The hydrogen sulfide in biogas is also hazardous. Hydrogen sulfide levels become life-threatening in 30 minutes at 300 ppm. Because hydrogen sulfide is heavier than air, sensors in buildings housing digester equipment should be placed near the floor. By adding large amounts of oxygen to the manure, naturally occurring bacteria will begin to break down the waste and reduce its odor in one to six months. Aerobic digestion is a one-step process.
Bacteria use oxygen to convert organic waste to carbon dioxide, water, and more bacteria.
Aerobic bacteria are sensitive to the lack of oxygen, so the lagoon must be managed carefully to ensure adequate oxygen is always present. Dilution water is needed from the start-up of the lagoon, and a steady, daily supply of manure is required. Slug loads will quickly use up the oxygen and result in a strong odor. Mechanical aerators are used in aerated lagoons to ensure constant mixing of the manure. The aeration system is intended to continuously bring the manure to the surface, where it is exposed to air.