The invention relates to a fixed bed for aerobic or anaerobic wastewater purification, comprising an upper frame having support elements fixed to opposite side rails and/or intermediate rails for attaching textile, foldable, non-degradable supports for microorganism, a lower frame having support elements for fixing the textile supports to the lower frame, and means connecting the frames at a corner region. The invention is a fixed bed where all parts of the fixed bed can be assembled to form a relatively small packaging unit that already contains the inserted textiles. Frames are provided with spacers at least in the corner regions, the height of which spacers corresponds to the material height of the folded supports between two frames and the supports are fixed by means of the supports members to the side rails and/or intermediate rails of the frames in a preassembly step before folding.