The purpose of primary treatment, also referred to as pre-treatment, is the removal of large dirt particles as well as settleable and suspended solids from the water. Larger particles are removed by a filtration unit, while settleable particles and fat are removed from the water by a flotation tank. Flotation allows for pollution to be removed in a physical or physico-chemical manner. This means with or without the addition of a coagulant or flocculant.
More about the individual steps: