Anaerobic biotransformation of organic micropollutants focused on the role of enzymes

Resumo

The increasing use of organic micropollutants (OMPs) such as pharmaceuticals, personal care products, hormones, industrial chemicals and pesticides and the lack of specific removal technologies lead to their presence in sewage, surface, ground, drinking water and in biosolid-amended soils. Although OMPs are present at trace concentrations, some of them pose a potential risk to human health and the environment due to their persistence and bioaccumulation. Mitigation strategies of OMPs in sewage treatment plants could be based on optimizing the removal capacity of technologies that have already been implemented in the several steps of wastewater treatment. Anaerobic digestion (AD), the main stabilization process of sewage sludge, could play a key role on removing OMPs and on reducing their toxic effects in sewage sludge, which is a sink where many OMPs accumulate. Due to the lack of knowledge regarding the fate of OMPs during AD, it is hard to develop efficient strategies to promote their complete removal. Therefore, this Thesis aims to go one step forward from just reporting removal efficiencies to fully understand the factors governing the anaerobic biotransformation of OMPs. For that purpose, the influence of different operational parameters on genotoxicity, estrogenicity and OMP removal from real (unspiked) sewage sludge was evaluated. Afterwards, the contribution of the acetogenic-methanogenic step on the anaerobic biotransformation of OMPs was investigated. Furthermore, the role of several enzymatic pathways on the anaerobic biotransformation of OMPs was studied. Finally, the mechanistic limitations that hinder a complete removal of many OMPs in anaerobic processes were addressed. AD has demonstrated the capacity to remove several OMPs and even to reduce their toxic effects of sewage sludge. Operating the digesters under thermophilic instead of mesophilic conditions improved the quality of digested sludge in terms of estrogenicity. In contrast, other operational parameters (i.e., sludge retention time and organic loading rate, in the usual range) did not have any positive impact on OMP elimination. It was proved that methanogenesis is an important anaerobic step for the biotransformation of some OMPs and that there is not a direct relationship between the phase distribution (liquid or solid) of the compounds and their biotransformation degree. Several enzymes active in the AD steps and belonging to different enzymatic classes (i.e., oxidoreductases, transferases, hydrolases and lyases) are proposed as candidates for the biotransformation of OMPs. Their action towards OMPs is strictly dependent on their affinity to the chemical structure of the compound. Finally, the reversibility of enzymatic reactions is proposed as the main restriction impeding a complete anaerobic biotransformation of many OMPs; yet, a decrease in the compound bioavailability by sequestration cannot be ruled out. The resulting insights from this Thesis intend to answer several research questions that need to be figured out to develop feasible and efficient strategies for a safer sludge disposal.