Biogas Digester Construction

Biogas digester construction using intermediate bulk containers (IBC) for home use is easy and often can be built within an hour once you’ve assembled the tools and materials.

Photos by T.H. and Enas Culhane

Biogas digester construction using intermediate bulk containers (IBC) for home use is easy and often can be built within an hour once you’ve assembled the tools and materials. All except the “uniseal” gaskets and bulkhead fittings can usually be sourced locally. Benefits of having a biogas digester include creating your own renewable fuel and fertilizer. Both are produced by biological “waste” from the homestead.

“Waste or pollution can be considered the right thing at the wrong place at the wrong time and/or in the wrong concentration,” Dr. T.H. Culhane says. By using our kitchen scraps and food “waste” and to a lesser extent animal excrement we can harness these organic residuals into something useful.

Dr. Culhane is an Associate Professor at the Patel College of Global Sustainability at the University of South Florida. In addition to being the Director of the Climate Mitigation and Adaptation concentration for the university, he is a co-founding director of the not-for-profit educational corporation “Solar CITIES Inc.” The organization helps community stakeholders solve urban ecology and development issues surrounding waste-water, solid waste, food security and create decentralized clean energy production.

“Solar energy is stored in the chemical bonds that photosynthesis has captured and stored,” Dr. Culhane explains. “We call those compounds “food” when we eat them, but there are other things that we don’t eat – but that microbes can eat. They will eat food waste, and animal and human waste.”

When you utilize a biogas digester you eliminate pollution because that “waste” is in the right place and at the right concentration. A biogas digester (“domestic dragons” Dr. Culhane calls them collectively), not only breaks down what your compost pile would, but it also can take care of meats, pet waste, oils, fats, grease – items that we are told not to combine into a composter. By putting these organic residuals into an artificial stomach (biogas digester) we can get the microbes to turn the “bads” into “goods.”

In addition to eating anything – like a dragon would – they also breathe fire. When the gas line is opened and a fire lit, the biomethane combusts. Biogas is a mixture of methane, carbon dioxide, and a small percentage of other gases. The carbon in biogas comes from plant matter that was formed into a chemical compound from atmospheric CO2. Therefore, biogas production is carbon-neutral and does not add to greenhouse gas emissions.

“It has the same properties as the natural gas that people are fracking for and that companies are flaring off in the desert,” Dr. Culhane explains. “It is functionally equivalent to the fossil fuels that microbes made thousands of years ago, but it is being produced here and now at a rate where it can be used and transformed into something useful without adding to the greenhouse effect.”

Biogas digester construction can be thought in terms of two structures – an artificial gut and a container that holds the gas that the microbes produce. The gut or the stomach needs a throat/mouth pipe, a ureter to pee out the liquid fertilizer, and a third pipe to belch and fart out the useful biomethane.

The first structure we will build is the artificial gut. IBC tanks, which are around 270 gallons, are perfect for the microbes and adequate for most families, since that size of a tank can digest about a paint bucket-sized (5 gallons) amount of food waste per day and produce about 2 hours of cooking gas from that bucket’s worth when temperatures get into the mid 80s- low 90s (Fahrenheit) and about an hour’s worth when temperatures are in the high 70s or low 80s. The closer to body temperature they run the more efficient they are.

To start, Place the IBC so the center opening hole is facing up. On the far side of the opening, drill or melt a 5-inch hole into the top of the tank. This will fit a 4-inch Uniseal® (which has a five inch flange) and a 4-inch feed pipe, which we consider the “mouth and throat” of the domestic dragon in biomimicry terms.

In the diagonal corner from the feed pipe, drill a 3-inch diameter hole for the liquid fertilizer overflow pipe (which we consider the “ureter” in biomimicry terms). This hole will fit the 2 inch Uniseal (which has a 3 inch flange) that accommodates a 2 inch pipe.

“For the gas outlet you have a couple of options,” Culhane explains. “If you can find a two-inch bulkhead fitting and the cap is smooth, make a 2 ¼ inch hole for the bulkhead and then fit it with a 2 inch pipe about 10 inches tall with a reducer to half an inch and put in a small section of ½ inch pipe, a half inch ball valve and a half inch barbed hose nipple.”

After the feed and liquid fertilizer holes are drilled, smooth the edges, and use a flexible silicone around the Uniseal®.To allow the food to slide into the tank, cut out a section near the bottom of the 4 inch pipe about 6 inches long and half the diameter of the pipe. Gently push the pipe through the Uniseal®. Twisting and smoothing out the edges of the pipe may be necessary. It helps if you lubricate with rubbing alcohol or grease. The opening at the bottom of the pipe should face the center of the tank and rest on the bottom. The feed pipe needs to be higher than the liquid fertilizer to prevent backups and spilling.

“Many people wonder how the tanks can always be overfilled with water and still work,” Culhane says. “In fact the feed pipe is filled a few inches above the outlet pipe so it pushes the fluid through and out as the opposite side tries to equal the height and then spills over, creating a pull action on the feeding side through the tank.”

A large 4-inch Y pipe or funnel can be placed on top of the feed pipe to allow you easy access to feed the domestic dragon.For the liquid fertilizer pipe, cut a hole in the center of the pipe about 2-inches wide. Undigested materials will sink or float. Only the fully digested materials will become neutrally buoyant, which will enter the pipe when new food is added to the system. The bottom of this pipe does not need to be covered since it will be clogged with sludge soon. When fully inserted the pipe’s 2-inch opening should face away from the feed pipe in the unlikely event that undigested food finds its way across the tank. The liquid fertilizer pipe should sit around 8 inches above the IBC. Add a 2-inch T with a short horizontal pipe and then a 2 inch elbow with a short vertical pipe to easily allow the liquid fertilizer to flow into a bucket or nearby garden. The open T (open to atmospheric pressure) keeps the tank from inadvertently siphoning and draining.

Culhane says to paint the tank black to keep out the light and to absorb solar heat.

Now we will need to build or buy a container that holds the gas that the microbes produce.One option is utilizing two additional IBCs. Whie the biogas digester construction above should only take about an hour or so, creating the contraption that holds the methane could be a little more complicated. Quick solutions include purchasing a 1,000-liter balloon or bladder. Culhane typically gets his from the Puxin Biogas company in China, but he also has found a Queen-size air mattress from Walmart® also works!

If purchasing a bladder, you will hook up a flexible rubber hose from the primary biodigester to a T-valve in the bladder.“Connect another hose from the other side of the gas holder T to your stove” Culhane explains. “We normally keep the valves connecting the primary and secondary digesters open all the time and keep the valve to the stove closed, but if you have another valve at the stove, you can open and close that and leave the valves at the digesters open all the time.”

Culhane also creates his own methane containers, using two IBCs (270 gallons each) with cages.The procedure to do that is as follows:

Using a grinder, remove the top side of one of the tanks.

This tank will hold the water in which the gas holder will float. The entire depth (height) is filled with water which is around 270 gallons. The other tank sinks down into it and then rises when filled with gas. Since gas can’t go down – it always rises – you don’t have to worry about it working its way down and out the sides. The lower tank is always filled with water, and the upper tank floats up and down in the water. It thus has a water seal.

“Turn the second tank on its side with the drain valve facing up, skyward, and lop off what now appears to be the bottom,” Culhane instructs. This is the far side opposite the drain. This tank needs to sit under the water line of the first tank. Typically this means cutting it at the second “bulge” if you don’t have a measuring tape. Of course, you could cut off a little, place it into the first container and keep trimming until it sits below the first tank.

“Make sure the lid of the gas holder tank has no holes and has an intact O-ring. If not, fill it with silicone and screw it on tight,” Culhane says.Drill a hole for a half inch bulkhead fitting on what is now to be considered the new “top” of the tank. This was previously the front side of the tank where the drain is. Insert the bulkhead fitting from the inside and apply silicone under the rubber washer for added security. Fit the locking nut on the top and connect your pipes.

Insert the bulkhead fitting from the inside and apply silicone under the rubber washer for added security.

Although they are the same size, due to the HDPE plastic sides, they are flexible. The problem with this is getting them back into the cages. This is important for long-term use.

Culhane recommends using a grinder to remove 10 center vertical bars, 5 from each long side to slightly enlarge the cage. Use proper personal protective equipment (PPE) as the metal bars have been known to fly. Culhane says to do this task away from windows, cars, children, and pets.

The cage needs to be large enough for the tanks to rise freely and go down depending on how much gas has been used.

Placing a second cage on top will keep the IBC that floats pending the gas in place as it rises out of the water. Use zip ties or wire to securely connect the two cages to prevent injuries.

“You can now fill your primary tank with animal manure or lake mud or RidX or any septic powder and with water,” Culhane says. “Make sure as you are doing this to soap test all the fittings (the lid, the uniseals, the pipes). Just make up a dish soap solution and pour it over the tank while you are filling while keeping the valves closed. This will compress the air in the tank, and you will see soap bubbles form if there are any leaks.”

Currently, 30-60% of our “waste” goes to landfills. By creating and using a home biogas digester, we can eliminate dangerous pathogens and use their high energy, nutrient-rich outputs. Culhane says that biodigesters kill 98-99% of dangerous pathogens.

“We have to work with nature now to keep up with the rate of energy and nutrient packages that we are producing,” Culhane says.A 5-gallon bucket filled with “waste” (food waste, spent flowers, fats, oils, meat, moldy foods, human and animal excrement, green yard waste) will give you approximately 1 kilowatt of energy if you put it into one IBC biodigester that we described above. Each system can take about one 5-gallon bucket of “waste” a day but no more to prevent indigestion. This will result in 45-60 minutes of energy if you run it through a generator to charge batteries for longer use. If you use it to power LED lights – it will probably last forever.

Traditionally the methane is used for cooking or water heating. A flame at medium height, like you would for stir fry or soups, will last around 2 hours based on feeding it the 5-gallon bucket during warm months. This is adequate for most households, especially those in developing countries. In addition to the cooking gas, one bucket into the system equals one bucket out of liquid fertilizer.Will you add biogas digester construction to your DIY list?