EPA-330/9-74-001e
NATIONAL FIELD INVESTIGATIONS CENTER
CINCINNATI
OPERATIONAL CONTROL PROCEDURES
for the
ACTIVATED SLUDGE PROCESS
PARTIII-B
CALCULATION PROCEDURES
FOR
STEP-FEED PROCESS RESPONSES
FEBRUARY 1975
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF ENFORCEMENT AND GENERAL COUNSEL
ov
33
O
\
O
-------
EQUIVALENTS USED FOR ACTIVATED SLUDGE CALCULATIONS
ft
inches
m
m
sq ft
sq m
cu ft
cu ft
cu ft
cu m
cu m
cu ra
gal
gal
liter
mgd
cu m/day
gpd/sq ft
cu m/day/sq m
Ib
Ib
kg
kg
lbs/1000 cu ft
g/cu m
cu ft (H20)
gal (H20)
liter (H20)
Ib/day
kg/day
Ib
kg
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
0.3048
2.540
3.28083
39.37
0.0929
10.7639
28.3170
0.028317
7.48052
1000.0
35.3145
264.179
3.785
0.003785
0.26417
3785
0.000264
0.0408
24.51
0.453592
453.592
2.20462
1000.0
16.0
0.0625
62.4
8.345
1.000
= mgd x mg/1
= cu m/day x
m
cm
ft
in
sq m
sq ft
liter
cu m
gal
liter
cu ft
gal
liter
cu m
gal
cu m/day
mgd
cu m/day/sq m
gpd/sq ft
kg
g
Ib
g
g/cu m
lbs/1000 cu ft
Ib (H20)
Ib (H20)
kg (H20)
x 8.345
mg/1 /1000
= English SLU x (WCR*/1198)
= Metric SLU
x (WCR/10)
English SLU = Metric SLU x 264.2
Metric SLU = English SLU x 0.003735
*WCR = sludge weight (mg/1)/centrifuged concentration (%)
-------
NATIONAL FIELD INVESTIGATIONS CENTER - CINCINNATI
OPERATIONAL CONTROL PROCEDURES
FOR THE
ACTIVATED SLUDGE PROCESS
PART III-B
CALCULATION PROCEDURES
FOR
STEP-FEED PROCESS RESPONSES
by
Alfred W. West, P.E.
Chief, Waste Treatment Branch
FEBRUARY 1975
UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF ENFORCEMENT AND GENERAL COUNSEL
-------
FOREWORD
The Waste Treatment Branch of the National Field In-
vestigations Center - Cincinnati is developing a series of
pamphlets describing Operational Control Procedures for the
Activated Sludge Process. The series will include Part I
OBSERVATIONS, Part II CONTROL TESTS, Part III CALCULATION
PROCEDURES, Part IV SLUDGE QUALITY, Part V PROCESS CONTROL
and an APPENDIX. Parts I and II were originally printed as
separate pamphlets dated April 1973. The May 1974 printing
combined the two Parts which includes some revisions
concerning use of the centrifuge and dilution settlometer
tests. Each part will be released for distribution as soon
as it is completed, though not necessarily in numerical
order. The original five-part series may then be expanded
to include case histories and refined process evaluation and
control techniques.
This pamphlet has been developed as a reference for
Activated Sludge Plant Control lectures I have presented at
training sessions, symposia, and workshops. It is based on
my personal conclusions reached while directing the
operation of dozens of activated sludge plants. This
pamphlet is not necessarily an expression of Environmental
Protection Agency (EPA) policy or requirements.
The mention of trade names or commercial products in
this pamphlet is for illustrative purposes and does not
constitute endorsement or recommendation for use by the EPA.
Alfred W. West
-------
TABLE OF CONTENTS
PAGE NO
INTRODUCTION
SUMMARY 1
Sludge Oxidation Pressures 1
Wastewater Treatment Pressures 1
Discussion 1
Example A 5
Example B 5
Conclusions 6
BASIC CALCULATION PROCEDURES 7
Data Sources & Text Organization 7
Use Of Calculated Relationships 9
Symbols & Data Used In The Examples 9
CALCULATION FORMS 11
FORM A 11
CALCULATION FORM A 12 ,13
TABLE A, Summary Of Calculations 14,15
Aeration Tank Wastewater Flow-In - AFI 17
CALCULATION FORM B 17
Discussion of Effects Of Switching To
Various Step-Feed Configurations 18
TABLE B, Comparison Between Plug-Flow &
Contact Stabilization 20,21
RATIONALE OF PROCEDURE DEVELOPMENT 23
AWDT, ADT, & ASDT 23
ATC X AWDT 25
Mean Aeration Tank Concentration - ATCm 26
FORMULAS FOR STEP-FEED CALCULATIONS 27
FORMULAS USED IN CALCULATION FORM A 27
ASDT 27
AWDT 27
ATCxAWDT 27
ASU 27
FORMULAS USED IN CALCULATION FORM B 28
AFIj 28
ATCm 28
ATCn 28
-------
INTRODUCTION
An activated sludge plant that has been designed to
permit operation in a plug-flow, step-feed, or contact-
stabilization mode provides great control flexibility and
can be operated many different ways. Calculations of step-
feed process characteristics, however, are more complex than
the previously illustrated calculations for aeration tanks
operating in the plug-flow mode. Though few operators will
perform all the step-feed calculations, all should be
generally aware of the oxidation and purification pressure
changes that occur when the process mode is shifted through
various combinations of step loading.
The Summary, which probably is the most useful part of
this section, illustrates the types of changes that occur
when a plug-flow system is switched to various step-feed
combinations.
Of nearly equal importance are the calculation
procedures used to determine the sludge and waste detention
times in a step-feed configuration. Then the additional
process parameters unique to step-feed are shown. Finally,
the rationale of the calculation procedures is included for
those who may be interested in the derivations.
The intent of this pamphlet is not to describe specific
step-feed locations that are most appropriate for all plant
loading and sludge quality combinations. The illustrations
and examples are intended to emphasize how the activated
sludge process reacts to changes in wastewater feed-point
locations. The Calculation Forms present an orderly
procedure to determine, and at times predict, process
response to various step-feed loadings. Other feed
configurations could at times be more beneficial than those
shown in the illustrations.
-------
SUMMARY
Treatment plants at which operators can switch waste-
water in-flow from one bay of an aeration tank to one or
more other bays (step-feeding) have additional ways to meet
the process demands of the activated sludge system.
Recognition of the process demands that call for such
control changes and knowledge of what happens when step-
feeding is employed provide the foundation for successful
operation of such plants. The curves on Figure 1 show how
shifting wastewater in-flow locations exerts forces on mixed
liquor sludge oxidation that are opposite to those exerted
on wastewater treatment. Knowledge of these facts alone
permits operators to shift step control in the proper
direction to correct sludge or final effluent deficiencies
and to restore best process balance.
SLUDGE OXIDATION PRESSURES
Oxidative pressures imposed on the activated
sludge increase as the wastewater enters farther
away from the head and closer to the exit end of
the compartmented aeration tanks,
WASTEWATER TREATMENT PRESSURES
Purification pressures exerted on the
wastewater decrease as it enters farther away from
the head end and closer to the exit end of the
aeration tanks.
DISCUSSION
Total aeration tank volumes and return sludge and waste
water flows shown in Figure 2 are similar to those used in
the calculation examples presented in Part III-A. The
aeration tank characteristics used in this Summary example
differ from those in Part III-A because the tank is divided
into four equal step-feed bays, but the flows differ only
slightly when the sludge wasting rate is set at zero.
In addition to the process changes induced directly by
switching step-feed inlet locations, the impact of such
changes will be further governed by any variation in the
sludge wasting rate.
-------
R SF
PLUG FLOW
100% API
TO BAY 1
30%
Fl
I
B AY
1
r
B AY
2
^J
>
B AY
3
BAY
4
\J-
AGE
TFL > ATP
* ATCm
ASDT
AWDT
ATCx
—
—
=
=
AWDT
6.0 days
5.0 %
4.2 hours
4.2 hours
21.0
R SF
100% AFI
TO BAY 2
100% AFI
TO BAY 3
BAY
1
V
r
BAY
2
>
>
BAY
3
^
B AY
4
/
RSF
100%
AFI
100%
AF I
BAY
1
V.
r
BAY
2
*
~\
BAY
3
V.
BAY
4
>
TFL
= 8.2 days
ATCm = 7.4 %
ASDT = 6.2 hours
AWDT = 3.1 hours
ATCx AWDT=15.7
AGE - 10.4 days
ATCm = 10.0 %
ASDT = 3.4 hours
AWDT = 2.1 hours
ATCx AWDT=10.4
R SF ^
CONTACT STAB.
100% API
TO BAY 4
BAY
1
r>
B AY
2
^
BAY
3
BAY
4
v^
TFI
-^->-AGE
ATCm =
ASDT -
AWDT =
* ATC x AWDT
100%
AFI
25% 25%
AFI AFI
1 1
R SF
STEP-FEED >
25% AFI
TO EACH BAY
BAY
1
C
BAY
2
^J
•>
BAY
3
B AY
4
^J
| TFL AGE
*" ATCm =
ASDT =
AWDT =
f f ATCx AWDT
25% 25%
AF 1 AF 1
12.5
12.5
10.5
1.1
= 5.2
7.8
7.1
6.0
3.3
= 22.3
days
%
hours
hours
days
%
hours
hours
WASTEWATER FEED INLET LOCATIONS FOR FIGURE 1
-------
20.0
15.0 _
200
— 150
INTERMEDIATE STEP LOADINGS
Figure 1
SLUDGE OXIDATION & WASTE TREATMENT PRESSURES
At Various Step-Aeration Loadings
-------
RSF
= 11,355 cu m/day
= 3.00 mgd
RSC = 15.00%
API
M^n^H«
= 22,710 cu m/day
= 6.00 mgd
AFI1
AFI3
BAY #1
V1 = 1,486 cu m
= 392,700 gal
V///////////////////7//77////77/
BAY #2
V2 = 1,486 cu m
= 392,700 gal
BAY #3
V3 = 1,486 cu m
= 392,700 gal
BAY #4
V4 = 1,486 cu m
= 392,700 gal
AFI2
TFL
maam
= 34,065 cu m/day
= 9.OO mgd
AFI4
Figure 2
FOUR BAY STEP-AERATION TANK
-------
EXAMPLE A
Sludge Wasting Rate Decreased
Normally increased sludge oxidation pressures can be
maximized and normally decreased waste treatment pressures
oan be improved by a coordinated reduction in the sludge
wasting rate when the step-feed in-flow location is shifted
toward the bays nearer the aeration tank outlet end.
This is the case discussed in the Summary and
illustrated in Figures 1 and 2. The reduced wasting rate,
in effect, increases the number of sludge units in the
system, eventually restores RSC to 15.0%, and increases
sludge age. The mixed liquor concentration in the last bay
(ATCn) would also be restored to 5.0% after a short-term sag
in both RSC and ATCn. Obviously, aeration devices must be
powerful enough to support the increased mixed liquor
concentrations, and final clarifiers must provide the depth
and surface area needed to permit proper compaction of the
slower-settling, high-concentration sludge mass.
This coordinated control procedure, shifting step-feed
toward the outlet end while simultaneously reducing sludge
wasting, is usually appropriate to restore balance when the
mixed liquor sludge settling rates have become too slow but
still do not approach the almost negligible rates associated
with true classic bulking. Such sludge quality degradation
can be caused, for example, by a short-term organic overload
that increases production of the new, underoxidized slow-
settling component of the mixed liquor mass. The altered
process requirements can then be met by moving the
wastewater in-flow nearer the aeration tank outlet to
increase oxidative pressures and, by decreasing the sludge
wasting rate, to increase sludge age slightly.
EXAMPLE B
Sludge Wasting Rate Held Constant
Sludge oxidation pressures will be increased only
nominally and waste treatment pressures will be reduced more
sharply if the sludge wasting rate is held constant or
increased after the in-flow location is shifted, as was done
in Example A.
This phenomenon will be detailed in following sections
where comparisons of other sludge and process responses to
-------
varied in-flow locations, wasting rates, and return sludge
flow percentages are discussed.
Although not illustrated in Figure 1, the mixed liquor
concentration of the outlet bay (ATCn), the number of sludge
units returned to the aeration tanks (RSU), and the waste
treatment pressure represented by RSU per 1000 gallons of
wastewater or per pound of incoming BOD all remained
constant throughout Example A but dropped in Example B when
the step-feed location was shifted toward the outlet end.
Holding the sludge wasting rate constant, as discussed
in Example B, will usually lower sludge blanket levels that
have risen too high in hydraulically overloaded final
clarifiers. Since identical quality mixed liquor sludges
(same AGE, WCR, SSC60, etc.) settle more rapidly as their
concentrations (ATC) are reduced, this particular response
is governed mainly by the reduced ATCn.
CONCLUSIONS
The following conclusions are based on both fundamental
theory and on the author's observations at step-feed plants
that were operated according to his direction.
1. Degraded sludge quality associated with decreasing
settling and compaction rates can usually be improved by
shifting the step-feed location toward the outlet end of the
aeration tanks.
In this case, sludge oxidation pressures can be
maximized by shifting all the way to the last bay to
approximate contact stabilization. The final effluent will
be temporarily degraded, but restoring proper sludge quality
will improve effluent quality to produce a long-term,
beneficial effect on receiving waters. The step-feed
location is then usually shifted back toward the plug-flow
configuration after sludge quality has improved
sufficiently-
2. Final effluent quality can usually be improved by
shifting the step-feed location toward the head end of the
aeration tanks.
In this case, the treatment pressures can be maximized
by shifting all the way back to the first bay in the
conventional plug-flow mode. This presupposes that the
plant has adequate capacity and that thorough and complete
mixing takes place in each bay-
-------
BASIC CALCULATION PROCEDURES
DATA SOURCES AND TEXT ORGANIZATION
Calculation procedures used by the Waste Treatment
Branch of the NFIC-C during technical support projects are
described in Part III of the Operational Control Procedures
for the Activated Sludge Process. The suggested types and
frequency of observations and control tests have been
described in Parts I and II.
Part III-A emphasizes calculation procedures for
conventional activated sludge plants. This Part III-B
utilizes the same text organization format and the same
plant geometry- The difference between the two pamphlets is
that this Part stresses the calculation procedures for the
facility that has been provided with the step-feed
capability. In addition to the calculation procedures,
comparisons are made to the plug-flow parameters of Part
III-A to emphasize those values which change as the process
is shifted from the plug-flow to the step-feed mode.
Flow meter readings and control test results comprise
the "Observed" data that are entered in the formulas to
determine the "Wanted" information.
All calculations are performed in step-by-step fashion
and in most cases tabular calculation forms are provided to
illustrate the proper sequence of calculation steps to be
used in obtaining intermediate and final results. All
examples are expressed in separate metric unit and English
unit sections to avoid confusion. A table of equivalents is
printed inside the front cover. Figure 3 identifies the
tank sizes, flow rates, and sludge concentrations used in
the calculation examples. For convenience, each example is
preceded by definitions of the symbols used in it. A
complete list of all symbols and their definitions is
included in the Appendix to this pamphlet series.
Though this Part requires numerical notation, the
reader need only remember that the number refers to the bay
of the aeration tank. For example, ATC2 means the
concentration of the mixed liquor (% by centrifuge) in the
second bay of the aeration tank. AVG3 means the volume of
the third bay expressed in gallons. TFLj means the total
flow through the "j th" bay, and finally, TFLj-1 means the
total flow through the bay preceding the "j th" bay.
-------
AFI1 = 2,272 cu m/d
= 0.60 mgd
AFI3 = 6,816 cu m/d
= l.oO mgd
RSF
= 11,360 cu m/d
= 3.00 mgd
RSC = 15.0%
API
= 22,720 cu m/d
= 6.00 mgd
AVM j (cu m) = ;
AVG[ {mill g) = ;
j
ATCj (%)
ASDT: (hrs) =
J '
TFL (mgd) = ;
ASUj(SLU) = ;
)
\ BAY 1
\ 1,189
\ 0.313
; 12.50
; 2.09
\ 36
; 39.250
\ \
>*
BAY 2 /'
1,308
0.346
9.38
1.73
4.8
32,450
•J
'
I
•s^
' \ BAY 3
1,665
0.440
6.82
1.60
6.6
30,000
\
S/tf//////// ,f
BAY 4 ^
/
/
/
1,783 \
0.471 ^
/
/
5.00 ;
1.26 ^
/
^
9.0 ^
23,550 ^
' !
i
AFI2 = 4,544 cu m/d
= 1.20 mgd
AFI4 = 9,088_cu m/d
= 2.40 mgd
TFL = 34,080 cu m/d
»
= 9.00 mgd
AVM = 5,945
AVG = 1.571
ATCm =7.97
ASDT = 6.68
(AWDT = 2.95)
ASU = 125,250
Figure 3
AERATION TANK CHARACTERISTICS FOR THE CALCULATION EXAMPLE
-------
USE OF CALCULATED RELATIONSHIPS
The Summary statements should help an operator
determine what to do when faced with deteriorating sludge or
effluent quality. They should help him start shifting
wastewater feed location in the proper direction along the
aeration tank flow path with greater assurance that he will,
in fact, be performing a corrective control adjustment.
But most operators will also wish to know if they are
shifting far enough and fast enough. Many will want to
determine the actual sludge oxidation and wastewater
treatment pressure changes that followed their process
control adjustments. And some will wish to trim up their
step-feed adjustments to achieve the best net result. To do
this, the operator needs more numbers. Although the
calculation of certain factors governing step-feed operation
is more complex than that in plug-flow operations, it is not
really too difficult if approached in an orderly manner.
Such numbers can be determined quite easily and rapidly with
the aid of a computer and fairly readily using a good desk
calculator. Finally, though more time consuming, they can
be determined by pencil-and-paper simple arithmetic.
The following tabular formats are geared to help an
operator post observed data, record intermediate calculation
results, and determine the process pressures and responses
without a computer. They should also help the more
fortunate few set up orderly computer programs.
If you have not performed step calculations before,
don't let the tables and methodology scare you. The
procedures are not nearly as formidable as they may appear
at first glance, even though you may have to plod
laboriously through your first few trials. After that the
logic will become more apparent and the procedures more
systematized. You may then wish to determine additional
process characteristics that can provide you with an even
greater insight into the reactions occurring throughout your
process. Above all, these efforts should help you produce a
better final effluent.
SYMBOLS AND DATA USED IN THE EXAMPLES
To calculate aeration tank characteristics for step-
feed or contact stabilization, it is necessary to average
the characteristics over the separate compartments of the
aeration tanks and, because the compartments may not all be
the same volume, a weighted average must be used.
-------
SYMBOLS
ADT - Aeration Tank Detention Time (Hours)
AFI - Aeration Tank Wastewater Flow-In
ASDT - Aeration Tank SJLudge Detention T_ime (Hours)
ASU - Aeration Tank SJLudge Units
ATC - Aeration Tank Concentration (% by Centrifuge)
ATCm - Mean Aeration Tank Concentration
ATCn - Aeration Tank Concentration (Final Bay)
AV - Aeration Tank Volume
AVG - Aeration Tank Volume (Gallons)
AVM - Aeration Tank Volume (Cubic Meters)
AWDT - Aeration Tank Waste Detention Time (Hours)
BODi - Five-day Biochemical Oxygen Demand of the
Wastewater Entering (in) the Aeration Tanks
BODo - Five-day Biochemical Oxygen Demand of the
Final Clarifier Effluent (out)
RSC - Return Sludge Concentration (% by Centrifuge)
RSF - Return Sludge Flow
TFL - Total Flow to Aeration Tank
EXAMPLES
Metric Units
Observed:
Bays =
ATC1 =
ATC 2 =
ATC 3 =
ATC 4 =
AVM1 =
AVM2 =
AVM 3 =
AVM 4 =
AVM =
AFI1 =
AFI 2 =
AFI3 =
AFI4 =
AFI =
RSF =
RSC =
4
12.50
9.38
6.82
5.00
1,189
1,308
1,665
1,783
5,945
2,272
4,544
6,816
9,088
22,720
11,360
15.0 %
%
%
%
%
cu m
cu m
cu m/d
cu m/d
cu m/d
English Units
Observed:
Bays =
ATC1 =
ATC 2 =
ATC 3 =
ATC 4 =
AVG1 =
AVG 2 =
AVG 3 =
AVG 4 =
AVG =
AFI1 =
AFI2 =
AFI3 =
AFI4 =
AFI =
RSF =
RSC =
4
12.50 %
9.38 %
6.82 %
5.00 %
314,000 gal
346,000
440,000
471,000
1,571,000 gal
0.600 mgd
1.200
1.800
2.400
6.000 mgd
3.000 mgd
15.0 %
10
-------
CALCULATION FORMS
FORM A
Calculation Form A is used to compute those step-feed
parameters that must be determined differently from the
calculation of plug-flow parameters that were illustrated in
Part III-A. The step-feed parameters involved include ASU,
ASDT, AWDT and ATCxAWDT. Once these specific values are
determined, however, calculation of other aeration tank
parameters and the use of these values to compute additional
relationships follow the simpler methods of Part III-A.
Calculation Form A is used directly when the mixed
liquor concentration (ATC) in each bay has been measured and
the wastewater flow into each bay (AFI) and the return
sludge flow (RSF) have been metered. The known and the
observed values in the example are italicized (0.314, 12.50,
3.0 etc.) for convenient reference. Intermediate calculated
values are shown in regular type (3.925, 39,250 etc.) and
the final results are shown in bold, large type ( 195 240 =
ASU, ASDT - 6,68 etc.).
Calculations are started by posting all observed values
in columns 1, 2, 5, 6, 7, 8, 9 and 12. Intermediate values
are then calculated, step by step, by following the
instructions printed in each column. The instruction of
"1x2" in column 3, for example, states that for the first
bay (j=l), the intermediate "AVxATC" value is obtained by
multiplying the 0.314 AVG in column 1 by the 12.50 ATC in
column 2, i.e., 0.314 x 12.50 = 3.925 as posted in column 3.
According to the "5+6+7+8+9" instruction in column 10:
TFL = 3.00 + 0.60 + 0.00 + 0.00 + 0.00 = 3.60 for Bay 1
TFL = 3.00 + 0.60 + 1.20 + 0.00 + 0.00 = 4.80 for Bay 2
TFL = 3.00 + 0.60 + 1.20 + 1.80 + 0.00 = 6.60 for Bay 3
TFL = 3.00 + 0.60 + 1.20 + 1.80 + 2.40 = 9.00 for Bay 4
After all intermediate calculations have been
performed, the desired process characteristic is determined
by following the printed instructions at the bottom of the
Table. The mean ATCxAWDT (shown below column 25), for
example, is determined by dividing the total of the four
column 25 values (121.930) by the total of four column 12
values (6.00), e.g., ATCxAWDT = 121.930/6.00 = 20.32
11
-------
CALCULATION FORM A
To Determine ASU, ASDT, AWDT and ATCxAWDT
From Observed: ATC, RSF, and API
Bay
No.
j = l
1 = 2
1 = 3
1 = 4
TOTAL
1
Obs
AVG
(mil g)
0. 314
0. 346
0. 440
0. 471
2
Obs
ATC
(%)
12. 50
9. 38
6. 82
5. 00
3
AVx
ATC
1x2
3.925
3.244
3.000
2.355
4
ASU
(SLU)
(10)4x3
39,250
32,440
30,000
23,550
5
Obs
RSF
(mgd)
3. 00
3. 00
3. 00
3. 00
1.571 = AVG 125,240 = ASU
Bay
No.
1 = 1
1 = 2
1 = 3
1 = 4
TOTAL
6
Obs
AFI1
(mgd)
0. 60
0. 60
0. 60
0. 60
7
Obs
AFI2
(mgd)
1. 20
1.20
1. 20
8
Obs
AFI3
(mgd)
•
1. 80
1. 80
9
Obs
AFI4
(mgd)
•
2. 40
10
TFL
(mgd)
5+6+7
+ 8+9
3.60
4.80
6.60
9.00
11
ASDTj
(hr)
(24)xl
10
2.09
1.73
1.60
1.26
ASDT = 6.68
Note: (10) and (24) are conversion factors, not Col. Nos
12
-------
CALCULATION FORM A (CONTINUED)
Bay
No.
j = l
j = 2
j = 3
j=4
TOTAL
12
Obs
AFIj
(mgd)
0. 60
1. 20
1. 80
2. 40
13
AS DTI
(hr)
14
ASDT2
(hr)
15
ASDT3
(hr)
16
ASDT4
(hr)
From Column 11
2.09
•
1.73
1.73
•
1.60
1.60
1.60
nn
1.26
1.26
1.26
1.26
17
SUM
(hr)
13+14+
15+16
6.68
4.59
2.86
1.26
6.00
18
PRO-
DUCT
12x17
4.008
5.508
5.148
3.024
17.688
Divide Col. 18 TOTAL by Col. 12 TOTAL
17.688/6.00 = AWDT = 2.95
Bay
No.
j = l
j = 2
j = 3
j = 4
TOTAL
19
ATCjx
ASDTj
2x11
26.125
16.219
10.909
6.300
20
ATClx
ASDT1
21
ATC2x
ASDT2
22
ATC3x
ASDT3
23
ATC4x
ASDT4
From Column 19
26.125
d
16.219
16.219
10.909
10.909
10.909
ill
6.300
6.300
6. 300
6.300
24
SUM
20+21+
22 + 23
59.553
33.428
17.209
6.300
25
PRO-
DUCT
12x24
35.732
40.114
30.976
15.120
121.930
Divide Col. 25 TOTAL by Col. 12 TOTAL
121.930/6.00 = ATCxAWDT = 20,32
13
-------
TABLE A
SUMMARY OF CALCULATIONS
TEST RESULTS
AVI
AV3
AV4
AV
AFI1
AFI2
AFI3
API 4
API
RSF
BODi
BODo
MLVSS
RSTSS
ATC1
ATC2
ATC3
ATC4
ATCm
ASU1
ASU2
ASUS
ASU4
ASU
METRIC UNITS
1,190 cu m
1,310
1,670
1,780
5^950 cu m
2,270 cu ra/day
4,540
6,810
9,080
22,710 cu in/day
11,360 cu m/day
160 mg/1
10 mg/1
3^000 mg/1
12,000 mg/1
12.50 %
9.38 %
6.82 %
5.00 %
7.97 %
149 SLU
123 SLU
114 SLU
89 SLU
474 SLU
ENGLISH UNITS
0.314 mil g
0.345
0.440
0.471
1.571 mil g
0.60 mgd
1.20
1.80
2.40
6.00 mgd
3.00 mgd
160 mg/1
10 mg/1
3,000 mg/1
12,000 mg/1
12.50 %
9.38 %
6.82 %
5.00 %
7.97 %
39,250 SLU
32,440 SLU
30,000 SLU
23,550 SLU
125,240 SLU
14
-------
TABLE A (CONTINUED)
RESULTS OF INTERMEDIATE CALCULATIONS
BODi
MLVSS
RSTSS
ASDT
AWDT
RFP
RSC
RSU
BODi/AV
BODi/ASU
BODi/MLVSS
(F/M)
METRIC UNITS
3,630 kg/day
17,840 kg
136,270 kg/day
6.68 hr
2.95 hr
50.00 %
15.00 %
1,700 SLU/day
ENGLISH UNITS
8,010 Ib/day
39,325 Ib
300,420 Ib/day
6.68 hr
2.95 hr
50.00 %
15.00 %
450,000 SLU/day
AERATION TANK LOADINGS
610 g/cu m
7,660 kg/lOOOASU
0.20 kg/kg
38.14 Ib/lOOOcu ft
63.96 Ib/lOOOASU
0.20 Ib/lb
PURIFICATION PRESSURES
ATCxAWDT 20.32
ATCxAWDT/1000mg/l BODi 127
20.32
127
RSU/1000AFI
RSU/BODi
RSTSS/BODi
75 RSU/lOOOcu m
0.47 RSU/kg
38 kg/kg
75 RSU/lOOOgal
56 RSU/lb
38 Ib/lb
15
-------
Explanatory Note; Some of the time-concen-
tration dependent parameters in step-feed (for
example, the ATCxAWDT factor described above) are
based on the accumulated sums of products of
factors for each specific bay of the aeration
tank. As such, the weighted mean answers cannot
be determined by the simple division of some of
the previously calculated mean values. This fact
need not be alarming because the instructions
printed on the Calculation Forms take care of
these special requirements.
The following explanation will help clarify
the values that might be obtained from different
calculation procedures.
The calculation procedure to determine
ATCxAWDT for step-feed cannot be simplified by
multiplying ATCm of 7.97 % (Table A) by the AWDT
of 2.95 hours (Form A), 7.97 x 2.95 = 23.51, which
does not equal the 20.32 ATCxAWDT oxidation
pressure shown in Form A.
The reason for this becomes more apparent
from the following more familiar example
calculation of the average number of pounds of
BODS entering a plant during a 3-day interval.
Flow(mgd) x BOD5(mg/l) x 8.345 = Ib of BOD5/day
Day 1
Day 2
Day 3
Sum
Avg.
2.4
1.5
3.5
7.4
2.467
x
x
X
152
140
250
542
180.67
x 8.345
x 8.345
x 8.345
3,044
1,752
7,301
12,097
4,032
But the BODS calculated from the average
Flow and BODS ( 2.467 x 180.67 x 8.345 = 3,713 )
does not equal the 4,032 average of the three
previously calculated BOD values.
16
-------
AERATION TANK WASTEWATER FLOW-IN - API
Calculation Form A can be used directly to determine
essential process relationships if flow rates (especially
AFI1, AFI2, etc.) have been metered and if mixed liquor and
return sludge concentrations (especially ATC1, ATC2, etc.)
have been determined. In all too many plants, however,
individual flow rates to each aeration tank bay cannot be
measured. In such cases, the AFI values needed for use in
Calculation Form A can be calculated from the measured RSF,
RSC, and ATC values. Calculation Form B can be used to
determine these AFI values.
CALCULATION FORM B
To Determine AFI
From Observed: RSF, RSC, and ATC
Bay
No.
j=l
j=2
j = 3
j=4
TOTAL
1
Obs
ATC
j-l
(%)
From 2
*15. 00
12.50*
9.38*
6.82^
2
Obs
ATC
j
(%)
12. 50
/
9.38
/
6.82
/
5. 00
3
Dif
(%)
1-2
2.50
3.12
2.56
1.82
4
TFL
j-l
(mgd)
From 6
**3.00 1
3.60-^
4.80-^
6.60 — -
5
AFI
j
(mgd)
3x4
T-
0,60^,
^f,2C^
"^Lsg^
"i^o
6
TFL
j
(mgd)
4 + 5
^3.60
_-4.80
_-6.60
9.00
AFI1 + AFI2 + AFI3 + AFI4 = AFI = 6,00
* ATC @ (j-l) for Bay 1 = RSC
** TFL @ (j-l) for Bay 1 = RSF
17
-------
The step-by-step calculation procedures in Form B are
self-explanatory. As emphasized in Calculation Form B, the
measured RSC is the "Observed ATC" for Bay j-1, and the
metered RSF is the "Observed TFL" for Bay j-1. It is
essential that all calculations for the first bay be
completed before starting calculations for the second bay.
As emphasized by the arrows on the form, the calculated TFL
through Bay 1 (3.60) must be posted in the "TFLj-1" column
in line "j=2" for use in calculating the API to the second
bay. Calculations can then proceed from bay to bay until
all AFI values are determined for use in Calculation Form A.
DISCUSSION OF EFFECTS OF SWITCHING
TO VARIOUS STEP-FEED CONFIGURATIONS
Use of the calculation procedures to estimate the
changes that could logically occur if the operational mode
were changed all the way from plug-flow (all wastewater
entering Bay #1) to contact stabilization (all wastewater
entering Bay #4) is discussed and illustrated in this
section.
Let's assume that a plant is operating in the plug-flow
mode, sludge quality has been deteriorating for a week or
more. The one-hour settled sludge concentration (SSC60),
for example, has finally fallen from 15.0% to a dangerously
low level of 6.0%. Let's further assume that the operator
has been increasing return sludge flow percentages according
to the calculated demands, but that he has finally reached
the maximum capacity of his return sludge pumps at a flow
rate equal to 100% of the incoming wastewater flow. Then
let's finally assume that he has been unable to improve
sludge quality by concurrent aeration intensity and sludge
wasting control efforts. Such occurrences are not uncommon
at plants suffering from either temporary or sustained
overloads.
In cases like this, final effluent quality frequently
remains excellent as long as the final clarifier sludge
blanket formed by the slowly settling mixed liquor sludge is
not forced up and out over the effluent weirs. It is,
therefore, imperative that the operator modify control
procedures before the decreasing sludge settling rates
induce classic sludge bulking with the accompanying drastic
deterioration of final effluent quality. Switching from
plug-flow to step aeration would increase sludge oxidation
18
-------
pressures and most probably improve mixed liquor settling
and concentration rates. Although final effluent quality
will probably sag somewhat because of the reduction in the
wastewater treatment pressures, such a sag will not nearly
approach that which might otherwise occur if the present
trend were permitted to continue right on to sludge bulking.
At this time, or preferably before the SSC60 had fallen
to 6%, the operator could calculate the wastewater and
sludge detention times that would result from step-feed
configurations and then change into the mode he believes
most appropriate. Shortly thereafter he should check the
actual effect of the switchover by observing the changes
reflected in the results of his operational control tests
and utilizing tabular Forms A and B to calculate process
parameters (ATCxAWDT, etc). Ultimately, he would modify the
percentages of wastewater flow into the various aeration
tank bays to best meet the actual plant loading and sludge
quality requirements.
The operator will obviously measure flow rates and
perform the normal operational control tests during and
after the switch from plug-flow to contact stabilization.
He should be able to observe distinctive changes in sludge
quality within 3 to 7 days after the switchover. By this
time better sludge quality, as indicated by increasing SSC60
values, can be expected. He should then be able to continue
reduced sludge wasting rates to further increase ATC, RSC,
sludge age, upgrade sludge quality, and improve process
performance.
The object of this mode switch and these process
control adjustments has been to improve sludge quality and
force the SSC60 value upward. When this objective is
reached or approached, the operator will then be primarily
concerned with final effluent quality. He will want to
maximize treatment pressures by shifting the step-feed
loading back toward the plug-flow configuration. Now that
the danger of sludge bulking has been removed, he can, for
example, readjust the wastewater distribution to send
approximately one-third of the waste flow to Bay 1 and
continue routing two-thirds to Bay 4. As conditions
improve, he can then decide to shift the two-thirds of the
waste loading from Bay 4 to Bay 3 to further increase
purification pressures. If all goes well, he should
continue backing up in this manner until he can once again
route all wastes to Bay 1, restore the process to plug-flow,
and maximize the waste treatment pressures.
19
-------
There are dozens of
be used to meet process
Summary Section, shifting
increase sludae oxidation
step-feed configurations that can
demands. As discussed in the
toward contact stabilization will
pressures and shifting toward
plug-flow will increase wastewater purification pressures.
TABLE B
Comparison Between Plug Flow & Contact Stabilization
@AFI=6.00 mgd, RSF=6.00 mgd, SSC60=6.0%,WCR=800, MLVSS=75%
TEST RESULTS (Measured or Calculated Values)
AVG (mil gal)
AVG-Contact Tk
AFI1 (mgd)
AFI2 (mgd)
AFI3 (mgd)
AFI4 (mgd)
AFI-Total (mgd)
RSF (mgd)
ATC1 (%)
ATC2 (%)
ATC3 (%)
ATC4 (%)
ATCm ( % )
RSC (%)
RSU/Day
RSTSS (mg/1)
RSTSS (Ib/day)
MLVSS (mg/1)
MLVSS (Ib/day)
BODi (mg/1)
BODi (Ib/day)
BODo (mg/1)
PLUG
FLOW
1.5708
1.5708
6.000
0
0
0
6.000
6.000
3.0
3.0
3.0
3.0
3.0
6.0
360,000
4,800
240,300
1,300
23,600
160
8,010
10
CONTACT STABILIZATION
@ CONSTANT TSU
% *
1.5708 100
0.3927 25
0
0
0
6.000
6.000 100
6.000 100
4.0 133
4.0 133
4.0 133
2.0 67
3.5 117
4.0 67
240,000 67
3,200 67
160,200 67
1,200 67
3,930 17
160 100
8,010 100
10 100
@ CONSTANT RSC
% *
1.5708 100
0.3927 25
0
0
0
6.000
6.000 100
6.000 100
6.0 200
6.0 200
6.0 200
3.0 100
5.25 175
6.0 100
360,000 100
4,800 100
240,300 100
1,800 100
5,900 25
160 100
8,010 100
10 100
* Percent of Plug-Flow Value
20
-------
ASU-Total
CSU-Total
TSU-Total
ASU-Contact
AERATION TANK
LOADINGS
BODi/lOOOAVF
BODi/ 1000ASU
BODi/MLVSS (F/M)
SLUDGE
OXIDATION
PRESSURES
ASDT (hrs)
CSDT (hrs)
SAH (hrs/day)
SAP
AGE (Days)
AAG (Days)
WASTEWATER
PURIFICATION
PRESSURES
AWDT (hrs)
ATCxAWDT
ATCxAWDT/
lOOOBODi
RSU/1000AFI
RSU/lb BODi
RSTSS/lb BODi
PLUG
FLOW
47,100
22,500
69,600
47,100
38
170
0.34
3.14
1.50
16.2
0.677
6.0
4.06
3.14
9.42
59.0
60.0
45.0
30.0
CONTACT STABILIZATION
@ CONSTANT TSU
% *
54,700 116
14,900 67
69,600 100
7,850 17
152 400
1,020 600
2.04 600
5.5 175
1.5 100
18.8 116
0.786 116
6.0 100
4.71 116
0.79 25
1.56 17
9.8 17
40.0 67
30.0 67
20.0 67
@ CONSTANT RSC
% *
82,500 175
22,500 100
105,000 150
11,730 25
152 400
680 400
1.36 400
5.5 175
1.5 100
18.8 116
0.786 116
9.0 150
7.1 175
0.79 25
2.36 25
14.7 25
60.0 100
45.0 100
30.0 100
* Percent of Plug-Flow Value
21
-------
-« —
^JJJSTEP FLOW
ffffffljl AWDT W
m [- 2.95^
(From Form A, Col.
;::::::::::::
18);::::::::::
-- i i
II [tt\\$ ° 6 x
2.4 X 1.26
= 3.024
1.8 X 2.86 - 5.148
6.68 = 4.008
- 2.8
< 126 *
6 __
4.59
6.68
U. OJ
< II
CO 00
U- r-
< II
_
< II
u. O
< II
AWDT (hrs.)
ADT (hrs.)
ASDT (hrs.)
Figure 4
AERATION TANK DETENTION TIME
-------
RATIONALE OF PROCEDURE DEVELOPMENT
The following diagrams were developed for those
interested in reviewing the rationale used in developing the
step-feed calculation procedures.
AWDT, ADT, & ASDT
The three shaded areas, representing detention times in
Figure 4, reveal at a glance the extent to which sludge and
wastewater aeration tank detention times are changed when
the process mode is switched from plug-flow to step-feed.
ADT, which is the same for both sludge and wastewater at
plug-flow, is indicated by the size of the shaded middle
sketch. The relative size of the shaded area of the upper
sketch shows that the time wastewater is subjected to
aeration (AWDT) was reduced to 70% of the plug-flow value
after the mode was switched to step-feed. The relative size
of the shaded area of the lower sketch shows that the time
that sludge was subjected to aeration (ASDT) was increased
to 160% of the former plug-flow value.
The ADT for plug-flow, illustrated in the middle
sketch, is the sum of the time that the combined return
sludge and wastewater (TFL) remained in each of the four
bays.
The ASDT for step-feed, which is the time that sludge
remains under aeration, (bottom sketch) is also the sum of
the time that the combined return sludge and wastewater
remained in each of the four bays. But in this case only a
fraction of the wastewater flow was directed into each of
the Bays. This reduced the total flow (TFL) through each of
the first three bays (Column 10, Form A) and therefore
increased the detention time in each of these bays (Column
11, Form A). Switching from plug-flow to step-feed
increases the time that sludge is subjected to aeration.
Wastewater detention time (AWDT in the upper sketch) is
calculated somewhat differently. The 0.6 mgd portion of the
wastewater introduced to Bay 1 flows through all four bays
and is therefore subjected to aeration for 6.68 hours. The
product of this portion of the flow multiplied by its
aeration detention time (0.6 x 6.68 = 4.003) is illustrated
by the size of the bottom rectangle in the upper sketch.
23
-------
14
12
10
ASDT (hrs.)
59.553
33.428
17.209
6.300
2.4 X
6.300
= 15.120
1.8 X 17.209 30.976
LL. CM
< „
m to
u. «-
< n
.
< II
UL O
< „
ATCj x ASDT,
Figure 5
ATC x AWDT from ATC & ASDT
-------
The 1.20 mgd portion of wastewater introduced into Bay
2, however, flows through only the last three bays and is
subjected to aeration for only 4.59 hours. This product
(1.2 x 4.59 = 5.508) is illustrated by the size of the
second lower rectangle in the upper sketch.
Similarly, the portions of wastewater flow introduced
into Bays 3 and 4 are subjected to even less aeration.
The shaded area of the upper sketch is equal to the sum
of the areas of the four separate horizontal rectangles.
The weighted mean wastewater detention time (AWDT) is
therefore equal to the sum of the products of the individual
flow portions times their respective aeration detention
times divided by the wastewater flow (AFI). (17.688 from
Column 18, Form A, divided by 6.0 from Column 12, Form A =
2.95 = AWDT)
ATC x AWDT
Calculation of the ATCxAWDT factor for step-feed is
also based on a progressively weighted mean value determined
somewhat similar to the previously described AWDT.
Wastewater flowing through each aeration tank bay is
subjected to the ATCj x ASDTj in each bay (upper sketch and
Column 19, Form A).
But, here again, only the 0.6 mgd portion of the
wastewater that enters Bay 1 is subjected to the sum of the
ATCj x ASDTj pressures in all four bays. This value (0.6 x
59.553 = 35.72) is represented by the lower rectangle in the
bottom sketch.
The 1.2 mgd portion of the wastewater that enters Bay 2
is subjected to the sum of the ATCj x ASDTj pressures in the
last three bays, etc. And so on.
The shaded area of the lower sketch in Figure 5 is
equal to the sum of the areas of the four horizontal
rectangles. The weighted mean ATC x AWDT for the entire
cycle is equal to the sum of AFIj multiplied by the
accumulated sum of ATCj x ASDTj; all are divided by the
total AFI entering the aeration tank. (121.930 from Column
25, Form A, divided by 6.0 from Column 17, Form A = 20.32 =
ATCxAWDT)
25
-------
MEAN AERATION TANK CONCENTRATION - ATCM
Since, in step-feed, the ATC will decrease from the
first to the last compartment, a weighted mean ATC replaces
the plug-flow ATC. To determine the weighted mean ATC,
(ATCm), multiply each compartment's ATC by the compartment's
volume, add these terms together, and divide by the total
aeration tank volume. Thus, for a four compartment aeration
tank with the individual compartment ATC's measured:
ATCM = (ATClxAVl + ATC2xAV2 + ATC3xAV3 + ATMxAWD / AV
Figure 6 graphically displays this calculation
procedure. The area outlined with the heavy line is the sum
of ATClxAVl + ATC2xAV2 + ATC3xAV3 -I- ATC4xAV4. This area
must and does equal the shaded area. ATCm is then
calculated by dividing the area by the total aeration tank
volume, AV.
14 I
ATCm =(12.50x0.314 + 9.38x0.346 + 6.82 x 0.440 + 5.00 x 0.471) AI.571 =7.97
AVG (million Gals)
Figure 6
26
-------
FORMULAS FOR STEP FEED CALCULATIONS
The formulas used to determine the various step-feed
relationships are provided for those who may wish to set up
their own special calculation procedures or program a
computer to do the work. All equations are set up on the
basis of a four-bay aeration system.
FORMULAS USED IN CALCULATION FORM A
The following formulas show the equations for the
process evaluation factors shown in Form A:
ASDTi =
J
24XAVG
TFL ;
J
" " AFIj x ASDTj
AWDT .T.Y.
n " AFI-, x (ATCj x ASDT:)
ATC x AWDT = V >, -
AFI
ASU = x ATCm
ASU 100
27
-------
FORMULAS USED IN CALCULATION FORM B
If the AFI to each bay is unmetered and unknown, each
AFIj or percent of AFI can be calculated from AFI, RSF, RSC,
and ATCj from the following:
AFI1 = (RSFKRSC-ATC1) / ATC1
AFI2 = (RSF+AFI1)(ATC1-ATC2) / ATC2
AFI3 = (RSF+AFI1+AFI2XATC2-ATC3) / ATC3
AFI4 = (RSF+AFI1+AFI2+AFI3)(ATC3-ATC4) / ATC4
ATCn & ATCN
The following two equations are used to calculate the
weighted mean aeration tank concentration (ATCm) and if it
should be required the concentration of the last bay in the
aeration tank (ATCn):
ATCm = V ATCi X Vi
The last compartment ATC is given by:
ATCN = RSFxRSC / (RSF+AFI) = RSFxRSC / TFL
28
-------
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D
D
D
a
a
OPERATIONAL CONTROL PROCEDURES for the
ACTIVATED SLUDGE PROCESS
PART I / PART II - Observations / Control Tests
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No. of Copies
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-------
AERATION TANKS
FINAL CLARIFIER
FROM PRIMARY
RECYCLE FROM
»
THICKENER & DRAINS
RSF @ RSC
XSF XSC
TO SLUDGE HANDLING
TFL
CFO
TYPICAL PLUG FLOW ACTIVATED SLUDGE PLANT
FLOW METER
SLUDGE PUMP
AERATION TANKS
FROM PRIMARY
RECYCLE FROM I
THICKENER & DRAINS
FINAL CLARIFIER
CFO
TO SLUDGE HANDLING
TYPICAL STEP-FEED ACTIVATED SLUDGE PLANT
------- |