Homemade Biogas

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    Biodigester efciency & the environmentCooking fuel is greatly needed in less industrialized nations, especially in

    rural areas. Several countries, such as India and Costa Rica, provide

    crucial government support for biodigester technology, but none more sothan China. More than 3 million biodigesters have been built there,

    supplying rene!able cooking and lighting gas for more than " million

    people.#naerobic composting and its biogas production have ma$or advantages

    compared to traditional aerobic composting and burning biomass for

    cooking and lighting. %hese advantages are opening the door to a more

    sustainable rural economy in China, especially in Sichuan province,

    !here the modern biogas movement began and government support

    and technical kno!&ho! is strongest.'robably the greatest advantage !ith biodigesters is in their e(ciency)

    biogas often achieves e(ciencies of *+, compared !ith about "+ forthe typical homemade biomass&burning cook stove. sing less biomass

    means more living trees, less air pollution and greenhouse gas

    emissions, and a vast improvement in household air -uality. ocal !ater

    -uality and sanitation is also greatly improved, as human and animal

    !astes can be composted in a sanitary manner. #nd lastly, the end

    product is a -uality fertilizer, rich in nitrogen and free of pathogens.

    /hile there is much in China to bemoan on the environmental front, the

    more than 0 million biodigesters being built in rural China each year

    certainly provide one of the bright spots in a nation lurching to!ard

    industrialization.

    Tank TroubleshootingTOOACIDIC:1enerally if there2s a problem, it2s that the slurry is too

    acidic p4 belo! 56. If there is a lot of ne!, ra!, green material placed in

    the digester or if too much material is added at once, the acid&forming

    bacteria have a 7eld day. %he methane bacteria are so annoyed by the

    high acid concentration, they simply can2t function. /hen this occurs, it

    can take a long time for the methane process to get under!ay naturally.

    o! p4 is a constant risk and must be countered by plenty of carbon

    !aste, such as leaves and stra! or !ood ashes.If a measured amount of ne! material)no more than one&fortieth of the

    total li-uid volume of the tank)is added, then the ne! material has to

    be dilute enough not to upset the balance. #t startup, though, there2s a

    lack of microorganisms and an inclination to!ard e8cessive acidity.

    nderstanding this, !e can see !hy some of the early literature on

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    making methane states that the startup time can be any!here from

    three !eeks to three months.I mentioned the acidity problem to a friend !ith !hom I !as !orking at

    the time. 4e said, 9I make a lot of !ine at home. :very once in a !hile, I

    have the same problem. /hen I do, I add a little baking soda. It

    straightens out the condition right a!ay.;%he baking soda added to my digester !orked like a charm. /ithin three

    days, I had methane on the !ay. %his is the secret for keeping your

    digester s!eet and happy. ou2ll need to kno! ho! hot the tank is, day to day, season toseason. %o eliminate the guess!ork, install sensors both inside and

    outside the tank. Record these temperatures over a period of time. %hen

    you !ill kno! ho! e(ciently the tank is retaining heat, at !hat rate the

    temperature drops !hen no heat is added, and ho! much energy is

    needed to raise the temperature. If this is done, then a reliable

    calculation can be made of ho! much heat is needed to maintain

    !orking temperature if 9free; heat is not available. 4eat conservation,

    more than any other factor, determines !hether a methane system !ill

    9?y; or not.Adapted from The Methane Process byAl Rutan(4'@A)

    BILDI!"T#$BIODI"$%T$4estia biodigesters are appro8imately B by 5 feet !ide by B feet deep,

    providing about 5 gallons of capacity. Slurry occupies about *

    gallons of this biodigester the remaining space is for the gas that2s

    produced. %he design is straightfor!ardD an insulated concrete vessel is

    topped !ith a steel frame that holds an :='M pond liner, !hich e8pands

    as gas is produced. %here2s an inlet for adding feedstock and an outlet

    for removing composted slurry. # closed loop of ':E tubing in the bottom

    of the tank is plumbed to an on&demand !ater heater to add heat !henthe slurry temperature drops belo! BFG)the temperature at !hich

    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 su(ciently !arm in !inter. #lternatively, the

    biodigester could be allo!ed to go dormant during the colder months.%he 7rst step in building the 4estia2s biodigester is to e8cavate @A inches

    belo! grade, !hich makes the height of the inlet right for easy addition

    https://homepower.com/profiles/al-rutanhttps://homepower.com/profiles/al-rutan
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    of feedstock by B&gallon bucket. /arren likes to make sure the digester is

    visible from the kitchen, since the in?ation of the rubber top indicates if

    there is su(cient gas available for cooking alternatively, a simple

    pressure gauge could be added to the gas line in the kitchen.#fter e8cavation comes building the !all forms for the @ cubic yards of

    aggregate&free concrete, !hich must be poured all at once to avoid leaksthrough the !alls. # 0&inch 'HC outlet pipe and any ':E tubing for

    adding hydronic heat must be set in place. %he ':E tubing !ill rest on

    the bottom of the ?oor of the tank, so t!o short pieces, one for entry and

    one for e8it, must be embedded in the !all so the rest of the radiant

    heating system can be attached later.# concrete truck !ith a pump is best to 7ll the forms in one pour. #

    concrete vibrator also called a 9stinger;6 !ill help remove air bubbles2

    !eak spots from the concrete. %he massive !eight of the !et concrete

    and the agitation of the stinger make it important to solidly secure the

    forms.

    #fter the concrete cures and the forms are removed, three coats of9moose&milk; 7nish)a mi8 of 'ortland cement and acrylic late8)is

    painted on to help prevent leaks. %he 7rst is a bonding coat of !atery&

    thin consistency. %he second coat is thicker like peanut butter6, !ith a

    higher ratio of 'ortland cement. %he 7nish coat is another thin

    application. #fter sealing, the tank is 7lled !ith !ater for a leak test. If

    this goes !ell, the outside of the tank can be insulated !ith 3 inches of

    rigid foam board insulation and then back7lled. Gor aesthetics, /arren

    !raps the e8terior of the biodigester in chicken !ire and then stuccos it.%he 0&mil pond liner and steel frame that serve as the tank top are held

    in place !ith "A anchor bolts inserted into the top of the concrete tank at

    the time of the pour. %he steel frame can be fabricated or purchased

    from 4estia.%he gas line is attached to a regular barbed 7tting secured !ith a hose

    clamp. %he rubber membrane is sand!iched bet!een t!o !ashers and

    nuts on the threaded end of the 7tting. %he gas line is a "@&inch ?e8

    hose, !hich is transitioned to a standard 'HC gas pipe !hen it goes

    underground. Jurying the 'HC line protects it from photodegradation and

    developing cracks that could lead to gas leaks.%he most common problem is !ater buildup in the gas line, !hich can

    interfere !ith gas ?o!. /hen this occurs, the gas line must be picked up

    and the !ater drained back into the digester. Ideally, a !ater separatorcould be combined !ith a pressure&relief valve on the bottom to

    eliminate e8cess moisture !hen the valve is tripped.

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    The outer forms are reinforced to withstand the pressures of the poured concrete.

    [Warren Weismann]

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    The interior forms, with floor and wall reinforcements visible. [Warren Weismann]

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    The finished concrete pour, with rubber bladder in place. [Warren Weismann]

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    Details of the frame, gasket, and rubber bladder assembly. [Warren Weismann]

    Rigid foam board is used to insulate the tank and help maintain the correct temperatu

    re for the bacteria. [Warren Weismann]

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    Flexible gas lines run from the top of the bladder to a valve and buried rigid

    plumbing. A stucco and brick facade helps the digester integrate with the garden

    landscape. [Isaac Marquez]

    CO!%TCTIO!TI'$& CO%T%Gor this particular biodigester, construction time !ill vary depending on a

    person2s construction e8perience or if a professional concrete contractor

    is hired to pour the digester tank. %ime !ill vary from several !eekends

    to several days, separated by a seven&day concrete&curing period.Materials !ill cost K", to K",@, depending on the price of ready&mi8

    concrete in your area and if the concrete company !ill charge you a

    9short&load; fee for ordering only @ yards of concrete. %he pro$ect can be

    broken do!n into e8cavation and concrete plumbing and gas piping

    and e8ternal masonry. %he concrete, rebar, battens, and anchor bolts run

    about KB. /arren highly recommends spending the e8tra [email protected] to

    K3 for a concrete pumper truck to avoid having to 9buck