A process for forming a biofilm carrier suitable for use in a moving bed biofilm reactor (MBBR) is presented. A blowing agent is mixed with a plastic material to form a blended material. The blended material is then heated to a temperature where the blowing agent liberates gas by decomposition. The heated blended material is extruded through a die to provide the extrudate with a profile of the biofilm carrier. Advantageously, biofilm carriers produced in this manner have increased effectiveness when compared to conventional biofilm carriers.