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

Abstract: 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.