Over the past decades in Europe, man-made ecosystems such as constructed wetland systems have been successfully harnessed to treat sewage and other pollutants in waste waters. This work presents a comparative Life Cycle Assessment (LCA) study between (i) a 2-stage vertical Reed Bed Filter, and (ii) a conventional activated sludge system, both designed following usual French guidelines in domestic sewage treatment. The first LCA applicability challenge is to get an equilibrated mass balance between wastewater inputs in Nitrogen, Phosphorus, Carbon and systems outputs. The second LCA challenge overcome in this work is to provide grounds for comparison in terms of treatment efficiency between the two systems. Results show the poor efficiency of vertical Reed Bed Filters in removing nitrates, organic nitrogen and phosphates which highlight the need to pursue other avenues in the all important reduction of eutrophication in waterways.