United States	Municipal Environmental Research
Environmental Protection	Laboratory
Agency	Cincinnati OH 45268
Research and Development	EPA-600/S2-84-039 Mar. 1984
Project Summary
Sale of Surplus Digester and
Landfill Gas to Public Utilities
Cynthia A. Cairns and Albert B. Pincince
Gas produced by anaerobic digestion
of wastewater can be upgraded and
sold to public utilities. Upgrading the
gas involves treatment to remove
carbon dioxide and hydrogen sulfide,
dehydration, and compression.
Upgrading digester gas is technically
feasible and less expensive than current
prices for natural gas. Several waste-
water plants have investigated the use
of digester gas, but no programs have
been instituted, primarily because
other gas-use alternatives (involving
on-site uses) are considered more eco-
nomical.
Gas generated in solid waste landfills
is similar to digester gas. Several
programs to use landfill gas have been
implemented and more are planned.
Because landfill gas cannot usually be
used on-site and because larger
quantities are typically produced,
landfill gas has a greater potential for
future development by public utilities
than digester gas.
This Project Summary was developed
by EPA's Municipal Environmental Re-
search Laboratory. Cincinnati. OH. to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Methane gas is generated as a
byproduct of (1) anaerobic digestion of
wastewater sludge and (2) solid waste
landfills. With proper recovery and treat-
ment, the gas can be upgraded and sold to
public utilities.
Anaerobic digestion is widely used in
wastewater treatment plants to stabilize
sludge. Sludge is maintained at about 90
°F for 10 to 30 days in the absence of free
oxygen. The organic matter decomposes,
and the pathogen content is reduced.
Decompositibn primarily produces
methane, carbon dioxide, and water. The
energy in the gas is greater than the
energy required to heat and mix the
sludge in the digester, and the surplus
gas is available for other uses.
Many wastewater plants flare the
surplus gas. With today s energy costs,
however, utilizing the gas has become
economically beneficial, particularly for
plants producing large volumes. The gas
can be sold to utilities and added to their
natural gas distribution systems. This
involves upgrading the gas and conveying
it to a distribution pipeline.
Solid waste, which decomposes in
landfills, produces a gas similar to
digester gas, and several programs have
already taken advantage of landfill gas.
These programs involve steps to recover
and upgrade the gas before it can be sold.
This study examines the sale of surplus
digester and landfill gas to public utilities.
It was conducted by contacting individ-
uals involved 
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Before gas can be injected into a
natural gas distribution system, it must
be upgraded to pipeline standards and it
must be pressurized. General require-
ments for purifying digester gas include
removing carbon dioxide to increase
heating value, removing hydrogen sulfide
to meet air quality standards and to elimi-
nate corrosion, and dehydration to elimi-
nate corrosion and the formation of
condensate that could restrict pipeline
flows. Although the requirements vary
significantly from one utility to another,
"purified" gas should have less than 3
percent carbon dioxide, less than 2.7 ppm
of hydrogen sulfide/:a,minimum heating.,
value of 36,300 kJ/stdcum (975 BtO/ ;
scf), and less than 0.1 mg/L (7 lb/106"cu
ft) of water. Hydrogen sulfide and water
are the most critical contaminants.; High
concentrations of either can cause severe '
corrosion problems.
Most public utility distribution systems
can accept whatever quantities of
digester gas are available. No minimum
natural gas quantity is required to satisfy
the demand, in that the ratio of digester
gas to natural gas in the distribution
system would be small.
The economics of gas treatment vary
from one plant to another. Factors to be
considered include the characteristics
and quantities of surplus digester gas,
variations in production, quality
standards required for pipeline injection,
and costs of treating, compressing, and
transporting the gas. The price the utility
will pay is another important factor. But
the price can vary greatly, depending on
the availability and cost of conventional
energy resources and on the utility's
assessment of the viability of the plan.
The potential revenue must also be
compared with the revenue generated
from selling the gas to a private user off-
site, or the energy savings from using the
gas on-site, to determine if selling to a
utility is economical.
One of the most significant factors
influencing a utility's decision to
purchase digester gas is the perceived
risk associated with using the energy
resource. Although the purification and
sale of digester gas for use in a natural
gas distribution system appears to be
technically an economically viable,
utilities have.been unwilling to commit
themselves, primarily because of the
financial risk for relatively small
quantities of gas.
Existing regulatory policies can
seriously constrain the sale of digester
gas because of price control, limited
government funding, lengthy permit pro-
cessing, and air pollution requirements
associated with gas processing facilities.
Moreover, no specific regulatory policy
governs the sale of digester gas to a local
utility. And numerous federal, state, and
local regulations overlap or conflict in
many cases.
Digester Gas Treatment
Before digester gas can be injected into
a utility's distribution system, it must be
upgraded to pipeline standards.
Purification usually involves removing
carbon dioxide, hydrogen sulfide, and
moisture from the gas. Gas purification
processes are often referred to as
scrubbing or sweetening processes.
"Sour" gas, containing impurities such as
hydrogen sulfide and carbon dioxide, is
converted to "sweet" gas, free of
contaminants.
Several commercial gas purification
processes have been proven technologi-
cally in the petroleum and energy indus-
tries. And many processes can be readily
adapted. The quantity of gas required to
make treatment with these processes
economical is, however, typically greater
than the amount of gas available.
Selecting appropriate processes for a
facility depends on many factors,
including the degree of purification
reqired, the need to recover impurities,
the quantity of gas, the presence of trace
contaminants, the plant's location, and
available financing.
Competing Uses for
Digester Gas
At wastewater treatment plants,
digester gas is usually used for fuel to
keep the digester at its required temper-
ature. Untreated digester gas is typically
fed to boilers for combustion, generating
steam or hot water. The steam or hot
water then heats the raw sludge by
pumping the sludge through heat
exchangers or by steam injection.
Oepending on ambient air temperatures,
one-third to one-half of the digester gas
produced is required to heat the digester.
The rest is available for other uses.
Surplus gas can be flared, used to
satisfy other energy needs within the
plant, or sold to off-site users, such as
public utilities or nearby industries.
Alternatives implemented at plants will
depend on local power costs, energy
requirements of the plant, quantities and
characteristics of the surplus gas,
proximity of off-site users, and existing
facilities. If the quantity of surplus gas is
small or if the projected energy saving
does not justify operating costs, the
surplus gas is flared.
In-plant uses include space heating
and cooling, driving internal combustioti
engines, generating power, incinerating
sludge, and process heating. Off-site
uses include selling the gas to private
industries, upgrading for use as vehicle
fuel, producing and selling excess steam
or hot water, selling electricity produced
by on-site cogeneration, and chemical
conversion.
Digester Gas Utilization
Programs
Most of the current interest in digester
gas use by public utilities has been
centered in California. The California
State Legislature adopted utility
regulations that, make off-site use of
digester gas eiasier to implement.
Moreover, because several wastewater
treatment plants serve large population
centers and because the warm climate
reduces in-plant energy demands, larger
quantities of digester gas are available for
sale.
In-plant gas utilization programs are
being conducted in Chicago, IL; Denver,
CO; East Bay Municipal Utility District,
CA; Fresno, CA; Livermore, CA; New
York, NY; Philadelphia, PA; Sacramento
Regional County Sanitation District, CA;
and S>an Diego, CA. Off-site gas
utilization programs are being conducted
in Los Angeles, CA; County Sanitation
Districts of Los Angeles, CA; New York,
NY;- Sanitation Districts of Orange
County, CA; and Washington, D.C.
Using Landfill Gas
Methane gas generated in sanitary
landfills is similar to digester gas. As with
digester gas, interest in using landfill gas
as an alternative energy source has
increased.
As municipal waste in landfills decom-
poses, methane is produced. In the past,
methane generation and migration in
landfills have created potential explosive
hazards. Several large landfills have had
to install gas collection systems to
prevent migration of gas to adjacent
property where explosive mixtures could
form. The collected gas can either be
vented to the atmosphere or incinerated.
With today's energy prices, it has become
economically beneficial for some larger
landfills to recover, process, and use the
gas.
Landfill gas can be used in various
ways: upgrading and then injecting the
gas into a natural gas pipeline; partially
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upgrading and selling to a nearby
industry; generating power; producing
steam; and conversion to liquified natural
gas or methanol.
Conclusions
Digester Gas
•	Selling upgraded surplus digester
gas to a public utility for injection
into a natural gas pipeline is both
feasible and economical. No
programs have been implemented in
the United States, however, mainly
because other gas-use alternatives
have been considered more viable.
•	Wastewater treatment plants usu-
ally choose on-site use of digester
gas for the following reasons: (1)
most of the energy made available by
digester gas can be used in-plant; (2)
it is relatively easy to implement in-
plant programs of gas use; (3) the
technology for in-plant gas use is
proven; (4) in-plant gas use requires
minimal gas treatment; (5) public
utilities, which represent the off-site
markets for digester gas, tend to
believe that using digester gas will
force them to accept disproportion-
ate risks in terms of reliability,
quality, and institutional factors,-
whereas the use will only offset a
tiny fraction of their natural gas
needs; and (6) complex and vague
regulatory policies concerning off-
site sale of digester gas at every level
of government can delay
implementation.
•	Alternatives to selling surplus
digester gas to utilities include: (1)
in-plant use, which is widely
practiced; (2) conversion of gas to
vehicle fuel and sale; and (3) upgrad-
ing and sale to a private user.
Landfill Gas
•	Gas generated at solid waste land-
fills is similar to digester gas, and it is
generated in large quantities.
•	Several programs of recovering and
using landfill gas have been
implemented or are in the planning
stages.
•	When compared with digester gas,
landfill gas has greater potential for
public utility development because
larger quantities are produced, thus
making the economics more
favorable, and because no
competing on-site uses exist.
The full report was submitted in fulfill-
ment of Contract No. 68-03-2803 by
Camp, Dresser and McKee, Inc., under
sponsorship of the U.S. Environmental
Protection Agency.
Cynthia A. Cairns and Albert B. Pincince are with Camp Dresser and McKee, Inc.,
Boston, MA 02108.
R. V. Villiers is the EPA Project Officer (see below).
The complete report, entitled "Sale of Surplus Digester and Landfill Gas to Public
Utilities," (Order No. PB 84-155 555; Cost: $10.00, subject to change) will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
it U.S. GOVERNMENT PRINTING OFFICE: 1984 — 759-015/7607
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United States	Center for Environmental Research	BULK RATE
Environmental Protection	Information	POSTAGE & FEES PAID
Agency	Cincinnati OH 45268	EPA
PERMIT No G-35
Official Business
Penalty for Private Use $300
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