Book excerpt: This book presents information that can be used for the design and operation of wastewater treatment plants that utilize biological nutrient removal processes, i.e., processes that utilize biological mechanisms instead of chemical mechanisms, to remove phosphorus and nitrogen from wastewaters. The book provides: basic fundamentals, concepts, and theories; design of prefermentation units, various types of BNR systems, and secondary clarifiers; retrofitting conventional activated sludge plants; modeling considerations; and special considerations for BNR systems. It includes full-scale and pilot plant case histories, design examples, and retrofit of existing plants.