Excess nutrients in lakes, rivers, and streams harm aquatic ecosystems.y Advanced treatment at wastewater treatment plants can reduce this pollutant load.y An innovative nutrient removal system, ANAMMOX, anaerobically converts ammonium and nitrite to N₂ gas.y Because this system requires a 1:1.31 NH₄⁺ : NO₂^– ratio, a partial nitrification system is coupled with the ANAMMOX system to provide the necessary substrates.y A hollow fiber membrane reactor was employed to create this partial nitrification system.y Gas mass transfer was first evaluated to determine the K_La oxygen transfer coefficient. Reactor parameters, such as mixing speed, membrane length, and gas pressure, were evaluated to determine how K_La was affected by these variables.y This hollow fiber membrane was then used to control oxygen delivery to the partial nitrification system, producing an effluent with ammonium and nitrite.y Controlling dissolved oxygen at low concentration selects for ammonium oxidizing bacteria, while nitrite oxidizing bacteria are suppressed.y This system effectively treated influent ammonium concentrations ranging from 50 mg/L NH₄⁺–N to 250 mg/L NH₄⁺–N.y Real–time polymerase chain reaction (qPCR) was employed to verify the system preference for ammonium oxidizing bacteria.y Hollow fiber membrane aeration is effective forcontrolling oxygen transfer.