Water and Wastewater Engineering
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10-30 WATER AND WASTEWATER ENGINEERING
Example 10-4. Design the settling tank(s) for the city of Stillwater’s water treatment plant ex-
pansion using the design overflow rate found in Example 10-3 . The maximum day design flow is
0.5 m
3
/ s. Assume a water temperature of 10 C.
Solution:
a . Find the surface area.
First change the flow rate to compatible units:
()( )0 5 86 400 43 200
33
., ,m /ss/dm/d
Using the overflow rate from Example 10-3 , the surface area is
A
s
43 200
32 5
1 329 233 1
3
3 2
,
.
,.
m /d
m /dm
or ,,330
2
m
b. Select the number of tanks.
Two tanks is the minimum number. For this flow rate make trial calculations using six
tanks.
1 330
6
221 66 222
2
2
,
.
m
tanks
or m /tank
c. Select a trial width for calculation.
The maximum width for the chain-and-flight sludge collector is 6 m increments. Assume
a width of 4 m.
d. Check length-to-width ratio.
L
m /tank
m/tank
m
L/W
m
m
222
4
55 5
55 5
4
1
2
.
.
33813 81..:or
This is larger than the ratio of 6:1 and is acceptable.
e. Select a trial depth.
Because the column depth used to calculate the overflow rate was 2 m, this is a starting
point for setting the design depth. An allowance for the sludge depth of 1 m is added to
this depth. In addition the tank should be provided with 0.6 m of freeboard. The total
depth of the tank i
s then
2106 3 6mm m m ..
Side water depth (SWD) 3.0 m.
SEDIMENTATION 10-31
If the sludge zone is not counted, the depth of the water is less than the design recom-
mendation of 3 m.
f. Check the length-to-depth ratios.
L/D
m
m
or
55 5
2
27 75 28 1
.
.:
The L:D ratio is acceptable.
g. Check the velocity and then check the Reynolds and Froude numbers.
v
Q
A
f
0 5
62 4
3
. m /s
tanksmdepth m widt()( )(
hh
m/s
)
0 0104.
This is within the acceptable range of 0.005 0.018 m/s.
R
A
P
h
x
w
()()24
242
10
mdeep m wide
mmm
m.
From Appendix A at a temperature of 10 C, the viscosity is 1.307 10
6
m
2
/ s and the
Reynolds number is
R
()()0 0104 1 0
1 307 10
7957
62
..
.
,.
m/sm
m /s
115 8 000or ,
This is less than 20,000 and is acceptable.
Fr
()
()()
0 0104
981 10
11 10
2
2
5
.
..
.
m
m/sm
This is greater than 10
5
and is acceptable.
h. Design the launders.
Provide launders for 1/3 of the tank length
L
Launder
m
m
55 5
3
18 5
.
.
Place them at 1 m intervals on center so that there are three in the tank.
i. Check the weir loading rate.
WL
43 200
6 3 18
3
, m /d
tanks launders/tank()( )(
..
.
5 2
64 86 65
3
m/launder sides
or m /d
)( )
mm
This is well below the limit of 250 m
3
/ d · m and is acceptable.
10-32 WATER AND WASTEWATER ENGINEERING
Summary:
Q
design
43,200 m
3
/ d 0.5 m
3
/ s
N umber of tanks 6
W i dth of each tank 4 m
Length of each tank 55.5 m
L:W 13.8:1
Depth including sludge 3.6 m
L:D 28:1 without sludge depth; 18.5:1 with sludge depth
v
f
0.0104 m/s
R e ynolds number 8,000
Froude number 1.1 10
5
L a unders 3 spaced evenly
L a under length 18.5 m
Weir loading 65 m
3
/ d · m
S l udge collector chain-and-flight
Comments:
1 . Not all of the design rec ommendations were met. This is, in part, due to the use of the
pilot column data to set the overflow rate and the water depth. In general, exceeding the
guidelines is acceptable. When the guidelines are not met, consideration should be given
to the importance of the gu
ideline in the function of the tank. In this case the depth of the
tank is quite shallow. Recognizing that deeper tanks are better for Type II settling, this
would be a reason for another design iteration. Likewise, the weir length is excessive for
the guideline, and an alternate scheme might be considered.
2. The design solution presented here is not the only one that is acceptable. For ex
ample,
4 tanks, 6 m wide, 3 m deep, divided into 2 channels with a baffle will also meet the
design criteria. An economic analysis is required to select the best alternative.
3. Numerous iterations may be required to balance the number of tanks, width, various
ratios and the Reynolds and F
roude number recommendations. A spreadsheet is recom-
mended for the iteration process.
Design Criteria for Small- to Medium-Sized Plants
Frequently, small to medium-sized plants will operate for only one or two 8-hour shifts and store
water for the remaining period (Walker, 1978). Thus, the flow rates are higher than the estimated
demand at the design life of the plant. For example, a 10,000 m
3
/ d demand could be met by oper-
ating one 8-hour shift at a flow rate of (24/8)(10,000 m
3
/ d) 30,000 m
3
/ d. The 30,000 m
3
/ d flow
rate would be used for setting the dimensions of the tank. The decision of the operating schedule
is an economic one because the capital costs will be higher, but the operating cost for personnel
and power will be less. If the operating schedule results in a flow rate above 40,000 m
3
/ d, then
the design criteria in Table 10-4 apply. The suggested design criteria in Table 10-5 may be used
for flow rates less than 40,000 m
3
/ d.
SEDIMENTATION 10-33
High-Rate Settler Modules
The guidance provided for the number of tanks for rectangular horizontal flow sedimentation
also applies to high-rate settler modules.
Inlet Zone. A diffuser is designed and placed in the tank inlet zone in the same fashion as it is
for a plain rectangular horizontal flow clarifier.
Flow Pattern. The three flow patterns, countercurrent, cocurrent, and crosscurrent, in theory
,
have little difference in performance. In practice the countercurrent pattern is the one most com-
monly employ ed because cocurrent designs often have trouble with resu spended sludge and
crosscurrent designs have trouble with flow distribution (MWH, 2005).
Plates versus Tubes. Although little differenc
e has been reported for various tube shapes, the
hexagon and v-shapes may have some advantage bec ause the sludge can collect in the notch.
TABLE 10-5
Typical design criteria for small to medium horizontal-flow rectangular
sedimentation basins
Parameter Typical range of values Comment
Number of tanks 1 1 spare < 10,000 m
3
/d
2 20,000 m
3
/d
Inlet zone
Distance to diffuser wall 4% of length up to 2 m
Diffuser hole diameter 0.10–0.20 m
Settling zone
Overflow rate 20 m
3
/d · m
2
< 10,000 m
3
/d
40 m
3
/d · m
2
>10,000 m
3
/d
Side water depth (SWD) 3–5 m
Length 30 m Wind constraint
60 m Chain-and-flight
Width 0.3 m increments Chain-and-flight
6 m maximum per train Chain-and-flight
L:W minimum of 4:1 6:1 preferred
L:D 15:1 Minimum
Velocity 0.005–0.018 m/s Horizontal, mean
Reynolds number < 20,000
Outlet zone
Launder length 1/3–1/2 length of ba
sin Evenly spaced
Launder weir loading 250 m
3
/d · m of launder
Sludge zone
Depth 0.6–1 m Equipment dependent
Slope 1:600 Mechanical cleaning
Sludge collector speed 0.3–0.9 m/min
Sources: AWWA, 1990; GLUMRB, 2003; Kawamura, 2000; MWH, 2005; Walker, 1978; Willis, 2005.
10-34 WATER AND WASTEWATER ENGINEERING
Current practice in Michigan is to use plates rather than tubes because of operation and mainte-
nance issues with the tubes.
Angle of Inclination. Typically, the plate inclination angle is 55 and the tube inclination is 60
from the horizontal.
Overflow Rate. Based on the tank area covered by the settler, the recommended range of over-
flow rates is from 60 to 180
m
3
/ d · m
2
. In cold regions, the maximum rate should be limited to
150 m
3
/ d · m
2
(MWH, 2005).
Based on the total projected area of the tubes, a typical overflow rate is 29 m
3
/ d · m
2
with a range
of 24 to 48 m
3
/ d · m
2
. For plate settlers, typical overflow rates range from 17 to 40 m
3
/ d · m
2
(Willis,
2005).
Velocities. In the tube settler, an average velocity of approximately 0.0025–0.0033 m/s is nor-
mally used in settling alum floc. An approach velocity of 0.6 m/min in the tank upstream of the
settler is recommended (Kawamura, 2000).
Depth. Because the sludge c ollec tion equ ip
ment must fit below the settler module, the mini-
mum depth below the tubes is 2 m. This also creates low velocities approaching the settler (Willis,
2005). Module heights range from 0.5 to over 2 m. Typical tank depths range from 3.6 to 5 m
(Kawamura, 2000, and MWH, 2005).
Placement. The module is placed in the
downstream end of the tank. For tube settlers, it is
common practice to have 75 percent of the tank area covered by the settler and the remaining
25 percent left as open space to settle heavy floc. F or plate settlers, up to 95 percent of the tank
area may be covered by the settler.
Detention Time. Tube settlers generally have a detention time of 3.5
to 5 minutes. The deten-
tion time in parallel plate modules is from 5 to 20 minutes (Kawamura, 2000).
Reynolds and Froude Numbers. As with horizontal flow rectangular tanks, the Reynolds
number and Froude number are used as a check on turbulence and backmixing. Equations 10-32
and 10-
33 apply. In lieu of manufacturer’s data, a working estimate of tube diameter of 50 to
80 mm may be used. A Reynolds number < 50 and a Froude number > 10
5
are recommended
(Kawamaura, 2005). Yao’s (1970) theoretical analysis suggests the Reynolds number may be as high
as 800. In both cases, the velocity is that of the water flowing between the plates or in the tubes.
Outlet Zone. Launders are placed above the settler module. Flow from tube settlers mus
t be
collected uniformly across the basin to equalize the flow through the tubes. Therefore, they are
spaced at not greater than 1.5 m on centers.
To provide a transition to the launders, a clear space of 0.6 to 1 m above tube settlers is pro-
vided. Flow is usually collected through submerged orific
es (Willis, 2005).
A proprietary alternative plate settler uses an effluent tube at the top of each plate to collect
the effluent. The launder is placed adjacent to the plate module.
Sludge Zone. The sludge zone extends along the length of the tank. Mechanical collection using
chain-and-flight collectors is common.
SEDIMENTATION 10-35
Operation and Maintenance. To facilitate maintenance, modules must be sufficiently inde-
pendent to allow removal of individual units. An overhead crane must be provided. Stainless
steel modules are preferred.
High-Rate Settler Module Design Criteria
T ypical design criteria for horizontal-flow rectangular sedimentation basins are summarized in
Table 10-6 . Example 10-5 illustrates the design of a high-rate settler in a rectangular sedimentation
basin.
TABLE 10-6
Typical design criteria for high-rate settler modules
Parameter Typical range of values Comment
Inlet zone
Distance to diffuser 2 m
Diffuser hole diameter 0.10–0.20 m
Settling zone
Overflow rate 60–180 m
3
/d · m
2
Alum floc
Side water depth (SWD) 3–5 m
Length 60 m Chain-and-flight
Width 0.3 m increments Chain-and-flight
6 m maximum per train Chain-and-flight
24 m maximum 3 trains per drive Chain-and-flight
Settler
Fraction of basin covered < 0.75 For plates 0.95
Height 0.5–2.0 m For plates 1.0 m
Plate angle
55
Tube angle 60
Tube hydraulic diameter 0.05–0.08 m
Tube velocity 0.0025–0.0033 m/s
Approach velocity 0.010 m/s Horizontal, mean
Reynolds number < 50
Froude number > 10
5
Outlet zone
Launder length Equal to length of settler
Launder spacing 1.5 m on centers
Launder elevation 0.6–1.0 m above top of settler For plates to top
Launder weir loading < 300 m
3
/d · m
Sludge zone
Depth 0.6–1 m Equipment dependent
Slope 1:600 Mechanical cleaning
Sludge collector speed 0.3–0.9 m/min
Sources: Kawamura, 2000, MWH, 2005, Willis, 2005.
10-36 WATER AND WASTEWATER ENGINEERING
Example 10-5. Design the settling tank(s) for the city of Stillwater’s water treatment plant
expansion using high-rate settlers. The maximum day design flow is 0.5 m
3
/ s. Assume a well set-
tling alum floc, a water temperature of 10 C, that the angle of the settler tubes is 60 , and that the
tubes have a hydraulic diameter of 50 mm.
Solution:
a . Find the surface area of the settler.
First change the flow rate to compatible units:
()( )0 5 86 400 43 200
33
., ,m /ss/dm/d
Using an overflow rate of 150 m
3
/ d
m
2
, the surface area is
A
s
43 200
150
288
3
3 2
2
, m /d
m /dm
m
b. Select the number of tanks.
Two tanks is the minimum number. For this flow rate make trial calculations using two tanks.
288
2
144
2
2
m
tanks
m /tank
c. Select a trial width for calculation.
The maximum width for the chain-and-flight sludge collector is 24 m in 0.3 m incre-
ments. Assume a width of 6 m. This is the maximum width per train.
d. Check length-to-width ratio.
L
settler
m /tank
m/tank
m
144
6
24
2
Setting the settler at 75% of the length of the basin, the tank length is
L
24
075
32
m
m
.
e. Select a trial side water depth (SWD).
Assume a depth of settler of 0.6 m. Provide allowance above the settler for laund er of
1.0 m (0.6 m clearance and 0.4 m depth of trough). Provide 2 m for sludge zone.
SWD m06 10 2 3 6.. .
The depth of the water plus the sludge zone is greater than the minimum design recom-
mendation of 3 m. With the addition of freeboard, the tank depth is 4.2 m.
SEDIMENTATION 10-37
f. Check the approach velocity.
v
Q
A
approach
m /d
tanksm
43 200
263 6
3
,
.()()(
mms/d
m/s
)( )86 400
0 0116
,
.
This is slightly high but acceptable.
g. Check the Reynolds and Froude numbers.
The angle of the settler tubes is 60 . The area of the settler module calculated in (b)
above is 144 m
2
. The velocity in the settler is
v
Q
A
fc
sin
m /s
tanksmsin
0 5
2 144
3
2
.
()()(
m/s
60
0 0020
)
.
This is a little low but acceptable. Assuming a 50 mm (0.05 m) hydraulic diameter of the
tube, the hydraulic radius is
R
A
P
d
d
d
h
x
w
2
4
1
4
005
4
0 0125
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎞
⎠
.
.
m
m
From Appendix A at a temperature of 10 C, the viscosity is 1.307 10
6
m
2
/ s. Using
Equation 10-32, the Reynolds number is
R
()()0 0020 0 0125
1 307 10
19
62
..
.
.
m/sm
m /s
112 19or
This is less than 50 and is acceptable. Using equation 10-33 with v
fc
for the velocity, the
Froude number is
Fr
()
()()
0 0020
9 81 0 0125
3 26 10
2
2
.
..
.
m
m/sm
5
This is greater than 10
5
and is acceptable.
h. Design the launders.
Provide launders over the length of the settler. L
Launder
24 m
Place them at 1 m intervals on center so that there are three in the tank.
i. Check the weir loading rate.
WL
m /d
tanks launders/tank
43 200
2 3 24
3
,
()( )(
m/launder sides
m /dm
)( )2
150
3
This is well below the limit of 300 m
3
/ d · m and is acceptable.
10-38 WATER AND WASTEWATER ENGINEERING
Summary:
Q
design
43,200 m
3
/ d 0.5 m
3
/ s
N umber of tanks 2
W i dth of each tank 6 m
L
settler
24 m
Length of each tank 32 m
S i de Water Depth including sludge 3.6 m
v
approach
0.0116 m/s
v
f c
0.0020 m/s
R e ynolds number 19
Froude number 3.26 10
5
L a unders 3 spaced evenly
L a under length 24 m
Weir loading 150 m
3
/ d · m
S l udge collector chain-and-flight
Comments:
1 . Not all of the design recommendations were met, but overall the design is satisfactory.
2. The number of tanks and their size is considerably less than in Example 10-4 . Initially,
this appears to be a very favorable alternative. However, the cost of the settler modules
may offset the reduced cost for less tankage. In addition, there is no redundant settling
basin at the maximum design flow, so an additional basin would have to be provided.
An economic analysis is required to select the best alternative.
10-5 OPERATION AND MAINTENANCE
Perhaps the most important O&M activity is the optimization of the sludge withdrawal process.
The proper sweep cycle and duration are determined by trial. The concentration and characteris-
tics of solids are used to adjust the cycle. This is, of course, dependent on the turbidity and flow
rate of the raw water,
so a range of conditions must be investigated.
Other operational characteristics, such as turbidity in the tank, equal hydraulic loading in the
tanks, and the number of tanks in service for a given flow rate must be observed with appropriate
adjustments. Maintenance is primarily focused on preventive maintenance on the collector an
d
observation for corrosion.
Hints from the Field. On new installations provided with plastic c hain collection systems,
frequent tension adjustment in the first year after installation should be expected. Stainless steel,
(305 ss), or very heavy gauge plastic, is recommended for tube settler
s to minimize corrosion
and/or deformation.
Pierpont and Alvarez (2005) offer the following suggestions for optimizing ballasted sedi-
mentation when surface water is characterized by high total organic carbon (TOC):
• U se larger tubes for the lamella settler: 90 mm instead of 40 mm.
SEDIMENTATION 10-39
• Use lower mixer speed (80 to 85 percent of maximum) and postion the blades a full diam-
eter from the floor.
• Use large sand grain sizes: 130–150 m instead of 80 or 100–120 m diameter.
Visit the text website at www.mhprofessional.com/wwe for supplementary materials
and a gallery of additional photos.
10-6 CHAPTER REVIEW
When you have completed studying this chapter, you should be able to do the following without
the aid of your textbooks or notes:
1 . Describe the difference between Type I, II, III, and IV settling and give an example of
where they are applied in water or wastewater treatment.
2. Show by derivation why overflow rate “controls” the efficiency of settling in both the
upflow clarifier and the horizontal flow clarifier.
3. Calculate the percent removal of a discretely settling particle in a vertical or horizontal
flow sedimentation bas
in.
4. Explain why a settling column study is probably not appropriate for design of a new
water treatment plant.
5. Identify four potential causes that you would investigate to explain the poor perfor-
mance of a settling tank.
6. Describe, for a client, the three types of sedimentation systems described in this text
and the reasons you would recommend one over another.
7. Explain the reason for calculating the Reynolds number and Froude number in check-
ing a horizontal-flow rectangular settling basin.
8. Given the dimensions of a horizontal-flow rectangular settling basin or high-rate set-
tler, recommend a mechanical collector.
W ith the use of this text, you should be able to do the following:
9 . Calculate the settling velocity of a discrete particle given its density, diameter, and the
water temperature.
10. Analyze settling column data to determine an overflow rate and detention time to
achieve a specified percent removal.
11. Calculate the settling time and particle travel time in a high-rate settler.
12. Des
ign a rectangular horizontal-flow sedimentation basin.
13. Design a high-rate settler including the horizontal flow tank.