Estudio técnico y biológico de un sistema MBBR con digestión bifásica en línea de fangos para la eliminación de contaminantes emergentes y control de nutrientes en aguas residuales urbanas

Résumé

During the last decades, the production of nutrient-rich wastewaters with a wide range of emerging contaminants such as personal care products, ultraviolet filters, synthetic fragrances and especially the active pharmaceutically compounds (PhACs) has been continuously increasing worldwide due to the exponential growth of urban populations, household consumption and industrial production. Nowadays, conventional wastewater treatment plants (WWTPs) have demonstrated many shortcomings to face more stringent discharge standards in terms of nutrient release and emerging contaminants removal. In this sense, the continues presence of different trace organic contaminants such as PhACs in treated urban wastewaters and treated sewage sludge is an issue of growing concern worldwide. Moreover, several studies have shown that PhACs can be accumulated into the soil, groundwaters and edible portions of plants irrigated with treated wastewater, representing an important exposure of pharmaceutical to humans, with potential health implications. To overcome these problems, the present PhD Thesis has studied the occurrence, fate and removal efficiency (RE) of 27 PhACs in a pilot-scale wastewater treatment plant operated with conventional and advanced processes in different experimental phases. The investigation was developed under a multidisciplinary approach, in search of interlinkages among the removal efficiencies of PhACs, the performance of the treatment system, the changes of operational parameters, and the shifts of the microbial communities’ structure in the sludge, in both the water line and sludge line of the pilot-scale plant. The pilot-scale plant is a scale-copy (1:25,000) of WWTP Murcia Este (Murcia, Spain) and treats up to 6 m3d-1 urban wastewater from the WWTP Murcia Este. The investigation was divided into three experimental phases (Phase I, II and III) in the water line. Phases I and II operated 104 days each with a conventional biological nutrient removal system (anaerobic/anoxic/aerobic, A2O), and with different operational parameters between phases; while in Phase III, the A2O bioreactor was transformed into a moving bed hybrid bioreactor (“integrated fixed-film activated sludge” IFAS system) filling the aerobic chamber 50% with the carrier AnoxKaldnes K5. In the sludge line, the primary and secondary sludge produced by the water line of the A2O system was thickened and treated by two digesters in series using the no conventional two-stage mesophilic digestion system (MAD). The two-stage MAD system was long-term operated during the first two experimental phases (Phase I and II), using two different sets of hydraulic retention times (HRTs) for the acidogenic (AcD) and methanogenic (MD) digesters (phase I, 2 and 12 days; and phase II, 5 and 24 days, in AcD and MD, respectively). Quantitative polymerase chain reaction (qPCR) and Illumina MiSeq sequencing analysis were used to quantify and to investigate the structure, diversity and population dynamics of bacteria, archaea and fungi communities in the activated sludge of the bioreactor and digested sludge of the AcD and MD. Multivariate analyses and Spearman correlation coefficients were used in search of significant links among the REs of the selected PhACs, the abundances of the microbial groups in the bioreactor and digesters, and the changes of environmental/operating variables in the A2O, A2O-IFAS bioreactors and two-stage anaerobic digesters. In the water line, the results showed that the A2O system improved the RE of organic matter, nutrients and 5 PhACs when the biomass concentration (MLSS) was improved and the F/M ratio was reduced in Phase II; accordingly, positive correlations were found among the REs of these PhACs and several operational/environmental variables (MLSS, F/M ratio, N-NH4 + influent concentration and total nitrogen removal rate). However, when the pilot-scale bioreactor operated with A2O-IFAS system in the experimental Phase III, the REs of organic matter, nutrients and especially 8 and 5 PhACs was significantly improved compared to the A2O system (Phase I and II, respectively). Besides, these performance improvements of the A2O-IFAS system occurred with a lower operational cost. In the sludge line, the optimum HRT ranges for an efficient two-stage MAD was found, based in the production of volatile free acid (VFA) and methane in both digesters, confirmed by the strong interlinkages found among operational/performance variables and the relative abundances of bacterial and archaeal groups. Finally, the extension of the HRT from 12 (Phase I) to 24 days (Phase II) in the MD was significantly linked with an improved removal of 7 PhACs. Non-metric multidimensional scaling (MDS) plots and Biota-environment (BIOENV) analysis revealed strong correlations between the bacteria and archaea communities’ structure in the A2O bioreactors in the water line, and the AcD and MD in the sludge line with the shifts of the operational/environmental variables of each bioreactor among the experimental phases, which were subsequently correlated with differences in the efficiency of nutrient removal in the water line and methane production in the sludge line. The BIO-ENV and MDS analysis also established robust interlinkages between the REs of 11 PhACs with the shifts of the relative abundances of different families of Bacteria and genera of Archaea in the water line, while robust correlations were also detected among the REs of 6 PhACs and the relative abundances of several bacterial and archaeal groups in the AcD and MD of the sludge line. These interlinkages pointed out their potential involvement in the biodegradation/biotransformation of these xenobiotics’ compounds in the wastewater and sludge treatment processes.