Biosafety assessment during anaerobic co-digestion of food waste and pig manure

Abstract

Antimicrobial resistance (AMR) spreading has been a public concern as it increases the mortality of infection, and some biowastes (such as pig manure, food waste and municipal waste sludges) have been reported as hotspots of antibiotic resistance. This PhD research aimed at comprehensively studying the profile of antibiotic resistomes in the biowastes of Ireland, and assessing the removal of antibiotic resistomes during anaerobic co-digestion of food waste and pig manure.

First, metagenomic analysis and high-throughput qPCR (HT-qPCR) were employed to investigate the profile of antibiotic resistance genes (ARGs), mobile genetic elements (MGEs) and microbial community in food waste (FW), pig manure (PM), aerobic activated sludge (AS) and anaerobic sludge (ANS). A total of 19 ARGs types (317 subtypes) were detected in these samples by metagenomic sequencing, and the abundances of total ARGs in the samples ranged from 1.90 ×10-1 to 4.03 ×10-1 copy of ARG/copy of 16S rRNA gene. HT-qPCR analysis showed that the total absolute abundances of ARGs in the FW, PM and ANS were approximately 2.2×107 to 1.6×1010 copies of gene/g wet sample, respectively. Both analysis methods revealed the presence of carbapenem resistance genes in the FW.

Following that, anaerobic digestion was operated to study the inactivation of carbapenemase-producing Enterobacterales (CPE), as well as the removal of ARGs, during mesophilic anaerobic co-digestion (AcoD) of FW and PM. The results showed that the elimination of CPE strains was more efficient at higher total solids (TS) contents. The survival time of CPE decreased from 9 days to 2 days with increasing TS contents from 5% to 20%. In addition, mesophilic dry AcoD with a TS content of 20% effectively reduced total ARGs by 1.24 logs copies/g wet sample. However, the fates of different types of ARGs varied during mesophilic AcoD.

Finally, the removal of ARGs and MGEs during thermophilic AcoD of FW and PM was assessed. Metagenomic sequencing results showed that the relative abundances of ARGs were reduced under all TS contents. However, the digesters operated at higher TS contents (TS=10%, 15% and 20%) did not show a noticeable improvement in ARG reduction, and there was little change in the final number of ARGs subtypes in all digesters. Because methane production and system stability were adversely impacted by high TS contents, the thermophilic dry AcoD with the current experimental setting may not be feasible for the treatment of FW and PM.

The results indicate that anaerobic co-digestion is an effective way to treat FW and PM and reduce ARGs. But the optimal initial TS contents were different at 37 ℃ and 55 ℃. A high initial TS content is recommended for mesophilic AcoD for a higher removal of ARGs and MGEs. TS content higher than 15% is not recommended for thermophilic AcoD, since it may result in failure.