Environmental Protection Technology Series
Activated Sludge Treatment Systems
With Oxygen
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Office of Research and Development
U.S. Environmental Protection Agency
Washington, D.C. 20460
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RESEARCH REPORTING SERIES
Research reports of the Office of 'Research and Monitoring, Environ-
mental Protection Agency, have been grouped into five series. These
five broad categories were established to facilitate further develop-
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are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
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This report has been assigned to the Environmental Protection Technology
Series. This series describes research performed to develop and
demonstrate instrumentation, equipment and methodology to repair or
prevent environmental degradation from point and non-point sources of
pollution. This work provides the new or improved technology required
for the control and treatment of pollution sources to meet environ-
mental quality standards.
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This report has been reviewed by the Office of Research and Monitoring,
EPA, and approved for publication. Approval does not signify that the
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EPA 670/2-73-073
September 1973
ACTIVATED SLUDGE TREATMENT SYSTEMS WITH OXYGEN
by
John B. Stamberg
Doll off F. Bishop
Alan B. Hais
Contract No. 14-12-818
Project 11010 EYM
Program Element 1B2033
Project Officer
Dolloff F. Bishop
Advanced Waste Treatment Research Laboratory
National Environmental Research Center
Cincinnati, Ohio 45268
Prepared for
OFFICE OF RESEARCH AND MONITORING
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, DC 20460
For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 - Price 90 cents
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ABSTRACT
The oxygen activated sludge system must be viewed as an entirely unique
approach, and compared on a total system basis with other alternative
systems. The system is composed of three interrrelated subsystems; a
biological reactor, a clarifier, and a solids handling system.
The unique gas-tight biological reactor normally with less than 2.5
hours of aeration time essentially completely insolubilized the bio-
degradable organics to less than 5 mg/1 of soluble BOD in the Washington,
D.C. wastewater. In D.C., the organisms in the mixed liquor were
maintained between 4000 and 8000 mg/1 and were visually similar to a
conventional system except that the rate of activity in the oxygen
volatile solids was greater than a conventional system above an SRT of
6 days. Greater than 95% of the oxygen supplied was consistently
utilized in the staged reactor which employed co-current liquid and gas
flow contacting.
The liquid solids separation was accomplished by conventional clari-
fication. The clarification efficiency was a function of mixed liquor
concentration, particle shape, particle density and seasonal variation
(i.e., temperature, metabolic changes, and load variation). During
the warmer temperature periods, the peak overflow rates of 1940 gal/day/
ft were observed. In colder temperature periods, steady state over-
flow rates 975 gal/day/ft^ were the maximum obtained. The underflow
solids varied with overflow rate, clarifier volume and recycle rate.
Under flow concentrations up to 2.5% were obtained.
The total production of solids was significantly less than the
similarly operated diffused air system above an SRT of 6 days. As
little as 0.35 Ib. of sol ids/1b. of BOD were produced at an SRT of
13 days.
This report was submitted in partial fulfillment of Project 11010 EYM
and Contract No. 14-12-818 by the Department of Environmental Services,
Government of the District of Columbia under the sponsorship of the
Environmental Protection Agency. Work was completed as of September 1971,
n
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CONTENTS
Page
Abstract ii
List of Figures iv
List of Tables v
Acknowledgments vi
Sections
I Conclusions 1
II Recommendations 3
III Introduction 4
IV Analytical Procedures 5
V Reactor 6
VI Clarification 23
VII Solids Handling 34
VIII References 35
IX Publications 36
m
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FIGURES
No. Page
1 Oxygen Aeration System 7
2 Oxygen Aeration Reactor 8
3 Diurnal Flow Variation 9
4 Biological Activity Relationships 11
5 Excess Biological Sludge Production 12
6 Photomicrograph of the Mixed Liquor 16
7 Initial Sludge Settling Velocity Profile for 24
Broad Mixed Liquor Concentration Range
8 Effect of Adjusted vs. Acclimated Wastewater 27
Temperatures on Sludge Settling Rates
9 Effect of Wastewater Temperatures on Initial 28
Batch Flux
10 Effect of Wastewater Temperature on oxygenated 29
Sludge Settling Rates (September-October 1971)
11 Effect of Wastewater Temperature on oxygenated 30
Sludge Settling Rates (November-December 1971)
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TABLES
No. Page
1. Organic Removal 14
2. Oxygen Usage 18
3. Reactor Variables and Operating Conditions 20
4. Clarifier Variables and Operating Conditions 32
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ACKNOWLEDGMENTS
The support of the Linde Division of the Union Carbide Corporation in
the maintenance and supply of the necessary instruments, equipment and
oxygen supplies is acknowledged. Likewise, the close cooperation of
their engineering staff provided the best possible results. The direct
on site support of Union Carbide engineers and technicians during the
period of January 1971 is especially appreciated.
-------
SECTION I
CONCLUSIONS
1. A gas-tight biological oxygen reactor with independent control of
dissolved oxygen and mixing, coupled with an aerobic final clarifier,
produces a good quality secondary effluent on District of Columbia
primary effluent with 1.5 to 2.5 hours average detention time (based
on raw flow) with MLSS concentrations between 4000 and 8000 mg/1.
2. Biodegradable organics in the District of Columbia primary effluent
are essential completely insolubilized by the oxygen process (less
than 5 mg/1 of soluble BOD). Total carbonaceous BOD removal depends
upon the amount of suspended solids in the effluent and, therefore, on
the ability to clarify.
3. Oxygen micro-organisms are visually the same as those in a
typical conventional system; however, the rate of activity of the
oxygen volatile solids is greater above an SRT of 6 days.
4. Oxygen activated sludge is subject to filamentous (Sphaerotilis)
growth as similarly observed in the air systems when operated below
an SRT of 5 days on D.C. primary effluent.
5. Sludge in the oxygen system underflow settles to approximately
1.0-1.4% solids in a clarifier with 1.9 hours of hydraulic retention
time and 2.0-2.4% with 2.8 hours.
6. Total production of excess biological solids is significantly
lower in the District of Columbia oxygen system than in a parallel
step aeration system at SRT's above 6 days with as little as 0.35 Ib.
of excess solids produced/1b. of BOD added at an SRT of 13 days.
7. When the oxygen clarifier is operated with a deep feed well or
with the mixed liquor sufficiently concentrated to settle in a
subsidence (zone) settling pattern, the blanket acted as a filter and
produced high quality effluent. In the summer and fall, 1970, the
clarifier operated at a peak rate of 1940 gpd/ft2. In the 1970-71
winter, oxygen clarifier rates could not exceed a sustained 975 gpd/ftS
A larger clarification area is required in the winter than in the
Summer on District of Columbia wastewater for a given MLSS concen-
tration for the same effluent quality.
8. With a shallow center feed well and with the mixed liquor
concentrations low enough (under 4500 mg/1) to permit discrete
particle settling, better settling rates are observed in the oxygen
clarifier than with the method of clarifier operation described in
No. 7 above. Only moderate decreases in effluent quality (increase in
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SS from 15 to 25 mg/1) in consecutive tests with similar operating
conditions are observed with this type of clarifier operation at the
District of Columbia.
9. Substantial nitrification is achieved in the oxygen aeration
system in the summer and fall at the District of Columbia.
10. Average effluent phosphorus residuals of 1.8 mg/1 as P with an
alum dosage of 1.4 A1+++/P, by weight, were achieved in the oxygen
system. Higher phosphorus removals are possible with higher alum
dosages, but in areas with moderate wastewater alkalinity, pH control
may be required to prevent the depletion of wastewater alkalinity
reserves in the oxygen system.
11. Based upon the influent and exhaust gas flows, over 95% of the
input oxygen is consistently utilized in the District of Columbia
oxygen reactor.
12. The oxygen activated sludge system must be viewed as an entirely
unique approach, and compared on a total system basis with other
alternative systems. Reactor and clarifier sizing must be
coordinated. As the reactor size is increased, a low MLSS concen-
tration is required for a given biological state (F/M ratio). The
lower the MLSS concentration, the smaller the required secondary
clarification area. The solids handling requirement of an oxygen
system depend on the biology established in the reactor/clarifier
combination. Thus, if minimum excess biological sludge production is
required, then more capacity is required in the reactor/clarifier
combination. Further, the concentration of the clarifier underflow
solids is dependent on clarifier volume and recycle rate.
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SECTION II
RECOMMENDATIONS
1. The oxygen activated sludge system fed with raw wastewater should
be evaluated over a wide range of operation conditions.
2. The oxygen activated sludge system, being fed either raw or
primary wastewater, should be evaluated as a high rate system with
high F/M ratios and lower SRT's than employed in this study.
3. Mineral addition (alum) for phosphorus removal should be further
evaluated especially using lime to restore pH and alkalinity.
4. Other methods of influent feeding such as step aeration and
contact stabilization should be evaluated with oxygen aeration.
5. The use of separate oxygen digestion of primary, secondary or a
combination of waste sludges should be investigated as a potential
system alternative.
6. A comprehensive thickening dewatering study is required on a
variety of equipment to define the best probable alternatives to
handle the waste oxygen solids.
7. Alternate liquid solids separation methods to gravity clarification
should be considered.
8. A study on designing of full scale circular and rectangular
clarifiers from pilot plant and batch test information is badly
needed not only for oxygen, but for all biological sludges.
9. Separate nitrification utilizing oxygen should be evaluated.
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SECTION III
INTRODUCTION
The use of pure oxygen within the activated sludge process dates back
some 20 years. Pirnie (1) proposed a system of predissolving pure
oxygen in high concentrations in the influent wastewater before
entering a non-aerated mixed reactor. The process was termed "a bio
precipitation system." Biological success was achieved by Okun (2) in
bench scale tests and later by Budd and Lambeth (3) on a pilot scale,
but oxygen utilization efficiencies of 20-25% were too costly. Okun
and Lynn (4) and later Okun (5) showed an increase in the effective
sludge activity in the mixed liquor by reducing or eliminating
anaerobic periods such as can occur in clarification.
McKinney and Pfeffer (6) more recently reviewed the use of oxygen in
activated sludge. Increased metabolism rates, produced by eliminating
periods of zero dissolved oxygen, would increase treatment efficiencies
in overloaded plants and reduce the size required for new plants. Thus
potential reductions in capital investment were viewed possible for
oxygen systems.
Union Carbide recently developed the UNOX System/7 which is an oxygen
aeration-activated sludge system with an oxygen utilization of over
90%. This oxygen-activated sludge process (UNOX) is presently being
piloted in several locations. The Environmental Protection Agency,
Washington, D.C. Pilot Plant has been evaluating the oxygen-activated
sludge process since May of 1970. The objectives of the study are to
determine process performance and operating requirements on the D.C.
primary wastewater, an average domestic metropolitan wastewater.
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SECTION IV
ANALYTICAL PROCEDURES
To evaluate the oxygen-activated sludge process, appropriate samples
were manually composited over a 24-hour period. Samples were stored
at 3°C to minimize biological activity.
The 5-day biological oxygen demand (BOD) of the composite samples was
determined by the probe method (9); the ammonia and nitrate-nitrite
on a Technicon Automatic Analyzer (9)(10). The total organic carbon
(TOC) was measured on a Beckman Carbonaceous Analyzer (11). The total
phosphorus was determined by the persulfate method (12). All other
analyses employed Standard Methods (13). Soluble phosphorus and
soluble BOD were filtered through a standard glass suspended solids
filter before analyses.
5
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SECTION V
REACTOR
The first and most unique aspect of the system (Figure 1) is the gas
tight biological reactor shown in Figure 2. In the EPA-DC Pilot Plant,
primary effluent from the District of Columbia's plant is fed to the
oxygen reactor either on steady state flow or on predetermined daily
cycle (diurnal variation), normally with a 2.3:1 peak to minimum
(45-105 gpm) daily flow variation (Figure 3).
Using all four available stages, the 8,100 gallon District of Columbia
oxygen reactor provides 1.95 hours of detention time at the nominal
influent flow of 100,000 gpd. At the peak daily flow, the detention
time is 1.29 hours. Using three of the available four stages, the
detention times are reduced to 1.50 hours and 1.00 hours, respectively,
at the nominal and peak daily flows.
The reactor is sealed to prevent loss of oxygen and includes submerged
hydraulic entrances and exits as well as simple water-sealed mixing
equipment. Internal spray equipment using tap water is provided to
suppress foam. Also, a partially submerged baffle plate before the
internal exit trough retains the foam until the baffle plate is raised
to allow the foam build-up to escape. The reactor is staged to provide
the proper tank geometry for efficient mixing and oxygen usage.
Efficient oxygen usage is achieved by co-current contacting of the mixed
liquor and oxygen gas through the various stages. The addition of
pure oxygen to the reactor is controlled by a pressure regulator. An
inlet oxygen control valve actuated by a pressure regulator maintains
the overhead gas at a selected pressure usually between 1" and 4" of
water. Even with large instantaneous fluctuations in oxygen consump-
tion, the oxygen control valve maintains the selected pressure. The
overhead gas pressure is normally selected to maintain the oxygen
concentration at approximately 50% in the exhaust gas from the last
reactor stage. Pure oxygen is introduced to the first stage where the
peak oxygen demand occurs. As the oxygen is used in biological
metabolism, respirated carbon dioxide and stripped inert gases reduce
the oxygen concentration in the overhead gas flowing co-currently with
the mixed liquor through the succeeding stages. The successive decrease
of both oxygen availability and oxygen demand produces efficient
oxygen use before the residual gas is exhausted from the reactor.
Mixed liquor dissolved oxygen levels in the District of Columbia oxygen
reactor are held between 4.0 and 8.0 mg/1 by adjusting the re-
circulation rate of the oxygen gas within the individual stages. The
compressor in each stage pumps the overhead gas through the rotating
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02 RECYCLE
"1
I
INFLUENT
fOa
CO
00
do
SLUDGE RECYCLE
EXHAUST GAS
IT
.15
TL
EFFLUENT
-~~ WASTE
FIGURE 1
OXYGEN AERATION SYSTEM
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oo
AERATION TANK
COVER
GAS RECIRCULATION
MIXER DRIVE COMPRESSOR
OXYGEN
FEED GAS
WASTE
LIQUOR
FEED
RECYCLE
SLUDGE
EXHAUST
GAS
MIXED LIQUOR
EFFLUENT TO
CLARIFIER
PROPELLER
SPARGER
Figure 2. Oxygen Aeration Reactor
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100 -
80 -
(average)
60 -
CO
tofi
2 40 -
20 -
I
NOON
I
I
3 6
TIME OF DAY
Figure 3. Diurnal Flow Variation
12
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submerged turbine-sparger to provide efficient dispersion and mixing
of the recirculated gas. The recirculation rate in each stage may
be set either manually on the basis of the dissolved oxygen analysis
or automatically using a control system with a dissolved oxygen sensor.
The gas recirculation rate in the first District of Columbia stage
typically is 3-7 cfm and 1-2 cfm in each of the last three stages.
Total recirculation requirements vary between O.JO and 0.20 ft^/gal.
of flow.
With its high oxygen transfer capabilities (which are essentially
independent of turbine mixing rates), the oxygen system is able to
operate at higher MLSS concentrations. These factors enable the
system to readily adsorb shock organic loads. Also, toxic shock
loads can be better handled, much as in a totally mixed activated
sludge system. Both types of systems initially expose the toxic
substrate to a large mass of active solids and the resulting "bio-
logical inertia" buffers the toxicity.
f
On the District of Columbia wastewater, as seen in Figure 4, the
volatile portion of the oxygen solids exhibit a much higher activity
for the SRT range above 6 days than the District's step aeration
pilot process. The F/M ratio is the pound of.BOD applied per dav
per pound mixed liquor volatile suspended solids (MLVSS) under aeration.
Figure 4 indicates that a lower total volatile mass under aeration is
required with oxygen than with air to obtain any given SRT above 6 days
for a similar influent BOD. Thus, shorter detention times are required
with oxygen than with step aeration at similar MLSS concentrations to
achieve any given SRT above 6 days. Further, at identical SRT's above
6 days, the oxygen system will produce less excess biological solids.
The most probable reason for the increased activity is attributed to
maintaining the mixed liquor dissolved oxygen between 4 and 8 mg/1.
The independently controlled mixing also minimized sludge pockets, dead
spots and shearing of the floe particles. Mixed liquor entering the
clarifier has a high dissolved oxygen content which permits a certain
amount of aerobic metabolism in the clarifier and greatly reduces the
time that the bio-mass is in an anaerobic condition.
The total production of solids in the oxygen system (Figure 5) per
pound of BOD added, including underflow waste and effluent solids, is
inversely related to the solids retention time (SRT) above an SRT of
1.3 days. The solids production with the oxygen aeration system is
significantly lower than in the conventional step aeration pilot system
above an SRT of 6 days. Indeed, the total solids production decreases
from 0.65 pounds of excess solids per pound of BOD added at an SRT of
6 days to 0.35 pounds of excess solids per pound of BOD added at an SRT
of 13 days with only a 33% increase in volatile solids concentration at
the higher SRT.
The parallel conventional system, operated as step aeration or contact
stabilization, exhibits increased solids production through an SRT of
9.5 days with a peak solids production of approximately 1 pound of
10
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1.0 r-
— 0.8
ts>
tS)
:= 0.6
0.4
0.2
STEP AERATION
I I I I I I I I
4 6 8 10 12
SRT (days)
FIGURE 4
BIOLOGICAL ACTIVITY RELATIONSHIPS
14 16
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1.0
S 0.8
0.6
0.4
0.2
0 2
OXYGEN AERATION
6 8 10
SRT (days)
12 14 16
FIGURE 5
EXCESS BIOLOGICAL SLUDGE PRODUCTION
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excess solids per pound of BOD added. In addition, an approximate
four-fold increase in volatile solids is required to raise the SRT from
6 to 13 days in the step aeration system to achieve reduced solids
production.
The reduction of BOD in the oxygen reactor is excellent. With an
influent BOD up to 130 mg/1 , a wide range of detention times from 1.5
to 2.5 hours, and SRT's that vary from 13 to as low as 2 days, the
effluent soluble BOD is consistently less than 5 mg/1 as described in
Table 1. This indicates virtually complete insolubilization of the BOD
in the primary effluent. Thus, BOD removal on the D.C. oxygen system
is a function of clarification.
The oxygen mixed liquor is similar visually to the micro-organisms in
conventional activated sludge (Figure 6). The mixed liquor biota is
normally very well bioflocculated with active stalked cilates growing
on the bacterial mass. Zooflagellates and free swiming cilates,
although few in number, remain adjacent to or within the flocculated
particles. Several varieties of large active rotifers are present in
abundance. A few nematodes exist in the sludge. Normally, filamentous
growth is not apparent. There is almost complete absence of fraqmented
debris or unflocculated bacteria between the discrete particles.
In the SRT range less than 5 days, both the oxygen activated sludge and
the conventional aeration systems exhibited filamentous growth on the
District of Columbia wastewater. Filamentous growth does not occur
during operation above an SRT of 5 days. Normally, when encountering
filamentous growth for a few days, reducing system influent flow to
increase the SRT reestablishes a filamentous free sludge in several
days. However, after extended periods of operation with filamentous
growth, the Sphaerotilis becomes firmly entrenched and can not be
quickly purged from the system by flow reduction techniques. Hydrogen
peroxide added to the recycle in two 24-hour periods approximately a
week apart at dosages of 200 mg/1 (based on influent flow) is then
required to purge the system of filamentous growth.
The recirculation of respirated carbon dioxide within the oxygen reactor
stages lowers the wastewater pH from 7.0-6.8 in the first stage and
to (6.1-6.4) in the final stage. With an average system pH of
approximately 6.5, the oxygen process more slowly establishes a
nitrifying population than the step aeration activated sludge system
operated at a pH of 7.0 to 7.4. However, during the warmer months
when the solids wasting is reduced to a level where the nitrifying
organisms propagate faster than they are removed, the Nitrosomonas and
Nitrobacter populations increase and substantial nitrification occurs
in the oxygen system. Nitrogen removal across the oxygen system
during periods of high nitrification and partial denitrification is as
high as 39-40%. Nitrogen removal decreased to a low of 9-10% during
periods without nitrification.
13
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TABLE 1
ORGANIC REMOVAL-
Operatinq Period
Month
Dates
Primary Effluent BOD (mg/1 )
Final Effluent BOD (mg/1)
Final Effluent Soluble BOD (mg/1)
Primary Effluent COD (mg/1)
Final Effluent COD (mg/1)
Primary Effluent TOC (mg/1)
Final Effluent TOC (mg/1)
Primary Effluent Suspended
1
June
12-30
89
18
--
250
45
75
14
113
2
July
87
19
--
244
70
65
24
101
3
August
1-25
89
12
2
245
49
77
15
102
4
September
106
13
2
252
51
100
15
107
5
October
3-11
116
14
3
284
51
106
15
120
6
November
131
27
3
275
63
91
21
92
7
January
1-16
124
11
3
250
59
83
21
98
Solids (mg/1)
Final Effluent Suspended
Solids (mg/1)
36
53
28
24
35
56
24
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CJ1
TABLE 1 (CONTINUED)
ORGANIC REMOVAL
8
January
17-31
134
32
3
256
99
87
26
100
58
9
March
1-18
121
27
--
251
76
88
22
104
49
10
April
107
10
4
267
48
81
17
83
18
11
May
140
7
4
278
51
92
18
120
12
12
June
110
8
—
238
45
74
18
100
13
13
July
129
14
--
235
35
78
14
103
11
14
August
110
15
—
219
32
69
13
97
16
15
September
149
15
4
239
35
79
15
95
15
16
October
120
14
4
224
37
69
14
81
15
17
November
125
20
5
244
54
75
17
90
23
18
December
1-21
125
20
5
238
59
76
19
95
23
19
Decembe
22-31
135
18
5
236
53
91
19
95
18
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FIGURE 6 PHOTOMICROGRAPH OF THE MIXED LIQUOR
16
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As in the parallel step aeration process, nitrification in the oxygen
system begins to decrease in the fall and becomes virtually nil during
the winter. At wastewater temperatures of about 63°F, 5 mg/1 of NCL-N
is still produced with operation of an SRT of 9.0 days.
Without alum addition, phosphorus is removed from the oxygen system
through metabolic uptake and by wasting of excess solids; thus, the
removals, vary with the metabolism of the mixed liquor. At high SRT's
(highly endogenous metabolism), total phosphorus removal averages only
about 15%. At lower SRT's with less endogenous respiration, phosphorus
removals increase to 20%.
With alum addition, phosphorus removal in the oxygen system increases
as the alum weight ratio (A1+++/P) increases. During experiments
conducted in the fall of 1970, for a dosage equal to an A1+++/P weight
ratio of 1.4/1, 80% of the phosphorus was removed to an average residual
of 1.8 mg/1 as P and only a slight decrease in wastewater alkalinity
and pH occurred. The filtered effluent (though 0.45 u) contained an
average of 1.6 mg/1 of soluble P. When the dosage was increased to a
ratio of 1.85/1 (A1+++/P), the residual total and soluble phosphorus
decreased to 0.62 and 0.53 mg/1 as P, respectively. At this higher
dosage, however, the buffering capacity of the oxygen mixed liquor was
further reduced and the average pH decreased from 6.5 to 6.0. The
oxygen biomass dispersed, necessitating termination of the alum
addition to allow the mixed liquor to recover. In areas with low
alkalinity wastewaters, additional alkalinity in the form of lime or
caustic may be required to control pH at a level which will prevent
floe dispersion. This pH adjustment may be necessary in either air or
oxygen systems but is more likely in an oxygen system because of the
increased dissolved C02 content of the mixed liquor. The addition of
alum and precipitation of Al(POd) and A1(OH)3 increases the inert
solids carried in the system ana adequate clarification capacity for
the higher solids concentration must be provided.
Consistently throughout the operation, vented gas from the fourth
oxygen reactor stage has been less than 10% of the input oxygen volume.
The vented stream is roughly 50% oxygen. Based upon the influent and
exhausted oxygen concentrations, the net utilization of oxygen in the
process is about 95%. The accountable oxygen consumption consisting of
COD removal, nitrification demand, exhaust gas, and effluent dissolved
oxygen is summarized in Table 2. The COD removed was calculated by
subtracting the COD in the underflow waste solids and that in the
process effluent from the primary effluent COD. With increasing SRT,
additional oxygen is required for endogenous respiration. Likewise
during periods with nitrification, additional oxygen is required.
Pertinent reactor variables and operating conditions are summarized
in Table 3 for approximately 1-1/2 years of operation on the District
of Columbia oxygen aeration pilot plant.
17
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TABLE 2 - OXYGEN USAGE
CD
Operating Period
Month
Dates
Primary Effluent COD
(1 fa/million gal . )
Final Effluent COD
(Ib/million gal .)
Waste Sludge COD
(Ib/million gal.)
COD Removed from System
(Ib/million gal . )
Nitrate Nitrogen Demand
(1 fa/million gal . )
Exhaust Oxygen
(Ib/million gal . )
Final Effluent D.O.
(Ib/million gal .)
Total
Oxygen Supplied
1
June
2080
375
188+
1517
14
85
10*
1626
1750
2
July
2030
584
42+
1404
69
54
10*
1537
1825
3
August
2040
408
150+
1482
128
75
10
1695
1775
4
September
2090
430
217+
1443
160
85*
10
1698
1900
5
October
3-21
2370
425
630+
1315
100
87
10
1512
1650
6
November
10-30
2290
525
247
1518
18
85
10*
1631
+
7
January
1-16
2080
488
233
1359
9
65
25
1458
1700**
(Ib/million gal.)
+ COD = 1.4 volatile solids
* Estimate
* Inlet meter malfunctioned
** Increase sampling and greater losses of
through sample ports
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TABLE 2 - OXYGEN USAGE _ CONT'D
8 9
January March
17-31 1-18
2140
826
336
978
14
65
25
1082
1800**
2030
600
160
1260
0
160
30
1450
1450
10
April
2180
380
700
1100
0
130
40
1270
1300
11 12 13 14 15 16
May June July August September October
2500
450
890
1100
0
40
50
1250
1260
1750
340
80
1330
0
80
60
1470
1600
1940
270
270
1400
60
300
60
1820
2200
1830 1920 1880
260 280 310
270 400 390
1300 1240 1180
70 270 200
260 200* 200
40 40 30
1670 1750 1610
1690 ± ±
17
November
1950
440
220
1290
220
200
40
1750
1740
18
December
1-21
1990
490
230
1270
190
200
40
1700
2000
19
December
22-31
1970
440
300
1230
70
150*
40
1490
1450
-------
ro
o
TABLE 3
REACTOR VARIABLES AND OPERATING CONDITIONS
Operating Period
Month
Dates
Flow Rate (gpm)
Aeration Time (hr. )
Recycle Rate
MLSS (mg/1)
MLVSS (%)
SRT (days)
F/M (Ib BOD/ day/1 b MLVSS)
Volumetric Loading (Ib BOD/
day /I, 000 ft3)
[/ ui — h v* "fck
Mi vor Pnu/or f *••« • MI . \
mxer Power Ilj000 gal<;
/ k w - hr \ **
tonpressor Power ( l^QQ ga] _ ;
Temperature (°F)
1
June
12-30
50-55
2.00
50
4140
74
7.7
0.333
57
1.27
0.39
74-80
2
July
80
1.66
50
5180
70
7.3
0.342
80
1.19
0.28
78-84
3
August
1-25
80
1.66
42
5250
73
11.8
0.296
80
0.98
0.39
82-85
4
September
70+
1.95
32
6000
78
10.7
0.304
96
0.92
0.41
79-83
5 6
October November
3-21 10-30
70+*
1.95
38
8120
67
5.5
0.283
106
1.00
0.35
70-79
70+
1.95
37
6350
73
5.5
0.355
108
1.00
0.26
56-69
7
January
1-16
53
2.50
77
5300
80
13.0
0.275
89
1.18
0.34
58-60
* Alum addition
2.3:1 diurnal variation
** Pilot plant equipment efficiency was not determined
-------
ro
TABLE 3 (CONTINUED)
REACTOR VARIABLES AND OPERATING CONDITIONS
8
January
17-31
53
2.50
80
3940
81
4.7
0.392
80
1.42
0.32
58-60
9
March
1-18
60-70
2.15
50-60
8070
77
3.7
0.580
90
-
-
50-62
10
April
31-67
3.30-1.55
90-60
2710
81
1.3-4.0
0.30-1.00
98
-
-
62-65
11
May
60
1.70
65
2750
78
2.0
0.970
157
-
-
65-71
12
June
30-70;
3.70-1.50:
100-50;
1000
73
13.0
0.400;
95
-
-
70-77
13
July
70
1.5
50
6600
70
12.6
0.430
160
-
-
77-80
14
August
70+
1.5
50
7500
70
10.0
0.32
131
-
-
77-81
15
September
70+
1.5
46
7400
72
7.5
0.39
185
-
-
76-81
16
October
70+
1.5
46
6000
73
9.5
0.31
146
-
-
76-71
17
November
70+
2.0
45
4600
78
9.8
0.39
111
-
-
71-65
-------
ro
ro
TABLE 3 (CONTINUED)
REACTOR VARIABLES AND OPERATING CONDITIONS
18
December
1-21
70+
2.0
40
4400
80
9.0
0.40
19
December
22-31
70+
2.0
40
4200
81
6.5
0.50
111 122
65-63 63-61
-------
SECTION VI
CLARIFICATION
The second important aspect in the oxygen aeration system is the
method of liquid/solids separation. As mentioned before, soluble
residual BOD in the effluent averaged less than 5 mg/1 in the test
periods indicating virtually complete BOD insolubilization. Thus,
most of the residual BOD in the District of Columbia oxygen system
effluent is associated with suspended solids. Overall removal of
suspended solids and its associated BOD is a functional of clarification
efficiency.
Clarifier efficiency is in turn a function of the basic settling
characteristics of the solids as well as of the actual design and
operation of the clarifier. With the normally higher mixed liquor
concentrations used in the oxygen aeration process, design criteria
for both clarifiers (i.e., overflow rates and volume) and thickeners
(i.e., solids loading - Ib/ft2/day) should be considered. The Ten
State Standards suggest that conventional activated sludge clarifiers
be designed for average overflow rates of 800 gpd/ft2. The Water
Pollution Control Federation Manual of Practice (1959) suggests that
the solids loading be held below a peak of 30 Ib/ft2/day. Overflow
rates and solids loading criteria should be better defined for high
solids systems such as oxygen aeration.
The basic settling characteristics of the mixed liquor typically have
been found to be a function of:
1. Concentration of the mixed liquor.
2. Particle shape.
3. Particle density.
4. Seasonal variation
a. Physical changes in water density and
viscosity with temperature.
b. Metabolic changes with temperature.
c. Seasonal loading variation.
As seen in Figure 7, the log of the initial settling rate is a function
of the log of the solids concentration. This curve illustrates that
two relationships exist. The first at lower MLSS levels corresponds
to free particle settling and is characterized by the absence of an
initial discrete subsiding interface and a zone of homogenous settling
solids. The second at higher MLSS levels had both an initial descrete
interface and a zone of homogenous settling particles (zone settling)
(14). Thus, the sizing of a clarifier is a function of the MLSS
concentration and must be coordinated with the reactor (and the sludge
23
-------
i
30
20
15
5 io
| 8
CD C
E 4
GO ^
^ 3
2
(D.C.-DEC.)
-n
Vj =lnitial Velocity
Cj =lnitial Cone.
A=lntercept Constant
n=Slope Constant
I II
I
1 2 3 4 6 8 10 15 20 30
INITIAL MIXED LIQUOR CONCENTRATION-C j (gm/l)
FIGURE 7
INITIAL SLUDGE SETTLING VELOCITY PROFILE
FOR BROAD MIXED LIQUOR CONCENTRATION RANGE
-------
handling facilities) to achieve the desired biological capabilities of
the system.
Another important factor is the particle shape. Normally, as shown in
Figure 6, the oxygen mixed liquor particles have rounded'shapes.
However, if filamentous growth exists, as experienced below an SRT of
5 days in D.C. (air and oxygen), both settling rates and compaction
deteriorate. The range, if any, that filamentous growth appears is
unique to each location and should be defined for that location.
Still another important factor in the basic settling characteristics
is the density of the particles in relationship to the density of the
water. It is the difference in density that is the driving force
for settling. The VSS/TSS ratio (or volatile %) is one relative
indication of density. There are several ways to improve the particle
density. One is to feed raw wastewater instead of primary effluent to
the oxygen aeration system thus incorporating the normally denser
particles captured in primary sedimentation into the biomass, such as
occurred at Batavia (7). Again, the sizing of the reactor oxygen
supply, etc., must be compatible with the increased organic loading.
In Washington, heavy rains and unusually high flows wash silt and clay
into the sewer system. These materials subsequently become incor-
porated in the mixed liquor solids and have increased sludge settling
rates 30% to 60%. Also, operation under different biological
conditions can alter sludge settling characteristics.
Another unique method of increasing the density of the sludge that was
evaluated at the EPA-DC Pilot Plant is by altering the method of
clarifier operation. Two major methods of clarifier operation are
possible. One is to use the blanket as a filter and the other is to
permit classification of the settling solids. The first method can
be accomplished in two different ways: (1) by providing sufficient
depth to the clarifier such that the mixed liquor passes up through
the clarifier blanket (which acts as a filter) (2) by carrying high
MLSS concentrations (usually above 4500 mg/1 in D.C.) such that the
particles settle in a subsidence (zone) settling pattern with discrete
interfaces existing between the homogeneous subsiding particles and
the decant. In the subsidence zone, the relatively uniform concen-
tration of particles are nearly homogeneously mixed by the counter-
current turbulence produced by water passing around the solids. The
homogeneous subsiding blanket does not allow classification of
individual particles because the settling blanket acts as a filter.
At the District of Columbia, with MLSS concentrations below 4500 mg/1,
subsidence (zone) settling does not occur during the initial portion
of settling. This provides for a second method of clarifier operation
where classification of the discrete settling particles can occur if
the mixed liquor is fed above the clarifier blanket level. The
lighter or unsettable particles, thus, can be purged from the system.
25
-------
The effluent suspended solids accordingly increased from 15 mg/1 to
25 mg/1 during this method of operation with a corresponding increase
in effluent BOD.
Seasonal variations also affect sludge settling characteristics in
oxygen as well as in air systems. These variations become critical
as the concentration of solids increases. The pure physical changes
in the wastewater density and viscosity contribute to slower settling
rates as the wastewater temperature decreases. As the density of the
water increases, the driving force for settling (which is the
difference in density between water and the settling particles)
decreases for a similar particle density. The drag force, viscosity,
also increases with decreasing temperature (A-25% from 80°F to 55°F)
again contributing to slower settling rates in colder waters. Figure 8
shows a series of liter batch settling tests conducted in June, 1971
by only altering the temperature of the mixed liquor (unacclimated
biota). As expected, the colder samples settled slower. These tests
are compared in Figure 8 to a similar series of tests in which the
mixed liquor was acclimated to the colder wastewater temperature of
January, 1971. In Figure 9, the batch flux (concentration multiplied
by settling velocity) or the solids loading in Ib/ft2/day is shown
for the previous tests. Again, the effect of wastewater temperature
is evident.
Besides the physical changes caused by seasonal variations, another
factor which must be considered is the metabolic change brought about
by changing wastewater temperature. Figure 10 shows that the settling
characteristics of oxygen mixed liquor change seasonably at D.C. The
clarifier was operated as a sludge blanket to capture the normally
unsettleable solids. At similar SRT's, the initial settling rate in
a 1 liter graduated cylinder test decreased from approximately 10 ft/
hr to 7 ft/hr at a concentration of 6000 mg/1 as the temperature
changed from 81°F to 71°F. In Figure 11, with the clarifier operated
as a slurry pool to purge the normally unsettleable particles; the
solids also showed a decreasing initial settling rate with decreasing
wastewater temperature for a similar biology. The initial settling
rate in the 1 liter test decreased from 14 ft/hr to 9 ft/hr at
4500 mg/1 as the temperature decreased from 70°F to 63°F. Similar
patterns of decreasing settling rates with decreasing wastewater
temperature have been observed in nitrifying and denitrifying mixed
liquors also.
Clarifier operation and design are equally important to the basic
settling characteristics of the solids in gravity clarification.
Besides selecting an overflow rate compatible with reactor sizing, the
depth and method of clarifier feed are important as discussed earlier
for either high solids capture or solids classification. Another
important design consideration is the volume or detention time of the
clarifier. At the District of Columbia, the oxygen system underflow
26
-------
30
20
15
\ 10
+-•
— 8
>-
t—
i 6
LiJ
i 4
t 3
LkJ
OO
^ 2
0.5
1
57° F (Actual)
D.C.Jan. 1971
I
I I
85°F (Adjusted) I
50°F (Adjusted) /
D.C.June 1971
I
30
2 34 6 8 10 15 20
INITIAL MIXED LIQUOR CONCENTRATION (gm/l)
Figure 8. Effect of Adjusted vs. Acclimated Wastewater
Temperatures on Sludge Settling Rates
27
-------
100
to
00
80
60
40
z 20 -
50°F (Adjusted)
D.C.June 1971
85°F (Adjusted)
D.C.June 1971
57°F (Actual)
D.C.Jan. 1971
) 5000 10,000 15,000
INITIAL MIXED LIQUOR CENTRATION (mg/l)
Figure 9. Effect of Wastewater Temperatures on Initial Batch Flux
-------
a
CO
30
20
15
10
8
6
4
3
2
1
D.C.-Sept 1971-(78-81°F)
D.C.-Oct1971-(71-73°F)
III II
1 2 34 6 8 10 15 20
INITIAL MIXED LIQUOR CONCENTRATION (gm/l)
FIGURE 10
EFFECT OF WASTEWATER TEMPERATURE ON
OXYGENATED SLUDGE SETTLING RATES
30
29
-------
30
20
_ 15
i; 10
g 8
S 6
cs
1 4
5 3
I 2
1
—
D.C. -Nov1971-(68-70°F)
/
V \
\ \
\\
\
D.C.-Dec1971-(63-64°F)
—
—
—
I II III II I
1 234 6 8 10 15 20
INITIAL MIXED LIQUOR CONCENTRATION (gm/l|
FIGURE 11
EFFECT OF WASTEWATER TEMPERATURE ON
OXYGENATED SLUDGE SETTLING RATES
30
30
-------
solids concentration varies between 1.0% and 1.4% with an average
clarifier detention time of 1.9 hours. With 2.8 hours average
detention time, the underflow solids concentration rises to 2.0%-
2.4%. The sludge recycle rate is then determined after an F/M ratio
is established for the reactor and the underflow concentration from
the clarifier is likewise established.
Inventory solids are another consideration in clarifier operation.
The total solids inventory is a result of both the build-up of solids
and the solids actually in the transition (or settling) process. The
build-up of solids can be removed by simply increasing the underflow
wasting. However, as the settling rates of the mixed liquor decrease
for a given concentration (i.e., with temperature) an increase in
inventory results as more solids are required in transition (or
settling).
In the summer of 1970 at an MLSS concentration in excess of 8000 mg/1,
peak clarifier overflow rates of 1940 gpd/ft^ were observed on the
District of Columbia oxygen system as shown in Table 4. During the
1970-71 winter, the peak sustained overflow rates which could be
maintained without the blanket coming over the weirs was 975 gpd/ft^
at MLSS concentrations that varied from 3900 to 5300 mg/1. The
causative agents which reduced the allowable overflow rates for
satisfactory operation from summer to winter were undoubtedly a
combination of all the above mentioned factors, not the least of which
was the decreased wastewater temperature. As expected with filamentous
growth in late spring 1971, allowable clarifier overflow rates were
markedly decreased as shown in Table 4. Again, during the
1971-72 winter, maximum overflow rates of 975 gpd/ft2 have been
demonstrated at the District of Columbia.
31
-------
CO
ro
TABLE 4
CLARIFIER VARIABLES AND OPERATING CONDITIONS
Operating Period
Month
Dates
9
Average Overflow Rate (gpd/ft )
At Surface
Above Feed Skirt
Below Feed Skirt
Peak Overflow Rate (gpd/ft )
At Surface
Above Feed Skirt
Below Feed Skirt
Average Solids Loading
(Ib/day/ft2)
SVI
Underflow Solids
% Dry Solids
% Volatile
Waste Solids (Ib/million gal.)
Volatile (Ib/million gal.)
Effluent Solids
(Ib/million gal . )
Volatile (Ib/million gal.)
1
June
12-30
- +
750
670
-
750
670
37
80
1.16
75
161
121
296
198
2
July
-
1210
1075
-
1210
1075
75
48
1.34
70
40
28
445
245
3
August
1-25
- +
1210
1075
- +
1210
1075
58
50
1.27
75
144
108
166
113
4
Sept.
1280
1050
940
1940
1580
1410
61
42
1.40
80
250
200
204
141
b
Oct.
3-21
1280+
1050
940
1940
1580
1410
88
33
2.14
65
680
441
290
189
6
Nov.
10-30
1280+
1050
940
1940
1580
1410
68
48
1.40
81
230
202
470
342
7
Dec.
1-16
975+
800
710
975
800
710
55
60
1.08
90
193
174
197
118
8
Jan.
17-31
975±
800
710
975
800
710
42
73
1.00
80
253
202
483
400
Peripheral feed - no center feed section
Area at surface 96 ft2 x 6 ft' x 5 ft. deep
Area below feed skirt 107 ft2 x 5 ft. deep
Total depth 11 ft. deep
± Center feed section:area at surface 78 ft. x
4 ft. deep; area above feed skirt 96 ft2 x
2 ft. deep; area below feed skirt 107 ft2 x
5 ft. deep.
-------
CO
CO
TABLE 4 (CONTINUED)
CLARIFIER VARIABLES AND OPERATING CONDITIONS
9
March
1-18
950±
780
700
950
780
700
35
81
0.79
77
130
100
410
375
10
April
-@
(290-620)
-
—
(290-620)
-
17
120-190
0.85
80
_
-
_
—
11
May
.
560
-
_
560
-
21
265
0.78
77
720
550
100
~
12
June
_
(280-650)
-
_
(280-650)
-
24
173
1.28
78
142
104
88
~
13
July
-@
975
-
_
650
-
54
50
1.92
70
168
118
92
™
14
Aug.
-(£>
975
-
_
650
-
61
30-35
2.22
70
253
178
133
100
15
Sept.
-e
975
-
_
650
-
58
33
2.58
70
460
323
160
104
16
Oct.
-
-------
SECTION VII
SOLIDS HANDLING
The other integral part of the oxygen system .is the excess solids
handling equipment. Of utmost importance is the relative ease with
which the oxygen activated sludge process can be operated in
endogenous respiration, thereby substantially reducing the quantity
of excess sludge to be handled. This factor will reduce the number
and/or size of the selected sludge handling and disposal facilities.
However, the increased operating costs resulting from the increased
oxygen necessary to oxidize ("burn-up") the excess sludge and the
larger reactor/clarifier capabilities needed to hold the increased
solids inventory required for endogenous respiration must tie balanced
economically with the reduction in size of the solids handling and
disposal units.
Another factor to be considered is that the larger the clarifier
volume, the thicker the underflow solids concentration. It may be
economically feasible to properly size the reactor/clarifier
combination to yield underflow solids sufficiently thick to be
dewatered directly without prior thickening or digestion. This would
be accomplished by selecting a small reactor and large clarifier.
An alternative to the above is to select a large reactor and a small
clarifier and provide additional thickener capabilities. In the
EPA-DC Pilot Plant, the excess solids are thickened separately by air
flotation or gravity thickening. These solids have been thickened to
over 4.5% without chemical additives by both gravity and air
flotation thickening.
34
-------
SECTION VIII
REFERENCES
1. Pirnie, M. Presentation at Twenty-First Annual Meeting of Sewage
Works Association, Detroit, Mich., October 18-21, 1948.
2. Okun, D.A., Sewage Works Journal, 21_, (1949).
3. Budd, W.E., and 6.F. Labeth, Sewage and Industrial Waste, 29,
253 (1957). ~
4. Okun, D.A. and W.R. Lynn, "Preliminary Investigation into the
Effect of Oxygen Tension on Biological Sewage Treatment", in
Biological treatment of Sewage and Industrial Wastes: Vol. 1
Aerobic Oxidation", Reinhold Publishing Corp., New York (1956).
5. Okun, D.A., Sewage and Industrial Wastes, 29_, 253 (1957).
6. McKinney, R.E., and J.T. Pfeffer, Water and Sewage Works,
October (1965).
7. Albertson, J.G., J.R. McWhirter, E.K. Robinson, and N.P. Walhdteck,
Investigation of the Use of High Purity Oxygen Aeration in
Conventional Activated Sludge Process", FWQA Department of the
Interior Program No. 17050 DNW, Contract No. 14-12-465, May (1970).
8. Barth, E.F., and M.B. Ettinger, Jour. Water Poll. Control Fed.,
39_, 1361 (1967).
9. "FWPCA Methods for Chemical Analysis of Water and Wastes", U.S.
Dept. of the Interior, Fed. Water Poll. Control Adm., Cincinnati,
Ohio, (November 1969).
10. Kamphake, L.,S. Hannah, J. Cohen, Water Res., 1_, 205 (1967).
11. Schaeffer, R.B., et al. , Jour. Water Poll. Control Fed., 37_, 1545
(1965).
12. Gales, M. E. Julian, and R. Kroner, Jour, of Am. Water Wks., Assoc.,
58, 1363 (1966).
13. "Standard Methods for the Examination of Water and Wastewater".
12th ed., American Public Health Association, New York (1965).
14. Rich, L.G., "Unit Operations of Sanitary Engineering", John Wiley
and Sons, Inc., New York (1961).
35
-------
SECTION IX
PUBLICATIONS
Stamberg, J.B., D.F. Bishop, and G. Kumke, "Activated Sludge Treatment
with Oxygen", AIChE Symposium Series 124, Water 71, 68, 25 (1972).
36
-------
SELECTED WATER
RESOURCES ABSTRACTS
INPUT TRANSACTION FORM
Report Na.
3. Accession ffo.
w
4, Title
'ACTIVATED SLUDGE TREATMENT SYSTEMS WITH OXYGEN"
7. Author(s)
Stamberg, John B.; Bishop, Dolloff F.; Hais, Alan B.
5. Report Date
6.
i, Performing Organization
Report No.
9. Organization EPA-DC Pilot Plant
Department of Environmental Services
Government of the District of Columbia
5000 Overlook Avenue SW, Washington, DC 20032
JO. Project No.
11010 EYM
11. Contract/Grant No.
14-12-818
Type c/ Repoi t and
Period Covered
12.
IS. Supplementary Notes
ENVIRONMENTAL PROTECTION AGENCY
Environmental Protection Agency report number EPA-670/2-73-073,
September 1973.
is. Abstract The gas-tight biological reactor with 2.5 hour detention time or less
insolubilized the biodegradable organics to less than 5 mg/1 of soluble BOD. The
organisms in the mixed liquor were maintained between 4000 and 8000 mg/1. Above an
SRT of six days, the rate of activity in the oxygen volatile solids was greater than
in a parallel step aeration system. Ninety-five (95) percent of the oxygen supplied
was consistently utilized in the staged reactor, which employed co-current liquid and
gas flow contacting.
The liquid solids separation was accomplished by conventional clarification.
The clarification efficiency was a function of mixed liquor concentration, particle
shape, particle density and seasonal variation (i.e., temperature, metabolic changes,
and load variation). During the wanner temperature periods, the peak overflow rates
of 1940 gal/day/ft2 were observed. In colder temperature periods, maximum steady state
overflow rates of 975 gal/day/ft' were obtained. The underflow solids varied with
overflow rate, clarifier volume and recycle rate. Underflow concentrations up to
2.5% were obtained.
Above an SRT of six days, the total production of solids was significantly
less than the solids production in a similarly operated step aeration system. At an
SRT of 13 days, 0.35 Ib. of solids/lb. of BOD were produced.
17a. Descriptors
Wastewater Treatment
Sedimentation
Phosphorus
Alkalinity
17b. Identifiers
*0xygen Activated Sludge
Suspended Solids
Step Aeration
Contact Stabilization
Solids Handling
17c. CO WRR Field & Group
*Activated Sludge
Oxi dati on
*0xygen
Oxygenation
Aerobic Conditions
Biodegredation
Biochemical Oxygen Demand
Dissolved Oxygen
05D
IS. Availability
A bstractor
Kent
19.
•1.
Kisenbauer
Security Class.
'Repot )
Se: rityCi >s.
(Page)
21.
3
rto.o/
Pages
Prn.0
Send To:
WATER RESOURCES SCIENTIFIC INFORMATION CENTER
U.S. DEPARTMENT OF THE INTERIOR
WASHINGTON. D. C. 2O24O
\ Institution
------- |