The invention provides an apparatus and method for purification of a fluid, the apparatus comprising: a vessel (1) for receiving, in operation, a filter bed (2) with granular filter material; a transport system (8) configured to, during operation, transport filter material from a cavity (5) underneath the filter bed (2) to a location (17) above the filter bed; a fluid inlet (14) for introducing fluid to be purified into the apparatus; a mixing section (3) adapted to, in operation, provide a mixture of fluid from the fluid inlet and granular filter material from the filter bed in a part of the vessel beneath the mixing section; and a plurality of gas lift injectors (4), wherein each gas lift injector (4) is configured to, in operation, transport the mixture of fluid from the fluid inlet (14) and granular filter material from the filter bed (2) to the cavity (5).