Flocs collect as sludge at the bottom of the sedimentation tanks from where it must be removed regularly. The clean water leaves the sedimentation tanks through collection troughs located at the top of the tanks. Up to here in the treatment process, the bulk of the particulate matter had been separated and removed from the water. Tiny particles and microorganisms may however still be present in the treated water. The water therefore has to proceed to the final process, namely filtration.
At Midvaal, a secondary chemicals-addition step is sometimes necessary, depending on the prevailing water quality. Relative small doses of flocculants and/or flocculant aid are applied after ozonation to coagulate the oxidised particulate matter into flocs. These flocs then have to be separated from the water, in specially designed sedimentation units. Three different types of sedimentation units are available at Midvaal, namely circular clari-flocculators, a horizontal-flow dam and a pulsator. Sedimentation is the process in which the flocs (formed during coagulation and flocculation) are allowed to settle from the water.