This chapter argues that the added anaerobic zone promotes the typical fermentation reactions and the substrate uptake/phosphorus release process. It discusses the A2/O process recycles the RAS to the anaerobic zone. The A/O process, which stands for anaerobic/oxic, is patented and marketed by Air Products and Chemicals, Inc. The chapter suggests the biological phosphorus removal processes are all capable of substantial reductions in effluent phosphorus concentrations if operated under the proper conditions. It discusses the sizing of the stripper and reactor-clarifier. These components are typically supplied as part of the Phostrip package, either using prepackaged metal tankage, cast-in-place concrete basins, or existing available tankage volume. The chapter includes descriptions of several actual operating full-scale biological phosphorus removal plants and pilot-scale studies. The corresponding increase in nitrate could likewise adversely affect the anaerobic zone operation for phosphorus removal. Operating Five-Stage Bardenpho systems reportedly achieve total phosphorus concentrations in the effluent of 3 mg/L or less.