This lecture introduces the fundamentals of water and wastewater treatment, covering topics such as the history of wastewater treatment in Switzerland, undesirable pollutants, typical concentrations of pollutants in urban wastewater, and the activated sludge process. It also delves into the characterization of wastewater, the removal of nitrogen and phosphorous, and the analysis of water pollutants. The instructor discusses the importance of safeguarding water quality and the treatment performance of Swiss wastewater treatment plants.