Life cycle assessment (LCA) was applied to evaluate duckweed ponds and constructed wetlands as polishing steps in pig manure liquid fraction treatment. Using nitrification-denitrification (NDN) of the liquid fraction as the starting point, the LCA compared direct land application of the NDN effluent with different combinations of duckweed ponds, constructed wetlands and discharge into natural waterbodies. Duckweed ponds and constructed wetlands are viewed as a viable tertiary treatment option and potential remedy for nutrient imbalances in areas of intense livestock farming, such as in Belgium. As the effluent stays in the duckweed pond, settling and microbial degradation reduce the remaining phosphorous and nitrogen concentrations. Combined with duckweed and/or wetland plants that take up nutrients in their plant body, this approach can reduce over-fertilisation and prevent excessive nitrogen losses to aquatic environments. In addition, duckweed could serve as an alternative livestock feed and replace imports of protein destined for animal consumption. The environmental performance of the overall treatment systems studied was found to depend greatly on assumptions about the possible avoidance of potassium fertiliser production through the field application of effluents. If it is assumed that the potassium contained in the effluent replaces mineral fertiliser, direct field application of the NDN effluent performed best. If the application of NDN effluent does not lead to mineral fertiliser savings or if the replaced K fertiliser is of low grade, duckweed ponds seem to be a viable additional step in the manure treatment chain. Consequently, whenever background concentrations of N and/or P in fields allow for effluent application and potassium fertiliser substitution, direct application should be favoured over further treatment. If direct land application of the NDN effluent is not an option, the focus should be on long residence times in duckweed ponds to allow for maximum nutrient uptake and feed production.