It is a process used to increase the amount of solid matter by removing excess water in biological or chemical sludge taken from settling ponds. Increasing the solid matter in the thickening of the sludge allows the systems of the dewatering steps to be selected smaller. Fixed Bridge Center Shaft Thickeners are suitable for use in sedimentation tanks up to 16 m in diameter. It is not economically viable for larger diameters. In such cases, rotating bridge concentrators should be preferred.