A 5 l aeration tank containing a separation apparatus composed of a rotary magnetic disk and a scraper was prepared for sewage treatment. A 100-d experiment was carried out using highly condensed activated sludge (7-12 gl-1 mixed-liquor volatile suspended solids) which is magnetized by adding magnetite powder, under the COD(Cr) loading rate of 0.81 gl-1 d-1 without withdrawal of excess sludge. The magnetized activated sludge was separated by the rotary magnetic disk from the treated water and returned to the aeration tank by means of a scraper. After the 10th d, COD(Cr) in the treated water as effluent became constant at around 10-20 mgl-1.