Dissolved-air flotation is a very effective solid-liquid separation process used in water treatment, as an alternative to sedimentation, to lower the solids concentrations before granular filtration. This enables filters to be operated more effectively and cost efficiently to produce a filtered water quality suitable, after disinfection, for potable supply. Dissolved-air flotation has been specifically developed for the treatment of algal laden and coloured waters. The flocs produced by the treatment of these waters have a low settling velocity, particularly when the water temperature is low and even when polyelectrolytes are added. Treating these waters, flotation requires smaller tanks and produces better treated water quality, compared to sedimentation. Dissolved-air flotation has become accepted as an alternative to sedimentation, in particular in the Scandinavian countries and the United Kingdom, with more than 50 plants in operation or under construction. The largest dissolved-air flotation plant currently being constructed is the 315 Ml/day Frankley water treatment works in the UK.