Abstract: A liquid is filtered through a bed 7 of granular material and cleaning of the bed is conducted, when necessary, by upflow of wash water through orifices 9 and air through orifices 12 positioned mid-way between the orifices 9. An overflow 13 defines a free water space 15 above the bed and a baffle formed of elements 18, 19 and 20 surrounds the overflow outlet. During cleaning the wash water and air flow upwardly at rates such that some granular material is entrained by upflowing air bubbles and could be carried to the overflow 13 but substantially all the entrained granular material is separated from the entraining air bubbles at or near the top of the free space 15.