Which two stages occur in an anaerobic digester?

The correct choice identifies the two primary stages involved in an anaerobic digestion process: acid formation and gas formation.

During the acid formation stage, organic materials are broken down by microorganisms in the absence of oxygen, leading to the production of volatile fatty acids, hydrogen, and other intermediate compounds. This stage is crucial as it prepares the substrate for the next step in the digestion process.

Following acid formation, the gas formation stage takes place, where the volatile fatty acids produced are further digested by specific microorganisms. This leads to the production of biogas, primarily composed of methane and carbon dioxide. This biogas can then be captured and utilized as a renewable energy source.

Understanding the sequential nature of these stages is essential for optimizing the anaerobic digestion process, as the efficiency of gas production is highly dependent on the initial breakdown of organic material into acids. The other choices do not describe the fundamental processes taking place within an anaerobic digester, as they either pertain to unrelated operations or technical concepts not directly tied to the digestion process.