Charles Fulhage, Dennis Sievers and James R. Fischer
 Department of Agricultural Engineering, College of Agriculture

     At first glance, the idea of generating methane gas has
considerable merit because it appears to offer at least a partial
solution to two pressing problems-the environmental crisis and the
energy shortage. Unfortunately, present-day large-scale methane
generation requires rather high investments in money and management
which considerably reduce the practicality of the idea for the
farmer. This Guide is intended to provide quantitative information
so that the feasibility of methane generation can be evaluated for
a given situation.

small version, large version; Figure 1. Anaerobic digestion

Anaerobic process

     Livestock manure contains a portion of volatile (organic)
solids which are fats, carbohydrates, proteins, and other nutrients
that are available as food and energy for the growth and
reproduction of anaerobic bacteria.

     The anaerobic digestion process occurs in two stages.
Initially the volatile solids in manure are broken down to a series
of fatty acids. This step is called the acid-forming stage, and is
carried out by a particular group of bacteria called acid formers.
In the second stage, a highly specialized group of bacteria, called
methane formers convert the acids to methane gas and carbon
dioxide. The biochemical reactions (greatly simplified) are shown
in Figure 1.

     The anaerobic process depends on methane formers because they
are more environmentally sensitive than acid formers. Methane
bacteria are strict anaerobes and cannot tolerate oxygen in their
environment. They function best at 950 F; therefore to obtain
maximum gas production, heat usually must be added to a digester.

     Methane bacteria are slower growing than acid-forming bacteria
and are extremely pH-sensitive (pH 6.8-7.4 optimum). Should an
excess of organic material be fed to a digester, the acid formers
will grow rapidly, producing an excess of volatile acids. The
accumulated acids will lower the pH inhibiting the methane bacteria
and stopping gas production.  To help buffer the system against
increases in acids, high alkalinity must be maintained. Lime has
been added to digesters during start up or periods of slug
loading to maintain pH control.

     A variety of materials can become toxic to anaerobic
bacteria-salts, heavy metals, ammonia and antibiotics.  Bacteria
require minimum amounts of salts for optimum growth. However, if
salts are allowed to accumulate beyond bacterial requirements, they
can become toxic and inhibit digestion.

     Soluble heavy metals (copper, zinc, nickel) may be toxic to
digester bacteria. Most heavy metals can be precipitated out with
sulfides and will cause no problems in the sludge.  Livestock feeds
containing significant amounts of heavy metals may require special

     Most livestock manures (particularly swine and poultry)
contain appreciable amounts of nitrogen which will be converted to
ammonia in the digester. Most of the ammonia will accumulate in the
digester material and will become toxic if not controlled. Ammonia
toxicity is a major concern in the anaerobic digestion of livestock
manures. To avoid the problem, loading rates must be carefully

Methane Production Potential

     The immediate and obvious benefit from methane production is
the energy value of the gas itself.  Hence, the question most
frequently asked concerning the process is: "How much gas will 1
get?" The answer to this question depends on several factors which
determine the efficiency of the operation.


                           SWINE     DAIRY     POULTRY     BEEF
                          150 lb    1200 lb   4 lb bird   1000 lb

Gas yield cu. ft. per
lb volatile solids
destroyed                  12        7.7        8.6         15
Volatile solids voided
lb/day                      0.7      9.5        0.044        5.0
Percent reduction of
volatile solids            49        31        56           41
Potential gas pro-
duction cu. ft. per
animal unit per day         4.1      22.7       0.21        31
Energy production
rate. BTU/hr/animal       103       568         5.25       775
Available energy
BTU/hr [after heating
digester)                 70        380         3.5        520

     In the anaerobic process, a certain amount of gas is produced
per pound of volatile solids broken down or destroyed by the
bacteria. This is referred to as "gas yield," and average values
are given for the various animal species in Table l. During
digestion, only a fraction of the volatile solids contained in the
raw manure are broken down or destroyed by the bacteria. Average
percentages for volatile solids breakdown for different animal
species are also given in Table 1.

     If the weight of volatile solids produced by an animal or bird
per day is estimated, the potential gas production for each species
on a daily basis can be calculated. For example, a 15 pound hog
will produce about 0.7 pound volatile solids per day. Of this, 49
per cent (or 0.34 lb) is broken down by bacteria. Since about 12
cubic feet of gas per pound of volatile solids are produced by
bacteria, a 15 pound hog has the potential to produce about 4.1
cubic feet of gas per day. These data are also summarized in Table
l for each species.

     The gas obtained in anaerobic digestion of animal wastes is a
mixture of carbon dioxide and methane with trace amounts of
hydrogen sulfide and hydrogen gas. Typically, the mixture is
composed of about 60 per cent methane and 40 per cent carbon
dioxide regardless of the type of waste. Pure methane has heat
value of about 1000 BTU per cubic foot, so we can expect the
methane carbon dioxide mixture to have a heat value of about 600
BTU per cubic foot. Heat value data for the various species on "per
hour basis is given in Table 1. Typically about one-third of the
energy in the manure gas is needed to maintain the necessary 950 F
temperature in the digester. Hence the energy available for other
uses is two-thirds of the total energy produced. These values are
listed in the last row of Table 1.


                Heat Requirement  Swine   Dairy   Poultry    Beef
                   BTU/hr        150 lb  1200 lb  4 lb bird 1000 lb
Kitchen range^1    65,000           77      14    1,547       11
Water Heater^2     45,000          107      20    2,143       15
Refrigerator^3      3,000           22       4      429        3
Heat 1500 sq ft^4
home               37,500          535      99   10,714       72
ln-bin grain
drying heater^5 2,000,000       14,285    2631  285,714    1,923
50 hp tractor
operating at
full load^6       637,000         4550    8311   91.000        612

1 Assumed to operate 2 hr/da, i.e. a 24 hr average of 5417 BTU/hr
2 Assumed to operate 4 hr/da, 24 hr average = 7500 BTU/hr
3 Assumed to operate 12 hr/da, 24 hr average  1500 BTU/hr
4 Assumed 25 BTU/hr/sq ft heat requirement
5 Assumes to operate 12 hr/da, during drying season. 24 1st average 
  1,000,000 BTU/hr
6 Assumed to operate 12 hr/da 24 hr average = 318.500 BTU/hr

     To be meaningful these heat values must be compared with some
typical heat requirements which might logically use the gas as an
energy source. Table 2 lists several typical farm heat requirements
that could possibly use manure gas as an energy source, and the
numbers of the various animals needed to supply energy at the
required rate. Obviously, the best possibilities for using manure
gas are the various heating requirements associated with the home.

     High energy requirements such as grain dryers and tractors arc
not compatible with methane generators because of the numbers of
animals required. It should be noted that most firm heat
requirements are seasonal, and the problem of how to best use the
gas in the "off" season exists.  Storage of the gas is one
possibility and will be discussed later.

     Data presented in Tables 1 and 2 were generated entirely from
laboratory experiments. In many cases there is a considerable loss
in efficiency when such an operation is conducted in a large-scale
field situation. For example, in calculating the potential gas
production per animal it was assumed that all the volatile solids
voided by the animal were introduced into the digester. In the
practical situation, a portion of the volatile solids will likely
be lost en route to the digester. Perhaps a more significant loss
would be the degradation of a portion of the volatile solids
between the time of excretion and introduction into the digester.
Such losses indicate the need for an efficient manure handling
system which effects continuous feeding of manure into the
digester. It is possible that the gas production values listed in
Table 1 could be decreased by as much as 50 percent, depending upon
the efficiency of the system.

Design and Equipment

     The design volume of an anaerobic digester is sized according
to the amount of volatile solids that must be treated daily, and
the period of time the material remains in the digester (detention
time). Loading rates are normally expressed in pounds of volatile
solids #VS) per cubic foot of digester volume, loading rates and
detention times for various livestock are presented in Table 3. The
loading rates as listed in Table 3 are designed to maintain the
necessary bacterial balance and prevent ammonia toxicity from


                   Swine        Dairy       Poultry   Beef

Loading rate,       0.14         0.37        0.12     0.37
lb volatile
solids per
cu. ft.
per day
Detention time.    12.5         17.5        10.      12.5
Digester            5.0         26.          0.37    13.5
cu. ft./
Digester      2500/20,000   1950/15,000  5550/42,000  4050/30,4000
cu. ft./gallons
 Swine-500 head
 Dairy-75 head
 Poultry-15.000 birds
 Beef-300 head

     The equipment necessary to generate usable quantities of
methane is not simple and requires a substantial investment. Figure
2 illustrates the basic components required to operate a continuous
mix, heated anaerobic digester.

     The primary structure consists of a digestion tank, usually
cylindrical in shape to promote better mixing. Most tanks are
constructed of concrete and must be strong enough to withstand the
weight and pressures of the contained liquid.  The bottom is
generally cone-shaped to facilitate sludge removal. The top can be
fixed or floating. A floating top provides expandable gas storage
with pressure control but costs more and is more difficult to
manage. Regardless of the tank shape or top used, the structure
must be airtight.  Methane gas when mixed with oxygen is highly
explosive. Most tanks are at least partially set below ground

small version, large version; Figure 2. Anaerobic Digester

     This helps support the structure and provides some insulation.
That portion of the tank above ground may have to be insulated to
minimize heat losses.

     Mixing aids digestion by continually bringing the bacteria in
contact with the waste material and by distributing the heat more
uniformly. Mixing can be accomplished by l( recirculating the gas
collected from the top of the tank, or 2) mechanical mixers. Gas
recirculation does a better job of mixing equipment costs are

     Heat may be added by circulating the digester material through
an external heat exchanger as shown in Figure 2 or by pumping
heated water through a heat exchange coil inside the digester (not
shown), External heat exchangers are difficult to use because of
the corrosive nature of the digester liquid. The special,
non-corrosive materials required are expensive and the heated water
approach is normally used.

     Pumping of digester contents and sludge removal will
necessitate using special solids (sludge) handling pumps. All
piping must be of sufficient size to prevent clogging. To use the
methane gas as an energy source requires some gas collection and
pressure regulation equipment including the necessary safety
devices to prevent explosions.

Digester Management

     A digester must be loaded with manure on a regular basis to
insure a continuous supply of food for the anaerobic bacteria.
Manure collection from the livestock production facility and
feeding to the digester should be done at least once daily.
Intermittent or slug loading of manure can cause acid build-up,
upsetting the bacterial balance and reducing gas production. Once
the bacterial populations are disrupted, several months may be
required to stabilize them.

     A manure slurry of the proper solids content is required to
maintain correct loading rates and detention times and to
facilitate mixing and pumping. Loading rates given in Table 3
result in solids content of 2 to 10 percent in the feed material.
Depending on the livestock enterprise and manure collection system.
some method of diluting and mixing the raw manure must be
incorporated into the manure-handling system.

     Sustained performance of an anaerobic digester depends heavily
on proper management of the chemical and physical environment
within the digester. The contents of the digester should be
monitored at regular intervals. The best indicators of digester
imbalance are 1) decreasing gas production, 2) decreasing pH, 3)
decreasing methane/carbon dioxide ratio, and 4) increase in
volatile acids.

     Imbalance in the system may be due to a change in a)
temperature, b) loading rate, c) nature of the waste.  The addition
of toxic materials such as antibiotics can also cause unbalance.

     When an unbalanced condition is discovered, maintain pH
control until the cause of the upset is discovered. The pH can be
controlled by reducing the feed to the digester and/or adding lime.
If the feed is reduced, the waste flow from the production
facilities will have to be handled in an alternative manner.

Management of Digester Gas

     Even with energy shortages, problems exist with how to best
handle or use gas generated from manure. Basically two things can
be done-store the gas as it is produced or burn the gas as it is
produced to fulfill some energy requirement. In reality, a
practical system would involve a combination of the two, but for
purposes of discussion we will consider them separately.

     Methane, unlike propane, does not liquefy under reasonable
pressures and temperatures. While propane can be liquefied at
pressures in the 130-250 psi range at ambient temperatures, methane
does not liquefy at any pressure if the temperatUre is greater than
-160 F. Fuel contains the greatest heat value per unit volume when
in liquid form. Methane has a relatively low heat value per unit
volume because it does not liquefy at normal storage pressures and
ambient temperatures.

     For example, assume that we wanted to store two months
accumulation of gas from a 500-head swine methane generation unit.
From Table l, we see that this amounts to 70 BTUs/hr. x 720 hr./mo. 
x 2 mo. = 100,800 BTUs/per hog.  At 600 BTUs/cu. ft. this amounts
to 168 cu. ft. per hog, or 84,000 cu. ft. for 500 hogs. If the
methane is compressed at 5500 psi, a 224 cu. ft. storage tank would
be required 3 ft. diameter by 32 ft. long).

     By comparison, an equal number of BTUs (energy) is contained
in 548 gallons of propane, and this would require a tank only 3
feet in diameter and 10 feet long at a pressure (130-250 psi) much
less than that of the methane.  Another important factor is the
strength required in the walls of a tank in which gas is stored at
very high pressures. For pressures greater than about 1000 psi,
storage tanks more than a foot or so in diameter must have extra
wall thickness, which makes such storage impractical.

     Because of the relatively low heat value of methane (compared
to propane and other liquid fuels) and its difficulty to liquefy
under reasonable pressures, methane is impractical to store in
large amounts. Hence most storage applications would likely involve
only short-term accumulations of methane.

     The other obvious alternative to storage of methane is to use
it as it is generated. The easiest way to accomplish this is to bum
the gas as it flows from the digester. Obviously this isn't
efficient use of the gas unless the flame is used to fulfill a heat
requirement. The most frequently proposed use is home heating with
the methane gas.

     From Table 2 notice that 535 hogs would be required to heat a
1500 square foot home, and almost 800 hogs would be required to
fulfill the heating, water heating, cooking range, and
refrigeration demands of a typical home.  This illustrates that it
may be feasible to use methane to some extent in fulfilling the
energy demands of the household.  Such demands are seasonal,
however, and gas utilization in the "off" season remains a problem.
For reasons already mentioned, storing "off season" (summer) gas
until winter probably is not feasible.

     Energy demands of grain dryers and tractors are high enough
that using methane is not practical.  Because of the relatively low
heat value of methane, tremendous volumes are required to fulfill
such demands, and most livestock operations simply do not have
sufficient numbers of animals available to meet those demands.

     An alternative often proposed which would solve the "seasonal"
problems of heating energy demands involves the burning of methane
in an internal combustion engine.  The engine, in turn, drives an
alternating current generator which supplies power to the farmstead
electrical system.  With proper regulating and controlling
equipment, this power could be used to its maximum availability,
and at the same time "power supplier" electricity need would be
proportionately reduced. However, in such a scheme there is a
significant energy loss in converting energy from the gas form to
the electrical form. Internal combustion engines are about 25
percent efficient, and generators are about 85 percent efficient.

     Consider, for example, a 500-hog production facility which
would yield about 103 BTU per hog per hour (Table 1). In this
example, we are using the higher BTU production rate (103 as
opposed to 70) because excess heat from the engine coolant can be
used to heat the digester. For 500 hogs producing energy at the
rate of 103 BTUs/hr we have a total energy production of 51,500
BTUs/hr. If we were able to convert this energy directly into
electricity with 100 percent efficiency, we would have
about 15 kilowatts of power, or about 15 horse power.  However,
since our total efficiency is only 0.25 x 0.85 = 0.21 we will
realize only 51,500 BTUs/hr. x 0.21 = 10,815 BTUs/hr. or about 3.2
kilowatts of electrical power from the original 15 kilowatt
equivalent.  Hence the advantages this system offers in eliminating
"seasonal" energy demands may be offset by the heavy loss in
efficiency in converting the energy from gas to electricity.

     Efficient management and use of digester gas will continue to
be a problem. Research is needed to investigate more efficient
methods of energy conversion and methods of concentrating the
energy in methane gas to eliminate storage difficulties.

Digesters as Waste Management Components

     The above discussion has been concerned with the energy
aspects of anaerobic digestion.  A digester will also be an 
integral part of the waste management system, and the advantages
and disadvantages should be reviewed from a waste management

     A primary advantage of an anaerobic digester is its ability to
nearly completely stabilize raw manure. As a result, the effluent
from a properly operating digester is relatively odor-free and odor
problems usually associated with production facilities and disposal
operations may be reduced.

     Another advantage of anaerobic digestion is nearly complete
retention of the fertilizer nutrients (N.P.K.) that were in the raw
manure. Nutrient losses may occur in subsequent handling of the
effluent. This advantage may become more significant in the future
if fertilizer shortages become more acute.

     Another advantage of the anaerobic digester is its ability to
stabilize more waste per unit volume than other treatment
facilities such as lagoons. This advantage is offset in most cases
by the fact that a lagoon will probably be required for storage of
digester effluent until such time that it can be used in irrigation
or otherwise distributed over the land.

     This brings up the point then, that a digester is not a
complete disposal tool in itself. The volume of liquid effluent
from a digester is the same as the liquid volume of waste
introduced into the digester. Hence, there is no reduction in
liquid volume of waste to be handled due to the action of the
digester. The digester does reduce the amount of solids to be
handled and provides relative odor-free treatment.

     Digester effluent is not suitable for discharge into streams;
there are usually less expensive systems which conform with the
pollution regulatIons set forth by the Missouri Clean Water


     Some energy can be extracted from manure through anaerobic
digestion. But this would likely comprise only a small fraction of
the total energy needs of a typical farm. High investments in money
and management along with difficulties in efficiently using methane
make anaerobic digestion a questionable venture for most farmers.

     If energy and fertilizer shortages become more acute, and
pollution regulations concerning odor become more strict, methane
generation may become a feasible process In waste management
systems. Research is needed to reduce capital costs of methane
generation systems and provide techniques for proper management of
such systems.

Issued in furtherance of Cooperative Extension Work Acts of May 8
and June 30, 1914 in cooperation with the United States Department
of Agriculture.  Carl N. Scheneman Vice President for Extension.
Cooperative Extension Service. University of Missouri and Lincoln
University Columbia, Missouri 65211

An equal opportunity institution.