Analysis of activated sludge aerated by membrane and monolithic spargers with ultrafast X-ray tomography

Up to 80% of the total energy budget of wastewater treatment plants is consumed by the activated sludge process. Current optimizations are mostly based on limited instrumentation for single points of measurement which cannot express the complex hydrodynamic and biochemical processes. Therefore, the ultrafast electron beam X-ray tomography system ROFEX of HZDR is used as a new measurement technique to capture the temporal evolution of the multiphase flow in the opaque active sludge. A detailed study has been carried out in a vertical column of 3.5 m height at HZDR to obtain an improved understanding of the hydrodynamics of aerated sludge and an evaluation of different aerators. The target parameters are bubble size distribution, equivalent Sauter mean diameter of the bubbles, bubble rise velocity and local gas hold-up under the variation of sparger type (rubber, monolithic material), gas flow rate, rheology of the fluid (deionized water, salty water, sludge) and height in the liquid column. Therefore, in-house developed advanced image analysis algorithms were applied to the reconstructed tomographic images, which are also presented in the paper. The experiments showed that with ROFEX reliable measurement data of opaque multiphase flows is produced and is expected to be used in further investigations for the validation of computational fluid dynamics (CFD) models. The different sparger types showed comparable hydrodynamic performance.