Improvement of Device Measuring Biogas Volume in Laboratory-Scale Methane Fermentation Experiments

—Experiments for countermeasures against CO₂ dissolution in water by simulated biogas—

Abstract

The authors, using the prototype biogas volume measuring device they developed, observed the dissolution of carbon dioxide (CO₂) in water; however, using the prototype, based on downward displacement of water, CO₂ dissolution lowered the height of the water column in the device and destabilized gas volume measurement.

Therefore, the authors improved the device to avoid the influence of CO₂ dissolution on biogas volume measurement. The accuracy and precision of the improved device was verified by injecting simulated biogas into the device using syringes. The simulated biogases, CO₂ mixed with methane gas to 30, 50, 75 and 100% of the CO₂ concentration, were applied to the verification of the device. The results were a difference of -2.8 to 4.7 mL and a standard deviation of 0 to 3.0 mL. Lowering of the rates of water-column height were also measured and indicated correlative relationship with the CO₂ concentration, depending on Henry's law. The correlative relationship would be useful for monitoring biogas production.

In addition, 3 mol/L of sodium hydroxide solution was added to the water in the device after simulated biogas volume measurement. After the sodium hydroxide solution absorbed all the CO₂ in the simulated biogas, the gas volume was remeasured. The CO₂ concentrations calculated with volumes before and after sodium hydroxide solution addition were approximately 2% smaller than the accurate concentrations. The addition of a sodium hydroxide solution to the device gives approximate CO₂ concentrations for convenience of measurements and supports making a relational expression between CO₂ concentration and the lowering rates of water-column height.