Integrated RTD − CFD Hydrodynamic Analysis for Performance Assessment of Activated Sludge Reactors

This work shows an integrated analysis method for hydrodynamic investigation of biological reactors for wastewater treatment in order to detect the amount and position of possible defects, such as bypass and dead volume, influencing process efficiency. To reach such a goal, the proposed methodology integrates Residence Time Distribution (RTD) analysis, providing global hydrodynamic information, with Computational Fluid Dynamics (CFD) analysis, showing local flow conditions. RTD analysis was performed through a timediscretized analytical model of in-series mixed-flow reactors with dead volumes and bypass. CFD analysis was carried out with a 3D finite volume model allowing the numerical solution of turbulent incompressible isothermal flow. The method was tested on a scale activated sludge pilot plant with pre-denitrification scheme made of two in-series tanks. Hydrodynamic tests were performed carrying out the stimulus-response experiment using clear water inside the reactor and lithium chloride as a tracer. Two operating conditions of practical interest were investigated: (i) no mixing and (ii) upstream mixing. The RTD analysis of the outflow curve of lithium concentration from the experiment allowed detecting: a) no bypass for both operating conditions, and b) 5% dead volume for condition (i). These results were confirmed by the CFD analysis that allowed localizing the position of the dead volume. The integrated analysis proved to be effective for detection of both types and position of hydrodynamic defects. Therefore the proposed method can be adopted for performance assessment of activated sludge reactors and subsequent improvement of their efficiency.