Which statement is a design characteristic of an aerobic digester?

• A. Dissolved oxygen level
• B. Temperature
• C. Solids retention time
• D. Volume

Answer: (C)

In aerobic digestion, how long the solids stay in the digester is the key design parameter. This solids retention time (SRT) determines how long the microorganisms have to oxidize and stabilize the material. A longer SRT means more complete digestion and a more stable biosolids product, but it also requires a larger reactor or slower throughput, while a shorter SRT reduces reactor size but may leave more volatile solids. Dissolved oxygen, temperature, and reactor volume are important operational or sizing factors you manage during operation, but the fundamental design decision centers on selecting an appropriate SRT to achieve the desired level of stabilization and effluent quality.