雨前干旱期对生物滞留系统去除氮性能的影响

Effects of Antecedent Dry Periods on Nitrogen Removal by Bioretention Systems

为应对传统生物滞留系统对雨水径流中氮元素控制效果不稳定的情况,通常在系统底部设置饱和区以强化对氮的去除,但设置饱和区后仍存在多种影响因素可能导致氮泄露。其中雨前干旱期长短关系着系统内部饱和区是否存在,直接影响下一次降雨时系统初期出水中含氮浓度,因此需要深入研究雨前干旱期对系统除氮效果的影响。通过设置带有40 cm饱和区的生物滞留系统,对比不同雨前干旱期条件下系统对污染物的去除效果并分析除氮机理。结果显示,持续进水和短雨前干旱期时系统由于初期出流硝酸盐和总氮浓度低,整体除氮性能良好;长雨前干旱期时系统初期出流硝酸盐和总氮浓度较高,虽然氨氮浓度低,但整体氮去除率较低。不同雨前干旱期条件下污染物去除率存在差异主要是因为干旱期长度会影响填料层和饱和区中氮元素的转化程度,进而影响去除效果。生物滞留系统在干湿交替的条件下基质中硝酸盐异化还原反应占优势地位,这与传统的认为在系统中硝酸盐主要发生反硝化反应有很大差异。

Aim at controlling nitrogen in stormwater runoff by traditional bioretention systems,a saturated zone is usually set up at the bottom of the system to strengthen the removal of nitrogen,but there are still a variety of influencing factors that may lead to nitrogen leakage after setting up the saturated zone.The length of the antecedent dry period is related to the existence of the saturation zone within the system,which directly affects the nitrogen concentration in the initial effluent of the system during the next rainfall,so it is necessary to examine in depth the influence of the antecedent dry period on nitrogen removal in the system.A bioretention system with a 40 cm saturated zone was set up to compare the pollutant removal of the system and analyze the mechanism of nitrogen removal at different antecedent dry period conditions.The results showed the nitrogen removal of the system was high due to low initial outflow nitrate level at continuous influent and short antecedent dry periods.Nitrate and total nitrogen concentrations were higher in the initial outflow from the system at long antecedent dry periods,and nitrogen removal was at a low level despite low ammonia concentration.The main reason for the differences in pollutant removal at different antecedent dry periods was that the length of the drought period affects the processes and degrees of nitrogen transformation in the media and saturated zone,which in turn affects nitrogen's removal.The predominance of dissimilatory nitrate reduction to ammonium in the media at alternate wet and dry conditions in bioretention systems differs significantly from the traditional belief that nitrate undergoes predominantly denitrification in the system.