The Lambro-Seveso-Olona system (L-S-O) is a effluent dominated watershed which derives from a strong urbanization process of natural river basins. The average population density in the L-S-O area is among the highest in Italy and Europe. Industry is also highly developed. Although at present the L-S-O system does not receive untreated wastewaters, depurated effluents constitute about half of its streamflow. This river has a long history of poor quality status, due to the high concentration of pollutant loads and the poor dilution. Recently new chemical quality standards have been set by the Italian legislation as support for the WFD Good Ecological Status. These standards are very restrictive, and make extremely challenging the achievement of the good ecological status. Aim of this study is to analyze the restoration possibilities of the L-S-O system. Elements are provided for a Cost-Effectiveness analysis.