This paper describes a study on “exergetic” design method to efficiently utilize nutrient matter contained by domestic wastewater and its results. The followings were found. An algorithm to purify domestic wastewater and to efficiently utilize its nutrient matter is derived. Only in the case of purifying wastewater by the combination of soil and vegetation such as small wetland, if the area of a vegetation site is 5-13m2 at least, exergy production by plants absorbing nutrient matter will exceed exergy consumption by a power plant and the other mechanical eguipment. Cascade use of domestic wastewater must be significant. A combined treatment septic tank or a multi-layering soil tank consumes “chemical exergy”, vegetation site fixes of chemical exergy and absorbs “concentration exergy”, and outdoor thermal environment is cooled by “wet exergy” consumption in summer. If the area of vegetation site is 20-40m2 at least, exergy production by palnts will exceed exergy consumption by power plant and vehcles transporting compost on 50km/year.
