Abstract
Dissolved air flotation (DAF) is an important flotation process in the production of potable water. During its use over the past 30 years, largely empirical data has been used to determine operational procedure. Some efforts have been made to construct numerical simulations of the DAF process by previous authors and these have been reviewed. This project adds to the body of knowledge surrounding the modelling of DAF by providing detailed experimental and simulated data. The work focuses on the flotation zone of a DAF tank where flow behaviour plays an important role in DAF tank performance. The model DAF tank used for experiments was constructed entirely of Perspex, allowing the use of non-intrusive, laser-based methods of flow measurement. Experimental data was used to verify the performance of computational fluid dynamic (CFD) simulations. Significant changes in flow behaviour were observed even at the lowest recycle ratio of 0. 6%; recycle ratio is related to volume fraction of air at the inlet for air-saturated water. As recycle ratio increased, the tendency was for the flow behaviour to change from a re-circulation-type current into a 'divided flow' where two distinct sub-paths emerged. This behaviour then changed further as the recycle ratio was increased. In the case of high main inlet flow, strong downward currents were observed which would be detrimental to DAF tank performance. At low-to-medium main inlet flow, these strong downward flows were not observed, and instead a 'stratified flow' was observed - this extends the residence time within the tank and is preferable in terms of performance. Interesting observations were made concerning the correlation between experimental and simulated results using FLUENT CFD code. The use of the more recent version 6. 0, which allowed more control over the specification of air release boundary conditions compared with version 4. 5, gave benefits in the degree of correlation. Further work using version 6. 0 should aim to improve the simulation of air dispersion, which was found to differ significantly from experimental observation. Water treatment consumes large quantities of electrical energy. The main energyconsuming step within the DAF process is the production and delivery of air-saturated water to the DAF tank. By careful design, the required quantity of air (expressed as the recycle ratio) may be reduced. Simulation by CFD, backed up by verification experiments, is one important tool to achieve this saving.