The providing of pure water for human needs and irrigation is one of the most important problems of many countries in hot climate. Moreover waste water is not purified at all or insufficiently. Purified waste water could be used by mankind e.g. for irrigation in order to establish a closed water circulation. Excellent solar- and wind conditions in the regions with hot climate suggest an energy supply with renewable energies for many applications. The feasibility of the combination of purification plants with a renewable energy supply is investigated. The investigations are carried out as follows: first of all the problematic of water and energy supply in developing countries is described. Then a general analysis of the system structure in view of the energy supply is made. A waste water treatment plant (WWTP) with a low energy demand which is adapted to a renewable power supply is designed. To present the investigations more clearly they refer to data representative for Jordan.