Assessing the role of microbial communities in the performance of constructed wetlands used to treat combined sewer overflows.

Combined sewer overflows are contaminated with various micropollutants which pose risk to both environmental and human health. Some micropollutants, such as carbamazepine and sulfamethoxazole, are very persistent and difficult to remove from wastewater. Event loaded vertical-flow constructed wetlands (retention soil filters; RSFs) have proven to be effective in the treatment of combined sewer overflows for a wide range of pollutants. However, little is known about how microbial communities contribute to the treatment efficiency, specifically to the reduction of micropollutants. To the best of our knowledge, this is the first study attempting to close this gap. Microbial communities in pilot-scale RSFs were investigated, which showed explicit grouping of metabolic activity at different filter depths with some differential abundance of identified genera. The highest microbial activity was found in the top layer of 0.75 m deep filters, whereas homogeneous activity dominated in a 0.50 m deep filter, indicating oxygen availability to be a limiting factor of the metabolic activity in RSFs. The removal efficiencies of all investigated organic trace substances were correlated to the utilization of specific carbon sources. Most notable is the correlation between the carbon source glucose-1-phosphate and the removal of metoprolol. The strongest correlations for other substances were the removal of diclofenac to the utilization of the carbohydrate i-erythritole; bisphenol A to carbohydrate α-d-lactose, and 1-H-benzotriazole to carbonic acid D-galacturonic acid. Those results are supported by positive correlations of specific microbial genera with both the utilization of the above mentioned carbon sources and the removal efficiency for the respective micropollutants. Most notable is correlation of Tetrasphaera and the removal of benzotriazole and diclofenac.