This book chapter on environmental biochemistry – an interdisciplinary field that deals with chemical processes in the environment – examines the interactions between chemical substances and biological systems in the environment and their effects on the environment and human health. A focus is on wastewater treatment, especially in densely populated urban areas. The technical challenges and the importance of resource availability for wastewater treatment are discussed. The process of wastewater purification is described in detail: from mechanical separation to biological purification by bacteria. Another topic is biogas production and methanogenesis, where organic waste is used in biogas plants for energy production. The complex biochemical reaction of methanogenesis is explained, including the importance of pH, temperature, and nutrients. Environmental biochemistry is a crucial field for understanding the interactions between chemistry and the environment. Wastewater treatment poses challenges to municipalities worldwide in terms of resource availability, cost, and public acceptance. Techniques such as mechanical separation and biological purification play a key role. Methanogenesis, as the final stage of anaerobic microbial degradation in biogas plants, is illuminated from a biochemical and technical perspective. The complex biochemical reaction and the technical processing of biogas for use are discussed. Understanding these processes is crucial for the development of measures to reduce environmental pollution and for sustainable energy production.