Aerobic integrated fixed-film activated sludge (IFAS) and moving bed biofilm reactors (MBBRs) use an engineered aeration system consisting of stainless-steel pipe diffusers, manifold (or submerged air header), down pipes, and manually operated air-flow control valves. The so-called medium-bubble diffuser has a large diameter (i.e., 4-mm) orifice that is situated along the underside of the stainless steel pipe diffuser. Therefore, medium-bubble diffusers are not as susceptible to scaling and fouling as fine-bubble diffusers, and operational experience has proven that medium-bubble diffusers require significantly less maintenance than fine-bubble diffusers. The oxygen transfer efficiency (OTE) of a medium-bubble diffuser is better than that of a typical coarse-bubble diffuser. The presence of free moving plastic biofilm carriers in a bioreactor improves OTE. Unfortunately, there is not a generally accepted design criteria for the aeration systems that are used in aerobic IFAS zones and MBBRs. This paper will describe a method for designing a state-of-the-art aeration system for aerobic IFAS zones and MBBRs. The methodology presented in this paper may be used to design aeration systems typical of state-of-the-art aerobic free moving plastic biofilm carrier-based reactors.