“…However, as the suspended phase is neglected in this approach, the use of neutral-density models may lead to over-prediction of the degree of mixing when translating the CFD results directly to fullscale agitated AS systems operating at high mixed liquor suspended solids (MLSS) concentrations, and where the occurrence of regions characterized by low liquid velocities has been identified. Therefore, work has been undertaken on the development of numerical tools based on a Eulerian twofluid model, which is able to assess the impact of the gas-liquid hydrodynamics on the global and local mass transfer coefficients in pilot-and full-scale oxidation ditches (one of several well-known modifications to the AS process) aerated with diffusers and agitated with slowspeed mixers (Cockx, Do-Quang, Audic, Liné, & Roustan, 2001;Do-Quang, Cockx, Liné, & Roustan, 1998;Fayolle, Cockx, Gillot, Roustan, & Héduit, 2007). The results from the simulations based on the fixed bubble size show that the gas holdup and oxygen transfer in closed-loop AS tanks depend on the axial velocity of the fluid imparted by the mixers.…”