A water treatment system including a media filter having a filter chamber with a volume to accommodate filter media and a sludge outlet and a charge chamber for storing air. The charge chamber is fluidly connected to the filter chamber and has: (i) an air outlet for admitting air into the filter chamber; and (ii) an air inlet. The system also includes a trigger for selectively allowing the passage of air from the charge chamber air outlet into the filter chamber upon reaching a trigger release point and a sludge concentration/storage volume having at least one wall extending upward to form an upper opening. This wall has a height substantially above the trigger release point and substantially isolates the sludge volume from the charge chamber, except for the upper opening. Finally, an inlet to the sludge volume is at a height which allows pressure within the media filter during a filtration stage to move water above the wall height and into the sludge volume.