What is the purpose of a clarifier overflow weir?

• A. To maintain a stable surface and allow clarified water to exit the basin at a controlled rate.
• B. To sludge remove from bottom of the tank.
• C. To monitor residual chlorine in the water.
• D. To inject air for mixing.

Answer: (A)

The main idea is that the overflow weir in a clarifier sets a fixed exit level so water spills over evenly along the tank. That keeps the surface level steady and lets the clarified water leave at a controlled, steady rate. With a stable surface and uniform discharge, solids have time to settle instead of being carried out with the effluent, and flow is less turbulent. This is exactly what makes the clarifier work effectively. Sludge removal happens at the bottom, chlorine monitoring happens later in disinfection, and air is injected for mixing—none of those are the purpose of the overflow weir.