Impact of rheology on mass transfer and bubble diameter in wastewater treatment systems

Mixing and mass transfer are critical to achieve the desired throughput in gas-liquid interactions and thus biological conversion efficiencies. Both mixing and mass transfer are significantly dependent on the size of the gas bubble. The non-Newtonian characteristics of wastewater result in a highly dynamic behaviour of these gas bubbles throughout the reactor volume. While studies so far have been conducted on gas-liquid hydrodynamic behaviour, the impact of wastewater rheology on gas bubble diameter and their population has been overlooked, leading to uncertainty in mass transfer rate prediction. To address this gap, we developed both computational, using coupled computational fluid dynamics (CFD) and population balance model (PBM), and experimental approaches, using high-speed imaging techniques. Unlike existing studies, these approaches were deployed on an industry representative system- a pilot-scale venturi mixed reactor- commonly applied for mixing and mass transfer in both low-viscosity Newtonian and high (and shear-dependent) viscosity non-Newtonian liquids. An excellent agreement was observed between the numerical and experimental results of bubble diameter and population, demonstrating a diverse range of bubbles resulting from bubble coalescence and breakage within the venturi and reactor in Newtonian and non-Newtonian liquids. The results identified that volumetric mass transfer rate (k L a) is highly variable throughout the reactor due to variations in bubble size. A larger average bubble size with higher viscosity results in a substantial decrease in k L a from 21 day − 1 (for water viscosity) to 3 day − 1 at higher viscosity, despite an increased gas holdup. • Validated CFD-PBM model for bubble dynamics in wastewater. • Non-Newtonian rheology reduces kLa by > 80%. • Bubble coalescence and breakup govern mass transfer reduction. • Venturi-ejection system enhances gas-liquid contact. • kLa varies substantially spatially.