| Summary: | Bioretention is considered one of the best management practices (BMPS) for managing stormwater quality and quantity. The bioretention system has proven good performance in removing total suspended solids, oil, and heavy metals. The nitrogen (N) removal efficiency of the bioretention system is insufficient, however, due to the complex forms of nitrogen. Therefore, this paper aims to review recent enhancement approaches to nitrogen (N) removal and to discuss the factors influencing bioretention efficiency. To improve bioretention efficiency, several factors should be considered when designing bioretention systems, including nitrogen concentration, climate factors, and hydrological factors. Further, soil and plant selection should be appropriate for environmental conditions. Three design improvement approaches have been reviewed. The first is the inclusion of a saturated zone (SZ), which has been used widely. The SZ is shown to have the best performance in nitrogen removal. The second approach (which is less popular) is the usage of additives in the form of a mixture with soil media or as a separated layer. This concept is intended to be applied in tropical regions with wet soil conditions and a short dry period. The third approach combines the previous two approaches (enhanced filter media and applying a SZ). This approach is more efficient and has recently attracted more attention. This study suggests that further studies on the third approach should be carried out. Applying amendment material through filter media and integrating it with SZ provides appropriate conditions to complete the nitrogen cycle. This approach is considered a promising method to enhance nitrogen removal. In general, the bioretention system offers a promising tool for improving stormwater quality.
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