Two-stage digestion combines a standard-rate stage followed by a high-rate stage.

• A. Two-stage digestion combines a standard-rate stage followed by a high-rate stage.
• B. Two-stage digestion uses only one stage.
• C. Two-stage digestion eliminates the need for decanting.
• D. Two-stage digestion requires continuous aeration.

Answer: (A)

Two-stage digestion is arranged so that different microbial processes happen in separate units to optimize performance. The first reactor handles standard-rate digestion, breaking down complex organics and producing soluble compounds and volatile fatty acids. The second reactor, operated at a high rate, then converts those acids into methane, allowing higher overall loading while keeping the system stable. This separation improves gas production and control because it prevents acid buildup from inhibiting the methanogens. The other options don’t fit because a two-stage system by definition has two stages (not one), decanting isn’t a defining feature, and digestion is anaerobic (no continuous aeration).