?/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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                                                                                           11

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12

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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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