阶梯式三维流生物滞留系统脱除山地城市降雨径流氮素机制研究

Unban stormwater runoff has been a leading contributor to nitrogen pollution of receiving water bodies in mountainous city for its higher intensity and shorter duration. Furthermore, the main composition of total nitrogen in urban stormwater runoff is inorgnic nitrogen whose removal efficiency and mechnism is still unclear in bioretention system to date due to its its high migration. The stepped bioretention system with three-dimensional flow is first proposed according to the topographic features of mountainous city in this project. We will run the experiments to identify the key effective factors of the nitrogen removal efficacy and to study the responding mechanism between denitrification performance and environmental factors based on the tempral and spatial distribution of nitrogen concentration and level of environmental factors within the system. This project focus on the tempral and spatial distribution of nitrogen concentration in the four-phase system of “rainfall runoff - plants - filler - microorganisms” with 15N isotope tracing methods to interpret the transformation characteristics of nitrogen among intersurfaces and to understand the removal process and pathways of nitrogen in rainfall runoff inputed. Based on the tempral and spatial distribution of micro-ecology factors within the bioretention system, in site occurrence of micro-ecology factors and nitrogen concentration in rainfall runoff is analyzed, and the relationship between removal of nitrogen and micro-ecology factors is also revealed. The results of this project will provide a theoretical basis and a new way for nitrogen removal in stormwater runoff in mountainous city.

山地城市降雨径流持续时间短、冲刷强度大，已成为城市受纳水体氮污染的主要来源。无机氮是降雨径流氮素的主要赋存形态，迁移性强，传统生物滞留系统对其难以稳定高效脱除，脱氮机制也不明晰。本项目首次提出一种适应山地城市特色的阶梯式三维流生物滞留系统，并围绕系统内降雨径流氮素浓度、氮素赋存形态和生境因子的沿程变化特征，阐释生境因子与系统脱氮效能、脱氮过程的协同响应机制，识别影响系统脱氮的关键生境因子；以15N同位素示踪为研究手段，围绕系统内“降雨径流-植物-填料-微生物”四相体系氮素浓度的时空动态分布特性，阐释降雨径流氮素在系统内的界面迁移转化规律，着重明晰输入径流氮素在系统内的去除途径及去除过程；以系统内微生态因子的沿程变化特征为核心，结合降雨径流氮素浓度和氮素形态的同步演变特点，阐释生物滞留系统脱氮与系统微生态因子的关联机制。项目研究结果将为山地城市降雨径流氮污染控制提供理论依据及新的技术途径。

生物滞留系统是海绵城市建设过程中普遍采用的主打技术之一，但该技术普遍面临氮素调控效能不稳定的技术缺陷。本项目根据山地城市的地形特征，首次提出了阶梯式生物滞留系统构建方式，并以同位素示踪为技术手段，系统研究了生物滞留系统的脱氮效能及脱氮机制，主要结论如下：.（1）种植绿宝、香根草、灯芯草、麦冬的小试系统对总氮、氨氮、硝态氮的平均去除率可分别达到52.8%-84.2%、69.7%-97.9%、42.5%-91.8%；相对于W型流态，U型流态更有利于系统脱氮。中试系统对总氮、氨氮、硝态氮的平均去除率分别为70.33%-85.71%、88.89%-96.15%、21.13%-66.67%。淹没区深度越大，进水氮素浓度越高，总氮去除率越高；降雨间隔时间越长，氨氮去除率越高。.（2）氨氮和总氮主要滞留在系统土壤层，分别占输入量的37.25%和49.64%；反硝化/植物吸收等途径去除的硝态氮占硝态氮总输入负荷的69.78%。在干旱期，土壤层氨氮、硝态氮、总氮反应速率方程分别为：y=8.28-0.08x (R2=0.94)、y=1.38+0.02x (R2=0.87)、y=70.55-0.24x (R2=0.39)；间隙水中硝态氮、总氮的反应方程分别为：y=2.69-0.09x+8.55x2 (R2=0.99)、y=3.72-0.09x+7.83x2 (R2=0.97)。.（3）输入系统的3.98 mg 15NH3-N，85.42%被滞留在土壤层（其中，1.10 mg以氨氮的形态存在，2.30mg转化为微生物量氮，分别占滞留15NH3-N的27.63%、57.79%），13.32%通过植物吸收/反硝化等途径离开系统。被系统土壤层滞留的15NH3-N在60小时后，主要以15NH3-N、15NO3-N、TO15N的形式存在，分别有0.56mg、0.10mg、1.62mg，截至此时，累计共有1.65 mg 15NH3-N被植物吸收或反硝化等途径去除，占输入系统15NH3-N总量的41.46%。.（4）生物滞留系统土壤微生物分析表明，放线菌门和变形菌门相对丰度最高，分别为33.7%-41.2%、26.3%-31.0%；异常球菌-栖热菌门则与含水率呈显著正相关关系，厚壁菌门与土壤有机氮、总氮含量呈显著正相关关系。