Two-stage anaerobic digestion involves which sequence?

• A. Two-stage digestion uses a high-rate stage followed by a standard stage.
• B. Two-stage digestion involves a standard stage followed by a high-rate stage, with focus on getting the supernatant liquor decanted.
• C. Two-stage digestion eliminates the need for decanting.
• D. Two-stage digestion is performed in a single tank.

Answer: (B)

In two-stage anaerobic digestion, two reactors are used in series to optimize different microbial processes. The feed first moves through a standard-rate stage, where initial hydrolysis and acidification occur in a stable, controlled manner. The effluent then enters a high-rate stage, where the remaining organics are rapidly degraded to produce more biogas. Focusing on decanting the supernatant liquor between stages helps remove the liquid portion from the solids, reducing carryover of solids and potential inhibitors into the high-rate stage, and keeping the high-rate stage operating under better, more consistent conditions. This sequence—standard-rate stage first, followed by a high-rate stage—with decanting the supernatant liquor, aligns with how the two-stage design can provide stable treatment and improved digestion performance.