The first basic courses for wastewater operators in Germany were conducted in the early 1960s. From the manuscripts of the presenters, a guide for entering the wastewater technology field was developed. This led to the first edition of the Wastewater Operator's Handbook in 1969. Thanks to the many suggestions from operational staff and the practical focus of the authors, it has been possible to seamlessly incorporate new technical insights and legal regulations into new editions without losing the easily understandable readability. The 20th edition of the Wastewater Operator's Handbook is a comprehensive work that describes both the fundamentals for beginners and the technical innovations of recent years, without neglecting the requirements for small wastewater treatment plants. The text has been streamlined, and the visual material has been improved and expanded. The extensive illustrations significantly contribute to the understanding of the texts.