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How to make a home biogas digester (maybe): if you've been thinking about ways to turn waste into fuel, these plans are for you.


MAKING METHANE cooking gas from waste and compost isn't new. Tinkerers in remote locations and farms around the world have been tapping into biogas for years with varying degrees of success. The natural gas produced can be enough to provide cooking fuel, heat, hot water and even gas for specially modified vehicles.

Environmentalists are turning to biogas as a means to help farmers stop the flow of runoff into rivers and to make power and fertilizer in a green way. But while YouTube clips abound with biogas success videos, few of them originate in the United States.

Biogas production is a tricky business dependent on keeping temperamental microorganisms alive and in perfect balance to keep the gas flowing. Also, U.S. codes for gas storage and sanitation are stricter for such endeavors. For many Americans, the process just isn't cost-effective enough to make it worthwhile.

But for the tinkerer at heart, biogas can be a good challenge, says Bob Crosby, a research and energy management consultant based in Willow, Alaska.

"It's a practical thing for people who might have a couple of animals or want to do it for the love of it," he says. "To get a working system in place really takes some dedication."

In a biogas digester, anaerobic bacteria (bacteria that thrive in an oxygen-free environment) decompose organic matter into methane, carbon dioxide and sludge. In the first stage, acidogens (acid-producing bacteria) break down the raw waste into simple fatty acids. In the second stage, methanogens (methane-producing bacteria) consume the acids and produce biogas as a byproduct. Acidogens are hardier critters than methanogens, which makes biogas production never a sure thing. The trick is that the two organisms must be in balance, or else the pH balance of the system will kill off the critters.

Crosby is among the handful of biogas enthusiasts who spend their time perfecting digester designs and discussing improvements online. He believes that would-be biogas digester constructors should build a tiny gallon-jug model before going all out with a small-scale or large-scale digester.

"This may be harder than you think," he says.

Most biogas digester veterans, including Crosby, will say there are no guarantees with any biogas digester design. Temperature variables and microorganism health are the twin nemeses of digester operatives.

"They're very fragile, finicky little bugs," Crosby says.

But for those who can't resist a challenge, Crosby shares with GRIT the basics for a plan of a small-scale digester, designed to handle the flushes of a family of six. Look for information about these plans and a lively discussion of biogas construction online at


A digester can be used to process kitchen waste, animal dung or human waste (local sanitation codes permitting). They can be designed either as a manually fed bucket in a yard or as part of an integrated waste system connected to a farm or home sewage system.

The trick with starting any digester is that the system will need minute attention in the beginning and steady attention throughout. Material should be mashed into a slurry and fed into the digester as close to 95 degrees Fahrenheit as possible.

The gas produced can be used either directly for cooking fuel or for hot-water heating. The gas can be stored several ways, including low-pressure gas bags (such as a truck tire or inner tube) and rigid tanks. However, remember that you are dealing with a flammable product. The safest bet is to burn the gas as soon as possible instead of storing long-term.

Crosby has built several digesters, although he's working on a new prototype for an upcoming book. With his new model, he hopes to keep temperatures more constant and speed up the starting time for the microorganisms to produce biogas, which can take more than a month.

Crosby offers these instructions under the concept of open copyright, meaning others are free to use and change his design, but that he offers the design without any warranty.


High-Density Polyurethane (HDPE) tank (for these plans
    a 90-gallon, open-top, 24-inches-by-48-inches tank)
76  inches of 3/4-inch HDPE tube
1   24-inch-by-24-inch sheet 1/4-inch HDPE
3/4-inch flexible hose
2-inch PVC pipe
3  2-inch bulkhead fittings
1  3/4-inch bulkhead fitting with male pipe threading
   and hose barb
20 3/4-inch self-tapping stainless steel screws
Silicone calk
1  4-inch-by-24-inch sheet 3/8-inch HDPE (or bigger to
   fit the flange on your tank)
1  4-inch ABS socket cap
1  4-by-4-by-2-inch ABS sanitary tee connection
1  4-inch PVC coupling
4-foot 4-inch cellular core ABS pipe cut to fit
Heat tape
Aluminum tape
Reflectix 1-inch rigid reflective insulation
Rubber for gasket
20 nylon hex cap screws and nuts
Special Tools Crosby Used: plastic welder, small air
   compressor, HDPE welding rod.

STEP 1: Before buying a high-density polyurethane (HDPE) tank, calculate how big your tank should be with Crosby's tank calculator (available at www.Biorealis. com/wwwroot/digesterrevised.html). For these instructions, Crosby used a 90-gallon open-top HDPE tank that was 24 inches in diameter and 48 inches tall.

STEP 2: Inside the tank, insert a 3/4-inch HDPE tube bent into a hoop. Weld tube onto the tank 12 inches from the bottom so that it acts as a blocking for a baffle that will separate two areas of the tank.

STEP 3: Assemble the baffle plate. Cut a disk to fit inside the tank from 1/4-inch HDPE, Once the disk is cut, cut three holes for the various tubing. You'll need holes to fit a 3/4-inch flexible hose plastic gas vent, a 2-inch PVC transfer pipe in the center, and a 2-inch effluent intake pipe.

STEP 4: Attach bulkhead fittings to the baffle plate, 3/4-inch bulkheads for bendable pipe and 2-inch bulkheads for PVC pipes. Then attach the hoses to the bulkheads. Glue the appropriate hoses to the appropriate bulkheads. Fit the PVC pipe only; do not glue. Let dry.

STEP 5: Once baffle plate is ready to attach, fix it to the blocking pipe with self-tapping stainless-steel screws. Seal the screws with silicone caulk.

STEP 6: Cut the digester tank lid from a 3/8inch HDPE sheet so that it fits to the inside flange of the open-top of the HDPE tank. (The actual diameter of your lid will depend on what design of tank you use.) Cut holes in the tank lid to fit your soon-to-be-assembled heat exchanger, your biogas outgoing pipe, your biogas recirculation pipe, and a gas vent.

STEP 7: Glue appropriate fittings to underside of tank lid. Caulk the underside of the hole around the heat exchanger before assembling the exchanger.

STEP 8: Make a simple heat exchanger from 4-inch ABS pipe with 2-inch PVC pipe fitted inside. The top of the heat exchanger should have a 2-inch PVC bulkhead fitted into a drilled hole through the center of a 4-inch ABS cap. Attach the exchanger to the appropriate hole. Attach to PVC coupling. Then, connect coupling to cellular core ABS pipe.

STEP 9: Wrap digester with heat tape. Attach the heat tape with aluminum tape.

STEP 10: Wrap digester with two layers of Reflectix 1-inch rigid reflective insulation bent around the tank and strapped.

STEP 11: Install lid so that it is both tightly sealed and easy to lift off. The lid should be gasketed and bolted to the tank flange. Cover with 2 inches of rigid-foam insulation, trimmed tightly around the pipes for a snug fit.

Once you have the basic digester constructed, you will then need to think about how it might be connected to a system. For many, the input will be placed manually and the output might be enough to light a gas-powered barbecue stove. Others might funnel the outgoing directly into their kitchen or add it to their hot water system. Although that might create a whole new host of design problems, free gas is a fun problem to have.

ON THE WEB Learn all you can about your septic system.

Reporter and editor Craig Idlebrook focuses on issues of sustainability and parenting. He lives in Somerville, Massachusetts, and can be contacted via email at craigidlebrook2@
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Title Annotation:In The Shop
Author:Idlebrook, Craig
Date:Nov 1, 2011
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