?/EPA
United States
Environmental Protection
Agency
Biosolids Technology Fact Sheet
Multi-Stage Anaerobic Digestion
DESCRIPTION
Anaerobic digestion is a naturally occurring bio-
logical process in which large numbers of
anaerobic bacteria convert organic matter into
methane and carbon dioxide (a mixture called
biogas) in the absence of air. It is a widely used
biological process for treating wastewater solids.
This process stabilizes the organic matter in
wastewater solids, reduces pathogens and odors,
and reduces the total solids/sludge quantity by
converting part of the volatile solids (VS) frac-
tion to biogas. Anaerobic digestion results in a
product that contains stabilized solids, as well as
some available forms of nutrients such as am-
monia-nitrogen.
The process of anaerobic digestion can be divided
into three separate steps, each of which is per-
formed by a different group of microorganisms:
• Hydrolysis, during which the proteins, cellu-
lose, lipids, and other complex organics are
broken down into smaller molecules and be-
come soluble by utilizing water to split the
chemical bonds of the substances
• Volatile acid fermentation, during which the
products of hydrolysis are converted into or-
ganic acids through the biochemical
processes of acidogenesis (where monomers
are converted to fatty acids) and acetogenesis
(the fatty acids are converted to acetic acid,
carbon dioxide, and hydrogen)
• Methane formation, during which the organic
acids produced during the fermentation step
are converted to methane and carbon dioxide.
The efficiency of each step is influenced by the
temperature and the amount of time the process is
allowed to react. For example, the organisms that
perform hydrolysis and volatile acid fermentation
(often called the acidogenic bacteria) are fast-
growing microorganisms that prefer a slightly
acidic environment and higher temperatures than
the organisms that perform the methane forma-
tion step (the methanogenic bacteria). The
acidogenic bacteria are also less sensitive than
the methanogenic bacteria to changes in organic
strength and composition in the incoming feed
stream. Therefore, although many wastewater
treatment plants have traditionally performed
anaerobic digestion processes in a single tank (in
a process called single-stage anaerobic digestion)
at a constant temperature, some facilities have
separated the process into multiple stages, by
physically separating the stages or by controlling
the process to separate the stages in time, or
both. This approach allows the facilities to opti-
mize the various stages of the anaerobic
digestion process to meet their needs.
The standard multi-stage anaerobic digestion
system is a two-stage acid/gas (AG)-phased
system, in which the acid-forming steps (hy-
drolysis and volatile acid fermentation) are
physically separated from the gas-forming step
(methane formation) by being conducted in sepa-
rate digestion tanks. The first stage, known as the
primary or acid phase digester, consists of the
hydrolysis and the first acid-production step, in
which acidogenic bacteria convert organic matter
into soluble compounds and volatile fatty acids.
The second stage, known as the secondary or
methane stage digester consists of further con-
version of organic matter to acetic acid through
acetogenesis, as well as the methane formation
step, in which methanogenic bacteria convert
soluble matter into biogas (primarily methane;
see Figure 1). The methanogenic step also pro-
duces other by-product gases, including
hydrogen sulfide, nitrogen gas, and several other
gases. In a typical two-stage system, the primary
digester is heated to optimize performance of the
hydrolytic and acidogenic bacteria. The secon-
dary digester is not normally equipped with
mixing or heating facilities because of the exo-
thermic (heat-producing) nature of the methane
formation reaction.
1
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Suspended
Solids •
Dissolved
" Solids
Organic
* Adds
Acetate
• Methane
Li quifl cation
Ac idfi cation
Acetate
Formation
Methane
Formation
Source: Wilson, et. al, 2005
Figure 1. Standard Multi-Stage Anaerobic Digestion System
An alternative method for designing the system
is to separate the stages over time by adding dif-
ferent levels of heating at different times in the
process by a process called temperature-phased
anaerobic digestion, or TPAD. As described ear-
lier, hydrolysis and acidogenesis can be
enhanced by increasing the operating tempera-
ture; however, acetogenesis is adversely affected
by high operating temperatures (Chang, et al.
2004). If the system is heated to enhance hy-
drolysis and acidogenesis, the resulting volatile
acid production can overwhelm the ability of the
slower-reacting acetogenic and methanogenic
bacteria to convert the volatile acids, resulting in
increased pH and inhibited acetogenesis and
methanogenesis (Chang, et al. 2004). Therefore,
controlling the temperature can be critical in
optimizing system performance.
Numerous facilities use some form of TPAD.
For example, in 2002 the wastewater treatment
facility in Waterloo, Iowa, rehabilitated its exist-
ing anaerobic digestion system to operate as a
TPAD system, in which the first digesters were
operated in the thermophilic range (50-60 °C
[122-150 °F]) to promote pathogen destruction
with the intent of producing Class A biosolids,
while subsequent digesters were operated in the
mesophilic range (30-38 °C [85-100 °F]) to re-
duce VS (Iranpour and Windau 2004). This type
of system can be abbreviated as a TPAD-TM,
where the T represents the thermophilic first
stage, and the M represents the mesophilic sec-
ond stage.
Facilities can separate these stages in both space
and time by operating multiple digesters in se-
ries, to increase control over the process and
enhance the results even further. Facilities in
Tacoma, Washington, Inland Empire, California,
and Calgary, Alberta, Canada, have gone to
three-phased processes. Table 1 provides several
examples of wastewater treatment facilities that
use different types of multi-stage processes (Wil-
son 2003 and personal communications).
APPLICABILITY
Multi-stage anaerobic digestion systems are po-
tentially applicable for all wastewater treatment
systems, provided that the solids can be delivered
to the system at an acceptable concentration.
These can include both new installations and ret-
rofits. In fact, much of the current research into
anaerobic digestion is directed toward retrofitting
multi-stage systems into facilities where single-
stage processes are already present (Cumiskey
2005; W. Parker, personal communication, 2006).
The primary factor in determining whether a
multi-stage anaerobic digestion process is feasi-
ble for a system is the feed solids concentration.
Because a multi-stage process can be sensitive to
changes in the feed solids, it might not be feasi-
ble if the characteristics of the feed solids
concentrations vary significantly. The VS con-
tent in the feed should preferably be at least 50
percent, and the feed should not contain sub-
stances at levels that may inhibit the biological
processes associated with anaerobic digestion
(see Table 2). Wastewater residuals containing
lime, alum, iron, and other substances can be
successfully digested as long as the VS content
remains high enough to support the growth of
microorganisms.
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Table 1. Example Wastewater Treatment Facilities with Multi-Stage Anaerobic Digesters
Plant
System Type
Woodridge WWTP, DuPage County, IL
Elmhurst, IL
Back River, Baltimore, MD (pilot)
Inland Empire (RP-1), Ontario, CA (farm manure)
Waterloo, IA
Waupun, Wl
Rockaway, NY
Pine Creek WWTP, Calgary, Alberta, Canada (pilot)
Tacoma, WA
Two-stage AG-MT
Two-stage AG-MM
Two-stage AG-MM
Three-stage AG-MTM
Two-stage TPAD-TM
Two-stage TPAD-TM
Two-stage TPAD-MT
Three-stage TPAD (multiple options being researched)
Heated aerobic stage (71° C [160° F]) + Three-stage
TPAD-TM M
Table 2. Substances with Potential to Cause Biological Inhibition in Anaerobic Digestion
Substance
Calcium
Magnesium
Sodium
Potassium
Ammonia Nitrogen
Copper
Chromium VI
Chromium
Nickel
Zinc
Moderately Inhibitive (mg/L)
1,500-4,500
1,000-1,500
3,500-5,500
2,500-4,500
1,500-3,000
—
—
—
—
—
Strongly Inhibitive (mg/L)
8,000
3,000
8,000
12,000
3,000
50-70 (total)
200-250 (total)
180-420 (total)
30 (total)
1.0 (soluble)
ADVANTAGES AND DISADVANTAGES
The major advantages of multi-stage anaerobic
digestion systems versus single-stage anaerobic
digestion systems is that multi-stage systems can
optimize the various steps in the process by sepa-
rating them in space or time and optimizing the
specific conditions under which the various steps
take place. As described above, they can also
allow a facility to adopt a specific system con-
figuration to meet its goals. For example, if the
facility wants to produce Class A biosolids, it
might require a thermophilic stage; however,
if volume reduction is its primary goal, only
mesophilic stages may be required (W. Parker,
personal communication, 2006).
The major disadvantage of multi-stage anaerobic
digestion systems is that they have higher opera-
tion and maintenance (O&M) requirements than
single-stage systems. In addition, they can be
more expensive than single-stage systems, al-
though this is more of a factor when retrofitting
into multi-stage systems.
An expanded discussion of the advantages and
disadvantages of multi-stage versus single-stage
anaerobic digestion systems follows:
Advantages
Gas Recovery and Storage. Multi-stage systems
can be optimized to maximize the amount of gas
they produce in the digestion phase. The gas pro-
duced from the anaerobic digestion of biosolids is
typically composed of 55 to 70 percent methane
and approximately 25 to 30 percent carbon diox-
ide, with the remaining fraction composed
primarily of nitrogen, hydrogen, and hydrogen
sulfide (USEPA 1979). Typical digester gas
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exhibits a heat content between 18,630 and
26,080 thousand Joules per cubic meter (kJ/m3)
or between 500 and 700 BTU/ft3, which is ap-
proximately two-thirds the heat content of the
natural gas delivered by gas utilities. Therefore,
digester gas can be an economical energy source
for plant operations. It can be temporarily stored
and/or mixed with natural gas through the pipe-
line system for in-plant use as a source for heat,
electricity, or steam. It is ideal as fuel to fire hot
water boilers, internal combustion engines, heat
drying equipment, and incinerators. Some plants
scrub their digester gas to reduce the levels of
carbon dioxide, hydrogen sulfide, siloxane, and
other gases and in several cases have marketed the
gas as a high-value natural gas source to their
local gas utility systems.
Biosolids Quality. Multi-stage anaerobic diges-
tion systems that use a thermophilic stage can
produce biosolids that meet Class A pathogen
reduction requirements. Much of the current re-
search into anaerobic digestion is devoted to
pathogen control through temperature phasing
and pretreatment of waste through processes like
enzyme hydrolysis prior to its anaerobic diges-
tion. For example, recent research by the City of
Los Angeles indicates that their product resulting
from systems operated at thermophilic tempera-
tures achieved Class A status and had lower odor
than the product produced by mesophilic proc-
esses. In addition, their results indicated that the
odor concentrations in solids digested using
mesophilic temperatures continued to increase as
the biosolids went through the digestion process
and even after they were applied on farmland.
(Material produced by digestion at mesophilic
temperatures and received at their land applica-
tion site had odor concentrations 10 times higher
than the material being introduced into the cen-
trifuges for dewatering.) In contrast, the odor
content of material subjected to thermophilic
digestion temperatures decreased by about 70
percent by the time it reached the land applica-
tion site (Haug et al. 2002). Enzyme hydrolysis
is being heavily researched in Europe. Additional
discussion of pretreatment through enzyme hy-
drolysis is presented later in the "Design"
section.
Other advantages of multi-stage anaerobic diges-
tion versus single-stage anaerobic digestion
processes include:
• Multi-stage systems require less digester
volume to handle the same amount of input
volume because they have lower retention
times and allow higher loading rates than
single-stage systems.
• Multi-stage systems have achieved VS re-
duction, which provides better odor control.
• A multi-stage system can be configured to
reduce foaming problems. (See discussion of
foaming in the "Operation and Maintenance"
section below.)
• Multi-stage systems reduce the short circuit-
ing of solids by separating the stages and
optimizing the retention time in each stage.
Disadvantages
The piping requirements for a multi-stage
system, operation, and maintenance are more
complex than those for a single-stage system.
DESIGN CRITERIA
Location in the Solids Processing Train
Multi-stage anaerobic digestion is typically lo-
cated in the solids processing train after
thickening but before dewatering. Thickening of
the solids prior to digestion is beneficial because
it reduces the biomass volume, digester size re-
quirement, supernatant volume, and heating
requirements (WEF 1998).
Solids Feed Rate
The solids feed rate is typically 5 to 6 percent of
the mixed solids retention range.
Organic Loading
Typical VS loading rates for both mesophilic and
thermophilic multi-stage systems are in the 482-
642 kg/m3/day (30-40 Ib/ft3/day) range, which is
significantly higher than the average of 2.57
kg/m3/day (0.16 Ib/ft3/day) for single-stage an-
aerobic digester systems (Sieger 2001).
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Solids Retention Time
As discussed earlier in the "Description" section,
solids retention time (SRT) is a critical factor in
the design of a multi-stage anaerobic digestion
system. High SRTs increase the digestion but
reduce the rate of throughput for the system.
Therefore, each facility must determine the op-
timum SRT to achieve the required amount of
digestion while also maximizing the facility
throughput.
Because the stages are optimized to maximize
digestion, the SRTs of multi-stage systems are
typically shorter than those of single-stage sys-
tems. For example, Sieger (2001) reported an
average SRT of approximately 20 days for
mesophilic single-stage systems, while the SRTs
for multi-staged systems typically ranged be-
tween 14 and 18 days.
In general, the SRT for a multi-stage system is
determined by the required end-product and the
sequence of the phasing. For example, if the fa-
cility is producing Class B biosolids, it might use
a lower SRT than a facility producing Class A
biosolids using a similar configuration.
A summary of typical SRTs and VS loading
rates is provided in Table 3.
Heat Exchangers
Temperature is important in determining the rate
of digestion. The design operating temperature
establishes the minimum SRT required to
achieve a given amount of VS reduction. As de-
scribed above, most anaerobic digesters currently
in operation are designed to operate in the meso-
philic temperature range, although many current
designs for multi-stage systems include phases
operated at thermophilic temperatures—through
TPAD systems with thermophilic processes.
Typical auxiliary heating methods include steam
injection, internal heat exchangers, and external
heat exchangers. External heat exchangers are
the most common because of their flexibility and
the ease of maintaining their heating surfaces.
Internal coils and heat-jacketed draft tube mixers
can become caked and effectively blocked, ne-
cessitating removing them or taking the digester
out of service to empty and clean the system.
Steam injection results in dilution of the digester
contents and can be energy-inefficient.
Table 3. Comparison of Anaerobic Digestion Processes
Digestion
Process
Single-Stage Meso-
philic
Staged or Extended
Thermophilic
TPAD
ATPC
Two-Phase
Pre-Pasteurization
SRT per Tank
at Max Month
(days)
20
15/1.5/1.5
5/10
1.5/15
2/12
30 min./15
Total SRT at
Max Month
(days)
20
18
15
16.5
14
15.02
Operating
Temperature
Regime
M
T
T/M
T/M
M/T; T/M; T/T;
orM/M
-70 C/M
VS Loading
Rate at Max
Month
(Ib/ft3/day)
0.16
0.30
0.30
0.30
0.40
0.40
Pathogen Level
Produced
Class B
Class Aa
Class Aa> b
Class A
Class Ad
Class A
Source: Adapted from Sieger 2001.
Notes:
a Believed to meet Class A requirements, but formal pathogen equivalency has not been approved by EPA.
b One process has been approved as a site-specific process by EPA, but the technology has not been approved
for national equivalency for Class A.
0 Aerobic Thermophilic Pretreatment.
d Testing may proceed on variations of feed and temperature of each phase.
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Mixing
Auxiliary mixing of the digester contents is
beneficial for reducing thermal stratification,
dispersing the biosolids for better contact with
the microorganisms, reducing scum buildup,
diluting levels of any inhibitory substances or
adverse feed characteristics, and retaining inor-
ganic material (grit) in suspension (WEF 1995).
Without adequate mixing, the digestion process
can be short-circuited and solids that have not
been sufficiently digested might be prematurely
discharged. Such solids will not be properly sta-
bilized and might not be suitable for the intended
end use.
The three mixing methods that have typically
been used are mechanical mixing, hydraulic mix-
ing, and gas recirculation.
Mechanical mixing includes the use of impellers,
propellers, and turbine wheels to mix the digester
contents.
Hydraulic mixing is accomplished by recirculat-
ing digester content through use of an external
pump network. The hydraulic mixing can pump
the digester contents from the lower half of the
digester to the top of the digester to potentially
stop the formation of a significant scum layer,
which can be a nuisance or detrimental to di-
gester operation.
Gas recirculation systems use the digester gas
produced by the anaerobic digestion process to
mix the digester contents. The gas is compressed
and recirculated through the tank to promote
mixing. The gas can be introduced into the tank
through one of several methods, including:
• Lances mounted on the inside of the tank
cover so they project down into the tank
• Diffusers mounted on the floor of the tank
• Draft tubes in the tank
• Bubble guns mounted inside the tank
The type of mixing device suitable for any di-
gester depends on the design (vessel and cover)
and size of the digesters.
Types of Covers
It is necessary to cover the digesters to maintain
anaerobic conditions. In addition to keeping am-
bient air out, the covers prevent digester gas from
being released and also reduce the amount of heat
loss to the atmosphere. Anaerobic digester covers
can be fixed or floating. Fixed covers are flat,
conical, or dome-shaped and are constructed of
reinforced concrete or steel. Floating covers can
rest directly on the liquid surface or float on the
gas and be supported by side skirts at the side of
the tank.
The appropriate type of cover for any given ap-
plication depends on the design and size of the
digester. Both fixed and floating covers have
advantages and disadvantages. For example,
floating covers rise and fall with the liquid level
in the digester and therefore prevent formation of
a vacuum, which could damage the vessel or the
cover. Floating covers also prevent air from be-
ing drawn into the digester during solids
removal. In contrast, a fixed cover is often easier
to design, requires less maintenance, and is less
prone to develop gas leaks.
Enzyme Hydrolysis Pretreatment
In January 2002 legislation was enacted in the
United Kingdom (UK) that required pathogen
reduction in municipal wastewater sludge for the
first time. This new requirement led many utili-
ties to search for methods to optimize their
existing anaerobic digestion systems (Cumiskey
2005), particularly mesophilic digesters, which
included the majority of operating systems in the
UK at that time. Investigations by United Utili-
ties (UU) in the UK indicated that the major
pathway for killing pathogens in mesophilic an-
aerobic digesters was solubilization or hydrolysis
(Mayhew et al. 2004). In anaerobic digestion,
hydrolysis occurs before the conversion of or-
ganic particulate matter to organic acids. UU
found that pathogen reduction could be im-
proved, and could be achieved at much lower
temperatures (mesophilic temperatures instead of
thermophilic temperatures) by separating the
hydrolysis stage from the mesophilic anaerobic
digestion stage (Mayhew et al. 2004). Therefore,
UU developed a specialized plug flow enzymic
hydrolysis process to pretreat the sludge before
anaerobic digestion. The enzyme hydrolysis step
breaks down cell wall lipoprotein structures
(Kelly 2003), enhancing the digestion process.
This process results in a better energy balance
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and the enhanced digestion increased biogas pro-
duction relative to other processes. UU uses a
plug-flow configuration that operates at 42 °C
(108 °F) with a 2-day hydraulic retention time.
UU began installing the enzyme hydrolysis
method in its facilities, including facilities in
Macclesfield, Bromborough, Crewe, and Black-
burn. Initial tests at the Macclesfield facility
show that the enzymic hydrolysis step results in
a 104 reduction in E. coli. The enzyme hydrolysis
process in Bromborough enables the plant to
operate at 4.0 kg VS/m3/day (250 Ib VS/ft3/day)
while also producing a high-quality product that
meets the new standards. The plant has also in-
creased its gas production from 4,500 mVday to
5,500 m3/day (158,916 ft3/day to 194,231
ft3/day) (Monsal 2004).
PERFORMANCE
Multi-stage anaerobic digestion can achieve su-
perior performance relative to single-stage
conventional digestion for most wastewater sol-
ids and for all loading rates. In addition, this
increased performance can be achieved with
smaller digester volumes because of the higher
loading rates that can be achieved with multi-
stage digesters. Compared to single-stage sys-
tems, the multi-stage process achieves higher VS
reduction with shorter residence times. Typical
VS reduction for a first-stage digester ranges
from 40 to 60 percent, and up to 5 percent addi-
tional reduction can occur in subsequent stages.
Multi-stage systems also produce more biogas of
a higher quality (as measured by its methane
content) than that produced by single-stage proc-
esses. Finally, these systems reduce, and
potentially eliminate, the foaming problem that
often occurs in single-stage systems.
Case studies highlighting the performance of
several multi-stage anaerobic digestion facilities
follow.
Woodridge WWTP, DuPage County, Illinois
The Woodridge wastewater treatment plant
(WWTP) was converted from its original single-
stage process to a two-stage AG-MT anaerobic
digestion system in the late 1980s in an attempt
to control foaming problems in the old system.
To convert the facility to a two-stage process, a
mesophilic acid-stage digester was added to the
existing digestion facility, which was converted
to a thermophilic gas-phase digester. The new
mesophilic acid-stage digester receives a feed of
46,000 GPD at a 4-5 percent solids content, with
approximately 11,325 kg/day (25,000 Ib/day) of
suspended solids and 9,060 kg/day (20,000
Ib/day) of volatile suspended solids. This stage
has a retention time of approximately 1 day. Af-
ter passing through this stage, the biosolids flow
to the methane-phase digester, which operates at
a thermophilic temperature of approximately 52
°C (126 °F) and produces approximately 190,000
standard cubic feet (SCF) of gas per day with an
average methane content of 64 percent.
The overall VS reduction averages approxi-
mately 65 percent. During the first 4 months of
2000, fecal coliforms were reduced by an aver-
age of 99.996 percent. The facility experiences
no foaming, and the digested sludge is highly
desirable as a soil enhancer for agricultural pur-
poses. The digester gas is recirculated to power
the digesters, and excess gas is used to produce
electricity.
Inland Empire Regional Water Recycling
Plant 1 (RP-1), Ontario, California
The anaerobic digestion system at the Inland
Empire Utility Agency's (IEUA) RP-1 was up-
graded in 2000 and went online as a three-stage
AG-MTM process in 2001. Before 2001 the fa-
cility had operated as a thermophilic single-stage
system. The system had experienced odor prob-
lems, however, and thus it had already gone to
separate acid and gas phases using both a semi-
batch and a continuous approach. After spending
2.5-3.5 days in a 32-40 °C (90-104 °F) meso-
philic acid digester, the biosolids can be diverted
to a semi-batch 56-58 °C (133-136 °F) thermo-
philic gas-phase digester, where they are retained
for 18-20 days, or can go to a 50-52 °C (122-
126 °F) thermophilic gas-phase digester, where
they are retained for 14-16 days. After the ther-
mophilic gas-phase digester, the biosolids are
sent to a mesophilic gas-phase digester. Flow
from the semi-batch process goes to a 42-48 °C
(108-118 °F) system for 13-17 days, while flow
from the continuous system goes to a 46-49 °C
(115-120 °F) system for 5-6 days.
7
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Overall, VS reduction improved for the facility,
from approximately 55 percent to 60-65 percent
with the AG-MTM process. Both processes
showed non-detects for helminth ova, enteric
viruses, and Salmonella. The semi-batch process
qualified through time and temperature as Class
A biosolids under alternative 1 of 40 CFR Part
503, while the continuous process received site-
specific EPA approval as Class A was granted
under alternative 3 of 40 CFR Part 503 (Wilson
et al. 2005).
Waterloo, Iowa
The City of Waterloo wanted to increase its bio-
solids treatment capacity and improve its VS
destruction and gas production. In 2002 the city
upgraded its anaerobic digestion process from a
single-stage mesophilic process to a TPAD-TM
system by converting two of its six digesters into
thermophilic digesters. The city began the pro-
ject by taking each of its six digesters out of
service one at a time and retrofitting them with
the necessary piping, heating equipment, and
mixers for the new system. This approach al-
lowed the plant to continue to operate while the
facility was upgraded. Once all the new equip-
ment was in place, two of the digesters were
sequentially transitioned to thermophilic tem-
peratures. First, the feed rate into the digester
was slowed, and then the temperature was raised
from 35 °C (95 °F) to 53 °C (131 °F) over a pe-
riod of 3 days, allowing the organisms to
stabilize until they were achieving good VS de-
struction. Once the first thermophilic digester
was stabilized, the second was transitioned the
same way. This quick transition from mesophilic
to thermophilic was important because it limited
the number of mesophilic organisms that might
survive in the thermophilic digester. During this
transition, it was also important to limit the load-
ing rate so that the digester would not be
overloaded as the thermophilic organisms grew.
The city's new system achieved its goals. VS re-
duction improved from approximately 47 percent
in the old system to approximately 60-64 percent
in the new system; gas production increased to
0.18-0.21 m3 per kg of VS destroyed (14-16
ft3/lb) (Wilson et al. undated).
Tacoma, Washington
The City of Tacoma, Washington, has operated
an anaerobic digestion system for many years,
but it has had a history of odor problems. In
1993 Tacoma transitioned from a single-stage
thermophilic system to a two-stage AG-MM
system, thereby improving the odor of its
TAGRO end-product so that it was more accept-
able to customers. Although the odor of the end-
product was acceptable, the hydrogen sulfide
odors in the plant's belt-filter press room were
extremely unpleasant to the workers and close to
dangerous levels. Therefore, the plant began ex-
perimenting with various temperature-phasing
approaches to try to reduce odors. Eventually,
the plant determined that a thermophilic- meso-
philic-low mesophilic approach of 55-38-32 °C
(131-100-90 °F) with a total retention time of 21
days was ideal. By lowering the middle digester
from 46 °C to 38 °C (115 °F to 100 °F), the plant
significantly reduced its odor problems. In addi-
tion, lowering the temperature from 38 °C to 32
°C (100 °F to 90 °F) in the final digester seems
to have improved dewatering. (Recent data show
that dewatering has improved from 22 percent to
24 percent). The facility uses the biogas gener-
ated by the digestion process to run its boilers.
The plant has been operating with this system
since 2004 (D. Thompson, City of Tacoma, per-
sonal communication, 2006).
Three-Stage TPAD (bench-scale)
Salasali et al. (2005) performed bench-scale tests
of several three-stage TPAD configurations to
evaluate the level of VS reduction and biogas
production in these configurations. These re-
searchers undertook these experiments to
determine whether modifying the operating prac-
tices for standard mesophilic digesters could
achieve high performance VS reduction and
Class A pathogen reduction so that facilities op-
erating mesophilic digesters could achieve high-
quality biosolids without going through the sub-
stantial costs of adding new digesters or
reconfiguring existing digesters. The authors
evaluated two three-stage configurations (35-35-
35 °C [95-95-95 °F] and 42-35-35 °C [108-95-95
°F]), as well as a two-stage system (35-35 °C
[95-95 °F]). The authors used 201 samples of a
mixture of primary and thickened waste-
activated sludge with a concentration of between
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4.0 and 5.2 percent solids from the City of Ot-
tawa, Canada, WWTP. The trials used a
hydraulic retention time of 15 days (5 days in
each stage for the three-stage systems, and 5
days in the first digester and 10 days in the sec-
ond digester for the two-stage system) and
measured conventional parameters (total solids,
VS, pH) as well as pathogen indicators (fecal
coliform bacteria, Escherichia coli, fecal Strep-
tococci, Salmonella spp., Cryptosporidium
perfringens). Each of the configurations was able
to achieve the 38 percent VS reduction required
for vector attraction reduction, although both of
the three-stage configurations achieved better VS
reduction and biogas production than did the
two-stage configuration. Bacterial results showed
that, with the exception of Salmonella spp.,
pathogens were reduced to the greatest extent in
the 42-35-35 °C (108-95-95 °F) configuration.
OPERATION AND MAINTENANCE
Because multi-stage anaerobic digestion systems
involve multiple stages, each having its own spe-
cific O&M requirements, these systems have
higher overall O&M requirements than do sin-
gle-stage anaerobic digestion systems.
Maintaining a stable operating temperature and
pH within the digesters is critical, particularly for
the methane formers, which are sensitive to
changes in temperature and pH (Dague 1968).
Changes in digester operating temperature
greater than -1.0 °C (~2 °F) per day can result in
process upset due to heat shock of microorgan-
isms. The optimum pH range for anaerobic
digestion is 6.8-7.2. A reduction in pH, which
can be caused by overloading the digester, inhib-
its methane formation. Methane formation is
further inhibited as the acid fermentation stage of
digestion continues, possibly leading to digester
upset and failure. Temperature control is also
important to ensure satisfactory operation of the
digestion system. Fluctuations in temperature
can result in the die-off of microorganisms and
process inefficiency. As discussed earlier, heat
exchangers are commonly employed to control
temperatures in the digester.
Chemical addition to anaerobic digesters might
occasionally become necessary for pH/alkalinity
control and to control the potential for metals
and other chemicals to inhibit the process (see
Table 2) (WEF 1995). Sodium bicarbonate, so-
dium carbonate, and lime can be used to provide
alkalinity. Ferrous chloride, ferrous sulfate, and
alum can be added to precipitate or coagulate
inhibitive chemicals or to control digester gas
hydrogen sulfide content.
A common operational problem with any an-
aerobic digestion system is foaming, which is the
trapping of fine bubbles of gas in the semi-liquid
digestion contents. Foam forms primarily when
the carbon dioxide-to-methane ratio is higher
than normal. This usually occurs during start-up
operations, but it can occur whenever a fresh
food supply suddenly contacts active microor-
ganisms. This is one reason continuous slow feed
of solids is preferred to batch feeding of digest-
ers. In addition, a common bacterium, Nocardia,
has a filamentous structure that traps gas, leading
to foaming. These bacteria should be eliminated
in aeration basins before the solids are fed to the
digesters. Two-stage AG anaerobic digestion
naturally overcomes this problem because the
first stage (acid phase) digester has low gas pro-
duction and low pH, along with higher volatile
acid concentrations, which together are detri-
mental to foam-causing microorganisms.
Another important operational concern is odor
control at the plant during the anaerobic diges-
tion process. As discussed previously, hydrogen
sulfide and ammonia are produced during an-
aerobic digestion. The most common way to
control odors from a digestion system is to use
covers, as discussed earlier.
Periodic clean-out of the digesters is necessary for
all digestion systems. The frequency of cleaning
is based on several factors, including the accumu-
lation of grit and scum (which can reduce the
effective volume of the tank); the condition of
internal mixing or heating equipment; the avail-
ability of backup solids handling equipment; and
tank structure (WEF 1998). Typically systems
require cleaning approximately every 5 years.
Because digesters are confined spaces, safety is a
primary consideration. Before personnel enter a
digester, the air composition inside the tank must
be monitored for oxygen levels and the presence
of hazardous gases.
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COSTS
The construction and operation and maintenance
costs of multi-stage anaerobic digestion depend
largely on the quantity and quality of the solids
to be stabilized, the size of the digesters, and the
type of mixing and heating equipment. Capital
items include digester tanks, piping and pumps,
digester heating and mixing systems, digester
gas-handling equipment, and chemical feed
equipment. Design and construction costs from
several example facilities follow.
The City of Grand Island, Nebraska, is in the
design stages for the construction of a $10.7 mil-
lion two-stage AG anaerobic digestion system.
The city's 12-MGD WWTP receives approxi-
mately 40 percent of its flow from large
industrial agricultural operations (including a
meat packing plant) and has had odor problems.
The city determined that replacing its open aero-
bic digesters with an anaerobic system will
reduce these problems and generate a usable end-
product for land application as fertilizer
(Overstreet 2006).
Western Lake Superior Sanitary District
(WLSSD) in Duluth, Minnesota, began operating
a new two-stage TPAD-TM anaerobic digestion
system at its 43-MGD regional WWTP in 2001.
The new system was the result of a multiyear
planning process that evaluated options for more
environmentally and fiscally responsible alterna-
tives to the existing sludge incineration process.
The committee recommended anaerobic diges-
tion, which would produce a usable end-product
as well as biogas.
The new anaerobic digestion facility had a con-
struction cost of $32.6 million and consists of
four 1-MG digesters. The solids are digested in
the first digester for 5 days at a temperature of 55
°C (131 °F). The thermophilically digested solids
are then transferred to one of the three meso-
philic digesters for an additional 15 days. The
facility markets the end-product as "Field Green"
and expects to produce approximately 8,000 dry
tons of fertilizer per year, providing local farm-
ers with an estimated $47,000 in no-cost
fertilizer annually. In addition, the facility directs
the biogas to a dedicated boiler, which provides
the heat for the digesters, as well as for the solids
processing building. By using the biogas from
the anaerobic digestion process to power the
boiler, the facility has reduced its peak electrical
demand by 706 kilowatts per month, a 14 per-
cent decrease (Western Lake Superior Sanitary
District 2001).
In addition to constructing new anaerobic diges-
tion systems, many facilities are upgrading
existing anaerobic digesters to multi-stage sys-
tems to produce high-quality biosolids, reduce
odor problems, or produce biogas to power plant
operations or sell. Depending on the configura-
tion of the current system (number of digesters,
piping configuration, capacity and location of
heating and mixing equipment, feed capabilities),
the costs of retrofitting existing anaerobic diges-
tion systems to multi-stage systems are typically
minimal and usually include only the cost of
installing new piping or reconfiguring existing
piping. For example, the IUEA RP-1 in Ontario,
California, was able to reconfigure its existing
system and add new variable speed pumps and
controls for $2.5 million (P. Cambiaso, IEUA,
personal communication 2006). Similarly, al-
though the exact cost figures were not readily
available, the city of Tacoma, Washington, was
able to transition from a single-stage thermo-
philic system to a two-stage AG-MM system at
"a very low cost" by re-plumbing its existing
system (D. Thompson, City of Tacoma, personal
communication, 2006).
Operation and maintenance costs include costs
associated with operating and maintaining mix-
ing, heating, and pumping equipment; operating
and maintaining gas-handling equipment; clean-
ing of digesters; and the purchase of chemicals.
Table 4 summarizes typical O&M costs in dol-
lars per dry ton of solids through the anaerobic
digesters.
10
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Table 4. Typical Operation and Maintenance Costs for Digesters
Range per Dry Ton
of Biosolids
Source: Multi-Agency Benchmarking Study 1999.
Average per Dry Ton of
Biosolids
Operation
Maintenance
Total
$5.30 -$41. 03
$4.09 -$10.48
$9.39-$51.51
$17.47
$7.44
$24.91
It should be noted that anaerobic digestion sys-
tems often pay for themselves through the
combination of reduced costs for biosolids dis-
posal (owing to a reduction in biosolids volume
through the digestion process), the potential
marketing of a Class A biosolids product, and
the recovery of usable biogas. For example, the
City of Tacoma markets the end-product from its
anaerobic digestion process, TAGRO, for $6.00-
$23/m3 ($8-$30/yd3), depending on its final
form (City of Tacoma Web site, June 2006).
Other Related Fact Sheets
Odor Control in Biosolids Management
EPA 832-F-00-067
September 2002
Centrifugal Thickening andDewatering
EPA 832-F-00-053
September 2002
Belt Filter Press
EPA 832-F-00-057
September 2002
Recessed Plate Filter Press
EPA 832-F-00-058
September 2000
Alkaline Stabilization of Biosolids
EPA 832-F-00-052
September 2000
Other EPA fact sheets can be found at
http://www.epa.gov/owm/mtb/mtbfact.htm
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ADDITIONAL INFORMATION
Earth Tech, Inc.
Tom Wilson
675 North Washington Street, Suite 300
Alexandria, VA 22314
Inland Empire Utility Agency
Patrick Shields
6075 Kimball Avenue
Chino, CA 91710
City of Tacoma
Dan Thompson
747 Market Street
Tacoma, WA 98402
The mention of trade names or commercial
products does not constitute endorsement or
recommendation for use by the U.S.
Environmental Protection Agency.
Office of Water
EPA832-F-06-031
September 2006
vvEPA
United States
Environmental Protection
Agency
13
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