Which type of digester has the longest detention time (DT)?

• A. Aerobic
• B. Mesophilic
• C. Anaerobic
• D. Thermophilic

Answer: (C)

Detention time is the length of time the sludge stays in the digester to reach stabilization and adequate digestion. Because stability and biogas production depend on how quickly the microbial processes proceed, oxygen availability and temperature play big roles.

Aerobic digestion uses oxygen, which generally speeds up breakdown, so stabilization occurs faster and the detention time is shorter. Thermophilic digestion runs at higher temperatures, which also speeds up microbial activity, often resulting in a shorter detention time than mesophilic anaerobic digestion. Anaerobic digestion, however, proceeds without oxygen and relies on slower microbial steps that must run to completion to stabilize the sludge and maximize methane production. This slower, multi-step process means it requires the longest detention time among these options.

Memorably, aerobic systems are typically in the days to a couple of weeks range, anaerobic mesophilic around two to four weeks, and thermophilic around one to a few weeks, with actual times depending on design and goals.