A method for treating wastewater containing organic contaminants is disclosed. Wastewater containing organic contaminants is fed into an outer pipe of a pipe-in-pipe assembly wherein the outer pipe concentrically surrounds an inner pipe. Oxygen is fed into the inner pipe which is rotatably mounted and is provided with openings thereby to provide different sizes of oxygen bubbles to the outer pipe. The oxygen is dispersed into an annular portion between the outer pipe and the inner pipe thereby contacting the wastewater with oxygen; and the thus treated wastewater is collected. The inner pipe may be a tunable membrane material and the outer pipe may have a biocatalyst material present on its inner surface.