Dissolved Air Flotation
Doktorsavhandling, 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.

two- and three-dimensional

Dissolved Air Flotation (DAF)

aggregation model

Computational Fluid Dynamics (CFD)

field study

surface forces

turbulence

single and multiphase flow

VG, Sven Hultins Gata 6, Chalmers tekniska högskola
Opponent: Dr Virginia Stovin

Författare

Mia Bondelind

Chalmers, Bygg- och miljöteknik, Vatten Miljö Teknik

Setting Up a Numerical Model of a DAF Tank: Turbulence, Geometry, and Bubble Size

Journal of Environmental Engineering, ASCE,; Vol. 136(2010)p. 1424-1434

Artikel i vetenskaplig tidskrift

Single- and two-phase numerical models of Dissolved Air Flotation: Comparison of 2D and 3D simulations

Colloids and Surfaces A: Physicochemical and Engineering Aspects,; Vol. 365(2010)p. 137-144

Artikel i vetenskaplig tidskrift

Dissolved Air Flotation (DAF) är en effektiv metod för att separera små partiklar från en vätska. Separationen utförs genom att man tillsätter mikroskopiska luftbubblor till vattnet. Luftbubblorna fastnar på partiklarna och bildar aggregat. Dessa aggregat har lägre densitet än vattnet och stiger därför upp mot vattenytan. På ytan bildas ett skum vilket kan skrapas bort. Det renade vattnet tas ut från botten av tanken. DAF används bland annat vid dricksvattenrening. En fältstudie visar att DAF kan fungera mycket bra, men att det kan vara svårt att finna orsaken till varför processen i vissa fall inte fungerar bra. DAF har visat sig vara utmanande att undersöka experimentellt då luftbubblorna stör mätutrustningen. Därför undersöker man nu möjligheterna att modellera processen numeriskt. I denna avhandling har DAF processen modellerats numeriskt och en modell som beskriver bildandet av aggregaten har tagits fram. Fältstudien har genomförts genom intervjuer av personal som arbetar med flotationsanläggningar vid dricksvattenverk i Sverige och Finland.

Dissolved air flotation is an efficient treatment process for separating small particles from a liquid. The separation is carried out by the addition of microscopic air bubbles to the liquid (in this case water). The air bubbles attach to the particles and form aggregates. The aggregates have lower density than the water and they will rise to the water surface. At the surface a foam layer develops which can be removed by scrapers. The clear water is removed from the bottom of the DAF unit. Dissolved Air Flotation is for example used for treating drinking water. A survey has demonstrated that DAF can efficiently remove particles such as algae; however, it can be challenging to detect the cause for a malfunctioning unit. The DAF process is challenging to examine experimentally due to the insertion of the measuring equipment into the flow and the turbidity caused by the dense air bubble concentration. Thus, the process is now examined by numerical modeling. In this thesis the process has been modeled numerically and a model describing the formation of the aggregates has been derived. The field study was carried out by interviewing personnel working closely with the flotation units at water treatment plants in Sweden and in Finland.

Styrkeområden

Building Futures

Ämneskategorier

Vattenteknik

Strömningsmekanik och akustik

Oceanografi, hydrologi, vattenresurser

Fundament

Grundläggande vetenskaper

ISBN

978-91-7385-574-7

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie

VG, Sven Hultins Gata 6, Chalmers tekniska högskola

Opponent: Dr Virginia Stovin