Date of Award

12-1-2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

RYOICHI S AMANO

Committee Members

DEYANG QU, J. RUDI STRICKLER, ISTVAN LAUKO, JOHN REISEL

Abstract

Wastewater treatment is considered one of the most common forms of pollution control in the united states. Considered as an integral part of the wastewater treatment, the aeration process is the most energy-consuming process among all the processes that take place in any wastewater treatment plant. According to the United States Environmental Protection Agency (EPA), a wastewater treatment plant is expected to remove at least 85% of the suspended solids and dissolved organic compounds from the wastewater before discharging it to a river or a lake. The normal operation of the aeration process is by compressing air continuously to basin diffusers for subsurface diffusion or agitate the water at the surface to create water droplets that can be mixed with atmospheric air. The goal of the aeration process is to bring the air in contact with the wastewater to provide the necessary oxygen for the biological flocculation and mixing in the wastewater. Aeration systems utilize subsurface diffusion employ diffusion equipment submerged at the bottom of water tanks called aeration diffusers. These diffusers introduce the air to the water in the aeration tank in the form of air bubbles. The rising of air bubbles in water is important for many engineering applications. This importance comes from the need to keep the oxygen in water at certain levels. When considering these applications, the first question that arises is how to increase the rate of oxygen transfer and make the process more efficient. In this study, it was shown that the extent of bubbles diffusion in water has a significant influence on the aeration efficiency. When rising in the water, these bubbles will convert the flow to rotational flow with high vorticity and circulation which will increase the mixing and the rate of oxygen transfer. Experimental and computational studies were conducted to obtain the standard oxygen transfer efficiency (SOTE), Standard aeration efficiency (SAE), vorticity, and circulation. The SOTE and SAE obtained from the experimental approach is considered as a measure of the aeration efficiency. While the vorticity and circulation from the computational fluid dynamics (CFD) as a measure of the extent of mixing in the aeration tank. The SOTE and SAE are considered important design parameters that can render the efficiency of the aeration process when studied experimentally. Experimental lab measurements to obtain the dissolved oxygen concentration under standard conditions were conducted at the UWM aeration lab using full scale aeration systems with all the required equipment are built and set up to complete the experimental measurements. High-speed camera with 2000 frames per second was also used for visualization of the bubble’s behavior. From both experimental and CFD results, it was proven that the method used to diffuse the air from sub-surface diffusion system can also alter the system efficiency. Normally, continuous air diffusion method is considered in typical aeration systems. A new method utilizes diffusing the air in pulsating order was proven to increase the aeration efficiency to up to 50%. When investigating for the effect of different pulsating times, it was obtained from the experimental results that when pulsating time is decreased to 0.5 S, the SOTE is increased. Also, experimental results of three water columns height showed that as water column height is increased, the SOTE is also increased. On the other hand, the computational (CFD) method was also implemented in this work. The CFD is a very useful tool in many applications and it can be used for modeling the aeration systems. Due to lab limitations to conduct experimental studies for large scale aeration systems (larger than 1.8 m), CFD method was used to investigate the effect of water column of 2, 2.5, and 3 m water columns. After solving the flow and mass transfer models, results of the dissolved oxygen evolution in time were processed to obtain the SOTE results, which showed the higher water column gives the higher SOTE. This is similar to the experimental study conducted for investigating the effect of water column height. Another study using the CFD method to investigate the effect of diffusion order considering inline and staggered orders showed that vorticity and circulation can render the effect of mixing on the rate of oxygen transfer and the potential to develop this method to consider more general cases without the need to evolve in time to obtain the dissolved oxygen time dependent profile and save a lot of computation time. In all experimental and CFD studies, the results were showing strong influence of the oxygen transfer rate on mixing and this conclude the consideration of increase the mixing in the aeration tank can lead to significant increase in the aeration efficiency and reducing the high energy cost characterized by the aeration systems.

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