Miljösystemanalys av VA-system i omvandlingsområden : fallstudie i Värmdö kommun

Abstract: Wastewater handling of private sewage systems has become an all increasing matter. In Stockholm region the 90 000 private sewage systems contribute with larger phosphorus flows to the Baltic Sea than the two biggest sewage plants in the area, Henriksdal and Käppala, contribute together, even though these two plants purify wastewater from more than one million inhabitants. When summer house areas turn into areas for permanent living – transition areas – the load on the existing systems increases, systems that already often are unsatisfying. In those areas it is not clear what the best system choice is; On-site systems for single households; Local collective systems or Connection to central systems. The VeVa-project was initiated in the year 2005. VeVa stands for Tool for environmental assessment of wastewater handling in transition areas. The aim of the project was to create a tool to support municipalities and consultants to evaluate what type of system is best suited, in environmental means, for a specific transition area. This Master Thesis is the base for the VeVa-tool and the aim was to compare six different sewage systems in an environmental assessment analysis, taking in consideration important environmental- and recourse aspects. A transition area in Värmdö community has been used for a case study. The following environmental- and recourse aspects have been studied; Flows of nitrogen, phosphorus, BOD7 and cadmium to recipient; The potential of the different sewage systems to bring plant available nitrogen and phosphorus by residues to arable land, and the amount of cadmium that is at the same time added to the field; Use of clean water; and Energy used for producing materials, construction and operation for the sewage systems, including taking care of the residues. In this study, six different sewage systems have been studied. They include three systems that are being used today in Värmdö community; Sand filter systems, Local waste water treatment plant and Conection to large scale waste water treatment plant. Three other systems considered to have a good purifying potential have also been studied; Reactive filter beds, Urine separation + reactive filter beds and Aerobe membrane bioreactor + reversed osmosis. The results show that all the studied sewage systems, except the Sand filter system, have at least 90 % reduction of phosphorus and can via the residues recycle a relatively large amount of phosphorous to arable land. The sewage systems with higher energy consumption can, according to the study, reach a better level of purification and get more concentrated and clearer residues than the systems using less energy. The discussion deals with the complexity of comparing environmental- and resource aspects, as well as other important aspects not included in this study, when choosing a sewage system. The conclusions to be drawn in this Master Thesis is that all the studied systems except for the Sand filter system can fulfil the limits of purification from the Swedish EPAs recommendations for small sewage systems (high as well as low level of protection). All systems apart from the Sand filter system have the potential to fulfil the national environmental goal of recycling phosphorus from residues. The used amount of clear water is hardly affected by the sewage system chosen. The energy use for the production of materials, system construction and operation varies much between the systems. For the two Reactive filter bed systems the energy for production of materials are considerably higher than for operation, while Aerobic membrane bio reactor + reversed osmosis has the opposite conditions with considerably more energy use for operations than for production of materials. For the remaining systems the amount of energy used fore production is about the same as for the operation.