Abstract: The invention concerns an acoustic sensor (44) for identifying obstruction of a circular grit trap for recycling fine sand in an industrial water clarifying plant. The circular grit trap comprises a cylindrical body having an outer wall and having at one first end a first sand-containing fluid inlet for receiving sludge and fine sand and a first overflow outlet of fluid not containing sand orthogonal to the fluid inlet, to evacuate said sludge, and at a second end a second sand-containing fluid underflow outlet coaxial with the first fluid outlet, to recover said sand. The acoustic sensor is sensitive to the noise radiated by the flow of the sand-containing fluid in the circular grit trap and is applied against the outer wall of the cylindrical body of the circular grit trap, in the plane of the fluid inlet but not coaxially therewith.