NOVELTY - The sewage treatment unit has anaerobic processing portion (1,2), aerobic treatment portion (3), solid-liquid separation portion (4) and sludge retention portion (5). The portion (1,2) carries out decompose-treatment of inflow wastewater (23) using microorganisms. The portion (4) separates processed wastewater (26) into sludge (27) and treated water (28). The portion (5) is divided into multi-layered sludge (29).    USE - For treating sewage.    ADVANTAGE - The unit efficiently enables the processing and decompose-treatment of sludge.    DETAILED DESCRIPTION - The sewage treatment unit has anaerobic processing portion (1,2), aerobic treatment portion (3), solid-liquid separation portion (4) and sludge retention portion (5). The portion (1,2) carries out decompose-treatment of inflow wastewater (23) using microorganisms. The portion (3) carries out decompose-treatment of processed wastewater (25) in presence of oxygen. The portion (4) separates processed wastewater (26) into processing sludge (27) and treated water (28). The portion (5) retains the processing sludge separated in portion (4). The portion (5) is divided into multi-layered sludge (29) having lower layer sludge (29b), intermediate layer sludge (29a) and upper layer sludge (29c). The specific gravity of sludge (29b) is higher than that of sludge (29a). The specific gravity of sludge (29c) is less than that of sludge (29a). The layered sludge from portion (5) is returned to portion (1,2) as inflow wastewater.    DESCRIPTION OF DRAWING(S) - The figure shows the system chart of sewage treatment unit and mixing portion of inflow wastewater and sludge. (Drawing includes non-English language text).    anaerobic processing portions (1,2)    aerobic treatment portion (3)    solid-liquid separation portion (4)    sludge retention portion (5)    inflow wastewater (23)    processed wastewater (25,26)    processing sludge (27)    treated water (28)    layered sludge (29)    intermediate layer sludge (29a)    lower layer sludge (29b)    upper layer sludge (29c)