Water reuse system based on self-purification in lake combined with advanced waste water treatment

A yearly increase of water consumption brought about a shortage of water and consequent pollution of waterways in all urban areas. Therefore, a regional metabolic water system with reuse of once used water was required for supporting various water consumption and pollution protection programmes. A metabolic water system for the Sapporo metropolitan area in Japan was investigated as a case study. Lake Barato located in downstrea- m basin of Sapporo city, has been heavily eutrophied. This study investigat- ed whether the introduction of advanced water treatment process could improve the water quality of the lake and whether the lake basin could be used as a nonpotable water resources. The application of iron coagulatio- n and sedimentation process to an urban river water containing the secondary sewage effluent which inflowed into Lake Barato, could decrease the eutrophication potential in terms of algal growth potential (AGP) expressed chlorophyll-a concentration from 100-200 µg/l to below 50 µg/l and suppress the algal growth between 2.0 and 10 µg/l in the reservoir- , less than AGP values. Consequently, the hybridization of advanced treatment of wastewater and impoundment of its effluent could enhance the natural self-purification ability due to obtaining a balance between photosynthesis and respiration over the whole of reservoir. Water quality, evaluated by the conventional pollution indicators such as SS, BOD and total phosphorus substances, could be much improved compared with impoundment of the untreated secondary effluent. The results indicated that the introduction of an advanced treatment system for secondary effluent enabled the recovery of the eutrophic lake to a clean recreational water area as well as a nonpotable water resource.