The purpose of this book is to present a concise summary of present theory, with emphasis on the application of that theory to both municipal and industrial wastewater treatment problems. This book, in large measure, is an updating of the author's previous books, Biological Waste Treatment, published by Pergaman Press, Industrial Water Pollution Control, published by McGraw-Hill, Water Quality Engineering for Practicing Engineers, published by Cahners Books. This book should be useful as a text for advanced undergraduate and graduate courses in Water Pollution Control, Industrial Waste Treatment and Water QualA-ity Management. The content of this book has been used as a text in Continuing Education Courses sponsored by Manhattan College, Vanderbilt University and the American Institute of Chemical Engineers. For in-depth study of specific topical areas the reader is referred to the cited book references at the end of the text.