System Alternatives In Oxygen Activated Sludge

EPA-670/2-75-008
April 1975
Environmental Protection Technology Series
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EPA-670/2-75-008
April 1975
SYSTEM ALTERNATIVES IN OXYGEN ACTIVATED SLUDGE
By
John B. Stamberg, Dolloff F. Bishop, Stephen M. Bennett
U.S. Environmental Protection Agency
and
Alan B. Hals
Department of Environmental Services
Government of the District of Columbia
Contract No. 68-01-0162
Project 11010 EYM
Program Element 1BB043
Project Officer
Dolloff F, Bishop
Advanced Waste Treatment Research Laboratory
National Environmental Research Center
Cincinnati, Ohio 45268
r »,1 sonant £ I 'Av-tactlon
Ro^icn Vr Library
230 Couth Pearborn Stree
Chicago 8 iilMlol* 60g£*»
NATIONAL ENVIRONMENTAL RESEARCH CENTER
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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REVIEW NOTICE
The National Environmental Research Center - Cincinnati has reviewed
this report and approved its publication. Approval does not signify
that the contents necessarily reflect the views and policies of the
U.S. Environmental Protection Agency, nor does mention of trade names
or commercial products constitute endorsement or recommendation for
use.
PROTBCTIO* 1GBK*
ii
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FOREWORD
Man and his environment must be protected from the adverse
effects of pesticides, radiation, noise and other forms of pollution,
and the unwise management of solid waste. Efforts to protect the
environment require a focus that recognizes the interplay between
the components of our physical environment—air, water, and land.
The National Environmental Research Centers provide this multi-
disciplinary focus through programs engaged in

o studies on the effects of environmental contaminants
on man and the biosphere, and

o a search for ways to prevent contamination and to
recycle valuable resources.

This report describes new technological advances with pure oxygen
in conventional biological treatment of wastewaters. These advances
in technology increase treatment reliability and thus reduce environ-
mental contamination.
A. W. Breidenbach, Ph.D.
Director
National Environmental
Research Center, Cincinnati
iii
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ABSTRACT
An oxygen activated sludge system consisting of a unique gas-tight
biological reactor, gravity clarification and solids handling equipment
was operated on District of Columbia primary effluent during a two-year
period over a wide range of loading (F/M 0.26 to 2.0) with Solids
Retention Times (SRT) from 2.0 to 3.0 days at the EPA-DC Pilot Plant.
The reactor detention times varied from a nominal 1.5 to over 3.0 hours
with the mixed liquor suspended solids between 2700 and 8100 mg/1.
The clarifier overflow rates varied between 300 and 1960 gpd/ft depend-
ing on the mixed liquor particle shape (the intensity of filamentous
growth) and water temperature. The average underflow solids varied
between 1.0% and 2.4% depending on clarifier volume and recycle rate.
The total solids production varied from 0.35 to 1.0 grams of solids
produced per gram of BODc applied with as little as 200 pounds of
underflow waste per million gallons of flow. The undigested underflow
solids thickened to 4-5% by either gravity or air floatation without
chemical additives and then dewatered by vacuum to 8-12% with polymers
and 13-21% with lime and ferric chloride addition.

This report is submitted in fulfillment of Project 11010 EYM and Contract
No. 68-01-0162 by the Department of Environmental Services, Government
of the District of Columbia under the sponsorship of the Office of
Research and Monitoring, Environmental Protection Agency. Work was
completed as of December 1972
iv
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CONTENTS

List of Figures vi

List of Tables viii

Acknowledgements ix

I Conclusions 1

II Recommendations A

III Introduction 5

IV Analytical Procedures 6

V Biological Reactor Performance of Oxygen 7
Activated Sludge

VI Clarification of Oxygen Activated Sludge 30

VII Solids Production and Handling of Waste
Oxygen Activated Sludge 51

VIII References 57

IX Publications 59
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FIGURES
No. Page

1 Oxygen Activated Sludge System 8

2 Photomicrograph of Oxygen Mixed Liquor 10

3 Biological Activity Relationships 18

4 Excess Sludge Production 20

5 Percent COD Oxidized as a Function of SRT 28

6 Nitrification Oxygen Demands 29

7 Initial Sludge Settling Velocity as a Function 32
of Concentration

8 Effect of Rains on the Initial Sludge Settling 34
Velocity

9 Effect of SRT on SVI 35

10 Effect of Temperature Adjustments on Initial 37
Sludge Settling Rates

11 Effect of Temperature Adjustments on Initial 38
Batch Flux

12 Effect of Wastewater Temperature on Oxygenated 39
Sludge Settling Rates (Sept. - Oct. 1971)

13 Effect of Wastewater Temperature on Oxygenated 40
Sludge Settling Rates (Nov. - Dec. 1971)

14 Effect of Wastewater Temperature on Oxygenated 41
Sludge Settling Rates (May - July 1972)

15 Initial Settling Velocities of Oxygen Sludges 42
as a Function of Concentration

16 Initial Settling Velocities of Oxygen Sludges 44
Compared with Air Sludges

17 Observed Initial Settling Velocities of Oxygen 45
Sludges Compared with Nitrification and
Denitrification Sludges
VI
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18 Effect of Solids Loading on the Concentration of 52
Oxygen Sludge in Air Floatation Thickening

19 Effect of the Secondary to Primary Solids Ratio 53
on Vacuum Filtration

20 Effect of the Secondary to Primary Solids Ratio 54
on Form Rate

21 Effect of Polymer Dosage on Sludge Yield 55

22 Oxygen Sludge Filtrate from a Filter Press as 56
a Function of Time
vii
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TABLES

1 Reactor Variables and Operating Conditions 12-14

2 Organic Removal - EPA-DC Pilot Plant 15-17

3 Nitrogen Summary - EPA-DC Pilot Plant 21-23

4 Oxygen Usage - EPA-DC Pilot Plant 25-27

5 Clarifier Variables and Operating Conditions- 46-48
EPA-DC Pilot Plant

6 Air Floatation Clarification 50
viii
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ACKNOWLEDGEMENTS
The efforts of Dr. Robert Samworth, Director of Research and
Development Projects, Department of Environmental Services,
Government of the District of Columbia were especially helpful.

On the EPA-DC Pilot Plant staff, Chief Operators George Gray and
Herbert Braxton and their crew chiefs Kenneth Graham, Jack Leech,
John Norton, Calvin Taylor and Kenneth Taylor were vital to a
successful pilot plant operation. The support from the instrument
staff headed by Walter Schuk and Paul Ragsdale and the mechanical
staff, headed by Robert Hallbrook and Charles Euth were equally
important. The efforts of Paul Warner and his analytical staff are
greatly appreciated.

The support by the Linde Division of Union Carbide to initiate the
study and to work with the pilot plant during the first six months
of operation is appreciated.
IX
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SECTION I

1. A gas-tight biological oxygen reactor with independent control
of dissolved oxygen and mixing and a gravity clarifier under aerobic
conditions produces a good quality secondary effluent from District
of Columbia primary effluent in operations with average reactor deten-
tion times between 1.5 to 2.5 hours (based on influent flow) and with
MLSS concentrations between 4000 and 8000 mg/1.

2. Biodegradable organics in the Washington, B.C. primary effluent are
essentially 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
clarification efficiency.

3. Oxygen micro-organisms are visually the same as those in a parallel
typical conventional step aeration system; however, the rate of activity
of the oxygen volatile solids is greater above an SRT of 6 days.

4. As also observed in the air system, oxygen activated sludge is
subject to filamentous (Sphaerotilus) growth when operated below an
SRT of 5 days on D.C. primary effluent.

5. At SRT's above 6 days, total production of excess biological solids
is significantly lower in the EPA-DC oxygen system than in the parallel
step aeration system. As little as 0.35 grams of excess solids are
produced per gram BOD added at an SRT of 13 days in the oxygen process.

6. Nitrification is achieved in the oxygen aeration system in the
summer and fall at EPA-DC Pilot Plant.

7. Based upon the influent and exhaust gas flows, over 9570 of the
input oxygen is consistently utilized in the EPA-DC oxygen system.

8. Oxygen requirements are based on the COD removal (influent COD
minus effluent COD and waste COD) and the nitrogenous oxygen demand.
As the SRT increases and endogenous respiration occurs, increased oxygen
is required to bio-oxidize the solids. Also at warm temperatures and
high SRT's, nitrification occurs with further oxygen demands.

9. Sludge in the oxygen system underflow settles to approximately
1.0-1.47o solids by weight in a clarifier with 1.9 hours of hydraulic
retention time and 2.0% to 2.47» in a clarifier with 2.8 hours of
hydraulic retention time.
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10. 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 acts as a filter and produces high
quality effluent. In the summer and fall with zone settling, the deep
well clarifier operates at a peak rate of 1940 gpd/ft . A larger clari-
fication area is required in the winter than in the summer on District
of Columbia wastewater for a given MLSS concentration.

11. With a shallow center feed-well and with the mixed liquor
concentrations low enough (under 4000 mg/1) to permit discrete particle
settling, better settling rates are observed in the clarifier than
with the sludge blanket method of clarifier operation (described in
No. 12 above). At Washington only moderate decreases in effluent quality
(increase in SS from 15 to 25 mg/1) are observed with the shallow center
well operation.

12. With the normally high mixed liquor concentrations used in the
oxygen aeration process, design of clarifiers should consider overflow
rates, volume, and solids loading.

13. In general, air floatation clarification is not a suitable
alternative to gravity clarification because, even though the concentra-
tion of the recycled solids are increased, the effluent suspended solids
are much higher than in a comparably loaded gravity clarifier.

14. In Washington, D.C., non-bulking (SVI less than 200) oxygen acti-
vated sludge settling characteristics are essentially similar to non-
bulking air activated sludges, nitrifying sludges and denitrifying
sludges. Likewise, bulking oxygen activated sludges settle similarly
to bulking air sludges.

15. Excess waste oxygen solids thicken to 4.57, solids by either
gravity or air floatation thickening.

16. Waste oxygen solids and primary sludge mixed at a 0.4:1 weight
ratio of secondary (oxygen) to primary solids gravity thickens up to
11.0% solids.

17. Excess waste oxygen solids alone dewater by vacuum filtration
to 8-12% solids with polymer conditioning and to 13-21% with FeCU and
lime conditioning. In a filter press with FeCl- and lime conditioning,
these solids dewater to 20.67o solids.

18. Proper coordination of the reactor, clarifier and solids handling
facilities permit many alternative designs of oxygen activated sludge.
In general, as the reactor size is increased, a lower MLSS concentration
is required for a given .biological state (F/M ratio). The lower the
MLSS concentration, the smaller the required secondary clarification area.
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19. The solids handling requirement of an oxygen system depend on
the biology established in the reactor/clarifier combination. If
minimum excess biological sludge production is required, then more
capacity is required in the reactor/clarifier combination and in the
oxygen supply. 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 D.C. wastewater
should be evaluated over a wide range of operation conditions.

2. The oxygen activated sludge system, fed either raw or primary
wastewater, should be evaluated as a high rate system with high F/M
ratios and very low SRT's.

3. The use of alum precipitation of phosphorus with chemicals such
as lime to restore pH and alkalinity should be evaluated.

4. Other methods of process operation such as step aeration and
contact stabilization should be evaluated with the oxygen activated
sludge process.

5. The use of oxygen digestion of primary, secondary or a combination
of waste sludges in a separate digester should be investigated as a
potential system alternative to operation of the oxygen reactor at low
F/M.

6. A comprehensive thickening-dewatering study is required on a
variety of equipment to evaluate the handling of the waste oxygen solids.

7. Liquid solids separation methods as alternates to gravity
clarification should be considered.

8. A method of designing full scale circular and rectangular clarifiers
from continuous 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.

10. An accurate method for measuring the bulk particle density of
biological solids is needed for defining settling characteristics.

11. A study using daily settling data should be undertaken to observe
the effect of grease accumulation and/or inerts (i.e. silt and clay) on
bulk particle density and on the corresponding settling velocity.
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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 20-257= 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 invest-
ment 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 907o.
This oxygen-activated sludge process (UNOX) is presently being piloted
in several locations. The Environmental Protection Agency participated
in the earlier studies in Batavia, New York and St. Louis, Missouri and
presently is participating in studies at Las Virgenes, California,
Newtown. Creek, New York City, New York and Washington, D.C. (8). In
Washington, D.C. the EPA-DC Pilot Plant has been evaluating the oxygen-
activated sludge process since May of 1970, nearly two and a half years.
This is the longest running pilot study and the only facility that has
operated for more than a full year at the same location.

The objectives of the study are to determine process performance and
operating requirements on D.C. primary wastewater over a wide range of
seasonal diurnal and operational conditions. The oxygen activated
sludge process should be and was viewed and evaluated as a system with
three interrelated subsystems, the oxygen biological reactor, the
clarifiers and the solids handling system.
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SECTION IV

ANALYTICAL PROCEDURES
To evaluate the oxygen-activated sludge system, appropriate samples
were manually composited over a 24-hour period. Samples were stored
at 3°C to minimize biological activity.

The five day biological oxygen demand (BOD) of the composite samples
was determined by the probe method (9); the ammonia (9) and nitrate-
nitrite (10) on a Technicon Automatic Analyzer. The total organic
carbon (TOC) (11) was measured on a Beckman Carbonaceous Analyzer.
The total phosphorus (12) was determined by the persulfate method.
All other analyses employed Standard Methods (13). Soluble phosphorus
and soluble BOD were filtered through a standard glass suspended solids
filter before analyses.
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SECTION V

BIOLOGICAL REACTOR PERFORMANCE OF OXYGEN ACTIVATED SLUDGE
The first and most unique aspect of the system is the gas tight bio-
logical reactor (Figure 1). 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 a predetermined daily cycle
(diurnal variation), normally with a 2.3:1 (45-105 gpm) daily flow
variation but occasionally with an alternate variation of 2.0:1
(40-80 gpm).

With all four available stages, the 8,100-gallon EPA-DC oxygen reactor
provides 1.95 hours or detention time at the nominal influent flow of
100,000 gpd. At the peak daily flow, the detention time is 1.29 hours.
With three of the available four stages, the detention times are
reduced to 1.50 hours and 1.00 hours, respectively, at the nominal and
normal 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. 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.
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Figure 1. Oxygen Activated Sludge System
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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 main-
tains the overhead gas at a selected pressure usually between 1" and
4" of water. Even with large instantaneous fluctuations in oxygen
consumption, the oxygen control valve maintains the selected pressure.
The overhead gas pressure is normally selected to produce an oxygen
concentration of approximately 50% in the exhaust gas from the last
reactor stage. The oxygen is introduced to the first stage where the
peak oxygen demand occurs. As the oxygen is used in biological metabo-
lism, 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 Blue Plains oxygen reactor
are held between 4.0 and 8.0 mg/1 by adjusting the recirculation rate
of the oxygen gas within the individual stages. The compressor in each
stage pumps the overhead gas through the rotating 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 stage typically is 3-7 cfm and 1-2 cfm in each of the
last three stages. Total recirculation requirements vary between 0.10
and 0.20 ft3/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 high MLSS concentrations. These factors enable the system
to readily adsorb shock organic loads. Also, toxic shock loads are
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 "biological inertia" buffers
the toxicity.

The oxygen mixed liquor at the EPA-DC Pilot Plant was similar visually
to the micro-organisms in conventional activated sludge (Figure 2). The
mixed liquor biota was normally very well bio-flocculated with active
stalked cilates growing on the bacterial mass. Zooflagellates and free-
swimming cilates, although few in number, remained adjacent to or within
the flocculated particles. Several varieties of large active rotifers
were present in abundance. A few nematodes existed in the sludge. Normally,
filamentous growth was not apparent. There was almost complete absence
of fragmented debris or unflocculated bacteria between the discrete particles.
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Figure 2. Photomicrograph of Oxygen Mixed Liquor
10
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In the sludge retention time (SRT) range below 5 days both the oxygen
activated sludge and the conventional aeration systems exhibited
filamentous growth on the District of Columbia wastewater. (SRT is
the inverse of growth yield). Filamentous growth did 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 reestablished a filamentous free sludge in several
days. However, after extended periods of operation with filamentous
growth, the Sphaerotilis became firmly entrenched and could 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 dosage of 200 tng/1 (based on influent flow) was then
required to purge the system of filamentous growth.

Pertinent reactor variables and operating conditions are summarized
in Table 1 for approximately 2-1/2 years of operation on the EPA-DC
oxygen aeration pilot plant. The food to micro-organism ratio (F/M)
varied from 2.0 to 0.27 grams BOD5 applied/gram of MLVSS. The Solids
Retention Time (SRT) varied from less than 2 days to 13 days. The
average volumetric loadings were as high as 185 Ib BOD,, applied
1,000 ft3 of tank.

The reduction of BOD in the reactor was excellent. With an influent
BOD up 130 mg/1, a wide range of detention times from 1.5 to 2.5 hours
and SRT that varied from 13 to as low as 2 days, the effluent soluble
BOD was consistently less than 5 mg/1 as described in Table 2. Virtually
complete insolubilization of the BOD in the primary effluent occurred
and, thus, BOD removal on the EPA-DC oxygen system was a function of
clarification efficiency.

On the District of Columbia wastewater, as seen in Figure 3, the
volatile portion of the oxygen solids exhibited a much higher activity
for F/M ratios in the SRT range above 6 days than the EPA-DC's step
aeration pilot process. Figure 3 indicates that a lower total volatile
mass under aeration was required with oxygen than with air to obtain
any given SRT above 6 days for a similar influent BOD. Thus, shorter
detention times were possible with oxygen than with step aeration for
similar MLSS concentrations to achieve any given SRT above 6 days.
Further, at identical SRT's above 6 days, the oxygen system produced
less excess biological solids. The most probable reason for the
increased activity was 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 had a high dissolved
oxygen content which permitted a certain amount of aerobic metabolism
in the clarifier and greatly reduced the time that the bio-mass was in
an anaerobic condition.
11
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SRT (days)

Figure 3. Biological Activity Relationships
12 14 16
18
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The total production of solids in the oxygen system (Figure 4) per
pound of BOD added, including underflow waste and effluent solids,
was inversely related to the solids retention time (SRT) above an
SRT of 1.3 days. The solids production with the oxygen aeration
system was significantly lower than in the conventional step aeration
pilot system above an SRT of 6 days (or than in a high rate modified
air aeration pilot process tested by the EPA-DC staff). Indeed, the
total solids production decreased 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, exhibited increased solids production through an SRT
of 9.5 days with a peak solids production of approximately 1 pound of
excess solids per pound of BOD added. However, an approximate four-
fold increase in volatile solids was required to raise the SRT from 6
to 13 days in this system and to begin to achieve reduced solids pro-
duction.

The recirculation of respirated carbon dioxide within the oxygen
reactor stages lowered the wastewater pH from 7.0-6.8 in the first
stage to 6.4-6.1 In the final stage. With an average system pH
of approximately 6.5, the oxygen process more slowly established 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 was reduced to a level where the nitrifying
organisms propagate faster than they were removed, the Nitrosomonas
and Nitrobacter populations increased and substantial nitrification
occurred in the oxygen system. Nitrogen removal across the oxygen
system during periods of high nitrification and partial denitrification
was as high as 35-43%. At wastewater temperatures of about 63°F, 5 mg/1
of NO§-N was still produced with an SRT of 9.0 days. Thus, as in the
parallel step aeration process, nitrification in the oxygen system
began to decrease in the Fall and became negligible during the Winter.
Nitrogen removal decreased to a low of 0-10% during periods without
nitrification (Table 3).

Without physical-chemical phosphorus removal, phosphorus was removed
from the oxygen system through metabolic uptake and by wasting of
excess solids; thus, the removals varied with the metabolism of the
mixed liquor. At high SRT's (highly endogenous metabolism), total
phosphorus removal averaged only about 15%. At lower SRT's with less
endogenous respiration, phosphorus removals increased to 20%.

With alum addition, phosphorus removal in the oxygen system increased
as the alum weight ratio (A1+++/P) increased. During experiments
conducted in the Fall of 1970, for a dosage equal to an Al++ /P
ratio of 1.4/1, 80% of the phosphorus was removed to an average residual
19
-------
1.0
^ 0.8
0.6
0.4
0.2
OXYGEN AERATION
0 2 4 6 8 10 12 14 16
SRT (days)

Figure 4. Excess Sludge Production
20
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23
-------
of 1.8 mg/1 as P and only a slight decrease in wastewater alkalinity
and pH occurred. The filtered effluent (though 0.45 /i ) contained an
average of 1.6 mg/1 of soluble P. When the dosage was increased to a
weight ratio of 1.85/1 (Al+"f+/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 bio-mass dispersed and required termination of the
alum addition to allow the mixed liquor to recover.

In the low alkalinity D.C. wastewater, additional alkalinity in the
form of lime or caustic is 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 CC>2 content of the mixed liquor.
The addition of alum and precipitation of Al(PO^) and Al(OH)o increases
the inert solids carried in the system and adequate clarification for
the higher solids concentration must be provided.

Consistently throughout the operation, vented gas from the fourth
system reactor stage contained less than 10% of the input oxygen
volume. The vented stream was roughly 50% oxygen. Based upon the
influent and exhausted oxygen concentrations, the net utilization of
oxygen in the process was about 95%. The accountable oxygen consumption
consisting of COD removal, nitrification demand, exhaust gas, and
effluent dissolved oxygen is summarized in Table 4. The COD removed
was calculated by substracting the COD in the underflow waste solids
and in the process effluent from the primary effluent COD. With
increasing SRT, additional oxygen was required for endogenous
respiration (Figure 5). Likewise, during periods with nitrification,
additional oxygen was required (Figure 6).
24
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29
-------
SECTION VI
CLARIFICATION OF OXYGEN ACTIVATED SLUDGE
The second important aspect in the oxygen aeration system is liquid/
solids separation. At the EPA-DC Pilot Plant, as at Batavia, gravity
clarification is employed. 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 Blue Plains oxygen system effluent was
associated with suspended solids and overall removal of suspended
solids and BOD were a function 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
of both clarifiers (i.e., overflow rates and volume) and thickeners
(i.e. solids loading - Ib/ft^/day) should be considered. The Ten
State Standards (15) suggest that conventional activated sludge
clarifiers be designed for average overflow rates of 800 gpd/ft . The
Water Pollution Control Federation Manual of Practice (1959) (16)
suggests that the solids loading be held below a peak of 30 Ib/ft /day.
Overflow rates and solids loading criteria must be better defined for
high solids systems such as oxygen aeration.

The settling of a particle in a fluid (such as mixed liquor solids
in water) is dependent on gravitational force. The magnitude of the
driving force for settling depends on the difference in the densities
of the bulk particle and the fluid. The density of the bulk mixed
liquor particle is determined by the composite of bacterial mass,
bound water and miscellaneous flocculated debris or chemical
precipitant. Furthermore, by definition, the driving force is affected
by changes in the specific gravity of water (caused either by varia-
tions of temperature of dissolved solids concentration).

Viscosity shear force is defined as the retardant of drag force in
settling. The magnitude of the drag force is dependent on the viscosity
of the fluid (in this case water) the shape of the particle and the
velocity of the particle relative to the fluid. Viscosity of water is
likewise dependent on temperature. The particle shape affects the
streamlines (hydraulic pattern) around the particle. Consequently, a
well rounded mixed liquor particle will settle faster than an irregular
shaped particle, such as those found in a filamentous culture. As
mentioned previously, the velocity of the particle relative to the fluid
also affects the drag force. The drag force increases as the velocity of
the fluid relative to the particles increases. Consequently as
concentration of the bulk particles increases, the net particle velocity
30
-------
is reduced due to the increased velocity of the fluid as it is forced
to pass through an increasingly smaller crossectional area.

In summary, the basic settling characteristics of the mixed liquor
typically are 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

Duncan and Kawata (17) and Dick (18) observed that in the range of
initial solids concentration when subsidence settling occurred, a
straight line relationship exists between the log of the initial
settling velocity and the log of the initial concentration.
Consequently an equation in the form of:

where Vi = initial settling velocity
Ci = initial concentration
a =• constant of magnitude
n = constant of rate of change

could be established for each sludge. Furthermore, a relationship of
this type can be determined for each of the three types of initial
settling conditions (free particle settling, subsidence and
consolidation settling).

As seen in Figure 7, the log of the initial settling rate is a
function of the log of the solids concentration. This sample 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
homogeneous settling solids. The second at higher MLSS levels has both
an initial discrete interface and a zone of homogeneous settling
particles (zone settling). Thus, the sizing of a clarifier is a
function of the MLSS concentration and must be coordinated with the
reactor (and the sludge handling facilities) to achieve the desired
biological capabilities of the system.

Another important factor is the particle shape. Normally, as shown in
Figure 2, 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
31
-------
30

§ 20
sT 15
I
i 10
UJ 0
CD
6
LU A
t/>

^ 3
(D.C.-DEC.)
I
I I
Vj =lnitial Velocity
Cj ^Initial Cone.
A=lntercept Constant
n=Slope Constant
I II
I I
I
2 3 4 6 8 10 15 20 30
INITIAL MIXED LIQUOR CONCENTRATION-C , (gm/l)
Figure 7. Initial Sludge Settling Velocity as a Function of
Concentration
32
-------
unique to each location and should be defined for that location.
The presence of industrial fibers also had an effect similar to
filamentous growth on mixed liquor settling characteristics.

Still another important factor in the basic settling characteristics
is the density of the particles in relationship to that 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 incorporated
in the mixed liquor solids and have increased sludge settling rates
30% to 60% (Figure 8).

In like fashion, operating under different biological conditions can
alter the sludge settling characteristics and the sludge volume index
(SVT). At SRT maintained below 5 days in Washington, D.C., filamentous
bulking occurs in oxygen and air systems and increases the SVI. In
addition, as the system is operated at SRT's above 10 days, bio-
flocculation decreases and some affects on settling and SVI are observed
(Figure 9). Also, in temperatures below 70°F at SRT's around 7 days,
the SVI's increased. Grab samples of MLSS analyzed for the hexane
soluble grease (13) reveal high levels around 6-97o by weight during
these periods. Conclusive evidence needs to be obtained on the
relation between settling rates, grease concentration, and temperature
(i.e. determine if and when grease accumulations occur and the effect
on the SVI and settling rate).

The mode of operation of the clarifier affects the density and settling
characteristics of the oxygen sludge. Two major methods of clarifier
operation are available. 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
4000 mg/1) 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 concentration of particles are nearly
homogeneously mixed by the countercurrent 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.
33
-------
LU
100

8
°
CD 6
S 5
«/>
I I I I I I I I I
I I I I I I T|
JUNE 22-JULY 3, 1972
(TROPICAL STORM)
JUNE 10-20, 1972
4567 8910
20 30 40 60 80 100
INITIAL MLSS CONCENTRATION (gm/l)

Figure 8. Effect of Rains on the Initial Sludge Settling
Velocity
34
-------
I I I I I I
i i i i i r
00
CO hH
c
o
o

4J
O
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CS CM —
O
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35
-------
At Washington, 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 unsettlable particles, thus, can be purged from the system. 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 effect sludge settling characteristics in
oxygen as well as in air systems. These variations become critical as
the MLSS of the mixed liquor increases. The pure physical changes in
the wastewater density and viscosity contribute to slower settling
rates as the wastewater temperatures 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 and contributes to slower
settling rates in colder waters. Figure 10 shows a series of liter
batch settling tests conducted in June, 1971 in which the temperature
of the mixed liquor was lowered. As expected, the colder samples
settled slower. In Figure 11, the batch flux (concentration multi-
plied by settling velocity) or the solids loading in Ib/ft /day is
shown for the previous tests. Solids loading is often used in
thickener design. Again, the effect of wastewater temperature is
evident.

Besides the physical changes caused by seasonal variation, another
factor which must be considered is the metabolic change brought about
by changing wastewater temperature. Figure 12 shows that the settling
characteristics of oxygen mixed liquor change seasonably at D.C. 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. The clarifier was being operated to capture unsettleable
particles at this time. In Figure 13, the clarifier was operated to
purge the unsettleable particles; but, again the solids 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 70°F to 63°F. Likewise, as the wastewater temperature
increased, the settling rates increased in the Spring and Summer
(Figure 14).

The D.C. settling characteristics of oxygen activated sludge conform
to the range of settling rates published by Wilcox (19) (Figure 15)
when non-filamentous cultures (less than 200 SVI) are tested on a
6" stirred column. With filamentous growth the settling rate markedly
36
-------
30

3
2
0.5
I
I I
85°F (Adjusted))

50°F (Adjusted) J D'C-June1971
I I
1 2 3 4 6 8 10 15 20

INITIAL MIXED LIQUOR CONCENTRATION (gm/l)

Figure 10. Effect of Temperature Adjustments on Initial
Sludge Settling Rates
30
37
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-IS ><
LO «
X
IX

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4->
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03
rt
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38
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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
I
1 2 34 6 8 10 15 20 30
INITIAL MIXED LIQUOR CONCENTRATION (gm/l)
Figure 12. Effect of Wastewater Temperature on Oxygenated Sludge
Settling Rates (Sept.-Oct. 1971)
39
-------
30

i; 10
t 8
CJ
LU 6
CD
1 4
5 3
—I
I 2
1
\
D.C. - Nov 1971-(68-70°F)
D.C.-Dec1971-(63-64°F)
I I
1 LA
\ \
1 234 6 8 10 15 20
INITIAL MIXED LIQUOR CONCENTRATION (gm/l)
30
Figure 13. Effect of Wastewater Temperature on Oxygenated Sludge
Settling Rates (Nov.-Dec. 1971)
40
-------
40

JUNE 10-
(70°F-74
JUNE 22-JULY 3,
F-76°F)
10

S 4
MAY 20-31,
(66°F-71°F)
L MAY 1-1
(63°F-65'
— 2
F)
JULY 11
-25,
F-79°F)
i
456 8 10
INITIAL MLSS CONCENTRATION gm/l
Figure 14. Effect of Wastewater Temperature on Oxygenated
Sludge Settling Rates (May-July 1972)
41
-------
CD
30
20
10

0.2
0.1
DEC., 1972.
- 61°F
JAN.,1972
62°F
FEB., 1972
; 50°F
I r
NOV., 1971
68°F
I IT
APRIL-MAY, 1971
64°-68°F
FILAMENTOUS BULKING
1-LITER TEST
OXYGEN SLUDGE
OBSERVED SETTLING LIMITS
UNION CARBIDE
i i i i
456 8 10
20
30 40 60 80 100
INITIAL MLSS CONCENTRATION (gm/l)

Figure 15. Initial Settling Velocities of Oxygen Sludges as a
Function of Concentration
42
-------
deteriorated to as little as 0.3 to 1.0 ft/hr at MLSS of 2500 mg/1
In the standard 1 liter settling test. Unfortunately, 6" stirred
settling data (minimized wall affects and particle bridging) was not
obtained.

The air activated and oxygen activated sludge settling rates are
comparable. As seen in Figure 16, the air activated sludge settling
rates observed by Agnew (2), Reed and Murphy (21), Dick (22),
Javaheri (23), Eckenfelder (24), Rudolfs (25), and Flischerstorm (26)
fell in the same settling range as Wilcox and EPA-DC oxygen sludges for
non-bulking sludges. Further, the settling rates observed in the
nitrification and denitrification sludges of a parallel EPA-DC multi-
stage mineral addition pilot plant fell in the same ranges as the air
and oxygen sludges (Figure 17), Similar patterns of decreasing
settling rates with decreasing wastewater temperature were also
observed in nitrifying and denitrifying mixed liquors,

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 high solids capture or for solids classification. Other
important design considerations are the volume or detention time of
the clarifier and recycle rate. At Blue Plains, the oxygen system
underflow 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% with similar recycle rates. For any selected F/M, the
clarifier volume will thus affect the under flow concentration and
thus the sludge recycle rate.

Inventory solids are another consideration in clarifier operation.
The total solids inventory is a result of both the build-up of solids
in the blanket and the solids actually in the transition (or
settling) process. A simple increase in the wasting rate reduces the
blanket level if the increase is caused by the build-up of solids.
However, as the initial settling rates decreases, as in Washington,
for a given MLSS concentration, the sludge inventory also increases in
the clarifier as more solids are in transition (or settling). In this
case, increased wasting rates does not reduce the solids inventory in
the clarifier without thinning the MLSS or altering the biology. It
appears that the increased inventory can be a result of slower settling
solids as well as a backlog of solids due to inadequate wasting.

In the summer of 1970 at an MLSS concentration in excess of 8,000 mg/1,
peak clarifier overflow rates of 1940 gpd/ft were observed on the Blue
Plains oxygen system as shown in Table 5. 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
43
-------
40
20
~ 10

< 1
E .8
z
~" .6

.4
.2
OXYGEN RATES (D.C.)
6 INCH STIRRED COLUMN
*
OXYGEN RATES
(WILCOX)
AIR RATES

1 LITER UNSTIRRED

3.5 INCH STIRRED

COLUMN
2 3 4 5 6 8 10 20 40

INITIAL CONCENTRATION (gm/l)
60
Figure 16. Initial Settling Velocities of Oxygen Sludges Compared

with Air Sludges
-------
40

0.1
I I
i i I I r
I i i i i i i r
SEPARATE DENITRIFICATION SLUDGE (BLUE PLAINS]
SEPARATE AIR NITRIFICATION SLUDGE (BLUE PLAINS)
OXYGEN SLUDGE OBSERVED SETTLING \
UNION CARBIDE

\
\
\
\
\
\
\ \
i i i
1 2 34568 10 2346

INITIAL MLSS CONCENTRATION (gm/lj

Figure 17. Observed Initial Settling Velocities of Oxygen
Sludges Compared with Nitrification and
Denitrification Sludges
lO
45
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•u in id
Q ^ 01
O -H
3 4J H
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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,
especially the decreased wastewater temperature. As expected with
filamentous growth in late spring of 1971, allowable clarifier overflow
rates were markedly decreased as shown in Table 5. Again, during the
current 1971-72 winter, maximum overflow rates of 975 gpd/ft2 were
demonstrated at Washington.

Air floatation was tried as an alternate method of clarification.
Initially the flow from the reactor was split between an air floatation
unit and a gravity clarifier. The solids thickened to 5-6% in air
floatation but averaged 3.82% in the recycle as additional water was
brought over with skimming (Table 6). The parallel gravity clarifier
recycle averaged 2.31%. The effluent suspended solids from air
floatation averaged 34 to 41 mg/1 at a solids loading of 14-15 lb/ft2/
day and the effluent suspended solids from gravity clarification
averaged 7 to 21 mg/1 at a solids loading of 36-37 Ib/ft2/day. Later
the loadings on the air floatation unit were increased to an average
of 25 Ib/ft2/day (400 gpd/ft2, overflow rate) with peaks of 36 lb/ft2/
day (580 gpd/ft2 overflow rate) and the suspended solids further
deteriorated to an average of 74 mg/1 even with 0.75 mg/1 of Dow C-31
polymer.

In general air floatation was not a suitable alternative to gravity
clarification because the effluent suspended solid were much higher
than a comparable gravity settler.
49
-------
TABLE 6

AIR FLOTATION CLARIFICATION
Overflow Solids Effluent Recycle
Kate Loading S.S. %
#/ftZ/day mg/1
July 24-31, 1972
Gravity Clarifier 520 37 7 2.31
Air Flotation 200 15 34 3.82*
August 1-20, 1972
Gravity Clarifier 520 36 21 2.40
Air Floatation 200 14 41 3.26*
August 21-31, 1972
Air Floatation 400 25 74 2.10*
*Grab samples before skimming were 5-6%, however, increasing skimming
to improve effluent suspended solids diluted the floated solids
concentration.
50
-------
SECTION VIJ
SOLIDS PRODUCTION AND HANDLING OF WASTE OXYGEN ACTIVATED SLUDGE
The other integral part of the oxygen system is the excess solids
handling equipment. The important factor in the solids handling
equipment is the relative ease with which the oxygen activated sludge
process can be operated in endogenous respiration, thereby substan-
tially reducing the quantity of excess sludge to be handled. This
capability permits the reduction in 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 bio-oxidize (burn-up) the excess sludge and the larger
reactor/clarifier capacities needed to hold the increased solids
inventory required for endogenous respiration must be balanced
economically with the reduction in size of the solids handling and
disposal units.

The first step in solids handling is thickening. The oxygen activated
sludge was thickened to 4-5% by either gravity or air floatation with-
out chemical additives. The Komline Sanderson HR air floatation
thickener (1 ft ) yielded greater than 4% solids at loadings up to
6 Ib/ft^/hr (Figure 18) with effluent suspended solids concentrations
less than 200 mg/1. When the oxygen activated sludge solids were
mixed with primary solids in weight ratios of 0.2 secondary to 1.0
primary and 0.4 secondary to 1,0 primary, the sludge concentration
gravity thickened to as high as 117, solids.

Dewatering studies were performed by vacuum filtration, centrifuga-
tion, and filter pressing. The oxygen waste sludge dewatered in
vacuum filter tests to 8-12% with polymer conditioning and to 13 -
21% with chemical conditioning (ferric FeCl3 at 5% and lime at
16-207> based on dry weight). Vacuum filter leaf tests showed that
the 35% solids, obtained at form rates of 21.0 Ib/ft^/hr for primary
solids alone, decreased with increasing activated sludge content to
20.7% solids at form rates of 11.0 Ib/ft2/hr for a 1:1 ratio of
primary to oxygen sludge (Figure 19 and 20). In the 3 ft by 1 ft
pilot filter study, polymer doses increased the yield as shown in
Figure 21 for 0.2:1 and 0.4:1 oxygen to primary sludge mixtures. In
the centrifuge study, a mixture of 0.2:1 oxygen-waste-sludge to
primary-sludge mixture was dewatered to 23.5% with polymer
conditioning in a solid bowl centrifuge with a 6.070 feed concentra-
tion. The centrate contained 3,840 mg/1 of suspended solids.

Dewatering of oxygen sludges were tried on a two-cell one foot square
Nichols filter press (Figure 22). The waste sludge conditioned with
4.7% ferric chloride and 14.4% lime by weight, dewatered to an
average of 20.6% with a bulk cake density of 64 Ib/ft3. The mixture
of sludge at 0.4:1 oxygen sludge to primary sludge, conditioned with
3.7% ferric chloride and 12.9% lime by weight, dewatered to 39.5%
with a bulk cake density of 67.0 lb/ft3.

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FORM TIME: 1 MIN.
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SOLIDS CONG. % WT.
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SECONDARY-4.8
POLYMER DOSAGE
26-28 16/TON
26.0 .
24.0 _
22.0 _
20.0
1/5 2/5 3/5 4/5 5/5
OXYGEN/PRIMARY SLUDGE RATIO
Figure 19. Effect of the Secondary to Primary Solids Ratio
on Vacuum Filtration
53
-------
10.0 -
9.0
2/5
3/5
4/5
5/5
OXYGEN/PRIMARY SLUDGE RATIO lbs/
Ib
Figure 20. Effect of the Secondary to Primary Solids Ratio
on Form Rate
54
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56
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SECTION VIII

1. Pirnie, M., Presentation at Twenty-First Annual Meeting of
Sewage Works Associations, Detroit, Mich., October 18-21, 1948.

2. Okun, D.A., "System of Bio-Precipitation of Organic Matter from
Sewage," Sewage Works Journal, 21, 1949.

3. Budd, W.E. and Lambeth, G.E., "High Purity Oxygen in Biological
Sewage and Industrial Wastes," 29, 1957.

4. Okun, D.A. and Lynn, W.R., Preliminary Investigation into the
Effect of Oxygen Tension on Biological Sewage Treatment," in
Biological Treatment of Sewage and Industrial Wastes, Vol. I,
Aerobic Oxidation. Reinold Publishing Corp., New York, 1956.

5. Okun, D.A., "Discussion of High Purity Oxygen in Biological
Sewage Treatment," Sewage and Industrial Wastes, 29. 1957.

6. Pfeffer, J.T. and McKinney, R.E., "Oxygen Enriched Air for
Biological Waste Treatment," Water and Sewage Works, October, 1965.

7. Albertson, J.G., McWhirter, J.R., Robinson, E.K. and
Walhdieck, N.P., "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. Stamberg, J.B., "EPA Research and Development Activities with
Oxygen Aeration," Presented at the Technology Transfer Design
Seminar, New York, New York, February 29, 1972.

9. "FWPCA Methods for Chemical Analysis of Water and Wastes,"
U.S. Dept. of the Interior, Fed. Water Poll., Control Adm.,
Cincinnati (November 1969).

10. Kamphake, L.J., Hannah, S.A. and Cohen, J.M. "Automated
Analysis for Nitrate by Hydrazine Reduction," Water Research
Journal of the International Association on Water Pollution Research
1, No. 3, March 1967.

11. Schaeffer, R.B., et al., "Application of Carbon Analyzer in Waste
Treatment," Journal Water Pollution Control Federation, 37,
No. 11, November 1967.

12. Gales, M.E., Julian, E.G. and Kroner, R.C., "Method of
Quantitative Determination of Total Phosphorus in Water," Journal
American Water Works Association, 58. No. 10, October 1966.

57
-------
13. Standard Methods for the Examination of Water and Wastewater,
12th Edition, American Public Health Association, New York, 1965.

14. Cole, C.A., Stamberg, J.B. and Bishop, D.F., "Hydrogen Peroxide
Cures Filamentous Growth in Activated Sludge,11 Proceedings of
the Fifth Mid-Atlantic Industrial Waste Conference,
Philadelphia, Pa., November 1971.

15. The Great Lakes and Upper Mississippi River Board of States,
"Recommended Standards for Sewage Works," 1960.

16. American Society of Civil Engineers and Water Pollution Control
Federation, "Sewage Treatment Plant Design,11 WPCF Manual of
Practice, No. 8, 1967.

17. Dick, R.I., "Thickening," in Water Quality Improvement by Physical
and Chemical Processes; Vol. II, Published by University of
Texas Press, (1970).

18. Duncan, J. and Hawata, K., "Evaluation of Sludge Thickening
Theories," Journal Sanitary Engineering Division, ASCE 94.
No. SA2, April 1968.

19. Wilcox, E., "Operating Experience and Design Criteria for "Unox"
Wastewater Treatment Systems," EPA Technology Transfer Seminar,
New York, New York, February 29, 1972.

20. Agnew, W.A., "A Mathematical Model of a Final Clarifier for the
Activated Sludge Process,11 FWQA Department of the Interior,
Contract No. 14-12-194, March 1970.

21. Reed, S.C. and Murphy, R.S., "Low Temperature Activated Sludge
Settling," Journal Sanitary Engineering Division, ASCE No. SA 4,
August 1969.

22. Dick, R.I. and Ewing, B.E., "Evaluation of Activated Sludge
Thickening Theories," Journal of Sanitary Engineering
Division, ASCE, August 1967.

23. Javaheri, A.R. and Dick, R.I., "Aggregate Size Variations During
Thickening of Activated Sludge," Journal Water Pollution Control
Federation, May 1969.

24. Eckenfelder, W.W. and Melbinger, N., "Settling and Compaction
Characteristics of Biological Sludges," Sewage and Industrial
Wastes, 29_, No. 10, 1957.

25. Rudolfs, W. and Lacy, 1.0., "Settling and Compacting of Activated
Sludge," Sewage Works Journal, 647, July 1934.

26. Fischerstorm, C.N., Isgard, F.E. and Larsen, I., "Settling of
Activated Sludge in Horizontal Tanks," Journal of Sanitary
Engineering Division ASCE, June 1967.

58
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SECTION IX

PUBLICATIONS AND PATENTS
Presented at the 45th Annual Conference of the Water Pollution
Control Federation, Atlanta, Georgia, October 1972.

No patents have been issued or requested as a result of the efforts
of this study.
59
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-670/2-75-008
3. RECIPIENT'S ACCESSI ON«NO.
4. TITLE AND SUBTITLE
SYSTEM ALTERNATIVES IN OXYGEN ACTIVATED SLUDGE
5. REPORT DATE
April 1975; Issuing Date
6. PERFORMING ORGANIZATION CODE
AUTHOR(S)
John B. Stamberg, Dolloff F. Bishop, Stephen M. Bennetl
and Alan B. Hais
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
EPA-DC Pilot Plant, Department of Environmental
Services, Government of the District of Columbia
5000 Overlook Avenue, SW
Washington, DC 20032
10. PROGRAM ELEMENT NO. JB3Q43

ROAP 21-ASR Task - 015
11. CONTRACT/B«AW*NO.
5-01-0162
12. SPONSORING AGENCY NAME AND ADDRESS
National Environmental Research Center
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final Report
14.
-inal Kep<
.SPONSORIN
G AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
An oxygen activated sludge system with co-current contacting of oxygen and
mixed liquor in a plug flow reactor was operated on District of Columbia primary
effluent during a two-year period over a wide range of loading (F/M 0.26 to 2.0)
with Solids Retention Times (SRT) from 2.0 to 13.0 days at the EPA-DC Pilot Plant.
The reactor detention times varied from a nominal 1.5 to over 3.0 hours with the
mixed liquor suspended solids between 2700 and 8100 mg/1. The clarifier overflow
rates varied between 300 and 1960 gpd/ft depending on the mixed' liquor particle
shape (the intensity of filamentous growth) and water temperature. The average
underflow solids varied between 1.0% and 2.4% depending on clarifier volume and
recycle rate. The total solids production varied from 0.35 to 1.0 pound of
solids produced per pound of BOD . The undigested underflow solids thickened to
4-5% by either gravity or air floatation without chemical additives and dewatered
by vacuum filtration to 8-12% with polymers and 13-21% with lime and ferric
chloride addition.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
COSATI Field/Group
*0xygen
Sedimentation
Waste water
Activated sludge process
Centrifuges
Filtration
*Settling
Aerobic "bacteria
Thickening
*0xygen activated sludge
Suspended solids
Sludge dewatering
Sludge production
13B
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
UNCLASSIFIED
21. NO. OF PAGES
70
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
60
U. S. GOVERNMENT PRINTING OFFICE: !975-657-592/53it8 Region No. 5-11
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