Designing Aeration Systems using Baseline Mass Transfer Coefficients

For Water and Wastewater Treatment

Johnny Lee

• Publisher: CRC Press
• Year: 2021
• ISBN:
  9781000366808
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Environmental Management
• Index: Technology & Engineering

The book is about the discovery of a Standard Specific Baseline Mass Transfer Coefficient (KLa0)20 that represents a revolutionary change in the understanding, designing, and operation of aeration equipment, as well as providing a baseline for future research and development for water and wastewater treatment systems. It discusses the use of the Standard Model for oxygen transfer to determine the baseline, and its major finding is to show that the gas transfer model is a consistent relativistic theory of molecular interactions. Previously, the challenge was the appearance of divergences in the mass transfer coefficient estimations that defies aeration design. This normalization to a baseline is a great achievement in physics and engineering.

1

Prologue

The book is divided into eight chapters. Chapter 2 below deals with the derivation of the 5th power model for temperature correction. Chapter 3 deals with the development of the model to determine the baseline mass transfer coefficients in aeration tanks. Chapter 4 is dedicated to the derivation and theoretical development of the Lee-Baillod model on which the subsequent depth correction model is based. Chapter 5 illustrates the functionality of the Baseline Mass Transfer Coefficient and Interpretation of Non-steady State Submerged Bubble Oxygen Transfer Data. Chapter 6 asks the question if Oxygen Transfer Rate (OTR) in Submerged Bubble Aeration is affected by the Oxygen Uptake Rate (OUR), concerning the use of the baseline for in-process field working conditions? Chapter 7 recommends further research to elucidate the question posed in Chapter 6, and Chapter 8 is the Epilogue that summarizes all the core findings. It is expected that this book would serve practitioners in the designing of aeration systems, as well as serve as Standard Guidelines for water and wastewater (both In-Process and non-In-Process) oxygen transfer testing, enhancing the current standards and guidelines, ASCE 2-06 [ASCE 2007] and ASCE-18-96 [ASCE 1997]

The US EPA in the 70s poured in substantial amount of money to fund fundamental research as they recognized the importance of the connection between clean water tests and wastewater tests. Although they have made substantial progress, the fundamental question of relating clean water and wastewater tests remains unresolved [Mahendraker et al. 20051. A new revolutionary finding may revive their interest.

This book is focused primarily on submerged bubble aeration. In aeration systems, diffused air is a simple concept which entails pumping (injecting) air through a pipe or tubing and releasing this air through a diffuser below the water’s surface. The submerged system has little visible pattern on the surface, and is able to operate in depths up to and exceeding 12 m (40 ft). The best aerators use quiet on-shore compressors that pump air to diffusers placed at a pond or tank bottom. From stone diffusers to self-cleaning dome diffusers, they release oxygen throughout the water column creating mass circulation that mixes bottom and top water layers, breaks up thermal stratification, and replenishes dissolved oxygen through molecular oxygen mass transfer by means of gas diffusion. Gas transfer is the exchange of gases between aqueous and gaseous phases. In a diffused aeration, gas exchange takes place at the interface between submerged air bubbles and their surrounding water. According to Lewis and Whitman (1924), these bubbles are each wrapped with two layers of films through which the gas must go through. The transfer rate is usually expressed by a mass transfer coefficient symbolized by K_La.

No one has seen the two films around a bubble, let alone measuring the thicknesses of these films based on which K_La can be quantified. The coefficient can only be determined by an indirect method, such as the one used by the current ASCE standard (ASCE 2007). The transfer rate can also be determined by mass balances-the gas depletion rate from the bubble must equal the oxygen uptake rate in the liquid. This concept of gas-side oxygen depletion is not as readily understood as it may seem:

The respiration determines Oxygen Uptake Rate (OUR) that equates to the Oxygen Transfer Rate (OTR) at steady-state. The understanding that “the respiration determines the OUR which is then matched by the oxygen transfer rate” concurs with my thesis in this book, and indeed is correct. But in submerged aeration, there is the phenomenon known as gas-side oxygen depletion, so that the oxygen transfer rate is affected by this effect and this effect (incorrectly) changes the value of K_La. To make the correction, the OTR is therefore given by K_La_f(C*_∞f— c)— R (where f means “under field conditions”) under the principle of superposition in physics (this concept is further explained in Chapter 6). This is then matched by the oxygen transfer rate OTR at steady-state, which is equal to respiration rate R.

Therefore,

$K_L{a}_f\left(C_{\infty }^{*}-c\right)-R=R$

Although R is not part of the oxygen transfer mechanism, gas-side oxygen depletion is. If R is non-variant within the test period, then it can be determined in a gas flow steady state, where R is matched by the gas depletion rate in the bubbles which affects the value of the OTR. This has led to the above equation where K_La_f is understood to be (alpha. K_La), where alpha is a function of the wastewater characteristics only. The current alpha as used in the conventional model treats it as a lumped parameter that envelops both effects (water characteristics and gas depletion), making it a highly variable parameter that is indeterminate. The concept of gas-side depletion of oxygen from air bubbles, at first glance, appears to be simple and straightforward, but is in fact less readily understood than it may seem. In ordinary air bubble aeration, the OTE is typically 10 ~ 20%, since oxygen gas is only slightly soluble in water. (In clean water, it can be as much as 40% depending on the aeration device and the mixing intensity). This 10 ~ 20% by weight is the actual amount of oxygen successfully being transferred to the liquid. This quantity is exactly equal to the quantity of gas depleted from the air bubbles, since ‘oxygen transfer’ and ‘gas-side gas depletion’ are one and the same.

In fact, gas-side gas depletion is the only means of oxygen transfer for bubbles rising to the water surface, when any other means of transfer such as the free water surface or the bubble formation at the diffuser are negligible. Therefore, any modeling of oxygen transfer into any liquid (tap water, sewage, industrial wastes, etc.) must include the gas depletion effect, otherwise, the model cannot be valid.

In the review paper by Uby (2019), in section 6.1, it was stated: “Among the CEN and DWA standard test methods, no result dependence on initial conditions (supersaturation or depletion of oxygen and nitrogen) has been detected (Wagner and Popel 1996), but a rigorous uncertainty analysis is lacking. This has been fully accepted in German and European practice (DWA 2007, CEN 2003). Gas side depletion of oxygen from air bubbles has been shown to be a minor concern under common conditions (Brown and Baillod 1982, Jiang and Stenstrom 2012), corroborating this approach bof Lars’ paper]. In the interest of standardization and uncertainty quantification, the difference should be quantified, though experience speaks for at most a minor impact.”

In my opinion, the above understanding by Lars (as well as the various standards) is absolutely incorrect. It should be taken only in the context of the results of a clean water test, where the parameters C_s (saturation value) and K_La (mass transfer coefficient) are estimated. The reason why no result dependence on initial conditions (supersaturation or depletion of oxygen and nitrogen) has been detected is because these effects have already been absorbed in the Standard Model. In other words, the calculated results of the two parameters have already included any dependence on these effects, even though such dependence is not detected. In the application of clean water results to sewage or other liquid, these effects will change in accordance with changes in the gas depletion rate which is the same as changes in oxygen transfer rate under a changed environment. The bacterial and other microbial composition and their metabolic functioning, in particular, constitute drastic changes in the oxygen gas depletion rate which then drastically affects the value of the gas transfer parameters, in particular the mass transfer coefficient K_La. Even if all means of standardization and uncertainty analyses are carried out, they will not significantly improve the clean water test results. On the other hand, if clean water test results are to be translated to other fluids with significant microbial cell content, mixed liquor for example, then the principle of superposition must be applied to the Standard Model to take into account this all-important gas depletion effect in diffused aeration, without which nothing in terms of oxygen transfer happens. Therefore, gas-side gas depletion is not a minor impact, but in fact is the ONLY impact in submerged diffused-air bubble aeration, the magnitude of which is a function of a myriad of variables.

The amount of gas depleted from the bubble at any time not only depends on the films, but also on the path taken by the bubble that follows a gas depletion curve which is a function of many variables. This curve would vary with different heights and depths. Also, this gas depletion curve in clean water is substantially different from that in wastewater. The loss rate of gas from the bubble is the gain rate of transfer in the case of clean water aeration at any time and place inside an aeration tank.

Given that the mass transfer coefficient (K_La) is a function of many variables, in order to have a unified test result, it is necessary to create a baseline mass transfer coefficient, so that all tests will have the same measured baseline. K_La is found to be an exponential function of this new coefficient and is dependent on the height of the liquid column (Z_a) through which the gas flow stream passes. DeMoyer et al. (2003) and Schierholz et al. (2006) have conducted experiments to show the effect of free surface transfer on diffused aeration systems, and it was shown that high surface-transfer coefficients exist above the bubble plumes, especially when the air discharge (Q_a) is large. When coupled with large surface cross-sectional area and/ or shallow depth, the oxygen transfer mechanism becomes more akin to surface aeration where water entrainment withair from the atmosphere becomes important. The water turbulence has a significant effect on oxygen transfer. The alternative to a judicious choice of tank geometry and/or gas discharge is perhaps another mathematical model that could separate the effect of surface aeration from the actual aeration under testing in the estimation of the mass transfer coefficient. This separate modelling for surface aeration is not a topic in this book. Nevertheless, a simple graphical method to take this effect into account in the establishing of the baseline coefficient is proposed in Chapter 6, Section 6.6.4.

In engineering, the mass transfer coefficient is a diffusion rate constant that relates the mass transfer rate, mass transfer area, and concentration change as driving force, using the Standard Model, typically stated in the form given by Eq. 4.1 in Chapter 4. This can be used to quantify the mass transfer between phases, immiscible and partially miscible fluid mixtures (or between a fluid and a porous solid). Quantifying mass transfer allows for design and manufacture of separation process equipment that can meet specified requirements and estimate what will happen in real life situations (chemical spill, wastewater treatment, fermentation, and so forth) if the effect of other factors, such as turbulence either due to the free surface exchange or due to mechanical mixing within the water body, can be isolated or eliminated or modelled separately.

Mass transfer coefficients can be estimated from many different theoretical equations, correlations, and analogies that are functions of material properties, intensive properties and flow regime (laminar or turbulent flow), all based on the Standard Model. Selection of the mos...

Table of contents

• Cover
• Title Page
• Copyright Page
• Preface
• Table of Contents
• 1. Prologue
• 2. Mass Transfer Coefficient and Gas Solubility
• 3. Development of a Model to Determine Baseline Mass Transfer Coefficients in Aeration Tanks
• 4. The Lee-Baillod Equation
• 5. Baseline Mass Transfer Coefficients and Interpretation of Non-steady State Submerged Bubble Oxygen Transfer Data
• 6. Is Oxygen Transfer Rate (OTR) in Submerged Bubble Aeration Affected by the Oxygen Uptake Rate (OUR)?
• 7. Recommendation for Further Testing and Research
• 8. Epilogue
• Index