Managing organic micropollutants in rivers : From monitoring to mitigation

The occurrence of organic micropollutants in the aquatic environment is ubiquitous. They comprise compounds from diverse application areas such as pharmaceuticals, personal care products, pesticides or compounds used during industrial processing each containing a variety of sub-classes and substances.Several micropollutants have negative impact on the environment due to their toxicity, persistence and bioaccumulation potential. For this reason, European legislations such as the Water Framework Directive require the observation of such harmful micropollutants. For some of them, environmental quality standards (legal thresholds) are already in place and it is expected that the number of micropollutants that will require monitoring and have environmental quality standards will grow in the near future.Therefore, water managers are confronted with new challenges: to measure and evaluate the occurrence of organic micropollutants in surface waters and to find adequate measures to mitigate them in the water cycle. Thus, an application-oriented research was accomplished with the aim to understand the origin, transport and fate of wastewater associated organic micropollutants and moreover to find a “ready-for-implementation” measure to reduce these emissions to the environment.The research was conducted in the Swist basin, a small and well-defined catchment area within the greater rhine river catchment, close to the city of Bonn in North Rhine-Westphalia, Germany.In Chapter 2, emissions entering the river are assessed. Concentrations of carbamazepine, diclofenac and metoprolol were studied in the effluent of the four sewage treatment plants within the Swist river catchment. It is found that the concentrations of these pharmaceuticals vary a lot within (and between) the treatment plants (33 – 55% standard deviation). This is related to the hydraulic retention time of the wastewater in the sewage treatment plants. The hydraulic retention time in turn is dependent on the amount of sewage treatment plant inflow and thus the hydraulic situation which is heavily affected by the precipitation in the catchment area. A linear relation of concentrations and hydraulic retention time is observed, showing the dilution effect which increases at high sewage treatment plant inflow volumes. Additionally, it was demonstrated that these dilution-induced concentration ranges can be reliably determined with inhabitant specific loads obtained from literature. At hydraulic retention times greater than 80 hours, the concentrations of biodegradable micropollutants (in this case metoprolol and diclofenac) are decreasing due to enhanced removal in the treatment plant. After dry periods and during subsequent storm events with high inflow volumes, it was revealed, that the dilution effect is delayed. This can be explained by incomplete mixing of wastewater in the tanks of the sewage treatment plant.In Chapter 3, micropollutants in the river are assessed. Besides the effluent of the four sewage treatment plants, the river Swist was investigated for the occurrence and short-term concentration variability of carbamazepine, diclofenac, 1H-benzotriazole and galaxolide. It was found that during dry weather conditions, with stable emission input concentrations, the concentrations in the river are fluctuating within the course of a day. Reasons for these diurnal concentration variations are varying sewage treatment plant effluent volumes and in situ removal in the river trajectory. The fate of micropollutants is described by combining mass balances studies and water quality modelling. By applying this method, it is demonstrated that carbamazepine and 1H-benzotriazole are persistent in the river, whereas diclofenac is removed mainly by photodegradation process and galaxolide is removed by diverse removal processes.In Chapter 4 and 5 retention soil filters, a mitigation measure to remove micropollutants from the effluent of a sewage treatment plant is presented. Retention soil filters are a special configuration of vertical flow constructed wetlands, and widely used in Germany and in particular in North Rhine-Westphalia for the treatment of combined sewer overflow and separate sewer outlets. Two kinds of retention soil filter have been investigated for a period of three years for their potential in micropollutant removal from a sewage treatment plant effluent: a conventional retention soil filters with sand, calcium carbonate, planted with reed and a retention soil filter with granular activated carbon as additive. In Chapter 4 it is demonstrated that retention soil filters with granular activated carbon can reduce concentrations of a broad range of different organic micropollutants from sewage treatment plant effluent to below limits of quantification (95% median removal). The conventional retention soil filters can remove micropollutants as well, with best results for galaxolide, diclofenac 4-hydroxy, metoprolol and clarithromycin (75 – 79%). By monitoring the different layers of the retention soil filters individually, it is revealed that the upper layer with highest organic matter content is most effective.In Chapter 5 the effluent polishing as studied in Chapter 4 is combined with the treatment of combined sewer overflow. These represents the intended application of a new generation of constructed wetlands: the retention soil filter for the combined usage of sewage treatment plant effluent treatment during dry weather and of combined sewer treatment in case of storm and overflow events. It is shown that total organic carbon, dissolved organic carbon, nutrients and several micropollutants can be efficiently removed from combined sewer. After the treatment with combined sewer, some compounds (e.g. metformin) were washed out from the filter material at subsequent treatment with sewage treatment plant effluent. It is determined that with a dry period of 18 h between the two treatment cycles, this wash out effect can be prevented. Finally, it is concluded that both combined sewer overflow and sewage treatment plant effluent can be treated with the same retention soil filter without any negative long-term effects on its performance.In Chapter 6 the advantages and challenges of combining scientific and application-oriented work is discussed with regard to monitoring, pilot plant studies and environmental fate research conducted at a whole river basin, all under the influence of uncontrollable environmental conditions. It is shown how the results of this work are implemented by the water board Erftverband, and how the results and conceptual findings can be translated to the wider context of the “science for impact” approach and more specifically the organic micropollutant management in river basins.