This book gives an overview of the modelling and design of biological wastewater treatment for undergraduate and graduate students in Environmental engineering. A real case study from one of the wastewater treatment plants in Norway will be presented in this manuscript. The first section of this book will describe the field of study and the different variants of biological processes. The second section will talk about the experimental setup. Presentation of the results and discussions are covered in the third section. Finally, simulation with AQUASIM software and design calculations of activated sludge and aerobic biofilm reactor will be elaborated in the two last sections.