Computational fluid dynamics tools for the design of mixed anoxic wastewater treatment vessels

Brannock, Matthew William Darcey (2003). Computational fluid dynamics tools for the design of mixed anoxic wastewater treatment vessels PhD Thesis, School of Engineering, The University of Queensland.

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Author Brannock, Matthew William Darcey
Thesis Title Computational fluid dynamics tools for the design of mixed anoxic wastewater treatment vessels
School, Centre or Institute School of Engineering
Institution The University of Queensland
Publication date 2003-08
Thesis type PhD Thesis
Supervisor Dr Tony Howes
Adjunct Prof Mike Johns
A/Prof Jurg
Total pages 181
Collection year 2003
Language eng
Subjects L
290700 Resources Engineering
760200 Environmental and Resource Evaluation
Formatted abstract Wastewater pollutants represent a threat to both aquatic and terrestrial environs. In river systems, and other aquatic environs, high concentrations of carbon and nutrient compounds from sewage can cause serious degradation, depleting the water of oxygen as well as promoting algal blooms. If our environment is to be conserved, the efficient and economic removal of these pollutants is a problem that needs to be addressed.

The performance of the wastewater removal process is strongly related to the design and operation of the wastewater facility. Most current methods for design of treatment vessels are based on empirical and heuristic techniques, and they cannot adequately predict how vessel configuration, such as the size and position of inlets, baffles or mixers, affects the hydrodynamics and the overall performance. Computational Fluid Dynamics (CFD) can provide a method for simulation of different designs to predict the effect on performance.

The biological reactions that are used to remove pollutants from the wastewater occur primarily within, what can conceptually be thought as, bio-catalytic particles. These particles suspended in the liquid form 'sludge'. The reactor design, through the hydrodynamics, influences the movement of the sludge and reactant species (pollutants) through the vessel and this determines the removal of pollutants from wastewater. The CFD model developed here, incorporates the three-dimensional hydrodynamics, sludge and species transport. The sludge transport has a slip velocity in the vertical direction to account for settling, and the density gradients it forms, which influence the hydrodynamics. The species that are transported in this system are soluble substrate (a carbon source) and nitrate, while the source and sink terms are the biochemical reactions relying on the local concentration of sludge. The biological reactions are derived from the Activated Sludge Model Number 1 (Henze et al 1987).

Experimental measurements were made on an existing system and were used for calibration and validation of the model. The system used for this purpose is an anoxic section of a bioreactor located at the Luggage Point Wastewater Treatment Facility, Brisbane, Australia. In this part of the reactor nitrate is biologically converted to nitrogen gas, through consumption of soluble substrate species. Calibration included the setting of the velocity profile, turbulence parameters and sludge concentration at the inlets with parameters of the phenomenological settling model. Validation included comparison of point velocities, predicted velocity profiles and residence time distribution curves. The velocity comparisons demonstrated the difficulty of its use in validation but the CFD model was generally able to predict experimentally measured data. The residence time distribution of the vessel examined was predicted very well by the CFD model, with a correlation coefficient of above 0.97.

Once the model was calibrated and validated it allowed the investigation the effect of various reactor modifications (baffle number and positions, mixer number, position and direction). Scenarios of differing internal configurations were developed to examine these effects, where performance measures were developed and applied to investigate each scenario. These investigations show that a flow type closer to plug flow gave greater removal of pollutants. The design modifications that aided in the producing this good flow type were the even distribution of mixers and baffles along the reactor length to induce several well mixed zones in series (approximating plug flow) and the redirection of mixers against the flow thus aiding in creating these well mixed zones.

This demonstrated that CFD along, in conjunction with the design methodology, experimental calibration and validation techniques and the performance measures provided, is a valuable design tool. The CFD tools developed provide an improvement to our fundamental understanding of mixed wastewater treatment vessels and fill a gap in the current state of understanding of this field of knowledge.

Keyword Sewage disposal plants
Fluid dynamics
Additional Notes

Variant title: CFD tool for design of mixed anoxic WWT vessels

Document type: Thesis
Collection: UQ Theses Collection (non-RHD) - Open Access
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Created: Fri, 24 Aug 2007, 18:16:37 EST