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The National Sustainable
Agriculture Information Service,
ATTRA (www.attra.ncat.org),
was developed and is managed
by the National Center for
Appropriate Technology (NCAT).
The project is funded through
a cooperative agreement with
the United States Department
of Agriculture’s Rural Business-
Cooperative Service. Visit the
NCAT website (www.ncat.org/
sarc_current.php) for
more information on
our other sustainable
agriculture and
energy projects.
1-800-346-9140 • www.attra.ncat.orgA project of the National Center for Appropriate Technology
Funding for the development
of this publication was provided
by the USDA Risk Management
Agency.
By Rich Dana
NCAT Energy Specialist
© 2010 NCAT
Contents
Micro-scale Biogas Production:
A beginner’s guide
Introduction
A
naerobic digestion is one of the most
basic processes of life on earth. Anaer-
obic means “in the absence of oxygen”
and anaerobic digestion takes place when biode-
gradable matter decomposes in a closed environ-
ment. By managing the digestion process and
capturing the biogas produced, we can power
boilers, burners and generators, and possibly
even refrigerators.
In the United States, there is a great deal of
interest in using large-scale anaerobic digesters
to control odor at livestock facilities and burning
the biogas produced to run electrical generators.
Commonly referred to as Methane Digesters,
these large-scale units actually digest manure
and produce biogas, which is made up of carbon
dioxide (CO2
) and methane (CH4
) with smaller
amounts of water vapor (H2
O), hydrogen
sulfide (H2
S), and in some cases ammonia
(NH3
). Methane is flammable gas that can be
used to make power, and is the primary compo-
nent of the fossil fuel natural gas in common use
to heat homes.
Although not widely used in the United States,
small-scale digesters are used successfully all
over the world to turn small amounts of manure,
plant waste and food scraps into usable energy.
For hundreds of years, biogas has been used in
China and India (and more recently in Africa
and Central America) to provide gas for cook-
ing, heating and lighting—utilizing very simple,
low-tech designs. This publication will exam-
ine several of the common designs for build-
ing micro-digesters that have been developed
around the world, and discuss how sustainable
farmers in American can take a lesson from their
counterparts in the developing world.
Using the Gas
If you decide to build the digester, how will you
use the gas? Biogas can take the place of pro-
pane or natural gas in many instances. The most
Anaerobic digestion is a simple biochemical
process by which waste products can be con-
verted to energy. Using manure, plant waste, crop
residue, food scraps or other waste products,
farmers can reduce their dependence on fossil
fuels while saving money, reducing greenhouse
gas emissions and solving odor problems (not
to mention producing high-quality compost
fertilizer as a by-product). This publication provides
a beginner’s guide to using anaerobic digestion
to produce usable biogas on a small scale with a
minimal investment.
For more information on biogas and a discus-
sion of larger-scale, manure-based digesters for
livestock operations, please see the ATTRA
publication Anaerobic Digestion of Animal Wastes:
Factors to Consider.
Workshop participants building a micro-scale digester
in Costa Rica. Photo by Ian Woofenden.
Introduction......................1
Using the Gas....................1
Small is Beautiful .............2
Biogas Basics:
The Chemistry of
Anaerobic Digestion......2
Feeding Your
Digester ..............................4
Micro-Digester
Designs ...............................5
Start-up, Operation
and Maintenance ............6
Side Streams......................8
Conclusion.........................9
References .........................9
Further Resources
Online..................................9
Page 2 ATTRA Micro-scale Biogas Production: A beginner’s guide
technology because of its low cost (and the abil-
ity to adapt salvage materials or materials that are
commonly at hand), simplicity, and environmen-
tal and health benefits. In places where wood is
used for cooking and heating, deforestation is a
major concern and burning wood can cause air
quality issues and respiratory problems for resi-
dents—especially small children. A household
digester can reduce the work of gathering wood
and provide a cheap and plentiful fuel source
where before there was none.
Biogas Basics: The chemistry
of anaerobic digestion
Anaerobic bacteria and archaea predate most
other forms of life on our planet, having devel-
oped in the period when the earth’s atmo-
sphere was mainly methane (CH4
), carbon
dioxide (CO2
) and water vapor (H2
O). Anaero-
bic digestion takes place in environments with
limited oxygen: underwater in a swamp or
marsh, at the bottom of a compost heap, or in
the gut of animals. Anaerobic digestion is used
in septic systems and sewage treatment plants
to break down waste.
The Stages of
Anaerobic Digestion
There are four stages to the digestion process—
hydrolysis, acidogenesis, acetogenesis and meth-
anogenesis. These stages need to be managed
early on in starting a digester, but if the pro-
cess is properly maintained, methane production
should require a minimum of chemical testing
and treatment. Bacteria must do their work in
the earlier stages to use up the oxygen in the
material, and then break it down into volatile
fatty acids and fermented alcohols, before the
methanogenic bacteria can start to make methane.
The process is as follows:
• Hydrolysis: Enzymes break down and
liquefy the smaller molecules and break
down large polymers in the material.
common use is for cooking. It can, of course,
be used as a heat source as well: to supplement
home heating, for heating a poultry house, far-
rowing house incubator or other small livestock
building, or for heating a greenhouse.
One unique use that has not been widely explored
in the U.S. (but has in China) is using the gas to
power an absorption refrigerator. This is a type
of refrigerator that uses a propane flame rather
than an electric compressor to create the evapo-
rative cooling cycle. These refrigerators are com-
monly found in RVs and in locations that have
no electricity or unreliable electricity supplies.
Though there is little research available on the use
of methane to power absorptive cooling, methane
has potential for use by farmers who may be look-
ing for refrigeration options in remote areas.
Small is Beautiful
There are several major technical issues to over-
come in order to operate a large-scale digester
successfully and use it to generate wholesale elec-
tricity. Maintaining proper bacterial health in a
40,000- to multi-million-gallon digester tank
can be difficult—and mistakes are expensive.
Large-scale digesters require hundreds—or thou-
sands—of head of livestock to provide adequate
manure, creating material handling issues. In
order to run large, sensitive electrical generat-
ing equipment, the biogas must be “scrubbed” to
remove most of the other gases, which can add
work and expense. Large digesters often require a
full-time manager. On the other hand, a micro-
digester can supply a family with cooking gas
for two meals a day (or add heat to a greenhouse
or workshop), from the manure of a few pigs,
cows or chickens, or food scraps and rotten or
unusable vegetables and fruit. Because the cost of
construction is low, the feedstock is free, and the
fuel is used at the source, economic risk can be
minimized and sustainability maximized.
In other parts of the world where energy resources
are not as readily available as they are in the
United States, the micro-digester is an important
Related ATTRA
publications
Disclaimer: This publication is meant as an introduction to micro-scale biogas production, and not a complete hands-on manual
on how to construct and operate a digester. There is a list of references at the end of this publication, and we suggest doing con-
siderable additional research before committing time and resources to a biogas project. Most importantly, remember that biogas,
though relatively safe, can be dangerous, flammable, and potentially explosive if handled improperly. We suggest that you exercise
all possible safety precautions and abide by all local, state and federal laws that might apply to a digester project in your location.
Anaerobic Digestion
of Animal Wastes:
Factors to Consider
Compost Heated
Greenhouses
Page 3ATTRAwww.attra.ncat.org
Pure water is said to be neutral, with a pH level
of about 7. Acids have pH numbers below 7,
while bases have numbers above 7. A digester
operates best near the neutral point, up to
a pH of about 8.5. In the early stages of the
anaerobic process, when the digester is making
acids, the pH can drop to 6 or below. When it
reaches methanogenesis, it will operate in the
7.5 to 8.5 range.
It is important to remember that the pH level
becomes less of an issue when the digester is
well-buffered, as it can take a certain amount of
acid or base material being added to it without
upsetting the digestion.(Fry, 1973) Buffering is
the digester’s ability to resist pH change. Since
pH is a “lagging indicator”, proper buffering
is important to avoid problems with acidity.
(see Figure 1).
In a large-scale digester, large variations in pH
can be catastrophic. In a micro-digester, how-
ever, high or low pH conditions may be cor-
rected relatively simply. A high-acid condition
can be treated with small amounts of lime,
ammonia or even bicarbonates like baking soda.
Carbon/Nitrogen Ratio (C:N)
Carbon and nitrogen are the two components
that anaerobic bacteria require for survival. They
need about 30 times more carbon than nitro-
gen, so the C:N ratio of your substrate should be
about 30:1. This is important when formulating
your slurry. When determining the C:N ratio
of feedstock like manure, the animal feed and
• Acidogenesis: The products of the
hydrolysis (soluble monomers) are fer-
mented to volatile fatty acids (or VFAs)
and alcohols.
• Acetogenesis: Acetogenic bacteria
break down the VFAs and alcohols, ace-
tic acid, carbon dioxide and hydrogen.
• Methanogenesis: The methanogenic
bacteria convert acetic acid and hydrogen
into CO2
and methane. (Note: The three
earlier stages can take place at lower tem-
peratures than Methanogenesis.)
Temperature and pH
Two of the most important factors for proper
anaerobic digestion and methane production are
temperature and pH. Failure to regulate these
two factors is among the most common prob-
lems with a digester.
Most anaerobic bacteria perform best in warmer
atmospheres. This is why micro-digesters are
most common in areas closer to the equator.
This does not mean a micro-digester will not
function in more northerly climates, but it does
affect design decisions, as the digester may need
to be insulated or even heated to make large
amounts of methane. Anaerobic digestion does
not stop completely at lower temperatures, but
methanogenesis is the stage most seriously hin-
dered by low temperatures, and this will reduce
methane production. There is evidence, how-
ever, that plant-based feedstocks may perform
better at low temperatures than do manure-
based systems. (House, 2006)
Although micro-digesters in Central America or
India often operate completely without added
heat or even insulation at temperatures ranging
from 60ºF - 100+ºF, the micro-digester design
most popular in China is an underground
design, which utilizes the earth’s geothermal
properties to maintain a healthy temperature.
This publication discusses design considerations
below, in the section entitled Micro-digester
Designs: Global D.I.Y. technology.
Along with temperature, pH is vital to digester
health. The pH is a measure of the acidity or
alkalinity (baseness) of the contents of the
digester. Just as in the human digestive sys-
tem, too much acid creates “indigestion” in the
anaerobic system.
Figure 1: Courtesy of David House.
Page 4 ATTRA Micro-scale Biogas Production: A beginner’s guide
amount of bedding material can heavily affect
the actual amounts of carbon and nitrogen—
you will need to adjust your feedstock accord-
ingly (see Table 1).
Table 1: Carbon Nitrogen Ratio
Substance C:N
Cow Manure—Alfalfa 16:1
Cow Manure—Dairy (with bedding) 21:1
Pig Manure 14:1
Sheep Manure 20:1
Chicken Manure 15:1
Oat Straw 48:1
Turnip Tops 19:1
Corn Stalks 53:1
Grass Clippings 19:1
Sunflower 30:1
Source: House, David. 2006. Biogas Handbook. p.37-40
Feeding Your Digester:
Substrates
In the proper feeding and care of your digester,
you must remember the “Seven S’s” of anaerobic
digester design: (see Table 2)
Table 2: The Seven S’s of Digester Design
1. Substrate: The organic material that you will be feeding your digester. This can be manure,
plant waste, paper pulp, or other biodegradable material.
2. Slurry: The slurry is the homogeneous mush that you will be passing through the digester. In
order for efficient digestion to take place, the substrate material should be mashed or ground
and mixed with water to make a uniform substance with 15–40% solids, depending on your
digester design.
3. Stratification: As your slurry breaks down in the digester, it will separate into layers. These lay-
ers—or “strata”—are biogas, scum, supernatant, sludge and solids. Mixing of the slurry prevents
excessive stratification, but some stratification will always occur, especially in “batch” digesters
(see below, Micro-digester Designs: Global D.I.Y. Technology).
4. Scum: The scum layer floats on the top of the material in the digester, just below the gas level.
The scum level is formed by hard-to-digest material like coarse straw and grease.
5. Supernatant: The spent liquid of the slurry. The Supernatant has a high content of solids,
making it of high value as fertilizer, similar to “compost tea.”(Diver, 2002)
6. Sludge: Below the liquid supernatant is the sludge layer. The sludge is the digested and semi-
digested organic solids. This sludge can provide excellent composted fertilizer, but depend-
ing on the feedstock, it may need to be dried in the sun to kill any surviving pathogens. This is
more of an issue with manure-based systems.
7. (Inorganic) Solids or Sand: The bottom layer consists of those non-digestible solids that find
their way into the digester. These could include dirt, sand, small rocks, plastic or metal—any
inorganic solid that may inadvertently be introduced into the system.
Page 5ATTRAwww.attra.ncat.org
The system consists of a long trench (approx.
20’), lined with a 48” diameter polyethylene
(poly) tube to form the “bag”. With an inlet at
one end, an outlet at the other and a gas out-
let fitting on the top, it is an extremely simple
design (see Figure 2). The poly tubing is available
by the roll, and is commonly used as packaging
material. A double layer of 6-8 ml poly is sug-
gested to avoid punctures, and a pressure release
allows excess gas to exit the system before over-
inflation of the bag. This design will produce 4
hours of usable cooking gas per day and requires
approximately 100 gallons of slurry to “charge”
the digester, and an additional 10 gallons of
slurry per day to maintain.(Brown, 2004)
In-ground dome
The in-ground dome design is used widely in
China, and may have considerable potential for
adaptation to northern climates in the United
States.(Koottatep et al., no date) This design
Micro-Digester Designs:
Global D.I.Y. (do-it-yourself)
technology
There are two basic types of digester designs:
continuous-flow and batch processors. In a contin-
uous-flow digester, new substrate is regularly fed
into the digester. The slurry moves through the
digester, pushing out digested material (efflu-
ent). The material may also be moved mechani-
cally through the continuous-flow digester with
augers or pumps. Batch digesters are loaded once
and then the material is allowed to digest. When
the digestion is complete, the effluent is removed
and the process is repeated. By adding an inlet
tube for feeding the digester and an outlet tube
for the overflowing effluent, a batch processor
can be fed some extra material after the initial
start-up to extend the cycle, making the unit a
semi-continuous batch processor. Each type has
its advantages. Continuous digesters produce
biogas without interruption. Batch digesters, on
the other hand, are simpler and less expensive to
build. Continuous-flow designs lend themselves
better to manure-based systems, while batch
processors may be more appropriate for unpro-
cessed plant-based substrates.
The types of micro-digesters we will be cover-
ing in this publication fall into three categories,
with some variation. These are the in-ground
bag or tube style (continuous flow), the in-
ground dome style (semi-continuous batch) and
the floating top/tank design (batch or semi-con-
tinuous). Because much of the micro-scale bio-
gas technology has been developed by individu-
als and non-governmental organizations, these
designs are simple and “open source”—mean-
ing that the designs are non-proprietary and
much of the information on these designs is free
and readily available on the Internet. (See the
Resources section at the end of this publication
for a list of books and websites to visit for more
detailed information.)
In-ground bag
The in-ground bag style digester is very popular
throughout Central America, and may be appro-
priate for use in the Southern U.S. It is a very
simple continuous-flow design especially well
suited to a family farm with just a few pigs or
cows. It is a very inexpensive design, with parts
that can be acquired for as little as $50.
Figure 2: In-Ground Bag
An in-ground bag digester in operation. Photo by Ian Woofenden.
Page 6 ATTRA Micro-scale Biogas Production: A beginner’s guide
This design is used widely in India in particu-
lar, where ARTI (Appropriate Rural Technology
Institute) has developed a very functional fam-
ily-sized design utilizing a 1000-liter (264 U.S.
gallons) polyethylene base tank with a 750-liter
(198 U.S. gallons) tank as the inner floating gas
chamber.(Voegeli et al., 2009) This design can
be adapted to use any size of tanks, with a larger
diameter for the outer and smaller diameter
for the inner tank, and can be an excellent and
inexpensive design to experiment with. Many
sizes of drums are readily available for free, and
a very small test digester can be made with a
couple of barrels and just a few standard plumb-
ing parts (see Figure 4).
Other designs
Several other designs have been developed that
are variations on these three basic styles. One is a
hybrid design that uses sealed 55-gallon drums as
modular digesters that are connected to a separate
floating top/tank gas storage container. Another
style uses the 275-gallon totes commonly used
to transport farm and industrial chemicals. In
general, plastic containers are preferable to steel
drums for the digester vessel and gas storage
tanks, because both the slurry and the biogas
itself can corrode steel drums fairly rapidly.
Start-up, Operation
and Maintenance
Start-up
The best way to get any anaerobic digester going
is by “seeding” or “inoculating” it with mate-
rial from another healthy, operating digester.
is not unlike a western-style septic tank, and
in fact, a conventional cast concrete or fiber-
glass septic tank could be modified to serve as
a digester (see Figure 3). In China, the latrine is
often connected to the digester, so it serves as a
septic system as well. In the U.S., zoning and
public health laws in most areas would not allow
“blackwater” from the home into the digester,
despite the obvious advantages to such a system.
This design involves a cast concrete, or concrete
block, or brick, tank and cap. While the expense
is much greater than that of a bag-style digester
(typical cost is $350 for 4-6 cu. meters), but by
using the earth’s natural insulation, the dome-
style digester can maintain a constant tempera-
ture, even in colder climates.(Nakagawa, 1981)
Floating top/tank
The floating top/tank design is one that adapts
itself particularly well to the materials at hand.
In this design, the slurry is contained in a bot-
tom tank, with another inverted tank or drum
serving as a cap, which is lifted by the biogas as
it is generated (see Figure 4).
Figure 3: In-Ground Dome
Figure 4: Floating Top/Tank
Asimplefloating-topdigesterbuiltfromscrapmaterials.
Photo by Rich Dana.
Page 7ATTRAwww.attra.ncat.org
scum layer can form and hinder the gas produc-
tion. Keeping substrate materials in suspension
will assist in the uniform breakdown of the mate-
rial, and will promote complete digestion.
Loading the digester will be the most common
operation, and this involves preparation of the
slurry. Manure must be liquefied, food scraps
must be mashed, cardboard or grasses must be
shredded. Remember, the smaller the particles
in your slurry, the happier your digester will
be. How much you need to feed your digester
is called the loading rate. The loading rate is
derived from the amount of organic matter
fed daily, divided by the size (volume) of the
digester. A healthier anaerobic environment can
handle a higher loading rate, but over-feeding
the digester can cause a rise in the acid content
and reduced gas production. Put more simply,
more feedstock is not always better.
Cleanout is necessary when gas production
drops below an acceptable level. This will be a
regular occurrence in a batch digester, but will
be less necessary in a continuous-flow system,
since much of the material will be leaving the
system as effluent. In both cases, at some point
during the operation, some digestate and inor-
ganic solids will need to be removed.
Depending on the climate, heating the digester
may be another consideration. Obviously, using
any kind of fossil-fuel-based heating like pro-
pane, oil or electricity doesn’t make economic or
environmental sense, but it may be possible to
use compost or solar heating, or capture waste
heat from another source, to keep a digester
operating efficiently in the colder months.
A word about safety
Remember, at all stages of construction, operation
and maintenance of a digester, you are exposing
yourself to potential hazards. Some feedstocks,
especially manures, contain pathogens or para-
sites. You can be exposed to them not only in the
loading phase, but also in clean-out, depending
on retention times. The digestion process may not
kill them all. In a dung-based system, special care
should be taken to keep yourself, your tools and
your work area clean.
Methane and the other gases produced can be
deadly. If you are a rural resident, you have
probably heard stories of individuals who have
been overcome by “septic gases” when cleaning a
Introducing live anaerobic bacteria into the
new digester will expedite start-up, and the
higher the ratio of inoculant material used, the
more likely you will have a successful start-up.
If only a small amount of seed material can be
obtained, then new substrate should be added
slowly over the course of several days, to pro-
mote growth of the bacteria. If material from an
existing digester is not available, active sludge
from a sewage treatment plant can be used.
Some DIY experimenters have used commercial
septic tank treatments to promote start-up, but
only anecdotal evidence is available on the effec-
tiveness of using septic starter. Of course, some
substrate material already contains anaerobic
bacteria. Manure or material from the bottom of
a compost pile will contain some anaerobic bac-
teria, and will begin the process more quickly
than will fresh vegetable or plant matter. In the
case of manure, the fresher the better. Anaerobic
material exists in the gut, and as it is exposed to
air it begins to die. Dried dung will likely con-
tain very little live bacteria.
A batch digester will generally be easier to
start than a continuous flow unit, because for
the latter, the hydraulic retention time (HRT)
is less than in a batch digester. HRT is the
time it takes for the slurry to move through
the digester. In a continuous or semi-continu-
ous flow arrangement, the digester should not
be “fed” for several days after initial loading
to allow the growth of the anaerobic bacteria.
During this start-up period, pH should be mon-
itored closely to make sure that the slurry is not
slipping into an acid state. This can easily hap-
pen by adding new material too quickly. If the
pH number begins to drop, lime or other “ant-
acids” may be needed to keep the pH level in
anaerobic operating range.
Operation and maintenance
If a healthy environment is established for your
anaerobic bacteria, very little effort is needed to
operate a micro-digester. In a continuous-feed
design, substrate should be added daily. A batch
digester is loaded and then left alone (or agitated
occasionally) until gas production falls below
an acceptable level. In a small continuous-flow
digester, agitation occurs as the material is added
and moves through the digester, although addi-
tional mechanical agitation can be added. Agita-
tion of some kind is generally desirable, because
without it, depending on the substrate, a thick
Page 8 ATTRA Micro-scale Biogas Production: A beginner’s guide
septic tank. The same can hold true for an
anaerobic digester. Remember, the digester is
designed to be an oxygen-free environment—
good for anaerobic bacteria, bad for humans!
Finally, of course, biogas is explosive. Never
smoke or use a torch or lantern in the pres-
ence of biogas, and use soapy water—NOT a
flame—to test for the presence of biogas, unless
it is fed to a properly regulated burner.
Gas quality
Because biogas is coming fresh out of a warm
biological environment, a fair amount of water
vapor is present in the gas. Systems for removing
moisture will need to be implemented for good
combustion. These can be as simple as pitching
the gas pipe to cause the moisture to run back
into the digester, or fashioning a moisture trap
made out of a glass jar or bottle.
Another potentially more serious issue is fil-
tering or “scrubbing” the gas. Because of the
corrosive nature of the hydrogen sulfide and
ammonia in the raw biogas, pipes, burners, and
especially generator engines can be damaged
over time. A number of designs for scrubbers are
available on the Internet, most using beakers or
vessels packed with steel wool, or bubbling the
gas through a solution of sodium carbonate.
Pressure
Achieving the proper pressure for combustion
of the biogas can be done in several very simple
ways. Rather than using a high-tech mechani-
cal compressor, most people use the very low-
tech method of adding weight to the top of the
gas storage unit, using bricks or other heavy
objects. This can be tricky with a bag digester,
and care must be taken not to snag or puncture
the plastic. Some trial and error is necessary, but
in combination with a regulator valve and a car-
buretor orifice to adjust the gas/air mixture, a
good, efficient flame can easily be achieved.
Environmental benefits
of using biogas
When biogas is burned, it does produce CO2
,
which is a greenhouse gas. However, using bio-
gas is still environmentally beneficial. Along
with CO2
, methane is one of the greenhouse
gases that concern scientists most in their study
of global climate change. According to the
U.S. Environmental Protection Agency (EPA),
although there is less methane in the environ-
ment than CO2
, methane is about 21 times
more powerful at warming the atmosphere than
CO2
(by weight). Methane’s chemical lifetime
in the atmosphere is approximately 12 years.
(U.S. EPA, 2010) Burning the methane (rather
than allowing it to escape into the atmosphere),
using the energy and turning the emissions into
CO2
reduces the potency of the greenhouse
gases being released, but more importantly, dis-
places fossil fuel use and prevents the release of
additional CO2
.
The EPA estimates that in large-scale dairy
digesters, for each 10 cows from which the
manure is anaerobically digested and the biogas
captured and used, greenhouse gas emissions are
reduced an amount equivalent to taking 4.3 pas-
senger cars off the road. Of course, this statistic
does not address the overall lack of sustainabil-
ity of large-scale confined animal feeding opera-
tions. Other sources estimate that small digest-
ers can reduce greenhouse gas emissions by the
equivalent of 5-7 metric tons of CO2
in house-
holds that currently burn wood.(BMU, 2009)
Side Streams:
Fertilizer and compost tea
Aside from biogas production, the other impor-
tant benefit of operating an anaerobic digester
is the high-quality fertilizer and compost that
it produces. This side stream of the digester can
provide your operation with a great deal of high
valuable soil amendments that can be used at
A farmer in Costa Rica exhibits the amount of gas available from his digester.
Photo by Ian Woofenden.
Page 9ATTRAwww.attra.ncat.org
composted before application to eliminate any pos-
sible surviving pathogens or parasites.
Conclusion
Building and operating a micro-scale biogas pro-
duction system is not right for everyone. It can
be messy and labor intensive. Your regional cli-
mate, available feedstock and ability to use the
gas at your site will determine if a project is even
viable. If you are looking for a good return on
investment (ROI) in a conventional farm busi-
ness model, it may be a hard case to make.
On the other hand, if you are a small-scale farmer
looking to increase the sustainability of your
operation by utilizing the resources at hand with
an investment of “sweat equity” rather than cash,
micro-scale biogas may be worth exploring.
the farm, or possibly sold for additional income.
Certified organic producers should check with
their certifiers for any restrictions.
The effluent and spent substrate that remain
after anaerobic digestion contain higher concen-
trations of potassium, phosphorous and nitrogen
than the raw manure or plant waste that went
into the digester. Unlike the raw feedstock,
the nitrogen is more readily available to plants,
because it is now in a mineralized form, rather
than an organic form.
In addition, weed seeds, parasites and pathogens
are much less of an issue than with raw manure.
The temperature and lack of oxygen in the anaero-
bic environment is sufficient to destroy many weed
seeds and pathogens. Nonetheless, particularly in
the case of manure-based digesters, it is strongly
advised that the spent substrate be sun-dried or
Bates, L. 2007. Biogas Technical Brief. Practical Action. 7 p.
http://practicalaction.org/practicalanswers
BMU—German Federal Ministry for the Environment,
Nature Conservation & Nuclear Safety. 2009. Mini Biogas
Plants for Households. Project number:
08_I_036_VNM_A_Biogas, 54 p.
Brown, Laura. 2004. Gas Bio-digester Information and
Construction Manual for Rural Families. Sustainable
Harvest International. 16 p.
www.wcasfmra.org/biogas_docs/6%20Biodigester%20manual.pdf
Diver, Steve. 2002. Notes on Compost Teas. ATTRA
Publication. National Center for Appropriate Technology.
19 p. www.attra.ncat.org/attra-pub/compost-tea-notes.html
Fry, L. John. 1973. Methane Digesters For Fuel Gas and
Fertilizer, With Complete Instructions For Two Working
Models. The New Alchemy Institute. 58 p.Available online
at: http://journeytoforever.org/biofuel_library/Methane
Digesters/MD1.html
Fulford, David. 1988. Running a Biogas Program. Practical
Action, Bourton on Dunsmore, UK. 200 p.
House, David. 2006. The Complete Biogas Handbook
(3rd Edition). House Press. 288 p.
Koottatep, Suporn, Manit Ompont and Tay Joo Hwa. No
date. Biogas: A GP Option For Community Development.
www.apo-tokyo.org/00e-books/GP-07_Biogas.htm
Nakagawa, Charles H. 1981. Chinese Biogas Digester. U.S.
Peace Corps Information Collection and Exchange. 92 p.
U.S. EPA. 2010. Methane. www.epa.gov/methane
Voca, N. et al. 2005. Digester residue as fertilizer after
mesophilic process of anaerobic digestion. Faculty of
Agriculture, University of Zagreb, Croatia. 265 p.
Voegeli, Y., C. Lohri, G. Kassenga, U. Baier and C.
Zurbrugg. 2009. Technical and Biological performance of
the ARTI compact biogas plant for kitchen waste. CISA
Publisher, Italy. 9 p.
Woofenden, Ian. 2009. Appropriate Technology for the
Developing World. Home Power Magazine. Oct./Nov.
80 p. www.homepower.com
Further Resources Online
ARTI Biogas Plant: A compact digester for producing
biogas from food waste. ARTI,
www.arti-india.org/content/view/45/52
Articles and videos by Al Rutan “The Methane Man”
www.rebelwolf.com/essn.html
www.green-trust.org/wordpress/2009/08/12/
free-bio-methane-articles
An introduction to biogas
Harris, P. University of Adelaide
www.adelaide.edu.au/biogas
A biogas plant for the digestion of fresh undiluted cattle
dung. Shyam, M.
http://practicalaction.org/docs/energy/docs47/BP47Shyam.pdf
Methane Digesters
www.green-trust.org/methane.htm
What is Biogas?
U.S. Department of Energy
www.afdc.energy.gov/afdc/fuels/emerging_biogas_what_is.html
References
Page 10 ATTRA Micro-scale Biogas Production: A beginner’s guide
Notes
Page 11ATTRAwww.attra.ncat.org
Notes
Page 12 ATTRA
Micro-scale Biogas Production: A beginner’s guide
By Rich Dana
NCAT Energy Specialist
© 2010 NCAT
Tracy Mumma, Editor
Amy Smith, Production
This publication is available on the Web at:
www.attra.ncat.org/attra-pub/biogas.html
or
www.attra.ncat.org/attra-pub/PDF/biogas.pdf
IP370
Slot 369
Version 082910

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Micro-Scale Biogas Production: A Beginner's Guide

  • 1. The National Sustainable Agriculture Information Service, ATTRA (www.attra.ncat.org), was developed and is managed by the National Center for Appropriate Technology (NCAT). The project is funded through a cooperative agreement with the United States Department of Agriculture’s Rural Business- Cooperative Service. Visit the NCAT website (www.ncat.org/ sarc_current.php) for more information on our other sustainable agriculture and energy projects. 1-800-346-9140 • www.attra.ncat.orgA project of the National Center for Appropriate Technology Funding for the development of this publication was provided by the USDA Risk Management Agency. By Rich Dana NCAT Energy Specialist © 2010 NCAT Contents Micro-scale Biogas Production: A beginner’s guide Introduction A naerobic digestion is one of the most basic processes of life on earth. Anaer- obic means “in the absence of oxygen” and anaerobic digestion takes place when biode- gradable matter decomposes in a closed environ- ment. By managing the digestion process and capturing the biogas produced, we can power boilers, burners and generators, and possibly even refrigerators. In the United States, there is a great deal of interest in using large-scale anaerobic digesters to control odor at livestock facilities and burning the biogas produced to run electrical generators. Commonly referred to as Methane Digesters, these large-scale units actually digest manure and produce biogas, which is made up of carbon dioxide (CO2 ) and methane (CH4 ) with smaller amounts of water vapor (H2 O), hydrogen sulfide (H2 S), and in some cases ammonia (NH3 ). Methane is flammable gas that can be used to make power, and is the primary compo- nent of the fossil fuel natural gas in common use to heat homes. Although not widely used in the United States, small-scale digesters are used successfully all over the world to turn small amounts of manure, plant waste and food scraps into usable energy. For hundreds of years, biogas has been used in China and India (and more recently in Africa and Central America) to provide gas for cook- ing, heating and lighting—utilizing very simple, low-tech designs. This publication will exam- ine several of the common designs for build- ing micro-digesters that have been developed around the world, and discuss how sustainable farmers in American can take a lesson from their counterparts in the developing world. Using the Gas If you decide to build the digester, how will you use the gas? Biogas can take the place of pro- pane or natural gas in many instances. The most Anaerobic digestion is a simple biochemical process by which waste products can be con- verted to energy. Using manure, plant waste, crop residue, food scraps or other waste products, farmers can reduce their dependence on fossil fuels while saving money, reducing greenhouse gas emissions and solving odor problems (not to mention producing high-quality compost fertilizer as a by-product). This publication provides a beginner’s guide to using anaerobic digestion to produce usable biogas on a small scale with a minimal investment. For more information on biogas and a discus- sion of larger-scale, manure-based digesters for livestock operations, please see the ATTRA publication Anaerobic Digestion of Animal Wastes: Factors to Consider. Workshop participants building a micro-scale digester in Costa Rica. Photo by Ian Woofenden. Introduction......................1 Using the Gas....................1 Small is Beautiful .............2 Biogas Basics: The Chemistry of Anaerobic Digestion......2 Feeding Your Digester ..............................4 Micro-Digester Designs ...............................5 Start-up, Operation and Maintenance ............6 Side Streams......................8 Conclusion.........................9 References .........................9 Further Resources Online..................................9
  • 2. Page 2 ATTRA Micro-scale Biogas Production: A beginner’s guide technology because of its low cost (and the abil- ity to adapt salvage materials or materials that are commonly at hand), simplicity, and environmen- tal and health benefits. In places where wood is used for cooking and heating, deforestation is a major concern and burning wood can cause air quality issues and respiratory problems for resi- dents—especially small children. A household digester can reduce the work of gathering wood and provide a cheap and plentiful fuel source where before there was none. Biogas Basics: The chemistry of anaerobic digestion Anaerobic bacteria and archaea predate most other forms of life on our planet, having devel- oped in the period when the earth’s atmo- sphere was mainly methane (CH4 ), carbon dioxide (CO2 ) and water vapor (H2 O). Anaero- bic digestion takes place in environments with limited oxygen: underwater in a swamp or marsh, at the bottom of a compost heap, or in the gut of animals. Anaerobic digestion is used in septic systems and sewage treatment plants to break down waste. The Stages of Anaerobic Digestion There are four stages to the digestion process— hydrolysis, acidogenesis, acetogenesis and meth- anogenesis. These stages need to be managed early on in starting a digester, but if the pro- cess is properly maintained, methane production should require a minimum of chemical testing and treatment. Bacteria must do their work in the earlier stages to use up the oxygen in the material, and then break it down into volatile fatty acids and fermented alcohols, before the methanogenic bacteria can start to make methane. The process is as follows: • Hydrolysis: Enzymes break down and liquefy the smaller molecules and break down large polymers in the material. common use is for cooking. It can, of course, be used as a heat source as well: to supplement home heating, for heating a poultry house, far- rowing house incubator or other small livestock building, or for heating a greenhouse. One unique use that has not been widely explored in the U.S. (but has in China) is using the gas to power an absorption refrigerator. This is a type of refrigerator that uses a propane flame rather than an electric compressor to create the evapo- rative cooling cycle. These refrigerators are com- monly found in RVs and in locations that have no electricity or unreliable electricity supplies. Though there is little research available on the use of methane to power absorptive cooling, methane has potential for use by farmers who may be look- ing for refrigeration options in remote areas. Small is Beautiful There are several major technical issues to over- come in order to operate a large-scale digester successfully and use it to generate wholesale elec- tricity. Maintaining proper bacterial health in a 40,000- to multi-million-gallon digester tank can be difficult—and mistakes are expensive. Large-scale digesters require hundreds—or thou- sands—of head of livestock to provide adequate manure, creating material handling issues. In order to run large, sensitive electrical generat- ing equipment, the biogas must be “scrubbed” to remove most of the other gases, which can add work and expense. Large digesters often require a full-time manager. On the other hand, a micro- digester can supply a family with cooking gas for two meals a day (or add heat to a greenhouse or workshop), from the manure of a few pigs, cows or chickens, or food scraps and rotten or unusable vegetables and fruit. Because the cost of construction is low, the feedstock is free, and the fuel is used at the source, economic risk can be minimized and sustainability maximized. In other parts of the world where energy resources are not as readily available as they are in the United States, the micro-digester is an important Related ATTRA publications Disclaimer: This publication is meant as an introduction to micro-scale biogas production, and not a complete hands-on manual on how to construct and operate a digester. There is a list of references at the end of this publication, and we suggest doing con- siderable additional research before committing time and resources to a biogas project. Most importantly, remember that biogas, though relatively safe, can be dangerous, flammable, and potentially explosive if handled improperly. We suggest that you exercise all possible safety precautions and abide by all local, state and federal laws that might apply to a digester project in your location. Anaerobic Digestion of Animal Wastes: Factors to Consider Compost Heated Greenhouses
  • 3. Page 3ATTRAwww.attra.ncat.org Pure water is said to be neutral, with a pH level of about 7. Acids have pH numbers below 7, while bases have numbers above 7. A digester operates best near the neutral point, up to a pH of about 8.5. In the early stages of the anaerobic process, when the digester is making acids, the pH can drop to 6 or below. When it reaches methanogenesis, it will operate in the 7.5 to 8.5 range. It is important to remember that the pH level becomes less of an issue when the digester is well-buffered, as it can take a certain amount of acid or base material being added to it without upsetting the digestion.(Fry, 1973) Buffering is the digester’s ability to resist pH change. Since pH is a “lagging indicator”, proper buffering is important to avoid problems with acidity. (see Figure 1). In a large-scale digester, large variations in pH can be catastrophic. In a micro-digester, how- ever, high or low pH conditions may be cor- rected relatively simply. A high-acid condition can be treated with small amounts of lime, ammonia or even bicarbonates like baking soda. Carbon/Nitrogen Ratio (C:N) Carbon and nitrogen are the two components that anaerobic bacteria require for survival. They need about 30 times more carbon than nitro- gen, so the C:N ratio of your substrate should be about 30:1. This is important when formulating your slurry. When determining the C:N ratio of feedstock like manure, the animal feed and • Acidogenesis: The products of the hydrolysis (soluble monomers) are fer- mented to volatile fatty acids (or VFAs) and alcohols. • Acetogenesis: Acetogenic bacteria break down the VFAs and alcohols, ace- tic acid, carbon dioxide and hydrogen. • Methanogenesis: The methanogenic bacteria convert acetic acid and hydrogen into CO2 and methane. (Note: The three earlier stages can take place at lower tem- peratures than Methanogenesis.) Temperature and pH Two of the most important factors for proper anaerobic digestion and methane production are temperature and pH. Failure to regulate these two factors is among the most common prob- lems with a digester. Most anaerobic bacteria perform best in warmer atmospheres. This is why micro-digesters are most common in areas closer to the equator. This does not mean a micro-digester will not function in more northerly climates, but it does affect design decisions, as the digester may need to be insulated or even heated to make large amounts of methane. Anaerobic digestion does not stop completely at lower temperatures, but methanogenesis is the stage most seriously hin- dered by low temperatures, and this will reduce methane production. There is evidence, how- ever, that plant-based feedstocks may perform better at low temperatures than do manure- based systems. (House, 2006) Although micro-digesters in Central America or India often operate completely without added heat or even insulation at temperatures ranging from 60ºF - 100+ºF, the micro-digester design most popular in China is an underground design, which utilizes the earth’s geothermal properties to maintain a healthy temperature. This publication discusses design considerations below, in the section entitled Micro-digester Designs: Global D.I.Y. technology. Along with temperature, pH is vital to digester health. The pH is a measure of the acidity or alkalinity (baseness) of the contents of the digester. Just as in the human digestive sys- tem, too much acid creates “indigestion” in the anaerobic system. Figure 1: Courtesy of David House.
  • 4. Page 4 ATTRA Micro-scale Biogas Production: A beginner’s guide amount of bedding material can heavily affect the actual amounts of carbon and nitrogen— you will need to adjust your feedstock accord- ingly (see Table 1). Table 1: Carbon Nitrogen Ratio Substance C:N Cow Manure—Alfalfa 16:1 Cow Manure—Dairy (with bedding) 21:1 Pig Manure 14:1 Sheep Manure 20:1 Chicken Manure 15:1 Oat Straw 48:1 Turnip Tops 19:1 Corn Stalks 53:1 Grass Clippings 19:1 Sunflower 30:1 Source: House, David. 2006. Biogas Handbook. p.37-40 Feeding Your Digester: Substrates In the proper feeding and care of your digester, you must remember the “Seven S’s” of anaerobic digester design: (see Table 2) Table 2: The Seven S’s of Digester Design 1. Substrate: The organic material that you will be feeding your digester. This can be manure, plant waste, paper pulp, or other biodegradable material. 2. Slurry: The slurry is the homogeneous mush that you will be passing through the digester. In order for efficient digestion to take place, the substrate material should be mashed or ground and mixed with water to make a uniform substance with 15–40% solids, depending on your digester design. 3. Stratification: As your slurry breaks down in the digester, it will separate into layers. These lay- ers—or “strata”—are biogas, scum, supernatant, sludge and solids. Mixing of the slurry prevents excessive stratification, but some stratification will always occur, especially in “batch” digesters (see below, Micro-digester Designs: Global D.I.Y. Technology). 4. Scum: The scum layer floats on the top of the material in the digester, just below the gas level. The scum level is formed by hard-to-digest material like coarse straw and grease. 5. Supernatant: The spent liquid of the slurry. The Supernatant has a high content of solids, making it of high value as fertilizer, similar to “compost tea.”(Diver, 2002) 6. Sludge: Below the liquid supernatant is the sludge layer. The sludge is the digested and semi- digested organic solids. This sludge can provide excellent composted fertilizer, but depend- ing on the feedstock, it may need to be dried in the sun to kill any surviving pathogens. This is more of an issue with manure-based systems. 7. (Inorganic) Solids or Sand: The bottom layer consists of those non-digestible solids that find their way into the digester. These could include dirt, sand, small rocks, plastic or metal—any inorganic solid that may inadvertently be introduced into the system.
  • 5. Page 5ATTRAwww.attra.ncat.org The system consists of a long trench (approx. 20’), lined with a 48” diameter polyethylene (poly) tube to form the “bag”. With an inlet at one end, an outlet at the other and a gas out- let fitting on the top, it is an extremely simple design (see Figure 2). The poly tubing is available by the roll, and is commonly used as packaging material. A double layer of 6-8 ml poly is sug- gested to avoid punctures, and a pressure release allows excess gas to exit the system before over- inflation of the bag. This design will produce 4 hours of usable cooking gas per day and requires approximately 100 gallons of slurry to “charge” the digester, and an additional 10 gallons of slurry per day to maintain.(Brown, 2004) In-ground dome The in-ground dome design is used widely in China, and may have considerable potential for adaptation to northern climates in the United States.(Koottatep et al., no date) This design Micro-Digester Designs: Global D.I.Y. (do-it-yourself) technology There are two basic types of digester designs: continuous-flow and batch processors. In a contin- uous-flow digester, new substrate is regularly fed into the digester. The slurry moves through the digester, pushing out digested material (efflu- ent). The material may also be moved mechani- cally through the continuous-flow digester with augers or pumps. Batch digesters are loaded once and then the material is allowed to digest. When the digestion is complete, the effluent is removed and the process is repeated. By adding an inlet tube for feeding the digester and an outlet tube for the overflowing effluent, a batch processor can be fed some extra material after the initial start-up to extend the cycle, making the unit a semi-continuous batch processor. Each type has its advantages. Continuous digesters produce biogas without interruption. Batch digesters, on the other hand, are simpler and less expensive to build. Continuous-flow designs lend themselves better to manure-based systems, while batch processors may be more appropriate for unpro- cessed plant-based substrates. The types of micro-digesters we will be cover- ing in this publication fall into three categories, with some variation. These are the in-ground bag or tube style (continuous flow), the in- ground dome style (semi-continuous batch) and the floating top/tank design (batch or semi-con- tinuous). Because much of the micro-scale bio- gas technology has been developed by individu- als and non-governmental organizations, these designs are simple and “open source”—mean- ing that the designs are non-proprietary and much of the information on these designs is free and readily available on the Internet. (See the Resources section at the end of this publication for a list of books and websites to visit for more detailed information.) In-ground bag The in-ground bag style digester is very popular throughout Central America, and may be appro- priate for use in the Southern U.S. It is a very simple continuous-flow design especially well suited to a family farm with just a few pigs or cows. It is a very inexpensive design, with parts that can be acquired for as little as $50. Figure 2: In-Ground Bag An in-ground bag digester in operation. Photo by Ian Woofenden.
  • 6. Page 6 ATTRA Micro-scale Biogas Production: A beginner’s guide This design is used widely in India in particu- lar, where ARTI (Appropriate Rural Technology Institute) has developed a very functional fam- ily-sized design utilizing a 1000-liter (264 U.S. gallons) polyethylene base tank with a 750-liter (198 U.S. gallons) tank as the inner floating gas chamber.(Voegeli et al., 2009) This design can be adapted to use any size of tanks, with a larger diameter for the outer and smaller diameter for the inner tank, and can be an excellent and inexpensive design to experiment with. Many sizes of drums are readily available for free, and a very small test digester can be made with a couple of barrels and just a few standard plumb- ing parts (see Figure 4). Other designs Several other designs have been developed that are variations on these three basic styles. One is a hybrid design that uses sealed 55-gallon drums as modular digesters that are connected to a separate floating top/tank gas storage container. Another style uses the 275-gallon totes commonly used to transport farm and industrial chemicals. In general, plastic containers are preferable to steel drums for the digester vessel and gas storage tanks, because both the slurry and the biogas itself can corrode steel drums fairly rapidly. Start-up, Operation and Maintenance Start-up The best way to get any anaerobic digester going is by “seeding” or “inoculating” it with mate- rial from another healthy, operating digester. is not unlike a western-style septic tank, and in fact, a conventional cast concrete or fiber- glass septic tank could be modified to serve as a digester (see Figure 3). In China, the latrine is often connected to the digester, so it serves as a septic system as well. In the U.S., zoning and public health laws in most areas would not allow “blackwater” from the home into the digester, despite the obvious advantages to such a system. This design involves a cast concrete, or concrete block, or brick, tank and cap. While the expense is much greater than that of a bag-style digester (typical cost is $350 for 4-6 cu. meters), but by using the earth’s natural insulation, the dome- style digester can maintain a constant tempera- ture, even in colder climates.(Nakagawa, 1981) Floating top/tank The floating top/tank design is one that adapts itself particularly well to the materials at hand. In this design, the slurry is contained in a bot- tom tank, with another inverted tank or drum serving as a cap, which is lifted by the biogas as it is generated (see Figure 4). Figure 3: In-Ground Dome Figure 4: Floating Top/Tank Asimplefloating-topdigesterbuiltfromscrapmaterials. Photo by Rich Dana.
  • 7. Page 7ATTRAwww.attra.ncat.org scum layer can form and hinder the gas produc- tion. Keeping substrate materials in suspension will assist in the uniform breakdown of the mate- rial, and will promote complete digestion. Loading the digester will be the most common operation, and this involves preparation of the slurry. Manure must be liquefied, food scraps must be mashed, cardboard or grasses must be shredded. Remember, the smaller the particles in your slurry, the happier your digester will be. How much you need to feed your digester is called the loading rate. The loading rate is derived from the amount of organic matter fed daily, divided by the size (volume) of the digester. A healthier anaerobic environment can handle a higher loading rate, but over-feeding the digester can cause a rise in the acid content and reduced gas production. Put more simply, more feedstock is not always better. Cleanout is necessary when gas production drops below an acceptable level. This will be a regular occurrence in a batch digester, but will be less necessary in a continuous-flow system, since much of the material will be leaving the system as effluent. In both cases, at some point during the operation, some digestate and inor- ganic solids will need to be removed. Depending on the climate, heating the digester may be another consideration. Obviously, using any kind of fossil-fuel-based heating like pro- pane, oil or electricity doesn’t make economic or environmental sense, but it may be possible to use compost or solar heating, or capture waste heat from another source, to keep a digester operating efficiently in the colder months. A word about safety Remember, at all stages of construction, operation and maintenance of a digester, you are exposing yourself to potential hazards. Some feedstocks, especially manures, contain pathogens or para- sites. You can be exposed to them not only in the loading phase, but also in clean-out, depending on retention times. The digestion process may not kill them all. In a dung-based system, special care should be taken to keep yourself, your tools and your work area clean. Methane and the other gases produced can be deadly. If you are a rural resident, you have probably heard stories of individuals who have been overcome by “septic gases” when cleaning a Introducing live anaerobic bacteria into the new digester will expedite start-up, and the higher the ratio of inoculant material used, the more likely you will have a successful start-up. If only a small amount of seed material can be obtained, then new substrate should be added slowly over the course of several days, to pro- mote growth of the bacteria. If material from an existing digester is not available, active sludge from a sewage treatment plant can be used. Some DIY experimenters have used commercial septic tank treatments to promote start-up, but only anecdotal evidence is available on the effec- tiveness of using septic starter. Of course, some substrate material already contains anaerobic bacteria. Manure or material from the bottom of a compost pile will contain some anaerobic bac- teria, and will begin the process more quickly than will fresh vegetable or plant matter. In the case of manure, the fresher the better. Anaerobic material exists in the gut, and as it is exposed to air it begins to die. Dried dung will likely con- tain very little live bacteria. A batch digester will generally be easier to start than a continuous flow unit, because for the latter, the hydraulic retention time (HRT) is less than in a batch digester. HRT is the time it takes for the slurry to move through the digester. In a continuous or semi-continu- ous flow arrangement, the digester should not be “fed” for several days after initial loading to allow the growth of the anaerobic bacteria. During this start-up period, pH should be mon- itored closely to make sure that the slurry is not slipping into an acid state. This can easily hap- pen by adding new material too quickly. If the pH number begins to drop, lime or other “ant- acids” may be needed to keep the pH level in anaerobic operating range. Operation and maintenance If a healthy environment is established for your anaerobic bacteria, very little effort is needed to operate a micro-digester. In a continuous-feed design, substrate should be added daily. A batch digester is loaded and then left alone (or agitated occasionally) until gas production falls below an acceptable level. In a small continuous-flow digester, agitation occurs as the material is added and moves through the digester, although addi- tional mechanical agitation can be added. Agita- tion of some kind is generally desirable, because without it, depending on the substrate, a thick
  • 8. Page 8 ATTRA Micro-scale Biogas Production: A beginner’s guide septic tank. The same can hold true for an anaerobic digester. Remember, the digester is designed to be an oxygen-free environment— good for anaerobic bacteria, bad for humans! Finally, of course, biogas is explosive. Never smoke or use a torch or lantern in the pres- ence of biogas, and use soapy water—NOT a flame—to test for the presence of biogas, unless it is fed to a properly regulated burner. Gas quality Because biogas is coming fresh out of a warm biological environment, a fair amount of water vapor is present in the gas. Systems for removing moisture will need to be implemented for good combustion. These can be as simple as pitching the gas pipe to cause the moisture to run back into the digester, or fashioning a moisture trap made out of a glass jar or bottle. Another potentially more serious issue is fil- tering or “scrubbing” the gas. Because of the corrosive nature of the hydrogen sulfide and ammonia in the raw biogas, pipes, burners, and especially generator engines can be damaged over time. A number of designs for scrubbers are available on the Internet, most using beakers or vessels packed with steel wool, or bubbling the gas through a solution of sodium carbonate. Pressure Achieving the proper pressure for combustion of the biogas can be done in several very simple ways. Rather than using a high-tech mechani- cal compressor, most people use the very low- tech method of adding weight to the top of the gas storage unit, using bricks or other heavy objects. This can be tricky with a bag digester, and care must be taken not to snag or puncture the plastic. Some trial and error is necessary, but in combination with a regulator valve and a car- buretor orifice to adjust the gas/air mixture, a good, efficient flame can easily be achieved. Environmental benefits of using biogas When biogas is burned, it does produce CO2 , which is a greenhouse gas. However, using bio- gas is still environmentally beneficial. Along with CO2 , methane is one of the greenhouse gases that concern scientists most in their study of global climate change. According to the U.S. Environmental Protection Agency (EPA), although there is less methane in the environ- ment than CO2 , methane is about 21 times more powerful at warming the atmosphere than CO2 (by weight). Methane’s chemical lifetime in the atmosphere is approximately 12 years. (U.S. EPA, 2010) Burning the methane (rather than allowing it to escape into the atmosphere), using the energy and turning the emissions into CO2 reduces the potency of the greenhouse gases being released, but more importantly, dis- places fossil fuel use and prevents the release of additional CO2 . The EPA estimates that in large-scale dairy digesters, for each 10 cows from which the manure is anaerobically digested and the biogas captured and used, greenhouse gas emissions are reduced an amount equivalent to taking 4.3 pas- senger cars off the road. Of course, this statistic does not address the overall lack of sustainabil- ity of large-scale confined animal feeding opera- tions. Other sources estimate that small digest- ers can reduce greenhouse gas emissions by the equivalent of 5-7 metric tons of CO2 in house- holds that currently burn wood.(BMU, 2009) Side Streams: Fertilizer and compost tea Aside from biogas production, the other impor- tant benefit of operating an anaerobic digester is the high-quality fertilizer and compost that it produces. This side stream of the digester can provide your operation with a great deal of high valuable soil amendments that can be used at A farmer in Costa Rica exhibits the amount of gas available from his digester. Photo by Ian Woofenden.
  • 9. Page 9ATTRAwww.attra.ncat.org composted before application to eliminate any pos- sible surviving pathogens or parasites. Conclusion Building and operating a micro-scale biogas pro- duction system is not right for everyone. It can be messy and labor intensive. Your regional cli- mate, available feedstock and ability to use the gas at your site will determine if a project is even viable. If you are looking for a good return on investment (ROI) in a conventional farm busi- ness model, it may be a hard case to make. On the other hand, if you are a small-scale farmer looking to increase the sustainability of your operation by utilizing the resources at hand with an investment of “sweat equity” rather than cash, micro-scale biogas may be worth exploring. the farm, or possibly sold for additional income. Certified organic producers should check with their certifiers for any restrictions. The effluent and spent substrate that remain after anaerobic digestion contain higher concen- trations of potassium, phosphorous and nitrogen than the raw manure or plant waste that went into the digester. Unlike the raw feedstock, the nitrogen is more readily available to plants, because it is now in a mineralized form, rather than an organic form. In addition, weed seeds, parasites and pathogens are much less of an issue than with raw manure. The temperature and lack of oxygen in the anaero- bic environment is sufficient to destroy many weed seeds and pathogens. Nonetheless, particularly in the case of manure-based digesters, it is strongly advised that the spent substrate be sun-dried or Bates, L. 2007. Biogas Technical Brief. Practical Action. 7 p. http://practicalaction.org/practicalanswers BMU—German Federal Ministry for the Environment, Nature Conservation & Nuclear Safety. 2009. Mini Biogas Plants for Households. Project number: 08_I_036_VNM_A_Biogas, 54 p. Brown, Laura. 2004. Gas Bio-digester Information and Construction Manual for Rural Families. Sustainable Harvest International. 16 p. www.wcasfmra.org/biogas_docs/6%20Biodigester%20manual.pdf Diver, Steve. 2002. Notes on Compost Teas. ATTRA Publication. National Center for Appropriate Technology. 19 p. www.attra.ncat.org/attra-pub/compost-tea-notes.html Fry, L. John. 1973. Methane Digesters For Fuel Gas and Fertilizer, With Complete Instructions For Two Working Models. The New Alchemy Institute. 58 p.Available online at: http://journeytoforever.org/biofuel_library/Methane Digesters/MD1.html Fulford, David. 1988. Running a Biogas Program. Practical Action, Bourton on Dunsmore, UK. 200 p. House, David. 2006. The Complete Biogas Handbook (3rd Edition). House Press. 288 p. Koottatep, Suporn, Manit Ompont and Tay Joo Hwa. No date. Biogas: A GP Option For Community Development. www.apo-tokyo.org/00e-books/GP-07_Biogas.htm Nakagawa, Charles H. 1981. Chinese Biogas Digester. U.S. Peace Corps Information Collection and Exchange. 92 p. U.S. EPA. 2010. Methane. www.epa.gov/methane Voca, N. et al. 2005. Digester residue as fertilizer after mesophilic process of anaerobic digestion. Faculty of Agriculture, University of Zagreb, Croatia. 265 p. Voegeli, Y., C. Lohri, G. Kassenga, U. Baier and C. Zurbrugg. 2009. Technical and Biological performance of the ARTI compact biogas plant for kitchen waste. CISA Publisher, Italy. 9 p. Woofenden, Ian. 2009. Appropriate Technology for the Developing World. Home Power Magazine. Oct./Nov. 80 p. www.homepower.com Further Resources Online ARTI Biogas Plant: A compact digester for producing biogas from food waste. ARTI, www.arti-india.org/content/view/45/52 Articles and videos by Al Rutan “The Methane Man” www.rebelwolf.com/essn.html www.green-trust.org/wordpress/2009/08/12/ free-bio-methane-articles An introduction to biogas Harris, P. University of Adelaide www.adelaide.edu.au/biogas A biogas plant for the digestion of fresh undiluted cattle dung. Shyam, M. http://practicalaction.org/docs/energy/docs47/BP47Shyam.pdf Methane Digesters www.green-trust.org/methane.htm What is Biogas? U.S. Department of Energy www.afdc.energy.gov/afdc/fuels/emerging_biogas_what_is.html References
  • 10. Page 10 ATTRA Micro-scale Biogas Production: A beginner’s guide Notes
  • 12. Page 12 ATTRA Micro-scale Biogas Production: A beginner’s guide By Rich Dana NCAT Energy Specialist © 2010 NCAT Tracy Mumma, Editor Amy Smith, Production This publication is available on the Web at: www.attra.ncat.org/attra-pub/biogas.html or www.attra.ncat.org/attra-pub/PDF/biogas.pdf IP370 Slot 369 Version 082910