除氮生物反应器净化农田排水的研究及应用潜力分析

农田排水氮素输出是造成水环境污染的重要原因。近年来涌现的生物反应器除氮技术通过在排水末端增设固体碳源装置,将农田排水部分或全部导入后,使其中的硝态氮通过反硝化反应得到去除。该文回顾了国际上现有生物反应器净化农田排水的研究进展,并分析了农田生物反应器在中国南方湿润区应用的潜力。现有研究结果表明,除氮生物反应器可以有效净化排水水质,平均每年可降低23%～98%的氮素负荷;是一种占地面积少且水质净化效率高的农田控污措施。生物反应器的除氮效果与农田排水过程密切相关,并受到介质特性、入流、出流条件以及环境因子等的影响。中国南方平原作物生长及排水过程相对集中,气象等环境条件非常适于生物反应器的反硝化反应。如何因地制宜确定反应器安装位置与设计尺寸,筛选填料介质,并通过对排水过程的调控来优化生物反应器的除氮效果是需要进一步研究的问题。生物反应器系统后期管理与内部反应动态监测是保证系统正常运行的重要手段。利用生物反应器来净化农田排水具有良好的应用前景,该文可为推动生物反应器在中国的应用研究以及排水污染治理提供理论依据与技术支撑。

Advances in denitrification bioreactor research and its potential applications in agricultural drainage water purification

Nitrogen loss with farmland drainage is a major cause for water quality degradation in aquatic environment. Denitrification bioreactors installed at field edges have been found effective in reducing nitrogen loss from agricultural fields; they have been widely investigated in recent years as an emerging technology to reduce agricultural nonpoint source pollution. Filled with carbon media, such as woodchip, rice husk and wheat straw etc., denitrification bioreactors may remove nitrate-nitrogen through denitrification process from drainage water, which was either partially or fully introduced into the device. This paper reviewed existing studies on the denitrification bioreactors that were designed to purify farmland drainage water; different factors that affect the denitrification processes were synthesized, and the potential application of such bioreactors in the humid regions of southern China was analyzed. The current research findings from both laboratory and field studies showed that denitrification bioreactors with variable sizes and designs can effectively improve drainage water quality; the reported annual NO3--N load reductions ranged from 23% to 98% in existing literature. Compared with other water treatment devices (e.g. wetlands) in agricultural landscape, denitrification bioreactors occupy only a small portion of land area but they exhibit high efficiency in reducing nitrogen load to receiving waters; they have been advocated as economical and effective water quality purification devices. The denitrification effect of bioreactors is closely related to the carbon media characteristics, drainage inflow and outflow conditions, and some environmental factors. Internal temperature control and carbon media selection are important factors for sustainable operation of field bioreactors. How to select carbon media to maintain their hydraulic conductivity and performance life in the bioreactors needs further study. Determining the appropriate drainage inflow and its hydraulic retention time based on the local drainage pattern is crucial for achieving certain nitrogen reduction goal. Nitrate reduction rate normally increases with the hydraulic retention time, but very high retention time is related to over reduction in the bioreactors. Installing a control structure at the inlet of bioreactor may help adjust hydraulic retention time of drainage water to prevent undesirable reaction processes. To size a bioreactor according to drainage system design remains a challenge due to variable weather, soil and cropping conditions; applicability of the existing methods proposed in different studies needs further examination. Field monitoring of bioreactor performances is needed to ensure proper system function; the advances of monitoring technologies may help provide better insights on internal processes in bioreactors. In the warm southern plain area of China, agricultural drainage processes are relatively concentrated in the summer growing season. Such environmental conditions are favorable for adopting denitrification bioreactors to treat drainage water discharge. Based on a regression analysis using reported data of nitrate reduction and the hydraulic retention time, the calculated land requirement for bioreactor installation in a case study area in southern China was only 0.117% of the drainage area to achieve 50% nitrogen reduction in drainage water. Findings from this research may help guide denitrification bioreactor research and their applications to reduce agricultural non-point source pollution from drainage water discharge.