NOVELTY - A biological-treatment apparatus comprises an aerobic-organism treatment tank (1), sedimentation tanks (2a,2b), an aerator, a separation-membrane immersion tank (3), and sludge return units (I,II), where the tank (3) has an aerator in the downward direction of a separation-membrane module (3a)    USE - For removing organic matter from wastewater containing a high concentration organic matter, such as garbage-containing wastewater, cooking refuse, foodstuffs manufacture wastewater, and liquor manufacture wastewater.    ADVANTAGE - The biological-treatment apparatus prevents clogging of immersion membrane, without suppressing the required volume of treatment tank for wastewater. The apparatus efficiently processes wastewater, and provides treated water of high quality. The apparatus solves the problems of fluid-bed carrier biological-treatment method and membrane-separation activated sludge method. The surplus sludge which is not removed by the sedimentation tank is later removed by the separation-membrane immersion tank. The washing frequency of the immersion tank is lowered.    DETAILED DESCRIPTION - A biological-treatment apparatus has an aerobic-organism treatment tank (1), two or more sedimentation tanks (2a, 2b) connected in parallel, an aerator, a separation-membrane immersion tank (3), and sludge return units (I, II). The treatment tank comprises a flow support which accepts organic wastewater and supports microorganisms. The sedimentation tank accepts the effluent of the treatment tank in batches, and carries out still-standing, supernatant-water ejection, and precipitate-sludge ejection. The immersion tank receives the supernatant water from the sedimentation tank, separates sludge accompanying the supernatant water, and ejects treated water. The immersion tank is equipped with an aerator provided in the downward direction of a separation-membrane module (3a) immersed in the tank. A sludge return unit (I) returns the precipitate sludge of the sedimentation tank to the treatment tank, and sludge return unit (II) returns the sludge of the immersion tank to the treatment tank.    DESCRIPTION OF DRAWING(S) - The figure shows the systematic diagram of the biological-treatment apparatus. (Drawing includes non-English language text).    aerobic-organism treatment tank (1)    sedimentation tanks (2a, 2b)    separation-membrane immersion tank (3)    separation-membrane module (3a)    diffuser (3b)