OPEN STORMWATER SYSTEMS FOR REDUCTION OF HEAVY METALS : AN EVALUATION OF COMMONLY USED DIMENSIONING METHODS

In Östersund there are a few stormwater ponds and oil separation units connected to the stormwater network, but mostly there is no systems for filtration of stormwater before it is released into a nearby lake which acts as Östersund’s source of drinking water. In the Industrial area in Lugnvik there is an oil separation unit connected to the stormwater network but no other means of filtration. This study will be conducted as a case study for the industrial part of catchment area 6 of the municipality of Östersund’s stormwater system, where this area acts as an example to apply the dimensioning methods on. Aside from grease (O/G) residues that might be removed by the oil separation unit, pollutants such as heavy metals, nutrients and suspended solids (SS) are present in the area. Which will make its way to the lake trough the existing stormwater system. One way to filtrate stormwater is to construct an open vegetated stormwater system, where the water is filtered as it passes through the vegetation and/or infiltrates to the ground and/or trough sedimentation. The purpose of this study is to evaluate some commonly used for Sweden relevant methods for dimensioning open stormwater systems. The evaluation is to see if the methods result in a system size that would give a satisfactory removal of heavy metals, or if the methods is not suitable to use for dimensioning a vegetated stormwater system if the purpose is to remove heavy metals. The study should give answers to if currently and commonly used methods for dimensioning open stormwater systems is suitable to use for dimensioning of open vegetated stormwater systems by relating the results to real examples when the purpose of the open stormwater system is to reduce heavy metal concentrations. Further, the study shall help to identify important factors that regulates the removal rate of heavy metals as well as determine what particle size that should be targeted to reach a satisfactory removal rate of heavy metals. A conclusion if the dimensioning methods is suitable to use or not is difficult to draw as the size of the system depends on what values that are used to calculate the stormwater flow. Therefore, there is a large variation in the resulting system size. To add to this uncertainty, the projection that is based on measurements on real systems is not accurate as this only use the size of the system in relation to the size of the impervious catchment area as a factor for heavy metal removal. In reality this is not the case, which is indicated by the R2-values of these projections. Other factors that in this study is confirmed to have an impact on the removal rate is particle size, surface load/flow, and in the case of vegetated filter strips the slope of the filter. The particle size that should be targeted is likely within the range of 45-65 μm.20181116