The water deficit in industrialized regions of the Republic of Poland, especially for cooling circuits and for heating purposes, led to the concept of application of water reuse from municipal wastewater in Cracow (Poland). Reasons for water reuse technology in Cracow＇s particular case have been presented. Cracow University of Technology together with the US Environmental Protection Agency has developed a reclamation technology for practical application. The first stage of experiments described was performed at the pilot wastewater reclamation installation operating with a capacity of 120 l/d. Experiences from the first stage were applied in construction of the water reuse demonstration plant with the capacity of 1680 m(3)/d, based on a multi-phase biological reactor. Operational results have been presented; they proved the applicability of the method. Due to low concentration of the organic carbon compounds in raw wastewater, various internal sources of organic carbon were tested, with the special emphasis on easy biodegradable COD load in wastewater entering the reactor.