Efficacité des couplages digestion anaérobie mésophile- réacteur thermophile pour la réduction de la production de boues, la production de biogaz et l’élimination des micropolluants

The management and valorization of sludge, a by-product of wastewater treatment in sewage treatment plants, is a major challenge today. The large amount of sludge produced, along with the presence of pathogens and adsorbed micropollutants, raises significant concerns. Anaerobic digestion offers considerable potential to reduce sludge volume and eliminate micropollutants (Gonzalez-Salgado et al., 2020). Furthermore, biogas, produced during anaerobic digestion, is a valuable by-product that can be injected into the natural gas grid or used as fuel. The MAD-TAR configuration, combining a mesophilic digester with a micro-aerated thermophilic digester, has been studied in previous research. This configuration significantly improved sludge reduction, with gains of 9.1% and 26.2% in terms of COD compared to the conventional mesophilic digester (Dumas et al., 2010; Gonzalez-Salgado et al., 2020).Experimental campaigns were conducted on three distinct configurations (MAD, MAD-TAD, which couples a mesophilic digester with a thermophilic digester, and MAD-TAR) with a liquid volume of 8 L for the MAD and 51 L for the MAD-TAD and MAD-TAR. The sludge retention time (SRT) was approximately 15 days for the MAD reactors and one day for the coupled reactors (TAD and TAR). A mixed sludge feed, which had already undergone a fermentation step (acidogenesis) in all three configurations, was used to validate the results against previous observations in the field. The main objectives were to study these configurations in terms of sludge reduction, biogas production, micropollutant removal, and the environmental impact of digestates regarding ecotoxicity and antibiotic resistance. Comparing these configurations allows us to differentiate between the effects of temperature and oxygen. An experimental approach (results of particulate matter hydrolysis, production of VFAs and biogas), combined with modeling using Aquasim, was adopted to study the dynamics of populations during the startup phase of the hybrid configurations (MAD-TAD and MAD-TAR).The first part of the thesis presents the experimental results of the three configurations. The MAD-TAR configuration showed a significant improvement in particulate matter hydrolysis (14%), in contrast to MAD-TAD, suggesting a key role for aeration. Additionally, biogas production was lower (-9%) in MAD-TAR compared to MAD and MAD-TAD. Changes in hydrolysis rates, transient VFA accumulations, and variations in biogas production were observed in MAD-TAD and MAD-TAR. In the second part of the thesis, modeling showed that these changes could result from disturbances in indigenous populations in the TAD and TAR. The hypothesis introducing two types of microbial populations, thermo-sensitive (Ts) and thermo-resistant (Tr), explored through modeling, demonstrated a better match with experimental data, suggesting the emergence of thermo-resistant populations when exposed to high temperatures.In the third part of the thesis, it was found that OFL is more effectively biodegraded in the MAD-TAD and MAD-TAR systems than in MAD, thanks to the high temperature. Moreover, the presence of oxygen in MAD-TAR enhances CBZ biodegradation compared to MAD. However, the raw digestate and the liquid fraction from MAD-TAD are more toxic than those from MAD and MAD-TAR, likely due to the higher ammonia concentration in TAD resulting from the elevated temperature. In contrast, the digestate from MAD-TAR is less toxic, probably due to aeration and ammonia stripping.