Microbial communities in reed and peat wetlands used for treatment of municipal wastewater

In wastewater treatment, wetlands are regarded as cost-efficient and sustainable purification systems. Currently, different types of wetland are used for year-round treatment of municipal wastewater (the polishing step after the secondary or tertiary units) in cold climate regions. However, there is a lack of understanding regarding the effect of freezing cold conditions on pollutant removal processes, in particular regarding those linked to nitrogen cycling. This study evaluated the effect of cold winter conditions on contaminant removal, with a focus on nitrogen transformation and removal in a reed-based surface flow (Reed-SF) wetland and a peat-based horizontal subsurface flow (Peat-HSSF) wetland. Year-round full-scale wetland monitoring was complemented with pilot-scale laboratory experiments, which allowed to follow the cold-climate induced effects on contaminant removal and nitrogen processes, along with other water quality, environmental, and microbial parameters. Overall, it was observed that the effect of cold climate conditions on nitrogen removal was dependent on the wetland type in combination with the quality of the inflow water. When treating NH4-N rich wastewater, removal of Ntot, as well as NH4-N, was higher in the Peat-HSSF than in the Reed-SF wetland. Under frozen conditions, NH4-N removal decreased slightly but remained positive in the pilot Peat-HSSF, whereas it declined and even turned into leaching in the pilot Reed-SF wetland. Conditions encountered in the Peat-HSSF wetlands (high abundances of active nitrifying bacteria, high levels of DO and redox potential) supported nitrification, which continued under cold conditions. Whereas in the Reed-SF, a low abundance of active nitrifying bacteria was found, especially in the water column under frozen conditions. DO here was low and decreased in cold conditions with ice cover.

在污水处理领域，湿地被视为兼具成本效益与可持续性的净化系统。目前，寒冷气候地区已采用多种类型的湿地对市政污水开展全年处理（即二级或三级处理单元后的深度抛光工序）。然而，当前学界对严寒环境对污染物去除过程的影响，尤其是与氮循环相关的过程，仍缺乏足够认知。本研究以芦苇表面流（Reed-SF）湿地与泥炭基质水平潜流（Peat-HSSF）湿地为研究对象，评估了冬季低温环境对污染物去除的影响，重点关注其中的氮转化与去除过程。本研究通过全年全尺寸湿地监测与中试实验室实验相结合的方式，追踪了寒冷气候对污染物去除、氮循环过程以及其他水质、环境与微生物参数的影响。整体观测结果表明，寒冷气候对氮去除的影响取决于湿地类型与进水水质的共同作用。当处理富含氨氮（NH4-N）的污水时，泥炭基质水平潜流湿地对总氮（Ntot）与氨氮的去除效果均优于芦苇表面流湿地。在结冰条件下，中试泥炭基质水平潜流湿地的氨氮去除率略有下降但仍保持正向去除效果；而中试芦苇表面流湿地的氨氮去除率则出现下滑，甚至转变为氨氮淋溶过程。泥炭基质水平潜流湿地具备的高丰度活性硝化细菌、高溶解氧（DO）与氧化还原电位条件，可支撑硝化过程在低温环境下持续进行；反观芦苇表面流湿地，其活性硝化细菌丰度较低，尤其是在结冰条件下的水体中，该湿地的溶解氧水平本就偏低，且在低温结冰环境下会进一步下降。