|As a unutilized energy, treated sewage water locates widespread near urban areas. From the previous survey, the sewage water is reported to hold energy potential up to 36,000 Tcal/year, which was 21.1% of the total domestic energy consumption and 9.7% of the energy usage in the household and business sector in 2006. Temperature of the sewage water differs locally, but its range is observed in a range of 20-25℃ in summer and 8-13℃ in winter. Since the temperature range of the sewage water has a better seasonal distribution about 5~10℃ compared to ambient air, it is a promising heat sink for summer or heat source for winter. The sewage water is also a high quality heat source from its abundant quantity and uniform temperature. Considering the ambient temperature of Korea is very low in winter, a heat pump system using the sewage water can be an alternative to prevent the problems of capacity deficiency and frost formation.