人工湿地的氮去除机理

Nitrogen removal mechanism of constructed wetland

湖泊等水环境的富营养化给人类带来诸多损害,如环境、生态和经济等方面的损害。富营养化的原因和控制途径引起了包括中国在内的很多国家的关注。我国针对水环境的富营养化问题开展了大量的工作。氮是引发水环境富营养化的主要营养物之一。外源氮负荷(分点源和非点源两部分)是水环境污染负荷的重要组成部分。传统污水处理技术应用于收集系统欠缺的非点源污染的治理时成本过高。人工湿地是有效削减水环境中外源氮负荷的重要技术手段,在处理非点源污染源带来的氮负荷时更是如此。人工湿地具有氮去除效果好、耐冲击负荷能力强、投资低和生态环境友好等优点。因此人工湿地非常适合于水环境富营养化的防治。阐明人工湿地中氮的去除机理对水环境的富营养化等具有重要的意义。防渗人工湿地的氮去除机理主要包括挥发、氨化、硝化/反硝化、植物摄取和基质吸附。未防渗的人工湿地中,周围水体与人工湿地的氮交换影响着人工湿地中氮的去除。一般情况下,人工湿地中硝化/反硝化是最主要的氮去除机理。pH值小于7·5时,氨挥发可忽略。pH值在9·3以上时,氨挥发很显著。处理生活污水的人工湿地中氮的去除主要是依靠微生物的硝化/反硝化作用。在进水负荷低、气候适宜、植物物种适宜和收割频率与时机适宜的条件下,植物收割可能成为主要的去氮途径。人工合理导向的湿地的氮去除效果通常优于天然湿地。合理的设计(填料的搭配、植物物种的配置以及布水和集水的优化)对人工湿地系统中氮去除的改善有重要影响。合理的运行,如有效的水位控制,正确的植物培育、合理的植物收割等,能有效地改善湿地中的氮去除。

Eutrophication of lakes and aquatic systems deteriorate the ecosystem and economy. The causes and controls of eutrophication have attracted attentions worldwide, including China. Extensive research has been carried out on problems related to eutrophication in China. Nitrogen is one of the major nutriments contributing to the eutrophication in aquatic environments. The external nitrogen load （including point source and nonpoint source） is an important component of the total water-pollutant load. The traditional wastewater treatment technology, which lacks in integrated collection systems, poses excessive investment costs on the control of nonpoint pollution. The constructed wetland is an important technology to reduce the external nitrogen load, especially from the nonpoint source. It is capable of effectively removing nitrogen and handling a high pollutant load; moreover, it is friendly to eco-system and requires low investment costs. Thus the constructed wetland is especially fit for controlling the eutrophication. Illustrating the nitrogen removal mechanisms in constructed wetland has an important meaning on the eutrophication in aquatic systems. The nitrogen removal mechanism in the constructed wetland with a leak-proof layer consists of volatilization,ammonification, nitrification/denitrification, plant uptake, and matrix adsorption, is reviewed in this paper. In a constructed wetland without a leak-proof layer, the nitrogen exchange with the surrounding environments has significant effects on the nitrogen removal rate. Generally, nitrification/denitrification is the major nitrogen removal mechanism in constructed wetlands. Volatilization is insignificant when the pH of the wastewater is less than 7.5. Volatilization is significant when the pH is higher than 9.3. Nitrification/denitrification is the major nitrogen removal mechanism in constructed wetlands treating domestic wastewater. Under low influent loads, suitable climate conditions, and appropriate plant species, the plant-harvest process maybe the major nitrogen removal pathway. Well-managed artificial wetlands usually show a better nitrogen-removing efficiency than do natural wetlands. Sound system designs （e. g., the medium configuration, the arrangement of helophytes, and the opitimization of water distribution and collection） have a direct impact on the nitrogen removal in constructed wetlands. Reasonable operations and maintenances, such as the effective control of the water level, the correct helophyte cultivation, and the sound helophyte harvest can effectively improve the nitrogen removal in a constructed wetland.