What is the purpose of mixing in an aeration basin?

• A. A step screen uses rotating blades to remove solids; a bar screen uses moving rods.
• B. Causes stratification to separate solids from water.
• C. Ensures uniform distribution of oxygen and solids, preventing localized anaerobic pockets and short-circuiting flows.
• D. Increases foam formation to measure aeration rate.

Answer: (C)

Mixing in an aeration basin is to keep oxygen and solids evenly distributed so every part of the tank experiences the same aerobic conditions and contact with the microbes. When mixing is effective, solids stay in suspension and oxygen is transferred uniformly, which prevents dead zones where conditions become anaerobic and slow down treatment. It also reduces short-circuiting, where water flows through the basin too quickly or along preferential paths and bypasses the bulk of the mixed area, leading to inefficient treatment. By maintaining uniform distribution, the biological processes can proceed at a steady, efficient rate.

The other statements describe functions not related to the mixing purpose: screens are for solids removal, not mixing; mixing tends to reduce stratification rather than cause it, since stratification is the problem mixing aims to prevent; and foam formation is not a purpose of mixing and isn’t used to measure aeration rate.