The complete mix version of the activated sludge process applies air and influent sewage at multiple points throughout the cross-section of the aeration tank.

• A. Plug-flow
• B. Batch
• C. Sequencing batch
• D. Complete mix

Answer: (D)

The main idea is complete mixing: the contents of the aeration tank are kept in near-uniform conditions so that every part of the tank has essentially the same substrate and oxygen levels. By feeding the sewage and introducing air at multiple points across the tank’s cross-section, mixing is thorough and there are no large concentration gradients. This uniformity is what defines a complete-mix activated sludge aeration system.

In contrast, plug-flow reactors rely on a steady progression of the wastewater with little mixing across the cross-section, so concentrations change along the flow path rather than across the entire tank. Batch and sequencing batch systems operate in defined time periods with fills, reacts, and decants, rather than maintaining a continuously uniform mixture throughout a continuously fed aeration tank.