A new recycling system by using paper debris and polymer has been developed for high water-content mud such as bed materials in the river and lake, and construction sludge. The new recycling system proposed in this study consists of two steps. In the first step, the paper debris are added for controlling water content of mud and strengthening the bonding force between mud clusters. In the second step, polymer materials are added to increase the bonding force of mud particles in the cluster. By using this system, 200- 500% water-content mud has been modified to landfill materials by 20-30 minutes mixing process. The modified soils have several features and one of the features of modified soils is "light-weighted". In this study, a theoretical equation to estimate the density of modified soils is derived. By using this equation, a design system to use light-weighted landfill materials is proposed.