The 1st edition of the textbook Biological Wastewater Treatment: Principles, Modelling and Design was published in 2008 and it went on to become IWA Publishing's bestseller to date. In 2020, the 2nd updated and extended edition of the textbook was published because, since 2008, the knowledge and understanding of wastewater treatment had advanced extensively and moved further away from empirically-based approaches to a fundamental first-principles approach based on chemistry, microbiology, physical and bioprocess engineering, mathematics and modelling. The updated edition has already been available for more than two years and the feedback from readers has been overwhelming - the textbook won the IWA Publishing Best Scientific Book Prize in 2022. This inspired the authors to embark on a new challenge - to prepare this complementary book Biological Wastewater Treatment: Examples and Exercises. This new book is an extension of the 2nd edition textbook; each chapter corresponds to a chapter in the textbook and is structured similarly around five sections, namely, Introduction, Learning objectives, Examples, and Exercises, with solutions provided in an annex. The overall objective of the book is to deepen, expand and test the knowledge of the reader through a set of worked out examples, followed up by exercises and questions with provided answers. Where applicable, the book is supplemented with MS Office Excel files. The book is open access and can be downloaded (together with supplements) at the publisher's website. The target readership of the book remains young water professionals, who will still be active in the field of protecting our precious water resources long after the aging professors who are leading some of these advances have retired. The authors are aware that cleaning dirty water has become more complex but also that it is even more urgent now than before, and offer this new book to help young water professionals engage with the scientific and bioprocess engineering principles of wastewater treatment science and technology with deeper insight, advanced knowledge and greater confidence built on stronger competence.