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                                TECHNICAL PAPER #4
                      UNDERSTANDING BIOGAS GENERATION                
                               Richard Mattocks
                             Technical Reviewers
                                 J.B. Farrell
                                 C. Gene Haugh
                                 Daniel Ingold
                                 Published By
                       1600 Wilson Boulevard, Suite 500
                         Arlington, Virgnia 22209 USA
                     Tel: 703/276-1800 . Fax: 703/243-1865
                        Understanding Biogas Generation
                             ISBN: 0-86619-204-2
                   [C]1984, Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in Technical
Assistance to provide an introduction to specific state-of-the-art
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 situations.
They are not intended to provide construction or implementation
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 illustrated
almost entirely by VITA Volunteer technical experts on a purely
voluntary basis.  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 layout,
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 biomass
products, and is currently researching various uses of biogas
digester effluent, particularly its use as an animal feed source.
Reviewers J.B. Farrell, C. Gene Haugh, and Daniel Ingold are also
specialists in the area.  Farrell is a chemical engineer by training
and chief of the Sludge Management Section of the U.S. Environmental
Protection Agency's Municipal Environmental Research
Laboratory.  Haugh 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 people
working on technical problems in developing countries.   VITA offers
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to their
situations.  VITA maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster of
volunteer technical consultants; manages long-term field projects;
and publishes a variety of technical manuals and papers.
              by VITA Volunteer Richard Mattocks
Biogas is a by-product of the biological breakdown--under oxygen-free
conditions--of organic wastes such as plants, crop residues,
wood and bark residues, and human and animal manure.   Interest in
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 million in
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 container,
it can be an important energy source.
Ancient Chinese experimented with burning the gas given off when
vegetables and manures were left to rot in a closed vessel.  More
recently, Volto, Beachans, and Pasteur worked with biogas-producing
organisms.  At the turn of the 20th century, communities in
England and Bombay, India, disposed of wastes in closed containers
and collected the resulting gas for cooking and lighting.
Germany, the United States, Australia, Algeria, France, and other
nations constructed such methane digesters to supplement
dwindling energy supplies during the two world wars.
Biogas generators or digesters yield two products: the biogas
itself, and a semi-solid by-product called effluent or sludge.
Biogas systems are most popular for their ability to produce fuel
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 fertilizer.  The
digestion process converts the nitrogen in the organic materials
to ammonium, a form that becomes more stable when plowed into the
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 wastes.  Simply
put, these systems are capable of destroying most bacteria and
parasitic eggs in human and animal wastes, enabling the digested
sludge to be applied safely to crops.   Tests have shown that
biogas systems can kill as much as 90 to 100 percent of hookworm
eggs, 35 to 90 percent of ascarid (i.e., roundworms and pinworms),
and 90 to 100 percent of blood flukes (i.e., schistosome
flukes, which are found in water snails that commonly live in
paddy fields and ponds).
Biogas systems are also capable of digesting municipal sewage,
which is a major source of pollution.   Using biogas systems in
this way substantially reduces the potential for environmental
Finally, agricultural and animal wastes, the major raw materials
for biogas production, are usually plentiful in rural areas.
People living in rural communities, who are often subjected to
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 production
of biogas for fuel, in some applications the gas is considered
to be the by-product of the process.   Some digesters in
China, for example, are used primarily for treating sewage and
producing fertilizer, and only secondarily for producing fuel.
Biogas generation is a process that takes place in an oxygen-free
environment.  It uses anaerobic bacteria--bacteria that live only
in the absence of oxygen--to break down complex organic compounds
in fairly well-defined stages.   The process is called anaerobic
digestion.  It produces biogas, a gas composed of approximately 50
to 60 percent methane, 40 to 50 percent carbon dioxide, as well
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, approximately
6.1 Calories per liter (around 600 BTU per cubic foot).   Compare
this with pure methane, which has a heat value of 995 BTU
per cubic foot, or natural gas with over 1,000.   Nevertheless,
biogas can be an important fuel source for many applications.
A biogas digester is the device in which the digestion process
occurs.  The organic feedstock, which is called the substrate, may
consist of night soil, manure, crop or kitchen residues, or
similar materials.  The substrate is usually diluted with water,
and is thoroughly mixed into a slurry; crop residues and vegetation
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 digester.
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 is a
three-stage process that leads to the production of methane
(Figure 1).

ubg1x3.gif (600x600)

In the first stage, numerous organisms release enzymes that
attack specific bonds in complex protein, carbohydrate, and lipid
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 fatty
acids to produce biogas (a mixture of methane and carbon dioxide).
The principles of anaerobic digestion are the same regardless of
the digestion vessel.  Organic material is loaded into a fairly
warm, temperature-controlled, oxygen-free environment and methane
is produced after acclimatization.   The makeup or quality of
incoming material to be digested, the vessel, and the surrounding
environment influence the digester efficiencies.   The production
of gases is greater when the digester is operated at a relatively
high temperature, when the substrate is stirred or otherwise
agitated, and when system conditions are kept fairly constant.  A
more detailed discussion of these and other factors influencing
digester efficiency follows.   In general, however, the important
objective to keep in mind when operating a biogas digester is the
production of the greatest volume of biogas in the shortest
possible time.
Researchers are only now gaining a better understanding of the
metabolic process in biogas digesters.   They do know, however,
that methane-producing organisms (called methanogens) "prefer" to
channel energy, or calories (derived by breaking down incoming
substrate), to methane rather than use the energy to construct or
satisfy internal cellular needs.   As such, methanogens do not
adapt well to changes in their environment that may require them
to increase their numbers or adjust their internal mechanisms.  If
the environmental changes are significant enough, the methanogens
may slow or even stop their work.
Changes that may affect the behavior of the bacteria and thus the
performance of the digester include variations in the substrate,
presence of certain toxic chemicals, gas pressure, temperature,
and the amount of time the material remains in the digester.
Other factors that could have a major impact on the operating
performance of a biogas digester include biological balance/
acidity, solids concentration, agitation, feedstock, pretreatment,
and the carbon-to-nitrogen ratio.
The primary factors that could affect the size of a biogas digester
include the type and amount of feedstock, the rate at which
it is loaded, and hydraulic retention time.
Factors Influencing Digester Operating Performance
Biological Balance/Acidity
Methanogens--methane-producing organisms--live in a syntrophic,
or complementary, relationship with certain other microorganisms
that consume the feedstock and produce simple acids as part of
their metabolism.  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 cooperate
by consuming these by-products in the methane-producing
Given sufficient time to establish the proper ratio of methane-producing
organisms to acid-producing organisms, a homeostasis,
or stability, will occur with a pH of about seven in a digester.
A digester fed poultry or high nitrogen waste may stabilize at a
pH of eight or greater.
The objective here is to create a stable working relationship
among the microbial population in the digester.   This implies the
need for fairly constant operating temperatures and feedstock
characteristics.  Conversely, any rapid variations of these conditions
will cause the microbial population to shift dramatically
and possibly upset the overall system balance in the digester.
For example, if the methane-producing organisms become dormant
due to, say, temperature fluctuations, the pH will drop so low
as to incapacitate them.
Maintaining a stable pH requires stabilizing the feedstock as
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 on a
case-by-case basis.
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
    either increase 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
    altogether, each day, will change the average age of the
    organisms in the digester.
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
    material removed each day.
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
Operating temperature is another factor influencing digester
efficiency.  A digester can operate in three temperature ranges:
(1) the low temperature, psycrophilic bacteria range, which is
less than 35[degrees]C (90[degrees]F); (2) the medium temperature, mesophilic
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 to
55[degrees]C (135 to 140[degrees]F).   Organic material degrades more rapidly at
higher temperatures because the full range of bacteria are at
work.  Thus, a digester operating at a higher temperature can be
expected to produce greater quantities of biogas.   The disadvantage
of an elevated-temperature digester is that even minor
changes in system conditions could offset digester efficiency or
productivity.  Moreover, an additional source of energy will likely
be required to maintain the digester contents at a constant
higher temperature.
Though operating temperature is critical, stabilizing the temperature
and keeping it stabilized are even more important.   Variations
of plus or minus 1[degree]C in a day may force the methane-producing
organisms into periods of dormancy.   These organisms
consume acids, and without them acids will accumulate and the pH
will fall, impeding the effectiveness of the whole biogas system.
In northern latitudes or colder climates, the volume of methane
will be substantially less unless specific provisions are made to
preheat the incoming substrate and maintain the digester temperature.
Thus, in colder climates, larger digesters will likely be
required.  Moreover, 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 (solar-heated)
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 well as
the operating temperature are two important factors governing
digester size.
Solids Concentration
The moisture content of the digestion liquor (waste that is
diluted) should be in the range of 5 to 12 percent total solids.
The percentage of total solids should include a minimum of inorganic
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 part
of relatively dry waste or one part water for every one part of
fresh manure.
Stirring the Digester Contents
The microorganisms degrading the waste material are living, metabolizing
creatures that produce their own metabolic by-products.
To prevent the bacteria from stagnating in their own waste products,
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 fibrous
scum on top of the digestion liquor.   Failure to break the scum
may result in excessive gas pressures forcing substrate out of
the openings instead of permitting the gas to escape through gas
transport lines.  The scum may also plug the digester.  Digesters
that are fed higher volumes of fibrous waste may require special
design considerations.
Feedstock Pretreatment
Feedstocks sometimes require pretreatment to increase the methane
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 are
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, when
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 nickel,
cobalt, and iron are required for methanogens to degrade organic
wastes more efficiently.  This is of little immediate consequence
to most farmers, as chemical analysis is required to determine
whether addition of these elements would be helpful.
Carbon-to-Nitrogen Ratio
If the carbon-to-nitrogen ratio is either too high or too low, or
fluctuates substantially, the digestion process will slow or even
stop.  To act efficiently on the substrate, microorganisms need a
20-30:1 ratio of carbon to nitrogen, with the largest percentage
of the carbon being readily degradable.   Digesters have efficiently
operated on poultry waste with a 5-7:1 ratio.   The key
here is to keep the quantity as well as the characteristics of
the incoming substrate constant.
One note of caution: some carbon compounds resist being broken
down.  Lignin, for example, which all land plants use to help
stiffen and support themselves, is the least readily degradable
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.   Remember, any
substrate that contains a high percentage of lignin will not
readily decompose in the biogas digester as well or as completely
as substrates that contain lesser amounts.   Thus, horse dung and
mature vegetative waste material are probably not good feedstocks,
because they contain a high fraction of non-degradable
Presence of Certain Toxins
Certain medications (e.g., antibiotics used in animal feeds or
injected into animals), feed additives, pesticides, and herbicides
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) are
two antibiotics that will harm these bacteria and immediately
halt methane production.
Factors Influencing Digester Size
Digester design depends basically upon the availability and type
of waste to be fed to the digester, as well as the amount of gas
and/or fertilizer required.  Large digesters are generally designed
after establishing system operating conditions through
laboratory analysis.  Small digestion plants are generally designed
based on past experiences with a particular substrate.
A distinct advantage of small digesters over large ones is that
their contents require less vigorous and less frequent stirring
(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 greater
Nevertheless, feeding a biogas digester--regardless of its size--any
number of individual or combined feedstocks or organic materials
will result in the production of biogas as long as the
proper conditions exist and are kept fairly stable.   These conditions
were researched initially for sewage treatment plants and
more recently are the subject of intense investigation toward
meeting the waste management needs of various agricultural and
specialized industries.
Type and Availability of Raw Waste Material
Husbandry practices can influence the quantities of manure available
for use in the digester.  For example, cattle in pasture will
scatter their waste over a large grazing area, making waste
collection difficult.  Conversely, a herd that spends most of the
day in a confined area (e.g., a corral) will deposit waste in a
concentrated area, making it possible to collect waste more
easily.  Moreover, manure deposited directly in the field will
likely contain a lot of soil or grit, which will eventually clog
the digester, and thus not be suitable for the production of
The amount of manure produced per animal per day varies.  For
example, one may expect about six pounds per day from a 1,000
pound beef or dairy cattle and about nine or 10 pounds per day
from 1,000 pounds of broiler chicken.   Remember, increased gas
production is directly proportional to the amount of volatile
solids in the raw waste used.
Under optimum collection conditions (i.e., when animal is confined),
you get:
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 adult
cattle will usually supply the gas required for cooking food for
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 amount
of vegetable waste in your digester, you need to know when such
material will be available in the greatest quantities.   For example,
water hyacinth may be available year round in some climates,
while grain straw or other crop residues will be most plentiful
only at harvest.
Wilted or semi-dried vegetation may require the addition of water
in order to maintain optimum solids concentration.   Freshly-cut
young vegetation may require less dilution than freshly cut older
plant material.
Organic Loading Rate
The organic loading rate refers to the number obtained when the
weight of the volatile solids loaded each day into the digester
is divided by the volume of the digester.   ("Volatile solids"
refers to the portion of organic material solids that can be
digested.  The remainder of the solids are fixed.  The fixed solids
and a portion of the volatile solids are non-degradable.  Organic
material may also contain a substantial amount of water.)
Loading rate is an important parameter, since it tells us the
amount of volatile solids to be fed into the digester each day.
At high loading rates, the feeding has to be more nearly continuous
(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 wastes
fed to the digester, 0.2 pounds of volatile solids per cubic foot
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 20
to 120 cubic feet of digester volume per 1,000 pounds of live
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 waste
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 per day
9 to 10 lb volatile solids per 1,000 pounds of poultry layer
The percentage of water in animal waste on a unit volume basis is
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 because
various crops have differing biomass production rates.
With time, constant temperature, and a uniform incoming substrate,
a digester will stabilize.  The rules of thumb for any
digester include:
1.  Incoming 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 determined
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 is
the means to measure gas production.   When reading literature on
biogas digesters, make sure that the gas production under discussion
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
methane volumes.  Other ways of quantifying gas include: gas
volumes per volume of digester, gas volumes per 1,000 pounds of
live weight of an animal species, gas volumes per pound of volatile
solids added, and gas volumes per pound of volatile solids
Hydraulic Retention Time
Hydraulic retention time (HRT) is the average number of days a
unit volume of substrate is to remain in the digester.   Put another
way, HRT is the volume of material already in the digester
divided by the average amount of incoming daily feedstock, or the
average age of the digester contents.   The HRT will vary from 10
to 60 days, and is an important parameter because it influences
the efficiency of the biogas digester.
Closely controlled digesters will average about 20 to 25 days
retention time.  Shorter retention times will create the risk of
washout, a condition where active biogas bacteria are washed out
of the digester at too young an age, making the population of
bacteria unstable and potentially inactive.   Daily conversion of
organic material to methane will continue to increase per unit
increase of weight (i.e., age) of bacteria up to a certain point.
Thereafter, methane production will drop off per unit weight (or
age) of bacteria.
Note that a longer retention time requires a larger digester and
more capital for its construction.   Recall, however, that the
smaller the digestion vessel, the less time the methane-producing
bacteria will have to act on the available substrate and thus the
more likely the biogas system could malfunction.   One should
consider all these factors carefully before choosing a system.
There are two general design characteristics of digesters: batch
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 can
be sealed and fitted with a means to collect the biogas.  The
continuous feed digester receives substrate on a continuous or
daily basis with a roughly equivalent amount of effluent removed.
There are many possible design variations for continuous feed
The design variations for continuous feed digesters can be divided
into four distinct types: the Indian design, the Chinese
design, the sewage treatment plant, and the hybrid design.  Each
of these types, along with cost and construction considerations,
is described in the sections that follow.
Indian Design
The Indian, or Khadi, design (Figure 2) is based on the principle

ubg2x12.gif (600x600)

that gas produced will lift a bell-shaped dome located above the
digestion vat.  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
Chinese Design
The gas storage chamber in the Chinese design characteristically
has a fixed top (Figure 3).  Substrate enters one side; effluent

ubg3x13.gif (600x600)

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, chamber.
The materials forced into the displacement chamber will, by
virtue of gravity, attempt to flow back into the digester.  The
attempt by the displaced liquor to flow back into the digestion
vessel creates the pressure to force the gas into the gas transport
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 industrial
wastes follow the same basic principles of the Indian and Chinese
designs, they are much more complex and more efficient.   The
digester content is stirred either by paddle or gas recirculation.
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 flared; it
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

ubg4x14.gif (600x600)

sewage treatment plant.
Hybrid Designs
Hybrid digesters imitate the principles employed in other designs,
except that digestion vessels conform to the least expensive,
most readily available construction materials.   They can be
built from available scrap materials, plastic bags, or covered
troughs.  A very simple design is the end-to-end welding of 55-
gallon oil drums to create a long, narrow, small-volume continuous
feed digester.  With hybrid digesters, care must be taken
not to let construction economy offset digester efficiency or
productivity.  Figure 5 shows a low-technology hybrid digester.

ubg5x16.gif (600x600)

Comparison of Continuous Feed Digesters
The more sophisticated biogas digesters require skilled people to
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 and
requires careful monitoring on a daily basis.
The Indian and Chinese designs are less expensive and easier to
build and operate, but those benefits are countered by fairly
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, it is
slightly more expensive and has the added maintenance requirements
associated with the floating dome.
Biogas can be burned directly as a fuel for cooking, lighting,
heating, water pumping, or grain milling, and can also be used to
fuel combustion engines.  In larger applications where scale and
skills warrant, biogas can be pressurized and stored, cleansed
for sale to commercial gas suppliers, or converted to electricity
and sold to power grids, to meet peak energy needs.
Gas transport lines are connected to the gas-collection chamber
of the digester (the floating dome of the Indian style digester).
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 the
moisture will flow down the line back into the tank.   If this is
not practical, it will be necessary to install a sump, or chamber,
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 a
highly corrosive acid when mixed with water.   For this reason, gas
transport lines must be corrosion resistant.   Polyvinyl chloride
(PVC) plastic pipe is a good choice for gas lines because it is
durable, corrosion resistant, and usually economical.   Because the
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 foot--and the
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 pressure.  To
stabilize the flame on a cookstove, for example, the jet forcefully
shoots the biogas up through and out of the burner.   Jets
can either be purchased or built easily from locally available
The amount of methane required daily per household will vary.
About 0.5 to 1.0 cubic meter of biogas is required per family
member for food preparation alone, and roughly one cubic meter of
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 150 to
250 pounds each), or over 500 chickens.   The amount of waste
material produced by these animals varies with their health and
diet and will influence the number of animals required.   Collecting
more than 30 to 40 pounds of waste daily per 1,000 pounds of
live weight per animal will increase the amount of gas produced
per animal.
The effluent leaving the digester at the end of the digestion
period is spread on farmland much as the undigested manure, etc.,
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 construction
depend upon the level of technology employed.   The basic Chinese
design requires cement, sand, clay, lime, and bricks.   Sulfate-resistant
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.   The higher
technology designs may require some specific machinery and electronics.
The following are generalized examples of the types and quantities
of materials required to build similar sized Chinese- or
Indian-style digesters.
A Brace Research Institute publication (1976) reports the following
materials for an Indian-style, 3-cubic meter digester that
should produce sufficient gas for the cooking needs of a family
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, India,
lists (in part) the following materials for a 3-cubic meter
horizontal digester:
    *   2,870 bricks
    *   3.2 cubic meters of sand
    *   1.9 cubic meters of 1/2" to 3/4" rock
    *   24 bags of cement
    *   7.5 meters of sheet steel
    *   various angle irons, pipes, reinforcing rods.
A masonry wall Chinese style digester of 8 cubic meters calls for:
    *   400 kg of cement
    *   1,000 kg of sand
    *   1,000 bricks
    *   various plastic tubes for gas delivery.
Small-scale, nonpermanent digesters can be constructed of oil
drums or uniformly-supported plastic bags.
The above materials are meant only for demonstration purposes.
Actual type and quantity of materials required depend on design.
Note, however, that smaller biogas digesters are generally built
with readily available materials.
The basics of a digester can be creatively adapted by competent,
local craftspeople working with locally available materials.
The Chinese design requires the skills of a competent mason.  The
Indian design requires the skills of a competent mason as well as
an iron worker and welder.
More sophisticated digesters for larger scale applications require
plumbers and electricians.  Careful planning is required
prior to building such facilities.
Once constructed, the digester requires the daily attention of a
semiskilled individual.  Each day, the digester must be fed and
agitated, and the effluent properly disposed of. Just as a caretaker
tends to a herd of animals, the individual responsible for
the digester must understand the operational procedures. This
person must maintain not only the digester's physical plant, but
also ensure that the gas transport line and gas utilization system
are operative and in good repair.
Costs for construction are governed by the level of technology
employed. They range from a few dollars for digesters built of
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 hundreds
of thousands of dollars for a large-scale operation.   A rule
of thumb for comparable sized digesters is that the Chinese-style
digester costs half that of a "drum"-style Indian digester.  A
more sophisticated digester will cost at least three times that
of an Indian-style digester of comparable volume.
Actual costs depend upon the availability of resources.   Large
numbers of semi-skilled laborers, for example, suggest that construction
of a Chinese-style digester would be more economical.
On the other hand, even though an Indian-style digester costs
more initially to construct, it is nevertheless more efficient,
requires less maintenance, and produces more gas than a Chinese-style
digester.  Larger, more sophisticated digesters require
markedly higher initial capital costs than smaller, less complex
units.  However, they are more efficient in terms of the total
volume of organic material that can be handled per unit volume of
digester, and they produce more gas per unit of organic material
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 replacing
conventional fuels (e.g., oil, gas) with biogas.
The amount of biogas varies from 30 to nearly 100 cubic feet per
1,000 pounds of live body weight.   Thus, there is no universal
formula to determine biogas efficiency.   To do so, one must consider
many factors.
For example, biogas efficiency varies, depending upon how the
biogas is used.  Biogas plants use organic wastes, which, if not
fed to a digester, are at best spread over land or at worst
directly burned.  Although direct combustion of dung or grasses
yields at best 10 percent of the available energy, the nutrient
values of such wastes are severely reduced.   Biogas systems yield
40 to 50 percent, or better, of the thermal potential of organic
wastes and yield a fertilizer of superior quality.   Composting
provides excellent fertilizer with no gas.   Other, much more
sophisticated procedures are also available for more efficient
removal of energy from waste.
Moreover, efficiency varies with the type of digester, the operating
conditions, and the type of material loaded into the digester.
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 units.  The
Chinese design, the Indian design, and the high-technology designs,
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 so
cubic feet of biogas per 1,000 pounds of live poultry weight)
than they do with cattle waste (25 to 30 cubic feet per 1,000
pounds of live cattle weight).
Apart from these factors, the key to maintaining efficiency is to
feed the digester a uniform feedstock daily, to maintain a constant
operating temperature, and to agitate the contents regularly.
Biogas digesters require careful maintenance.   Operators should be
responsible for the following maintenance activities:
     *   Daily Activities: Collect and prepare the feedstock, and
        load it into the digester.  Collect the liquid effluent
        from the digester.  It may be spread over fields, used to
        fertilize fish ponds, or dried for later use.
    *   Periodic (at regular intervals) Activities: Remove the
        digester contents, including any solids that have accumulated
        at the bottom of the digester.  Because of the
        potentially corrosive nature of the digester contents
        (slurry as well as gas), check all metal components of
        the digester to see whether they need to be resurfaced
        (e.g., the metal dome of the Indian-style digester).
    *   Occasional (at irregular or infrequent intervals) Activities:
        Check the digester, particularly Chinese-style
        digesters, for any gas leaks.  Also, examine components in
        high-technology units such as pumps and mixers, which
        require occasional repair or replacement.
Finally, preventing sand, dirt, and gravel from mixing with dung
as it is being collected, and protecting the dome of the digester
with a metal or asphalt coating, will lengthen time between
Biogas Generation Technology
Extensive research continues with the various biogas generation
plants operating worldwide.  Various institutions throughout the
world are conducting research toward making maximum use of the
biogas produced.  This involves matching energy needs to gas
production, and using equipment that burns or converts the gas
more efficiently.  Additional research deals with digester designs
and design parameters; here, heat losses and maintaining an
adequate, stable temperature in the digester are of prime interest
to researchers in their efforts to maximize methane production.
Other research efforts focus on improvements in the use of
digester effluent to promote maximum growth of algae, fish,
aquatic vegetation, and farm animals.
Competing Technologies
More sophisticated and expensive biomass conversion technologies
exist to convert organic material to charcoal, producer gas,
crude oil, simple sugars, alcohol, plastics, or other chemicals.
Pyrolysis, which may be used to produce crude oil, for example,
or distillation, which yields ethyl alcohol, are examples of
these technologies.  These technologies have been introduced in
many developing countries, but further research is required before
they can be widely applied.
This paper focuses on biogasification as a means of producing
fuel from material that might otherwise be wasted or that has
only a single end use, for example, as fertilizer.   The alternative
biomass conversion technologies are burning raw waste to get
rid of it, composting, distillation, burning raw waste to provide
process or other beat, gasification, and pyrolysis.   To compare
all of these technologies, you must examine each technology
separately, weighing its advantages and disadvantages and taking
into account such factors as the availability and cost of capital,
energy costs, the relative value of a particular raw waste
and the end products it produces, the availability of human and
material resources, and the impact of the technology on the
environment.  The discussion below presents some examples of the
kinds of factors you need to consider in balancing one technology
against another.
If the sole objective is to reduce waste, burning raw waste may
be a good choice, provided it is sufficiently dry, air pollution
is controlled, and there is a means to dispose of the ash.  One
disadvantage of burning raw waste for disposal is that it is a
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 more
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.   One disadvantage
of composting is that some of the nutrients in the raw waste--
particularly nitrogen, phosphorus, and potassium--convert to a
gas, evaporate, and are lost to the atmosphere, or they leach out
through the soil.  Moreover, composting is limited to producing
only fertilizer.
If you want to do more with raw waste than composting or just
getting rid of it--that is, if you want to harness the energy
from the raw waste material to produce fuels or other products--
you will need to make additional investments in capital, materials,
and labor.  As we have seen in this paper, a biogas digester
yields both a fuel gas and a high quality fertilizer.   Unlike
composting, the digestion process retains and even improves the
nutrient value of the original feedstock.   With biogasification,
raw wastes can be digested, and returned to the environment in
the form of fertilizer and fuel, without degrading the environment.
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 with
a loader, a manure spreader) necessary for composting.
The remaining four biomass conversion technologies--distillation,
controlled burning to provide process or other heat, gasification,
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 other
biofuels such as low- and medium-energy gas (often called producer
gas).  These four technologies differ chiefly in their
equipment requirements (i.e., depending on the technology, the
hardware can be as simple as a cookstove or retort or as intricate
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 another
must be based on what end products you want from the technology,
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.
Economics are a major factor in deciding whether or not to introduce
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, labor
costs, energy availability/needs/cycles, material availability
and costs, and anticipated returns.   Remember, also, to factor
into the cost analysis inflation and capitalization expenses.  All
cost factors and the resulting analysis will vary from country to
Certain social/cultural questions need to be addressed.   For example,
is daily waste handling acceptable or taboo? Moreover, to
succeed, a biogas technology must interface with existing practices:
can existing waste management practices be adapted, for
example, to include a digester and effluent disposal? What happens
to the very poor who have traditionally collected cattle
dung freely to use for fuel when the dung is used in a digester
and the fuel is available only to those who can pay for it? Who
controls the distribution of the gas in a community system?
Technical resource considerations include taking into account the
availability of a constant, high-quality supply of organic material,
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 whether
the space is sufficient for effluent disposal and usage.  Moreover,
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 that
may prevent you from building or using a biogas generator.  On the
positive side, some laws might work in your favor.   For example,
the governments of some developing countries provide investment
incentives, grants, or low-interest loans to people who want to
introduce a biogas plant.  Such governments are actively pursuing
national policies that would reduce dependence on imported fuels
and so encourage the production of biogas as an environmentally
safe fuel source.
Chinese- and Indian-style biogas generators can generally be
built in-country, since plant components are usually available
locally.  Certain components, i.e., the dome and guide mechanism
of an Indian digester, can be manufactured on a larger scale and
sold to users.
Subsistence farmers who depend on firewood for cooking and heating
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 technologies require
        investments of capital usually available only to a few
        people in society;
     *   cultural norms may not permit waste handling or gas
        usage, or may limit availability of organic material if
        animals are pastured rather than confined; and
     *   biogas generation must be accepted and learned, a process
        dependent on motivated, knowledgeable extension agents
        or others who can point to successful applications of the
        technology, or who can demonstrate it effectively.
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 Chemistry
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
Ajitmal, Etawah
Uttar Pradesh, INDIA
Director, National Environmental Engineering Research Institute
World Health Organization
1211 Geneva 27, SWITZERLAND
Economic and Social Commission for Asia and the Pacific (ESCAP)
Division of Industry, Housing, and Technology
United Nations Building
Bangkok 2, THAILAND
Bangladesh Academy for Rural Development
Appropriate Technology Development Organization
Planning Commission
Government of Pakistan
Islamabad, PAKISTAN
Apartado 1160
Guatemala, GUATEMALA
Casilla 119
Volunteers in Technical Assistance (VITA)
1815 North Lynn St., Suite 200
Arlington, VA 22209 USA
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Lichtman, R.J. Biogas Systems in India. Arlington, Virginia:
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Pohland, F.G., ed. Anaerobic Biological Treatment Processes.
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Shuler, M.L., ed. Utilization and Recycled Agricultural Wastes
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Subramanian, S.K. Bio-gas Systems in Asia. New Delhi, India:
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Targanides, P. "Anaerobic Digestion of Poultry Waste." World
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                          SUPPLIERS AND MANUFACTURERS
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