a hollow , polymeric sludge collector flight is provided having a sigma - shaped cross - section , including a wedge - shaped transverse indentation on its forward face , a vertically extending wear lip on its scraping edge , and a transverse gap on its rear face . the gap allows free circulation of liquid within the non - buoyant flight . the indentation structurally stiffens the mid - span section of the flight . where additional stiffness is required , filler blocks may be placed between mounting blocks .