Which is a design characteristic used to evaluate an aerobic digester's performance?

• A. Solids retention time
• B. Color of digestate
• C. External temperature
• D. Gas production rate

Answer: (A)

Solids retention time is the key design parameter that determines how long the solids stay in the aerobic digester, giving the microbial community enough time to break down organic matter. In an aerobic process, keeping solids in the reactor long enough ensures sufficient contact with microbes and oxygen to achieve stabilization and treatment goals. A longer SRT generally improves the extent of digestion and odor control, though it requires more reactor volume or slower feed rates, impacting cost and throughput.

Gas production rate isn’t a primary performance measure for aerobic digestion, since most of the digestion is aerobic and does not generate substantial Biogas as in anaerobic systems. The color of the digestate isn’t a reliable indicator of performance, and external temperature affects reaction rates but isn’t a design measure used to evaluate how well the digester is performing.