Which configuration distributes air across the cross-section of the aeration tank?

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

Answer: (C)

Uniform oxygen transfer across the tank requires keeping the liquid well mixed. In a completely mixed aeration tank, vigorous mixing and diffused air keep the mixed liquor uniformly homogeneous in temperature and concentration throughout the entire cross-section. This means air bubbles introduced anywhere in the tank have access to the whole liquid, so oxygen is distributed evenly from wall to wall and from top to bottom, avoiding zones with low DO.

If it were plug-flow, the liquid moves as a relatively unmixed stream along the flow path. Air transfer would mainly occur along that path and near the inlet, with little mixing across the tank’s cross-section, leading to gradients rather than uniform aeration.

Sequencing batch reactors can operate with good mixing during aeration, but their defining feature is batch operation rather than ensuring cross-sectional air distribution. Fixed-film systems rely on biofilm on media for treatment rather than distributing air evenly through the liquid volume.

So the design that best distributes air across the cross-section of the aeration tank is the completely mixed configuration.