Sludge Rising and Critical Time Prediction for Denitrification in Secondary Clarifiers: Experimental and Computational Fluid Dynamics Studies

In this study, we investigated the sludge rising process during denitrification, especially its hydrodynamics, with both experiments and computational fluid dynamics (CFD) modeling. The rising process was observed and divided into three stages: bubble-forming, sludge particles rising with isolated bubbles, and sludge blanket rising. A new solid phase denoted as lighter sludge was introduced in the CFD simulation to bridge the gap between the simulation and the reality. We simulated the three-phase (wastewater, normal activated sludge, and lighter sludge) turbulent flow in the secondary clarifier. CFD simulation agreed with the experiments and math-model results. Lighter sludge dominated the effluent quality during the denitrification process. Sludge blanket rising started at scattered points in the bottom hopper. A 30-min window occurred between the rising of the sludge blanket and the worsening of effluent quality, which could be regarded as a critical time to prevent sludge from rising.