The invention relates to a method and an apparatus for cleaning the sand beds of slow sand filters, infiltration basins and pools of other types having sand beds in water treatment works and similar plants, while the filter is in service, by removing a few centimeters of the bed surface, cleaning the sand of the sludge or filter film formed on the bed surface, returning the cleaned sand to the bed, and discharging the separated sludge, suspended in water to drain or to a suitable disposal site. This is achieved using a tracked cleaning machine equipped with a submersible pump (7) and nozzle (8) to remove the top layer of sludge-bearing sand from the bed, cleaning the sludge from the sand thus removed in a hydrocyclone (13), returning the cleaned sand to the bed via a spreader plate (15) and discharging the water-suspended sludge to a drain or to a suitable disposal site through a hose (17).