Roughing filters are used to separate solid matter from water. Some solid particles suspended in water are so small and light that they do not settle in a normal sedimentation tank. The same sedimentation tank can be improved by filling it with rough filter material such as gravel. Roughing filters usually consist of sedimentation tanks filled with differently sized filter material decreasing successively in size in the direction of flow. The bulk of solid particles is separated by the coarse filter medium located next to the filter inlet. The subsequent medium and fine filter media further reduce the concentration of suspended solids. Roughing filters are operated with small hydraulic loads - filtration velocity is usually about 0.3-1.5 m/h.