United States
Environmental Protection
Agency
Municipal Environments! Research EPA-600/2-80-147
Laboratory —August 1980
Cincinnati OH 45268
&EIPA
Research and Development
Waste Activated
Sludge Processing
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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 nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental 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 Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-80-147
August 1980
WASTE ACTIVATED SLUDGE PROCESSING
by
Scott R. Austin
Jack R. Livingston
Liberate Tortorici
County Sanitation Districts of Los Angeles County
Whittier, California 90607
Contract No. 14-12-150
Project Officer
Irwin 3. Kugelman
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.
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FOREWORD
The Environmental Protection Agency was created because of increasing
public and government concern about the dangers of pollution to the health and
welfare of the American people. Noxious air, foul water, and spoiled land are
tragic testimony to the deterioration of our natural environment. The
complexity of that environment and the interplay between its components
require a concentrated and integrated attack on the problem.
Research and development is that necessary first step in problem solution
and it involves defining the problem, measuring its impact, and searching for
solutions. The Municipal Environmental Research Laboratory develops new and
improved technology and systems for the prevention, treatment, and management
of wastewater and solid and hazardous waste pollutant discharges from
municipal and community sources, for the preservation and treatment of public
drinking water supplies, and for minimizing the adverse economic, social,health,
and aesthetic effects of pollution. This publication is one of the products
of that research; a most vital communications link between the researcher and
the user community.
Federal laws mandating that all POTW plants provide at .least
secondary treatment will result in significant increases in quantities of
waste activated sludge. This report covers a study of a variety of methods of
dewatering and stabilizing waste activated sludge.
Francis T. Mayo, Director
Municipal Environmental Research
Laboratory
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ABSTRACT
This research program was conducted to determine the most effective means
of handling waste activated sludge from the future secondary treatment facili-
ties at the Joint Water Pollution Control Plant in Carson, California. Various
methods of thickening, stabilization, conditioning, dewatering, and drying have
been evaluated.
Gravity thickening was found to be too sensitive to plant upsets to be a
viable thickening option. Dissolved air flotation effectively thickened waste
oxygen activated sludge to 3.5% total solids at polymer dosages below 4 Ib/ton
(2g/kg). Basket and scroll type centrifuges achieved higher thickened sludge
solids, but the necessary polymer dosages were higher and the thickened sludge
was too viscous to allow mixing during subsequent processes. Disc-nozzle cen-
trifuges were found to be operationally unsuitable for sewage sludge thickening.
Both aerobic and anaerobic digestion were evaluated. Aerobic digestion
achieved lower volatile solids destruction than anaerobic digestion and demon-
strated no advantages over anaerobic digestion. Thermophilic (120°F or 45°C)
anaerobic digestion achieved higher volatile solids destructions than mesophil-
ic (94°F or 34°C) anaerobic digestion, but the increased heating demands more
than offset the increase in gas production. No other benefits of thermophilic
digestion were found, so mesophilic anaerobic digestion appears to be the most
attractive stabilization process.
Vacuum and pressure filtration were evaluated for dewatering the digested
waste activated sludge. These processes required extremely high lime and fer-
ric chloride dosages (700 to 800 Ib/ton (350 to 400 g/kg) CaO and. 250 to 400
Ib/ton (125 to 200 g/kg) Fed3), and polymer conditioning was ineffective for
filtration. The filter press produced cakes up to 40% total solids, but the
vacuum filter gave wet cakes with poor discharge characteristics.
Scroll centrifugation of anaerobically digested waste activated sludge pro-
duced 15% cakes at a 15 Ib/ton (7.5 g/kg) polymer dosage. This cake was plas-
tic in nature, but it was conveyable. Basket centrifugation achieved compara-
ble results, but the run times were extremely short. Combining the digested
waste activated sludge with digested primary sludge before dewatering may re-
duce the chemical costs and will result in more manageable cakes. A 70% waste
activated sludge - 30% primary ratio is optimum, and separate digestion before
combining the sludges seems to be better than digesting the two sludges togeth-
er.
Thermal conditioning of waste activated sludge greatly improved its dewa-
tering characteristics. Thermally conditioned sludge produced vacuum filter
cakes of 31 to 37% total solids and filter press cakes of 34 to 51%. These
cakes were solid in texture and easily conveyable. Centrifuge dewatering of
iv
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thermally conditioned waste activated sludge, however, produced 20 to 22% cakes
which were too fluid to be conveyable.
- Thermal conditioning produces a sidestream containing concentrated dis-
solved organics. Soluble COD's in this sidestream averaged about 15,000 mg/1,
and the total dissolved solids ran over 12,000 mg/1. Studies concerning the
handling of this liquor were not completed for inclusion in this report. Other
problems encountered with the thermal conditioning system included odor gener-
ation and mechanical failures.
Successful composting of the dewatered digested waste oxygen activated
sludge required the recycling of large volumes of dried compost product to ad-
just the initial moisture content of the cakes. Indirect steam drying was less
successful because of the tendency of the sludge to agglomerate into balls
which would not dry on the inside.'
One of the two most cost effective sludge disposal systems incorporated
dissolved air flotation, mesophilic anaerobic digestion, centrifuge dewatering,
compost drying, and sale to a fertilizer manufacturer. The other most cost ef-
fective system was dissolved air flotation thickening, thermal treatment, fil-
tration dewatering, and landfill disposal.
This report was submitted in fulfillment of Contract No. 14-12-150 by the
Sanitation Districts of Los Angeles County under the partial sponsorship of the
U. S. Environmental Protection Agency. This report covers a period from June,
1973, to September, 1976. Further studies are being conducted as of this writ-
ing.
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CONTENTS
Forev/ord iii
Abstract iy
Figures I ... .viii
Tables xv
Acknowledgments xix
1. Introduction 1
Background 1
Purpose and Scope 2
Study Location 3
2. Conclusions 5
Thickening 5
Stabilization 6
Digested Sludge Conditioning and Dewatering 7
Thermal Conditioning and Dewatering 9
Sludge Drying 11
Systems Evaluation 11
3. Recommendations ,. 13
4. Process Results . 14
Thickening 14
Stabilization 24
Digested Sludge Conditioning and Dewatering 31
Thermal Conditioning and Dewatering 45
Waste Activated Sludge Drying 57
5. Systems Evaluation 60
Process Considerations 60
Economic Analysis 62
References 65
Tables and Figures 67
Appendices
A. Economic Analysis Calculations 224
B. Unit Conversions 233
vn
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FIGURES
Number Page
1 Research Program Schematic .............. ... 129
2 District 26 WRP Layout .............. ..... 130
3 22" Diameter x 72" Gravity Thickener ......... « . 131
4 Rectangular Dissolved Air Flotation Unit ....... ... 132
5 Float Solids vs. Polymer Dosage for Sludge Thickening on
the 14 Ft2 Rectangular Dissolved Air Flotation Unit . . 133
6 Underflow Quality and % Removal vs. Polymer Dosage for
Sludge Thickenina on the 14 Ft2 Rectanoular Dissolved
Air Flotation Unit .......... " ....... . . 134
7 Float Solids vs. Solids Loading for Sludge Thickening on
the Rectangular Dissolved Air Flotation Units ..... 135
8 Underflow Quality & % Removal vs. Solids Loading for
Sludge Thickening on the Rectangular Dissolved Air
Flotation Units ............ . ........ 136
9 Float Solids vs. Polymer Dosage for Sludge Thickening
on the 14 Ft2 Dissolved Air' Flotation Unit ....... 137
10 Float Solids vs. Polymer Dosage for Sludge Thickening on
the 14 Ft2 Dissolved Air Flotation Unit ... ..... 138
11 Float Solids vs. Polymer Dosage for Sludge Thickening on
the 14 Ft2 Dissolved Air Flotation Unit; Flotation and
Oxygen System Operating Parameters ........... 139
12 Underflow Quality vs. Polymer Dosage for Sludge Thickening
on the 14 Ft2 Dissolved Air Flotation Unit; Flotation
and Oxygen System Operating Parameters ........ . 140
13 Suspended Solids Recovery vs. Polymer Dosage for Sludge
Thickening on the 14 Ft2 Dissolved Air Flotation Unit;
Flotation and Oxygen System Parameters ........ . 141
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Number
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
28 Ft2 Circular Dissolved Air Flotation Unit
Float Solids vs. Polymer Dosage for Sludge Thickening on
the 28 Ft2 Circular Dissolved Air Flotation Unit
Underflow Quality vs. Polymer Dosage for Sludge Thickening
on the 28 Ft2 Circular Dissolved Air Flotation Unit ....
Basket Centrifuge
Cake Solids vs. Polymer Dosage for Sludge Thickening on
' the 48" Basket Centrifuge
Centrate Quality and % Removal vs. Polymer Dosage for Sludge
Thickening on the 48" Basket Centrifuge
Cake Solids vs. Polymer Dosage for Sludge Thickening on the
48" Basket Centrifuge
Centrate Quality & SS Recovery vs. Polymer Dosage for Sludge
Thickening on the 48" Basket Centrifuge
Tapered Bowl Scroll Centrifuge
Cake Solids vs. Polymer Dosage for Sludge Thickening on the
32" x 100" Scroll Centrifuge
Centrate Quality vs. Polymer Dosage for Sludge Thickening on
the 32" x 100" Scroll Centrifuge
Suspended Solids Recovery vs. Polymer Dosage for Sludge
Thickening on the 32" x 100" Scroll Centrifuge
Cake Solids vs. Polymer Dosage for Sludge Thickening on the
"20" x 62" Scroll Centrifuge
Centrate Quality and % Removal vs. Polymer Dosage for Sludge
Thickening on the 20" x 62" Scroll Centrifuge
Disc-Nozzle Centrifuge ....
Anaerobic Digestion Volatile Solids Destruction vs. Waste
Activated Sludge/Primary Sludge Ratio
Digester Response to Change from flesophilic to Thermophilic
Temperature Ranges at the JWPCP
Thermal Requirements for Mesophilic Digestion for 100 MGD of
Secondary Treatment
Page
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
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Number
Page
32 Thermal Requirements for Thermophilic Digestion of the
WAS from 100 MGD of Secondary Treatment 160
33 Filter Press '. . .' 161
34 Effects of Operating Pressure on Filter Performance on
Filtrate Volume vs. Time Relationship for Dewatering
Digested Oxygen Sludge on the 0.33 Ft2 Pressure Filter. ... 162
35 Rotary Drum Vacuum Filter ..'..... 163
36 Cake Solids and Filter Yield vs. Cycle Time for Dewatering
Aerobically Digested Waste Activated Sludge on the 3' x 1'
Rotary Drum Vacuum Filter 164
37 Filtrate Quality vs. Cycle Time for Dewatering Aerobically
Digested Waste Activated Sludge on the 3' x 1' Rotary
Drum Vacuum Filter 165
38 Cake Solids and Yield vs. Cycle Time for Dewatering Aerobi-
cally Digested Waste Activated Sludge on the 3' x 1'
Rotary Drum Vacuum Filter 166
39 Filtrate Quality vs. Cycle Time'for Dewatering Aerobically
Digested Waste Activated Sludge on the 3'- x~ 1' Rotary Drum
Vacuum Filter .......... J .. 167
40 Cake Solids and Yield vs. Cycle Time for Dewatering Digested
Blend on the 3' x 1' Rotary Drum Vacuum Filter ....... 168
41 Filtrate Quality vs. Cycle Time for Dewatering Digested Blend
on the 3' x 1' Rotary Drum Vacuum Filter. .;........ 169
42 Cake Solids vs. Polymer Dosage for Dewatering Aerobically
Digested Waste Activated-Sludge on the 20" x 62" Scroll
Centrifuge 170
43 Centrate Quality vs. Polymer Dosage for Dewatering Aerobi-
cally Digested Waste Activated Sludge on the 20" x 62"
Scroll Centrifuge 171
44 Cake Solids vs. Polymer Dosage fpr Dewatering Aerobically
Digested Waste Activated Sludge and Anaerobically Digested
Primary Sludge on the 20" x 62" Scroll Centrifuge 172
45 Centrate Quality vs. Polymer Dosage for Dewatering Aerobically
Digested Waste Activated Sludge and Anaerobically Digested
Primary Sludge on the 20" x 62" Scroll Centrifuge 173
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Number Page
46 Cake Solids vs. Polymer Dosage for Digested Blend
Dewatering on the 32" x 100" Scroll Centrifuge 174
47 Centrate Quality vs. Polymer Dosage for Digested Blend
Dewatering on the 32" x 100" Scroll Centrifuge 175
48 Cake Solids vs. Polymer Dosage for Digested Blend Dewatering
on the 14" x 48" Scroll Centrifuge . . . 176
49 Centrate Quality vs. Polymer Dosage for Digested Blend
Dewatering on the 14" x 48" Scroll Centrifuge 177
50 Cake Solids vs. Polymer Dosage for Digested Blend Dewatering
on the 20" x 62" Scroll Centrifuge 178
51 Centrate Quality vs. Polymer Dosage for Digested Blend
Dewatering on the 20" x 62" Scroll Centrifuge 179
52 Cake Solids vs. Polymer Dosage for Digested Blend and
Digested Primary Sludge Dewatering on the 20" x 62"
Scroll Centrifuge ,: 180
53 Centrate Quality vs. Polymer Dosage for Digested Primary Sludge
Dewatering on the 20" x.62" Scroll Centrifuge 181
54 Cake Solids vs. Polymer Dosage for Digested Blend and
Digested Primary Sludge Dewatering on the 20" x 62"
Scroll Centrifuge 182
55 Centrate Quality vs. Polymer Dosage for Digested Blend and
Digested Primary Sludge Dewatering on the 20" x 62" Scroll
Centrifuge 183
56 Cake Solids vs. Polymer Dosage for Digested Waste Activated
Sludge and Digested Primary Sludge Dewatering on the 20" x 62"
Scroll Centrifuge. 184
57 Centrate Quality vs. Polymer Dosage for Digested Waste Activated
Sludge and Digested Primary Sludge Dewatering on the 20" x 62"
Scroll Centrifuge 185
58 Cake Solids and Polymer Dosage vs. Sludge Fraction for Digested
Waste Activated Sludge and Digested Primary Sludge Dewatering
on the 20" x 62" Scroll Centrifuge 186
59 Comparison of Separate and Combined Digestion; Cake Solids vs.
Polymer Dosage for Digested Sludge Dewatering on the 20" x 62"
Scroll Centrifuge 187
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Number
Page
60 Comparison of Separate and Combined Digestion; Centrate Quality
vs. Polymer Dosage for Digested Sludge Dewatering on the
20" x 62" Scroll Centrifuge 188
61 Cake Solids vs. Polymer Dosage for Dewatering Digested Oxy-
gen Sludge on the 18" x 54" Scroll Centrifuge 189
62 Centrate Quality & SS Recovery vs. Polymer Dosage for Dewater-
ing Digested Oxygen Waste Sludge on the 18" x 54" Scroll
Centrifuge 190
63 Cake Solids vs. Polymer Dosage for Dewatering Digested WAS
on the 18" x 54" Scroll Centrifuge. 191
64 Centrate Quality & SS Recovery vs. Polymer Dosage for Dewater-
ing Digested WAS on the 18" x 54" Scroll Centrifuge 192
65 Cake Solids vs. Polymer Dosage for Dewatering Digested Oxygen
Sludge on the 18" x 54" Scroll Centrifuge 193
66 Centrate Quality & SS Recovery vs. Polymer Dosage for Dewatering
Digested Oxygen Sludge on the 18" x 54" Scroll Centrifuge . . 194
67 Cake Solids vs. Polymer Dosage for Dewatering Digested Primary
Plus Digested Oxygen Sludge on the 18" x 54" Scroll Centri-
fuge 195
68 Centrate Quality & SS Recovery vs. Polymer Dosage for Dewater-
ing Digested Primary Plus Digested Oxygen Sludge on the
18" x 54" Scroll Centrifuge 196
69 Cake Solids and Polymer Dosage vs. Sludge Fraction for Dewater-
ing Digested Combined Primary and Oxygen Sludge on the
18" x 54" Scroll Centrifuge 197
70 Cake Solids vs. Polymer Dosage for Dewatering Aerobically
Digested Waste Activated Sludge on the 48" Basket Centrifuge 198
71 Cake Solids vs. Polymer Dosage for Dewatering a Digested Blend
on the 48" Basket Centrifuge 199
72 Centrate Quality vs. Polymer Dosage for Dewatering a Digested
Blend on the 48" Basket Centrifuge ." 200
73 Cake Solids vs. Polymer Dosage for Dewatering a Digested Blend
on the 48" Basket Centrifuge 201
74 Centrate Quality vs. Polymer Dosage for Dewatering a Digested
Blend on the 48" Basket Centrifuge 2Q2
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Number Page
75 Cake Solids vs. Polymer Dosage for Dewatering Digested Oxygen
Sludge on the 48" Basket Centrifuge '....... 203
76 Centrate Quality & SS Recovery vs. Polymer Dosage for Dewater-
ing Digested Oxygen Sludge on the 48" Basket Centrifuge ... 204
77 Thermal Conditioning Schematic 205
78 Cake Solids and Yield vs. Cycle Time for Dewatering LPO
Thermal Conditioned Sludge on the 3' x 1' Rotary Drum
Vacuum Filter 206
79 Cake Solids and Yield vs. Cycle Time for Dewatering LPO
Thermal Conditioned Sludge on the 3' x I1 Rotary Drum
Vacuum Filter 207
80 Cake Solids and Yield vs. Cycle Time for Dewatering LPO
Thermal Conditioned Sludge on the 3' x 1' Rotary Drum
Vacuum Filter 208
81 Cake Solids and Filter Yield vs. Cycle Time for Dewatering
LPO Thermal Conditioned Sludge on the 3' x I1 Rotary
Drum Vacuum Filter 209
82 Cake Solids and Yield vs. Cycle Time for Dewatering LPO Thermal
Conditioned Sludge on the 3' x 1' Rotary Drum Vacuum Filter 210
83 Cake Solids vs. Polymer Dosage for Dewatering H. T. Thermal
Conditioned Sludge on the 20" x 62" Scroll Centrifuge .... 211
84 Centrate Quality vs. Polymer Dosage for Dewatering H. T. Thermal
Conditioned Sludge on the 20" x 62" Scroll Centrifuge .... 212
85 Odor Control Schematic. 213
86 Compost Parameters vs. Drying Time for Composting Dewatered
Digested Primary Sludge 214
87 Compost Parameters vs. Drying Time for Composting Dewatered
WAS 215
88 Compost Parameters vs. Drying Time for Composting Dewatered
Digested Primary Plus Dewatered Digested WAS 216
89 Economic Analysis: Sludge Handling Scheme 1 217
90 Economic Analysis: Sludge Handling Scheme 2 218
91 Economic Analysis: Sludge Handling Scheme 3 219
xm
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Number
92
93
94
95
Economic Analysis:
Economic Analysis:
Economic Analysis:
Economic Analysis:
Page
Sludge Handling Scheme 4 220
Sludge Handling Scheme 5 221
Sludge Handling Scheme 6 222
Sludge Handling Schemes 7 and 8 223
xiv
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TABLES
Number
Page
I Flotation Performance and Oxygen System Parameters
for the 14 Ft2 Dissolved Air Flotation Unit 67
II Sludge Thickening Summary - Oxygen WAS 68
III Aerobic Digestion Summary ........ . 69
IV Aerobic Digestion Summary - November 1974 70
V Aerobic Digestion Summary - December 1974 71
VI Aerobic Digestion Summary - January 1975 72
VII Aerobic Digestion Summary - February 1975 73
VII Aerobic Digestion Summary - March 1975 74
IX Aerobic Digestion Summary - April 1975 75
X Anaerobic Digestion Summary 76
XI Mesophilic Digestion Operating Parameters
(November 1975 - January 1976) 77
XII Mesophilic Digestion Sludge Description
(November 1975 - January 1976) 78
XIII Mesophilic Digestion - Heavy Metals Analysis 79
XIV Thermophilic Digestion Operating Parameters
(March 1976 - June 1976) 80
XV Thermophilic Digestion Sludge Description
(March 1976 - June 1976) 81
XVI Heavy Metal Analysis for Thermophilic Digestion .... 82
XVII Specific Filtration Resistance Determinations on
Mesophilically Digested Oxygen Activated Sludge 83
XVIII Specific Filtration Resistance Determinations on
Digested Oxygen Activated Sludge . 84
xv
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Number
Page
XIX
XX
XXI
XXII
XXIII
XXIVA
XXIVB
XXVA
XXVB
XXVI
XXVII
XXVIII
XXIX
XXX
XXXI
XXXII
XXXIII
XXXIV
XXXVA
XXXVB
Summary of Pressure Filtration Dewatering of Mesophilic
Digested Oxygen Activated Sludge at 223 PSIG Opera-
ting Pressure
85
Pressure Filtration of Combined Digested Sludge 86
Specific Filtration Resistance Determinations on
Thermophilically Digested Oxygen Activated Sludge 87
Pressure Filtration of Thermophilic Digested Oxygen
Activated Sludge 88
Digested Combined Sludge Dewatering - 3' x I1 Rotary
Drum Vacuum Filter
Mesophilically Digested Oxygen Sludge Dewatering On
The 3' x 1' Vacuum Filter
89
90
Mesophilically Digested Oxygen Sludge Dewatering On
The 3' x 1' Vacuum Filter (con't) 91
Mesophilically Digested Oxygen Plus Digested Primary
Sludge Dewatering On The 3' x 1' Vacuum Filter. 92
Mesophilically Digested Oxygen Sludge Plus Digested
Primary Sludge Dewatering On The 3' x 1' Vacuum Filter (con't). 93
Primary Sludge Digester Operating Parameters 94
Low Pressure Wet Oxidation Operating Summary 95
Low Pressure Wet Oxidation Data Summary 96
Low Pressure Wet Oxidation Data Summary (con'.t) 97
Low Pressure Wet Oxidation Operating Summary 98
Low Pressure Wet Oxidation Data Summary 99
Low Pressure Wet Oxidation Data Summary (con't) . . . 100
Theoretical and Measured COD Oxidation for LPO
Conditioning 101
Coliform Reduction Data for LPO Conditioning. 102
Dewatering LPO Conditioned WAS On The 3' x 1' Rotary
Drum Vacuum Filter 103
Dewatering LPO Conditioned WAS On The 3' x 1' Rotary
Drum Vacuum Filter (con't) 104
xvi
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Number
XXXVI
XXXVIIA
XXXVIIB
XXXVIII
XXXIX
XL
XLI
XLII
XLIII
XLIV
XLV
XLVI
XLVII
XLVIIIA
XLVIIIB
XLIX
L
LI
LII
LIII
Dewatering LPO Conditioned WAS On The 8.4 Ft2
Filter Press
Dewatering LPO Conditioned WAS On The 8.4 Ft2
Filter Press
Dewatering LPO Conditioned WAS On The 8.4 Ft2
Filter Press (con't) .
Heat Treatment Operating Summary
Heat Treatment Operating Summary . . . .
Heat Treatment Data Summary
Heat Treatment Data Summary
Heat Treatment Data Summary
Heat Treatment Data Summary (con't) . . .
Heat Treatment Conditioning Coliform Reduction Data.
Dewatering Heat Treated WAS - 3' x I1 Rotary Drum
Vacuum Filter
Dewatering HT Conditioned WAS On The 3' x 1' Rotary
Drum Vacuum Filter
Dewatering Heat Treated WAS On The 8.4 Ft2 Filter
Press
Dewatering HT Conditioned WAS On The 8.4 Ft2
Filter Press
Dewatering HT Conditioned WAS On The 8.4 Ft2
Filter Press (con't) .
Intermediate Pressure Wet Oxidation - Operational
& Performance Summary
Operation and Performance Summary for Intermediate
Pressure Wet Oxidation
Dewatering IPO Conditioned WAS On The 3' x I1 Rotary
Drum Vacuum Filter
Dewatering IPO Conditioned WAS On The 8.4 Ft2 Filter
Press
Dewatering IPO Conditioned WAS On The 3' x 1' Rotary
Drum Vacuum Filter
xvii
Page
. 105
. 106
. 107
. 108
. 109
. 110
. Ill
. 112
. 113
. 114
. 115
. 116
. 117
. 118
. 119
. 120
. 121
. 122
. 123
. 124
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Number Page
LIV Mechanical Drying with the 1.5' X 3.0' Rotary Drum Dryer .... 125
LV Systems Evaluation 126
LVI Economic Analysis-Cost Estimate Summary for WAS Sludge
Handling Alternatives 127
LVII Economic Analysis - Summary of Most Cost Effective
Alternatives . 128
xvm
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ACKNOWLEDGMENTS
In addition to the sponsorship of the U.S. Environmental Protection
Agency and the County Sanitation Districts of Los Angeles County, this work
was supported by the State of California and the federal government through
the construction grants program.
Supervision and guidance for the program were provided by Mr. James F.
Stahl and Mr. Robert P. Miele of the Sanitation Districts.
Libby Tortorici is now employed by CM Engineering Associates in Vista,
California.
xix
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SECTION 1
INTRODUCTION
BACKGROUND
In July 1972, the California State Water Resources Control Board (SWRCB)
adopted the State Ocean Plan, followed in October 1972 by the passage in Con-
gress of Public Law 92-500, the Federal Water Pollution Control Act Amendments
of 1972. PL 92-500 already established secondary treatment as the minimum
treatment level for all publicly-owned treatment works discharging to navigable
waters of the United States. The State Ocean Plan, while not specifically re-
quiring secondary treatment as a minimum treatment level, advanced such restric-
tive effluent standards that secondary treatment was the only cost-effective re-
course for the Joint Water Pollution Control Plant (JWPCP).
In compliance with certain provisions of the State Ocean Plan, the Los An-
geles County Sanitation Districts submitted on January 15, 1973, the "Ocean
Plan Technical Report." The Technical Report outlined measures that had to be
taken to achieve compliance with the State Ocean Plan. While the Technical Re-
port was primarily intended to address itself to complying with the State Ocean
Plan requirements, recognition was also given to the federal requirement that
all wastewaters receive secondary treatment. Thus, it was stated in the Tech-
nical Report that the Districts would construct a diffused air biological sec-
ondary treatment process at the Joint Water Pollution Control Plant (JWPCP), in
conjunction with a comprehensive industrial waste source control program, to
achieve compliance with the Ocean Plan and federal requirements.
The Districts acknowledged in the Technical Report that other processes
such as physical-chemical treatment, mechanical aeration-air activated sludge,
and pure oxygen activated sludge might result in a more cost effective system
for compliance with standards, but at the time of the Technical Report there
was insufficient information on their performance, costs, design criteria and
environmental impacts to consider proceeding with them in lieu of the recommend-
ed biological treatment process.
Subsequent to the issuance of the Technical Report, the Districts began
pilot plant studies at the JWPCP to evaluate the alternative secondary treatment
processes and to confirm the suitability of the recommended process. Two rela-
tively small activated sludge pilot plants (a coarse bubble diffused air system
and a high purity oxygen system) and a small physical-chemical treatment pilot
plant were established for these studies. All of the pilot plant work has been
done with the specific intention of obtaining a cost effective full scale sys-
tem that will achieve an effluent quality in compliance with the Ocean Plan and
Federal requirements.
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The results of the physical-chemical pilot work were contained in a Dis-
tricts' summary report entitled Physical-Chemical Treatment Pilot Plant Inves-
tigations at the Joint Water Pollution Control Plant. The report concluded
that conventional physical chemical treatment was cost prohibitive in compari-
son to biological activated sludge treatment. This comparison was made neglect-
ing the reclamation of chemical coagulants from sludge, the potential air qual-
ity problems associated with the regeneration of carbon, the inability of car-
bon adsorption to produce an effluent of 30 mg/1 BODs, and assuming the suita-
bility of the present JWPCP solids processing scheme.
The results of the evaluation of the biological units are contained in the
summary report entitled Evaluation of Activated Sludge Pilot Plants at the Joint
Water Pollution Control Plant, April 1974. In general, it can be said that the
small scale units acknowledged the biological treatability of the JWPCP primary
effluent; however, both units proved to be too small in scale to provide mean-
ingful design and operation data for a full scale treatment plant at the JWPCP.
To provide this essential information, it was decided that biological pilot
units of a significant scale (approximately 0.5 MGD*) would be constructed at
the JWPCP. The two 0.5 MGD pilot units consisted of a deep tank (20-25' SWD)
air activated sludge system utilizing a mechanical submerged turbine for air
diffusion, and a high purity oxygen aeration system. The accompanying clarifi-
ers were rectangular in shape and were of the Districts' conventional clarifier
design. It was planned that the evaluation of these units would have provided
the vital information required for complete design of a cost-effective full
scale biological treatment system. However, in early 1975 the SWRCB embarked
upon a greatly accelerated grant construction program, and a major impact was
the requirement for completion of plans and specifications for 100 MGD of sec-
ondary treatment capacity at the JWPCP by September 1976, with the design of an
additional 100 MGD to be completed nine months later. With such an imposing
deadline, the data from yet to be completed pilot plant investigations was com-
bined with available information from the literature and the results of a na-
tionwide field investigation by District personnel. The total information de-
veloped indicated that the optimum secondary treatment system should be a high
purity oxygen activated sludge system utilizing surface aeration equipment and
a cryogenic air separation plant for oxygen generation.
While a process selection was made for the aeration system, considerable
uncertainty existed concerning the choice of a waste activated sludge processing
system. Clearly, the composite evaluation of a biological .treatment process for
the JWPCP must address the questions associated with the processing of excess
biological solids. Handling and disposing of sludge is a major expense in waste-
water treatment and the problem becomes particularly acute with the activated
sludge process. Historically, waste activated sludge has been a difficult com-
modity to thicken or dewater because of the large amount of bound water in the
cellular mass and the highly concentrated electrostatic charge on the cell wall.
This report was prepared with U.S. customary units and then modified to
EPA format. Conversions from U.S. to metric units will be found in Appendix
B.
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PURPOSE AND SCOPE
It is the purpose of this report to set forth the program that has been con-
ducted to determine the optimum method(s) of handling waste activated sludge at
the JWPCP, and to report the findings of those studies, which were conducted
from June 1973 to September 1976.
A system flow diagram illustrating the assorted unit processes that were
investigated in the waste activated sludge processing study is shown in Fig-
ure 1. The initial efforts of the study were directed toward thickening of the
waste activated sludge. Gravity thickening, centrifugation, and air flotation
have been investigated for this purpose. Aerobic digestion, anaerobic diges-
tion, and thermal conditioning were evaluated for their abilities to reduce the
solids mass for disposal and to improve the dewaterability of the sludge. De-
watering of digested sludge, thermal conditioned sludge, and thickened waste
activated sludge was accomplished by centrifugation and filtration following
appropriate chemical conditioning. One possible solution is to add the waste
activated sludge to the existing sludge processing system at the JWPCP. The
effects on dewatering of combining the primary and waste activated sludge before
and after digestion were investigated. Heat drying and air drying/composting
of the dewatered sludge has been evaluated.
Certainly, it is to be recognized that there are other unit processes than
those shown in Figure 1, such as those involving multiple effect evaporators:
sludge/solvent mixtures; pyrolysis and various types of incineration. A major-
ity of these processes are being scrutinized on paper by the Districts and most
certainly all by the Los Angeles - Orange County Regional Sludge Management
Study. It would seem though, that at the writing of this report either exist-
ing local air pollution control standards preclude their use or that their state
of development is such that their feasible full scale implementation would be at
a date far beyond that required for the initial 200 MGD of secondary treatment
at the JWPCP.
STUDY LOCATION
The ideal location for the conduct of these investigations would be at the
JWPCP, using the waste activated sludge generated from the 0.5 MGD activated
sludge pilot plants. However, when the need for these studies was evident, the
biological pilot plants were in the conceptual design stage and far from oper-
ational, and the accelerated CSWRCB construction grants program was certainly
not anticipated. Moreover, even if the pilot units had been operational it was
felt that the amount of sludge to be generated from the pilot plants would not
be of a sufficient quantity so as to allow for the examination of sludge pro-
cessing at a significant enough scale that the results could be the sole cri-
teria for a full scale design. Considering the time problem with the construc-
tion of the activated sludge pilot plants and the limitation on the scale of
the sludge processing that could be examined with the amount of waste activated
sludge that would be generated, it was decided that the Saugus-Newhall Water
Reclamation Plant (District 26 Water Renovation Plant) should be used as the
site for the initial phase of the waste activated sludge processing studies.
With the facilities at this plant, it is possible to conduct thickening, diges-
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tion (aerobic and anaerobic), and dewatering studies of waste activated sludge
on a large enough scale for the development of significant operational and de-
sign parameters.
The Saugus-Newhall WRP is located in Saugus, California. The present
plant average flow is 3.2 MGD. The treatment system as shown in the attached
Figure 2 involves grit removal, primary sedimentation, activated sludge second-
ary treatment utilizing a step feed aeration pattern, and chlorination of the
secondary effluent. The present solids handling system consists of centrifugal
thickening of the waste activated sludge followed by anaerobic digestion (pri-
mary and secondary) of the primary and waste activated sludge in two separate
sets of digesters. One set of digesters was isolated from daily plant opera-
tion and used in the research investigation of the anaerobic digestion of waste
activated sludge. The various thickening, conditioning, and dewatering equip-
ment investigated was located in the area labeled "Research Site", while the
necessary project staff was housed in the mobile office facility. Preparation
of the Research Site included the purchase and installation of appurtenant
equipment such as pipes, pumps, and power supply.
Even considering the benefits of scale and digestion facilities existing at the
Saugus-Newhall WRP, a legitimate concern arises as to the results obtained from
a study using the waste activated sludge at this plant and their direct appli-
cation for the design of a waste activated sludge handling system at the JWPCP.
In addition to the differing influent characteristics of the two plants, the
activated sludge system at the Saugus-Newhall WRP is of the conventional Dis-
tricts' design of step aeration using coarse bubble air diffusion; while the
activated sludge plant at the JWPCP will be of a high rate nature, employing
pure oxygen gas. How these differences would reflect themselves in the process-
ing of the waste activated sludges is at the least extremely difficult to anti-
cipate. The waste activated sludge processing studies were, therefore, trans-
ferred to the JWPCP when the 0.5 MGD biological treatment pilot plants were
operational. The work at the JWPCP emphasized those processes which had demon-
strated feasibility in the Saugus-Newhall WRP studies.
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SECTION 2
CONCLUSIONS
THICKENING
A. Concentrating waste activated sludge can effectively be accomplished by
either dissolved air flotation or centrifugation through basket or scroll type
centrifuges.
1. Dissolved air flotation will effectively thicken oxygen waste activated
sludge to a concentration of 3.5 percent TS with 99+ percent suspended solids
recovery. Polymer dosages of 2 to 4 Ib/ton are required and solids loadings as
high as 4 Ib SS/hr-ft2 can be applied to the flotation cell.
2. Basket centrifugation of oxygen waste activated sludge through a 48"
unit will yield cake solids of 5 to 8 percent TS with 95 percent suspended sol-
ids recovery. Polymer dosages in excess of 5 Ib/ton are required and the aver-
age basket run time approximates 13 minutes. Deceleration and knife insertion
must be employed due to the dryness of the cake near the basket wall. The ef-
fective solids loading has been determined to be 280 Ib SS/hr.
3. Scroll centrifugation was not employed to thicken oxygen waste activated
sludge at the JWPCP. Small daily sludge quantities and the limited availability
of an 18" x 54" scroll centrifuge did not allow for thickening studies to be con-
ducted. Data collected at the Saugus-Newhall WRP and extrapolated to the JWPCP
through comparison with the basket centrifuge data indicate that with the addi-
tion of 7-11 Ib/ton of cationic polymer discharge solids will fall between 7 and
9 percent TS with 95 percent solids capture. The effective solids loading should
approximate 600 Ib SS/hr through a 32" x 100" unit.
B. Concentrating waste activated sludge via gravity thickening is feasible on-
ly when the sludge exhibits good settling characteristics. Bulking or rising
sludge conditions adversely affect gravity thickening.
C. Concentration of waste activated sludge via a disc-nozzle type centrifuge
requires prescreens and excessive operator attention.
D. Waste activated sludge concentrated to greater than 6% TS is extremely vis-
cous and contains very little free moisture. At concentrations greater than 6%
TS, difficulties may arise in pumping the sludge because of its plastic and vis-
cous nature.
E. The operating parameters of the pure oxygen activated sludge system affect
sludge thickening characteristics. The most notable parameter was determined to
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be power input to the system reactor. An input power reduction of approximate-
ly 70 percent to the fourth stage reactor significantly enhanced the thickening
characteristics of the waste activated sludge.
STABILIZATION
A. Aerobic digestion of waste activated sludge is an alternative to anaerobic
digestion which allows substantial reductions in detention period, but the re-
duction in volatile solids on Saugus-Newhall WRP waste activated sludge approx-
imated only 25% at hydraulic detention periods of 8 to 13 days and volatile sol-
ids loadings of 0.070 to 0.105 Ib/ft3-day. Foam problems continually hampered
the digestion process posing severe operational problems. Increasing the air
rate so as to maintain a residual D.O. in excess of 1 mg/1 did not alleviate the
foam problem but did increase the population of nitrifying bacteria, accelerat-
ing the nitrification process and depressing the digester pH.
B. Anaerobic digestion of thickened oxygen waste activated sludge was success-
fully accomplished under mesophilic and thermophilic conditions.
1. Under mesophilic operation (93°F), anaerobic digestion destroyed an
average of 32 percent of the applied total volatile solids and yielded 14.8 cu-
bic feet of total gas per pound of volatile solids destroyed. The gas consisted
of 61 percent methane. An average hydraulic detention period of 22 days and a
daily volatile solids loading of 0.085 pounds per cubic foot were maintained.
Operation of the digester was stable and no major operational problems were en-
countered.
2. Under thermophilic operation (120°F) an average of 39 percent of the ap-
plied total volatile solids were destroyed. Unit total gas production was mea-
sured at 17.0 cubic feet per pound of volatile solids destroyed with a methane
content of 60 percent. An average hydraulic detention time of 21 days and a
volatile solids loading of 0.074 pounds per cubic foot per day were maintained.
No major operational problems were encountered and operation of the digester
was stable after the transition from the mesophilic to thermophilic temperature
regime.
3. Ammonia nitrogen generation under thermophilic operation may result in
concentrations that inhibit biological activity. The average ammonia nitrogen
concentration encountered while thermophilically digesting oxygen waste activat-
ed sludge approximated 1500 mg/1.
4. For the JWPCP thermophilic digestion of oxygen waste activated sludge
cannot be justified when the fuel requirements necessary to sustain thermophilic
operation are considered. Based on 100 M6D of secondary treatment capacity,
mesophilic digestion of' oxygen waste sludge will yield a surplus of approximate-
ly 9xl06 BTU/day, while thermophilic digestion will require the addition of
45xl06 BTU/day.
5. Anaerobic digestion of waste activated sludge requires that the digester
biological mass first be acclimated in order to assimilate the waste activated
sludge feed. Acclimation can be accomplished by gradually increasing the amount
of activated sludge feed to the digester and decreasing the primary sludge feed
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to the digester.
C. Thermal conditioning also accomplishes stabilization but the process is pri-
marily intended for conditioning.
DIGESTED SLUDGE CONDITIONING AND DEWATERING
Aerobically Stabilized Sludge, Saugus-Newhall WRP
A. Basket centrifugation through a 48" unit yielded maximum cake solids of 10%
TS and 99+% suspended solids capture with the addition of 5 to 18 Ib/ton of cat-
ionic polymer.
B. Centrifugation through a 20" x 62" scroll centrifuge with the addition of
4 to 26 Ib/ton of cationic polymer yielded cake solids of 7% to 10% TS. Sus-
pended solids recoveries varied from 80% to 95% at polymer dosage of 15 to 26
Ib/ton.
C. Vacuum filtration of the aerobically digested waste activated sludge with
the addition of ferric chloride (0 to 300 Ib/ton) and lime (0 to 600 Ib/ton)
produced cakes of 11% to 15% TS while filter yields varied from 0.5 to 1.0 lb/
hr-ft2. Suspended solids recoveries varied from 87% to 97%.
D. Blending of aerobically digested waste activated sludge and anaerobically
digested primary sludge followed by centrifugation via a 20" x 62" scroll cen-
trifuge produced discharge cakes from 11% to 15% TS with cake moisture decreas-
ing as the primary sludge ratio increased. Depending on the percentage of
digested primary sludge, polymer requirements varied from 7 to 17 Ib/ton to pro-
duce effluents with suspended solids concentrations of 1500 mg/1 or less.
Anaerobically Stabilized Sludge, Sau'gus-Newhall WRP
The anaerobic decomposition of primary-secondary treatment sludges was car-
ried out under two modes of operation. These were to combine waste activated
sludge with primary sludge followed by digestion and to digest a straight waste
activated sludge.
A Various ratios of combined sludge were digested and subsequently dewatered.
These ratios varied from 23% WAS - 77% primary to 70% WAS - 30% primary. As
the percentage of waste activated sludge increased, the resulting cake solids
via centrifugation contained more moisture and centrate quality deteriorated.
Maximum cake solids obtained for any of these combined sludge ratios via cen-
trifugation (basket and scroll type) approximated 13-14% TS and required approx-
imately 15 Ib/ton of cationic polymer to produce a centrate containing less than
1500 mg/1 of suspended solids. Vacuum filtration of digested combined sludge
met with very little success. Maximum obtainable cake solids approximated 8%
TS and exhibited extremely poor discharge characteristics. Chemical condition-
ing prior to vacuum filtration included the addition of cationic polymer
(5 Ib/ton), ferric chloride (150 Ib/ton) and/or lime (400 Ib/ton) with result-
ing filter yields of approximately 0.5 lb/hr/ft2.
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B. Separate sludge digestion followed by blending and centrifugation was also
investigated. Blends ranging from 100% WAS - 0% primary to 0% WAS - 100% pri-
mary v/ere subjected to centrifugation preceded by polymer conditioning. Dis-
charge cakes decreased linearly from 22% TS to 12% TS as the amount of digested
primary sludge decreased and the amount of digested waste activated sludge in-
creased in the blend. Polymer requirements necessary to achieve less than 1500
mg/1 of suspended solids in the centrate. increased as the ratio of waste acti-
vated to primary sludge increased. Polymer requirements for a straight waste
activated sludge approximated 35 Ib/ton while a straight digested primary sludge
required 6 Ib/ton.
Mesophilically Digested Oxygen Waste Activated Sludge, JWPCP
A. The vacuum filtration of this particular sludge was unsuccessful. With the
addition of 1200 Ib/ton of lime and 350 Ib/ton of ferric chloride, the maximum
obtainable discharge solids approximated 14 percent TS while maximum filter
yields approximated 3.4 lb/hr-ft2. Captured solids had to be manually scraped
from the filtering media, necessitating full time operator attention.
B. Successful pressure filtration required the addition of 700 to 900 Ib/ton
of lime and 240 to 400 Ib/ton of ferric chloride. At these dosages, discharge
solids varied from 34 to 40 percent total solids, with corresponding filter
yields from 0.25 to 0.44 lb/hr-ft2. Precoating the filter with 10 Ib/ton of
diatomaceous earth is an optional operation that improves cake discharge charac-
teristics and may reduce maintenance costs. Suspended solids recovery in excess
of 99 percent were consistently obtained.
C. Basket centrifugation met with little success. Flow rates of 50 and 35 gpm
were applied to a 48" basket with resultant run times of 5 and 8 minutes, re-
spectively. In both situations the effective flow rate approximated 25 gpm.
With the addition of 5 to 15 Ib/ton of cationic polymer cake solids varied from
5 to 9 percent TS while the resultant centrates contained 2500 to 1200 mg/1 of
suspended solids. Approximately 15 percent (2.4 ft3) of the solids retained in
the basket had to be skimmed out and were unconveyable. The remaining solids
were easily plowed and although plastic in nature were conveyable.
D. Centrifugation through a pilot scale (18" x 54") scroll centrifuge yielded
discharge solids of 15 percent TS with the resultant centrate containing 1200
mg/1 of suspended solids. Chemical conditioning with 15 Ib/ton of cationic
polymer was required while flow rates of 10 and 15 gpm were evaluated. Centri-
fugation of digested waste activated sludge from the Saugus-Newhall WRP through
the above unit, under the same operating conditions, yielded cake solids of 10
percent TS with centrate suspended solids approximating 2000 mg/1.
Combination of Mesophilically Digested Primary and Mesophilically Digested
Oxygen Waste Activated Sludge
A. Dewatering of digested primary and digested waste activated sludge is im-
proved when the sludges have been digested separately instead of blended prior
to digestion. Separate dewatering may also give higher overall solids content
than if the sludges are combined prior to dewatering; however, dewatered waste
activated sludge by itself is very difficult to handle, and it is desirable to
blend in some digested primary prior to dewatering in order to produce a sludge
8
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that can be efficiently handled.
B. Vacuum filtration of various blends of digested primary and digested waste
activated sludge resulted in cake solids ranging from 9 to 18 percent TS. All
of the cakes from the blended sludge mixture required manual scraping of the
filtration media, rendering the use of vacuum filters unsuccessful for this ap-
plication. Chemical addition prior to filtration included 190 to 250 Ib/ton of
ferric chloride and 700 to 1050 Ib/ton of lime.
C. Blended sludge ratios varying from 100 percent primary - 0 percent WAS to 0
percent primary - 100 percent WAS were dewatered via the 18" x 54" scroll cen-
trifuge. The 100 percent digested primary sludge feed required 4 Ib/ton of cat-
ionic polymer to produce a centrate containing 1500 mg/1 or less of suspended
solids, and yielded a discharge cake of 25 percent TS. As the ratio of oxygen
waste sludge was increased to 35 percent, cake solids decreased linearly to 17
percent TS and polymer requirements increased to 6 Ib/ton. From 65 percent pri-
mary - 35 percent WAS to a 0 percent primary - 100 percent WAS ratio the resul-
tant cakes continued to decrease in a linear fashion while the polymer require-
ments increased. At the 0 percent primary - 100 percent WAS ratio approximately
15 Ib/ton of polymer was needed while the cake solids approximated 15 percent
TS.
D. Pressure filtration of various blended sludge ratios indicated that the addi-
tion of digested primary sludge improves the handleability of the digested oxygen
waste activated sludge. With the addition of 30 percent digested primary, total
filter cake solids averaged 28 percent TS whereas a 100 percent digested WAS de-
watered to 22 percent TS. Chemical addition for each of the above sludges ap-
proximated 200 Ib/ton of ferric chloride and 700 Ib/ton of lime.
Thermophilically Digested Oxygen Waste Activated Sludge
A. Thermophilic anaerobic digestion of oxygen waste sludge did not improve
sludge dewaterability beyond that obtained with mesophilic digestion.
B. Vacuum filtration yielded cake solids that had to be manually scraped from
the cloth media. Chemical conditioning with 1000 Ib/ton of lime and 300 Ib/ton
of ferric chloride resulted in cake solids of 11.5 percent TS, suspended solids
capture of 82 percent and a filter yield of 0.84 lb/hr-ft2.
C. Successful pressure filtration required 800 to 1200 Ib/ton of lime and 200
to 300 Ib/ton of ferric chloride. At these dosages, cake solids approximated
23 to 31 percent .total solids with filter yields approximating 0.25 lb/hr-ft2-.
THERMAL CONDITIONING AND DEWATERING
A. Thermal conditioning of undigested oxygen waste activated sludge under the
wet oxidation and heat treatment modes of operation resulted in high degrees of
solubilization of the particulate organic material. Under wet oxidation condi-
tioning soluble COD concentrations increased from an average of 1550 mg/1 to an
average of 14,200 mg/1 while the dissolved solids concentration increased from
2200 mg/1 to an average of 12,200 mg/1. Heat treatment conditioning resulted
in an increase in the soluble COD concentration from 1650 mg/1 to 15,300 mg/1,
-------
while the dissolved solids concentration increased from an average of 2000 mg/1
to 12,500 mg/1.
B. The measured degrees of oxidation were inconsistent with the thermal unit
operating conditions. No correlation between the thermal operating parameters
(temperature, detention time, and air supply) and total COD reductions was es-
tablished. The inconsistencies observed indicate that accurate performance pre-
dictions for a full scale system will be difficult, if not impossible.
C. The measured reductions in total and fecal coliforms through the thermal
conditioning process were erratic. The problems surrounding the coliform kill
data remain unanswered at this time.
D. Dewatering of thermally conditioned undigested oxygen waste activated sludge
produced vacuum filter cakes from 31 to 37 percent and filter yields from 2.3 to
6.6 lb/hr-ft2. Pressure filtration yielded discharge solids from 34 to 51 per-
cent TS and filter yields of 0.50 to 1.03 lb/hr-ft2. Dewaterability was ob-
served to increase with increases in reactor temperature and detention time and
no significant differences in dewaterability was observed between wet oxidation
and heat treatment conditioning.
E. Dewatering of heat treated sludge through a 20" x 62" scroll centrifuge at
the Saugus-Newhall WRP required the addition of approximately 8 Ib/ton of cat-
ionic polymer for the centrate to contain less than 1500 mg/1 of suspended sol-
ids with resulting discharge cakes averaging 20% - 22% TS.
F. The addition of raw primary sludge prior to thermal conditioning and dewa-
tering did not enhance dewaterability.by vacuum filtration. A blend of 23%
primary - 77% waste activated sludge from the Saugus-Newhall WRP was subjected
to LPO conditioning and vacuum filtration and produced cakes of 30% to 33% TS
with filter yields varying from 2 to 3.5 lb/hr-ft2. Suspended solids recovery
approximated 97%.
G. Anaerobic digestion of oxygen waste sludge prior to thermal conditioning
adversely affected sludge dewaterability. Pressure filtration of the anaerobi-
cally digested thermally conditioned sludge produced cakes of 30 percent TS
with filter yields approximating 0.60 lb/hr-ft2. This is compared to a filter
press cake of 39 percent TS and a filter yield of 0.94 lb/hr-ft2 for undigested
thermally conditioned oxygen waste activated sludge. A fine line exists between
the solids content of a dewatered thermally conditioned sludge and the handle-
ability of the resultant cake solids. At a solids concentration of 30 percent
TS, the filter press cakes are dry on the outer surfaces but contain a liquid
core which is not conveyable. At solids concentrations in excess of 34 percent
TS, the filter cake is consistently dry and firm and easily conveyable.
H. Numerous operational problems were encountered with the pilot thermal con-
ditioning unit. These included, scaling of the heat exchange surface, compres-
sor failures, boiler malfunctions and corrosion of the air lines. Odor genera-
tions at the JWPCP were less severe than encountered at the Saugus-Newhall WRP
because of the use of a wet scrubber and carbon adsorber. However, even with
this equipment for vent gas treatment, strong odors were detectable.
10
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SLUDGE DRYING
A. Composting of dewatered digested oxygen waste sludge was successful but be-
cause of the high moisture content of the cake solids, the addition of large
volumes of compost material was required to bring the initial moisture content
of the compost pile to 65 percent or less. The required drying time and final
product moisture content approximated that for digested primary sludge.
B. Indirect steam drying of digested dewatered and undigested dewatered oxygen
waste activated sludge will require 1300 to 1500 BTU's per pound of water evap-
orated. Problems were encountered while drying dewatered biological solids be-
cause these sludges tended to agglomerate into 2" to 4" balls that would dry on
the outside but remained moist and spongy on the inside. The final moisture
content after four hours of drying at a jacket temperature of 297°F approxi-
mated 65 percent and odors normally associated with thermal conditions were de-
tectable during these drying studies.
SYSTEMS EVALUATION ' .
A. Based on pilot and full scale data and engineering judgment as to the feas-
ibility of certain processes, twenty (20) alternate waste activated sludge
handling schemes were established and analyzed for cost effectiveness. For a
system to be considered feasible, the overall suspended solids removal had to
be in excess of 95 percent and the dewatered discharge cakes had to be of such
a consistency to be conveyable and handleable.
It must be realized that all of these systems were selected without significant
regard for any future system considerations or ultimate disposal projects that
would result from the studies presently being initiated by the Los Angeles Coun-
ty/Orange Metropolitan Area Sludge Management Study.
B. Of these twenty alternatives, the four most cost effective systems are
listed below. Cost estimates reflect a consumer price index of 170 and an ENR
index of 2400.
1. Flotation-Anaerobic Digestion-Centrifuge-Compost-Fertilizer Mfr. ($95
to $97/ton)
Composting of dewatered digested oxygen waste activated sludge and de-
watered blended digested oxygen plus digested primary sludge was successfully
accomplished during these studies. If sufficient land area exists for compost-
ing and the final product is acceptable to a fertilizer manufacturer, the most
cost effective system would include flotation thickening, anaerobic digestion,
centrifugation of a 70 percent WAS - 30 percent primary sludge blend, compost-
ing and disposal to a fertilizer company. Basket or scroll centrifuges can be
employed for the dewatering process. If basket centrifuges are used, the unit
cost for this sludge train would be $95 per day ton of solids processed while
a unit cost of $97 per dry ton will be incurred if scroll centrifuges are em-
ployed.
11
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2. Flotation-Anaerobic
S102/ton).
Digestion-Centrifuge-Composting-Landfill (S100 -
If composting is practiced but a sufficient market does not exist or
the final compost product is not acceptable to a fertilizer manufacturer, this
final product would be disposed of at a sanitary landfill. The sludge train
would then be flotation thickened, anaerobic digestion, centrifugation of a 70
percent WAS - 30 percent primary sludge, composting and landfill disposal. The
unit cost for this system would be $100 and $102 per dry ton of solids pro-
cessed for basket and scroll centrifugation, respectively.
3. Flotation-Anaerobic Digestion-Dewater-Landfill ($112 - $123/ton)
If composting cannot be accomplished because of land restriction, the
most cost effective systems involving digestion would include flotation thicken-
ing, anaerobic digestion, pressure filtration or scroll centrifuge dewatering
and landfill disposal. The unit cost for the system involving pressure filtra-
tion of the digested oxygen waste activated sludge would be $123.per ton of sol-
ids processed. The unit cost for the scheme incorporating scroll centrifugation
of a 70 percent WAS - 30 percent primary blended digested sludge would be $117
per dry ton. This figure may be reduced to $112 per dry ton processed pending a
tiling as to whether the 15 percent TS cake to be disposed of is classified as
a liquid or a solid. The current practice at the Districts'landfill site is to
charge $2.50 per ton for sludges with total solids concentration greater than
25 percent and $3.50 per ton for sludge with solids concentrations less than 25
percent TS.
4. Flotation-Thermal Treatment-Dewater-Landfill ($96 - $97/ton)
The most cost effective sludge handling schemes involving thermal treat-
ment would include flotation thickening, thermal treatment, vacuum or pressure
filtration and sanitary landfill disposal. Anaerobic treatment of the liquid
side streams associated with thermal treatment would also be incorporated in
this particular sludge train. The unit cost associated with these schemes have
been estimated at $97 and $96 per ton of solids processed for vacuum and pres-
sure filtration, respectively, and reflect the cost of anaerobic filtration of
the liquid side streams.
Composting of dewatered thermally treated waste activated sludge was not
considered at this time. The composting characteristics of thermally treated
sludge remains for evaluation and there are serious concerns regarding the prob-
able production of odors upon turning of this particular compost material. Addi-
tionally, it is not known if the final product'would be acceptable to a ferti-
lizer manufacturer if in fact the thermally treated sludge were amenable to
composting.
12
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SECTION 3
RECOMMENDATIONS
Based on the results of the studies reported herein, it has been recom-
mended that dissolved air flotation be adopted for thickening of waste activated
sludge. Additional work is needed to optimize operating parameters for sludge
from the pure oxygen process.
Mesophilic anaerobic digestion of 100% waste activated sludge has been re-
commended for stabilization. Future work should include repetition of the
startup procedure and long-term steady state operation to address potential
problems such as foaming and scale formation.
The thermal conditioning studies raised many questions that must be answered
before the process could be effectively applied. Further studies should inves-
tigate the mitigation of odors, treatment of the high COD side stream, and solu-
tions to operational problems such as corrosion and scaling.
Scroll centrifuges are the most promising of the dewatering devices tested.
However, performance of this type of equipment is known to be dependent on the
size of the machine, and evaluation of full scale machinery is needed. Entirely
new equipment, in particular belt filter presses, have entered the market and
deserve thorough evaluation.
Composting plays a major part in current disposal practices at the Joint
Water Pollution Control Plant, but composting of waste activated sludge has re-
ceived only cursory evaluation. Further study of the windrow composting method
for processing waste activated sludge, as well as evaluation of new methods such
as the static aerated pile, is necessary before a disposal system incorporating
composting could be recommended.
The Los Angeles/Orange County Regional Sludge Management Study is still in
progress, and further work may be dictated by the results of this study.
13
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SECTION 4
PROCESS RESULTS
THICKENING
As illustrated in Figure 1, three separate unit operations—dissolved air
flotation, gravity thickening, and centrifugation--were investigated for thick-
ening waste activated sludge. The waste activated sludge from the Saugus-Newhall
WRP was taken from the reaeration tank instead of the return sludge line. The
construction of the Saugus-Newhall WRP system would have made obtaining the waste
activated sludge from the return sludge line costly and mechanically difficult,
and while no evaluation was conducted at that time to determine the relative
merits of thickening reaerated sludge as opposed to return sludge, it is believed
that the differences are minimal.
The efforts at the JWPCP were focused on thickening waste activated sludge
from the 0.5 MGD high purity oxygen activated sludge (UNOX) pilot plant, although
some work was conducted using the sludge from the 0.5 MGD mechanical air acti-
vated sludge pilot plant. Waste oxygen activated sludge was obtained either
from the final stage of the reactor or from the return sludge line. Waste air
activated sludge wass obtained from the return sludge line.
Gravity Thickening
Gravity thickening is commonly accomplished in a sedimentation tank in
which solids separate from the liquid phase by gravity forces and the settled
solids are concentrated by the action of gravity and by virtue of the weight of
the overlying solids (compaction). Conventional sludge collecting mechanisms
with vertical pickets are employed to stir the sludge gently, thereby opening up
channels for the release of water and promoting densification.
A 22" diameter by 72" high gravity thickener with vertical pickets, Figure
3, which was operated in a batch manner, was evaluated at the Saugus-Newhall WRP.
Sludge bulking is a recurring operational problem at that plant, and a bulking
sludge does not favor compaction in gravity settlers. No data were obtained re-
garding the ability of gravity settlers to thicken waste activated sludge (WAS),
but the process was shown to be extremely sensitive to plant upsets such as bulk-
ing or rising sludge. Because of process instability, no gravity thickening
studies were conducted at the JWPCP.
Dissolved Air Flotation
The flotation process has long been employed in industry especially in min-
ing and refineries for two-phase separation. Generally, the process is applied
to systems where there is a large concentration of insoluble or immiscible par-
ticles suspended in a bulk liquid. Either the suspended particles are quite
small and nearly colloidal or they have a density comparable to that of the bulk
liquid. Air bubbles are introduced into the system to combine with the parti-
cles resulting in an aggregate with a density sufficiently less than the bulk
density to effect flotation and concentration. Flocculation aids such as poly-
14
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electrolytes are often used to aid in clarification and concentration. The
flotation unit may be either rectangular or circular in design and a dissolved
air system may employ either pressurization of the waste stream and/or recycled
effluent.
Three dissolved air flotation units were obtained for these studies. Two of
these units were rectangular in design and had flotation areas of 14 and 50 ft2.
The third unit had a 6 ft diameter flotation cell.
The two rectangular units were similar (length/width = 2.3) except that the
50 ft2 unit had more sophisticated controls and a higher head recirculation pump.
As shown in Figure 4, the influent WAS solids enter the unit at the bottom via
a distribution box where they are blended with a pre-pressurized recycled efflu-
ent stream. The recycled stream is pumped to a retention tank that is maintained
at 45 to 55 psig in the 14 ft2 unit and 55 to 70 psig in the 50 ft2 unit. Air
is introduced into the retention tank via an air compressor and the entire con-
tents are continually recycled by a reaeration pump that augments the dissolu-
tion of air into the liquid. Following a short retention period, the pressurized
air-saturated liquid is discharged to the distribution box through a back-
pressure regulator valve and released at atmospheric pressure. The pressurized
stream and influent blend in the distribution box with the minute air bubbles
adhering to the WAS solids and causing the solids to rise to the surface and be
skimmed by the scraper arms.
The first part of this evaluation was conducted at the Saugus-Newhall WRP
and consisted of optimizing the 14 ft2 unit with regard to retention tank pres-
sure as controlled by the recycle rate. With a WAS feedrate of 20 gpm and poly-
mer added in the range of 4 to 16 Ib/ton, recycle rates of 16 and 32 gpm (80%
and 95% recycle) corresponding to retention tank pressures .of 55 and 45 psig
respectively, were evaluated. Data for optimization of the unit is shown in
Figures 5 and 6. As shown in Figure 5, varying the recycle rate from 16 to 32
gpm had very little effect on the concentration of the float solids but suspend-
ed solids recovery was greatly affected by the recycle rate as shown in Figure 6.
At a recycle rate of 16 gpm, suspended solids (SS) recovery in excess of 99% were
consistently obtained whereas at a recycle rate of 32 gpm SS recovery ranged
from 92% to 95%.'
It can be argued that a poorer solids capture obtained at the higher re-
cycle rate (32 gpm) was attributed to excessive turbulence through the unit due
to an increased hydraulic loading but data presented by Hayes1 provides a fur-
ther explanation.
According to Hayes, a linear relationship exists between the percent satu-
ration of air in water and the detention time in the pressurized retention tank.
As the retention tank pressure increases, the recycle flow decreases and conse-
quently the detention time in the pressurized retention tank increases. As the
detention time increases, the percent saturation increased by virtue of the re-
aeration pump and consequently a greater number of minute air bubbles are re-
leased when the pressurized recycle flow is released to atmospheric pressure.
With this increase in the number .of minute air bubbles, the attachment of air
to solids in the distribution box is enhanced because as Ettelt2 has pointed
out, the smaller bubbles have less liquid to displace from the surface of the
solids to which they must attach and, therefore, they attach more readily than
15
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larger bubbles. Additionally, because their terminal velocities are less than
those of larger bubbles, the detention time is also increased which appreciably
enhances the opportunity for contact with the solids thus allowing for more
solids to float to the surface of the unit and be removed by the scraper mecha-
nism.
With the unit operating at the lower recycle rate of 16 gpm, a retention
tank pressure of 55 psig and 0.6 Ib/hr of air added to the retention tank, the
second part of the evaluation was conducted. The effects of solid and hydrau-
lic loadings on float solids concentration and solids capture were investigated
over a wide range of polymer dosages. These results are shown in Figures 7. and
8. It should be noted that with this particular unit the maximum amount of air
added to the retention tank was limited to 0.6 Ib/hr because of limitations on
the air injection system. As seen in Figure 7, the float solids concentration
did not vary as the solids loading increased from 2.3 to 6.2 lb/hr-ft2 corres-
ponding to hydraulic loadings (excluding recycle) of 0.75 to 1.5 gpm/ft2, re-
spectively. As the solids loading increased to 9.3 lb/hr-ft2 (2.6 gpm/ft2 ex-
cluding recycle), a pronounced drop in float solids concentration is o'bserved
at polymer dosages of 7-10 Ib/tqn, but at polymer dosages of 11-13 Ib/ton the
variation in float solids concentration is insignificant. With respect to
solids capture, increases in solids loading and hydraulic loadings over the
range investigated had very little effect on suspended solids removal.
As shown in Figure 8, the fact that a total hydraulic loading of 3.8 gpm/
ft2 including a recycle of 16 gpm still produced suspended solids removal in
excess of 99% adds support to the theory presented earlier. For example, at a
total hydraulic loading of 3.8 gpm/ft2 (Figure 8), solids recovery in excess of
99% were obtained because the lower recycle rate yielded a longer retention tank
detention time and a degree of air saturation.
The efforts extended towards evaluating the 50 ft2 rectangular dissolved
air flotation unit were limited because the unit was being utilized strictly as
an operations tool and as such only a limited number of operating parameters
were investigated. In fact, only one waste activated sludge feedrate was ap-
plied to the unit. At a feedrate of 63 gpm, corresponding to a hydraulic load-
ing of 1.2 gpm/ft2 and a solids loading of 5 lb/hr/ft2, polymer was added in
the range of 4 to 7 Ib/ton and the A/S ratio was maintained at 0.018. At a
total hydraulic loading of 3 gpm/ft2 (including recycle) the unit consistently
removed 99% of the influent suspended solids (Figure 8) and. the resultant float
solids obtained are shown in Figure 7.
During the flotation studies, the Saugus-Newhall WRP sludge volume index
(SVI) averaged 360 ml/g with a range of 215 ml/g to 615 ml/g. These high SVI's
are indicative of poor sludge thickening qualities.
In September 1975, the 14 ft2 rectangular flotation unit was relocated to
the JWPCP research site. The flotation studies conducted at the Saugus-Newhall
WRP utilized waste activated sludge taken directly from an aeration tank. The
relative merits of thickening mixed liquor as opposed to return waste sludge
could not be evaluated at that time, so the initial phase of the JWPCP studies
included a series of tests on both pure oxygen mixed liquor from the final
stage of the UNOX reactor and waste sludge from the return sludge line.
16
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In addition to comparing the flotation characteristics of mixed liquor and
return waste sludge, these initial studies investigated the effects of storing
return waste sludge for up to 12 hours prior to flotation. The relatively low
wasting rate from the 0.5 MGD pure oxygen pilot plant and occasional operation-
al problems which caused variations in the wasting rate necessitated storing
sufficient quantities of excess sludge to ensure a constant loading rate to the
flotation cell. It, therefore, became necessary to determine if this practice
of storing sludge prior to flotation was detrimental to its flotation character-
istics.
The results from this evaluation are summarized in Figures 9 and 10 which
reflect the data collected at feed rates of 5 and 8 gpm, respectively, to the
flotation cell. For these runs, the flotation unit was operated at a retention
tank pressure of 55 psig and a recycle rate of 25 gpm with the addition of 0 to
11 Ib/ton of cationic polymer. The results indicate that the oxygen sludge
thickening characteristics did not vary significantly with sludge origin of stor-
age up to 12 hours. More important, these results confirm that the differences
in flotation performance between a return sludge feed and a mixed liquor or re-
aerated sludge would have been minimal at the Saugus-Newhall WRP.
Evaluation of the rectangular dissolved air flotation unit on oxygen waste
activated sludge continued on a regular basis for approximately eight months af-
ter these initial studie^.. During the duration of the studies, the performance
of the flotation unit fluctuated considerably and performance criteria were es-
tablished to help characterize the flotation characteristics of the sludge as
operational parameters varied within the pure oxygen system. Based on float
solids concentration, SS recovery and chemical requirements, flotation perform-
ance was categorized as either good, marginal, or poor. Those runs which met the
criteria of 3.0 to 4.0 percent float solids, and 99 percent SS recovery with 0
to 4 Ib/ton of polymer were characterized as good, while marginal or poor per-
formance indicates that one or more of the performance criteria were not met.
The initial operation of the air flotation unit was conducted during the
start-up period of the 0.5 MGD pure oxygen activated sludge system. Because of
the start-up problems with the pilot plant and the many operating variables in-
herent to any secondary biological system, it was extremely difficult to corre-
late any single operating parameter with subsequent flotation of the waste acti-
vated sludge. On a very general basis, the waste activated sludge exhibited good
floating characteristics when the unstirred SVI was below 85, the MCRT was less
than six days, the sludge blanket level in the final clarifier was less than
five feet, and the total system solids approximated 3600 pounds or less. Periods
of marginal or poor performance were normally encountered when the above parame-
ters exceeded the specified limits-. The results from the operation of the air
flotation unit during this period can best be described as sporadic, and in gen-
eral inferior to what has been reported in the literature by pure oxygen system
manufacturers. Figures 11, 12, and 13 represent flotation performance during
periods of good, marginal, and poor operation, and include the summary of the
pure oxygen system and the flotation unit operation parameters.
On April 30, 1976, the speed of the surface aerator in the fourth stage of
the oxygen system was reduced from 68 to 45 rpm, resulting in an input power re-
duction of approximately 70 percent. By May 6, 1976, the flotation unit perfor-
mance was visually observed to improve and on May 7, 1976, a program was set up
17
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to ascertain If the oxygen waste sludge would consistently concentrate to with-
in the limits of the established performance criteria. For 19 days in a six-
week period, the flotation unit was closely monitored. The results from this
evaluation are presented in Table I which includes a summary of the pure oxygen
system and flotation unit operating conditions. The established performance
criteria were consistently met during these evaluations and a review of the oxy-
gen system operating parameters indicates that the power density change in the
fo.urth stage reactor enhanced the flotation characteristics of the waste sludge.
The oxygen system parameters encountered during these studies ranged over the
full spectrum of conditions encountered prior to the power reduction; yet the
flotation performance was consistently categorized as good. The power change
reduced shearing forces within the fourth stage reactor resulting in a waste
sludge more amenable to flotation thickening and less sensitive to system oper-
ating variations.
A comparison between the data presented in Table I and Figure 7 indicates
that the oxygen WAS from the JWPCP will flotation thicken better than the WAS
from the Saugus-Newhall WRP. At polymer dosages of 4-6 Ib/ton, the Saugus-
Newhall sludge yielded float solids at about 3.5 percent TS. The oxygen WAS
gave an average float solids of 4.1 percent TS and a minimum of 3.3 percent TS
at polymer dosages less than 3.2 Ib/ton.
The six-foot diameter circular flotation cell, as furnished by the manu-
facturer, was divided into three separate compartments or flotation cells by
vertical steel baffles that were slotted at the bottom and terminate approxi-
mately 6" below the liquid surface at the top. Each cell contained a one horse-
power pump, a back-pressure regulator valve, and an air aspirator system which
was the sole source of air for solids flotation.
This was the only pilot circular flotation unit available for rental and
it was certainly unique and would not be considered typical of a standard cir-
cular flotation unit for use on waste activated sludge.
To make the unit suitable for waste activated sludge, changes were made by
Districts personnel and the unit operated under these changes will be herein-
after referred to as the "modified" flotation unit. Since the modified unit
more closely resembles a standard circular flotation cell, only data collected
under the modified mode of operation will be presented and discussed. The mod-
ified system, as shown in Figure 14, employed coupling pumps 1 and 3 in a se-
ries arrangement along with the addition of a retention tank downstream of the
pumps. The WAS stream entered the suction of pump 3 where it combined with the
underflow of Cell 3. Air was then added at the discharge side of Pump 3 by
means of an air compressor which replaced the venturi aspirator system. The
air-liquids-solids mixture then passed through Pump 1 and was discharged to a
retention tank operating at approximately 60 psig. Following a short retention
period, the pressurized mixture was discharged into Cell 1 at atmospheric pres-
sure through a modified inlet works which included a perforated clay distribu-
tion box.
With all the modifications made, the unit was still limited by the surface
sludge scraper system. It was visually ascertained that at the solids loadings
in excess of approximately 3.6 lb/hr-ft2 (22 gpm) the scraper mechanism was not
able to recover solids at the same rate they were captured. To correct this
18
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would have required extensive alterations and, as such, the unit was operated
with this limitation. In addition, in view of the fact that Cell 3 was utilized
only as the draw off of clear underflow, it can be concluded that only two-
thirds of the unit was utilized for flotation. Therefore, the effective surface
area used was 18.67 ft2 instead of 28 ft2, and the actual maximum solids loading
was 5.4 lb/hr-ft2 instead of 3.6 lb/hr-ft2. With an air to solids (A/S) ratio
averaging 0.023 and cationic polymer added in the range of 4 to 16 Ib/ton for
conditioning, composite samples of various runs were taken and the results are
shown in Figures 15 to 16.
It should be noted that although there was a pressurized retention tank to
aid in the dissolution of air, this system was quite inferior to the air injec-
tion system on the rectangular units evaluated. Specifically, the retention
tank pressure could not be easily controlled and the recycle rate could only be
estimated to be between 100% and 200% of the feedrate. In addition, the lack of
a mixing device in the retention tank limited the degree of saturation obtain-
able. As presented by Eckenfelder3, the use of mixing in a pressurized reten-
tion tank can produce 90% of saturation whereas 50% of saturation is usually ob-
tained in an unmixed pressurized retention tank.
The plot of float solids versus polymer dosage, Figure 10, shows that a
maximum float solids concentration of 3.5% TS was obtained on this modified unit
with the addition of 13 Ib/ton of polymer. SS recoveries in excess of 99% were
obtained at all the polymer dosages as shown in Figure 16, but at the lower
range of 4 Ib/ton, float solids of only 2.3% TS were obtained.
The circular dissolved air flotation unit was not evaluated on oxygen WAS.
Centrifugation
The centrifuge is not new to wastewater treatment; sanitary engineering
literature since the beginning of the century is sprinkled with reports of
sludge centrifugation. A perforated basket-type was used in Germany to dewater
raw primary sludge as long ago as 1902; and in Milwaukee a centrifuge was eval-
uated in 1920 but operating results were disappointing.^ Only in recent years
have centrifuges come into fairly common use, however, and factors which have
contributed to the increase in the number of centrifuge installations include
alteration of centrifuge design to make the machines more suited to use with
the types of solids encountered in waste treatment and the availability of syn-
thetic organic polyelectrolytes for sludge conditioning.
Basically, centrifuges separate solids from the liquid through sedimenta-
tion augmented by centrifugal force. Sludge is fed into the rotating bowl at a ,
constant feedrate where it separates into a dense cake containing the solids and
a dilute centrate stream containing fine, low-density solids. Three different
classes of centrifuges were evaluated for sludge thickening: 1) basket centri-
fuge, 2) horizontal scroll, and 3) disc-nozzle. The results obtained are given
below:
Basket Centrifuge—
Three 48" diameter, imperforated bowl, basket centrifuges were examined
for WAS thickening. Each of these units rotated at approximately 1380 rpm which
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is equivalent to 1300 gravities at the bowl wall. Except for minor variations
in the chemical injection system and the feed inlet works, the units were iden-
tical baring differences in the drive mechanisms.
The basket centrifuge is a solid bowl which rotates along a vertical axis
and operates in a batch manner. A schematic of a basket centrifuge is shown in
Figure 17. The feed material is introduced at the bottom of the unit and is
accelerated radially outward to the wall of the basket through centrifugal
force. Cake continually builds up within the basket until the quality of the
centrate, which overflows a weir at the top of the unit, begins to deteriorate.
At that point, feed to the unit is stopped and a skimmer enters the bowl to re-
move its contents. The total solids concentration of the cake increases in an
outward radial direction, and as a result cake solids concentrations near the
basket wall can be of such a magnitude as to prevent their being removed by the
skimmer. The field operations indicated that when the WAS is thickened to a
composite basket cake concentration of less than 6% TS full depth skimming is
possible while the basket is revolving at full speed (1380 rpm). At this speed,
the basket acts as a centrifugal pump and the skimmings are discharged through
a hose. Upon completion of the skimming sequence, which takes one minute, the
feed sequence is again initiated. When the composite cake is greater than 6%
TS, those solids near the basket wall are too thick to be skimmed and, as a re-
sult, to remove this material the machine must be decelerated and the remaining
cake plowed out. The need to plow out the cake is a major detraction of this
unit, when compared to either the air flotation or horizontal scroll continuous
discharge centrifuge.
In total, the three machines produce competitive results and., as such, dis-
tinctions were not made as to manufacturer in the discussion of data. The ma-
chines were operated at hydraulic feedrates of 50 gpm, 65 gpm, and 70 gpm, with
cationic polymer added in the range of 0 to 27 Ib/ton. As can be seen from
Figure 18, when the cake solids concentration is plotted as a function of poly-
mer dosage, a well defined single curve is obtained. The thickened solids ranged
from approximately 4% TS with no chemical conditioning to a maximum of approx-
imately 8% TS with 25 Ib/ton of cationic polyelectrolyte. As noted previously,
in general observation, when the composite thickened solids concentration was
less than 6% TS, the material could be completely removed from the machine via
the skimmer nozzle; however, in excess of 6% TS the solids near the basket wall
had to be knifed. The majority of the data points shown in Figure 18 were dupli-
cated several times, and particularly for the single point shown for 65 gpm. On
a great number of occasions the machines were run at 65 gpm for 'extended periods
of time to supplement the existing sludge thickening mechanism at the Saugus-
Newhall WRP. The results from these runs were very consistent and, as such,
were averaged and shown as the single data point. The corresponding solids re-
coveries and centrate SS concentrations are shown in Figure 19. As can be seen,
rather than define a single function, the data appears to divide hydraulically
between those runs at 70 gpm and those at 50 and 65 gpm. For 50 gpm, the solids
recovery ranged from 80% (centrate = 1200 mg/1 SS) with no polymer addition, to
in excess of 95% (centrate = 300 mg/1 SS) at polymer dosages greater than 15 lb/
ton. The duplicated runs at 65 gpm yielded results that were equivalent to the
50 gpm data. For 70 gpm, the results at polymer dosages in excess of 15 Ib/ton
were, for all practical purposes, equivalent to those for 50 gpm. However, as
the polymer dosage was decreased the effluent quality for the 70 gpm condition
decreased at a faster rate than for the 50 gpm condition. It is difficult to
20
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elude if the deteriorating effluent quality is a.result of either hydraulic or
solids loading rate limitations, or a combination of both.
One of the 48" basket centrifuges was moved to the JWPCP to evaluate vari-
ous polymers in conjunction with the second stage sludge dewatering station.
Scheduling difficulties, operational problems with the test centrifuge, and a
limited WAS supply greatly restricted the thickening studies on the oxygen WAS,
but such data as were collected are summarized in Figures 20 and 21.
The unit was operated at a constant bowl speed of 1380 rpm, corresponding
to an acceleration force of ISOOg's and was loaded at a constant hydraulic rate
of 50 gpm. Polymer addition in the range of 0 to 5 Ib/ton yielded composite
cake solids of 6 to 8 percent TS while SS recovery varied from 77 to 95%. Be-
cause the composite cake was greater than 6% TS, full depth skimming could not
be employed. The volume of sludge skimmed out is proportional to the variation
in solids content as the skimmer moves into the captured solids. When the skim-
mer reaches a point where the solids concentration is in excess of approximately
7% TS, it will no longer advance into the cake and the machine has to be decel-
erated and the ploy inserted to remove the remaining solids. The deceleration
and plow insertion sequence in .conjunction with average run times of 13 minutes
yielded an effective flow rate of 38 gpm and suspended solids loadings of 280
Ib/hr. These data were collected when the oxygen WAS exhibited good floccula-
tion and flotation characteristics and, therfore, reflect optimum performance.
The Saugus-Newhall WAS required 10-12 Ib/ton of polymer, whereas the oxygen
WAS required only 5 Ib/ton to obtain 95 percent SS recovery. The data indicate
that these differences in performance are not due to differences in sludges as
much as the SS recovery really does not describe the performance of a centrifuge
as well as centrate SS does. Both sludges gave centrate SS concentrations of
about 700 mg/1 at the 5 Ib/ton polymer dosage. With the oxygen WAS (1.47 per-
cent) this centrate corresponded to a 95 percent recovery; but with the Saugus-
Newhall WAS (0.55 percent), the recovery was only 87 percent. A SS recovery of
95 percent with the Saugus-Newhall WAS requires a centrate with only 300 mg/1
SS.
Horizontal Scroll Centrifuge--
Two horizontal scroll, concurrent flow, tapered bowl centrifuges were eval-
uated as WAS thickening devices. Both machines were manufactured by the same
company; however, they were of different sizes (32" bowl diameter x 100" bowl
length vs. 20" x 62") and had different hydraulic capacities. A schematic of
the basic characteristics of each machine is shown in Figure 22.
The scrolls of each machine rotate along a horizontal axis and operate in
a continuous manner. Sludge is fed to the unit through a stationary tube along
the centerline of the inner screw which accelerates the sludge and minimizes
turbulence. The sludge passes through-ports in the inner conveyor shaft and is
distributed to the periphery of the bowl. Solids settled through the liquid
pool in the separating chamber are compacted by centrifugal force against the
wall of the bowl and are conveyed by the outer screw conveyor to the opposite
end of the inlet works. Separated liquid (centrate) is discharged continuously
over an adjustable weir at the inlet end.
21
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The machines were evaluated at different time periods during the study.
For all data presented utilizing the 32" x 100" centrifuge, the bowl speed was
maintained at 1280 rpm (750g's). The relative scroll speed was held constant
at 16.5 rpm's and the pool depth was maintained at maximum. When utilizing the
20" x 62" centrifuge, speeds of 2070 gpm (1200g's) and 19 rpm were maintained
for the bowl and relative scroll, respectively, and the pool depth was main-
tained at maximum. Preliminary^testing with each unit governed the relative
scroll speed, while the desire to obtain maximum solids recovery set the pool
depth. The machines were operated at mid and low pool depths to ascertain the
effect on centrate quality and cake solids, but the change was negligible.
The results from the operation of the 32" x 100" unit are presented in
Figures 23, 24, and 25. The hudraulic flow rate ranged from 70 to 90 gpm while
the resulting solids loading rates varied from 185 to 280 Ib SS/hr. As shown
in Figure 23, these variations in loading rates did not affect the cake solids
concentration, nor did the polymer dosage have a significant effect on the con-
centrations obtained. The cake solids concentration varied from approximately
7% TS with no polymer addition to 8% TS with the addition of polymer at a rate
of 20 Ib/ton. However, for the corresponding solids recovery and centrate con-
centration, as shown in Figures 24 and 25, the hydraulic and solids loading
rates, as well as the polymer dosage had a significant effect. In general, at
cationic polymer dosages greater than 15 Ib/ton there was no significant differ-
ence in centrate quality or solids recovery for the hydraulic and solids loading
rates encountered. However, at dosages less than 15 Ib/ton it can be seen that
as the solids loading rate increased the centrate quality and solids recovery
decreased.
In the operation of the 20" x 62" unit, the flow rate was varied from 40 to
60 gpm, while the corresponding solids loading rate ranged from 72 to 108 Ib
SS/hr. As shown by the results in Figures 26 and 27, the same general operation-
al response was observed in this unit as compared to the larger 32" x 100" unit.
While no data was obtained at polymer dosages of less than 5 Ib/ton, the results
indicated that a cake of 7.5% TS could be obtained at 7 Ib/ton of polymer and
could be increased to approximately 9% TS with a polymer dosage of 25 Ib/ton.
These results were slightly better than that achieved with the larger machine,
although quite obviously at much reduced hydraulic and solids loading rates. As
illustrated in Figure 27, the suspended solids recoveries obtained with the smal-
ler machine approximated those achieved with the larger unit and demonstrated
the same response in regard to increased solids loading rate. Both machines re-
quired cationic polymer dosages in excess of 10 Ib/ton to achieve adequate sol-
ids recoveries (95%), with the corresponding cake solids in the range of 7% to
9% TS. As such, the choice of which machine to implement on a full scale appli-
cation would result from the flow and quantity of solids to be handled in combi-
nation with an economic analysis of the thickening system.
Scroll centrifuge thickening of the oxygen WAS was not evaluated. Assuming
that the centrate SS vs. polymer dosage curves for the two WAS's will be similar
for the scroll centrifuges as they were for the basket centrifuges, it is possi-
ble to extrapolate the Saugus-Newhall data and estimate the results of scroll
centrifuge thickening at the JWPCP. In order to obtain 95% SS recovery on the
oxygen WAS, a centrate with 600 mg/1 SS is required. The 32" x 100" scroll cen-
trifuge required 7 Ib/ton of polymer and the 20" x 62" machine required 11 Ib/
ton to meet this criterion with the Saugus-Newhall WRP WAS. These same dosages
22
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would be expected at the JWPCP. The resultant cake solids would be at least as
high as the 7-9% TS cakes produced in the Saugus-Newhall WRP studies.
Disc-Nozzle Centrifuge--
The disc centrifuge has a perforated bowl which rotates along a vertical
axis at approximately 6000 rpm and operates in a continuous manner. A schematic
of the disc centrifuge is shown in Figure 23. The feed material is introduced
at the top of the unit and flows through a set of some 50 conical discs which
are utilized for stratification of the waste stream to be clarified. The discs
are fitted quite closely together and centrifugal force is applied to the rela-
tively thin film of liquor and solids between the discs. This force throws the
denser solid material to the wall of the centrifuge bowl where it is subjected
to additional centrifugal force and concentrated before it is discharged through
nozzles located on the periphery. The clear liquid continually flows over a
weir at the top of the bowl and exits via the centrate line. The bowl is
equipped with 12 nozzle openings, but various numbers and sizes of discharge
nozzles can be utilized depending on the feed liquor and the desired results.
The number and size of discharge nozzles used directly influences the sludge
concentration for any given feed condition.
Historically, prescreening of the feed material has been of necessity, be-
cause of the machine nozzle size (0.07 in. to 0.08 in.). As such, screens with
openings of .030" and .027" were installed upstream of the centrifuge but failed
to successfully remove small sand particles that eventually clogged the nozzles
and continuously interrupted operation of the unit. It was the intent to study
the machine's thickening capabilities over a full range of WAS flows with and
without polymer conditioning. However, because of the prescreening problems,
the evaluation of the machines was greatly curtailed. Perhaps with the imple-
mentation or development of adequate prescreening mechanisms the machine could
realize its potential; however, for this investigation it was decided that the
disc-nozzle mechanism was simply not competitive with the previous systems eval-
uated.
Process Selection
The thickening performance data are presented in Table II. Table II re-
flects data collected at the JWPCP on oxygen WAS for dissolved air flotation
and basket centrifugation. The scroll centrifuge data are projected to the oxy-
gen WAS based on the scroll centrifuge results on the Saugus-Newhall WAS and the
basket centrifuge data from both research locations. No usable data could be
obtained for gravity thickening or disc-nozzle centrifugation.
Centrifugation'will consistently produce cakes of 6 to 8 percent TS. WAS
solids at that concentration are very plastic and viscous in nature, and it is
the opinion of the authors that conventional gas recirculation mixers will not
provide adequate mixing for the digestion process if the digesters are fed 6%
WAS.
Dissolved air flotation is the most attractive alternative. It will pro-
vide sludge thickened to 3.5 percent TS at polymer dosages less than 4 Ib/ton.
The SS recoveries will be greater than 99 percent. The high SS recovery is es-
pecially beneficial for the operation of the secondary treatment system since
23
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it will allow more accurate control of the system solids. Also, those solids
which would escape from the thickening system will be the most difficult to han-
dle, and returning them to the main treatment stream would only contribute to
operational problems. Dissolved air flotation will minimize those problems.
Thickening systems will be further discussed in the cost analysis section
of this report.
STABILIZATION
The satisfactory disposal of the concentrated organic solids removed from
sewage in the primary sedimentation tanks and excess biological solids from the
activated sludge process frequently requires that the solids first be stabi-
lized. The original objective of stabilization was to reduce the objectionable
qualities of the sludge such as putrescibility and odors. In practice, some of
the side benefits of stabilization may become the primary objectives. The ba-
sic benefits of stabilization that were of interest in these studies were vola-
tile suspended solids destruction, energy production, and improved dewaterabil-
ity of the sludge.
The most common and widely used method of sludge stabilization is anaero-
bic digestion where the decomposition of organic and/or inorganic matter is per-
formed by microorganisms in the absence of molecular oxygen.
Other unit processes commonly employed for the stabilization of sewage
sludges include aerobic digestion and oxidation ponds. Thermal treatment in-
volves heating the sludge for short periods of time at elevated temperatures and
pressures. Since this process is primarily intended for conditioning it will be
discussed in a separate section.
Aerobic Digestion
Aerobic digestion may be defined as the destruction of degradable organic
sludges by aerobic, biological mechanisms and has essentially evolved from the
extended aeration version of the activated sludge process. The process may be
used for either primary sludge, excess biological sludge, or mixtures of the
two. Generally, aerobic digestion is most applicable to excess biological
sludges because in the absence of an external substrate, microorganisms enter
the endogenous phase of the life cycle, resulting in a net decrease in the de-
gradable portion of the microbial or sludge mass.
As an integral part of the biological sludge treatment studies, aerobic
digestion of thickened waste activated sludge was investigated at the Saugus-
Newhall WRP. A 13,000 gallon, coarse bubble, diffused air system was installed
at the research site in November 1974 and served as an aerobic digester for
approximately six months. Initially, it was the intent of this evaluation to
investigate aerobic digestion of waste activated sludge over a full range of
volatile solids loadings, detention periods, and air loadings; but disappointing
results prematurely ended the study.
24
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The operating parameters that were investigated are presented in Table III.
Detention times of 8 and 12.7 days were investigated while volatile solids load-
ing rates varied from 0.070 to 0.105 Ib VSS/ft3-day with the air input main-
tained at 0.043 or 0.060 cfm/ft3. .
During the first month of operation, (November 1964) severe foaming in the
digester hampered its operation and foam spillage occurred daily, causing a por-
tion of the digester solids to be washed out of the system. It was impossible
to calculate the amount of solids leaving the system via the foam. Hence, a
true assessment of the volatile solids destruction efficiency was not possible
during this period. No direct measures were taken to eliminate the foaming
problem but with the addition of approximately 1500 ml per day of defoaming a-
gents the problem was alleviated and the foam confined to the digester.
Tables IV through IX represent the operational and performance data col-
lected during each month of the study and, as seen in Table IV, the residual
dissolved oxygen (DO) maintained in the digester during November was only 0.18
mg/1. Relatively little design or operational data are presented in the litera-
ture for aerobic digestion but minimum DO concentrations of 1.0 to 2.0 mg/1 are
normally recommended.
In December 1974, the air loading rate was increased to 0.060 cfm/ft2, re-
sulting in an average residual DO concentration of 1.05 mg/1. As presented in
Table V, at a hydraulic detention time of 8 days and a volatile solids loading
of 0.090 pounds per cubic foot per day, an average of 26.8% of the applied vola-
tile solids were degraded. During this period a notable decrease in pH was
observed and it dropped as low as 4.9 before two pounds of NaOH were added to
suppress the pH decline.
The most plausible explanation for the decline in pH is that excessive ni-
trification was occurring in the digester. As nitrification becomes more com-
plete, the acidity increases because of a greater number of hydrogen ions going
into solution. The increase in nitrification was believed to be related to the
higher air rate being maintained to the aerobic digester. When the air rate
was increased to 105 cfm, the digester residual DO increased to approximately
1.0 mg/1 as opposed to a residual DO of .2 mg/1 or less during the month of No-
vember. This substantial increase in DO, in conjunction with the other favor-
able conditions such as higher NHs concentrations in the feed, resulted in an
inordinate growth or bloom of the nitrifying bacteria and an increased rate of
nitrification.
During the next three months; January, February, and March, 1975; the di-
gester was operated at a hydraulic detention time of 8 days, an air loading
rate of 0.060 cfm/ft3 and a volatile solids loading varying from 0.089 to 0.105
Ib/ft3-day. The data collected during this period are summarized in Tables VI,
VII, and VIII.
Volatile solids destruction declined slightly during this three-month peri-
od to an average of 23%. The residual DO concentration and oxygen uptake rates
approximated each other during the months of December and January but during the
first of February the DO uptake began to steadily decrease while the digester
residual DO began to steadily increase. During this time microscopic examina-
tions revealed that the biological population in the digester was also declining.
25
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By February 20, 1975, the oxygen uptake rate had declined to 14.7 mg/l/hr
and the number of rotifers and ciliates (stalked and free swimming) had de-
clined drastically. It should be noted that during this period the plant aera-
tion tank solids had declined significantly and the foam problem had become
particularly acute. In fact, the MLSS concentration in Aeration Tank No. 1 had
fallen to below 0.3% SS. On February 21, 1975, the step aeration flow pattern
was changed. The MLSS concentration in Aeration Tank No. 1 immediately began
to increase while the aeration foam problem was alleviated. These changes im-
mediately affected the aerobic digester. By February 25, 1975, the residual DO
decreased to 0.2 mg/1, the oxygen uptake rate increased to 45.5 mg/l/hr, roti-
fers and ciliates again became predominant and the foam in the aerobic digester
was alleviated.
During the month of March (Table VIII) the digester operating parameters
approximated those of the previous three months and the volatile solids destruc-
tion leveled off at an efficiency of 22%. The DO uptake rate increased slightly
and the residual DO concentration decreased to,0.20 mg/1 while the recorded rate
of nitrification decreased from the rates recorded previously. The reason(s)
'for the decrease in nitrification and residual DO are not apparent but are
thought to be related to the step changes made in the aeration system at the end
of February 1975.
The digester operating parameters were changed during the month of April
1975, and the data recorded during this period is presented in Table IX. The
digester was operated at a hydraulic detention time of 12.7 days, a volatile
solids loading of 0.070 Ibs VSS/ft3-day and an air loading rate of 0.060 cfm
/ft3. Under these operating conditions, volatile solids destruction increased
slightly to a value of 26.4%. The residual DO increased to 3.5 mg/1 while the
DO uptake decreased to 37.5 mg/l/hr, indicating that the system could have been
operated at a lower air loading rate than the 0.060 cfm/ft3.
Aerobic digestion studies were not conducted at the JWPCP.
Anaerobic Digestion
In modern practice, anaerobic digestion is usually accomplished in heated
reactors maintained within the mesophilic temperature range (90° to 100° F)..
The elevated temperature has been found to speed up the digestion process and to
improve the process stability. Anaerobic digestion can also be conducted in the
thermophilic temperature range (120 to 135°F). The organisms involved in ther-
mophilic digestion are not the same as are involved in mesophilic digestion, so
the process results may be different.
Both mesophilic and thermophilic anaerobic digestion were evaluated in
these studies.
Mesophilic Digestion—
In May of 1973, an unsuccessful attempt was made at the Saugus-Newhall WRP
to anaerobically digest straight waste activated sludge. The failure was attri-
buted to not allowing the bacteria time ,to acclimate. Waste activated sludge
was pumped to the digester while completely halting primary sludge pumpings and,
as a result, the digester failed and anaerobic digestion of straight waste acti-
26
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vated sludge was thought to be impractical.. In August 1973, the Research Sec-
tion at the Saugus-Newhall WRP began monitoring a 125,000 gallon, gas-mixed,
heated primary digester which was receiving a combined sludge of approximately
20% waste activated and 80% primary sludge. This digester was isolated for the
biological sludge treatment research studies and the waste activated to pri-
mary sludge ratio was gradually increased until the digester was fed 100% waste
activated sludge.
By April 1974, the digester wa,s successfully digesting a 73% - 27% mixture
of waste activated and primary sludge but operational problems which developed
at the plant led to a digester failure in July 1974. As a result, the digester
had to be reseeded and the ratio of waste activated to primary sludge had to
again be gradually increased.
Table X summarizes the anaerobic digestion operating parameters evaluated
at the Saugus-Newhall WRP. Included in Table X is a summary of data collected
on combined sludge anaerobic digestion at the Valencia WRP. Figure 29 is a plot
of volatile solids destruction versus the percentage of waste activated and pri-
mary sludge. Volatile solids destruction in excess of 50% were consistently ob-
tained for all of the combined sludge ratios. Digestion of 100% waste activat-
ed sludge produced volatile solids destructions of 45-50%.
Unfortunately, valid gas data were not collected in the Saugus-Newhall
studies because of the plant's practice of continually hauling stored sludge
from the 125,000 gallon secondary digester and an inadequate gas metering sys-
tem. The gas lines from all four of the digesters at the Saugus-Newhall WRP are
interconnected and it was virtually impossible to collect isolated gas data from
any of the digesters. In addition, excessive hauling from the secondary digest-
er which often resulted in a blown seal and gas leakage through the seal. Even
after an additional gas meter was installed in January 1975, the blowing of
seals continually hampered the collection of gas data.
In August 1975, the 13,000 gallon diffused air aerobic digester was trans-
ported to the JWPCP and converted to a 12,000 gallon anaerobic digester. The
unit served to digest thickened waste activated sludge and was operated in the
mesophilic and thermophilic temperature ranges.
In mid-September 1975, the pilot digester was seeded with 5,000 gallons of
digested waste activated sludge from the Saugus-Newhall WRP and immediately be-
gan receiving air flotation thickened waste activated sludge from the JWPCP pi-
lot plants. Due to various start-up and operational problems, the digester did
not reach steady state conditions until the latter part of October 1975. Tables
XI and XII summarize the operational parameters maintained and the performance
achieved during a 61-day period from November 1975 to January 1976. During this
steady state period, the digester received an average of 551 gpd of thickened
oxygen and air waste activated sludge. It was the intent of this study to di-
gest oxygen sludge only, but fluctuations in the wasting rate from the oxygen
system necessitated that waste sludge from the mechanical air system be added so
as to ensure a uniform loading rate to the digester. An exact ratio of oxygen
to air sludge added to the digester is difficult to determine and the best esti-
mate is that 20-30 percent of digester feed contained waste sludge from the air
system. The volatility of the two waste sludges approximate each other and it
is not felt that the addition of air sludge had any significant effects on the
27
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project objectives.
At an average hydraulic detention period of 22 days and a volatile solids
loading of 0.085 pounds per cubic foot per day, a volatile solids destruction
efficiency of approximately 32 percent was recorded in the digester. Total gas
production averaged 14.8 cubic feet per pound of volatile solids destroyed and
consisted of approximately 61 percent methane. Operation of the digester dur-
ing this period was stable and no major operational problems were encountered
other than normal start-up difficulties.
Subsequent to the collection of data for this report, an inadequacy in the
volatile acids analysis at the JWPCP which caused low results was discovered.
The average volatile acids concentration reported was less than 10 mg/1 for
mesophilic digestion.
In addition to the routine analysis reported in Tables XI and XII, one set
of samples were taken and analyzed for heavy metal concentrations. These sam-
ples consisted of the feed oxygen waste activated sludge and the digester efflu-
ent. The results from these analyses are presented in Table XIII. It is im-
portant to note that the feed sample consisted of a single grab sample and
while the digester effluent sample was also a grab, it actually reflects the
accumulation of approximately 20 days worth of storage in the digester. It is
the author's opinion that the influent and effluent total metals concentration
should be essentially equivalent. That there are some substantial differen-
tials is thought to be reflective of the lack of statistical significance in a
single sample. Even considering the sampling procedure it is significant to
note that the soluble metals concentrations remained virtually unchanged through
the digestion process.
Thermophilic Digestion—
*
On January 5, 1976, the temperature of the pilot digester began to be in-
creased from 94°F to a targeted thermophilic temperature of 120°F. By Janu-
ary 13, 1976, the digester temperature had reached 103°F with no appreciable
changes in any of the performance or operating parameters. In an attempt to re-
tard a digester upset, the digester feed rate was lowered on January 13 to re-
duce the solids loading rate and to increase the hydraulic detention time from
19 to 29 days. On January 15 the temperature had reached 108°F and except for
a slight decrease in methane quality, the performance and operating parameters
remained constant until January 22 when the digester temperature was recorded
at 113°F. This caused an immediate rise in volatile acids, a further decrease
in methane quality and mild foaming conditions. To avert an ultimate failure,
digester feed was halted on January 23, resulting in a sharp drop in gas pro-
duction. The digester was closely monitored with respect to pH, volatile
acids, alkalinity and methane gas quality, and the temperature was again in-
creased, to 120°F, on January 26. On the following day, approximately 10 per-
cent of the digester volume was displaced with thermophilically digested sludge
from the Hyperion Treatment Plant to establish a healthy thermophilic bacteria
population in the pilot digester.
On January 28 the digester began to again receive oxygen waste activated
sludge with continued close monitoring of the vital operating parameters. Fig-
28
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ure 30 shows the digester response (ph, volatile-acids, and methane gas quality)
to temperature increases during the transition period from mesophilic to thermo-
philic operation. Due to the analytical problem mentioned earlier, the vola-
tile acids data are probably inaccurate, but since all analyses were conducted
by the same method, changes in the results are significant.
Steady state conditions were achieved by the end of February 1976, and
Tables XIV and XV summarize the operational and performance parameters obtained
through June 1976. Volatile solids destruction averaged 39.4 percent while the
unit gas production was measured at 17.0 cubic feet per pound of volatile sol-
ids^ destroyed. An average hydraulic detention time of 21 days and a volatile
solids loading of 0.074 pounds per cubic foot per day were maintained during
this four month period. No accurate volatile acids data are available for ther-
mophilic digestion, but the available data and a characteristic odor indicate
that thermophilic digestion will result in higher volatile acids than mesophil-
ic digestion. The reported volatile acids concentration averaged 90 mg/1,
but due to the previously mentioned laboratory problem, the actual"volatile
acids concentration is unknown. A characteristic volatile acids odor was de-
tected during the thermophilic study, but not during mesophilic digestion,
which confirms the qualitatively higher volatile acids resulting from thermo-
philic digestion. Again, it was the intent of this program to solely digest
oxygen waste activated sludge but due to the problems mentioned previously, the
digester feed sludge contained approximately 20 - 30 percent of waste sludge
from the 0.5 MGD mechanical aeration air system.
Of interest is the fact that all of the anaerobic digestion studies con-
ducted to date on waste activated sludge, at both the Saugus-Newhall WRP and
the JWPCP, indicate that these waste sludges are not destroyed or converted as
readily as raw sludge solids in anaerobic digestion. The difference in vola-
tile solids destruction between the two sludge types is most likely related to
the relative amount of degradable solids contained in each of the respective
sludges and the effects of ammonia toxicity as outlined by McCarty and McKinney.5
Ammonia is usually formed in anaerobic treatment from the degradation of
wastes containing proteins or urea. Inhibitory concentrations may be approached
in industrial wastes containing high concentrations of these materials or in
some highly concentrated municipal waste sludges.
Ammonia may be present during treatment in the form of the ammonium ion
) or as dissolved ammonia gas (NH3). These two forms are in equilibrium
with each other, the relative concentration of each depending upon the pH or
hydrogen ion concentration.
According to McCarty,6 the following ammonia nitrogen (sum total of the
ammonium ion plus ammonia gas) concentrations which may have an adverse effect
on anaerobic digestion are listed below.
29
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- Concentration
(mg/1)
Effect on Anaerobic Treatment
15
200
1500
Above
200
1000
3000
3000
Beneficial
No Adverse Effect
Inhibitory at Higher pH Values
Toxic
If the concentration is between 1,500 and 3,000 mg/1, and the pH is greater
than 7.4 to 7.6, the ammonia gas concentration can become inhibitory. A review
of the thermophilic digestion data collected at the JWPCP (Table XV) indicates
the digester operated within this region of inhibited biological activity. Un-
fortunately, nitrogen analyses were not conducted during previous mesophilic
digestion studies but it is the author's opinion that higher than normal ammonia
concentrations would have been encountered.
If the assumption is made that excess biological solids are not as amenable
to anaerobic treatment as raw sludge solids solely because of the buildup of
ammonia nitrogen, it would then seem that two solutions are available to alle-
viate the problem. These are: to dilute the biological sludge feed prior to
digestion or to blend concentrate biological solids with primary sludge solids
prior to digestion.
In April 1976, a laboratory scale study was initiated to study the effects
of biological sludge feed concentrations on subsequent ammonia nitrogen genera-
tion in the anaerobic treatment process. At the time of this writing, the labo-
ratory scale digesters were approaching steady state conditions and the results
from these studies will be reported at a later time.
In regard to total volatile solids destruction, a review of the digestion
data collected at the Saugus-Newhall WRP indicates that approximately 40 percent
of the applied volatile solids are destroyed by anaerobic digestion whereas meso-
philic digestion of excess activated at the JWPCP yielded 32 percent volatile
solids destruction and 39% for thermophilic digestion. The variation in meas-
ured efficiency might be related to the fact that the Saugus-Newhall WRP treats
primarily a domestic waste while a combination of industrial and domestic waste
is treated at the JWPCP. The industrial fraction of the waste treated may be
less suited to anaerobic digestion than domestic wastes.
One set of samples consisting of digester feed sludge and digester effluent
were collected during the thermophilic studies for heavy metal analysis. These
results are presented in Table XVI and indicate a higher degree of solubiliza-
tion than under mesophilic conditions. It should again be noted that although
both grab samples were taken at the same time, the effluent sample reflects 20
days of detention in the digester and may be regarded as a composite while the
feed sample was collected over an eight-hour period.
30
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Process Selection
When comparing aerobic digestion and anaerobic digestion, it becomes evi-
dent that from a solids reduction standpoint there are no advantages to aero-
bically digesting waste activated sludge. Additionally, aerobic digestion re-
quires an energy input whereas anaerobic digestion is self-sustaining and pro-
duces a usable source of energy. Furthermore, as will be discussed in the
following section, the, dewaterability of waste activated sludge was not en-
hanced by aerobic digestion when compared to anaerobic digestion.
Thermophilic anaerobic digestion appears attractive because of a 23 per-
cent increase in volatile solids destruction and a 15 percent increase in unit
gas production (ft3/pound destroyed) when compared to mesophilic digestion, but
it cannot be justified when fuel requirements for heating are considered.
Figures 31 and 32 summarize the theoretical heating requirements for meso-
philic and thermophilic digestion. These calculations are based on a second-
ary treatment capacity of 100 MGD and incorporate the average digestion perfor-
mance results achieved with the JWPCP research digester. Assuming a total sys-
tem thermal efficiency of 50 percent mesophiTic digestion will yield a surplus
of 9 x 106 BTU/day, while thermophilic digestion will require an addition of
45 x 106 BTU/day. Additionally, the dewatering properties of thermophilic di-
gested sludge were not observed to increase over those of mesophilically di-
gested oxygen sludge. A complete cost analysis for the various stabilization
options will be presented in the system analysis section of this report.
DIGESTED SLUDGE CONDITIONING AND DEWATERING
Subsequent to stabilization, jsludges can be conditioned and dewatered so
so that their moisture content is considerably reduced. As a result of de-
watering, the transportation costs to final disposal are reduced; or if inciner-
ation or mechanical drying is practiced, reductions in fuel requirements may be
realized. The dewatering system must also achieve an effluent low enough in
suspended solids that it can be either returned to the head end of the treatment
plant without causing any adverse effects on the operation of the plant or it
can be combined with the plant effluent without appreciably affecting the ef-
fluent quality.
Two basic mechanisms are employed to dewater sludges. In filtration, a
matrix is established which allows water to pass but retains the solids. The
driving force can be provided by pressure pumps on the feed sludge or vacuum
pumps on the filtrate side. In centrifugation, sedimentation is enhanced by
centrifugal force. Chemical conditioning with polymer and/or inorganic chemi-
cals (lime, alum, and ferric chloride) can improve the performance of dewater-
ing equipment.
The research digesters provided aerobically and anaerobically digested WAS
and anaerobically digested blends of primary sludge and WAS. In addition to
these sludges, blends of anaerobically digested WAS and anaerobically digested
•primary sludge were available for the entire study, so each machine was tested
on a limited selection of these sludges.
31
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Filtration
Pressure Filter (Filter Press)—
Pressure filtration is a batch operation and consists of vertical steel
plates (trays) which are held rigidly in a frame and pressed together. A sche-
matic of the plate and frame pressure filter is shown in Figure 33. The sludge
is fed into the press and passes through feed holes in the trays along the
length of the press. As filtration proceeds, the liquid passes through the
fiber of the cloth media and the solids are retained. Sludge feeding is stopped
when the cavities between the trays are completely filled. Drainage ports are
provided at the bottom of each press chamber, and the filtrate is collected and
discharged to a common drain. The dewatering step is complete when the filtrate
flow is near zero. The plates are then disengaged and the filter cake is dis-
charged.
Pressure filtration studies at the Saugus-Newhall WRP were confined to
thermally conditioned sludge and will be discussed in another section.
The manufacturer of the pilot filter press evaluated at the JWPCP provided
a specific filtration resistance meter, a modified Buchner funnel, for prelim-
inary testing of the sludge and its reaction to conditioning chemicals. The
specific filtration resistance (R) determinations on mesophilically digested WAS
are summarized in Tables XVII and XVIII.
Four series of "R" test were run. The first utilizing a 3.8 to 1 ratio of
lime (CAO) to ferric chloride, a ratio that the filter press manufacturer had
frequently found to be cost-effective. The following three series of runs were
conducted using either ferric chloride, lime, or cationic polymers for condition-
ing. The equipment manufacturer suggested that an "R" reading of 2 x 1012 cm/g
or less is required for effective dewatering on a full scale basis.
As seen in Tables XVII and XVIII, the addition of lime, ferric chloride, or
polymer alone did not reduce the R value to the desired range. The addition of
16 percent (320 Ib/ton) ferric chloride and 61 percent (1220 Ib/ton) lime did
reduce the specific filtration resistance reading to below 2 x 1012 cm/g. Al-
though ferric chloride and lime are the standard conditioning agents used in con-
junction with vacuum and pressure filtration, an evaluation of polymer condition-
ing was considered appropriate at least on a laboratory scale level.
The pressure filter manufacturers indicated that polymers "break down" at
the high pressures encountered in pressure filtration and as seen in Table XVII,
the R readings did increase significantly as the pressure was increased from 100
psig to 225 psig. The increase in the R value was minimal as the pressure in-
creased from 45 psig to 100 psig, indicating that the polymer is strong,enough
to withstand pressures at least up to 100 psig.
The 30 ft2 pilot pressure filter was operated at lime dosages (CaO) of 30
percent (600 Ib/ton) to 50 percent (1000 Ib/ton) and ferric chloride dosages of
8 percent (160 Ib/ton) to 20 percent (400 Ib/ton) while dewatering mesophilical-
ly digested oxygen WAS. The unit was operated at a pressure of 225 psig for
filtration periods of 2 to 3 hours. The results of these studies, all of which
were run at a 30 mm cake thickness, are summarized in Table XIX. The average
32
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feed solids concentration during these studies approximated 3.0 percent TS: The
cake solids data shown in Table XIX ranged up to 40 percent at the higher chemi-
cal dosages and three-hour run times. It was found that discharge cakes con-
taining less than 30 percent total solids would not discharge completely. Cakes
below this point, although firm next to the filter, had a fluid core which al-
lowed the cake to split down the middle when the filter press was open. As a
result, the fluid portion would discharge while the firm portion remained at-
tached to the filter media, requiring manual removal and cleaning.
In order to meet or exceed 30 percent total solids in the filter press cake,
this particular sludge required lime dosages of 35 percent (700 Ib/ton) to 40
percent (800 Ib/ton) CaO and ferric chloride dosages of 12 (250 Ib/ton) to 20%
(400 Ib/ton). A precoat application of approx. 10% (200 Ib/ton) diatomaceous
earth preceded the filtration sequence. Precoating is an optional operation
that improves the discharge characteristics and reduces maintenance costs. At
these dosages, discharge solids varied from 34 to 40 percent total solids with
corresponding filter yields ranging from 0.25 to 0.44 lb/hr-ft2. For all of
these runs, the suspended solids capture exceeded 99 percent.
One filtration run was made with the addition of 20 Ib/ton of cationic poly-
mer but as was the case with polymer addition to vacuum filters, its use in this
application was unsuccessful and was not further evaluated.
A number of filtration experiments were also conducted on digested primary
sludge and mixtures of digested primary and digested waste activated sludge.
The results from these evaluations are presented in Table XX. The three runs on
digested primary sludge again indicate that the filterability of this sludge is
poorer than had previously been recorded.7 The improvement in handleability of
digested waste activated sludge by blending with digested primary sludge prior
to filtration was again verified in these tests. A combined mixture of 70 per-
cent WAS - 30 percent primary (Table XX) dewatered to 28 percent total solids
with the addition of 720 Ib/ton CaO and 200 Ib/ton of ferric chloride. At these
same chemical dosages, straight digested waste activated sludge (Table XIX) de-
watered to 22 percent total solids.
Prior to dewatering of the thermophilically digested oxygen WAS, R measure-
ments were made to indicate the sludge response to conditioning agents. Table
XXI summarizes the R measurements with and without preconditioning with lime and
ferric chloride. When comparing this data with the R measurements made on meso-
philically digested waste activated sludge (Table XVII), it becomes evident that
no advantages in dewaterability are realized by digesting in the thermophilic
temperature range. Included in Table XXI is the result of conditioning the di-
gested sludge with a body feed of 100 percent (1 ton/ton) diatomaceous earth.
This measurement was made at the request of the filter manufacturer based on
their success at other installations. The resulting R reading approximated
100 x 1012 cm/g and was well above the recommended maximum value of 2 x 1012
cm/g. Further investigation into the use of diatomaceous earth was not consid-
ered justified because of the high R value and the resultant increase in total
mass of solids to be disposed of in subsequent dewatering processes.
The results of filter pressing the thermophilic sludge on the pilot (30 ft2)
filter are presented in Table XXII. The data are presented in chronological or-
33
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der because of observed changes in sludge filterability with time. It should
be noted that degradation in filterability of mesophilically digested sludge
was not observed, and the change in filterability of the thermophilic sludge
may have been attributed to not allowing sufficient time for the digester to
stabilize after it had reached its targeted temperature of 120°F. The first
six runs presented in Table XXII were conducted prior to two hydraulic deten-
tion times and as such, the digester may still have contained a significant
quantity of mesophilic sludge. Runs 7 through 12 were conducted after the
digester had gone through two detention periods. As seen in Table XXII, the
filterability increasingly degraded for runs 7 through 10 as evidenced in the
filter yield and cake solids. Although the filter performed satisfactorily at
the higher lime dosages used in runs 11 and 12, these dosages were much higher
than were required for the mesophilic digested oxygen waste sludge. Additional-
ly, these higher filter yields were attributed to operating at a 2-1/4 hour
filtration time as opposed to 3 hour runs for the remaining filter tests. Sus-
pended solids removal for all of these filtration experiments were consistent-
ly in excess of 99 percent.
The manufacturer of the 30 ft2 pilot filter press used for these studies
is the only company manufacturing presses that operate at 225 psi. Numerous
companies manufacture presses with operating pressures of 100 to 125 psi and
investigations to determine the effects of pressure on filtration performance
were considered appropriate. These pressure evaluations were conducted on a
0.33 ft2 prototype filter press that was manufactured and supplied by the same
company that supplied the 30 ft2 unit. The feed sludge for these studies con-
sisted of thermophilically digested waste oxygen activated sludge that had been
chemically conditioned with 62 percent lime (1240 Ib/ton) and 16 percent ferric
chloride (320 Ib/ton). As seen in Figure 34, insignificant variations in fil-
ter performance are observed when operated at pressures of 225 or 125 psi. The
filtrate volume with time was essentially equal for the two operating pressures
and the total amount of solids retained were within 5 percent for both operat-
ing conditions. Although the cake total solids were higher for the run operated
at 125 psi, the discrepancy is most likely due to sampling. The lack of benefit
from the higher pressure is attributed to the compressibility of the biological
sludge solids. The increased pressure causes an increase in the water's veloc-
ity through the. cake, but at the same time it compresses the cake and reduces
the size of the interstices, and no net increase in total flow is realized.
Vacuum Filter-
Vacuum filtration is a continuous process and consists of a rotating drum
which continuously passes through a trough containing the feed sludge. A sche-
matic of the rotary drum belt-type vacuum filter is present in Figure 35. The
cylindrical drum which is covered with a cloth media is submerged approximately
20% to 40% in the trough. Radial partitions divide the drum into compartments,
each alternately subjected to a vacuum. As a vacuum of 20 to 25 inches of mer-
cury is applied, a sludge mat forms on the filtration media while the filtrate
or effluent is discharged. As a point on the filter drum rotates out of the
trough, the vacuum is decreased and the captured solids are subsequently re-
moved via the discharge roller.
At the Saugus-Newhall WRP, vacuum filtration studies using the 3' x I1
rotary drum filter were conducted on both aerobically and anaerobically digest-
34
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ed sludges. Only one operating condition of the aerobic digester was evaluated.
During the period from December 1974 through March 1975, the aerobic digester
was operated at a hydraulic detention time of eight days, on average solids
loading of 0.095 Ib VSS/ft3 day, and an air loading' rate of 0.060 cfm/ft3.
Chemical conditioning with ferric chloride in the range of 0 to 300 Ib/ton
and lime in the range of 0 to 600 Ib/ton was the only method of condition em-
ployed prior to vacuum filtration of the aerobically digested waste activated
sludge. The results from this evaluation are presented in Figures 36 through
oy •
At a drum cycle time of approximately 2-1/2 minutes, an applied vacuum of
22" Hg and utilization of a tightly woven nylon cloth, cake solids, and filter
yields increased with increasing additions of ferric chloride (Figure 36). With
no chemical addition, cake solids approximated 11.5% TS with a filter yield of
0.5 lb/hr-ft2. Cake solids approximated 15% TS and the filter yield increased
to 1.0 lb/hr-ft2 with the addition of 300 Ibs/ton of ferric chloride. Solids
recovery decreased with increasing amounts of ferric chloride. As seen in
Figure 37, solids recoveries of 97% were recorded with no chemical addition
while a recovery of 87% was obtained with the addition of 300 Ib/ton of ferric
chloride. Increases in drum cycle time yielded increased cake solids while fil-
ter yields and solids recoveries decreased.
Preconditioning with 0 to 600 Ib/ton of lime prior to vacuum filtration was
also investigated. Applied vacuums of 21.5 to 22" Hg and drum cycle times from
2 to 7 minutes were employed. As seen in Figure 38, as cycle time increased
cake solids increased and filter yields decreased at each of the applied lime
dosages. Cake solids varied with chemical dosage and cycle time but little dif-
ference in cake solids was observed with chemical dosages from 100 to 600 Ib/ton
and cycle times between 2.25 to 5.75 minutes. At a cycle time of 2.25 minutes
and no chemical addition, cake solids of 11.7% TS and filter yields of 0.48 Ibs/
hr-ft2 were observed. The addition of lime from 100 to 600 Ibs/ton increased
cake solids to 12.5 to 14% TS and increased filter yields to a maximum of 0.55
lbs/hr-ft2. Increasing the cycle time to 5.75 minutes increased cake solids to
14.5% TS while decreasing the filter yield to 0.3 lb/hr-ft2. The data collected
on solids recovery was scattered with respect to cycle time and lime dosages.
At a cycle time of 2.25 to 2.75 minutes, recovery varied from 97.5% with 100 Ib/
ton of lime to 87% with the addition of 200 to 600 Ib/ton of lime. It should be
noted that during all these vacuum filtration experiments the solids retained on
the filter media exhibited extremely poor discharge characteristics. The depth
of solids buildup was small and the cloth had to be manually scraped in order
for the solids to be removed from the filter.
The vacuum fi'lter was evaluated on two digested blends of WAS and primary
sludge. These blends resulted from the anaerobic digester at the Saugus-Newhall
WRP being converted incrementally from primary sludge feed to WAS feed, so the
two sludges were blended before digestion.
The 43 percent WAS - 57 percent primary blend was evaluated using two dif-
ferent nylon cloths and various cycle times. Chemical addition ranged from no
chemicals to 5 Ib/ton of cationic polymer, 125 Ib/ton of ferric chloride and 200
Ib/ton of lime. The results from this evaluation are presented in Figures 40
and 41. Cake solids varied from 6 to 8.5% TS over the range of parameters inves-
35
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tigated and, as expected, cake solids increased slightly and the filter yields
(lb/hr-ft2) decreased with increasing drum cycle times. The filter yields were
extremely poor (0.5 to 0.15 lb/hr-ft2) while solids recovery was consistently
greater than 93% and the filtrate contained less than 1500 mg/1 of suspended
solids. The filter cloths used were tightly woven, which may account for the
high moisture content in the discharged cake because of the inclusion of fine
solids on the cloth. Coarser cloths would have produced slightly drier cakes
but fine solids would have passed through and increased the amount of solids re-
cycled with the filtrate stream. It should be noted that none of the operating
conditions produced readily dischargeable cakes. In fact, during each of these
runs the cloths had to be manually scraped in order to remove the captured sol-
ids.
A similar vacuum filtration evaluation was conducted on the 70 percent WAS
- 30 percent primary blend. As shown in Table XXIII, the chemical dosages
tested were higher than with the 43 - 57 blend (up to 400 Ib/ton of lime and
150 Ib/ton of ferric chloride), but the process results were comparable. The
filter yields were less than 0.6 lb/hr-ft2, the cakes were 8.7 percent TS or
less, and the cake discharge was poor, requiring the media to be scraped.
The vacuum filter used in the JWPCP studies was similar in construction
and the same size, 3' x I1, as the unit tested at the Saugus-Newhall WRP. The
mesophilically digested oxygen waste activated sludge exhibited extremely poor
vacuum filter dewatering characteristics. The results obtained are summarized
in Tables XXIVA and XXIVB. The vacuum filter was operated at cycle times vary-
ing from 2 to 6 minutes with an applied vacuum of approximately 23 inches Hg.
The maximum obtainable discharge solids approximated 14 percent TS with the ad-
dition of 1200 Ib/ton of lime and 350 Ib/ton of ferric chloride. Filter yields
peaked at approximately 3.4 lb/hr-ft2. Chemical dosages less than those listed
above yielded lower discharge solids and reductions in filter yields. The addi-
tion of 100 Ib/ton of alum and 10 Ib/ton of cationic polymer preceded a number
of filter runs but poor results renders their use unsuccessful for this applica-
tion.
In an effort to improve the handling characteristics of digested oxygen
waste activated sludge, various dewatering tests were conducted on combinations
of digested primary sludge and mesophilically digested oxygen sludge. The di-
gested primary sludge was generated at the JWPCP and three runs were made on
this sludge for the sake of background information. The data generated are
summarized in Tables XXVA and XXVB. Chemical addition prior to filtration in-
cluded 190 to 250 Ib/ton of ferric chloride and 700 to 1050 Ib/ton of lime.
Cycle times were varied from 2 to 6 minutes while applied vacuums of 23 inches
Hg were maintained.
Resultant cake solids on the digested primary sludge feed approximated 25
percent TS while filter yields varied from 2.5 to 3.7 lb/hr-ft2 over the range
of parameters investigated. The three runs at 100 percent digested primary
sludge indicate that the filterability of the digested primary sludge is poorer
than in 1973.7 This decline in filterability has been attributed to an unex-
plained shift toward smaller particles in the particle size distribution of the
incoming sewage and, consequently, the raw and digested primary sludges. The
smaller particles have more surface area per unit mass and therefore require
higher chemical dosages for coagulation. The digested primary sludge, however,
36
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did still dewater better than the digested waste- activated sludge.
Combined digested sludge feeds resulted in discharge solids ranging from
9 to 18 percent TS while filter yields varied from 0.80 to 4.5 lbs/hr-ft2. It
should be noted that for all runs made on the combined sludges, the captured
solids exhibited extremely poor discharge characteristics and had to be manual-
ly scraped from the filter cloth. In those runs classified as "poor", consid-
erable cleaning of the media was also required. The digested primary sludge
exhibited "fair" discharge characteristics indicative of self-discharging cakes
but considerable cleaning of the media was still required. Runs listed as
"good" are indicative of self-discharging cakes with media cleaning not re-
quired.
One set of vacuum filtration tests were conducted on thermophilically di-
gested oxygen waste activated sludge, and poor results rendered this dewatering
technique unsuccessful. The thermophilic sludge was conditioned with 50 per-
cent lime (1000 Ib/ton) and 15 percent ferric chloride (300 Ib/ton) prior to
filtration. The filter was operated under a vacuum of 18 inches Hg and a cycle
time of 4:45. The resultant cake solids were measured at 11.5 percent TS and
had to be manually scraped from the belt. The filter yield was calculated at
0.84 lb/hr-ft2 while 82 percent of the applied suspended solids were captured.
A comparison of this data with that presented for mesophilic sludge (Table XXIV)
indicates that thermophilic digestion offers no dewatering advantages and fur-
ther vacuum filtration studies were not considered justified.
Centrifugation
Only basket and scroll centrifuges were evaluated for digested sludge de-
watering. These machines were described in the "Thickening" section of this
report. Primary sludbe and WAS were combined in various ratios both before and
after digestion, so, for clarity, the following notation will be used in this
section:
1. Sludges which were blended prior to digestion will be referred to
as (% WAS): (% Primary) digested blend.
2. When digested primary sludge was combined with digested WAS or a
digested blend, the product will be referred to as (% WAS): (% Primary)
combined digested sludges.
Horizontal Scroll Centrifuge—
Three scroll centrifuges were evaluated in the dewatering studies at the
Saugus-Newhall WRP. Two of these described as "tapered bowl", scroll centri-
fuges, were different size models provided by the same manufacturer and measured
32" (bowl diameter) x 100" (bowl length) and 20" x 62". The third machine was
provided by a competing manufacturer and measured 14" x 48". The 20" x 62"
scroll centrifuge was the only scroll centrifuge available for dewatering the
aerobically digested WAS.
From December 1974 through March 1975, the aerobic digester was operated
at a hydraulic loading of eight days, an average volatile solids loading of
37
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0.095 Ib VSS/ft3-day and an air loading rate of 0.060 cfm/ft3. Polymer addi-
tion was the only conditioning method employed to aid in centrifugal dewatering.
Cationic polymer was added in the range of 4 to 26 Ib/ton, while various feed
rates (20 to 40 gpm), relative scroll speeds (19 and 24 rpm) and the maximum and
middle liquid pool depths were evaluated. For all test runs, the unit was oper-
ated at a bowl speed of 2070 rpm's (1200 g's) and the results are presented in
Figures 42 and 43. Cake solids varied from 7.5 to 10% TS and were insignifi-
cantly affected by the various centrifuge operating parameters. Suspended sol-
ids recoveries were more significantly affected by variations in feed rate, rel-
ative scroll speed and liquid pool depth. With the exception of the runs made
at 20 gpm, which may have received an improperly prepared mixture of chemicals,
polymer dosages had to be increased with increases in feed rate and relative
scroll speed to produce centrates with suspended solids concentrations of 1500
mg/1 or less. A comparison of the data collected at feedrate of 30 gpm and the
different pool depths illustrates the effect of pool depth on centrate quality.
At corresponding polymer dosages, the runs made at the maximum pool depth con-
sistently recovered more solids than those runs made at the middle pool depth.
During April 1975, the aerobic digester was operated at a hydraulic deten-
tion time of 12.7 days, a volatile solids loading of 0.070 Ibs VSS/ft3-day and
an air loading rate of 0.060 cfm/ft3. Cationic polymer was used for condition-
ing. The data collected is included in Figures 44 and 45. Additionally, vari-
ous mixtures of aerobically digested waste activated sludge and anaerobically
digested primary sludge were combined in a 2000 gallon holding tank and then
centrifugedJ The waste activated to primary sludge ratios investigated were 70%
WAS - 30% primary and 50% WAS - 50% primary. The data from these evaluations
are also included in Figures 44 and 45.
For all test runs, the centrifuge was operated at a bowl speed of 2070 rpm
(1200 g's) and a relative scroll speed of 19 rpm.
A maximum pool depth was maintained for the straight waste activated sludge
and the 70% WAS - 30% primary mixture while three pool depths were evaluated on
the 50% WAS - 50% primary sludge mixture. As seen in Figure 44, cakes ranging
from 8% to 10.5% TS were obtained on aerobically digested waste activated sludge
with the addition of 9 to 31 Ib/ton of polymer. Centrate quality deteriorated
rapidly at polymer dosages less than 20 Ib/ton and a minimum dosage of 17 Ib/ton
was required for effluent suspended solids concentrations to be 1500 mg/1 or
less (Figure 45). When comparing these two curves (Figures 44 and 45) with the
two developed when the digester was operated at an eight-day detention time
(Figures 42 and 43), it is readily seen that the sludge aerobically digested for
eight days exhibited better dewatering characteristics than the sludge aerobi-
cally digested for 12.7 days. This same relationship existed in the work done
by Parker et a!8 whereby the effects of aerobic digestion detention time on
waste activated sludge filterability were investigated. They showed that "aera-
tion produced an initial improvement in sludge filterability, with a maximum
improvement in 4 to 6 days. However, greater periods of aeration caused an in-
crease in filtration time with the result that after two weeks of digestion the
sludge was almost as difficult to dewater as it was before aerobic digestion."
The effects of combining anaerobically digested primary sludge with the
aerobically digested waste activated sludge are also presented in Figures 44 and
38
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45. With the centrifuge operating at a feedrate of 30 gpm and the maximum
pool depth setting, the resultant cake solids increased to 11%'- 12% TS on
70:30 combined digested sludges, and with the 50:50 combined digested sludges,
the cake solids increased to 13% - 15% TS. Suspended solids recoveries also
increased with increases in the amount of digested primary sludge. The effects
of pool depth on the 50:50 combined digested sludges are also shown in these
two figures. Cake solids increased "slightly and suspended solids recovery de-
creased slightly as the liquid pool depth was varied from maximum to minimum.
A wider range of pool depths and its subsequent effects on cake solids and
recovery could not be investigated because this particular centrifuge does not
have the capability of varying pool depths beyond those investigated.
As the anaerobic digester was gradually converted to WAS feed, various
ratios of waste activated and primary sludge have been digested. These blend
ratios ranged from 23% WAS - 77% primary to 100% WAS - 0% primary. Centrifuge
data were collected on the 23:77, 31:69, 43:57, and 70:30 digested blends.
Centrifugation of anaerobically digested waste activated sludge (100% WAS - 0%
primary) was also evaluated. In addition, centrifuge data were collected on
various combinations of digested primary and digested waste activated sludge
and various combinations of digested primary plus 70:30 digested blend.
As a matter of interest, the average performance and operating parameters
maintained in the primary sludge digester during periods when digested primary
sludge was combined with either a digested blend or digested waste activated
sludge are presented in Table XXVI.
Data collected on scroll centrifuge dewatering of a 23:77% digested blend
is presented in Figures 46 and 47. A 32" x 100" tapered bowl scroll centrifuge
was used to dewater this particular sludge mixture from the Saugus-Newhall WRP
and the Valencia WRP. The Valencia WRP utilizes secondary digesters for separa-
tion; hence, secondary digester subnatant (SDS) and primary digester supernatant
(PDS) were subjected to dewatering through the 32" x 100" scroll centrifuge.
The maximum cake solids obtained for the Saugus-Newhall WRP sludge approxi-
mated 11% TS while cake solids of 15% TS were obtained on both sludge sources
from the Valencia WRP. Centrate suspended solids of 1500 mg/1 or less were con-
sistently obtained at polymer dosages in excess of 13 Ib/ton as shown in Figure
47. It should be noted that the 32" x 100" scroll centrifuge had been optimized
with regard to pool depth prior to the collection of this data. Resultant cake
solids did not vary significantly with variations in the liquid pool depth but
the desire to obtain a relatively clear centrate (1500 mg/1 SS) required that
the unit be operated under maximum pool depth conditions. Upon the recommenda-
tion of the manufacturer, the unit was operated at a bowl speed of 1280 rpm .
(750 g's) and a relative scroll speed of 16.5 rpm.
A digester at the Valencia WRP was fed a blend of 31 percent WAS - 69 per-
cent primary sludge. The data from dewatering the 31:69 digested blend on a
14" x 48" scroll centrifuge are presented in Figures 48 and 49. The cake solids
ranged from 12 percent to 14 percent TS at: polymer dosages of 6 to 13 Ib/ton,
but a minimum dosage of 8 Ib/ton of polymer was required to produce a centrate
with less than 1500 mg/1 SS.
39
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Dewatering experiments on the 43:57 digested blend from the Saugus-Newhall
WRP were conducted utilizing the 20" x 62" tapered bowl scroll centrifuge. The
centrifuge was operated at constant bowl and relative scroll speeds of 2070 rpm
(1200 g's) and 19 rpm, respectively, while the pool depth was varied from its
maximum depth to its mid-depth setting.
As seen in Figure 50, the effect(s) of pool depth on cake solids was very
slight over the range of polymer dosages investigated (15 - 33 Ib/ton). At
the maximum pool depth setting, cake solids of approximately 14% TS were ob-
tained whereas cake solids of approximately 15% TS were obtained at the midpool
depth setting. The variations in pool depth had a more pronounced effect on
centrate solids and solids recovery as shown in Figure 51. Under maximum pool
depth conditions, centrate solids of less than 500 mg/1 were obtained over the
polymer range investigated whereas at a polymer dosage of 22 Ib/ton and midpool
depth conditions the centrate stream contained 10,000 mg/1 of suspended solids.
Polymer dosages less than 15 Ib/ton were not investigated because problems de-
veloped within the centrifuge and the investigation had to be terminated.
Dewatering data collected on the 70:30 digested blend utilizing the 20" x
62" scroll centrifuge are included in Figures 52 through 55. In addition to
dewatering this digested blend, combining with various amounts of digested pri-
mary sludge prior to centrifugation were also investigated. This data is also
included in Figures 52 through 55.
Figures 52 and 53 represent the data collected while the centrifuge was
operated at a bowl speed of 2070 rpm (1200 g's), a relative scroll speed of 19
rpm and a maximum pool depth. With the addition of 8 to 17 Ib/ton of cationic
polymer, the combined digested sludge dewatered to 11 - 13% TS arid a minimum of
17 Ib/ton of polymer was required for the centrate to contain less than 1500
mg/1 of SS. As the amount of digested primary sludge was increased in the
blended mixtures, the dryness of the resultant cakes was observed to also in-
crease while the polymer requirements to achieve 95% solids recovery decreased.
It should be noted that as the percentage of digested primary sludge increases
and an increase in handleability is realized, the final dryness of the resultant
sludge mixtures are equivalent to that which would be achieved if the sludges
are dewatered separately and then combined.
Figures 54 and 55 summarize the data collected while the centrifuge was
operated at its midpool depth. The bowl and relative scroll speeds were respec-
tively 2070 rpm and 19 rpm. Resultant cake solids for the combined digested
sludge varied from 12 - 15% TS with the addition of 11 to 23 Ib/ton of cationic
polymer.
A minimum polymer requirement of 17 Ib/ton was again required to produce a
centrate containing less than 1500 mg/1 of SS but at the midpool depth setting,
the deterioration in centrate quality with decreasing polymer dosages was more
rapid than under maximum pool depth conditions. Again, as the amount of digest-
ed primary sludge increased, the handleability of the resultant sludge mixtures
also increased, while the degree of dewatering was the same as had the sludges
been blended after dewatering.
40
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The successful digestion of 100% waste activated sludge enabled dewater-
ing data to be collected, utilizing the 20" x 62" scroll centrifuge. The anaer-
obically digested waste activated sludge exhibited poor dewatering character-
istics and required a minimum of 35 Ib/ton of cationic polymer in order for the
centrate flow to contain 1500 mg/1 or less SS. Three different cationic poly-
mers were used for pre-conditioning, but in each case a minimum dosage of 35
Ib/ton was required and, as seen in Figure 56, the maximum cake dryness obtained
approximated 12% TS. The physical appearance or characteristics of the cakes
obtained on the digested waste activated sludge suggests that little or no free
moisture exists.
In fact, the discharge cakes for this particular digested sludge are plas-
tic in nature and appear as dry as 20 - 25% TS cakes obtainable upon centrifu-
gation of digested primary sludge.
Various mixtures of anaerobically digested primary and anaerobically di-
gested waste activated sludge were combined (on a solids basis) in a 2,000 gal-
lon holding tank prior to dewatering through the 20" x 62" scroll centrifuge.
During these dewatering experiments, the centrifuge operating parameters were
held constant with a bowl speed of 2,070 rpm (1200 g's), a relative scroll speed
of 19 rpm, and a maximum pool depth setting, while the feed rate was maintained
at 30 gpm. The data collected is included in Figures 56 and 57. As presented
in the plot of cake solids versus polymer dosage, cake dryness increased with
increasing amounts of digested'primary sludge and, as seen in Figure 57, the
polymer requirements necessary for the centrate' to contain less than 1,500 mg/1
SS decreased significantly with increasing quantities of digested primary sludge.
The effects of blending various ratios of digested primary sludge and di-
gested waste activated sludge on resultant centrifuge cake solids, and polymer
requirements to achieve 1,500 mg/1 SS or less in the centrate, are more clearly
defined in Figure 58. A linear relationship exists'between maximum obtainable
cake solids and the ratio of anaerobically digested waste activated to digested
primary sludge. The 100% digested primary sludge dewatered to 22% TS while cake
solids decreased linearly to 12% TS upon centrifugation of 100% digested waste
activated sludge. As the quantity of digested primary sludge increased, the
dryness of the resultant centrifuge discharge solids also increased; but not
beyond that which would be achieved if the dewatered sludges are blended follow-
ing separate centrifugation. Polymer requirements increased linearly with in-
creasing amounts of waste activated sludge up to a waste activated to primary
sludge ratio of 40:60%. From 40% WAS - 60% primary to a 70% WAS - 30% primary
sludge mixture, the polymer requirements for the centrate to contain 1,500 mg/1
or less of suspended solids remained at 18 Ib/ton, then increased linearly to
35 Ib/ton for 100% digested waste'activated sludge.
Combined Digestion and Dewatering vs. Separate Digestion and Dewatering--
Consideration should be given to the dewaterability of digested blends as
compared to separate digestion followed by combining and dewatering. It might
be assumed that there is no difference in dewaterability if the sludges are
blended prior to digestion and dewatering, or combined and dewatered subsequent
41
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to separate digestion. Data contrary to this assumption are presented in Fig-
ures 59 and 60. The 70:30 and 43:57 digested blends were dewatered in the
20" x 62" tapered bowl, scroll centrifuge. For comparison purposes, the same
waste activated to primary sludge ratios were combined following separate anaer-
obic digestion and dewatered through the same unit. Although dewatering data
were collected on various sludge blends, the two ratios listed above are the
only ones that can be isolated for direct comparison, because the 20" x 62"
scroll centrifuge was the only unit on site when anaerobically digested waste
activated sludge was available. In both cases, separate digestion followed by
blending and centrifugation produced dryer discharge cakes than the flow scheme
incorporating combined digestion followed by centrifugation. Polymer require-
ments to produce centrate SS of 19500 mg/1 or less approximated each other for
the two sludges and were in the range of 15-18 Ib/ton. At these polymer dosages,
cake solids for the 70% WAS - 30% primary mixtures were 12 - 13% TS and 14 - 15%
TS, respectively, for digested blends and combined digested sludges. The de-
watering benefits realized by separate digestion followed by combining and cen-
trifugation were more apparent on the 40% WAS - 60% primary sludge blends. As
shown in Figure 59, centrifuge discharge solids approximated 18 - 20% TS for
separately digested combined sludge while cake solids of 14 - 15% TS were ob-
tained on the digested blend.
When comparing the centrifuge data collected on aerobically digested waste
activated sludge (Figures 44 and 45) with that collected on anaerobically di-
gested waste activated sludge at the Saugus-Newhall WRP (Figures 56 and 57),
it can be seen that the dewaterability of the aerobically and anaerobically
digested sludge approximate each other. To further substantiate that aerobic
digestion of sludges does not enhance dewaterability, aerobically digested
sludges from two other wastewater treatment plants were hauled to the Saugus-
Newhall WRP and dewatered via centrifugation. Each of the sludges consisted of
approximately 70% primary - 30% WAS that has been aerobically digested for
approximately fifteen (15) days. In each case, the dewaterability was equiva-
lent to that of combined anaerobically digested sludge. For centrifugation
cake solids were in the range of 10% - 13% TS and approximately 15 Ib/ton of
polymer were required for a centrate suspended solids concentration of less
than 1500 mg/1.
The scroll centrifuge evaluations at the JWPCP were conducted on an 18" x
=54" machine. This unit's dewatering data on mesophilically digested oxygen
WAS are presented in Figures 61 and 62'. Because of the small daily volume of
WAS produced, relatively few centrifuge runs were conducted on digested oxygen
sludge alone. Flow rates of 10 and 15 gpm were evaluated while the unit was
operated at its maximum pool depth (3.94"). A constant bowl speed of 1550 rpm,
corresponding to an acceleration force of 650 g's, was maintained while the
differential scroll, speed was varied from 1.0 to 3.3 rpm. Cationic polymer
addition ranged from 8.5 to 15 Ib/ton and, as seen in Figure 61, within this
range of dosages the discharge solids were consistently in the range of 15 to
17 percent TS. Cake solids increased with increasing chemical dosage and de-
creasing differential scroll speeds while suspended solids removal decreased
at the lower relative scroll speeds. As seen in Figure 62, SS recovery varied
from 60 to 95 percent over the range of parameters investigated. With the ex-
ception of the run made at a differential scroll speed of 1.0 rpm, polymer re-
42
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quirements varied from 10 to 13 Ib/ton for the centrate stream to contain 1500
to 2000 mg/1 SS.
A review of centrifuge data collected at the Saugus-Newhall WRP on digested
WAS indicated that either the JWPCP digested oxygen WAS has better dewatering
characteristics, or the 18" x 54" centrifuge was more suited for dewatering
these sludges than the 20" x 62" scroll centrifuge. To resolve this question,
5000 gallons of digested WAS from the Saugus-Newhall WRP were hauled to the
JWPCP research site and dewatered via the 18" x 54" scroll centrifuge. The re-
sults from this evaluation are presented in Figures 63 and 64. With the addi-
tion of polymer only, cake solids approximated 10 percent TS while previous test-
ing with the 20" x 62" scroll centrifuge at the Saugus-Newhall WRP yield cake
solids from 10 to 11-percent TS. The fact that both centrifuges dewatered the
digested WAS from the Saugus-Newhall WRP to approximately 10 percent TS and the
digested oxygen waste activated sludge from the JWPCP was dewatered to 1.5 to 17
percent TS (Figure 61) indicates that the oxygen WAS exhibits better dewatering
characteristics.
A series of tests were also conducted on the digested WAS from the Saugus-
'Newhall WRP with the addition of 100 Ib/ton of alum and 9-to-15 Ib/ton of poly-
mer. These .results are included in Figures 63 and 64 and indicate that alum
addition substantially enhanced the dewaterability of the sludge. Based on these
few runs, it was decided to evaluate alum and polymer addition on the JWPCP
digested oxygen waste sludge. The results from this evaluation are presented in
Figures 65 and 66 and indicate that, alum addition decreased the dewatering char-
acteristics of this particular sludge. The results obtained on the 18" x 54"
scroll centrifuge for the sludges generated at the JWPCP and the Saugus-Newhall
WRP (Figures 14 through 17) are indicative of the unpredictable responses en-
countered when dewatering sludges generated at different treatment plants. Pri-
or to terminating the mesophilic digestion studies, various combinations of the
JWPCP digested primary sludge and digested oxygen WAS were dewatered through the
18" x 54" scroll centrifuge. The data presented in Figures 67 and 68 show the
effects of increasing the ratio of digested WAS on cake solids, centrate quality,
and chemical requirements.
To more clearly show the above effects, Figure 69 is a plot of the required
chemical dosage necessary to achieve 95 percent SS recovery for the various WAS
to primary sludge ratios. Also included in Figure 69 is the resultant cake sol-
ids obtained at these polymer dosages. When comparing this data with that col-
lected at the Saugus-Newhall WRP (Figure 58), it becomes evident the digested
WAS generated at the JWPCP dewatered more readily than the sludge encountered at
the Saugus-Newhall WRP.
Basket Centrifuge—
A number of basket centrifuges were evaluated during this study. All were
similar in design, and all had 48" diameter bowls, so no distinction will be
made in this report between the machines.
Only a very limited basket centrifuge study was conducted on aerobically-
digested sludge because of equipment scheduling conflicts. During November 1974,
when the aerobic digester was operated at a hydraulic detention time of eight
43
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days, a volatile solids loading of 0.081 Ib VSS/ft3 day, and an air loading of
0.043 cfm/ft3, one set of centrifuge operating conditions was evaluated for de-
watering the aerobic digester mixed liquor. Rotating at 1,380 rpm (1300 g's),
the centrifuge was fed aerobically digested waste activated sludge with a sus-
pended solids concentration of 0.90 percent at a feedrate of 25 gpm. Cationic
polymer was added in the range of 5 to 18 Ib/ton. The results from this brief
evaluation are shown in Figure 70. The maximum cake solids obtained was approx-
imately 10% with the addition of 18 Ib/ton of polymer. Suspended solids recovery
was 99% or better for each of the chemical dosages evaluated.
No other dewatering equipment was available for evaluation when the aerobic
digester was operated under the same parameters. Some comparison may be made
between the basket centrifuge data and the scroll centrifuge data from December
1974 through March 1975 (Figures 42 and 43). The digestion detention time and
solids loadings were comparable during the two periods, but the air rate was al-
most 50 percent higher during the period when the scroll centrifuge data were
obtained. The two types of centrifuges produced comparable cakes at 8 to 10 per-
cent TS, but the basket centrifuge obtained much better SS recoveries. It is
not possible to estimate, however, what effects the change in air rate had in the
dewaterability of the digested WAS.
The data collected at the Saugus-Newhall WRP utilizing a 48" basket centri-
fuge to dewater the 23:77 digested blend are presented in Figures 71 and 72.
The maximum cake solids obtained approximated 11% TS while the centrate suspended
solids were less than 1500 mg/1 at polymer dosages in excess of 10 Ib/ton. Visu-
al observations while loading the unit in excess of 400 Ibs/hr indicated the cen-
trate was of extremely poor quality and for this reason the maximum applied sol-
ids loading was 400 Ibs/hr. Lower solids loadings were not investigated because
the feed pump to the centrifuge had a minimum capacity of 35 gpm. The cake sol-
ids and SS recoveries of the basket centrifuge and the scroll centrifuge (Fig-
ures 46 and 47) were equivalent on this digested blend.
The data collected on dewatering,the 31:69 digested blend from the Valencia
WRP are presented in Figures 73 and 74. The Valencia WRP employs two-stage di-
gestion and both primary digester supernatant and secondary digester subnatant
were dewatered on the basket centrifuge. With cationic polymer added in the
range of 5 to 30 Ib/ton, the maximum cake solids obtained approximated 13-15% TS
for both sludge sources. Centrate suspended solids were consistently less than
1500 mg/1 over the polymer range investigated but higher solids recoveries were
recorded while feeding secondary digester subnatant because of a higher solids
concentration in this feed source.
These data are comparable to the scroll centrifuge except that the 1500 mg/1
centrate SS criterion was met at a lower polymer dosage with the basket centri-
fuge.
At the JWPCP, basket centrifugation of the digested oxygen WAS with the
addition of 5 to 15 Ib/ton of cationic polymer yielded total cake solids from 5
to 9 percent TS with 90 to 95 percent recovery of the applied SS. The 48" bas-
ket was operated at a bowl speed of 1380 rpm and hydraulically loaded at 50 to
35 gpm. The effect of polymer dosage and hydraulic loading rate on discharge
solids and suspended solids recovery are shown in Figures 75 and 76, respective-
ly. The cake solids data shown represents a composite of the basket solids and
44
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includes approximately 2.5 cubic feet of skimmings and 13.5 cubic feet of
plowed solids. It should be noted that the basket run times approximated 8
minutes at the 35 gpm feed rate and 5 minutes for feed rates of 50 gpm. These
short run times in conjunction with the time required to decelerate and knife
out the basket contents yield an effective flow rate of approximately 25 gpm.
Further indication that the digested oxygen WAS from the JWPCP was less
difficult to dewater than the WAS from the Saugus-Newhall WRP can be found in
the basket centrifuge data. The same 48" basket centrifuge was used at the
Saugus-Newhall WRP to dewater the 23:77 digested blends. With the addition of
5 to 15 Ib/ton of cationic polymer, total cake discharge solids varied from 8 to
11 percent TS while suspended solids recoveries varied from 90 to 97 percent.
Realizing that this sludge consisted of only 23 percent waste activated sludge
and responded almost identically to the 100 percent oxygen waste activated
sludge from the JWPCP, it becomes more evident that the biological sludge gen-
erated at the JWPCP is less difficult to dewater.
Process Selection
Centrifugation seems to be the best process for dewatering digested WAS.
The filtration options required extremely high chemical dosages. The chemicals
involved are comparable in expense, but considerable materials handling prob-
lems will be encountered. Basket centrifuges require less chemical than the
scroll centrifuges, but, since they operate in a batch mode, basket centrifuges
require more operator attention. These factors will be discussed in more de-
tail in the cost analysis section of this report.
THERMAL CONDITIONING AND DEWATERING
Thermal conditioning of WAS was evaluated as an alternative to digestion
and chemical conditioning. Thermal treatment is basically a continuous pres-
sure cooking process. The excess sludge is heated under pressure so that the
proteinaceous material composing the cell walls is hydrolyzed and the bound wa-
ter is released, thereby permitting the sludge to dewater more readily. Addi-
tionally, under normal conditions of thermal conditioning, all pathogenic
organisms should be destroyed due to the high temperatures and detention times
maintained. Typically, the sludge temperature is elevated to 350°F - 400°F,
the pressure is raised to 300-400 psi and the retention time is between 20 and
40 minutes.
A 40' mobile trailer-mounted continuous flow thermal sludge conditioning
pilot plant was operated at the Sa.ugus-Newhall WRP and at the JWPCP. A sche-
matic of the process is shown in Figure 77. The feed sludge is passed through
a grinder to reduce all particles to 1/4" dimensions before pumping. Sludge
(with air when operated under the wet oxidation mode of operation) is then
pumped_to the system where it is passed through heat exchangers and brought to
the initiating reaction temperature as it enters the reactor. Oxidation takes
place in the reactor and the oxidized products leaving the reactor are cooled
in the heat exchangers against the entering cold sludge. The oxidized liquid
and remaining suspended solids are released to a decant tank for separation and
compaction while the gases are released through a pressure control valve to an
odor control system. The overflow from the thickener or decant tank may be re-
45
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turned to the head end of the treatment plant while the thickened subnatant is
pumped to subsequent dewatering units. For start-up and whenever the process
is not thermally self-sustaining, heat is added from an outside source such as
the steam generator incorporated in the pilot trailer.
Prior to dewatering, the thermally conditioned sludge flowed to a decant
tank. The decant tank installed on the mobile trailer was oversized and would
have yielded detention times of 4.4 to 6.5 hours had the unit been operated con-
tinuously. In actuality, the thermal conditioning unit was operated in a batch
manner and after a sufficient amount of sludge had accumulated in the decant
tank the thermal conditioning process was aborted and the decant tank underflow
dewatered. Because of the intermittent mode of operation, the actual solids
retention time in the decant tank approximated 24 to 48 hours prior to dewater-
ing. The manufacturer of the thermal unit agreed that the decant tank detention
time was excessive and indicated that under normal practice detention times of
11 to 24 hours are common.
Three modes of thermal conditioning were investigated over the course of
this evaluation. These were, low pressure wet oxidation (LPO), intermediate
pressure wet oxidation (IPO), and heat treatment (HT). Under the low pressure
wet oxidation mode of operation, the thickened waste activated sludge was
reacted with air while temperatures of 380°F to 400°F and a pressure of 400 psig
were maintained in the reactor. Under the intermediate pressure wet oxidation
mode of operation, the sludge was again reacted with air while a temperature of
450°F and a reactor pressure of 500 psig was maintained. Heat treatment pro-
ceeded with the same operating conditions as LPO with the exception that no air
was introduced into the reactor. Various sludge feedrates and, consequently,
different reactor detention periods or "cooking times" were also investigated.
The results obtained in the operation of the unit and the subsequent dewater-
ing of the sludge are discussed below.
Low Pressure Wet Oxidation
Under the LPO mode of operation the sludge feedrate to the unit was varied
from 3 to 6 gpm corresponding to reactor detention periods between 24 and 48
minutes. The reactor pressure was held constant at 400 psig while reactor tem-
peratures of 380°F and 400°F were investigated. Composite samples of the feed
sludge, oxidized sludge, herein referred to as "heatrate" and thickener overflow
were collected and analyzed at the treatment plant laboratories.
Summaries of the operating conditions, solids data, and COD and BOD data
from the Saugus-Newhall Studies are presented in Tables XXVII, XXVIII, and XXIX,
respectively. Eight separate runs were made while incorporating the LPO mode
of thermal conditioning and these are designated SNLPO 1 through SNLPO 8. The
first seven were made on a 100% waste activated sludge feed source while a com-
bination of 77% waste activated and 23% primary sludge was fed to the reactors
for run number 8.
Thickened waste activated sludge feed solids varied in concentration from
2.16% TS to 3.28% TS for runs 1 through 7, and volatile solids destruction ranged
from 26 to 49 percent with no apparent correlation to reactor temperature or
cooking time. The greatest reduction of solids was achieved at a cooking time
of 29 minutes with reactor temperatures of 380°F - 400°F. A volatile solids
46
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destruction of 55% was achieved on the combined feed source (77% WAS - 23%
primary) at a cooking time of 29 minutes and a reactor temperature of 390°F.
Regardless of the cooking time or pressure investigated, the low pressure
wet oxidation process effected an increase in dissolved solids, a decrease in
total and suspended solids, and an increase in soluble organics. In fact, as
seen in Table XXIX, soluble COD increased from 38 to 63 times and soluble BOD
increased from 11 to 32 times as the waste activated sludge was thermally con-
ditioned. The thermal solubilization of organics is one of the major disad-
vantages to this type of stabilization because of the increased organic load
on any treatment works when supernatant liquor is returned to the inlet works
of the treatment system. Another problem encountered with thermal conditioning
is odor production. For lack of a more descriptive name, the odors generated
from the thermal unit were classified as having the characteristics of "burnt
coffee" or "burnt plastic", and even with the catalytic combustion unit for
odor control the odors were distinctive and highly offensive.
Summaries of the operating conditions, solids data, COD data, and BOD data
collected at the JWPCP are presented in Tables XXX, XXXI, and XXXII. Six sep-
arate runs were made under LPO conditions and are designated JLPO 1 through
JLPO 6. The third low pressure oxidation run (JLPO 3) was made on thermophil-
ically digested oxygen waste activated sludge while the remaining five runs
utilized thickened oxygen WAS or a combination of thickened oxygen plus air WAS.
Solubilization of solids and organics are evidenced by an increase from
2000 mg/1 to 12,000 mg/1 in the dissolved solids concentration and an average
increase from 1400 mg/1 to 14,000 mg/1 in coluble COD concentrations as the
oxygen waste activated sludge was subjected to low pressure wet oxidation. The
degree of solubilization could not be predicted by the process operating param-
eters and no correlations exist between solubilization obtained and operating
temperatures or reactor detention times maintained. Thermal runs JLPO 1 and
JLPO 2 were made at an operating temperature of 380°F and reactor detention
times of 29 and 41 minutes, respectively. The longer retention time should
yield a higher degree of solubilization but the data shows the reverse. Con-
versely, runs JLPO 4, 5, and 6 were conducted at 400°F and detention times of
41, 29, and 48 minutes, respectively, and the most solubilization occurred at
41 minutes of reactor detention time (JLPO 4). These same inconsistencies were
observed at the Saugus-Newhall WRP and indicated that problems will exist in
predicting process results for a full scale installation. A more substantial
indication of the unpredictable performance of the thermal conditioning unit is
given by the COD oxidation values. For each of the LPO conditions the theoreti-
cal percent of COD oxidation can be predicted based on the amount of air intro-
duced into the reactor. Table XXXIII summarizes the theoretical and actual
oxidation values obtained for each of the low pressure oxidation runs. As pre-
sented, the actual COD oxidation varied from 65 percent below to 93 percent
above the theoretical value with no apparent correlation to system operating
parameters. For those runs in which the measured reductions were greater than
the theoretical values, it is conjectured that dilution of the processed sludge
with steam occurred within the system reactor. Steam is injected into the
reactor whenever the system is not thermally self-sustained and depending on
the amount added the degree of dilution will vary. For those runs with measured
reductions, lower than the theoretical estimates, it can be concluded that the
47
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oxygen transfer efficiency was low and indicative of plugging or scaling condi-
tions within the reactor. In view of the above concerns; namely, steam dilu-
tion; the remaining data presented in Tables XXXI and XXXII must be reviewed,
with caution because the exact dilution factors are not known and the data does
not reflect corrections for steam dilution.
Under normal thermal conditioning operation, complete pathogenic organism
destruction should be accomplished due to the high temperatures and detention
times obtained. The measured reductions in total and fecal coliforms, as pre-
sented in Table XXXIV, were erratic during these low pressure wet oxidation
studies. The reason(s) for the high coliform counts in the thermally treated
sludge are not thought to be related to either sampling or laboratory technique.
As suggested by the manufacturer of the thermal conditioning unit, the high coli-
form counts are thought to be the result of regrowth in the pipes connecting the
heat exchanger outlet and the decant tank. The sample tap for the heatrate
stream is located downstream of the reactor and heat exchangers, but upstream of
the decant tank, and the manufacturer contends that regrowth is occurring within
this line. The manufacturer suggested that samples of the heatrate be taken on
the reactor discharge line prior to flowing through the heat exchangers and sub-
sequently to the decant tank. Due to the high temperature and pressure of the
reactor effluent stream, a special sampling container had to be manufactured and
at the time of this writing had yet to be supplied. The problems surrounding
the coliform kill data are indeed perplexing but not uncommon to the host of
other unexplained phenomena surrounding the thermal treatment pilot studies.
Dewatering of LPO conditioned WAS was accomplished on an 8.4 ft2, filter
press and a 3' x I1 rotary drum vacuum filter. Data collected on dewatering
via the vacuum filter are presented in Figures 78 through 82. Selection of the
cloth media was made by the manufacturer of the thermal conditioning unit and
three cloths accompanied the vacuum filter. Initially, data were collected on
all three cloths but based on process performance data collected at the Saugus-
Newhall WRP, it was decided to use one cloth for the filtration studies. A
tightly woven nylon fabric served as the filtration media and the data presented
herein were all collected on the same cloth.
Vacuum filtration data on thickened waste activated sludge that had been
subjected to LPO conditioning at 380°F and three different detention periods
are presented in Figure 78. The sludge conditioned for 36.4 minutes yielded
higher filter yields than that conditioned for 26.5 minutes. These two sludge
sources were thickened to 8.3% SS and 8.5% SS, respectively, for the 36.4 and
26.5 detention periods prior to dewatering. Filter yields' increased with lower
cycle periods and the cake solids data exhibited a random effect with various
drum cycle times. Cake solids were consistently greater than 31% TS and it
should be noted that no chemicals were added for preconditioning prior to de-
watering and the captured solids exhibited good discharge characteristics. The
sludge conditioned for 29.2 minutes at 380°F yielded the highest filter yields
at all of the cycle times investigated but the sludge was thickened to 12.61%
SS prior to dewatering and it is believed that the yields increased because of
the high solids loading and not the result of a 29.2 minute detention period.
Suspended solids recovery through the vacuum filter for each of these runs ex-
ceeded 97% while the overall suspended solids removal through the thermal unit
and vacuum filter consistently exceeded 93%.
48
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The effects of varying reactor temperature from 380°F to 400°F and main-
taining a constant reactor detention period on cake solids and filter yields is
presented in Figures 80 and 81. With a reactor detention time of 36.4 minutes,
no significant changes in dewaterability were noted by increasing the reactor
temperature from 380°F to 400°F. At a reactor detention time of 29.2 minutes,
the sludge exhibited somewhat better dewaterability when the reactor was oper-
ated at 380°F, but the difference is probably attributed to the higher feed
solids concentration to the filter and not the variation in temperature.
The effects of blending primary sludge wizh thickened waste activated
sludge (23% RAW - 77% WAS) prior to low pressure wet oxidation on dewaterability
is presented in Figure 82. Only one run was made on thermal conditioning of a
combined sludge. The reactor was operated at a temperature of 380°F, a pressure
of 400 psig and the sludge was retained in the reactor for 29.2 minutes. Fol-
lowing LPO conditioning and decant thickening, the sludge was fed to the vacuum
filter. As seen in Figure 82, cake solids increased slightly and filter yields
decreased for the combined sludge as compared to thermally conditioned waste
activated sludge.
At the JWPCP, vacuum filtration of LPO conditioned oxygen WAS yielded cake
solids varying from 31 to 34 percent TS and filter yields of 3.0 to 6.6 Ib/hr-
ft2. Drum cycle times were varied from 2 to 8-1/2 minutes while applied vacuums
varied from 15 to 20 inches Hg. The resultant suspended solids removal efficien-
cies were erratic and varied from 61 to 95 percent and, as seen in Tables XXIII
A and B, the discharge characteristics were consistently good, indicative of
self-discharging and minimal media cleaning required. The vacuum filter was not
available for runs JLPO 1, JLPO 2, and JLPO 3.
Data collected on filter pressing of LPO conditioned Saugus-Newhall WAS was
disappointing with regard to filter yields. Two nylon cloths (recommended and
supplied by the manufacturer) were employed as filtration media and pressures
varying from 102 to 120 psi were applied to the press. Filter yields averaged
0.50 lb/hr-ft2 while cake solids varied from 36 to 56% TS. It should be noted
that the particular filter press used during these studies could sustain a maxi-
mum pressure of only 120 psi while other manufacturers can supply presses con-
structed to withstand pressures in excess of 220 psi. This latter type of press
has been used in previous Districts' programs but was not available during the
time the thermal conditioning unit was on site. A summary of the data collect-
ed on pressure filtration of LPO conditioned Saugus-Newhall WRP is presented in
Table XXXVI.
Data collected on the pressure filter for each of the low pressure oxida-
tion runs at the JWPCP are summarized in Tables XXXVII A and B. For each of
these runs the filter was operated at a pressure of 100 psig and utilized a
cloth previously used in conjunction with the Saugus-Newhall WRP thermal stud-
ies.
JLPO 1 and JLPO 2 sludges were conditioned at 380°F for respective periods
of 29 and 41 minutes prior to dewatering. The cake solids and filter yields for
the JLPO 2 were higher than those obtained for a 29-minute thermal conditioning
period, indicating a slight advantage to increased conditioning time. The con-
sistency of the filter cakes for these two sets of runs were listed as good,
indicating the cakes were consistently firm throughout. The discharge charac-
49
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teristics were poor, indicating that the cakes did not fall by gravity when the
press was opened, and considerable cleaning of the media was required.
Pressure filtration of LPO conditioned digested oxygen WAS (JLPO 3) pro-
duced cake solids in the range of 29 to 32 percent TS that were, for the most
part, unconveyable. The consistency of the discharge solids were poor, indi-
cating that an "egg shell" effect was observed. The portion of solids nearest
the filter plates were firm but when these plates were disengaged the cake would
split along the center!ine and discharge a liquid center. The thermal condition-
ing operating parameters were identical for runs LPO 1 and LPO 3 and the result-
ant filter press data collected on these two sets of thermal runs is indicative
of the adverse effects that digestion has on dewaterability of thermally condi-
tioned WAS.
The dewatering advantages afforded by LPO conditioning at 400°F as opposed
to a reactor temperature of 380°F are evident by the pressure filtration data
shown in Table XXXVII, B. Thermal runs LPO 4 and 5 were conducted at a tempera-
ture of 400°F and the resultant filter press cakes were consistently in excess
of 47 percent TS while the filter yields (lb/hr-ft2) were higher than those con-
ducted on JLPO 1 and 2 sludges. For each of these experiments the cake consis-
tency was good while the discharge characteristics were poor. The exception was
for the run which employed an application of approximately 10 Ib/ton of diato-
maceous precoat prior to the filtration sequence. For this riri, the cake dis-
charge was fair, indicating it did not discharge by gravity; but it left the
filter media clean, requiring minimal cleaning.
Thermal run JLPO 6 employed a temperature of 400°F and a reactor detention
time of 48 minutes. This was the longest thermal conditioning time employed
and, as seen in Table XXXVII B, a slight increase in dewaterability was observed.
Cake solids approximated 50 percent TS with good consistency and even without
precoating had fair discharge characteristics.
Heat Treatment
Under the heat treatment mode of operation, reactor temperatures of 380°F
and 400°F were'maintained while the reactor operating pressure was held constant
at 400 psig. The sludge feed rate was varied from 3.5 to 6.2 gpm, corresponding
to detention times of 41 to 24 minutes. Tables XXXVIII and XXXIX summarize the
operating conditions maintained and the sludge source conditioned for each of
the eight heat treatment runs.
The solids, BOD, and COD data from the Saugus-Newhall studies are presented
in Tables XL and XLI. For each of the three runs, thickened waste activated
sludge served as the feed source with feed solids concentrations ranging from
2.36 to 2.91% TS. Volatile solids reduction varied from 24 to 41%, again with
no apparent correlation to reactor temperature or detention time. As was the
case with LPO conditioning, heat treatment affected an increase in dissolved
solids and soluble organics and a decrease in total and suspended solids. As
seen in Table XLI, thermal solubilization of organics is evident by an increase
in soluble COD of 17 to 65 times and an increase in soluble BOD of 8 times.
During these heat treatment studies, offensive "burnt coffee" odors were again
present and easily detectable.
50
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Thermal run JHT 1 was made on digested oxygen WAS while the remaining heat
treatment runs at the JWPCP utilized undigested oxygen or oxygen plus air WAS.
Summaries of the solids data and COD and BOD data are presented in Tables XLII
and XLIII, respectively. Solubilization of solids and organics are evidenced
by an increase from 200 mg/1 to 12,500 mg/1 in dissolved solids and from 1600
mg/1 to 15,300 mg/1 in the soluble COD concentrations as the oxygen waste acti-
vated sludge was heat treated.
It is interesting to note that under heat treatment conditions, little, if
any, reduction in total COD should occur because of the omission of air addition
into the thermal reactor. As seen in Table XLIII, measured reductions in total
COD varied from 9 to 35 percent, indicating that steam dilution is occurring but
the exact amount is not known. The problems of coliform reduction encountered
under LPO conditions were again manifested during the heat treatment studies,
and as shown in Table XLIV, the measured reductions were extremely erratic.
The underflow sludge from the thickener was dewatered via the vacuum filter
and filter press. The data collected on vacuum filtration of the Saugus-Newhall
sludge are presented in Table XLV. A comparison of the data collected on fil-
tration of heat treated sludge at 380°F and a detention time of 29.2 minutes
(SNHT1) with that collected under LPO conditions at the same temperature and
detention time (Figure 78) indicates that LPO conditioning slightly enhances
sludge dewaterability. Under LPO conditions cake solids approximated 32% TS and
a filter yield of 3.5 lb/hr-ft2 was obtained. For heat treatment conditioning,
the cake solids approximated 30% TS while the filter yield decreased to 0.72
lb/hr-ft2. Suspended solids removal through the filter approximated 97% while
the net removal through the thermal conditioning unit and filter approximated
Vacuum filtration of the heat treated oxygen WAS, however, was very compar-
able to the results obtained under LPO conditioning. As seen in Table XLVI,
cake solids varied from 33 to 37 percent TS while filter yields were measured
at 2.3 to 5.9 lb/hr-ft2. Suspended solids recoveries were consistently in ex-
cess of 91 percent. In this regard it should be noted that prior to the heat
treatment studies the vacuum filter manufacturer supplied a new filter cloth.
This new cloth was identical to the one previously used in conjunction with the
LPO studies but employed a rubber gasket to ensure a good seal between the fil-
ter drum and filtration media. During the LPO experiments at the JWPCP an ade-
quate seal may not have developed; hence, solids may have passed through the
outer edges of the drum instead of through the cloth. The suspended solids data
presented on vacuum filtration of LPO conditioned sludge (Table XXXV) may there-
fore be erroneous, but the cake solids and yield data are thought to be reliable.
Pressure filtration was conducted on heat treated Saugus-Newhall WAS condi-
tioned at 400°F, 400 psig for 29.2 minutes. These results are presented in
Table XLVII. Cake solids approximated 45% TS but, as experienced previously
with LPO conditioned Saugus-Newhall sludge, the filter yield was extremely low
at a value of 0.27 .lb/hr-ft2. When comparing this data with that collected un-
der the same operating conditions and the LPO mode of treatment (Table XXXVI),
it is again evident that LPO conditioning yields a sludge slightly easier to de-
water for the Saugus-Newhall WRP sludge.
51
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Further indication that digestion adversely affects the dewaterability of
sludge was encountered at the JWPCP. Thermal runs JHT 1 and JHT 2,..Table
XLVIIIA, were made under identical reactor operating conditions but the di-
gested sludge JHT* 1 did not dewater as readily as the undigested waste sludge
JHT 2. The digested sludge dewatered to 29 to 30 percent TS and exhibited the
same "egg shell" effect previously discussed. The undigested sludge was de-
watered to approximately 40 percent TS and was consistent throughout.
With regard to the effect of temperature, sludge heat treated at 380°F
JHT 2 and JHT 3 yielded filter press cakes of 34 to 45 percent TS while that
heat treated at 400°F JHT 4 and JHT 5 produced filter cakes of 46 to 51 percent
TS and were competitive to the results obtained on pressure filtration of LPO
conditioned sludge. The cake consistency and discharge characteristics obtained
by pressure filtration are included in Table XLVIII A, B.
The only centrifuge data collected on dewatering of thermally conditioned
WAS was a series of tests made on heat treated WAS at the Saugus-Newhall WRP.
The thermal unit was operated at 400°F, 400 psig, and a reactor detention time
of 29.2 minutes. The heatrate, following a short detention time in the decant
thickener was fed to the 20" x 62" scroll centrifuge at a rate of 30 gpm with
a suspended solids concentration of 1.85%. The centrifuge was operated at its
maximum pool depth setting, a bowl speed of 2070 rpm and a relative scroll speed
of 19 rpm. Cationic polymer was added in the range of 0 to 11.5 Ib/ton for con-
ditioning and the results are shown in Figures 83 and 84. Regardless of poly-
mer dosage, the discharged cakes averaged between 20 and 21% TS, but a minimum
of 7.5 Ib/ton of polymer was required to obtain a centrate containing less than
1500 mg/1 of SS. Of extreme importance is the fact that even though the cakes
were in excess of 20% TS they were not conveyable and contained large quantities
of free moisture. In fact, from a visual standpoint, the .discharge cakes were
more fluid than the 6% - 7% TS cakes achieved when thickening waste activated
sludge.
No dewatering data were collected on run SNHT 3 because problems developed
with the sludge transfer pump, and the decant tank contents had to be drained.
Intermediate Pressure Wet Oxidation
Under the intermediate pressure wet oxidation a feed source of thickened
WAS from the Saugus-Newhall WRP was fed to the unit at a rate of 1.5 gpm. The
feed to the unit had to be drastically reduced because of the limited capacity
of the air compressor which supplied the oxygen necessary for oxidation. To
maintain a feed of 5-6 gpm under IPO, the amount of air required for oxidation
was approximately 60 cfm which was beyond the capacity of the compressor in-
stalled on the mobile facility. Consequently, the reactor detention period of
cooking time approximated 97 minutes. The reactor temperature and pressure were
held constant at 450°F and 500 psig,"respectively.
As shown in Table XLIX, volatile solids destruction equalled 41% with an
increase in dissolved solids and a decrease in suspended and total solids as
the sludge was subjected to intermediate wet oxidation. Thermal solubilization
of organics is again evident by an increase in soluble BOD and soluble COD by
a factor of ten. Again, odors were present and easily detectable during this
phase of the thermal treatment studies.
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At the JWPCP, the IPO process was fed at a sludge flow of 3.0 gpm and an
air flow of 17.5 cfm. Table L summarizes the operating parameters maintained
and the performance data collected. Total COD reduction was measured at 21 per-
cent while the theoretical reduction was calculated to be 37 percent, based on
the pounds of oxygen added per pound of COD introduced into the reactor. Solu-
bilization of organics and solids was again evidenced by an increase in the sol-
uble COD concentration from 1600 mg/1 to 11,000 mg/1, and an increase in dis-
solved solids from 2000 mg/1 to 10,000 mg/1. Compared to LPO and heat treatment
conditioning, the degree of solubilization for this intermediate pressure wet
oxidation run was lower and the fact that the measured COD reduction was only 21
percent adds support to the statement made earlier concerning the unpredictable
performance of this thermal conditioning unit.
Dewatering of conditioned sludge via vacuum and pressure filtration followed
IPO conditioning at the Saugus-Newhall WRP. The data collected are shown in
Tables LI and LII. Comparing the results obtained on pressure filtration with
those obtained on pressure filtration of LPO and heat treated waste activated
sludge, it becomes evident the IPO conditioning enhances the dewaterability of
the sludge. Filter press yields increased from 0.50 to 1.05 lb/hr-ft2 for IPO
conditioning while cake remained relatively unchanged at approximately 43% TS.
The cake solids obtained under vacuum filtration of the IPO conditioned sludge
approximated those obtained for the LPO and heat treated sludges with solids in
the range of 27 - 29% TS, but the filter yields increased significantly after
IPO conditioning. Filter yields from 12 to 14 lb/hr-ft2 were obtainable for LPO
and heat treatment conditioning.
Vacuum filtration was the only dewatering method employed at the JWPCP be-
cause the IPO mode of thermal conditioning was performed during a period when
not enough sludge could be processed to run both the vacuum and pressure filters.
The data which were collected are included in Table LIU. The vacuum filter was
operated at cycle times of 2 to 7 minutes with applied vacuums of 12 to 17 inches
Hg. The lower vacuums maintained for these series of tests can only be attri-
buted to physical characteristics of the conditioned sludge. With the vacuum
pump operating at full capacity, a vacuum of 20-23 inches Hg is usually achieved,
but depending on how well the sludge is picked up by the drum and the porosity
of the sludge mat, the vacuum will vary. Resultant cake solids approximated 30
percent while filter yields varied from 2.9 to 4.8 lb/hr-ft2. These lower cake
solids, when compared to the discharges obtained on LPO and heat treated sludges,
are probably attributed to the lower concentration of solids in the feed sludge
which is the direct result of conditioning a relatively small quantity of sludge
during this experiment. The small volume of sludge applied to the decant tank
did not allow for a sufficient sludge blanket to develop and, .consequently, maxi-
mum compaction of the underlying solids was not achieved.
Treatment of the Thermal Liquor
The previous discussion was concerned with the dewaterability of the heat
treated sludge after it had been concentrated in a gravity thickener. While
this thickened sludge exhibited excellent dewatering characteristics, consider-
ation must at the same time be given to the heat treatment liquor overflowing
•from the thickener. This liquor has inordinately high concentrations of COD and
BOD. The COD approximated 15,000 mg/1 with 95 percent being soluble. The BOD
approximated 5300 with 95 percent being soluble. The rather small ratio of BOD
53
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to COD is both an indication of the exotic nature of the waste and its low rate
of biodegradation, and also the lack of an acclimated seed in the BOD test.
The degradability of a thermal liquor with activated sludge has been reported
in the literature9'10 along with the numerous problems9 that have been asso-
ciated with its direct recycle to the aeration system. These problems are not
unexpected, as the difficulties associated with the recycle of the effluent
from sludge processing schemes that are heavily laden with suspended solids
fines and/or high organic concentrations have continually plagued the success-
ful operation of biological wastewater treatment plants. Certainly some of
these problems can be traced to the lack of consideration of the recycle effects
on the treatment plant design. However, such accountability in the case of
thermal treatment is weakened by the reported problems in predicting the organic
solubilization that will be obtained in a full-scale system. Even if one as-
sumes that good scale up can be obtained, it would seem that aerobic treatment
of thermal liquor, whether sidestream or recycle to the main plant, would not
be the preferred mode. Not only is power spent to transfer oxygen for waste de-
gradation, but an additional biological sludge is generated. Anaerobic treat-
ment would seem preferable over aerobic treatment as not only can energy be re-
covered in the form of methane gas but the sludge production is minimized.
Operating Experiences with Thermal Conditioning Systems
The Districts' staff experience with thermal conditioning experiments at
the Saugus-Newhall WRP and the JWPCP indicates numerous operational problems
associated with this process. The particular problems encountered during the
Waste Activated Sludge Processing Studies on the pilot scale thermal unit in-
cluded scale buildup on the walls of the heat exchanger surfaces, odor genera-
tion, and various operational failures.
Scaling of the heat exchanger surfaces necessitated an acid flushing after
119 hours of intermittent operation at the Saugus-Newhall WRP. Operation of
the same unit at the JWPCP required an acid flushing for scale removal after
101 hours of intermittent operation. The time required to flush the system
approximated 8 hours indicating that for the JWPCP at least 5 percent addition-
al capacity must be provided for a- full-scale installation to allow for acid
flushing downtime.
As mentioned above, a "burnt plastic" or "burnt coffee" odor persisted in
and around the pilot plant during these thermal conditioning studies. Odor
generation during the Saugus-Newhall WRP studies was more intense than that en-
countered during the most recent JWPCP studies because the prior studies did not
incorporate the use of odor control equipment. During the previous studies the
manufacturer indicated that odor control is not a problem provided adequate odor
removal devices are installed to treat the vent gases. When the thermal condi-
tioning unit was operated at the JWPCP it was furnished with odor control equip-
ment which significantly reduced process odors, but it was our finding that the
characteristic odors were also incorporated in the processed sludge and liquid
side-streams. In fact, upon centrifugation of thermally conditioned sludge at
the Saugus-Newhall WRP, intense odors were detectable. The turbulence and mix-
ing action in the centrifuge causes a release of the odorous gases, and at
times these odors were extremely noxious.
54
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The specific odor control devices utilized during the JWPCP studies in-
cluded a wet scrubber operated in series with an activated carbon adsorber. The
process flow schematic is present in Figure 85. The decant tank vent gases are
first processed through a water scrubber which utilizes 20 to 30 gallons of
water per 100 gallons of sludge processed. The gases exiting the scrubber are
then processed through an activated carbon adsorber which required regeneration
after approximately every 12 hours of operation corresponding to a total pro-
cessed sludge flow of about 4000 gallons.
The liquid waste streams generated by the two-stage odor system were rou-
tinely collected and analyzed for total and soluble COD. The average total and
soluble COD concentrations of the scrubber waste stream were respectively mea-
sured, at 35 and 32 mg/1 and represent an insignificant COD load if this stream
were to be recycled to the plant head works. The carbon regeneration system pro-
duced approximately 5 gallons of condensate per regeneration with an average
soluble COD concentration of 600 mg/1. This regeneration waste stream consti-
tutes a recycle soluble COD load, of less than one pound per ton of solids pro-
cessed and may be regarded as insignificant.
Samples of the decant tank vent gases and the scrubber and carbon adsorber
effluent gas streams were also collected on two separate occasions and quantita-
tively analyzed for odor intensity. The first set of samples were collected,
while the thermal conditioning unit was operated under the heat treatment mode
of operation of digested oxygen sludge (JHT 1). The decant tank vent gases had
a very strong odor with an average of 17,400 OU/SCF while the exiting gas
streams from the scrubber and adsorber had moderate and slight odors, respective-
ly. The average number of odor units measured in the scrubber and adsorber gas-
es were 37 OU/SCF and 15 OU/SCF. Realizing the large volume of water used in
the scrubber (20 to 30 percent of the total processed sludge flow), it is not
known at this time if a properly sized scrubber will reduce the odors as effec-
tively as the one evaluated. Additionally, for comparison purposes it should
be noted that the average emission recorded at the head works of the JWPCP ap-
proximates 200 OU/SCF. :
The second set of gas samples were^collected during intermediate pressure
wet oxidation (JIPO) of thickened oxygen waste activated sludge. The decant
tank vent gases were measured at 29,000 OU/SCF and were characterized as very
strong. The scrubber and adsorber effluent gases were characterized as having
"strong" and "very strong odors", while the respective odors were measured at
10,600 and 3,200 OU/SCF. Keeping in mind that the head works to the JWPCP have
measured odors of 200 OU/SCF, it becomes apparent that the oxidation process
may pose serious odor problems.
Other operational difficulties encountered during the JWPCP studies in-
cluded: (1) pressure control valve problems after 66 hours of total processing
time, (2) compressor failure after 140 hours of sludge processing time, (3)
boiler failure and difficulties in maintaining the desired reactor temperature
after 200 hours of operation and, (4) corrosion of air lines. In all fairness
to the manufacturers, it must be noted that the pilot unit was used at other
installations prior to being set up at the JWPCP and the actual processing times
incurred prior to these failures may have been substantially longer than those
reported. However, the unit was refurbished by the manufacturer prior to its
installation at the JWPCP.
55
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There is no reason to suspect these problems were unique to the particular
system evaluated, or to the facility at which the tests were conducted. A re-
view12*13 of various thermal treatment installations indicates similar difficul-
ties. At a number of these installations there were excessive operating costs
due to corrosion and scaling conditions.
Corrosion of metals is related to the aggressive properties of water which,
in turn, are identified with the solubility relations of calcium carbonate.
Calcium carbonate is only slightly soluble in water, but in the presence of car-
bon dioxide it becomes much more soluble through the formation of bicarbonate.
Under given pH conditions, there is an equilibrium among calcium salts, carbon-
ate and bicarbonate radicals, and free carbon dioxide. If the water is over-
saturated with calcium, it carries an excess of calcium carbonate which tends to
form a protective coating on wetted metallic surfaces. On the other hand, if
carbon dioxide is in excess, the tendency is in the reverse, allowing potential
solution of the metal.
In general, if a water or sludge has a low pH, it would be expected that
some corrosion would occur. Thermal oxidation usually results in low pH condi-
tions due to production of carbon dioxide and organic acids. However, other
factors are also important in determining the rate of corrosion. Generally, the
corrosion rate will be proportional to the conductivity, which is a measure of
the total dissolved minerals. Also, certain anions exert considerable influence
on the rate of corrosion; and chloride, in particular, is a strong corrosion
catalyst. Sulfate is less corrosive, while bicarbonate tends to reduce corro-
siveness of the other two by an inhibitory action. Corrosion is basically a
materials problem and should be solvable, or at least controllable, although the
cost may be high. In some cases, high corrosion rates have necessitated use of
titanium heat exchanger elements.
A notable installation of the wet oxidation process was the Chicago Sanitary
District installation which was operated from 1962 to 1972 but has now been re-
placed by a land disposal system. Operational and maintenance problems were en-
countered with the process at the Chicago installation, including odor produc-
tion, scaling in the heat exchangers, and problems of maintaining high pressure
equipment.
A summary15 dealing with the problems associated with thermal treatment fol-
lows: "Heat treatment processes have been used for several years in Europe and
the United States. It is only within the past several years that significant
United States operating and cost data on heat treatment processes have become
available. Results from the United Kingdom are now in technical journals. Dif-
ficulties with plants in the United Kingdom are generally attributed to the prob-
lems of maintaining such items of equipment as high pressure pumps, compressors,
and high temperature and pressure reaction systems. Plant difficulties in the
United Kingdom were in some cases attributed to the installation of systems at
older plants. However, some plants that have ceased operation were specifically
designed with new liquid treatment facilities which could accommodate the heat
treatment system recirculation loads. The principal cause of process cost and
effluent quality problems appears to be a much higher degree of sludge solubili-
zation with heat treatment than was predicted. Available information indicates
high costs of operation, maintenance, and effluent quality problems are associ-
56
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ated with heat treatment systems. Several United States plants have ceased
operating heat treatment systems due to those problems, including Golden
Coors, Colorado; Santee, California; and Chattanooga, Tennessee."
Certainly, it must be recognized that research and development of thermal
conditioning systems is ongoing, and the operational problems experienced to
date may be resolved in the future. However, the alleged problems associated
with thermal treatment in the past are authentic and do pose serious operational
and maintenance concerns.
WASTE ACTIVATED SLUDGE DRYING
The decision to dry sewage sludges beyond the level attained by normal de-
watering methods usually includes the assumption that the dried product will be
marketed as a soil conditioner. However, lack of a sufficient market should not
necessarily preclude the drying process. The possibility exists, especially for
difficult to dewater sludges, that drying might be a viable and required step to
render sludges suitable for landfill disposal. Drying is a dehydration process
and is accomplished by a variety of methods including direct and indirect heat
drying, composting, solvent extraction dehydration, and oil-emersion dehydra-
tion. The two methods employed for the JWPCP sludge studies were windrow com-
posting and indirect rotary kiln heat-drying. The operating and process param-
eters encountered with these two drying processes are discussed below.
Composting
The composting process attempts to create a suitable environment for thermo-
philic facultative aerobic microorganisms, and in so doing, several criteria
must be met to insure successful composting. First, organic solids should be
well-mixed. Composting of sewage sludge alone requires that the sludge be
blended with previously composted material or bulking agents such as sawdust,
straw, wood shavings, or wood chips. This blending process should produce a
homogenous porous solids structure in the composting material to enhance aera-
tion. Second, aeration must be sufficient to maintain aerobic conditions in the
material. Third, proper moisture content must be maintained. Microorganisms
require moisture to function and a moisture content between 45 and 65 percent
is generally considered desirable in the composting mixture.
In order to obtain a composting mixture with an initial moisture content of
45 to 65 percent, sludge to be composted must be dewatered prior to blending with
compost material. At the time of this study, the practice at the JWPCP was to
blend dewatered digested primary sludge at a moisture content of approximately
70 percent with compost material at a moisture content of approximately 30 per-
cent to bring the initial moisture content of the mixture to 60-65 percent.
Figure 86 shows temperature, moisture content, and volatility against time from
initial blending of dewatered digested primary sludge and previously composted
material. . Following 10 to 15 days of composting, the mixture obtained a mois-
ture content of approximately 30 percent while the volatility leveled off at 40
percent.
Figure 87 shows the same parameters presented in Figure 86, but reflects
compost performance on dewatered digested waste activated sludge. The waste
57
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activated sludge was generated at. the. Saugus-Newhall WRP and was dewatered to
a moisture content of 88 percent following anaerobic digestion. To obtain an
initial compost moisture content of 65 percent, the dewatered waste activated
sludge was blended with previously composted material at a moisture content
of 49 percent. After 15 days of composting, the moisture content of the mix-
ture approximated 40 percent while the volatility leveled off at 35 percent.
Monitoring of the compost performance continued for 35 days beyond the initial
mixing and at the termination of the study the mixture had a moisture content of
20 percent.
Additional composting studies included blending dewatered digested primary
and dewatered digested waste activated sludge prior to blending with compost
material. The dewatered digested primary and waste activated sludges were mixed
at a ratio of 50/50 (solids bases) to bring the combined moisture content to 83
percent. This mixture was then blended with compost material at a moisture con-
tent of 49 percent to yield an initial moisture of 65 percent in the compost
pile. After fifteen days of operation, the recorded moisture content was 20
percent with a volatility of 30 percent.
It should be noted that all of these experiments reflect summertime opera-
tion, and it is expected that the composting time during inclement conditions
will approximately double those reported. Nonetheless, it has been shown that
waste activated sludge can successfully be composted provided it is thoroughly
mixed with enough compost material to yield an initial homogenous moisture of
65 percent or lower.
Mechanical Drying
The class of dryers designated as indirect dryers employ indirect contact
of sludge with preheated gases. Common types include rotary dryers and rotary
vacuum dryers, and these units can be operated as either batch or continuous
type processes. In the batch mode, the dryer is charged with material to be
dried and then sealed. 'A vacuum (approximately 26" Hg) is then applied to the
internally charged compartment. Recirculating steam (50-100 psig) from a boil
is passed through a jacketed hollow in the outer shell wall and, in some cases,
through the internal central portion of an agitator assembly. The agitator
assembly rotates at about 4 rpm and consists of spiral blades which turn the
charged material, thereby providing frequent contact of all wetted particles
with the heated surfaces. Vapor is removed by vacuum pumps and passed through
a condenser prior to discharge.
In the continuous mode no vacuum is applied and the drying process takes
place at atmospheric pressure. Material is introduced continuously at one end
of the dryer and is discharged continuously at the opposite end. All other
process functions are the same except for exhaust gas temperatures, which are
much higher in the continuous flow process. This is due to the differences in
vaporization temperature requirements at atmospheric pressure versus a vacuum of
6 inches Hg.
A pilot scale 18" x 36" rotary drum vacuum dryer was utilized at the
OWPCP16 to dry four different sludge types. As shown in Table LIV, feed mater-
ial included digested primary sludge, digested and undigested oxygen waste
sludge from the JWPCP, and digested air waste activated sludge from the Saugus-
58-
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Newhall WRP. In each case, the sludges were dewatered via scroll centrifuga-
tion prior to drying and the initial solids content applied to the dryer is
included in Table LIV.
For each test run, the dryer was operated in a batch manner at a jacket
temperature of 297°F, an applied vacuum of 28" Hg and allowed to run for approx-
imately four hours. Included in Table LIV are the measured heat requirements
expressed in BTU's per pound of water vaporized and the surface heat transfer
coefficient (h) for each run.
Depending on the initial moisture -content, the initial heat transfer co-
efficient varies and increases with increasing moisture content. As the sludge
dries, the transfer coefficient decreases; effecting an increase in the required
drying time and an apparent increase in the' unit BTU requirement. Additionally,
depending on the sludge characteristics, "balling" or the agglomeration of
sludge particles into 2" to 4" balls may occur within the dryer, preventing max-
imum sludge-dryer contact. The "balling" phenomenon was observed for each of
the digested waste activated sludges and causes a decrease in the heat transfer
coefficient, an increase in the required drying time and an apparent increase
in the unit BTU requirement.
Final moisture content following four hours of drying approximated 3 per-
cent for the digested primary sludge and 60 to 65 on the various biological
sludges. It should additionally be noted that during the course of these exper-
iments strong odors were commonly encountered and were characteristic of the
odors generated during thermal conditioning studies.
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SECTION 5
SYSTEMS EVALUATION
Basically, three waste activated sludge handling schemes have been estab-
lished as viable alternatives. These are:
1. Flotation thickening, followed by anaerobic digestion, mechanical
dewatering, and disposal.
2. Scroll centrifuge thickening, followed by anaerobic digestion,
mechanical dewatering, and disposal.
3. Flotation thickening, followed by thermal conditioning, mechanical
dewatering, and disposal.
Because of the options available for mechanical dewatering and.sludge dis-
posal, the actual number of schemes set up for analysis was twenty. These are
presented in Figures 89 through 95.
PROCESS CONSIDERATIONS
Selection of the twenty possible alternatives for subsequent cost effective-
ness were based on pilot and full scale data collected by the LACSD and engineer-
ing judgment concerning the workability of certain processes. Basket centrifu-
gation was not considered to be a workable thickening process because of the
logistics involved when the composite cake is thickened to concentrations of
approximately 6'percent TS. At this concentration, approximately half of the
basket contents can be skimmed out while the remaining solids have to be plowed
out. The skimmed and plowed solids would then have to be blended and mixed prior
to pumping to a digester. These procedures coupled with the short run times (13
minutes) encountered with basket centrifuge thickening would present difficult
control problems.
Scroll centrifuge thickening was not considered prior to thermal treatment
because in our judgment the resultant cake solids of 6 percent TS would be too
viscous for optimum thermal efficiency in the heat exchangers and reactor. Such
problems have been encountered at Fort Lauderdale, Florida17 where a heat treat-
ment system was incorporated to condition a disc centrifuge thickened waste
activated sludge. At this installation, the centrifuge solids approximated 6
percent TS, and the desired reactor temperatures could not be maintained because
of the poor thermal transfer characteristics of the thickened waste activated
sludge. Doubling of the heat exchanger capacity was proposed to alleviate the
problem, but it remains an untried solution.
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A most recent installation of a thermal conditioning unit at Louisville,
Kentucky18 incorporates the use of dissolved air flotation for thickening
waste activated sludge prior to treatment. The fact that centrifuges were not
employed for thickening adds support to our judgment that concerns exist re-
garding the mixing and thermal transfer characteristics of the thickened
sludge.
Vacuum filtration of digested oxygen waste activated was not considered
because of the unsuccessful operation and performance encountered during pilot
scale investigations. Pressure filtration, and basket and scroll centrifuga-
tion were successful in pilot and/or full scale investigations and were con-
sidered workable alternatives for dewatering digested oxygen waste activated
sludge.
The disposal methods analyzed included direct landfill ing of the dewatered
sludge, composting followed by landfilling, and composting followed by dispos-
al to a fertilizer manufacturer. Although the treatment train economics involv-
ing pressure filtration, followed by composting and landfilling or delivery to
a fertilizer manufacturer were analyzed, a number of uncertainties surround
this scheme. Successful pressure filtration requires approximately 800 Ib/ton
of lime, and it is not known if the dewatered solids are amendable to compost-
ing because of the high pH (11.5) obtained. Even if the pressure filter solids
can successfully be composted, it is not known if the compost product will be
acceptable to a fertilizer manufacturer because of the large amounts of lime
included in the product. Utilization of digested centrifuged and composted
waste activated sludge by a fertilizer manufacturer was assumed to be a viable
ultimate disposal alternative. An analysis of the heavy metals concentration
of such a product has indicated that the constituents are equivalent to, if not
somewhat lower in concentration than, that presently encountered in the exist-
ing primary digested sludge. However, it has been found that the composted
waste activated sludge lacks the fiber or bulk characteristics of the composted
primary sludge and basically has much finer particle sizes. The finer particle
sizes of the waste activated sludge pose a question as to its use as a ferti-
lizer base or soil amendment. Realizing this possible limitation, it was still
assumed that the product could be used by a fertilizer manufacturer.
Vacuum and pressure filtration of thermally conditioned sludge were con-
sidered for dewatering. Centrifugation was not considered. Thermally condi-
tioned waste activated sludge was dewatered via a 20" x 62" scroll centrifuge,
and although the discharge solids approximated 22 percent TS, the nature of
these solids were such that they were unconveyable and would pose serious han-
dling problems. If the thermally-conditioned sludge can be centrifuged to yield
conveyable discharge solids, the odor problems encountered at the Saugus-Newhall
WRP would pose seritius environmental concerns. Upon centrifugation of thermally
conditioned waste activated sludge, the characteristic thermal odors were inten-
sified by virtue of turbulence in the centrifuge, and at times were extremely
noxious. The only disposal method considered was sanitary landfilling of the
dewatered thermally treated sludge. Neither composting nor delivery to a ferti-
lizer manufacturer were considered viable alternatives at this time. The com-
posting of thermally conditioned dewatered sludge has not been attempted, but
it is probable that odors can emanate from the processed solids when it is
stirred or turned in a fashion simulating the action of a mechanical composter.
These odors may pose serious environmental concerns.
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At the writing of this report significant work had not been conducted re-
garding the suitability of the thermally treated sludge as a product for use by
a fertilizer manufacturer. It has been reported that a thermal sludge is ster-
ile, but concerns are expressed regarding the apparently very fine particle
sizes of the sludge, its odor characteristics, and its unknown heavy metal con-
centrations. Thus, for the purposes of this economic analysis it was assumed
that the sludge would not be used by a fertilizer manufacturer.
Treatment of the liquid side-streams inherent to thermal conditioning
systems will have to be incorporated in the total processing scheme. If these
side-streams were to be recycled to the aeration system, an approximate 12 per-
cent increase in the organic load would occur, necessitating a 12 percent in-
crease in the capacity of the cryogenic unit and the oxygen diffusion equipment*
The current secondary system under construction will not allow for this increase
in capacity and, as such, it was assumed that anaerobic treatment of the liquid
streams would be incorporated.
ECONOMIC ANALYSIS
The data used in preparing these waste activated sludge handling costs
were derived from several sources. Equipment manufacturers provided estimates
of their respective equipment, and, where possible, costs generated by the LACSD
were incorporated in this analysis. The purchase prices and construction costs
were all standardized to a consumer price index of 170 and an ENR index of 2400,
respectively, and all costs reflect the expenditures necessary for a secondary
treatment capacity of 200 MGD. The expected quantity of oxygen waste sludge
from this system approximates 106 dry tons per day at a total solids concentra-
tion of 1.5 percent.
For simplicity, cost components common to all alternatives such as trans-
fer pumps, conveyors, chemical pumps, etc., are not reflected in these estimates,
and as such these are relative rather than actual costs. Power costs were based
on $0.03 per KWH while operation and maintenance labor were based on $1,050 per
man-month.
Chemical costs were based on the following unit prices: (1) polymer at
$4,000 per dry ton, (2) lime as CaO at $40 per dry ton, (3) ferric chloride at
$200 per dry ton, and (4) diatomaceous earth at $100 per dry tori. Amortization
of capital expenditures were based on an interest rate of 7 percent for a peri-
od of 15 years for all equipment with the exception of anaerobic treatment facil-
ities and composting and hauling equipment. Anaerobic digesters and filters
were amortized over a period of 25 years while composting and sludge hauling
equipment were amortized for 5 years.
The daily chemical requirements (dry tons) for each of the twenty schemes
are shown in Table LV. The final solids content and daily wet tons of disposal
solids, along with the combined suspended solids concentrations of the process
side-streams, are also presented in Table LV.
\
If composting is feasible after anaerobic digestion and mechanical dewater-
ing, the least amount of solids (wet tons/day) for final disposal would be gen-
erated by centrifuging of the digested sludge. The large quantities of lime and
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diatomaceous earth needed for successful pressure'filtration would increase the
daily volume of sludge for disposal from 110 wet tons/day (for centrifugation)
to 200 wet tons/day. If composting is not possible, then thermal treatment,
followed by pressure filtration, will yield 230 wet tons/day of sludge for dis-
posal. Digestion followed by pressure filtration will require the daily dis-
posal of 390 wet tons, while scroll centrifugation of the digested oxygen waste
activated sludge will produce 520 wet tons/day for disposal.
A complete summary of the relative costs for each of the twenty schemes is
presented in Table LVI. Summarized in Table LVI are the capital expenditures,
operation and maintenance costs, and the total annual and unit cost, based on
106 influent dry tons of waste activated sludge. A complete breakdown of the
costs associated with each of the unit processes is presented in Appendix A.
Included in these appended tables are the total capital and annual capital costs,
the amortization period and interest rates used, operation and maintenance labor,
maintenance materials, and power, water, and chemical costs. Table A-7 includes
the cost of anaerobic filters to treat the liquid side-streams associated with
thermal treatment. The anaerobic filter was sized for a 2-day hydraulic deten-
tion period, and the costs include those for purchase and installation of filter
media. Where applicable, credits were given for the production of methane gas
and subtracted from the calculated power costs. A breakdown of composting and
land disposal costs is not given in the Appendix because unit costs generated by
the operations section of the LACSD for these two unit operations were used.
Total annual composting costs were based on $1.80 per dry ton composted, with
33 percent of the total annual cost constituting capital expenditure at an in-
terest rate of 7 percent and an amortization period of 5 years. Hauling or land-
fill disposal costs were based on a unit price of $.08 per wet ton mile and a
dumping price of $2.50 per ton for disposal solids in excess of 25 percent TS
and $3.50 per ton for sludges with less than 25 percent TS. Twenty-three (23)
percent of the total annual hauling and disposal cost was used to determine the
capital expenditure at an interest rate of 7 percent and an amortization period
of 5 years for equipment.
A summary of the eight most cost-effective systems is presented in Table
LVII. The first four alternate schemes involve composting prior to final dis-
posal. If composting is possible and the product is acceptable by a fertilizer
manufacturer, the most economical schemes would incorporate basket or scroll
centrifugation after digestion and prior to composting. The unit cost for these
alternatives would respectively be $95 and $97 per dry ton of solids processed.
If disposal to a fertilizer manufacturer is not possible but composting is still
possible, the most effective systems would again involve basket and scroll cen-
trifugation of the digested sludge prior to composting and landfill disposal.
The unit cost for these alternatives would respectively be $100 and $102 per dry
ton of solids processed.
The last four most cost effective schemes are based on the assumption that
composting prior to disposal will not be feasible. In this case, thermal treat-
ment followed by pressure or vacuum filtration and landfill disposal would result
in respective unit costs of $96 and $97 per dry ton of solids processed, includ-
ing the costs associated with anaerobic filtration of the liquid side-streams.
It should be noted that the cost estimates for thermal treatment were generated
by manufacturers of thermal equipment. Although the capital, power, and operat-
ing costs are thought to be reliable, there are serious questions in the mind of
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the author as to the reliability of the maintenance labor and maintenance
material costs supplied.
If digestion and mechanical dewatering prior to landfill disposal were incor-
porated, then unit costs of $117 and $123 per dry ton would be incurred, re-
spectively, for scroll and centrifugation and pressure filtration. The
sludge train involving basket centrifugation of digested waste activated
sludge followed by landfilling was not included in this summary because the
solids content of the disposal solids would only approximate 11 percent TS and
would require a Class I site for disposal. Although the train involving scroll
centrifugation of digested oxygen waste activated and digested primary sludge
would produce final sludge solids of only 15 percent TS, it was included in the
summary pending a ruling as to whether this final product will be accepted at
a Class II site. The nature of these solids at a concentration of 15 percent
TS are such that no free moisture exists, and they exhibit plastic character-
istics.
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2. Ettelt, G. A. Activated Sludge Thickening by Dissolved Air Flotation.
Proceedings of 19th Industrial Waste Conference, Purdue University, 1964.
3. Eckenfelder, Jr., W. W. Industrial Water Pollution Control. McGraw-Hill,
Inc., New York, New York, 1966.
4. Burd, R. S. A Study of Sludge Handling and Disposal. Grant Number P. H.
86-66-32, Federal Water Pollution Control Administration, 1968.
5. McCarty, P. L. and McKinney, R. E. Salt Toxicity in Anaerobic Digestion.
Journal Water Pollution Control Federation, April 1961.
6. McCarty, P. L. Anaerobic Waste Treatment Fundamentals - Part Three. Public
Works Journal, November 1964.
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Summary Report: Pilot Plant Studies on Dewatering Primary Digested Sludge.
Prepared for the EPA, Contract No. EPA-670/2-73-043, August 1973.
8. Parker, D. G., Randall, C. W., and King, P. H. Biological Conditioning for
Improved Sludge Filterability. Journal Water Pollution Control Federation,
November 1972, p 2066.
9. Erickson, A. H., and Knopp, P. U. Biological Treatment of Thermally Condi-
tioned Sludge Liquors. Advances in Water Pollution Research, Pergamon
Press, New York, 1972, p 11-33.
10. Boyle, J. D., and Gruenwald, D. D. Recycle of Liquor from Heat Treatment
of Sludge. Journal Water Pollution Control Federation, October 1975,
pp 47, 2482.
11. Haug, R. T., Raksit, S. K., and Wang, G. G. Anaerobic Filter Treats Waste
Activated Sludge. Water and Sewage Works, February 1976.
12. Report on An Evaluation of the Sludge Disposal System at the Consolidated
Regional Wastewater Treatment Plant. Prepared by UTN Consolidated, Inc.
for Gloucester County Sewage Authority, Woodbury, New Jersey, April 1975.
13. Phase I Report of Technical Alternatives to Ocean Disposal of Sludge in the
New York City - New Jersey Metropolitan Area. Prepared for Interstate Sani-
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June 1975.
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14. McCarty, P. L. Anaerobic Process. Presented at the Birmingham Short
Course on Design Aspects of Biological Treatment, International Association
of Water Pollution Research, Birmingham, England, September 1977.
15. Black, Crow, and Eidsness. Process Design Manual for Sludge Treatment and
Disposal. EPA-625/1-74-006, U. S. Environmental Protection Agency, Cincin-
nati, Ohio, October 1974.
16. Livingston, J. R. Memorandum - Performance Characteristics of a Pilot
Rotary Drum Dryer: Phase I. Los Angeles County Sanitation Districts,
February 1976.
17. Units of Expression for Wastewater Treatment. WPCF Manual of Practice
No. 6, Water Pollution Control Federation, Washington D.C., 1976.
66
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TABLE XVII. SPECIFIC FILTRATION'RESISTANCE DETERMINATIONS ON
MESOPHILICALLY DIGESTED OXYGEN ACTIVATED SLUDGE
Ve Clz
Dosage
Ib/ton
0
160
200
240
320
200
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340
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12
83
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TABLE XVIII. SPECIFIC'FILTRATION RESISTANCE DETERMINATIONS ON MESOPHILICALLY
DIGESTED OXYGEN ACTIVATED SLUDGE
tolymer*
Dosage
Ib/ton
10
20
10
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10
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psig
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1012 cm/g
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TABLE XXI. SPECIFIC FILTRATION RESISTANCE DETERMINATIONS ON
THERMOPHILICALLY DIGESTED OXYGEN ACTIVATED SLUDGE
Ferric
Chloride
Dosage.
Ib/ton.
u
200
200
200
200
300
300
300
300
400
400
400
400
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Ib/ton
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0
620
620
780
940
620
780
780
940
620
780
780
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TABLE XXXIII. THEORETICAL AND MEASURED COD OXIDATION* FOR LPO CONDITIONING
THERMAL
RUN
JLPO 1
JLPO 2
JLPO 3
JLPO 4
JLPO 5
JLPO 6
THEORETICAL
OXIDATION + .
11.2
20. S
22.9
21.6
14.8
23.6
MEASURED
OXIDATION
21.1
13.1
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^Theoretical I Oxidation =
scf/min AIR x gal x 0.075 Ib AIR x 0.21 Ib 02 x
gpm 'IbCOD' scf ib Air
* Effect of steam dilution not considered. See text.
101
-------
TABLE XXXIV. COLIFORM REDUCTION DATA FOR LPO CONDITIONING
THERMAL
RUN
JLPO 2
JLPO 3
JLPO 4
JLPO. 5
JLPO 6
INFLUENT
TOTAL
COLIFORM
2.3x108
9.3x105
4.3x108
2.3x108
2.3xl08
FECAL
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2.3xl08
2.3x108
HEATRATE
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9.3x105
4.3xl03
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FECAL
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4.3xl03
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102
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TABLE XLIV. HEAT TREATMENT CONDITIONING COLIFORM REDUCTION DATA
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RUN
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JHT 5
INFLUENT
TOTAL
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9.3xl04
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FECAL
COLIFORM
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TABLE L. OPERATION AND PERFORMANCE SUMMARY FOR INTERMEDIATE PRESSURE WET.
OXIDATION
OPERATING SUMMARY
Thermal Run
Designation
JIPO
Feed Sludge
Type
02+Air WAS
GPM
3.0
Air Rate
(cfm)
17.5
Reactor Parameters
Terap.(°i-J
430
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460
Td (MinJ
48
DATA SUMMARY
CONSTITUENT
Total Volatile Solids
Volatile Suspended Solids.
COD - Total
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BOD - To tal
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pH
Alkalinity
%
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%
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%
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2.83
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2.63
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1,630
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580
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1.89
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WASTE ACTIVATED SLUDGE
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Figure 1. Research program schematic.
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Sample Taps —
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Symbol'
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on
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216
-------
LANDFILLING
Scheme 1A
DISSOLVED AIR
FLOTATION
ANAEROBIC
DIGESTION
PRESSURE
FILTRATION
COMPOSTING
LANDFILLING
FERTILIZER
MANUFACTURING
Scheme IB
Scheme 1C
Figure 89. Economic Analysis: Sludge handling scheme I.
217
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LANDFILLING
Scheme 2A
DISSOLVED AIR
FLOTATION
ANAEROBIC
DIGESTION
BASKET
CENTRIFUGATION
COMPOSTING
LANDFILLING
FERTILIZER
MANUFACTURING
Scheme 2B
Scheme 2C
Figure 90. Economic Analysis: Sludge handling scheme 2,
218
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LANDFILLING
Scheme 3A
DISSOLVED AIR
FLOTATION
ANAEROBIC
DIGESTION
SCROLL
CENTRIFUGATION
COMPOSTING
LANDFILLING
FERTILIZER
MANUFACTURING
Scheme 3B
Scheme 3C'
Figure 91. Economic Analysis: Sludge handling scheme 3.
219
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LANDFILLING
Scheme 4A
SCROLL
CENTRIFUGATION
ANAEROBIC
DIGESTION
PRESSURE
FILTRATION
COMPOSTING
LANDFILLING
FERTILIZER
MANUFACTURING
Scheme 4B
Scheme 4G
Figure 92. Economic Analysis: Sludge handling scheme 4.
220
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LANDFILLING
Scheme 5A
SCROLL
CENTRIFUGATION
ANAEROBIC
DIGESTION
BASKET
CENTRIFUGATION
COMPOSTING
LANDFILLING
FERTILIZER
MANUFACTURING
Scheme SB
Scheme 5C-
Figure 93. Economic Analysis: Sludge handling scheme 5.
221
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SCROLL
CENTRIFUGATION
ANAEROBIC
DIGESTION
SCROLL
CENTRIFUGATION
COMPOSTING
LANDFILL-ING
LANDFILLING
FERTILIZER
MANUFACTURING
Scheme 6A
Scheme 6B
Scheme 6C-
Fig'ure 94. Economic Analysis: Sludge handling scheme 6,
222
-------
I
VACUUM
FILTRATION
LANDFILLING
Scheme 7
DISSOLVED AIR
•FLOTATION
THERJiAL
CONDITIONING
PRESSURE
FILTRATION
LANDFILLING
Scheme 8
Figure 95. Economic analysis: Sludge handling schemes 7 and 8,
223
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APPENDIX A
ECONOMIC ANALYSIS CALCULATIONS
TABLE A-l. COST ESTIMATE SUMMARY FOR AIR FLOTATION THICKENING
CONDITIONS ,.«.«. * •>
1. Suspended Solids Loading 3 lb SS/hr-ft2
2. Sludge Conditioning polymer dosage of 3 lbs/to:
(318 Ibs/day)
3. Underflow Suspended Solids.. 50 rog/1 „ ., _ __
4. Float Solids 0.73 mgd 6 3..S* TS
CAPITAL COST
17 Air flotation-purchased $ 840,000
2. Installation § Housing 210,000
3. Contingencies 210,000
4. Contractors Profit »... 110,000
5. Engineering Fee ^70.000.
Total Capital $l,MU,ui)0
Annual Capital
(7*-15 yrs) $ 170,000/yr
OPERATION § MAINTENANCE COSTS
1. Labor $
2. Power 3
3. Water ,
4. Maintenance Materials 15,000/yr
5. Polymers 240.00p/yr
Total 0 § M ?344,300/yr
Total Annual $ 514,300/yr
$/ton $ 13.30/ton
224
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TABLE A-2. COST ESTIMATE SUMMARY FOR SCROLL CENTRIFUGE THICKENING
CONDITIONS
TT Suspended Solids Loading 600 Ibs/hr - 32" x 100" unit
2. Sludge Conditioning... polymer dosage of 10 Ibs/ton
(1060 Ibs/day)
3. Centrate Suspended Solids 600 rag/1
4. Thickened Sludge 0.40 MGD @ 6% TS
CAPITAL COST
T~. 15 Scroll Centrifuges-purchased. $2,550,000
2.
3.
4.
5.
Installation § Housing.
Contingencies.
Contractors Profit.
640,000
640,000
320,000
Engineering Fees .' ' " 510,000.
Total Capital
Annual Capital
(7%-lS yrs)
$4,6007000
$ 510,000/yr
OPERATION § MAINTENANCE COSTS
Labor. $
I.
2.
3.
4..
5.
Power.
Water
Maintenance Materials.
Polymers
Total 0 § M
Total Annual
S/ton
114,000
170,000
4,600
46,000
780,000
$l,114,600/yr
$l,624,600/yr
$ 42.00/ton1
225
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TABLE A-3. COST ESTIMATE SUMMARY FOR ANAEROBIC DIGESTION
CONDITIONS
XI Detention Time.... .........20 days
2. Volume............. ..,...^..1.75 MCF for flotation thickening,
1.07 MCF for centrifuge thickening
Thick'e'rii'ng Process
OPERATION COST
Total Capital.. ............
Annual Capital (7% -2 5 yfs)
OPERATION 5 MAINTENANCE COST
4 . Maintenance Materials .....
Total 0 § M
Totail Annual
$/ton,
Flotation
$ 8,940,000
- 1,790,000
§10,750,000
$ 922,780/yr
$ 114,000/yr
55,700/yr
3,800/yr
103,500/yr
§ 277,000/yr
$ 1,199, 800/yr
$ 31.00/ton
Centrifugatibn
$5,500,000
1,100,000
$6,60!di,COQ
$' 568,000/yr
$ 114,000/yr
. 55,700/yr
2,400/yr
: 67,500/yr
$ 259,600/yr
$ 807,600/yr
$. 20.90/ton
* Includes contingencies, contractors profit, control building,
heating, gas and electrical equipment.
•••'Includes credit for Digester Gas.
226
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TABLE A-4. COST ESTIMATE SUMMARY FOR BASKET CENTRIFUGATION DEWATERING OF BLENDED
DIGESTED SLUDGES
CONDITIONS
17 Sludge Quantity 116 tons/day
2. Sludge Blend ..... ...^. 70% WAS - 30% Primary
3. Solids Loading . 550 Ibs/hr - 48" unit
4. Sludge Conditioning............. Polymer Dosage of 11 Ibs/ton
(1276 Ibs/day)
5. Gentrate Suspended Solids....... 1500 mg/1
6. Total Cake Solids..... 11% TS
CAPITAL COST
T.20 Basket Centrifuges-purchase.. $1,600,000
2. Installation § Housing. '..... 400,000
3. Contingencies.......... 400,000
4. Contractors Profit... 200,000
5. Engineering Fee.... ...... 320,000
Total Capital §2,320,000
Annual Capital(7%-15 yrs) 320,000/yr
OPERATION § MAINTENANCE
T.—Labor $ 114,000/yr
2. Power , 150,000/yr
3. Water .- .H0,0//*
4. Maintenance Materials 29,200/yr
5. Polymer* .- S6a'0.°.G/7T
Total 0 5M *l,ib&,8UO/yr
Total Annual $1,476,800/yr
$/ton 34.90/ton
*Includes credit for sludge taken from Primary Sludge station.
227
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TABLE'A-5. COST ESTIMATE SUMMARY FOR SCROLL CENTRIFUGE DEWATERING OF BLENDED
DIGESTED SLUDGES
CONDITIONS
HSludge Quantity r.116 tons/day
2. Sludge Blend 70% WAS - 30% Primary
Si slllds Sing. 1500 Ibs/hr - S2».x 100" unl
4. Sludge Conditioning ....polymer dosage of 16 Ibs/ton
(1856 Ibs/day}
5. Centrate Suspended Solids. ......1500 mg/1
6. Total Cake Solids . 15% TS
CAPITAL COST
TI—7-ScrolT Centrifuges-purchase $1,190,000
2. Installation § Housing 297,500
3. Contingencies 297,500
4. Contractors Profit..^ ... 148,800
5. Engineering Fee "?>?P°
Total Capital $2,l/i,suu
Annual Capital (7% - 15 yrs).$ 239,000
OPERATION S MAINTENANCE •
1. Labor '.... $ Z
2. Power 8
3. Water ,
4. Maintenance Materials 2l,700/yr
B: Polymer*....' '•',?'?"r*'?°?OrT-
Total 0 5 M ?l,49Z,900/yr
Total Annual $l,731,900/yr
•$/ton $ 40.90/ton
Includes credit for sludge taken from Primary Sludge Station.
228
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TABLE A-6. J^^ESTIMATE SUMMARY FOR PRESSURE FILTRATION DEWATERING OF DIGESTED
CONDITIONS 1
T*Sludge Quantity............... 83 tons/day
2. Solids Loading............;.-..- 0.31 lbs/hr-ft2
3. Sludge Conditioning 240 Ibs/ton FeCLs (9.96 tons/day)
800 Ibs/ton CaO (33.2 tons/day)
150 Ibs/ton DE (6.5 tons/day!
4. Filtrate Suspended Solids 50 mg/1
5. Total Cake-Solids 34% TS
CAPITAL COSTS
373-7500 ft"2 presses-purchase... $3,350,000
2. Installation $ Housing.' 838,000
3. Contingencies 838,000
4. Contractors Profit 418,800
5. Engineering Fee 670>00p
Total Capital $6,114,800
Annual Capital(7% - 15 yrs) 670,500
OPERATION § MAINTENANCE
T;; Labor.... $ 114,000/yr
2. Power.. ,i.... 100,000/yr
3. Water... 2,000/yr
4. Maintenance Material 61,200/yr
5. Chemical............ 1,440,000/yr
Total 0 § M §l,717,200/yr,
Total Annual $2,387,700/yr
$/ton. $ 78.80/ton
229
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TABLE A-7.. COST ESTIMATE SUMMARY FOR THERMAL TREATMENT
GONDItlONS
17 Detention Time ............... 30 min
Z. Volume.,..............:.,..... 2100 CF
3 . Temperature « ...'...... . ....... 40 OAF
CABITAL COST;
Thermal tTn.lt: , .
T:—Installed Cost* .., $ 7,800,000
2. Housing..................... 600,000
3. Contingencies..;., ........* 1,500,000
4. Engineering Fee ... 1,200,OOO
5. Contractors Profit..,.. „......„ 750,000
Side Stream Treatment?
XTConstruction cost*.......... $ 2,167,000
2. Engineering Fee.. ........ 455,000
.Total Capital $l4,45u,uuu
Annual Capital++ $ 1,.521,500
OPERATION' g MAINTENANCE
1. Labor .... $ 1
2. Power*** -..--» 44,000/yr
3'. Fuel .......'...,..,...... 300,000/yr
4. Water... ..^....., 5,000/yr
S. Chemical CHN03)..*.* ... .MS0/,5^
6. Maintenance Materials....... I97,ffOO/yr
Total 0 § M ^721,000/yr
Total Annual $ 2,242*540/yr
$/ton $ 57.98/ton
•¥
#
**
Manufacturers estimate for purchase and installation of
boiler, titanium exchangers § reactor decant tank, odor
control equipment.
Anaerobic Filter 2 day detention time.
7* - 15 yrs Thermal.Unit, 7% - 25 yrs Anaerobic Filter.
Includes credit for filter gas.
230
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TABLE A-8. COST ESTIMATE SUMMARY- FOR PRESSURE FILTRATION OF THERKAL CONDITION-
ED WASTE ACTIVATED SLUDGE
CONDITIONS nf .,
1. Sludge Quantity. . •.. "tons/day
2. Solids Loading ... 0.92 lbs/hr-ft^
3. Filtrate Suspended Solids 250 mg/1
4. Total Cake Solids... ..".... 42% TS
CAPITAL COST . . . . t. inn nnn
T.—2-4300 £t2 presses - purchase.:... $1,100,000
2. Installation § Housing • 5,1'nnn
3. Contingencies....... ?«'nnn
4. Contractors Profit ^8,000
5. Engineering Fees «• 13&'K-
Total Capital ^ 52,««»»"""
Annual Capital (7% - 15 yrs) 220,000
OPERATION § MAINTENANCE nx'nin/w
T r aho-r —— $ 114,000/yr
|- p*Ser? :::::::.IM... 24,ooo/yr
Si' Maint^nancr Materials-. -,»-nnn/vr
Total 0 § M : $ 158,000/yr
Total Annual . { 378000/yr
$/ton 5 10.90/ton
'-.-231
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TABLE A-9. COST ESTIMATE SUMMARY FOR VACUUM FILTRATION OF THERMAL
CONDITIONED WASTE ACTIVATED SLUDGE
CONDITIONS . .,
1. Sludge Quantity 95 ton/day
2. Solids Loading 3.0 lbs/hr-ft'
3. Filtrate Suspended Solids 5000 mg/1
4. Total Cake Solids 35* TS
CAPITAL COSTS .
T;—3-960 '±t'2' filters - purchase §478,uuu
2. Installation § Housing.. Ho cnn
3. Contingencies '
4. Contractors Profxt
5. Engineering Fee -.
g Total Capital
Annual Capital (7% - 15 yrs) 95,700
MAINTENANCE $n4,000/yr
::::::*.*.".i:iini". 84,ooo/yr
3. Maintenance Material ™'?™F*
Total 0 § M 5208,000/yr
Total Annual i303?7^7/^
$/ton * 8.80/ton
232
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APPENDIX B
UNIT CONVERSIONS
TABLE B-l. UNIT CONVERSIONS 17
Customary Unit
BTU/lb
BTU/hr/ft2
cfm o
cfm/ftj
OF
fto
ft3/lb
gal
GPD
gpm
in
in
in . Hg
Ib
Ib/dy
Ib/ft3-dy
lb/hr
lb/hr-ft2
Ib/ton
MGD
psi
ton
ton/dy
Conversion
X2.326
X3.154
X0.4719
X1.667X10-5
(°F-32)/1.8
X0.3048
XO. 06234
XO. 003785
XO. 003785
XO. 06308
X25 . 4
X0.0254
X3.38
X0.4536
X0.4536
X16.02
X0.4536
X4.883
X0.500
X0.0438
X6.895
X907 . 2
X907.2
Metric Unit
kJ/kg
J/m2 • s
1/s
/•»
I/up • s
°C
m
m3/kg
m3
m3/d
1/s
mm
m
kPa
kg
kg/dy
kg/mj • d
kg/hr
kg/m2 -h
g/kg
mj/s
kPa
kg
kg/d
233
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TECHNICAL REPORT DATA .
(Please read Instructions on the reverse before completing)
REPORT NO.
EPA-600/2-80-147
3. RECIPIENT'S ACCESSION-NO.
TITLE AND SUBTITLE
WASTE ACTIVATED SLUDGE PROCESSING
5. REPORT DATE
August 1980 (Issuing Date)
6. PERFORMING ORGANIZATION CODE
, AUTHOFUS)
Scott R. Austin, Jack R. Livingston and
Liberate Tortorici
8. PERFOI
. PERFORMING ORGANIZATION NAME AND ADDRESS
County Sanitation Districts of Los Angeles County
Whittier, CA 90607
10. PROGRAM ELEMENT
1BC611 SOS#1
11. CONTRACT/GRANT NO.
Contract No.
14-12-150
2.SPQNSPRING AGENCY NAME AND ADDRESS Tin OH
Municipal Environmental Research Laboratory--Cm.,OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, OH 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final - 6/73-9/76
14. SPONSORING AGENCY CODE
EPA/600/14
15. SUPPLEMENTARY NOTES
Project Officer: Irwin J. Kugelman (513) 684-7633
6. ABSTRACT
A study was made at pilot scale of a variety of processes for dewatering and
stabilization of waste activated sludge from a pure oxygen activated sludge system.
Processes evaluated included gravity thickening, dissolved air flotation thickening,
basket centrifugation, scroll centrifugation, aerobic digestion, and anaerobic
digestion (mesophillic and thermophillic). In addition combinations of processes
were evaluated including: scroll centrifugation after anaerobic digestion, basket
centrifugation after anaerobic digestion, centrifugation of mixtures of anaerobically
digested primary sludge and anaerobically digested waste activated sludge, centri-
fugation after thermal conditioning and composting after digestion and centrifugal
dewatering. Data are presented on all of the processes above and optimum economic
combinations are identified.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
COS AT I Field/Group
Sludge Disposal
Dewatering
Thickening
Composting
Digestion
Centrifugation
Filtration Vacuum
Waste Activated Sludge
13B
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19.
CLASS (This Report}
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
SPA Form 2220-1 \C9-73)
234
U.S. GOVEBNMENT PRINTING OFFICE: 1980 -657-165/0145
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