An environmental approach to compare on-site sanitation systems

About 700,000 on-site wastewater treatment systems (OWTS) have been installed in Sweden, serving both year-round and summer houses (Olshammar et al., 2015). Their treatment efficiency is often questioned, because they may contribute considerably to eutrophication of natural surface waters. It is estimated that the phosphorus (P) discharge of all the OWTSs accounts for 15% of the total anthropogenic P emissions to Swedish surface waters; the same amount as all municipal wastewater treatment plants combined (Ejhed et al., 2011). About half of the existing facilities are not built according to the guidelines (Ek et al., 2011) and discharge higher levels of nutrients into the receiving waters than suggested by the current recommendations. When new OWTS need to be built, or old ones upgraded because of facility ageing, or population dynamics or the need for legal compliance, it is often difficult for operators (usually house owners) and the supervising authority (the municipalities) to discern among the different available options and their sustainability.