曝气对垂直流湿地处理水产养殖废水脱氮的影响

人工湿地作为一种有效的污水处理技术,现已被逐渐拓展到水产养殖业中。鉴于其与养殖竞争有限土地资源的弊端,如何构建节地高效型湿地成为未来研究的重点。曝气增氧是强化潜流湿地净化效能的重要措施之一,但是关于曝气强度以及净化效率与影响因素的关系仍缺乏深入系统的研究。为此,该文设计构建了7组不同要素组合的垂直流湿地小试系统,同步或分阶段探讨了曝气强化对垂直流湿地脱氮的影响。研究结果表明,无论曝气与否,构建的7组湿地系统于试验运行工况下都存在明显的硝化过程,且空气复氧和植物根系泌氧足以弥补硝化作用耗氧量。曝气增氧进一步强化了湿地内部的矿化和硝化过程;鉴于养殖废水不缺乏碳源(该研究各组湿地进水碳氮比在28.4~30.6之间),湿地内部的反硝化几率增大,导致曝气后总氮的去除效率提高。但是曝气条件下过高的溶解氧又会进一步抑制反硝化过程,从而也会导致系统总氮去除速率的下降。因此,对垂直流湿地而言,曝气强度不是愈高愈好。为了获得更高的脱氮效率,建议可以通过延长水力停留时间或者在垂直流湿地尾部增设水平潜流湿地来补充反硝化过程,进而提高系统对总氮的去除效果。

Impact of artificial aeration on nitrogen removal from aquaculture wastewater treated by vertical-flow constructed wetland

Abstract: Due to the serious trend of water pollution across the country, the problem of aquaculture wastewater discharge must be solved appropriately to achieve sustainability. As a novel technology for sewage treatment, constructed wetland (CW) has been gradually expanded to aquaculture. In view of the disadvantages in land dispute with pond aquaculture, how to develop or design a land-saving, high-efficiency CW will be the focus of future study. It is widely accepted that artificial aeration can enhance the purification efficiency of CW's subsurface flow on wastewater due to its capacity to improve the oxidation conditions, which is beneficial for organic matter degradation and nutrient removal. Nevertheless, more detailed studies on the impact of aeration intensity on treatment performance and the associated relationships with influencing factors are still lacking. In the present study, 7 pilot-scale vertical-flow CWs with different combinations of substrates and plants were configured and then systematically investigated in field for treating low-strength aquaculture wastewater with or without artificial aeration. An attempt to explore the impact of the aeration on nitrogen (N) removal or transformation within wetland bed was made. After a thorough comparison between aerated and non-aerated states investigated simultaneously or by stages, the results were depicted as follows: under the operating conditions characterized by high hydraulic loading(HLR) (mean value 1.85 m/d), short hydraulic retention time(HRT) (mean 4.6 h), strong aeration intensity (air flow rate 30 m3/(m2·d), air-water ratio 16.2:1) and low inflow dissolved oxygen (DO) (mean 2.34 mg/L), nitrification occurred obviously within all the systems no matter with or without aeration. DO replenished from atmospheric reoxygenation and plant roots appeared enough to cover the quantity consumed by nitrification and organic matter degradation. Artificial aeration enhanced the intensity of internal mineralization and nitrification. In virtue of no lack of available carbon source (for instance in the present case, the influent ratio of chemical oxygen demand to nitrogen (COD/N) ranged from 28.4 to 30.6), the probability for denitrification under the aerated state increased compared to the non-aerated state, which finally led to the elevation of purification performance on total N (TN). Nevertheless, if too much DO was presented under the aerated state, denitrification would further be restrained, which would conversely lead to the reduction of removal efficiency on TN. Therefore, for complete-drain vertical-flow systems, it is not the truth that the higher aeration intensity is, the better efficacy will be obtained. To obtain higher stable purification efficiency, a good suggestion is decreasing HLR or extending HRT or supplementing horizontal flows to vertical-flow systems to ensure complete denitrification. That strategy will be especially beneficial for TN removal.