Jämförelse mellan våtkompostering och andra VA-system i omvandlingsområden
en fallstudie i Norrtälje kommun.
Private Sewage SystemsEutrophicationRecyclingLiquid CompostingVeVa-toolEnskilda avloppÖvergödningKretsloppVåtkomposteringVeVa-verktyg
Eutrophication of the Baltic Sea and Swedish lakes remains a major problem despite many years of effort to reduce the emissions of nutrients. One major source is waste water systems, especially private sewage systems. These sewages accounts for a very large share of nutrients per capita in comparison with citizens connected to larger sewage treatment plants. Norrtälje community has the greatest number of private sewage systems in Sweden. Discharges of nutrients, especially phosphorus, to the Baltic Sea from these sewage systems are significant. That is why Norrtälje is working actively to improve the standard of these sewage systems.The purpose with this Master Thesis was to compare from an environmental and cost perspective, different sewage systems in an area of Norrtälje. One of these sewage systems was liquid composting. To make this easier a tool called VeVa, which is an instrument for assessment of water and sewage systems in transition areas, was used. Since liquid composting was not included in VeVa in the beginning of the study one purpose was also to implement this option in the tool. Another objective of the work was to evaluate whether liquid composting is still a good sewage options in the future. The result of this study is based on a literature review, contact with Norrtälje community and modeling in VeVa.The four sewage systems that where included in the study where connection to a large scale waste water treatment plant, local sewage treatment plant and liquid composting. The fourth system was a new construction of the already existing private sewage system in the area. This system was a sand filter system with a phosphorus trap and a separation and collection of toilet water.Results from the study showed that all systems, with some exceptions for local wastewater treatment plant, can meet the Swedish Environmental Protection Agency high demand of phosphorus and nitrogen reduction for small sewage systems. All systems in the study, except for sand filter with phosphorus traps, have a high reversal potential of phosphorus. However it is only the liquid compost that can recycle nitrogen back to arable land to a greater extent. A comparison between manure sludge from waste water treatment plants and liquid composts showed that the liquid composts had a much lower level of cadmium content and higher proportion of plant available nutrients. The energy use for these systems varied widely where connection to a large scale wastewater treatment plant have the highest, while the liquid compost have the lowest energy use. The cost analysis showed that a municipal sewage treatment plant is the most expensive and liquid composting system is the next most expensive system. However, the differences in cost between the studied systems are not particularly large when uncertainties in the input are accounted. The conclusions of this Master Thesis were that liquid composting is a good sewage solution, especially when recycling of nutrients is an important factor. However there are important social and economic considerations that must be taken into account when deciding which sewage system should be implemented.