Effects of biochar supported nano zero-valent iron with different carbon/iron ratios on two-phase anaerobic digestion of food waste

Fe oxides nano-modified pumice enhances hydrogenotrophic methanogenesis in anaerobic digestion: Performance and mechanism of microbial community

Anaerobic digestion (AD), as an eco-friendly biological process, shows potential for the decomposition of leachate produced by waste incineration power plants. In this study, the effects of Fe oxides nano-modified pumice (FNP) were investigated on the fresh leachate AD process. Firstly, a simple hydrothermal method was used to prepare FNP, then introduced into the UASB reactor to evaluate its AD efficiency. Results showed that the inclusion of FNP could shorten the lag phase by 10 days compared to the control group. Furthermore, cumulative methane production in the FNP group was enhanced by 20.11%. Mechanistic studies suggested that hydrogenotrophic methanogenesis in the FNP group was more pronounced due to the influence of key enzymes (i.e., dehydrogenase and coenzyme F420). Microbial community analysis demonstrated that FNP could enhance the abundance of Methanosarcina, Proteobacteria, Sytrophomonas, and Limnobacter, which might elevate enzyme activity involved in methane production. These findings suggest that FNP might mediate interspecies electron transfer among these microorganisms, which is essential for efficient leachate treatment.

Anaerobic treatment of nitrogenous industrial organic wastewater by carbon-neutral processes integrated with anaerobic digestion and partial nitritation/anammox: Critical review of current advances and future directions

Anaerobic digestion combined with partial nitritation/anammox technology holds promising potential for the carbon-neutral treatment of nitrogenous industrial organic wastewater, boasting remarkable advantages in effective removal of both organic matters and nitrogen, bio-energy recovery and carbon emission reduction. This study provides a concise overview of the development and advantages of anaerobic digestion combined with partial nitritation/anammox technology for treating nitrogenous industrial organic wastewater. The process excels in removing organic matter and nitrogen, recovering bio-energy, and reducing carbon emissions, compared to traditional physicochemical and biological methods. Case studies highlight its energy-saving and efficient attributes, especially for carbon-neutral nitrogen removal. Challenges for achieving stable operation in the future are discussed, and the study offers insights into the broader application of this integrated process in industrial wastewater treatment.

Enhancement of anaerobic digestion by adding elemental sulfur

In this study, a new approach to enhance methane (CH4) production from organic substrates in anaerobic digestion (AD) has been discovered. That is, the addition of elemental sulfur (S0) particles into the AD system promotes the synergistic growth of elemental sulfur disproportionation bacteria, acidogenic bacteria and methanogenic archaea, thus facilitating hydrolysis, acidogenesis and methanogenesis. The efficacy of this AD enhancement pathway was confirmed in AD experiments with glucose as a model organic substrate. The results demonstrated that CH4 production in the AD system increased considerably with S0 dosages ranging from 20 mg/L to 300 mg/L. Two gas production peaks appeared at dosages of 20 mg/L and 180 mg/L, where the total CH4 production increased by 2.1 times and 2.5 times, respectively compared with the control group. However, inhibitory effect was observed for S0 dosages above 300 mg/L. The chemical states of S, the microbial community and the abundance of key functional enzymes in the AD system were analyzed. The results showed that S0 addition increased the relative abundance of Dethiobacteraceae, Caldatribacterium, Anaerolineaceae, Methanobacterium and Methanosaeta and considerably increased the abundance of key functional enzymes, such as dehydrogenase, D-glucosidic glucosidase, pyruvate synthase and acetyl-CoA deacetylase. The enrichment of these microorganisms and functional enzymes was strongly positively correlated with the production of volatile fatty acids and CH4, demonstrating that S0 addition effectively enhances methanogenesis during AD.

Effect of D-limonene on volatile fatty acids production from anaerobic fermentation of waste activated sludge under pH regulation: performance and mechanisms

D-limonene extracted from citrus peels possesses an inhibitory effect on methanogenic archaea. This study is aimed to bridge the research gap on the influence of D-limonene on volatile fatty acids (VFA) production from waste activated sludge (WAS) and to address the low VFA yield in standalone anaerobic fermentation of WAS. When the initial pH was not controlled, 1.00 g/g TSS D-limonene resulted in a VFA accumulation of 1175.45 ± 101.36 mg/L (174.45 ± 8.13 mgCOD/gVS). When the initial pH was controlled at 10 and the D-limonene concentration was 0.50 g/g TSS, the VFA accumulation reached 2707.44 ± 183.65 mg/L (445.51 ± 17.10 mgCOD/gVS). The pH-regulated D-limonene treatment enhanced solubilization and acidification, slightly inhibited hydrolysis, and significantly suppressed methanogenesis. D-limonene under alkaline conditions can increase the relative abundance of Clostridium_sensu_stricto, significantly enhancing acidification. Moreover, it markedly inhibited methanogenesis by particularly reducing the relative abundance of Methanothrix that was responsible for acetate consumption, thus favoring the accumulation of VFA. The research reveals the potential mechanism of pH regulation and D-limonene on anaerobic fermentation acid production, providing a theoretical basis for improving the acid production performance of the anaerobic fermentation of WAS.

The potential role of iron-carbon micro-electrolysis materials in curtailing lag-phase stimulates kitchen waste anaerobic digestion at different solid contents: Performance, synergistic effect and microbial response

High-solid anaerobic digestion (HSAD) of kitchen waste was generally faced to the common problems such as systemic acidification, prolonged lag-phase time and low methane production. Iron-carbon micro-electrolysis (ICME) materials exhibited advantages that porous structure, large specific surface area and excellent conductivity. It was beneficial for organic compounds to hydrolysis. Moreover, ICME materials could establish direct interspecies electron transfer (DIET) pathway between bacteria and methanogens. ICME materials were commonly used to enhance the AD of wastewater, but they were rarely applied to HSAD of kitchen waste. In this study, ICME materials were utilized to enhance HSAD of kitchen waste at different solid content conditions. The results showed that the highest cumulative biogas yield (705.23 mL/g VS) was obtained in the experimental group (TS = 10%), which was 94.15% higher than that of the control group. At the same time, the addiction of ICME could shorten lag-phase time. Electrochemical characteristics and XPS analysis showed that ICME materials promoted the release of Fe2+ in the AD system and acceleration of direct interspecies electron transfer between microorganisms. Microbial community analysis showed that ICME materials enriched electroactive bacteria (Proteiniphilum), Methanosarcina, Methanobrevibacter and Methanofollis. Functional gene prediction revealed that ICME materials increased the relative abundance of carbohydrate transport and metabolism and coenzyme transport and metabolism. It provided a potential measure to treat kitchen waste.

Metagenomic analysis reveals the mechanisms of biochar supported nano zero-valent iron in two-phase anaerobic digestion of food waste: microbial community, CAZmey, functional genes and antibiotic resistance genes

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.

Boosting Volatile fatty acids (VFAs) production in fermentation microorganisms through genes expression control: Unraveling the role of iron homeostasis transcription factors

Iron (Fe0, Fe (II), and Fe (III)) has been previously documented to upregulate the expression of key genes, enhancing the production of volatile fatty acids (VFAs) to achieve waste/wastewater resource recovery. However, the precise mechanism by why iron influences gene expression remains unclear. This study applied iron-assisted fermentation systems to explore the behind enhancing mechanism by constructing regulon networks among genes, microbes, and transcription factors. In iron-conditioned systems, a significant enhancement in VFAs production and upregulation of genes expression (1.19-3.92 folds) related to organic conversion and the electron transfer chain was observed. Besides, gene co-expression network and Procrustes analysis identified ten hub transcription factors (e.g., arsR, crp, iscR, perR) and their major contributors (genus) (e.g., Paludibacter, Acinetobacter, Tolumonas). Further analysis suggested that most of hub transcription factors were implicated in iron homeostasis regulation, which speculated that the induced iron homeostasis transcription factors probably effectively regulated the expression of genes encoding enzymes involving in VFAs production and electron transfer of functional microbes, in the case of Paludibacter, Acinetobacter, and Tolumonas while regulating the iron homeostasis, resulting in the efficient production of VFAs in iron-conditioned systems. This study might contribute to an enhanced understanding of the underlying genetic mechanisms by why iron influences gene expression regulation of microbes, which also provides a genetic theoretical basis for improving system VFAs production and resource recovery.

Review in anaerobic digestion of food waste

Due to the special property of food waste (FW), anaerobic digestion of food waste is facing many challenges like foaming, acidification, ammonia nitrogen and (NH4+-N) inhibition which resulted in a low biogas yield. A better understanding on the problems exiting in the FW anaerobic digestion would enhance the bio-energy recovery and increase the stable operation. Meanwhile, to overcome the bottle necks, pretreatment, co-digestion and additives is proposed as well as the solutions to improve biogas yield in FW digestion system. At last, future research directions regarding FW anaerobic digestion were proposed.

Enhanced anaerobic digestion of human feces by ferrous hydroxyl complex (FHC): Stress factors alleviation and microbial resistance improvement

Anaerobic digestion (AD) offers a reliable strategy for resource recovery from source-separated human feces (HF), but is limited by a disproportionate carbon/nitrogen (C/N) ratio. Ferrous hydroxyl complex (FHC) was first introduced into the HF-AD system to mediate methanogenesis. Mono-digestion of undiluted HF was inhibited by high levels of volatile fatty acids (VFAs), ammonia, and hydrogen sulfide (H2S). FHC addition at optimum dosage (500-1000 mg/L) increased the cumulative methane (CH4) yield by 22.7%, enhanced the peak value of daily CH4 production by 60.5%, and shortened the lag phase by 24.7%. H2S concentration in biogas was also greatly decreased by FHC via precipitation. FHC mainly facilitated the hydrolysis, acidification, and methanogenesis processes. The production and transformation of VFAs were optimized in the presence of FHC, thus relieving acid stress. FHC elevated the activities of alkaline protease, cellulase, and acetate kinase by 32.3%, 18.2%, and 30.3%, respectively. Microbial analysis revealed that hydrogenotrophic methanogens prevailed in mono-digestion at high HF loading but were weakened after FHC addition. FHC also enriched Methanosarcina, thereby expanding the methanogenesis pathway and improving the resistance to ammonia stress. This work would contribute to improving the methanogenic performance and resource utilization for HF anaerobic digestion.

Feasibility of anaerobic co-digestion of biodegradable plastics with food waste, investigation of microbial diversity and digestate phytotoxicity

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/gVSremoval, respectively which are nearly 58 % higher than the control group (58.86 mL/gVSremoval). 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.