This book is designed for use by wastewater professionals and researchers. The book covers the effects of HRT on different wastewater plant parameters and design of activated sludge process for municipal and industrial wastewater. A Professional advisory committee evaluated this material and approved as a professionally sound work. The committee is composed of licensed engineers, a Microbiologist and wastewater experts. They have read and commented on all of the chapters. The book starts with an overview of industrial wastewater, its characterization, impacts, and the design and construction process involved in pilot and full scale wastewater treatment plant. Each subject in each chapter is introduced with a discussion of the theoretical principles that are to be applied in the design of activated sludge process. In addition, in each chapter, appropriate design criteria from the different books as well as alternative approaches from the literature are addressed. A highlight of the book is the inclusion of full scale wastewater design requirements as well as operation and maintenance activities involved in treatability studies of industrial and municipal wastewater.