In the Stockholm region there are around 90,000 households with single systems. These households cause larger phosphorus flows to the Baltic Sea than the 1.8 million people connected to four advanced large-scale treatment plants in the same region. According to city plans, some of these areas with on-site systems in transition to permanent living shall be connected to central systems. The problem is however that this sewer system will take decades to complete. It is also not ensured whether or not all peripheral areas with on-site systems could be connected to the central systems. To help support environmental decisions for the selection of wastewater systems for these areas, an Excel-based model has been developed where the cost for the systems can be assessed and evaluated in relation to their environmental impact. The model deals with two types of environmental issues: substance flow analysis and energy analysis. The cost part considers investigations, investments, design, operation, maintenance and supervision, and calculates total annual cost for the water and wastewater system per person.