Impact of sludge density and viscosity on continuous stirred tank reactor performance in wastewater treatment by numerical modelling

Background: Wastewater treatment is crucial for maintaining a sustainable environment and managing water resources. Unprocessed wastewater contains pollutants that damage ecosystems and trigger eutrophication. The continuous stirred tank reactor plays a pivotal role in wastewater treatment due to its effective mixing, stability, and versatility. In this study, a multiphase CSTR model was developed using a numerical scheme. Methods: The Reynolds-Averaged Navier-Stokes model was utilized for turbulence, considering two phases: water-liquid as the primary phase (Phase-1) and sludge as the secondary phase (Phase-2). Computational fluid dynamics facilitated the creation of an Eulerian model using a dispersed particle distribution approach. Four CSTR variations were explored by altering sludge density, ranging from 1200 kg/m(3) to 1400 kg/m(3). Significant Findings: Adaptive mesh refinement improved result reliability and sludge dispersion near the tank walls. Mesh refinement near to the impeller is high to capture the vortices magnitude and the turbulence kinetic energy. The study highlights the pivotal role of the relationship between wastewater and microorganisms in effective treatment. Optimizing sludge density and viscosity enhances CSTR mixing efficiency. High sludge density contributes to dead zone formation within the CSTR, while increased viscosity affects overall performance. Increasing the sludge density affects the turbulent viscosity of the operating fluids. Added to above, there is significant change in the velocity distribution for the phase 2. Insights from this research contribute to understanding CSTR performance linked to impeller speed, sludge density, and viscosity for sludge of the wastewater.