Water and energy are two of the most important resources of the 21st century. Water is required to supply energy and, at the same time, energy is required to supply water. In urban water management, the key factor is warm water heating. Depending on the quality of the raw water, the provision of drinking water requires the application of different process technologies; the more complex the methods, the higher the energy demand. As in metropolitan areas, in particular, water consumption exceeds local availability, water pipelines are necessary with respective energy demand. The reuse of water can contribute significantly to conserve water and energy resources. Usually, the water to be reclaimed is supplied locally, making long-distance transport dispensable. By adjusting the process technology to the intended function (fit for purpose), it is possible to minimize the energy demand as well. Water use implies the input of energy (heat, chemically bound energy in form of organic matter) as well as nutrients (nitrogen, phosphorus, etc.). In the context of implementing water reuse technologies, they can also be reclaimed.