The invention relates to a filter unit for purification of impure water, which could be seawater intended for ballast water, pool water, waste water or the like. The purification is performed by means of filter modules comprising filter means with dead end filter elements of a sintered material, and the filter unit facilitates alternating backwashing of the filter means at the same time as the filter unit may perform filtration of the impure water. The invention likewise relates to use of a dead end filter element in a de-gassing version, the filter element comprising a top end, a bottom end and a plurality of substantially parallel arranged channels with blockings in different ends.