Conventional bioretention column with Fe-hydrochar for stormwater treatment: Nitrogen removal, nitrogen behaviour and...

An FeCl₃-modified rice husk hydrochar ('Fe-hydrochar') was used as the filler in a conventional bioretention column to remove nitrogen from synthetic stormwater. When the ammonia nitrogen (NH₄-N) and nitrate nitrogen (NO₃-N) concentrations of the influent were both 20 mg/L, the average removal rates of NH₄-N and total nitrogen (TN) were approximately 97% and 50%, respectively. Nitrogen was mainly removed by microbial nitrification and denitrification, with 25% of NH₄-N being adsorbed by the Fe-hydrochar. The remaining NH₄-N was converted into NO₃-N by nitrification in the upper layer, and NO₃-N was mainly converted to nitrogen gas (N₂) by denitrification in the lower layer. The organic matter released by the Fe-hydrochar was degraded and used as the carbon source for denitrification. The dominant bacteria were Pseudomonas, Rhizobium, and Flavobacterium at the genus level. Pseudomonas and Rhizobium were responsible for heterotrophic nitrification-aerobic denitrification, while Flavobacterium was related to the degradation of organic matter.