the present invention provides apparatus and a process for manufacturing a bio - oxidation and nitrification module useful in treating sewage and industrial wastewater . paired thermoplastic flat sheet and corrugated sheet are nested in a cradle and advanced beneath a bank of ultrasonic welders to produce a sonic weld along those interfacial zones of the sheets which are in contact . welded units of flat and corrugated sheets are stacked to a height equal to the height of the module and the edges of opposite sides of the stack are softened by heated platens to form an edge - seam which bonds the welded units together to form the finished module .