The values affect the maximum treatment effect that can be achieved by stopping the treatment when these minimum concentrations are reached. The difficulty in achieving lower concentrations is due to e.g. the decomposition of plants, sedimentation due to high flows and that vegetation beds (soil) and filters contain materials that can leak nutrients and pollutants. You can test different facilities and combinations of facilities with different irreducible concentrations if lower outlet concentrations are required.
The update is a result of a comprehensive investigation and literature review with new outlet concentrations and a comparative analysis of minimum outlet concentrations from similar types of facilities.