This book is intended to provide an introduction to the technical concepts of wastewater treatment. It assumes that the reader has at least a first-year undergraduate level of scientific understanding but not necessarily any previous knowledge of wastewater treatment. A reading list is provided at the end of the book for more in-depth information on the topics that are introduced here. The authors are based in the United Kingdom and therefore the scientific units and terminology that are used in this book are those which are common in that country