The used design methods for storm water treatment facilities (STF:s) are often based on compiled field data of relative reduction of a pollutant. However, relative reduction is bias in regard to the influent runoff quality. In addition, the bulk of removal occurs between storm events in the STF:s water volume. In this study field data from eight Scandinavian STF:s was elaborated on. The STF:s had a ratio between permanent volume and runoff volume (Vp/Vr) between 0.14 and 7.0. This study suggests that the use of Vp/Vr or the ratio between permanent area and reduced watershed area (Ap/Ared) is less applicable as a design tool for STF:s with inflow TSS concentrations approaching the irreducible concentrations. The range of irreducible outflow concentrations was suggested to be 5-32 mg/l (TSS), 44-144 μg/l (P) and 6-34 μg/l (Cu). Assessment of TSS reduction suggested that the major removal occurs within 6-12 hours under quiescent conditions. Based on data from the field studies we suggest the design methods to be complemented with e.g. inflow and outflow concentrations, detention volume and share of vegetation. The model StormTac is being updated with these factors for further elaboration on case studies.
Larm T and Hallberg M (2008). Design methods for stormwater treatment – Site specific parameters. 11th International Conference on Urban Drainage, Edinburgh, Scotland, UK, 2008.