The relationship between ecosystem services and purchased input in Swedish wastewater treatment systems — a case study

Abstract There is an increasing awareness of ecosystem services and use of ecological engineering in today's search for more sustainable wastewater treatment systems, but there is still great uncertainty about how to compare and evaluate the ecological sustainability of different systems. The aim of this study was to analyze the use of resources in three different wastewater treatment systems: (i) conventional three-step treatment (WWTP), (ii) conventional mechanical and chemical treatment complemented with a constructed wetland (TP+CW), and (iii) treatment in a natural wetland (NW), and to investigate to what extent increased space, time, and dependence of ecosystem services could substitute for purchased input in wastewater treatment. In order to compare resources of different quality we used emergy analysis, assessing the amount of solar energy required, directly and indirectly, for a certain production. Emergy use due to purchased inputs and to local renewable inputs differed substantially between the three treatment system studies, with emergy ratios of purchased to local renewable inputs of 9:1, 141:1, and 3056:1 for NW, TP+CW, and WWTP, respectively. Total use of emergy per person equivalent (p.e.) and kg phosphorus was similar in all three systems, strengthening our hypothesis of unchanged total emergy use in systems of different purchased input and land use. However, in the present study, purchased input was not fully substituted, while the natural wetland, apart from requiring a large land area, also needed a lot of purchased resources to fulfil the strict rules for wastewater treatment prescribed by society.

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