Andrea Frangiosa, La Merced Tarija, B-38
What is Bio-gas?
Bio-gas, also referred to as digester gas, swamp gas, or marsh gas, is the by
product of decomposing organic matter. Bio-gas is made up of about 1%-80% methane,
carbon dioxide, and trace amounts of nitrogen, hydrogen, and hydrogen sulfide. Bio-gas
mixtures containing more than 50% methane are combustible. The gas that is produced
can be used as fuel to cook, heat, and light households.
Bio-gas is produced when organic material decomposes under anaerobic
conditions. This anaerobic digestion occurs naturally in wetlands, lake bottoms, and deep
in soils. Anaerobic digestion and the production of methane as a by product occurs in a
two-stage process due to the presence of bacteria which thrive in an oxygen deprived
Acid-forming bacteria break down organic matter into volatile fatty acids.
Proteins are broken down into amino acids which are then broken down into volatile fatty
acids. Carbohydrates are broken down into simple sugars and then into volatile fatty
acids. Fats and oils are broken down into long chain fatty acids and then into volatile
Acetogenic bacteria use the volatile fatty acids to form acetic, propionic, and
lactic acids; as well as hydrogen and carbon dioxide gas.
Methane-forming bacteria (methanogens) use the acids formed in Stage 1 from
the breaking down of proteins, carbohydrates, fats, and oils to produce methane. Other
bacteria that are present in Stage 1 use some of the hydrogen and carbon dioxide to
produce methane as well.
The production of Bio-gas is initiated by putting organic material into a
biogenerator, or bio-gas digester. The biogenerator provides an environment where the
necessary anaerobic reactions can take place. The biogenerator, or unit, also captures and
stores the biogas for latter use as combustible fuel. The unit produces two by products,
the bio-gas and the digestate. Digestate is the sludge like material left over after the
organic material has undergone anaerobic decomposition. Digestate can be used as an
agricultural fertilizer which can aid in increased soil moisture retention, as well as
improved soil fertility.
The biodigestion process does reduce the amount of disease causing pathogens in
the manure. This is especially true if the unit reaches higher temperatures. However,
caution still needs to be exercised when ever working with animal waste, as e coli is
present as well as other bacteria that can cause illness.
Organic Materials that can be used to produce bio-gas.
* cows, pigs, or chicken
Ideally pig and chicken manure is most affective in the production of bio-gas.
* agricultural wastes, as long as they are organic
* sugar cane, material left over after the extraction of the cane juice
* straw and material left over from the harvesting of wheat
* corn stalks left over from the harvesting of corn
* weeds from the vegetable garden
* coffee bean shells and pulp
You can use only pure manure, or only pure plant material, or you can mix the two
together. However you will more than likely be able to produce higher quantities of bio-
gas if you use pure manure or manure mixed with a low percentage of plant material.
Why is Bio-gas beneficial?
Why is the production of bio-gas a good idea? There are various reasons why
bio-gas is beneficial to implement, both economically and environmentally.
Here in the rural areas of Bolivia many people use wood as their main source of
fuel for cooking. They often use only wood because buying gas tanks and gas stoves are
cost prohibited. As a result an extra strain has been placed on the surrounding rural
environment, as well as forested areas. The cutting down of trees for a fuel source leads
to deforestation and degradation of forest ecosystems. Gathering wood is often a time
consuming task that can involve walking long distances with heavy loads of wood.
Take into consideration that in many rural areas food is usually prepared using an
open wood fire in confined roofed areas. These open fires usually have no chimneys and
produce large quantities of smoke. The smoke produced by these fires, in confined
closed areas, can lead to respiratory irritation, infections, and illness.
Another important fact about bio-gas is that it is a renewable energy resource, and
is not a fossil fuel; it is a non-petroleum based product. In other words you are not
contibutating to the further depletion of non renewable resources by using bio-gas as a
When a Household uses Bio-gas they:
Will use less:
* natural gas, kerosene, wood, and coal
They will become less dependant on these materials as a result.
Due to the time saved by not looking for wood, families will have more time to:
* do domestic tasks
* do agriculture
* do trades such as artesania to help earn more income
* have a little bit more free time to spend with family and friends
Other benefits of using bio-gas
* People will have better air quality within the kitchen because bio-gas does not
produce smoke like wood or coal when burned.
* People will diminish their impact on the environment that surrounds them by
decreasing deforestation and the use of non renewable resources.
* They will not only produce fuel for cooking, heating, and light, but they will
also be producing digestate which can be used as organic fertilizer.
How to Build a Small Scale Biogenerator
It must be noted that it is neither easy, nor straight forward to build a biogenerator
in order to produce bio-gas. It is a commitment and investment of time, work, money,
and experimentation. Therefore, it is suggested that you make a small scale unit first to
learn how the system works. There is a level of experimentation involved with this
project. Using a small scale unit will make it easier to find out what works best to
produce the largest quantity of bio-gas from the organic material.
Where to build a biogenerator
Look for a place that is close to the area where you will be using the gas. There
needs to be a distance of at least 10 meters between the area where you are going to use
the gas and the unit. Keep in mind that the larger the distance between the two areas, the
more gas hose you will need to buy, and the more expensive the unit will become to
construct. Also the larger the distance the more gas line you will need to maintain, and
repair to prevent gas leaks. Make sure that the gas line will be in an area that is
protected, away from chickens and other animals. This needs to be done in order to
prevent puncturing of the line. This could mean that you will need to bury the gas line.
The unit needs to be at least 15 meters away from water sources. This is to
prevent contamination of the water by the manure that you will be using in the unit to
Biogenerators perform best between the temperatures of 32°C to 37°C. 35°C is
the ideal temperature for anaerobic digestion. As temperature decreases so does bacteria
activity, thus resulting in lower production of bio-gas. As temperature increases to a high
level, above 37°C, some bacteria begin to die and as a result bio-gas production decreases
or stops completely. Since temperature regulation is such an important factor in bio-gas
production, it is extremely important to place the unit in a suitable location.
It is not advisable to place the unit in an area that has direct sunlight all day long,
especially in hot climates. You need a location that experiences a good mix of both
sunlight and shade through out the day. Trying to maintain a unit temperature of 35°C is
key and will greatly aid in the success of the project.
In colder climates it may be necessary to bury the unit in a compost pile above
ground. The compost pile naturally generates heat due to the decomposition of the
organic matter present in the compost. This heat can help regulate the temperature of the
unit and avoid allowing the unit to fall below 32°C, which would lower the amount of
bio-gas being produced. It is not advisable to bury the unit below ground. It makes it
more difficult to maintain as well as reload manure and empty the digestate.
For the construction of the unit you will need the following:
One old oil barrel that is at least 200 liters in volume. Look for an oil barrel that
is in good condition, NOT rusted. You want to create an air tight unit, or you will
lose the bio-gas you are trying to produce through leaks from the barrel. Make
sure the barrel has two openings on the top, a larger one that has a screw down
cap, and a smaller one that has a diameter of about 2 cm.
One metal pipe with the length of 15 cm and the diameter of 2 cm. Make sure the
pipe threaded at one end so you can screw it to the T pipe.
One metal T pipe with diameter of 2cm.
Two metal pipes with 10 cm of length, and diameter of 2 cm that are threaded at
2 metal gas valves
2 hose adapters
Gas hose that is either made of rubber or plastic and at least 12 meters in length
and 2 cm in diameter. Make sure to take note of the distance between the unit and
the stove. Make sure that the hose fits snuggly to the hose adapters.
2 or more large inner tubes to store the gas.
1 small metal T to connect hose to multiple inner tubes.
A least 7 hose clamps to tighten and secure the hose to the gas exit on the unit.
Teflon tape to seal connections between pipes.
To make the catalyst you will need:
A small container that has a volume of at least 15 liters that has a secure screw
Rubber or plastic hose with a length of about 50 cm.
empty wine or pop bottle
Materials need to prepare unit:
Welder, or access to a welding shop
Large container to mix manure and water together. An old oil barrel cut in half is
useful for this project.
Detergent, Omo works great
Funnel with a diameter equal to the opening in the oil barrel to which you will
pour the manure mixture.
Cleaning the Barrel
You will need to clean the barrel both inside and out, making sure to remove all
petroleum residue. If you do not clean the barrel sufficiently the oil residue could inhibit
the production of bio-gas.
1) First clean the inside of the barrel by heating water in the half barrel container.
You can do this by building a small fire under the container that is propped up on
rocks. Pour the hot water into the large oil barrel when the barrel is in the
horizontal position. Then pour powdered detergent into the oil barrel.
2) Now place the oil barrel with the water/detergent mixture on its side and roll the
barrel back and forth. This will clean the oil residue off from the inside of the
barrel walls. Do this until you think you have washed the inside of the barrel
3) Pour out the dirty water and repeat the process until the water that you pour out of
the unit is relatively clean.
4) Remember that this is an excellent time to look for leaks on the barrel. If there
are leaks, mark them and fix them by welding them closed. You can do this with
or without using a metal patch. It all depends on the severity of the leak. The unit
needs to be air tight in order to produce and save the bio-gas effectively.
5) Once the inside is clean, pour clean water, without detergent, into the barrel and
rinse out all the remaining soap. It is very important to rinse the barrel thoroughly
because the detergent can also affect the production of bio-gas. The soap could
kill the beneficial bacteria you are trying to cultivate.
6) Now wash the outside of the barrel with hot water and detergent. It is best to
scrub the outside with a brush until all oil residue is cleaned off. Make sure to the
clean the top of the barrel well, especially the two holes that you will use in the
future to fill the barrel with manure.
7) Rinse the outside of the barrel well and then turn it upside down, resting the barrel
on a few rocks. This is to help all the water to drain out of the barrel. Once the
barrel is dry, inside and out, you can start the construction of the unit.
Constructing the Unit
1) Take the 15 cm pipe and weld it to the smaller of the two holes on the top of the
barrel. This smaller hole is where the gas produced will exit the unit. The larger
of the two holes will be used to fill the unit with manure. It is important that the
larger hole has a cap that you can screw down tightly. It is very important that the
unit be as air tight as possible.
2) After you have welded on the 15 cm pipe attach the T pipe, 10 cm pipe, hose
adapters, and the two metal gas valves as shown in the drawing.
3) Now take the inner tubes and make sure that they are without holes or tears in the
rubber. To do this pump up the inner tube with air and then submerge the inner
tube in water. Look for small bubbles escaping from the inner tube, this signifies
4) Mark the holes and patch them like you would a bike tire with a rubber patch and
5) Once the inner tube is ready make sure that the inner tube has a two way valve.
The valve should allow gas to enter and leave freely from the inner tube.
6) -Now lay the inner tube flat and roll it up using a stick that is at least the width of
the inner tube. Roll the inner tube using the stick, trying to get as much air out of
the inner tube as possible.
7) Clamp a portion of the hose length that you are going to use to attach the inner
tubes to the unit, making sure that no air will be able to pass through the hose.
8) Take this hose and attach the hose to the inner tube, while still rolled up, at the
two way valve. Make sure the connection is secure; you may need to use a hose
clamp or wrap wire around the connection. Repeat the above process with each
inner tube and inner tube attachment.
9) Take the inner tubes and secure them with sticks. This is to immobilize the inner
tubes. If you have more than one inner tube you can stack them on top of one
10) Take the hose that is attached to the inner tubes and attach it to the unit at the hose
adapter. You are now ready to fill the unit with organic matter.
Making the Catalyst
About a month to two months before you fill the unit you should make a catalyst.
The catalyst will jump start the production of bacteria in the unit and speed up the
production of bio-gas inside the unit.
1) Take the screw cap of the 15 liter container you will be using and punch a hole in
2) Take the rubber hose and secure one end to the screw cap, securing and sealing
the sides of the hose to cap with cement or sealing putty. Make sure that the
connection between the rubber hose and the cap is air tight and that no air escapes.
3) Take 5 liters of manure and 5 liters of water and mix them together in a bucket.
Let it stand for 20 minutes, mixing periodically, making sure that the mixture is
some what smooth and the thickness of paint.
4) Then pour the manure-water mixture into the 15 liter container and secure the cap
with the hose attached to it.
5) Take an empty wine or soda bottle and fill it half way with water. Then take the
free end of the hose that is attached to the 15 liter container and place it into the
bottle of water. Make sure that the hose is submerged completely in the water.
6) Leave the manure mixture to ferment for at least 4 weeks. Agitate the container,
mixing and moving the manure, at least 3 times a week. Bio-gas will be produced
in small quantities. You can lift the hose in the bottle and see bubbles being
released from the hose when gas is being produced. You can also experiment and
try and light the gas that is escaping from the end of the hose.
Filling the Unit
1) Using an old oil barrel that is cut in half or any other large volume container, mix
one part manure with one part water. For example, mix three buckets manure
with three buckets of water.
2) Mix in a small amount of catalyst, and let the mixture stand for about 20 to 30
minutes. Mix every 5 minutes until the mixture is smooth and the consistency of
3) Once the mixture is ready pour it into the unit, repeat steps 1 and 2 until the barrel
is almost full. Slowly add the catalyst to the manure mixture, little by little, until
all used up. Make sure to leave 10 cm between the top level of the manure and
the top of the barrel. It is a good idea to have the gas valve that runs to the stove
open so that you allow air to escape from the barrel while you are filling it with
4) Once the unit is full to capacity, secure the unit by capping the larger of the two
holes. Make sure that the main gas valve is closed, the one that leads to the stove.
Make sure that all connections are air tight and you have no leaking of gas.
5) Now you can open the hose that runs from the unit to the inner tubes. This will
allow the bio-gas that is produced to be stored in the inner tubes.
6) To help facilitate the production of bio-gas twice a week you can shake the unit,
this turns the mixture within the unit, which can aid in the production of bio-gas.
It maybe necessary to wait three weeks to one month until there is any production
of bio-gas. You will note the production of gas when the inner tubes start to inflate.
Production of bio-gas can last for up to eight weeks. Half of the gas will be produced
in the first two to three weeks, and the rest will be produced between the fourth and
8th weeks. If you are producing little to no gas towards the end of the 6th week,
empty the unit and repeat the process again.
Do not try to burn the gas that is produced at the beginning of production. Instead,
once the inner tube is full, open the valves and release the gas. In order to empty the
inner tubes you may need to place some heavy objects on top of the inner tubes,
smooth rocks, wooden boards, to aid in pushing the gas out of the inner tubes.
The first gas produced usually has a high amount of water and air mixed with it;
this is why it is a good idea to release it. Make sure that there are no open flames
close to the unit, as this is a combustible gas that is very volatile. After you release
the first batch close the valves and let the unit fill the inner tubes a second time, you
can use this gas as fuel.
Temperature is a key factor in bio-gas production. The best temperatures for bio-
gas production are between 32°C and 37°C. When the unit drops to 15°C, the
production of bio-gas stops completely. As mentioned earlier you can prevent the
unit from dropping below the optimal temperatures by burying the unit in a compost
pile. However, it is not a good idea to bury the unit underground because it makes
maintenance and reloading of the unit very difficult to impossible.
Alkalinity and pH
Alkalinity and pH are two important factors in bio-gas production. pH is used to
indicate whether a solution is acidic or basic. An acidic solution has more hydrogen
or hydronium ions than hydroxide ions. A basic solution has more hydroxide than
hydronium ions. At a pH of 7 there are equal amounts. A pH greater than 7 is
classified as a basic solution and a pH less than 7 is classified as an acidic solution.
Alkalinity is the measure of the amount of carbonate in a solution. Alkalinity is
important because as an acid is added to a solution, carbonates will contribute
hydroxide ions, which tend to neutralize the acid, thus affecting the pH of a solution.
This is known as the buffering effect of alkalinity.
pH is important to bio-gas production because the bacteria responsible for bio-gas
production flourish within a narrow range of pH, between 6.5 and 8.0. As the acid-
forming bacteria produces acid, the methane-forming bacteria use the acid and aid in
maintaining a neutral pH in the unit. It is important to know that the reaction rate for
the acid-forming bacteria is much faster than the reaction rate involving the methane-
forming bacteria. Therefore it is important that a good balance is maintained between
the two in order to avoid acidification of the manure. This means that you need to
maintain and nurture a higher population of methane-forming bacteria.
In order to facilitate a large population of methane-forming bacteria you can raise
the pH to about 7.5 by adding baking soda. The baking soda increases the alkalinity
or buffering capacity of the unit. This may or may not be necessary to do. If you are
not producing bio-gas test the pH of the unit, and if you have an acidic solution you
know to add baking soda the next time you mix the manure and water together.
When the initial reactions occur in the unit there is a large production of acid-
forming bacteria. The methane-forming bacteria population may not be large enough
to maintain a neutral pH in the unit. If the unit drops below 6.5 the methane-forming
bacteria will begin to die, pushing the bacteria populations further out of balance.
This results in an acidic solution within the unit, which can not produce bio-gas.
Formation of Foam
Some times the unit will form a cap of foam on the top of the manure mixture
inside of the barrel. This can diminish the production of bio-gas and stop the flow of
bio-gas to the inner tubes. If the organic matter and water was mixed well before
introduction to the unit this problem can usually be prevented. It is also less likely to
occur if the organic material you are using is 100% manure and has no plant matter
mixed with it. To prevent the formation of foam in the unit it is a good idea to agitate
the unit twice weekly as mentioned before.
Bio-gas can be used with almost any small stove. The simpler the better, meaning
you may want to construct your own. You can do this simply by using plastic tubing
and a simple metal frame.
Or you can use the small stoves available here in Bolivia. Which ever type of
stove you use, make sure that there is not too much air mixing with the gas before it is
lit. If you are producing a small amount of methane and you mix the bio-gas with too
much outside air, it may not ignite. The opposite is true as well; if you have too much
bio-gas and not enough outside air the gas may not ignite.
So make sure that you can adjust the air mix on the stove, it may take some
experimentation to get the bio-gas to light. When the bio-gas/air mix is right the gas
will ignite and a blue flame will result, try to obtain as blue of a flame as you can by
adjusting the mixture.
Over time your stove may become clogged, or the flame may turn from blue to
yellow. This means that your line is dirty and that you need to clean the stove. You
can clean the stove with a brush, warm water, and soap. Make sure the stove is
completely dry before reassembling it and using it to cook with once again.