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United States September
Environmental Protection 1983
Agency
SEPA An Alternative
Technology
Methane
Recovery
An Energy
Resource
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Methane Recovery - An Energy Resource
The Role of Methane, Recovery '
Meeting wastewater treatment discharge standards'
in a cost-effective and energy-efficient manner is a
challenge facing many communities today..
Accordingly, wastewater treatment technologies that
reduce' energy consumption and recover energy are
in demand. ' '. '..-.'.
One,suqh technology is the recovery and utilization
of methane from the anaerobic digestion of
wastewater treatment sludges.
Process Description
Anaerobic digestion of sewage solids is one of the
oldest forms of biological sludge treatment and was
used extensively in the 1930s and 1940s. However,
this time consuming and relatively sensitive process
fell into disfavor during the energy abundant '50s
and '60s. Digester systems failed due in part to
inadequate understanding of the processes and the
_increased attention given to quicker, non-biological,
'energy intensive methods of sludge stabilization. In
the last 20 years basic research has led to
.improvements in the reliability and performance of
anaerobic digesters.
Anaerobic digestion utilizes microorganisms,that
live in the absence of air to convert unstable
organics to a more stable form. The process
reduces the sludge volume by destroying 25 to 45
percent of the raw sludge solids and yields a solids
residue suitable for use as a soil conditioner. One
byproduct of this conversion process is a gas
(digester gas) which typically contains 60 to 70
percent methane and has a heating value of 550 to
650 btu/cu.ft. By comparison, the heating value of
natural gas is approximately 1000 btu/cu.ft.
Typically, about 15 standard cubic feet (scf) of
digester gas is produced per pound of volatile
solids destroyed, although this may range from as
low as 6 scf to as much as 18 scf depending upon
the type of sludge and the digester efficiency. This
will result in about 6.5 million BTUs of digester gas
from anaerobic digestion of the sludge produced by
conventional activated sludge treatment of 1 million
gallons of municipal wastewater. Various processes
such as thermal conditioning prior to anaerobic
digestion may offer a potential for improving the
energy production from sewage sludges.
The majority of plants that employ anaerobic
digestion use a two vessel system for sludge
stabilization (Figure, 1). The first tank (primary
digester) is used for digestion and is heated and
rnixed; most of the sludge stabilization and gas
production occurs in the primary digester. The
second tank (secondary digester) is used for sludge
storage, gravity concentration.of digested sludge;
and decanting of supernatant.
Figure 1 Two-Stage, High-Rate Anaerobic
Digester System
Utilization options depend on.the quantity and
quality of digester gas. The characteristics of
digester gas from a typical anaerobic digester are
shown in Table 1. Gas characteristics from a
specific digester depend on the nature of the
sludge, the rate at which the sludge is fed to the
digester, and the activity of the anaerobic
microoganisms.
Quantity
Sf"B' to 1 aV/lb'voiatiie solids added"
,12 to I
y 0 6 to 1
lb of volatile solids destroyed
ft/gaplta/day
Gas. Quality
(yiethane (CH«)
Carbon dioxide (CO2)
Nitrogen (N2)
Hydrogen (Ha)
Hydrogen suifide (H2S)
Water vapor
BTU'FT3
65-70%
25-30%
Trac;e
Trace
Trace
Trace
550 to 600
Table 1 Characteristics of Digester Gas
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Figure 2 Energy Flow Through an Anaerobic
Sludge Digestion System
The energy value of the methane generated from
the anaerobic digestion process (Figure 2) exceeds
the energy requirements of the digestion process
for mixing and heating the sludge. It is this excess
energy in the form of methane gas which can be
used to supply other energy needs including:
Production of steam or hot water
Fuel for internal combustion engines or gas
turbines
Domestic or industrial gas supply
Digester gas may require treatment depending on
ultimate use of the gas. It is generally preferable to
use recovery systems that can operate on
untreated digester gas. Minimal treatment is
required for combustion in boilers or internal
combustion engines. However, gas sold to local
gas utilities requires treatment to upgrade the gas
to pipeline quality.
Treatment of digester gas to remove impurities
such as water or hydrogen sulfide is routine. Simple
water traps are located upstream of recovery
equipment such as boilers and are installed in the
system at low spots to collect condensed water
vapor. The combustion products of hydrogen sulfide
can be corrosive in the presence of moisture by
forming sulfuric acid; removal of this constituent is a
key consideration in system design. Hydrogen
sulfide is most commonly removed by iron-sponge
scrubbers consisting of wood shavings impregnated
with iron oxide. The iron oxide reacts with hydrogen
sulfide to form a non-volatile compound and is
removed from the gas stream.
Present Use
The processes used to recover and utilize methane
from sludge digestion are quite varied. Table 2:lists
four selected facilities, the digester gas recovery
processes employed at each plant, and the energy
uses of the recovered energy.
,; /Facility
Chaitotle, Mi
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feukesha, WL
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Whitewater, Wl
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Energy
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designed to operate on digester gas with natural
gas as a backup fuel are one of the most
commonly employed energy conversion processes.
Figure 3 is a photograph of one such boiler. The
hot water or steam from these boilers can be used
to heat sludge, water, buildings, or generate
electricity.
Potential Limitations
Possible limitations of methane recovery include:
Applicability - A variety of sludges from municipal
wastewater treatment can be stabilized by
anaerobic digestion; however, decreased plant
performance may result from additions of some
chemical sludges and activated sludges because
the additional solids do not readily settle after
digestion.
System Reliability - The microorganisms that
generate the methane are sensitive and do not
function well under fluctuating operating
conditions. The process must be carefully
evaluated for use at treatment plants where wide
variations in sludge quantity and quality are
common. A backup source of fuel is necessary to
assure continuity of operation.
Gas Characteristics - Impurities in digester gas
can cause operational problems, increase
maintenance costs, and give rise to air emission
problems. Hydrogen sulfide and its combustion
by-products can cause corrosion in energy
recovery systems if effective treatment of the
digester gas is not achieved.
Costs
Although the basic mechanical components needed
to recover and utilize methane gas are relatively
standard for each project, the economic feasibility
can be highly variable for specific projects. The
characteristics of the raw sludge and the digester
gas can have a major impact on costs. Also, the
type and cost of other available energy alternatives
at the project location will be important in assessing
the cost-effectiveness of utilizing the methane
recovery process.
An example of the cost savings of using methane
recovery can be found at the Charlotte, Michigan,
wastewater treatment plant (Figure 4). The
Figure 4 Charlotte, Michigan Digesters
construction bid price for the complete wastewater
treatment plant was $4,895,000 in 1978 for a
design flow of 1.2 MGD. Included in that cost was
an estimated $573,000 for all of the energy
recovery facilities, including the digester installation
and other components which would have been
installed even if methane recovery was not
practiced. The expenditures related only to the
methane recovery portion of the project amounted
to approximately $50,000 (in 1979 dollars). It has
been estimated that this additional expenditure for
methane recovery resulted in an energy savings of
at least $11,000 per year in 1982.
Summary
Digester gas is a viable source of energy and can
be made to perform a variety of useful functions
which significantly reduce operating energy costs.
Digester gas can be used for digester and
building heating or as fuel for internal combustion
engines that pump sewage, operate blowers, and
generate electricity.
Research and development on anaerobic
digestion of municipal wastewater treatment
sludges has led to increased process control and
reliability, making methane recovery an attractive
energy source.
The anaerobic digestion of sludge reduces the
quantity of sludge by conversion to methane,
carbon dioxide, and water. The residue that
remains is stabilized and can be land applied as
a soil conditioner.
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