Author(s): Boyang Chen; Bruno Fraga

Linked Author(s):

Keywords: Ctivated sludge; Point-particle Eulerian-Lagrangian; Large-eddy simulation; Mass transfer; Three-phase flows

Abstract: In environmental engineering, wastewater must be collected and conveyed to a treatment facility to remove pollutants. With regards to energy consumption, the largest expense in wastewater treatment occurs in secondary treatment, particularly for activated sludge processes requiring a fundamental understanding in terms of hydrodynamics and biological kinetics. We develop a universal numerical tool to enhance and access the cost-effectiveness of real aeration systems in wastewater treatment. Aeration in an activated sludge process is based on pumping air into a tank, promoting the mixing and redistribution of the dissolved oxygen to facilitate the microbial growth. The microbial feeds on organic material, forming flocks which are normally called as sludge and can easily settle out in the wastewater. A well-designed aeration system is key to an effective and sustainable wastewater treatment. However, the design of these facilities often obeys to rules-of-thumb that usually lead to an excessive energy expense. A novel Eulerian-Lagrangian based on large-eddy simulation approach applicable to simulate activated sludge process is employed where the wastewater transport equations are resolved in Eulerian framework, suspension of solid particles and buoyant plume are represented as discrete Lagrangian particles. The algorithm is developed based on the in-house finite-difference-based large-eddy simulation code BubLPT. The solver uses a four-way coupling point-particle Eulerian-Lagrangian (PP-EL) model to couple dispersed and carrier phases. The interaction between particles is handled by a soft-sphere collision model. The transport of a passive tracer is simulated to visualize the redistribution of dissolved oxygen induced by the dispersed phase in carrier fluid. The biological reaction is modelled in the solver, including mas transfer between the three phases. The parallelization of the code is supported by a Message Passing Interface (MPI) implementation. The accuracy of the solver predicting mass transfer between the phases is validated versus laboratory measurement in a bubble column with five different gas flow rates and shown a well agreement with experimental data. Simulations of solid-gas-liquid three-phase flow are carried out for a real case of wastewater treatment plant in order to explore the impact of selected physical parameters (e.g., weather condition, blowers’ power and injected air bubble size) on oxygen uptake by sludge in aeration tank. The results show that weather condition has a significant influence on the performance of consuming and transferring dissolved oxygen in wastewater. The prediction also presents the comparison of bubble screens - at different flow rates – with discrete plume, exhibiting significant differences in energetic efficiency for different aerator setups. Ongoing work on the integration of sludge rheology in the activated process is being currently implemented.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022762

Year: 2022