[Nitrogen Removal Efficiencies from Road Runoff by Dry Grass Swales with a Shallow Substrate Layer]

To investigate nitrogen removal efficiencies and mechanisms from road runoff by dry grass swales with a shallow substrate layer, we constructed six dry grass swale columns with different structures and media composition. In order to enhance the nitrogen removal efficiencies during the whole process, fermented woodchips were added into the substrate layer, and saturated zones were established. Semi-synthetic road runoff was used as the influent water. The influent and effluent quality were analyzed, and the change in volumetric water content and ORP of the media were monitored. The results showed significant nitrogen removal by these columns under unfavorable conditions. The range of the average removal rate of TN by the dry grass swales with saturated zones was 67%-78%. The nitrogen removal process mainly occurred during the wet period of the substrate layer. The saturated zones enhanced nitrogen removal efficiencies during the dry period, and also promoted the quick establishment of anoxic conditions in the substrate layer during the wet period. The water-holding transition layer with organic matter was effective at providing a carbon source for denitrification in the saturated zone, and for avoiding the leaching of pollutants caused by organic decomposition.