Nitrogen removal from urban stormwater runoff by stepped bioretention systems

•Stepped bioretention systems were designed based on the terrain of mountainous cities.•18 stepped bioretention systems were constructed and operated.•Performance of systems and influence factors were investigated last for two years.•Several kinds of plants and composition media were discovered efficient in N removal.•Layer materials and flow pattern were also given to enhance denitrification.

Nitrogen excess is a key trigger for the eutrophication of water bodies. Stormwater is an important nitrogen source in urban environments and thus requires effective treatment, especially in mountainous cities due to their stronger runoff flushing. However, the design method of bioretention systems focusing on mountainous cities is still rare in China and nitrogen is often released due to the lack of denitrification environments. In this study, the stepped bioretention systems were designed based on the terrain characteristics in mountainous cities by two columns in the stair-stepping connection. From 2015-2016, 18 replicates of stepped bioretention systems, which included a 400 mm deep planting layer, a 200 mm deep transition layer and a 200 mm gravel layer planted with Radermachera hainanensis (Merr.), Juncus effusus (L.), Vetiveria zizanioides (L.), Ophiopogon japonicus (Linn. f.) and Medicago sativa (L.)., respectively, were tested under certain operational conditions (e.g., the simulated rainfall intensity was 3.5, 5.3 and 14 mm/h, respectively.). The stepped columns planted with Medicago sativa (L.). showed poor nitrogen removal (e.g., the RRTN (removal rate of TN) was from − 29.8% to − 123.0%), while those planted with Radermachera hainanensis (Merr.), Juncus effusus (L.), Ophiopogon japonicus (Linn. f.) or Vetiveria zizanioides (L.) performed well without additional carbon sources (e.g., the RRTN was from 52.8% to 84.2%). Nitrogen would be released from the systems (e.g., the average RRTN was −178.0%) when peat soil was mixed in the planting layer at a ratio of 20%. After retrofitting the flow pattern within the stepped columns, U flow pattern was more advantageous for the improvement of nitrogen removal. In the 1st, 2nd, 4th, 5th, 10th and 11th systems, the average RRTN and RRNO3N (removal rate of NO3N) in the systems with U flow pattern were 1.2 − 7.0% and 5.0 − 5.8%, both higher than those with W flow pattern. Through leaching dynamics analysis, NH3NC (NH3N Concentration) of all effluent samples was < 0.5 mg/L, but the concentrations of TN and NO3N were increased with the duration of rainfall events. Generally, RRTN was significantly correlated to TNC (TN Concentration), NH3NC, NO3NC (NO3N Concentration) and D (Depth of saturated zone), while RRNH3N (Removal Rate of NH3N) was associated with ID (Interval Days between rainfall events). Our results suggested that the stepped bioretention systems should be recommended in mountainous cities if nitrogen discharges posed a potential threat to the receiving environments.

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