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The addition of carbonaceous materials into anaerobic digestion (AD) has gained widespread attention due to their significant effects on anaerobic performance and antibiotic resistance gene (ARG) removal. This study selected graphite, graphene, and graphene oxide (GO) as additives to investigate variations in AD performance, ARG removal, microbial community diversity and structure in wet AD systems. The results indicated that the addition of graphite-based materials in wet AD systems could increase degradation of solid organic matters by 0.91%-3.41% and utilization of soluble organic fractions by 10.43%-13.67%, but could not stimulate methane production. After the addition of graphite and graphene, ARG removal rates were effectively increased to 90.85% and 94.22%, respectively. However, the total ARG removal rate was reduced to 77.46% with the addition of GO. In addition, the microbial diversity in the wet AD process was enhanced with the addition of GO only, graphite and graphene led to a reduction in it. As for bacterial community, graphite and graphene increased the abundance of Thermotogae from 43.43% to 57.42% and 58.74%, while GO increased the abundance of Firmicute from 49.90% to 56.27%. For the archaeal community, the proportion of hydrogenotrophic methanogens was improved when adding each graphite-based material; however, only GO increased Methanosaeta that was acetoclastic methanogens. Finally, methanogens were found as the ARG host, and ARGs that belong to the same subtype might exist in the same host bacteria. [ABSTRACT FROM AUTHOR]

In anaerobic digestion (AD) system, the C/N ratio can affect utilization efficiency of organic matter and microbial community structure, thereby influencing methane production. This study investigated that the C/N ratio in AD of sewage sludge (SS) suitably enhanced by adding food waste (FW) to improve methane production efficiency. The effects of different C/N ratios (9, 11, 13, and 15) on dry thermophilic anaerobic co-digestion of SS and FW and the microbial community structure were researched. The cumulative methane production at C/N ratio of 11 reached 165.68 ± 11.88 mL/g VSadded which increased by 1.23 times compared with the mono-digestion of SS; and the cumulative methane production increased by 8.15 – 63.55% at C/N ratios of 9, 13, and 15. The methanogenic pathway changed from acetoclastic methanogenesis process to hydrogenotrophic methanogenesis process with the increasing of C/N ratio. Redundancy analysis explained that the C/N ratio had obvious effects on the variation of the microbial phylum and genus, which contributed 21.63% and 29.27%, respectively. The C/N ratio of 11 enriched the dominant bacterial phyla (Firmicutes and Thermotogae) and increased the methanogens diversity at genus level. This research found an approach that could improve methane production in the lower C/N range. [ABSTRACT FROM AUTHOR]

Anaerobic digestion (AD) is one of the effective methods to treat the food waste (FW). In this study, biochar supported nano zero-valent iron (BC-nZVI) was prepared by carbon thermal reduction process. The effects of concentrations (1, 2 and 3 g/L) of BC-nZVI on methane production, microorganisms and metabolic mechanism during the AD of food waste were investigated. The results showed that 1 g/L of BC-nZVI was the best concentration for methane production. The cumulative methane production was enhanced by 29.54% at BC-nZVI 1 g/L. Microbial community analysis showed that BC-nZVI mainly increased the abundances of hydrolytic bacteria (including Bacteroidetes_vadinHA17 , Proteiniphilum , Syntrophobacter) and acetoclast (Methanosaeta). Further exploration of the mechanism using PICRUSt2 version based on 16 S rRNA gene sequence analysis indicated that the addition of BC-nZVI promoted the abundances of enzymes related to methane metabolism such as the phosphate acetyltransferase and the methyl-coenzyme M reductase alpha subunit, which made the AD system stable. [Display omitted] • Methane production of AD system increased by 29.54% with1g/L of BC-nZVI. • BC-nZVI decreased the concentrations of SCOD, NH 4 +-N, TVFA in AD system. • BC-nZVI enriched the hydrolytic bacteria and aceticlastic methanogens in AD system. • Genes related with methane production were enhanced with BC-nZVI addition. [ABSTRACT FROM AUTHOR]

Acidification is a key technical problem in the dry anaerobic digestion system. The effect of bioaugmentation of butyric-resistant microorganisms (BRM) on an unstable acidified anaerobic digestion (AD) system was studied in this work. BRM were enriched in an AD system fed with butyric acid for 36 days. The slurry containing BRM was collected from the fermenter peak on the 24th day when methane production reached the peak, and transferred to an acidified fermenter fed with food waste. After the bioaugmentation, the acidified system was restored obviously. The concentration of butyrate decreased from 16.57 g/L to 5.06 g/L in the acidified system and the methane production increased by 52.2 mL/g-VS in the bioaugmentation treatment, compared to the fermenter without bioaugmentation. Syntrophomonas (Firmicutes) and Proteiniphilum (Bacteroidetes) were enriched in BRM, which contributed to maintain the interspecies electron transfer relationship with Methanosaeta. Therefore, bioaugmentation of BRM restored the integrity of microorganisms in AD and relieved pressure of VFA accumulation. This study provided an experimental and theoretical foundation for the management of acidification in dry AD of food waste. [Display omitted] • Potential of biogas production from dry anaerobic digestion system was explored. • Inoculation of butyric-resistant microorganisms (BRM) enhanced CH 4 production by 98.76%. • BRM eliminated the accumulation of VFAs and restored the acidified AD system. • Firmicutes, Bacteroidetes, and Proteobacteria were enriched during the AD of food waste. • Bioaugmented BRM maintained the interspecies electron transfer with Methanosaeta. [ABSTRACT FROM AUTHOR]

The mechanisms of biochar supported nano zero-valent iron (BC/nZVI) on two-phase anaerobic digestion of food waste were investigated. Results indicated that the performance of both acidogenic phase and methanogenic phase was effectively facilitated. BC/nZVI with the amount of 120 mg/L increased methane production by 32.21%. In addition, BC/nZVI facilitated direct interspecies electron transfer (DIET) between Geobacter and methanogens. Further analysis showed that BC/nZVI increased the abundance of most CAZymes in acidogenic phase. The study also found that BC/nZVI had positive effects on metabolic pathways and related functional genes. The abundances of acdA and ackA in acidogenic phase were increased by 151.75% and 36.26%, respectively, and the abundances of pilA and TorZ associated with DIET were also increased. Furthermore, BC/nZVI mainly removed IMP-12 , CAU-1 , cmeB , ErmR , MexW , ErmG , Bla2 , vgaD , MuxA , and cpxA from this system, and reduced the antibiotic resistance genes for antibiotic inactivation resistance mechanisms. [Display omitted] • Adding BC/nZVI resulted in a 32.21% increase in methane production. • BC/nZVI facilitated DIET between bacteria and methanogens. • BC/nZVI improved most CAZymes in acidogenic phase. • AcdA and ackA related to acetic acid production were upgraded by 151.75% and 36.26%. • Adding BC/nZVI to AD could remove ARGs by 38.35% and 21.77% in two-phase AD. [ABSTRACT FROM AUTHOR]

[Display omitted] • Biodegradable plastics (30 μm) improved the methane production of the anaerobic system (> 57%). • The digestate with biodegradable plastics (40 μm) inhibited the elongation of seed roots. • Biodegradable plastics increased abundance of Methanobacterium and Methanosaeta. • Biodegradable plastics (≥ 40 μm) should be shredded to less than 20 × 20 mm. The effects of biodegradable plastics of different thicknesses (30 and 40 μm) and sizes (20 × 20, 2 × 2, and 1 × 1 mm) on anaerobic digestion of food waste and digestate phytotoxicity were investigated. Methane productions (38 days) for the groups with 20 × 20, 2 × 2, and 1 × 1 mm of 30 μm plastics were 92.46, 138.27, and 259.95 mL/gVS removal , respectively which are nearly 58 % higher than the control group (58.86 mL/gVS removal). Methane production in 40 μm plastics groups was lower than in 30 μm groups of equal size. All sizes of 30 µm plastics promoted substrate hydrolysis, acidification, and relative abundance of key hydrolytic bacteria and methanogens. Phytotoxicity tests results showed that seed root elongation was inhibited in groups with 40 μm plastics. In conclusion, 30 μm biodegradable plastics were more suitable for anaerobic digestion with food waste than 40 μm. [ABSTRACT FROM AUTHOR]

Anaerobic digestion is considered biotechnology that is following greenhouse gas reduction policies and circular economy development goals. However, the activity and quality of inoculum are critical factors in guaranteeing the successful start-up and stable operation of an anaerobic digestion reactor. This review summarized the literature on relevant the categories of anaerobic digestion inoculum reflecting its merits and drawbacks, with a particular focus on livestock manure, digestate, and rumen fluid. The inoculum contains hydrolytic bacteria that may break down lignocellulose, such as Firmicutes, Bacteroidetes and Proteobacteria as well as methanogenic archaea like Euryarchaeota, Methanosaeta and Methanomicrobia. However, there is a risk that heavy metals and antibiotics in livestock manure and digestate may inhibit anaerobic digestion. The anaerobic digestion reactor in most situations lacks a cost-effective, ready-to-use, dedicated methanogenic culture when required. The inoculum is frequently stored for the start-up of a new reactor and bioaugmentation of an under-performing anaerobic digestion system. Numerous laboratory-scale evaluations have been carried out on preservation strategies for inoculums, including simple preservation in wet and dehydrated storage by centrifugation, heat drying, and freeze-drying. Temperature and time are the main factors affecting preservation effectiveness. However, preservation of anaerobic inoculum by freeze-drying with an effective cryoprotectant may provide the best results. Furthermore, the potential mechanisms by which dehydration techniques and storage conditions alter the chemical composition and microbial activity of inoculum, and the development of waste-based cryoprotectants and novel dehydration preservation techniques in the future should be made more investigations. [Display omitted] • The profile of digestion products is closely related to inoculum type. • Balanced microbial community and nutrients are key quality parameter for inoculum. • Bacteroidota, Firmicutes and Proteobacteria are dominant bacteria in inoculum. • The effect of temperature and time on the mass of the inoculum is not linear. • Freeze-drying may provide positive effect on methanogenic activity. [ABSTRACT FROM AUTHOR]

This research investigated the impact of polyethylene terephthalate (PET) microplastics on the anaerobic co-digestion of sewage sludge and food waste. The performance of anaerobic digester was investigated with different particle sizes (30 µm and 250 µm) PET at concentrations of 0, 0.45, 1.35, and 2.70 mg/g total solid (TS). Results suggested that the CH 4 production at 2.70 mg/g TS level decreased by 21.63% (30 µm) and 15.87% (250 µm) compared with the control group, respectively. PET microplastics exerted negative influence through leaching toxic diisobutyl phthalate (DIBP) and dibutyl phthalate (DBP). Microbial community analysis and redundancy analysis showed that DIBP and DBP reduced the abundances of key hydrolysis bacteria (Bacteroides vadin HA17) and acidification bacteria (Clostridium and Sphaerochaeta). Structural equation model analysis showed that DBP and DIBP were the key factors that reduced the methane production, and they had stronger impact on daily methane production in the 30 µm group compared with the 250 µm group. [Display omitted] • PET microplastics had negative effect on methane production. • PET microplastics-30 µm inhibited the methane production more obviously. • PET microplastics inhibited the hydrolysis and acidification in AD systems. • Leached DBP and DIBP reduced key acidogens and methanogens. [ABSTRACT FROM AUTHOR]

[Display omitted] • BC/nZVI with carbon/iron of 3:1 was found to be optimum for this system. • VFAs concentration was increased by 31.4% in acidogenic phase with BC/nZVI added. • The addition of BC/nZVI enhanced cumulative methane production by 24.8%. • BC/nZVI promoted the abundance of Methanothrix in methanogenic phase. • BC/nZVI stimulated methanogenesis through the acetoclastic methanogenic pathway. The promotion effects of biochar supported nano zero-valent iron (BC/nZVI) with different carbon/iron ratios on two-phase anaerobic digestion (AD) of food waste (FW) were studied. Results suggested that when the carbon/iron ratio was 3:1 AD system showed the best performance, with the concentration of volatile fatty acids (VFAs) in acidogenic phase (AP) and the cumulative methane production in methanogenic phase (MP) increased by 31.4% and 24.8%, respectively. Metagenomic analysis demonstrated that BC/nZVI increased the relative abundance of Defluviitoga in AP, and promoted the growth of Methanothrix in MP. Metabolic pathway analysis in AP indicated that BC/nZVI mainly promoted the abundances of acetate kinase and butyrate kinase to enhance acid production. Methane metabolism pathway analysis in MP revealed that BC/nZVI increased methane production by promoting the module of M00357 and activating related enzymes. The results of this sutdy showed that BC/nZVI promoted AD of FW mainly through acetoclastic methanogenic pathway. [ABSTRACT FROM AUTHOR]

[Display omitted] • Graphene improved methane yield the most, followed by graphite and graphene oxide. • Graphene removed macrolide resistance genes in an anaerobic digestion system. • Graphene oxide addition effectively attenuated sulfonamide resistance genes in AD. • The potential hosts of mefA , ermB , and tetO were different than other ARG hosts. This study explored and compared the influence of graphite, graphene, and graphene oxide (GO) on the performance of anaerobic co-digestion fed with sewage sludge and food waste, the variations of antibiotic resistance genes (ARGs), and the evolution of microbial community. Graphene exhibited the best performance for improving methane production and organic degradation, which increased by 36.09% and 23.07% compared with control group. The experimental results showed that graphene had the greatest influence on the removal efficiency of blaOXA-1 , macrolide resistance genes (ermF and ermB), and some tetracycline resistance genes (tetQ and tetX); however, the removal efficiency of sulfonamide resistance genes (sul1 and sul2), intI1 , and some tetracycline resistance genes (tetM , tetO, and tetW) were highest when GO was added. Network analysis indicated that the host cells of mefA , ermB , and tetO were different from other ARG host cells; moreover, graphene controlled the horizontal transfer of ARGs between microbial communities. [ABSTRACT FROM AUTHOR]

[Display omitted] • The fate of ARGs in biogas slurry and residue were investigated. • Bentonite improved methane production in mesophilic and thermophilic condition. • ARGs and intI1 in biogas residue were effectively removed by using bentonite. • Bentonite and temperature were the main driver factors effecting host of ARGs. Anaerobic digestion (AD) processes of food waste (FW) have potential risk on environments due to the prevalence and dissemination of antibiotic resistance genes (ARGs). This study investigated the effect of bentonite on methane production and the abundance of ARGs in biogas slurry and residue during AD of FW. Results showed that methane production increased by 68.52% and 56.79% with 3 g/L and 5 g/L of bentonite in mesophilic and thermophilic digestion, respectively. Adding 5 g/L of bentonite effectively reduced the genes of ermB , ermF , tetQ , tetX , sul1 , sul2 and intI1 with a range of 80.82% - 100.00% in biogas residue under mesophilic reactor. The abundance of ARGs and intI1 in biogas residue were lower than in slurry under both temperatures with 5 g/L of bentonite. Statistical analysis indicated that bentonite and temperature were main driver factors which could impact ARGs by influencing the abundance and structure of microbial communities. [ABSTRACT FROM AUTHOR]

• nZVI enhanced biogas production and facilitated ARGs reduction. • The abundance of tetracycline genes reduced significantly in nZVI amended digesters. • Microbial community was the main factor that influenced the fate of ARGs. The effects of nanoscale zero-valent iron (nZVI) on the performance of food waste anaerobic digestion and the fate of antibiotic resistance genes (ARGs) were investigated in thermophilic (TR) and mesophilic (MR) reactors. Results showed that nZVI enhanced biogas production and facilitated ARGs reduction. The maximum CH 4 production was 212.00 ± 4.77 ml/gVS with 5 g/L of nZVI in MR. The highest ARGs removal ratio was 86.64 ± 0.72% obtained in TR at nZVI of 2 g/L. nZVI corrosion products and their contribution on AD performance were analyzed. The abundance of tetracycline genes reduced significantly in nZVI amended digesters. Firmicutes, Chloroflexi, Proteobacteria and Spirochaetes showed significant positive correlations with various ARGs (p < 0.05) in MR and TR. Redundancy analysis indicated that microbial community was the main factor that influenced the fate of ARGs. nZVI changed microbial communities, with decreasing the abundance bacteria belonging to Firmicutes and resulting in the reduction of ARGs. [ABSTRACT FROM AUTHOR]

• Thermal temperature significantly affected digestibility of FO-recovered FW. • Hydrolysis of starch at 100 °C induced rapid growth of carbonate-fermenting bacteria. • Soluble organics transformed to recalcitrant humic acid-like organics at 160 °C. • Protein hydrolysis and cellulose H-bonds cleavage occurred at 120–160 °C. • Protein and cellulose alteration accelerated growth of syntrophic bacteria. The intrinsic reason determining digestion performance of 100–160 °C preheated food waste after recovering floatable oil (FO-recovered FW) was investigated using two-dimensional correlated infrared spectroscopy, three-dimensional fluorescence spectroscopy and high-throughput 16S rRNA amplicon sequencing. The results indicated that thermal temperature significantly affected CH 4 production of FO-recovered FW due to different structural alteration degree of starch, protein, cellulose and lipid components. Fragmentation of starch mainly occurred at 100 °C. The hydrolytic and acidogenic rate of starch was promoted and accordingly induced rapid growth of carbohydrate-fermenting bacteria, which resulted in severe acidification. Protein hydrolysis and cellulose H-bonds cleavage occurring at 120–160 °C accelerated the accessible sites interacting with microbial hydrolytic enzymes, and growth of Cloacimonetes and Syntrophomonas enhanced CH 4 production. Non-degradable humic acid-like organics remarkably formed at 160 °C caused a carbon loss and digestion inhibiting/deteriorating. Pretreatment at 120 °C was feasible for promoted methane production based on energy assessment. [ABSTRACT FROM AUTHOR]