Dose-response and type-dependent effects of antiviral drugs in anaerobic digestion of waste-activated sludge for biogas production.

In the context of the COVID-19 pandemic, antiviral drugs (AVDs) were heavily excreted into wastewater and subsequently enriched in sewage sludge due to their widespread use. The potential ecological risks of AVDs have attracted increasing attention, but information on the effects of AVDs on sludge anaerobic digestion (AD) is limited. In this study, two typical AVDs (lamivudine and ritonavir) were selected to investigate the responses of AD to AVDs by biochemical methane potential tests. The results indicated that the effects of AVDs on methane production from sludge AD were dose- and type-dependent. The increased ritonavir concentration (0.05-50 mg/kg TS) contributed to an 11.27-49.43% increase in methane production compared with the control. However, methane production was significantly decreased at high lamivudine doses (50 mg/kg TS). Correspondingly, bacteria related to acidification were affected when exposed to lamivudine and ritonavir. Acetoclastic and hydrotropic methanogens were inhibited at a high lamivudine dose, while ritonavir enriched methylotrophic and hydrotropic methanogens. Based on the analysis of intermediate metabolites, the inhibition of lamivudine and the promotion of ritonavir on acidification and methanation were confirmed. In addition, the existence of AVDs could affect sludge properties. Sludge solubilization was inhibited when exposed to lamivudine and enhanced by ritonavir, perhaps caused by their different structures and physicochemical properties. Moreover, lamivudine and ritonavir could be partially degraded by AD, but 50.2-68.8% of AVDs remained in digested sludge, implying environmental risks.

Effects of hydrothermal treatment on organic compositions, structural properties, dewatering and biogas production of raw and digested sludge.

The effects of hydrothermal treatment (HTT) under different temperatures and time (120 °C to 250 °C, 10 min to 60 min) on organic matter solubilization and structure changes of secondary sludge (SS) and digested sludge (DS), as well as downstream dewatering and anaerobic digestion were investigated. The results showed that organic matter solubilization increased significantly at 120 °C to 170 °C, then decreased at 200 °C to 250 °C. The organic matter solubilization during HTT showed no obvious difference for two sludge, but for the different organic components. The polysaccharides are easier to be dissolved than protein, which was manifested by the higher dissolution rate at low temperature. The protein was the main soluble component for both of hydrothermal SS and DS, which accounted for 44 % to 64 % of soluble chemical oxygen demand (SCOD). The decrease of residual extracellular polymeric substances (EPS) content and increase of N-acetylglucosamine and DNA concentrations indicated that sludge EPS and cell wall structure were damaged at 170 °C, which contributed to the high organic matter solubilization. Nitrogen balance and molecular weight distribution indicated the concentrations of soluble organic components were the combined result of dissolution and hydrolysis reaction. The hydrolysis and polymerization reaction were intensified at 170 °C to 250 °C, which was verified by the COD balance and molecular weight transformation. The hydrothermal time could further facilitate the organics dissolution and hydrolysis based on the effect of hydrothermal temperature. The EPS structure damage also contributed to the high percentage of free moisture, resulting in enhanced dewaterability. The highest methane production was 298.1 mL CH4/g VSadd for DS hydrothermally treated at 170 °C, which were 125 % and 9.8 % higher than SS and SS-HTT, respectively. This study provided an insight into the general mechanism of HTT and the application of different HTT and AD configurations.