TECHNICAL PAPER #4
UNDERSTANDING BIOGAS GENERATION
C. Gene Haugh
1600 Wilson Boulevard, Suite 500
Arlington, Virgnia 22209 USA
Tel: 703/276-1800 . Fax: 703/243-1865
Understanding Biogas Generation
[C]1984, Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in
Assistance to provide an introduction to specific
technologies of interest to people in developing countries.
The papers are intended to be used as guidelines to help
people choose technologies that are suitable to their
They are not intended to provide construction or
details. People are
urged to contact VITA or a similar organization
for further information and technical assistance if they
find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and
almost entirely by VITA Volunteer technical experts on a
Some 500 volunteers were involved in the production
of the first 100 titles issued, contributing approximately
5,000 hours of their time.
VITA staff included Leslie Gottschalk
as primary editor, Julie Berman handling typesetting and
and Margaret Crouch as project manager.
Richard Mattocks, author of this paper, is an environmental
scientist with Draper-Aden Associates, Inc.
He specializes in the
management of solid waste materials and the recovery of
products, and is currently researching various uses of biogas
digester effluent, particularly its use as an animal feed
Reviewers J.B. Farrell, C. Gene Haugh, and Daniel Ingold are
specialists in the area.
Farrell is a chemical engineer by training
and chief of the Sludge Management Section of the U.S.
Protection Agency's Municipal Environmental Research
heads the Department of Agricultural Engineering
at Virginia Polytechnic Institute.
Ingold, a biophysicist, is
a research engineer at Appropriate Technology Corporation.
VITA is a private, nonprofit organization that supports
working on technical problems in developing countries.
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to
maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster
volunteer technical consultants; manages long-term field
and publishes a variety of technical manuals and papers.
UNDERSTANDING BIOGAS GENERATION
Volunteer Richard Mattocks
Biogas is a by-product of the biological breakdown--under
conditions--of organic wastes such as plants, crop residues,
wood and bark residues, and human and animal manure.
biogas as a viable energy resource has spread throughout the
globe in the past two decades.
Biogas generators or digesters
operate throughout Asia, for example, with more than 100,000
reported in India, about 30,000 in Korea, and several
China. Many more are
operating in the Middle East, Africa,
Oceania, Europe, and the Americas.
Biogas is known by many names--swamp gas, marsh gas,
"will o' the
wisp," gobar gas.
It contains about 50 to 60 percent methane, the
primary constituent of natural gas.
Biogas is produced naturally
from the degradation of plants in such situations as rice
paddies, ponds, or marshes.
Because it can also be produced and
collected under controlled conditions in an airtight
it can be an important energy source.
Ancient Chinese experimented with burning the gas given off
vegetables and manures were left to rot in a closed
recently, Volto, Beachans, and Pasteur worked with
organisms. At the
turn of the 20th century, communities in
England and Bombay, India, disposed of wastes in closed
and collected the resulting gas for cooking and lighting.
Germany, the United States, Australia, Algeria, France, and
nations constructed such methane digesters to supplement
dwindling energy supplies during the two world wars.
NEEDS SERVED BY THE TECHNOLOGY
Biogas generators or digesters yield two products: the
itself, and a semi-solid by-product called effluent or
Biogas systems are most popular for their ability to produce
from products that might otherwise be wasted--crop residues,
manures, etc. The
fuel is a flammable gas suitable for cooking,
lighting, and fueling combustion engines.
The digested waste--sludge--is a high quality
digestion process converts the nitrogen in the organic
to ammonium, a form that becomes more stable when plowed
soil. Ammonium is
readily "fixed" (bonded) in soil so that it can
be absorbed by plants.
In contrast, raw manure has its nitrogen
oxidized into nitrates and nitrites, which do not
"fix" well in
soil and are readily washed away.
Moreover, biogas systems offer a means to sanitize
put, these systems are capable of destroying most bacteria
parasitic eggs in human and animal wastes, enabling the
sludge to be applied safely to crops.
Tests have shown that
biogas systems can kill as much as 90 to 100 percent of
eggs, 35 to 90 percent of ascarid (i.e., roundworms and
and 90 to 100 percent of blood flukes (i.e., schistosome
flukes, which are found in water snails that commonly live
paddy fields and ponds).
Biogas systems are also capable of digesting municipal
which is a major source of pollution.
Using biogas systems in
this way substantially reduces the potential for
Finally, agricultural and animal wastes, the major raw
for biogas production, are usually plentiful in rural areas.
People living in rural communities, who are often subjected
the price and supply fluctuations of conventional fuels and
fertilizers, can benefit directly from biogas systems.
It should be noted that, while this paper focuses on the
of biogas for fuel, in some applications the gas is
to be the by-product of the process.
Some digesters in
China, for example, are used primarily for treating sewage
producing fertilizer, and only secondarily for producing
BASIS OF THE TECHNOLOGY
Biogas generation is a process that takes place in an
environment. It uses
anaerobic bacteria--bacteria that live only
in the absence of oxygen--to break down complex organic
in fairly well-defined stages.
The process is called anaerobic
produces biogas, a gas composed of approximately 50
to 60 percent methane, 40 to 50 percent carbon dioxide, as
as water vapor and a small quantity of nitrogen, sulfur, and
other trace compounds.
Biogas is flammable, which is what makes
it useful, but it has a relatively low heat content,
6.1 Calories per liter (around 600 BTU per cubic foot).
this with pure methane, which has a heat value of 995 BTU
per cubic foot, or natural gas with over 1,000.
biogas can be an important fuel source for many
A biogas digester is the device in which the digestion
occurs. The organic
feedstock, which is called the substrate, may
consist of night soil, manure, crop or kitchen residues, or
The substrate is usually diluted with water,
and is thoroughly mixed into a slurry; crop residues and
are usually cut or chopped into small, fairly uniform
pieces. It is then
fed into the digester and permitted to undergo
degradation in a sealed oxygen-free chamber.
When digestion is
completed, the material is discharged, or removed from the
The biogas is collected for direct usage or pressurized for
subsequent use. The
discharged material is called effluent, or
The actual breakdown of organic material inside the digester
three-stage process that leads to the production of methane
In the first stage, numerous organisms release enzymes that
attack specific bonds in complex protein, carbohydrate, and
compounds in the incoming substrate.
This stage of degradation
converts the compounds into simpler molecules.
Another set of
organisms further degrades the molecules to form short-chain
volatile fatty acids.
At this point, various methane-producing
organisms (or methanogens) use carbon dioxide or volatile
acids to produce biogas (a mixture of methane and carbon
The principles of anaerobic digestion are the same
the digestion vessel.
Organic material is loaded into a fairly
warm, temperature-controlled, oxygen-free environment and
is produced after acclimatization.
The makeup or quality of
incoming material to be digested, the vessel, and the
environment influence the digester efficiencies.
of gases is greater when the digester is operated at a
high temperature, when the substrate is stirred or otherwise
agitated, and when system conditions are kept fairly
more detailed discussion of these and other factors
digester efficiency follows.
In general, however, the important
objective to keep in mind when operating a biogas digester
production of the greatest volume of biogas in the shortest
FACTORS INFLUENCING PERFORMANCE AND SIZE OF BIOGAS DIGESTERS
Researchers are only now gaining a better understanding of
metabolic process in biogas digesters.
They do know, however,
that methane-producing organisms (called methanogens)
channel energy, or calories (derived by breaking down
substrate), to methane rather than use the energy to
satisfy internal cellular needs.
As such, methanogens do not
adapt well to changes in their environment that may require
to increase their numbers or adjust their internal
the environmental changes are significant enough, the
may slow or even stop their work.
Changes that may affect the behavior of the bacteria and
performance of the digester include variations in the
presence of certain toxic chemicals, gas pressure,
and the amount of time the material remains in the digester.
Other factors that could have a major impact on the
performance of a biogas digester include biological balance/
acidity, solids concentration, agitation, feedstock,
and the carbon-to-nitrogen ratio.
The primary factors that could affect the size of a biogas
include the type and amount of feedstock, the rate at which
it is loaded, and hydraulic retention time.
Factors Influencing Digester Operating Performance
Methanogens--methane-producing organisms--live in a
or complementary, relationship with certain other
that consume the feedstock and produce simple acids as part
The simplest acids are essential to the metabolic
processes of the methanogens.
As acid-producing organisms
tend to choke in their own acetic by-products, methanogens
by consuming these by-products in the methane-producing
Given sufficient time to establish the proper ratio of
organisms to acid-producing organisms, a homeostasis,
or stability, will occur with a pH of about seven in a
A digester fed poultry or high nitrogen waste may stabilize
pH of eight or greater.
The objective here is to create a stable working
among the microbial population in the digester.
This implies the
need for fairly constant operating temperatures and
Conversely, any rapid variations of these conditions
will cause the microbial population to shift dramatically
and possibly upset the overall system balance in the
For example, if the methane-producing organisms become
due to, say, temperature fluctuations, the pH will drop so
as to incapacitate them.
Maintaining a stable pH requires stabilizing the feedstock
well as the operating temperature in the digester.
If this proves
impractical, adding lime or other buffering compounds to the
digester will prevent the pH from falling.
Note that the correct
amount and type of buffering compound can be determined only
Four additional factors that could affect the overall system
balance in the digester are:
1. The concentration
of the incoming solid waste could vary and
or decrease the amount of food to be consumed
by the digester.
2. Removing the
slurry (the mixture of water and substrate
added to the
digester) from the digester or replacing it
day, will change the average age of the
organisms in the
3. The average
characteristics of the material being consumed
by the microbial
population in the digester will change in
response to any
fluctuations in the amount of feedstock
4. The temperature,
as well as the contents of the water used
to dilute the incoming
waste, will alter the nature of the
food to be
consumed by the digester.
Operating temperature is another factor influencing digester
digester can operate in three temperature ranges:
(1) the low temperature, psycrophilic bacteria range, which
less than 35[degrees]C (90[degrees]F); (2) the medium
bacteria range, which is 29 to 40[degrees]C (85 to
105[degrees]F); and (3) the
high temperature, thermophilic bacteria range, which is 50
55[degrees]C (135 to 140[degrees]F).
Organic material degrades more rapidly at
higher temperatures because the full range of bacteria are
work. Thus, a
digester operating at a higher temperature can be
expected to produce greater quantities of biogas.
of an elevated-temperature digester is that even minor
changes in system conditions could offset digester
Moreover, an additional source of energy will likely
be required to maintain the digester contents at a constant
Though operating temperature is critical, stabilizing the
and keeping it stabilized are even more important.
of plus or minus 1[degree]C in a day may force the
organisms into periods of dormancy.
consume acids, and without them acids will accumulate and
will fall, impeding the effectiveness of the whole biogas
In northern latitudes or colder climates, the volume of
will be substantially less unless specific provisions are
preheat the incoming substrate and maintain the digester
Thus, in colder climates, larger digesters will likely be
the amount of digester surface constructed
above ground should be reduced when temperatures are low.
One way to overcome the problem of lower temperatures is to
dilute the daily incoming waste material with preheated
water. Or you can
construct a greenhouse or compost pile
around the digester.
Note that the amount and type of waste to be degraded as
the operating temperature are two important factors
The moisture content of the digestion liquor (waste that is
diluted) should be in the range of 5 to 12 percent total
The percentage of total solids should include a minimum of
sands and soils.
Incoming waste products may have to be
diluted to a consistency of slightly thick cream.
A rule of thumb
for diluting cattle waste is 2.5 parts water for every one
of relatively dry waste or one part water for every one part
Stirring the Digester Contents
The microorganisms degrading the waste material are living,
creatures that produce their own metabolic by-products.
To prevent the bacteria from stagnating in their own waste
and thus to promote a more rapid digestion, stir or agitate
the digester contents by paddle, Scraper, piston, or in more
sophisticated settings, by gas recirculation.
Agitation also helps to minimize the build-up of internal
scum on top of the digestion liquor.
Failure to break the scum
may result in excessive gas pressures forcing substrate out
the openings instead of permitting the gas to escape through
transport lines. The
scum may also plug the digester.
that are fed higher volumes of fibrous waste may require
Feedstocks sometimes require pretreatment to increase the
yield in the anaerobic digestion process.
Pretreating the feedstock
(with alkaline or acid treatments, for example) breaks down
the complex organic structures into simpler molecules that
then more susceptible to microbial degradation.
Thus, you may want to pretreat any incoming substrate whose
volatile solids are not readily degradable.
Note that microorganisms
do not readily act upon rice hulls or sawdust.
Fibrous wastes also require special handling.
Wastes with long
fibers such as straw should be chopped or broken.
Any given waste
will digest more rapidly, and possibly even more completely,
broken into bits.
Thus, the finer the waste is shredded, ground,
or pulped, the easier the digestion process will be.
Scientific research has determined that minimum levels of
cobalt, and iron are required for methanogens to degrade
wastes more efficiently.
This is of little immediate consequence
to most farmers, as chemical analysis is required to
whether addition of these elements would be helpful.
If the carbon-to-nitrogen ratio is either too high or too
fluctuates substantially, the digestion process will slow or
stop. To act
efficiently on the substrate, microorganisms need a
20-30:1 ratio of carbon to nitrogen, with the largest
of the carbon being readily degradable.
Digesters have efficiently
operated on poultry waste with a 5-7:1 ratio.
here is to keep the quantity as well as the characteristics
the incoming substrate constant.
One note of caution: some carbon compounds resist being
down. Lignin, for
example, which all land plants use to help
stiffen and support themselves, is the least readily
carbon compound. The
amount of lignin increases proportionally
with plant age.
Thus, old grass contains more lignin than new
grass, and wood contains more of it than do leaves.
substrate that contains a high percentage of lignin will not
readily decompose in the biogas digester as well or as
as substrates that contain lesser amounts.
Thus, horse dung and
mature vegetative waste material are probably not good
because they contain a high fraction of non-degradable
Presence of Certain Toxins
Certain medications (e.g., antibiotics used in animal feeds
injected into animals), feed additives, pesticides, and
may have adverse effects on anaerobic bacteria, particularly
the methanogens. For
example, lincomycin (frequently used in
treating swine) and monensin (often used in treating cattle)
two antibiotics that will harm these bacteria and
halt methane production.
Factors Influencing Digester Size
Digester design depends basically upon the availability and
of waste to be fed to the digester, as well as the amount of
and/or fertilizer required.
Large digesters are generally designed
after establishing system operating conditions through
Small digestion plants are generally designed
based on past experiences with a particular substrate.
A distinct advantage of small digesters over large ones is
their contents require less vigorous and less frequent
(only several times a day) to prevent scum buildup and thus
increase the production of biogas.
A principal disadvantage of
these digesters, on the other hand, is that their operating
temperatures tend to fluctuate more often and to a much
Nevertheless, feeding a biogas digester--regardless of its
number of individual or combined feedstocks or organic
will result in the production of biogas as long as the
proper conditions exist and are kept fairly stable.
were researched initially for sewage treatment plants and
more recently are the subject of intense investigation
meeting the waste management needs of various agricultural
Type and Availability of Raw Waste Material
Husbandry practices can influence the quantities of manure
for use in the digester.
For example, cattle in pasture will
scatter their waste over a large grazing area, making waste
Conversely, a herd that spends most of the
day in a confined area (e.g., a corral) will deposit waste
concentrated area, making it possible to collect waste more
manure deposited directly in the field will
likely contain a lot of soil or grit, which will eventually
the digester, and thus not be suitable for the production of
The amount of manure produced per animal per day
example, one may expect about six pounds per day from a
pound beef or dairy cattle and about nine or 10 pounds per
from 1,000 pounds of broiler chicken.
Remember, increased gas
production is directly proportional to the amount of
solids in the raw waste used.
Under optimum collection conditions (i.e., when animal is
4 lb of manure per 100-lb sheep
80 lb of manure per 1,000-lb dairy cattle
60 lb of manure per 1,000-lb beef cattle
10 lb of manure per 200-lb pig
45 lb of manure per 1,000-lb horse
0.2 lb of manure per 4-lb poultry layer
The rule of thumb here is that the waste material from two
cattle will usually supply the gas required for cooking food
a family of four.
Comparable quantities of other waste may produce
slightly more or slightly less gas.
If you are considering relying on the use of a significant
of vegetable waste in your digester, you need to know when
material will be available in the greatest quantities.
water hyacinth may be available year round in some climates,
while grain straw or other crop residues will be most
only at harvest.
Wilted or semi-dried vegetation may require the addition of
in order to maintain optimum solids concentration.
young vegetation may require less dilution than freshly cut
Organic Loading Rate
The organic loading rate refers to the number obtained when
weight of the volatile solids loaded each day into the
is divided by the volume of the digester.
refers to the portion of organic material solids that can be
remainder of the solids are fixed. The
and a portion of the volatile solids are
material may also contain a substantial amount of water.)
Loading rate is an important parameter, since it tells us
amount of volatile solids to be fed into the digester each
At high loading rates, the feeding has to be more nearly
(perhaps hourly). At
lower loading rates, the biogas
digester needs to be fed only once a day.
Digesters are designed to receive and treat from 0.1 to 0.4
pounds of volatile solids per cubic foot of digester volume.
Although the actual loading rate depends on the type of
fed to the digester, 0.2 pounds of volatile solids per cubic
of digester volume (approximately 3 kg per cubic meter) is a
frequently used design parameter.
This means a digester used to
process mainly manure should be designed to accommodate from
to 120 cubic feet of digester volume per 1,000 pounds of
animal. (The actual
amount varies from species to species.)
Here, it is important to remember that a digester must be designed
on the basis of the amount of waste that can be collected
and actually fed to the digester, not on the quantity of
For illustration, the following estimates are useful:
1 lb of volatile solids per 200-lb pig per day
1 lb of volatile solids per 1-lb sheep per day
0.04 lb of volatile solids per 4-lb poultry layer per day
6 lb of volatile solids per 1,000-lb beef or dairy cattle
9 to 10 lb volatile solids per 1,000 pounds of poultry layer
The percentage of water in animal waste on a unit volume
around 75 to 95 percent.
Of the solids in the waste, about 70
percent are volatile.
Percentage of water in vegetable and plant
wastes varies from 40 to 95 percent.
Of that, the percentage of
volatile solids varies from 50 to 95 percent.
The amount of
biogas produced from vegetable and plant waste varies
various crops have differing biomass production rates.
With time, constant temperature, and a uniform incoming
a digester will stabilize.
The rules of thumb for any
substrate 5 to 12 percent total solids;
2. 0.2 to 0.5 pounds
volatile acids per cubic foot of digester
3. 1 to 2 pounds raw
manure per cubic foot of digester space
per day; and
4. 0.2 to 1.0 unit
volume of biogas produced per unit volume of
The actual amount of biogas that will be produced can be
by experimentation under conditions similar to those at the
site. One should
experiment with various types of waste, the
amount of water used to dilute an incoming waste, operating
temperature, and feeding (loading) frequency.
A source of potential confusion in determining digester size
the means to measure gas production.
When reading literature on
biogas digesters, make sure that the gas production under
is in comparable units.
Gas produced in a digester is biogas,
of which 50 to 60 percent is methane; the remainder is
carbon dioxide and other gases.
Biogas volumes are distinct from
Other ways of quantifying gas include: gas
volumes per volume of digester, gas volumes per 1,000 pounds
live weight of an animal species, gas volumes per pound of
solids added, and gas volumes per pound of volatile solids
Hydraulic Retention Time
Hydraulic retention time (HRT) is the average number of days
unit volume of substrate is to remain in the digester.
way, HRT is the volume of material already in the digester
divided by the average amount of incoming daily feedstock,
average age of the digester contents.
The HRT will vary from 10
to 60 days, and is an important parameter because it
the efficiency of the biogas digester.
Closely controlled digesters will average about 20 to 25
Shorter retention times will create the risk of
washout, a condition where active biogas bacteria are washed
of the digester at too young an age, making the population
bacteria unstable and potentially inactive.
Daily conversion of
organic material to methane will continue to increase per
increase of weight (i.e., age) of bacteria up to a certain
Thereafter, methane production will drop off per unit weight
age) of bacteria.
Note that a longer retention time requires a larger digester
more capital for its construction.
Recall, however, that the
smaller the digestion vessel, the less time the
bacteria will have to act on the available substrate and
more likely the biogas system could malfunction.
consider all these factors carefully before choosing a
There are two general design characteristics of digesters:
feed and continuous feed.
The batch digester is loaded, sealed,
and after a period of gas collection, emptied.
A batch digester
can essentially be any suitably sized container or tank that
be sealed and fitted with a means to collect the
continuous feed digester receives substrate on a continuous
daily basis with a roughly equivalent amount of effluent
There are many possible design variations for continuous
CONTINUOUS FEED DIGESTERS
The design variations for continuous feed digesters can be
into four distinct types: the Indian design, the Chinese
design, the sewage treatment plant, and the hybrid
of these types, along with cost and construction
is described in the sections that follow.
The Indian, or Khadi, design (Figure 2) is based on the
that gas produced will lift a bell-shaped dome located above
Substrate enters one side of the digester and
displaces effluent out the other side.
As gas is produced, it is
collected under the dome, forcing it to rise.
The dome descends
as gas is forced out of the digester into the gas transport
The gas storage chamber in the Chinese design
has a fixed top (Figure 3).
Substrate enters one side; effluent
exits the other side.
Gas produced accumulates under the dome and
above the vessel contents.
Increases in gas volume displace
digester contents into the displacement, or overflow,
The materials forced into the displacement chamber will, by
virtue of gravity, attempt to flow back into the
attempt by the displaced liquor to flow back into the
vessel creates the pressure to force the gas into the gas
line. As the gas is
used, materials displaced into the
displacement chamber will flow back into the vessel.
Sewage Treatment Plant
Though the designs associated with treating sewage or
wastes follow the same basic principles of the Indian and
designs, they are much more complex and more efficient.
digester content is stirred either by paddle or gas
Temperature controls are much more stringent and digester
content may be heated.
The effluent exits the plant and is thickened
prior to final disposal.
Gas is tapped from the digester,
possibly pressurized, and used for heating purposes or
may be used for process heat in the digester.
The sewage treatment
plant principles may be employed on a much smaller scale
with lower levels of technology.
Figure 4 shows a high-technology
sewage treatment plant.
Hybrid digesters imitate the principles employed in other
except that digestion vessels conform to the least
most readily available construction materials.
They can be
built from available scrap materials, plastic bags, or
troughs. A very
simple design is the end-to-end welding of 55-
gallon oil drums to create a long, narrow, small-volume
feed digester. With
hybrid digesters, care must be taken
not to let construction economy offset digester efficiency
5 shows a low-technology hybrid digester.
Comparison of Continuous Feed Digesters
The more sophisticated biogas digesters require skilled
build, operate, and maintain them.
Such digesters will likely be
more economically feasible if they are used to process large
quantities of waste.
Although a high-technology digester does
produce considerably more gas than either the Indian or the
Chinese design, it has higher capital and operating costs
requires careful monitoring on a daily basis.
The Indian and Chinese designs are less expensive and easier
build and operate, but those benefits are countered by
inefficient gas production.
Moreover, leakage may become a problem
if the digesters are not maintained well.
Although the Indian
design produces slightly more gas than the Chinese design,
slightly more expensive and has the added maintenance
associated with the floating dome.
Biogas can be burned directly as a fuel for cooking,
heating, water pumping, or grain milling, and can also be
fuel combustion engines.
In larger applications where scale and
skills warrant, biogas can be pressurized and stored,
for sale to commercial gas suppliers, or converted to
and sold to power grids, to meet peak energy needs.
Gas transport lines are connected to the gas-collection
of the digester (the floating dome of the Indian style
The gas has a high moisture content.
It is necessary to devise a
way to remove the moisture before the gas is used.
One way is to
slope the transport line back toward the digester so that
moisture will flow down the line back into the tank.
If this is
not practical, it will be necessary to install a sump, or
in the gas line to collect the moisture.
Biogas is also very corrosive.
It may contain dangerous amounts
of hydrogen sulfide, a poisonous flammable gas that produces
highly corrosive acid when mixed with water.
For this reason, gas
transport lines must be corrosion resistant.
(PVC) plastic pipe is a good choice for gas lines because it
durable, corrosion resistant, and usually economical.
gas is so corrosive, it may have to be cleansed before it is
used, particularly in engines.
While biogas is an excellent fuel, it does have a fairly low
energy value for its volume--500-600 BTUs per cubic
pressure in the distribution lines may be low.
Lamps, stoves, refrigerators,
and other appliances require specially designed jets
to offset the low energy value and the low gas
stabilize the flame on a cookstove, for example, the jet
shoots the biogas up through and out of the burner.
can either be purchased or built easily from locally
The amount of methane required daily per household will
About 0.5 to 1.0 cubic meter of biogas is required per
member for food preparation alone, and roughly one cubic
biogas is produced per 1,000 pounds of animal.
Meeting one family
member's cooking requirements, then, requires two or three
healthy dairy or beef cows, or eight to 10 pigs (weighing
250 pounds each), or over 500 chickens.
The amount of waste
material produced by these animals varies with their health
diet and will influence the number of animals required.
more than 30 to 40 pounds of waste daily per 1,000 pounds of
live weight per animal will increase the amount of gas
The effluent leaving the digester at the end of the
period is spread on farmland much as the undigested manure,
is used. Research
has been performed on using the digester effluent
to feed cattle or to promote algal growth in fish ponds,
as is done in some Chinese aquaculture installations.
The equipment and materials required for digester
depend upon the level of technology employed.
The basic Chinese
design requires cement, sand, clay, lime, and bricks.
cement should be used if available due to the corrosive
nature of the gas and slurry.
The Indian design requires these
same materials, plus some welding and iron works.
technology designs may require some specific machinery and
The following are generalized examples of the types and
of materials required to build similar sized Chinese- or
A Brace Research Institute publication (1976) reports the
materials for an Indian-style, 3-cubic meter digester that
should produce sufficient gas for the cooking needs of a
of six to eight members:
9 meters galvanized iron sheet
3,200 small construction bricks
25 50-kg bags of cement
12 cubic meters of sand
various angle irons, iron pipes, etc.
The Khadhi and Village Industries Commission in Bombay,
lists (in part) the following materials for a 3-cubic meter
3.2 cubic meters of sand
1.9 cubic meters of 1/2" to 3/4"
24 bags of cement
7.5 meters of sheet steel
various angle irons, pipes, reinforcing
A masonry wall Chinese style digester of 8 cubic meters
400 kg of cement
1,000 kg of sand
various plastic tubes for gas delivery.
Small-scale, nonpermanent digesters can be constructed of
drums or uniformly-supported plastic bags.
The above materials are meant only for demonstration
Actual type and quantity of materials required depend on
Note, however, that smaller biogas digesters are generally
with readily available materials.
SKILLS REQUIRED TO PRODUCE AND OPERATE A BIOGAS DIGESTER
The basics of a digester can be creatively adapted by
local craftspeople working with locally available materials.
The Chinese design requires the skills of a competent
Indian design requires the skills of a competent mason as
an iron worker and welder.
More sophisticated digesters for larger scale applications
plumbers and electricians.
Careful planning is required
prior to building such facilities.
Once constructed, the digester requires the daily attention
Each day, the digester must be fed and
agitated, and the effluent properly disposed of. Just as a
tends to a herd of animals, the individual responsible for
the digester must understand the operational procedures.
person must maintain not only the digester's physical plant,
also ensure that the gas transport line and gas utilization
are operative and in good repair.
Costs for construction are governed by the level of
employed. They range from a few dollars for digesters built
readily available scrap to a few hundred dollars for a small
family, Chinese-style digester, and from several hundreds of
dollars for a small-scale Indian-style digester to several
of thousands of dollars for a large-scale operation.
of thumb for comparable sized digesters is that the
digester costs half that of a "drum"-style Indian
more sophisticated digester will cost at least three times
of an Indian-style digester of comparable volume.
Actual costs depend upon the availability of resources.
numbers of semi-skilled laborers, for example, suggest that
of a Chinese-style digester would be more economical.
On the other hand, even though an Indian-style digester
more initially to construct, it is nevertheless more
requires less maintenance, and produces more gas than a
more sophisticated digesters require
markedly higher initial capital costs than smaller, less
units. However, they
are more efficient in terms of the total
volume of organic material that can be handled per unit
digester, and they produce more gas per unit of organic
handled. To do a
thorough cost analysis one must take into
account such factors as inflation, interest rates, operating
costs, maintenance expenses, labor costs, and the value of
conventional fuels (e.g., oil, gas) with biogas.
The amount of biogas varies from 30 to nearly 100 cubic feet
1,000 pounds of live body weight.
Thus, there is no universal
formula to determine biogas efficiency.
To do so, one must consider
For example, biogas efficiency varies, depending upon how
biogas is used.
Biogas plants use organic wastes, which, if not
fed to a digester, are at best spread over land or at worst
Although direct combustion of dung or grasses
yields at best 10 percent of the available energy, the
values of such wastes are severely reduced.
Biogas systems yield
40 to 50 percent, or better, of the thermal potential of
wastes and yield a fertilizer of superior quality.
provides excellent fertilizer with no gas.
Other, much more
sophisticated procedures are also available for more
removal of energy from waste.
Moreover, efficiency varies with the type of digester, the
conditions, and the type of material loaded into the
All else equal, the Chinese-style digester produces about
half as much gas as the Indian-style digester, which in turn
yields less than half the gas of more sophisticated
Chinese design, the Indian design, and the high-technology
respectively, yield about 0.2 to 0.3, 0.5 to 0.7, and 1.0
to 2.0 volumes of biogas per volume of digester.
And, in general,
digesters produce more gas with poultry waste (about 100 or
cubic feet of biogas per 1,000 pounds of live poultry
than they do with cattle waste (25 to 30 cubic feet per
pounds of live cattle weight).
Apart from these factors, the key to maintaining efficiency
feed the digester a uniform feedstock daily, to maintain a
operating temperature, and to agitate the contents
Biogas digesters require careful maintenance.
Operators should be
responsible for the following maintenance activities:
Daily Activities: Collect and prepare the
load it into
the digester. Collect the liquid
digester. It may be spread over fields,
fish ponds, or dried for later use.
Periodic (at regular intervals) Activities:
contents, including any solids that have accumulated
at the bottom
of the digester. Because of the
corrosive nature of the digester contents
well as gas), check all metal components of
to see whether they need to be resurfaced
metal dome of the Indian-style digester).
Occasional (at irregular or infrequent
digester, particularly Chinese-style
for any gas leaks. Also, examine
high-technology units such as pumps and mixers, which
occasional repair or replacement.
Finally, preventing sand, dirt, and gravel from mixing with
as it is being collected, and protecting the dome of the
with a metal or asphalt coating, will lengthen time between
IV. COMPARING THE ALTERNATIVES
CURRENT RESEARCH AND DEVELOPMENT
Biogas Generation Technology
Extensive research continues with the various biogas
plants operating worldwide.
Various institutions throughout the
world are conducting research toward making maximum use of
This involves matching energy needs to gas
production, and using equipment that burns or converts the
Additional research deals with digester designs
and design parameters; here, heat losses and maintaining an
adequate, stable temperature in the digester are of prime
to researchers in their efforts to maximize methane
Other research efforts focus on improvements in the use of
digester effluent to promote maximum growth of algae, fish,
aquatic vegetation, and farm animals.
More sophisticated and expensive biomass conversion
exist to convert organic material to charcoal, producer gas,
crude oil, simple sugars, alcohol, plastics, or other
Pyrolysis, which may be used to produce crude oil, for
or distillation, which yields ethyl alcohol, are examples of
These technologies have been introduced in
many developing countries, but further research is required
they can be widely applied.
COMPARISON OF TECHNOLOGIES
This paper focuses on biogasification as a means of
fuel from material that might otherwise be wasted or that
only a single end use, for example, as fertilizer.
biomass conversion technologies are burning raw waste to get
rid of it, composting, distillation, burning raw waste to
process or other beat, gasification, and pyrolysis.
all of these technologies, you must examine each technology
separately, weighing its advantages and disadvantages and
into account such factors as the availability and cost of
energy costs, the relative value of a particular raw waste
and the end products it produces, the availability of human
material resources, and the impact of the technology on the
discussion below presents some examples of the
kinds of factors you need to consider in balancing one
If the sole objective is to reduce waste, burning raw waste
be a good choice, provided it is sufficiently dry, air
is controlled, and there is a means to dispose of the
disadvantage of burning raw waste for disposal is that it is
very inefficient use of energy.
The energy produced by burning is
wasted. In some
situations, simply making the waste material
available to people who can use it for cooking fuel may be a
effective means of disposal.
And it does help assure that the
heat energy will be put to use.
Composting is an excellent way to turn waste products into a
commodity--fertilizer--simply and economically.
of composting is that some of the nutrients in the raw
particularly nitrogen, phosphorus, and potassium--convert to
gas, evaporate, and are lost to the atmosphere, or they
through the soil.
Moreover, composting is limited to producing
If you want to do more with raw waste than composting or
getting rid of it--that is, if you want to harness the
from the raw waste material to produce fuels or other
you will need to make additional investments in capital,
and labor. As we
have seen in this paper, a biogas digester
yields both a fuel gas and a high quality fertilizer.
composting, the digestion process retains and even improves
nutrient value of the original feedstock.
raw wastes can be digested, and returned to the environment
the form of fertilizer and fuel, without degrading the
Keep in mind, however, that the equipment (e.g., a digester,
systems, pumps) necessary for biogasification will generally
be more expensive than the equipment (e.g., a wagon equipped
a loader, a manure spreader) necessary for composting.
The remaining four biomass conversion
controlled burning to provide process or other heat,
and pyrolysis--collectively produce an even wider range of
products than biogasification.
Distillation of raw wastes produces
sugar and alcohol, for example; controlled burning produces
heat to, say, a boiler.
Pyrolysis produces biofuels such as
charcoal and crude oil; and gasification produces still
biofuels such as low- and medium-energy gas (often called
gas). These four
technologies differ chiefly in their
equipment requirements (i.e., depending on the technology,
hardware can be as simple as a cookstove or retort or as
as a distillation plant), in their techniques (i.e., some
techniques are more complex than others, resulting in higher
product yields), and in costs.
In sum, comparing one biomass conversion technology with
must be based on what end products you want from the
end product user how much you are willing to spend, relative
economies of scale, skill levels, availability of raw waste
materials, environmental impact, and many other factors.
V. CHOOSING THE TECHNOLOGY RIGHT FOR YOU
Economics are a major factor in deciding whether or not to
a biogas system. To
determine the economics of such a system,
you need to consider such factors as availability and cost
of biogas (based on BTU), cost of equipment, capital costs,
costs, energy availability/needs/cycles, material
and costs, and anticipated returns.
Remember, also, to factor
into the cost analysis inflation and capitalization
cost factors and the resulting analysis will vary from
Certain social/cultural questions need to be addressed.
is daily waste handling acceptable or taboo? Moreover, to
succeed, a biogas technology must interface with existing
can existing waste management practices be adapted, for
example, to include a digester and effluent disposal? What
to the very poor who have traditionally collected cattle
dung freely to use for fuel when the dung is used in a
and the fuel is available only to those who can pay for it?
controls the distribution of the gas in a community system?
AVAILABILITY OF RESOURCES
Technical resource considerations include taking into
availability of a constant, high-quality supply of organic
the suitability of the ambient temperature, the availability
of good-quality water with which to dilute the feedstock,
whether the biogas produced can be used efficiently, and
the space is sufficient for effluent disposal and
keep in mind the need for a biogas plant, whose construction
and operation depend upon the availability of capital,
personnel (skilled and semiskilled), and materials.
Consult local officials about any local regulations and laws
may prevent you from building or using a biogas
generator. On the
positive side, some laws might work in your favor.
the governments of some developing countries provide
incentives, grants, or low-interest loans to people who want
introduce a biogas plant.
Such governments are actively pursuing
national policies that would reduce dependence on imported
and so encourage the production of biogas as an
safe fuel source.
Chinese- and Indian-style biogas generators can generally be
built in-country, since plant components are usually
components, i.e., the dome and guide mechanism
of an Indian digester, can be manufactured on a larger scale
sold to users.
SCALE OF PRODUCTION AND POTENTIAL MARKET
Subsistence farmers who depend on firewood for cooking and
comprise a substantial percentage of the world's population.
Though biogas generation seems likely to at least supplement
their current energy supplies, there are several reasons why
biogas may not totally replace firewood:
raw waste from the equivalent of several
cows is required
to meet a
family's cooking needs;
nearly all of the biomass conversion
of capital usually available only to a few
cultural norms may not permit waste handling
usage, or may
limit availability of organic material if
pastured rather than confined; and
biogas generation must be accepted and
learned, a process
motivated, knowledgeable extension agents
or others who
can point to successful applications of the
or who can demonstrate it effectively.
SOURCES OF INFORMATION ON BIOGAS
Director, Gobar Gas Scheme
Khadi and Village Industries Commission
Irla Road, Vile Parle (West)
Bombay 400 056 INDIA
Head of the Division of Soils Science and Agricultural
Indian Agricultural Research Institute
New Delhi 110 012 INDIA
Farm Information Unit
Directorate of Extension
Ministry of Agriculture and Irrigation
New Delhi, INDIA
Gobar Gas Research Station
Uttar Pradesh, INDIA
Director, National Environmental Engineering Research
World Health Organization
1211 Geneva 27, SWITZERLAND
Economic and Social Commission for Asia and the Pacific
Division of Industry, Housing, and Technology
United Nations Building
Bangkok 2, THAILAND
Bangladesh Academy for Rural Development
Appropriate Technology Development Organization
Government of Pakistan
Volunteers in Technical Assistance (VITA)
1815 North Lynn St., Suite 200
Arlington, VA 22209 USA
Barnett, A.; Pyle, L.; and Subramanian, S.K. Biogas
the Third World:
A Multidisciplinary Review. IDRC-103e.
Canada: International Development Research
Brace Research Institute. MacDonald College of McGill
A Handbook on
Appropriate Technology. Ottawa, Ontario, Canada:
Hills, D.J., and Roberts, D.W. "Basic Fundamentals of
Agricultural Waste." Extension service
University of California, Davis, 1980.
House, D. The Compleat Biogas Handbook. Aurora, Oregon,
McGarry, M.G., and Stainforth, J. Compost, Fertilizer, and
Human and Farm Wastes in the People's Republic
IDRC8e. Ottawa, Ontario, Canada: International
Research Center, 1978.
Lichtman, R.J. Biogas Systems in India. Arlington, Virginia:
Pohland, F.G., ed. Anaerobic Biological Treatment Processes.
Chemistry Series 105. Washington, D.C.: American
Shuler, M.L., ed. Utilization and Recycled Agricultural
Boca Raton, Florida: CRC Press, Inc., 1978.
Subramanian, S.K. Bio-gas Systems in Asia. New Delhi, India:
Development Institute, 1977.
Targanides, P. "Anaerobic Digestion of Poultry
Poultry Science Journal 19 (1962):252-61.
Tatom, J.W. "Pyrolysis Experience in the Developing
Big-Energy '80 World Congress and Exposition.
D.C.: Bio-Energy Council, 198, pp. 180-85.
SUPPLIERS AND MANUFACTURERS
OF BIOGAS PLANT EQUIPMENT AND ACCESSORIES
Patel Gas Crafters Private Limited
1/2, Shree Sai Bazar Centre
Bombay 54, INDIA
11 Swami Vivekananand
Nagar Varanasi 221 002 INDIA