Ozonation of trace organic compounds in wastewater : identification and environmental impact of transformation products

• Ozonierung organischer Spurenstoffe im Abwasser : Identifizierung und Umweltauswirkung von Transformationsprodukten

Diehle, Miriam; Pinnekamp, Johannes (Thesis advisor); Schäffer, Andreas (Thesis advisor); Linnemann, Volker (Thesis advisor)

Aachen : RWTH Aachen University (2020, 2021)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2020

Abstract

Trace organic compounds are substances that are present in wastewater or water bodies in low concentrations (ng to µg/l) because of anthropogenic activities. These substances are ubiquitously distributed in the aquatic environment and it is unclear how they affect the ecosystem in a long-term perspective. One point source for entry of trace organic compounds in water bodies is wastewater treatment plants. To minimize the entry due to effluents, advanced wastewater treatment techniques are currently tested and implemented. One technique for the removal of trace organic compounds from wastewater is ozonation, which is installed after the biological treatment step. During this treatment, ozone is passed through the wastewater and reacts, directly or indirectly via the formation of hydroxyl radicals, with the trace organic compounds present. In most cases, trace organic compounds are not mineralized during this reaction, leading to the formation of transformation products (TPs). The structure and thus the environmental properties of these TPs are unknown. For structure elucidation of TPs of trace organic compounds and the understanding of possible reaction pathways, laboratory-scale ozonation experiments with single trace organic compounds can be used. Additionally, it is important to investigate the actual occurrence of TPs in wastewater treated with ozone. For this purposes, two approaches using high-performance liquid chromatography coupled to high-resolution mass spectrometry (HPLC-HRMS) were pursued in the present thesis. Moreover, potential effects of detected TPs on the aquatic environment were investigated for both approaches using ecotoxicological tests or quantitative structure-activity relationships (QSAR). 1. Identification of transformation products resulting from the ozonation of single trace organic compounds: Since it is impossible to determine the source and the reaction pathway of single TPs due to the complex composition of wastewater, laboratory-scale ozonation experiments using single substances were necessary. In this thesis, two substances, valsartan and candesartan, from the substance class of sartans were ozonated in laboratory experiments. Sartans are used for the treatment of hypertension and showed a significant increase in prescription rates and consequently an increased release into wastewater during the last years.For both valsartan and candesartan multiple TPs are formed during ozonation. While the detected TPs of valsartan are solely formed due to the reaction with hydroxyl radicals, the TPs of candesartan are partly generated via the direct reaction with ozone. Overall, 51 and 24 TPs were analytically detected for valsartan and candesartan, respectively. Possible structures and one definite structure could be proposed via structure elucidation for about half of these substances. Reactions that lead to the formation of TPs occur at different sites of the investigated molecules and for some TPs, reaction pathways were postulated. Except for one TP, the ozonation of valsartan and candesartan led to different TPs enabling no predictions about the behavior of other substances from the class of sartans during treatment with ozone. Ozonation experiments were performed at different specific ozone dosages. For most TPs, maximum formation occurred at ozone dosages that are used for the treatment of municipal wastewater. Consequently, it is expected that the detected TPs are also formed under actual treatment conditions. In the future, this should be investigated including these TP in suspect screening procedures of real wastewater samples. Concerning the environmental behavior of TPs, on the one hand, acute and chronic tests with luminescent bacteria were performed. For this purpose, ozonated samples, containing the mixture of all TPs, were investigated. Like the initial valsartan and candesartan solutions, these samples showed no effects on luminescent bacteria. Tested concentration levels were already above concentrations expected to occur in the aquatic environment. On the other hand, lipophilicity, biodegradability under aerobic conditions and acute and chronic toxicity towards fish, green algae and daphnia were investigated for those TPs, which were assigned a definite structural formula, using QSAR models. Most TPs show lower lipophilicity compared to the parent substance valsartan or candesartan and thus, a decreased tendency for bioaccumulation. Moreover, most TPs show an increased biodegradability. Toxicity estimations indicate similar or lower toxicity compared to the parent substance concerning the species mentioned before. Overall, these results allow the conclusion that the discharge of the investigated TPs does not lead to unacceptable effects on the aquatic environment. However, this leaves potential mixture toxicities out of consideration. 2. Identification of transformation products in real wastewater samples: To detect TPs in real wastewater samples, a suspect screening procedure using HPLC-HRMS as the analytical method was developed. This procedure was used for the investigation of samples from the in- and effluent of a pilot-scale ozonation unit of a municipal wastewater treatment plant and its biological post-treatment unit. Die method developed was suitable for multiple trace organic compounds and showed method detection limits down to the low ng/l range. Wastewater samples were analyzed for a total of 188 TPs of trace organic compounds, which were compiled in a database from peer-reviewed literature. In the samples from a total of eight samplings, 57 TPs included in the database were potentially detected. The detected TPs originate from 20 different parent substances. High identification confidence was obtained for one third of the detected TPs, whereas one TP of the β-blocker metoprolol was unequivocally identified via a reference standard. The occurrence of TPs dependent on the applied specific ozone dosage shows a trend towards a higher number of TPs at higher ozone dosages. However, the formation of TPs is not only dependent on the ozone dosages, but also on other parameters such as the hydroxyl radical exposition or the composition of the wastewater matrix. The analysis of the samples after biological post-treatment revealed high elimination capacity for one-third of the detected TPs. This leads to the conclusion that during biological post-treatment TPs are only partly removed and thus released into the receiving waters. Consequently, the need for biological post-treatment should further be investigated. QSAR models for the assessment of environmental behavior were also applied for the TPs that were identified via suspect screening. Compared to their parent compounds, TPs show an increased biodegradability under aerobic conditions. Moreover, their potential of causing toxic effects towards aquatic organisms remains on a similar level as those of the parent substances. In total, these estimations show that the ozonation of trace organic compounds in wastewater will presumably not lead to adverse effects on the aquatic environment.