Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02707777 2012-07-11
DENSITY BAFFLE FOR CLARIFIER TANK
Field of the Invention:
This application relates to a baffle and baffle system for use in a solids-
precipitating clarifier tank. More particularly, the application relates to a
baffle and
baffle system having a plurality of inter-engaged individual baffles secured
to the clarifier
tank peripheral wall.
Prior Art Discussion:
Passive baffle devices, also known in the art as a lamella gravity separators
or
settlers, are used in clarifier tanks for waste treatment for gravitationally
separating
suspended solids from solids containing carrier liquid or fluid suspensions.
The clarifier
tanks, with which such baffles are typically used, are circular or
rectangularly configured
tanks in which a centrally mounted radially extending arm is slowly moved or
rotated
about the tank at or proximate the surface of the carrier liquid.
Specifically, in waste water treatment facilities utilizing secondary
clarifiers, the
clarifier's effectiveness in removing solids is perhaps the most important
factor in
establishing the final effluent quality of the facility. A major deterrent to
effective
removal is the presence of sludge density currents which cause hydraulic short
circuits
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within the tank. These short circuits, in turn, allow solids concentrations to
unintentionally bypass the tank's clarification volume and enter the effluent.
In the prior art, peripheral baffles are attached to the tank wall and
directed
downward at an angle into the tank. These baffles help to interrupt the
density currents
and properly redirect the flow of solids away from the effluent and into the
main
clarification volume (center) of the tank.
However, although these density baffle systems work to significantly reduce
solids from entering the effluent, under greater load conditions these baffle
systems
occasionally fail, allowing for the above described short circuits.
Summary:
The present arrangement overcomes the drawbacks associated with the prior art
providing for a density current baffle and installation employing the same,
with an
inclined surface, having a modified angle of attachment, dimensioned to
minimize the
density currents and properly redirect the flow of solids away from the
effluent and into
the main clarification volume (center) of the tank.
To this end, a baffle system is used in a clarifier tank having a tank bottom
and a
periphery and a substantially vertical peripheral wall bounding the interior
of the tank.
The baffle system has a plurality of baffles mounted on the peripheral wall of
the clarifier
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tank. Each baffle has a baffle surface with a lower end and an upper end. The
upper end
of the baffle surface is coupled to the side wall of the clarifier tank wall.
The lower end
of the baffle surface portion is disposed, at a substantially 600 angle away
from the side
wall of the clarifier tank such that the baffle surface slopes downwardly and
away from
the side wall.
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Brief Description of the Drawings:
The present invention can be best understood through the following description
and accompanying drawings, wherein:
Figure 1 shows a clarifier tank and density baffle in accordance with one
embodiment;
Figure 2 shows the density baffle within a clarifier tank in cross section
view, in
accordance with one embodiment;
Figure 3 shows a close up view of a density baffle surface from Figure 1 in
accordance
with one embodiment;
Figure 4 shows a schematic diagram of the baffle of Figure 1, in accordance
with one
embodiment;
Figure 5 shows a first set of exemplary test results;
Figure 6 shows a second set of exemplary test results;
Figure 7 shows a third set of exemplary test results;
Figures 8A and 8B show a fourth set of exemplary test results;
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Figure 9 shows a fifth set of exemplary test results, in accordance with one
embodiment;
Figure 10 shows a sixth set of exemplary test results, in accordance with one
embodiment;
Figure 11 shows a seventh set of exemplary test results, in accordance with
one
embodiment;
Figure 12 shows an alternative baffle surface, in accordance with one
embodiment; and
Figure 13 shows an alternative baffle surface coupled to a tank, in accordance
with one
embodiment.
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Detailed Description:
In one arrangement, as shown in Figure 1, a density current baffle 10 is shown
attached to a tank wall T. Density baffle 10 is made from a plurality of
connected baffle
surfaces 12, each of which forming a portion of baffle 10 about the
circumference of tank
wall T.
Bracket elements 14 are positioned under baffle surfaces 12, preferably at the
connection points between adjacent baffle surfaces as shown in Figure 1. In
one
arrangement, an upper mounting flange 18 is located at the top edge of each of
baffle
surfaces 12 for coupling baffle surfaces 12 to tank wall T. Also as shown in
Figure 1, an
end flange 20 projects downward from each of baffle surfaces 12, substantially
perpendicular to tank wall T. Bracket element 14 and baffle surfaces 12 can be
molded
as an one piece fiberglass baffle.
Figure 2 shows a cut away view of baffle 10 within a typically circular type
clarifier tank C, having an influent I, tank wall T, a spillway effluent
channel and a weir
W. Sludge blanket S is shown at the bottom of clarifier tank C, referring to
the settled
solids.
In one embodiment, as shown in Figure 3, a close up view is shown of a single
baffle surface 12 of baffle 10. As shown in Figure 2, baffle surface 12 may
optionally
have one or more vent openings 22 located at the top surface. In one
arrangement, vents
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22 are formed as convex deformations of upper mounting flange 18. As noted
above,
baffle 10 is configured to prevent solids carried by density currents from
flowing
upwards and out of the clarifier tank However, because of the downward sloping
design
of baffle surfaces 12, some solids may become trapped and produce a buildup of
gases
damaging baffle surfaces 12 and possibly reducing their functionality. Vents
22 allow
these gases to escape to the surface without harming baffle 10.
Using the basic design as set forth above for baffle 10 and baffle surfaces
12, it
has been found by the inventor that by implementing certain advantageous
arrangements
of baffle surfaces 12, including the deflection angle of baffle surfaces 12
from tank wall
T, the length of projection of the bottom of baffle surfaces 12 from Tank wall
T into the
center of tank C and the position of baffle surfaces 12 at certain heights on
tank wall T,
the relative concentration of solids in the effluent may be substantially
reduced over the
prior art designs. The following description sets forth the salient features
of the baffle
10/baffle surfaces 12 in those respects.
As shown in Figure 4, the schematic drawing identifies the measurements that
define the size and positioning of baffle surfaces 12.
D = distance from weir (water level)
L = Length of baffle surface 12
a = angle from wall T
t = size of end flange
P = Projection distance from wall T (based on a and L)
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following equation(s)It is noted that the desired minimum horizontal
projection is ideally based on the
In metric units:
Minimum Horizontal Projection = 440mm + a(d ¨ 9.15) m,
Where Horizontal Projection is in millimeters
a = 16.7 millimeters per meter, and
Or in English Units d = tank diameter in meters
Minimum Horizontal Projection = 18 + a(d ¨ 30),
Where Horizontal Projection is in inches
a = 0.2 inches per foot, and
d = tank diameter in feet
In some calculations ¨ it is recommended to increase the minimum horizontal
projection by increasing the value of a to 0.3 inches/foot (25mm/m). The
coefficient "a"
could be set greater than 0.3 inches per foot.
In view of the above, an exemplary series or modeling tests were performed to
simulate sample baffle (of similar basic design to baffle 10 but with varying
dimensions)
performance in an exemplary 70-foot diameter clarifier C with 10-foot side
water depth
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(height from bottom of tank to weir/water level). The dimensions of the
exemplary
clarifier C are given in the following Table 1.
Tank Diameter Table 1: Circular Clarifier Dimensions 70 ft
Side Water Depth 10 ft
Bottom Slope 1:12
RAS Well Diameter 6 ft
Inlet Pipe Diameter 2 ft
Influent Baffle Diameter 23 ft
Influent Baffle Height Variable
Effluent Launder Type Outboard
Simulations data was conducted using no baffle, a prior art baffle and several
alternative designs. The simulations are carried out for a period of 110 to
220 minutes
(real-time). During this period of time, effluent solids concentrations and
the calculated
velocity field were continuously recorded.
In the present instance, seven different baffle configurations were defined.
Referring to Figure 4 and Table 2 below, seven baffle configurations are set
forth. Not
identified in Table 2 is an eighth configuration, referred to as Case 0 which
is a "no-
baffle" or baseline configuration.
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Case 1 is a prior art design for a baffle for this size clarifier, namely a
baffle with
45 degree inclination angle and a 26 inch horizontal projection (18" + 0.2
(70'-30') =-
26".
Table 2: Density Current Baffle Design Variations
(Length dimensions are inches, angle measures are degrees)
Case Number D L a P t
1 36.0 37.0 45 26 3.0
2 36.0 52.4 30 26 3.0
3 12.0 37.0 45 26 3.0
4 36.0 48.0 45 34 3.0
5 36.0 24.0 45 17 3.0
6 60.0 37.0 45 26 3.0
7 36.0 30.3 60 26 3.0
In case 2, the inclination angle relative to wall T is a steep 30 , meaning it
projects sharply downward. To maintain the projection distance P, the length L
of the
baffle surface was increased to 52.4 inches.
In case 3, the standard prior art baffle of case 1 is positioned 1 foot below
the
weir/water level instead of the typical 3 feet below as in case 1.
In case 4, the angle from tank wall T is kept at 45 degrees, but the baffle's
horizontal projection into the tank is increased by 8".
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In case 5, the angle from tank wall T is kept at 45 degrees , but the baffle's
horizontal projection is decreased by 8".
In case 6, the prior art baffle of case 1 is positioned five feet below the
weir
instead of the normal three feet (of case 1).
In case 7, the inclination angle relative to wall T is a lowered to 600,
meaning it
projects only slowly downward. To maintain the projection distance P (26"),
the length L
of the baffle surface was decreased to 30.3 inches.
Using the above seven (plus blank ¨ case 0) dimensions for the baffles, the
simulations were run assuming a three foot (.914 m) deep blanket (settled
solids on the
bottom of clarifier C) and a Surface Overflow Rate (SOR) of 1300 gpd/sq ft
(gallons per
day per square foot of water surface area). This high SOR value was selected
to examine
the effectiveness of the baffles under what is generally considered a high
flow rate or
stress condition. While this is higher than typical clarifiers operate in
general, it insures
that active density currents are created and that the baffle designs are fully
operating.
Computed effluent solids concentrations in the below test results are output
for
each Case scenario and normalized with respect to the maximum carry-over
concentration, calculated for the baseline (Case 0 ¨ No Baffle) computation.
The test
results are done on a "better than-worse than" basis, with the results being
within the
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range of prior recorded "real world" values and were consistent with one
another and
with the operating conditions.
Figure 5 shows a comparison of Cases 0-3 with relative effluent solid
concentration being measured over 110 minutes of operating time. Figure 6
shows a
similar comparison of cases 0 and 4-7.
The results of Case 0 show that no baffle quickly results in a very high
relative
effluent concentration. The prior art baffle arrangement of Case 1 appears to
do an
effective job of reducing effluent solids under these operating conditions.
Case 2, where the inclination angle relative to wall T is a steep 30 , appears
to
have some effectiveness, but the widely oscillating results show that the
steep angle
likely creates an unsteady flow of solids preventing the generating of
constant reduced
flow.
In Case 5, the angle from tank wall T is kept at 45 degrees, but the baffle's
horizontal projection is decreased by 8". As seen from Figure 6, this was only
marginally
effective at reducing solids in the effluent, but did not even achieve the
same results as
the prior art Case 1.
The Case 3 and Case 6 baffles were located one foot and five feet below the
weir,
respectively. As noted above, in the simulation, the clarifier C had a 10-foot
side water
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depth and the sludge blanket was assumed to be three feet deep from the bottom
of
clarifier C. Judging from the performance of the Case 3 baffle, it may be
concluded that it
was positioned too far from the blanket, while the wave-like variations in the
presentation
of the Case 6 baffle results would appear to indicate that it was positioned
too close to the
blanket. The findings of Cases 3 and 6 confirm that baffles appears to be most
effective
when positioned midway between the blanket and the launder channel so as to
allow
sufficient clearance for solids to be deflected without impacting the blanket
or being
drawn into the weir current.
Finally, it is noted that Case 4 baffle (45 degrees with extended projection),
and
the Case 7 baffle (60 degrees with basic 26" projection), were both effective
at reducing
the total suspended solids in the effluent, at least as well, if not better
than the prior art
Case 1.
In view of the above, another exemplary series or modeling tests were
performed
for Cases 1, 4 and 7 to simulate baffle performance in an exemplary 100-foot
diameter
clarifier C with 14-foot side water depth (height from bottom of tank to
weir/water
level). Boundary conditions in the simulation were also maintained, including
the influent
solids concentration and the return flow compared to total flow.
In this testing, four sets of operating conditions were established, namely a
low
blanket with low flow, high blanket with low flow, low blanket with high flow
and high
blanket with high flow.
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In the operating conditions the low blanket and high blanket were 4.3 feet and
6.3
feet, respectively. The low flow and high flow were defined as SOR =
900gpd/ft2 and
SOR = 1200gpd/ft2, respectively.
Each of the three baffle configurations was scaled for the 100-foot clarifier,
as follows:
Case 01 (prior art): D = 36.0 inches; L = 45 inches; a = 45'; P = 32 inches
(based
on 18+0.2(100-30)); and t = 3.0 inches
inchesCase 04: D = 36.0 inches; L = 57 inches; a = 45'; P = 37.75 inches and t
= 3.0
Case 07: D = 36.0 inches, L = 37 inches, a = 60 , P = 32 inches and t = 3.0
inches
For the case where the sludge blanket was low, and the flow was less than SOR
=
700, vertical currents near the outer wall of the clarifier did not form
strongly. In fact,
under these circumstances, these currents tend to rise up and then fall back
on
themselves. In this case, maximum effluent solids concentrations are
calculated during
model spin-up (around t = 40 minutes) and then they moderate somewhat. While
it does
appear that maximum effluent solids concentrations are reduced with baffles
(cases 1, 4
and 7) in place, the influence of the baffles is minimized. The best
performance is
achieved for the Case 07 variation. The Case 7 has a basic horizontal
projection, but the
inclination angle from the wall is increased to 60 degrees. As a result, the
end of this
baffle design is positioned further above the sludge blanket than the standard
baffle.
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Figure 7 shows the results of the simulation with a high blanket and low flow.
Here, the difference between the low blanket and high blanket is only 2 feet,
and the top
of the blanket is 2 feet closer to the baffle. As shown in Figure 7, in this
case it appears
that the vertical currents were stronger near the clarifier wall and the Case
0 effluent
solids concentrations remained high even after model spin-up. In this same
scenario, the
Case 4 baffle configuration performed best. As before, however, in these low
flow
scenarios, the effectiveness of the baffle was proportionally small.
In all of the low blanket simulations, particularly those with low flow, only
weak
density currents appeared at the outer wall of the clarifier, and those lacked
sufficient
energy to climb the wall and reach the effluent trough. Later simulations
showed that
SOR in the range of 600 to 800gpd/sqft were required to produce short-
circuiting
currents. Blanket depth is also a contributing factor. This is consistent with
field results
that suggest that at or below design flow, density currents are not
significant and the
effect of the baffle is lessened.
Finally, a set of calculations was carried out for a high flow (1200 SOR)
condition
with a high (6.3 foot) blanket. The results are shown in Table 3 below. In
this scenario,
the Case 01 and 07 baffles performed best. From the flow patterns it appeared
that the
Case 4 baffle was positioned too close to the blanket and created a
disturbance there that
affected its performance.
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Table 3: Percent Solids Reduction, High Flow-High Blanket
Case No. Relative Effluent
Solids Concentration
vs. Case 00 (%)
01 33%
04 63%
07 30%
The difference between Case 01, 04 and 07 baffles is the fact that the
inclination angle of
the Case 07 baffle (600 versus 45 ) has been made more shallow, in effect
increasing the
relative distance between the baffle and the top of the sludge blanket. The
calculated flow
patterns in Figures 8A and 8B show the development of a short-circuiting
pattern in this
area.
Based on the above test results, a third set of testing was done using the 100
foot (by 14
foot tall) clarifier arrangement using three samples:
- Case 8 (Prior art with 8" projection extension) 45 inclination angle and 32
inch
horizontal projection.
- Case 9 extended prior art baffle with 45 inclination angle with 39 inch
horizontal
projection (based on 18"+(0.3 (100'-30'))
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- Case 10 baffle 10 of the present arrangement having a combination of a 600
inclination
angle from wall T and an extended 39 inch horizontal projection.
The flow patterns that resulted from these calculations are shown in Figures 9
and
10. The calculated relative solids concentrations show that the case 8 baffle,
although
effective was surpassed by the extended baffle (case 9 ¨ (20% more effective))
and even
more so by the case 10, where baffle 10 exhibited, such as that shown in
Figures 1-3,
30% more effective than the prior art baffle designs.
As such, the results in Figures 9 and 10 confirm that increasing the length of
the
horizontal projection of baffle surfaces 12 improves their ability to deflect
wider density
currents. The width of the current varies with clarifier dimensions, solids
settling
characteristics, flow and other parameters. Moreover, simultaneously
increasing the
inclination angle to substantially 60 degrees raises the bottom of the baffle,
by end flange
20 further from the blanket which limits the sludge blanket's interference
with the
operation of baffle surface 12.
For example, this arrangement also opens a wider path for the solids under
baffle
10 to be deflected from wall T. The flow patterns in Figure 9 indicate that
the velocity
vectors emerging from beneath baffle 10 are aligned horizontally, toward the
center of
the clarifier C, which keeps higher solids concentrations further from the
effluent weir
currents. In contrast, the motion vectors at the top of both of the other
baffles (Case 8 and
9) appear more vertically aligned. Accordingly, one feature that is
determinative of the
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effectiveness of baffle 10, is the velocity vector generated by the 60 degree
angle as
shown in case 10 of Figure 9. In an alternative arrangement, the 60 degree
angle can be
lowered to 55 degrees or increased to 70 degrees, so long as the velocity
vector of solids
can be aligned substantially horizontal to the clarifier tank. As shown in
Figure 9 case 10
as compared to case 9 and case 8, the velocity vector of case 10 is closer to
horizontal
that either of the other cases.
Regarding the positioning of baffle 10 relative to the sludge blanket, the
first test
(70-foot clarifier diameter) implied that there is a range of suitable values.
In that test, the
clarifier C is 10 feet deep and the top of the blanket was at 3 feet. The
Cases 1, 4 and 7
baffles, all of which performed relatively well, were mounted 3 feet below the
weir, but
the bottom position of the 3 baffles were 19 inches, 11 inches and 33 inches,
respectively,
above the blanket. Of the other configurations, the Case 2 baffle was 3 inches
above the
blanket and apparently too close; the Case 6 baffle was into the blanket; the
Case 3
baffle, at 43 inches, appears to be too far from the blanket.
It is noted that the Case 5 baffle was 28 inches above the blanket, (within
the
distance range defined by Cases 1, 4 and 7), but it did not perform well.
However, it is
noted that the Case 5 baffle had a shorter horizontal projection than the
other baffles, and
particularly shorter that the present baffle 10. The Case 4 baffle, on the
other hand, was
the closest to the blanket of the three best performing test cases, at 11
inches. Case 4 had
the longest horizontal projection of all of the test baffles. The case 7
baffle, at 33 inches,
was almost twice as far from the blanket as the case 1 baffle (19 inches), and
the two
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baffles had the same horizontal projection, but the case 7 baffle had a 60
degree
inclination angle.
Accordingly, the test results show that the position of baffle 10 within
clarifier C
relative to the top of the sludge blanket is not simply a matter of distance,
but a
combination of distance from the blanket as well as inclination angle and
horizontal
projection. With the baffle 10 having the dimensions as set forth above in
Case 10 (60
degrees with 39 inch projection) the best performance relative to the height
of the sludge
blanket (to the bottom of the baffle 10 at end flange 20) is 2 feet + or ¨ 6
inches.
It is noted that the sludge blanket height is not always a definable position
as it is
constantly changing in height based on the flow and sediment conditions within
the
clarifier. Thus, the optimum position for baffle 10 is based on an estimate of
typical
blanket heights and variations that might occur in normal operations in each
particular
clarifier C. In one arrangement, in order to address this issue, baffle 10 may
be positioned
at a point midway between the average blanket height and the weir in order to
estimate
the 2 foot from blanket height.
In any event, in the present arrangement, baffle 10, in order to function
effectively
across the range of operating conditions of any clarifier C, is positioned at
a low enough
point that its proximity to the blanket allows it to perform effectively
whenever density
currents carrying solids are able to reach it, and not so low that there is a
danger that the
blanket could rise above it.
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In an alternative arrangement, as shown in Figure 12 and 13, a baffle surface
12 is
shown with a modified bracket element 14. This bracket element 14, extends
backward
to provide an additional mounting surface beyond the upper mounting flange 18.
Such an
arrangement, allows for baffle surface 12 to be mounted directly below the
effluent
channel under the weir.
In this arrangement, the substantially 60 degree angle is maintained. Also,
the
horizontal projection, as measured from the wall of the effluent channel
(instead of the
tank wall) allows for additional projection of the lower end of baffle surface
12 into the
center of clarifier C. This positioning of baffle 10 directly below the
effluent channel,
ensures a sufficient distance from the average height of the sludge blanket
and ensures
that the lower end of baffle surfaces 12 do not fall below the periodically
rising sludge
blanket.
In one arrangement, for typical twelve-fourteen foot deep clarifiers C, baffle
10 is
optimally positioned five feet below the weir (water level), which generally
places it
approximately midway between the typical blanket and the launder channel.
As shown in Figure 11, further calculations of solids concentrations in the
effluent
are performed using the parameters of the third test comparing these two
baffle
configurations (Case 8 prior art versus Case 10 (baffle 10 of the present
arrangement) )
over a broad range of increasing SOR values.
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In order to further demonstrate the effectiveness of baffle 10 relative to the
prior
art baffle arrangements, tests were again simulated with even larger 130
diameter
clarifiers C. In these cases, as with the above described samples in the third
test, baffle
10 of the present arrangement, with a 60 degree angle and extended horizontal
projection
shows a 30% improvement in reduction in solids (in the effluent) relative to
the model
predictions based on prior art baffle designs. As can be expected, baffle 10
in the range of
55 to 70 degrees with a similar extended horizontal projection, would achieve
similar
results to the third test.
While only certain features of the invention have been illustrated and
described
herein, many modifications, substitutions, changes or equivalents will now
occur to those
skilled in the art. It is therefore, to be understood that this application is
intended to cover
all such modifications and changes that fall within the true spirit of the
invention.
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