Vacuum and Pressure Filtration works most efficiently if the feed solids concentration is high. Therefore low concentration slurries can first be thickened in a thickener-clarifier with optional use of polymers to agglomerate the fine solids for faster settling and clearer overflow. The solids settle on the conical bottom of the thickener and can reach concentrations which are 10-20 times higher than the feed solids concentration. The thickened sludge is fed to the filter where the solids are dewatered and can be washed efficiently and discharged. The thickener overflow is usually very clear.