What are the detention times for standard-rate and high-rate anaerobic digestion?

• A. Standard rate: 30-60 days.
• B. Standard rate: 60-90 days.
• C. High rate: 60-90 days.
• D. High rate: 30-60 days.

Answer: (A)

Detention time in anaerobic digestion is how long the sludge stays in the digester to allow microorganisms to stabilize the waste. Standard-rate digesters are designed for conventional digestion with lower loading, so they require a longer residence time to achieve stabilization. High-rate digesters handle more sludge in a shorter period by operating at higher loading and often higher temperatures, which shortens the necessary detention time. In practice, standard-rate digestion is typically in the 30 to 60 days range, making that option the best match. Longer times like 60–90 days aren’t typical for standard-rate, and high-rate digestion is expected to be shorter, not longer, than standard-rate.