VA-försörjning i Spakbackenoch Östuna : Multikriterieanalys av enskilda-, lokala- och centraliserade VA-lösningar utifrån miljö, hälsa och ekonomi

Sammanfattning

Many rural areas in Sweden rely on on-site water and wastewater (VA) systems that may contribute to environmental impacts and risk to human health. Regarding these areas municipalities must assess whether existing systems meet the requirements of the Swedish Water Services Law (LAV). This study investigates the current and future VA supply in the areas of Spakbacken and Östuna in Knivsta municipality. The aim of the study was to evaluate whether the existing VA systems pose risks to the environment and human health and to compare three alternative scenarios for future water and wastewater management. The evaluated scenarios consisted of continued use of existing on-site and shared systems, a local wastewater treatment plant in Spakbacken combined with existing systems in Östuna and connection to the municipal water and wastewater system in Knivsta through transfer pipelines. The scenarios were evaluated and compared using a multi-criteria analysis based on environmental-, health risk- and economic criteria. Nutrient emissions to recipients, microbial health risk, investment costs, and operational- and maintenance costs were assessed. Nutrient transport and retention were estimated through mass balance calculations. Microbial health risks were assessed by a quantitative microbial risk analysis for drinking water wells. The results show that the third scenario provided the lowest nutrient emissions and microbial health risks but also the highest costs. The first scenario resulted in both the highest environmental impact and microbial risk, but the lowest costs. Scenario 2 achieved the highest overall score in the weighted multi-criteria analysis. Sensitivity analysis showcased that the ranking of scenarios was influenced by how criteria were prioritized. The results indicate that local water and wastewater solutions may represent a viable alternative to centralized systems in smaller rural areas where transfer pipelines result in high costs. However, the best fitting solution depends on how environmental-, health risk- and economic criteria are valued in decision making.