Strategies to enhance the removal of organic micropollutants during wastewater post-treatment

Resumo

The presence of pharmaceuticals, hormones and personal care products in the aquatic environment is an issue of increasing concern since, even though these compounds are detected at environmental concentrations ranging from ng L-1 to µg L-1, their particular properties (bioaccumulative, potentially toxic, endocrine disrupter potential…) can entail several risks on the aquatic organisms, such as antibiotic resistance and estrogenic activity. Wastewater treatment plants (WWTPs) are considered one of the main input sources of these organic micropollutants (OMPs) in the environment as their removal during the conventional treatment is only partial. Consequently, the application of post-treatment technologies emerges as a promising solution to increase the removal of OMPs, and consequently, reducing the potential risk associated to the discharge of WWTPs effluents in rivers, lakes and oceans. The main objective of this thesis is to assess the application of different strategies, based on the evaluation of several post-treatment technologies and/or on the selection of operating conditions, with the purpose of maximizing the removal of OMPs (mainly pharmaceuticals, cosmetics and hormones) from secondary wastewater effluents. The post-treatment technologies assessed are focused on both advanced oxidation processes (UV treatment and heterogeneous photocatalysis) and adsorption/filtration processes (granular activated carbon (GAC) and sand (bio)filtration systems). The evaluation of operational factors is mainly based on the influence of wastewater composition and the treatment time. The results of this thesis provide interesting information about the potential of the selected post-treatment technologies for the removal of different types of OMPs and the operational factors limiting their application. Regardless the technology, it was demonstrated that the efficiency for OMPs abatement is strongly linked to the secondary effluent characteristics, being the dissolved organic matter and suspended solids content identified as the main parameters affecting to the capacity of these systems. The efficiency of UV systems is associated to the application of UV doses much higher than those often used for disinfection in WWTPs, which supposes an economic limitation. Consequently, the application of novel supported catalysts, such as those based on immobilized TiO2 on PVDF hollow fibre membranes, emerges as a promising option to increase the phototransformation rate of OMPs, even showing a higher efficiency than that obtained when using a similar concentration of the conventional suspended TiO2 Degussa catalyst. In the case of (bio)filtration systems, only the application of GAC as filtering material was demonstrated to be efficient to remove selected OMPs (including compounds poorly eliminated during the biological treatment, such as carbamazepine, diazepam and diclofenac) employing the typical empty bed contact time values used in WWTPs. Additionally, GAC filters were identified as versatile technologies to treat secondary effluents of different quality, being their application specially interesting for the post-treatment of effluents coming from membrane bioreactors due to their low maintenance and extended lifespan compared to the filters treating settler effluents. Finally, the application of an integrative approach, which combines chemical analysis and biological assays, was identified as a necessary tool to evaluate the potential of post-treatment technologies towards OMPs and their related toxicity abatement. To sum up, this thesis contributes to the broadening of the current state of knowledge concerning the behaviour and fate of OMPs during the application of several post-treatment technologies and it provides information about the strategies that should be applied for OMPs abatement during the treatment of different types of secondary wastewater effluents.