Feed the Biogas Digester: Biodigesters trap the methane gas from decomposing organic waste, such as manure and table scraps, and use it for producing electricity or cooking fuel. Methane is composed of carbon and hydrogen — CH4. It has an octane rating of 110 and produces about 1,000 Btu of heat per cubic foot of gas.
Photo ©2013 Isaac Marquez, firstname.lastname@example.org
The following article is posted with permission ©2013 Home Power Inc. Visit www.HomePower.com for information on renewable energy projects for your farm or home.
About five years ago, writer and renewable energy aficionado
Warren Weismann was researching ancient Greece for his novel when he
stumbled across information that the Greeks had built anaerobic digesters to produce
methane. He then read about similar archaeological evidence in ancient Syria and China. But it was the modern biogas
boom in China that got him
most excited and distracted him from his writing career: Tens of millions of
home-scale biodigesters have been built in China over the last century, with
the pace of construction still accelerating. Warren wanted one for himself.
After a few years of further research, including conversations
with colleagues in India and
Nepal, where small-scale
biogas production is prevalent, Warren
modified traditional designs to create a plan for his own 700-gallon biodigester.
He was living at Maitreya Ecovillage, a threeblock community and
green-building-oriented neighborhood near downtown Eugene, Oregon.
After building his first biodigester last year, he’s become increasingly
excited about the possibilities for home-scale biogas, and has established Hestia
Home Biogas to build biodigesters locally and consult on biodigesters across
Back from Obscurity
Biogas has been used for lighting for at least a century,
and possibly millennia. But it was mostly abandoned in the United States
after cheap and abundant fossil fuel was harnessed in the early 20th century.
Home-scale biodigesters have remained on the sidelines in the developed world,
but are poised for a comeback as interest in a replacement fuel increases.
There are good reasons to consider building biodigesters for
a community, small farm, or even home. Biodigesters yield two products that are
extremely useful for the home and garden—high-nitrogen compost and flammable gas.
Biodigesters anaerobically (without air) break down organic
matter in a slurry held in a tank. The nitrogen remains in the composted slurry
as ammonia, a vital plant nutrient. The flammable gas produced by biodigesters
is about twothirds methane and one-third carbon dioxide—very similar to natural
gas—making it a good cooking fuel. Cooking requires intense direct application
of heat on demand, and renewable options for accomplishing this are limited.
Solar energy is dispersed and not consistently available, making solar cooking challenging,
and burning wood contributes to particulate pollution and further depletes
diminishing resources in the developing world. Cooking is not a huge consumer
of energy in the industrialized world, but doing it more sustainably is challenging. Unlike cooking with solar electricity,
biodigesters can be assembled with readily available materials by a handy homeowner.
Any type of propane or natural gas stove will run on biogas. For maximum
efficiency, propane stoves will require a larger air inlet.
Biodigester Efficiency and the Environment
Cooking fuel is greatly needed in less industrialized
nations, especially in rural areas. Several countries, such as India andCosta Rica, provide crucial government
support for biodigester technology, but none more so than China. More than 30 million
biodigesters have been built there, supplying renewable cooking and lighting
gas for more than 100 million people.
Anaerobic composting and its biogas production have major advantages
compared to traditional aerobic composting and burning biomass for cooking and
lighting. These advantages are opening the door to a more sustainable rural economy
in China, especially in Sichuan province, where
the modern biogas movement began and government support and technical know-how
Probably the greatest advantage with biodigesters is in their
efficiency—biogas often achieves efficiencies of 60%, compared with about 10%
for the typical homemade biomass-burning cook stove. Using less biomass means more
living trees, less air pollution and greenhouse gas emissions, and a vast
improvement in household air quality. Local water quality and sanitation is
also greatly improved, as human and animal wastes can be composted in a
sanitary manner. And lastly, the end product is a quality fertilizer, rich in
nitrogen and free of pathogens. While there is much in China to bemoan on the environmental front, the
more than 4 million biodigesters being built in rural China each year certainly provide
one of the bright spots in a nation lurching toward industrialization.
Inside a Biodigester
A biodigester is a sophisticated way to harvest fuel from
the complex carbon chains of organic matter—energy collected by plants from the
sun as they grow—without combusting them directly. Direct combustion of carbon
causes air pollution, a loss of much of the nutrient value of the biomass, and
a poor energy harvest—especially when used for cooking or lighting, as most of
it goes up in smoke. Burning wood, even in an EPA-certified woodstove, can
produce more than 500 times the fine particle emissions of burning natural gas.
As new plant material is fed into the digester, it is first attacked
by acidogenic bacteria, which break the chains holding together some of the
more complex plant matter, especially cellulose—the structural backbone of most
plants. Ammonia and acetates (mostly acid) are produced, lowering the pH and using
up any oxygen in the process. Acetates are the perfect food for methanogenic
bacteria, as long as the slurry they reside in is not too acidic and all oxygen
has been removed. They consume the acetates and produce methane (CH4) and carbon
dioxide (CO2), along with a lesser amount of other gases and residues depending
on the original feedstock.
In consuming these acids, the methanogenic bacteria raise the
pH and keep it hospitable for both the acid-formers and themselves, both of
which would perish if the pH dropped too low. The high-nitrogen ammonia (a
byproduct of the breakdown of plant proteins) remains dissolved in the slurry, unlike
in aerobic composting where it is released as a gas. Although both the
acid-formers and the methanogens can suffer from rapid changes in living
conditions created by the addition of feedstock, the methanogens are especially
vulnerable to low pH and the introduction of too much oxygen. For this reason,
biodigesters generally work on the principle of steady applications of new
feedstock in regular intervals, rather than adding large amounts of biomass at
There are several different types of methanogenic bacteria that
will colonize a digester, depending upon the slurry’s temperature range. The
two ranges of interest to homescale biodigesters are the cryophilic (50°F to
80°F) and the mesophilic (95°F to 125°F). There is a dead zone between these
two temperature ranges that must be avoided. Warren’s biodigester operates in the
cryophilic range. While mesophilic methanogens can break down material several
times as quickly as their cryophilic counterparts, consistently maintaining high
temperatures consumes a great deal of energy—which can make a net energy loss
for smaller biodigesters.
Building a Biogas Biodigester
Hestia biodigesters are approximately 5 by 7 feet wide by 5
feet deep, providing about 700 gallons of capacity. Slurry occupies about 600 gallons
of this biodigester; the remaining space is for the gas that’s produced. The
design is straightforward: an insulated concrete vessel is topped with a steel
frame that holds an EDPM pond liner, which expands as gas is produced. There’s an
inlet for adding feedstock and an outlet for removing composted slurry. A
closed loop of PEX tubing in the bottom of the tank is plumbed to an on-demand
water heater to add heat when the slurry temperature drops below 50°F—the temperature
at which cryophilic methanogenic bacteria go dormant and stop producing gas. If
the climate is mild, it may be enough to build a hoop house over the tank to
keep the slurry sufficiently warm in winter. Alternatively, the biodigester
could be allowed to go dormant during the colder months.
The first step in building the Hestia’s biodigester is to
excavate 28 inches below grade, which makes the height of the inlet right for easy
addition of feedstock by 5-gallon bucket. Warren
likes to make sure the digester is visible from the kitchen, since the
inflation of the rubber top indicates if there is sufficient gas available for
Alternatively, a simple pressure gauge could be added to the
gas line in the kitchen.
After excavation comes building the wall forms for the 2
cubic yards of aggregate-free concrete, which must be poured all at once to avoid
leaks through the walls. A 4-inch PVC outlet pipe and any PEX tubing for adding
hydronic heat must be set in place. The PEX tubing will rest on the bottom of
the floor of the tank, so two short pieces, one for entry and one for exit,
must be embedded in the wall so the rest of the radiant heating system can be
A concrete truck with a pump is best to fill the forms in
one pour. A concrete vibrator (also called a “stinger”) will help remove air bubbles’
weak spots from the concrete. The massive weight of the wet concrete and the
agitation of the stinger make it important to solidly secure the forms.
After the concrete cures and the forms are removed, three
coats of “moose-milk” finish—a mix of Portland cement and acrylic latex—is
painted on to help prevent leaks. The first is a bonding coat of watery-thin
consistency. The second coat is thicker (like peanut butter), with a higher
ratio of Portland cement. The finish coat is another thin application. After
sealing, the tank is filled with water for a leak test. If this goes well, the
outside of the tank can be insulated with 3 inches of rigid foam board
insulation and then backfilled. For aesthetics, Warren wraps the exterior of the biodigester
in chicken wire and then stuccos it.
The 40-mil pond liner and steel frame that serve as the tank
top are held in place with 18 anchor bolts inserted into the top of the
concrete tank at the time of the pour. The steel frame can be fabricated or
purchased from Hestia.
The gas line is attached to a regular barbed fitting secured
with a hose clamp. The rubber membrane is sandwiched between two washers and
nuts on the threaded end of the fitting. The gas line is a 1/2-inch flex hose,
which is transitioned to a standard PVC gas pipe when it goes underground.
Burying the PVC line protects it from photodegradation and developing cracks
that could lead to gas leaks.
The most common problem is water buildup in the gas line,
which can interfere with gas flow. When this occurs, the gas line must be picked
up and the water drained back into the digester. Ideally, a water separator
could be combined with a pressure-relief valve on the bottom to eliminate
excess moisture when the valve is tripped.
The released CO2 and CH4 gases percolate through the slurry to
the top of the tank. Once enough biogas accumulates, the pressure created
inside the expanding rubber top reaches 0.25 to 0.5 psi, enough to move the gas
through the delivery pipes and use it for cooking or lighting. As new material
is added to the tank through the inlet, it displaces an equal amount of slurry
through the outlet, which can be applied directly to the garden. Once applied,
covering the slurry with soil helps keep the ammonia from turning to gas and
losing its coveted nitrogen.
Operating the Digester
biodigester at Maitreya primarily uses kitchen waste once in operation. Getting
it up and running, however, requires a whole mess of ruminant manure—about 300 pounds’
worth. There happened to be an alpaca farm nearby, and so he used alpaca
manure, but any ruminant manure will be loaded with plenty of methanogenic
bacteria. He also added a few gallons of kitchen compost and tree leaves, and then
filled the tank to 600 gallons.
It takes a minimum of two to three days before a biodigester
begins to produce gas, since the acid-forming bacteria need to do their work
before the methane-producing bacteria can go to work. About 10 to 15 pounds of
kitchen scraps are collected and added to the biodigester daily, producing an
average of about 70 cubic feet of biogas, which is the only source of cooking
fuel for the community’s kitchen. When the methane content begins to get low,
the flame will begin to burn orange. This is remedied by feeding the digester.
The scraps are shoved into the digester with a pole, and
this slight agitation helps mix the slurry, exposing the material to the
bacteria so it can be thoroughly composted. The tank can also be topped off
with water at this time if the slurry level is low. As new feedstock is added,
it displaces an equal volume of composted slurry through the outlet, which is
captured in a 5-gallon bucket and then added to nearby gardens. Biodigesters prefer
a carbon-to-nitrogen ratio similar to a conventional aerobic compost pile, with
about 25:1 being ideal. Too much carbon-rich material (grass clippings,
newspaper, etc.) will slow the digester. Manure is the most common nitrogenous material
to add to rebalance a slow digester.
The biodigester at Maitreya has been operating for more than
a year with consistent results. Warren and Hestia Biogas are in the final
stages of getting city permits for new biodigesters, to bring an official stamp
of approval to a renewable energy technology with great promise for any
Biogas Biodigester Tank Alternatives
Besides building your own biodigester tank, repurposing old
food storage or other tanks is a possibility for biogas generation. You’ll just
need to size it correctly and make sure it’s leakproof. A general rule is that
the tank needs to be 50 times the size of the daily input to allow for some
space for gas to collect. If your input is 15 gallons of material per day, you’d
need a 750-gallon tank.
Construction Time and Costs
For this particular biodigester, construction time will vary
depending on a person’s construction experience or if a professional concrete
contractor is hired to pour the digester tank. Time will vary from several
weekends to several days, separated by a seven-day concrete-curing period.
Materials will cost $1,000 to $1,200, depending on the price
of ready-mix concrete in your area and if the concrete company will charge you
a “short-load” fee for ordering only 2 yards of concrete. The project can be
broken down into excavation and concrete; plumbing and gas piping; and external
masonry. The concrete, rebar, battens, and anchor bolts run about $500. Warren highly recommends
spending the extra $200 to $300 for a concrete pumper truck to avoid having to
“bucket brigade” the concrete. The plumbing and gas piping will be an
additional $200, and external insulation and masonry, another $300. (To
purchase a complete plan set for $89, visit www.HestiaHomeBiogas.com.)
Biodigester Maintenance, Costs and Risks
Biodigesters are living things, and just like a vegetable
garden or flock of chickens, they require regular maintenance to function
properly. It’s important to avoid adding materials that can overwhelm or clog
up the digester, including wood or plant stalks thicker than your finger; large
amounts of meat; bones (unless they are ground up); fresh citrus or apples; and
fresh chicken manure (which is OK only if it’s allowed to dry first). A few
5-gallon buckets of digested slurry will need to be removed every few days for
a 700-gallon digester like the one at
Maitreya, but otherwise the digester shouldn’t ever need to be cleaned out.
Regular use of the gas is important to avoid explosion
hazards. Letting the digester sit for more than a week can create an abundance
of biogas. Operating the digester at its natural pressure, without further gas
pumps to pressurize it, in addition to the appropriate check- and
pressure-relief valves, helps ensure that an unsafe buildup of gas doesn’t
A biodigester of this size should be cleared with your local
fire marshal, who may or may not be sympathetic to its construction. Biodigesters
are not common in North America, and some education
of officials will likely be required. Warren has
been working with the City of Eugene
for formal approval, and the case is pending.
Author and builder Stephen Hren (StephenHren@gmail.com)
lives in Durham, North Carolina. His latest book is Tales
from the Sustainable Underground: A Wild Journey with People Who Care More
About the Planet Than the Law.
Hestia Home Biogas
Biodigesters in China
Methane: Planning a Digester by Peter-John Meynell
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