PhD student Katharina Lange tested a new method to investigate the metal treatment performance of stormwater bioretention systems. By further subdividing the dissolved metal fraction (commonly defined as the fraction obtained by filtration through a 0.45 µm filter), she could show that a high proportion of the metals in the <0.45 µm fraction of the bioretention effluent is in the colloidal fraction, and thus not “truly” dissolved. The findings of her column experiment have implications on further investigations of bioretention sytems and support understanding of metal treatment and environmental impact of stormwater treatment using bioretention systems.
Metal treatment performance of stormwater bioretention systems is usually investigated by measuring total metal concentrations; in fewer cases also dissolved metal concentrations were measured. Earlier studies showed that total metal treatment is usually very efficient while dissolved metal treatment can be more variable. Dissolved metals (<0.45 µm) are a mixture including free metal ions and colloidal metals, i.e. basically defined as everything which passes through a 0.45 µm filter. Transportability, treatment capacity, bioavailability and toxicity of the different metal species are variable. Therefore, we integrated an extra filtration step to further distinguish between colloidal and truly dissolved metals. We found that the colloidal fraction is more pronounced in the effluent of bioretention systems. The reason for that will be investigated in further studies but results from previous studies suggest that the organic matter in the bioretention system could be an important factor.
The subdivision of dissolved metals does not only help to better understand the mechanism inside the bioretention systems but also ties on legal demands to improve water quality assessment. Based on guidance of the European Water Framework Directive, Sweden defines e.g. bioavailable threshold concentrations in their water quality guidelines (MKN). Instead of directly measuring the bioavailable fraction to date a modeling approach is used to determine the bioavailable fraction. The economically feasible approach tested by LTU can complement these modeling data.