Numerical modelling of oxygen mass transfer in diffused aeration systems: A CFD-PBM approach. (April 2021)

• Record Type: Journal Article
• Authors: Mohan .T, Reshma; Kumar, M.S. Mohan; Rao, Lakshminarayana
• Is Part Of: Journal of water process engineering. Volume 40(2021)
• Publication Date: 2021-04
• Subjects: Bioreactor -- Diffused aeration system -- Oxygen mass transfer -- Computational fluid dynamics -- Population balance model; Water-supply engineering -- Periodicals; Saline water conversion -- Periodicals; Seawater -- Distillation -- Periodicals; Sanitary engineering -- Periodicals; Sewage -- Purification -- Periodicals; 627
• DOI: 10.1016/j.jwpe.2021.101920
• Languages: English
• ISSNs: 2214-7144

Abstract

Highlights: Multiphase CFD model coupled with discrete PBM equation. Effective air bubble size reduces with height of water column. Bubbles have a wide distribution, with larger diameter near bottom. Bubbles have a narrow distribution with small bubble size at the top. Higher H/B ratio result in increased rate of DO saturation at fixed airflow rate. Abstract: The diffused aeration process is the most energy-intensive operation of bioreactor treatment, amounting to 45–75 % of the plant energy costs. To improve its efficiency, it is essential to measure the oxygen transfer rate from the aerators to wastewater. In this study, a multiphase mixture computational fluid dynamics (CFD) model is developed using k- ε turbulence closure equations along with a discrete population balance model (PBM) add-on with specific bubble classes, to predict the oxygen mass transfer. The transfer of oxygen species from air to water is modeled using the species transport model. The PBM is used to analyze the formation, growth, breakage, and coalescence of air bubbles. The validated model is then extended for sensitivity analysis for a diffused aeration system in a bench-scale aeration tank. Results show that, the volumetric oxygen mass transfer coefficient increases by 15 %, with a decrease of air bubble size by 10 %. The air bubbles have a wider distribution, with a larger diameter near the bottom of the bioreactor and a narrow distribution with a smaller bubble size at the top. Results show that, in the bioreactor, the dissolved oxygen concentration reaches the equilibrium or saturation value when the height by breadth ratio is 2.5 and does not increase further with increase in height of the water column. Also, the air bubble size of 6 mm was the efficient bubble size for a fixed airflow rate of 1.45 m 3 h −1 .