Biogas Systems Powerpoint Notes Document – February 2013
Title Slide: We need to start with a quick definition. Biogas systems rely on a process called “anaerobic
digestion.” This means a process of breaking something down without oxygen. Stomachs breakdown our
food without using oxygen and biogas systems breakdown our food and food waste without using
oxygen. One very nice thing about anaerobic digestion is that, without the oxygen, it doesn’t smell so
bad!
Slide 2: Here is anaerobic digestion happening in an animal. A cow’s stomach is a perfect example of an
anaerobic digester. A cow eats. The bacteria in her stomach digests her food. She produces methane gas
(farts) and manure. We can build simple machines that work like a cow stomach to transform human
and other waste products into gases that we can use as fuel and fertilizers that enrich our land and our
gardens.
Slide 3: We build “Anaerobic Digesters” to copy the natural system of a cow’s stomach. We input
organic waste products like animal dung, human sewage and food waste. Our anaerobic digester
transforms these smelly waste products into valuable energy and fertilizer.
Slide 4: No notes
Slide 5: We are going to take a few minutes to talk about this slide because there are many important
benefits to discuss.
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Sanitation: Human waste is dirty. It can make people and animals sick and it can poison the
ground so that people cannot grow plants. Many households can contribute their waste to a
biogas system. All of the sewage that they would have put in the ground or near the community
is then contained and used. Instead of creating little areas of bad smelling and toxic land, the
community can bring its waste to one location and benefit from it.
Fuels: Another immediate benefit of biogas is replacing other fuels for cooking. Biogas burns
clean and without odor. It can be used for cooking and heating. In rural areas, biogas usually
replaces fuel wood, which can be difficult to collect or expensive. Studies by BSP-Nepal show
that households with biogas plants save three hours per day on average, because collecting
dung and feeding it to a biogas plant takes much less time than collecting fuel wood or
preparing a cooking fire.
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Air pollution: Because biogas burns with a clean flame, users of biogas do not have wood smoke
in their homes. Wood smoke is a major cause of respiratory and eye disease and it is responsible
for an estimated 1.6 million deaths each year.
CO2 and deforestation: In many places, gathering fuel wood is slowly destroying the natural
resources around the human community. A biogas system reduces the stress on surrounding
forests and vegetation—letting these systems regenerate to serve the community and its
animals in more useful ways. CO2 is also saved when biogas replaces kerosene or LPG for
cooking.
Fertilizer: The residue from dung-based biogas plants makes a good fertilizer with minimal smell.
The fertilizer value can be improved by composting the residue with crop waste, and feeding the
compost to earthworms for additional processing (vermicomposting).
Slide 6: Small-scale biogas digesters are reactors typically designed to produce biogas at the household
or community level in rural areas.
The airtight reactors are filled with animal manure, kitchen and agricultural waste and human waste.
These systems do not work with human sewage alone. A biogas system is not a toilet. It also makes use
of animal waste, agricultural waste and food waste.
Typically these systems are airtight, round underground chambers in which anaerobic digestion takes
place during a couple of weeks and sometimes up to several months, depending on the local
temperature. Digestion occurs between 20-45 C and 50-65 C and should remain stable to sustain gas
production; higher temperatures are more effective in killing pathogens in waste.
Slide 7: Here we see a drawing that illustrates a biogas digester receiving waste products and producing
energy and electricity.
Slide 8: Organic matter such as animal and human manure, kitchen waste and agricultural waste can all
be added to the system as they are produced. Water needs to be fed into the digester daily, so toilets
can be connected directly.
Since toilet waste, especially urine, has a high nitrogen content, feedstocks with a high carbon content,
such as sawdust, should be added to maintain the ideal carbon:nitrogen ration of 20:1 to 30:1. This
means that people using a biogas system need to be adding dry, carbon-rich materials to the digester on
a regular basis in addition to their waste.
Care should be taken that mixes of feedstock maintain proper C:N ratio (if there is too much nitrogen,
the system will yield less biogas. If there is too much carbon, the digester will slow down).
The system needs some methane-producing bacteria to get started. Once it’s started, bacteria will
reproduce and keep the process going. Cow dung contains suitable bacteria, and a small amount is often
used as a starter.
Slide 9: Cattle manure by itself has a low gas yield, but combined with higher yielding feedstocks—
especially agricultural waste or sawdust—it has a high energy yield. Different inputs take different
amounts of time to break down and contribute different amounts of useful gases.
Slide 10: When a community is managing their biogas system, it is almost like they are cooking together.
They need to aim for a carbon/nitrogen ratio between 20 and 30. Too much carbon (high ratio) causes a
rapid consumption of nitrogen and results in lower gas production. If there is too much nitrogen,
digestion slows down and nitrogen loss will decrease the quality of fertilizer. A community will aim for
an ideal ratio by mixing waste of low and high C:N ratio, such as agricultural waste mixed with sewage or
animal manure.
Slide 11: With just cow manure, this is how much gas is produced per day in biogas digesters of a few
different sizes. You can see the potential for very high amounts of released energy. This can save
families and communities lots of money!
Slide 12: If you have a cubic meter of biogas, here is how it compares to other fuel and energy sources.
You can see that biogas produces a useful amount of energy.
Slide 13: Small rural biogas systems can eliminate a family’s need for other cooking fuels and sometimes
even light their living space.
In urban settings where there is not easy access to agricultural waste, the food waste breaks down and
produces gas more quickly. Plants that use mostly food waste are therefore smaller and more suitable
for urban homes.
Large-scale: Larger-scale biogas schemes can produce sufficient gas to generate electricity. This is
frequently done in sewage treatment plants in the UK, and there are a number of large farm-based
plants in Germany and elsewhere.
Slide 14: No notes.
Slide 15: Here is the most common type of small-scale biodigester. The slurry container and gas
container are combined, so that the gas collects under a rigid dome over the slurry. As the slurry breaks
down, the biogas which is produced pushes some of the slurry into a separate reservoir. When the
biogas is taken, the slurry flows back. These systems are always installed below ground.
The cheapest, fixed-dome biogas plants are made mainly of masonry, either brick or concrete. A 6
square meter plant using cattle dung, to provide gas for a single family costs about US$500 (according to
Ashden Awards).
Fixed-dome biogas plants have relatively low construction costs and do not contain rusting steel (as the
floating-drum reactors). Thus, if they are well constructed, they have a long life span. The underground
construction saves space and protects the digester from temperature changes. However, temperatures
are generally relatively low. Fixed-dome plants must be covered with earth up to the top of the gas-filled
space to counteract the internal pressure (up to 0,15 bar). The earth cover insulation and the option for
internal heating makes them suitable for colder climates. Due to economic parameters, the
recommended minimum size of a fixed-dome plant is 5 m3
The construction provides opportunities for skilled local employment. However, problems of gastightness caused by poor brickwork and cracking can cause heavy losses of biogas. Fixed-dome plants
are, therefore, recommended only where experienced biogas technicians can supervise construction.
Fixed dome plants produce just as much gas as floating-drum plants, if they are gas-tight. However,
utilization of the gas is less effective as the gas pressure fluctuates substantially. Burners and other
simple appliances cannot be set in an optimal way. If the gas is required at constant pressure (e.g., for
engines), a gas pressure regulator or a floating gas-holder is necessary (according to Energypedia).
Slide 16: This community in Nepal had a problem: pit latrines were expensive to empty, and filled up
quickly due to a high water table. People couldn’t afford to empty the latrines one or two times each
year, so open defecation became the norm and led to outbreaks of disease.
The community found this solution: All 37 households’ grey and black wastewater and kitchen waste is
digested in the biogas plant, which generates enough biogas for five households in the community. The
constructed wetland (reed bed) treats excess wastewater discharged from the biogas plant and the
anaerobic baffle reactor benefits farmers by allowing the treated water to be used for irrigation of their
fields. Slurry is dried and composted to be used as fertilizer.
All pay a small monthly fee for the convenient waste disposal. This fee goes towards biogas plant
maintenance. Households who receive biogas pay slightly more than households that do not. In this
community, the households do not generate enough waste to power a high number of households, but
the biogas system is keeping their community clean and producing useful fertilizers.
Slide 17: Here is a smaller, family-use variety of biogas system. The gas container gradually rises up as
biogas is produced, and sinks back down as the biogas is used—maintaining pressure in the smaller
system.
A typical Indian floating dome generator for use by a family would consist of a digester with a depth of
about 3.6 meters and a diameter of 1.6 meters. A generator of this size would be supplied with 40-50kg
of dung (produced by approximately 5 cows) per day and would produce 3m3 of biogas with 55%
methane content per day (Read 16).
Steel and plastic are used in some floating-dome plants. These can be pre-fabricated and thus installed
very quickly. 1m3 plant costs about US$220 (Ashden Awards)
Floating dome reactors have the advantage that the gas pressure remains constant as it depends on the
weight of the gasholder. The construction is relatively easy and construction mistakes do not lead to
major problems in functioning and gas yield. However, the material costs of the steel drum are very high
and all the steel parts are susceptible to corrosion. Because of this, floating drum plants have a shorter
life span than fixed-dome plants, and regular maintenance costs for the painting of the drum arise.
Slide 18: Domestic plants: have a volume of one square meter and produce about one square meter of
biogas per day, with a maximum input of 5kg solids and 20 liters organic waste water per day.
This quantity of gas can be used for two hours of cook time.
Biogas plants for schools and hostels range in capacity from about ten square meters to twenty-five
square meters and each serve an average of 200 people.
The larger scale installations use food waste at markets and municipal sites, and are made from one or
two twenty-five square meter biogas digesters. Biogas used to generate electricity for lighting. Market
and municipal systems reduce health risks of food disposal onto streets.
One square meter household plants cost about US$220, with government subsidy available of up to
60%. An average family can pay back their contribution to the cost of the plant in about three years
through savings in LPG use. Meanwhile, the family benefits from high quality fertilizer, free of charge.
Slide 19: Another variation on the biogas system is currently being piloted in Antananarivo, Madagascar.
The Loowatt toilet uses a simple, patent-protected mechanical sealing unit to contain human waste
within biodegradable film in the most efficient way possible, with a unique odor-inhibiting system. The
waste is then stored in a cartridge beneath the toilet, for periodic emptying, which can be weekly or
daily, depending upon level of usage and capacity. The cartridge (on left) is emptied into floating dome
biodigester (on right).
Slide 20: The flexible balloon installation is a larger variety. Balloon plants consist of a huge common
plastic bag (e.g. PVC) in which the sludge settles on the bottom (there is no expansion chamber) and the
gas is collected in the upper part from where it is withdrawn. Inlet and outlet are attached directly to
the plastic skin of the balloon.
This variant is extremely low-cost and simple, this type of small-scale biogas digester has also the
advantage of higher temperatures under sunny sky and its ease for cleaning and emptying. However, its
life span can be relatively short as it is susceptible to damage.
Slide 21: Here is a variant on the balloon installation. The tubular polyethylene film (two coats of 300
microns) is bent at each end around a 6 inch PVC drainpipe and is wound with a rubber strap of recycled
tire-tubes. With this system a hermetically isolated tank is obtained.
Tubular polyethylene is produced in most countries. The choice of supplementary fittings and related
materials has been limited to those available locally on farms or in rural markets; they are the basic
components of sanitary installations which are similar all over the world. The polyethylene tubular film
biodigester technology is a cheap and simple way to produce gas. It has been applied in: Bolivia (Peru,
Ecuador, Colombia and Mexico). Disadvantages are the lower gas pressure and the need to protect the
film from sunlight and animals.
Slide 22: No Notes.