Water and Wastewater Engineering
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23-68 WATER AND WASTEWATER ENGINEERING
Solution:
a . The solu tion procedure is iterative. The following assumptions are made based on the
discussion of design practice. These are checked in the following calculations and sub-
sequent examples.
(1) Assume two tanks preceded by an equalization basin
(2) Assu
me a 6-hour cycle time
(3) Select the following phase times to achieve a total of 6 hours:
A n o xic fill = 135 min
Aerated fill = 45 min
React = 90 min
Settle = 45 min
D e cant = 30 min
I dle = 15 min
Total cycle time = 360 min or 6 h
b.
Estimate the fill volume ( V
F
) for one SBR.
Cycles/d
h/d
h/cycle
cycles/d
24
6
4
Divide the flow between two tanks.
22 700
2
11 350
3
3
,
,
m /d
tanks
m /d per tank
Divide the flow per tank by the number of cycles to find the fill volume.
V
F
11 350
4
28
3
,
,
m /d per tank
cycles per day
337 5
3
. m /fill
c. Estimate the fill fraction using Equation 23-62, the assumed MLSS of 3,000 mg/L, and
an assumption that the settled sludge will achieve a concentration of 6,000 mg/L.
V
V
S
T
3 000
6 000
0 5
,
,
.
mg/L
mg/L
To provide a zone for decanting without disturbing the sludge blanket, assume 35% clear
liquid volume above the sludge blanket based on Schroeder (1982) assumptions.
V
V
S
T
1 35 0 5 0675.. .()
SECONDARY TREATMENT BY SUSPENDED GROWTH BIOLOGICAL PROCESSES 23-69
Recognizing that V
F
V
S
= V
T
, determine the fill fraction:
V
V
V
V
F
T
S
T
10.
and
V
V
V
V
F
T
S
T
10 10 0675 0 325 0 33.....or
d. Determine the volume of the tank.
V
V
T
F
/tank m
or
0 33
2837 5
0 33
8 598 5 8
3
.
,.
.
,. ,6600
3
m
e. Assume a tank depth of 6 m, and estimate the dimensions the tank.
8 600
6
1433 33
3
2
,
,.
m
m
m
A ssuming a square tank, the length and width are
LW ()1433 33 3786 38
212
,. .m or m
/
f. The overall dimensions of the tank are then 38 m 38 m 6 m plus 0.6 m of free-
board.
Example 23-14. Check the assumed react plus aerated mix time used to design the SBR for the
town of Quintuple using the data from Example 23-13.
Solution:
a . The time required for nitrification must be less than the assumed reac
t aeration plus fill
aeration in Example 23-13 :
Reacttime Fill time min min m in90 45 135 oor h225.
b. Estimate the biomass using parts A, B, and C of Equation 23-40. Use the WEF (1998)
a ssumption that S 0. From Example 23-13 assumptions: NO
x
0.8(24 g/m
3
)
19.2 g/m
3
. Assume that f
d
0.15 and that Table 23-13 applies.
Part A
P
x,
,.
bio
m /d g VSS/g COD
()( )(11 350 0 40 220
3
gg/m kg/g
dd
33
1
010
1 0 12 20
998
)( )
()().
..
.
.
8
3 4
293 76 kg/d
23-70 WATER AND WASTEWATER ENGINEERING
Part B
()( )( )(015 012 11350040
1 3
.. , .dm/d gVSS/gCOD g/md kg/g
d
)( )( )( )
(
220 20 10
1012
33
.
11
20
359 57
3 4
105 76
)( )d
kg/d
.
.
.
Part C
()( )()(11 350 0 12 19 2
33
,. .m /d g VSS/g NO g/m
x
110
1 0 08 20
26 15
26
10 0
3
1
kg/g
dd
)
()().
.
.
.66
293 76 105 76 10 06
kg/d
kg/d kg/d kg/d...4409 6. kg/d
c. Determine the amount of NO
x
to be oxidized using the nitrogen balance (Equation 23-45).
NO g/m g/m
gVSS/gNH N
x
24 1 0
012 4
33
4
.
.()(
009 6 10
11 350
24 1 0 4 3
3
3
.
,
..
kg/d g/kg
m /d
)( )
33 18 67
3
.g/m
This is close to the assumed value of 19.2 g/m
3
so another iteration is not required.
d. Determine the amount of N to be oxidized at the beginning of the cycle. This is the
amount added during fill plus the amount remaining in the tank before fill.
Oxidizable added per cycle g/mN ()(18 67 2
3
.,, )837 552 976 1
3
.,.m /cycle g/cycle
The NH
4
-N remaining before fill = (volume settled)(nitrogen in effluent) ( V
s
)( N
e
)
where V
s
( V
T
V
F
). N
e
is the design effluent standard from Example 23-13.
NH -N remaining before fill
4
10
NV V
eT F
()
(. gg/mm m
g
33 3
8 600 2 837 5
5 762 5
)( ),,.
,.
Total oxidizable N 52,976.1 g 5,762.5 g 58,738.6 g
e. Determine the oxidizable NH
4
-N concentration, N
0
.
N
0
5873
Mass of oxidizable N
Volume of tank
, 886
8 600
68
3
3
.
,
.
g
m
g/m
f. Determine the nitrifying bacteria concentration with kinetic coefficients from Table 23-14 .
At the design temperature of 20
C, the kinetic coefficients are as follows: Y
n
0.12 g
SECONDARY TREATMENT BY SUSPENDED GROWTH BIOLOGICAL PROCESSES 23-71
VSS/g NH
4
-N; k
dn
0.08 g/g · d. Note that X
n
can be estimated with Equation 23-15
rewritten as:
X
QY
kV
X
n
nc
dn c T
n
()( )()
[ ( )( )]
(
NO
x
1
11 350, mm /d g VSS/g NH -N g/md
3
4
3
012 1867 20)( )( )(..))
[ ( )( )]( )1 0 08 20 8 600
508
3
.,
,57
g/g dd m
00
2 6 8 600
22 75
3
()( ).,
. /mg
g. Using the kinetic coefficients from Table 23-14 and an assumed DO concentration
equal to the minimum recommendation of 2.0 mg/L, solve Equation 23-23 for t. At the
design temperature of 20
C, the kinetic coefficients are as follows:
m
0.75 g VSS/g
VSS · d; K
n
0.74 g NH
4
-N/m
3
; Y
n
0.12 g VSS/g NH
4
-N; K
0
= 0.50 g/m
3
.
LHS g/m ln
g/m
g/m
() (074
68
10
6
3
3
3
.
.
.
⎡
⎣
⎢
⎤
⎦
⎥
.. . . 2
.
.
810 7
22 75
07
33
3
g/m g/m
RHS g/m
)
()
55
012
20
0 5020
3
g/g d
g/g
g/m
g/
.
.
..
⎡
⎣
⎢
⎤
⎦
⎥
mm
RHS
3
113 75
72
113 75
0
⎡
⎣
⎢
⎤
⎦
⎥
t
t
t
.
.
t
LHS
.
.6633 1 52d or h.
h. The aeration period provided by React and Fill is 2.25 h. This is greater than the 1.52 h
required, so the design assumptions for React and Fill time aeration do not need to be
changed.
Comments:
1 . If the React plus Fill aeration time is not sufficient, the times must be adjusted and
the check must be repeated. If d
enitrification is to be performed, as in the following
example, the adjustment should be made after the next step as excess time in the denitri-
fication step may provide more flexibility in adjusting the react plus fill times.
2. Because the waste characteristic s assumed for de
sign are not constant, the operator
should be provided with a computer system that allows reprogramming the timing of
phases and, perhaps, the cycle to account for changes.
3. The iterative nature of the des ign suggests that a spreadsheet be used to facilitate
recalculation.
Example 23-15. Check the assumed anoxic fill time us
ed to design the SBR for the town of
Quintuple using the data from Examples 23-13 and 23-14.
Solution:
a . The anoxic fill time must be greater than the time required for denitrification.
23-72 WATER AND WASTEWATER ENGINEERING
b. From Example 23-14 , the NO
3
-N concentration at the end of aeration with the tank full
is 6.8 g/m
3
.
c. The volume remaining after decant is the settled volume:
VVV
sTF
( ) 8 600 2 837 55762 5
33 3
,,.,.mmm
d. The mass of nitrate remaining in the tank after decant is
Mass NO N g/mm g
3
33
68 5 762 5 39185-. ,. ,()( )
e. Determine the SDNR in the fill period by computing the F/M ratio and using the graphs in
Figure 23-12 . Because the SBR is a batch operation, the value of X must be computed from
X
V
c
T
()()Active biomass
The active biomass is Part A of Equation 23-40 computed in Example 23-14. Therefore,
at full tank volume
X
()()()293 76 20 10
8 600
683
3
3
.
,
kg/ddg/kg
m
g/m
3
The biomass in the system (683 g/m
3
)(8,600 m
3
)(10
3
kg/g) 5,873.8 kg
The BOD feed rate ( Q
fill
)( S
0
)
Converting the fill volume to a flow rate,
V
t
fill
fill
m
h
h/d
2837 5
225
24 30 266
3
,.
.
,.() 667
30 266 67 220
3
0
3
m /d
m /d g/
fill
()()( )(QS ,. mm
kg/d
kg/d
33
10
665 87
665 87
5 873
)( )
,.
,.
,
F
M
..
.
8
113
kg
g/g d
From the data in Example 23-13 , calculate the fraction of rbCOD:
Fraction of rbCOD
g/m
g/m
50
220
023
3
3
.
From Figure 23-12 at F/M 1.13 in the range 20–30, find SDNR 0.21 g NO
3
/g bio-
mass · d. Because the design temperature is 20
C, no temperature correction is required.
f. Determine the removal capacity during the fill period. Note that the biomass in the
tank ( X )( V
T
).
NO SDNR
gNO /gbiomass d
x
()()()
(
XV
T
021
3
...,,,)( )( )683 8 600 1 233 498
33
g/mm g/d
SECONDARY TREATMENT BY SUSPENDED GROWTH BIOLOGICAL PROCESSES 23-73
At a fill time of 2.25 h
NO at h
g/d h
h/d
x
,.
,, .
225
1233 498 2 25
24
()()
115 640,g
From step (d), the NO
x
available for denitrification 39,185 g. Therefore, all of the
NO
3
-N can be removed during the fill period.
Comment. The extra removal capacity for denitrification may be useful in adjusting the phase
timing and/or the cycle time.
Example 23-16. Estimate the number of jet aerators for the SBR being designed for the town of
Quintuple using data from Examples 23-13, 23-14, and 23-15. Assum
e that 60% of the theoretical
oxygen released in denitrification is available for oxidation. Assume the following data for the
jet aerators:
SBR is at sea level
O
t
19%
0.50
0.95
F 1.0 because jets are not prone to fouling
Depth of aerator is at 5.6 m
W a stewater temperature 20 C
Manufacturer’s SOTR 1,240 kg/d at 5.6 m depth
Manufacturer’s air flow rate at standard conditions 1,800 m
3
/ d · jet
Solution:
a . The required oxygen may be estimated by modifying Equation 23-44 to account for
the oxygen credit of 2.86 g O
2
/g NO
3
-N for denitrification and the assum ption of 60%
recovery:
MQSS P Q
xO x
kg/g NO
20
3
10 1 42 4 33
()( ) () ().. (()()()060 286..NO
x
b. Using the data from Examples 23-13, 23-14, and 23-15, the estimated mass of oxygen
for one tank is:
M
O
m /d g/m kg/g
2
333
11 350 220 0 10 1
()( )(),.442 409 5
4 33 11 350186
3
()
()(
.
., .
kg/d
m /d 7710
060 286 39185
33
g/m kg/g)( )
()()(
.. ,gg/cycle cycles/d kg/g
kg/d
)( )( )410
2 497
3
,
581 5 917 5 268 96
2 564
...
,
kg/d kg/d kg/d
orr kg/d2 600,
This is the required AOTR. This is designated AOTR
req
for this problem.
23-74 WATER AND WASTEWATER ENGINEERING
c. Solve Equation 23-50 for SOTR. This is the required SOTR (SOTR
req
).
SOTR
AOTR
req
req
a
()()()
()(
,
1 024
20
20
.
T
s
F
C
C
vvg
C
L
)
⎛
⎝
⎜
⎞
⎠
⎟
d. From Appendix A, find C
s, T, H
9.17 mg/L or 9.17 g/m
3
at 20
C.
e. As in Example 23-5 , P
d
i s the pressure at the depth of air release. P
d
P
atm, H
P
water
.
Converting P
atm, H
to meters of water,
P
Hatm
Atmospheric pressure
Spec ific weight
,
oof air
kN/m
kN/m
m
101 325
98
10 34
2
3
.
.
.
From the assumed depth of the aerator,
P
d
10 34 5 615 9...mm m
f. Calculate C
avg
.
C
avg
mg/L
m
m
()()917 05
15 9
10 34
19
21
..
.
.
⎛
⎝
⎞
⎠
11 2. mg/L
g. Calculate SOTR
req
using DO required 2.0 mg/L as assumed in Example 23-14.
SOTR
kg/d
req
2 600
1 024 0 5010
9
20 20
,
...
.
()()()
117
095 11 2 2 0
26
mg/L
mg/L mg/L()(.. .
,
⎛
⎝
⎜
⎞
⎠
⎟
000
0 50
917
874
5
kg/dmg/L
mg/L.
.
.
,
⎛
⎝
⎞
⎠
⎛
⎝
⎜
⎞
⎠
⎟
4455 8 5500.,or kg/d
h. Calculate the ratio of SOTR
manuf
/SOTR
req
.
SOTR
SOTR
kg/d
kg/d
manuf
req
1 240
5500
0225
,
,
.
i. The required air flow rate is found from the following relationship:
AOTR
Density of air Mass O in air
req
()( )%
2
⎛
⎝
⎜⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
SOTR
SOTR
req
manuf
SECONDARY TREATMENT BY SUSPENDED GROWTH BIOLOGICAL PROCESSES 23-75
The density of air at standard conditions is 1.185 kg/m
3
. Air contains about 23.2%
o xygen on a mass basis. The required air flow rate is
2 600
1185 0232
1
0225
42
3
,
..
.
kg/d
kg/m()()
⎛
⎝
⎞
⎠
,, ,032 42 000
3
or m /d
j. The number of aerators required is
42 000
1 800
2333 24
3
3
,
,
.
m /d
m /d jet
or jet
ss
Comment. Note that the assumed value for i s based on the Rosso and Stenstrom (2007)
values presented after Equation 23-48.
A
2
/O
™
The A
2
/O
™
process is shown schematically in Figure 23-5b . This is an example of processes
used for carbonaceous BOD oxidation, nitrification, denitrification, and phosphorus removal.
Other processes that perform these treatment functions are listed in Table 23-3 . Although it is
not listed in the table, the oxidation ditch process can be modified with a “front-end” anoxic/
anaerobic s tage to provide biological phosphorus removal (Yonker et al., 1998; Curley, 2007).
Operation of biological nutrient rem oval (BNR) systems requires substantial operator
knowledge, skill, and oversight. To optimize the process, a sophisticated analytical laboratory
capability is required. As of 1997, very few BNR plants had design flow rates less than 20,000 m
3
/ d
(WEF, 1998).
With the exception of the A/O process that was not designed to remove nitrogen, the BPR
removal plants can achieve total nitrogen limits of 10 mg/L and total phosphorus limits less than
2 mg/L. Some plants can achieve nitrogen concentrations a low as 8 mg/L and phosphorus con-
centrations on the order of 1 to 2 mg/L. With chemical addition, these plants can a
chieve phos-
phorus concentrations less than 1 mg/L.
The A
2
/O
™
discussion is organized as follows: u pstream processes, general considerations,
design practice for for each stage in the A
2
/O
™
process flow, appurtenances, and downstream
process considerations.
Preliminary Treatment. Bar screens, mechanical s creens, grinders or shredders, and grit
chambers may be used. Equalization is highly recommended. Typically municipal wa
stewater
flow rates and rbCOD concentrations follow a diurnal pattern that results in large changes in the
mass flow rate of rbCOD to the wastewater treatment plant. This, in turn, results in large changes
in the production of volatile fatty acids (VFA) and polyhydroxybutyrate (PHB). The number of
phosphate accumulating organisms (PAOs) in the system will be representative of the average
VFA load
. The rate of phosphate release and uptake are more rapid than the growth of the PAOs.
The result is that the PHB content of the PAOs becomes saturated and the effluent concentration
of phosphorus increases. Equalization dampens this effect and decreases the amount of phospho-
rus discharged by a factor of 4 to 8 depending on the V
FA concentration produced by degradation
of rbCOD (Filipe et al., 2001).
23-76 WATER AND WASTEWATER ENGINEERING
It is recommended that the equalization volume be greater than or equal to 20 percent of
the volume of the average daily flow. Equalization volumes up to 40 percent do not appreciably
i mprove the results, but they do not adversely affect the results either (Filipe et al., 2001).
Primary Treatment. Clarifiers or screens may be used for primary treatment. While introduc-
tion of ret
urn activated sludge to the primary clarifier is generally discouraged, this practice may
be selected to increase the production of VFAs for phosphorus removal. Likewise, screens may
be selected in place of settling to allow more COD to pass to the BPR process.
Redundancy. A minim
um of two units are provided for redundancy. For plants having design
flow rates in the range of 19,000 to 38,000 m
3
/ d, three units are preferred to allow for one unit to
be out of service at the maximum flow rate. In the range of 38,000 to 190,000 m
3
/ d, four or more
tanks are often provided to allow operational flexibility and ease of maintenance.
Design Flow Rate and Loading. Unlike grit removal, primary settling, and secondary settling,
suspended growth biological treatment systems are not hydraulically limited. They are process
limited. The total tank capacity must be determ
ined from the biological process design. There-
fore, the loading (flow rate concentration) is an important design parameter.
It is recommended that the maximum month and peak daily loadings based on daily flow and
concentration data be used for design (WEF, 2006a).
Type of Reactor. Both plug-flow and c
omplete-mix reactors have been used. The anaerobic,
anoxic, and aerobic regimes must be physically separated to be effective. This may be accomplished
by dividing a tank into compartments. Although a theoretical plug-flow reactor will require less
tank volume than a complete-mix reactor to a
chieve the same efficiency, actual plug-flow reactors
seldom achieve ideal plug-flow. As a result it has been found that staging of complete-mix reactors
is the best method of approximating plug-flow efficiency. As a practical matter, three or four reac-
tors or stages in series will ad
equately approximate plug flow (WEF, 1998).
Modeling Equations. If completely mixed reactors are used, then Equations 23-14 and 23-15
are applicable for heterotrophic and nitrification biokinetics. BPR microorganism growth kinetics
fall in the same order of magnitude as that of other heterotrophic bacteria. A maximum specific
growth rate at 20
C is given as 0.95 g/g · d by Barker and Dold (1997). Kinetic coefficients for
removal of bCOD by heterotrophic bacteria are given in Table 23-13 . The kinetic coefficients for
design of nitrification are given in Table 23-14 .
Design Practice for Phosphorus Removal. The following paragraphs outline the design prac-
tice for those portions of the BPR process that affect phosphorus removal.
rbCOD. The available rbCOD determines the amount of pho
sphorus that can be removed by
the BPR mechanism. Metcalf & Eddy (2003) estimates that 10 g of rbCOD is required to remove
1 g of phosphorus.
Observations of the influent BOD to phosphorus ratio at operating plants as a function of
their design SRT are shown in Table 23-17 . Although these data do not include rbCOD, they
give an indication of the trend with respect to SRT and their relation
ship to the types of BRP
processes.
SECONDARY TREATMENT BY SUSPENDED GROWTH BIOLOGICAL PROCESSES 23-77
Nitrate reduction in the anaerobic tank will proceed before the BPR mechanisms. This will
reduce rbCOD that is available for BPR. In processes like A
2
/O
™
where RAS is returned to the
anaerobic tank, the plant effluent nitrate concentration (which also appears in the RAS) must be
minimized to maximize the amount of phosphorus that can be removed by the BPR mechanism.
Likewise, the nitrate in the return flow from the anoxic tank will lim it phosphorus removal for
the same reason.
Hydraulic residence time (HRT). Despite advance
s in understanding the biochemistry of BPR,
current practice for sizing the anaerobic tanks is based on empirical observations. The required
HRT is dependent on the MLSS, which is dependent on the SRT of the system. This will be dif-
ferent for different BPR systems. While VFA uptake is relatively rapid, corresponding to an HRT
of 0.7
5 hours or less, fermentation of rbCOD is slower. It typically requires an HRT of one to
two hours or more. However, in some system s, such as the UCT ™ process, two hours or more
may be required (WEF, 2006a). Typical HRTs are in the range one-half to two hours (Metcalf &
Eddy, 2003).
Longer detention times improve phosphorus removal. However, sy
stems with excessive
anaerobic contact times and without significant VF A production will experience phosphorus
release with no uptake in acetate. This is called “secondary release.” Because this phosphorus
release is not associated with PHB storage, no energy is available in the aerobic zone for
subseq
uent uptake of the released phosphorus, and the full potential for phosphorus removal will
not be achieved (Barnard, 1984; Stephens and Stensel, 1998).
Tankage. All the tanks in the BPR processes are typically built of concrete. For the anaerobic
stage, a complete mix system with three tanks provides the most efficient tank arrangement. The
design HRT is divid
ed into thirds for the volume estimate. The tanks may stand alone or share a
common wall. When a common wall is used, a broad-crested weir allows flow to move to succes-
sive tanks. The tanks typically will have the same depth as the anoxic and aerobic tanks. This is
on the order of 4.5 to 7.5 m with a freeboard of 0.
3 to 0.6 m. The plan of the tanks is square.
M ixing. Submersible mechanical mixers keep the biomass in suspension. The mixing should be
just sufficient to keep the biomass suspended without entraining air. Typical power requirements
are in the range of 3 to 13 W/m
3
.
Solids retention time (SRT). BPR systems with longer SRTs are less efficient than those with
shorter SRT designs. Two adverse effects on phosphorus removal efficiency are associated with
lightly loaded , long SRT processes. First, because the final amount of phosphorus removed is
Type of BPR process g BOD/g P g COD/g P SRT, d
Phoredox (A/O
™
), VIP 15–20 26–34 8
A
2
O
™
, UCT 20–25 34–43 7–15
Modified Bardenpho
™
25 43 15–25
TABLE 23-17
BOD/P and COD/P ratios for phosphorus removal
Sources: Grady et al. (1999); Metcalf & Eddy, 2003.