Graduation Year

2021

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Civil and Environmental Engineering

Major Professor

Andrés Tejada-Martinez, Ph.D.

Co-Major Professor

Aydin Sunol, Ph.D.

Committee Member

Joel Ducoste, Ph.D.

Committee Member

Marian Hernández-Viera, Ph.D.

Committee Member

Mahmood Nachabe, Ph.D.

Keywords

Activated Sludge Model - 1, sanitation, sludge stratification, Fluent, multiphase

Abstract

An oxidation ditch is a biological wastewater treatment unit process where microorganisms are utilized to transform organic matter, and nitrogen from wastewater to meet the desirable effluent concentrations. Mechanical aerators are part of these biological reactors facilitating oxygen transfer for the aerobic microorganisms, while agitating and circulating the phase constituents within the ditch. Modeling the performance of an oxidation ditch involves combining hydrodynamics, oxygen mass transfer, and bio-kinetics and is one of the grand challenges that is of paramount importance in optimization of design and operations of these facilities in an energy efficient way while meeting the targeted effluent concentrations. Several computational fluid dynamics (CFD) models were developed and used to understand the hydrodynamics, flow patterns, and bio-kinetics for the design and operation of these oxidation ditches.This work developed a 3-D computational fluid dynamics (CFD) model of a full-scale oxidation ditch. The 3-D model was first used to investigate the impact of unsteady effects induced by variable aerator speed and time-dependent influent velocity on the hydrodynamics of the ditch. Subsequently, the CFD model is coupled with the Activated Sludge Model (ASM)-1, a popular bio-kinetics model, to provide a unique study that allow analysis of the hydrodynamics on the spatial and temporal distribution of the ASM-1 components, and ultimately the performance of the ditch. Simultaneously, two 1-D models were developed to determine extent which simplified models can be utilized. Lastly, a two-phase (wastewater-sludge) 3-D CFD model was developed to explore the effects of sludge stratification on the hydrodynamics and the temporal evolution and spatial distribution of ASM-1 components in the ditch. While the influent velocity does not greatly affect the hydrodynamics in the oxidation ditch, the aerator speed has a major impact on the hydrodynamics in the 3-D model. Additionally, the hydrodynamics in the oxidation ditch are mostly steady except for about an hour after adjusting the aerator speed. Due to the simplification of the flow, the 1-D model of the oxidation ditch was not able to predict spatial heterogeneities in dissolved oxygen observed in the 3-D model. As a result, the 1-D model was not able to predict comparable steady-state volume averaged concentration of soluble nitrate nitrite nitrogen (???) to the 3-D model. However, it did predict steady-state volume averaged concentration values of the other ASM-1 components in good agreement with the 3-D model. Furthermore, soluble ammonia ammonium nitrogen (???) and soluble nitrate nitrite nitrogen (???) were found to be more sensitive to changes to the volumetric mass transfer coefficient, ??? than readily biodegradable substrate (??) in both models. Finally, sludge stratification in the ditch had minimal impact on the mean flow spatial patterns and thus, the temporal evolution and spatial distribution of ASM-1 components. This was due to the small sludge volume fraction of the water-sludge mixture entering the ditch (0.004), relative (for example) to sludge volume fractions found in previous computational studies (0.02).

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