Water quality improvement of highway runoff by the filter material D-Rainclean : A five-month field trial

Abstract: Highway runoff and its pollutants are of growing concern. These pollutants require treatment as their concentration often are increased. A common removal method is the use of sedimentation ponds that removes particulates through gravity. However, sedimentation does not remove the pollutants inside very fine particulate material or the dissolved pollutants. An additional treatment step is required to remove these pollutants and necessary to reach more stringent discharge demands. In this report, the removal of metals was studied on stormwater after sedimentation in a dam in the existing treatment facility at Lilla Essingen. The runoff reaching the facility largely comes from the highway Essingeleden, which has an annual average daily traffic of 140 000 vehicles. The commercial filter material D-Rainclean was used in this study, which started in late August 2021 and ended in November 2021. The filter material was filled equally into two separate filter wells. After sedimentation, the runoff was pumped to each filter well with a constant flow of around 600 L/h. This load caused a hydraulic retention time of about 41 minutes. The stormwater flowed through the filter material from the bottom towards its surface, i.e. upward flow, creating saturated conditions. The removal of dissolved zinc (Zn) was just above 90% while dissolved copper (Cu) was in average removed by 50 %. The effluent concentrations of Cu were generally above the regulation limits set by Järfälla municipality and Gothenburg city. Dissolved calcium (Ca) and potassium (K) was released by the filter material itself through the entire trial and dissolved vanadium (V) was released initially but decreased continuously and reached close to zero at the end of the trial. Influent stormwater concentrations of sodium (Na), magnesium (Mg), chromium (Cr) and molybdenum (Mo) was unaffected by the filter as was the dissolved fraction of nickel (Ni), iron (Fe) and aluminium (Al). Particulate material was removed by the filters. However, at the end of the trial a release of particulate material was observed. This is likely caused by filter erosion that occurred through the combination of clogging and the high surface load. Surface loading and retention time was elevated during the trials. The sharp increase of suspended solids loads in November followed by increased filter pressure drop shows the importance of having a functional sedimentation to prevent physical clogging of the filter media.