A water treatment vessel for removing immiscible fluid and solid contaminants from contaminated water and producing clean water, having - a plurality of chambers (A, B, C, D) with at least one treatment chamber (A, B, C) and a last chamber interconnected in series, with a baffle wall separating adjacent chambers, each chamber comprising a central void providing a dynamic residence time for separation of gas, liquid and solid phases; - an injection line in fluid communication with an inlet of the central void of a first chamber of the plurality of chambers (A, B, C, D) for passing contaminated water to the central void of the first chamber, said injection line having a bubble generation means arranged to inject gas bubbles into the injection line; - a clean water outlet pipe extending from the central void of the last chamber for producing the clean water; - an inlet weir mounted within each chamber (A,B,C,D) and arranged adjacent the inlet for deflecting the inlet fluid in an upwardly direction; - an immiscible fluid weir mounted within each treatment chamber (A, B, C) and spaced from the inlet weir; a trough for collecting immiscible fluid flowing over said immiscible fluid weir, said trough allowing the immiscible fluid to flow out of each chamber through an immiscible fluid outlet; characterized by: a plurality of vertical riser conduits within each treatment chamber (A, B, C), each riser conduit having a lower receiving end adjacent a bottom of each treatment chamber (A, B, C) for receiving entry of contaminated water and an upper outlet end in fluid communication with the inlet of an adjacent downstream chamber in the series; and a bubble conduit connected on one end to a gas bubble stream source and on the other end to a lateral header extending across the plurality of vertical riser conduits adapted to inject gas bubbles into said lower portion of said vertical riser conduits. - a plurality of chambers (A, B, C, D) with at least one treatment chamber (A, B, C) and a last chamber interconnected in series, with a baffle wall separating adjacent chambers, each chamber comprising a central void providing a dynamic residence time for separation of gas, liquid and solid phases; - an injection line in fluid communication with an inlet of the central void of a first chamber of the plurality of chambers (A, B, C, D) for passing contaminated water to the central void of the first chamber, said injection line having a bubble generation means arranged to inject gas bubbles into the injection line; - a clean water outlet pipe extending from the central void of the last chamber for producing the clean water; - an inlet weir mounted within each chamber (A,B,C,D) and arranged adjacent the inlet for deflecting the inlet fluid in an upwardly direction; - an immiscible fluid weir mounted within each treatment chamber (A, B, C) and spaced from the inlet weir; a plurality of vertical riser conduits within each treatment chamber (A, B, C), each riser conduit having a lower receiving end adjacent a bottom of each treatment chamber (A, B, C) for receiving entry of contaminated water and an upper outlet end in fluid communication with the inlet of an adjacent downstream chamber in the series; and a bubble conduit connected on one end to a gas bubble stream source and on the other end to a lateral header extending across the plurality of vertical riser conduits adapted to inject gas bubbles into said lower portion of said vertical riser conduits.