A method of purifying raw water comprises the steps of supplying the raw water to a generally vertical, open-ended inlet cylinder (23) in a reservoir (20), exposing the water in the inlet cylinder (23) to air for accomplishing oxidizing of matter, such as iron, manganese, and hydrogen sulphide in the water and/or agitation of the water, allowing the oxidized matter to fall down by gravity to the bottom of the reservoir (20), allowing the purified water to flow upwards past the inlet cylinder (23) and through a distribution disc (24) in the reservoir (20), and removing the purified water from the upper part of the reservoir (20).