The invention relates to a water-oil-sludge separation plant comprising: a slop tank (0) for water/oil/sludge, an outlet for slop water to a vertical separator (1) comprising a vertical, substantially cylindrical tank (11), wherein the vertical separator (1) leads to an upper oil outlet (27); a middle, horizontal, tangential outlet (22) for oil-polluted water to a membrane filtration plant (6); and a lower pipe (44) for sludge outlet to a decanter/centrifuge (5); wherein the decanter / centrifuge (5) is arranged -to separate dehydrated solids, and - to lead water treated in the centrifuge back to the vertical separator (1) or the slop tank (0); wherein the membrane filtration plant (6) is arranged - to lead the concentrated retentate back to the vertical separator (1) or to the slop tank (0) or remove from the process; - wherein the permeate comprising cleansed water is let out; wherein the sludge outlet (44) from the vertical separator (1) comprises a control valve (3) controlled by a control unit (41) that, based on a sensor (4) for density of sludge in a lower portion of the vertical separator (1), continuously adjusts outlet of sludge to the decanter/centrifuge (5).
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