Applications of computational fluid dynamics modelling in water treatment (2005)

Funded by: Water Research Commission

Partner organisations: Partners in Development, eThekwini Water and Sanitation

Project description
Almost all water and wastewater treatment equipment rely on continuous through flow of water. Some equipment requires this flow to be well-mixed, whereas other equipment requires plug-flow. Examples of well-mixed systems are activated sludge plants, chemical dosing zones and anaerobic digesters while sand filters (in both filtration and back-wash modes), clarifiers, adsorption columns (ozone, activated carbon and ion exchange) and dissolved air flotation cells are examples of the plug-flow systems. Some processes such as nutrient removal activated sludge plants require the combination of both plug flow and
completely mixed reaction zones.

The laboratory-scale experiments that are used to obtain design data for a plant are usually operated under ideal flow conditions; unfortunately it is usually not feasible to carry this through to full-scale plants, due to the greater difficulties and expense of achieving similar ideal conditions on a large scale. The complexity of the flow patterns, and the uncertainties about how they affect the relevant performance indicators for the process involved have led designers of equipment to use safety factors based on experience to ensure that the process achieves its required objectives. This means that equipment that is installed is often larger and more expensive that it needs to be.

Computational fluid dynamics (CFD) is a numerical procedure to calculate the properties of moving fluid. Most water treatment processes involve the movement of water. This motion is often complex and difficult or very expensive to observe. The prediction of the flow patterns and other properties of flowing fluids would provide insight into processes which otherwise would not have been possible. A previous WRC project (No 648) indicated the value of CFD modelling of clarifiers and an anaerobic compartment. It was able to both logically explain the unexpected behaviour of the clarifier and in designing features to modify the undesirable flow pattern.

Apart from its use in design of water treatment equipment, CFD modelling can also assist in research into water treatment processes. The project on which this report was based was unusual in that it was initiated to provide a CFD modelling service to assist water researchers who felt that it could enhance their investigations. As a result the project did not have a specific research focus of its own, but adapted to the objectives of each research project that it became involved with. Furthermore, not all the collaborations that were started were fruitful. The main content of the report is a series of case studies, each corresponding to a different investigation. To give the report some thematic consistency, it has been compiled with a view to illustrating the kinds of situations where CFD modelling is useful. To this end, the case studies that were selected for inclusion in the report are those which best fitted this purpose, i.e. they each involved an appropriate application of CFD, and yielded some useful conclusions. Various investigations which for one reason or another did not fit these criteria have been left out. The investigations which are presented have also been cleaned up to reflect a logical development which did not always take place in reality; i.e. the various misapplications, misconceptions and dead ends which occurred along the way have been removed from the narratives.

Project Objectives
  • To provide a service to water researchers by undertaking modelling exercises on proposed and existing equipment so that they may design more efficient experimental equipment and to better understand their experimental results.
  • To promote the use of computational fluid mechanics by water authorities, consultants and water researchers.
  • To model wastewater treatment secondary settling tanks.
  • To assist in the training of academics and students in the practical use of computational fluid mechanics.