As part of the Tahuna Wastewater Treatment Plant Stage 2 Upgrade, Pump Systems Ltd was awarded a contract to design and supply two Rotamix sludge mixing systems to mix one 1,000 m3 Thickened Waste Activated Sludge (TWAS) Storage Tank and one 300 m3 Pre-Gravity Belt Thickener (GBT) Flow Balance Tank. The new process at the plant meant that sludge levels in each tank would continuously vary throughout the day. To accommodate this, extra-low-profile Rotamix nozzles were selected and variable speed drives recommended to control the flow output from each Vaughan mixing pump. Detailed CFD simulations carried out prior to installation predicted excellent mixing of both tanks. The results showed that the mixing systems would achieve active volume mixing rates of between 91.5 and 97.8 percent, surpassing Vaughan Co.'s requirement of 90% or greater active volume mixing for all systems.