&EPA
United States
Environmental Protection
Agency
Office of Research and
Development
Washington, DC 20460
Technology Transfer
EPA/625/10-90/007
September 1990
Environmental
Regulations and
Technology
Autothermal Thermophilic
Aerobic Digestion of Municipal
Wastewater Sludge
-------
-------
Technology Transfer
EPA/625/10-90/007
Environmental
Regulations and
Technology
Autothermal Thermophilic
Aerobic Digestion of Municipal
Wastewater Sludge
September 1990
This report was prepared by the Risk Reduction Engineering Laboratory
and the Center for Environmental Research Information,
U.S. Environmental Protection Agency, Cincinnati, OH 45268
Printed on Recycled Paper
-------
Acknowledgments
This document was produced by the U.S. Environ-
mental Protection Agency's (EPA's) Risk Reduction
Engineering Laboratory (RREL) and Center for En-
vironmental Research Information (CERI) in Cincin-
nati, Ohio. A significant portion of this document is
based upon a study developed in the Federal
Republic of Germany (FRG) at the Universitat
Karlsruhe under a contract with the EPA. Professor
H. Hahn and D. Leonhard (Dipl. Ing.) of the Institut
fur Siedlungswasserwirtschaft prepared the initial
draft report based upon site visits, interviews with
owners, operators, engineers, researchers, manufac-
turers, and literature sources. Mr. Kevin Deeny of
Junkins Engineering in Morgantown, Pennsylvania,
oversaw the FRG effort, participated in site visits
and interviews in the FRG, and contributed to the
development of the final report. Dr. James Heidman
(EPA RREL) and Dr. James E. Smith, Jr. (EPA
CERI) provided overall technical direction and were
major contributors to the final report. This document
was edited by Jan Connery and Elizabeth Collins of
Eastern Research Group in Arlington, Mas-
sachusetts.
This document was reviewed by several technical
experts including: Robert Brobst (EPA Region VIII),
John Bush (New Hampshire Water Supplies and Pol-
lution Control Division), Joseph Farrell (EPA RREL),
Walt Jakubowski (EPA Health Effects Research
Laboratory), Bruce Jank (Canadian Center for In
land Waters), Norm Kowal (EPA Environmental
Criteria and Assessment Office), Henryck Melcer
(Environment Canada-WTC), George Neill (New
Hampshire Water Supplies and Pollution Control
Division), Burnell Vincent (EPA Office of Technology
Transfer and Regulatory Support), and Thomas
Gleason (EPA Office of Health and Environmental
Assessment).
Information from a parallel Canadian study prepared
by Harlan Kelly of Dayton & Knight Ltd. for Environ-
ment Canada's Water Technology Centre has been
utilized in preparing this document. Results of re-
search conducted at Darmstadt University by doc-
toral candidate J. Glasenapp under the direction of
Dr. K. Bau and Professor Popel were made avail-
able to the authors at Karlsruhe. These contributions
are gratefully acknowledged. '
Figures 3-2,3-3,3-4, 7-2, and 7-3 were reprinted
courtesy of Fuchs Gas und Wassertechnik Com-
pany.
Notice
This document has been reviewed by the U.S. En-
vironmental Protection Agency and approved for
publication. Mention of trade names or commercial
products does not constitute endorsement or
recommendation for use.
Cover Photograph: Autothermal Thermophiiic Aerobic Digestion System at Gemmingen, Federal Republic of Ger-
many.
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Table of Contents
Acknowledgments
Notice
Table of Contents
Abbreviations and Acronyms
Conversion Factors
CHAPTER 1 INTRODUCTION
CHAPTER 2 PROCESS CONCEPTS AND
DEVELOPMENT
2.1 Autothermal Digestion
2.2 ATAD Process Development
2.2.1 European Experience
2.2.2 North American Experience
CHAPTER 3 ENGINEERING AND DESIGN
CRITERIA
3.1 Introduction
3.2 Process Configurations
3.3 Sludge Feed Source and
Thickening Requirements
3.4 Detention Time
3.5 Feed Cycle and Isolated Reaction Time
3.6 Aeration and Mixing
3.7 Temperature and pH
3.8 Foam Control
3.9 Post-Thickening/Dewatering
3.10 Sludge Storage and Disposal
3.11 Construction Features
3.11.1 Tank Construction
3.11.2 Heat Exchange
CHAPTER 4 DESIGN EXAMPLE
4.1 Introduction
4.2 Plant Information
4.3 Solids Balance
4.3.1 Primary Solids Removal
4.3.2 Secondary Sludge Production
4.3.3 Digester Feed
4.3.4 Digestion
4.3.5 Sludge Storage and Disposal
4.4 Reactor Sizing
4.5 Process Oxygen Requirement
4.6 Aeration and Mixing Requirements
4.6.1 Discussion
4.6.2 Empirical Aeration/Mixing Sizing
4.7 Sludge Storage
4.7.1 Mixing
4.8 Cost Estimate
CHAPTER 5 PERFORMANCE DATA
5.1 Introduction
5.2 Volatile Solids Reduction
ii
ii
iii
iv
v
3
3
4
4
4
5
5
5
5
7
7
7
8
8
9
10
10
10
11
13
13
13
13
13
13
15
15
16
16
16
16
16
17
18
18
18
19
19
19
5.3 Pathogen Reduction
5.4 Dissolved Oxygen Concentrations and
Respiration Activity
5.5 Oxygen Transfer Efficiency (OTE)
5,6 Odor Control
5.7 Operation and Maintenance
CHAPTER 6 ABILITY TO MEET u.s.
REGULATORY STANDARDS
6.1 Introduction
6.2 Current U.S. Federal Regulations
(as of 1990)
6.2.1 Processes to Further
Reduce Pathogens
6.2.2 Processes to Significantly
Reduce Pathogens
6.3 Proposed U.S. Regulations
6.4 Ability of ATAD to Meet the Proposed U.S.
Standards for Pathogen Reduction
6.5 Ability of ATAD to Meet the Proposed U.S.
Standards for Reduction of
Vector Attraction
CHAPTER 7 CASE STUDIES
7.1 General Description
7.2 Overview of Existing Facility Applications
7.3 Highlighted Facilities
7.3.1 Fassberg
7.3.1.1 General Facility Design Basis
7.3.1.2 ATAD
7.3.2 Gemmingen
7.3.2.1 General Facility Design Basis
7.3.2.2 ATAD
7.3.3 Salmon Arm
7.3.3.1 .General Facility Design Basis
7.3.3.2 ATAD
CHAPTERS COSTS
8.1 Introduction
8.2 Source Cost Data
8.3 Literature Source Cost Data
CHAPTER 9 THERMOPHILIC PRE-STAGE
PROCESS
9.1 Introduction
9.2 System Design and Operation
9.3 Process Evaluation
20
23
23
26
27
31
31
31
32
32
33
34
35
37
37
37
37
40
41
41
42
42
42
44
44
44
49
49
49
50
57
57
57
60
APPENDIX CRITERIA FOR AGRICULTURAL
USE OF SLUDGE IN THE FEDERAL REPUBLIC
OF GERMANY 61
REFERENCES
63
HI
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Abbreviations and Acronyms
AOR process oxygen requirement
ATAD autothermal thermophilic aerobic
digestion
BODs 5-day biochemical oxygen demand
Blu British thermal unit(s)
°C degrees Centigrade
CFR Code of Federal Regulations
CPU colony-forming unit(s)
COD chemical oxygen demand
d day(s)
DM Deutsche mark
ENR-CCI Engineering News Record - Construction
Cost Index
EPA U.S. Environmental Protection Agency
F/M food-to-mass ratio
FRG Federal Republic of Germany
ft foot/feet
ft3 cubic foot/feet
g gram
gal gallon
h height
H/D ratio filling-depth-to-diameter ratio
hp horsepower
hr hour(s)
HRT hydraulic residence time
k decay coefficient
kg kilogram(s)
kJ kilojoule
kW kilowatt
kWh kilowatt-hour(s)
IAWPRC International Association on Water
Pollution Research and Control
L liter(s)
!b pound(s)
m meter(s)
m3 cubic meter(s)
MCD Municipal Construction Division
mg milligram(s)
MG million gallons
MGD millions of gallons per day
ml milliliter(s)
MPN most probable number
NVSS nonvolatile suspended solids
Oa oxygen
OSS Ordinance of Sewage Sludge
OTE oxygen transfer efficiency
P.E. population equivalent
PEC EPA's Pathogen Equivalency Committee
PFRP process to further reduce pathogens
PFU plaque-forming units
PSRP process to significantly reduce
pathogens
SOUR specific oxygen uptake rate
s seconds
TS total solids
TSS total suspended solids
TVS total volatile solids
TVSS total volatile suspended solids
t ton (metric)
UTB Umwelttechnik Buchs
VS volatile solids
VSS volatile suspended solids
W .. watt(s)
yr year(s)
$ U.S. dollars
IV
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Conversion Factors
Unit
°C
g/L
kg
kg/m3
kJ
kJ/kg
kg/m3
kW
kWh/m3
L
m
m3
m3
m2
m3/s
m3/s
mg/L
W
W/m3
Multiply by
(°Cx1.8)+32
8.345
2.205
6.243 x10~2
0.9486
0.430
1.686
1.341
96
0.2642
3.281
35.32
1.308
10.76
2,119
22.83
8.345
3.4129
9.66 X10"2
To get
°F
lb/1,000gal
Ib
Ib/ft3
Btu
Btu/lb
Ib/yd3
hp
Btu/ft3
gal
ft
ft3
yd3
ft2
ft3/min
MGD
lb/1 ,000,000 gal
Btu/hr
Btu/hr/ft3
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Chapter 1.
Introduction
This document describes a promising technology —
autothermal thermophilic aerobic digestion — for
meeting the current and proposed U.S. federal re-
quirements for pathogen controJ and land applica-
tion of municipal wastewater sludge. Autothermal
thermophilic aerobic digestion, or ATAD, has been
studied since the 1960s and significantly developed
since the mid-1970s. It is currently widely and suc-
cessfully implemented in Europe, particularly the
Federal Republic of Germany (FRG), where there
are over 35 full-scale operating facilities. Facilities
can also be found in Great Britain, France, and Italy.
Four full-scale facilities are operational in Canada,
and one is planned for Connecticut. The growing in-
terest in the technology has evolved following in-
creased scientific and regulatory concern for
sludge-borne pathogens. Full-scale development of
the technology in the FRG closely parallels advan-
ces made by a German aerator manufacturer.
ATAD systems are normally two-stage aerobic
processes that operate under thermophilic tempera-
ture conditions (40°C to 80°C) without supplemental
heat. Typical ATAD systems operate at 55°C and
reach 60°C to 65°C in the second stage. They rely
on the heat released during digestion to attain and
sustain the desired operating temperatures.
The ATAD process has many benefits: a high disin-
fection capability, low space and tankage require-
ments, and a high sludge treatment rate. It is a
relatively simple technology that is easy to operate
(automatic monitoring or control equipment and full-
time staff are not required) and economical, par-
ticularly for small facilities. It provides a proven,
cost-effective way to achieve aerobic digestion and
to produce sludge that can be applied to land in the
U.S. without any management restrictions for
pathogen control.
This document provides detailed information con-
cerning the history, design, operation and main-
tenance requirements and performance of ATAD
systems. Limited information is also provided on
another thermophilic aerobic digestion system cur-
rently operating in Europe — the pre-stage system.
Pre-stage systems are typically retrofitted into exist-
ing facilities already equipped with anaerobic
digesters. They are used to enhance treatment and
pathogen destruction. Unlike ATAD systems, they
are not completely autoheated. Supplemental heat,
provided through an auxiliary heat exchanger, is re-
quired to achieve the desired operating temperature
of 65°C. Pre-stage systems are operating primarily
in Switzerland and Germany.
The document is intended primarily for owners and
operators of treatment plants and their consulting en-
gineers who are interested in using ATAD treatment
to achieve the regulatory requirements for land ap-
plication of municipal sludge. Regional, state, and
local government officials responsible for implement-
ing and enforcing the land application regulations
will also find the document useful.
The information in this document is primarily from a
1989-1990 EPA study that collected and analyzed
design and operating data from ATAD systems in
the FRG. Information was obtained from several
sources: a review of German and U.S. literature,
telephone contact with FRG facilities, site visits to
selected facilities to observe operating and design
conditions and to interview plant operating person-
nel and design engineers, telephone contact and
meetings with system manufacturers, and contact
with researchers at Darmstadt University to discuss
completed and ongoing research. In addition, this
document includes information obtained from three
of the Canadian facilities operating in 1990. Note
that key design data are highlighted in italics.
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Chapter 2.
Process Concepts and Development
2.1 Autothermal Digestion
With an adequate supply of oxygen, microor-
ganisms, nutrients, and biodegradable organic
material, autothermal aerobic digestion can degrade
complex organic substances into end products in-
cluding carbon dioxide and water. Some of the ener-
gy released by microbial degradation is used to form
new cellular material; much of it is released as heat.
Typical biological heat production values reported or
assumed range from 14,190 to 14,650 kJ/kg Oa (i,
2,3). The carbonaceous oxygen requirements vary,
but are often considered to be 1.42 kg Oa/kg volatile
suspended solids (VSS) oxidized. In autothermal
thermophilic aerobic digestion, the heat released by
the digestion process is the major heat source used
to achieve the desired operating temperature.
References 1,2, and 4 provide detailed discussions
of the heat and mass balance equations necessary
for describing aerobic thermophilic systems opera-
tion. The same basic analytical approach has also
been utilized in the FRG (5). Figure 2-1 shows the
various inputs, outputs, and heat production items to
be included in a heat balance. Autothermal condi-
tions result from an adequately thickened sludge
feed, a suitably insulated reactor, good mixing, and
an efficient aeration device that keeps the latent
heat loss to an acceptable level. ATAD systems cur-
rently operating in Europe and Canada do not use
heat exchange between reactor feed and effluent to
warm the incoming sludge because sufficient heat is
generated and sustained in the process. Heat ex-
HEAT LOSS TO
SURROUNDINGS
MIXING
HEAT INPUT
FEED SLUDGE
V
0*0
BIOLOGICAL HEAT
PRODUCTION
INFLUENT GAS
(AIR)
SENSIBLE AND LATENT
WATER VAPOR HEAT
LOSS IN GAS EFFLUENT
HEAT LOSS IN
SLUDGE EFFLUENT
Figure 2-1. Heat Balance Schematic of a Thermophilic Aerobic Digester.
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Process Concepts and Development
change is occasionally practiced for waste heat
recovery such as to provide building heat.
2.2 ATAD Process Development
2.2.1 European Experience
Much of the developmental work leading to the
ATAD process has been described by Popel (6,7,
8,9,10). These references include detailed discus-
sions of the heat and mass balances, biochemical
cell composition, kinetics of the metabolic reactions,
kind of aerating device needed, and the research
results.
The early studies used a self-aspirating aeration
device (known as the Umwalzbelufter) developed by
Herr Fuchs and manufactured by Alfa Laval. Using
this aerator, the process was demonstrated both
with animal manure and with sludge having high con-
centrations (10 to 60 kg/m3) of volatile suspended
solids. Temperatures of 50°C to 60°C were
achieved in 20-m3 reactors. These studies showed
the importance of periodic feeding in relation to both
reactor temperatures and the degree of stabilization,
and relationships were developed to determine the
required reactor size for the degree of decomposi-
tion and temperature desired. Pope! reported (8)
that a properly designed system could reach
temperatures of 60°C to 70°C, kill pathogens,
produce an odor-free humus, and produce a readily
dewaterable product.
Fuchs continued to improve its aeration devices and
to revise its overall ATAD system design. It sold the
license for its previous aeration system to Alfa Laval
(DeLaval in the USA) in the mid-1970s.
The first full-scale municipal ATAD system in the
FRG was a Fuchs system installed in Vilsbiburg in
1977, which continues to operate to this day. As of
1989,35 ATAD systems were installed or under con-
struction in the FRG, 30 of which were supplied by
Fuchs.
Significant advances have been made in optimizing
and adapting ATAD technology since the original
development work 20 years ago. Much of the ex-
perimental work on which the present ATAD sys-
tems are based was performed by BreitenbOcher (5)
at the full-scale ATAD plant at Gemmingen, FRG.
Current European design practices, process perfor-
mance, and implementations of ATAD technology
are discussed in the subsequent chapters of this
document.
2.2.2 North American Experience
As indicated above, the thermodynamic require-
ments for ATAD systems along with process simula-
tion predictions have been described in the U.S.
literature for many years. Self-aspirating aerators
manufactured by DeLaval, Inc. were marketed in the
1970s for high-strength industrial and liquid manure
waste treatment in the U.S. in a patented process
called the LICOM (liquid composting) system.
Several short-term batch tests with various industrial
and animal wastes were reported to produce
autoheating to 50°C to 60°C (11). Studies on dairy,
beef, and swine wastes demonstrated that autoheat-
ing to thermophilic temperatures was possible (12,
13).
In the early 1970s, Union Carbide was active in
developing an ATAD system that utilized high-purity
oxygen. Pilot plant studies were conducted under
autothermal thermophilic conditions at Tonawanda,
New York (14). The researchers concluded that an
ATAD system operating at a 5-d hydraulic residence
time (HRT) could achieve volatile solids destruction
equivalent to conventional aerobic digestion
operated at a 15- to 20-d HRT. The thermophilic con-
ditions inhibited nitrification, thus eliminating this fac-
tor as a source of oxygen demand. By maintaining
the temperature at 50°C for 5 or more hours,
pathogenic organisms could be reduced to less than
detectable levels.
The most extensive U.S. study of ATAD using air
aeration was conducted at Binghamton, NY in 1977
and 1978 (2). The design was based on the system
marketed by DeLaval, Inc. Two aerators were
studied: an LFE Corporation Midland-Frings self-
aspirating aerator and a DeLaval centri-rator self-
aspirating aerator. Regardless of air and sludge
temperatures, reactor temperatures after startup
were always in the thermophilic zone and often
above 50°C. No nitrification occurred, but some am-
monia was lost due to stripping. The study reported
a high degree of pathogen control with respect to
bacteria, viruses, and parasites. Practical operation
and maintenance problems were minimal, although
problems with poor mixing were noted. Currently the
only ATAD systems for municipal sludge stabiliza-
tion on the North American continent are in Canada,
where three systems have been installed in British
Columbia and one in Alberta. Two installations use
Fuchs-supplied equipment: One is a pump and ven-
turi-air-supplied plant, and one is a retrofit facility
that uses aspirator-type aerators developed in
Canada (15,16). Details of the last plant (Salmon
Arm) appear in this report. As of 1990, no ATAD sys-
tems were operating in the U.S.
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Chapter 3.
Engineering and Design Criteria
3.1 Introduction
Several single- and multiple-stage versions of ATAD
are under development or operating in the FRG,
Canada, and Great Britain. Systems in the FRG in-
clude those manufactured and/or distributed by Bab-
cock, Fuchs, Limus, and Thieme. In Great Britain,
the Water Research Centre has been developing a
version of the process. Canadian facilities include
two equipped by Fuchs and two retrofitted plants
with locally supplied equipment.
The Fuchs system of the FRG is by far the most
widely used system (17), and it is therefore the sys-
tem for which the greatest amount of full-scale
operating experience exists — exceeding 10 yr at
some facilities. For these reasons, this system is
used as the design standard described here. Key
design parameters for this system are summarized
Table 3-1 Design Parameters for ATAD Systems1
Reactors:
Reactor type:
Sludge type:
Feed TS range:
Required VSS:
Detention time:
Minimum reaction
time:
Temperature and
pH:
Air input:
Specific power:
Energy
requirement:
Heat potential
for recovery:
Two or more stages, depending on plant
size; tanks of equal volume operating in
series; daily batch operation
Cylindrical; height/diameter ratio 0.5-1.0
Primary
Secondary activated sludge, trickling filter
Gravity or air flotation thickened
Mixture of primary and secondary
Domestic, industrial origin
Manure
40-60 g/L (4-6%)
>25 g/L (2.5%)
5-6 d
20 hr/stage
Reactor I: 35 -50°C , pH >7.2
Reactor II: 50 -65°C , pH -8.0
4 m3/hr/m3 active reactor volume
85-105 W/m3 active reactor volume
9-15 kwh/m3 of sludge
20-30 kWh/m3 of sludge
1 These parameters are based on Fuchs-equipped facilities.
in Table 3-1 and discussed in this section. Alterna-
tive systems must be evaluated on an individual
basis.
3.2 Process Configurations
Figure 3-1 illustrates configurations for three ATAD
systems in the FRG. Typical features include pre-
thickening, two enclosed and insulated reactors con-
figured in series, mixing/aeration and foam control
equipment, and a final storage/post-thickening tank.
Like most digestion processes, ATAD systems exist
as both single- and multiple-stage processes that
can be configured with a multitude of pre- and post-
thickening arrangements. Single-stage systems can
achieve similar volatile solids reduction to multiple-
stage systems, but cannot reduce pathogens to the
same extent. Therefore, single-stage configurations
are more suitable for disposal options (such as
landf illing or restricted-use land application) that do
not require enhanced pathogen reduction. The multi-
ple-stage process has been recommended as a
design standard for European systems where the
sludge may be applied to land. Two-reactor systems
are typical.
3.3 Sludge Feed Source and Thickening
Requirements
Most digestion processes thicken the sludge prior to
digestion to minimize the size of the digestion tanks
and to limit the energy requirements for mixing and
heating. This is also true for ATAD systems.
Like composting, ATAD systems rely on conserving
the heat released during digestion to attain and sus-
tain an operating temperature of 55°C to 65°C.
Thus, sludge is both the material being treated and
the "fuel" that drives the process.
A sludge in a Fuchs system is adequate to support
process temperature requirements if it is thickened
to 4-6% TSS, of which at least 2.5% is mostly
biodegradable volatile solids. Gravity thickening can
usually achieve this concentration. Some plants also
successfully co-thicken the waste-activated sludge
with the primary solids arid thereby avoid the need
for a separate sludge thickener.
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Engineering and Design Criteria
SLUDGE
TO LAND
-*• APPLICATION
THICKENER
ATAD REACTORS SUMP SLUDGE STORAGE
B
•-Q-
SLUDGE
I
| POLYMER
I
^3 ULJ-QJ
MAZORATOR
T
U
TO LAND
APPLICATION
ROTATING SUMP
SCREEN
HOLDING TANK CONDITIONING ATAD REACTORS
TANK
SLUDGE
STORAGE
EXHAUST AIR
TO LAND
APPLICATION
THICKENER
ATAD REACTORS
SLUDGE STORAGE
Rgure 3-1. A) Fuchs ATAD System, B) Thieme ATAD System, C) Limus ATAD System.
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Engineering and Design Criteria
Although some ATAD systems receive only waste-
activated sludge (e.g., Gemmingen), most ATAD
facilities in the FRG have both primary and secon-
dary sludge sources.
Sludge from plants without primary clarifiers and
with activated sludge food-to-mass ratio (F/M) load-
ings as low as 0.1 to 0.15 (kg BODs/d) /kg TVSS still
seems to be suitable for ATAD. No data are avail-
able from extended air plants, although one plant,
Mundelsheim, is known to have taken the diges-
tion system off line as a result of low loading condi-
tions (F/M < 0.1).
Two nonmunicipal applications by Thieme have
used rotary screens to pre-thicken the sludge to 7.5-
10% volatile solids (VS) prior to digestion. Aeration
and mixing energy requirements are reportedly high
for these systems; limited performance data exist.
3.4 Detention Time
To satisfy the FRG process requirements for
destruction of pathogens and total organic solids,
the design hydraulic detention time for Fuchs sys-
tems is established at5to 6 d (2.5to 3d per reac-
tor). Sixty percent of the volatile solids destruction
reportedly occurs in the first reactor.
3.5 Feed Cycle and Isolated Reaction
Time
Batch feeding is established by design intent and is
reflected in the sizing of the sludge feed pump(s).
The feed sludge pumping system is sized to deliver
the daily thickened sludge volume to the reactor in
less than 1 hr. Since all sludge is pumped within a 1-
hr period, the reactors are isolated for the remaining
23 hr during each day. This undisturbed reaction
time is considered an important factor in attaining a
high degree of pathogen destruction.
The series reactor configuration and a reverse-order
filling procedure further isolate treated sludge from
partially treated or raw sludge to prevent reinocula-
tion of the final treated product. The operating
strategy is best understood by referring to the
process flows in Figure 3-1. Each day, aeration and
mixing are stopped, and stabilized sludge is dis-
charged from reactor II into the storage tank, thus
lowering the level in that reactor. Sludge is then al-
lowed to flow by gravity from reactor I to reactor II
until both tanks reach equilibrium. Raw sludge is
added to reactor I to raise the level and displace
sludge into reactor II. When both reactors reach full
operating level, aeration and mixing are resumed.
The entire daily feeding process is typically ac-
complished in 30 minutes, resulting in an undis-
turbed reaction time of 23.5 hr in both tanks. In
addition, the displaced volume should be adjusted in
such a way that the average residence time will vary
by less than approximately 20% (5).
3.6 Aeration and Mixing
Aeration efficiency and mixing efficiency are probab-
ly the two most important factors to consider when
designing an ATAD system. A highly efficient aera-
tion system is needed for two process purposes
(aside from minimizing energy requirements): to
keep pace with the extremely high oxygen demand
that results in the process, and to minimize the
latent heat loss from the reactors that occurs when
air is exhausted. Both the air flow rate and oxygen
transfer efficiency depend on system geometry
(height/depth, location of aerators), substrate charac-
teristics (e.g., TSS, viscosity)-, and turbulence condi-
tions that are specific to the procedure as well as to
the substrate.
The aeration systems of several early pilot opera-
tions failed to attain the necessary efficiency and/or
reliability to support the process requirements. Much
research and development continues today concern-
ing ATAD aeration and mixing systems. However,
the aeration system used for the Fuchs ATAD sys-
tems throughout the FRG is not developmental,
since considerable successful operating experience
has been amassed. The Fuchs ATAD systems util-
ize aspirator aerators of the type illustrated in Figure
3-2. Two aerators are side-mounted into each reac-
tor; depending on reactor size, a third aerator can
also be mounted in the center of the reactor.
Typical design ranges for empirical aeration/mixing
parameters include:
Specific power:
Air input:
Energy requirement:
85-105W/m3of
active reactor volume
4 m3/hr/m3 of active
reactor volume
9-15kWh/m3of sludge
throughput
The specific power range of 85-105 W/m3 is com-
parable to a mixing energy range of 430-530 hp/MG,
which represents a high mixing intensity.
The air input value of 4 m3/hr/m3 of active reactor
volume is premised upon a feed TVSS range of 2.5-
5.0%. This value relates to a design process air
-------
Engineering and Design Criteria
requirement of 1.42 kg of Oz/kg VSS destroyed.
(See Chapter 4 for a design example.)
The energy requirement of 9-15 kWh/m3 of sludge
throughput also includes the energy required for
foam cutters (minimum of two per tank, requiring
from 0.3 to 0.75 kW each).
3.7 Temperature and pH
An average temperature of55°C in the second reac-
tor is used for design purposes. This is consistent
with FRG design recommendations. The tempera-
ture in the second reactor can exceed 55°C (and, ac-
cording to observations made during site visits,
often does). However, to prevent resolubilization of
organics, the temperature should not exceed 65°C
(5).
During batch feeding, which results in a 33% reactor
volume displacement, the temperature drop ranges
from 5°C to 10°C in reactor I with a recovery rate of
about 1°C/hr. To avoid biological adaptation
problems, temperatures in reactor I should not be al-
lowed to fall below 25°C (13). The temperature drop
in reactor II is about 4°C to 6°C, with a higher
recovery rate.
The design areas that most affect operating
temperature include the efficiency of the aeration
system, reactor insulation, foam management in the
reactor, and sludge pre-thickening.
Generally, process pH does not have to be control-
led by special design considerations. The ther-
mophilic operating temperatures of the reactors
suppress nitrification in the process. Consequently,
the pH depressions that could occur in a nitrifying
environment are not experienced. With a feed
sludge pH of 6.5, pH values in the first reactor are
typically near 7.2 and may approach 8,0 in the
second reactor.
3.8 Foam Control
The foam layer in the treatment reactor plays an im-
portant role in the ATAD process, though this role
has not been fully evaluated. The foam layer ap-
pears to improve oxygen utilization, enhance the
biological activity, and provide insulation, but it
retards the amount of air entering the reactor. The
amount of foam should be optimized and not
eliminated.
Treatment reactors are sized to accommodate about
0.5 to 1.0m of freeboard, which is partially used as
volume for foam development and control. Control
consists of densifying the foam (i.e., breaking up the
large foam bubbles into small bubbles) to form a
AIR
AIR
Figure 3-2. Examples of Fuchs Aerators Used for ATAD Systems.
-------
Engineering and Design Criteria
NON-CONTACT AIR
FOR MOTOR COOLING
./T/i/Z/2/Z/Z/Z/
REACTOR
Figure 3-3. Example of Fuchs Foam Cutter.
compact layer floating above the liquid surface of
the reactor. In Fuchs systems, foam control is
achieved by mechanical foam cutters installed in the
reactor at fixed elevations. These units cut through
foam that rises above the predetermined level (see
Figure 3-3).
The design basis for foam cutters is determined em-
pirically and is a function of the surface area of the
reactor. All tanks have at least two foam cutter units.
3.9 Post-Thickening/Dewatering
In general, the gravity-thickening performance of the
hot effluent sludge is poor immediately after treat-
ment, due to the thermal convection currents that
occur in a thickening tank. If the sludge is allowed to
cool in the post-thickening/storage tank or additional
heat exchangers exist that cool the sludge down,
thickening performance is usually satisfactory (18,
19). Old Imhoff tanks have been very suitable for
post-thickening. Values of 6-9% TS are typically
achieved. Values up to 10-14% TS and even 18%
TS have often been reported (e.g., from Ellwangen,
Gemmingen, and Kirchberg). High thickened con-
centrations can, in unfavorable cases, lead to
problems with sludge pumping if not considered
during design.
Limited data exist on dewaterability since most
plants land apply sludge as a liquid. However, the
limited data that do exist from the FRG indicate
that dewaterability is generally comparable to
that of anaerobically stabilized sludge (20, 21).
Preliminary Canadian experience confirms
similar dewaterability, but with significantly
higher polymer dosages (16).
-------
Engineering and Design Criteria
Figure 3-4. ATAD System Installed at Gemmingen, FRG.
3.10 Sludge Storage and Disposal
Agricultural use of sludge requires sufficient
capacity to store sludge during those periods when
sludge cannot be land applied. A 3-month capacity
is standard in Switzerland and recommended in Ger-
many. Storage tanks are typically uncovered and
equipped with a mixer. Few problems with odor
nuisance, reactivation of damaged pathogens, or
reinfection during storage have been reported in sys-
tems equipped with storage tank mixing. Aeration
during storage does not appear to be necessary if
the sludge is fully stabilized. Usually, sludge storage
tanks are mechanically mixed once a day for about
an hour.
Odor problems have been experienced either as a
result of ATAD overloading or when storage tanks
remained un-mixed for significant periods. See Sec-
tion 5.6 for a discussion of odor control.
3.11 Construction Features
A complete sludge-handling system may include pre-
thickening, post-storage/thickening, and dewatering.
When utilized with the Fuchs ATAD process, these
process units are typical of and comparable to U.S.
engineering and construction practice. This section
emphasizes other construction features that are uni-
que to ATAD systems.
3.11.1 Tank Construction
The system shown in Figure 3-4 is typical of ATAD
systems installed throughout the FRG. Due to the
reduced residence time requirement (6 d), ATAD
reactors are much smaller than their conventional
counterparts. ATAD reactors are usually cylindrical
and constructed of carbon steel with an interior coal-
tar-based coating. These systems tend to be'
aboveground installations requiring base concrete
slab construction and piping and utility connections.
Concrete tanks are not used for new construction for
cost reasons. One concrete tank retrofit using Fuchs
equipment is known to exist in the FRG. Two retrofit
facilities are operating in Canada using local equip-
ment.
One manufacturer (Limus) has proposed conical bot-
tom areas to avoid grit deposits. However, since
only two plants reported operating trouble as a
result of grit deposits, such measures do not seem
necessary unless the waste sludge contains a sig-
nificant amount of grit.
The reactor must be insulated. ATAD tanks are insu-
lated with mineral wool (~10-cm thick) around the
tank walls and styrene foam material on the cover,
resulting in a typical overall heat transfer coefficient
of 0.3-0.4 W/(rr? °C) (19,22). The entire tank is
covered with a metallic skin (corrugated steel or
aluminum) to protect the insulation and enhance the
appearance of the tanks.
10
-------
Engineering and Design Criteria
With near-surface aerators, such as the Fuchs spiral
ejector or circulation aerators, the filling-depth-to-
diameter (H/D) ratio of the reactor must be relatively
small to ensure complete mixing. The H/D ratio typi-
cally varies between 0.5 and 1. This type of tank is
relatively common in the U.S., since it can be used
for many different applications. Total tank height in-
cludes foam layer thickness, typically 20-30 cm, and
freeboard. The total free space above the filling level
varies between 0.5 and 1 m.
When using jet aerators, diffused air, or submerged
turbine aerators (Roediger, Oswald-Schulze), the
H/D ratio has to be higher than 1. The typical design
range for the H/D ratio is 2-5.
3.11.2 Heat Exchange
In principle, no heat exchangers are needed to
operate an ATAD process. However, economic- of
process-specific reasons could recommend their in-
stallation. Economically, heat exchangers may
reduce the operating costs by utilizing the excess
heat for building heating or other uses. From a
process standpoint, heat exchangers could be re-
quired if the feed sludge has insufficient
biodegradable solids to sustain process tempera-
tures. Due to the lack of data on extended aeration
waste sludge applications, each such application
must be evaluated, including an assessment of the
need for heat recovery or supplementation. Here,
the term "heat exchange" will be limited to heat
recovery. However, the explanations are also valid
for auxiliary heating.
Heat exchange is occasionally used in two basic
ways to recover surplus heat from the exothermic
aerobic digestion process: (1) cooling the effluent,
usually coupled with heating up the influent in
sludge/sludge heat exchangers, and (2) cooling the
reactor, i.e., direct recovery from the process itself
by sludge/water heat exchangers. The latter allows
continuous recovery as well as direct heating of the
process whenever necessary. For this reason ex-
changer designs may involve double-mantle reac-
tors and intermediate heat-exchanging coils.
There are many kinds of sludge/sludge and
sludge/water heat exchangers. The sludge/sludge
exchangers (e.g., two concentric chambers) have
better direct heat transfer but are more difficult to
operate. They are used for heat exchange between
effluent and influent. Conversely, sludge/water type
exchangers are easier to operate but less efficient
due to the double heat transfer via water (warm
sludge/water/cold sludge). They are used for imme-
diate recovery from the tank or, as in pre-stage
plants, for heating the recirculating sludge volume.
According to literature reports, energy equivalent to
15-30 kWh/m3 sludge can be recovered (23,18).
The feasibility for energy recovery strongly depends
on local conditions, such as the heat potential of
sludge, the heat demand of the process itself,
climatic conditions, and the potential proximate uses
for low-temperature heat.
11
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-------
Chapter 4.
Design Example
4.1 Introduction
This example illustrates the design approach for an
ATAD facility. It is specifically suited for the FRG ver-
sion equipped with Fuchs aerator/mixers and foam
cutters. Many of the design concepts would also
apply to other systems. Caution should be used in
extrapolating the aeration power requirements and
air flow rates to other systems since these are
specific to Fuchs equipment.
4.2 Plant Information
Figure 4-1 illustrates the process flow diagram for
the example wastewater treatment plant. Major treat-
ment units include primary clarification, conventional
activated sludge-secondary treatment, gravity thick-
ening, sludge digestion (ATAD), sludge storage, and
land application of treated sludge. Design conditions
for the plant include:
Parameter Design
Average
Flow
BODs
TSS
TVSS
Value
7,570 m3/d
200 mg/L
200 mg/L
150 mg/L
Design
Loading
1,51 4 kg/d
1,51 4 kg/d
1,1 36 kg/d
(2.0 MGD)
(3,330 Ib/d)
(3,330 Ib/d)
(2,500 Ib/d)
The wastewater treatment facility is expected to
meet typical secondary limits for BODs and TSS of
30/30 mg/L.
4.3 Solids Balance
The table in Figure 4-1 summarizes the solids
balance for the overall treatment system.
Sidestream solids quantities have been neglected.
The treatment system configuration will result in two
major sludge streams requiring treatment: primary
sludge and waste-activated sludge.
4.3.1 Primary Solids Removal
Primary clarification is anticipated to remove ap-
proximately 65% of the typical influent suspended
solids. These solids will become the primary sludge,
with an expected solids content of about 50 g/L
(5%). The estimated primary sludge mass and
volume are therefore:
Mass
Influent TSS x 0.65 = primary sludge TSS
1, 51 4 kg/d x 0.65 = 984 kg/d
Volume
(984 kg/d)/(0.05 kg/L)
1,OOOL/m3
_
Of the 530 kg of TSS that remains in the effluent
from the primary clarifiers, 25% is expected to be
nonvolatile (inert) solids and the remaining portion
volatile solids.
Approximately 30% of the influent BODs will also be
removed through primary clarification. The resultant
overflow from the primary clarifiers can be charac-
terized as:
Flow =
BODs =
TSS =
VSS =
NVSS =
7,550.3 m3/d
1,060kg/d
530 kg/d
397.5 kg/d
132.5 kg/d
4.3.2 Secondary Sludge Production
The effluent from the primary clarifiers represents
the loading to the secondary activated sludge
process. Excess sludge production is estimated
using methods generally adopted by the IAWPRC
(24), which are represented by equation 1.
= (NVSS + NBVSS)..+ Xa + Xe (1)
XT
where:
XT = total secondary sludge production
NVSS = nonvolatile suspended solids in feed
13
-------
Design Example
Stream
(1) (2) (3) (4) (5)
Parameter Influent Side- Primary Primary Primary
streams1 Feed Sludge Effluent
Row(m3/d) 7.570 7,570 19.7 7,550.3
BODs 200 200 — 140
(mgfl.)
BODs 1,514 1,514 454 1,060
(kg/d)
TSS(mg/L) 200 200 50,000 70
TSS(kg/d) 1.514 1.514 984 530
VSS(mg/L) 150 150 37,500 52.5
VSS(1
-------
Design Example
where:
fd = degradable fraction of active biomass
For this estimate, the following conditions or as-
sumptions have been adopted:
NBVSS
fd
b
SRT
Y
40% of VSS
= 0.8
0.24 d'1
5d
0.45 VSS/g chemical oxygen
demand (COD)
COD/BOD = 2.0
For sizing purposes, 100% substrate removal has
been assumed, which reduces equation 2 to:
Y(So) (4)
Xa=
(1.0 + bSRT)
Therefore:
NBVSS= 397.5x0.4 =159kg/d
0.45x1,060x2.0
Xa
Xe
[1.0+(0.24x5.0)]
: 433.6 x (1.0 -.8) x
0.24 x 5.0
. 433.6 kg/d
=104kg/d
For design purposes, no credit has been taken for
the loss of suspended solids to the effluent.
Although the facility permit limitation is 30 mg/L, the
facility is expected to operate consistently at lower
levels and occasionally at much lower levels. Ignor-
ing the credit for effluent loss will provide sufficient
capacity during periods of exceptional treatment ef-
ficiency.
Therefore, total secondary sludge production is:
XT= 132.5 + 159 + 433.6 + 104 = 829.1 kg/d
(~ 830)
The volatile portion is:
VSS= 159 + 433.6 + 104 = 696.6 kg/d
(-697)
The nonvolatile (fixed) portion is:
NVSS . = 132.5 kg/d
(~ 133)
This excess sludge will be wasted from the process
at a concentration approximately equivalent to the
secondary clarifier underflow concentration. For es-
timating purposes, this concentration has been as-
sumed to be 9 g/L (0.9%).
The volume of waste-activated sludge, Qw, is:
Qw = (830 kg/d)/(9 kg/m3) = 92.2 m3/d
4.3.3 Digester Feed
Primary sludge and waste secondary sludge are dis-
charged to a gravity thickener. The underflow con-
centration from the thickener is estimated to be 40
g/L (4%). For sizing purposes, 100% solids capture
is assumed in the thickener. Sludge will be pumped
directly from the gravity thickener to the ATAD sys-
tem on a daily basis.
The ATAD feed conditions are:
TSS .= 984 + 830 =1,814 kg/d
VSS = 738 + 697 =1,435 kg/d
NVSS = 246 + 133 = 379 kg/d
Qd
(1,814 kg/d)/(40 g/L) = 45.3 m3/d
Note that the VSS concentration (1,435/45.3 = 31.7
g/L or 3.17%) satisfies the minimum feed require-
ment of 2.5%.
Sludge feed pumping is sized to accomplish sludge
pumping during a 30-minute period. From the above
estimates, the daily sludge volume required to be
pumped is 45.3 irr/d. To deliver the daily volume of
sludge in 30 minutes, the pumping rate is:
Feed pump rate = (45.3 m3/d)/(30 min/d) x
1,OOOL/m3
= 1,500 L/min (400 gpm)
Depending upon site elevations, additional pumping
may be required after ATAD to deliver sludge to a
holding tank. If post-ATAD pumping is required, the
pump sump must be sized to contain the daily feed
volume that will be displaced from the process.
Pump sizing is not dictated by process conditions
and can be sized according to typical methods.
4.3.4 Digestion
The digestion process is expected to attain 35-45%
reduction in overall VSS when treating a mixture
of primary and waste-activated sludge. A 40%
reduction has been assumed for this estimate.
Approximately 60% of the total VSS reduction repor-
15
-------
Design Example
tedly occurs in the first-stage reactor. The resultant
digester performance is:
Reactor I Effluent:
VSS= 1,435 x [1.0 - (0.4 x 0.6)] = 1,090 kg/d
NVSS = 380 kg/d
TSS-1,090 + 380 =1,470 kg/d
Reactor II Effluent:
VSS= 1,435 x [1.0 - 0.4 ] = 860 kg/d
NVSS = 380 kg/d
TSS= 860 + 380 =1,240 kg/d
4.3.5 Sludge Storage and Disposal
Digested sludge will be discharged to a storage tank
that will be equipped with the capability to decant su-
pernatant. Although higher sludge concentrations
can be achieved, this estimate assumes that the
final sludge product, after storage and decant, will
thicken to 60 g/L (6%). Neglecting solids loss with
the decant stream, the final daily sludge volume is:
Qfs= (1,240kg/d)/(60g/L) =20.7m3/d
4.4 Reactor Sizing
Reactor sizing involves a multi-step process that
begins with determining the active reactor volume,
selecting dimensions for circular tanks with a
height/diameter (H/D) ratio of 0.5 to 1.0, and adding
additional height for foam management and
freeboard.
The minimum active volume is based on hydraulic
detention time. A 6-d total system hydraulic deten-
tion time is used as the design basis. With two tanks
operating in series, each will have half the system
detention time (3 d each). Based on the solids
balance determined above, the active volume reac-
tor sizes are:
Actual active volume: 141.3 m
VR=
45.3m3/dx3d =140m3
The typical range of 0.5 to 1.0 for reactor H/D ratio
is considered during reactor sizing. Through trial
and error, the active volume reactor dimensions are
determined to be:
Actual H/D:
0.83
Added height is required for foam management and
freeboard. An additionaM .0 m is added for this pur-
pose. The resultant final theoretical dimensions of
the reactors are:
Reactor dimensions:
6.0-m diameter,
6.0-m height
(The above tank dimensions equate to a 18.8-ft
diameter, 18.8-ft height, which represents odd-size
dimensions for U.S. manufacturers. Adjustments in
dimensions can usually be made to accommodate
standard size tank dimensions. As an example, the
tank diameter can be set at a standard 20 ft and the
operating height determined accordingly [16-ft
operating depth]).
4.5 Process Oxygen Requirement
The process air requirement is determined using
conventional methods for aerobic digestion. How-
even one significant difference in the design ap-
proach is that the oxygen requirement for
nitrification is not included because nitrification is
suppressed at the higher operating temperatures of
thermophilic systems. The process oxygen require-
ment (AOR) is estimated by the following:
AOR = 1.42kgO2/kgVSSd
Where:
VSSd = volatile suspended solids destroyed
Volatile solids destruction is estimated to be 575
kg/d (40%). The process oxygen requirement is:
AOR = 1.42x575
<815kgO2/d
Active volume
dimensions:
6.0-m diameter,
5.0-m height
4.6 Aeration and Mixing Requirements
The aeration and mixing requirements for ATAD sys-
tems are crucial design elements. However, this part
of the design process is the least defined theoretical-
ly and therefore requires further discussion before
proceeding with the design example.
4.6.1 Discussion
A typical procedure for sizing aeration equipment is
to adjust aerator standard transfer rates to field con-
ditions (equation 5) and then to calculate total aera-
tion requirements using the field transfer rate
(equation 6).
16
-------
Design Exampfe
OTRf = a(SOTR) x 1.024(Tf-2°) x (tf 0 wf C*20 -C)
Aip
where:
OTRf
- AOR/OTRf
field oxyg
C*20
ien transfer rate,
(5)
(6)
SOTR =
a
P
C*20
Tf
tf
Wf
Atp
kg O2/kWh
standard oxygen transfer rate,
kg O2/kWh
oxygen transfer coefficient
oxygen saturation coefficient
equilibrium dissolved oxygen
concentration at 20°C, mg/L
operating dissolved oxygen
concentration, mg/L
field temperature, °C
temperature correction factor
(Csat*/Cs20*)
barometric pressure correction
total aeration power required, kWh/d
Using the above method requires some knowledge
of specific conditions, such as alpha (a) and beta
( P), that are relevant for ATAD applications.
Although these coefficients have been measured in
wastewater environments, no coefficients are known
to exist that are specific to ATAD systems. As can
be seen from equation 5, the alpha value is an im-
portant coefficient that materially affects the calcu-
lated OTRf value. Accurate estimates of oxygen
transfer require representative alpha coefficients.
As an example, for a 5-m liquid depth, 0*20 would
be about 10.4 mg/L. On the other hand, CSAT*
varies as a function of temperature (5.93 mg/L at
45°C; 5.05 mg/L at 55°C). Under these conditions
the field oxygen transfer rate from equation 5 is
OTRf
Temperature (°C) 0 mg/L DO 1 mg/L DO*
45 1.18 a p SOTR 0.98 a SOTR
55 1.27 ap SOTR 1.03 a SOTR
* assumes |3 = 0.98
According to this table, the decrease in oxygen
transfer that would be expected at higher tempera-
tures because of decreased oxygen solubility is
offset by the increase in the term 1.024 (Tf-20).
Hence a is the key coefficient when translating
clean water test performance into field transfer ef-
ficiencies in the ATAD environment.
An additional complicating condition, although a
positive one, relates to observations by various re-
searchers that oxygen utilization efficiency appears
to be significantly enhanced in the ATAD environ-
ment (5,25, 26). This enhancement is believed, in
part, to be related to the presence and extent of
foam in the reactors. The foam itself can have a
high oxygen demand, probably as a result of
entrained biology (26). Air entering into the foam
layer from the liquid phase may provide significant
oxygen transfer into the overlaying foam. This addi-
tional transfer would not be included with traditional
computational procedures (equation 5).
This observation indicates that the efficiency of aera-
tion is inextricably linked with the process operating
conditions and that typical methods for estimating
field efficiency from standard conditions cannot be
used with confidence until process factors have
been fully defined and quantified. These and other
factors are the subjects of ongoing research in the
FRG that will aid in developing the theoretical
design basis for ATAD aeration and mixing.
Although not all mechanisms that may be contribut-
ing to oxygen transfer are understood, ATAD sys-
tems are being routinely designed using empirical
methods developed from extensive pilot- and full-
scale operations. This approach is illustrated in the
following example.
4.6.2 Empirical Aeration/Mixing Sizing
As a result of full-scale research at Gemmingen
(20), direct estimates of field oxygen transfer rates
were made in ATAD systems equipped with Fuchs
aerators. Values ranged from 1.5 to 3.7 kg Oa/kWh.
The manufacturer utilizes 2.1 kg Oa/kWh for design
purposes.
Using the manufacturer's design value assuming a
peaking factor of 1.5 (see Section 5.3), the aeration
requirement is determined by:
815kgO2/dx1.5
24 hr/d
51 kg O2/h
2.1 kg O2/kWh
= 51 kg O2/hr
= 24.3 kWh
(32.6 hp)
17
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Design Example
A minimum of two foam cutters is required for
each ATAD reactor and, depending on reactor
geometry, as many as three can be installed.
Recently designed facilities with a reactor diameter
less than or equal to 6 m have been equipped with
two units, each rated at 0.75 kW. Reactor sizes
greater than 6 m have been equipped with three
units at 0.75 kW. For this example, a 6-m diameter
reactor size has been selected. Therefore, each
reactor will be equipped with two units rated at 0.75
kW each.
Mixing and energy requirements are checked to
determine that they are within design ranges given
below:
Specific power:
Throughput energy:
85-105 WArf of
active volume
9-15kWh/m3of sludge
throughput
Theoretical specific power requirement:
Aeration: 24.300 W =87W/m3
(140m x2reactors)
Foam Cutter: 3,000 W
(140 m3x 2 reactors)
10.7W/m3
Total
= 97,7 W/m3
Theoretical throughput energy requirement:
27.3kWx23.5hr/d ^OI1AIU, 3
=14.2kWh/rrr
45.3 m3/d
(Note that 23.5 hr/d is used in the above calculation
because the aeration equipment is turned off for one-
half hour per day during feeding.)
The above values do fall within the typical design
ranges.
4.7 Sludge Storage
Sludge storage requirements will vary for each
application. This estimate assumes that the final
sludge will be land applied as a liquid and that, due
to frozen or snow-covered conditions, 90 consecu-
tive days of storage will be required. Note that more
northern climates (e.g., New England, Canada) may
consider additional storage capacity to suit local
conditions. Other factors, such as warmer climates,
rainfall, crop rotation, and seeding/harvesting
schedules, may impact the total storage volume
required for a specific site.
The required storage volume is:
Vs= 20.7 m3/d x 90 d =1,863 m3
The storage tank must be equipped with a mixer
and the ability to decant supernatant.
4.7.1 Mixing
The sludge storage tank must be mixed for about an
hour each day to limit the potential for storage odors
and reactivation of pathogens. Submerged mechani-
cal mixers are typically used for this purpose. The
procedure for mixer sizing is typical of mixer sizing
procedures for other sludge storage applications.
However, the potential exists to thicken beyond the
6% solids concentration used for the volume es-
timates. Mixer sizing should be capable of handling
about 12% solids in the storage tank.
4.8 Cost Estimate1
For planning purposes, a cost estimate is presented
here using the data summarized in Chapter 8. The
cost data are presented on the basis of a sludge
throughput of 1,814 kg/d (see Section 4.3.3). Figure
8-1 is a plot of cost data summarized in Table 8-3
and includes estimated costs for a prethickener,
ATAD system, and post-storage tank (20-d storage
basis). The projected cost for 1,800 kg/d sludge
throughput is $480,000, of which $50,000 is for a
sludge storage tank with a 20-d capacity. The
scaled up storage tank cost (for 90-d storage
capacity) is $225,000, resulting in a total system
cost of $655,000. The actual ATAD component cost
is $330,000, or about one-half of the total system
cost.
1The cost estimate summarized here is included as an example for planning purposes. Each facility will have
site-specific factors to take into account in assessing system cost, including site conditions, disposal limitations,
climate, and availability of existing equipment.
18
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Chapter 5.
Performance Data
5.1 Introduction
The proliferation of ATAD systems throughout the
FRG has given researchers the opportunity to study
and collect operating data on full-scale operating
systems over the last decade. The system located in
Gemmingen was studied extensively by Breiten-
bucher, resulting in a comprehensive summary of
performance levels (5). Other studies have focused
on particular issues related to ATAD, such as
pathogen destruction and oxygen transfer efficiency
(25, 27). In the United Kingdom, studies at the
Water Research Centre have led to commercial ap-
plications of ATAD technology utilizing venturi aera-
tion (26). Four full-scale facilities are located in
Canada, three of which have been studied under the
sponsorship of the Environment Canada's Waste-
water Technology Centre. These comprehensive
studies include data and experience concerning the
retrofit of mesophilic aerobic digesters to achieve
autothermal thermophilic conditions (16). In the
U.S., several laboratory and pilot studies have been
conducted.
This chapter discusses and summarizes perfor-
mance data and experience related to several fac-
tors that characterize digestion systems in general
and ATAD systems in particular.
5.2 Volatile Solids Reduction
ATAD, like other digestion systems, is intended to
reduce the total mass of sludge requiring disposal
and to produce a stabilized final product suitable for
disposal or reuse. Digestion processes reduce
sludge mass by destroying some of the volatile
suspended solids. The percent VSS reduction
achievable by digestion varies depending on the
feed sludge make-up and operating conditions in the
overall treatment plant. For example, VSS reduction
can be influenced by process residence time, operat-
ing temperature, VSS concentration of raw sludge,
and VSS mass loading. Long-term controlled
studies by Breitenbucher indicate that VSS removal
efficiency is strongly related to both reactor deten-
tion time and temperature (5).
Data on VSS reductions obtained from nine ATAD
facilities in the FRG are summarized in Table 5-1.
The data reflect a range of HRTs (5-12 d) and raw
sludge feed concentrations. These values are in
agreement with minimum VSS removal efficiencies
of 25-35% (27) and 35-45% (28, 29), and COD
removal efficiencies of 40-45% (30) for ATAD sys-
tems with HRTs of 6 or more days. Generally, VSS
reduction efficiencies for ATAD systems are com-
parable to those of other forms of digestion.
Changing operational modes as well as seasonal
influences make it difficult to define precise design
and/or operational relationships for some plants. For
example, the loading of the Kirchberg plant fluc-
tuates between 350 and 900 kg VSS/d, with reduc-
tion efficiencies between 66% and 43%. This is also
the case in Gemmingen, where a variation in load-
ing from 130 to 420 kg VSS/d leads to varying reduc-
tion efficiencies between 30% and 45%.
Additionally, the Gemmingen facility treats only
waste-activated sludge, whereas the Kirchberg
facility treats a combination of primary, trickling filter,
and waste-activated sludges.
Data from the Tonisberg plant show how strongly
sludge degradability affects the average degradation
efficiency. Because the plant had unused free
capacity, it was fed with sludge from other plants.
Table 5-1. Summary of VSS Reduction Observed at
Nine ATAD Facilities Where Data Were Available
Facility
VSS Reduction (%)
Backnang
Gemmingen1
Kirchberg
Neckarwestheim
Nette2
Rheinhausen3
Romersberg
Tonisberg4
Vilsbiburg
48
25
43
35
30
• 44
41
33
41
...
-40
-66
-56
-44
1 Waste-activated sludge only.
Industrial waste contribution - 50%.
3During a 14-week test period.
4During 1- and 2-stage operation.
19
-------
Performance Data
After extraneous sludge was introduced, the
average degradation efficiency dropped from 56% to
33%. This drop occurred even though the operation-
al mode was changed from one to two stages during
the same period, which should have compensated
for the increased loading and allowed the plant to
sustain a similar performance level. Even with the
decreased VSS reduction, the second-stage
temperatures were still > 50°C.
5.3 Pathogen Reduction
Several studies have been performed at pilot- and
full-scale plants with different substrates and various
indicator organisms. Table 5-2 summarizes results
from selected published studies and the respective
conditions that have been identified as necessary to
achieve disinfection. These data indicate that a
properly operated ATAD process can achieve the
FRG requirements for the disinfection of sewage
sludge or manure (see Appendix for a discussion of
the FRG regulations). (Chapter 6 discusses the
ability of ATAD to meet proposed U.S. standards for
land application of sludge.)
The actual disinfection mechanisms are still largely
unclear. Various organisms are probably destroyed
by different, partly synergistic, combinations of
mechanisms. For example, Ascaris egg cytoplasm
is destroyed by temperatures > 50°C (31). Reduc-
Tabte 5-2. Investigations of the Influence of Time, Temperature, and pH on Disinfection at Pilot- and Full-Scale
ATAD Plants in the FRG
tion of enterovirus appears to require a combination
of temperature and basic pH. Martin (32) has deter-
mined a strong relationship between temperature
conditions and enterovirus reduction. Other data indi-
cate that the chemical or biochemical reactions in-
itiated by increasing pH values (e.g., release of
virucide ammonia) are important factors in
enteroviruses destruction (33). Jakob et al. (34)
have proposed that disinfection may also occur
through an inhibiting ("antibiotic") effect of microbial
metabolism products; Strauch (35) reports that this
has been proven experimentally while treating pig
manure. Still other phenomena that have not been
quantified are believed to participate in disinfection
(36). During investigations in Gemmingen, or-
ganisms in closed ampules that had been fed to the
reactor and that were not in direct contact with the
substrate survived much longer than equivalent free-
swimming organisms (see Table 5-3).
Tables 5-3 to 5-6 summarize the results of disin-
fection studies for various organisms at four full-
scale plants: Gemmingen, Kirchberg, Ellwangen,
and Nette. Since these are exclusively Fuchs plants,
all conclusions derived from these studies are valid
only for this type of plant. The investigations were
carried out under operational conditions. At Ellwan-
gen (Table 5-5) the temperature in reactor II was
maintained by a heat exchanger within a narrow
Authors
(33, 37)
(38, 37)
(39, 37)
(40, 37)
Scale
pilot
pilot
pilot
pilot
Investigated
Pathogens
bovine enterovirus
rhinovirus, foot-and-mouth
disease
pseudorabies virus
reovirus
Inactivation
Substrate
pig manure
pig manure
pig manure
pig manure
pH
6.5
8.0
8.0
9.0
°C
55
50
40
48
t(hr)
48
48
50
78
enterovirus
adenovirus
(37) pilot swine vesicular disease
(41) pilot Salmonella
Salmonella
(31,41) pilot Salmonella
Ascaris eggs
(36,42) full Salmonella,
enterobacteriaceae
pig manure
pig manure
cattle manure
sewage
sludge
sewage
sludge
9.0
9.0
8.0
8.5
8.7
8.7
8.5
7.5
8.0
48
48
45
40
45
40
45
50
50
48
87
48
10
48
48
10
57
48
20
-------
Performance Data
Table 5-3. Pathogen Reduction at the Gemmingen Plant (4,18,19,49)
Parameter2
Temperature (°C)
Influent
Rlmin
Rl max
Rll min
Rll max
PH3
Influent
Rl
Rll
Coliform (ml/1)
Influent
Rl
Rll
Salmonella4
S. senftenberg
Rl
A '
K
Rll
A
K
S.typhimurium
Rl
A
K
Rll
A
K
Contact Time (hr)
Rl
Rll
HRT(days)
A
10
36
45
46
47
6.4
7.9
8.3
2.7E+06
6.1E-1-03
9.4E+04
+
-t-
+
+
. +
+
+
+
96
96
8
B
13
34
39
45
47
6.6
7.8
8.2
1.6E+06
2.3E+04
2.4E+04
+
+
• +
+
+
+
+
+
96
96
8
C
15
45
53
53
53
6.2
7.6
8.4
3.1E+07
• -
-
+
+
-
-
+
-
-_, .
-
-
96
96
8
D
16
44
52
55
58
6.5
8
8.4
9.7E+04
-
-
+
+
-
-
+
-
-
-
96
96
8
E F G
17 16 18
49 48 48
62 61 63
63
65
6.2 6.3 6.3
8 8 8.2
8.4
1.3E+06 1.6E+06 8.3E+04
.
-
.
+
-
-
-
-
-
-
96 96 48
96 96 10
884
H 1 J
16 12 9
57 39 35
67 46 43
52 48
59 54
6.2 6.4 6.3
8.2 7.6 7.7
8.5 8.2
1.1E+05 2.3E+06 3.4E+06
-
-
-
-
-
-
-
• -
24
48
666
Notes:
1A-H test series performed with coliform organisms in a closed container, I-J series with coliforms in direct contact with substrate.
2RI = Reactor I; Rll = Reactor II; HRT = hydraulic residence time.
3pH values are average over test period.
4A = Salmonella, in a closed container; no substrate contact.
K = Salmonella in contact with substrate. " ~~,
+ = detected.
- = not detected.
range between 47.5°C and 50°C. These relatively
low temperatures reduced all tested pathogens as
effectively as the higher temperatures in the other
plants.
Most indicator organisms are significantly reduced in
the first reactor. An exception is Salmonella, which
is only reliably eliminated during the second stage
(34,35). This observation supports the two-stage
design concept.
Figures 5-1 and 5-2 show the pathogen reduction
performance at two operating facilities: Kirchberg
and Ellwangen. At Kirchberg, a 4.5-log reduction in
21
-------
Performance Data
10
10
10
101J
RAW
ATAD
TOTAL
COLIFORM
PATHOGEN REDUCTION
KIRCHBERG
sss
RAW
/X/x
ATAD
\
RAW
ATAD
ENTERO- FECAL
BACTERIACEAE STREPTOCOCCI
Figure 5-1. Pathogen Reduction: Plant at Kirchberg (34,35).
10'
106-d
10^
104,
io3:
io2-:
10N
10V
,o-
RAW
PATHOGEN REDUCTION
ELLWANGEN
ATAD
RAW
ATAD
RAW
ATAD
RAW
ATAD
X
N/
TOTAL ENTERO- FECAL ENTEROVIRUSES
COLIFORM BACTERIACEAE STREPTOCOCCI
Figure 5-2. Pathogen Reduction: Plant at Ellwangen (34,35).
22
-------
Performance Data
enterobacteria and a 3.0-log reduction in fecal strep-
tococci were observed at an average temperature of
53°C and an 8-d residence time. At Ellwangen, a
5.5-log reduction in enterobacteria, a 3.5-log reduc-
tion in fecal streptococci, and 1.0- to 3.5-log reduc-
tion in enterovirus were observed at an average
temperature of 49°C and a residence time of 5 d.
Salmonella and Ascaris (not shown) were not
detected at either facility in the treated sludges.
5.4 Dissolved Oxygen Concentrations
and Respiration Activity
Dissolved oxygen measurements in some ATAD
studies have varied from 0.7 mg/L to more than 3
mg/L for about 80% of the reactor height (5). Other
recent measurements in a single-stage ATAD sys-
tem have shown negligible dissolved concentrations
ranging between 0 to 0.2 mg/L (25). These recent
measurements suggest that oxygen concentration
can be a limiting factor for a portion of the batch
cycle after new sludge feed has been introduced
and is not uniform throughout.
Figure 5-3 illustrates the course of the respiration
rate in an experimental ATAD system over several
days. These measurements were taken from a
single-stage ATAD plant with the following basic
design features: reactor volume of 27 m3, two spiral
aerators (2.2 and 4.0 kWh), specific power input 230
W/m3 (note: the specific power value of 230 W/m3
for this experimental observation period is over
twice the normal design power input for Fuchs full-
scale systems), specific air input 6.9 (m3/hr air)/m3,
feed solids concentration between 6 and 6.5% and
reactor solids between 4 and 4.5% (33).
Figure 5-3 indicates the cyclic nature of the specific
oxygen uptake rate (SOUR), which is consistent
with the batch feeding strategy. The SOUR values
observed during this study averaged 287 (g O2/d)/kg
VSS [11.96 (mg Oa/hr)/g VSS], which is equivalent
to an oxygen uptake rate of 476 (mg/L)/hr. The maxi-
mum and minimum values were about 140% and
35%, respectively, of the average value.
5.5 Oxygen Transfer Efficiency (OTE)
Earlier evaluations of ATAD processes that used air
aeration emphasized the need for an efficient aera-
tion system to limit the heat loss through the ex-
haust of latent heat from the reactor. This factor was
considered crucial for the process to achieve the
autoheated state. Several attempts at developing
ATAD systems have been affected by factors re-
lated to aeration and mixing.
The first known full-scale ATAD system using air
aeration was developed by Fuchs. So many of these
systems are now operating successfully that there is
a general belief that aeration and mixing in an ATAD
system must be efficient and effective to support the
500
O)
=* 400
1
O) 300
200
100
ui
EC
r
A.
05-JUN
07-JUN
09-JUN
11-JUN
13-JUN
15-JUN 16-JUN
18-JUN
20-JUN
Figure 5-3. Repiration Rate of Mixed Liquor in a Single-Stage ATAD System During Operation (25).
23
-------
Performance Data
Table 5-4. Pathogen Reduction
Parameter
Residence time (d)
Temperature (*C)
No. of samples
pH - value
Total bacteria count (mL~1)
Enterobacteriaceae (ml_~1)
Fecal streptococci (mL~1)
>
Salmonella (% positive)
NA = not available.
NO = not detected.
av. = average.
at the Kirchberg Plant (34, 35)
min
max
av.
min
max
av.
min
max
av.
min
max
av.
min
max
av.
Raw
NA
NA
NA
8
5.9
6.3
6.1
1.7 107
4.4 109
7.3 108
1.0 105
1.8 107
4.8 106
2.1 105
2.6 106
6.3 105
100
ATAD
5
44 __
60 .
53
8
8.6
8.9
8.8
5.8 104
3.3 108
4.7 107
NA
3.4 102
8.6 101
4.010°
1.0 103
2.2 102
ND
process. This section describes the present level of
understanding concerning these factors.
The Fuchs aeration devices are best described as
aspirator-type aerators (see Figure 3-2). In a typical
ATAD system, aerators are installed in ports through
the reactor side walls. An additional aerator, central-
ly located in the reactor, may also be required
depending on the diameter of the tank. An average
of 2.1 kg Oz/kWh is used by the manufacturer to es-
timate the oxygen transfer capacity in the sludge en-
vironment. However, sizing is normally based on
mixing energy requirements.
Breitenbucher observed an apparent relationship
between the oxygen utilization efficiency (analogous
to oxygen transfer efficiency), reactor solids con-
centrations, and the presence or absence of foam in
the reactor. Specifically, he indicated that oxygen
utilization efficiency increases with increased solids
concentrations (to about 5% TS) and that it also in-
creases with the presence of a foam layer in the
reactor (43). Later studies by Wolenski and Bruce
(with an ATAD system using an exterior venturi for
oxygenation), confirmed that the foam layer that
developed in their reactor enhances oxygen utiliza-
tion efficiency (26). The foam layer was shown to
24
-------
Performance Data
100 -i
90 -
80 -
70 -
60 -
^ 50 -
°E
•— ' 40 -
'55
30 -
20
10 -
0 -
14:
24
.
^^
19:12
^\
00:00
Time (h)
^^
^^
04:40
PHASE I
PHASE II
ui
-------
Performance Data
100 •
90 •
80 •
70 -
60 -
i— i
gs OU -
111
I— Jir\
b 40 "
30 -
20 -
10 -
14:24
/-"
/
-- — "
_^- —
- '
•
—
IU
i
0.
->
H asvHd i
S
I PHASE III
>
ut
tn
<
a.
M
\J
19:12 00:00 04:40 09:36
Time (h)
14:24
Figure 5-5. Daily Variation of OTE in an ATAD Plant (25). Phase Ml: Stopping, Cleaning, Restarting Aerator 1.
Phase III-IV: Stopping, Cleaning, .Restarting Aerator 2.
are not adequately defined, which explains the em-
pirical approach to aeration and mixing design.
5.6 Odor Control
ATAD odors are characterized mainly as humus-
like, with some gaseous ammonia. At the Nette
plant in the FRG, triethylamine was also detected.
Ammonia is released by thermophilic aerobic
degradation during digestion and cannot be
avoided. Ammonia is not subsequently nitrified due
to the suppression of nitrification at thermophilic
temperatures. Depending on the pH of the reactor,
ammonia can be stripped into the exhaust. ATAD
systems typically exhibit an elevated pH, particularly
in the second-stage reactor (5), which enhances the
stripping potential for ammonia.
Many of the early facilities did not include provisions
for odor control. Exhaust air was generally charac-
terized as having a humus-like odor that did not re-
quire treatment. With an increasing number of
facilities located in nonrural areas, provisions for
odor control have become routine.
Sufficient odor control can usually be accomplished
by returning the exhaust air to the activated sludge
aeration tank or into trickling filters. Odor control
devices are used for situations requiring long pipe
runs or more precise control.
Seven plants, representing about 16% of all ATAD
plants, reported odor problems, and seven plants
reported using control devices. These two groups of
seven facilities are not identical, i.e., not all plants
with problems have control devices and vice versa.
Site visits indicated that most odors were short-term
events that occurred when raw sludge was pumped
into the reactor and when raw sludge odors were
stripped out into the exhaust. One plant reported
odors when the temperature in the second-stage
reactor approached 70°C. Another experienced
odor problems (along with the entire treatment plant)
during an organic overload condition (50% above
design) that resulted during peak vineyard process-
ing periods. This plant was not equipped with odor
controls.
Common odor control devices are biowashers and
biofilters. Not enough experience has been gained
26
-------
Performance Data
EXHAUST
AIR IN
I(11( It
11111 11
x A A x A /
-
x X x x" KXX X ;
V. .V. x/... ^ ' ^1 .X^L ^S'^J
X >
sx sx
ttttt ttttttt ttttttt tt
PRE-WASHING DEVICE
FAN
LATTICE GRATING
CONDENSATE
Figure 5-6. Biofliter.
with either device to compare their performance.
The biowashers are designed as tower-like counter-
current washers filled with filter elements and
equipped with a water recirculation system. The con-
densate is removed from the air by a drop
separator. At the Nette plant in the FRG, a system
has been installed for a maximum of 2,000 m3/hr ex-
haust air with a 2 x 30 m2 filter area. No odor
problems have been reported for this facility (23).
The biofilter (see Figure 5-6) consists of a simple
tank containing a filterbed of biologically active
material (compost, bark, or similar material). Odors
are removed by adsorption and digestion. The filter
bed should have a moisture content between 40%
and 60%.
A water scrubber precedes the actual filter to pre-
humidify the air to a moisture level of about 95%. Ad-
ditional parameters for biofilter design are hydraulic
filter load (m3 air/(m2 filter hr), void volume (< 80%),
reaction time (> 50 s), specific bipmass load, VSS in
the odor control device, respiration activity, and pH
(25). No odor-related problems have been reported
at the Walheim facility in the FRG, which uses a
biofilter-type system for odor control.
5.7 Operation and Maintenance
All ATAD facilities in the FRG are reportedly easy to
operate and require very little process control and
maintenance. In most cases, process control con-
sists of performing periodic suspended solids and
pH analyses, monitoring reactor temperatures, and
controlling the pumping of specific volumes of
sludge to the ATAD reactors on a batch basis. The
normal control parameter for sludge volume
management is the filling level.
Most operators of ATAD plants prefer to run their
plants manually or semi-automatically, even if they
are equipped with the necessary equipment for full-
automatic control. "Semi-automatic" operation
means that the program for withdrawal and filling is
started manually.
Aeration control can be optimized through the use of
Programmable Logic Controllers (PLCs) or timers.
Automatic aeration control could provide important
energy savings, even with simple timer-controlled
strategies such as running on full speed for 4 to 5 hr
after batch filling followed by a downshift to half
speed.
At least three FRG facilities have been equipped
with two-speed drive motors for the aerators be-
cause of the extremely variable seasonal loading. It
is not known if the operators have attempted to op-
timize aeration on a daily basis. The use of respira-
tion activity as the control variable has recently been
proposed as a more direct control concept for the
process (25).
Maintenance requirements are reported to be low—
from 2 to 6 hr/week (excluding sampling or
analysis). The most frequently reported main-
tenance requirement is flushing the aerator hollow
shafts, which requires a few minutes for each
aerator. One plant reports the practice of flushing
the shafts twice per day, while others report twice
per week requirements. Flushing of the aerator hol-
low shafts is an important practice that must be con-
ducted on a scheduled basis. The importance of the
27
-------
Performance Data
Table 5-5. Pathogen Reduction at the Ellwangen Plant (34, 35)1
Parameter
Residence time (d)
Temperature (*C)
No. of samples
Total bacteria count (mL"1)
Enterobacteriaceae (mL"1)
Enteroviruses (mL"1)
Fecal streptococci
Salmonella (% positive)
NA = not available.
NO = not detected.
1Seconcl-stage reactor cooled to
min
max
av.
min
max
av.
min
max
av.
min
max
av.
min
max
av.
50°C
Period 1
Raw
NA
NA
NA
9
4.0 106
1.8 107
1.1 107
4.0 105
9.0 106
6.3 106
6.0 102
6.6 103
6.3 103
4.8 104
1.3105
8.1 104
78
during test period.
ATAD
5
48.5
50
49
9
3.3 105
1.21 107
1.5 107
ND
200
33
ND
660
330
ND
150
39
ND
Period 2
Raw
NA
NA
NA
10
3.6 106
3.6 107
1.6 107
1.1 105
6.6 106
1.9 106
4.6 103
1.4 104
1.1 104
3.3 104
2.1 105
8.7 104
90
ATAD
5
43
5
49
10
7.0 105
5.3 107
1.2 107
ND
300
77
ND
ND
ND
ND
490
140
ND
flushing is emphasized by the findings illustrated in
Figure 5-4: Significant air volume reductions occur
under fouling conditions.
One Canadian facility experienced erosion of
aerator impellers after a 1 -yr period due to ac-
cumulation of grit in the sludge (16).
28
-------
Performance Data
Table 5-6. Pathogen Reduction at the Nette Plant (23)
Parameter
Raw
Residence time (d)
No. of samples
Temperature (°C)
pH - value
Enterobacteriaceae
(ml.-1)
Salmonella (% positive)
100
NA = not available.
ND = not detected.
ATAD
min
max
av.
min
max
av.
min
max
av.
9
NA
NA
NA
NA
NA
NA
1.0 106
1.0 107
8.0 106
46
66
59
7.3
8.7
8.2
100
1.0 104
1.2103
ND
29
-------
-------
Chapter 6.
Ability to Meet U.S. Regulatory Standards
6.1 Introduction
Regulations governing the land application of sludge
have been in force since 1979 (16); new regulations
were proposed in 1989 (46). This chapter describes
the current and proposed regulations, and presents
data indicating that the ATAD process can be
operated to meet the most stringent U.S. regulatory
requirements. ATAD sludges that meet the require-
ments can be applied to agricultural and forage land
without access and use restriction.
As of publication, the new regulations were
scheduled for promulgation in October 1991. The
final regulations almost certainly will differ somewhat
from the proposed regulations discussed here. The
reader should obtain a copy of the most recent
proposed or final regulations in order to fully assess
the operational requirements for ATAD to meet the
future U.S. regulations for land application of
municipal sludge.
6.2 Current U.S. Federal Regulations
(as Of 1990)
U.S. federal regulations governing land applica-
tion of municipal wastewater sludge are currently
defined in 40 CFR Part 257 Criteria for Classifica-
tion of Solid Waste Disposal Facilities and Prac-
tices. These are technology-based regulations
that specify acceptable technologies for treating
sludge. Other technologies can be used if they
can be demonstrated to provide the same level of
treatment.
The 40 CFR Part 257 regulations require that waste-
water sludge be treated prior to land application to
reduce pathogen levels and to reduce its attractive-
ness to disease vectors (flies, mosquitos, rodents,
etc.). The regulations list acceptable treatment tech-
nologies and divide them into two categories based
on the level of pathogen control they can achieve:
processes to further reduce pathogens' (PFRPs)
and processes to significantly reduce pathogens
(PSRPs) (see Tables 6-1 and 6-2).
6.2.1 Processes to Further Reduce Pathogens
PFRPs are processes capable of reducing
pathogens to below detectable levels. No site
management restrictions regarding biological and
microbiological contaminants apply to land applica-
tion of sludges treated by a PFRP. Thermophilic
aerobic digestion is listed as a PFRP at a residence
time of 10 d and a temperature of 55°C to 60°C (see
Table 6-1), on the assumption that a single tank is
being operated with some interior baffeling to
prevent short circuiting. However, the ATAD •
process, as designed by Fuchs and operated in Ger-
many, has two tanks and generally has a residence
time of 5 to 6 d at a temperature of 55°C. Because
of its lower residence time, ATAD is considered an
unlisted process. The EPA's Pathogen Equivalency
Committee (PEC) has developed performance
criteria concerning reduction of pathogens and vec-
tor attraction to judge whether an unlisted technol-
ogy is equivalent to PFRP (47). These criteria vary
depending on the type of process and the type of
sludge. For the ATAD process, the relevant criteria
are:
1. Pathogen Reduction
100 mL treated sludge at 5% solids must
contain:1
< 3 MPN Salmonella spp.
< 1 PFU total enteroviruses
< 1 viable Ascaris spp. ovum
Studies'discussed in Section 6.3 indicate that
enteroviruses are more resistant to the ATAD
process than are Salmonella spp. or Ascaris ova.
Therefore, according to the criteria set forth by the
PEC, PFRP equivalency can be demonstrated simp-
ly by showing that total enteroviruses are not detec-
table in 100 mL treated sludge at 5% solids. It is not
necessary to test for the other two organisms.
Anyone applying for PFRP equivalency must
demonstrate that the organisms tested for are in fact
present in the untreated sludge at certain levels:
1,000 MPN Salmonella spp./g TSS, 1,000 PFU total
enteroviruses/g TSS, and 100 viable Ascaris sppJg
Indicator organisms may be substituted in some
cases. See (47) for further discussion.
31
-------
Ability to Meet U.S. Regulatory Standards
TSS. If the untreated sludge does not contain the
test organisms at these levels, then it must be
spiked to those levels to demonstrate the reduction.
2. Reduction of Vector Attraction
To be equivalent to a PRFP, the ATAD process
must achieve:
Table 6-1. Regulatory Definition of Processes
to Further Reduce Pathogens (PFRPs)
Composting: Using the within-vessel composting
method, the solid waste is maintained at a temperature of
55'C or greater for 3 d. Using the static aerated pile
composting method, the solid waste is maintained at a
temperature of 55*C or greater for 3 d. Using the windrow
composting method, the solid waste attains a temperature
of 55'C or greater and maintains it for at least 15 d during
the composting period. Also, during the high temperature
period, there is a minimum of five turnings of the windrow.
Heat Drying: Dewatered sludge cake is dried by direct or
indirect contact with hot gases, and moisture content is
reduced to 10% or lower. Sludge particles reach
temperatures well in excess of 80'C, or the wet bulb
temperature of the gas stream in contact with the sludge
at the point where it leaves the dryer is in excess of 80'C.
Heat Treatment: Liquid sludge is heated to a
temperature of 180*C for 30 minutes.
Thermopnillc Aerobic Digestion: Liquid sludge is
agitated with air or oxygen to maintain aerobic conditions
at a residence time of 10 d and a temperature of 55'C to
60"C, with a volatile solids reduction of at least 38%.
Other Methods: Other methods or operating conditions
may be acceptable if pathogens and vector attraction of
the waste (volatile solids) are reduced to an extent
equivalent to the reduction achieved by any of the above
methods. Any of the processes listed below, if added to a
PSRP, further reduce pathogens.
Beta Ray Irradiation: Sludge is irradiated with beta rays
from an accelerator at dosages of at least 1.0 megarad at
room temperature (ca. 20"C).
Gamma Ray Irradiation: Sludge is irradiated with
gamma rays from certain isotopes, such as 60Cobalt and
Cesium, at dosages of at least 1.0 megarad at room
temperature (ca. 20'C).
Pasteurization: Sludge is maintained for at least 30
minutes at a minimum temperature of 70'C.
Other Methods: Other methods or operating conditions
may be acceptable if pathogens are reduced to an extent
equivalent to the reduction achieved by any of the above
add-on methods.
Source: 40 CFR 257, Appendix II.
EITHER:
A 38% reduction in VSS in the treated sludge
compared to the untreated sludge
OR:
A specific oxygen uptake rate (SOUR) of less
than 1/mg Oa/hr/g TSS at 20°C in the treated
sludge
OR:
A total suspended solids concentration in the
treated sludge of 75% or greater (which must be
maintained until land application)
6.2.2 Processes to Significantly Reduce
Pathogens
PSRPs effectively reduce (but do not eliminate)
pathogenic viruses and bacteria; they are less effec-
tive in reducing helminth eggs. This level of
pathogen reduction — comparable to that which can
be achieved with a well-run anaerobic digester — is
currently the minimum requirement if sludge is to be
applied to land (see Table 6-2). Since PSRPs
reduce but do not eliminate pathogens, the regula-
tions require site management practices to reduce
the potential for human contact with the sludge.
Specifically, the regulations control public access,
the growing of human food crops, and grazing by
livestock at sites where PSRP-treated sludges have
been applied.
To be classified as a PSRP, ATAD must achieve the
following two criteria (57):
1. Pathogen Reduction
EITHER:
A 2-logio reduction (i.e., the difference between
the treated and untreated sludge) in either:
—fecal conforms and fecal streptococci or
—fecal coliforms and enterococci
OR:
An average log density (no/g TSS) of less than
6.0 for fecal conforms and fecal streptococci in
the treated sludge
2. Reduction of Vector Attraction
EITHER:
A 38% reduction in VSS
OR:
A SOUR of less than 1 mg Oa/hr/g TSS at 20°C
32
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Ability to Meet U.S. Regulatory Standards
6.3 Proposed U.S. Regulations
In February 1989, the EPA proposed new standards
(Subpart F, Part 503 Standards for Disposal of
Sewage Sludge — see Reference [46]), which are
intended to replace all aspects of the existing Part
257 regulations except for co-landfilling of municipal
sludge and solid waste. These proposed regulations
are performance-based and would require routine
sampling and analysis.
The proposed standards for pathogen management
are classified into three categories (A, B, and C).
Class A standards correspond with PFRP perfor-
mance levels. Class B standards correspond with
PSRP performance levels. Class C standards cor-
respond with PSRP levels established for treatment
systems without primary .clarification and with a long
mean cell residence time. These standards are sum-
marized as follows:
Class A - Pathogen Reduction Requirements
To achieve Class A, one of the following two groups
of standards must be met:
Group 1
—Pathogen densities in the treated sludge must
be:
< 3 Salmonella spp./g VSS
< 1 plaque-forming virus unit/g VSS
< 1 protozoan organism/g VSS
< 1 helminth egg/g VSS
Group 2
—One of these three time-temperature
conditions must be met:
tmin>120hr@530C
tmin>72hr@55°C
train > 0.5 hr @ 70°C
—And densities of indicator organisms in the
treated sludge must be:
< 2 logio fecal coliform/g VSS and
< 2 logio fecal streptococci (enterococci)/g VSS
Class B - Pathogen Reduction Requirements
To achieve Class B, one of the following two groups
of standards must be met:
Table 6-2. Regulatory Definition of Processes to
Significantly Reduce Pathogens (PSRPs)
Aerobic Digestion: The process is conducted by
agitating sludge with air or oxygen to maintain aerobic
conditions at residence times and temperatures,
, respectively, ranging from 60 d at 15'C to 40 d at 20°C,
with a volatile solids reduction of at least 38%.
Air Drying: Liquid sludge is allowed to drain and/or dry
on underdrained sand beds, or on paved or unpaved
basins in which the sludge depth is a maximum of 9
inches. A minimum of 3 months is needed, for 2 months
of which temperatures average on a daily basis above O'C.
Anaerobic Digestion: The process is conducted in the
absence of air at residence times and temperatures,
respectively, ranging from 60 d at 20'C to 15 d at 35*C to
55'C, with a volatile solids reduction of at least 38%.
Composting: Using the within-vessel, static aerated pile,
or windrow composting methods, the solid waste is
maintained at minimum operating conditions of 40'C for
5 d. For 4 hr during this period the temperature exceeds
55°C.
Lime Stabilization: Sufficient lime is added to produce a
pH of 12 after 2 hr of contact.
Other Methods: Other methods or operating conditions
may be acceptable if pathogens and vector attraction of
the waste are reduced to an extent equivalent to the
reduction achieved by any of the above methods.
Source: 40 CFR 257, Appendix II.
Group 1
—2 logio reduction of Salmonella spp./g
wastewater plant influent VSS
—2 logio reduction of viruses/g wastewater plant
influent VSS
Group 21
—Densities of indicator organisms in the treated
sludge must be:
1This group applies only if the influent to the treat-
ment plant is processed by a physical or biological
method and when the sewage sludge from those
methods is treated in a physical, biological (e.g.,
ATAD), or chemical additional method, or is stored
for at least one day.
33
-------
Ability to Meet U.S. Regulatory Standards
£ 6 logio fecal coliform/g VSS and
£ 6 logio fecal streptococci (enterococci)/g VSS
Class C - Pathogen Reduction Requirements
To achieve Class C, one of the following two groups
of standards must be met:
Group 1
—1.5 logio reduction of Salmonella spp./g
wastewater plant influent VSS
—1.5 logio reduction of viruses/g wastewater
plant influent VSS
Group 2
—Densities of indicator organisms in the treated
sludge must be:
^ 6.3 logio fecal coliform/g of VSS and
^ 6.7 logio fecal streptococci (enterococci)/g VSS
Class B and C Use and Access Restrictions
Use and access restrictions apply for Class B and
Class C sludges. These requirements restrict crops,
foraging, and public access. Class C restrictions are
more stringent than Class B due to the lower level of
pathogen reduction (see reference [48] for more
detailed information on operational restrictions).
Class A, B, and C Vector Attraction Reduction Re-
quirements
All three classes of treated sludge must also
demonstrate reduced potential for vector attraction
by meeting at least one of several additional criteria.
For aerobic processes, these are:
• A 38% VS reduction
• A SOUR <, 1 mg Og/hr/g TS
• TSS > 75% prior to mixing
Sewage sludge is subsurface-injected with
no evidence of the sewage sludge on the
land surface 1 hr after injection
6.4 Ability of ATAD to Meet the
Proposed U.S. Standards for
Pathogen Reduction
Most of the data concerning ATAD performance
come from the FRG. There the process has been
shown to meet the FRG criteria for agricultural use
of sludge, described in the Appendix. The perfor-
mance criteria and goals for land-applied sludge in
the U.S. differ from those in the FRG. Figure 6-1 il-
lustrates the time/temperature functions proposed
by each country. The U.S. proposed 503 criteria
generally require a significantly longer detention
time (almost seven times as long at 55°C) than the
proposed FRG criteria. The differences between the
two diminish with increasing temperature. The time-
temperature functions eventually converge at
temperatures above 65°C. An important difference
between the two criteria is that the FRG time-
temperature function specifies time during isolated
conditions, whereas the proposed U.S. time-
temperature function does not include this distinc-
tion.
FRG and U.S. studies generally differ in terms of
time, temperature, isolation of reaction conditions,
and indicator organisms studied. However, where
possible, organism-specific performance data from
FRG studies have been compared to the existing
U.S. 257 regulations and the proposed 503 regula-
tions. In addition, data from U.S. and Canadian
studies are useful in evaluating ATAD performance.
Studies of ATAD-treated sludges in the FRG, U.S.,
and Canada indicate that ATAD can reduce fecal
coliform, Salmonella and Ascaris ova to nondetec-
table densities (5, 23,35,16, 32). One FRG study
reported that enterovirus was generally reduced to
nondetectable densities at 50°C (35). Recent U.S.
studies observed enterovirus densities to be < 100
PFU/g TSS in most cases within a relatively low
temperature range (29.0°C to 39.8°C) and with a
system residence time between 10 and 20 d (32).
The study noted a much stronger relationship bet-
ween enterovirus reduction and temperature than
between enterovirus reduction and residence time,
which suggests that the higher temperature and
lower residence times characteristic of the FRG sys-
tems could yield a similar enterovirus reduction per-
formance.
Fecal streptococci reduction was significant but vari-
able in several studies where temperatures ex-
ceeded 50°C (34, 35). Reductions to nondetectable
densities were observed during some tests; how-
ever, densities were also observed to exceed 100 .
CFU/g TSS under similar detention time and
temperature conditions. The reason for this
variability is not .known. .
In summary, under appropriate operating conditions,
ATAD systems can reduce Salmonella and Ascaris
ova to nondetectable levels; thus, the process
meets the current criteria for PFRP equivalency and
the proposed Class A Group 1 standards. Additional
34
-------
Ability to Meet U.S. Regulatory Standards
1000 -q
100-
f/\
rv
~~>
o
^ 10--
n _
LJ
2
1—
1-
_
•i
REGULATORY TIME-TEMPERATURE FUNCTIONS
i
i^.
^
FRf
k
- — ^»c-
--n
3 - ATAD
v
\
\
X ""
\-<
\
•^(-1
TJ
^v
X.
X
""s,
'
EPA
CLA
\
\
/
503
SS A
-.
— -¥-
:RG - PR
' *-^-
ESTAGE
— JK^-
_^r-
^-^_
^
• in
50 53 56 59 62 65 68 71 74 77 80
TEMPERATURE (C)
Figure 6-1. Regulatory Time-Temperature Functions for Pathogen Destruction.
the proposed Class A Group 1 standards. Additional
data on enterovirus reduction at temperatures repre-
sentative of operating ATAD systems (55°C) are re-
quired to confirm the ability of ATAD systems to
meet the current PFRP equivalency requirement to
reduce enterovirus to nondetectable levels and the
U.S. Class A requirement to reduce enterovirus to
<1 PFU/g VSS. No data are currently available on
the ability of ATAD to reduce protozoan organisms
to the levels required under the proposed 503
regulations. Data indicate that ATAD reduces fecal
coliform to below the proposed limits required for
PSRP equivalency and under Class A, B, and C
Group 2 standards. The ability of ATAD to consis-
tently meet either the current PSRP requirements or
the proposed Group 2 requirements for fecal strep-
tococci reduction is less certain. However, there is
no need to demonstrate PSRP equivalency if the
process already achieves the more stringent PFRP
equivalency status, and there is no need to meet
Group 2 requirements if Group 1 requirements can
be met for Classes A, B, and C. Thus, based on the
temperatures reached and the available pathogen
data; ATAD systems can apparently meet the exist-
ing and proposed U.S. requirements for pathogen
destruction.
6.5 Ability of ATAD to Meet the
Proposed U.S. Standards for
Reduction of Vector Attraction
The current PSRP and PFRP equivalency criteria
and the proposed 503 standards for reduction of vec-
tor attraction are almost identical. The major dif-
ference is that, under the proposed 503 standards,
reduction can be achieved by injecting sludge below
the soil surface. Both the current and proposed
regulations accept a suitably dried sludge (75%
35
-------
Ability to Meet U.S. Regulatory Standards
TSS) as satisfactorily reducing vector attraction.
Drying is an additional treatment process that would
follow ATAD and therefore is not discussed here. A
reduction in the SOUR is also accepted as satisfying
the vector attraction reduction requirement. Insuffi-
cient data are available at this time to comment on
ATAD's ability to meet this requirement. At present,
therefore, the option for demonstrating reduction of
vector attraction that is most generally relevant to
the ATAD process is reduction of volatile solids.
Limited data are available on the ability of ATAD to
reduce VSS. The literature shows minimum removal
efficiencies of 25-35% (27), 35-45% (28,29) (based
on VSS), and 40-45% ([30] based on COD) with a
minimum detention time of 6 d.
Table 5-1 in Chapters summarizes VSS reduction
data for nine plants in West Germany. These nine ATAD
plants reduced VSS by 29-56%. This relatively wide
range suggests that degradation is influenced by
design differences such as the wastewater charac-
teristics, the presence or absence of primary
clarification, and F/M conditions on various
mainstream biological processes, each of which
would impact the nature of the sludge fed to the
digestion process. These sludges had low heavy
metals load, indicating that industrial influences
are not responsible for the differences. The exist-
ing PFRP and PSRP and the proposed Class A,
B, and C standards require at least a 38% reduc-
tion in volatile solids. Most of the nine plants in
Table 5-1 can meet this requirement. Thus, the
data indicate that ATAD systems, if properly
operated, can achieve the necessary reduction.
36
-------
Chapter 7.
Case Studies
This chapter summarizes available data on FRG
two-stage facilities. Data were obtained through con-
tact with system manufacturers, engineers, owners,
operators, and researchers; site visits; and publish-
ed sources.
7.1 General Description
Table 7-1 summarizes selected design data and
identifies the system manufacturer for 35 existing
ATAD plants in FRG. Reference lists from manufac-
turers only provide data either on the basis of sludge
throughput or population equivalent (P.E.). Where
necessary in Table 7-1, the facility P.E. and sludge
throughput quantity have been calculated using
design values typical for FRG (49): 1.87 L/P.E./d of
sludge at 4.5% TS (primary + secondary sludge) for
plants employing primary clarification, and 2.07
L/P.E./d of sludge at 1.5% TS (secondary sludge)
for plants without primary clarification. The calcu-
lated values in Table 7-1 are marked with an
asterisk (*). Table 7-2 similarly summarizes operat-
ing conditions for the same facilities.
Figure 7-1 shows the location of ATAD facilities in
the FRG. The installation numbers are keyed to
Table 7-1. Most ATAD plants are located in central
and southern West Germany. The southern plants
are situated almost exclusively in regions where
sludge can be utilized for agriculture. However, not
a single plant is located in Schleswig-Holstein, a
state in northern Germany that also has a mostly
agricultural economic structure and a comparatively
low population density.
Over 85% of ATAD systems are supplied by Fuchs.
Thieme and Babcock have supplied systems in the
past but seem to be no longer active in the market.
The Babcock plant is a single-stage system and
thus does not correspond to current FRG criteria
that recommend at least two-stage operation. The
Thieme system, which uses a rotary screen to
prethicken sludge to 10% VSS concentrations, has
been sold by the Rapiddrain Company, Emmendin-
gen. Except for the two nonmunicipal Thieme sys-
tems listed in Table 7-1, no other Thieme
installations could be found.
7.2 Overview of Existing Facility
Applications
Table 7-1 shows the growing importance of ATAD
plants in the German market during the last 15
years. The ATAD technology is considered to be
state of the art, as evidenced by the proliferation of
facilities in the past decade. Thirteen years ago, no
full-scale plants existed; today there are over 35 full-
scale operating facilities.
ATAD plants were installed in facilities ranging in
capacity between 4,000 P.E. (700 m3/d; 0.18 MGD)
and 80,000 P.E. (14,005 m3/d; 3.7 MGD). More than
60% are in plants with capacities less than 15,000
P.E. (2,650 m3/d; 0.7 MGD). Over 25% of all plants
have a daily sludge throughput of about 20 m /d,
corresponding to an installation size of about 10,000
P.E.
FRG cost evaluations indicate that ATAD is cost-ef-
fective over a range of 5,000 to 100,000 P.E. with
an optimal range between 10,000 and 50,000 (19,
50). Actual installations generally fall within the
recommended range. These cost comparisons were
based on digestion performance and did not con-
sider the value attributed to enhanced pathogen
reduction.
In 1988, about 86% of operating ATAD facilities ap-
plied the stabilized sludge to agricultural land, 7%
utilized landfilling exclusively, and 7% alternated
landfilling with agricultural disposal. The large num-
ber of facilities that reported the practice of land ap-
plying sludge is consistent with the distribution
shown in Figure 7-1.
7.3 Highlighted Facilities
Three facilities are highlighted here to provide ex-
amples of current ATAD technology: the Fassberg
and Gemmingen facilities in the FRG and the Sal-
mon Arm facility in Canada. There is no special sig-
nificance to the selection of these facilities. Detailed
information on a wider range of facilities is available
in several sources (51, 52,16).
37
-------
Case Studies
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38
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Case Studies
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39
-------
Case Studies
Figure 7-1. ATAD Plants In FRG. Installation Numbers Are Keyed to Table 7-1.
7.3.1 Fassberg
Fassberg exemplifies an ATAD installation that is
equipped with a heat exchanger for heat recovery
and that practices co-removal of primary and waste-,
activated sludge in the same unit process: a flota-
tion/sedimentation unit.
Fassberg is located in north-central FRG in an
agricultural area north of Hannover. The ATAD sys-
tem was commissioned in 1983 and was the first
system in the FRG to use a heat exchanger for heat
recovery. The treatment plant serves two com-
munities (Fassberg and Munden) and receives no in-
40
-------
Case Studies
Figure 7-2. ATAD System Installed at Fassberg, FRG.
dustrial waste. Sludge from the facility is land ap-
plied for agricultural utilization.
7.3.1.1 General Facility Design Basis
The design conditions for the overall treatment sys-
tem include:
Population
Flow
BOD
TSS
22,000
6,000 m3/d
1,320 kg/d
1,760kg/d*
*Estimated using FRG standard 80 g/day/P.E.
The^major unit processes utilized at the plant in-
clude:
• Mechanical screening
• Primary sludge removal - flotation/sedimenta-
tion with co-removal of waste-activated solids
• Activated sludge - conventional complete mix
• Final polishing lagoons
• Autothermal thermophilic aerobic digestion
Heat exchange - energy recovery for building
heat
Sludge storage (liquid)
7.3.1.2 A TAD
Figure 7-2 illustrates the ATAD facilities at Fassberg.
Design conditions include:
Feed Conditions
• Sludge feed source:
• Feed strategy:
• Feed volume, m3/d:
• Feed TSS, %:
• Feed mass loading, kg/d:
Reactors
• Number:
• Materials of construction:
Primary + waste-
activated
Batch, once/day
40
4
1,600
2 (w/bituminous
base coating)
Steel
41
-------
Case Studies
• Dimensions:
• Active volume, m3 ea.:
Equipment
Aerators
• Number of side-mounted
per reactor:
• Number of center-mounted
per reactor:
• Connected power
per reactor, kW:
Foam Cutters
• Number per reactor:
• Power rating, kW:
Performance Factors
• Total detention time, days:
• Batch isolation time, hours:
• Volumetric loading rate,
(m3 sludge/d)/m3:
• Mass loading rate,
(kgsludge/d)/m3:
• Power density,
W/m3 active volume:
• Feed temperature, °C:
• Reactor I temperature, °C:
• Reactor II temperature, °C:
Sludge Holding Tank
• Volume, m3:
• Holding time, days:
7.0-m diam.,
3.2-m SWD
120
2
1
2@3.4+1@2.2
2
0.5
6
23
0.17
6.66
83.3
6-8
45
55
620
56
The Fassberg ATAD facility has been in operation
since 1983. Site visits have confirmed reactor II
temperatures at or above 55°C under autothermal
conditions. The facility is not equipped with an odor
control system, and no complaints have been noted.
Operating personnel have reported that the system
requires minimal operation and maintenance. The
hollow aerator shafts occasionally became plugged
due to rags passing through the plant screening
facilities.
7.3.2 Gemmingen
Gemmingen is one of the earliest full-scale ATAD
systems treating municipal sludge. Located in
southern Germany in an agricultural area, the facility
has been operational since 1980. It is one of the
smallest facilities, serving an equivalent population
of 4,000. A significant amount of research has been
conducted there.
The facility design is basic and not as sophisticated
as later designs. Nonetheless, the facility
demonstrates the fundamental operating charac-
teristics for ATAD.
7.3.2.1 General Facility Design Basis
The design conditions for the overall treatment sys-
tem include:
Population
Flow
BOD
TSS
4,000
1,600m3/d
290 kg/d
320 kg/d*
*Estimated using FRG standard 80 g/d/P.E.
The major unit processes utilized at the plant in-
clude:
• Storm water retention
• Screening and grit removal
• Activated sludge - oxidation ditches
• Sludge thickening
• Autothermal thermophilic aerobic digestion
• Sludge storage (liquid)
7.3.2.2 ATAD
Figure 7-3 illustrates the ATAD facilities at Gemmin-
gen. Design conditions include:
Feed Conditions
• Sludge feed source:
Waste-
activated
42
-------
Case Studies
gL.^* t.
-*-& f
f* A^,
^W^^Wir'jf
inns
MP ! ! ,,fai ,. 4
A
Mm j ;
i*c •'
•i K» •«
|i '!;;;r * 1
i\'
^ *f
_,uJJ!
4-
Figure 7-3. ATAD System Installed at Gemmingen, FRG.
• Feed strategy:
• Feed volume, m3/d:
• FeedTSS.%:
• Feed mass loading, kg/d:
Reactors
• Number:
• Materials of construction:
• Dimensions:
• Active volume, m3 ea.:
Equipment
Aerators
• Number per reactor:
• Connected power
per reactor, kW:
Batch,
once/day
8
5
400
Steel (coated)
3.5-m diam.,
2.5-m SWD
24
1@2.5
Foam Cutters
• Number per reactor:
• Power rating, kW:
Performance Factors
• Total detention time, days:
• Batch isolation time, hours:
• Volumetric loading rate,
(m3 sludge/d)/m3:
• Mass loading rate,
(kg sludge/d)/m3:
• Power density, W/m3:
• Feed temperature, °C:
• Reactor I temperature, °C:
• Reactor II temperature, °C:
Sludge Holding Tank
• Volume, m3:
• Holding time, days:
2
0.3
6
23
0.17
8.33
129
6-14
45
55
280
35
43
-------
Case Studies
40
§ 30
£g 20
10
70
60
0 50
ul
EC
m 30
Q-
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10
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9
X 8
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6
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9 12 d 15
TIME(d)
Figure 7-4. Data from the Gemmingen ATAD Facility (5).
This facility was studied extensively and docu-
mented in a doctoral thesis on aerobic thermophilic
digestion (5). Figure 7-4 illustrates typical results of
various operating parameters during these studies.
The system achieved approximately 33% reduction
in VSS for the waste-activated sludge feed source
during the study period. A "sawtooth" temperature
curve is evident in both reactors and is the result of
batch reactor feeding. During treatment, the pH in-
creased from approximately 6.5 to 8.0.
Since the study, the Gemmingen facility changed its
operation to feed the ATAD system once a week.
This results in a complete displacement of the ATAD
reactors on a weekly basis, which effectively con-
stitutes a weekly restart of the process. Limited
operating data are available to characterize this
mode of operation. Both ATAD reactors achieve
temperatures over55°C. Odors are experienced
during start-up.
7.3.3 Salmon Arm
Salmon Arm is an example of a retrofitted ATAD sys-
tem and is one of two such facilities studied in
Canada. The facility utilizes rectangular, concrete
reactors and a locally supplied aeration system for
the ATAD system. The plant is equipped with trick-
ling filters operated with both fixed and suspended
growth for biological phosphorus removal. As is typi-
cal for many trickling filter systems, secondary
waste solids are returned to the primary clarifierfor
co-settling and removal.
Salmon Arm is located in British Columbia, Canada,
and serves an equivalent population of 6,000. The
facility partitioned and insulated an existing
mesophilic digester for ATAD operation.
7.3.3.1 General Facility Design Bas/s
The design conditions for the overall treatment sys-
tem include:
Population
6,000
44
-------
Case Studies
Flow
BOD
TSS
2,300 m-
425 kg/d
400 kg/d
The major unit processes at the plant include:
• Primary clarification
• Trickling filters
• Autothermal thermophilic aerobic digestion
• Sludge storage (liquid)
7.3.3.2 ATAD
Figure 7-5 illustrates the ATAD facilities at Salmon
Arm. Design conditions include:
Feed Conditions
• Sludge feed source:
• Feed strategy:
• Feed volume, m3/d:
• Feed TSS, %:
• Feed mass loading, kg/d:
Primary +
trickling filter
Batch,
once/day
8.8-14.6
1.2-9.3
178-1,362
| SCRE
S>n
&
en
U
WBS
T" PRIMARY
1 TANK
8
ATAD
t
'
*u{
HMp)
J
*-
f
I
T
—- —
td
•=•
ss
1
1
®/Oi
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1
A
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1 FA '
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1
^
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xz
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ER
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ai
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-/ p
FA
CS
LPA
FB
TA
DS
SS
TU
TMP
ORP
/T S T
i \J— -/ F
FA OPTIONAL
STORAGE
TANK
f
""=F"
Tlfl
DS ^^
LPA
LEGEND
AIR
DENSITY PROBE
SCREENED RAW SEWAGE
WASTE BIOLOGICAL SOUDS
PUMP
FOUL AIR
CRUDE SLUDGE
LOW PRESSURE AIR
FOAU BREAKER
TURBORATOR
DIGESTED SLUDGE
SETTLED SEWAGE
TIMER
TEMPERATURE PROBE
OJtP. PROBE
RJCKUNG \
1LTER ^X
T
-| STORAGE ^>
Figure 7-5. ATAD Facility at Salmon Arm, Canada (16).
45
-------
Case Studies
Reactors
• Number:
• Materials of construction:
• Dimensions, m:
• Active volume, m3 ea:
Equipment
Aerators
• Number per reactor:
2
Concrete
(insulated)
2.59W, 3.96L,
3.2 SWD
32.82
1
• Volumetric loading rate,
(m3 sludge/d)/m3:
• Mass loading rate,
(kg sludge/d)/m3:
• Power density, W/m3
(aeration only):
• Feed temperature, °C:
• Reactor I temperature, °C:
• Reactor II temperature, °C:
Sludge Holding Tank
• Volume, m3:
• Holding time, days:
0.27 - 0.44
5.42-41.5
174
8-21
46-66
52-66
unspecified
unspecified
Connected power
per reactor, kW: 1 @ 5.7
Foam Cutters
• Number per reactor: 1
• Power rating, kW: 0.75
Performance Factors
• Total detention time, days: 4.5-7.5
• Batch isolation time, hours: unspecified
This plant was studied extensively under the spon-
sorship of Environment Canada, British Columbia
Ministry of the Environment, District of Salmon Arm
and Dayton & Knight Ltd. Detailed information is
available in reference (16).
Thermophilic operating temperatures were achieved
and sustained in the process as illustrated in Figure
7-6. Volatile solids reduction ranged from a high of
55% to a low of 12% experienced during low deten-
tion time (washout) conditions. Although variable,
significant reductions in pathogens were ex-
.perienced at detention times at or greater than 5.3 d.
46
-------
Case Studies
a.
80 C
70 C
60 C
SO C
a 4oc
30 C
20 C
10 C
0 C
TEMPERATURE IN REACTORS
AT SALMON ARM. MARCH 89 - DECEMBER 89
CHECK VALVE THIN SLUD
STUCK AND FEED
WASHOUT
OCCURRED
1—7.1
•4.&-
TC-m. HYDRAULIC DETENTION TIME IN DAYS
I MARCH I APRIL I MAY I JUNE I JULY I AUG 1 SEPT I OCT ! NOV I DEC I
REACTOR *1
REACTOR *2
RAW SLUDGE
• ELECTRICAL FALURE AND SUBSEQUENT WASHOUT DUE TO PUMP PROGRAM ERRORS
•» DIGESTER CLEANING
•*» BEARING FAILURE ON TURBORATOR
Figure 7-6. Temperatures in Reactors at Salmon Arm (16).
47
-------
-------
Chapter 8.
Costs
8.1 Introduction
The AT AD cost information presented in this chapter
comes from two sources: system suppliers and
literature sources. Since literature sources do not
always identify the facilities to which the cost data
apply, these sources have been evaluated and
presented separately to avoid double counting of
facilities that may be represented in both data
bases. The cost information summarized here
applies only to ATAD systems.
The majority of components required for an ATAD
system (similar to the Fuchs example) are available
locally in the U.S. These would include reactors, val-
ves, interconnect piping, controls and instrumenta-
tion. Unique requirements include the aerator/mixer
apparatus and foam cutters that have been es-
timated to be about 25% of the ATAD systems cost.
8.2 Source Cost Data
The ATAD systems marketed in the FRG are sup-
plied as package systems and, therefore, the cost
information provided is limited to the extent of the
supplied equipment. Typically, the manufacturers
provided information on total package costs. Oc-
casionally, more detailed cost information was made
available. Fuchs and Thieme were the only manufac-
turers that provided cost information.
To provide a basis for comparing system costs,
Table 8-1 lists items that are or are not included
in the manufacturer's package cost. Table 8-2
summarizes cost data from the facility engineers or
manufacturers of the ATAD system. For comparison
purposes, these costs have been converted and
adjusted to 1990 U.S. dollars (January 90 ENR-CCI
= 4701). Unit costs, i.e., $/kg of sludge throughput,
are also presented.
Cost estimates have been prepared for unit proces-
ses (pre- and post-thickener) that were not typically
included in the manufacturer's cost information.
These estimated costs are presented with the ATAD
system costs in Table 8-3 as a total cost for the
sludge,handling system. Total system unit costs are
also presented. Figure 8-1 illustrates the ATAD
sludge-handling system cost data as a function of
designed sludge throughput.
Table 8-1. ATAD System Component Comparison
Component Number
Pre-ATAD thickener 1
ATAD digestion 2
reactors
Aerators
Foam cutters
Interconnect piping
Locally mounted
electrical controls
Exhaust blower4
Aeration blower
Sludge transfer
pump
Sludge feed pump
Post-ATAD
thickener/holding
tank
Concrete slab for
reactors
Included in
Fuchs Cost
No
Yes; covered
steel tanks
with insulation
Yes
Yes
Yes
Yes
Yes
N/A
N/A
No
No
Yes
Included in
Thieme Cost
Yes1
Yes; enclosed
FRP tanks with
insulation2
Yes
N/A3
Yes
Yes
N/A
Yes
Yes
No
No
No
1 Pre-thickener is actually a flocculation/straining system referred
to as the "Rapid Drain" system.
2 Thieme reactors are reported to be one-third of Fuchs reactor
volume due to increased feed concentration.
3 Thieme does not use a foam cutter. Foam overflows aeration
inductor cone and is educted into aerator.
4 Exhaust blower is used to carry off gas to odor control process.
Annual operating costs have been estimated for
nine Fuchs systems. Labor requirements were
reported by the facility operators to be low and did
not appear to vary significantly based upon the size
of the facility. Labor requirements were estimated at
1.5 hr/d as shown in Table 8-4, based on discus-
sions with operating personnel and review of the
facilities.
49
-------
Costs
Table 8-2. ATAD Capital Cost Summary1
Facility Population
Equivalent
Isenbuttel
Schontal
Fassberg
Nettetal-Viersen
Gemmingen
Vitsbiburg
Meinersen/Leiferde
Walheim
Trebur
Neckarwestheim
Wesendorf
Lachendorf
Rheinhausen8
Romersberg8
12,000 '
12,500
22,000
80,000
6,000
25,000
4,000
5,000
8,000
10,000
12,000
15,000
5,000
20,000
Cost (DM)
242,000
130,000
366,000
450.0007
166,000
285,000
338.00010
362,500
410.50010
350,500
393.00010
601.30010
280,000
450,000
Cost Year/
(ENR-CCI)3
1982(3721)
1979 (2886)
1981 (3384)
1983 (4006)
1980(3159)
1977(2513)
1989 (4568)
1989 (4568)
1989 (4568)
1989 (4568)
1989 (4568)
1990(4701)
1984(4118)
1984(4118)
1990
Cost ($)2'4
191,000
132,000
318.0006
330,000
154,000
333,000
217,000
233,000
264,000
225,000
253,000
376,000
200,000
321,000
MARCH 1989
Through- Cost
put(kg/d) ($/kg)5
800
1,000
1,600
2,540
425
700
450
400
1000
1050
800
1350
291 9
8999
239
132
199
130
362
476
482
582
.264
214
316
278
687
357
1 Based on data received from system engineers or manufacturers. Costs for pre- or post-thickening and storage not included in Fuchs cost
data. Fuchs costs confirmed as installed costs. Post-thickening and storage not included in Thieme costs.
2 Assumes an exchange rate of 1.60 DM/$.
3 Engineering News Record-Construction Cost Index, for March of indicated year.
4 ENR-CCI =4701 (March 1990).
5 Nonamortized unit costs based on annual throughput capacity.
8 Includes cost of heat recovery equipment.
7 Cost does not Include tankage. This project was a retrofit into existing tankage.
8 Industrial applications.
9 These facilities loaded three times per week. -Weekly loading rates have been divided by 7 to develop a daily loading rate for
comparison purposes.
10
'Includes "biofilter" odor control system cost.
Table 8-5 includes a summary (translated from early
1980s [5]) of costs determined for the Gemmingen
facility during the period of study (early 1980s).
These costs (1980 base) compare favorably with the
estimated costs in Table 8-2.
8.3 Literature Source Cost Data
Capital cost data have been derived from several
sources (25,53,54) and updated to 1989 basis. Fig-
ure 8-2 illustrates the ATAD capital costs as a func-
tion of installed reactor volume.
50
-------
Costs
CO
T5
C
O
w
D
O
JC
O
O
600
540
480
420
360
300
240
180
120
SYSTEM COST
TOTAL
SYSTEM
THICKENER
0
I I I Tn^ TIII
300 600 900 1200 1500 1800 2100 2400 2700 3000
SLUDGE THROUGHPUT, kg/Day
Figure 8-1. ATAD Sludge Handling Systems Estimated Capital Costs.
51
-------
Costs
7QQ
C TfO
630 -
c cr\
560
ro
o AQn
x 49°"
vr A on
43 42°"
1)
Q TCCO
Q 350 -
-H
C oon
u 280 -
"t« O -1 n
w 210-
>
4 A Hi
1 40 -
~7r\
/U
&
^
o
o
0 /
SQ
0
o
s
^
o
/
s
*r^ *•
^
s> ^
^
o
s*^>
,
•
0 70 140 210 280 350 420 490 560 630 700
Reqctor Volume, [m3]
Figure 8-2. ATAD System Capital Costs (22,53,54).
52
-------
Costs
Table 8-3. Estimated Sludge Handling Capital Cost
Facility Throughput
(kg/d)
Isenbiittel5
Schontal
Fassberg5
Nettetal-
Viersen5
Gemmingen
Vilsbiburg
Meinersen/
Leiferde
Walheim
Trebyr
Neckar-
westheim
Wesendorf
Lachendorf
Rhein-
hausen6
Romersberg6
800
1,000
1,600
2,540
425
700
450
400
1,000
1,050
800
1,350
291
899
ATAD Cost
($)1
199,000
132,000
318,000
330,000
154,000
332,000
217,000
233,000
264,000
225,000
253,000
376,000
200,000
321,000
Pre-
Thickener
Estimated
Cost($)2
71,000
76,000
105,000
161,000
71,000
71,000
71,000
65,000
76,000
76,000
71,000
95,000
N/A
N/A
Post-
Thickener
Storage
Cbst($)3
33,000
36,000
47,000
62,000
27,000
32,000
28,000
27,000
36,000
36,000
33,000
42,000
29,000
39,000
Total ($) Unit Cost
($/kg)4
295,000
244,000
470,000
553,000
252,000
436,000
316,000
325,000
376,000
337,000
357,000
513,000
229,000
360,000
369
'244
218
218
593
623
702
813
376
321
446
380
787
400
1 ATAD costs from Table 8-2.
2 Cost estimated using MCD-53 constructioacost curve, adjusted, to ENR 4701.
3 Cost estimated assuming the following condition: 40% VSS destruction. 20-day storage volume, 5% TSS concentration for Fuchs, 9%
TSS concentration for Thieme, circular tank design (1-ft-thick concrete side walls, 1.2-ft-thick slab, 15-ft tank height), $342 per cubic yard
(installed) concrete cost. Cost includes $4,600 for pump station to discharge final product to vehicles.
4 Nonamortized unit costs based on annual throughput capacity.
5 These facilities do not include dedicated pre-thickening units, but are costed for example and comparison purposes.
6 Cost does not include tankage for the ATAD system.
53
-------
Costs
Table 8-4. Estimated Annual Operating Costs for Nine Fuchs Systems in the FRG1
Facility Through- Labor ($)2
put (kg/d)
Isenbuttel
Fassberg
Nettetal-
Viersen
Gemrningen
Vilsbiburg
Schontal
Walheim
Neckar-
westheim
800
1,000
2,540
425
700
1,000
400
1,050
5,500
5,500
5,500
5,500
5,500
5,500
5,500
5,500
Power ($)3
9,500
19,200
38,600
5,100
13,800
—
4,500
to 8,200
14,500
Miscel-
laneous
Parts and
Supplies
($)
600
1,200
2,200
2,800
900
—
600
1,700
Total ($)
15,600
25,900
46,300
13,400
20,200
—
10,600
to 14,300
21 ,700
Unit Cost
($/kg)4
19.5
16.2
18.2
31.5
28.9
26.5
to 35.8
20.7
21.4
1 Cost does not include disposal.
2 Labor cost estimated at 1.5 hr/d labor cost at $7.5/hr.
3 Design power consumption (calculated) with power cost at 6.5 cents/kWh. Power cost estimate is based on aeration and foam cutter
power requirements.
4Nonamortized unit costs based on annual throughput capacity.
54
-------
Costs
Table 8-5. Summary of Costs for the Gemmingen ATAD System1
Cost Item
Cost-1980 DM
($)2
Remarks
Capital Cost
- 2-24 m3 reactors
- Installation
TOTAL INSTALLED
Operating Cost
- Power
- Personnel
- Upkeep
TOTAL OPERATING
133,000 DM (48,900)
62,700 DM (23,050)
195,700 DM (71,950)
6,132 DM/yr (2,250)
3,650 DM/yr (1,340)
2,584 DM/yr ( 950)
12,366 DM/yr (4,540)
Mechanical-electrical (concrete slab not
included in cost). Pre- and post-thickeners
not included.
112 kWh/d x 0.15 DM/kWh; 0.5 k/g
x 20 DM/hr; 1% structures; 2 %
mechanical/electrical
Fixed Cost
- Depreciation
- Interest
TOTAL FIXED
TOTAL ANNUAL COST
Cost Factors
- DM/Pop. eq.
- DM/m3 of sludge/yr
-DM/Pop. eq.
- DM/m3 of sludge
- DM/Pop. eq.
- DM/m3 of sludge
- DM/I TS (metric)
9,709 DM/yr (3,570)
9,785 DM/yr (3,600)
19;494DM/yr(7,170)
31 ,860 DM/yr (11, 710)
28.40 DM (10.44)
67.02 DM (24.64)
1.79 DM/yr (0.66)
4.23 DM/yr (1.56)
4.62/yr(1.70)
10.91/yr(4.01)
1 81. 85/yr (66.86)
4% structures, 7% mechanical/electrical;
i = 10%
Capital cost - 6,900 P.E.
Capital cost - 8 m3/day
Operating cost
Operating cost
Total annual cost
Total annual cost
Total annual cost (480 kg/day)
" translated from Reference [5].
2Assumes an exchange rate of 2.72 DM/$.
55
-------
-------
Chapter 9.
Thermophilic Pre-Stage Process
9.1 Introduction
Pre-stage systems also provide thermophilic diges-
tion and are normally incorporated in the treatment
process ahead of conventional anaerobic digestion.
Pre-stage systems differ from ATAD systems in
several respects, including:
• Normally only one prestage reactor is used. If
more than one reactor is installed, they operate in
parallel.
• Detention time is 1 d or less, with sludge added
several times per day.
• Pre-stage systems require supplemental heating.
Heat exchangers are used in all systems.
• Aeration is limited.
• Sludge leaving the prestage system has not been
stabilized.
Pre-stage systems are classified as a process that
meets the FRG criteria for the production of a disin-
fected sludge (see Appendix). Pre-stage installa-
tions in the FRG are summarized in Table 9-1 and
located as shown in Figure 9-1.
In 1989, 31 pre-stage systems were being con-
structed or operated in the FRG. Design capacities
range from 3,500 to 120,000 P.E. (0.16 to 5.5
MGD). Twenty systems were furnished by Um-
welttechnik Buchs (UTB), a Swiss company, and
eleven by German companies: seven by Roediger,
three by Oswald-Schulze, and one by Fuchs.
The Swiss company has a relatively strong market
position because pre-stage systems have been
used in Switzerland for some time. The Swiss or-
dinance regulating the land application of sewage
sludge and the required pathogen reductions was
enacted in 1981 (55). The UTB system was original-
ly developed for the purpose of pathogen reduction
and weed seed control (56). UTB reports that more
than 70 UTB installations are under construction or
in operation; most of these are in Switzerland and
the FRG.
In the United States the UTB pre-stage system is
marketed by CBI Walker.
9.2 System Design and Operation
Atypical pre-stage configuration (see Figure 9-2)
consists of a storage/thickening tank, a heat ex-
changer (sludge/sludge), the pre-stage reactor with
aeration, mixing and foam-controlling equipment, an
equalizing tank to adjust the pre-stage effluent and
the anaerobic feed cycle, and an auxiliary heating
device (usually a sludge/water heat exchanger). The
basic design parameters for the pre-stage system
are shown in Table 9-2.
Pre-stage conversion of COD is limited by the sys-
tem HRT and the availability of oxygen. Air input into
the process is typically less than stoichiometric re-
quirements for digestion, and at no time is a dis-
solved oxygen residual maintained in the reactor.
Although pre-stage systems are referred to as
aerobic, "aerated" is probably a better descriptor. Be-
cause of the limited oxygen supply, the concentra-
tion of soluble degradable organic compounds such
as volatile acids is normally high (57, 58, 59).
Most systems use an injector system (venturi) to pro-
vide air to the process, and an external recycle
pumping loop to accomplish mixing. In UTB and
some other systems, waste gas from the reactor is
partially recirculated back through .the venturi injec-
tor to increase oxygen transfer and minimize vent
gas heat loss. No full-scale measurements to deter-
mine oxygen transfer efficiency are known (60), but
values of 70-80% have been claimed (60, 61).
The exhaust air can be particularly foul due to the
presence of large quantities of organic (volatile)
acids. As a result, it has to be treated and is typically
diffused into the activated sludge aeration tank.
The design limits on detention time and oxygen
transfer limit the amount of heat generated by the
microbial reactions in the pre-stage. COD reduction
is reported to range from 5% to 15%. As a result,
heat development is not sufficient to achieve a
temperature level above 60°C, and an auxiliary heat-
ing device is required. The UTB system typically has
50% of the thermal input supplied with external heat.
Supplemental heat is supplied by the auxiliary heat
exchanger fueled by methane produced in the
anaerobic digesters. This additional heat require-
57
-------
Thermophlllc Pre-Stage Process
Table 9-1.
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
Summary of Pre-Stage
Name
Bad Aibling
Bad Tolz
Chiemsee
Detmold
Dilligen/Bayern
Dilligen/Saar
Duisburg
Ebersdorf
Griineck
Bundelfingen
Gunzburg
Heroldsberg
Holzkirchen
Huckelhoven
HQnfeld
Hzgrund
Linkenheim
Marl-Ost
Mitterteich
Nabburg
Nellingen
Oberteuringen
Raubling
Rosenheim
Saarlouis
Schuttorf
Schwabmunchen
Schwarzach
Weiershagen
Werne
Wilhelmstal
Installations In the FRG
Size
(P.E.)
47,000
52,000
62.000
92,000
15.000
24.000
6,500
17.500
60,000
12,500
35,000
57.000
, 36,000
4,500
8.000
19.000
13,500
42.000
4.500
15.000
120,000
86,000
29,000
27,500
20.000
85,000
3,500
Vendor
Roediger
UTB
Roediger
UTB
UTB
UTB
OS
UTB
Roediger
UTB
Roediger
UTB
UTB
UTB
Fuchs
UTB
UTB
OS
UTB
UTB
Roediger
UTB
Roediger
UTB
UTB
OS
UTB
UTB
UTB
Roediger
UTB
Design Load
(nVVd)
94
105
125
185
30
49
13
135
35
120
25
70
115
43
9
16
140
38
27
85
9
30
240
173
120
58
55
40
170
7
Years of
Operation
1
0.5
3
1
4
3
3
5
3
1
1
0.5
0.5
1
0.5
3
. 3
3
0.5
58
-------
Thermophilic Pre-Stage Process
MUNICH
13 1 243
2 9 23
Figure 9-1. Pre-Stage Plants in FRG. Installation Numbers Are Keyed to Table 9-1.
ment is reportedly satisfied by the digester methane
production under most operating conditions. Dual-
fuel boilers (typical for anaerobic digester installa-
tion) provide startup and backup heating capacity as
required.
The operation cycle is discontinuous. A normal
operating cycle begins when hot effluent from the
aerated pre-stage is discharged to the
sludge/sludge heat exchanger. Thickened, cold raw
sludge is fed to the second chamber of this ex-
changer. There the raw sludge is preheated while
the pre-stage effluent cools down to mesophilic
temperatures (40°C). Oswald-Schulze installs an
emergency cooler to protect the mesophilic
anaerobic digester. After temperature equalization,
the preheated raw sludge is transferred to the pre-
59
-------
Thermophillc Pre-Stage Process
EXHAUST AIR
TO TREATMENT
TO GAS/STORAGE/RECOVERY
TO LAND
APPLICATION
THICKENER
SLUDGE/SLUDGE PRE-STAGE AUXILUARY
HEAT EXCHANGER (THERMOPHILJC) HEAT
ANAEROBIC DIGESTERS
(MESOPHILJC)
STORAGETANK
Figure 9-2. Typical Configuration of a Pre-Stage Plant.
stage reactor where aeration and mixing start again.
No additional raw sludge is fed for 1 to 2 hr in order
to maintain the undisturbed detention required for
disinfection. Temperature in the pre-stage reactor is
controlled to 60°C to 65°C as a function of the time-
temperature requirements for disinfection.
Typically the displaced volume in the pre-stage sys-
tem amounts to 10% of the reactor volume, resulting
in 10 cycles per day. Variations in the daily waste
sludge quantity can be compensated for by adjust-
ing the number of cycles. The batch volume itself is
fixed by the compartment volume of the heat ex-
changer.
9.3 Process Evaluation
As of 1990, the EPA has not thoroughly reviewed
the cost, performance and benefits of pre-stage sys-
tems. Clearly, these systems offer a process
suitable for pathogen reduction where existing
anaerobic digestion facilities provide most of the
sludge stabilization. Since the discharge from pre-
stage systems is not stabilized, further sludge
processing is required whenever the pre-stage
process is used.
Some reports (56, 62) claim that the pre-stage
process improves sludge thickening and dewatering,
increases sludge degradation, and increases
methane production during anaerobic digestion. The
reported improvement in thickenability and
dewaterability is controversial. Swiss investigations
at full-scale plants showed distinct improvements as
compared to a one-stage anaerobic digestion sys-
tem: 9-13% TS with gravity thickening and 35-45%
TS with belt filters (57, 63). German sources do not
entirely confirm these results (62, 21,52) and
believe the Swiss results to be influenced by the use
of iron salts for phosphorus removal, which may con-
tribute a much denser chemical sludge to the sys-
tem.
Table 9-2. Design Parameters for Pre-Stage Systems
Reactors:
Type of sludge:
Usual TS range:
Required VSS:
Detention time:
Minimum
reaction time:
Temperature and pH:
Reactor:
Air input:
Specific power:
Energy requirement:
One or more stages, depending
on plant size, operating in parallel
semi-continuous operation
— Primary or secondary
(activated sludge, tricking filter)
— Sedimentation or flotation
— Mixed or separate
— Domestic, industrial origin
— Manure
40-60 g/L (4-6%)
> 25 g/L (2.5%)
Average 12-24hr
1-2 hr
60'C - 65'C, pH > 7.2 in final sludge
Cylindrical; height/diameter ratio: 2-5
1 m3/(m3 reactor hr) [1-3 (37)]
~ 100 W/m3 reactor (50)
No design data available; reported
are 3.5-12 (29) and 5 kWh/m3 sludge
(54); measured value for a plant with
21-m3 sludge loading (design load
31 m3): 4.9 kWh/m3
60
-------
Appendix
Criteria for Agricultural Use of Sludge in
the Federal Republic of Germany
Agricultural utilization of sewage sludge in the FRG
is regulated by the Ordinance on Sewage Sludge
(OSS) enacted on April 1,1983. As of January 1,
1987, only "hygienically safe" sewage sludge can be
applied to fields used for vegetable and fruit growing
or for pasture or forage land. The OSS defines a
sludge to be "hygienically safe" if appropriate tech-
nologies have eliminated all pathogenic organisms
(64). The Ordinance does not cite the relevant
pathogens nor does it define the appropriate tech-
nologies to use. The federal OSS is regulated on a
state (Lander) level through incorporation in state
decrees. Here the term "hygienically safe" has been
interpreted to mean "disinfected," but the state
decrees provide no clear definition of this term.
A joint working group of the ATV (Abwassertechnis-
che Vereinigung, Water Pollution Control Federa-
tion) and the VKS (Vereinigung kommunaler
Stadtereinigungsbetriebe, Association of Public
Cleansing Enterprises) assembled for the first time
in 1986. The working group is composed of en-
gineers and human and veterinary medicine ex-
perts. The working group has proposed a definition
of "disinfected" and has recommended appropriate
sanitizing technologies (65, 66). It is expected that
the working group's proposals will be incorporated
into an administrative regulation for "disinfection"
and will then become legally binding (66, 67, 68).
According to the working group, a sewage sludge
can be considered disinfected if it meets the criteria
of the following three-stage control concept.
1. Treatment Process Qualification
The sludge must be treated by an approved tech-
nology. An approved technology is one in which
adequate investigations have shown that:
• The indigenous or seeded Salmonella have
been reduced by at least 104.
• Indigenous or seeded eggs of Ascaris are
rendered noninfectious.
2. Treated Sludge Performance Criteria
Directly after treatment there shall be:
• No Salmonella in 1 g of sludge.
• Not more than 1,000 enterobacteriaceae/g of
treated sludge.
3. Process-Specific Operational Control Criteria
Operational control criteria must be established
specifically for each process. These criteria define
those conditions that result in disinfection as re-
quired above. As of 1989, six treatment processes
had been evaluated and control criteria established.
The operational control criteria for ATAD and pre-
stage systems are summarized below.
a) Autothermal Thermophilic Aerobic Digestion
(ATAD)
m At least two stages must be used to avoid
short-circuiting.
• The undisturbed reaction time, tmin, for given
temperature T must be:
tmin > 23 h @ T = 50°C
tmin>10h@ T = 55°C
tmin>4h@ T=60°C
The temperature requirements take into con-
sideration the drop in temperature resulting from
intermittent (batch) feeding occurring once per
day.
• Total detention time (td) must be > 5 d (two
equal-sized tanks).
Operational controls include:
• Continuous on-line measurement and
recording of the temperature of each tank in at
least two locations.
• Measurement of raw and final sludge pH
values.
• Continuous on-line measurement of sludge
feed volumes to allow for calculation and
control of detention time.
b) Aerated Thermophilic Digestion Pre-Stage Before
Anaerobic Digestion (PREST):
The temperature in the aerobic stage must be
maintained with additional heating. The require-
ments are the same as for a pre-pasteurization:
tmin>10min@T = 80°C
tmin > 20 min @ T = 75°C
61
-------
Appendix
tmin £ 25 min @ T « 70°C
tmin 2:30 min @ T = 65°C
Operational control includes the measurement of:
• The time of undisturbed reaction (contact
time) tmin.
• The temperature in at least two locations, in
the aerobic as well as in the anaerobic stage.
In addition, the temperature of the subsequent
anaerobic stage must be kept above 30°C to
maintain disinfection.
In summary both the ATAD and pre-stage proces-
ses have a sufficient data base and operating his-
tory to be classified as processes that meet the FRG
criteria for agricultural use of sludge. The relation of
documented ATAD performance and the proposed
U.S. regulations is discussed in detail in Chapter 6.
62
-------
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65
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