A long term study elucidates the relationship between media amendment and pollutant treatment in the stormwater bioretention system: Stability or efficiency?

Media amendment has been more and more frequently tested in stormwater bioretention systems for enhanced runoff pollutant treatment. However, few studies systematically evaluated the amended system over a long time span, which hindered the further optimization of the proposed amended media. In this study, biochar-pyrite system (PB), conventional sand system (SB), and biochar-woodchip system (WB) were established and operated for 26 months. Media amendment greatly enhanced the dissolved nutrient removal, the highest total dissolved nitrogen removal in PB and WB were 65.6±3.6% and 68.2±2.5%, respectively. Compared with PB, WB could maintain excellent nitrogen removal under long-term operation. In contrast, PB demonstrated stable and more effective total dissolved phosphorus removal during all stages (73.1±3.1%-80.3±4.1%). A high content of phosphorus and organic matter was leached in WB especially at initial operation, while the initial pollutant leaching in PB and SB is much lower, about one-third of WB. Microbial and metabolic function analysis indicated that the microbial community in the bioretention system is complicated and stable. Media amendment enhanced microbial diversity and the relative abundance of functional genera related to nitrogen (Nitrospira, Thauera, Denitratisoma, etc.), sulfur (Thiobacillus, Geobacter, Desulfovibrio, etc.), and carbon cycles (cellulomonas, saccharimonadales, and SBR1031, etc.), which well explained the enhanced pollutant removal and by-product leaching in different systems. Overall, the current study indicates that although media amendment is conducive to enhanced dissolved nutrient removal in bioretention systems, it can hardly maintain both stability and efficiency from initial set-up to long-term operation. In practical application, catchment characteristics, prioritized pollutants, meteorological factors, etc. should all be considered before choosing suitable amended media and its design factors, thereby maximising the stability and efficiency of the bioretention system.