Department of Environmental Science and Engineering, Ewha Womans University, Seoul 03760, Republic of Korea.
Department of Chemical Science and Engineering, Tokyo Institute of Technology, Yokohama 226-8502, Japan.
Water Res. 2020 Dec 15;187:116417. doi: 10.1016/j.watres.2020.116417. Epub 2020 Sep 19.
Although eutrophic urban rivers receiving loads of wastewater represent an important anthropogenic source of NO, little is known as to how temperature and other environmental factors affect temporal variations in NO emissions from wastewater treatment plants (WWTPs) and downstream rivers. Two-year monitoring at a WWTP and five river sites was complemented with available water quality data, laboratory incubations, and stable isotopes in NO and NO to explore how wastewater effluents interact with seasonal changes in environmental conditions to affect downstream metabolic processes and NO emissions from the lower Han River traversing the megacity Seoul. Water quality data from four WWTPs revealed significant inverse relationships between water temperature and the concentrations or fluxes of total N (TN) in effluents. Increased TN fluxes at low temperatures concurred with NO surges in WWTP effluents and downstream rivers, counteracting the long-term decline in TN fluxes resulting from enhanced wastewater treatments. Incubation experiments with river water and sediment, in isolation or combined, implied the hypoxic winter sediment as a large source of NO, whereas the anoxic summer sediment produced a smaller amount of NO only when it was added with oxic water. For both WWTP effluents and downstream rivers, bulk isotope ratios and intramolecular distribution of N in NO distinctly differed between summer and winter, indicating incomplete denitrification in the hypoxic sediment at low temperatures as a primary downstream source adding to WWTP-derived NO. Winter surges in wastewater TN and sediment NO release highlight temperature variability as an underappreciated control over anthropogenic NO emissions from increasingly urbanized river systems worldwide.
尽管富含营养的城市河流接收大量废水是人为产生硝酸盐的重要来源，但对于温度和其他环境因素如何影响污水处理厂（WWTP）和下游河流中硝酸盐排放的时间变化，人们知之甚少。在一个 WWTP 和五个河流站点进行了为期两年的监测，同时还结合了可用的水质数据、实验室培养和硝酸盐中的稳定同位素，以探讨废水与环境条件季节性变化的相互作用如何影响下游代谢过程以及穿过首尔大都市的下汉江的硝酸盐排放。来自四个 WWTP 的水质数据显示，水温与废水中总氮（TN）的浓度或通量之间存在显著的反比关系。低温下 TN 通量的增加与 WWTP 废水中和下游河流中的硝酸盐激增同时发生，抵消了由于强化废水处理而导致的 TN 通量长期下降。对河水和沉积物进行的单独或组合的培养实验表明，缺氧冬季沉积物是硝酸盐的一个重要来源，而只有当缺氧沉积物与好氧水一起添加时，夏季缺氧沉积物才会产生少量的硝酸盐。对于 WWTP 废水和下游河流，硝酸盐中氮的整体同位素比值和分子内分布在夏季和冬季之间明显不同，这表明在低温下缺氧沉积物中不完全反硝化是 WWTP 衍生硝酸盐的主要下游来源。冬季废水 TN 和沉积物硝酸盐释放的激增凸显了温度变化作为对全球日益城市化河流系统中人为硝酸盐排放的控制作用被低估。
