A water purifier includes a base, an outer casing mounted on the base, a fresh water conduit demountably mounted in the outer casing, a plurality of filtering units demountably mounted on the fresh water conduit and a cover demountably mounted on the outer casing. Each filtering unit includes a membrane bag and multiple supporting members. The membrane bags are sandwiched by the supporting members and are arranged in a cascading manner. Gaps are formed between each pair of adjacent membrane bags so that the filtering bags can be cleaned directly.
