be a critical factor in the
choice of the specific type of combined sewer regulator to be used. The head
loss must be considered for two conditions: 1) For periods of dry-weather
flow, and 2) for periods of wet-weather flow. The available head during dry-
weather flow will depend on the difference in elevation between the combined
sewer and the interceptor that will convey the flow to the wastewater treat-
ment plant. The available head during wet-weather flow will depend on the
difference in elevation between the combined sewer and the water surface of
the receiving stream. A further consideration in the latter case is whether
the existing combined sewer is to be used to convey the overflow from the re-
gulator to the receiving stream or to any holding or treatment facilities
involved.
First, consider the case where there is to be no surcharge on the inlet
during design discharge.
93
-------
In the helical separator the transition will have a level top. The
drop in the invert of the transition will be 1 D. Therefore, the drop in the
invert from the inlet to the foul outlet will be 1 D (neglecting the slope of
the channel through the regulator). The loss in the hydraulic gradient will
also be 1 D. The invert of the clear outlet will be at approximately the
same elevation as the invert of the separator, as explained previously in the
discussion of the weir overflow spillway channel. Therefore, the drop in the
invert between the inlet and the clear outlet will also be 1 D. The loss in
the hydraulic gradient may be the same as the drop in the invert or it may be
slightly different depending on outlet design.
When the inlet sewer is surcharged different hydraulic conditions will
exist in the helical separator. If the inlet is surcharged an amount equal
to D, the transition invert will be level, as shown in Figure 45. The
drop in the invert from the inlet to the foul outlet will be zero (neglecting
the channel slope through the separator). The drop in hydraulic gradient
will also be zero. Likewise, the drop in the invert from the inlet to the
clear outlet will be zero. However, the loss in hydraulic gradient for this
case will be 1 D.
CONSTRUCTION DETAILS
Means of access must be provided to the curved section of the separa-
tor for maintenance purposes, including possible washing down after each
storm event. The provision of a superstructure over this section is desir-
able for safety and aesthetic reasons and for confining possible odors. The
type of superstructure used will depend on the character of the locality. As
a minimum and for purposes of this report, the walls are assumed to be of
concrete block and the roof of precast concrete units. For roof spans ex-
ceeding about 8.5 m (28 ft), it will be necessary to provide structural steel
framing. The facility could be constructed of poured concrete, Gunite or
fiber glass.
A cross section of the helical separator with a superstructure is shown
in Figure 52.
In this Figure, for cost estimating purposes, the width of structure is
indicated as 4.1 D and the width of spillway channel as 0.67 D. For any
specific case the width and elevation of the spillway channel may vary from
that shown in Figure 52 as explained previously.
The hydraulic conditions require that the transition section be pro-
vided with a roof. These conditions do not apply to the straight section,
having a length of 5 D, preceding the curved section. It is believed that
this section will not require the same maintenance as the curved section.
Accordingly, there appears to be no need to make this section accessible.
For purposes of costing it has been assumed that the straight section
will have walls 2.5 D high and will be provided with a concrete roof at that
elevation.
94
-------
Precast Concrete Roof
IVin.0.3m. (1ft
Figure 52 Typical Cross Section — Helical Bend Separator
Other construction details considered necessary or desirable are as
follows:
A. Provide concrete walls with a minimum thickness of 0.3 m (1 ft)
extending above grade a minimum height of 0.3 n (1 ft).
B. Coat all concrete surfaces with an epoxy paint. This will reduce
maintenance by decreasing deposition of solids on the walls of the
structure.
95
-------
C. Provide a concrete walk 1.2 m (4.0 ft) wide.
D. Provide a stainless steel railing on each side of the walk.
E. Provide a fiber glass scum baffle hung from the beams or supported
from the weir.
F. Provide a flushing water pipe on the channel side of the scum
baffle and hung from the beams. Connect this line to the public
supply with a backflow device if this is permitted by local code.
If this is not permissible, provide a storage tank to store over-
flow from the weir and a submersible pump to use for washing down.
The usual criteria of 3.1 I/sec (50 gpm) at 28.120 N sq m (40 psi)
for flushing purposes at treatment plants should be applicable to
the helical separator facility. Hose connections should also be pro-
vided in case the stream from the wash water pipe is not effective.
Provide additional nozzles at the end of the structure to assist in
the removal of floatables which will be concentrated at this loca-
tion as flow returns to dry-weather conditions.
G. Provide concrete block walls with a height of 2.4 m (8.0 ft).
H. Provide a precast concrete roof.
I. Provide adequate electric lights.
J. Provide roof ventilators.
K. Provide doors at both ends of the structure for ventilation and
access.
Cost estimates of the helical separator were made for two purposes:
1) to indicate the probable construction cost of the facility; and 2) to
compare its costs with that of the swirl separator used as a combined
sewer regulator.
The cost estimates are considered to be reasonable engineers' estimates.
However, during periods of economic inflation, it is not unusual for con-
tractors' bids to materially exceed engineers' estimates.
In making a choice between the helical separator and the swirl sep-
arator, it is possible that other factors such as space available or depth of
the combined sewer, related to the specific site of the facility, greatly
influence construction costs.
QUANTITIES COST ESTIMATE
The estimated quantities are based on the following:
A. The transition will be constructed with a drop in the invert equal
to D as shown in Figure 45 so that the sewer upstream of the tran-
sition will not be surcharged.
B. The straight section preceding the curved section will have walls
2.5 D high and concrete roof.
96
-------
C. The superstructure over the curved section will be as shown in
Figure 51.
D. The width of the curved section is assumed to be 4.1 D: the width
of the spillway channel is assumed to be 0.67 D.
E. The cover on the sewer at the transition inlet will be 2.44 m
(8 ft).
F. The ground is level and the subsurface is earth with no groundwater
problems.
G. All concrete walls will have a minimum thickness of 0.3 m (1 ft)
except the weir.
H. Sheet piling will be required about 0.6 m (2 ft) outside the
structure.
I. Transverse concrete beams will be required at 4.5 m (15 ft) intervals
with a cross section 0.45 m (1.5 ft) square.
J. The continuous concrete walk will be 1.22 m (4 ft) wide and 0.20 m
(0.67 ft) thick.
COST CALCULATION
The costs are based on the following:
A. The Engineering News-Record Construction Cost Index average for the
United States is 3140.
B. Unit prices are as follows:
Steel Sheet Piling $ 129/sq m $ 12/sq ft
Excavation $ 24/cu m $ 18/cy
Reinforced Concrete $ 490/cu m $375/cy
Concrete Block Walls $ 129/sq m $ 12/sq ft
Roof $ 150/sq m $ 14/sq ft
C. Miscellaneous costs are assumed to be 25 percent of the foregoing
items and to include a manual sluice gate and manhole handrail,
flushing water facilities, scum baffle, electrical work, roof
ventilators and doors.
D. The estimated cost of the bypass sewer during construction is based
on providing a sewer of the same diameter as a transition inlet
around the proposed separator, plus an allowance for temporary
connections at each end.
E. Contingent and engineering costs will be 35 percent of the fore-
going items.
Table 15 presents the estimated costs for units with 1.42, 2.83 and
4.67 cu m/sec (56, 100 and 165 cfs) flow.
97
-------
Table 15
Construction Cost of Helical Bend Separator
Construction Cost of Helical Bend Regulator
Capacity 1.42 m3/s (50 cfs)
Item
Sheet piling
Excavation
Reinforced concrete
Concrete block walls
Roof
Miscellaneous costs
Bypass sewer
Contingent and engineering costs
Capacity 2.83m3 (100 cfs)
Item
Sheet piling
Excavation
Reinforced concrete
Concrete block walls
Roof
Miscellaneous costs
Bypass sewer
Contingent and engineering costs
Capacity 4.67 m3/s (165 cfs)
Item
Sheet piling
Excavation
Reinforced concrete
Concrete block walls
Roof
Miscellaneous costs
Bypass sewer
Contingent and engineering costs
Quantity
420m2
950m3
250m3
114m2
85m2
(4,550 sf)
d,240cy)
( 330 cy)
(1,230 sf)
( 910 sf)
Subtotal
25%
Subtotal
35%
Total
Quantity
710m2
2,200 m3
475m3
160m2
170m2
(7,700 sf)
(2,800 cy)
( 620 cy)
(1,740sf)
(1,800sf)
Subtotal
25%
Subtotal
35%
Total
Quantity
950m2
3,200 m3
679m3
200 nm2
250m2
(10,200 sf)
(4,180 cy)
( 888 cy)
(2,130sf)
(2,700 Sf)
Subtotal
25%
Subtotal
35%
Total
Amount
$ 54,600
22,320
123,750
14,760
18,200
233,630
58,400
30,000
322,030
112,710
$434,740
Amount
$ 92,400
50,400
232,500
20,880
25,200
421,380
105,350
58,300
585,030
204,760
$789,790
Amount
$122,400
75,240
333,000
25,560
37,800
594,000
148,500
83.700
826,200
289,170
$1,115,370
98
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PROTOTYPE
The helical bend separator was tested extensively in Nantwich, England.
The first full size unit has been built in Boston, Massachusetts. A purpose
of the demonstration project is to compare the efficiency of the unit as
compared to a swirl separator/regulator. The unit will be tested on combined
sewer overflows and stormwater discharges. Test results were not available
at the time of preparation of this manual. Construction costs were very low
due to the fact that the unit was prefabricated from wood and is not intended
for permanent use. Figure 53 shows the completed facility in place. Design
details are given in Table 16.
Figure 53 Helical Bend Regulator/Separator Prototype, Boston, MA
99
-------
Table 16
Design Details — City of Boston
Helical Bend Combined Sewer Overflow
Regulator/Separator Prototype
Inlet diameter m 0.45
(ft) (1-5)
Overall length m 18.3
(ft) (60)
Outlet diameter
Overflow m 0.6
(ft) (2)
Weir length m 9.15
(ft) (30)
Outlet to plant diameter m 0.24
(ft) (0.66)
Design flow l/s 170
(cfs) (6)
Maximum flow l/s 340
(cfs) (12)
Maximum underflow l/s 7.6
(cfs) (0.27)
100
-------
Section IV
COMPARISON OF SWIRL REGULATOR/SEPARATOR
AND HELICAL BEND COMBINED SEWER OVERFLOW REGULATOR/SEPARATOR
Design and cost information have been presented in the preceding two
sections for different types of combined sewer overflow regulators. Both
regulators have been designed to accomplish both the control of quantity and
quality of the discharge to receiving waters. This section has been prepared
to offer a basis for comparing the two separators.
It appears that the principal advantages of the helical bend separator
are the low head requirements and the discharge to treatment of the captured
solids at the end of the storm event. The swirl regulator/separator in turn
requires less space and should be less expensive to construct. Use of the
swirl regulator/separator where insufficient hydraulic head is available for
its normal mode of operation may require dry weather bypassing the device.
As both units have been designed to minimize the cost of operation and
maintenance problems, they are considered comparable.
SITE REQUIREMENTS
The site requirements for both the helical and the swirl separator are
shown in Figure 50. The required lot dimensions and area for three sizes of
each facility are shown in Table 17.
The site dimensions are based on a helical separator to remove 100
percent of grit and a swirl separator to remove 90 percent of grit.
It is evident from Table 17 that the site requirements for the helical
bend are greater than for the swirl separator and that the larger the design
flow the greater the difference. For the design flows of 1.42 cu m/sec
(50 cfs), 2.83 cu m/sec (100 cfs), and 4.67 cu m/sec (165 cfs); the helical
bend requires a site 63 percent, 115 percent, and 145 percent greater,
respectively, than the swirl separator.
HEAD LOSSES
A discussion of the computation of head losses for each regulator has
been presented in the previous sections.
In the following comparisons, the head losses are given as a multiple
of the inlet dimension: D, the inlet diameter of the helical separator; and
DI, the side of the square inlet of the swirl separator. To show that D and
D are approximately the same for the same discharge, their values for
three discharges are given in the following section on Design.
101
-------
Table 17
Site Dimensions and Areas for Helical Bend and
Swirl Regulator/Separator
Swirl Regulator
Capacity 1.42 cu m/s (50 cfs)
Site size 38.0 m x 38.0 m
(124.5 ft x 124.5 ft)
Site area 1,440sqm
(15,500 sf)
Relative area 1.00
Capacity 2.83 cu m/s (100 cfs)
Site size 40 m x 40 m
(131.5 ft x 131.5 ft)
Site area 1,600sqm
(17,300 sf)
Relative area 1.00
Helical Separator
43.0 m x 54.6 m
(141 ft x 179 ft)
2,340 sq m
(25,200 sf)
1.63
48.5 m x71.5 m
(159 ft x 234 ft)
3,460 sq m
(37,200 sf)
2.15
Capacity 4.67 cu m/s (165 cfs)
Site size 42 m x 42 m
(138 ft x 138 ft)
Site area 1,770sqm
(19,000 sf)
Relative area 1.00
52.0 m x 82.8 m
(171 ft x 272 ft)
4,300 sq m
(46,500 sf)
2.45
DESIGN
First, consider the case where there is to be no surcharge on the inlet
during design discharge.
Discharge
cu m/sec
(cfs)
m
(ft)
m
(ft)
1.42
(50)
1.07
(3.5)
0.90
(3.0)
2.83
(100)
1.52
(5.0)
1.52
(5.0)
4.67
(165)
1.83
(6.0)
1.83
(6.0)
In the helical regulator the transition will have a level top. The
drop in the invert of the transition will be 1 D. Therefore, the drop in the
invert from the inlet to the foul outlet will be 1 D (neglecting the slope of
the channel through the regulator). The loss in the hydraulic gradient will
102
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also be 1 D. The invert of the clear outlet will be at approximately the
same elevation as the invert of the separator, as explained previously in the
discussion of the weir overflow spillway channel. Therefore, the drop in the
invert between the inlet and the clear outlet will also be 1 D. The loss in
the hydraulic gradient may be .the same as the drop in the invert or it may be
slightly different depending on outlet design.
In the swirl regulator, if there is to be no surcharge on the inlet
sewer, the crown must be at a distance above the invert of the chamber equal
to H, (the height of weir above the chamber invert), plus the head on the
weir. The drop in the invert of the sewer will be this distance less Di , the
dimension of the inlet. The foul outlet is located below the chamber bottom.
Assuming a foul outlet diameter of 0.31 m (1 ft) and concrete cover over the
outlet to the same amount, the distance from the chamber invert to the outlet
invert is 0.61 m (2 ft).
Excluding the channel slope through the separator, the drop in the
invert from the inlet to the foul outlet is, therefore, 0.8 DI to 1.5 D, ,
plus 0.61 Hi (2 ft). The foul outlet pipe diameter may exceed 0.31 m (1 ft)
diameter for larger flows, thus increasing the total drop somewhat. The
hydraulic gradient will have a similar drop.
The clear outlet is also located below the chamber floor and, if a
0.31 m (1 ft) concrete cover is provided over the outlet, the vertical
distance from the chamber invert to the invert of the clear outlet will be
1 D-^, plus 0.31 m (1 ft). Combining this with the entrance drop of 0.8 D-^
to 1.5 D-^, will result in a total drop in the invert from the inlet to the
clear outlet of 1.8 D^ to 2.5 D^, plus 0.31 m (1 ft). The drop in the
hydraulic gradient in this case will be different. The circular weir is
set a distance -equal to the head on the weir below the top of the inlet
sewer. If there is no submergence of the weir then the loss in the
hydraulic gradient will be equal to this head. Trial computations indicate
the head on the weir is about 0.2 D-, . Allowing for friction losses in the
outlet pipe and some freeboard downstream of the weir, the drop in hydraulic
gradient is about 0.4 D^.
When the inlet sewer is surcharged, different hydraulic conditions will
exist in the helical separator. If the inlet is surcharged an amount equal
to D, the transition invert will be level; as shown in Figure 45, the drop
in the invert from the inlet to the foul outlet will be zero (neglecting the
channel slope through the separator). The drop in hydraulic gradient will
also be zero. Likewise, the drop in the invert from the inlet to the clear
outlet will be zero. However, the loss in hydraulic gradient for this case
will be 1 D.
In the case of the swirl separator if a surcharge of D-, is permitted,
then the crown of the sewer can be set a distance of D-^ below the water
surface of the chamber. The drop from the chamber invert to the foul outlet
will be 0.61 m (2 ft), as previously computed. Therefore, the drop in the
103
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invert from the inlet to the foul outlet will be 0 to 0.5 Di , plus
0.61 m (2 ft). The drop in hydraulic gradient will be the same. The drop
from the chamber invert to the clear outlet invert will be 1 Di, plus
0.31 m (1 ft), as before. Therefore, the total drop from the inlet invert to
the clear outlet invert will be 1 D-, to 1.5 D-^ plus 0.31 m (1 ft). The drop
in hydraulic gradient will be about 0.4 D^ as computed previously.
The data relative to the foregoing discussion are shown in Table 18.
Table 18
Typical Head Losses in Helical Bend and Swirl Regulator/Separator
Dry-weather flow — drop in invert
Helical separator
Transition invert level
Transition roof level
Swirl regulator
Wet-weather flow
Helical separator
Transition invert level
Hydraulic grade
Drop in invert
Transition roof level
Hydraulic grade
Drop in invert
Swirl regulator
Hydraulic grade
Drop in invert
Note: Friction losses not included in above table
Swirl
Helical Concentrator
Separator w/o Surcharge
none
1 D
1 D
none
1 D
1 D
0.8to1.5Dj+ 61cm(2ft)
Surcharged
NA
°-4Dl
1.8to2.5D]+ 30cm(1ft)
0.4Dj
1 to 1.50!+ 30cm(1ft)
From the Table, it is apparent that the drop in the invert is always
greater in the swirl separator than in the helical.
When the inlet sewer is not surcharged, the drop to the foul outlet is
only slightly greater, but the drop to the clear outlet is about twice as
great. When the inlet sewer is surcharged an amount equal to D or D\> the
drop in the invert is zero in the helical separator, compared to the minimum
drop of 0.61 m (2 ft) to the foul outlet and of 1 D,, plus 0.31 m (1 ft) to
the clear outlet in the swirl separator. For dry-weather flows, the drop
in hydraulic gradient is similar to the drop in the invert. For wet-weather
flows, the drop in hydraulic gradient from the inlet to the clear outlet in
the swirl separator is about one-half that for the helical separator.
104
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CONSTRUCTION COSTS
Utilizing the cost data developed in Sections II and III, Table 19
compares construction costs for flows of 1.42, 2.83 and 4.67 cu m/sec
(50, 100, and 165 cfs).
Table 19
Comparison of Costs of Helical Bend and Swirl Regulator/Separator
Flow Capacity
Efficiency
cu m/s cfs
1.42 50
2.83 100
4.67 165
Helical Bend Swirl Regulator
100% 90%
370,000
687,000
972,000
152,000
246,000
340,000
Note: Cost based upon total project — land cost not included
These costs are shown graphically in Figure 54 as explained in
Section II and III. Although the two regulators are sized for the same
discharge, the helical separator will remove 100 percent of the grit compared
to 90 percent for the swirl regulator, the usual design efficiency.
The costs of swirl regulator to remove 100 percent of grit were
estimated and the results shown in Figure 54.
i
r 4
o
u
Helical separator
100% removal
Swirl separator
100% removal
Swirl separator
90% removal
SO
1.4
100
2.8
150
4.2
200
5.6
els
Discharge
Figure 54 Estimated Construction Costs Helical Bend and Swirl Regulator/Separator
105
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SECTION V
SWIRL DEGRITTER
The swirl degritter was originally conceived as an auxiliary combined
sewer overflow treatment device to protect pumps, wet wells and downstream
facilities from the large amount of grit removed by the swirl separator/
regulator. Accordingly, the device was designed without moving parts for
ease in unattended operation and maintenance. Conventional grit washing and
removal equipment was deemed the most practical method of removing collected
grit from the swirl. The unit's high efficiency and essential lack of moving
parts has made its use suitable for use with stormwater, sanitary sewage,
and raw potable water.
DESCRIPTION
An isometric view of the swirl degritter is shown in Figure 55.
The principal difference in the configuration is the shape of the floor.
A conical hopper is required to concentrate the solids for discharge. The
principal features of the unit include:
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 the flow, inlet dimension, and chamber width. The flow is
directed tangentially so that a "long path" pattern, maximizing solid
separation in the chamber, may be developed.
B. Deflector: 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.
106
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B
Inlet
Deflector
Weir and weir plate
Spoiler
Floor
Conical hopper
Figure 55 Isometric View, Swirl Degritter
107
-------
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.
The weir diameter is equal to two-thirds 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 times 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 flow which may be anticipated if the unit is used on a
continuous basis.
E- Floor; The floor of the unit is level and is in effect a shelf,
the width of the inlet.
F. Conical Hopper: The conical 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 one
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 minimum hydraulic interference for
settling particles.
Figure 56, General Design Dimensions, lists the various important
dimensions, which are given as a function of the inlet diameter, D-^. The
ratio of D2 to Dj_ is given on Figures 57 to 59-
FACILITY FACTORS TO BE CONSIDERED
Before using the swirl concentrator as a degritter, designers
should make a comparison of the various alternatives. For large flows the
swirl degritter with a cone-shaped hopper may require a depth greater than
the more conventional grit chambers. The presence of high groundwater or bed
rock may affect cost estimates appreciably if the deeper structure is used.
108
-------
• Downpipt
tlbcm
ELEVATION
Section A-A
Note: Inlet and foul outlet should be parallel
for optimum hydraulic conditions within conical hopper.
D3 = 2/3 D2
D4 = D1 (larger will spoil flow outside pipe)
H, = 2 D,
H 2 = 1/4 D,
H3 = D, or larger
Inlet transition = 3 D,
D, and D2 to be determined from design curves, Figs. 57-59
Figure 56 General Design Dimensions, Swirl Degritter
Another major factor is the head available and the effect of the swirl
degritter 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 degritter 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 con-
structed so that the removal of grit can be continued when one unit is taken
out of service if the unit is used at the wastewater treatment plant. For
combined sewer overflow use, only one unit should be considered.
When used at a wastewater treatment plant 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. For combined sewer
overflow use it would appear desirable to bypass the dry-weather flow
and not use the degritter to prevent septic conditions from developing.
109
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80
70
60
50
40
30
20
jft
"o
Q>
S>
o
I 10
8
D1 = 1.2m(4tt)
+ -—
D,= 1.07m (3.5 ft)
,= 0.45m (1.5 ft)
, = 0.3 m (1 ft)
Note: 1 cfs = 28.32 l/s
6 78 9 10 11 12
Ratio D2/D,
Figure 57 D2/D, vs Discharge for 95% Efficiency
110
-------
100
80
60
50
40
30
20
E>
1C
w 10
_ D, = 1.2m (4 ft)
Note: 1 cfs = 28.32 l/s
D1= 1.07m (3.5 ft)
D, = Q.9m (3 ft)
= 0.76m (2.5 ft)
D, = 0.6 m (2 ft)
= 0.45m (1.5 ft)
D, = 0.3m (1ft)
_L
8 9 10 11 12
Ratio D2/D,
Figure 58 D2/D, vs Discharge for 90% Efficiency
111
-------
0)
(0
u
CO
90
70
60
50
30
20
10
8
7
6
5
4
3
-D,= 1.2m(4ft)
Note: 1 cfs = 28.32 l/s
D,=J.07m(3.5ft)
Di=J).9m(3fti
D, = 0.76 m (2.5 ft)
^__^--— •
D, = 0.6m(2ft)
, ——•
D,= 0.45m (1.5 ft)
= 0.3m (1ft)
J I
9 10 11 12
Ratio D2/D,
Figure 59 D2/D, vs Discharge for 80% Efficiency
112
-------
The area of the space above the screw should be designed so that the
velocity of the wash water flowing upward will be between 0.045 to
0.075 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 of the grit washer 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 cu m/min/sq m (13.5 gpm/sq ft).
DESIGN
The following sequence is recommended for the design of the swirl
degritter.
1. Select Design Discharge: The design engineer must select the
design discharge appropriate to each project based on the design
criteria for the project.
One application of the swirl degritter 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 degritter considered in connec-
tion with this study was as a grit removal device for the foul
flow from a swirl regulator/separator. In that case the
design flow for the grit chamber should be based on the foul flow
discharge from the overflow regulator. A third application would be
as a combined sewer overflow or stormwater treatment plant unit
process. The swirl unit can also be used where grit is a problem
prior to syphons or pumping stations within the collection system.
Therefore, the designer must select the design flow based on the
particular application.
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, D,: Having selected the desired
recovery rate and the design discharge, the corresponding figure in
the series of Figures 57 to 59 would be used. Enter the figure with
the design discharge and go horizontally to the curve which most
closely represents the supply sewer diameter. It might be advantageous
to select a larger or smaller D-^ 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.
113
-------
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 D-, (3D-,). 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 swirl degritter. The
distance from this manhole offtake to the square inlet discharge in
the chamber should also be a minimum of 3 times Di (3D,). This
arrangement could be used to provide the transition in directions,
levels or sizes between the supply sewer and the square inlet.
It may be noted that a smaller diameter of inlet sewer, D^, results
in a higher inlet velocity, a larger diameter, D2? and a smaller
chamber depth, H,.
4. Find Grit Chamber Diameter, D ; The intersection point found in
(3) above defines the chamber diameter, D2 on the abscissa scale,
inasmuch as D-^ is known and the scale is the ratio D2/D]. • In the
consideration for choosing D^ it might be a valuable aid to check the
D£ size as well. Taking a smaller D^ means a larger D2 is necessary;
there could well be an economical or practical optimum relation
between the two dimensions.
5. Determine Head Over Weir: Using Figure 60, determine head over
the weir by entering the flow and proceeding to the appropriate
DT/DI curve.
6. Example: Assume the designer decides to remove 90 percent of the
grit over an effective diameter of 0.2 mm size: The average daily
sanitary sewage flow varies from 85 to 425 I/sec (3 to 15 cfs) with
an inlet pipe diameter of 61 cm (2 ft).
Enter Figure 58 with 425 I/sec (15 cfs)
at: D-^ = 61 cm (2 ft), D2/D, = 8
or: D2 = 4.88 m (16 ft)
On Figure 60, the intersection of 0.43 cu m/sec (15 cfs) and the
D /D curve for 8 lies wi
is about 24 cm (0.78 ft).
D /D curve for 8 lies within the curves and the head on the weir
Interpolation between curves on Figures 57 through 59 can be
done without extreme care as slight changes in the ratio are not
critical to the structures.
7. Find Dimensions of Complete Unit Using D and E-; Use Figure 56
to compute dimensions of all pertinent elements in 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.
114
-------
em
60
50
a 40
I30
a
a
b
|20
^
o
•o
o
£ 10
ft
I 8
16
1.4
12
- 10
OB
0.6
04
0.2
D2/D, .
_L
10 20 30 40 50 60 70 80 cfs
0.5
1.0 I 5
Discharge
2.0
2 5 mVs
Figure 60 Approximate Stage and Discharge Curves over Weir
CONSTRUCTION COST
For comparative purposes estimates were made of construction and
annual operation costs of the swirl degritter and the standard aerated grit
chamber. Estimates were made for three sizes of each type for average flows
of 43.8, 131.4 and 438 I/sec (1, 3, and 10 mgd). Present worth was deter-
mined for each size and type based on a 20 year period and 7-1/8 percent
interest rate.
The principal diameter of the chamber, D was obtained from Figure 58
for 90 Percent Recovery and H /D = 2, using a ratio of D9/D-, of 6. The
remaining dimensions were obtained from Figure 56. The derived dimensions
are as shown in Table 20.
The type of unit used for estimate purposes was similar to that shown
in Figure 61, with the following revisions: (1) the exterior wall of the
grit separator was assumed to be of concrete with a vertical exterior face,
(2) a horizontal passage through the concrete assumed to provide access for
lubricating the bottom fitting of the inclined screw conveyor and (3) a
manhole, 0.91 m (3 ft) sq, was provided to give access to the bottom fitting
of the screw conveyor.
115
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INLET
CRIT
CHAMBER
WASHWATER
OVERFLOW WEIR
WASHWATER
OUTLET
CRIT WASHER
AND ELEVATOR
SECTION A-A
Figure 61 Grit Chamber with Inclined Screw Conveyor
116
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Table 20
Swirl Degritter Dimensions for 3 Flowrates
43.8 I/S
(1 mgd)
2.67m
(8.75 ft)
0.30 m
(1.0ft)
1.22m
(4.0 ft)
0.61m
(2.0 ft)
0.08 m
(0.025 ft)
0.30m
(1.0ft)
131 .4 l/s
(3 mgd)
3.90 m
(12.8 ft)
0.40 m
(1.33ft)
1.62m
(5.33 ft)
0.81m
(2.67 ft)
0.10m
(0.33 ft)
0.40 m
(1.33 ft)
438 l/s
(10 mgd)
5.95 m
(19.5ft)
0.71 m
(2.33 ft)
2.85m
(9.34 ft)
1.42m
(4.67 ft)
0.17m
(0.58 f)
0.71 m
(2.0 ft)
Average flow
D,&D4
H,
H2
H, min
Cost estimates of the swirl degritter were made for two purposes:
(1) to indicate the probable construction cost of the facility; and (2) to
compare its cost with that of a conventional aerated grit chamber.
The cost estimates are considered to be reasonable engineer's esti-
mates. However, during periods of economic inflation, it is not unusual for
contractor's bids to materially exceed engineers' estimates.
Cost Basis
The costs are based on the following:
A. Engineering News Record Construction Cost Index average for
United States is 3140.
B. Unit prices as follows:
Steel Sheet Piling $129/sq m $12/sq ft
(for temporary use during construction)
Excavation $ 24/cu m $18/cy
Reinforced Concrete $490/cu m $375/cy
C. Contingent and engineering costs are assumed to be 35 percent
of the foregoing items.
The swirl degritter dimensions are derived in the previous section. It
is assumed that the ground surface is 0.61 m (2 ft) above the crown of the
inlet pipe and the top of tank is 0.30 m (1 ft) above the crown of the inlet
pipe, this will provide 0.61 m (2 ft) of freeboard above the weir.
Aerated Grit Chamber
The aerated grit chamber was sized to provide a detention period of
3 minutes at the maximum rate of flow. Peak flow factors were based on
Figure 4 in American Society Civil Engineers Manual No. 37 (16). The
resultant dimensions are shown in Table 21.
117
-------
The conventional aerated grit chamber is set to provide a freeboard
0.46 m (1.5 ft) with a top of wall 0.30 m (1 ft) above ground surface.
The following assumptions are made for both structures:
A. Excavation is all earth. The unit price includes cost for back-
filling and crushed stone under the structures.
B. Temporary steel sheet piling is required to 0.61 m (2 ft) outside
the exterior walls of the structures. Sheeting assumed to extend
0.61 m (2 ft) below lowest point of excavation and 0.30 m (1 ft)
above the existing ground elevation.
C. Equipment costj for .the aerated grit chamber include the cost of
bucket elevator, screw conveyor, transverse baffle, diffuser
piping, motors, and electrical work.
D. Miscellaneous costs for the aerated grit chamber include the cost
of the longitudinal and effluent baffles, compressors, slide gates,
baffle supports, and grating for by-pass channel.
E. Equipment costs for the swirl degritter include the cost of a grit
wash screw.
F. Miscellaneous costs for the swirl degritter includes the cost of
piping, skirt, weirs and plates.
Table 21
Aerated Grit Chamber Dimensions for 3 Flowrates
Average flow
Peak flow factor
Maximum flow
Required volume
Selected depth
Selected width
Selected length
Selected volume
43.8 I/sec
(I mgd)
3.0
131.4 I/sec
(3 mgd)
23.6 cu m
(835 cf)
2.44m
(8.0 ft)
2.29 m
(7.5 ft)
4.27 m
(14.0 ft)
23.65 cu m
(835 cf)
131.4 I/sec
(3 mgd)
2.5
328.5 I/sec
(7.5 mgd)
59.2 cu m
(2,090 cf)
3.05m
(10.0 ft)
3.05 m
(10.0 ft)
6.41 m
(21.0 ft)
59.08 cu m
(2,085 cf)
438 I/sec
(10 mgd)
2.0
876 I/sec
(20.0 mgd)
157.9 cu m
(5.560 cf)
3.66 m
(12.0)
4.27m
(14.0 ft)
10.06 m
(33.0 ft)
157.09 cu m
(5.544 cf)
Cost of Swirl Degritter
The estimated construction cost of a swirl degritter with a capacity of
43.8 I/sec (1 mgd) is $72,980, for 131.4 I/sec (3 mgd), $84,090, and for
118
-------
438 I/sec (10 mgd), $97,830. The breakdown of these costs is shown in
Table 22.
Cost of Aerated Grit Chamber
The estimated construction costs of a conventional aerated grit chamber
with a capacity of 43.8 I/sec (1 mgd) is $87,240, for 131.4 I/sec (3 mgd),
$112,530, and for a 438 I/sec (10 mgd), $155,650, as shown in Table 23.
OPERATION AND MAINTENANCE COSTS
The estimated operation and maintenance costs for the swirl degritter
and the aerated grit chamber for capacities of 43.8 I/sec (1 mgd), 131.4 I/sec
(3 mgd) and 438 I/sec (10 mgd) are shown in Table 24. For units with capa-
city of 43.8 I/sec ( 1 mgd) the annual expenses are estimated at $7,020
for the aerated chamber and $6,355 for the swirl degritter. For capacity of
131.4 I/sec (3 mgd) the annual expenses are $11.720 for the. aerated chamber
and $10,450 for the swirl degritter. For capacity of 438 I/sec (10 mgd) the
annual expenses are $22,280 for the aerated chamber and $18,630 for the
swirl degritter.
The operator labor is assumed to be 1.5 hours per day for the
131.4 I/sec (3 mgd) unit. This assumes 1 hour for operation of the equip-
ment and 0.5 hours for disposal of the grit. This is based on the actual
experience at a unit with the capacity where the daily operation ranges from
0.5 to 1 hours with occasional periods of 1.5 hours following storm periods.
The labor rate used of $10.00 per hour is intended to include the actual
labor cost plus all benefits but excludes administration and general expenses
of the overall plant.
Based on the results shown in Table 24, the annual operation costs of
the aerated grit chamber will exceed the annual costs of the swirl degritter
by about 10 percent for each size unit.
Present Worth
The present worth of the grit removal units is shown in Table 25. The
present worth is based on a life of 20 years and an interest rate of 7-1/8 per-
cent. Hence the present worth of the operation and maintenance costs for a
20 year period is 10.49 times the annual cost.
For the unit with capacity of 43.8 I/sec (1 mgd) the present worth of
the aerated chamber is $160,940 and the swirl degritter is $139,980. Thus
the present worth of the aerated chamber is 15 percent greater than that of
the swirl degritter.
For the unit with capacity of 131.4 I/sec (3 mgd) the present worth of
the aerated chamber is $235,530 compared to $194,090 for the swirl de-
gritter. Thus the present worth of the aerated chamber is 20 percent greater
than that of the swirl degritter.
119
-------
Table 22
Construction Cost of Swirl Degritter
Item
Capacity 43.8 l/s (1.0 mgd)
Sheet piling
Excavation
Reinforced concrete
Equipment
Miscellaneous and bypass
Subtotal
Contingent and engineering costs
Total
Capacity 131.4 l/s (3.0 mgd)
Sheet piling
Excavation
Reinforced concrete
Equipment
Miscellaneous and bypass
Subtotal
Contingent and engineering costs
Total
Capacity 438 l/s (10.0 mgd)
Sheet piling
Excavation
Reinforced concrete
Equipment
Miscellaneous and bypass
Subtotal
Contingent and engineering costs
Total
Quantity
72 sq m
(780 sq ft)
115 cu m
(150 cy)
12 cu m
(16 cy)
Job
job
35%
Amount
$ 9,360
2,700
6,000
24,700
11,300
54,060
18,920
$72,980
89 sq m
(960 sq ft)
142 cu m
185 cy
15 cu m
(20 cy)
Job
Job
35%
$11,520
3,330
7,500
27,380
12,560
62,290
21,800
$84,090
102 sq m
(1,100sqft)
184 cu m
(240 cy)
20 cu m
(26 cy)
Job
Job
35%
$13,200
4,320
9,750
31,400
13,800
72,470
25,360
$97,830
120
-------
Table 23
Construction Cost of Aerated Grit Chamber
Item
Capacity 43.8 l/s (1.0 mgd)
Sheet piling
Excavation
Reinforced concrete
Equipment
Miscellaneous
Subtotal
Contingent and engineering costs
Total
Capacity 131.4 l/s (3.0 mgd)
Sheet piling
Excavation
Reinforced concrete
Equipment
Miscellaneous
Subtotal
Contingent and engineering costs
Total
Capacity 438 l/s (10.0 mgd)
Sheet piling
Excavation
Reinforced concrete
Equipment
Miscellaneous
Subtotal
Contingent and engineering costs
Total
Quantity
67.5 sq m
(725 sq ft)
78 cu m
(101 cy)
11 cu m
(14cy)
Job
Job
35%
Amount
$ 8,700
1,820
5,250
38,680
10,170
64,620
22,620
$ 87,240
98 sq m
(1066 sq ft)
99 cum
(127cy)
21.2 cu m
(27 cy)
Job
Job
35%
$ 12,800
2,290
10,120
45,720
12,430
83,360
29,170
$112,530
157 sq m
(1,710sqft)
276 cu m
(361 cu m)
34.2 cu m
(44.7 cy)
Job
Job
35%
$ 20,500
6,500
16,800
56,500
15,000
115,300
40,350
$155,650
121
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Table 24
Operation and Maintenance Costs for Grit Removal
Capacity 43.8 l/s (1.0 mgd) Aerated Swirl
Chamber Separator
Labor
Operation 1.5 hr/day at $10/hr $ 5,480 $ 5,480
Maintenance 0.2 hr/day at $10/hr 730 730
Materials and supplies 250 130
Power
1 Compressor at 1 hp, 24 hr/day x $0.06/kwh 530 —
1 Screw conveyor at Vt hp, 1 hr/day x $0.06/kwh 15 15
1 Bucket conveyor at 1/2 hp, 1 hr/day x $0.06/kwh 15 —
Total Annual Costs $ 7,020 $ 6,355
Capacity 131.4 l/s (3.0 mgd)
Labor
Operation 2.5 hr/day at $10/hr $9,130 $9,130
Maintenance 0.3 hr/day at $10/hr 1,100 1,100
Materials and supplies 380 190
Power
1 Compressor at 2 hr, 24 hr/day x $0.06/kwh 1,050 —
1 Screw conveyor at Vi hp 2 hr/day x $0.06/kwh 30 30
1 Bucket conveyor at Vi hp, 2 hr/day x $0.06/kwh 30. —
Total Annual Costs $11,720 $10,450
Capacity 438 l/s (10.0 mgd)
Labor
Operation 4.5 hr/day at $10/hr $16,430 $16,430
Maintenance 0.5 hr/day at $!0/hr 1,830 1,830
Materials and supplies 750 310
Power
1 Compressor at 6 hr, 24 hr/day x $0.06/kwh 3,150 —
1 Screw conveyor at Vi hr, 4 hr/day x $0.06 kwh 60 60
1 Bucket conveyor at 1/z hp, 4 hr/day x $0.06 kwh 60 —
Total Annual Costs $22,280 $18,630
Capacity 43.8 l/s (1.0 mgd)
Table 25
Present Worth Grit Removal Units
Aerated
Chamber
Construction cost $ 87,240
Operation and maintenance cost 73,700
Cost Total Present Worth $160,940
Capacity 131.4 l/s (3.0 mgd)
Construction cost
Operation and maintenance cost
Total Present Worth
Capacity 438 l/s (10.0 mgd)
Construction cost
Operation and maintenance cost
Total Present Worth
$112,530
123,000
$235,530
$155,650
233,300
$388,950
Swirl
Degritter
$ 72,980
67,000
$139,980
$ 84,090
110,000
$194,090
$ 97,830
195,000
$292,830
122
-------
For the 438 I/sec (10 mgd) unit, the present worth of the aerated
chamber is $388,950 compared to $292,830 for the swirl degritter, or
33 percent greater.
PROTOTYPE INSTALLATIONS
Three units have been constructed. Results of the extensive testing of
the unit at the Metropolitan Denver Sewage Disposal District No. 1 have been
published. (5) The unit was tested using sanitary sewage and sewage spiked
with fine sand. A swirl degritter has been constructed at Lancaster,
Pennsylvania, to remove grit from the tank underflow of a swirl regulator/
separator to protect downstream pumping facilities. This unit is identical
in construction to the Denver unit. Operating results are not presently
available.
A third unit has been constructed in the City of Tamworth, New South
Wales, Australia. The unit has been designed to protect raw water treatment
.equipment. Again, operating results are not yet available.
Details of the three installations will be reviewed.
1. Metropolitan Denver Sewage Disposal District No. 1, Denver,
Colorado. The test program was designed to determine the grit
removal efficiency of the test system and to compare the results
with the removal performance of the plant's conventional aerated
grit chamber (AGC). Figure 62 shows the layout for the test pro-
gram and Figure 63 is a plan of the test unit while Figure 64
is a photograph of the test installation.
The 43.8 I/sec (1 mgd) swirl degritter was constructed in 1974 at
a cost of $4,500 exclusive of pumps, valves, and grit washer ele-
ments which the district had available. The cost of a conventional
AGC for the same flow was estimated to be $57,000.
Extended tests were made at flows of 43.8, 87.6 and 131 T I/sec
(1, 2, and 3 mgd). Grit ash was used as a basis of efficiency
comparison. Grit ash was used as it represents the inorganic,
heavier material that a grit chamber is designed to remove. The
testing program found that the recovery of grit less than 0.2 mm
was 10 percent or less. Therefore, tests were run with the flow
spiked with fine blasting sand (0.25 mm diameter) at concentrations
ranging from 288 gm/cu m (2,400 Ib/MG) at flows of 21.9 I/sec (0.5 mgd)
to 48 gm/cu m (400 Ib/MG) at flows of 131.4 I/sec (3 mgd).
It was found that the percent dry grit removal in the AGC for raw
sanitary sewage was consistantly higher (77.3 percent) than
accumulated in the swirl degritter (66.4 percent), the AGC retained
an undesirably higher percentage of organic particles (volatile
solids) than the swirl (12-30 percent for the AGC as compared to
3-10 percent for the swirl). The swirl degritter when tested under
conditions which might be encountered in removing grit from a
combined sewer overflow and overflow concentrate had a removal
efficiency of 50 to 87 percent.
123
-------
Plant
Influent
Point for adding
sand (spiking)
Parshall
Flume
Aerated
Influent
Channel
Aerated
Grit
Chambers
Pump
Chasick
Sampler
No. 3
Effluent to
Primary Tanks
Swirl
Degritter
Chasick
Sampler
No. 2
Figure 62 Layout for Denver Tests
124
-------
48.3cm
Inlet
B
Leg position
I
J J Outlet for effluent to Chasick Sampler
Plan
Figure 63 Plan of Swirl Degritter, Denver
125
-------
Figure 64 Photograph, Denver Test Facility
The swirl degritter remained effective at flows twice the
design flow with a marked decrease in performance at three
times design flow, which is similar to other types of grit
removal facilities.
The residual concentration of grit in the effluent is a major
factor in evaluating performance. Limited studies of wastewater
treatment plants have shown concentration of grit removal in all
subsequent treatment processes.
Thus there is some, as yet undefined, concentration in the
effluent which can be tolerated. The results of the Denver
tests tend to point to a relationship whereby with an increase
in grit concentration in the influent an increase in the effi-
ciency ratio may also be observed.
126
-------
Lancaster, Pennsylvania. The swirl degritter, of the same size
as Denver, was constructed to remove grit from the foul under-
flow from the swirl regulator/separator. Modeling studies
prior to construction indicated for the six storms studied a
maximum suspended solids concentration of 380 kg/cu m (172 Ib/MG)
could be expected. The swirl degritter was built to protect a
wet well and pump needed to discharge the foul flow into the
interceptor sewer. Dry weather flow is diverted directly to
the wet well and is not treated.
The sampling program at Lancaster has been delayed and detailed
results are not yet available. One important operating factor
that is readily apparent is the need to provide a means to
easily dewater the facility when it is not in use. An
intermittant use, such as Lancaster, will have many long periods
when there is no flow and the contents of the chamber will
rapidly become septic.
Tamworth, New South Wales, Australia. The Department of Public
Works of New South Wales has recently constructed a 5 m
(16.4 ft) diameter chamber to remove grit from a raw river water
supply subject to intermittant high solid loads from a normally
dry water course. Again details of the operating performance
are not now available.
127
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Section VI
SWIRL PRIMARY SEPARATOR
DESCRIPTION
The swirl primary separator was designed to incorporate the use of
secondary motions to hasten the settling of suspended solids to achieve
primary treatment. Figure 65 is an isometric view of the unit.
It was found that removal efficiency sufficient to meet the standard of
primary sewage treatment could only be obtained at flows of less than 6.5 I/sec
(0.15 mgd). Without chemical additives the swirl was not found useful for
removal of more than 50 percent suspended solids due to the size and cost of
the required units. Thus, application of the swirl primary unit with the
design given is limited. The constraints of no moving parts and minimum
hydraulic head loss severely limited the capability of the present
design.
The key components as shown on Figure 65 include:
A. Inlet: tangential access to the chamber.
B. Baffle: acts to force flow in outside area of the chamber down
below the incoming flow and traps floatables,
C. Skirt: a circular baffle which separates the outer chamber where
the flow becomes organized from the central area and where essen-
tially quiescent settling takes place. The distance from the
bottom of the skirt to the chamber wall has a major effect on
removal efficiency.
D. Weirs: conventionally-designed notched weirs to allow discharge
of treated effluent.
E. Clear Effluent Outlet: draw off of clear effluent for discharge
to additional treatment or to receiving waters.
F. Sludge Baffle: a baffle at the bottom of the sludge collecting
cone to assist draw off of concentrated sludge.
G. Sludge Discharge: a valve-controlled line to allow periodic draw
off of settled sludge.
128
-------
Sludge discharge
A Inlet
B Baffle
C Skirt
D Gutters
E Clear effluent outlet
F Baffle
G Sludge discharge
Figure 65 Isometric: Swirl Primary Separator
129
-------
DESIGN BASIS
Conventional primary sedimentation tanks are generally designed on the
basis of overflow rate and, to a lesser extent, on detention time. The term
"overflow rate" or "surface settling velocity" is the unit volume of flow
per unit of time divided by the unit of tank surface area. In U.S. customary
units this is expressed as gallons per day square foot (gpd/sq ft) and in metric
units may be expressed as cubic meters per day per square meter (cu m/day/sq m).
The American Society of Civil Engineers Manual of Engineering Practice
Number 36 (17) lists data on various primary settling tanks which indicate
removal of suspended solids ranging from 20 to 80 percent. Figure 6 of
that publication indicates the relation between removal of suspended solids
and overflow rate. Many tanks fall in the range of 60 to 70 percent removal
of suspended solids. If we accept 60 percent removal of suspended solids as
a desirable objective then Figure 6 indicates the necessary overflow rate
is 36.67 cu m/day/sq m (900 gpd/sq ft). The ASCE manual's curve in this
range of suspended solids removal has been verified by recent analyses of
field data by Smith. (18) Detention time is no longer considered as the
only factor in design of primary settling tanks. However, the use of tanks
with liquid depths of 2.13 to 3.66 m (7 to 12 ft) combined with accepted
overflow rates will result in nominal detention times of 1 to 2 hours. For
instance, the use of a 3.05 m (10 ft) liquid depth with an overflow rate of
36.67 cu m/day/sq m (900 gpd/sq ft) will result in a detention time of
2 hours.
The equation developed by Smith (18) from the analyses of field data
can be used to estimate the removal efficiency (percent) of suspended solids,
r? as a function of overflow rate (OVFRA) in gpd/sq ft as follows:
„= 0.82e-°VFRA/2'780
For OVFRA = 36.67 cu m/day/sq m (900 gpd/sq ft) = 54.3 percent. This value
is in reasonable agreement with the 60 percent removal estimated by the use
of the ASCE figure for a 36.67 cu m/day/sq m (900 gpd/sq ft) overflow rate.
The design of the swirl separator is based neither on overflow rate nor
detention time, but on the results of model tests. However, these two
parameters are useful in comparing the size of the swirl separator with a
conventional tank.
DESIGN PROCEDURE
Figures 66, 67, and 68 are used for design.
As indicated, the swirl separator cannot economically achieve conven-
tional suspended solids removal of 60 percent. Therefore, the following
design example is based upon 45 percent suspended solids removal, which is
near the upper level of its efficiency for sanitary flow. Settling charac-
teristics of combined sewer overflow solids are usually better than for
sanitary sewage.
130
-------
OS
T-
IO SO 40 »O
Discharge I/sec
-* I •
COS
O.I 0.2 03 0.40-5 10"
Discharge — mgd
-1 - - •
O.CZ
. I ,
005
... 1
oi 0.15
Discharge —
O2
cfs
OS
0.4
1 , . .
0.5
. 1
1.0
to
JL.
10 30 40 SO 100
Discharge — I/sec
00 SOO 400 500
I . . . .ll
SJD 10
0.5 IX) t£>
Discharge — mgd
I ... .1 , I I i I i i i t I
0.2 C« 0.4 05
I tO S.O 4.0 6.0
Discharge — cfs
Figure 66 Predicted Prototype Solids Removal Efficiency for Sanitary Sewage
131
-------
2
o
Q>
0)
Q
1.0 * 3 4 6 • 7 »10
100
500 I/sec
1
0.05
. i
i i
0.1
1 1
1 1 1
0.5
. i i
i
1.0
,
1 III
5
I iii
i
10
, i
Flow rate
cfs
10 mgd
Figure 67 Detention Times
132
-------
Plan
Figure 68a General Design Dimensions, Swirl Primary Separator
133
-------
Overflow outlet.
Skirt
Overflow outlet
D = Diameter
D, = 0.066 D
D2 = 0.67 D
D4 = 0.58 D
D5 = 0.056 D
D6 = 0.042 D
E, = 0.028 0
E2 = 0.028 D
H, = 0.056 D
H2 = 0.07 D
H3 = 0.125 D
H4 = 0.2 D
H5 = 0.04 D
H6 = 0.04 D
H7 = 0,19 D
He = 0.8 D
Elevation
of chamber (from Fig. 67)
Inlet
Skirt
Gutter
Sludge draw off
Outlet
Weir gutter width
Slot width
Slot height
Invert elevation
Circular gutter height
Circular gutter top elevation
Gutter top above weir lip
Weir gutter depth
Gutter depth at outlet
Cone height
Figure 68b General Design Dimensions, Swirl Primary Separator
134
-------
Normal practice is to provide a minimum of two plant units of each type
in a plant. Thus the initial construction phase would include at least two
primary separators.
From the tests which were conducted, the low efficiency of solids re-
moval and long detention times make large units impractical with the present
design. Table 26 gives the flow and detention times for several size units
as taken from Figures 66 and 67. The table indicates that the swirl separator
has less detention time than conventional settling tanks over a small range of
flows. At a diameter of 5.5 m (18 ft) the detention time necessary to achieve
40 to 50 percent suspended solids removal is approximately that of convention-
al units.
From Table 26 it is obvious that if the detention time is to be less
than that of a conventional unit, the diameter will be less than 5.5 m (18 ft),
Thus, for 40 percent suspended solids removal, the maximum flow would be
10 I/sec (0.22 mgd). Since in conventional practice two tanks are used, the
maximum plant design capacity would be 20 I/sec (0.44 mgd) or less.
The design of a swirl primary separator follows:
A. Plant design average daily flow is 15 I/sec (0.34 mgd)
B. Removal efficiency of suspended solids desired is 45 percent
C. Use two swirl primary separators. Design flowrate per unit is
7.5 I/sec (17 mgd). Peak flowrate is 11.2 I/sec (0.26 mgd).
D. Enter Figure 66 with design flowrate. For 45 percent effi-
ciency, select "n = 3.7 m (12 ft). Surface area is 10.5 sq m
(113 sq ft). Overflow rate is 61,295 1/day/sq m (1,505 gd/sq ft).
E. Enter Figure 67 with design flowrate of 7.5 I/sec (0.17 mgd) and
D of 3.7 m (12 ft). Detention time is 37 minutes.
Note: For conventional settling units, the detention time would be
51 to 63 minutes.
F. Enter Figure 66 with peak flow of 11.2 I/sec (0.26 mgd) and D of
3.7 m (12 ft). Read recovery is 38 percent.
G. Enter Figure 67 with peak flow of 11.2 I/sec (0.26 mgd) and D of
3.7 m (12 ft). Read detention time is 25 minutes.
H. Determine dimensions of structure from Figure 68, as follows:
D = 3.7 m (12 ft) inside diameter of tank
D,= 0.24 m (0.8 ft) inlet (side of square)
D = 2.4 m (8 ft) skirt diameter
D,= 2.1 m (7 ft) gutter diameter
From the values given for D0 and D, the circular gutter width is 0.3 m
(1 ft). D, does not appear to be^a critical dimension insofar as the tank
135
-------
Table 26
Comparison of Diameter, Detention Time, and Suspended Solids Removal for
Swirl Primary Separator and Detention Time for Conventional Settling for
Various Overflow Rates
30
Swirl % SS Removal
40
50
60
Diameter
m
1.8
3.6
5.5
6.1
«
6
12
18
20
Flow
Msec
4.5
15
27
28
mgd
0.1
0.34
0.60
0.62
Detention
Time
mln
8
19
30
35
Flow
Usec
2.8
9.8
15
mgd
0.06
0.22
0.33
Detention
Time
mln
13
30
54
Flow
Usec
2
6.5
10
mgd
0.05
0.15
0.22
Detention
Time
min
18
45
75
Detention
Flow Time
I/sec mgd min
1.6 0.04 24
CONVENTIONAL SETTLING TANKS
%SS Removal
% SS
Removal
60
50
40
30
^ V Wl 111
l/day/m2
36,653
57,017
51,543
114,034
tf W¥ I1C* H*
gal/day/ft2
900
1,400
2,000
2,800
Detention Time
(min)
3.05m (10 ft) depth or over
120
77
54
38
Overflow Rate Comparison for Swirl Separator
Flow
Diameter 1.8 m (6 ft)
Diameter 3.6 m (12 ft)
Diameter 5.5 m (18 ft)
Diameter 6.1 m (20 ft)
l/s
4.5
2.8
2
1.6
15
9.8
6.5
27
15
10
28
mgd
0.1
0.06
0.05
0.04
0.34
0.22
0.15
0.6
0.33
0.22
0.62
Overflow Rate
l/day/m2 gal/day/ft2
144,170
86,340
72,085
57,625
122,585
79,415
54,165
96,110
52,945
35,230
80,435
3,540
2,120
1,770
1,415
3,010
1,950
1,330
2,360
1,300
865
1,975
136
-------
performance is concerned and therefore we assume the gutter width could be
changed if greater width is necessary to carry off the weir discharge.
D,- is not a critical dimension. Suggest D = 0.2 m (0.67 ft).
D, is not a critical dimension and designer may select size depend-
ing on hydraulics. Suggest D = 0.2 m (0.67 ft).
ttl = 0.2 m (0.67 ft) slot height.
H = 0.25 m (0.80 ft) vertical distance from invert to junction of
tank slope and tank side.
H = 0.45 m (1.5 ft) height of circular gutter.
H, = 0.73 m (2.4 ft) vertical distance from top of circular gutter
to junction of tank slope and tank side.
H- = 0.15 m (0.48 ft) vertical distance from circular gutter top to
overflow weir.
H, = 0.15 m (0.48 ft) depth of weir gutter. Designer should check
to make sure this depth is adequate.
ti, = 0.69 m (2.28 ft) vertical distance from gutter top to invert of
outlet pipe.
Hg = 2.92 m (9.6 ft) depth of chamber with sloping sides. The
horizontal dimensions of sludge hopper bottoms are usually no larger
than 0.61 m (2.0 ft). If the bottom is given this width then HR
will be reduced by 0.53 m (1.73 ft). Hence H0 = 2.4 m (7.9 ft).
o
E = 0.1 m (0.33 ft) weir gutter width.
E? = 0.1 m (0.33 ft) slot width at right angles to slope.
The size of the resultant structure for an average design flow of 7.5
I/sec (0.17 mgd) is shown in Figure 69.
The design and size of the overflow weirs and effluent gutters should be
based on principles used in conventional primary tanks and should be revised
from the values derived from Figure 68 as required.
CONSTRUCTION COSTS
A conventional round tank designed to handle the same flow and at the
same suspended removal efficiency 7.5 I/sec (0.17 mgd), 45 percent suspended
solids, would have essentially the same diameter, but less depth.
Cost estimates of the swirl primary separator were made for two purposes:
1) to indicate the probable construction cost of the facility; and 2) to com-
pare its costs with that of a conventional primary settling tank designed for
the same efficiency.
137
-------
Capacity
7.5 I/sec (0.17 mgd)
Inlet 0.25m (10 in.)
Sludge
manhole
j A Outlet 0.2m (8 in.)
i •
Note Provide surface skimming device for floatables
Telescopic
valve
0.24 m (0.8 ft)
0.25 m (0.84 ft)
Sludge pipe
0.61m (2 ft)
0.2m (8 in.) diai
Section AA
Figure 68 Swirl Primary Separator
138
-------
The cost estimates are considered to be reasonable engineer's estimates.
However, during periods of economic inflation, it is not unusual for con-
tractors' bids to materially exceed engineers' estimates.
COST BASIS
The costs are based on the following:
a. Engineering News Record Construction Cost Index average for
U.S. is 3140.
b. Unit prices as follows:
Steel Sheet Piling $129/sq m $ 12/sq ft
(for temporary use during construction)
Excavation $ 24/cu m $ 18/cu yd
Reinforced concrete (swirl) $390/cu m $300/cu yd
Reinforced concrete (conventional) $490/cu m $375/cu yd
Note: The concrete for the swirl unit will require less reinforcing
steel, thus a lower cost.
c. Contingency and engineering costs 35 percent of the foregoing items.
The estimated quantities of materials are based on the dimensions shown
in Figure 69.
The swirl separator dimensions are derived in the previous section. It
is assumed that the ground surface is 0.6 m (2 ft) above the crew., of the
inlet pipe and the top of tank is 0.3 m (1 ft) above ground surface. Since
the top of overflow weir is 0.2 m (0.7 ft) above crown of inlet pipe, this
provides 0.7 m (2.3 ft) of freeboard above the weir.
The conventional primary settling tank dimensions are inside diameter
of 3.6 m (12 ft) and side water depth of 2.44 m (8 ft). These dimensions
provide an overflow rate of 61,260 1/day/sq m (1,500 gal/day/sq ft) and a
detention time of 57 minutes. The tank is set to provide a freeboard of
0.7 m (2.3 ft) with top of wall 0.3 m (1 ft) above ground surface.
The following assumptions are made for both structures:
a. Excavation is all in earth. The unit price includes cost of
backfilling.
b. Temporary steel sheet piling is required 0.61 m (2 ft) outside
exterior walls of structure.
139
-------
c. Equipment cost for conventional settling tank includes cost of
rake-type sludge collector with fixed bridge and center drive,
scum collector, weir plates, telescopic valve, and electrical work.
d. Miscellaneous costs for swirl separator includes cost of skirt,
weirs, gutters, telescopic valve, center support for weir gutters,
piping, and railing around tank.
e. Miscellaneous costs for conventional settling tank includes piping
within limits of structure, gratings, and railing around periphery
of tank.
COST OF SWIRL PRIMARY SEPARATOR
The estimated construction cost of a swirl separator with a capacity of
7.5 I/sec (0.17 mgd) is §117,090. The breakdown of this cost is shown in
Table 27.
Table 27
Construction Cost of Swirl
Primary Separator
Capacity 7.5 I/sec (0.17 mgd)
Item Quantity Amount
Sheet Piling 128 sq. m $ 16,560
(1,380 sq ft)
Excavation 340 cu m 7,920
(440 cu yd)
Reinforced Concrete 125 cu m 48,750
(162cuyd)
Miscellaneous Costs Job 13,500
Subtotal $ 86,730
Contingent and
Engineering Costs 35% + 30,360
Total $117,090
COST OF CONVENTIONAL PRIMARY SETTLING TANK
The estimated construction costs of a conventional primary settling
tank with a capacity of 7.5 I/sec (0.17 mgd), based on the dimensions shown
in Table 28 is $129,370 . The breakdown of this cost is also shown in Table 28.
As the capacity of the swirl unit increases, there is a rapid increase
in cost as compared to the cost of conventional units, due mostly to the
increased excavation sheeting and amount of reinforced concrete. Similar
construction calculations were made for comparable units having a capacity of
21.9 I/sec (0.5 mgd) with a suspended solids efficiency of 60 percent. The
construction cost of the swirl unit was estimated to be $458,000 and the
conventional unit $207,000. Figure 70 is a plot of the cost comparisons made.
140
-------
(0
o
u
450
400
350
300
250
200
150
100
50
$458,000
65% ss
$129,370
45%SS
Conventional
Swirl
3
10
6 9 12m
20 30 40 ft
Diameter
Figure 70 Cost vs Diameter Swirl and Conventional Primary Treatment
141
-------
Table 28
Construction Cost of Conventional
Primary Settling Tank
Capacity 7.5 Msec (0.17 mgd)
Item Quantity Amount
Sheet Piling 96 sq m $ 12,600
(1,050 sq ft)
Excavation 345 sq m 8,100
(450 cu yd)
Reinforced Concrete 40 cu m 19,130
(51 cu yd)
Equipment Job 53,000
Miscellaneous Job 3.000
Subtotal $ 95,830
Contingent and
Engineering Costs 35% + 33,540
Total $129,370
Table 29
Comparison of Operation and Maintenance Costs
for Primary Treatment Units
Conventional Swirl
1. Labor operation, 1 hr/day at $10/hr $ 2600 —
maintenance, 0.54 hr/day at $10/hr 1,400 1,000
2. Materials and supplies 1,000 —
3. Power. 2 pumps at 1/2 hp, 1 hr/day $0.06/kwh 400 400
4. Annual maintenance at 3% of capital cost
Primary tank sludge collections 150
Raw sludge plunger pumps 120 120
TotalannualO&M $5,670 $1,520
Operating and maintenance costs for a 43.8 I/sec (1 mgd) unit, the
smallest size for which USEPA data is available, can be estimated as shown
in Table 29.
COMPARISON OF COSTS
From the foregoing it is seen that the construction cost of the swirl
separator will be $117,090 compared to $129,370 for a conventional settling
tank of 7.5 I/sec (0.17 mgd). Annual operating and maintenance costs may be
$4,000 less with the swirl unit. The surface area required for units of this
low volume does not appear to warrant a comparison of land cost savings.
142
-------
This comparison assumes that the two structures will produce equal
results in removal of suspended solids in raw sewage. The sizing of the
conventional primary settling tank is based on standard design criteria.
The sizing of the swirl primary separator is based on model results in the
laboratory using IRA-93 resin as representative of suspended solids in raw
sewage.
Cost comparison of large size units do not appear warranted at this
time. A different configuration is obviously needed for large units to
avoid the adverse construction costs of such a deep structure. A flat
bottom with scrapers sacrificing the principal of no moving parts appears
reasonable.
The present worth of the swirl separator units is shown in Table 30.
The present worth is based on a life of 20 years and an interest rate of
7-1/8 percent. Hence the present worth of the operation and maintenance
costs for a 20-year period is 10.49 times the annual cost.
For the unit with capacity of 7.5 (0.17 mgd) at 45 percent removal the
present worth of the conventional unit is $188,850 and the swirl separator is
$133,035. Thus the present worth of the conventional unit is 42 percent
greater than that of the swirl separator.
For the unit with capacity of 21.9 Isec (0.5 mgd) and 60 percent
removal the present worth of the conventional unit is $266,480 compared to
$473,945 for the swirl separator. Thus the present worth of the conventional
unit is 44 percent less than that of the swirl separator.
Table 30
Present Worth Swirl Primary Separator Treatment Units
Conventional Swirl
Tank Separator
Capacity 7.5 I/sec (0.17 mgd) (45% ss removal)
Construction cost $129,370 $117,090
Operation and maintenance cost 59,480 15,945
Cost total present worth $188,850 $133,035
Capacity 269 I/sec (0.5 mgd) 60% ss removal
Construction cost $207,000 $458,000
Operation and maintenance cost
-------
PROTOTYPE TEST
The Municipality of Metropolitan Toronto, Ontario, constructed a 3.66 ra
(12 ft) diameter unit at the Number Wastewater Treatment Plant (Figure 71) to
evaluate treatment efficiency.
I
Figure 71 Swirl Primary Separator, Toronto, Ontario
144
-------
Flowrates were 0.79 cu m/min (0.3 mgd). These studies indicated that
the device closely matched the treatment efficiency of conventional primary
sedimentation at an overflow rate of 65.2 cu m/sq m/d (1600 gpd/sq ft) which
is 2.67 times conventional design (17). Figure 72 gives a comparison of time
to achieve treatment between the swirl and the conventional system at Toronto.
Its height and diameter are equal, thus providing a relatively deep structure
which enhances sludge thickening.
For small treatment facilities the relatively high overflow rates or
lower detention times used with swirl concentrator design at various levels of
suspended solids removal make the device potentially less costly to construct
with less space required, thus enhancing its use in wastewater plant
expansion and combined sewer overflow treatment. Its static sludge collection
system enhances appeal because of lower operation and maintenance costs.
re
O
E
o>
oc
V)
60-
50-
40-
30-
20-
10-
0
0
= Conventional
= Swirl
60
Time, in minutes
720
Figure 72 Comparison of Time to Achieve Primary Treatment
145
-------
Section VII
THE SWIRL CONCENTRATOR FOR EROSION RUNOFF TREATMENT
The swirl secondary flow pattern to separate solids from a moving stream
makes its use appropriate for separating settleable erosion sediment from
surface runoff. The unit has been designed to concentrate the heavier soil
particles. The unit cannot be used alone. Rather good construction management
practices are needed throughout the project. The swirl unit can provide
additional pollution control before discharge of flows to receiving waters or
storage ponds.
BASIC ASSUMPTIONS
Several basic assumptions have been made concerning how and when the
swirl device will be used. It is assumed that temporary units will be small,
i.e., generally 3.7 m (12 ft) in diameter. Multiple units will be used to
treat flows requiring larger capacity.
A permanent facility will require a flow-splitting diversion device
where multiple units are used, and bar screens to protect the unit from coarse
debris. A solids basin or other facility will be needed for settling the
solids in the 5 to 14 percent range of the concentrated underflow. The
clarified flow may be discharged into a detention pond, or directly into
receiving waters, based upon the degree of protection against erosion solids
required by water quality standards.
The unit is designed to be self-cleansing. However, the flow field is
not strong enough to move gravel or heavy loadings of sand through the under-
flow. Thus it is imperative that good erosion and sediment control construc-
tion practices be used. This treatment device does not take the place of
conventional methods of on-site control.
DESCRIPTION
The swirl concentrator for erosion runoff treatment can be characterized
as a shallow tank with a tangential inlet at the bottom, a circular overflow
weir with a central downshaft discharge pipe, and a small underflow drain for
removal of concentrated solids.
146
-------
Figure 73 is a schematic view of the unit. The essential features, as indicated
in the figure, include:
A. a square inlet
B. a baffled inlet to ensure the development of the swirl flow field
C. flow spoilers to improve the efficiency of the circular discharge
weir
D. an internally supported clear water overflow weir with a central
downshaft discharge pipe
E. a flat weir plate
F. a central downshaft for the clarified overflow
G. a concentrate discharge take off to a settling or thickening basin
H. a flat floor
DESIGN GUIDELINES
The design procedure is developed in accordance with the various ele-
ments formally required for a complete system. These elements are:
Hydrological considerations
Solids analysis
Swirl unit design
Efficiency computation
Assessment of retention volumes
Other design considerations and details
A typical site situation is shown in Figure 74 . This plan shows a
large drainage area with a stormwater retention facility. The swirl unit and
soil collection pond intercept this flow ahead of the stormwater detention ponds.
It is assumed that all runoff from the basin is detained on the property and
passed through the swirl unit or units.
Hydrological Considerations
For purposes of determining the quantity of runoff to be expected from
the drainage area any of a number of methods can be used. In reference to a
survey conducted by APWA, (19) rainfall runoff predictions in practice are
based primarily on unit hydrographs and the Rational Method. In general,
maximum erosion will occur under conditions of peak runoff. For a device such
as the swirl when used as a temporary treatment device, the unit should be
designed for a rainfall intensity of less than a one-year recurrence interval,
although in many cases the choice of a design rainfall is determined for a
specific project by the requirements of the local or state public agency having
jurisdiction.
147
-------
Legend
A — Inlet
— Flow deflector
— Spoilers
— Overflow weir
— Weir plate
— Overflow (clear)
— Underflow (solids)
H — Floor
Figure 73 Schematic View, Swirl Concentrator for Erosion Runoff
148
-------
UNDERFLOW
DRAINAGE
AREA
SOLIDS
LAGOON
OR
FOREBAY
RETENTION POND
OR
RECEIVING WATER
OVERFLOW
SWIRL
CONCENTRATOR
PIPE
OR DITCH
Figure 74 Typical Application, Swirl Concentrator as an Erosion Runoff
Treatment Device
A second part of the hydrologic analysis required to design an erosion
control facility involves an estimate of the peak volume of runoff for a
given storm. This volume will be used to size the retention pond and the
soil collector pond. Obviously, the high-intensity, short-duration storm may
contribute a high flow rate for a short period but it would represent only a
portion of the total volume of runoff that could be expected from a storm of
longer duration.
The use of the Rational Method C factor will result in an estimate of a
larger flow than would ordinarily be anticipated for all but the most intense
storms.
Perhaps the most accurate method for determing the volume of runoff
would be to integrate the area on a hydrograph determined for this watershed.
For the determination of this volume, the use of a unit hydrograph would be
advantangeous.
Various other methods are also available for computing the storage
volume necessary to hold the total runoff.
149
-------
The final hydrologic determination deals with an annual estimate of the
total quantity of sediment to be expected. This volume will be used to esti-
mate the total amount of settleable solids to be collected in the two ponds.
Reference to a chart of expected annual rainfall in the project area, will
provide the annual precipitation rate. It is probably not necessary to re-
duce these values for precipitation occurring as snowfall for the purpose of
this e s timate.
In summary, these three calculated quantities will be used in the
following manner:
A. The peak runoff rate will be used to size the swirl concentrator
erosion control device or devices, the main drainage trench conducting
flow to the device and any inlet conduit that must be used.
B. The single storm volume will be used to size the solids basin.
C. The annual storm runoff volume will be used to estimate the quantity
of settleable material which will accumulate in the retention basins.
This represents material for which storage capacity must be provided
within the solids basin, or the volume of material which must be
removed.
Solids Analysis
The next step in the design procedure is to determine the quantity,
type and size of material that is likely to be found in stormwater runoff.
Table 31, presents an analysis of a sample of storm runoff from a construc-
tion site which was sieved and separated into groups having similar specific
gravities. Such an analysis is used to determine the type and specific gravi-
ties of the material present, thus enabling a reasonable estimate of tne type
and quantity of material that can be removed in a swirl erosion control unit.
This example should be viewed as merely an indication of the type of investi-
gation that should be conducted. There may be many sites for which more
elaborate and complete analyses may be desirable.
Table 31
Sieve Analysis, Sample from Construction Site
% Retained
According
Material to SG
Sieve Size Size Retained % Retained SG/2.65 SG/1.20 SG/1.01
mm (in) gm (oz)
10 2.000 (.080) 4.0(0.14) 1.14 1.04 — 0.1
20 0.840 (.030) 6.5(0.23) 1.86 1.66 — ^0.2
60 0.250 (.010) 39.0(1.40) 11.14 8.64 2.0 *0.5
100 0.149 (.006) 100.5(3.50) 28.71 23.61 tJ5.g 0.1
120 0.125 (.005) 77.0(2.70) 22.00 21.00 1.0 —
200 0.074 (.003) 44.0(1.50) 12.57 12.07 0.5 —
PAN - 79.0(2.80) 22.57 22.57 - -
TOTAL 90.59 8.5 0.9
150
-------
Assuming that it is desirable to remove as much settleable material as
possible in the swirl unit, the smallest particle that is predicted from the
model studies to be removed is a grit particle, SG 2.65, 43 microns in dia-
meter, having a settling velocity of 0.14 cm/sec (0.055 in. /sec) as shown in
Figure 75. A design incorporating the removal of this size of grit parti-
cle will also remove larger size particles of lighter specific gravity. For
example, a particle of SG 2.65 and a settling velocity of 0.14 cm/sec
(0.055 in. /sec) is 0.043 mm (0.002 in.) in diameter. Particles having a SG
of 1.20 with a diameter of 0.14 ram (0.006 in.) will settle at the same rate
as particles having a SG of 1.01 (organic material) with a diameter of
0.6 mm (0.02 in.). Particle sizes larger in diameter than those quoted are
expected to be removed. In the sieve analysis shown in Table 31, the materi-
al expected to be removed in part by the swirl concentrator is shown in the
specific gravity columns at the right side of the table above the asterisk
marks, considering that the settling velocity is 0.14 cm/sec (0.055 in. /sec)
for a particle having an SG of 2.65.
Hydrometer analysis using pan material, or 22.5 percent of the total
sample, showed:
Percent particle size greater than 0.052 mm (0.002 in.) - 16%
Percent particle size less than 0.052 mm (0.002 in.) - 6.57%
From design data using a particle settling velocity of 0.14 cm/sec
(0.055 in/sec) it was determined that the following percent of material will
be subject to removal in the swirl chamber:
SG 2.65 90.59 - 6.57 = 84.02%
SG 1.20 8.5 -1.5 = 7.0 %
SG 1.01 0.9 - 0.6 = 0.3 %
(These quantities are shown in the table as the percent in each SG column
above the asterisk)
Total material subject to removal by swirl concentrator - 91.33%
Total material not subject to removal - 8.6%
The percent of removed material shown here will be multiplied by the re-
covery efficiencies of the chamber from the design curves as explained next.
Unit Design
This part on design makes frequent reference to the following listed
Figures .
76 Prototype Particle Sizes Represented by Gilsonite - SG 1.06
77 Recovery Rates in Model as Function of Particle Settling Velocity
and Discharge with 5 Percent Draw Off
78 Recovery Rates in Model as Function of Particle Settling Velocity
and Discharge with 10 Percent Draw Pff
151
-------
o
o
•2
o
E
o
o
o
"3
o>
c
Q>
V)
Range of proto-
type grit settling
velocities
frt—H
.012? (.0047)
~ Other prototype
particle sizes
simulated
.001 .0004)
.012
(.008)
.052 0.1 .22
(.0193) (37.9) (83.27)
Particle diameter in mm (in)
Figure 75 Prototype Particle Sizes Represented
152
-------
7.0 mm (2.76 in) —
6.0 mm (2.36 in) —
5.0 mm (1.97 in) —
4.0 mm (1.57 in) -
3.0 mm (1.18 in) —
2.0 mm (.787 in) —
t? 1.0 mm (.40 in)
-------
Qm = 3 I/sec
(0.79 gal/sec)
Qm = 5 I/sec
(1.3 gal/sec)
Q = 7 I/sec
TTl
(1.8 gal/sec)
10
0.07
(.03)
0.1
(-04)
2.0 3.0
(-79) (1.18)
Particle settling velocity, cm/sec (in/sec)
Figure 77 Recovery Rate on Model as Function of Particle Settling Velocity and
Discharge with 5% Drain-Off
154
-------
100
90
80
70
c 60
o>
a
0)
Q.
0)
o
u
0)
oc
50
40
30
20
10
Qm = 3 I/sec(0.79gal/sec)
Qm = 5 I/sec (1.3 gal/sec)
Q = 7 I/sec (1.8 gal/sec)
0.07
(.03)
0.1
(.04)
0.2
(.08)
0.5
(.20)
1.0
(.39)
2.0
(.79)
3.0
(1.18)
Particle settling velocity, cm/sec (in/sec)
Figure 78 Recovery Rate on Model as Function of Particle Settling Velocity and
Discharge with 10% Drain-Off
155
-------
79 Recovery Rates in Model as Function of Particle Settling Velocity
and Discharge with 14 Percent Draw Off
80 Predicted Prototype Recovery Rates with 5 Percent Draw Off
81 Predicted Prototype Recovery Rates with 10 Percent Draw Off
82 Predicted Prototype Recovery Rates with 14 Percent Draw Off
83 General Design Dimensions
The procedure described in this section is relevant to a standard
3.66 tn (12 ft) diameter tank as the swirl chamber. The dimensions of the
structure are determined from Figure 83.
Under operating conditions it is assumed that the user has a situation
ch the discharge Q is known as well as the
V , of the materials to be removed from the flow.
in which the discharge Q is known as well as the particle settling velocity,
1. Enter Figure 80 (5 percent draw off) where the expected discharge
appears on the abscissa
2. Move up in the graph until the given particle settling velocity
curve (or particle size) is found
3. Check whether or not this intersection gives an acceptable rate
of recovery
4. If the recovery is not high enough, try Figures 81 or 82 in
which draw off is increased, respectively, to 10 and 14 percent of the
inflow
5. If conditions are still not acceptable, even with the larger draw
off rates, then reduce the expected discharge per unit by providing
multiple swirl chambers
6. If this gives too many standard 3.66 m (12 ft) units, try larger
chambers, making reference directly to Figures 77, 78, and 79;
the recovery curves for the 0.914 m (3 ft) diameter model
7. Select an approximate new chamber diameter, Dn and divide this by
the model diameter to find the new scale:
l/\n = 0.914/Dn m = 3/Dn ft
Where:
Xn = scale factor
Next calculate:
S / ?
new discharge scale = I/ X n ' 1
new settling velocity scale = I/ X n ^
156
-------
100
0.07 0.1 0.2 0.5 1.0 1.5 2.0
(0.03) (0.04) (0.08) (2.0) (0.39) (0.59)(0.79)
PARTICLE SETTLING VELOCITY, cm/sec (in/sec)
Figure 79 Recovery Rates on Model as Function of Particle Settling Velocity
and Discharge with 14% Drain-Off
157
-------
100
o
u
o>
oc
Particle settling velocity
Prototype grit particle
(gal /sec)
PROTOTYPE DISCHARGE
Opm3/sec
(ft3 /sec)
i
0.07
(2.5 )
1
0.08
(3.0 )
i
0.10
(3.5 )
I
0.11
(4-0 )
i
0.14)
(5.0 )
i
0.17
(6.0 )
i
0.2
(7.0)
i
0.23
(8.0 )
i
0.25
(9-0 )
Qpm3/day
(mgd)
i
6,435
(1.7)
i
7,570
(2.0)
i
9,463
(2.5)
i
11,355
(3.0)
i
13,248
(3.5)
i
15,140
(4.0)
i
18,925
(5.0)
i
22,710
(6.0)
Figure 80 Predicted Prototype Recovery Rates with 5% Drain-Off
158
-------
100
o>
B
0)
a
o>
5
u
a
cc
80
60
40
20
0
I/sec
(gal /sec)
Particle settling velocity cm/sec
SIZE SG=265
I 1 I
Prototype grit particle
60
(21.1)
Prototype discharge
Q nv/sec
(ft3 /sec)
Qp m3/day
(mgd)
1
0.07
(2.5 )
i
6,435
(1.7)
i
0.08
(3.0 )
i
7,570
(2-0)
0
(3
_L
i
.10 0.11
-5 ) (4.0 ]
i
9,463
(2-5)
I
1
0.
(5.
11,355
(3.0)
I
I i
14 0.17 0.2
0 ) (6.0 ) (7.0)
i i
13,248
(3-5)
15,140
(4.0)
I
0.23
(8.0 )
i
18,925
(5.0)
I
0.25
(9.0 )
i
22,710
(6.0)
Figure 81 Predicted Prototype Recovery Rates with 10% Drain-Off
159
-------
0)
i»
0)
Q.
o
u
100
Particle settling velocity cm/sec
Prototype grit particle
(gal/tec)
Prototype discharge
m'/sec
(ft3 /sec)
ro'/day
(mgd)
j
0.07
(2.5 )
I
6,435
(1.7)
I
0.08
(3.0 )
i
7,570
(2.0)
i
0.10
(3.5 )
I
0.11
(4.0
i
9,463
(2
.5)
)
l
11,355
(3.0)
!
0.14
(5.0 )
I
13,248
(3.5)
I
I
0.17
(6.0
)
i
* r
I
I
»
I
0.20 0.23 0.25
(7.0
15,140
(4.0)
) (8.0
i
18,925
(5.0)
) (9-0 )
i
22,710
(6.0)
Figure 82 Predicted Prototype Recovery Rates with 14% Drain-Off
160
-------
Four flow spoilers
0.86m (34 in.) long
- Pipe
0.61 m (2 ft)
0.61 mxO.61 m
(2 ft x 2 ft)
3.65m (12 ft) 90°
0.10m(4in.)60«
Foul outlet discharge
10 cm (4 in.)
sheargate
Figure 83 General Design Dimensions, Swirl Concentrator for Erosion
Runoff Treatment
161
-------
8. Multiply:
Qp x l/\n 5'2 = Qm model discharge
v x I/X n2 = vsm model particle settling velocity
9. Go into Figures 77, 78, or 79 with these model values, inter-
polating as necessary between the discharge curves, to find the
corresponding recovery.
10. If the recovery is too low, try progressively larger chambers, each
time following the procedure in steps 7, 8, and 9 above, until a satis-
factory recovery rate is obtained
11. Use Figure 83 to find the dimension of the new chamber. Since
the chamber shown on the figure is the standard 3.66 m (12 ft) unit
studied at scale 1:4, each dimension must be multiplied by the
factor An/4
12. The bottom orifice must be large enough to prevent clogging by
solids which may be carried by the stormflow into the chamber.
For purposes of illustrating the procedure for the application of this
swirl unit to the problem of soil erosion, two examples will be given. The
first is based upon an engineering approach where a permanent facility is to
be designed for a required level of efficiency. The second example is for
the case where a developer must provide temporary facilities at a construc-
tion site.
For a permanent erosion control facility the use of the swirl concen-
trator may be envisioned as an auxiliary treatment device installed ahead of
a stormwater retention/detention facility. The primary purpose of the unit
would be to concentrate the larger soil particles in order to retard the
siltation of the retention facility or downstream receiving waters. To this
end, the concentrated underflow could be directed to a readily cleanable auxi-
liary sediment trap where conventional equipment such as a backhoe, Gradall,
or even a bucket loader—assuming that the area could be dewatered—could
be used to remove the collected soil.
Such a facility would minimize the total maintenance cost and improve
the efficiency of the major storage facility or receiving waters.
For this example, let it be assumed that a 80.9 ha (200 ac) drainage
basin is selected with a time of concentration of 45 minutes. Assuming that
it is desired to find the peak runoff at a time when equilibrium conditions
are established for this site, the duration of the storm is taken as the time
of concentration. From a duration-intensity relationship established for this
site, it is determined that the intensity is 1.27 cm/hr (0.5 in./hr). Further
information on the site indicates that 20 percent of the basin is occupied by
buildings for which a runoff coefficient of 0.9 is selected; 15 percent is
roadways with a runoff coefficient of 0.9; and the remainder is grassed yard
areas for which a runoff coefficient of 0.3 is assumed. An average coefficient
for this site can be calculated as:
162
-------
(40 ac -f 30 ac) x 0.9 + 130 ac x 0.3
C = 200
ave
C = °-51
^
For this example a simplified method of calculation of rainfall and
runoff will be used. In practice, each agency should use models or methods
which present a better representation of what can be expected to occur.
The peak runoff calculated for this storm, using the Rational Method,
is:
Q C i A
Q =0.51 (1.27 cm/hr) 80.9 ha = 52.4 cra/ha/hr
52.4 x 27.8 = 1,460 I/sec
Q = 0.51 (0.5 in./hr) (200 ac) = 51 cfs
This will be the flow to the swirl treatment facility—next an estimate
of the peak volume must be made to size the retention pond and solids
collector pond. With reference to a set of intensity-duration curves, it was
observed that for the same recurrence frequency that was used in the deter-
mination of the peak flow rates, a storm of longer duration than 4 hours would
yield an intensity of 1.02 cm/hr (0.4 in/hr). The peak rate of flow for this
storm can be estimated in the same manner as previously:
Q = 0.51 (1.02 cm/hr) 80.9 ha = 42.1 cm/ha/hr
42.1 x 27.8 = 1170 I/sec
Q = CiA = 0.51 (0.4 in./hr) 200 ac
40.8 cfs
Various methods are available for computing the necessary storage
volume. Using one of these methods, assume the resultant volume is
8,420 cu m (297,226 cu ft). This yields a larger volume than that associated
with a short-duration, higher-intensity storm.
The final determination is to estimate the annual total quantity of
sediment to be expected. Charts of estimated annual rainfall in the project
area should be consulted. Assume that this value is 76.2 cm (30 in.) per
year. It is also assumed that the area of the retention pond(s) is small
compared to the total area, although this fact may not always be true.
Neglecting the reduced volume resulting from the cumulative effects of
smaller storm events, the maximum runoff volume per year is then:
V = 0.51 (76.2 ™] x -r^-r.. * 80.9 ha
= 314,000 cu m/yr
U . Jl
0.51
VI U . i. IX "i r, ri '
yr I 100 cm
10,000 sq
x • • - •
ha
ju in/ yr ) x - • • .
12 in
x 200 ac x
m
43,500
ac
11,100,000 cu ft/yr = 411,000 cu yd/yr
163
-------
These three calculated quantities will be used in the following manner:
A. Peak runoff rate will be used to size the swirl devices and
the drainage conduits to and from the facility.
B. Single storm volume will be used to size the retention basin(s),
and
C. Annual storm runoff volume will be used to estimate the quantity
of settleable solids in the solids basin. This material must
either be stored or removed.
With reference to Figure 82 it is seen that for a 3.66 m (12 ft)
diameter chamber, the highest efficiency is obtained when the flow rate does
not exceed 96 I/sec (3.4 cfs). Dividing the flow by a factor of 15 would
give 96 I/sec (3.4 cfs) as the design flow for each of the chambers, and
this flow in Figure 82 is at the left end of the curve at the highest
possible removal efficiency for this particle size. The use of
15 chambers would also mean that higher intensity storms would still be
handled by these chambers with only a small reduction in efficiency. In fact,
the design runoff could be more than doubled in each chamber. It should be
noted that if the 14 percent draw off rate is excessive for the volume of
storage desired, Figures 80 and 81 should be used with smaller draw off
rates and corresponding reductions in efficiencies.
Efficiency Computation
Using the efficiencies given in Figure 82 and the percent of each
size material given in Table 31, the efficiency of the 3.66 m (12 ft)
diameter chamber can be determined as shown in Table 32.
For specific gravities less than 2.65 an equivalent particle size for
that particle can be obtained from Figure 76. As an example with refer-
ence to Table 32, a particle for specific gravity of 1.20 is taken as
0.25 mm (0.01 in.). In Figure 76, find this size along the abscissa:
go vertically upward to the curve marked SG 1.20
then left or horizontally to the curve marked SG 2.65
then downward to the abscissa.
The values read,
settling velocity is 0.5 cm/sec (0.19 in./sec) for a SG 1.20
and
particle size 0.25 mm (0.01 in.):
an equivalent particle of SG 2.65 having this settling velocity is a
particle of 0.082 mm (0.003 in.)
Refer back to Figure 82 for this size particle of 82 microns and
settling velocity of 0.5 cm/sec (0.019 in./sec).
164
-------
Table 32
Swirl Efficiency Analysis
Sieve
Size
10
20
60
100
120
200
HYD
Col.1
Particle
Size mm(in)
2.00 (.08 )
0.84 (.03 )
0.25 (.01 )
0.149(.006)
0.1251.005)
0.074(.003)
0.052(.002)
Col. 2
Percent
Retained
SG 2.65
1.04
1.66
8.64
23.61
21.00
12.07
16.00
Col. 3
Percent
Eff . from
Fig. 82
100
100
92
82
79
69
59
Col. 4
= 1x2
100
1.04
1.66
7.95
19.36
16.59
8.32
9.44
Col. 5
Percent
Retained
SGI. 20
2.0
5.0
1.0
0.5
Col. 6 Col. 7
= 4x5
Percent
Eff. from
Fig. 82 & 85
69
56
1.38
2.8
64.36
4.18
Sieve
Size
10
20
60
100
120
200
HYD
Col. 8
Percent
Retained
SG1.01
0.1
0.2
0.5
0.1
Col. 9
Percent
Eff. from
Fig. 82 & 85
60
Col. 10
= 7X8
0.1
0.12
0.22
Total percent of removal material removed in
swirl unit = 64.36% + 4.18% + 0.22%
= 68.76%
Total percent of settleable material removed
by swirl concentrator
= 68.76% x 91.3% = 62.7%
For a flowrate of 96 I/sec (3.4 cfs) this yields an efficiency of
69 percent.
This procedure is continued for other particle sizes. It is seen then
an efficiency of 69 percent is predicted for this material if a set of 15
swirl concentrators were used.
165
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Alternate Chamber Design
The design discharge Q is 1,460 I/sec (51 cfs). With reference
to Figure 79 the smallest particle shown is one having a settling velocity
of 0.07 cm/sec (0.03 in./sec). Figures 77 and 78 could be used for 5 percent
and 10 percent draw off, respectively. Figure 79 was selected since the best
recovery occurred with a draw off of 14 percent. Assume that four prototype
chambers will be used, each having a diameter of 6.4 m (21 ft). This sets the
model scale at:
A = L /L =6.4 m/0.914 m (21 ft/3ft) = 7
P m
where L = diameter prototype unit and L = diameter of hydraulic model
From the Froude Law the velocities of settlement can be related as:
v /v =
sp sm Y p m
where V = settling velocity of solids in the prototype and V = settling
velocity of model solids
The model discharge is also found from the Froude Law as:
, /L ;
p m
Q /Q - (L /L )5/2 = (l)'J/2= 129.64
p in om
Qm = Qp/129.64 = X _____ =2.81 1/sec
where Q = flow through the prototype and Q = flow through the model
Referring now to Figure 79, the discharge line for 2.81 I/sec
(0.74 gps) must be interpolated between the 3 I/sec (0.80 gps) line and zero
at 100 percent recovery. Assume it crosses the 0.07 cm/sec (0.03 in/sec)
settling velocity line at about 60 percent recovery.
This model settling velocity corresponds to a prototype settling
velocity of:
0.07 cm/sec x 2.65 = 0.185 cm/sec (0.073 in./sec)
166
-------
From Figure 76, this gives a particle size of 0.05 run (0.002 in.)
for SG = 2.65 material.
Another approach to selecting the chamber size would be to decide to
use 3 I/sec (1.106 cfs) in the required recovery curx'e, either Figures 77,
78, or 79. Working with Froude ' s Law, the scale can be found:
x i - 121.7
p m 4 3
X = 6.83
The corresponding chamber diameter would be:
d = 6.83 x 0.94 m = 6.4 m (21 ft)
The velocity scale becomes:
1 1 1
2.61
It is now possible to prepare a new operating curva for this unit at
the 365 I/sec (12.89 cfs) discharge by taking recovery rates from the
3 I/sec (0.8 gps) in either Figures 77, 78, or 79 and multiplying the
corresponding settling values by 2.61 to find the settling velocities that
would be recovered.
The dimensions of the individual swirl units would be A/4, or 6.83/4,
or 1.70 times for each dimension shown in Figure 83, since the dimensions
shown in Figure 83 are for a model scale of 1:4.
Keeping the same scale relations, similar calculations could be carried
out for the 5 and 7 I/sec (1.32 and 1.85 gps) lines on Figures 77, 78,
and 79. The resulting three operating curves could then be interpolated at
selected settling velocity values to yield data that could be plotted in the
same manner as shown in Figures 80, 81, and 82 but for the chosen chamber
size.
In addition to the settleable solids, a considerable quantity of light
suspended or colloidal solids is present in storm erosion runoff. It is
anticipated that none of these lighter solids would be removed in the swirl
unit, but there would be almost complete removal of such solids in the second
retention pond if sufficient settlement time occurred between storms.
Assessment of Retention Volumes:
The volume per storm was determined to be 8,420 cu m (297,226 cfs).
Using an underflow drain-off rate of 14 percent, the volume to be handled
in the solids basin is:
167
-------
8,420 cu m (297,226 cu ft) x 0.14 = 1,180 cu m (41,696 cu ft) while the
stormwater retention pond would be:
8,420 cu m (297,226 cu ft) - 1,180 cu m (41,696 cu ft) = 7,240 cu m
(255,830 cu ft).
These pond volumes are sized to retain all of the treated runoff from
the design storm. In practice, most ponds are designed to allow flow-through
for the normal runoff before construction development. For the 80.9 ha
(200 ac) site, with a runoff coefficient of 0.2, after full development the
outflow would be 566 I/sec (20 cfs). Various methods are available for
computing the required storage based on an outflow of 566 I/sec (20 cfs).
An estimate of the volume of settled material to be expected can be
obtained from information provided in a study for APWA by the firm of Beak
Consultants, Ltd. (8) Among figures quoted for suspended solids in storm-
water, these settleable solids vary from 0 to 7,640 mg/1, with an average of
687 mg/1. The concentration of solids can vary widely and is dependent upon
the character and the use of the land from which the storm flow is generated.
Using an average value of 700 mg/1, an estimate of the settleable solids per
storm is:
V = 8,420 cu m X 1,000 1 X 700 ES. X
cu m ;[
kg Y cu m
1,UUU,UUU A 1,6'UU kg
= 3.68 cu m (130 cu ft)
On an annual basis the volume of settleable solids is:
V = 314,000 cu m X 1,000 _J: X 700 m§ X
cu m ~T
kg cu m
1,000,000 g 1,600 kg
= 137, cu m (4,841 cu ft)
Assume cnat 100 percent of all settleable solids will be retained in
the ponds.
Temporary Facility at Construction Site
Another application of the swirl separator is as a temporary facility
for erosion control at a construction site. For this purpose the foul sewer
underflow, conveying most of the settleable solids, would discharge into a
soil collector pond and the overflow would discharge into a drainage ditch or
channel, or flow directly into a watercourse.
The riser pipe, shown in Figure 83 as 0.67 m (2.2 ft), could be changed
to 0.61 m (2 ft) to utilize standard size pipe. The clarified overflow out-
let could be attached directly to the underside of the chamber and could be
made rectangular in shape. Dimensions of 0.61 m (2 ft) wide and 0.22 m
(0.75 ft) high would provide a waterway having an area equivalent to the
163
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riser pipe. The underflow or foul outlet could likewise be made in rectangu-
lar or square shape, attached to the bottom of the box. The outlet should
probably be at least 15 cm (6 in.) square to prevent problems with clogging.
The outlet could terminate at the outside wall of the chamber, with a 15 cm
(6 in.) standard pipe flange for attaching the pipe to convey flow to the
solids basin.
Assume the following conditions:
Site area tributary to chamber is 3.12 ha (8 ac)
Runoff coefficient C is 0.4
Time of concentration is 15 min
Rainfall intensity is 6.35 cm/hr (2.5 in/hr)
Again, using a set of simplified calculations, for
Q = CiA:
= 0.4 x 6.35 cm/hr x 3.12 ha
= 8 cm/ha/hr
= 8 x 27.8 = 222.4 I/sec
Q = 0.4 x 2.5 in/hr x 8.0 ac
= 8 cfs
From Figures 80, 81, and 82 it is apparent that the largest allow-
ble flow through one chamber is 222.4 I/sec (8.0 cfs). Therefore, under
the above assumed conditions the largest site that can be served by one
chamber is 3.12 ha (8.0 ac). The greatest recovery of solids will occur if a
14 percent draw off (Figure 82) is used rather than 10 percent (Figure 81)
or 5 percent (Figure 80).
From Figure 82, the percentage of various size solids to be recovered
will be as follows:
Size Solids Percentage
Recovery
85
73
53
37
31
A 14 percent draw off means that this percentage of the peak flow will
pass through the underflow outlet to the soil collection pond. This amounts
to 0.14 x 222.4 I/sec (8 cfs) = 32 I/sec (1.1 cfs). The head or
depth of water above the underflow outlet will be 0.61 m (2 ft) when the
outlet weir starts overflowing. At peak flow this head may increase to
0.76 m (2.5 ft). Approximate hydraulic computations indicate that this head
is too small to permit use of a 10 cm (0.33 ft) diameter underflow outlet.
If an outlet pipe 15 cm (6 in) in diameter is used, the head is sufficient to
force the flow through about 15 m (50 ft) of outlet.
169
-------
To meet the recovery performance shown in Figure 82 it is necessary to
keep the underflow to about 32 I/sec (1.1 cfs). To prevent decreasing
the rate of underflow due to backwater, the maximum water level in the soil
collection pond should be below the top of the underflow pipe. The most
practical way of regulating the underflow rate would be to provide a shear
gate at the outlet pipe and to determine the actual setting of the gate from
measurements of the volume in the collection pond during actual Storm condi-
tions .
A further design consideration is the volume of the soil erosion
collection pond. Obviously the foul sewer discharge from the swirl chamber
underflow will outlet into the selected drainage ditch or the designated
watercourse during a storm period. However, whenever the rate of flow into
the swirl chamber is not sufficient to fill the chamber to the overflow weir
crest, all of the storm runoff will discharge through the foul sewer into the
soil collection pond. Thus, the rate of flow into the pond will vary from
0 to 32 I/sec (1.1 cfs). Hence, if it is desired to provide storage
for all underflow in a 4-hour storm the required storage would be 32 I/sec
(1.1 cfs) x 4 x 60 x 60, or 447 cu m (15,800 cu ft). This would re-
quire a pond 1.2 m (4 ft) deep and 18.9 m (62 ft) square. If a 2-hour deten-
tion time is considered adequate to settle out the suspended solids, then the
depth could be reduced to 0.61 m (2 ft) or the surface dimensions of the pond
reduced. An overflow weir should be provided to pass 32 I/sec (1.1 cfs)
when the pond becomes filled to the designed depth.
The chief advantage of such a temporary facility is that it is portable
and has no mechanical parts. Thus, the chamber could be moved about on the
construction site, as required, or moved to other sites. Multiple units could
be used to meet requirements of larger sites or to remove higher percentages
of suspended solids.
CONSTRUCTION COST
Site preparation is minimal, consisting of the leveling of about 25 sq m
(24 sq yd) for each 4 m (12 ft) diameter unit
The unit will ordinarily be fabricated of steel off-site and delivered
intact. This site work would consist of leveling the unit and connecting the
inlet and two discharge lines.
The cost estimate is as follows:
170
-------
Cost Estimate Per 4 m (12 ft) Diameter Unit
Site preparation $ 400
Material and fabrication 6,500
Setting and field connection 1,100
Sub Total 8,000
Engineering and contingency at 25% 2,000
Total $10,000
PROTOTYPE INSTALLATION
A prototype was tested in South Carolina at a newly constructed
highway site which was known to be actively eroding and contributing
abundant sediment to receiving waters adjacent to the site.
A standard unit of 3.6 m (12 ft) was constructed. The area served was
1.5 ha (2.1 ac) with an estimated peak discharge for a one in two years
precipitation event of 165 I/sec (5.8 cfs).
The calculated efficiency of the unit based upon a grain size analysis
of a composite of six samples from the bed material at the base of the
drainway was 98,7 percent with 10 percent foul flow.
During the desired test periods, it did not rain. Tests were
eventually run with tank trucks discharging water to synthesize runoff from
the roadway. It appears that there were no erosion control devictb to protect
the swirl and thus the device received a concentrated flow of sediment with
particles as large as gravel. The unit essentially failed under the test
conditions. The heavier'larger size particles settled rapidly on the floor of
the swirl unit and the flow was of such a short duration that the solids were
not moved to the foul outlet. The test procedures used indicated essentially
the same particle size distribution in the overflow and the foul outlet.
It is apparent from the test results that there was a large bedload
flow and that suspended solids as they entered the unit joined the bedload.
Thus, in the absence of moving the bedload to the foul outlet, only minimal
particle size variances were noted. The outlet was frequently clogged.
The test results are interesting and point towards the need to use the
unit as a part of an erosion control system. Prior to the publication of
the report on the test facility, additional tests are planned on another
site.
171
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Section VIII
CHARACTERIZATION OF SOLIDS
The efficiency of secondary flow motion devices is essentially deter-
mined by the degree that solids-are removed from the influent. The devices
have been considered efficient in the model studies when either standard or
arbitrary amounts of the particular size and weight of material have been
separated to the foul outlet. Thus for the purpose of the hydraulic model
studies it was necessary to l) select a "typical" solids composition for
each type of waste flow to be treated, and 2) select an appropriate synthetic
medium to represent the solids. Due to problems of scale up and handling,
the use of actual waste streams was not practical.
In addition to the assumptions that were made from the published litera-
ture, the firm of Beak Consultants, Ltd. was engaged to conduct additional
literature and laboratory tests to characterize solids in combined sewer
overflows, stormwater and sanitary sewage (8).
In this section the basic assumptions used for each type of pollutant
stream will be reviewed and compared to the basis of efficiency established
for particular devices.
COMBINED SEWER OVERFLOWS
Studies by Others
In common with other non-industrial pollutional loads, combined sewer
overflows have been found to vary widely in the concentrations and composition
of solids and pollutants. Table 33 reports overall results and cummulative
particle size distribution and results are shown in Figure 84 for comparison
with some other waste streams.
Total suspended and settleable solids concentrations found by various
investigators is shown in Table 34.
Table 35 presents the results of a study of the size of solids found
on streets, potential solids in stormwater runoff. The average distribution
is also shown in Figure 84. The solids particles are larger than found in
combined sewer overflows and should be more susceptible to treatment than
combined sewer overflows.
The settling velocity of the organic and grit solids for combined sewer
overflows was assumed to be as represented in Figure 85.
172
-------
Table 33
Particle Size Distribution of Suspended Solids in Combined Sewer Overflow
Source of Figures
(reference number)
Envirogenics Co.24
San Francisco, Cal
Meridian Engineers25'
Lancaster, Pa.
Size Range
(microns)
>3,327
991-3,327
295-991
74-295
<74
> 9,525
4,760-9,525
2,000-4,760
1,190-2,000
590-1,190
420-590
210-420
149-210
74-149
44-74
<44
Distribution
(percent)
5.1
8.8
15.9
21.8
48.3
1.77
1.06
1.40
1.88
3.10
2.78
7.01
5.19
20.1
23.8
31.91
* \The material tested represents those sotids retained in a catch basin. Sampling took .place
the week following the storm the week following the storm event. Thus, results are noftiirectly
applicable to all solids in combined sewer overflows. The particle sizes could be higher than in
the actual flow as some fractions of the smaller size ranges could have been carried through
the basin.
173
-------
Table 34
Solids Concentrations in Combined Sewer Overflows
Source of Figures Total Volatile
(Reference Number) Settleable Solids Suspended Solids Suspended Solids
ml/1 mg/l mg/l mg/l
Avg, Max. - Min. Avg. Max. Min. Avg. Max. Min. Avg. Max. Min.
Envirogenics Company" 2.58 4.0 0.05 67.6 426 4 52.2 373 4
Rex Chainbelt. Inc.26
a) Extended overflows 166 ± 90 ±
26 14
b) First flushes 522 ± 308 ±
(95% confidence level for a & b) 150 83
Hydrotechnic Corporation27
a) Spring storms (1971) 6.98 14.0 1.5 411 976 177
b) Summer and fall storms (1970) 5.26 19.0 0.2 2341.560 28
Envirogenics Company28
Winter 19681969
a) Start of storm 178.2 488 28230.5 502 56166.2 311 51
b) 3 hours after start 77.3 142 0106.3 186 47 91.7 186 26
c) 12-18 hours after start 112.2 210 28145.5 241 30 99.5 221 26
Symposium on Storm and
Combined Sewer Overflows29
Portland. Oregon 3.1 5.0 1.5 146 325 70 90 166 57
Milwaukee. Wisconsin
a) Extended overflows 133 58
174 87
b) First flushes 330 221
(95% confidence level) 848 495
Detroit Michigan
a) 1968 average of daily grab
samples — 59 locations 1.350 53
b) 1969 average of daily grab
samples — 59 locations 1.005 70
Bucyrus. Ohio — 3 sewer loca-
tions24 533 2.440 20 182 440 70
430 990 90 238 570 80
477 1.050 120 228 640 70
Engineering Science. Inc.30
San Francisco. Selby Street 145.0 <0.3 1.067 27 1.260 24 886 4
Laguna Street 40.0 2.0 483 53 264 28
Benzie and Courchaine31
Detroit. Michigan (1964) 150 1.398 23
Burm et al32
Detroit. Michigan (1965) 238 656 274 804 117 452
Dunbarand Henry33
Buffalo. New York 1.220 172
Buffalo. New York 544 158
Buffalo. New York 436 126
Detroit. Michigan 250
Toronto. Ontario 930 130
Toronto. Ontario 580 17
Wetland. Ontario 426 168
Weibeletal"
Cincinnati. Ohio (1962-1963) 210 1.200 5 53 290 1
174
-------
Table 35
Particle Size Distribution of Solids — Selected City Composites
Particle Distribution (Percent by Weight)
Range Milwaukee Bucyrus Baltimore Atlanta Tulsa
(microns)
>4800 120 — I7-4 ~~ ~~
2,000-4,800 12.1 10.1 4.6 14.8 37.1
840-2,000 40.8 7.3 6.0 6.6 y.4
246-840 20.8 20.9 22.3 30.9 16.7
104-246 5.5 15.5 20.3 29.5 17.1
43104 1.3 20.3 11.5 10.1 12.0
30-43 4.2 13.3 10.1 5.1 3.7
1430 2.0 7.9 4.4 1.8 3.0
414 12 4.7 2.6 0.9 0.9
<4 0.5 - 0.9 0.3 0.1
Note Columns may not total 100°c due to rounding
Source DBS Research Company (35)
Assumptions for Swirl Regulator and Helical Bend Separators
On the basis of available data, the concentrations, by size and speci-
fic gravity shown in Table 36, were selected. This selection is shown
graphically in Figure 86.
A design objective for both separator designs was for the capture of
all grit particles of 0.35 mm diameter. Capture of this amount will result
in the separation of various amounts of other sized particles. From
Figures 85 and 86 it can be readily determined that with the 100 percent
capture of 0.35 mm grit, 82 percent of the grit and 68 percent of the
organics will be captured at 100 percent efficiency.
SANITARY SEWAGE
Various studies have indicated a rather wide range of particle sizes
in sanitary sewage. Table 37 indicates the results of five studies. In
Table 38 the classification of solids is shown by total and volatile sus-
pended solids as determined by a variety of investigators.
Settling velocity tests were made at the Northeast Water Pollution
Control Plant in Philadelphia, Pennsylvania. Figure 87 indicates the average
values found for the three samples. The percent settleable solids ranged
from 63 to 84 percent and the median settling velocity observed was
0.054 cm/sec (0.0017 ft/sec).
The conventional method for establishing efficiency of primary
settling facilities has been to set the overflow rate, i.e., the liters
per day per square meter (gal/day/sq ft) with a minimum depth. Although
performance requirement varies widely, 60 percent suspended solids
removal is assumed as normal.
Available data on the mechanical analysis of grit removed from repre-
sentative wastewater treatment plants were compared to establish criteria
for grit sizes for this study.
175
-------
Table 36
Specific Gravity, Size, and Concentration of Settleable Solids
for Combined Sewer Overflows
Material
1) Settleable
excluding grit
2) Grit
3) Floatable
solids
Specific Concentration
Gravity (mg/l)
Particle
Size
Particle Size Distributed
1.05-1.2
2.65
0.9—0.998
200-1550
20-360
10-80
0.2—5mm Particle size (mm) 0.2 0.5 1.0 2.5 5.0
% by weight 10 10 15 25 40
0.2—2mm Particle size (mm) 0.2 0.5 1.0 1.5 2.0
% by weight 10 10 15 25 40
5—25mm Particle size (mm) 5 10 15 20 25
% by weight 10 10 20 20 40
Table 37
Particle Size Distribution of Suspended Solids in Sanitary Sewage
Source of Figures
Hunter & Heukelekian36
(average of two studies)
a) Winter-spring 1959
b) Fall-winter 1959-1960
Huekelekian & Balmat"
Meridian Engineers25
Painter, Viney & Bywaters38
*Note Remainder passed No 200 mesh
Particle Size Range
(microns)
> 100 (Settleable)
1 — 100 (supracolloidal)
0.2- 1.0 (colloidal)
>100
1 — 100
0.2 - 1.0
> 1,190 (0.047 in.)
590 - 1,190
420 — 590
210 — 420
<149
>100
1 - 100
0.2 — 1.0
Distribution
(percent)
49.4
31.4
19.2
47.0
34.0
19.0
4.42
1.38
3.46
3.09
86.9
37.1
44.8
18.1
176
-------
10,000
1,000
-------
U.S. Standard Sieve Numbers
400 300 140 70 40 30
0.01
0.03
0.04 0.08
0.60 1.0
Particle diameter, mm
10.0
8.0
1 in. = 2.54 cm
Figure 85 Particle Settling Velocities for Grit and Organic Material in Still Water
178
-------
3 4
U.S. Standard Sieve Numbers
8 10 16 20 30 40 50 70 IOO 140
1
\
\
1
\
\
1
\
ORGANI
\ I
\
\
;
cs^
SG 1.20
v
\
^
I
I
*•
\
\
*>**
\
\
-• —
->
\
\
\
G
-1
y
\
\
ill
G 2.
^
\
65
\
V^v
1 1
\.
IUU
Q/\
if\J
on
OU
7O
«n £
bU o>
'55
50 -°
0)
c
il
40 ^
*rU
30
orv
C.\J
in
\\j
B 4 2 1 0.6 0.4 Q2 O.I v
Grain size in mm
Finel Medium | Fine
1
U.S. SIEVE SIZE
4
10
20
40
50
70
SIZE
mm in.
5.0 (0.020)
2.0 (0.08)
0.84 (0.034)
0.42 (0.017)
0.30 (0.0121
0.20 (0.008)
% FINER BY WEIGHT
GRIT
IOO
IOO
63
31
18
0
ORGANICS
IOO
53
31
17
14
10
Figure 86 Typical Gradation for Grit and Organic Material
179
-------
ft/Sec
331
X101
•o
c
a
£
o
o
o
33
X10»
10.
9 •
8 .
7 .
6 •
5 •
01.
9.
33 0-01
x103
98
95
90
80 70
60 50 40
20
J I
III!
TTi i i i
20 30 40 50 60 70 80
95
_L
98
Percent less than or equal to
Figure 87 Settling Velocity Distribution of Solids in Sanitary Sewers
180
-------
Table 38
Sieve Analysis of Samples from Grit Chambers
Percentage Finer by Weight
Sieve
(D
(2) (3) (4)
(5)
Designation Green
mm
6.3
4.75
3.35
2.36
2.00
0.850
0.600
0.425
0.300
0.212
0.180
0.150
0.075
U.S.
Sieve No.
1.2 mm (0.5 in.)
4
6
8
10
20
30
40
50
70
80
100
200
Bay
Wis.
1/
96.3
90.9
80.2
70.4
48.3
21.8
3.9
Kenosha Tampa St. Paul
Wis. Fla Minn.
1/ 1/ 1/
99.0
95.0
88.0
88.0
30.0
97.7 80.0
5.0
40.7 3.0
0.5
St. Paul
Minn.
1/
93.0
80.0
47.0
33.0
0.1
(6)
(7)
(8)
Winnipeg Winnipeg Denver
Manitoba Manitoba Colo
21
96.9
83.2
44.3
19.2
4.4
21
77.1
46.3
38.9
14.7
6.3
3.5
1.3
21
94.9
89.2
75.2
6.7
0.7
Notes:
1/ adapted (com data in ASCE Manual No 36,1959 edition
2-8/all data adapted from correspondence, 1973
(4) upper range
(5) lower range
(6) inlet end
(7) outlet end
Data from eight existing plants located in the United States and Canada
are tabulated in Table 38.
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 88.
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 0.2 mm.
Based on the foragoing, 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 39 and shown graphically in
Figure 89.
181
-------
_IM I/I I* 1/4 4 •
U.S. Standard Sieve Numbers
• « II )• MM 40 M TO 100 MO
»0 4.0
tO 1.0 0.«
Grain size in mm
0 4
O.I
100
O.I 0.0«
FINE
GRAVEL
COARSE I MEDIUM
SAND
FINE
Legend
Green Bay
Kenosha
Tampa
St. Paul (lower range)
St. Paul (upper range
Winnipeg (inlet end)
Winnipeg (outlet end)
Metro Denver
1
2
3
4
5
6
7
8
Figure 88 Gradation Curves of Samples from Grit Chamber
182
-------
Table 39
Typical Grit Gradation
Size
mm
2.000
0.850
0.420
0.300
0.212
in.
0.080
0.034
0.017
0..012
0.008
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
EROSION PRODUCTS
The types of solids found in construction site stormwater runoff is
even more site-specific than the other pollution streams which have been
discussed. Hazen (19) has reported that settling velocities of soil
materials can range from 0.015 cm/sec (0.0005 ft/sec) for silt 10 microns in
size to 10 cm/sec (0.33 ft/sec) for coarse sand, 1,000 microns in size. The
smallest size particles that the swirl unit can remove at its design flow is
43 microns in diameter with a Specific Gravity of 2.65 and a settling velocity
of 0.14 cm/sec (0.0046 ft/sec). Larger particles of lighter specific gravi-
ty will also be removed as described in Section VII.
A standard of efficiency for the swirl unit is not feasible. Rather,
the amount of polishing of flow can be determined based upon an analysis of
the soil particles which will reach the unit.
183
-------
U.S. Standard Sieve Numbers
Size opening — inches
I* k/t M 1/4 4 • t 10 It l« |« M 40 M TO 100 MO MO ^
10
70
•0
•0
40
to
to
10
{
•
I i
RO CX
\
t
10 4
T
l
0 t
V
\
\
\
0 1.
1
^y
Si
V
>
0 C
\
1
\
• 0
\
\
1
\
\
4 C
1 1
t 0 I
0
o
TO Z
0
Uf
to
to
X
40 U.
*
to
to
•0
0
M
QftAIN SIZE IN MM
FINE
Gravel
COAR-SE 1 MEDUM I FINE
Sand
Gradation
Size U.S. Sieve % Finer
mm No. by Weight
2.000 10 100
0.850 20 63
0.425 40 31
0.300 50 18
0.212 70 0
Figure 89 Gradation Curve of Typical Grit
184
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Section IX
GLOSSARY
Comb ined Sewer — A pipe or conduit which collects and transports sanitary
sewage, with its component commercial and industrial wastes and
infiltration and inflow during dry-weather conditions, and which, in
addition, serves as the collector and conveyor of stormwater runoff
flows from streets and other sources during precipitation and thaw
periods, thus handling all of these types of wastewaters in a
"combined" facility.
Concentrate — The portion of the inflow directed to the interceptor sewer
which carries the bulk of the settleable solids.
Concentrate Outlet — The outlet in the floor of the chamber in which the
concentrate enters the foul sewer. Also see Foul Sewer.
Concentric Skirt — A vertical sheet or panel, constructed in circular form
concentric with the outer diameter and the overflow downshaft pipe in
a swirl chamber for the purpose of separating flow zones and acting as
a suppressant of any short-circuiting of flow patterns or the overflow
of floating solids with the effluent.
Deflector — A plate or plane structure which diverts and directs flows in a
swirl separator chamber into desired patterns and thus prevent flow
kinetic conditions which would interfere with optimum swirl motion.
Depth of Chamber — The vertical distance between the floor level of the
swirl separator and the crest of the overflow weir.
Diameter of Swirl Chamber — The internal diameter of the separator chamber.
Dip Plate — A vertical plate or baffle which is partially immersed in
flowing liquid in a manner that will prevent the discharge of surface
or floating materials over an outlet weir in the swirl and helical bend
regulator/separator.
Dry-Weather Flow — The flow in the combined sewer during periods without
precipitation, normally sewage and groundwater infiltration.
185
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Erosion — The washing or scouring action of stormwater on the land,
resulting in the displacement and movement of grit, silt and other
indigenous solids with the wastewater flow; the type of solids which
are intended to be removed from the flow by the swirl separator
chamber.
Exterior Liquid Mass — The liquid induced to flow in the outer zone of the
circular swirl separator chamber, by use of the skirt, wall structural
configuration or other built-in devices, where the higher velocities of
flow produce a longer liquid trajectory which allows adequate time for
heavier solids to settle to the floor of the chamber.
Fleatables — Solid and liquid matter which is lighter than water and float
on the surface of the wastewater flowing in the swirl and helical
regulator/separator.
Floatables Trap — A device or structural configuration in a swirl separator
chamber which intercepts floatable solids, prevents them from overflow-
ing from the chamber with clarified wastewater, and retains these
materials at a desired location until removed and disposed of by pre-
determined means.
Flow Spoiler — Vertical energy dissipating baffle or plate installed on the
weir disc or elsewhere in the swirl separator chamber for the purpose
of preventing excessive flow disturbances and dampening the development
of free vortex flow patterns and other undesired flow conditions in the
chamber.
Foul Sewer — A sewer line, from the bottom of the swirl separator chamber to
some point of discharge to an interceptor sewer, a catchment basin or
other point of solids disposal, installed for the purpose of drawing
off the solids concentrate flow from the swirl chamber due to the
recovery efficiency of the device.
Grit — Solids, predominantly mineral in character, in the combined sewer
flow which are larger than 0.2 mm (0.008 in.) and with a specific
gravity 2.65.
Gutter — A structural configuration in the floor of the swirl separator
which provides a channel for the desired flow of sanitary wastewater
during dry-weather conditions from the chamber inlet to the foul sewer
(concentrate) outlet, and during wet-weather for conducting the foul
concentrate to the foul sewer.
Helical Bend — A physical configuration of a pipe or open channel which
results in a bend or radius through which a liquid flow occurs in a
manner that produces helical, or secondary flow phenomena, inducing the
rapid separation of solids from the liquid and the deposition of the
solids along the inner diameter of the radius; in the study, the total
helical bend combined sewer overflow regulator/separator consists of a
transition section, a straight section, and the bend section.
186
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Helical Flow — The pattern of liquid flow induced by the helical bend
combined sewer overflow regulator/separator characterized by a
helical configuration, or secondary motion, created in the liquid flow.
Hydraulic Head Loss — The lowering of the hydraulic grade line through a
pipeline, device, chamber or other facility, due to dynamic conditions
which produce friction, turbulence or other conditions that are
translated into loss of pressure, or head, or free water gradient
surface level.
Inlet Baffle — A structural plate installed from the inlet to the overflow
weir for the purpose of producing or inducing the desired flow pattern
in a swirl chamber; a device to serve as a guide for the incoming flow
and to place it in circulatory action to take full advantage of the
swirl secondary flow pattern in the chamber.
Inlet Size — The diameter or square dimensions of the sewer which enters the
swirl separator at its floor level and thereby, serves to create the
flow pattern which produces the solids-liquid separation which the
chamber is intended to induce.
Interior Liquid Mass — The liquid induced to flow in the inner zone of the
circular swirl separator chamber—by use of the weir skirt, wall
structural configuration or other built-in devices which induce
exterior liquid mass flows—where the lower velocity permits lighter
solids to settle out of the wastewater flow and to deposit on the
chamber floor and to be drawn to the foul sewer outlet. The principle
of the swirl separator is to organize the flow patterns and cause the
liquid mass to pass through the exterior and interior liquid mass
zones to optimize solids separation and removal.
Long-Flow Pattern — The swirl flow pattern through the swirl separator,
induced by proper baffling which causes the liquid to travelse the
circular chamber more than once, and prevents the incoming flow from
being diverted or short-circuited directly to the overflow weir,
thereby inducing the solids to discharge into the foul gutter and foul
sewer outlet.
Organic Solids — Solids of a non-grit, or lighter weight, contained in the
combined sewer flow, which can decompose and become oxygen-demanding
in receiving waters.
Overflow Weir — The structural member of the swirl chamber, which is built
as a central circular wall with a proper form of overflow edge over
which the clarified wastewater can discharge to the downshaft outlet
leading to receiving waters or to holding or treatment facilities.
Regulator — A device or apparatus for controlling the quantity of sewage and
stormwater admitted from a combined sewer collector sewer into an
interceptor sewer, pumping or treatment facility. The secondary flow
motion regulators described in this manual also improve the quality of
the overflow to receiving waters.
187
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Scaling — The principle of ascertaining dimensions and capacities of
hydraulic model test units and mathematical analysis systems to
evaluate the performance of swirl chambers, and to scale up such sizes
to provide actual field design and construction criteria or parameters.
Scum Ring — A circular plate or baffle encircling the overflow weir, located
at a predetermined distance from the weir and at a depth that will cause
it to retain floatables and scum and prevent them from passing over the
weir crest with the clarified liquid.
Settleable Solids — That portion of the solids contained in the wastewater
flow into a swirl separator chamber which will subside and be collected
in the chamber due to gravity and other liquid-solids kinetic conditions
induced by the controlled swirl flow pattern. (Note: Not all suspended
solids are settleable solids, nor are so-called colloidal solids or
other finely dispersed solids settleable solids.)
Spoiler (Energy Dissipating Ba_ff_le) — A plate or structural plane constructed
from the scum ring to the downshaft on the weir plate in a swirl
separator chamber for the purpose of preventing or dampening the
development of free vortex flow conditions, minimizing agitation and
rotational flow over the discharge weir, and increasing the capacity of
the downshaft.
Static Regulator — A regulator device which has no moving parts, or has
movable parts which are insensitive to hydraulic conditiors at the
point of installation and which are not capable of adjusting themselves
to meet varying flow or level conditions in the regulator-overflow
structure.
Storm Frequency — The time interval between storms for which storm sewers
and combined sewers, and such appurtenant structures as swirl separator
chambers, are designed to handle or treat without flooding and or for
desired treatment efficiency.
Straight Section — The part of the helical bend combined sewer overflow
regulator/separator structure which precedes the bend section and
delivers the flow uniformly and without velocity interferences into the
helical section. In the studies of the helical bend principle, it was
determined that the straight section having a length of five times the
diameter of the sewer pipe will be required for effective solids
recovery in the helical bend section.
Spillway Channel — The channel or conduit which receives the overflow
effluent from the helical bend weir section and delivers it to a pipe
or conduit leading to receiving waters, or facilities for the retention
and/or treatment of the clarified wastewater discharge.
188
-------
Suspended Solids — 1) The quantity of material deposited when a quantity of
water, sewage, or other liquid is filtered through an asbestos mat in
a Gooch crucible or a 0.35-0.45 micron millipore fiberglas filter. (39)
2) Solids that either float on the surface of, or are in suspension,
in water, wastewater, or other liquids, and which are removable by
laboratory filtering as described above.
Swirl Chamber — A cylindrical tank or chamber, in which the shape, method of
inflow and overflow, and internal appurtenant structures induce a
secondary motion flow pattern which produces the desired separation of
solids from the liquid flow.
Swirl Combined Sewer Overflow Regulator/Separator — In the context involved
in this study and report, a chamber with necessary appurtenant
structural configurations which will kinetically induce a rotary motion
to the entering wastewater flow from a combined sewer, resulting in
secondary motion phenomena which will cause a concentration of solid
pollutional materials at a predetermined location, from which it can
be diverted into the foul sewer, thereby producing a partially clarified
waste for decantation or overflow into receiving or storm overflow
treatment facilities.
Transition Section — That portion of the helical bend combined sewer overflow
regulator/separator which carries the combined sewer flow from the
entering sewer pipe section and delivers it to the straight section and
thence to the bend section; the transition section in the studies had
a length of at least fifteen times the inlet sewer diameter and expanded
the flow cross section to three times the inlet diameter.
Underflow — The concentrate, containing the recovered solids, which is
withdrawn from the bottom of the swirl concentrator for erosion runoff
treatment; the converse of the clarified overflow.
Weir Plate — A plate or surface constructed contiguous with the outlet over-
flow weir of a swirl chamber. In the swirl combined sewer overflow
regulator/separator, a weir skirt hanging below the weir traps float-
ables and holds them until released for removal from the chamber.
Weir Skirt — A plate hanging below the weir plate, to assist in retaining
floatable solids under the weir plate and in inducing the shearing of
the chamber flow into an exterior liquid mass and an interior liquid
mass, thereby optimizing the solids separation effectiveness of the
swirl concentrator principle.
WWF (Wet-Weather Flow) — The flow in the combined sewer caused by rainfall
or snow melt and the dry-weather flow.
189
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SECTION X
REFERENCES
1. American Public Works Association - "The Swirl Concentrator as a
Combined Sewer Overflow Regulator Facility" - EPA Report No. EPA-
R2-72-008, NTIS No. PB 214 134, September, 1972.
2. Sullivan, R.H., et al - "Relationship Between Diameter and Height
for the Design of a Swirl Concentrator as a Combined Sewer Overflow
Regulator" - EPA Report No. EPA-670/2-74-039, NTIS No. PB 234 646,
July, 1974.
3. Sullivan, R.H., et al - "The Helical Bend Combined Sewer Overflow
Regulator" - EPA Report No. EPA-600/2-75-062, NTIS No. PB 250 619,
December, 1976.
3a. Drehwing, F.J., et al - "Disinfection/Treatment of Combined Sewer
Overflows" - Syracuse, New York. EPA-600/2-79-134, U.S. Environmental
Protection Agency, Cincinnati, Ohio, August 1979.
4. Sullivan, R.H., et al - "The Swirl Concentrator as a Grit Separator
Device" - EPA Report No. EPA-670/2-74-026, NTIS No. PB 233 964,
June, 1974.
5. Sullivan, R.H., et al - "Field Prototype Demonstration of Swirl
Degritter" - EPA Report No. EPA-600/2-77-185, NTIS No. PB 272 668,
September, 1977.
5a. Drehwing, F.J., et al - "Combined Sewer Overflow Abatement Program" -
Rochester, New York - Volume II. Pilot Plant Evaluations. EPA-600/2-
79-031b, U.S. Environmental Protection Agency, Cincinnati, Ohio,
August 1979.
6. Sullivan, R.H., et al - "The Swirl Primary Separator: Development
and Pilot Demonstration" - EPA Report No. EPA-600/2-78-122, August,
1978.
7. Sullivan, R.H., et al - "The Swirl Concentrator for Erosion Runoff
Treatment" - EPA Report No. EPA-600/2-76-271, NTIS No. PB 266 598,
December, 1976.
8. Dalrymple, R.J., et al - "Physical and Settling Characteristics of
Particulates in Storm and Sanitary Wastewaters" - EPA Report No.
EPA-670/2-75-011, NTIS No. PB 242 -01, April, 1975.
8a. EPA Technology Transfer. Capsule Report entitled'Swirl Device for
Regulating and Treating Combined Sewer Overflows." EPA-625/2-77-012,
U.S. Environmental Protection Agency, Cincinnati, Ohio, 1977.
190
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8b. Field, R. , "Design of a Combined Sewer Overflow Regulator/Concentrator."
Journal Water Pollution Control Federation, 46(7): 1722-1741,
July, 1974.
8c. Field, R. and Traver, R.P., "Urban Runoff Flow Regulator/Concentrators."
National Conference on Environmental Engineering, American Society of
Civil Engineers. San Franc-isco, CA, July 10, 1979.
9. Smisson, B., "Design Construction, and Performances of Vortex Over-
flows," Proceedings, Symposium on Storm Sewage Overflows, Institution
of Civil Engineers, May 4, 1967, pp. 99.
10. Prus-Chacinski, T.M., and Wielgorski, J.W., "Secondary Motions Ap-
plied to Storm Sewage Overflows," Proceedings, Symposium on Storm
Sewage Overflows, Institution of Civil Engineers, May 4, 1967, pp. 89.
11. Ackers, P., Harrison, A.J.M. and Brewer, A.J., "Laboratory Studies
of Storm Overflows with Unsteady Flow," Proceedings, Symposium on
Storm Sewage Overflows, Institution of Civil Engineers, May 4, 1972,
p. 37.
12. Sullivan, R.H., et al - "Nationwide Evaluation of Combined Sewer
Overflows and Urban Stormwater Discharge," Volumes 1,2,3. EPA Re-
port No. EPA-600/2-77-064a.b,c, NTIS No. PB 273 133, PB 266 005,
PB 272 107, September, 1977.
13. American Public Works Association, "Combined Sewer Regulator Overflow
Facilities," 110022DMU 07/70, U.S. Environmental Protection Agency,
1970, pp. 139.
14. Prus-Chacinski, T.M., "The Secondary Flow in a Meandering Channel."
Ph.D. Thesis, University of London, 1955.
15. American Public Works Association, "Combined Sewer Regulation and
Management," 11022DMU 08/70, U.S. Environmental Protection Agency,
1970, pp. 134.
16. American Society of Civil Engineers and The Water Pollution Control
Federation. "Design and Construction of Sanitary and Storm Sewers."
ASCE-Manual and Reports on Engineering Practice -No. 37, WPCF Man-
ual of Practice No. 9, 1969, pp. 33.
17. American Society of Civil Engineers and the Water Pollution Control
Federation. "Sewage Treatment Plant Design," Manual No. 36, ASCE-
WPCF, New York, 1959.
191
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18. Smith, R. "Preliminary Design of Simulation of Conventional Waste-
water Renovation Systems Using the Digital Computer" U.S. Depart-
ment of the Interior, FWPCA, 1968.
19. Poertner, H., et al. "Practices in Detention of Urban Stormwater
Runoff," APWA Special Report No. 43, Chicago, 1974.
20. Lager, J.A., et al. "Urban Stormwater Management and Technology, An
Assessment," EPA-670/2-74-040, U.S. Environmental Protection Agency,
Cincinnati, Ohio, NTIS No. PB 240 687.
21. Lager, J.A., et al. "Urban Stormwater Management and Technology
Update and Users' Guide." EPA-600/8-77-014, U.S. Environmental
Protection Agency, Cincinnati, Ohio, NTIS No. PB 275 654, September,
1977.
22. White, R.A., "A Small Scale Swirl Concentrator for Storm Flow,"
Masters Thesis, University of Wisconsin, Milwaukee, May, 1974.
23. Benjes, H.H. Jr., et al. "Cost Estimating Manual - Combined Sewer
Overflow Storage Treatment." EPA-600/2-76-286, U.S. Environmental
Protection Agency, Cincinnati, Ohio, NTIS No. PB 266 359, December,
1976.
24. Envirogenics Co., Div. of Aerojet General Corp., "In-Sewer Fixed
Screening of Combined Sewer Overflows." 11024FKJ 10/70, NTIS No.
PB 213 118, U.S. Environmental Protection Agency, October, 1970.
25. Krantz, J., et al. Lancaster Silo Project: "Particle Sizing and
Density Study." Meridan Engineers, Philadelphia, Pennsylvania,
January, 1973.
26. Ecology Div., Rexchainbelt, Inc., "Screening/Flotation of Combined
Sewer Overflows." 11020FDC, 01/72, U.S. Environmental Protection
Agency, January, 1972.
27. Nebolsine, R., et al. "High Rate Filtration of Combined Sewer
Overflows." 11023EYI04172, U.S. Environmental Protection Agency,
April, 1972.
28. Envirogenics Co., Div. of Aerojet General Corp., "Urban Storm Runoff
and Combined Sewer Overflow Pollution, Sacramento, California,"
11024FKM12171, NTIS No. PB 208 989, U.S. Environmental Protection
Agency, Cincinnati, Ohio, December, 1970.
192
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29. "Combined Sewer Overflow Technology," 11024—06170, NTIS No. PB 193
939, U.S. Environmental Protection Agency, Washington, D.C., June, 1970.
30. Engineering - Science, Inc.,"Characterization and Treatment of Combined
Sewer Overflows." Div. of Research and Training Grants, WPD-113-01-66,
U.S. Environmental Protection Agency, November, 1967.
31. Benzie, W.J., and Courchaine, R.J., "Discharges from Seperate Storm
Sewers and Combined Sewers," Journal, Water Pollution Control
Federation, 38:410, 1968.
32. Burm, R.J., et al - "Chemical and Physical Comparison of Combined and
Seperate Sewer Discharges," Journal, Water Pollution Control Federa-
tion, 40:112, 1968.
33. Dunbar, D.D., and Henry, J.G.F.."Pollution Control Measures for Storm-
water and Combined Sewer Overflows," Journal, Water Pollution Control
Federation, 38:19, 1966.
34. Weibel, S.R., et al. "Urban Land Runoff as a Factor in Stream
Pollution," Journal, Water Pollution Control Federation, 36:914, 1964.
35. Sartor, J.D., and Boyd, G.B., "Water Pollution Aspects of Street
Surface Contaminants," EPA-R2-72-081, NTIS No. PB 214 408, U.S.
Environmental Protection Agency, Cincinnati, Ohio, November, 1972.
36. Hunter, J. V., and Heukelekian, H., "The Composition of Domestic Sewage
Fractions," Journal, Water Pollution Control Federation, 37:8:1, 142-
151, 163, 1965.
37. Heukelekian, H., and Balmat, J., "Chemical Composition of the Particu-
late Fractions of Domestic Sewage," Sewage and Industrial Wastes,
31:4:413, April, 1956.
38. Painter, H.A., and Viney, M., "Composition of a Domestic Sewage,"
Journal of Biochemical and Microbiological Technology and Engineering,
1:143, 1959.
39. Puch, A.B., "Aquaculture Pollutants and Their Potential Treatment,"
Masters Thesis, Clemson University, 1977.
40. Veliglu, S.G., "Vortex Type Sedimentation Tank," Masters Thesis,
Bogazici Universitesi, Turkey, 1972.
193
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SECTION XI
APPENDIX - OTHER APPLICATIONS AND DEVICES
The fundamental work which was used to develop the family of secondary
flow solids separation devices fostered interest by other researchers
interested in treating other wastewater streams. Such devices are in various
stages of development and testing. Reference is made to them in this manual
to indicate other applications of the flow principle.
The following listed devices will be highlighted:
1. An advanced primary treatment unit,
2. A device to use when bypassing excess sewer flow, and
3. A treatment unit for aquaculture wastes.
(R)
In addition, a short description of the Hydro-Brake will be given.
This unit, while not used for treatment, uses secondary motion, and has no
moving parts and acts as an effective foul sewer flow control.
ADVANCED PRIMARY TREATMENT
Bernard Smisson, the developer of the swirl principle for combined
sewer overflow regulations and treatment has continued his work to other
applications including primary treatment. Following the hydraulic model
studies which led to the design of the unit described in Section VI of this
manual, Smisson introduced moving scrapers to hasten sludge collection and
reduce the overall size of the unit. Laboratory results need to be con-
firmed with a larger scale unit and closer control of the solids used to
represent wastewater solids before a prototype unit is constructed. However,
the general approach appears promising.
SEWER BYPASS POLLUTION CONTROL
Russell Allen White, while a graduate student of the University of
Wisconsin, Milwaukee, was concerned with the pollution problems resulting
from the discharge from pumps when overloaded sewers were relieved to minimize
basement flooding. He developed, through hydraulic model tests, a small unit
through which the pump could discharge. (22) The foul flow was then returned
to the sewer, and only the clarified effluent discharged to national waterways,
194
-------
Such a device represents a practical interim measure to treat part of the
flow until such time as the system has infiltration and inflow removed or
sufficient transport capacity established.
TREATMENT FOR AQUACULTURE WASTES
Andrew B. Buch, while a graduate student at Clemson University exten-
sively studied the methods and problems associated with treatment of fish
rearing wastes (39) Many forms of aquaculture are being advanced as a means
of providing protein to the world population. Such facilities use a large
flow-through volume of water which must be treated before discharge. A
variation of the swirl separator was adapted to allow partial treatment of
the flow prior to recycling to minimize total quantities needed. The wastes
of interest are solids, BOD,-j and ammonia. A primary finding of a comparative
study of several treatment systems found that "the swirl primary unit requires
one-third the volume of normal retention basins with more effective treatment.
Since reduced construction and maintenance costs also make this unit attrac-
tive, it should be seriously considered for aquaculture pollution abatement."
CR")
HYDRO-BRAKEV '
A Hydro-Brake is a patented flow controller made of stainless steel.
It is self-regulating and has no moving parts. It requires no power, but
uses the static head of stored water to operate its own "energy" to retard
the flow. The movement of water through a Hydro-Brake involves a swirl
action, dissipating energy to control the rate of discharge. Although the
function of a Hydro-Brake is somewhat similar to an orifice, it has certain
important advantages:
1. It permits a much larger opening for passage of the same amount of
water. This is particularly important where clogging is a
possibility, such as, for instance, in catch basins. It is also
important where sanitary or combined sewage flows are being regu-
lated.
2. The flow rate of a Hydro-Brake is not significantly affected by a
variation in head. This is important where it is desirable to
maintain a relatively large passage for the water, yet also main-
tain a fixed maximum rate of flow during peak conditions.
3. The outflow from a Hydro-Brake does not create a high velocity jet
stream as an orifice will, thus avoiding scouring inside sewer pipe.
The Hydro-Brake was invented in Denmark about 15 years ago and has,
since 1975, been marketed in North America by Hydro Storm Sewage Corporation
of New York.
The flow control device has been used successfully on units at Boston,
Massachusetts and Lancaster, Pennsylvania. The Hydro-Brake must be designed
for the specific application and flow condition. The design is patented and
units are'available only through the Hydro Storm Sewage Corporation,
195
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New York City. Units may be very small, say for a 10 cm (4 in.) diameter
pipe, or several meters in diameter.
OTHER WORK
A vortex type solid-liquid separator was tested in 1972 by
S. Giray Veliglu while a graduate student at Bogazici University,
Istanbul, Turkey. (40) The unit was designed to be used as an inter-
mediate unit in water and wastewater treatment. Available data does
not allow direct comparisons to the units described in this manual.
The Turkish report concluded, however, that the unit met their
design objectives.
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a US GOVERNMENT PRINTING OFFICE 1982 -559-092/0414
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