Thermal hydrolysis of sludge for process intensification and mitigation of oxidative stress of nanoplastics in anaerobic digestion
Creators: Mirsoleimani Azizi, Seyed Mohammad
Publisher: name: University of Alberta Library
Year: 2023

<p>Anaerobic digestion (AD) stands as a pivotal process for sludge stabilization, resource reclamation, pathogen elimination, and biogas production. The integration of thermal hydrolysis (THP) as a pre-treatment for sludge has shown promise in bolstering methane yield and reducing solids content in AD. However, practical application encounters challenges, and the presence of microplastics and nanoplastics (MPs/NPs) in sewage sludge introduces oxidative stress to the anaerobic microbiome. This thesis investigates the impact of low-temperature THP on AD under varying conditions and explores its influence on MPs/NPs-induced stress.<br>
First, the co-digestion of fermented primary sludge (FPS) and thickened waste-activated sludge (TWAS) through low-temperature THP was performed. Two schemes were evaluated: scheme-1 (THP of TWAS + FPS) and scheme-2 (THP of TWAS only). Scheme-2 exhibited superior solubilization of chemical oxygen demand (up to 20.4% increase) and various macromolecular compounds. Conversely, scheme-1 excelled in volatile suspended solids (VSS) solubilization (up to 26.12%). Methane production saw a notable boost in scheme-1 over scheme-2, highlighting its potential benefits. Economic assessment favored THP at 90°C (90 min) under scheme-1, projecting substantial savings compared to control conditions.<br>
Second, the impact of THP (80 and 160 °C) on AD of sewage sludge exposed to different levels (50–150 μg/L) of polystyrene nanoplastics (PsNPs) was investigated. Compared to the control, higher PsNPs levels of 100 and 150 μg/L decreased methane yields by 17.98 and 29.34%, respectively. Moreover, reactive oxygen species (ROS) levels increased by 17.18 and 34.84%. Our results demonstrated that THP counteracted the suppression of methane production imposed by such PsNPs concentrations, with decreased ROS levels. Also, THP reduced antibiotic resistance gene (ARG) propagation that can be encouraged by PsNPs, thus minimizing the ARG transmission risks of digestate biosolids. These findings suggest that THP holds a high promise to further develop as a remediation method for MPs/NPs in WWTPs.<br>
Third, although few recent reports indicated that the THP could alleviate oxidative stress of MPs/NPs in AD, little is known about how different solids contents of sludge would influence its effectiveness. This study scrutinized how the THP (160 °C, 60 min) affects AD of primary sludge with 4, 8, and 12% total solids (TS) when exposed to PsNPs. The presence of PsNPs (150 μg/L) substantially enhanced ROS levels at lower TS (4 and 8%) compared to a higher TS of 12% (16.20–16.71% vs 8.79%). Consequently, methane production decreased by 7.25–15.07% for 4–8% TS. Nonetheless, applying the THP could effectively mitigate ROS-induced stress and the propagation of most ARGs. Moreover, a positive correlation was observed between the changes in extracellular polymeric substances due to the THP and the impact of PsNPs. These results provide new insights into understanding the significance of the sludge solids content in the THP for coremediation of PsNPs-induced oxidative stress and ARGs propagation in AD.<br>
Fourth, previous research has shown that THP and FPS play a crucial role in improving methane production during the AD process. However, it&#39;s essential to consider that the chemistry of continuous or semi-continuous fed digesters may differ significantly from batch tests. For the most accurate representation of industrial AD conditions, this study investigated the impact of THP (at 90 °C for 30 min) on the mixture of FPS+TWAS and TWAS alone, under semi-continuous mode. The digesters were operated at different solid retention times (SRT) of 20, 15, and 10 d. The findings revealed that applying THP to the mixture of FPS+TWAS led to a 34.5%-37.9% increase in methane production compared to the control. Similarly, when TWAS was exposed to THP under the same conditions, it resulted in a methane production enhancement of 25.6%-31.2%. Further, The presence of PsNPs resulted in a higher reactive oxygen species (ROS) production and a higher abundance of antibiotic resistance genes (ARGs). Additionally, their presence caused a significant inhibition of methane production by 28.2%, 29.3%, and 38.8% for SRTs of 20, 15, and 10 d, respectively. Nevertheless, the application of THP proved to be effective in mitigating ROS-induced stress and curbing the propagation of ARGs during the AD process. These results provide new insights into the semi-continuous AD of pretreated FPS+TWAS and TWAS and the impact of THP on mitigating the negative impact of PsNPs on the AD process.</p>