MODEL RELEASED. Water treatment. Worker checking the rapid gravity filter tanks at a water treatment works. Water is sent here after the clarification process (see T825/224). Each tank contains a gravel base and a bed of Granular Activated Carbon (GAC). GAC is a highly porous adsorbent material that has a positive charge. It removes negatively charged organic compounds from the water such as ozone, chlorine and fluorides, which impair the flavour of drinking water. Photographed at the River Itchen Treatment Works, Gaters Mill, Southampton, UK.