Dissolved Air Flotation

Doctoral thesis, 2011

Dissolved Air Flotation (DAF) is a well-established treatment method for water containing low density particles. The density-driven process is operated by the injection of water saturated with air under high pressure. At the pressure reduction microscopic air bubbles are formed. The bubbles attach to the particles and the aggregates created rise to the surface of the unit where they are removed. Researchers working with DAF units struggle with disturbances in their measurements caused by air bubbles and the intrusion of the measuring equipment into the flow. In order to increase the knowledge and efficiency of the process researchers are now turning to Computational Fluid Dynamics (CFD). The aim of this thesis is to investigate the applicability to use CFD for modelling DAF. The work is carried out by examining single and multiphase, steady-state numerical models of a pilot tank where the flow is simulated with ANSYS Fluent. The options of making a two- or a three-dimensional model and the choice of turbulence and multiphase models are examined. The simulations are compared to experimental measurements for validation. A model describing the formation of the aggregates in the flotation unit is derived and a field study on flotation units in operation in Finland and Sweden is carried out. The work demonstrates that a single-phase, two-dimensional model can capture the flow in the separation zone reasonably well, but that a three-dimensional model is required if the flow in the contact zone is to be studied. A two-phase flow can be captured in a two-dimensional geometry if it carries the characteristics of a stratified flow, suggesting that a two-dimensional model should be used with caution. A three-dimensional, two-phase model will reflect the flow in a DAF tank more truthfully, but requires more computer capacity and results indicate that a transient solver is required. The implementation of the aggregate model demonstrates the creation and motion of the aggregates within the contact zone of the DAF unit. The field study proves the acceptance of the DAF process and indicates that a numerical model would be valuable for investigating the performance of a flotation unit in operation.