Basic Wastewater Operator Practice Exam

Which gas is predominantly produced in an anaerobic digester?

1. Ethanol

2. Alcohol

3. Methane

4. Hydrogen sulfide

The correct answer is: Methane

Anaerobic digestion is a biological process that occurs in the absence of oxygen, where microorganisms break down organic matter. The predominant gas produced during this process is methane. Methane is a simple hydrocarbon (CH4) that results from the decomposition of organic materials, such as food waste, animal manure, and other biological feedstocks. In anaerobic digesters, the specific microorganisms known as methanogens play a key role in the final stages of digestion, converting intermediate products like volatile fatty acids and carbon dioxide into methane. This gas is valuable not only as a renewable energy source but also for its role in reducing greenhouse gas emissions when compared to untreated waste decomposition. Ethanol and alcohol, while related to fermentation processes, occur under different conditions and involve aerobic or microaerophilic environments rather than strict anaerobiosis. Hydrogen sulfide is a byproduct as well but is not the predominant gas; it often results from the breakdown of sulfur-containing compounds in the presence of certain microorganisms and indicates the presence of anaerobic conditions rather than being the main product of digestion. Thus, methane stands out as the primary and most significant gas produced in an anaerobic digester, making it the correct answer.