The components in wastewater treatment processes may be conveniently categorized as physical, chemical and biochemical unit operations. A thorough understanding of the principles governing their behavior is a prerequisite for successful process design. This "unit operations approach" to the study of process engineering has been widely accepted in the field of environmental engineering, just as in chemical engineering where it was developed, and a number of books are now available presenting the principles of physical and chemical operations and illustrating their applications. The purpose of this book is to provide similar coverage for the biochemical unit operations, replacing Biological Wastewater Treatment: Theory and Applications, which was published in 1980.

Much has happened in the field of biological wastewater treatment since 1980. A particularly significant occurrence was recognition of the many events that can happen simultaneously in biological processes and the role that the designer has in determining which predominate. The impact on process design of this recognition demanded that new environmental engineers be trained from the outset to think in terms of multiple events, rather than compartmentalizing them as they had in the past. In addition, because of the efforts of task groups within the International Association on Water Quality (IAWQ), process simulation has been widely adopted as a tool by design engineers, allowing them to visualize how those events will interact in a proposed design. There have been other significant changes in practice as well, with the development or maturation of several new processes. Consequently, to incorporate all of these changes, it was necessary to completely rewrite—not just revise, as is customary in second editions.

Another change that has occurred since the appearance of the first edition is the background and preparation of environmental engineering students. Most have much better preparation in microbiology and reactor engineering than their predecessors. Consequently, the two sections dealing with these topics have been eliminated. However, background information essential to an understanding of the text is presented in the three chapters of Part I. Elimination of the two background sections provided space for expansion of later sections to include many of the new developments in biological wastewater treatment that have evolved since 1980.

The book is organized into six parts: Part I, Introduction and Background; Part II, Theory: Modeling of Ideal Suspended Growth Reactors; Part III, Applications: Suspended Growth Reactors; Part IV, Theory: Modeling of Ideal Attached Growth Reactors; Part V, Applications: Attached Growth Reactors; and Part VI, Future Challenges.

Part I seeks to do three things. First, it describes the various "named biochemical operations" in terms of their treatment objectives, biochemical environment, and reactor configuration. This helps to remove some of the confusion caused by the somewhat peculiar names given to some biochemical operations early in their history. Second, it introduces the format and notation that will be used to present the models describing the biochemical operations. Finally, it presents the basic stoichiometry and kinetics of the various microbial reactions that form the key for quantitative description of biochemical operations. In Part II, the stoichiometry and kinetics are used in mass balance equations to investigate the theoretical performance of biological reactors containing microorganisms growing suspended in the wastewater as it moves through the system. Part II is at the heart of the book because it provides the reader with a fundamental understanding of why suspended growth reactors behave as they do. In Part III, the theory is applied to the various named suspended growth biochemical operations introduced in Part I. In that application, however, care is taken to point out when practical constraints must be applied to ensure that the system will function properly in the real world. In this way, the reader obtains a rational basis for the design of biological wastewater treatment operations that incorporates knowledge that has been obtained through practice. In other words, we have sought to make Part III as practical as possible. Parts IV and V parallel Parts II and III in organization, but focus on biochemical operations in which the microorganisms grow attached to solid surfaces. This mode of growth adds complexity to the analysis, even though the operations are sometimes simpler in application. Finally, Part VI looks briefly at the use of biochemical operations to remove xenobiotic organic chemicals from wastewaters. The intention is to introduce this topic so the reader can continue learning with the rest of us as we seek to solve the world's environmental problems.

Our plan in preparing this new edition was to provide a text for use in a graduate-level environmental engineering course of three semester-hours credit. In reality, the amount of information provided is more than can be covered comfortably. This provides some latitude for the instructor, but also makes the book a resource for the student interested in knowing more than the minimum. It is our hope, furthermore, that our professional colleagues will find the book to be worthwhile as a reference and as a resource for self-guided study.

At this point, we would like to add a note of caution to the students using this book. It relies heavily upon modeling to provide a conceptual picture of how biochemical operations function. Although these models are based on our best current ideas, one must always remember that they are simply just someone's way of trying to describe in simple terms very complex phenomena. Their purpose is to help the reader learn to think about the processes described by providing "experience." One should not fall into the trap, however, of substituting the models and their simulated experience for reality. Engineering requires the application of judgment in situations lacking sufficient information. The reader can use the background provided by this book to help gain sound judgment, but should not hesitate to discard concepts when real-world experience indicates that they are incorrect or don't apply. Theories are constantly evolving, so be prepared to change your ideas as our knowledge advances.

As with any book, many people have had a hand in its preparation, either directly or indirectly. First and foremost, we would like to thank Henry C. Lim, coauthor of the first edition. Although his career has followed another path, his thoughts and ideas continue to permeate the work. For example, much of the material on modeling of attached growth systems, written by him for the first edition, has been retained in this edition. Second, CPLG owes a great deal to M. Henze of the Technical University of Denmark, W. Gujer of the Swiss Federal Institute for Environmental Science and Technology, G. v. R. Marais of the University of Cape Town (now retired), and T. Matsuo of the University of Tokyo for all that he learned through long discussions and arguments about the modeling of suspended growth biological reactors as he studied with them on an IAWQ task group. Third, this book would not have been possible had it not been for the hundreds of researchers who generated the knowledge upon which it is based. In the first edition we asked the forbearance of those we overlooked or neglected. This time, we must ask for that tenfold because the information explosion since 1980 has been phenomenal! It simply is not possible for two individuals to have read every article of merit. Fourth, we thank the thousands of practitioners (both designers and operators) who have had the foresight and faith to use biological processes to treat such a wide variety of wastewaters. Their observations and factual documentation of the performance and operational characteristics of these processes have provided both a sound basis for process design and operation and the development of new process options. It is the combination of thoughtful and creative research and application that has provided the factual basis for this book. Finally, we would like to express our appreciation to the many people directly involved in the preparation of this book. The students in ESE 804 at Clemson University, who suffered through early drafts of the manuscript, did yeoman's service in helping us write more clearly. Professorial colleagues at other universities provided very constructive criticism. In particular we would like to thank Nancy G. Love at Virginia Tech, Lutgarde Raskin at the University of Illinois, Barth F. Smets at the University of Connecticut, Timothy G. Ellis at Iowa State University, Robert M. Cowan at Rutgers University, and Wen K. Shieh at the University of Pennsylvania. And last, but far from least, we would like to recognize the hard work of three wonderful people who provided valued assistance: Patsy A. Phillips for her word-processing skills, Rebecca E. Laura for her artwork, and Joni K. Grady for preparing the index.

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