Figure 7 To the left is a vertical cylinder which narrows at the bottom. There is an arrow above the cylinder pointing vertically down to the cylinder, labelled contaminated water. At the bottom of the cylinder is another vertical arrow pointing downwards, labelled purified water. The cylinder is shaded blue. Inside are four large grey spheres. The key to the right identifies these as ion-exchange medium. Initially there are a number of small green squares representing chloride and small green triangles representing other contaminants scattered randomly throughout the cylinder. Small red circles representing nitrate are then seen to enter the cylinder and travel down it. These red circles each come to rest on one of the large grey spheres. The small green squares on the grey spheres come off and travel down and out of the cylinder. A label ion-exchange column regenerated with chloride solution then appears. Small green squares enter the cylinder and gradually replace the small red circles attached to the grey spheres. The small red circles travel down and out of the cylinder.