The fundamental objective of wastewater treatment is to reduce the concentration of contaminants in the wastewater to such a degree that safe discharge to a receiving water, either surface water or groundwater, can be accomplished. Achieving that goal requires the application of several fundamental principles of engineering. Among those are chemistry, biology, hydraulics, fluid mechanics and mathematics of varying types. This book provides a synopsis of the basic fundamentals of those disciplines, as well as an outline of the use of those principles to solve specific wastewater engineering problems. This is the second in a series of volumes designed to assist with mastering the principles of environmental engineering. Inside this volume, the author addresses the process of wastewater treatment; not the mechanics or the machinery and reactors used to do the work. No amount of machinery and reactor vessels will ever treat wastewater effectively unless the process of using the equipment is properly developed first and properly utilized afterwards. A separate volume will address new and emerging technologies, updated regularly to cover those changes to the practice of wastewater treatment.