The invention relates to a reactor unit for treatment of a liquid. The reactor unit comprises at least one plate (30) of a permeable material, said at least one plate being arranged to be stationary within the reactor unit, at least one disc (12), the disc(s) being arranged to be placed adjacent to one of the at least one plate, but leaving a gap between said one plate and the disc member, and in parallel to said one plate; and means (16) for rotating the at least one disc member around a central axis through the at least one disc member. Preferably, the at least one plate comprises a first number of filtering plates of a first material, said first material being permeable to liquid, each filtering plate having outlet means for outletting liquid which has permeated into the plate, and/or a second number of aeration plates of a second material, said second material being permeable to air, each aeration plate having inlet means for inletting air, said aeration plate(s) being placed in parallel to the filtering plate(s).