Abstract: 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.