September 1975 Environmental Protection Technology Series
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development,
U.S. Environmental Protection Agency, have been grouped into
five series. These five broad categories were established to
facilitate further development and application of environmental
technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in
related fields. The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to
develop and demonstrate instrumentation, equipment and methodol-
ogy 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 environmental quality standards.
This document is available to the public through the National
Technical Information Service, Springfield, Virginia 22161.
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EPA-600/2-75-035
September 1975
AEROBIC STABILIZATION OF WASTE ACTIVATED SLUDGE
An Experimental Investigation
by
David B. Cohen
Metropolitan Denver Sewage Disposal District No. 1
Commerce City, Colorado 80022
Donald G. Fullerton
F.M.C. Corporation - MAROX Systems
Englewood, Colorado 80110
Contract No. 68-03-0152
Project Officer
James E. Smith, Jr.
Wastewater Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Municipal Environmental
Research Laboratory, U.S. Environmental Protection Agency, and
approved for 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.
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 com-
ponents of our physical environment—air, water, and
land. The Municipal Environmental Research Laboratory
contributes to this multidisciplinary focus through
programs engaged in
• studies on the effects of environmental
contaminants on the biosphere, and
• a search for ways to prevent contamination
and to recycle valuable resources.
The research reported here was performed for the
Ultimate Disposal Section of the Wastewater Research
Division to determine the effects of different opera-
tional parameters on and develop design criteria for
the aerobic digestion process. In addition, various
benefits were demonstrated on a pilot plant scale for
a pure oxygen digestion process. Large cost savings
have been realized at Metro Denver with aerobic digestion
of their waste activated sludge.
iii
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ACKNOWLEDGEMENTS
This project was funded by the Environmental Protection Agency
(Contract No. 68-03-0152) under the direction of Dr. J.E.
Smith, Jr., Municipal Environmental Research Laboratory, Cin-
cinnati, Ohio.
The investigation of the full scale aerobic digestion system
was carried out by the staff of the Metropolitan Denver Sew-
age Disposal District No. 1. Mr. John Puntenney was the Pro-
ject Director; Dr. David Cohen was Project Coordinator. In
particular, the following individuals contributed significant-
ly to the implementation of the project:
Mr. William Martin - Field Supervisor
Mr. Harry Harada - Laboratory Supervision
Ms. Mary Ann Tavery - Laboratory Analysis
and Coordination
Mr. Steve Pearlman - Laboratory Analysis
and Coordination
Mr. William Broderick - Graphics
Mrs. Pam Stover - Stenographic Support
The pure oxygen pilot plant phase of the project was jointly
carried out by Metro staff and F.M.Cc Corporation, Marox
Diffusion Systems, Englewood, Colorado. The advice and
assistance of the following individuals are particularly
acknowledged: Mr. Richard B. Weber - Subcontract Director,
Mr. Donald G. Fullerton - Field Coordinator for the oxygen
system and Mr. Gordon Drew - Field Engineer.
iv
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CONTENTS
PAGE
SUMMARY AND CONCLUSIONS 1
INTRODUCTION 5
MATERIALS AND METHODS 12
EXPERIMENTAL RESULTS - DIFFUSED AIR SYSTEM 18
EXPERIMENTAL RESULTS - PURE OXYGEN BATCH TESTS 86
EXPERIMENTAL RESULTS - PURE OXYGEN FLOW THROUGH
(FAD) TESTS 119
EXPERIMENTAL RESULTS - PURE OXYGEN FLOW THROUGH
(RAD) TESTS 140
COMPARISON OF AIR AND OXYGEN PERFORMANCE 145
BENEFIT - COST ANALYSIS I65
SYMBOLS AND ABBREVIATIONS 169
REFERENCES 172
APPENDIX 174
v
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TABLES
NO. PAGE
1. Diffused air digester field data - monthly averages 22
2. Dissolved oxygen uptake rate in diffused air digester 23
3. Diffused air digester - laboratory data - monthly 36
averages
4. Diffused air digester - laboratory data - percent 37
change influent versus effluent
5. Diffused air digester - performance calculations 38
6. Monthly variation of SRT and VSS reduced in diffused 48
air digester
7. Effect of cold shock on nitrification parameters 50
8. Monthly variation of invertebrate biomass in diffused 67
air digester (volumetric standard units - g/1 wet
weight)
9. Monthly variation of invertebrate biomass percent 68
distribution in diffused air digester (VSU basis)
10. Monthly variation of invertebrate biomass as a per- 69
cent of VSS in diffused air digester
11. Vacuum filter leaf test results of diffused air diges- 80
ter influent and effluent
12. A comparison of sand filtration results with aerobic 83
and anaerobic digested sludge
13. Effect of loading rate to diffused air digester on 84
sand filtration rate (volume filtered/volume applied)
14. Pure oxygen digester batch test no. 1 - laboratory 87
data (mg/1 unless other units indicated)
15. Pure oxygen digester batch test no. 1 - field data 88
16. Pure oxygen digester batch test no. 1 - inventory 89
mass reduction
17. Pure oxygen digester batch test no. 2 - laboratory 92
data (mg/1 unless other units indicated)
18. Pure oxygen digester batch test no. 2 - field data 93
19. Pure oxygen digester batch test no. 2 - inventory 94
mass reduction
vi
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NO
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35.
36.
97
98
99
101
102
103
104
105
111
123
125
126
127
128
130
131
132
TABLES (continued)
Pure oxygen digester batch test no. 3 - laboratory
data (mg/1 unless other units indicated)
Pure oxygen digester batch test no. 3 - field data
Pure oxygen digester batch test no. 3 - inventory
mass reduction
Pure oxygen digester batch test no. 4 - laboratory
data (mg/1 unless other units indicated)
Pure oxygen digester batch test no. 4 - field and
inventory mass reduction data
Pure oxygen digester batch test no. 5 - laboratory
data (mg/1 unless other units indicated)
Pure oxygen digester batch test no. 5 - field
data
Pure oxygen digester batch test no. 5 - inventory
mass reduction
Pure oxygen digester batch tests 1-5 - biomass
reduction summary
Pure oxygen digester flow through pilot plant (FAD)
field data
Pure oxygen digester flow through pilot plant (FAD)
performance data
Pure oxygen digester flow through pilot plant (FAD)
laboratory data - influent loading (mg/1 unless
other units indicated)
Pure oxygen flow through pilot plant (FAD) - labora-
tory data effluent waste
Pure oxygen digester flow through pilot plant (FAD)
laboratory data influent loading versus effluent
waste percent change
Pure oxygen digester flow through pilot plant (FAD)
invertebrate biomass in tank A - VSU (ml/1)
Pure oxygen digester flow through pilot plant (FAD)
invertebrate biomass in tank B - VSU (ml/1)
Pure oxygen digester flow through pilot plant (FAD)
invertebrate biomass in tank A and B averages - VSU
(ml/1)
vii
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37
38
39
40
41
42
43
44
45
46
47
133
134
137
138
141
142
143
146
147
168
168
TABLES (continued)
Pure oxygen digester flow through pilot plant
(FAD) invertebrate biomass percent distribution
VSU basis
Pure oxygen digester flow through pilot plant (FAD)
invertebrate biomass as percent of VSS under oxy-
genation (dry weight basis)
Pure oxygen digester flow through pilot plant - ATP
data summary
Pure oxygen digester flow through pilot plant influent
and effluent vacuum filter leaf performance
Pure oxygen digester flow through pilot plant (RAD)
field data
Pure oxygen digester flow through pilot plant (RAD)
influent and effluent laboratory data
Pure oxygen digester flow through pilot plant (RAD)
biomass reduction performance
Diffused air digester laboratory data - organic
loading versus percent change between influent and
effluent
Diffused air digester laboratory data - SRT versus
percent change between influent and effluent
Cost savings of Metro diffused air aerobic digestion
(1970-1974)
Potential benefit of converting existing holding tank
to pure oxygen aerobic digester
viii
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2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17.
8
11
14
15
16
19
20
21
25
26
27
29
30
31
32
33
34
FIGURES
Schematic flow diagram of Metro Denver Sewage
Treatment Plant
High purity oxygen flow through pilot plant with
fixed active diffuser
Oxygen bubble diameter versus water depth required
for 100% dissolution
Fixed active diffuser (FAD) showing bubble shear
method
High purity oxygen pilot plant with experimental
rotating active diffuser
Simplified plant flow diagram of Metro Denver Sewage
Treatment Plant
Monthly variation of biomass temperature in diffused
air digester
Monthly variation of biomass oxygen uptake rate and
DO in diffused air digester
Effect of sludge loading rate on oxygen requirement
of biomass in diffused air digester
Monthly variation of air supply and 02 transfer
efficiency in diffused air digester
Oxygen transfer efficiency in diffused air digester
as a function of liquid depth/temperature
Monthly ZSV and SVI variation of biomass in diffused
air digester
Correlation of SVI versus solids weighted ZSV in
diffused air digester
Isosettling rates of biomass in diffused air digester
as a function of temperature and loading rates
Monthly variation in diffused air digester average
field data
Monthly variation in volatile solids loadings to
diffused air digester
Monthly variation in diffused air digester hydraulic
detention time
ix
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NO
18
19
20
21,
22.
23,
24,
25,
26,
27,
28,
29,
30.
31,
32,
33.
34,
35
40
41
43
44
45
46
51
53
54
55
56
58
59
60
61
63
FIGURES (continued)
Schematic inter-relationships of solid forms
undergoing aerobic digestion
Spectrum of temperature and loadings versus VSS
reductions in diffused air digester
Correlation coefficients for various factors in-
fluencing VSS reduction in diffused air digester
Temperature-time factor versus percent VSS reduced
in diffused air digester
Monthly variation in percent VSS between influent
and effluent of diffused air digester
Monthly variation of influent and effluent TSS and
TDS in diffused air digester
Monthly variation in solids conversion to dissolved
and gaseous end products in diffused air digester
Temperature effects on growth rate of nitrifying
bacteria (after Downing)
Influence of DO on nitrification in diffused air
digester
Effect of temperature-time factor on nitrification
in diffused air digester
Monthly variation of nitrates in diffused air diges-
ter influent and effluent
Monthly variation of TKN and NH4-N in diffused air
digester influent and effluent
Monthly variation of pH and alkalinity in diffused
air digester influent and effluent
Monthly variation of conductivity in diffused air
digester influent and effluent
Percent VSS reduction versus percent conductivity
change in diffused air digester
Monthly variation of COD influent and effluent in
diffused air digester
Monthly variation of influent and effluent phosphate
in diffused air digester
x
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Aijr.
64
66
71
72
74
76
77
79
82
91
100
108
110
113
114
116
122
150
FIGURES (continued)
Monthly variation of selected lab data (influent
versus effluent) in diffused air digester
Total invertebrates and rotifer fraction of biomass
versus percent VSS reduced
Monthly variation of invertebrate biomass (VSU) in
diffused air digester
Aerobic and anaerobic bacterial reduction versus
detention time in digester
Percent volatile solids reduction under aerobic and
anaerobic conditions
A comparison of supernatant quality under aerobic and
anaerobic conditions
Diffused air digester supernatant quality monthly var-
iation
Odor panel results for aerobic and anaerobic sludge
Air flotation polymer demand versus diffused air
digester loading rate
Pure oxygen batch test no. 2 DO profile
Pure oxygen batch test no. 4 biodegradable COD re-
duction versus detention time
Pure oxygen batch test no. 5 specific oxygen uptake
rate versus detention time
Pure oxygen batch tests biodegradable VSS reduction
versus detention time
Pure oxygen batch test 3 and 5 biodegradable COD and
VSS reductions versus detention time
Comparison of Batavia and Denver pure oxygen batch
test VSS reduction versus detention time
Pure oxygen batch test no. 5 oxygen uptake rate as
a function of biodegradable solids concentration
Pure oxygen flow through test biomass and oxygen
temperature versus time
Air versus oxygen performance-percent change in solids
forms as a function of organic loadings and SRT
xi
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FIGURES (continued)
NO. PAGE
53. Air versus oxygen performance-percent change in VSS, 151
COD and TKN as a function of organic loadings and
SRT
54. Air versus oxygen performance-percent change in con- 154
ductivity and TDS as a function of organic loadings
and SRT
55. Air versus oxygen performance-percent change in NH4 155
and NO3-N as a function of organic loadings and SRT
56. Air versus oxygen performance - alkalinity and pH 156
change as a function of organic loadings and SRT
57. Temperature-time factor versus percent VSS reduced 157
in air and oxygen digesters
58. Air versus oxygen performance-percent VSS reduction 159
as a function of loading rate
59. Air versus oxygen performance-oxygen supplied/VSS 160
reduced as a function of loading rate
60. Air versus oxygen performance-filter leaf yield per 162
unit chemical cost
61. Sludge processing cost as a function of sludge 166
quality
62. Metro unit cost of sewage treatment versus time 167
xii
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SUMMARY AND CONCLUSIONS
Metro Denver Sewage Disposal District No. 1 (Metro)'s aerobic
digestion system consisted of four converted secondary aerators
(8 million gallons) using diffused air. The digesters were
fed waste activated sludge (WAS) and were operated in a plug
flow mode. Aerobic digestion performance averaged 32.0% VSS
reduced with a minimum of 11.2% during low temperature high
loading conditions and a maximum of 47.2% during warm weather,
low loading conditions. Based on a 13 month evaluation of the
diffused air digester performance, it was determined that
a well stabilized sludge should have the following char-
acteristics :,"^-a) temperature standardized specific res-
piration rate K20 of less than 5.0 mg/hr/g VSS, b) reduc-
tion in volatile fraction between WAS influent and diges-
ted effluent greater than 6%, c) NO3-N/NH4-N greater
than 1.0 in effluent, d) effluent/influent conductivity
ratio greater than 1.5, e) effluent/influent alkalinity
ratio less than 0.60, f) pH reduction between influent
and effluent greater than 0.3 units, g) a supernatant
quality with TSS concentration less than 30 mg/1, BOD
less than 100 mg/1 and NH4-N less than 100 mg/1. Best
digestion performance was found to coincide with rotifers
comprising the majority of the invertebrate biomass on a
volumetric standard unit basis (VSU).
In cold climates with biomass less than 20°C, sludge loading
must be corrected for cold temperature inhibition of meta-
bolic activity. A new design parameter in units of degree-
days (SRT x temperature °C) is suggested as a practical ap-
proach to digester performance predictability. A temperature-
time factor in excess of 150' degree-days should be maintained
to ensure a VSS reduction rate of greater than 40%. If the
biomass is subjected to cold temperature shock (i.e. 5°C or
greater drop in 5 days), the temperature-time factor should
be increased to 250 degree-days^ Nitrification was inhibited
in the diffused air digester when shock chilling occurred.
1
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The best solid/liquid separation performance was obtained at
loadings between 0.8 - 1.28 kg VSS/m^/day (0.05 - 0.08 lb
VSS/ft3/day). Because of low sludge subsidence rates it was
impossible to maintain constant supernatant removal. The batch
approach of shutting off air flow temporarily to create a qui-
escent settling zone frequently led to anaerobiosis, denitri-
fication, and floating sludge. Clogging of fine bubble diffu-
sers was also increased when batch supernating was attempted.
If the digested sludge is to be applied to land, the concept
"stabilization" must be operationally defined in relation to
odor potential. If a mixture of anaerobically and aerobically
digested sludges are applied to land, objectionable odors may
result unless the volatile fraction of both sludges is less
than 60%.
For aerobically digested sludge with SRT between 3 and 13 days,
sand bed drainability on a volumetric basis was three times
faster than well digested anaerobic sludge. On a mass basis
however, the anaerobic sludge drained 2.5 times faster than
aerobically digested sludge. Dewaterability of the aerobically
digested sludge as determined by specific resistance (of the
aerobically digested sludge) was not significantly different
than that of waste activated sludge prior to digestion. For
an equivalent chemical cost, however, better vacuum filter
leaf test performance was obtained with the aerobically di-
gested sludge compared with undigested sludge. Polymer demand
for air flotation of the digested sludge increased in direct
proportion to increasing SRT because of increased particle
breakdown creating additional surface area for polymer attach-
ment.
Air supply required to maintain 1.0 iuq/1 dissolved oxygen
averaged 0.5 l/sec/m^ (30 cfm/1000 ft ) with a range of 0.3
to 0.75 1/sec/m^ (18 to 45 cfm/1000 ft^), the minimum occurring
during cold weather. Specific oxygen uptake rates Kr averaged
7.1 mg/hr/g VSS ranging from 3.4 to 11.1. Oxygen transfer
efficiency averaged 10% ranging between 5% during warm weather
to 19% during cold weather.
A comparison of operations and maintenance costs for the
diffused air aerobic digestion system with costs of conven-
tional sludge disposal methods at Metro indicated a benefit/
cost ratio of 3.5:1 and a cumulative savings in sludge disposal
costs in excess of $1,000,000 for the years 1970-1974.
2
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Concurrent with evaluation of the diffused air aerobic diges-
tion system, a parallel investigation of pure oxygen digestion
of concentrated waste activated sludge (5% TS) using the open
tank fine bubble MAROX system was conducted. The diffused air
system was not capable of satisfying the oxygen respiration re-
quirements of a polymer floated waste activated sludge. The
high oxygen demand (greater than 200 mg/l/hr) was consistently
satisfied with the oxygen system. Two different pure oxygen
diffusion devices were investigated, the slot type Fixed Action
Diffuser (FAD) which had a tendency to plug with screened in-
fluent and a Rotating Active Diffuser (RAD) which required no
screening. Aerobic digestion performance with the FAD averaged
42.7% VSS reduced with a minimum of 38.8% at the highest load-
ing rate of 6.94 kg VSS/m^/day (0.433 lb VSS/ft^/day). Aerobic
digestion performance with the RAD averaged 33.7% VSS reduced
at a loading range between 6.88 and 9.62 kg VSS/m3/day (0.43 to
0.60 lb VSS/ftVday) .
Oxygen uptake (Rr) averaged 176 mg/l/hr with a maximum of 562
mg/l/hr. Rr during the RAD test period averaged 218 mg/l/hr
with a maximum of 453 mg/l/hr. Specific O2 uptake Kr averaged
6.0 mg/hr/g VSS with the FAD and 7.3 mg/hr/g VSS with the RAD.
During both the FAD and the RAD test periods, a significant
temperature increase occurred in the biomass compared with
surrounding atmosphere and was directly proportional to the
organic loading rate. Temperature differential increased from
1°C at 1.33 kg VSS/in3/day {0.083 lb VSS/ft3/day) to 23<>C at
9.63 kg VSS/nrr/day (0.6 lb VSS/ft3/day) .
The higher sludge concentration and temperatures in the pure
oxygen system resulted in declining ecological diversity of
the digester biomass with bacteria predominating and stalked
ciliates and rotifers absent.
During the pure oxygen batch tests, the total VSS digestion rate
coefficient k equalled 0.07, while the readily biodegradable VSS
rate coefficient equalled 0.27. No correlation was observed be-
tween rate of digestion and dissolved oxygen concentration above
a minimum of 1.0 mg/1. No solid-liquid separation was observed
in the polymer thickened sludge, before or after digestion.
At loading rates greater than 2.4 kg VSS/nrVday (0.15 lb VSS/
ft-Vday)/ the VSS reduction rate was significantly greater with
the pure oxygen flow-through system. At lower loading rates.
3
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VSS reduction rates were equal or greater in the diffused air
system. The diffused air system required 15 kg 02/kg VSS re-
duced compared with 2.3 kg 02/kg VSS for the FAD and 1.4 kg
02/kg VSS for the best performance with the RAD. The tempera-
ture-time factor (°C x SRT) for predicting digester performance
was found to be valid for the pure oxygen system as well.
Advantages of pure oxygen digestion compared with diffused air
digestion include (a) ability to digest thickened waste acti-
vated sludge having high oxygen uptake demand, (b) reduced
space requirements, (c) reduced gas flow requirements, (d)
exothermic waste heat production which can be utilized to acce-
lerate digestion reaction rates in the mesophilic temperature
range.
Future research in this area should be directed towards invest-
igation of pure oxygen thermophilic digestion of thickened
waste activated sludge. Conservation of the waste heat pro-
duced by insulating the reaction vessel could raise the biomass
temperatures to the 50-65°C range. Potential benefits of ther-
mophilic aerobic digestion include greater degree of volatile
solids fraction reduction, reduced detention time requirements
and increased pathogen destruction.
4
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INTRODUCTION
The purpose of this research project was to provide information
on the aerobic stabilization process by studying the effects of
diffused air and high purity oxygen on dilute and thickened
waste activated sludges both on a plant and pilot scale basis.
Plant scale testing involved the diffused air aerobic stabil-
ization of dilute waste activated sludge (0.5 - 1% total solids)
while pilot scale testing involved pure oxygen stabilization of
thickened sludge (4 - 5% total solids).
Scope of the Study
Variables investigated in relation to aerobic stabilization
performance included:
1. Time of stabilization.
2. Volatile solids loading.
3. Temperature.
4. Amount of oxygen required per unit volatile solids
reduced.
5. Dissolved oxygen concentration.
6. Oxygen uptake rates.
7. Digested sludge thickening.
8. Settled sludge supernatant quality.
9. Odor levels. -
10. Dewaterability.
11. Other physical, chemical and microbiological character-
istics .
5
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In addition, existing plant records in the solids handling unit
processes were utilized to enable correlation of changes in
aerodigestion operation with changes in other solids handling
unit processes, e.g. dissolved air flotation and vacuum filtra-
tion. Finally, information which indicated economic promise on
a pilot scale was used as a basis for recommending plant scale
process modification for the Metro Denver Sewage Disposal Dis-
trict No. 1 (Metro).
Early Metro Operational Experience
Metro, since its inception in 1966, has been confronted with
the problem of handling and disposing of waste activated sludge
(WAS). Original design for the plant envisaged a WAS/total
sludge ratio of 0.50, whereas by 1970 the WAS/total sludge ra-
tio was 0.65. The inordinate proportion of difficult to de-
water waste activated sludge compounded the total sludge dis-
posal effort. Massive doses of conditioning chemicals were re-
quired for vacuum filter dewatering, which in turn adversely
affected the operation and maintenance of the flash dryer-
incinerator equipment. Further, insufficient incineration ca-
pacity resulted in sludge accumulation and subsequent deterior-
ation in secondary removal efficiencies. As a consequence of
these difficulties, incineration was abandoned in 1971 in favor
of land spreading and incorporation of vacuum filter cake.
The sludge disposal problem was subjected to a detailed invest-
igation with particular emphasis on reduction of sludge volume.
Metro staff initiated a number of lab and field research pro-
jects which included:
a) Preflocculation of primary influent flow using anionic
polymers and ferric chloride.
b) pumping of Metro waste activated sludge to a neigh-
boring primary treatment plant for anaerobic sludge
digestion.
c) Aerobic digestion of waste activated sludge using
diffused air.
The first two projects were not pursued further either because
of excessive ccsts or capacity limitations. The aerobic diges-
tion project however showed promise of economic mass reduction
6
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through the use of excess aerator and blower capacity. Modifi-
cation of the existing secondary facilities to aerobic diges-
ters required only installation of "v" notch weirs to monitor
return sludge flows to the aerobic digesters. Existing drain
piping was used to transfer the digested sludge to air flota- ¦
tion units for further concentration.
In June 1970 two previously empty secondary aeration basins
were converted to aerobic digesters. Strict monitoring of
flows and sampling of influent and effluent enabled a solids
material balance to be calculated. No additional compressor
horsepower was required for aerobic digestion during 1970.
The compressed air 37800 1/sec (80,000 cfm) was redistributed
more equitably between the secondary aerators and the aerobi'c
digesters to ensure at least 1.0 mg/1 of dissolved oxygen in
all basins.
The immediate relief which these modifications provided in
sludge handling and disposal convinced Metro staff of both the
feasibility and desirability of subjecting all the waste acti-
vated sludge to aerobic digestion. Consequently, in August
1970, two additional basins which had previously been used as
preaeration and grease skimming tanks were drained, and con-
verted to aerobic digesters. This provided each quadrant of
the secondary facility with two aeration basins 15140 m^ (4.0
mil gal), one aerobic digester 7570 m^ (2.0 mil gal) and three
secondary clarifiers 11350 m^ (3.0 mil gal). (Figure 1).
During 1971, (the first full year of plant scale aerobic diges-
tion) an average daily loading to the digesters of 48 tons TSS
and an average digested sludge loading to the air flotation
units of 31 tons TSS represented a 35% reduction in volatile
suspended solids. Solids loadings averaged 1.44 kg VSS/m^/day
(0.09 lb VSS/ft^/day), while air consumption averaged 0.667
l/sec/m^ of aeration capacity (40 cfm/1000 ft^) .
Aerobic digestion operations and maintenance costs averaged
$15 to $20 per ton, compared with a total sludge handling and
disposal cost of $50 per ton by conventional methods of vacuum
filtration, incineration or land disposal. Sludge disposal
costs were therefore reduced in 1971 by approximately $200,000,
by converting one-third of the activated sludge capacity to
aerobic digesters.
7
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AREA 1
AREA 3
N»»T» HOC irriMNt
WKiistm »ircK'
MTWIM PIIW *
AREA 2
AREA 4
AEROBIC DIQeSTCRS
¦•O
rMM CMWI HKTNSIOC HUKC)
Fig. 1. Schematic flow diagram of Metro Denver Sewage Treatment Plant
-------
Open Tank Oxygenation Research
As hydraulic and organic loading to Metro's facilities increased,
part of the secondary aeration capacity that was used for di-
gestion during 1970-1972 had to be converted back to secondary
aeration basins. This fact led to a joint research and deve-
lopment effort between Metro and Martin Marietta Company to
investigate the potential for using pure oxygen in the aerobic
stabilization process. Preliminary research data indicated
that high oxygen transfer efficiencies could be achieved by
applying a unique fine bubble oxygen diffuser in an open tank
system. Although Metro's air diffusion system could not sup-
port the high oxygen demand of thickened waste activated sludge
(4-5% TS), pure oxygen fine bubble diffusion could meet this
demand. It was immediately evident that the benefits of aero-
bic digestion could continue to be obtained in much smaller
space, provided that this digestion was accomplished after
thickening of the waste activated sludge.
The investigation of diffused air and pure oxygen aerobic di-
gestion of waste activated sludge reported here began in August
1972. The plant scale diffused air aerobic digestion phase was
completed by August 31, 1973. The pure oxygen digestion phase
began on November 1, 1972 and continued until May 5, 1974.
Plant Scale Testing
The plant scale diffused air tests were conducted in one of
the 7570 m^ (2 mil gal) aerobic digesters that were modified
for this purpose in 1970. The test digester (No. 7) was iso-
lated in such a way as to provide for a wide range of sludge
loading rates. Each operational mode was held at a constant
loading rate for thirty days with one week intervals between
modes during which loadings were adjusted to the new rate.
By operating the digester at the same loading rate at differ-
ent seasons of the year, the effect of temperature on the
stabilization process was evaluated. Solids loading rates
were controlled within the sludge retention time range compa-
tible with Metro's solids handling needs and capabilities.
Essentially, five different loading rates were applied during
the 13 month period. They ranged from a minimum of 0.417 to a
9
-------
maximum of 3.0 kg VSS/m3/day (0.026 to 0.187 lb VSS/ft3/day)
Monthly temperatures ranged from 15.8°C during February - April
to 28.7 C during August - September. Sludge retention times
(SRT) ranged between 3.0 and 29.8 days. Due to limitations
diffused air oxygen transfer capabilities, it was not possible
to investigate a wide range of dissolved oxygen concentrations.
A minimum DO of 1.0 mg/1 was however maintained throughout the
plant scale test period.
Because of flood conditions at the plant site during May 1973,
the data for this period were non-representative of normal vol-
atile solids concentrations. Monitoring of the plant scale air
system was therefore continued until the end of August 1973, to
obtain twelve full months of representative data.
Oxygen Pilot Plant Testing
The oxygen investigation was carried out in three stages.
Stage one included a series of five batch tests for evaluation
of diffuser hardware and system performance. Data obtained
were also used in the operation and control of subsequent flow-
through tests. Both undigested and diffused air digested WAS
were used in these batch tests after air flotation concentra-
tion to 4-5% TS. Stage two of the oxygen investigation con-
sisted of a flow-through system (Figure 2) using the slot type
fixed active diffuser (FAD) for determining the effects of
loading rates on system performance. During the five phases of
stage two, solids loadings ranged from 1.33 to 6.94 kg VSS/m^/
day (0.083 to 0.433 lb VSS/ft3/day). Stage three duplicated
the flow-through system of stage two but used a rotating active
diffuser (RAD) in a single tank. The loading rates were 6.89
to 9.61 kg VSS/m3/day (0.43 to 0.60 lb VSS/ft3/day).
10
-------
DISSOLVED
OXYGEN
METER
SLUDGE
SOURCE
SUPPLY
ES3^S-SCREEN
(TYP)
TANK
TAN K
DIFFUSER
PUMP
DIFFUSER
PUMP
TRANSFER
PUMP
DIFFUSER
DIFFUSER-
DISPOSAL
Fig. 2.
High purity oxygen flow through pilot plant with fixed
active diffuser
-------
MATERIALS AND METHODS
Metro Diffused Air System
The plant scale diffused air system employed existing aeration
equipment consisting of precision tube diffusers. The preci-
sion tube diffusers provide an average bubble diameter of ap-
proximately 2 - 5 mm. The only innovation used in the plant
scale study was the shutting off of diffused air in the third
and last pass of the aeration basin once a day to allow for
solids/liquid separation. This action was followed by decant-
ing of clear supernatant to the secondary influent. Supernat-
ant transfer was accomplished with portable 31.5 1/sec (500 gpm)
pumps. A water level indicator was installed at the head of
"C" pass in No. 7 aerobic digester (Figure 1) to enable accur-
ate solids and hydraulic balances to be calcula ted before and
after decanting.
Oxygen System
The oxygen system utilized a unique method for dissolution of
oxygen in the liquid sludge. The oxygen diffusion hardware is
configured so that oxygen bubbles that are formed from a mul-
tiplicity of very small orifices (4 micron or less) are sheared
from the forming surface by a high velocity sludge stream 305
cm/sec - (L0 ft/sec) to develop bubble sizes that average 50 to
100 micron in diameter. The sludge stream flows perpendicular
to the gas emitting surface and is pumped by an externally
mounted pump recycling the tank contents. The sludge flow
through each of three vertical slots of the diffuser used in
this project provides the necessary mixing requirements to cir-
culate the tank contents (Figure 4). This particular hardware
(slot type) is identified as the fixed active diffuser (FAD).
The slot gap for sludge of this solids concentration was 0.203
to 0.254 cm (0.080 to 0.100 inch) to maintain the proper flow
for both bubble shear requirements and tank mixing. The slot
12
-------
/idth required prescreening of the tank influent sludge through
).317 cm (1/8 inch) mesh screen. Two existing Moyno positive
lisplacement pumps were used to recycle the sludge through the
slot diffusers, one for each of the sludge digestion tanks.
?he oxygen diffuser (FAD) used during the first stage (batch
:ests) and the second stage (flow-through tests) had three ver-
:ical slots 30.5 cm (12 inches) long mounted on a water box
L0 cm (4 inches) square and approximately 43.2 cm (17 inches)
Long. The pumped sludge entered the water box at the top
;hrough a 5.08 cm diameter (2 inch diameter) opening. The dif-
fuser was attached to a flexible hose that was connected to the
sludge manifold at the top of the tank. A gas manifold around
:he top of the diffuser distributed oxygen to each of the 6 gas
>ars.
?he importance of this small bubble size is shown in Figure 3
tfhich illustrates the maximum allowable bubble size at certain
depths of unsaturated tap water to obtain 100% dissolution of
:hat bubble. As an example, a bubble of 100 microns diameter
tfould require a water depth of at least 1.22 meters (4 feet) to
obtain 100% dissolution before reaching the air/water interface.
rigure 3 was a theoretical line later verified in clean water
;esting.
)uring the third stage of the oxygen test program, an experi-
mental gas transfer device developed by FMC - Marox systems be-
:ame available. This device, called a rotating active diffuser
(RAD), employed the same shear principle for small bubble deve-
lopment used in the FAD. While the shear for the FAD was ob-
:ained by recirculation of sludge through a narrow slot between
:he gas bars, the RAD shear was accomplished by revolving gas
)ars mounted on a cylindrical drum through the sludge at a per-
ipheral velocity of approximately 6.1 m/sec (20 ft/sec). A
propeller mounted beneath the gas diffusion drum providedthe
nixing requirement for the tank contents. The major advantage
>f the RAD compared to the FAD is the former device does not
require prescreening since the sludge is not required to pass
ihrough any narrow passages. (Figure 5). The pilot scale sys-
;em included two 6.8 m^ (1800 gallon) tanks that were adapted
:or these tests. Piping, pumps and other accessories were
idded to accomplish the necessary sludge transfer into and out
)f the system.
13
-------
25
20
.15
INCOMPLETE DISSOLUTION
COMPLETE DISSOLUTION
.10
.05
20
30
40
WATER DEPTH ft (X 0.305 = meters)
Fig. 3. Oxygen bubble diameter versus water depth required for 100%
dissolution
-------
SLOT KNIFE
ACTUATING
ROD
LIQUID SLUDGE INLET
OXYGEN GAS MANIFOLD
OXYGEN GAS
INLET
GAS BAR (TYP.)
"GAS PLENUM
LIQUID
SHEAR
m
J?o\
•50 MICRON
BUBBLE (TYP.)
SLOT KNIFE
GAS BAR CAPILLARIES
CLOSE UP OF BUBBLE FORMING
METHOD
Fig. 4. Fixed active diffuser (FAD) showing bubble shear method
15
-------
ROTATING SEAL
SHEAVE
-e- OXYGEN SUPPLY
LIQUID LEVEL
OXYGEN DIFFUSER
GAS BAR
O0O
10" PROP
Fig. 5. High purity oxygen pilot plant with experimental
rotating active diffuser
16
-------
The stage one batch test program in a single tank used a com-
bination of one to three FADs, each with four slots. Valving
capability was provided to remove any diffuser from the sludge
manifold at the top of the tank. During this stage, it was de-
termined that one - three slot diffuser would provide adequate
oxygen dissolution and mixing for the next stage of testing.
During stage one, second batch test, a serious foaming problem
was experienced. For this reason a spray system was installed
using a separate recycle pump and spray nozzles to eliminate
this avenue of solids loss. A second foam suppression system
was installed in "B" tank before start up of stage two testing.
During stage one and the first two phases of stage two, slot
cleaning of the FAD was remotely accomplished by knife blades
in the slots moving up and down manually to clear the accumu-
lated debris. For phases three to five of stage two, a valving
arrangement was installed to provide backflushing of the dif-
fuser slots to prevent debris accumulation. The second tank
"B" provided separation between undigested and digested solids
so that short circuiting would be minimized.
Transfer of the material from "A" tank to "B" was accomplished
by one of the spray pumps, since gravity flow could not be
maintained with the concentrated material. Sludge loadings
were sequential with the operator first wasting from "B" tank,
then transferring an equivalent volume from "A" tank to "B"
tank, followed by loading of "A" tank with concentrated feed
sludge. Digested sludge was collected in a calibrated 55 gal-
lon drum for material balance purposes. When high VSS load-
ings were required, the removal of digested sludge had to pro-
ceed in several steps because of the requirement to maintain
a minimum liquid level above the suction to the Moyno pumps.
Thirty centimeters (1.0 ft) of freeboard were provided in each
of the two tanks for a net volume of 6.52 m^ (1,722 gallons)
per tank.
A core sampling method was developed whereby the exterior sur-
face of the sampler was wiped during removal from the tank to
obtain a representative composite sample for lab analysis.
While automatic dissolved oxygen sensor equipment was available
during part of the test program, it was determined that the DO
sensor used did not provide an adequate signal to control the
system. This necessitated manual DO control.
17
-------
EXPERIMENTAL RESULTS - DIFFUSED AIR SYSTEM
Field Data
Figure 1 shows the aeration basins in each of the four quad-
rants that were converted to aerobic digesters. Figure 6
is a simplified flow diagram depicting how the aerobic diges-
tion unit process fits into the total solids handling and
disposal scheme. The aerobically digested sludge is blended
with raw primary and anaerobically digested sludges, and the
mixture is vacuum filtered prior to land disposal. A summary
of the average monthly field data for the test period is pre-
sented in Table 1.
Figure 7 depicts the average monthly temperature (°C) of the
biomass undergoing aerobic stabilization, as well as the maxi-
mum and minimum monthly values. The annual average tempera-
ture was 22.2°C with an observed range of 11.5 - 32.2°C. Fig-
ure 8 compares average monthly dissolved oxygen concentrations
(mg/1) with specific oxygen uptake rates Kr (mg 02/hr/g VSS).
Dissolved oxygen concentrations ranged between 1.0 and 3.5 mg/1
averaging 1.9 mg/1, while oxygen uptake rates ranged between
3.5 and 11.1 averaging 7.1 mg/hr/g VSS for the entire test
period. The Kr values are higher than the 3-4 mg/hr/g VSS
reported in the literature as "normal" for the endogenous res-
piration phase,d) and may be accounted for by re-synthesis
O2 requirements. The data show that oxygen uptake is higher
at the higher temperatures and the coefficient of correlation
was +0.63. No meaningful correlation was observed between O2
uptake and nitrification rates. As SRT in the digester in-
creased beyond 15 days, Kr declined to its lowest level of
less than 5.0 mg/hr/g VSS.
When Kr was temperature corrected to 20°C/ the relationship
between oxygen uptake and temperature changed from a positive
to a negative correlation. Using the equation:
log Rr(20) = lo9 Rr(t) -03156 t
18
-------
RAW
WASTE
PRIMARY
EFFLUENT
DENVER
NORTHSIDE
PLANT
CU
PRIMARY
TREATMENT
ACTIVATED
SLUDGE
SECONDARY
CLARIFIER
RETURN
SLUDGE
~l
WASTE
ACTIVATED
SLUDGE
PLANT
EFFLUENT
AEROBIC
DIGESTION
FLOTATION
RAW PRIMARY SLUDGE
VACUUM
FILTRATION
LAND
INCORPORATION
DIGESTED
SLUDGE
DENVER
NORTHSIDE
PLANT
Fig. 6. Simplified plant flow diagram of Metro Denver Sewage
Treatment Plant
19
-------
33
32
11
30
29
28
27
26
25
24
23
o
22
V
21
CL
20
s
u
19
H
18
17
16
15
14
13
12
II
10
9
8
7
6
5
\
as
MIN.
\
\
\
LEGEND
AV6- —
MIN
MAX
AVG=
22.2
\ /
\ /
V
r
+
/
AUG. SEPT. OCT. NOV. DEC.
1972 *
JAN. FEB. MAR. APR. MAY JUNE JULY AUG
1973
Fig. 7. Monthly variation of biomass temperature in diffused
air digester
20
-------
LEGEND
02 uptake
0, UPTAKE
RATE
LU
DO.
CD
X
AUG. SEPT. OCT. NOV. DEC
JAN. FEB. MAR. APR. MAY JUNE JULY AUG.
1972
1973
Fig. 8. Monthly variation of biomass oxygen uptake rate and
DO in diffused air digester
21
-------
Table 1. Diffused air digester field data - monthly averages
to
NJ
MONTH
FLOU
TEMP
°C
nvvnvf r
AIR SUPPLY
02 EFF1
Xtransfer
CIENCY
DIGESTION
AIR SUPPLY
LIQUID VOLUME
SLVOGB
SETTLEA3ILITY
H
n fl)
CONC UPTAKE RATE
USED
Oi USED
0-) USKD
cfm/lO^it^/'day
/30 min
ZSV
in/!ir
INF.
EFF.
ma/1
"r
T/DAY
ftJ/dau x 106(2)
TONS 0-,/DAY
¦ 0-3 SUPPLIED
VSS RKn
AUG.
1972
.371
.336
28.0
1.2
56,0
n.i
7.8
9.9
77.4
10.0
2.1
34.0
"SPLIT"
NA
SEPT.
.339
.336
28.7
3.0
42.0
8.6
6.4
10.0
78.0
8.2
1.6
34.2
527
22
OCT.
.228
.232
25.1
1.9
36.7
S.2
5.5
9.6
74.8
7.4
2.2
33. J
416
33
NOV.
.178
.178
22.0
2.1
33.5
6.6
S.l
5.1
39.7
12.8
2.3
17.6
572
31
DEC.
.508
.507
18.4
1.6
53.2
7.4
9.2
6.2
48.2
19.3
3.2
18.7
789
23
JAN.
1973
.633
.633
16.4
1.4
51.2
7.1
8.3
6.5
50.4
16.7
3.7
21.0
86 S
IS
FEB.
.706
.692
15.8
2.0
38.1
6.5
5.7
7.9
61.6
9.3
1.4
27.4
889
17
MARCH
.5 07
.505
17.1
1.1
38.8
6.1
5.8
8.8
68.8
8.5
2.2
30.9
83 5
11
APRIL
.074
.072
15.9
3.2
21.2
3.4
2.9
7.1
55.2
5.2
2.9
24.7
730
11
HAY
.233
.230
20.2
1.4
28.9
4.8
4.4
8.8
65.S
7.3
1.2
29.3
433
28
JUNE
.300
.298
24.3
1.0
38.9
6.8
6.0
11.5
89.6
6.7
1.7
38.9
"SPLIT"
"SPLIT"
JULY
•402
.480
27.6
1.3
45. J
6.3
7.4
13.t
loo.n
7.6
2.4
42.6
850
3
AUGUST
.644
.640
28.5
3.6
4 8.0
8.8
8.3
13.4
104.S
8.0
2.6
45.0
742
12
AVG.
.400
.395
22.2
1.9
40.9
7\1
6.4
¦9.1
fo.i
9.8
2.3
30.6
700
20
(1) MGD x 3785 = m3/day
(2) ft3/day x 0.0283 = m3/day
(3) CFM/103ft3 = m3/min/103m3
-------
developed by Wuhrmann,^ and correcting for specific VSS con-
centration to obtain K20 a very significant coefficient of
correlation -0.924 between percent VSS reduced and K20 was
obtained for the period September 1972 through June 1973. It
appears that a sludge having a K20 of less than 6.0 mg/hr/g
VSS shows a high level of VSS stabilization. Conversely, if
the value for K20 is greater than 6, the degree of VSS reduc-
tion is relatively small. Thus, by correcting Kr for tempera-
ture, standardized to 20°C, this parameter can be used as a
measure of the degree of stabilization of the aerobically di-
gested sludge. A useful criteria would be K20 °f less than 5
to indicate a sludge of good stability.
In order to determine whether the oxygen uptake test run on a
sample collected at the end of "B" pass was representative of
the uptake rate for the entire aeration basin, a series of
samples at the end of "A", "B" and "C" passes were collected
once per shift on two separate days. Temperature was measured
in all three passes and the standard deviation from the mean
temperature throughout the digester was found to be less than
1.0°C. The highest average uptake rate was experienced on the
midnight shift. The results for August 23, 1972 indicated that
02 uptake rates at "A" pass were 17.4% higher than the daily
averages, while the uptake rates at "B" pass were 1.4% lower
and those at "C" pass were 16% lower than the daily average.
The results for August 26, 1973 showed the uptake rates at "A"
pass to be 9.4% higher, those at "B" pass 3% lower and those
at "C" pass 5% lower than the daily average.
Table 2
DATE
Oxygen uptake rates in three passes of No. 7
aerobic digester (mg/l/hr)
TIME
A PASS
B PASS
C PASS
DAILY
AVERAGE
8/23/72 10:00 A.M. 96
6:00 P.M. 72
2:00 A.M. 76
AVERAGE 81
60
66
79
68
36
72
66
58
64
70
73
69
8/26/72
10: 00 A.M,
6:00 P.M,
2:00 A.M.
AVERAGE
63
63
83
70
60
56
71
62
51
51
80
61
58
57
78
64
23
-------
A sample collected at the end of "B" pass was therefore con-
sidered representative of the average daily uptake rate in the
aerobic digestion basin as a whole. This assumption was veri-
fied on several other occasions.
Figure 9 indicates that when air is used as the oxygen source,
the most efficient utilization of oxygen per unit biomass re-
duced is obtained with a loading of 1.28 kgVSS/m^/day (0.08
lb VSS/ft^/day) with biomass temperatures above 20°C. In order
to aerobically digest one ton of VSS# three times as much oxy-
gen was required at the high loading rate of 3.0 kgVSS/m^/day
(0.187 lb VSS/ft3/day) as was required at the optimal loading
rate. At the low loading rate of 0.417 kgVSS/m^/day (0.026
lb VSS/ft^/day) the high O2 requirements may be related to ni-
trification as well as the diminishing amount of readily bio-
degradable biomass remaining in the system. At very high
loading rates, sludge retention time is so short that the solu-
bilization of VSS barely begins before the active biomass is
wasted from the system.
Figure 10 shows the relationship between compressed air supply
per unit volume under aeration and the oxygen transfer effi-
ciency of the system. Calculation of oxygen transfer efficiency
in the diffused air system required converting oxygen respired
(Kr) to tons 02 utilized per day on the basis of total biomass
VSS inventory in the digester. This value was then compared
to the tons per day oxygen equivalent of the compressed air
supplied to the digester. Air utilization ranged between 0.293
and 0.75 1/sec/m^ (17.6 and 45.0 cfm/1000 ft^) with an average
annual value of 0.51 (30.6). Oxygen transfer efficiency was
highest when temperature was lowest, averaging 9.8% for the
entire test period and ranging from a minimum of 5.2% in April
1973 to a maximum of 19.3% in December 1972. The diffused air
system is a relatively inefficient method for transferring oxy-
gen from the gaseous to the liquid phase. Oxygen transfer
efficiency was found to vary inversely with liquid temperature
and directly with side wall depth of liquid in the digester.
Figure 11 shows the relationship between O2 transfer and the
above variables (ft/°C x 0.305 = m/°C). An expression of this
relationship having correlation coefficient of +0.87 is:
Y = 0.12 + 0.31X
where Y = ft/°C units
X = percent O2 Transfer Efficiency
24
-------
5.0-
4.5-
4.0-
to
GO
3.0-
CM
O
2.5-
c
o
+->
2.0-
LU
1.0-
L±J
0.5-
.01
.02 .03 .04 £5 .06 j07 .08 .09 .10 .11 .12 .13 .14 .15 .16 .17 .18 .19
LOADING RATE lb VSS/ft 3/day (X 16.02 = kg/m3/day)
Fig. 9. Effect of sludge loading rate on oxygen requirement of biomass in diffused
air digester
-------
X
40
LEGEND
Al R SUPPLY—
OgTRANS.—
AIR SUPPLY
o 35
o
o
30
25
20 -
18 .
cc
LU
02TRANSrER ErFt6lENCY
o
o
AVG
=9.8
cc
LU
2 -
AUG. SEPT. OCT. NOV- DEC.
JAN- FEB. MAR. APR. MAY JUNE JULY AUG.
1972
Month!
1973
Fig. 10. Monthly variation of air supply and 02 transfer efficienqy
of
on
ai
in diffused air digester
26
-------
o.»
0.8-
0.7—
0.6-
0.5-
0.4-
0.3-
/
/
/
/
't
V
p.
V
/
/
/
/
/
/
/
0.2-
/
/
/
0.1-
I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19 20 21. 22 23 24 25 26
02 USED
% 02 TRANSFER EFFICIENCY
02 SUPPLIED
xlOO
Fig. 11. Oxygen transfer efficiency in diffused
air digester as a function of liquid
depth/temperature
27
-------
Figure 12 indicates the settleability of the aerobically stabi-
lized sludge measured both as sludge volume index (SVI) and as
zone settling velocity (ZSV). SVI was measured in a 2 litre
Mallory jar while ZSV was determined in a 2.45 meter (8.0 ft)
deep, 0.153 meter (0.5 ft) diameter column stirred at 10 rph.
Excluding those months when the stabilized sludge tended to
"split" into a settled and a floated fraction as a result of
inadequate DO leading to denitrification, the ZSV averaged
50.8 cm/hr (20 in/hr). For the entire test period, the SVI
averaged 114 ml in 30 min/g TSS. A poor correlation (-0.09)
was found to exist between ZSV and SVI. At the high solids
concentrations maintained in the aerobic digester, SVI calcula-
tions are a less sensitive measure of the changing settleability
rates than ZSV. When ffiV is multiplied by TSS concentration,
a solids weighted settling rate (Civi) having dimensions g/cm^/
hr is obtained. Figure 13 shows a better correlation (-0.54)
between CiVi and SVI than the previously discussed correlation
between ZSV and SVI. Figure 14 was obtained by plotting daily
solids-weighted settling rates CiVi on temperature versus load-
ing rate coordinates, and then constructing iscsettling rate
curves. This figure indicates that the best settleabilities
were obtained at temperatures of 20-21 °C, and at a loading
rate of 1.28 kg VSS/m^/day (0.08 lb VSS/ft^/da^. A summary of
most important field data appears in Figure 15 where average
monthly values for SRT, temperature, Kr, C>2 respired/VSS re-
duced, percent and tons/day VSS reduced are shown in relation
to each other. Figures 16 and 17 show monthly variations in
VSS and hydraulic loading.
Laboratory Data
Monthly averages of laboratory data for the test period are
summarized in Table 3. Table 4 includes the percent change
between the influent loading and effluent waste from the di-
gester. Table 5 summarizes aerobic digestion performance data.
Solids Data
TSS and VSS were analyzed by a modification of the Gooch crucible
method.^ Figure 18 shows a schematic interrelationship of
the various solids forms undergoing aerobic digestion. The
28
-------
60
E
(J
50
n
v
40
\
> CM
CO
M X
150 -
140 -
130 -
120 "
110 -
114
sPLrr
too
90
80
o
CO
60
50
40
30
20
JAN. FEB MAR APR. MAY JUNE JULY AUG
AUG SEPT OCT NOV. DEC.
1972
1973
Fig. 12. Monthly ZSV and SVI variation of biomass in diffused
air digester
29
-------
0.7
0.6
cr>
o
0.5
0.4-
UJ
0.3
o
oo
0.2
100
50
75
125
150
25
SVI ml/30 min
g TSS
Fig. 13. Correlation of SVI versus solids weighted ZSV in
diffused air digester
30
-------
30
29
28
27
26
25
TEMR 24
Pc)
23
103
22
=25 ) g/l : in/h
CIV
:!00 50
21
20
19
18
17
16
15
14
.02 .04 .06 .08 .10 .12 .14 .16 .18 .20
LOADING RATES
lb VSS/ft3/day (X 16.02 = kg/m3/day)
Fig. 14. Isosettling rates of biomass in
diffused air digester as a func-
tion of temperature and loading
rates
31
-------
SLUDGE
DIGESTED
(tons VSS/
day)
PERCENT
VSS
REDUCED
O2 UPTAKE
(mg/hr/g
VSS) „
LB 02 USED *
LB VSS DIG- „
ESTED
TEMP (°C)
30
SLUDGE
RETENTION 20
TIME
(days) ,0
WG*3.
V
AV6» 3
AVG. = 75
LVG.a2
AV 5 22.2
AVG.S9B
AUG SEPT OCT NOV DEC JAN FEB MAR APRIL MAY JUNE JULY AUG
1972-
¦1973
Fig. 15. Monthly variation in diffused air digester average
field data
32
-------
LEGEND
AVG
M IN./MAX.
AUG. SEPT.
OCT. NOV. DEC.
JAN. FEB. MAR. APR. MAY JUNE JULY AUG
Fig. 16. Monthly variation in volatile solids loadings to
diffused air digester
33
-------
30
LEGEND
28
AVG —
MIN /MAX
26
24.
22
20
18
16
I- o
AVG.
iii
AUG. SEPT OCT. NOV. DEC. JAN. FEB. MAR. APR. MAY JUNE JULY
1973
1972
Fig. 17. Monthly variation in diffused air digester
hydraulic detention time
34
-------
LEVEL I
LEVEL II
LEVEL III
LEVEL IV
T S
Soluble
Particulate
(Minerals)(Humic Acid) (Sugars)
(Lipids) (Cellulose) (Clays)
Proteins
COD Total
BOD Total
Enzymatic
Bio oxidations
Catalase: Dehydrogenase
Oxidase t Decarboxylase
Hydrolase:
CO
etc
Non
Biodegradable
Residue
Resynthesized
Biomass
LEGEND
T S
D 8
8 S
Total Solids
Dissolved Solids
Suspended Solids
I D
V D
V S
I S
Inert D S
Volatile D
Volatile S
Inert S S
N B VD S » Non
Biodegradable V D
B V D S ¦ Biode-
gradable V D S
B V S S ¦ Biode-
gradable V S S
N B V S S - Non
Biodegradable V S
Fig. 18. Schematic inter-relationships of solid forms undergoing aerobic
digestion
-------
Table 3. Diffused air digester laboratory data - monthly average (mg/1 unless other units
indicated)
TOTAL SOLIDS
SUSPENDED SOLIDS
DISSOLVED SOLIDS
COD
NITROGEN
-V
po4-p
COND.
nho/cm*
pH
ALK.
FECAL
COLJ.
HOUTH
SAMPLE
TS
VS
FS
TSS
VSS
FSS
TDS
VDS
FDS
NO j
nh4
TKN
(units)
as
CaCOv
106/ua
tva.
1972
IUF
EFF
8,4 72
6,198
6,491
4,177
1,981
2,021
7,582
5,054
6,275
3,839
1,307
1,215
890
1,144
216
338
674
806
10,100
5,942
.34
27.5
36
55
617
438
218
228
1,407
1,626
6.8
7.0
479
424
NA
NA
SEPT.
IUF
EFF
8,266
6,193
6,354
4,136
1,912
2,057
7,366
4,688
6,160
3,568
1,206
1,120
900
1,505
194
568
706
937
10,120
5,790
.25
94.0
29
70
600
398
179
168
1,263
1,870
6.9
6.6
409
229
17.0
0.5
XT.
IUF
EFF
7,921
5,364
6,287
3,644
1,634
1,720
7,146
3,545
6,121
2,793
1,025
752
775
1,819
166
851
609
968
10,796
5,460
.06
128.4
36
80
641
396
158
141
1,232
2,154
7.0
6.5
437
104.
17.0.
0.9
•IOV.
INF
EFF
8,661
5,825
6,833
4,155
1,828
1,670
7,813
4,449
6,626
3,488
1,187
961
848
1,376
207
667
641
709
12,601
6,033
.05
93.0
35
91
615
446
171
151
1,188
2,045
7.0
6.6
490
237
17.0
0.6
DEC.
ihf
EFF
10,225
8,640
8,149
6,571
2,076
2,069
9,386
7,607
7,964
6,32 5
1,422
1,282
839
1,033
185
24 6
654
787
14,616
10,500
.03
.15
41
23
691
628
14 7
148
1,183
1,303
7.0
6.9
563
344
6.1
1.7
JAN.
1973
IUF
EFF
9,804
9,225
7,789
7,130
2,015
2,094
8,926
8,104
7,604
6,891
1,322
1,213
878
1,121
185
23 9
693
881
14,416
11,985
.03
.06
39
19
653
574
199
203
1,205
1,226
7.0
6.9
516
362
20.0
17.3
FEB.
IUF
EFF
8,755
7,420
6,610
5,400
2,145
2,020
7,775
6,385
6,470
5,195
1,305
1,190
980
1,035
140
205
840
830
10,292
8,313
.02
.03
48
85
620
567
242
196
1,251
1,543
6.9
7.0
430
595
SA
KA
HAS.
INF
EFF
9,240
7,811
7,274
5,797
1,966
2,014
8,296
6,785
7,092
5,552
1,204
1,233
944
1,026
182
245
762
781
11,452
8,548
.04
.06
60
101
632
626
126
147
1,395
1,723
6.8
7.0
487
601
8.1
5.3
UPRIL
IUF
EFF
10,534
7,509
7,920
5,159
2,614
2,348
9,513
6,089
7,751
4,704
1,762
1,385
1,021
1,418
169
455
852
963
12,370
7,269
.05
67.8
44
78
738
480
259
213
1,377
2,011
7.0
6.8
524
333
NA
NA
HAY
IUF
EFF
13,183
9,107
8,200
5,048
4,989
4,059
12,047
6,847
8,022
4,116
4,025
2,731
1,142
2,260
178
932
964
1,328
12,525
6,892
.04
178.0
33
73
784
433
271
282
1,555
2,680
6.9
6.5
563
225
11.0
1.9
JUNE
' tNt
EFF
10,199
7,699
7,404
4,705
2,795
2,993
9,202
6,300
7,198
4,275
2,004
2,025
997
1,398
206
430
791
968
11,597
6,700
.03
68.9
38
55
664
443
232
270
1,182
1,706
6.7
6.7
520
450
17.0
0.7
JULY
IUF
EFF
11,21$
9,395
8,415
6,509
2,860
2,886
10,311
8,237
8,186
6,178
2,125
2,059
964
1,158
224
331
735
82 7
12,352.
8,969
.06
7.9
28
73
776
612
233
251
1,196
1,475
6.9
7.0
582
520
50.5
7.3
AUG.
IUF
EFF
8,517
7,514
6,193
5,269
2,324
2,245
7,575
6,450
6,009
4,905
1,566
1,545
942
1,064
209
364
758
700
9,277
7,326
.02
3.5
28
38
602
505
20 5
197
1,130
1,302
7.0
7.0
435
453
24.8
4.0
AVG.
INF
EFF
9,620
7,531
7,225
5,208
2,395
2,323
8,688
6,195
7,037
4,756
1,651
1,439
932
1,335
169
452
745
883
11,731
7,671
.08
51.S
38
65
664
504
202
200
1,274
1,743
6.9
6.8
499
37 5
14.2
J.n
-------
Table 4. Diffused air digester laboratory data - percent change influent versus effluent
to
-J
Month
TOTAL SOLIDS
SUSPENDED SOLIDS
DISSOLVED SOLIDS
COD
NITROGEN-N
po4-p
CONDUCTIVITY
fmho/cm2
pH
(Uni ts)
ALK.
as
C&CO
FECAL
CCLI
x 10^/ml
TS
vs
FS
TSS
vss
FSS
TDS
VDS
FDS
NOi
NJfrf
TKN
AUGUST
1972
-26.8
-35.6
+ 2.0
-33.3
-38.8
- 7.0
+28.5
+56.5
+19.6
-41.2
+ 8.0
x 103
+52.8
-29.0
+ 4.6
+15.6
+0.2
¦11.5
N.A.
SEPTEMBER
-25.J
-34.9
+ 7.6
-36:4
-42.1
- 7.1
*67.2
+192.8
+32.7
-42.8
+ 37.5
x 103
+141.4
-33.7
- 6.2
+48.1
-0. 3
¦44.0
-97.1
OCTOBER
-32.3
-42.0
+ 5.3
-5 0.4
-5 4.4
-26.6
+ 134.7
+412.7
+59.0
-49.4
+214.0
x 103
+122.2
-38.2
-10.8
+74.8
-0. S
•7 6.2
-94.7
NOVEMBER
-32.7
-39.2
- 6.6
-43.1
-47.4
-19.0
+62.3
¥222.2
+10.6
-52.1
+186.0
v 103
+160.0
-27.5
-il.7
+72.1
-0.4
¦51.6
-96.2
DECEMBER
-15.5
-19.4
- 0.3
-19.0
-20.6
- 9.9
+23.1
+33.0
+20.3
-28.2
+ 0.4
x 103
-43.9
- 9.1
+ 0.7
+10.1
-0.1
¦38.9
-7 2.1
JAI.VAPr
1973
- 5.9
- 8.5
+ 3.9
- 9.2
- 9.4
- 8.3
+27.7
+29.2
+27.1
-16.9
-51.3
-12.1
+ 2.0
+ 1.7
-o.l
¦29.8
-13.5
FEBRUARY
-15.2
-18.3
- 5.8
-17.9
-19.7
- 8.8
+ 5.6
+46.4
- 1.2
-19.2
+77.1
- 8.5
-19.0
+23.3
+0.1
¦38.4
N.A.
MARCH
-15.5
-20.3
+ 2.4
-18.2
-21.7
+ 2.4
+ 8.7
+34.6
+ 2.5
-25.4
*+i
+68.3
- 1.0
+16.7
+23.5
+0.2
¦23.4
-93.5
APRIL
-38.7
-34.9
-10.2
-36.0
-39.3
-21.4
+38.9
+169.2
+13.0
-41.2
m
+77.3
-35.0
-17.8
+46.0
-0.2
¦36.5
K.A.
HAY
(flrytrl)
-31.0
-38.4
-18.6
-43.2
-48.7
-32.1
+97.9
+423.6
+ 37.8
-45.0
Mb
+121.2
-44.8
+ 4.1
+72.3
-0.4
60.0
-82.7
JUNE
-24.5
-36.5
+ 7.1
-31.5
-40.6
+ 1.0
+40.2
+108.7
+22.4
-42.2
+230
x 103
+44.7
-33.3
+16.4
+44.3
+ 0
¦13.5
-96.2
JULY
-16.7
-22.7
+ 0.9
-20.1
-24.5
- 3.1
+20.1
+ 44.5
+12.5
-27.4
+0.13
x 103
+160.7
-21.1
+12.6
+23.3
+ 0.1
¦10.0
-85.5
AUGUST
-li.a
-14.9
- 3.4
-14.9
-18.3
- 1.3
+13.0
+ 74.2
- 7.7
-21.0
+0.17
x 103
+35.7
-16.1
- 3.9
+15.2
+ 0
• 6.6
-96.1
avg.
-21.7
-27.9
- 3.0
-28.7
-32.4
-12.8
+43.2
+139.0
+18.5
-34.6
+0.64
x 103
+ 71.1
-24.1
- 1.0
+36.8
- 0.1
¦24.8
-83.9
-------
Table 5.. Diffused air digester performance calculations
10HTH
S.V.I.
ml/30 mln/g
SUPERNATANT
TURBIDITY
LOADING
vnuiMFTPrr.
(1)
1h VSS/ft3/dav
iATES
ORGANIC
VSS INF.
DET&TICH
HYDftAULIC
TIME
SRT
SOLIDS LOADING
TONS/DAY
SOLIDS WASTED
TONS/DAY (2)
AEROBIC
nTGFSTTnN
AEROBIC
DIGESTION
JTU
van in v.
IDAYS)
(DAYS)
TSS
VSS
TSS
VSS
% VSS RED
fSST/OAY(2)
AUG.
1972
HA
Nh
0.092
.36
4.3
5.6
11.4
9.6
7.3
5.5
39.8
3.8
SEPT.
154
NA
0.085
.29
4.5
6.9
10.6
8.8
6.4
4.9
47.0
4.1
OCT.
119
NA
0.054
.20
7.0
10.9
6.3
5.4
3.4
2.7
47.2
2.5
NOV.
120
NA
0.049
.16
8.6
13.4
5.8
4.9
3.3
2.6
46.2
2.3
DEC.
104
73
0.147
.3X
3.7
4.3
20.1
17.0
16.3
13.6
16.7
2.8
JAN.
1973
107
76
0.167
.42
2.6
3.1
23.5
20.0
21.5
18.2
11.2
2.2
FEB.
126
55
0.187
.52
2.2
2.7
22.6
18.8
18.3
14.9
22.4
4.2
VAffCW
123
73
0.150
.38
3.1
3.7
17.3
14.8
14.5
11.9
18.1
2.7
APRIL
lie
149
0.026
.07
19.8
29.8
3.0
2.4
2.0
1.7
41.5
1.0
HAY
68
161
0.078
.20
7.0
18.2
11.7
7.8
5.9
4.0
45.8
3.6
JUNE
"SPLIT"
179
0.086
.24
5.2
8.6
11.3
8.8
7.1
5.5
39.6
3.5
JULY
101
131
0.150
.33
3.5
4.3
20.6
16.4
16.7
12.5
19.8
3.1
AUGUST
114
109
0.140
.41
2.8
3.3
20.0
15.8
17.3
13.0
20.2
3.2
114
112
0.110
.30
5.7
8.8
14,2
11.6
10.8
8.S
32.0
3.0
(1) lb VSS/ft3/day x 16.02 = kg/m3/day
(2) Tons/day x 0.907 = tons (metric)/day
-------
total solids (TS) are subdivided into particulate and soluble
solids (suspended solids - SS and dissolved solids - DS). The
soluble solids are further subdivided into volatile dissolved
solids (VDS) and inert dissolved solids (IDS). The particulate
suspended solids are also subdivided into volatile suspended
solids (VSS) and inert suspended solids (XSS). On a fourth
level of differentiation, the VDS are subdivided into biode-
gradable volatile dissolved solids (BVDS) and non-biodegradable
volatile dissolved solids (NBVDS). Similarly, the volatile
suspended solids can be divided into biodegradable and non-bio-
degradable fractions. Considerable confusion exists in the
literature with regard to the solids form that is aerobically
digested or reduced.^ Some references base their calcula-
tions of mass reduction on total solids,(5) others use total
volatile solids,and still others use volatile suspended
solids. This study used VSS reduction as the criterion for de-
termining degree of aerobic digestion or stabilization. Collo-
dial materials ranging in size between the suspended and the
dissolved fractions were not dealt with separately in this
analysis and are assumed to be part of the dissolved solids
passing through the average pore diameter of the Gooch filter.
Although VSS is the major criterion used for determining the
diffused air system performance, biodegradable VSS and COD are
also considered when discussing the pure oxygen batch tests.
Several lab samples were analyzed separately for suspended,
dissolved, and total solids and compared with calculated values.
The most reliable results were obtained by directly analyzing
suspended and dissolved solids, and adding these values to ob-
tain a calculated total solids value.
Volatile Solids Reduction
Figure 19 shows the volatile solids reduction achieved within
the spectrum of temperature and loading conditions encountered
during the diffused air study. VSS reductions ranged from
11.2% to 47.2%. Attempts to relate this performance to a single
variable were unsuccessful. A complicating factor in correla-
tion analysis was the fact that beyond a certain limiting value
for sludge detention time, VSS reduction was asymptotic, approa-
ching but rarely exceeding 50%. When coefficients of correla-
tion were calculated for several environmental-operational
functions within the limits previously observed, the most sig-
nificant correlation +0.93 was observed between VSS reduction,
and SRT x temperature (Figure 20). This time-temperature
39
-------
.200
.175
.150
.125
.100
.075
.050
.025
LEGEND
% VS.S.
reduced
//A INSUFFICIENT
DATA RANGE
7777
ssss
77 V r.
y///'
'////
'S///
////s
7777
~ ///
50%
40%
10 12 14 16 18 20 22 24 26 28 30
TEMP (• c)
Fig. 19. Spectrum of temperature and load-
ings versus VSS reductions in dif-
fused air digester
40
-------
+0.93
rSHTxt""
+ 0.85
[(0-25UMIH
(0-12 DAYS . l_t QJJL
UMIT> fsa t !
SRI XT .
(days°C)
+0.67
1(0-12 DAYS
LIMIT)
SatxT
/g^hr°C
SRT
(DAYS)
+0.33
TEMR
(°C)
V.S.S
-0.75
ENVIRONMENTAL OPERATIONAL FACTORS
Fig. 20. Correlation coefficients for various factors.
influencing VSS reduction in diffused air di-
gester
41
-------
factor was within the limits of an SRT of 0 - 12 days and a
temperature of 12 - 22°C. (Figure 21)
Volatile Solids Residue Change
The VSS/TSS ratio of the WAS influent averaged 81.6% compared
with 76.7% in the digested effluent. Thus/ a 5% decline in
volatile solids residue accounted for an average 32% VSS re-
duction (Figure 22). If the volatile solids.reduction per-
formance were based on a VS/TS ratio as is commonly used when
calculating anaerobic digestion performanceC) rather than a
VSS/TSS ratio, a 7% decline in volatile solids residue would
account for a 30% VS reduction.
A definition of solids reduction under aerobic conditions must,
therefore, take into account both solubilization of particulates
as well as carbon loss in a gaseous form. Changes in kinetic
equilibrium between particulate biomass undergoing enzymatic
solubilization and soluble substrate being resynthesized back
to particulate biomass, may account in part for these differ-
ences in calculated biomass reductions. Suspended solids
undergoing aerobic stabilization are converted to dissolved
solids, water and gas (mainly carbon dioxide). Figure 23 and
24 indicate that as TSS conversion increases, the rate of solu-
bilization also increases. For example, during October, 1972
when TSS conversion performance was at a maximum, increase in
the effluent TDS accounted for approximately 30% of the TSS
converted. In March 1973, however, when TSS reduction was mini-
mal, TDS increase accounted for only 5% of the TSS converted.
The volatile fraction of Metro WAS must be reduced from an aver-
age 80% to less than 60% in order to avoid potentially obnoxious
odors, particularly if the stabilized sludge is to be spread on
land. This degree of volatile solids reduction has been im-
possible to achieve at Metro for up to thirty days SRT in this
study (excluding the nonrepresentative May 1973 "flood period").
The volatile fraction of the aerobically stabilized sludge can
however be further reduced to less than 60% by either chemical
oxidation (ozonation) or subsequent anaerobic digestion of the
aerobic digester effluent.
42
-------
500
450
400
350
t/i
>>
i—
200
s:
LlI
I—
150
100 -
50
40
70
20
30
PERCENT VSS REDUCED
50
60
Fig. 21. Temperature-time factor versus percent VSS reduced
in diffused air digester
43
-------
98
96
94
92
90
66
66
84
82
80
78
76
74
72
70
68
66
64
62
•I
-2
-3
-4
-5
• 6
-7
•8
-9
LEGEND
INFLUENT
EFFLUENT
A % —*—*
FLOOD"
22.
Monthly variation in percent VSS between influent and effluent
of diffused air digester
-------
15000
LEG
•ND
14000
rL,UEN"
13000
12000
11000
10000
9000
TSS AVG.
eooo
£ 7000
TSS
z 6000
W 500C -
8 4000
300C -
2000
TDS AVG.
1000
¦ t
S>3 2
TOS AVCi,
JAN. FEB. MAR. APR. MAY. JUN. JULY. AUG.
i 1973
AUG. SEPT. OCT. NOV. DEC.
1972 1
Fig. 23. Monthly variation of influent and effluent TSS and TDS in
diffused air digester
45
-------
£7000
Id
D
^6500
ll_
Ul
Z6000
UJ
3
2 5500
g>5000
<1
^4500
q; 4000
UJ
>
q 3500
O
CO3000
Q
q 2500
CO
2000
1500
1000
750
500
250
0
aTSS
_^GAS
H20 9P%
00 %%
=77%
s7l %
=64%
-89 %
= 96%
=70%
= 4%
=95%
=5 %
=88%
= 78 %
= 86%
7_Z7.
= 91%
ST'"
= 89%
^TP»/0
AUG. SEPT OCT NOV. DEC.
1972 »
JAN. FEB. MAR. APR. MAY JUNE JULY AUG
1973
Eig., 2.4. Monthly variation in solids conversiof> to dissolved and
gaseous end products in diffused air digester
46
-------
Digester Performance as a Function of SRT
R. C. Loehr in his paper "Aerobic Digestion - Factors Affecting
Design"wrote, "Different rates of sludge oxidation and
oxygen utilization are due to different starting points ... (but)
few authors report the sludge ages of solids entering the aero-
bic digester. The percent volatile solids reduction of a sludge
with a high sludge age will be less than that of a sludge with
a low sludge age. For waste sludges with a high sludge age,
much of the sludge oxidation has taken place in the activated
sludge (secondary) system." In order to test this hypothesis
against Metro data, the average monthly SRT of the waste acti-
vated sludge in the secondary system prior to loading to the
digester was calculated. These data are shown in Table 6.
For an SRT range in the activated sludge system between 4.2 and
25.6 days, very little effect on VSS reduction was observed.
For example, during April 1973 when the highest secondary SRT
occurred, VSS reductions were 41.5%. When the lowest secondary
SRT of 4.2 days occurred in March 1973, VSS reduction was only
18.1%.
For equivalent conditions of total SRT (sludge age in secondary
aerator plus aerobic digester), the degree of volatile solids
reduction should, according to Loehr, have been almost identical.
However, the 10°C drop in temperature between September (28.7°C
and SRT 12.8 days) and December (18.4°C and SRT 12.6 days) ac-
tually resulted in a three fold drop in digestion efficiency.
In September VSS reduction averaged 47% compared to only 16.6%
in December. The results obtained at Metro did not support the
contention that SRT in the activated sludge system alone is a
major influence on VSS reduction in the aerobic digester. A
better correlation (+0.53) was found between total SRT and per-
cent VSS reduced. VSS reduction is apparently most sensitive
to environmental conditions during digestion rather than to
sludge prehistory.
VSS Materials Balance
Analysis of Metro operational data indicated that a good approxi-
mation of the average biomass concentration to be used in cal-
culation of solids inventory was obtained by using the formula:
47
-------
Table 6. Monthly variation of SRT and VSS reduced in diffused air digester
00
MONTH
SRT (DAYS)
PERCENT VSS REDUCED
PERCENT VOLATILE SOLIDS
Before
Diqestion
During
Diaestion
Total
Before
Diqestion
After
Diqestion
8/72
4.8
5.6
10.4
39.8
82.8
76.0
-6.8
9
5.9
6.9
12.8
47.0
83.6
76.1
-7.5
10
9.2
10.9
20.1
47.2
85.7
78.8
-6.9
11
11.1
13.4
24.5
46.2
84.8
78.4
-6.4
12
8.3
4.3
12.6
16.7
84.8
83.1
-1.7
1/73
6.0
3.1
9.1
11.2
85.2
85.0
-0.2
2
5.0
2.7
7.7
22.4
83.2
81.4
-1.8
3
4.2
3.7
7.9
18.1
85.5
81.8
-3.7
4
25.6
2S.8
55.4
41.5
81.5
77.3
-4.2
5 (Flood)
7.6
18.2
25.8
45.8
66.6
60.1
-6.5
6
6.4
8.6
15.0
39.6
78.2
67.9
-10.3
7
5 .0
4.3
9.3
19.8
79.4
75.0
-4.4
8
6.2
3.3
9.5
20.2
79.3
76.0
-3.3
AVG.
8.1
8.8
16 .9
32.0
81.6
76.7
-4.9
-------
Inventory Concentration = Influent + Effluent
2
i.e. If influent solids concentration = 8000 mg/1
effluent solids concentration = 4000 mg/1
inventory concentration
(calculated) = 6000 mg/1
Spot checks using a Biospherics suspended solids meter in all
three passes of the test basin showed this to be a valid approxi-
mation. All inventory calculations were, therefore, based on
this formula. The VSS balance in the test digester over the
thirteen month period 8/1/72-8/31/73 indicate the following:
Influent loading = 4600 Tons VSS
Effluent wasting = 3360 Tons VSS
Initial inventory - 44 Tons VSS
Final inventory = 34 Tons VSS
VSS reduction due to
aerobic digestion
= Inf - (Eff + &lnv)
= 4600 - (3360 - 10)
= 1250/4600
= 27.2%
Nitrogen Balance
Using the VSS materials balance, a nitrogen balance was calcu-
lated. The inventory change in total nitrogen forms (TKN plus
NO3-N) over the test period was negligible (-0.93 tons N). The
change between influent and effluent of NH4-N was +21.2 tons,
of NO3-N was +36.4 tons, and of TKN was-76.7 tons. The differ-
ence between the total nitrogen forms loaded to the digester
(434.0 tons) and the nitrogen leaving the system in the effluent
(393.2 tons) represents an unaccounted for loss of 40.8 tons.
This nitrogen loss is attributed to denitrification (9.6% of the
total nitrogen loading to the system). It is assumed that de-
nitrification occurred when periods of maximum nitrification
coincided with periods of insufficient oxygen due to the shutting
off of air in "C" pass for dewatering purposes. This resulted
in conversion of NO3 to N2 gas. The "split"-float phenomenon
during August 1972 and June 1973 tends to substantiate this
supposition, as calculated denitrification losses during these
49
-------
months averaged 23% of the total nitrogen load to the system.
Because of the denitrification-flotation problem resulting
from dissolved oxygen depletion, the decanting operation was
discontinued in September 1972.
The effects of cold temperature on aerobic digestion performance
were most pronounced during the last two weeks of November 1972,
In particular, the inhibition of organic nitrogen mineralization
to ammonium and nitrate ions was evident during this period. A
decline in calculated denitrification to less than 3% of the
total nitrogen loaded to the digester occurred during November
1972. The dramatic impact of sudden cold temperature change is
illustrated in Table 7 which compares at a constant loading rate
various parameters associated with nitrification, two weeks
"before" and two weeks "after" the sudden onset of the first
winter snows (November 15, 1972).
Table 7. Effect of "Cold Shock" on Nitrification Parameters
EFFLUENT
PARAMETER
UNITS
INFLUENT
"BEFORE"
"AFTER"
Nitrate - N
mg/1
0.05
174
0.08
Ammonium - N
mg/1
35
145
29
Organic - N
mg/1
580
320
390
Alkalinity
as CaC03
mg/1
490
110
364
PH
Units
7.0
6.0
7.0
Temperature
°c
-
25.0
19.0
According to
somonas is 0.
Downing(9)
, 33/day at
the growth constant max) for nitro-
20°C and this compares with 0.448/day at
25°C (Figure 25). Downing also determined that the rate of
nitrification is independent of ammonium concentrations above
3.0 mg/1 NH4. In the diffused air digester, the WAS influent
ammonium concentration averaged 38 mg/1 with a minimum concen-
tration of 28 mg/1 and thus did not limit the nitrification
rate.
50
-------
^ 30
ID
< 20
cc
uj
o.
0.1
0.2
0.3
0.4
0.5
0.6
MAXIMUM GROWTH RATE, ^ MAX (doy"')
Fig. 25. Temperature effects on growth rate of nitri-
fying bacteria (after Downing)
51
-------
The observed decline in nitrification rate with decreasing tem-
perature (-7% per 1°C) should theoretically have required a
14°C drop in the biomass temperature to account for the drop
in nitrification during the latter half of November 1972. In
order to determine whether other factors beside temperature
could account for the cessation of nitrification, the aerobic
digestion system was analyzed for heavy metals. No unusually
high concentrations of nickel, copper, zinc, etc., were noted.
No other signs of toxic effects such as a decline in O2 uptake
rates were observed during this period. It is, therefore,
assumed that the cumulative drop in liquid temperature of approxi-
mately 1°C per day over a four day period (11/8 - 11/12/72) re-
sulted in a "cold shock" to the sensitive nitrifying bacteria.
Nitrification was reestablished in April 1973 even though the
average monthly temperature declined in April to a minimum of
15.9°C indicating gradual adaptation of the nitrifying biomass
to the colder temperature. During April the high SRT in the
digester of 29.8 days was sufficient to maintain high nitrifica-
tion rates despite the cold temperature. In July 1973, when
biomass temperature rose 28.0 °C, and SRT in the digester dropped
to 4.3 days, nitrification inhibition was again observed but
this time because of the low SRT.
Figure 26 shows that a poor correlation -0.28 existed between
dissolved oxygen concentration and nitrification. Both the high-
est and lowest nitrification rates occurred at DO concentration
of 1.4 mg/1. No significant correlation was observed between
temperature standardized oxygen uptake rate (K2q) and nitrifi-
cation rates. In fact, the highest degree of nitrification
occurred when K20 was below 5.9 mg/hr/g VSS. By applying Down-
ing' s temperature correction to nitrifying bacteria activity,
the important effect of SRT on nitrification rate is apparent.
The most significant correlation +0.96 between nitrification
rates and environmental conditions was found for the temperature-
time factor SRT (days) x temperature (°C) (Figure 27). Nitrate
levels in excess of 100 mg/1 NO3-N were observed when the tem-
perature-time factor exceeded 200. Figure 28 and 29 show the
influent and effluent monthly variations for nitrate, ammonium
and TKN concentrations. Detection limit for NO3-N was 0.01
mg/1.
52
-------
5.0
4.5
4.0
3.5
3.0
2.5
-------
500
450
400
350
300
250
200
150
100
50\
25
0
nitrification onset
25 50 75 100 125 150 175 20
N03-N EFFLUENT (mg/1)
ig* 27. Effect of temperature-time factor on nitrifi-
cation in diffused air digester
54
-------
LEGEN
NFLUiNT
EFFLLENT
1000
500
100
50
10
5.0
1.0
50
.10
AVG.
O.Oi
AUG. SEPT OCT. NOV DEC. JAN. FEB. MAR. APR. MAY JUNE JULY AUG
1973
1972 —*
.fig* 28.. Monthly variation of nitrates .in diffused air digester
influent and effluent
55
-------
800
LEGEND
750
700-
TKNAVG=66
650
600
550
[KN_ AVG^ 504^
500
450
400
350
100
75
NH4 \AVGa >5
50
25
AUG. SEPT. OCT NOV. DEC.
JAN. FE8. MAR. APR. MAY JUNE JULY AUG
1973
<972
Fig. 29. Monthly variation of TKN and NH4-N in diffused air
digester influent and effluent
56
-------
Alkalinity and pH
Figure 30 shows the parallel trends between alkalinity and pH
changes as they relate to nitrification rates. During the
months August - November 1972,the effluent pH was approximately
0.5 units lower than the influent pH, and the alkalinity was
approximately 200 mg/1 less in the effluent than in the influent.
At the end of November 1972 when nitrification ceased, the pH
and alkalinity of the influent remained virtually unchanged
compared to the effluent. The onset of nitrification in April
1973 coincided with a reduction in the effluent of pH and alka-
linity. The best performance, measured as percent VSS reduction
(Table 5) occurred when nitrification was highest, and pH and
alkalinity differential between the influent and the effluent
was maximal. Measurement of pH in the influent and the effluent
can indicate the degree of stabilization achieved during periods
of nitrification.
Conductivity
Figure 31 shows the change in electrical conductivity between
the influent and the effluent. As was the case with pH and
alkalinity, the best performance (August - November 1972, and
April - June 1972) coincided with the highest differential in
conductivity between the influent and the effluent. Figure 32
shows a very high degree of correlation +0.94 between the change
in conductivity and the percent VSS reduced. When the conduc-
tivity change between influent and effluent was less than 25%,
the percent VSS reduced was also less than 25%. Conversely,
when the conductivity change was greater than 40%, the percent
VSS reduced was also greater than 40%. Electrical conductivity
differential measures the degree of mineralization. The corre-
lation between TDS and electrical conductivity was therefore
very high. Thus, another simple method for estimating the de-
gree of stabilization achieved is measuring the change in elec-
trical conductivity between the influent and effluent.
COD
The percent COD reduction between influent and effluent (Table
4) correlated very well with the VSS reduction. Figure 33 shows
57
-------
600
550
500
K>
O
o
03
C_J
450
(/>
400
350
>-
300
5 250
-------
2600
2500
2400
LEGEND
2300 -
2200
2100
2000
1900
1800
1700
1600
1500
1400
1300
AV
1200
1100
AUG. SEPT OCT. NOV. DEC.
JAN. FEB. MAR.. APR. MAY JUNE JULY AUG
1972
Fig. 31. Monthly variation of conductivity in diffused air digester
influent and effluent
59
-------
o
LcJ
O
ID
Q
LiJ
or
CO
c_>
ct
VSS red
i >40% w
uced
hen
flange
! % cond.
40 %
< 25
change
duced
cond.
100
PERCENT CONDUCTIVITY CHANGE (|j^ - 1 x lOO)
Fi,g. 32. Percent VSS reduction versus percent conduc-
tivity change in diffused air digester
60
-------
15000
14000
13000
12000
IL73I
11000
I000Q
9000
O 7000
UIQEMD
5000
IFFLl ENT- -
4000
3000
2000
1000
JAN. FEB MAR APR MAY JUN. JULY AUG
1973
AUG. SEPT OCT NOV DEC.
1972
Fig. 33. Monthly variation of COD influent and effluent in diffused
air digester
61
-------
the monthly variation in COD concentration between influent and
effluent. The decline in COD was 34.6% and compared well with
a 32.4% reduction for VSS. The COD reduction was slightly
greater than VSS reduction, as dissolved volatile solids are
also oxidized during the aerobic stabilization process.
Phosphate
No correlation between VSS reduction and phosphate removal was
observed. Figure 34 indicates that the percent change between
influent and effluent averaged less than 1%. No significant
changes in phosphorous uptake or release to the digester efflu-
ent occurred. Phosphorous was obviously not a limiting factor
in the growth and reproduction of the aerobic digester micro-
organisms .
Summary of Physical and Chemical Laboratory Data
On the basis of changes that occurred between the influent and
effluent (Table 4) the variables measured may be classified in-
to three groups. In the first group, there was no correlation
between influent and effluent differential and the degree of
aerobic digestion. Phosphate was the only variable in this
group. In the second group, there was a positive correlation
between the influent and effluent differential and the aerobic
digestion rate. In this group were found total, volatile and
fixed dissolved solids, conductivity, nitrates and ammonium.
In the third group, there is a negative correlation between the
influent and effluent differential and the aerobic digestion
rate. This group included pH, alkalinity, TKN, COD, TSS and
VSS. Figure 35 shows the trends of the three major groups as
previously discussed.
Invertebrate Analysis
Samples of aerobic digester biomass were collected, immediately
diluted (1:10) with plant effluent, and placed in a Sedgwick
Rafter cell. Ten fields were counted for each sample at a mag-
nification of 150x, and the numerical count (number x 10^/1)
calculated. The numerical counts were in turn converted to a
62
-------
250
INFLUENT Avq/?02
lDeni *~AVG.="200
200
150
legenc
FLUEjNT
100
5 D.
AUG. SEFT OCT NOV DEC.
1972 —*
JAN. FEB. MAR. APR. MAY JUNE JULY AUG
*— 1973
Fig. 34. Monthly variation of influent and effluent phosphate in
diffused air digester
63
-------
500
legend
VDS
CONDUCTIVITY—S-
N03 -CX3GCQjCGG6G&-
r.nn _ xxxxxx/7y"
450
400
350
o
o
x
300
o
250
u_
y—1
Ll_
UJ-
200
150
0
LlJ
o
o;
UJ
100
90
60
50
40
30
20
JAN. FEB. MAR. APR. MAY JUNE JULY AUG.
AUG. SEPT. OCT. NOV. DEC.
1973
Fig. 35. Monthly variation of selected lab data (influent
versus effluent) in diffused air digester
64
-------
volumetric standard unit (VSU) using previously determined
dimensions for each particular organism observed. The conver-
sion factors from no./liter to VSU (itim^/1) are listed in Appen-
dix A. No attempt was made to include bacterial biomass in the
enumerization because of size considerations. On a numerical
basis, the smaller motile flagellates and ciliates comprised
the great majority of organisms. When the numerical counts
were converted to VSU, rotifers were found to comprise the
great bulk of the invertebrate biomass. Table 8 summarizes the
invertebrate biomass VSU data for the period August 1972 -
August 1973. Table 9 analyzes the data in Table 8 with the
organisms in each of the six major taxonomic groups observed
listed as a percent of the total VSU. In Table 10 the inver-
tebrate VSU data is analyzed as a percent of the VSS under
aeration.
Changes in invertebrate populations appear to be related to
environmental stresses, particularly temperature change and
solids loadings. During August - November 1972, when temper-
atures were above 22°C and volumetric loadings were less than
1.6 kg VSS/m^/day (0.1 lb VSS/ft-Vday) / invertebrate diversity
was high with the rotifer population assuming nearly half of
the total dry weight biomass. During December 1972 when load-
ings increased to 3.0 kg VSS/m^/day (0.187 lb VSS/ft^/day) and
temperature declined to 16°C, rotifers disappeared and ecolo-
gical diversity declined. During this same period the percent-
age of flagellates and ciliates increased dramatically. Amoeba
disappeared with the disappearance of rotifers and reappeared
in May 1973, with the reappearance of rotifers. Nematodes were
absent during most of the test period with the exception of the
period May - June 1973 when several nematodes were observed.
The calculated dry weight of total invertebrate biomass as a
percent of the VSS under aeration (Table 10) approached a maxi-
mum of 54% during November 1972 and dropped to a low of less
than 3% during April of 1973.
The highest VSS reductions were observed to occur during those
months when invertebrate organisms comprised a significant
fraction of the VSS under aeration. No one group of organisms
was found to be an absolute indicator of ecosystem performance,
but the rotifer population appeared to have the most signifi-
cant correlation with VSS reduction (Figure 36). The coeffi-
cient of correlation between the rotifer population and the
percent VSS reduced was very high +0.87. Figure 36 shows that
the relationship between the percent VSS reduced and the total
invertebrate biomass had a lower coefficient of correlation
65
-------
LEGEND
© ROTIFERS
A TOTAL
INVERTEBRATES
30 40 50
PERCENT VSS REDUCED
Fig. 36. Total invertebrates and rotifer fraction of bioniass
versus percent VSS reduced
66
-------
Table 8. Monthly variation of invertebrate biomass in diffused air digester
(volumetric standard units - g/1 wet weight)
-JEAXONOMIC GROUP
MONTH" —.
FLAGELLATE
MOTILE
CILIATE
SESSILE
CILIATE
ROTIFER
AMOEBA
NEMATODE
TOTAL
ORGANISMS
AUGUST 1972
0.40
0.94
0.17
12.50
. 03
0
14. 04
SEPTEMBER
0.22
0.29
0.24
20.80
.006
0
21.56
OCTOBER
0.30
1.00
0.50
14.60
.02
0
16.42
NOVEMBER
1.30
0.50
0.50
25.00
.10
0
27.40
DECEMBER
2. 30
1.10
0.70
0
.08
0
4.18
JANUARY 197 3
5.00
0.60
3.00
0
0
0
8.60
FEBRUARY
1.80
4.00
5.00
0
0
0
10.80
MARCH
0.90
2.50
0.80
0
0
0
4.20
APRIL
0.30
0.70
0.80
0
0
0
1.80
MAY
0.72
0.16
0.35
8.30
.12
.03
9.68
JUNE
0.45
1.00
0.36
8.30
.05
.03
10.19
JULY
0.34
0.64
1. 02
1.16
.05
0
3.21
AUGUST
0.62
1.72
0.75
1.56
.06
0
4.71
AVG.
1.13
1.17
1.09
7.09
.04
.005
10.52
-------
Table 9. Monthly variation of invertebrate biomass percent distribution
in diffused air digester (VSU basis)
TAXONOMIC
_^ROUP
MONTff __
FLAGELLATE
MOTILE
CILIATE
SESSILE
CILIATE
ROTIFER
AMOEBA
NEMATODE
ECOLOGICAL
DIVERSITY
INDEX
OBSV / TOTAL
AUGUST 1972
2.85
6.70
1.20
89.00
0.25
0
5/6
(0.83)
SEPTEMBER
1.02
1.35
1.11
96.47
0.05
0
5/6
(0.83)
OCTOBER
1.83
6.09
3.05
88.92
0.11
0
5/6
(0,83)
NOVEMBER
4.74
1.82
1.82
91.24
0.38
0
5/6
(0.83)
DECEMBER
55.02
26.32
16.75
0
1.91
0
4/6
(0.67)
JANUARY 1973
58.14
6.98
34.88
0
0
0
3/6
(0.50)
FEBRUARY
16.67
37.04
46.30
0
0
0
3/6
(0,50)
MARCH
21.43
59.52
19.05
0
0
0
3/6
(0.50)
APRIL
16.67
38.89
44.44
0
0
0
3/6
(0.50)
MAY
7.44
1.65
3.62
85.74
1.24
0.31
6/6
(1.00)
JUNE
4.42
9.81
3.53
81.45
0.49
0.30
6/6
(1,00)
JULY
10.59
19.94
31.78
36.14
1.55
0
5/6
(0.83)
AUGUST
13.16
36.52
15.92
33.12
1.28
0
5/6
(0.83)
AVG.
16.46
19.43
17.19
46.31
0.56
0.05
-
(0.74)
-------
Table 10. Monthly variation of invertebrate biomass as a percent of VSS in diffused air
digester
TAXONOMIC
——___GROUP
month"""
FLAGELLATE
MOTILE
CILIATE
SESSILE
CILIATE
ROTIFER
AMOEBA
NEMATODE
TOTAL
PERCENT
OF VSS
VSS
INV.
fa/1.)
AUGUST 1972
.8
1.86
.34
24.70
.06
0
27.75
5.06
SEPTEMBER
.45
.60
.49
42.80
.01
0
44 .36
4.86
OCTOBER
.67
2.24
1.12
32.74
.04
0
36.82
4.46
NOVEMBER
2.57
.99
.99
49.41
.20
0
54.15
5.06
DECEMBER
3.22
1.54
.98
0
vll
0
5.85
7.14
JANUARY 1973
6.90
.83
4.14
0
0
0
11.86
7.25
FEBRUARY
3.09
6.86
8.58
0
0
0
18.52
5.83
MARCH
1.42
3.96
1.27
0
0
0
6.65
6.32
APRIL
.48
1.12
1.28
0
0
0
2.89
6.23
MAY
1.19
.26
.53
13.67
.20
.05
15.95
6.07
JUNE
.78
1.74
.63
14.46
.09
.05
17.75
5.74
JULY
.47
.89
1.42
1.62
.07
0
4.47
7.18
AUGUST
1.14
3.15
1.37
2.86
.11
0
8.63
5.46
AVG.
1.73
2.00
1.78
14.02
.07
0.008
19.67
5.90
-------
+0.73 than for rotifers only. Changes in taxonomic group pop-
ulation densities are depicted in Figure 37. In order to con-
vert the wet weight volumetric standard units listed in Table
8 to the total organism percent of VSS in Table 10, the inver-
tebrate counts were converted to a dry weight basis by assuming
that the biomass has the density of water with approximately
10% of the wet weight being equivalent to the dry weight.
The highly significant correlation between rotifer VSU and
aerobic digester performance helps to explain a major differ-
ence between aerobic and anaerobic digestion systems. In the
anaerobic ecosystem the dominant organisms are bacteria (vola-
tile acid and methane producers), whereas in the aerobic eco-
system higher trophic levels of organisms consume lower forms,
e.g. protozoa feed on bacteria and in turn are fed upon by the
metazoan rotifers. In the aerobic system energy is dissipated
as heat, whereas in the anaerobic system most of the energy is
lost as carbon dioxide and methane. In the aerobic system, the
newly synthesized biomass h&s virtually the same VSS/TSS ratio
as the biomass originally loaded to the system. Autotrophic
organisms in the aerobic system can use the waste CC>2 to pro-
duce new biomass. In the anaerobic system, the resynthesis
rate is lower than in the aerobic system with consequent greater
decline in the VSS/TSS ratio of the residue.
Bacterial Reductions
Figure 38 compares aerobic and anaerobic bacterial reductions
versus detention time in the digester. The diffused air diges-
ter data represents optimal temperature conditions at an SRT of
8 days. The anaerobic digester data was obtained from a neigh-
boring treatment plant (Denver Northside anaerobic digesters)
during 1971 when SRT averaged 28 days. At an equivalent SRT
(8 days) the anaerobic total bacteria count declined by only
25% (10®/ml) while the aerobic total bacterial count dropped
by 97% (10^/ml). If bacteria were the only life forms in both
systems, one would expect a significantly greater ratio of ac-
tive mass/VSS in the anaerobic system after 8 days SRT.
Analysis of ATP as a measure of active biomass did not show any
significant difference in 'the ATP/VSS ratio between the two
systems.
70
-------
30
29
28
27
QUH3I
26
ROTIFERS
25
FLAGELLATES
24
23
MOTILE CtLIATES
SESSILE CILIATES
20
AMOEBA
GO
NEMATODES
O
CO
o
D£
10 —
o
3 —
2 —
DEC. ' JAN
AUG
SEP
OCT
NOV
FEB MAR
JUN
JUL
AUG
1972 1973
Fig. 37. Monthly variation of invertebrate biomass (VSU) h*
diffused air digester
71
-------
100—I
9
CO
LEGEND
ANAEROBIC DIGESTION (PRIMARY!
30 DAY DETENTION TIME
AEROBIC DIGESTION (OF WAS)
6 DAY DETENTION TfME
AVG. BACTERIAL CONC.-
UNDIGESTED SLUDGE NO./«nl
\c
TOTAL
T. COLl
F. COLl
5.4x10
LOxlO
8.6 *10
WAS
7.0 x.10
£?x 10
.5 x 10
26
26
20
22
24
DETENTION TIME (DAYS)
Fig. 38. Aerobic and anaerobic bacterial. reduction versus deten-
tion time in digester
72
-------
The concentration of ATP as determined by the luciferase tris-
buffer extraction method (10) was 0.34 mg/1 for the aerobic
digester effluent and 0.60 mg/1 for the anaerobic digester
effluent. The VSS concentration of the aerobic sludge was 6.0
g/1 versus 15.0 g/1 for the anaerobic sludge. The ratio of
ATP/VSS x 10^ was therefore 58 for the aerobic sludge and 40
for the anaerobic sludge. The fact that the aerobic ATP/VSS
ratio was greater than the anaerobic ratio can be explained on
the basis of invertebrate ATP being significantly higher in the
aerobic system. As previously discussed, data in Table 10
indicated that invertebrate biomass in the aerobic digester
could exceed 50% of the VSS under aeration on a dry weight
basis.
Fecal Coliform Bacteria
The concentration of fecal coli in the WAS fed to the aerobic
digester was found to range between 6 and 50 x 10 /ml, averaging
14.2 x 106/ml (Table 3). Fecal coli in the aerobic digester
effluent ranged between 0.5 and 17.3 x 10^/ml, averaging 3.6 x
106/ml. Fecal coli reductions ranged between 13.5% in January
1973 and 97.1% in September 1972. As temperature declined and
organic loadings to the system increased, fecal coli reductions
tended to decline.
Comparison of Aerobic and Anaerobic Stabilization
When Metro activated sludge is anaerobically digested for
thirty days, the VSS/TSS ratio is reduced from approximately
80% to 60%. The amount of carbon lost from the anaerobic
digester as methane and carbon dioxide is greater than the car-
bon dioxide lost from the aerobic system. When Metro WAS is
aerobically digested for thirty days, the VSS/TSS ratios typi-
cally decline by less than 10% from approximately 80% to 72%.
The concept "stabilization"nust therefore be operationally de-
fined in relation to biodegradation and odor potential parti-
cularly when digested sludges are to be disposed of on land.
If aerobically stabilized sludge with a volatile solids residue
of about 76% is loaded to an anaerobic digester (SRT = 18 days)
the VSS/TSS ratio can be further reduced to 62% (Figure 39).
73
-------
100
95
90
85
80
75
70
65
60
50
40
30
20
10
0
AEROBIC
DIGESTER
INFLUENT
f/////7j
€FFJ_UENT
ANAEROBIC
DIGESTER
INFLUENT
//J LU
LEGEND
I-AEROBIC DIG.(RAW WAS)
II-ANAEROBIC DIG OF
(AEROB.DIG.WAS)
HI-ANAEROBIC DIG. OF
CRAW PRIMARY")
(.RAW WAS J
33Z-ANAEROBIC DIGESTION
OF RAW PRIMARY
A? I2«X
EFFLUENT^
EFFLUENT,
n
ANAEROBIC
DIGESTER ANAEROBIC
INFLUENT DIGESTER
INFLUENT
,E££LyEN"&
ni
rz
DIGESTION SLUDGE TYPES
|g. 39. Percent volatile solids reduction under aerobic
and anaerobic conditions
74
-------
This "double digested" material can be spread on land without
fear of subsequent odor problems which might develop if the
sludge was spread too heavily on land or otherwise allowed to
go septic. This double digested sludge is approximately equiv-
alent to the end product obtained after 20 days of anaerobically
digesting primary sludge. Figure 40 contrasts the quality of
aerobic and anaerobic digester supernatants. Metro aerobically
digested supernatant recycle loadings are much less concentrated
than anaerobic digestion supernatants from the neighboring Den-
ver Northside plant.
A comparison of aerobic digester supernatant quality with anaer-
obic digester supernatant from a Metro pilot plant operated
during 1973 indicated that COD, TKN, and NH4 concentrations in
the aerobic supernatant averaged less than 15% of the anaerobic
concentrations. The BOD and TSS concentration in the aerobic
supernatant averaged less than 10% of the anaerobic concentra-
tion. Figure 41 shows the monthly variations in the diffused
air digester supernatant quality for the period February -
August 1973.
Odor Panel Results
The most important indicator of sludge stability from an aesth-
etic viewpoint is odor. In order to define this problem quan-
titatively, a seven person panel (composed of five male and two
female Metro staff personnel) was formed to periodically moni-
tor the odor potential of a 190 litre (50 gal) sample spread
over a 0.93 m (10 ft^) plot.
The panelists individually listed their reactions based on the
following scale;
0 - Not Detectable 3 - Detectable - Ob-
jectionable
1 - Very Faintly Detectable
4 - Detectable - Very
2 - Detectable - Not Objectionable Objectionable
On October 25, 1972 three separate sludge test plots were pre-
pared consisting of:
1. Anaerobically Digested Primary Sludge
75
-------
5000
4000
3000
CD
£
O
c
C£
o
CJ
2000
1500
1000
500
N H/t-N
T.S.S
C.O.D
LEGEND
AEROBIC
ANAER08IC EZZZZ3
1
7/
TOTAL
SOLIDS
^ORG^N. P04-P
Fig. 40. A comparison of supernatant quality under
aerobic and anaerobic conditions
76
-------
AV6=I>0
AVG * 40
AVG =85
AV5«II5
3000
2000
1500
1000
AVG- 700
AVG s
3000
f800 vN.
AVG -
«- 1000
Fea
MAR.
APRIL
MAY
1973
JUNE
JULY
AUG.
Fig. 41. Diffused air digester supernatant quality
monthly variation
77
-------
2. Aerobically Digested Sludge
3. Mixture (1:1 Ratio) of Sludges 1 and 2
Figure 42 summarizes odor results for the three sludge mixtures
during a twenty-eight day period. The aerobically digested
sludge (VSS/TSS ratio of 75%) had the least offensive odor
initially. When this sludge was mixed in a 1:1 ratio with anae-
robically digested sludge (VSS/TSS ratio of 60%) however, a
definitely objectionable odor occurred. The resultant "pig pen"
odor was so objectionable that large scale plans to land spread
a mixture of aerobically and anaerobically digested sludges had
to be cancelled.
These results, which were repeated both indoors and outdoors,
indicated that the so called "non-biodegradable" fraction of
the aerobically digested VSS can be utilized by a heterogeneous
anaerobic bacterial culture. Thus offensive odors are a defi-
nite possibility if either aerobic or anaerobic sludge residues
have a VSS/TSS ratio above 60%.
Sludge Dewaterability
The major impetus for investigating the aerobic stabilization
of waste activated sludge arose from the need to reduce handling
and disposal costs. Although the existing solids handling unit
processes at Metro included dissolved air flotation, vacuum
filtration and land application, sand drying bed dewatering was
also investigated, as this method is commonly used in small
treatment plants. Specific resistance of sludges to vacuum fil-
tration as determined from Buchner Funnel tests, indicated no
significant difference in dewaterability between sludges before
and after aerobic stabilization (rs=10^ sec2/g) where SRT ranged
between 3.1 and 13.4 days.
Vacuum filter leaf tests were run on dissolved air flotation
thickened samples of influent and effluent from the test di-
gester to obtain information on relative loading rates and
chemical demand. Table 11 summarizes the filter leaf data, com-
paring relative performances based upon chemical cost ($/ton),
filter cake (percent TS) and loading rates kg/m2/hr (lb/ft^/hr).
The results obtained indicate that for an equivalent chemical
cost, better vacuum filter performance was obtained with the
aerobically digested sludge. For example, on July 10, 1973
78
-------
LEG£NO
D-AEROBIC SLUOGE
2)' l-l MIX SLUDGE
3)-ANAER08lC SLUDGE
1973
Fig. 42. Odor panel results for aerobic and anaerobic sludge
-------
Table 11. Vacuum filter leaf test results of diffused air digester influent
and effluent
PERCENT TS
CONC.
FACTOR
(1)
PERCENT CHEMICALS
CHEMICAL
COST
(1) S/ton
LOADING
RATE (3)
(lb/ft /hr
VACUUM FILTER
PERFORMANCE (2)
DATE
SAMPLE
Feed
Cake
FeCl-j
Lime
Total
3/27/
73
EFF
4.1
II
If
15.1
15.0
13.6
3.7
3.7
3.3
7.4
6.2
9.9
26.8
22.4
33.6
34.2
28.6
43.5
14.10
11.80
18.30
2.1
1.9
2.5
1'8*
1.7 *
2.2
7/10
INF
4.6
II
10.1
11.9
13.1
2.2
2.6
2.8
5.0
5.0
9.9
4.9
9.9
19.8
9.9
14.9
29.7
6.23
7.48
14.85
0.5
0.9
1.9
5*7*
3.2
2.8
EFF
4.6
ii
«
10.2
12.2
13.3
2.2
2.7
2.9
4.9
4.9
9.9
4.9
9.8
19.7
9.8
14.7
29.6
6.13
7.35
14.83
0.6
0.9
3.0
4.6
3.0
1.7 *
8/8
INF
4.9
•i
ii
12.9
13.5
15.4
2.6
2.8
3.1
3.1
5.1
8.1
6.1
10.2
16.3
9.2
15.3
24.4
4.63
7.65
12.18
0.8
1.3
3.0
2.2
2.1
1.3*
EFF
4.7
ii
12.0
13.2
14.0
2.6
2.8
3.0
4.3
5.3
8.6
8.6
10.8
17.2
12.9
16.1
25.8
6.45
8.00
12.90
1.0
1.4
2.9
2.5
2.0
1.5*
8/22
INF
4.0
ii
12.9
14.9
3.2
3.7
6.5
9.3
12.9
18.6
19.4
27.9
9.73
13.95
0.5
1.1
6.1
3.1*
EFF
5.1
ii
H
11.8
12.3
14.3
2.3
2.4
2.8
4.5
5.2
6.7
8.9
10.3
13.3
13.4
15.5
20.0
6.73
7.78
11.70
1.4
1.6
2.5
2.1
2-°*
1.7
(1)FeCl3 = $100/Ton;Lime = $25/Ton* optimal dose
(2) Cone. FactorTxnLoad Rate (best Performance = low«t va^)
(3) lb/ft^/hr x 4.88 = kg/m^/hr
-------
the vacuum filter production rate after aerobic digestion was
50% greater at an equivalent chemical dosage than before aero-
bic digestion.
In order to obtain additional data concerning the effects of
aerobic digestion on dewatering characteristics of the sludge,
four model size sand drying beds were built according to the
description of Randall and Kochi^*^ To determine the relative
dewaterability of aerobic and anaerobic sludge mixtures, 20
liters of each sludge sample were applied to separate sand fil-
ters and cumulative filtrate volumes recorded for 7.5 days
(180 hours). The 180 hour drainage time was chosen as a perfor-
mance standard and calculated on the basis of volume filtered/
volume applied. Table 12 summarizes the results obtained. On
a direct volumetric measurement basis, the filtration rates of
the aerobically digested and undigested WAS sludges were approxi-
mately equal. Anaerobically digested sludge drained slowest,
while the 1:1 volumetric ratio of aerobic and anaerobic sludges
showed intermediate drainability. Aerobically digested sludge
drained three times as fast as anaerobically digested sludge on
a volumetric basis. Since a waste treatment plant must process
a given amount of sludge mass per day, the comparison of drain-
age rates should be made on a mass basis. When this comparison
is made, the anaerobically digested sludge was found to drain
2.5 times faster than the aerobically digested sludge. Table
13 summarizes the effect of VSS loading rates on the relative
drainability of aerobically digested and undigested sludges.
On a volumetric comparison basis, there is little difference
in drainability between aerobic digester influent and effluent.
On a solids weighted basis, the undigested sludge drains 20%
faster than the digested sludge. Although the aerobic digester
effluent appears to drain slower on a weight basis, there is
less mass to filter due to mass reduction in the digester.
The thickening of aerobically digested sludge by air flotation
was affected by the particle surface area available for catio-
nic polymer conditioning. An inverse relationship was observed
between polymer demand during air flotation and sludge loading
rates to the aerobic digester. The effect of varying loading
rates on dissolved air flotation polymer demand is illustrated
in Figure 43.
81
-------
90
80
LEGEND (POLYMER)
AMERICAN CYANAM1D 575C
70-
60
cn
*3-
LO
«5j"
50
O
X
40
c
o
+¦>
30-
UNDIGESTED
25
20
>-
i
o
Q_
0.10
0.20
0.25
0.05
0.15
DIGESTER LOADING RATE lb VSS/ft3/day (X 16.02 - kg/
m 3/day)
Fig. 43. Air flotation polymer demand versus diffused air
digester loading rate
82
-------
Table 12. A comparison of sand filtration results with aerobic and
anaerobic digested sludge
•
FILTERED RATIO
U)
RATIO AER0B- (3)
KA11° ANAER.
DATE
A. DIG1
INF
:stion
EFF
ANAEROBIC
EFFLUENT
MIXED
EFFLUENT (2)
VOLUMETRIC
BASIS
MASS
BASIS
1/3/7 3
0.65
0.61
-
-
-
-
1/17
0.60
0.69
-
-
-
-
1/31
0.630
0.650
-
-
-
-
2/13
0.510
0. 595
0.240
0.501
2.48
0.32
3/19
0.615
0.615
0.270
0.450
2.28
0.31
4/19
0.445
0. 755
0.315
0.650
2.40
0.29
5/9
0.685
0.600
0.125
0.075
4.80
0.65
5/29
0.650
0.735
0.110
0.360
6.68
0.91
6/8
0.650
0.740
0.0
0.300
-
-
6/19
0.660
0.735
0.300
0.540
2.45
0.31
6/29
0.590
0.715
0.290
0.495
2.47
0.31
7/9
0.625
0.690
0.325
0.550
2.12
0.35
7/24
0.700
0.705
0.355
0.605
2.00
0.26
8/2
0.690
0.690
0.240
0.425
2.38
0.31
8/13
0.745
0.740
0.0
0.115
-
-
AVG.
0.630
0.684
0.218
0.422
3.00
0.40
(1) volume filtrate collected 9180 hrs
volume digested sludge applied to sand filter
(2) 1:1 ratio of aerobic and anaerobic digester effluent
(3) volume aerobic digester effluent filtrate
volume anaerobic digester effluent filtrate
83
-------
Table 13. Effect of loading rate to diffused air digester on sand filtration rate
(volume filtered/volume applied)
VSS LOADING
1 (1)
lb/ft' /day
DIGESTER
INFLUENT
DIGESTER
EFFLUENT
VOLUMETRIC RATIO
EFF
INF
VSS RATIO
EFF
INF
MASS BASIS
RATIO
EFF/INF
.026
0.445
0.755
1.70
0.61
1.02
.078
0.668
0.668
1.00
0.51
0.51
. 086
0.633
0.730
1.15
0.59
0.68
.140
0.718
0.715
o
o
0.82
0.82
.150
0.647
0.670
1.04
0.77
0.80
.187
0.598
0.636
1.06
0.86
0.91
AVG. -
-
-
1.10
-
0.79
(1) lb/ft^/day x 16.02 = kg/m^/day
-------
Undigested WAS had an average cationic polymer demand of 11.4
kg/ton (25 lb/ton). Average daily production of WAS before
aerobic digestion was 40 tons/day. Cationic polymer cost at
Metro without aerobic digestion averaged $91,000/year. When
aerobic digestion of WAS was initiated at Metro, it was found
that polymer demand increased from 13.7 kg/ton (30 lb/ton) at
a loading to the digester of 1.6 kg VSS/m^/day (0.10 lb vSS/ft^/
day) to 41.1 kg/ton (90 lb/ton) at a loading of 0.8 kg VSS/m^/day
(0.05 lb VSS/ft^/day). If the optimal loading to the digester of
1.6 kg VSS/m^/day was maintained, 16 tons VSS/day were digested
leaving a residue of 24 tons VSS/day requiring air flotation at
a cost of $66,000/year. When the sub-optimal loadings to the
digester of 0.8 kg VSS/m-^/day were maintained 18 tons VSS/day
were digested leaving a residue of 22 tons/day requiring air
flotation at a cost of $181,000 per year. Strict control of
loadings to the aerobic digester must be maintained if the ad-
vantages of mass reduction are not to be lost in the subsequent
air flotation thickening process.
85
-------
EXPERIMENTAL RESULTS - PURE OXYGEN BATCH TESTS
Batch Test No. 1
The first pure oxygen batch test with concentrated WAS (5,1% TSS)
was run between November 30 and December 20, 1972. Only one of
the 6.8 m (1800 gal) tanks was used to determine the mechanical
and operational adequacy of the system. Three FAD diffusers
were used in the single tank.
Experimental data for Batch Test 1 are presented in Tables 14,
15 and 16. Table 14 summarizes data from laboratory analysis.
Table 15 presents average operating data and Table 16 indicates
the calculated values for TSS and VSS reductions.
Cyclical variations in solids concentrations and calculated TSS
and VSS reduction are evident. The sampling and laboratory analy-
ses were checked to verify the accuracy of the analytical tech-
niques. The methods were found to be accurate.
The cyclical results were attributed to fluctuations between
soluble and insoluble VS and TS. Soluble VS and TS were not
measured frequently enough in this batch test to verify this
conjecture.
The sludge temperature increased from (19.4°C) initially to a
maximum value of (44.5 C). This uncontrolled temperature in-
crease may have been responsible for significant changes in
microorganism populations during the test period. Installation
of a water jacket heat exchanger was recommended to correct
this situation. A leak in the sludge recirculation pump pack-
ing was noted on December 7, 1972 but not until approximately
15.3 cm (6 in.) of sludge had been lost from the system. A
float measuring device was provided to accurately indicate
liquid level for detection of this type of failure. The dis-
solved oxygen concentration remained below 1.0 mg/1 for the
first nine days, because of very high O^ uptake rates (in
excess of 200 mg/l/hr).
86
-------
Table 14. Pure oxygen digester batch test no. 1 - laboratory data (mg/1 unless
other units indicated)
>ate
P«
(Unitrtl
CONDUCTIVITY
iiwho/cin^
SUSPENDED SOLIDS
COD
NITRO
CEN-N
ALKALINITY
(.18 CiiC03>
FECAL
COL I
(flO./ml)
TSS
vss
TDS
EDS
VDS
%
no3
nii3
T K N
Ll/30
6.5
1,090
51,300
44,400
2,220
1,464
756
82,200
1,130
"C.Ol
136
3,730
2,460
1.8 x 107
12/1
6.6
1,270
42,500
37,700
2,420
12/2
6.5
4,850
38,900
33,100
2,900
12/3
6.8
5,310
37,200
30,800
3,380
12/4
7.0
6,000
38,800
31,400
3,620
12/5
7.1
6,060
45,200
37,200
3,970
12/6
7.2
5,240
30,600
26,400
3,750
12/7
7.2
6,292
24,600
20,000
3,690
12/8
7.3
5,140
32,800
26,800
4,050
12/9
7.3
6,410
20,400
17,000
12/10
7.3
6,410
26,400
22,800
12/11
7.6
7,660
28,200
22,400
12/12
7.6
5,620
24,400
16,600
12,200
1,700
10,500
3,940
L2/13
7.6
5,880
28,200
22,200
12/14
7.5
5,970
29,800
22,200
12/15
7.2
5,150
N.A.
N.A.
12/16
N.A.
N.A.
37,000
30,200
12/17
7.1
8,040
33,200
24,800
L2/18
7.4
5,800
20,300
16,000
4,690
12/19
7.3
9,190
25,400
18,000
12/20
7.8
8,100
26,300
22,200
9,800
5,800
4,000
41,100
744
1.0
N.A.
3,540
5,490
42.0 x 105
-------
Table 15. Pure oxygen digester batch test no. 1 - field data
DATE
AVERAGE D.O.
faK/l)
AVERAGE SLUDGE
TEMP. f°F) (1)
AVERAGE 0. UPTAKE
mg/i/hr
ns/hr/g (V'SS)
11/30/72
0.8
72 (Initial 67°)
N.A.
N.A.
12/1/72
0.5
82
N.A.
N.A.
L2/2
0.6
91
N.A.
N.A.
12/3
0.6
94
N.A.
N.A.
L2/4
0.6
88
N.A.
N.A.
12/5
0.7
88
N.A.
N.A.
L2/6
0.6
87
N.A.
N.A.
12/7
0.5
90
N.A.
N.A.
12/8
0.4
95
N.A.
N.A.
12/9
0.9
99
N.A.
N.A.
12/10
1.5
106
N.A.
N.A.
12/11
3.7
108 (high 112°)
N.A.
N.A.
12/12
13.3
97
34
2.05
12/13
16.9
94
36
1.62
12/14
17.7
92
52
2.34
12/15
2.0
91
52
6.67
12/16
8.2
91
54
1.79
12/17
11.5
89
39
1.57
12/18
2.7
90
44
2.75
12/19
2.6
90
47
2.61
12/20
2.8
90
61
2.75
(1) (°F-32) 5/9 = °C
88
-------
Table 16. Pure oxygen digester batch test no. 1 - inventory
mass reduction
LIQUID
SOLIDS INVENTORY
PERCENT
INVENTORY
TSS
VSS
TSS
VSS
MASS REDUCTION
DATE
(lb) (1)
feg/l)
feg/l)
(lb)(1)
( lb )(1)
TSS
VSS
11/30/72
14276
51,300
44,400
732.4
633.9
—
—
12/1/72
14019
42,500
37,700
595.8
528.5
18.6
16.6
L2/2/72
13983
38,900
33,100
543.9
462.8
25.7
27.0
L2/3/72
13946
37,200
30,800
518.8
429.S
29.2
32.2
L2/4/72
13873
38,800
31,400
538.3
435.6
26.5
31.3
L2/5/72
13836
45,200
37,200
625.4
514.7
14.6
18.8
12/6/72
13800
30,600
26,400
422.3
364.3
42.3
42.5
L2/7/72
13727
24,600
20,000
337.7
274.5
53.9
56.7
L2/8y72
13690
32,800
26,800
449.0
366.9
38.7
42.1
L2/9/72
13653
20,400
17,000
278.5
232.1
62.0
63.4
L2/10/72
13617
26,400
22,800
359.5
310.5
50.9
51.0
L2/11/72
13544
28,200
22,400
381.9
303.4
47.8
52.1
12/12/72
13507
24,400
16,600
329.6
224.2
55.0
64.6
12/13/72
13470
28,200
22,200
379.9
299.0
48.1
52.8
L2/14/72
13434
29,800
22,200
400.3
298.2
45.3
52.9
12/15/72
HA
NA
NA
NA
NA
NA
NA
12/16/72
13361
37,000
30,200
494.4
403.5
32.5
36.3
12/17/72
13324
33,200
24,800
442.4
330.4
39.6
47.9
12/18/72
13287
20,300
16,000
269.7
212.6
63.2
66.5
L2/19/72
13251
25,400
18,000
336.6
238.5
54.0
62.4
12/20/72
13241
26,300
22,200
347.5
293.4
52.5
53.7
(1) lb x 0.454 = kg
89
-------
On December 12, 1973, IX) concentration rose suddenly accompanied
by a sudden drop in temperature to 32.8 C , and a drop in DO
uptake rates. These three changes indicated that the initial
high rate of metabolic activity had altered qualitatively, as
well as quantatively on the twelfth day of the test.
The pH rose from an initial 6.5 to 7.6,then gradually declined
to 7.1, and then increased again to a maximum of 7.8 on the
final day. These fluctuations correlated with the increasing
alkalinity (+223%) of the system. Volatile organic acids
which can be easily driven off at the high temperatures en-
countered up to December 12, 1973 would account for this in-
crease in alkalinity. Conductivity increased eight fold between
initial and final sampling. The cyclical nature of this increase
may be related to resynthesis of new bacterial biomass, utiliz-
ing metabolic intermediates. The VSS concentration declined by
50% after 20 days. This reduction was almost identical with that
achieved at the end of 10 days. This cyclical phenomenon was
observed in all of the subsequent pure 0^ batch tests. TDS and
VDS increased more than four fold between the initial and final
samples (+441% and +529% respectively). The COD reduction of
50% correlated well with the VSS reduction previously noted.
Nitrification did not occur to any substantial degree. The
nitrifying bacteria may have been inhibited by the high temper-
ature experienced during this test. The very slight decrease
in TKN might reflect accelerated resynthesis at elevated tem-
peratures. Fecal coliform bacteria declined by 98.9%. The VSS
reduction rate fell below 3% per day after 16 days detention
time.
Batch Test No. 2
Undigested concentrated WAS was used (3.63% TSS) and improve-
ments in sampling and temperature control were instituted to
avoid problems encountered during the first batch test.
Experimental data for Batch Test 2 are presented in Tables 17,
18 and 19. Table 17 summarizes data from laboratory analysis.
Table 18 presents average operating data and Table 19 indicates
the calculated values for TSS and VSS reductions.
Figure 44 is a DO profile taken at fifteen separate locations
within the test tank to demonstrate the homogenity and adequate
mixing of the biomass undergoing oxygenation.
90
-------
E
N
W
I-
«-
PLAN ELEVATION
LOCATION P.O. (mq/1) LOCATION P.O. tmg/l) LOCATION AA(m|/1)
N-l
0.8
S- 1
0.6
M- 1
0.55
N ~2
0.4
S-2
0. 6
M.2
0.55
NE-3
0.4
SW-3
0.6
M-3
0.60
E- 1
0.3
w-l
0.7
e-z
0.3
W-2
0.4
SE-3
0.4
NW-3
0.5
Fig. 44.
Pure oxygen batch test no. 2 DO profile
91
-------
Table 17. Pure oxygen digester batch test no. 2 - laboratory data (mg/1
unless other units indicated)
DATE
PH
Units
CONDUCTIVITY
jMtnhp/cm^
SUSPENDED
SOLIDS
DISSOLVED
SOLIDS
NITROGEN-N
ALKALINITY
*9 C«C03
FECAL
COLI
no./ml
TSS
vss
TDS
VDS
COD
TO4
«°3
m3
TKN
INITIAL
L/lO/73
6.0
570
36,300
30,000
1,200
353
51,500
888
0.14
66
3,070
2,000
6.1 x 107
1/10
6.3
900
33,600
28,700
2,425
1,405
l/U
6.2
2,500
31,000
26,600
3,410
2,085
1/12
6.6
2,500
29,300
25,300
3,800
2,455
1/13
6.9
NA
23,900
19,100
NA
2,790
2,420
1/14
6.1
1,100
23,200
18,500
3,420.
2,000
1/15
7.4
6,960
24,700
20,100
3,640
2,120
1/16
7.2
7,200
26,700
23,300
3,250
1,850
1/17
7.6
6,300
20,200
16,400
3,300
1,870
1/18
6.3
4,700
19,700
15,800
1,660
1,980
1/19
6.7
4,850
17,600
15,800
3,650
2,000
1/20
6.3
4,640
17,800
15,600
3,710
2,140
1/21
6.4
4,600
19,400
13,300
4,130
2,430
1,220
1/22
L.A.
L.A.
22,100
16,100
4,110
2,260
1/23
6. S
4,180
17,300
13,400
4,360
2,740
L/24
TINAL
6.6
6,250
21,200
15,900
3,970
2,480
24,700
NA
NA
700
2,220
1,400
2.0 x 107
-------
Table 18. Pure oxygen digester batch test no. 2 - field data
DATE
OXYGEN PRESS.
(2">
OXYGEN FLCM '
OXYGEN nv
°2 SUPPLY
D.O.
&IR/1)
SLUDGE (3)
TEMP. (°n
1 & 2
3
1 & 2
3
TEMP. (°fS
(scfd )(4)
(lb/day) (5)
1/10/73
22.6
23.0
1.72
0.35
90.1
2,393
199
3.6
82.1
1/11
20.6
20.5
1.33
0.26
83.9
1,706
142
3.4
91.6
1/12
20.6
20.6
1.30
0.26
68.7
1,706
142
N.A.
93.6
1/13
21.0
21.0
1.21
0.25
72.5
1,555
129
13.4
95.0
1/14
17.2
18.1
0.65
0.18
72.9
787
65
7.3
91.0
1/15
25.2
21.2
0.61
0.23
78.4
989
82
5.2
89.4
1/16
20.4
19.2
0.42
0.21
79.7
659
55
6.2
79.5
1/17
17.3
17.5
0.32
0.20
73.1
493
41
5.4
76.2
L/18
15.9
15.8
0.27
0.18
67.2
400
33
12.6
75.2
1/19
15.2
14.6
0.25
0.18
68.2
371
31
16.7
77.7
1/20
15.0
15.0
0.25
0.16
64.8
352
29
13.8
70.6
L/21
14.9
14.0
0.25
0.16
63.3
345
29
15.1
69.6
1/22
15.1
16.2
0.25
0.15
83.4
362
30
16.7
72.2
1/23
15.9
—
0.25
--
78.4
227
19
15.9
76.5
(1) psig (diffusers 1-2 and 3) x 0.0703 = kg/cm2
(2) cfm (diffusers 1-2 and 3) x 0.472 = ]/sec
(3) (°F-32) 5/9 = °C
(4) scfc x 0.0283 = m3/day at STP
(5) lb/day x 0.454 = kg/day
-------
Table 19. Pure oxygen digester batch test no. 2
inventory mass reduction
DATE
LIQUID
INVENTORY
db)fl)
TSS
frg/1 )
VSS
4=8/1)
SOLIDS INVENTORY
PERCENT
MASS REDl'CTIOJ
TSS
(lb) (1)
VSS
(lb)(1)
TSS
VSS
INITIAL
1/10/73
14,349
36,300
30,000
520.9
430.5
—
—
1/10 COK?
14,130
33,600
28,700
474.8
405.5
8.9
5.8
1-/11
12,227
31,000
26,600
379.0
325.2
27.2
24.4
1/12
12,044
29,300
25,300
352.9
304.7
32.2
29.2
1/13
11,883
23,900
19,100
284.0
227.0
45.5
47.3
1/14
11,692
23,200
18,500
271,2
216.3
47.9
49.8
1/15
11,531
24,700
20,100
284.8
231.8
45.3
46.1
1/16
12,995
26,700
23,300
347.0
302.8
33.4
29.7
1/17
12,922
20,200
16,400
261.0
211.9
49.9
50.8
1/18
12,812
19,700
15,800
252.4
202.4
51.5
53.0
1/19
12,702
17,600
15,800
223.6
200,7
57.1
53.4
1/20
12,629
17,800
15,600
224.8
197.0
56.8
54.2
1/21
12,519
19,400
13,300
242.9
166.5
53.4
61.3
L/22
12,446
22,100
16,100
275.1
200.4
47.2
53.4
1/23
12,373
17,300
13,400
214.0
165.8
58.9
61.5
1/24
"INAL
12,263
21,200
15,900
260.0
195.0
50.1
54.7
(1) lb x 0.454 = kg
94
-------
An air calibrated DO meter accurate to ± 0.2 mg/1 was used for
this purpose. The use of a water jacket heat exchanger for the
second test kept the sludge temperature below 37.8°C (100°F),
the overall average being 26.9°C (81°F).
Excessive foaming was observed during the second day of this
test and continued to be a problem for the duration of the test.
The foaming caused the loss of solids from the system which
lowered the sludge level to the vicinity of the pump suction.
The lowered liquid level accentuated the foaming problem even
more requiring 623 liters (165 gal) of water to be added on
January 16, 1973.
DO Uptake
A YSI model 54 oxygen meter with a model 5420A BOD polarographic
probe in a standard 300 ml BOD bottle was used to measure the
oxygen respiration rate. The sample was oxygenated prior to
analysis by diffusing pure O2 into the liquid using a carborun-
dum stone diffuser. Measurement of DO decline versus time was
taken once per minute until a DO concentration of less than 1.0
mg/1 was observed. This method, although satisfactory for dil-
ute WAS (0.5 to 1.0% TSS) was found to be inadequate for the
more concentrated sludges (4.0 to 5.0% TS) used in the pure oxy-
gen batch tests. The viscous nature of the polymer conditioned
and air flotation thickened sludge caused erratic readings from
gaseous oxygen collecting within the sample and on the probe.
Therefore no attempt was made to calculate O2 transfer efficien-
cies for this test.
Although a relatively high DO was maintained (up to 15.6 mg/1)
no difference was observed in the rates of solids digestion
that could be related to the DO concentrations. Increasing pH
followed by a decline was observed during this test. The differ-
ence between initial and final pH values during the second batch
test was much less (0.6 pH units) than for the first test (1.3
units). High temperatures were not a factor during the second
test. Volatile organic acids losses were therefore less than
during the first test. Conductivity increased eleven fold be-
tween initial and final samples. VSS were reduced by 53% after
14 days. This percent reduction was also observed after 7 days.
TDS and VDS values increased rapidly during the first four days
of the test, with final values on the fourteenth day closely
95
-------
approximating values obtained on the fourth day.
COD was reduced by 52%, correlating well with the VSS reduction
previously noted. A very high deamination rate resulted in a
more than ten fold increase in NH^ concentration between initial
and final samples. TKN was reduced by 27.7%.
The lab data in Table 17 have not been corrected for evapor-
ation losses. On a total inventory basis, the percent reduc-
tion is higher than indicated in this table. Fecal coliform
reduction during this test was 67.2%.
Batch Test. No. 3
Diffused air aerobically digested sludge was used in this test.
The small VSS reduction observed in this test is attributed to
the relatively low initial VSS/TSS ratio. The foaming problem
experienced during batch test No. 2 was corrected during this
test by the installation of a foam suppression system consist-
ing of a recycle pump and spray nozzles.
Experimental data for Batch Test 3 are presented in Tables 20,
21 and 22. Table 20 summarizes laboratory analyses. Table 21
presents average operating data and Table 22 shows calculations
of TSS and VSS reductions.
Because of the relatively low respiration rates and resultant
high oxygen concentration that occurred midway through this test,
two of the three diffusers installed in the system were removed.
The previously mentioned problems with the method of DO uptake
measurement were not resolved during this test. Virtually no
change was noted between the initial and final pH. The 2.5
fold conductivity increase was less than for previous tests.
VSS concentration declined by only 22.2% and the VSS/TSS ratio
of the final sample was 2.7% higher (82.6%) than the initial
sample (79.9%). TDS increased approximately 3.6 fold.
Ammonium-N in the final sample was 20% higher than the initial
sample and the alkalinity declined by 52.2%. From the above
data it was apparent that this test was not typical of pure
oxygen digestion performance with undigested WAS.
96
-------
Table 20. Pure oxygen digester batch test no. 3 - laboratory data (mg/1
unless other units indicated)
pll
COND.
jimho/cm
SUSPENDED
SOLIDS
DISSOLVED
SOLIDS
NITROGEN
ALKALINITY
FECAL
COL I
DATE
(Units;
TSS
vss
TDS
VDS
COO
P°4
N03
nh4
TRN
(as CaC03)
no./ml
INITIAL
6.4
1440
45700
36500
1610
533
57700
1150
0.26
250
3130
1160
1.3x10*
1/31/73
6.4
1770
42100
33400
2715
1525
2/1
6.8
2440
36500
30000
3580
2345
2/2
7.1
2200
35000
28000
3920
2560
2/3
6.7
2570
44400
32400
3705
2230
2/4
6.8
3400
33300
24500
4120
2630
2/5
6.8
1250
33600
25000
4130
2H50
5.0 * 106
2/6
6.8
3390
32300
24900
4480
3040
2480
2/7
6.3
2520
33900
25200
4700
3010
41300
2/8
6.8
4500
32200
23700
4 (.00
2720
2/9
6.2
2690
33900
25800
5200
3020
2/10
6.2
3430
32300
24400
4820
2620
2/11
6.1
4380
28400
25100
4940
2630
2/12
6.0
4020
29300
21600
5590
3140
1640
2/13
6.2
4430
30700
26200
5510
3050
2/14
6.5
3760
31200
23500
5180
2940
i/15
6.4
3500
31900
24800
5120
2920
i/16
7.2
4600
29900
22500
5290
3010
2/17
N A
N A
31600
24300
4890
2850
1/18
7.0
3660
32800
24000
5200
3070
2/19
6.3
3610
34800
25700
5340
3120
J/20
5.8
3660
34500
29300
—
-
?INAL
6.2
3600
34400
28400
5780
3360
N k
N A
N A
300
N A
55S
NA
-------
Table 21. Pure oxygen digester batch test no. 3 - field data
CO
OXYGEN PRESS.^
OXYGEN FLOW*2)
OXYGEN(3)
TEMP. (°f\
°2 SUPPLY
D.O.
(nw/t)
SLUDGE (3)
temp.
DATF.
I t. 2
3
1 & 2
3
(gcfirO(4)
fl h/(l.iv1(5)
1/31/73
30.3
29.3
1.34
0.37
70
2,313
192
7.5
72
2/1
25.6
25.8
0.98
0.33
81
1,617
134
0.5
80
112
27.5
27,9
1.14
0.33
76
1,894
157
1.4
81
If 3
26.3
25.8
1.01
0.30
73
1,616
134
1.4
82
2/4
26.0
25.2
0.95
0.28
69
1,528
127
2.9
82
115
25.2
25.2
0.95
0.27
73
1,466
122
1.7
82
2/6
25.7
25.2
0.96
0.28
66
1,485
123
2.5
76
2/7
22.7
22.7
0.85
0.26
58
1,210
100
3.8
2/8
19.6
20.1
0.63
0.23
59
858
71
2.2
76
2/9
16.9
REMOVED*
0.44
REMOVED*
66
376
31
7.7
75
J/10
15.0
--
0.28
—
66
240
20
8.0
75
2/11
14.7
--
0.26
--
67
210
17
12.1
72
2 / X 2
17.0*
-
0.21*
--
65
193
16
7.7
6$
2/13
15.0
—
0.16
62
137
11
2.6
68
2/14
15.1
—
0.15
-
64
130
U
2.0
68
2/15
15.4
—
0.14
--
58
121
10
2.0
66
2/16
16.5
--
0.14
—
65
127
10
1.2
64
2/17
16.2
--
0.14
—
62
125
10
1.4
64
2/18
16.5
—
0.14
—
65
127
10
1.1
66
1/19
18.5
—
0.17
—
60
163
13
1.6
64
2/20
20.0
"
0.20
--
59
202
17
5.5
63
(1)
(2)
(3)
(4)
(5)
psfg (dfffusers 1-2 and 3) x 0.0703
cfm (diffusers 1-2 and 3) x 0.472 =
(0F-32) 5/9 - °C -
scfd x 0.0283 * m /day at STP
lb/day x 0.454 - kg/day
= kg/cm'
1/sec
-------
Table 22. Pure oxygen digester batch test no. 3 - inventory
mass reduction
DATE
LIQUID
INVENTORY
(lb) (1)
TSS
(og/1)
vss
(ag/1)
SOLIDS INVENTORY
PERCENT
MASS REDUCTION
TSS
(lb)(1)
VSS
(lb)(1)
TSS
VSS
INITIAL
1/31/73
14386
45700
36500
657.4
S25.1
L/31/73 COMP.
14284
42100
33400
601.4
477.1
8.5
9.1
2/1
14166
36500
30000
517.1
425.0
21.3
19.1
2/2
14049
35000
28000
491.7
393.4
25.2
25.1
2/3
13932
44400
32400
618.6
451.4
5.9
14.0
2/4
13830
33300
24500
460.5
338.8
29.9
35.5
2/5
13727
33600
25000
461.2
343.2
29.8
34.6
2/6
13610
32300
24900
439.6
338.9
33.1
35.4
2/7
13508
33900
25200
457.9
340.4
30.3
35.2
2/8
13391
32200
23700
431.2
317.4
34.4
39.5
2/9
13288
33900
25800
450.5
342.8
31.5
34.7
>/IO
13171
32300
24400
425.4
321.4
35.3
38.8
2/11
13054
28400
25100
370.7
327.6
43.6
37.6
!/12
12951
29300
21600
379.5
279.7
42.3
46.7
5/13
12834
30700
26200
394.0
336.2
40.1
36.0
J/14
12732
31200
23500
397.2
299.2
39.6
43.0
!/15
12615
31900
24800
402.4
312.8
38.8
40.4
1/16
12512
29900
22500
374.1
281.5
43.1
46.4
i/17
12410
31600
24300
392.1
301.6
40.3
42.6
J/18
12293
32800
24000
403.2
295.0
38.7
43.8
2/19
12175
34800
25700
423.7
312.9
35.5
40.4
2/20
12073
34500
29300
416.5
353.7
36.6
32.6
J/21/73
?IKftL
11970
34400
28400
411.8
339.9
37.4
35.3
(1) lb x 0.454 = kg
99
-------
—i—l—i—I—i—J—i—!—i———l—i———\—) > *—I ——I—i—I—i—J
o 2 4 6 8 10 12 14 16 13 20 22 24 26 28
DETENTION TIME (DAYS)
Fig. 45. Pure oxygen batch test no. 4 biodegradable
COD reduction versus detention time
100
-------
Table 23. Pure oxygen digester batch test no. 4 - laboratory data (mg/1
unless other units indicated)
DATE
SUSPENDED SOLIDS
DISSOLVED SOLIDS
COND. 2
^imho/cm
PH
Units
ALK.
COD
N03
nh4
TKN
TO4
TSS
vss
TDS
VDS
INITIAL
38300
32900
2080
585
1610
6.4
60800
.22
495
2900
748
3/8/73
34600
28J00
2110
745
1970
6.3
1020
50500
3/9
35800
31500
3500
1830
2550
6.6
46400
3/10
33800
2 7 LOO
3940
2320
2800
6.6
46900
3/LI
31200
24900
5800
3700
2850
7.0
2140
NA
3/12
30000
24L00
NA
NA
NA
NA
NA
NA
3/13
28800
23500
4600
1900
3470
7.3
36100
3/14
30200
24900
2800
100
3700
7.5
2340
37600
3/15
28400
23000
3400
500
4520
6.8
34200
3/16
29200
22400
2800
1300
4320
7.0
31600
3/17
25000
20900
3940
2420
4180
7.0
33900
3/18
22800
18600
8100
4200
3910
6.3
35700
3/19
21000
16600
9700
6000
3890
6.2
1520
29600
3/20
26800
21600
3400
440
3780
6.7
28300
3/21
25500
18700
4300
3100
3930
6.8
33300
3/22
31700
24600
4260
2560
2890
6.9
29000
3/23
27100
22 700
4460
2580
3450
6.7
28600
3/24
27200
20800
4460
2600
3370
6.7
28500
3/25
29500
22000
4220
2380
2920
6.4
29700
3/26
28200
23800
4060
2050
2800
6.7
1095
26100
3/27
29200
26400
4180
2200
2870
6.2
28000
3/28
TINAL
27000
22800
4070
2130
NA
NA
713
26500
712
90
2020
1120
-------
Table 24. Pure oxygen digester batch test no. 4 - field
and inventory mass reduction data
SLUDGE
LIQUID
SOLIDS INVENTORY
PERCENT
MASS
D.O.
TEMP.
INVENTORY
TSS
VSS
TSS
VSS
REDUCTION
DATE
ag/1
Cf)(1)
(lb) (2)
)
)
at>)(2)
(lb) (2 )
TSS
VSS
INITIAL
1.6
66
14385.9
38300
32900
551.0
473.3
—
"
3/8/73
1.6
66
14312.7
34600
28300
495.2
405.0
10.1
14.4
3/9
1.8
74
14224,9
35800
31500
509.2
448.1
7.6
5.3
3/10
1.6
77
14093.1
33800
27100
476.3
381.9
13.5
19.3
3/11
2.1
77
13888.1
31200
24900
433.3
345.8
21.4
26.9
3/12
2.4
79
14166.3
30000
24100
424.9
341.4
22.9
27.9
3/13
2.5
80
14371.3
28800
23500
413.9
337.7
24.9
28.6
3/14
2.8
81
14385.9
30200
24900
434.4
358.2
21.2
24.3
3/15
2.2
82
14415.2
28400
23600
409.4
340.2
25.7
28.1
3/16
2.3
83
14444.5
29200
22400
421.3
323.6
23.4
31.6
3/17
4.0
81
14371.3
25000
20900
359.3
300.4
34.8
36.5
3/18
8.7
76
14166.3
22800
18600
323.0
263.5
41.4
44.3
3/19
N.A.
72
14342.0
21000
16600
301.2
238.1
45.3
49.7
3/20
5.7
74
14349.3
26800
21600
384.6
309.9
30.2
34.5
3/21
1.6
76
14385.9
25500
18700
366.8
269.0
33.4
43.2
3/22
1.1
76
14517.7
31700
24600
460.2
357.1
16.5
24.5
3/23
0.8
79
14458.1
27100
22700
391.8
328.2
28.9
30.7
1/24
0.8
76
14312.7
27200
20800
389.3
297.7
29.3
37.1
J/25
0.5
73
14137.0
29500
22000
417.0
311.0
24.3
34.3
3/26
1.7
70
14224.9
28200
23800
401.1
338.5
27.2
28.5
3/27
4.7
70
14400.5
29200
26400
420.5
380.2
23.7
19.7
3/28
?1KAL
10.7
70
14422.5
27000
22800
389.4
328.8
29.3
30.5
(1) (°F-32) 5/9 = °C
(2) lb x 0.454 = kg
102
-------
Table 25. Pure oxygen digester batch test no. 5 - laboratory data
(mg/1 unless other units indicated)
SUSPENDED SOLIDS
DISSOLVED SOLIDS
COND.
PH
ALX.
as
NtTROGEN-N
DATE
TSS
VSS
TDS
VDS
^.eho/cm^
Units
CaC03
COD
no3
SH4
TKN
P04
[NITIAL
35600
29500
1560
605
1760
7.3
1167
44500
.46
12
2580
716
i/5/73
33300
26100
2480
1200
2560
6.9
1398
40400
'*16
34500
27300
3110
1770
2420
6.6
37000
27700
21700
3590
2290
2570
6.8
33800
*/s
2S100
20600
3290
1960
2740
7.4
1525
34000
+/9
27300
21200
3550
2210
3600
7.3
30000
4/10
23400
18400
3080
1640
3100
7.0
26500
i/11
25000
18500
2840
1440
2570
6.6
2 7900
4/12
22400
17900
2490
1200
2440
6.8
1254
27000
4/13
19400
18400
2700
1060
2270
6.9
24800
4/14
20100
15600
3000
1240
2430
5.9
25200
4/15
22200
15200
3020
1340
2390
6.6
599
23300
4/16
22100
16700
3240
1430
2550
6.0
24800
4/17
23500
16400
3190
1400
2760
6.0
20280
4/18
FINAL
22300
16600
3300
2460
2640
5.9
435
20580
148
104
1560
660. fi
103
-------
Table 26. Pure oxygen digester batch test no. 5. - field data
5ATE
OXYGEN
DO
Or/1)
DO UP
AKE
SLUDGE
TEMP.
°2
RESPIRED
(lb/day) <5)
J'ERCrNT
oxy<;en
TRANSFER
EFFICIF.NCY
(cfm)(1)
(I'hIrXZ
(°F)(3)
(ac fd)(4
ll>/dny(5)
K
r
K
r
V/5/73
0.53
30
67
742,7
61.6
1.0
156
6.0
73
53.8
87.3
4/6
0.51
30
71
712.0
59.1
1.0
173
6.3
82
59.7
101.0
4/7
0.44
28
61
605.2
50.2
5.2
111
5.1
85
38.3
76.3
4/8
0.33
26
55
445.0
36.9
2.0
82
4.0
82
28.3
76.7
4/9
0.32
27
56
436.7
36.2
1.0
94
4.4
81
32.4
89.5
4/10
0.32
27
68
431.8
35.8
1.2
114
6.2
85
39.3
109.8
4/11
0.30
27
73
40 2.9
33.4
0.8
88
4.8
85
30.4
88.4
4/12
0.29
27
59
394.7
32.8
1.3
118
6.6
83
40.7
124.1
4/13
0.29
27
69
390.9
32.4
2.3
90
4.9
79
31.1
96.0
4/14
0.25
22
69
314.7
26.1
15.2
57
3.7
73
19.7
5.0
4/15
0.22
18
68
260.4
21.6
11.9
12
0.8
73
4.1
-
4/16
0.21
16
70
239.7
19.9
17.9
21
1.3
72
7.2
—
4/17
0.17
15
62
192.0
15.9
18.0
19
1.2
71
6.6
—
4/18
0.21
15.5
68
79.3
6.6
18.8
12
0.7
70
4.1
--
1) cfm X 0.^
72 = 1/sec
(2) psig x 0.0703 = kg/cm2
(3) (°F-32) 5/9 = °C
(4) lb/day x 0.454 = kg/day
(5) scfd x 0.0283 = m3/day at STP
-------
Table 27. Pure oxygen digester batch test no. 5 - inventory
mass reduction
DATE
LIQUID
INVENTORY
(lb)(1)
TSS
fag/1 )
VSS
es/i>
SOLIDS INVENTORY
PERCENT MASS
REDUCTION"
TSS
(Lb) (1)
VSS
(lb)(1)
TSS
VSS
INITIAL
14349.3
35600
29500
510.8
423.3
4/5/73
14298.1
33300
26100
476.1
373.2
6.8
11.8
4/6
14283.4
34500
27300
492.8
389.9
3.5
7.9
4/7
14283.4
27700
21700
395.6
309.9
22.5
26.8
4/8
14254.1
25100
20600
357.8
293.6
29.9
30.6
4/9
14400.5
27300
21200
393.1
305.3
23.0
27.9
4/10
14371.3
23400
18400
336.3
264.4
34.2
37.5
4/11
14298.1
25000
18500
357.4
264.5
31.2
37.5
4/12
14371.3
22400
17900
321.9
257.2
37.0
39.2
4/13
14254.1
19400
18400
276.5
262.3
45.9
38.0
4/14
14283.4
20100
15600
287.1
222.8
43.8
47.4
4/15
14166.3
22200
15200
314.5
215.3
38.4
49.1
4/16
14283.4
22100
16700
315.7
238.5
38.2
43.6
4/17
14371.3
23500
16400
337.7
235.7
33.9
44.3
4/18 FINAL
14349.3
22300
16600
320.0
238.2
37.3
43.7
(1) lb x 0.454 = kg
105'
-------
Batch Test No. 4
Undigested WAS (3.83% TSS) was used in this test. In order to
compensate for evaporation losses during the test period,
make up water was added daily to maintain the initial volume of
6500 liters (1722 gal). Because of the wide fluctuations in
solids data previously noted, the biomass was analyzed daily
for COD.
Experimental data for Batch Test 4 are presented in Tables 23
and 24. Table 23 summarizes laboratory analyses and Table 24
presents operating data and calculations of TSS and VSS reduc-
tions .
Oxygen uptake calculations were not listed in these tables
because of continuing difficulty in measurement.
Figure 45 shows the reduction of biodegradable COD during the
21 day test period. Biodegradable COD was defined as that
fraction of the total COD reduced by the end of each test. The
biodegradable COD reduction rate coefficient was equal to 0.161
(16.1% per day).
TSS and VSS concentrations were reduced by 29.5 and 30.7% res-
pectively. TDS and VDS increased by 95.7 and 264.1% respec-
tively. COD reduction between the initial and final sample was
56.4%. The pH reduction was minimal (0.2 units) while alkalin-
ity was reduced by 30%. TKN was reduced by 30.3%, ammonium-N
was reduced by 81.8% and nitrates increased over 3000 fold to
712 mg/1 in the final sample.
Bath Test No. 5
Mechanical and operational problems encountered in the previous
batch tests were solved prior to start of this test. The data
from this test were considered to be more representative of
open tank aerobic digestion than data from the previous batch
tests and will be referred to as the standard for pure oxygen
batch test performance0
Experimental data for Batch Test 5 are presented in tables 25,
26 and 27. Table 25 summarizes laboratory analyses. Table 26
presents average operating data and Table 27 shows calculations
of TSS and VSS reductions.
106
-------
The oxygen uptake rate method was improved by injecting
hydrogen peroxide (3% solution) into the biomass within the
500 ml respiration chamber at a controlled application rate to
provide the oxygen source for respiration. Homogeneous mixing
was accomplished by a two propeller mixer operating at 700 rpm.
A YSI dissolved oxygen probe mounted on top of the respiration
chamber transmitted a signal through the DO meter to a strip
chart recorder. The oxygen uptake rate was calculated from the
constant slope section of the chart recording. The optimal
mixing speed and the hydrogen peroxide injection rate were
determined by trial and error to avoid toxic peroxide effects.
Reliability of this method within + 10% was verified by testing
several aliquots of the same sample for reproducibility. Less
than 5 minutes elapsed between sample collection and analysis
to prevent biomass anoxia.
Figure 46 shows the relationship of specific oxygen uptake K
to detention time. During the first nine days, K varied
between 4 and 6 mg/hr/g VSS. This data appears to substantiate
the previous observations concerning cyclical digestion and re-
synthesis. On the tenth day, K dropped to less than 1.0
mg/hr/g VSS. It is suggested tEat for batch aerobic digestion,
a K of less than 3.0 can be used to define satisfactory
stabilization*,
During the first 9 days of this test, K averaged 5.4, DO con-
centration averaged 1.7 mg/1 and oxygen transfer efficiency
averaged 93.7%. During the last 4 days of the test, when K
averaged 1.0, DO concentration rose to an average of 16.2 mg/1
with a consequent reduction in oxygen transfer efficiency. In
order to ensure a consistently high oxygen transfer efficiency
in an open tank system, the DO must be controlled below the air-
liquid saturation limit of 7.0 mg/1 at 20 C and Denver altitude.
TSS and VSS reductions averaged 37.4% and 43.7% respectively,
while TDS and VDS increased by 212% and 241.3% respectively.
Conductivity increased 50%, COD declined 53.8% and pH declined
by 1.4 units from 7.3 to 5.9 in the final sample. This decline
in pH was accompanied by a 62.7% decrease in alkalinity and a
three hundred fold increase in nitrate concentration to 248
mg/1. Ammonium-N increased 8.7 fold while TKN declined 39.5%.
107
-------
-------
Summary of Pure Oxygen Batch Test Results
The five pure oxygen batch tests indicated that the biodegrad-
able VSS reduction rate levels off after approximately 15 days.
Figure 47 shows that by the fifteenth day all 5 tests had
reached a 5%/day VSS reduction rate, compared with initial
rates of 15 to 25%/day. Biodegradable VSS was defined as that
fraction of the total VSS reduced by the end of each test.
The final sample did not always have the lowest VSS concentra-
tion. Although triplicate analysis of TSS and VSS were done to
reduce the possibility of error, sampling procedure may account
in part for this variability. The repeatability of the cyclical
solids concentration phenomenon in all of the batch tests
suggest that alternating periods of digestion and resynthesis
by heterotrophic and autotrophic organisms may be involved.
The rate of endogenous respiration of biodegradable VSS is rep-
resented by the first order reaction equation:
dS =-kS
dt
where k = rate of decay constant
(aerobic digestion rate
coefficient)
S = concentration of biodegradable
cell material at any time
t = detention time (days)
Integration of the above equation gives:
In
S(t=t)
- -kt
_S(t=0)-
If the system follows first order kinetics, the plot on semi-
logarithmic paper of the ratio of the biodegradable VSS at any
time (t) to the VSS at time zero versus detention time will
yield a straight line.
Table 28 summarizes the mass reduction data for all of the batch
tests including values for aerobic digestion rate coefficients
on a VSS and COD basis: Batch test No. 3 was unique in that a
previously air digested material was used as the starting sludge
109
-------
H'
O
LEGEND
O BATCH TEST No. I
X BATCH TEST Na 2
A BATCH TEST No. 3
P BATCH TEST No. 4
BATCH TEST Na 5
RUN No. 3
RUN No. 4
JN No.
8 9 10 II 12
DETENTION TIME (DAYS)
Fig, 47. Pure oxygen batch tests biodegradable VSS reduction versus detention time
-------
Table 28. Pure oxygen digester batch tests 1-5 - biomass reduction
summary
SAMPLE
BATCH
TEST NO.
DETENTION
TIME
DAYS
VSS/
initial
CSS RATIO
final
A I
PERCENT
VSS
REDUCED*
BIODEGRADABLE
as
UNDIGESTED
1
21
0.865
0.844
-2.1
53.7
0.143
-
MAS
UNDIGESTED
2
15
0.826
0.750
-7.6
54.7
0.175
-
WAS
UNDIGESTED
4
21
0.859
0.844
-1.5
30.5
0.204
0.174
HAS
UNDIGESTED
5
14
0.829
0.744
-8.5
43.7
0.273
0.190
AVG (UNDIGESTED)
0.845
0.796
-5.0
45.7
0.200
0.182
WAS i
AIR DIGESTED 3
1
21
0.799
0.826
+2.7
35.3
0.182
-
* Based on initial versus final VSS concentration.
Ill
-------
for the test. Tests 1, 2, 4 and 5 were loaded with previously
undigested WAS.
VSS/TSS ratio was higher for the undigested sludge (84.5%) than
it was for the air digested WAS (79.9%). The final VSS/TSS
ratio for tests 1, 2, 4 and 5 averaged 79.6% for test 3. The
average of the four tests using undigested material showed a
net reduction of 5% in the VSS/TSS ratio.
The percent VSS reduced based on the difference between the
initial and final sample averaged 45.7% for tests 1, 2, 4 and 5
compared with 35.3% for test 3. The VSS remaining plotted
against detention time showed an aerobic digestion rate co-
efficient of 0.071 with a coefficient of correlation of -0.920
for batch test 5.
A comparison of digestion rates in batch test 5 compared with
batch test 3 appears in Figure 48. The biodegradable VSS reduc-
tion rate was 50% higher in batch test 5. The reduction rate
coefficient was lower on a COD basis than on a VSS basis in
batch test 5. The most efficient utilization of pure oxygen for
aerobic digestion was obtained by using an undigested WAS with
a high initial VSS/TSS ratio.
Comparison of Metro Denver and Batavia pure Oxygen Batch Tests
One study on aerobic digestion reported in the literature where
pure oxygen was used appears in E.P.A. report No.
17050DNW02/72. The aerobic digestion experiments in this
study at Batavia, New York were performed entirely on an oxygen
WAS with low VSS/TSS ratios (67.6% - 74.1%). Figure 49 compares
results for runs 6 and 8 at Batavia with batch test 5 results
of this study. The biodegradable VSS digestion rate coefficient
in the Batavia study was 0.12 compared to 0.27 in the Metro
Denver batch test 5. The coefficient of correlation for the
Batavia and Metro Denver tests are both significant at the 95%
confidence level. For SRT of 6 days, 20% of the biodegradable
VSS remained in the Metro Denver batch system compared with 48%
remaining in the Batavia system.
112
-------
LEGEND
BATCH TEST 3-VSS
BATCH TEST5-VSS
BATCH TEST 5-C00
.6 • ¦
.5 •
.4 ••
.3 «•
•K« O.J 82
\
2
3
4
5
8
6
7
9 iO II 12 13 14 15 16 17
DETENTION TIME (DAYS)
Fig. 48. Pare oxygen batch test 3 and 5 biodegradable COD and
VSS reductions versus detention time
113
-------
LEGEND —
G BATAVtA RUN No.8 U9?0>*
O BATAV1A RUN No.6 (1970)*
• DENVER RUN No. 5
\ < '
* REF: E.P.A. REPORT No. 17050 ONW
02/72
BATAVJA VSS/TSS RATIO AVERAGED:
OENVER VSS/TSS RATIO AVERAGED
I 82 9 %
BATAV1A
- *.» 0.12
09
20
22
24
26
DETENTION TIME (DAYS)
Fig. 49. Comparison of Batavia and Denver pure oxygen batch
test VSS reduction versus detention time"
114
-------
Oxygen Uptake Rate
If all of the data are included, a rather poor linear relation-
ship is found to exist between oxygen uptake rates (OUR) and
biodegradable VSS or total VSS in both the Batavia and Metro
systems. Figure 50 however, shows an excellent correlation
+0.99 between biodegradable VSS having a concentration greater
than 2,000 mg/1 and OUR for the Metro data. The endogenous
respiration, therefore, must be proportional to the active mass
rather than the TSS. Below an active VSS of 2,000 mg/1, chang-
ing metabolic states make this level of activity non-linear.
In this study, the equation for OUR for solids concentrated
above 2,000 mg/1 was found to be OUR = 0.0127*VSS (biodegradable)
+ 39.7, with a correlation coefficient of +0.99. On a biodegrad-
able COD basis the equation was found to be OUR = 0.0066 COD +
35.6,with a correlation coefficient of +0.766. The poor linear
relationship that was found initially to exist in the Batavia
study also became a very significant relationship when the bio-
degradable VSS above 2,000 mg/1 were substituted for the total
VSS.
The results of the pure oxygen batch test portion of this study
yielded the following conclusions:
1* A stabilized sludge and 40 to 50% VSS reduction was
obtained after one to three weeks of detention time.
These values are similar to diffused air digestion rates
under ideal laboratory conditions, and are significantly
higher than the VSS reduction rates in the Batavia study.
2. The batch test 5 rate coefficient for total VSS reduction
was 0.071 compared with 0.038 in the Batavia study. The
difference in rates may be due to CO^ stripping as well
as a more effective oxygen-sludge contacting system in
the Metro Denver digester.
3. No correlation was observed during any of the tests
between DO concentration and VSS digestion rates. The
highest DO concentrations occurred when the OUR was at
a minimum and the VSS digestion rate was at its lowest.
It appeared that above the minimal concentration required
to sustain aerobic metabolism, DO concentrations are a
result rather than a cause of aerobic digestion reaction
rates.
115
-------
LEGEND
O COD BIODEGRADABLE
X VSS BIODEGRADABLE
200
180
£ 140
r» + 0.99 (>2000m®/l VSSao)
* 120
fc 100
80
60
40
1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11,000 12,000 13,000 14,000 15,000 16,000 15,000 16,000 19,000 20,000
BIODEGRADABLE SOLIDS CONCENTRATION (mg/l)
Fig. 50. Pure oxygen batch test no. 5 oxygen uptake rate as a function of biodegradable
solids concentration
-------
4. A "high degree of oxygen utilization (approximately 92%)
was demonstrated in batch test No, 5„
5. The rate of decline in the VSS/TSS ratio in the batch
test can be correlated with the percent volatiles of
the initial sample. The variations in VSS/TSS ratios
during the various batch tests may be explained by the
cyclical periodicity of alternate digestion followed
by resynthesis of biomass using previously solubilized
nutrients (cryptic growth).
6. Microscopic analysis of invertebrate populations revealed
that the high concentration of suspended solids results
in a stressful "crowding" situation that was inimical to
successful growth and reproduction of these organisms in
the batch test digester. The addition of cationic high
molecular weight polymers to the sludge during dissolved
air flotation might also adversely affect the ecological
diversity of this system (quaternary ammonium compound
polymers have been shown to exert a biostatic effect on
numerous microorganisms). At the end of batch test No.
1, only one of the commonly observed invertebrate groups
(micro-flagellates) was observed in a viable condition,
while no motile invertebrate organisms were observed at
the end of batch test No. 2.
In the pure oxygen batch tests mass decay and endogenous
respiration were accomplished almost entirely by bacteria
rather than higher invertebrate organisms that were
prevalent in the diffused air digestion system under
optimal conditions.
Several possible reasons were advanced to explain the relative
differences in performance between the aerobic digestion oxygen
studies at Batavia and this study. These hypotheses included
mixing energy, initial volatile solids concentration, and the
different methods of transferring oxygen from the gaseous to the
liquid phase. Another possible theory considered the carbon
dioxide/ph differences between the initial and final samples.
A masters thesis by Thomas J. Weston concluded that "the oxygen
unit achieved lower solids reduction and exhibited lower oxygen
uptake rates than the air unit." Weston mentioned that in his
closed tank system, oxygen digested sludges had significantly
lower pH value (4.9), and concluded that pH toxicity may have
affected the biological activity of the system thus inhibiting
117
-------
solids reduction. The average volatile solids reduction in
Weston's pure oxygen digester was only 23.7% for ten days
detention time compared with 41.3% in the air digester for an
equivalent detention time. The unique nature of the Marox
open tank diffuser system used at Metro allowed for purging of
carbon dioxide and volatile organic acids. Thus the final pH
was more alkaline than in the Batavia study. The continuous
recycling of the liquid sludge through the Metro diffuser to
create minute gas bubbles not only aided in purging CO^ from
the system, but also contributed a high degree of mixing energy
which could also be related to the degree of volatile solids
reduction.
Dissolved oxygen levels were substantially higher in the
Batavia experiments and yet the digestion rate coefficient was
higher in the Metro Denver experiment suggesting that the effec-
tiveness of oxygen transfer was not a major factor affecting
digestion performance.
In summary, a comparison of the relative performance of pure
oxygen versus diffused air digestion involves a multiplicity of
factors. Only when all of the important factors and their
interrelationships are identified can one predict which system
will provide the most effective treatment.
118
-------
EXPERIMENTAL RESULTS - PURE OXYGEN FLOW-THROUGH (FAD) TESTS
Operational Limitations
On the basis of stage one batch test data which indicated that
satisfactory stabilization was achieved between 1 and 3 weeks
detention time, the second stage flow-through tests were de-
signed to investigate detention times in this range. The com-
mencement of the flow-through tests was delayed, however, due
to the May 6, 1973 flood of the South Platte River. This
caused an abnormal decline in the VSS/TSS ratio of the Metro
WAS from 85% to less than 60%. This effect was noted for sev-
eral months after the floods subsidence.
At the start of the second stage phase one testing on June 11,
1973, the VSS/TSS ratio was 77.4%. The second phase starting
July 14 through August 22, 1973 had essentially the same VSS/
TSS ratio. With commencement of phase three testing on October
20, 1973, the VSS/TSS ratio reached the previously normal lev-
els of 82.5%. During phases four and five from November 1973
through January 1974 the VSS/TSS ratio was 83.5%. The effluent
waste solids leaving the pilot plant had an abnormally Low TSS/
VSS ratio during the first two phases (67.1 - 68.9%). In this
respect, these two phases were deemed non-typical.
The major objective of the second stage testing was to deter-
mine the breakdown loading rate to the system. The first
three phases were designed to duplicate the loading range that
had been applied to the diffused air system 1.34 to 3.14 kg
VSS/m^/day (0.83 to 0.196 lb VSS/ft^/day). The last two phases
of this test investigated the performance at much higher loading
rates 5.22 to 6.93 kg VSS/m^/day (0.326 to 0.433 lb VSS/ft^/day).
During phase one numerous diffuculties were experienced with
the sludge recirculation pumps. After conversion of the pilot
plant from batch to flow-through, the drive and screw mechan-
isms of the Moyno pumps failed. Since spare parts were not
available, it was decided to use a diaphragm type pump. After
119
-------
start up of the diaphragm pumps several malfunctions occurred
requiring shut down of these pumps as well. In order to keep
the pilot plant running, two self priming centrifugal pumps
were installed. The flow and pressure from these pumps was
insufficient to create the requisite gas bubble size. Entrained
gases also caused pump cavitation. For this reason the pilot
plant was shut down on August 22, 1973. Two new Moyno pumps
were utilized in October 1973 and performed satisfactorily dur-
ing phases 3, 4 and 5 of the second stage testing. The pro-
blems with recirculating pumps resulted in DO sag at times dur-
ing phases one and two due to an inability to create the requi-
site bubble size.
Concentrated WAS from the air flotation unit was available only
during 5 days of the week for phase one of this stage as the
process was shut down on weekends. In order to avoid a situa-
tion of alternate starve and over-feed, it was decided on July
7, 1973 to save sufficient concentrated WAS over the weekend to
enable continuous loading of the pilot plant 7 days a week. No
detrimental effects were noted from the loading of slightly sep-
tic sludge on weekends, other than an increase in oxygen uptake
rates. In October 1973, the air flotation process was converted
to a 7 day per week operation, obviating the need to save sludge
during weekends.
Measurement of oxygen transfer efficiency was difficult during
the initial study phases because of foaming, pump maintenance
and our measurement problems. Because of difficulties in DO
sensor membrane coating in the concentrated sludge, automatic
DO control was not feasible requiring manual control. DO con-
centrations were determined every 2 hours with a YSI probe.
DO' concentrations above 7 mg/1 occurred on occasion resulting
in loss of oxygen to the atmosphere and reduced oxygen transfer
efficiency. Prior to phase three, a valving system was in-
stalled to allow for flow reversal to the PAD and release any
material bridging the slot from the inside. Although the back
flush cleaning method did reduce slot bridging, it was not a
complete solution to the problem. Even though the influent
sludge was processed through a 3 mm screen, much fibrous, pulpy
material remained. The ultimate solution to this problem was
found to be the substitution of the RAD for the FAD.
Oxygen transfer efficiencies were calculated using average
daily OUR based upon two grab samples taken during the morning
and afternoon shifts. These values were used to calculate a
24 hour theoretical oxygen demand. The average of twelve oxygen
120
-------
flow and pressure readings per day were used to calculate the
daily oxygen supply. A comparison of the oxygen respired per
day based on the calculated average OUR with the daily oxygen
supply provided a daily O2 transfer efficiency.
The accuracy of the oxygen flow and pressure measurement varied
by less than + 5%. Diurnal fluctuations in oxygen respiration
rates were significantly greater than oxygen supply variations.
This variability led to inaccuracies in calculated oxygen trans-
fer efficiencies, particularly in phases one and two.
Field Data
A summary of field data for the five phases of the FAD test pro-
gram is included in Table 29. Figure 51 shows the increasing
temperature differential between the air temperature and the
biomass temperature with increasing loadings. At the lowest
loading rate 1.34 kg VSS/m^/day (.083 lb VSS/ft^/day) there was
virtually no difference between air and biomass temperatures.
At the highest loading rate 6.93 kg VSS/m^/day (0.433 lb VSS/
ft^/day) the temperature differential increased to 20°C (i.e.
ambient temperature averaged 8.4°C while the biomass in tank B
averaged 28.6°C).
Hydraulic balance through the system took into consideration
liquid volume fed, wasted, spilled and/or evaporated. The av-
erage for the entire 15 9 day period showed 2.1% losses as spills
and an evaporation loss of 22.8%. DO concentrations averaged
1.4 mg/1 in tank A and 1.6 mg/1 in tank B for the entire per-
iod. As loading rates increased, average DO concentrations
tended to decline, but a positive DO residual was maintained
at all times. OUR ranged between 27 and 660 mg/l/hr. Average
uptake rates for the last three phases were 176 in tank A and
110 mg/l/hr in tank B representing an average daily respiration
rate of 45 kg 02/day (99 lb 02/day) for both tanks. Oxygen
supplied to the system averaged 37 kg/day (82 lb/day) in tank A
and 26 kg/day (57 lb/day) in tank B. The oxygen transfer effi-
ciency for the last three phases was 67% with a minimum of 49%
in tank A during phase five and a maximum of 85% in tank A dur-
ing phase three.
121
-------
LOAOING ,
I b V S S/f t^da\P
(x 16.02® hg/m^Joy)
A TEMP. 0.6.
0 TANK-AMBIENT °C
0.139-
0.0
0.196
14.S
0.326*
18.3 •
LE
iEND
x— x — x-
"A" TANK
"b" TANK
AMBIENT (Ot)
0.433'
20.0'
Atl\XjlpL
r \-
V
Kh
"1
10
X
6/11
7/1 7/13
PHASE I I
8/11
PHASE 2
8/22
10/19
ll/l
PHASE *3
11/19 12/1 12/13 1/1/74 1/13
I PHASE 4 t| PHASES J
Fig. 51. Pure oxygen flow through test biomass and oxygen temperature versus time
-------
Table 29. Pure oxygen digester flow through pilot plant (FAD) - field data
VOLUMETRIC DATA fCAL/OAYXn
TEMPERATURE "C
OXYGEN DATA
EVAP.
DO
•w/J)
UPTAKE me/l/hr
lb/DAY RESPIRED^2*
lb /DAY SUPPLIED
X TRANSFER EFFICIENCY
PHASE
FEED
WASTE
SPILLS
CALC.
TANK A
TANK B
OXYGEN
TANK A
TANK B
TANK A
TANK B
TANK A
TANK B
TOTAL
TANK A
TANK B
TOTAL
TANK A
TANK B
TOTAL
I ME AH
129
60
26
43
25.1
26.8
26.3
2.1
3.2
N.A.
N.A.
N.A.
N.A.
N.A.
63
61
124
N A
N A
N K
MIN.
0
0
0
-
16.5
17.5
15.7
0.1
0.2
-
-
-
-
-
20
46
-
-
-
-
MAX.
250
225
615
-
31.7
33.3
33.9
8.7
19.5
-
-
-
-
-
105
115
-
-
-
-
II MEAN
220
161
4
55
29.3
30.1
21.9
2.1
1.9
N.A.
N.A.
N.A.
N.A.
N.A.
49
54
103
N \
N A
N A
MIN.
0
0
0
-
24.7
23.7
13.5
0.2
0.1
-
-
-
-
-
33
33
-
-
-
-
MAX.
330
374
150
-
40.3
39.5
26.7
4.3
4.4
-
-
-
-
-
94
137
-
-
-
-
Ill MEAN
335
252
8
75
25.8
30.3
15.5
1.8
2.0
145.1
87.4
50
30
80
59
63
122
85
48
66
MIN.
157
154
0
-
22.7
24.5
10.7
0.3
0.3
27
12
9
4
-
47
39
-
-
-
-
MAX.
539
459
139
-
31.3
33.5
20.6
14.6
15.0
562
660
195
226
-
90
94
-
-
-
-
IV MEAN
514
415
0
99
23.9
29.0
10.7
0.4
0.3
169.8
124.2
59
42
101
88
53
141
67
79
72
MIN.
452
367
0
-
19.7
24.0
5.1
0.2
0.2
108
42
38
14
-
32
28
-
-
-
-
MAX.
623
597
3
-
26.7
33.2
16.0
0.8
1.0
276
270
96
92
-
165
75
-
-
-
-
V MEAN
640
525
2
113
22.3
28.6
8.4
0.5
0.5
211.9
118.6
74
41
115
152
55
207
49
75
56
MIN.
539
399
0
-
17.7
23.5
-2.9
0.3
0.3
126
54
44
18
-
118
43
-
-
-
-
MAX.
704
689
55
-
25.2
32.0
15.6
1.3
1.7
375
216
130
74
-
178
98
-
-
-
-
AVG
159 DAYS
377
283
8
86
25.3
29.0
16.6
1.4
1.6
176
L10
61
36
99
82
57
139
67
67
67
(1) gal/day x 3.785 = 1/day
(2) lb/day x 0.454 = kg/day
-------
Solids Data
Table 30 summarizes VSS reductions in relation to loading rates.
The average loading rate for all five phases was 3.76 kg VSS/m^/
day (0.234 lb VSS/ft"^/day) . SRT declined from 63.3 days during
phase one to 7.9 days in phase five, averaging 13.7 days. Hy-
draulic detention time declined from 22.3 days in phase one to
5.4 days in phase five averaging 9.1 days. VSS to the system
increased from an average 7.3 kg/day (38 lb/day) in phase one
to 90.8 kg/day (200 lb/day) in phase five, averaging 49.0 kg/
day (108 lb/day). Waste VSS from the system including spills
and foaming losses averaged 8.6 kg/day (19 lb/day) in phase one,
increasing to 54.9 kg/day (121 lb/day) in phase five and aver-
aging 28.8 kg/day (63.4 lb/day). VSS inventory in both tanks
remained relatively constant, averaging 388 kg (855 lb) during
phase one and increasing to 418 kg (921 lb) during phase five
for an average increase of 0.16 kg/day (0.36 lb/day).
The amount of oxygen respired per lb VSS reduced declined from
1.94 in phase three to 1.49 in phase five, averaging 1.7. The
amount of oxygen supplied per lb of VSS reduced which reflects
the oxygen transfer efficiency, declined from 6.93 lb during
phase one when severe foaming and plugging problems were exper-
ienced to a minimum of 2.3 7 in phase foir,averaging 3.1 lb.
VSS reductions in the digester increased from 8.13 kg/day (17.9
lb/day) in phase one to 35.1 kg/day (77.3 lb/day) in phase five,
averaging 20.2 kg/day (44.6 lb/day). Percent VSS reduced de-
clined from 47.1% at the lowest loading rate to 38.8% at the
highest loading rate averaging 42.7%.
Laboratory Data
Experimental data for the flow-through test with the FAD are
presented in Tables 31, 32 and 33. Table 31 summarizes influ-
ent loadings to-foe digester, Table 32 summarizes effluent wast-
ing from the digester and Table 33 compares the percent change
between influent and effluent. TS declined by 8.1%, TVS de-
clined by 14.9%, TSS declined by 21.3%, VSS declined by 27.2%
while TDS increased by 173%.
124
-------
Table 30. Pure oxygen digester flow through pilot plant (FAD) - performance data
PHASE
LOADING
lb VSS/ftVday(l)
FEED
WASTE
INVENTORY
RETENTION TIME (DAYS)
oxygen/vss reduced
AEROBIC DIGESri®
lb/day<2)
lb/day (2)
Aib/day('2.
lb (2)
S R T
HYDRAULIC
RESPIRED
SUPPLIED
lb/dav(2)
percent
I
MEAN
KIN.
MAX.
0.083
O
0.135
38.0
0
81.9
19.0
0
171.5
+ 1.1
855
63.3
22.3
18.8
28.2
N.A.
6.93
17.9
47.1
II
MEAN
KIN.
MAX.
0.139
0
0.188
64.3
0
92.4
35.8
0
76.8
+ 1.1
884
25.3
15.7
11.0
23.3
N.A.
3.76
27.4
42.6
III
MEAN
MIN.
MAX.
0.196
0.092
0.293
90.3
42.3
134.9
52.9
31.2
98.0
- 3.8
806
16.6
10.7
6.4
21.9
1.94
2.96
41.2
45.6
IV
MEAN
MIN.
MAX.
0.326
0.230
0.421
150.1
127.0
193.8
88.3
59.4
138.4
+ 2.4
866
10.2
6.7
5.8
12.8
1.70
2.37
59.4
39.6
V
MEAN
MIN.
MAX.
0.433
0.301
0.579
199.2
152.3
266.2
120.9
90.9
186.2
+ 1.0
921
7.9
5.4
5.0
6.4
1.49
2.68
77.3
38.8
159 DAYS
0.235
108.4
63.4
+ 0.36
866
13.7
9.1
1.70
3.10
44.6
42.7
(1) lb VSS/ft3/day x 16.02 = kg/m3/day
(2) lb/day x 0.454 = kg/day
-------
Table 31. Pure oxygen digester flow througn pilot plant (FAD) - laboratory data - influent
loading (mg/1 unless other units indicated)
PHASE
SOLI
JS
COD
N
1TROCEN
TOTAL
P
ALK.
38
CaCOi
pH
{Units'!
COND.
jtnho/cm^
GREASE
PERCENT
FECAL COLI
no./ml
ts
TVS
TSS
vss
, TDS
(calc.)
PERCENT
VSS/TSS
TKN
NH4-N
NO3-N
I MEAN
MIN.
MAX.
47,930
61,240
62,440
37,150
45,640
38,700
60,000
35,350
27,200
44,500
2,290
77.5
55,010
42,900
76,800
3,450
2,930
3,980
180
60
285
0.90
0.01
2.86
1,150
754
2,680
2,320
1,740
3.330
6.5
6.1
7.4
1,790
1,060
2,520
-
-
II MEAN
MIN.
MAX.
48,630
34,180
58,860
37,740
45,680
31,000
56,500
35,470
24,500
50,600
2,950
77.6
51,120
34,600
70,800
3,430
2,380
4,420
210
50
350
0.32
0.01
1,59
1,280
684
2,780
2,230
1,560
3,430
6.6
6.4
7.4
1,680
1,130
2,S80
:
Ill MEAN
MIN.
MAX.
43,070
36,200
62,300
35,530
39,230
32,900
56,500
32,360
27,600
47,600
3,840
82.5
51,840
43,200
72,800
3,590
3,070
5,050
127
36
320
0.23
0.01
1.12
1,880
800
2,910
1,670
1,370
2,160
6.5
6.1
6.8
1,870
1,150
3,110
7.4
37.0
x 106
IV MEAN
MIN.
MAX.
45,350
38,600
50,000
37,960
41,830
35,200
49,200
35,020
31,800
43,100
3,520
83.7
56,380
45,600
69,400
3,940
3,420
4,760
120
40
260
0.21
0.01
0.65
1,120
900
1,300
1,970
1,340
2,120
6.6
6.4
6.9
1,410
980
2,060
17,8
16.0
x 106
V MEAN
MIN.
MAX.
48,140
36,800
62,800
40,000
44,860
30,500
58,900
37,290
26,700
47,000
3,280
83.1
61,360
46,600
70,100
3,990
3,680
4,330
165
39
385
0.10
0.01
0.15
1,110
600
1,460
1,710
1,330
2,860
6.5
6.2
6.9
1,620
1,070
2,760
7.4
15'7*
x 10
AVG.
159 DAYS
46,620
37,675
43,450
35,100
3,180
80.0
55,140
3,680
160
0,35
1,310
1,980
6,6
1,670
10.9
23.0
v in6
-------
Table 32. Pure oxygen digester flow through pilot plant (FAD) - laboratory data
effluent waste
SOLID
S
NITROGEN
ALK.
FECAL COLI
no./ml
PHASE
TS
TVS
TSS
vss
. TDS
(calc.)
PERCENT
VSS/TSS
COD
TKN
NH4-N
N03-N
TOTAL
P
as
CaC03
, pH ,
.Units.
COND.
yraho/c'B'
GREASE
PERCENT
I
MEAN
44,440
31,830
37,040
24,870
7,400
67.1
40,180
3,060
860
3.60
1,350
2,450
6.8
4,550
_
_
MIN.
45,900
-
31,200
20,400
-
-
33,000
2,230
200
0.01
608
1,840
6.2
2,840
-
-
MAX.
49,600
-
43,400
29,000
-
-
53,300
3,680
1,600
14.8
2,080
3,410
7.4
7,280
-
-
II
MEAN
45,180
31,130
37,950
26,140
7,230
68.9
42,430
3,040
805
0.60
1,270
4,200
7.2
3,720
-
KIN.
42,400
-
28,900
21,400
-
-
34,700
2,750
290
0.01
528
3,040
6.4
3,000
-
-
MAX.
52,300
-
43,500
31,200
-
-
49,900
3,360
1,300
4.48
2,600
4,820
7.9
5,620
-
-
III
MEAN
38,180
30,390
30,780
24,510
7,400
79.6
41,410
2,990
300
0.22
1,930
1,710
6.5
3,040
7.8
4.7
x 106
MIN.
36,100
-
24,400
20,800
-
-
35,600
2,140
145
0.01
1,340
1,430
6.1
2,480
-
MAX.
41,800
-
38,600
30,900
-
-
50,600
3,500
650
0.60
2,880
2,500
7.0
4,600
-
17
MEAN
41,310
33,710
30,980
25,280
10,330
81.6
47,100
3,590
650
0.47
1,000
2,890
6.9
3,860
12.9
MIN.
39,800
-
21,000
17,600
-
-
36,000
3,030
250
0.01
970
2,760
6.4
2,620
-
MAX.
42,400
-
36,600
29,500
-
-
53,800
4,110
970
1.14
1,420
3,010
7.2
5,710
-
V
MEAN
42,150
33,420
34,130
27,070
8,020
79.3
52,300
3,690
680
0.36
1,300
3,120
6.8
4,560
7.4
5*i6n*
MIN.
39,700
-
26,500
22,100
-
-
43,600
2,240
490
0.01
600
2,760
6.6
3,820
-
MAX.
49,100
-
41,000
32,400
-
-
59,700
4,030
960
1.17
1,520
3,770
7.0
5,790
-
AVG.
159 DAYS
42,650
32,050
34,180
2S,570
8,680
74.8
44,680
3,270
660
1.05
1,370
2,880
6.8
3,950
9.4
5-* fi
x in6
-------
Table 33. Pure oxygen digester flow through pilot plant (FAD) - laboratory data influent
loading versus effluent waste percent change
H
N)
CO
PHASE
SOLIDS
COD
NITROGEN
TOTAL
P
ALK.
as
CaCOi
PH
units
COND.
¦
-------
The VSS/TSS ratio declined by an average 6.0% for all phases
with a range of 2.1% to 10.4%. COD reduction averaged 19.0%.
Nitrogen forms showed a decrease of 11.1% for TKN while amition-
ium-N increased by 312.5% and nitrates increased by 200%. Al-
though the increase in ammonium-N solubilization was substan-
tial, the absolute concentration of nitrates in the efflu-
ent never exceeded 3.6 mg/1. Total phosphorus increased by
only 4.6%. Alkalinity increased by +45.5% and pH increased by
0.2 units. Conductivity increased by +136.5% and TDS by 173.0%.
The change in grease content between influent and effluent for
the three periods that were analyzed indicates a reduction of
-13.8%. Fecal coliforms declined by an average of 76.5%.
Invertebrate Analysis
Tables 34 and 35 present the invertebrate biomass concentrations
(VSU) for tanks A and B respectively. Tank A was analyzed dur-
ing phases 1, 2 and 3 only, whereas tank B was analyzed for all
five phases. Table 36 summarizes the average invertebrate con-
centrations for each of the five phases in both tanks. Table
37 expresses the invertebrate biomass inventory as a percent
distribution while Table 38 expresses the invertebrate biomass
as a percent of the VSS under oxygenation. It is evident that
the invertebrate biomass declined considerably after being trans-
ferred from tank A to tank B. Tank A data reflect the biomass
population distribution of the activated sludge loaded to the
system, whereas tank B biomass is more representative of the
stabilized populations that are attained after substantial de-
tention times. The total invertebrate wet weight in tank A
averaged 13.8 g/1 and declined by 83% to 2.4 g/1 in tank B.
Rotifers and amoeba were observed in tank A but never in tank
B. Flagellates increased on a percent distribution basis from
5.9% in tank A to 12.9% in tank B, motile ciliates averaged
55% of the total invertebrate biomass in both tanks, sessile
ciliates increased from 14% in tank A to 32.5% in tank B, and
rotifers, amoeba and nematodes did not appear at all in tank B.
Flagellates as a percent of VSS declined from 0.18% in tank A
to 0.08% in tank B, sessile ciliates declined from 1.03% in
tank A to 0.35% in tank B, rotifers declined from 2.4% in tank
A to zero in tank B, and amoeba declined to zero in tank B.
Whereas 4.17% of the VSS in tank A consisted of invertebrates,
they declined to 0.95% in tank B, representing a decrease of
11%. This decrease indicates that the conditions of crowding
129
-------
Table 34. Pure oxygen digester flow through pilot plant (FAD) - invertebrate biomass
in tank A - VSU (ml/1)
N. TAXONOMIC GROUP
FLAGELLATES
MOTILE
SESSILE
ROTIFERS
AMOEBA
NEMATODES
TOTALS
ECOLOGICAL DIVERSITY INDEX
PHASE/DATeS.
CILIATES
CILIATES
OBSERVED/TOTAL
FLKLfcJU'
I 6/4/73
0.50
0
0
0
0
0
o.so
1/6
16.7
6/12 - 6/16/73
0
0
0
0
0
0
0
0
0
6/20/73
0.10
0
0
0
0
0
0.10
1/6
16.7
6/25/73
0.90
0
36.0
0
0
0
36.9
2/6
33.3
6/27/73
0. 30
0
0
0
0
0
0.3
1/6
16.7
7/2/73
0
0
0
0
0
0
0
0
0
7/10/73
0.5
0
0
0
0
0
0.5
1/6
16.7
II 7/16/73
2.90
0
0
0
0
0
2.9
1/6
16.7
7/1B/73
0.20
0
0
0
0
0
0.2
1/6
16.7
7/23/73
0.90
3.3
10.0
0
0
0
14.2
3/6
50.0
7/25/73
0
0
0
0
0
0
0
0
0
7/30/73
0.50
0
0
0
0
0
0.50
1/6
16.7
8/1/73
0
2.2
2.0
0
0
0
4.2
2/6
33.3
8/6/73
0.40
13.0
0
0
0
0
13.4
2/6
33.3
8/8/73
0
0
0
0
0
0
0
0
0
8/13/73
0.40
1.1
0
0
0
0
1.5
2/6
33.3
8/15/73
0
7.60
0
0
0
0
7.60
1/6
16.7
8/22/73
0
0
0
0
0
0
0
0
0
8/24/73
0
0
0
0
0
0
0
0
0
III 10/19/73
0
3.3
6.0
0
0
0
9.3
2/6
33.3
10/24/73
0.30
13.0
2.0
0
0
0
15.3
3/6
50.0
10/26/73
0.80
0
14.0
0
0.50
0
15.3
3/6
50.0
10/29 - 10/31/73
0
0
0
0
0
0
0
0
0
11/7/73
5.40
1.10
6.0
0
0
0
12.5
3/6
50.0
11/12/73
0
0
4.0
167.0
0
0
171.0
2/6
33.3
11/14/73
2.20
0
0
0
0
0
2.2
1/6
16.7
-------
Table 35. Pure oxygen digester flow through pilot plant (FAD) - invertebrate biomass
in tank B - VSU (ml/1)
u>
H
\
V TAXONOMIC GROUP
FLAGELLATE
MOTILE
SESSILE
ROTIFERS
AMOEBA
NEMATODES
TOTALS
RCOl.OfiTCAt. TVTVRRSTTY INDEX
PHASE/DATE
\
CILIATES
CILIATES
OBSERVED/TOTAL
PERCENT
I
6/4/73
0.50
0
0
0
0
0
0.5
1/6
16.7
6/l2 - 6/27/73
0
0
0
0
0
0
0
0
0
7/2/73
0.10
0
0
0
0
0
0.1
1/6
16.7
7/10/73
0
2.2
2.0
0
0
0
4.2
2/6
33.3
II
7/16/73
2.3
0
0
0
0
0
2.3
1/6
16.7
7/18/73
0.20
0
0
0
0
0
0.2
1/6
16.7
7/23 - 7/25/73
0
0
0
0
0
0
0
0
0
7/30/73
0.30
0
0
0
0
0
0.3
1/6
16.7
8/1/73
.10
3.8
0
0
0
0
3.9
2/6
33.3
8/6 - 8/8/73
0
0
0
0
0
0
0
0
0
8/13/73
2.40
2.2
0
0
0
0
4.6
2/6
33.3
8/15 - 8/22/73
0
0
0
0
0
0
0
0
0
III
10/19/73
0
3.3
6.0
0
0
0
9.3
2/6
33.3
10/24/73
0.2
4.3
2.0
0
0
0
6.5
3/6
50.0
10/26 - 11/12
0
0
0
0
0
0
0
0
0
11/14/73
0.3
0
0
0
0
0
0.3
1/6
16.7
IV
11/20/73
0
0
6.0
0
0
0
6.0
1/6
16.7
11/23 - 11/25/73
0
0
0
0
0
0
0
0
0
11/27/73
0
6.5
0
0
0
0
6.5
1/6
16.7
12/3/73
0.10
0
0
0
0
0
0.10
1/6
16.7
12/7 - 12/12/73
0
0
0
0
0
0
0
0
0
V
12/14/73
0
3.3
4.0
0
0
0
7.3
2/6
33.3
12/19/73
0
2.2
0
0
0
0
2.2
1/6
16.7
12/27 - 12/31/73
0
0
0
0
0
0
0
0
0
1/5/74
0
5.4
0
0
0
0
5.4
1/6
16.7
1/8/74
0.10
1.1
0
0
0
0
1.2
2/6
33.3
1/10/74
0
0
0
0
0
0
0
0
0
-------
Table 36. Pure oxygen digester flow through pilot plant (FAD) -
invertebrate biomass in tank A and B averages -VSU (ml/1)
MOTILE
SESSILE
PHASE
FLAGELLATE
CILIATE
CILIATE
ROTIFER
AMOEBA
NEMATODE
TOTALS
PHASE I
Tank "A"
.3
0
5.1
0
0
0
5.4
Tank "B"
.2
.6
.5
0
0
0
1.3
PHASE II
Tank "A"
.3
2.3
1.0
0
0
0
3.6
Tank "B"
.7
.8
0
0
0
0
1.5
PHASE III
Tank "A"
1.2
2.5
4.6
23.9
0.1
0
32.3
Tank "B"
.1
1.9
2.0
0
0
0
4.0
PHASE IV
Tank "B"
0
1.3
1.2
0
0
0
2.5
PHASE V
Tank "B"
0
2.0
0.7
0
0
0
2.7
\V£RAG£
Tank "A"
0.6
1.6
3.6
8.0
0.03
0
13.8
Tank "B"
0.2
1.3
0.9
0
0
0
2.4
132
-------
Table 37. Pure oxygen digester flow, through pilot plant (FAD) -
invertebrate biomass percent distribution - VSll basis
MOTILE
SESSILE
PHASE
FLAGELLATE
CILLATE
CILIATE
ROTIFER
AMOEBA
NEMATODE
10TALS
?HASE I
Tank "A"
5.6
94.4
0
0
0
0
100
Tank "B"
15.4
46.2
38.5
0
0
0
100
PHASE II
lank "A"
8.3
63.9
27.8
0
0
0
100
Tank "B"
46.7
53.3
0
0
0
0
100
^HASE III
Tank "A"
3.7
7.7
14.2
74.0
3.1
0
100
Tank "B"
2.5
47.5
50.0
0
0
0
100
•"HASE IV
Tank "B"
0
52.0
48.0
0
0
0
100
PHASE V
Tank "B"
0
74.1
25-9
0
0
0
100
AVERAGE
Tank "A"
5.9
55.3
14.0
23.8
1.0
0
100
Tank "B"
12.9
54.6
32.5
0
0
0
100
133
-------
Table 38. Pure oxygen digester flow through pilot plant (FAD)
invertebrate biomass as percent of VSS under oxygen-
ation (dry weight basis)
MOTILE
SESSILE
PHASE
FLAGELLATE
CILIATE
CILIATE
ROTIFER
AMOEBA
NEMATODE
TOTALS
PHASE I
Tank "A"
.085
0
1.4
0
0
0
1.5
Tank "B"
.080
0.24
0.20
0
0
0
0.5
PHASE II
Tank "A"
.085
0.65
0.28
0
0
0
1.0
Tank "B"
.27
0.31
0
0
0
0
0.6
PHASE III
Tank "A"
0.37
0.77
1.42
7.4
0.03
0
10.0
Tank "B"
0.04
0.78
0.82
0
0
0
1.6
PHASE IV
Tank "B"
0
0.51
0.47
0
0
0
1.0
PHASE V
Tank "B"
0
0.74
0.26
0
0
0
1.0
AVERAGE
Tank "A"
0.18
0.47
1.03
2.47
0.01
0
4.17
Tank "B"
0.08
0.52
0.35
0
0
0
0.95
134
-------
in this system vere not conducive to growth and reproduction of
these organisms.
Viable Biomass (ATP) Analysis
The viable portion of the aerobic digester biomass was quanti-
fied as the volatile portion of the suspended solids in the
initial phases of this study. The biodegradable VSS was deter-
mined by an arbitrary calculation for the batch tests only. No
simple, accurate method of distinguishing between viable and
non-viable organic matter was available at the start of this
project. Previous attempts to determine a specific measure of
active biomass such as standard plate counts, organic carbon
analysis, DNA, organic nitrogen or protein, rate of oxygen
utilization or dehydrogenase enzyme activity were limited in
their application by time consuming and sophisticated techni-
ques. Recent investigations (14) have shown adenosine tri-
phosphate (ATP) to be a specific measurement for biological
activity in the activated sludge process. New instrumentational
methods for ATP analysis have proven to be both rapid and sim-
ple .
ATP measurement is based on the firefly reaction where
luciferin (reduced) + ATP + O2 > luciferin (oxidized) +
pyrophosphate + AMP + H2O + light.
Two instruments that were used for ATP analysis included a Du-
pont luminescence biometer (15), and JRB Inc. ATP photometer
(16). Concentrations as low as 10~® mg/1 could be analyzed
directly after proper sample preparations. The JRB technique
recommends extraction of the sample with tris buffer, while the
Dupont instrument technique recommends the use of DMSO extrac-
tion. The DMSO extraction method yielded higher results than
did the tris buffer extractions. For example, identical sam-
ples of aerobic digester influent yielded an average ATP con-
centration of 15.8 mg/1 with the DMSO extraction versus 2.4
mg/1 with the tris buffer extraction. Similarly, analysis of
the aerobic digester effluent from tank B yielded 7.7 mg/1
using DMSO as compared with 0.8 mg/1 using tris buffer. Al-
though the absolute concentration of ATP was significantly
higher with DMSO, the rate of ATP reduction between the in-
fluent and effluent samples was similar with either method.
Table 42 summarizes the ATP data for the period 12/14/73 -
135
-------
3/15/74. At the completion of the pure oxygen FAD test on
January 13, 1974, operation of the pilot plant was continued
for additional ATP data collection. ATP analysis phases 6 and
7 using the FAD during the period January 14 - March 15, 1974
are included in Table 39.
ATP reduction during phase five averaged 64.5% compared with
69.9% in phase six and 65.5% in phase seven. ATP reduction
during phases 5, 6 and 7 averaged 66.6% compared with 37.0%
VSS reduced for the same three phases. ATP reduction/VSS re-
duction averaged 1.8. The consistency of these results is
all the more remarkable in view of the fact that no good co-
efficient of correlation was obtained between VSS and ATP con-
centrations in any of the phases studied. Similarly, no sig-
nificant correlations were found between OUR and ATP concen-
trations in the aerated biomass. The utility of ATP, as a
measure of aerobic digestion performance, should be further
investigated in future aerobic digestion studies.
Sludge Dewaterabilitv
Table 4o presents vacuum filter leaf test data run on raw in-
fluent and digested effluent WAS samples from the pure oxygen
pilot plant for the period 7/3/73 to 1/7/74. Vacuum filter
performance was calculated on the basis of the chemical cost,
divided by the concentration factor x the loading rate. The
average influent TS averaged 4.8% as compared with 4.2% in
the effluent. Optimal chemical dosage is defined as the low-
est vacuum filter performance factor at the highest filter
cake concentration. At the optimal dosage, the filter cake TS
averaged 14.2% in the influent compared with 13.3% in the efflu-
ent. This represented an influent concentration factor of 3.0
compared with 3.2 for the effluent.
The chemical demand for optimum dewatering of the influent
solids was 9.0% FeCl3 and 19.2% lime compared with 13.7% FeCl3
and 19.0% lime for the effluent.
At an average cost for FeC^ of $100/ton and $25/ton for lime,
influent WAS chemical cost was $13.80 compared to $20.95 for
digester effluent chemical cost. The loading rate at the opti-
mal chemical dosage was 13.7 kg/m^/hr (2.8 lb/ft2/hr) in the
influent WAS and 13.2 kg/m2/hr (2.7 lb/ft2/hr) in the effluent
WAS. The vacuum filter performance which summarizes all of
136
-------
Table 39. Pure oxygen digester flow through pilot plant - ATP
data summary
ATP CONCENTRATION (eg/1)
PERCENT ATP
ATP REDUCTION
PERCENT VSS
PHASE
INFU'WJT
EFFLUENT
REDUCTION
VSS REDUCTION
REDUCED
V MEAN
3.24
1.15
64.5
1.67
33.8
KIN.
1.40
1.01
MAX.
5.07
1.28
VI MEAN
2.16
0.65
69.9
1.98
35.4
MIN.
0.69
0.46
MAX.
4.54
0.86
VII MEAN
18.04
6.23
65.5
1.78
36.8
MIN.
7.42
1.89
MAX.
32.16
11.32
Phase V - 12/14/73 - 1/13/74
Phase VI - 1/14/74 - 2/13/74
Phase VII - 2/14/74 - 3/15/74
(Tris-Buffer Extraction Method)
(Tris-Buffer Extraction Method)
(D.M.S.O. Extraction Method)
137
-------
Table 40. -pure oxygen digester flow through pilot plant influent and
effluent vacuum filter leaf performance
LOAD RATE
PERCENT TS
CONC.
PERCENT CHEMICALS
COST
DATE
SAMPLE
FEED
CAKE
FACTOR
Feci j
LIME
TOTAL
(S/TON)
lb/ft2/hr
FACTOR (2)
7/3/73
Influent
4.8
14.5
3.0
4.7
20.0
24.7
9.70
1.1
2.9
¦*
14.9
3.1
9.5
20.0
29.5
14.50
2.3
2.0
"
15.5
3.2
9.5
30.0
39.5
17;00
3.8
1.4*
Effluent
4.4
14.9
3.4
4.8
20.0
24.8
9.80
0.8
3.6
"
14.7
3.3
9.5
20.0
29.5
14.50
1.4
3.1
"
15.5
3.5
9.5
30.0
39.5
17.00
2.1
2.3*
8/1/73
Influent
5.1
13.5
2.6
4.6
9.8
14.4
7.05
1.1
2.5
"
15.6
3.1
5.5
13.7
19.2
8.93
1.9
1.5
"
15.5
3.0
7.1
15.7
22.8
11.03
2.6
1.4*
Effluent
4.7
13.2
2.8
5.0
10.7
15.7
7.68
0.3
9.1
"
15.0
3.2
7.7
17.1
29.8
11.98
1.3
2.9
"
15.0
3.2
9.6
21.4
31.0
14.95
1.7
2.7*
11/10/73
Influent
4.4
10.4
2.4
4.6
9.1
13.7
6.88
0.8
3.6
"
13.5
3.1
9.1
18.1
27.2
13.63
2.5
1.8*
"
13.7
3.1
11.9
23.6
35.5
17.80
2.9
2.0
Effluent
3.7
11.1
3.0
7.9
15.5
23.4
11.78
0.7
5.6
"
10.9
2.9
11.8
23.4
35.2
17.65
1.4
4.3*
"
10.8
2.9
9.8
19.6
29.4
14.70
0.9
5.6
11/13/73
Influent
5.7
12.9
2.3
4.3
8.7
13.0
6.48
1.4
2.0
"
14.2
2.5
6.2
12.4
18.6
9.30
1.9
2.0*
*'
14.7
2.6
8.1
16.1
24.2
12.13
2.2
2.1
Effluent
3.9
10.8
2.8
13.6
27.1
40.7
20.33
1.7
4.3
"
10.9
2.8
16.3
32.6
48.9
24.45
1.8
4.9
"
11.2
2.9
18.6
36.2
54.8
27.65
2.7
3.5*
11/26/73
Influent
4.4
11.6
2.6
6.8
13.5
20.3
10.18
1.5
2.6
"
12.3
2.8
9.0
18.1
27.1
13.53
2.6
1.9*
"
12.6
2.9
10.1
20.3
30.4
15.18
2.5
2.1
Effluent
4.1
11.7
2.9
10.3
20.6
30.9
15.45
1.3
4.1
"
12.3
3.0
14.6
29.2
43.8
21.90
2.1
3.5
"
12.1
3.0
18.9
37.8
56.7
28.35
2.8
3.4*
12/10/73
Influent
4.6
13.1
2.8
6.5
12.9
19.4
9.73
1.4
2.5
"
14.5
3.2
10.7
21.6
32.3
16.10
2.3
2.2
"
14.3
3.1
16.2
32.3
48.5
24.28
4.6
1.7*
Effluent
4.1
14.7
3.6
15.4
31.0
46.4
23.15
2.5
2.6
"
14.4
3.5
17.5
36.9
54.4
26.73
3.7
2.1*
"
14.1
3.4
22.7
45.4
68.1
34.05
3.9
2.6
12/20/73
Influent
4.9
11.9
2.4
10.2
19.4
29.6
15.05
5.8
1.1*
"
13.3
2.7
8.5
15.4
23.9
12.35
2.2
2.1
"
12.6
2.6
5.9
11.7
17.6
8.83
1.6
2.1
Effluent
4.5
11.6
2.6
14.1
27.8
41.9
21.05
2.6
3.1
"
13.3
3.0
16.0
31.4
46.4
23.85
3.6
2.2*
"
12.6
2.8
11.3
22.4
33.7
16.90
2.1
2.9
1/2/74
Influent
5.0
14.2
2.8
9.6
19.2
28.8
14.40
3.1
1.7
"
13.8
2.8
7.7
15.4
23.1
11.55
2.5
1.7
"
14.5
2.9
5.8
11.5
17.3
8.68
2.1
1.4*
Effluent
4.4
13.0
3.0
13.7
27.5
41.2
20.58
2.6
2.6*
12.6
2.9
9.1
18.3
27.4
13.68
1.0
4.7
"
13.0
3.0
11.4
22.9
34.3
17.13
1.8
3.2
1/7/74
Influent
4.1
15.1
3.7
9.8
19.7
29.5
14.73
2.5
1.6*
"
12.1
3.0
7.8
15.8
23.6
11.75
2.0
2.0
Effluent
4.2
10.0
3.4
9.5
19.0
28.5
14.25
0.6
7.0
"
14.7
3.5
15.0
30.0
45.0
22.50
2.7
2.4
"
15.0
3.6
20.0
40.0
60.0
30.00
4.1
2.0*
AVC, ,
(optimal
Influent
4,8
14.2
3.0
9.0
19.2
28.2
13.80
2.8
1.6
dosage)
Effluent
4.2
13.3
3.2 |
13.7
29.0
42.7
20.95
2.7
2.4
(1) lb/ft2/hr x 4.88 = kg/m2/hr
(2) Vacuum filter performance factor = ¦- $/ton——
r conc. factor x load rate
(lowest factor = best performance)
138
-------
these factors was 1.6 for the influent versus 2.4 in the efflu-
ent, (the smaller the factor, the better the performance). This
represents a 50% increase in chemical costs of the effluent sam-
ple in order to obtain equivalent filter cake concentrations and
loading rates.
If the data for chemical costs are plotted against loading rates
as shown in Figure 60, two straight line equations are obtained:
y (effluent chemical cost) = 5.83 x loading rate +9.8 and
y (influent chemical cost) = 5.19 x loading rate -0.6.
139
-------
EXPERIMENTAL RESULTS - PURE OXYGEN FLOW-THROUGH (RAD) TESTS
Stage 3 testing with RAD in a single tank commenced on March 6,
1974 and continued until May 6, 1974. The objectives of these
tests were:
1. To demonstrate the capability of the RAD to aerobi-
cally digest thickened WAS without prescreening.
2. To demonstrate a high degree of O2 transfer efficiency.
3. To demonstrate a high degree of VSS reduction.
The first objective was consistently achieved. No debris accumu-
lation appeared, despite the removal of prescreening which was
required with the FAD. The other two objectives were realized
and are discussed in test runs 1 and 2. Experimental data for
the flow-through RAD test are presented in Tables 41, 42 and 43.
Table 41 summarizes the operating data, Table 42 summarizes the
influent and effluent laboratory data, and Table 43 presents
calculations of biomass reduction performance.
Run No. 1 of the stage 3 test series was loaded at 9.60 kg VSS/
m^/day (0.60 lb VSS/ft^/day) while the loading for run No. 2
was reduced to 6.88 kg VSS/m^/day (0.43 lb VSS/ft^/day). DO
concentration averaged 1.2 mg/1 with range of 0.5 to 6.1 mg/1.
OUR was substantially higher during the test run at the higher
loading rate. The lb O2 supplied/lb VSS reduced averaged 1.54
with a range of 1.36 to 1.71. This performance should be com-
pared favorably with the flow-through FAD tests which averaged
3.1 lb O2 supplied/lb VSS reduced. The RAD test results re-
presented a 50% reduction in oxygen supply requirements com-
pared with the FAD test results at identical loadings. The
pounds of oxygen respired compared to pounds of oxygen supplied
indicates an oxygen utilization of greater than 100%. This
anomaly is attributed to oxygen uptake sampling and calculation
methods previously discussed.
140
-------
Table 41 . Pure oxygen digester flow through pilot plant (RAD) -
field data
(1)
OXYGEN
VOLUMETRIC DAIA fcal /day)
TEMPERATURE f'O
D 0
UPTAKE! lb/day
lb/day(2]
OXYGEX/VSS DIG
TEST NO.
FEED
WASTE
TANK B
OXYGEN
ag A
ng/l/hr
RESPIRED
SUPPLIED
RESPIRED
SUPPLIED
L. MEAN
474
448
33
10
1.5
247
85
83
1.75
1.71
MIN.
365
348
27
2
0.5
141
48
39
-
-
MAX,
748
643
43
17
6.1
453
155
192
-
-
2. MEAN
397
346
32
19
0.8
189
65
43
N.A.
1.36
MIN.
313
296
25
9
0.4
111
38
33
-
-
MAX.
848
383
35
26
1.3
249
85
52
-
-
AVG.
57 Days
436
397
32. 5
14.5
1.2
218
75
63
-
1.54
(1) gal/day x 3.785 = 1/day
(2) lb/day x 0.454 = kg/day
141
-------
Table 42. Pure oxygen digester flow through pilot plant
(RAD)-influent and effluent laboratory data
TEST RUN
TSS (tng/1)
VSS (mg/1)
1. INFLUENT
MEAN
A3,200
35,200
KIN.
35,400
29,300
MAX.
51,400
42,200
EFFLUENT
MEAN
31,850
24,600
MIN.
22,900
17,700
MAX.
42,200
31,700
PERCENT CHANGE
INF vs EFF
- 26.3
- 30.1
2. INFLUENT
MEAN
38,200
31,300
KIN.
31,500
26,900
MAX.
44,000
35,400
EFFLUENT
MEAN
29,600
23,100
KIN.
23,700
20,000
MAX.
34,000
•26,900
PERCENT CHANCE
JNF vs EFF
- 22.5
- 26.2
142
-------
Table 43. Pure oxygen digester flow through pilot plant (RAD) -
biomass reduction performance
TEST RUN
LOADING U>
lb VSS/ft3/day
feed(2)
lb/day
WASTE z-*
lb/dayw
IJJVENTOf
lb/day
IY
lb
RETENTIO
SRT
S TIME (DVi'S)
HYDRAULIC
AEROB. I
lb/da\<2)
IG.
Z
1. MEAN
MIN.
MAX.
0.60
0.47
0.93
137
108
213
92
64
130
-3.0
424
4.7
3.1
6.2
3.7
2.3
4.7
48.4
35.3
2. MEAN
MIN.
MAX.
0.43
0.34
0.51
99
79
118
66
59
82
+0.7
386
5.8
5.2
6.6
4.5
4.1
5.4
31.5
32.0
£yG
57 Days
0.515
118
79
-1.6
405
5.3
4.1
40.0
33.7
(1) lb VSS/ft3/day x 16.02 = kg/m3/day
(2) lb/day x 0.454 = kg/day
143
-------
The average loading for both test periods was 8.25 kg VSS/m^/day
(0.515 lb VSS/ft^/day). VSS loadings averaged 53.6 kg/day (118
lb/day) with an average wasting rate of 36.8 kg/day (79 lb/day).
Inventory decreased by an average 0.73 kg/day (1.6 lb/day) for
the 57 day period. During the test run 1, inventory declined by
an average of 1.36 kg VSS/day (3.0 lb VSS/day) with an increase
of 0.32 kg/day (0.7 lb/day) during the second test run. The SRT
and hydraulic detention time averaged 5.3 days and 4.1 days re-
spectively, indicating the feasibility of high volumetric load-
ings and low space requirements. VSS digestion averaged 33.7%.
The temperature differential between ambient oxygen and biomass
was 13°C at the lower loading and 23°C at the higher loading.
These temperature differentials are similar to those observed
during the flow-through FAD test program.
The data obtained with the RAD represents a significant break-
through in the technology of pure oxygen aerobic digestion. No
other literature references are known indicating the existence
of pure oxygen digestion systems capable of achieving a high
degree of biomass reduction in thickened WAS at very high VSS
loading rates and low oxygen utilization levels.
144
-------
COMPARISON OF AIR AND OXYGEN PERFORMANCE
In order to make a comparison of the plant scale diffused air
digestion performance with the pure oxygen pilot plant per-
formance, the Metro diffused air data wore recalculated by-
averaging data for months of approximately equivalent SRT and
VSS loading rates. Table 44 summarizes the organic loadings
versus the percent change between influent and effluent labor-
atory data. Table 45 summarizes the percent change between
influent and effluent versus SRT.
While there is some overlapping of loading rates, ^etween the
ranges 7.9 to 29.8 days S^T or 1.34 to 3.0 XgVSS/m /day
(0.084 to 0.187 lb VSS/ft /day), the extreme ranges of less
than 8 days SRT for the air system and greater than 30 days SRT
for the oxygen system do not overlap.
A major difference between the air and oxygen systems relates
to the temperature ranges experienced with each system, whereas
air system temperatures were subject to sudden changes (between
11.5 and 32.2 C) the pure oxygen pilot plant system was conduct-
ed indoors and therefore not subject to cold temperature shock.
Significant increases in biomass temperature occurred with the
oxygen system, particularly during the initial batch tests.
During batch test 1, the temperature increased from an initial
19.4°C to a high of 44 c, for an increase of 24.5 C. The sudden
decline in OUR with an increase of temperatures above 40 C in-
dicates that the mesophilic biomass may have been replaced by
an incipient thermophilic bacterial culture. The intermediate
temperature range of 40 to 50 C is apparently an inefficient
range for accomplishing rapid VSS reduction because of time re-
quired for reestablishment of new dominant bacterial cultures.
During the flow-through pure oxygen testing using the FAD, the
biomass temperatures increased in direct relation to the loading
rates, with the maximum increase of 20 C being experienced at
the highest loading rate. A temperature increase of 23 C was
attained when using the RAD at the highest loading rate. The
ability to maintain high loading rates in thickened WAS suggests
the possibility of thermophilic aerobic digestion with
145
-------
Table 44. Diffused air digester laboratory data - organic loading versus percent change
between influent and effluent
V (1
lb/vss/ft /day
TOTAL
SOLIDS
SUSPENDED SOLIDS
TOTAL DISSOLVED
COD
NITROGEN-N
CONDUCTIVITY
/jmo/cn2
PH
units
ALK.
as CaCO-t
TSS
vss
SOLIDS
NOixlO0
NH/,
TKN
0.026 (a)
-28.7
-36.0
-39.3
+38.9
-42.1
+135
+77.3
-35.0
+46.0
-0.2
-36.5
0.052 (b)
-32.5
-46.8
-50.9
+98.5
-50.8
+200
H41.1
-32.9
+73.5
-0.45
-63.9
0.085 (c)
-26.9
-36.1
-42.6
+58.5
-42.8
+180
+90.0
-35.2
+45.1-
-0.13
-32.3
0.147 (d)
-14.9
-18,1
-21.3
+16.2
-25.5
+0.19
+55.2
-11.8
+18.0
+0.05
+11.4
0.187 (e)
-10.6
-13.6
-14.6
+16.7
-18.1
+0.07
+12.9
-10.3
+12.5
0
+4.3
Data Averaged From: (T) lb VSS/ft3/day x 16.02 = kg/m3/day
(a) April 1973
(b) October. November 1972
(c) Augusti September 1972, May, June 1973
(d) December 1972, March, July, August 1973
(e) January, February 1973
-------
Table 45. Diffused air digester laboratory data - SRT versus percent change between
influent and effluent
SRT
(DAYS)
TOTAL
SOLIDS
SUSPENDED SOUUS
TOTAI. DISSOLVED
COD
NITROGEN - N
CONDUCTIVITY
pmlio/cm^
PH
units
ALK.
as CjiCOt
TSS
vss
SOLIDS
NOixlO-5
NH&
T.K.N.
3.0 (a)
-11.0
-14.0
-15.8
+15.4
-19.0
+0.11
+20.5
-12,2
+13.4
0
+0.7
4.1 (b)
-15.9
-19.1
-22.3
+17.3
-27.0
+0.19
+61.7
-10.4
+19.0
+0.07
-8.5
6.3 (c)
26.0
-34.9
-40.5
+47.9
-42.0
+22.8
+97.1
-31.4
+31.9
-0.1
-27.8
8.6 (d)
-24.5
-31.5
-40.6
+40.2
-42.2
+230
+44.7
-33.3
+44.3
0
-13.5
12.2 (e)
-32.5
-46.8
-50.9
+98.5
-50.8
+200
+141
-32.9
+73.5
-0.45
-63.4
18.3 (f)
-31.0
-43.2
-48.7
+97.9
-45.0
+445
+121.4
-44.8
+72.3
-0.40
-60.0
29.8 (g)
-28.7
-36.0
-39.3
+38.9
-41.2
+135
+77.3
-35.0
+46.0
-0.20
-36.S
Data Averaged From:
(a) January, February, August 1973 (d) June 1973
(b) December 1972, March, July 1973 (e) October, November 1972
(c) August, September 1972 (f) May 1973
(g) April 1973
-------
accelerated rates of VSS reduction if the oxygen system were to
be insulated to conserve the heat generated. It would, however,
be necessary to ensure that the thermophilic cultures were con-
sistently maintained above the minimum temperatures required
for optimal growth and development in order to avoid cycling
between mesophilic and thermophilic conditions with subsequent
unpredictability of biological performance.
Another difference between the air and oxygen system relates to
settleability and the possibility of decanting. While the
settling rate of the aerobically digested sludge from the
diffused air system was relatively slow <0.92m/hr, the subsequent
handling of the WAS with cationic polymers to obtain a thicken-
ed float precluded any additional solid-liquid separation by
gravity settling in the pure oxygen system. No significant
separation of the oxygen digested sludge was observed after
several weeks settling time. Comparison of settleability rates
between the air and oxygen systems was, therefore, not possible.
A further difference between the air and oxygen systems relates
to the influence of SRT prior to aerobic digestion or the ulti-
mate rate of VSS reduction. While the SRT of the activated
sludge prior to loading the aerobic digester appeared to have
little influence on the rate of VSS reduced, the opposite was
the case during the batch tests with pure oxygen. During batch
test 3 when diffused air digested sludge was loaded to the pure
oxygen pilot plant, a much lower rate of VSS reduction was ob-
served than when undigested sludge was loaded to the digester.
It appears that a major factor determining VSS reduction rates
in the oxygen digester was the initial VSS/TSS ratio. The
initial VSS/TSS ratio of the undigested WAS loaded to the oxygen
digester averaged 82% with little seasonal variability. A sig-
nificant reduction in the VSS/TSS ratio was noted in the diffus-
ed air digested sludge loaded to the oxygen pilot plant with
subsequent reduction- in digestion performance. The activated
sludge system gets continual replenishment of organic substrate,
with biosynthesis resulting in relatively high volatile fractions.
During aerobic digestion of WAS, however, there is no external
replenishment of soluble organic substrate. Endogenous respir-
ation, thus ensures that the effluent from the aerobic digester
will have a lower volatile solids ratio than the original sample.
There does not appear to be any advantage to aerobic digestion
in two stages, that is diffused air digestion followed by pure
oxygen digestion. If a pure oxygen system is available, the
best use of plant resources would indicate that the WAS be
loaded directly to the oxygen system without an intermediary
diffused air step.
148
-------
No difference in odor potential between the sludges from the
diffused air and pure oxygen systems was observed. The final
volatile solids ratio obtained would determine the potential for
odor generation, particularly if land spreading were the ultimate
sludge disposal method. The possibility of anaerobic conditions
causing odors should not be overlooked if the VSS fraction of
the digested sludge is greater than 60%. A more quantitative
measure of sludge stability than the evaluation of odor poten-
tials is the specific oxygen uptake rate (K ). Whereas in the
oxygen batch tests K of less than 1.0 was achieved in 10 days,
the flow through air and oxygen systems K ranged between 4 to
6. The high OUR above the endogenous respiration level was
attributed to metabolic resynthesis oxygen requirements using
lysed metabolites. No significant correlation between K and
nitrification rate was observed with either the air or tSe
oxygen system. Similarly, no relation was observed between
increasing DO concentration and the rate of aerobic digestion up
to 16.6 mg/1 in the oxygen system and up to 6.0 mg/1 in the
diffused air system. Contrary to some statements in the liter-
ature regarding the influence of DO concentration on digestion
levels, it appeared that DO concentration was an effect rather
than a cause of changing OUR. When OUR declined, DO concentra-
tions tended to increase if the oxygen supply was constant.
The specific oxygen uptake rate (K ) averaged 7.1 for the air
system compared with 5.0 in "A" tank and 4.3 in "B" tank of the
pure oxygen system. On the basis of the differential between
initial and final VSS/TSS ratios achieved in both the air and
the oxygen digesters, an empirical standard for a stabilized
sludge of Kr<^ appeared reasonable.
The percent reduction in the different solid forms achieved for
both the air and oxygen flow-through systems is^depicted in
Figure^52. At loading rates above 2.4 kgVSS/m /day (0.15 lb
VSS/ft /day), the percent reduction of VSS was greater.with the
oxygen systen than^the air system. Conversely, at loading rates
below 1.60 kgVSS/m /day (0.10 lb VSS/ft /day), the reduction of
all solids forms was greater with the air system than the oxygen
system. The degree of solubilization represented by TDS percent
change between influent and effluent was greater at all loading
rates for the oxygen system than for the air system. The oxygen
system was, therefore, better able to maintain a high degree of
VSS reductions at very high loading rates that could not be
maintained in the air system because of oxygen transfer limita-
tions. Figure 53 shows the change between influent and effluent
VSS compared with COD and TKN for both air and oxygen as a
function of organic loading rates and SRT. A very close
149
-------
MR) —
TS S
VS S (
T 0 S (0»M*
0)
^C.
TS (0»)
UlR)
.TS S (Oj)
V S S (0»)
6, S 3
O O o
® f- f-U>
2s £ffl
8
ORGANIC LOADINGS |jb VSS/ft3/day (X 16.02 = kg/m3/day|
/
^
/
/
T 0
S (AIR)
/
)
y
N
1
t—T D S (OzHxIO)
>
¦
<
J. — — — '
/
-T
(a
IR)
T S <0«)
>
b
\
¦
~ —•
_^_TSS (04)
"5
i
"t
y'
¦
.
^-V S S (0*)
N
•/— •
T S S
(A)
R1
vj
—
v—VS
3 (AIR)
O — ^ 0 M W KJ K>
10 V » o s; <£ 2?
ro «
m ci
(M CM
SRT (days)
Fig. 52. Air versus oxygen performance-percent change in
solids forms as a function of organic loadings
and SRT
150
-------
- TKN (AIR) ^-TKN
(0.)
»
&
>
""
_ — —
>
/ /I j;
/ / X
•<
' .
\ C00(0*)
//n
VSS
(AIR)
^—VSS (0*)
c—.
— . -i '
^-COO(Oi)
A
\\
1-1
1
)—o—
\
\
1
• — —
y-
U
&
(
KN (AIR) —
J —VSS(Oi)
-J-r^S===="* 1
\ 1 "** VSS l(AIR)
-COO (AIR )[
'
<0 £
SRT (days)
Fig. 53. Air versus oxygen performance-percent change
in VSS, COO and TKN as a function of organic
loadings and SRT
151
-------
correlation was observed in percent reduction of all three
parameters for "both the air and oxygen systems. The highest
rate of VSS gr COD reduction occurred below loadings of
1.60 kgVSS/m /day (0.1 lb VSS/ft /day) for the air system,
whereas performance with the oxygen system was uniformly high
throughout the entire loading range. For both the air and
oxygen systems at identical loadings, TKN" reductions were
usually less than COD reductions, which in turn were generally
less than VSS reductions. The ultimate potential of the
oxygen system for reducing VSS was exhibited in the batch tests.
During batch test 1 53.7% VSS reduction was achieved after 20
days compared with 53.4% in 14 days during batch test 2. In
both these tests, VSS approached the maximum limit in half of
the ultimate detention times. The high degree of correlation
noted in the diffused air digester between percent VSS reduced
and the time-temperature factor was also observed for the oxygen
system (Figure 57). No additional benefit appeared to be gained
by detaining sludge beyond 12 days at temperatures between
15-30°C.
The VSS/TSS reduction in the air digester averaged 5% compared
with 6% for the pure oxygen flow-through tests. These volatile
solids ratio reductions correlated well with the percent VSS
reduced, which averaged 32% for the air digester and 41% for the
oxygen flow-through FAD digester. Figure 54 shows the change
between influent and effluent in conductivity and TDS for both
air and oxygen systems as a function of organic loadings and SRT.
For both systems, the best performance was directly correlatable
with the highest differential in conductivity and TDS. Solubil-
ization of suspended solids to TDS reached a peak at SRT of 18
days in the air system, while solubilization continued to increase
in the oxygen system up to 63 days SRT. At identical organic
loadings, the solubilization rate was always higher in t^e oxygen
system than the^air system. At loadings of 1.39 3cg-vss/m /day
(0o19 lb VSS/ft /day), conductivity increased by 15% in the air
digester compared with 60% increase in the oxygen digester.
Conductivity appeared to be a simple and accurate analytical
field method for measuring degree of aerobic stabilization
achieved.
Conversion rates of organic nitrogen to nitrates, ammonium-N and
nitrogen gas were significantly different in the oxygen and air
digesters. During periods when temperature conditions were
favorable for nitrification, most of the organic nitrogen con-
version occurred as an increase in NO^ in the air digester.
152
-------
Approximately half of the organic nitrogen was converted to
nitrate, while a fourth of the conversion appeared as an in-
crease in ammonium-N concentration. The remainder of the con-
version was due to denitrification during periods of low DO
concentration. Figure 55 shows the change between influent
and effluent in ammonium and nitrate percent increases in re-
lation to organic loadings and SRT for both the oxygen and air
digesters. Whereas a very good correlation was observed be-
tween nitrification and time-temperature factor above 20°C for
the diffused air digester +0.96, no such correlation was ob-
served for the oxygen digester. High nitrification rates were
observed in several of the oxygen batch tests, but minimal
nitrification occurred in the flow-through tests. It is assumed
that the conditions of crowding due to the high biomass concen-
trations as well as air flotation polymer conditioning and high
mixing energy did not provide a suitable environment for rapid
growth and reproduction of nitrifying bacteria in the oxygen
digester. In the air digester, the highest rate of nitrifica-
tion was observed when the temperature corrected OUR (K20) was
at a minimum. No such correlation was observed, however, with
the oxygen digester. Nitrate concentrations in the air digester
increased by a factor of 10^ between influent and effluent at
an SRT of 8.6 days. The highest nitrate levels observed in any
of the oxygen phases was less than 5 mg/1 NO3-N. Ammonium con-
centrations were significantly higher and NO3-N concentrations
were significantly lower in the oxygen digester than the air
digester at equivalent loading rates. At organic loadings of
3.0 kg vSS/m^/day (0.187 lb VSS/ft^/day), ammonium-N increased
in the air system by only 12% versus 150% in the oxygen system.
Increases in nitrate concentration in the diffused air digester
were observed to occur concurrently with increases in ammonium-
N concentration up to a SRT of 10 days and then declined in
parallel up to a SRT of 30 days. The oxygen system appeared
to convert more organic nitrogen to ammonium-N than the air
system at equivalent loadings, while more of the ammonium-N
was oxidized to nitrates in the air digester.
The effect of nitrification on changes in alkalinity and pH
are apparent in Figure 56. In the air system for organic
loading rates of (<2.24 kg VSS) m^/day (CO.14 lb VSS/ft^/day) ,
alkalinity and pH were lower in the effluent than the influent.
At loadings above 2.4 kg vSS/m^/day (0.15 lb VSS/ft^/day), ni-
trification ceased in the air digester causing both alkalinity
and pH to increase in the effluent sample. As nitrification
essentially did not occur in the flow-through oxygen digester,
153
-------
lu
<0
5
*
*
§
5
-
y
/
/
/
s
—C(
NOUCTl
MT
' (AIR)
A
/
/
\
//
O - IO ©V<£» OJ CM <0 K>
fr> o ^ u> ®
SRT (days)
Fig. 54. Air versus oxygen performance-percent change
in conductivity and TOS as a function of or-
ganic loadings and SRT
154
-------
NH4-N(Oi)
NH.-N (AIR)
NOi-N (Ot)
200
ORGANIC LOADINGS [lb VSS/ft3/day (X 16.02 = kg/m3/day]]
NH. -N (Ot)
NOs-N(AIR)
NH.-NUIR)
NO»-N (Oc) (x 10* )
o — 10 All) N OJ
ri ~ <0 n® g N
isi 8
S R T (dayt)
Fig. 55. Air versus oxygen performance - percent change
in NH4and N03-N as a function of organic loadings
and SRT
155
-------
ALKAl
; ¦"
ALKALINITY (0*) /
/
/
.pH (x 10
")(0t)
IHITY (AIR)
i g i a* m
ORGANIC LOADINGS [Tb VSS/ft3/day (X 16.02 = kg/m3/dayj]
\
¦^r
N
-ALKALINITY (0*)
V
\
\
:£
pHUIO-'XOt)
V
* ¦>
~a
-pH (xic
/
•AL
KAI fNITY (AIR )
O — m o<£> eg
fi yf <£> O CJ
10 rt
«b CD
s a
S, R T., {d oy • )
5
Fig. 56. Air versus oxygen performance - alkalinity and
pH change as a function of organic loadings and
SRT
156
-------
w.ooo
900O
eooo
7O00-
eooo
SOOO
4000
3000
O-DIFFUSED AIR
X—OXYGEN - SLOT DIFFUSER
A-OXYGEN-ROTARY DIFFUSER
PERCENT VSS REDUCED
Fig. 57. Temperature-time factor versus percent VSS reduced in
air and oxygen digesters
157
-------
alkalinity, and pH were always greater in the effluent than
the influent. Variability in pH and alkalinity changes were
related to stripping of CC>2 and volatile organic acids. For
the air digester, the best performance coincided with the
highest rate of nitrification and highest pH and alkalinity
differential between influent and effluent. The measurement
of pH could be used in the air digester under conditions of
nitrification as a good indicator of stability. In the oxygen
digester nitrification did not occur and pH could not be used
as an indicator of stabilization.
Figure 58 shows VSS reduction as a function of loading rates
for both the oxygen and air digesters. At loading rates below
1.28 kg VSS/m^/day (0.08 lb VSS/ft^/day) the air system results
were equal to or better than the oxygen digester. At loadings
of 2.24 tej VSS/m^/day (0.14 lb VSS/ft^/day) , the performance
of the oxygen and air digesters were approximately equal. Above
2.24 kg VSS/irrVday (0.14 lb VSS/ft^/day) the performance of the
air system declined rapidly, while the oxygen digester continued
to perform well up to loading rates as high as 9.6 kg VSS/m^/day
(0.6 lb VSS/ft3/day).
Figure 59 shows the oxygen utilization efficiency expressed as
lb 02 supplied per lb VSS reduced versus loading rates for all
air and oxygen f low^-through tests. The ranges of oxygen effi-
ciency were related to the three different oxygen transfer
systems used. The best performance with the diffused air sys-
tem required 15 kg O2 supplied/kg VSS reduced. The best per-
formance using the pure oxygen FAD required 2.3 kg O2 supplied/
kg VSS reduced. The pure oxygen RAD required only 1.36 kg O2
supplied/ ^9 VSS reduced at its best performance. The optimal
loading rate for high oxygen transfer efficiency was 1.28 kg
VSS/m^/day (0.08 lb VSS/ft^/day) for the diffused air digester.
The optimal loading range for the pure oxygen system was not
determined, as the highest loading of 9.6 kg VSS/m^/day (0.6 lb
VSS/ft^/day) did not cause system failure. The change in oxygen
transfer efficiency in the diffused air digester varied with
temperature and liquid depth, ranging between 5.2% and 19.3%.
With the FAD, oxygen efficiencies as high as 93% were experienced
in one of the batch tests. This efficiency could not be con-
sistently maintained during the flow-through testing. With the
substitution of the RAD for the FAD, oxygen transfer efficien-
cies in excess of 90% were consistently achieved for several
months of continuous operation. In order to ensure high oxygen
transfer efficiencies with an open tank oxygen digester, it is
158
-------
60;
h-
s:
LU
"PURE OXYGEN DIFFUSION
CONCENTRATED W.A.S. {> 4.5 %
TSS)
O
q:
lu
Q_
30-
ETRO DENVE
R AtF DIFF
\s (c;0.e%
USION
20
.10 .15 .20 .25 .30 .35 .40
LOADING RATE-"lb VSS/ft^/day (X 16.02 = kg/m^/day)
.45
£5
.05
.50
55
.60
Fig. 58. Air versus oxygen performance-percent VSS reduction as
a function of loading rate
159
-------
MET
RO DIFFUSED
AIR SY
STEM
\
PURE Oi- SL<
T OlFl
USER
SYSTEM
PURE 0
ROTATir
- Jt
G 01FF
SY3
ISER
TEM
.05 .10 .15 .20 .25 .30 .35 40 .45 .50 .55 £0 £5 .70
LOADING RATE-lb VSS/ft3/day (X 76.02 = kg/m3/day)
Fig. 59. Air versus oxygen performance-oxygen supplied/VSS. reduced
as a function of loading rate
160
-------
necessary that the DO never exceed the saturation concentration
at the air-liquid interface.
Comparison of the dewaterability of undigested and aerobically
digested sludges from the diffused air and oxygen digesters
indicated no significant differences in vacuum filter specific
resistance (rs= 10^ sec^ /gr) for SRT between 3 and 13 days.
Differences in filter leaf performance tests were observed.
Figure 60 indicated that for the air digester, dewaterability
by vaccum filter comparing chemical costs with filter yields
improved after aerobic digestion. For the oxygen digester a
50% increase in chemical costs was required in order to obtain
equivalent vacuum filter performance with the digested sludge
compared with undigested sludge. The chemical demand of the
digested sludge from the air digester was approximately equiva-
lent to that of the air floatation thickened WAS loaded to the
oxygen digester. It appears that for dilute sludge (<1.0% TS) ,
aerobic digestion improves vacuum filter performance by reducing
the volatile fraction. After digestion of air flotation thick-
ened WAS (4 to 5% TS), there is an adverse effect on vacuum
filter performance which is related to the decrease in solids
concentration. The effect of SRT in the digester on air flo-
tation polymer demand for the diffused air system is directly
related to sludge particle size and TSS/VSS ratio* The higher
SRT in the digester results in higher polymer demand
Microscopic analysis of invertebrates revealed some interesting
differences between the air and oxygen digesters. The air di-
gester VSU ranged between 1.8 and 27.4 g/1, averaging 10.5 g/1.
For the pure oxygen flow-through digester, the VSU in "A" tank
averaged 13.8 g/1, compared with only 2.4 g/1 in tank "B". The
biomass loaded to the pure oxygen digester represented by tank
"A" was approximately equivalent to the VSU results for the
diffused air digester. The reduction in VSU observed in tank
"B" represented equilibrium conditions in the pure oxygen di-
gester. Ecological diversity, expressed as the number of
different taxonomic groups appearing in any particular sample,
was higher in the air digester than tank "A" of the oxygen di-
gester. Diversity in "B" tank was lower than observed in tank
"A". During the batch tests, invertebrate diversity declined
with increasing detention time. The sludge loaded to the oxygen
batch tests system had 3 to 4 out of 6 taxonomic groups ini-
tially, but the final sample after the 21 days SRT had 0 to 1
out of 6 taxonomic groups. Rotifers represented the major tax-
onomic group by weight and volume in the air digester with the
161
-------
32.0
¦C 30 JO
k.
§
vl
2
§
S
LEGEND
EFFLUENT (0»)
INFLUENT COt)
EFFLUENT (AIR)
INFLUENT — (AIR)
INFUJ&"
i {k\Rl
4.0
FILTER LEAF YIELD [Tb./ft 2/hr (x 4.88 = kg/m2/hri]
Fig. 60. Air versus oxygen performance-filter leaf yield per
unit chemical cost
162
-------
exception of shock loading or temperature periods. Rotifers
were absent in all samples observed from "B" tank of the pure
oxygen digester. Flagellates and ciliates comprised the ma-
jority of the invertebrate biomass in the oxygen digester.
The total invertebrates as a percent of VSS ranged between 2.9%
and 54.2% in the air digester, averaging 19.7%. In the pure
oxygen digester, the total invertebrates as a percent of VSS
ranged between 1.0 and 10.0% averaging 4.2% in "A" tank and
1.0% in "B" tank. The invertebrate population as a part of
the total VSS biomass was reduced between the diffused air
digester effluent and "A" tank of the oxygen system by 80%, and
was further reduced by 77% between "A" tank and "B" tank. Tem-
perature changes and loading stresses influenced population
dynamics in the diffused air digester. It is assumed that the
conditions of crowding and the high mixing energy in the pure
oxygen digester created an environment which was inimical to
growth and reproduction of higher invertebrate forms. In the
pure oxygen digester, VSS reduction at high loadings and solids
concentrations was dependent almost entirely upon mesophilic
bacteria. In the air digester, the rotifer population had a
significant/correlation with VSS reduction +0.87. During the
periods of optimal performance, rotifers made up as much as 50%
of the dry weight biomass.
The concentration of ATP in the diffused air digester effluent
averaged 0.34 mg/1 compared with 0.65 to 1.2 mg/1 in the efflu-
ent from the oxygen digester, using the tris buffer extraction
method. The ratio of ATP/VSS was 58 x 10^ in the air digester,
and 18 to 31 x 10^ for the oxygen digester (phases 5 and 6).
The percent reduction of ATP was not measured in the air digester,
but averaged 65% in the oxygen digester. The ratio of ATP/VSS
reduced was quite uniform, averaging 1.8 for the pure oxygen
digester. Fecal coliform reductions in the air digester ranged
between 13.5% and 97.1% increasing with higher temperatures and
lower loading rates. In the pure oxygen batch tests, fecal
coliform reduction was directly related to detention time, reach-
ing a maximum of 98.8% during batch test 1 (20 days SRT) versus
67.2% for batch test 2 (14 days SRT). The higher fecal coliform
reductions in batch test 1 may also be related to the elevated
temperatures experienced during that test.
The ecology of the pure oxygen digester was directly influenced
by the crowding induced by the air flotation polymer condition-
ing of the biomass. Quaternary ammonium compounds which consti-
tute the base of some of the cationic polymers used for dissolved
163
-------
air flotation may have caused a biostatic or biocidic toxicity
effect expressed as a reduction in ecological diversity. The
mixing energy required through the FAD to maintain the optimal
oxygen bubble size was also inimical to growth and reproduction
of the larger organisms such as rotifers that were observed to
be predominant during the best performance periods in the diffused
air digester.
164
-------
BENEFIT - COST ANALYSIS
Figures 61 and 62 show the relationship between percent WAS in
the total sludge mixture and sludge processing costs. The
dramatic decrease from $60 to $40 per mil gal in sludge pro-
cessing costs between 1970 and 1972 was directly attributable
to reduction of WAS by aerobic digestion. The capital costs
involved in this conversion were negligible. Since full scale
aerobic digestion was initiated in July 1970, this upgrading
modification has saved Metro Denver in excess of $900,000
(Table 46). The major cost factor for diffused air digestion
was electric power for compressed air. At a peak load cost of
7 mils/kwh, electric power costs $1.98/mil liters/sec ($4.20/
mil cfd). Average air supply for the 13 month period was 4.3
mil liters/sec (9.1 x 106 cfd.) Aerobic digestion performance
averaged 3.0 tons VSS reduced per day. Electric power costs
for aerobic digestion averaged $12.67 per ton of VSS reduced.
Maintenance costs were very low involving primarily diffuser
cleaning at an average cost of $1.33 per ton for a total oper-
ation and maintenance cost of $14 per ton. Prior to inaugura-
tion of aerobic digestion, WAS was disposed by air flotation,
vacuum filtration and truck haul or incineration. The operation
and maintenance costs of this alternative method averaged $50
per ton. Aerobic digestion of WAS reduces the mass of sludge
requiring further treatment and disposal. At an equivalent
vacuum filter cost per dry ton of undigested and aerobically
digested sludges, the savings as a result of digestion are $50-
$14 = $36/ton (1974 costs). The benefit-cost ratio of aerobic
digestion compared with conventional sludge disposal methods is
$50/14 = 3.5. This economic comparison does not consider amori-
tization of capital equipment cost. In order to continue to
obtain the benefits of VSS reduction resulting from aerobic di-
gestion while utilizing the secondary capacity for its origi-
nally intended purpose, Metro staff is at present considering
the conversion of an existing - 3755 m^ (1 mil gal) holding
tank to a pure oxygen digester utilizing RAD. Table 47 summar-
izes the estimated economic benefits to Metro of such a con-
version. The cost estimates were based on the on-site cryogenic
generation of pure oxygen at operation and maintenance cost of
165
-------
90
80
70
60
50
40
30
20
10
0
©
•)
10 20 30 40 50 6T
SLUDGE PROCESSING COSTS $^iil gal {X 3.79 - $/l X TO6;
Fig. 61. Sludge processing cost as a function of sludc
166
-------
8 118.67**
8 127.65
969
MANAGEMENT
6 SUPPORT
0EBT SERVICE
3 124.74
i:-no.es-;i\
v „ ;
8.70
:io.s6
13.03
15.00
^ ;ik&
52.70
40. 57
4 7 . 80
SI60.50
19.30
35.56
33.99
tesMty/i
^CJI2-50W;
•.'! 12.19 .
mwm
•:i, ¦ i
c7pr.-r,V7V
/W (7 80)',/0
1972
1971
- -IMPLIES
OEFICIT
FIN ANCIN3
* * (I 970)
*(1969)
119.70
126.42
OTHER FUNDS
113.01
110 67
. WW
WAS
was i;?»v*
y\. SAW'
>«'«¦
WAS
W» S
/5>siHN
-------
Table 46. Cost savings of Metro diffused air aerobic digestion
(1970-1974)
1968
1969
1970
1971
1972
1973
1974
Total WAS
Generated (X 1000 Tons)
6.5
17.9
19.9
13.0
15.3
17.2
19.3
Cone .WAS
(X 1000 Tons)
6.5
17.9
17.4
9.3
10.2
12 .5
14.7
tfAS Reduced by Aerobic
digestion (X l000 Tons)
0
0
2.S
3.7
5.L
4.7
4.6
Savings due to Aerobic
112
167
230
212
207
Total Cost Savings
$928,
000
Table 47. Potential benefit of converting existing holding tank to
pure oxygen aerobic digester
PARAMETER
UNITS
AEROBIC DIGESTION
(:» 40% vss
Reduction)
* WITHOUT
** WITH
A
Sludge for Disposal
Tons/Yr
(1)
38,975
31.730
7,245
Chemicals
Tons/Vr
(1)
13,640
9,520
4,120
Sludge + Chemicals to Soil
Tons/Vr
(1)
52.615
41.250
11,365
cost ***
S/Yr
*2.340.000
**1.430.000
910.000
•Without Aerobic Digestion - Sludge Disposal Costs = $60/ton
**With Aerobic Digestion - Sludge Disposal Costs = $45/ton (assuming pure 02 available @ $10/ton)
*** Costs do not include capital expenditure for conversion
(1) Tons/yr x 0.907 = metric tons/year
approximately $10 per ton. The capital cost of this conversion
has been estimated to equal approximately 50% of the capital
costs that would be required for building anaerobic digesters
to handle an equivalent amount of WAS. Operating costs at a
oxygen demand of 1.3 tons oxygen per ton VSS reduced would
approximately equal $15 per ton. This cost is equivalent to
the operating and maintenance costs anticipated for anaerobic
digestion.
168
-------
SYMBOLS AND ABBREVIATIONS
Aerob. - aerobic - in the presence of oxygen
AMP - Adenosine mono phosphate
Anaerob. - in the absence of oxygen
ATP - adenosine tri phosphate
AVG - arithmetic mean
BOD - biochemical oxygen demand
BVDS - biodegradable volatile dissolved solids
BVSS - biodegradable volatile suspended solids
°C - degrees centrigrade
Calc. - calculated value
Cfm - cubic feet per minute
Cfd - cubic feet per day
Ci - concentration of settling sludge - g/1
CiVi - solids weighted settling velocity - g/cm^
COD - chemical oxygen demand
COD - biodegradable chemical oxygen demand
Coll - coliform bacteria
Comp - composite sample
Cone. - concentration
DIG. - digested
DMSO - dimethyl su3.foxide
DNA - deoxy ribonucleic acid
DO - dissolved oxygen concentration
DS - dissolved solids
EFF - effluent waste from digester
FAD - fixed active diffuser
FDS - fixed dissolved solids
FS - fixed solids
FSS - fixed suspended solids
ft^- foot
ft - cubic foot
g - gram
gal - gallon
gpm - gallon per minute
hr - hour
INF - influent feed to digester
INV - inventory
169
-------
JTU - Jackson turbidity units
k - aerobic digestion rate coefficient
Kcod " biodegradable COD rate coefficient - day
KySS - biodegradable VSS rate coefficient - day
K - specific oxygen uptake rate mg/hr/g VSS
- reaction rate coefficient at temgerature T°C
K20 " reaction rate coefficient at 20 C
kg - kilogram
1 - liter
lb - pound
m - meter
m - cubic meter
MAX - maximum
Metro - Metropolitan Denver Sewage Disposal District No. 1
mg - milligram
MGD - million gallons per day
mil - million
mil. gal. - million gallon
MIN - minimum
ml - milliliter
N - nitrogen
N.A. - data not available
NH4-N - ammonium
No. - number
NO^-N - nitrate
O- - pure oxygen gas
obsv. - observed
OUR - oxygen uptake rate mg/l/hr
pH - negative log of hydrogen ion concentration
PO4 - phosphate
press. - pressure
psig - pounds per square inch of gauge pressure
R - oxygen uptake rate
rs " specific resistance to vacuum filtration
RAD - rotating active diffuser
RED - reduced
RPH - revolutions per hour
RPM - revolutions per minute
S - concentration of biodegradable cell material
Sa-t - conceiitration-tirae factor-g*hr/l
sec - second of time
SRT - sludge retention time-days
STP - standard temperature and pressure - 20 C and 760 mm
SVI - sludge volume index-ml per 30 min/g TSS
170
-------
T - temperature
t - hydraulic detention time in digester
TDS - total dissolved solids
TKN - total Kjeldahl nitrogen (organic + ammonium)
TS - total solids
TSS - total suspended solids
Vi - settling velocity - cm/hr
VDS - volatile dissolved solids
vs - versus
VS - volatile solids
VSS - volatile suspended solids
VSU - volumetric standard units - ml/1
WAS - waste activated sludge
yd3~ yard
yd - cubic yard
ZSV - zone settling velocity
171
-------
REFERENCES
1. Eckenfelder, w. w. and D. J. O'Connor - 1961 - Biological
Waste Treatment, Pergamon Press
2. Wuhrman, K. - 1954 - High-rate activated sludge treatment
and its relation to stream sanitation. Sewage and Indus-
trial Wastes 26; 1-27.
3. Harada, H. M., G. H. Reid, E. R. Bennett and K. D. Linstedt,
1973 - A modified filtration method for the analysis of
wastewater suspended solids. Journal water Pollution Control
Federation 45: 1853-1859.
4. Burd, R. S., - 1968 - A study of sludge handling and disposal.
Water Pollution Control Publication WP - 20 - 4, U.S. Depart-
ment of the Interior.
5. Reynolds, T. D., - 1973 - Aerobic digestion of thickened
waste activated sludge. Water and Sewage Works, Reference
NO. R: 118-123.
6. Graves, Q. B. _et ^1 - 1971 - Aerobic digestion of organic
waste sludge. Water Pollution Control Publication 17070
DAU. U.S. Environmental Protection Agency.
7. Bargman, R. D. jst cil - 1968 - Anaerobic sludge digestion.
Manual of Practice No. 16. Water Pollution Control Federa-
tion, Washington, D.C.
8. Loehr, R.C. - 1965 - Aerobic digestion: factors affecting
design. Water and Sewage Works, Ref. No. R: 166-169.
9. Downing, A.L., Painter, H.A., and Knowles, G. - 1964 -
Nitrification in the activated sludge process. Journal
Institute of Sewage Purification 2: 130-151.
10. Patterson, J.W., Brezonik, P.L. and Putnam, H.D. - 1970 -
Measurement and significance of adenosine triphosphate in
172
-------
activated sludge. Environmental Science and Technology
4: 569-572.
11. Randall, C.W. and C.T. Kock - 1969 - Dewatering character-
istics of aerobically digested sludge. Journal Water
Pollution Control Federation, 41: R215-238.
12. McDowell, M.A., et .al - 1972 - Continued evaluation of
oxygen use in conventional activated sludge processing.
Water Pollution Control Publication 17050DNW, U.S. Environ-
mental Protection Agency.
13. Weston, T.J. - 1972 - The application of high purity oxygen
to aerobic sludge digestion. 149p. Masters Thesis, Mich.
Tech. Univ.
14. Weddle, C.L. and D. Jenkins - 1971 - The viability and
activity of activated sludge. Water Research 5: 621-640.
15. D'evstachio. A.J., and G.V. Levin - 1968 - Rapid assay of
bacterial populations. Bacteriological Proceedings. E.I.
DuPont de Nemours & Co. (Inc.) Wilmington, Delaware.
16. Wise, R.H. - 1973 - Personal Communications, U.S. Environ-
mental Protection Agency, Cincinnati, Ohio.
17. Standard Methods for the Examination of Water and Wastewater
1971 - 13th Edition, American Public Health Association.
173
-------
APPENDIX
VOLUMETRIC STANDARD UNITS (V.S.U.)
CONVERSION FACTORS
TAXONOMIC GROUP
Flagellates
DIMENSIONS
(MICRONS)
SPHERICAL VOLUME
(M3) = 0.5xCUBE
CONVERSION
FACTOR
Small
Large
8 x S
30 x
~ x 8
20 x 15
256
4500
2.4 !
Avg
C .256
^ 4.5
Amoeba
30
X
20 x 40
12,000
12
Rotifer
1200
x 40 x 40
1,000,000
1,000
Nematodes
1000
x 15 x 15
100,000
100
Unidentified
Ciliates
Motile
Average of All Motile
Ciliates
26
Unidentified
Ciliates
Stalked
Average of All Stalked Ciliates
54
Chilodonella
60
X
45 x 40
54,000
54
Acineta
40
X
40 x 40
32,000
32
Euplotes
70
X
50 x 40
70,000
70
Aspidisca
40
X
30 x 25
15,000
15
Lionotus
Small
35
X
15 x 15
4000
10
i 4
Large
50
X
25 x 25
16,000
Avg
1_16
Suctoria
60
X
40 x 40
48,000
48
Vorticella
60
X
40 x 40
48,000
48
Opercularia
20
X
50 x 50
174
88,000
83
-------
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO. 2.
EPA-600/2-75-035
3. RECIPIENT'S ACCESSIOr+NO.
4. TITLE AND SUBTITLE
AEROBIC STABILIZATION OP WASTE ACTIVATED SLUDGE -
An Experimental Investigation
5. REPORT DATE
September 1975 (Issuinq Date)
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
David B. Cohen and Donald G. Fullerton
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Metropolitan Denver Sewage Disposal District No. 1
3100 East 60th Avenue
Commerce City, Colorado 80022
10. PROGRAM ELEMENT NO.
1BB043 (ROAP 21ASD, Task 17)
11. CONTRACT/GRANT NO.
68-03-0152
12. SPONSORING AGENCY NAME AND ADDRESS
Municipal Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati» Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Metro Denver Sewage Disposal District No. 1 (Metro) in 1970 converted excess secondary
aerators to aerobic digesters. The plant scale diffused air system was compared with
a pilot scale open tank oxygen system using very fine bubble fixed and rotating dif-
fusers. For the air system volatile suspended solids (VSS) reductions ranged between
11.2% and 47.2%. A significant correlation was observed between VSS reduction and
detention time-temperature factor. Cold shock eliminated nitrification for a five
month period. When invertebrates, particularly rotifers, comprised a significant
fraction of the biomass, digestion was maximal. The pollutant concentration in the
supernatant from the aerobic digester averaged 10% of that from an anaerobic digester.
For the oxygen batch tests, biodegradable VSS digestion rate coefficient k averaged
0.27. No correlation was observed between DO concentration and VSS reduction rates.
The temperature of the oxygen digested biomass increased with increased loadings. At
loadings greater than 2.25 kg VSS/m^/day (0.14 lb VSS/FT-^/day), oxygen performance was
superior to the diffused air system. Loadings as high as 9.6 kg VSS/m-Vday (0.60 lb
VSS/ft3/day) were successfully employed with the rotating diffuser oxygen system. To
continue the aerobic digestion economic benefits of reduced sludge disposal costs,
Metro is considering conversion of a 3,785 cubic meter (1 million gallon) sludge
holding tank to an oxygen digester.
17. KEY WORDS AND DOCUMENT ANALYSIS
21. DESCRIPTORS
b.IDENTIFIERS/OPEN ENOED TERMS
c. COSATl Field/Group
Activated sludge process, *Aerobic
processes, Oxygenation, Respiration,
Biochemical oxygen demand, Metabolism,
Biomass, *Design criteria, High tempera-
ture tests, Hygiene, Thickening, Aerobic
bacteria. Flotation, *Sludge digestion.
Vacuum filtration
*Biodegradability,
Water pollution control,
Biodegradable solids
destruction, Odor
reduction, Pathogenic
organisms elimination
13B
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RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
UNCLASSIFIED
21. NO. OF PAGES
187
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73) 175 ®USGPO: 1975-657-695/5315 Region 5-11
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