Roles of extracellular polymeric substances on the fate of antibiotic resistance genes in anaerobic digestion of thermally hydrolyzed sewage sludge
Creators: Haffiez, Nervana
Publisher: name: University of Alberta Library
Year: 2022

<p>Antibiotic-related contaminants have recently been detected globally in natural and engineered ecosystems. Due to high concentrations of residual antibiotics in wastewater and organic feedstocks used for anaerobic digestion (AD), wastewater treatment plants (WWTPs) and sludge are substantial antibiotic resistance genes (ARGs) reservoirs. The inappropriate disposal of antibiotics-contaminated sludge led to the continuous emergence of antibiotic resistant bacteria (ARB), ARGs, and residual antibiotics, which pose severe environmental and human public health threats. AD is a widely applied technology for sludge treatment in WWTPs; however, conventional mesophilic AD is often ineffective in efficiently removing ARGs. On the contrary, thermal hydrolysis pre-treatment (THP) of sludge was found to reduce ARGs abundance in AD efficiently. Extracellular polymeric substances (EPS) in sludge play a critical role in ARG propagation. Particularly, EPS-associated ARGs abundance and their propagation in AD are overlooked. However, the detailed characterization of EPS components and their impact on ARGs propagation is still unclear. This master’s thesis focuses on understanding the roles of THP, and EPS components on ARGs abundance in AD by investigating the fate of EPS-associated ARGs and evaluating their effect on the total ARGs in AD of thermally pretreated sludge. <br>
First, the positive impact of the THP of sewage sludge on ARGs removal during AD has been reported in the literature. However, little information is available on how changes in different EPS due to THP can influence ARG propagation during AD. This study focused on systematically correlating EPS components and ARG abundance in AD of sewage sludge pretreated with THP (80 °C, 110 °C, 140 °C, 170 °C). THP under different conditions improved sludge solubilization, followed by improved methane yields in the biochemical methane potential (BMP) test. The highest methane yield of 275 ± 11.5 ml CH4/g COD was observed for THP-140 °C, which was 40.5 ± 2.5% higher than the control. Increasing THP operating temperatures showed a non-linear response of ARG propagation in AD due to the rebound effect. The highest ARGs removal in AD was achieved with THP at 140 °C. The multivariate analysis showed that EPS polysaccharides positively correlated with most ARGs and integrons, except for macrolides resistance genes. In contrast, EPS protein was only strongly correlated with β-lactam resistance genes. These results suggest that manipulating THP operating conditions targeting specific EPS components will be critical to effectively mitigating the dissemination of particular ARG types in AD. <br>
Second, recent studies have found that EPS in sludge may be an important ARGs reservoir. Although intracellular ARGs have gained considerable attention since they could be transferred by transduction or conjugation in sludge, extracellular ARGs are overlooked. However, no information is available on the fate of EPS-associated ARGs and their effect on the total ARGs abundance during AD of TH pretreated sewage sludge. The abundances of intracellular ARGs and extracellular including EPS-associated and cell-free ARGs were investigated under low and high temperature THP (90° C and 140° C). Also, the EPS components (proteins, polysaccharides, and eDNA) were correlated with the intracellular and extracellular ARGs abundance before and after AD. THP at both temperatures could increase the sludge solubilization, hence enhancing methane production in AD. The maximum methane yield of 305.7±4.7 ml CH4/g COD was observed for 140°C-AD. The lowest EPS-associated ARGs abundances were detected at 140°C-AD, where the highest abundances were observed at 90°C-AD and control-AD. The EPS-associated ARGs represented a higher ratio of the total extracellular ARGs (EPS-associated and cell-free ARGs) in all samples. The multivariate analysis showed that EPS-polysaccharides, proteins, and eDNA were strongly positively correlated with intracellular and EPS-associated ARGs. On the contrary, cell-free ARGs exhibited a fairly weak positive correlation with EPS components. These results suggested that there is a functional link between intracellular and extracellular ARGs; however, they are correlated with EPS components. Also, EPS-associated ARGs reduction in AD of pretreated sludge was reflected in intracellular and cell-free ARGs reduction as well.</p>