The improved circular secondary clarifier of the present invention requires less surface area and low HRT and provides efficient solids-liquid separation. The improved clarifier has better SS and BOD reduction and provides high under flow solids concentrations, as compared to conventional secondary clarifiers. The improved clarifier is even capable of treating wastewaters containing low MLSS concentration. One of the biggest advantages of improved clarifier is that it does not require a separate sump cum pump house as it removes the settled sludge using suction mechanism thereby reduces capital and recurring cost. Further, it provides natural flocculation, which is essential for agglomeration of solids and increases particle size referred to as `floc` and eliminates provision for a separate flocculation facility thereby reduces capital and recurring costs.