Abstract
Although the occurrence, fate and behaviour of micropollutants in the urban water cycle is reported extensively in literature, the biotransformation of micropollutants in the sewer network has only received little attention. Rising main (RM) and gravity main sewer biofilms (GS) are capable of transforming sulfur species and organic compounds biochemically. Moreover, first laboratory-based and full-scale studies demonstrated the capability of sewer biofilms to transform illicit drugs, human drug metabolites and other human biomarkers, such as caffeine. However, the in-sewer biotransformation of pharmaceutically active compounds (PhACs) and pesticides has not received much attention. The aim of this thesis was to understand the role of sewer biofilm during the removal of micropollutants by specifying the removal mechanisms, identifying the contributing microbial communities and elucidate in-sewer transformation products (TPs) and pathways. Thus, the fate and behaviour of 30 PhACs of various therapeutic groups and 4 pesticides was investigated in RM and GS sewer biofilm reactors, as well as pilot full-scale RM and GS sewers.The capability of RM biofilm to remove the investigated micropollutants and corresponding removal mechanisms were assessed during laboratory-based batch experiments at elevated initial concentrations of 50 µg L-1. Biotransformation was the dominant removal mechanism, with significant sorption of cationised and/or lipophilic compounds (by 80-100%, i.e. propranolol, enrofloxacin, erythromycin, lincomycin, ranitidine, sertraline, fluoxetine and sulfasalazine). A comparison of the overall removals of the investigated micropollutants at different concentration levels, i.e. 50 µg L-1 and 5 µg L-1, exhibited significantly higher removal of sorbed compounds at lower initial concentrations. Furthermore, negative removals of sulfadiazine, dapsone and erythromycin were observed at lower initial concentrations, likely due to deconjugation of human metabolites in the utilised sewage or the release of faeces-bound compounds into the aqueous phase. A comparison of removal efficiencies in RM with GS at 50 µg L-1 indicated higher capability of RM to transform compounds that are persistent to aerobic treatment, such as carbamazepine, trimethoprim or macrolide antibiotics and halogenated compounds.The contributions to transformation of micropollutants by the dominant RM microbial biofilm communities, i.e. sulfate-reducing bacteria (SRB) and methanogenic archaea (MA) were investigated during laboratory-scale experiments after the inhibition of either SRB, MA or both simultaneously. Significantly lower micropollutant removal efficiencies were yielded when SRB and MA were inhibited simultaneously compared to uninhibited RM. Furthermore, inhibition of either SRB or MA yielded similar removals to those obtained without inhibition. The results demonstrate the vital role of SRB and MA during the in-sewer biotransformation of micropollutants, with both microbial communities capable to transform many compounds. In particular, atrazine, iopromide, DEET and metolachlor were removed by SRB, however not by MA. Trimethoprim and atenolol, on the other hand, were only removed by MA. While no clear relationship was found between removal by the functional organism and the compounds’ molecular properties, the functional groups of a micropollutant may be the determining factor for the preferred transformation by SRB or MA.Analysis of the in-sewer metabolic pathways of the antibiotics roxithromycin and trimethoprim revealed the formation of 8 in-sewer TPs. Roxithromycin mainly underwent microbial inactivation through hydrolysis of the lactone ring or phosphorylation of the desosamine moiety, yielding previously unreported anaerobic in-sewer TPs. Also, four anaerobic in-sewer TPs of trimethoprim were elucidated, three of which with preserved antibiotic activity due to an intact diamino pyrimidine moiety. These results highlight the capability of SRB and MA to transform pharmaceuticals into TPs with potentially preserved active antibiotic activity.Experiments in pilot full-scale RM and GS sewer pipes were conducted at ambient initial concentrations. Removal efficiencies in RM and GS were up to 60% and 70%, respectively, however with readily degradable sulfasalazine and cephalexin yielding up to 100% removal. Furthermore, full-scale RM exhibited negative removal of 10 compounds, including diclofenac, while only 3 compounds exhibited negative removal in GS. The overall removal of the majority of micropollutants was lower than observed during laboratory-based experiments, especially in RM, likely due to i) different flow conditions, ii) possibly different microbial biofilm community between full-scale and laboratory biofilms and iii) lowered removal of parent compounds due to their introduction into the bulk sewage by deconjugation of human metabolites or release of faeces-bound compounds. In-sewer biotransformation was confirmed, as ~3% of removed trimethoprim was transformed into 3-desmethyl-trimethoprim and 48% of the removed N4-acetyl-sulfamethoxazole was deconjugated into the parent compound sulfamethoxazole. The low transformation ratio between these compounds and their TPs strongly suggest the formation of other unknown in-sewer transformation products and pathways.The outcomes of this thesis enhance our understanding of the fate and behaviour of selected micropollutants in sewers. The biotransformation kinetics and pathways reported here can be of great interest when predicting the pharmaceutical loads in the wastewater treatment plant inlet after sewer passage, but also in sewage-based epidemiology, where neglected biotransformation of drugs leads to wrong estimations of consumption in a target population.
Published Version
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