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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

DeGroot, Christopher T.

2nd Supervisor

Straatman, Anthony G.

Co-Supervisor

Abstract

Numerical modelling of wastewater management systems is crucial for investigating alternative designs and developing strategies for operation and control that improve the performance of the treatment stage (such as improving the aeration systems in activated sludge systems). Modelling can also help operators to mitigate common problems that arise from unwanted biochemical conversions in the sewer networks (such as the production of sulfide and methane). Therefore, this work focuses on two major areas related to modelling wastewater management systems. First, it seeks to develop more accurate models for aeration systems in activated sludge reactors. Second, it seeks to study the mathematical formulation and sensitivity/uncertainty related to input parameters for biochemical models of the sewer systems. Based on the uncertainty analysis of the sewer models, an improved biochemical model for the biological oxidation of sulfide using nitrate dosing is developed. This allows for the investigation of different dosing strategies and to propose optimized experimental plans for lab-scale experiments. A CFD model integrated with the population balance model (PBM) was developed to simulate the aeration of a bubble column of clean water operating in a homogeneous flow regime. This study aims at investigating the influence of the liquid phase flow field on the evolution of the bubble size distribution (BSD) from a fine pore air diffuser to the free surface of the water. Moreover, the local oxygen mass transfer rate is calculated based on the bubbles’ relative velocity and interfacial area, which is deduced from the modelled BSD, between the air bubbles and water. The model is validated by experimental data obtained from the literature. The validation is based on the BSD and the values of the oxygen mass transfer coefficient (KLa). A comparison between different PBM closure models for the bubble breakup and coalescence rates is conducted to determine the proper closure models for this flow regime. The study shows the influence of the flow field, especially near the free surface of the water, on the BSD and KLa. Moreover, a comparison is conducted with the simulation results of the constant bubble size (CBS) approximation to show the inaccuracy that accompanies this approximation. The results show that the widely used constant bubble iii size approximation can predict the global gas holdup reasonably, but poor matching with the oxygen mass transfer is obtained. Uncertainty analysis of the Wastewater Aerobic/Anaerobic Transformation in Sewers (WATS) biochemical model is conducted. The analysis is concerned with the uncertainty of the biochemical model parameters and its mathematical form. The WATS model is implemented in 1-D (CSTR-in-series) and CFD frameworks. The 1-D model is used to study the uncertainty/sensitivity analysis by the Monte Carlo technique, and standardized regression coefficients (SRC) are determined to quantify the importance of the different biochemical model parameters. The CFD model is used to study the influence of two assumptions that are used in the 1-D model; homogenization of the reactions that occur in the biofilm and neglecting the non-uniform distribution of the particulate matters due to the settling of solids. It is concluded that the 1-D model approximations are reasonable in the case of simple pressure mains. Modelling of a lab-scale experiment, intended to replicate the behaviour of a sewer pipe, is conducted to determine the optimal nitrate dosing strategy in the system. The WATS model is extended to include the biological oxidation of sulfide by nitrate dosing. The experiment is modelled as a series-of-CSTRs. The developed model is calibrated and validated using experimental data collected from the system. A dosing strategy is developed to be used in the planning of experiments.

Summary for Lay Audience

Wastewater management systems, which involve the collection system (sewer networks) and wastewater treatment plants, are critical for the protection of human health and the environment. Mathematical modelling of wastewater management systems is useful for predicting their performance. Through modelling, the design of different processes can be made more efficient or their performance can be improved. The biological and chemical conversions that happen in these systems can be beneficial, such as the reactions that occur in activated sludge reactors, which help to degrade waste organic material before it is discharged into the environment. On the other hand, these conversions could be harmful to humans and the environment, for example to the formation of sulfide and methane in sewer systems. Therefore, mathematical modelling is crucial to study the different sections of the wastewater management system and is more efficient than conducting expensive and time-consuming experiments and measurement campaigns in full-scale utilities. However, the most common mathematical models in this field rely on approximations that are necessary to simplify the complexity of the real world. These approximations could carry high uncertainty that may affect the accuracy of the model predictions which could subsequently result in wasted resources or substandard designs. Therefore, more sophisticated mathematical models such as computational fluid dynamics (CFD) could be used either directly in the design and operation process or indirectly by verifying the more simplified models. This doctoral thesis is divided into two parts. The first part is concerned with a common approximation used in modelling the aeration of the biological treatment reactors where either the oxygen transfer rate from air bubbles is assumed to be homogeneous (uniform) throughout the reactor or a non-uniform distribution is considered but with the assumption of all the bubbles have the same size. This approximation is studied, and an alternative model based on CFD framework integrated with a statistical model is proposed. The study shows that the simplified models cannot address the strong influence of the water flow field and the evolution of the bubble size on the oxygen mass transfer rate. In the second part, uncertainty/sensitivity analysis is conducted on the widely used WATS biochemical model for the reactions in sewer systems. The biochemical model is also implemented in a CFD framework to study the approximations of the 1-D models. The studied assumptions are the homogenization of the reactions that occur in the biofilm only and neglecting the settling of v particulate matter. It is concluded from this investigation that the 1-D model can predict the biological and chemical conversion satisfactorily and the approximations are valid to be used in the case of simple pressure mains. Therefore, the 1-D framework is used to implement the WATS model that was extended to include one of the most common control strategies for sulfide levels in the sewer system, which involves nitrate dosing to stimulate a certain population of microorganisms that exist in the biofilm to oxidize the produced sulfide biologically. The developed model is used to determine the optimal dosing strategy that will be followed in a lab-scale experiment.

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Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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