Fate and removal of emerging contaminants in water and wastewater treatment plants

Organic MicroPollutants (OMPs) – also called Emerging Contaminants or Contaminants of Emerging Concern – include a wide number of chemicals belonging to different classes, e.g. pharmaceuticals and personal care products (PPCPs), drugs of abuse and their metabolites, steroids and hormones, endocrine-disrupting compounds, surfactants, perfluorinated compounds, phosphoric ester flame retardants, industrial additives and agents, siloxanes, artificial sweeteners, and gasoline additives (Barbosa et al., 2016; Bletsou et al., 2015; Chiavola et al., 2019). In the last two decades, increasing attention has been dedicated to OMPs, as a matter of high risk for public health and environment. (Naidu et al., 2016; Rodriguez-Narvaez et al., 2017; Thomaidi et al., 2016; Vilardi et al., 2017). OMPs are characterized by low environmental concentrations (about ng/L or µg/L), high toxicity, very low biodegradability and resistance to degradation and to conventional treatments. Consequently, they tend to be bioaccumulated in aquatic environments, and to enter the food chain through agriculture products and drinking water (Clarke and Smith, 2011). Measurement of OMPs in the aquatic medium became possible only in the last 20 years, thanks to the improvement of sensitivity and accuracy of the analytical methods; among the different methods, liquid chromatography coupled with high-resolution tandem mass spectrometry (LC-HRMS/MS) is increasingly applied for the analysis of some known and unknown emerging contaminants in water. However, for a number of OMPs, the optimization of analysis conditions and procedures is still insufficient to allow routine monitoring (Boni et al., 2018). The scientific community established that one of the main source of release in the environment is represented by the wastewater treatment plants (WWTPs), which are not specifically designed and operated to remove OMPs (Sousa et al., 2017). Therefore, improvements of the WWTPs performance are needed to reduce the load of OMPs released into the environment through either the final effluent and wasted sludge (Trapido et al., 2014). Extent of their removal/transformation in the different units of the WWTPs is still not completely known and depends on numerous parameters and conditions. Therefore, it would be very useful to assess the removal efficiency achievable in the main treatment units, and particularly in the biological process which often represents the core of the plants; it is also important to assess if efficiency can be enhanced by properly modifying the operating conditions (e.g. the sludge retention time). Among the treatment processes investigated so far for the removal of OMPs from wastewater, the biological treatments provided interesting and promising possibilities, in terms of costs and environmental impact with respect to physical-chemical processes, at least for a number of OMPs (Ahmed et al., 2017). Assessment of effective removal in the biological processes is made more complicated because various OMPs transformations can take place in the reactor, determining new compounds release (transformation by-products, TPs) which, to some extent, differ in the environmental behaviour and ecotoxicological profile from the original substance (parent compound, PCs) (Hollender et al., 2017). Furthermore, TPs may be more toxic, persistent and less biodegradable of their parents and are usually unknown and unpredictable. These issues highlight the needs of further investigation which must be based also on non-target screening (NTS) approach (Schollée et al., 2018). In an attempt to fill some of the gaps in the knowledge of OMPs behaviour in water treatment plants, various aspects of the subject were approached in the present Ph.D. thesis. In order to contribute to fill some of the gaps in the knowledge about OMPs in water treatment plants, different aspects of this problem were addressed in the present Ph.D. thesis. Among the wide class of OMPs, the first step of the present study focused on some drugs of abuse, specifically Benzoylecgonine (BE), 11-nor-9carboxy-Δ9-THC (THC-COOH) and Methamphetamine, and on the most abundant perfluorinated compounds present in the environment, which are Perfluorooctanoic acid (PFOA) and Perfluorooctanesulfonic acid (PFOS). The double purpose of this part of the thesis, carried out through laboratory scale investigations, was (1) to optimize the analytical method for the detection of these compounds in wastewater and sludge of a WWTP and (2) to determine the removal rate through abiotic and biotic processes in the biological reactor of the WWTP. The results obtained allowed to assess the optimal conditions of the analytical method: therefore, under these conditions, the method is suitable for rapid and reproducible measurements, minimizing the interferences due to the other compounds always found in wastewater and sludge. About goal (2), contribution of biodegradation and other processes (e.g. adsorption and volatilization) was quantified and the kinetic parameters determined. Furthermore, it was evaluated through a standard respirometric procedure (n. 209 OECD) if the presence of these contaminants at increasing concentrations can negatively affect the microbial activity in the biological reactor, and particularly the nitrification and COD oxidation processes. Complementary to the assessment of the removal achieved by the activated sludge processes was the in-depth analysis of the enzyme biocatalytic activity with the aim to enhance the efficiency of the OMPs degradation in the biological reactor. This investigation was carried out at the Auckland University, New Zealand, during a 6-months period of research. Particularly, this innovative approach can induce the synthesis of OMPs degrading enzymes by exposing microbes to cycles of stressing and non-stressing environmental conditions. In the present study, stimulation of oxidoreductase production by microbial cells was favoured by varying the dissolved oxygen concentration within the reactor. This strategy showed to be successful, being capable of enhancing the removal rate of some OMPs; furthermore, its implementation at full-scale would contribute to reduce the energy cost of the aeration system and also allow simultaneous nitrification-denitrification within the same tank (Han et al., 2018; He et al., 2018). As mentioned above, in parallel to the concern about OMPs removal processes, a new issue was highlighted in the past ten years: the formation of transformation products (TPs) from wastewater and water treatment. These substances are often unknown and can be more toxic than their precursors (Li et al., 2017). Several studies focused on TPs produced by wastewater treatment and on their environmental risk assessment (Bletsou et al., 2015; Schollée et al., 2018, 2016; Schymanski et al., 2015). However, the knowledge and scientific data concerning TP monitoring in drinking water are still limited.