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This textbook is intended for undergraduate students in their junior and senior years in environmental, civil, and chemical engineering, and students in other disciplines who are required to take the course in physical-chemical treatment of water and wastewater. This book is also intended for graduate students in the aforementioned disciplines as well as practicing professionals in the field of environmental engineering. These professionals include plant personnel involved in the treatment of water and wastewater, consulting engineers, public works engineers, environmental engineers, civil engineers, chemical engineers, etc. They are normally employed in consulting firms, city and county public works departments, and engineering departments of industries, and in various water and wastewater treatment plants in cities, municipalities, and industries. These professionals are also likely to be employed in government agencies such as the U.S. Environmental Protection Agency, and state agencies such as the Maryland Department of the Environment.

The prerequisites for this textbook are general chemistry, mathematics up to calculus, and fluid mechanics. In very few instances, an elementary knowledge of calculus is used, but mostly the mathematical treatment makes intensive use of algebra. The entire contents of this book could be conveniently covered in two semesters at three credits per semester. For schools offering only one course in physical-chemical treatment of water and wastewater, this book gives the instructor the liberty of picking the particular topics required in a given curriculum design.

After the student has been introduced to the preliminary topics of water and wastewater characterization, quantitation, and population projection, this book covers the unit operations and unit processes in the physical-chemical treatment of water and wastewater. The unit operations cover flow measurements and flow and quality equalization; pumping; screening, sedimentation, and flotation; mixing and floccu-lation; conventional filtration; advanced filtration and carbon adsorption; and aeration, absorption, and stripping. The unit processes cover water softening, water stabilization, coagulation, removal of iron and manganese, removal of phosphorus, removal of nitrogen, ion exchange, and disinfection.

The requirements for the treatment of water and wastewater are driven by the Safe Drinking Water Act and Clean Water Act, which add more stringent requirements from one amendment to the next. For example, the act relating to drinking water quality, known as the Interstate Quarantine Act of 1893, started with only the promulgation of a regulation prohibiting the use of the common cup. At present, the Safe Drinking Water Act requires the setting of drinking water regulations for some 83 contaminants. The act relating to water quality started with the prohibition of obstructions in harbors as embodied in the Rivers and Harbors Act of 1899. At present, the Clean Water Act requires that discharges into receiving streams meet water quality standards; in fact, regulations such as those in Maryland have an antidegradation policy. In recent years, problems with Cryptosporidium parvum and Giardia lamblia have come to the fore. Toxic substances are being produced by industries every day which could end up in the community water supply. These acts are technology forcing, which means that as we continue to discover more of the harmful effects of pollutants on public health and welfare and the environment, advanced technology will continue to be developed to meet the needs of treatment.

The discipline of environmental engineering has mostly been based on empirical knowledge, and environmental engineering textbooks until recently have been written in a descriptive manner. In the past, the rule of thumb was all that was necessary. Meeting the above and similar challenges, however, would require more than just empirical knowledge and would require stepping up into the next level of sophistication in treatment technology. For this reason, this textbook is not only descriptive but is also analytical in nature. It is hoped that sound concepts and principles will be added to the already existing large body of empirical knowledge in the discipline. These authors believe that achieving the next generation of treatment requirements would require the next level of sophistication in technology. To this end, a textbook written to address the issue would have to be analytical in nature, in addition to adequately describing the various unit operations and processes.

This book teaches both principles and design. Principles are enunciated in the simplest way possible. Equations presented are first derived, except those that are obtained empirically. Statements such as "It can be shown..." are not used in this book. These authors believe in imparting the principles and concepts of the subject matter, which may not be done by using "it-can-be-shown" statements. At the end of each chapter, where appropriate, are numerous problems that can be worked out by the students and assigned as homework by the instructor.

The question of determining the correct design flows needs to be addressed. Any unit can be designed once the flow has been determined, but how was the flow determined in the first place? Methods of determining the various design flows are discussed in this book. These methods include the determination of the average daily flow rate, maximum daily flow rate, peak hourly flow rate, minimum daily flow rate, minimum hourly flow rate, sustained high flow rate, and sustained low flow rate.

What is really meant when a certain unit is said to be designed for the average flow or for the peak flow or for any flow? The answer to this question is not as easy as it may seem. This book uses the concept of the probability distribution to derive these flows. On the other hand, the loss through a filter bed may need to be determined or a deep-well pump may need to be specified. The quantity of sludge for disposal produced from a water softening process may also be calculated. This book uses fluid mechanics and chemistry without restraint to answer these design problems.

Equivalents and equivalent mass are two troublesome and confusing concepts. If the chemistry and environmental engineering literature were reviewed, these subjects would be found to be not well explained. Equivalents and equivalent mass in a unified fashion are explained herein using the concept of the reference species. Throughout the unit processes section of this book, reference species as a method is applied. Related to equivalents and equivalent mass is the dilemma of expressing concentrations in terms of calcium carbonate. Why, for example, is the concentration of acidity expressed in terms of calcium carbonate when calcium carbonate is basic and acidity is acidic? This apparent contradiction is addressed in this book.

As in any other textbook, some omissions and additions may have produced some error in this book. The authors would be very grateful if the reader would bring them to our attention.

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