This project utilizes hydrocyclones in wastewater treatment to select for heavier solids, and has been used before in multiple small-scale systems. This is the first implementation of hydrocyclones in a full-scale plant for the purpose of increased settleability, while also achieving enhanced biological phosphorus removal without the use of an anaerobic selector. Hydrocyclones receive mixed liquor tangentially and separate light solids from more dense solids through their tapered shape, increasing the velocity of liquid as it moves downward and allowing for selection of a certain solids fraction. The hydrocyclones receive flow from the waste stream, selecting for dense solids to recycle through the process while light solids are wasted, creating a balance of granules and flocs with superior settling characteristics in which phosphorus is removed through phosphorus accumulating organisms (PAO). This project was implemented at a wastewater treatment plant rated at 20 MGD utilizing a 4-stage Bardenpho configuration with an IFAS system. This plant routinely experienced moderate settleability issues with an average SVI of 141 and a 90th percentile SVI of 179. Over time data was collected to characterize settleability and activity of PAO, GAO, and filaments. Using an external selector to achieve biological phosphorus is significant in that most wastewater treatment plants cannot do this without the use of an anaerobic selector. This has the potential to apply external selectors to existing infrastructure throughout plants worldwide to achieve not only biological phosphorus removal, but also improved settleability with a very minor capital investment. / Master of Science