The effectiveness of water filtration in gravel-sand bed filters has been studied in the rehabilitating swimming pool treatment plant. Apart from instrumental analysis of the water the investigation considered additionally particle size distribution as well as thermal analysis of the sediment collected in the sand bed of depth filters and removed during the process of washing. Variable value of TOC in the washings for each filter indicates indirectly their non-uniform loading. It has been proved that the size of particles in the suspension easily changes as a result of operation of shear stress. Derivatographic research has indicated approximately 30 % presence of organic substances in the sediment. In neither of the investigated samples loss of mass over >460oC has been observed.