A water-oil-sludge separation plant comprising: - a slop tank for water/oil/sludge, with an outlet for slop water; - a vertical separator comprising a vertical, substantially cylindrical tank having a tangential intake, an upper oil outlet at the top of the vertical tank, a middle, horizontal, tangential outlet for oil-polluted water, and a lower pipe for a sludge outlet at the bottom of the vertical tank, wherein an upwards-tapering conical frustum is provided in the interior of the vertical tank and the tangential outlet is at a lower end of the conical frustum; - a membrane filtration plant; - a control valve for the sludge outlet; - a sensor for sensing the density of the sludge in a lower portion of the vertical separator; - a control unit; wherein the water-oil-sludge separation plant is arranged in such a way that: - the slop water is directed from the outlet of the slop tank to the vertical separator and enters through a wall of the vertical separator and subsequently through the tangential intake, horizontally in the interior of the upwards-tapering conical frustum surrounded by the vertical tank, - water with oil exits via the top of the conical frustum, whereas the sludge is separated by the conical frustum and subsides in the interior down to the lower pipe, - oil and possibly air are led out through the upper oil outlet, - oil-polluted water continues to flow downwards, externally with respect to the conical frustum, to the tangential outlet and then to the membrane filtration plant; - the membrane filtration plant is arranged to let out the permeate comprising cleansed water and to lead the concentrated retentate back to the vertical separator or to the slop tank or to remove it from the process; wherein the water-oil-sludge separation plant is characterized in that: - it comprises a decanter centrifuge arranged to separate dehydrated solids and to lead water treated in the decanter centrifuge back to the vertical separator or the slop tank; - the control valve is controlled by the control unit that, based on the input of the sensor continuously adjusts outlet of sludge to the decanter centrifuge. - a slop tank for water/oil/sludge, with an outlet for slop water; - a vertical separator comprising a vertical, substantially cylindrical tank having a tangential intake, an upper oil outlet at the top of the vertical tank, a middle, horizontal, tangential outlet for oil-polluted water, and a lower pipe for a sludge outlet at the bottom of the vertical tank, wherein an upwards-tapering conical frustum is provided in the interior of the vertical tank and the tangential outlet is at a lower end of the conical frustum; - a membrane filtration plant; - a control valve for the sludge outlet; - a sensor for sensing the density of the sludge in a lower portion of the vertical separator; - a control unit; wherein the water-oil-sludge separation plant is arranged in such a way that: - the slop water is directed from the outlet of the slop tank to the vertical separator and enters through a wall of the vertical separator and subsequently through the tangential intake, horizontally in the interior of the upwards-tapering conical frustum surrounded by the vertical tank, - water with oil exits via the top of the conical frustum, whereas the sludge is separated by the conical frustum and subsides in the interior down to the lower pipe, - oil and possibly air are led out through the upper oil outlet, - oil-polluted water continues to flow downwards, externally with respect to the conical frustum, to the tangential outlet and then to the membrane filtration plant; - the membrane filtration plant is arranged to let out the permeate comprising cleansed water and to lead the concentrated retentate back to the vertical separator or to the slop tank or to remove it from the process; - the slop water is directed from the outlet of the slop tank to the vertical separator and enters through a wall of the vertical separator and subsequently through the tangential intake, horizontally in the interior of the upwards-tapering conical frustum surrounded by the vertical tank, - water with oil exits via the top of the conical frustum, whereas the sludge is separated by the conical frustum and subsides in the interior down to the lower pipe, - oil and possibly air are led out through the upper oil outlet, - oil-polluted water continues to flow downwards, externally with respect to the conical frustum, to the tangential outlet and then to the membrane filtration plant; - the membrane filtration plant is arranged to let out the permeate comprising cleansed water and to lead the concentrated retentate back to the vertical separator or to the slop tank or to remove it from the process; wherein the water-oil-sludge separation plant is characterized in that: - it comprises a decanter centrifuge arranged to separate dehydrated solids and to lead water treated in the decanter centrifuge back to the vertical separator or the slop tank; - the control valve is controlled by the control unit that, based on the input of the sensor continuously adjusts outlet of sludge to the decanter centrifuge. - it comprises a decanter centrifuge arranged to separate dehydrated solids and to lead water treated in the decanter centrifuge back to the vertical separator or the slop tank; - the control valve is controlled by the control unit that, based on the input of the sensor continuously adjusts outlet of sludge to the decanter centrifuge.