Final Result: Successful Methane Production|
After two and a half weeks, the digester was producing enough methane to burn. The simple burner tip did not carburet well, so I needed a pilot light to keep it burning. Slight pressure on the storage bag made a nice flame. Later I found that a 1/2" deep socket inverted over the burner tip made a simple burner that would stay lit on its own.
Discussion of Methane Production
The process of producing methane from organic matter relies on bacteria. Initially the digester contains organic matter, water, and oxygen. The first step is for oxygen-loving bacteria to consume all of the oxygen in the digester. CO2 is produced during this phase, which must be collected and released without introducing more oxygen to the system. This was accomplished by using the water manometer which allowed the CO2 to escape while preventing the reintroduction of outside air.
After the oxygen has been removed from the system, anaerobic processes can begin to digest the organic matter. First, the acid-producing bacteria break down the organic matter into simple compounds, including volatile acids such as acetic acid. The volatile acids are then used by the methane producing bacteria to produce methane, CO2, water, and other gases. The methane, CH4, is produced in this reaction:
4H2 + CO2 = CH4 + 2H2O
The trickiest part of methane production is establishing and maintaining the methane producing bacteria. Since they feed on the acid produced by the acid-producing bacteria, this acid must be present. But the methane producing bacteria are also sensitive to pH levels, which should optimally be between 7.5 and 8.5. Initially, the acid phase can lower the pH to below 6. Slowly the pH increases as acids are digested, and methane production eventually takes over.
Notes for next time
Next time I'll definitely start with a thinner mixture. I don't think you need a lot of solids to get this process going. Also, the setup did not allow for easy stirring of the slurry, which is supposed to accelerate the process. Ideally the heater would have kept the slurry at 95 degrees Fahrenheit, production systems typically use methane heaters to keep the slurry at the optimum temperature. I'm not sure what would have happened if I didn't use the baking soda, I'm guessing that the process would have started on its own, but maybe it would have taken a little longer. The biggest thing I was missing was a way to actually use the methane. You can utilize standard appliances that are designed for natural gas, but storage of the gas would require tanks.