This article does not get into gas yields or what biogas can be used for, it is a basic introduction to the five necessary conditions to create flammable biogas in the first place and hopefully encourages a few folks who have failed before to try again.
I can guarantee the reader on my life biogas works, and it works great. The ancient Assyrians used biogas to heat their baths in 3,000 BC, the famous gas lamps of Victorian England were fueled with biogas, Sweden runs all of its city buses with biogas and today there are an estimated 50 million households in China using biogas.
There are no technical reasons every home in the world is not already using biogas for cooking energy and some light electric. The failure of any biogas project big or small are a result of violating one or more of these five easy-to-remember steps.
The microscopic organisms that produce biogas, known as Archaea, are among the oldest life forms on Earth. They predate the planet’s oxygen atmosphere much less oxygen-breathing and CO2-absorbing plant life by a cool 3.5 billion years. That’s billion with a “B.” Archaea are not bacteria, they are genetically closer to humans and other animals (eukaryotes), and form their own animal kingdom.
As the Earth’s atmosphere became predominantly oxygen about 500 million years ago, archaea became isolated in the few remaining airless places, such as stagnant swamps, deep oceans, caves and hot springs, and of course the stomachs of vertebrates. To create biogas, we must recreate the conditions in which Archaea thrive in nature.
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5 Steps to Making Homemade Biogas
The following table outlines the five steps to creating flammable biogas and I will get into further detail with each one. Biogas is reproduced in a special airtight tank called an anaerobic digester. The design of the anaerobic digester determines the first three steps.
Airtight Environment. A Ziploc baggie can be used for an anaerobic digester. The difficulty arises from trying to add fresh material without allowing oxygen into the system. The most common method of creating a continuous flow digester is the “teapot” or “P-trap” shape. Most biogas digesters are some variation of this teapot shape.
Archaea love water. When loading a digester, the water content in the material put in it should be taken into consideration. A head of lettuce, for example, looks very solid to us, however, it is 98% water. Dried rice is only 14% water.
Regardless of the size of your digester, the “40-50-10 Rule” is simple rule of thumb to follow to get the correct volume: Forty percent material, fill the rest of the digester with water except for 10% headspace.
A good analogy to think about regarding temperature and anaerobic digestion is your temperature is like the gas pedal of your car. The more you step on it, the faster your digester will convert waste into gas.
However, also just like stepping on the gas pedal, there are consequences for it. The warmer your digester is, the archaea that decompose your waste get more fragile and susceptible to an unexpected crash.
Temperature can be controlled a few different ways. In China, digesters are typically buried underground and built much larger than they need to be. This way they can be overloaded in winter months to maintain consistent gas production. Other designs employ a greenhouses or hoop house over them.
More advanced systems integrate some kind of heat exchanger, which can be heated with solar collectors. Regardless of your design, avoid using biogas or any other fuel to heat your digester. Make sure energy you use is excess energy on its way to being wasted.
Neutral pH is an important parameter in anaerobic digestion, just as it is for aerobic composting. If pH is measured at the inlet, it will be slightly lower than neutral usually around 5.5 as fresh material is converted into acids.
The pH will neutralize as these acids are converted into methane gas. By the time the liquid biofertilizer comes out the digester, it should be 7. If the pH of the biofertilizer is lower than this, it is an indicator the digester has been over-fed and is at risk to “sour,” or stop working due to low pH.
If the pH at the inlet goes below 5.5, it is necessary to add some wood ashes or lime to buffer the digester. A soured digester has no bubble activity and instead of producing gas, instead it draws air into it.
The top will be sucked in tightly against the surface of the liquid and if a brewer’s airlock is being used, the water in the airlock will be sucked into the digester. Restarting a soured digester is time consuming, and in most cases it is simpler to dump it out and start over.
Biogas production is best at the same 25:1 C:N ratio as aerobic composting. The reason cattle manure is far and away the most common feedstock for biogas is cattle manure is naturally the perfect 25:1 carbon-to-nitrogen ratio.
Cattle manure makes an excellent feedstock to begin experimenting with biogas with. Other wastes need to be combined as a compost pile is.
After these five steps, it is important to know that for the first 48 hours for a small digester or up to a couple of weeks for a larger system, the digester will only produce carbon dioxide (CO2).
Carbon dioxide is of course used in fire extinguishers. When you put a match to the gas to test for flammability, it will be blown out with an audible “hiss” and a wisp of black smoke. As the biogas begins to come on, the hiss and black smoke will be gone and you will smell the distinct “rotten eggs” scent of the hydrogen sulfide (H2S).
This odor is the signal to begin capturing your gas, as it is either flammable or soon will be. This “CO2 Phase” has caused many people to abandon DIY projects that might have been flammable if they had waited a short time longer.