but the lid 40 is attached hermetically sealing the cell operation of a volumetric pump 29 extracts air from the cell and pulls liquid from the inlet line 27 into the cell through the inlet opening 18 treated liquid from the cell is pumped by the pump 29 along an exit line 32 of say 10 mm diameter at say 1 liter min an optical level sensor 39 is provided in a cavity adjacent the exit line 32 at a position above the level of the volumetric pump 29 this sensor stops the circulation pump 50 each time too much air passes through the cavity the volumetric pump 29 continues to operate however by this arrangement de aeration of the cell proceeds quickly due to the action of the circulation pump 50 a vortex is formed above the outlet passage 21 the air of the vortex is sucked in by the volumetric pump 29 this air is sensed by the sensor 39 which causes the circulation pump 50 to stop the vortex remains for about one second allowing even more air to leave the cell once the sensor 39 detects liquid the circulation pump 50 is caused to re start further pumping not only continues to fill the cell but.