Exploring the roles of zero-valent iron in two-stage food waste anaerobic digestion

Mitigated dissemination of antibiotic resistance genes by nanoscale zero-valent iron and iron oxides during anaerobic digestion: Roles of microbial succession and regulation

Nanoscale zero-valent iron (ZVI) and the oxides have been documented as an effective approach for mitigating the dissemination of antibiotic resistance genes (ARGs) during anaerobic digestion (AD). However, the mechanism of ARGs dissemination mitigated by nanoscale ZVI and iron oxides remain unclear. Here, we investigated the influencing mechanisms of nanoscale ZVI and iron oxides on ARGs dissemination during AD. qPCR results indicated that nanoscale ZVI and iron oxides significantly declined the total ARGs abundances, and the strongest inhibiting effect was observed by 10 g/L nanoscale ZVI. Mantel test showed ARGs distribution was positively correlated with physiochemical properties, integrons and microbial community, among which microbial community primarily contributed to ARGs dissemination (39.74%). Furthermore, redundancy and null model analyses suggested the dominant and potential ARGs host was Fastidiosipila, and homogeneous selection in the determinism factors was the largest factor for driving Fastidiosipila variation, confirming the inhibition of Fastidiosipila was primary reason for mitigating ARGs dissemination by nanoscale ZVI and iron oxides. These results were related to the inhibition of ARGs transfer related functions. This work provides novel evidence for mitigating ARGs dissemination through regulating microbial succession and regulation induced by ZVI and iron oxides.

Cation exchange resins enhance anaerobic digestion of sewage sludge: Roles in sequential recovery of hydrogen and methane

The recovery of renewable bioenergy from anaerobic digestion (AD) of sludge is a promising method to alleviate the energy problem. Although methane can be effectively recovered through sludge pretreatment by cation exchange resin (CER), the simultaneous enhancement of hydrogen and methane generation from AD using CER has not been extensively investigated. Herein, the effect of CER on the sequential recovery of hydrogen and methane and the corresponding mechanisms were investigated. When CER is introduced, the maximum increases for the hydrogen and methane production are 104.7 % and 35.3 %, respectively, confirming the sequential enhancement effects of CER on the hydrogen and methane production. Analyses of the variations in the main biochemical components with and without the effect of CER demonstrate that CER promotes sludge organic solubilisation, hydrolysis, and acidification in both hydrogen- and methane-production stages. Moreover, investigations of variations in the solid-liquid interfacial thermodynamics and removal rates of main multivalent metals of sludge reveal that the ion exchange reactions between the CER and sludge in the hydrogen-production stage provide the direct driving force of effective contact between bacteria and organic particulates. Additionally, the residual effect of the CER during methane production reduces the energy barrier for mass transfer and provides a driving force for this transfer. Further analyses of the microbial community structure and metagenomics indicate that CER directly drives the enrichment of hydrogen-producing bacteria (+ 15.1 %) and key genes encoding enzymes in the hydrogen-production stage. Moreover, CER indirectly induces the enrichment of methane-producing anaerobes (e.g. Methanosaeta: + 16.7 %, Methanosarcina: + 316.5 %); enhances the bioconversion of different substrates into methyl-coenzyme M; and promotes the metabolism pathway of acetoclastic process and CO2 reduction in the methane-production stage. This study can provide valuable insights for simultaneously enhancing the production of hydrogen and methane from AD through sequential recovery.

Enhancing methane production from food waste with iron-carbon micro-electrolysis in a two-stage process

A two-stage process, consisting of a leach-bed reactor (LBR) and an up-flow anaerobic sludge blanket reactor (UASB), has been commonly adopted to improve food waste anaerobic digestion. However, its application is limited due to low hydrolysis and methanogenesis efficiencies. This study proposed a strategy of incorporating iron-carbon micro-electrolysis (ICME) into the UASB and recirculating its effluent to the LBR to improve the two-stage process efficiency. Results showed that the integration of the ICME with the UASB significantly increased the CH4 yield by 168.29%. The improvement of the food waste hydrolysis in the LBR mainly contributed to the enhanced CH4 yield (approximately 94.5%). The enrichment of hydrolytic-acidogenic bacterial activity, facilitated by the Fe2+ generated through ICME, might be the primary cause of the improved food waste hydrolysis. Moreover, ICME enriched the growth of hydrogenotrophic methanogens and stimulated the hydrogenotrophic methanogenesis pathway in the UASB, contributing partially to the enhanced CH4 yield.

A review of application of combined biochar and iron-based materials in anaerobic digestion for enhancing biogas productivity: Mechanisms, approaches and performance

Strengthening direct interspecies electron transfer (DIET), via adding conductive materials, is regarded as an effective way for improving methane productivity of anaerobic digestion (AD). Therein, the supplementation of combined materials (composition of biochar and iron-based materials) has attracted increasing attention in recent years, because of their advantages of promoting organics reduction and accelerating biomass activity. However, as far as we known, there is no study comprehensively summarizing the application of this kind combined materials. Here, the combined methods of biochar and iron-based materials in AD system were introduced, and then the overall performance, potential mechanisms, and microbial contribution were summarized. Furthermore, a comparation of the combinated materials and single material (biochar, zero valent iron, or magnetite) in methane production was also evaluated to highlight the functions of combined materials. Based on these, the challenges and perspectives were proposed to point the development direction of combined materials utilization in AD field, which was hoped to provide a deep insight in engineering application.

Evaluation of different 16S rRNA gene hypervariable regions and reference databases for profiling engineered microbiota structure and functional guilds in a swine wastewater treatment plant

High-throughput 16S rRNA gene amplicon sequencing technology is widely applied for environmental microbiota structure analysis to derive knowledge that informs microbiome-based surveillance and oriented bioengineering. However, it remains elusive how the selection of 16S rRNA gene hypervariable regions and reference databases affects microbiota diversity and structure profiling. This study systematically evaluated the fitness of different frequently used reference databases (i.e. SILVA 138 SSU, GTDB bact120_r207, Greengenes 13_5 and MiDAS 4.8) and primers of 16S rRNA gene in microbiota profiling of anaerobic digestion and activated sludge collected from a full-scale swine wastewater treatment plant (WWTP). The comparative results showed that MiDAS 4.8 achieved the highest levels of taxonomic diversity and species-level assignment rate. For whichever sample groups, microbiota richness captured by different primers decreased in the following order: V4 > V4-V5 > V3-V4 > V6-V8/V1-V3. Using primer-bias-free metagenomic data results as the judging standard, V4 region also best characterized microbiota structure and well represented typical functional guilds (e.g. methanogens, ammonium oxidizers and denitrifiers), while V6-V8 regions largely overestimated the archaeal methanogens (mainly Methanosarcina) by over 30 times. Therefore, MiDAS 4.8 database and V4 region are recommended for best simultaneous analysis of bacterial and archaeal community diversity and structure of the examined swine WWTP.

Co-digestion of food waste and hydrothermal liquid digestate: Promotion effect of self-generated hydrochars

Hydrothermal treatment (HTT) can efficiently valorize the digestate after anaerobic digestion. However, the disposal of the HTT liquid is challenging. This paper proposes a method to recover energy through the anaerobic co-digestion of food waste and HTT liquid fraction. The effect of HTT liquid recirculation on anaerobic co-digestion performance was investigated. This study focused on the self-generated hydrochars that remained in the HTT supernatant after centrifugation. The effect of the self-generated hydrochars on the methane (CH₄) yield and microbial communities were discussed. After adding HTT liquids treated at 140 and 180 °C, the maximum CH₄ production increased to 309.36 and 331.61 mL per g COD, respectively. The HTT liquid exhibited a pH buffering effect and kept a favorable pH for the anaerobic co-digestion. In addition, the self-generated hydrochars with higher carbon content and large oxygen-containing functional groups remained in HTT liquid. They increased the electron transferring rate of the anaerobic co-digestion. The increased relative abundance of Methanosarcina, Syntrophomonadaceae, and Synergistota was observed with adding HTT liquid. The results of the principal component analysis indicate that the electron transferring rate constant had positive correlationships with the relative abundance of Methanosarcina, Syntrophomonadaceae, and Synergistota. This study can provide a good reference for the disposal of the HTT liquid and a novel insight regarding the mechanism for the anaerobic co-digestion.

Deciphering different effects of ZVI and NaOH on metabolic characteristics in the process of methanogenesis recovery from VFA suppression

Dosing zero valent iron (ZVI) or sodium hydroxide (NaOH) is the common method of addressing acidification in anaerobic digestion (AD) systems; however, few studies have discussed and compared their effects on microbial metabolism. In the present study, microbial syntrophy and metabolic pathways under ZVI and NaOH regulation are comparatively analyzed through microbial network analysis and metagenomic/metaproteomic analyses. CH₄ yield in the ZVI reactor was 414 mL/gVS, an increase of 23% when compared with that in the reactor with NaOH dosing (336 mL/gVS). The methanogenesis recovery period in the ZVI reactor (37 days) was shorter than that in the NaOH reactor (48 days). Co-occurrence networks indicated that ZVI promoted Methanoculleus and Methanosarcina to establish a complex syntrophic association with SAO bacteria (Syntrophaceticus and Aminobacterium) and syntrophic acetogens (Syntrophomonas), strengthening SAO-hydrogenotrophic methanogenesis (HM) and acetoclastic methanogenesis (AM) pathways simultaneously. Metagenomic analysis showed that the relative abundance of mcrA and fwdB in the ZVI reactor was higher 27% than that in the NaOH reactor. Furthermore, through metaproteomics analysis, much more enzymes related to glucose degradation, bioconversion of butyric acid and pyruvate, conversion of formate and acetate to CO₂, and production of CH₄ from acetate and CO₂ were significantly upregulated under ZVI regulation than under NaOH regulation (fold change relative to control [FC] > 1.5, p < 0.05). The results of the present study enhance our understanding of methanogenic mechanisms under the regulation of ZVI, providing a theoretical basis for its practical application in AD systems experiencing VFA suppression.

Effect of Addition of Zero-Valent Iron (Fe) and Magnetite (Fe3O4) on Methane Yield and Microbial Consortium in Anaerobic Digestion of Food Wastewater

Direct interspecies electron transfer (DIET), which does not involve mediation by electron carriers, is realized by the addition of conductive materials to an anaerobic digester, which then activates syntrophism between acetogenic and methanogenic microorganisms. This study aimed to investigate the effect of the addition of two conductive materials, zero-valent iron (ZVI) and magnetite, on the methane production and microbial consortium via DIET in the anaerobic digestion of food wastewater. The operation of a batch reactor for food wastewater without the addition of the conductive materials yielded a biochemical methane potential (Bu), maximum methane production rate (Rm), and lag phase time (λ) of 0.380 Nm3 kg−1-VSadded, 15.73 mL day−1, and 0.541 days, respectively. Upon the addition of 1.5% ZVI, Bu and Rm increased significantly to 0.434 Nm3 kg−1-VSadded and 19.63 mL day−1, respectively, and λ was shortened to 0.065 days. Simultaneously, Methanomicrobiales increased from 26.60% to 46.90% and Methanosarcinales decreased from 14.20% to 1.50% as the ZVI input increased from 0% to 1.50%. Magnetite, at an input concentration of 1.00%, significantly increased the Bu and Rm to 0.431 Nm3 kg−1-VSadded and 18.44 mL day−1, respectively. However, although magnetite improves the efficiency of methanogenesis via DIET, the effect thereof on the methanogen community remains unclear.

Mechanisms, performance, and the impact on microbial structure of direct interspecies electron transfer for enhancing anaerobic digestion-A review

Direct interspecies electron transfer (DIET) has been received tremendous attention, recently, due to the advantages of accelerating methane production via organics reduction during anaerobic digestion (AD) process. DIET-based syntrophic relationships not only occurred with the existence of pili and some proteins in the microorganism, but also can be conducted by conductive materials. Therefore, more researches into understanding and strengthening DIET-based syntrophy have been conducted with the aim of improving methanogenesis kinetics and further enhance methane productivity in AD systems. This study summarized the mechanisms, application and microbial structures of typical conductive materials (carbon-based materials and iron-based materials) during AD reactors operation. Meanwhile, detail analysis of studies on DIET (from substrates, dosage and effectiveness) via conductive materials was also presented in the study. Moreover, the challenges of applying conductive materials in boosting methane production were also proposed, which was supposed to provide a deep insight in DIET for full scale application.

The Effects of Nanoparticles- Zerovalent Iron on Sustainable Biomethane Production through Co-Digestion of Olive Mill Wastewater and Chicken Manure

The impacts of nanoparticles-zerovalent iron (NP-ZVI) on anaerobic co-digestion (AcoD) were assessed. The production of biogas and methane (CH4), as well as the removal efficiency of volatile solids (VS) and contaminants were investigated in the AcoD of chicken manure (CM) and olive mill wastewater (OMWW) with the addition of NP-ZVI at different concentrations (10–50 mg/g VS) and different sizes resulting from various mixing volume ratios (MVR) of NaBH4:FeSO4.7H2O. The results show that NP-ZVI ≤ 30 mg/g VS at MVR-2:1, MVR-4:1, and MVR-6:1 improves the AcoD. In contrast to 40–50 mg/g VS of NP-ZVI, which caused an inhibitory impact in all of the AcoD stages, as well as a decrease in the contaminant’s removal efficiency, the concentration of 10–30 mg NP-ZVI/g VS at MVR-4:1 achieved a maximum improvement of CH4 by 21.09%, 20.32%, and 22.87%, respectively, and improved the biogas by 48.14%, 55.0%, and 80.09%, respectively, vs. the 0 additives. Supplementing AcoD with NP-ZVI at a concentration of 30 mg/g VS at MVR-4:1 resulted in maximum enhancement of the contaminant removal efficiency, with a total oxygen demand (TCOD) of up to 73.99%, turbidity up to 79.07%, color up to 53.41%, total solid (TS) up to 59.57%, and volatile solid (VS) up to 74.42%. It also improved the hydrolysis and acidification percentages by up to 86.67% and 51.3%, respectively.

Synergistic effects of paper mill sludge and sulfonated graphene catalyst for maximizing bio-hydrogen harvesting from sugarcane bagasse de-polymerization

In this study, hydrogen harvesting from fermentation of sugarcane bagasse (SCB) was promoted by maintaining synergism between sulfonated graphene (SGR) catalyst and paper mill sludge (PMS). The sulfonic acid (-SO₃H) groups in the catalyst played a major role in destructing the β-1,4 glycosidic bonds of sugarcane bagasse, releasing readily biodegradable sugars into the fermentation medium. The cellulose, hemicellulose, and lignin conversion efficiency were improved by 127.5%, 495.0%, and 109.2%, respectively with 20 mgSGR/gVS catalyst addition, compared with the control samples. These values were also higher than those obtained by non-sulfonated graphene catalyst. The hydrogenation of sugarcane bagasse was maximized at a sulfonated graphene catalyst dosage of 60 mgSGR/gVS, providing the highest hydrogen harvesting of 4104 ± 321 mL. This was associated with an increase of the Proteobacteria phyla up to 52.0%, Firmicutes phyla to 13.9%, and Acinetobacter sp. to 39.8% compared with only 37.0%, 11.3% and 11.1% in the control assay respectively. Moreover, sulfonated graphene catalyst supplementation promoted the acetate fermentation reaction pathway by increasing the acetate/butyrate ratio up to 4.1. Nevertheless, elevating the catalyst dosage up to 120 mgSGR/gVS reduced the hydrogen harvesting (1190 ± 92 mL) due to the release of furfural (1.76 ± 0.02 g/L) in the fermentation cultures, deteriorating the microbes' internal composition and metabolism bioactivities. Finally maximizing the hydrogen productivity from sugarcane bagasse is feasible by incorporation of paper mill sludge and sulfonated graphene catalyst at dosage not exceeding 60 mgSGR/gVS. However, investigating the recyclability and disposal of digestate containing sulfonated graphene catalyst and the associated economic feasibility needs more attention in the future.

Roles of zero-valent iron in anaerobic digestion: Mechanisms, advances and perspectives

With the rapid growth of population and urbanization, more and more bio-wastes have been produced. Considering organics contained in bio-wastes, to recover resource from bio-wastes is of great significance, which can not only achieve the resource recycle, but also protect the environment. Anaerobic digestion (AD) has been proved as one of the most promising strategies to recover bio-energy from bio-wastes, as well as to realize the reduction of bio-wastes. However, the conventional interspecies electron transfer is sensitive to environmental shocks, such as high ammonia, organic pollutants, metal ions, etc., which lead to instability or failure of AD. The recent findings have proved that the introduction of zero-valent iron (ZVI) in AD system can significantly enhance methane production from bio-wastes. This review systematically highlighted the recent advances on the roles of ZVI in AD, including underlying mechanisms of ZVI on AD, performance enhancement of AD contributed by ZVI, and impact factors of AD regulated by ZVI. Furthermore, current limitations and outlooks have been analyzed and concluded. The roles of ZVI on underlying mechanisms in AD include regulating reaction conditions, electron transfer mode and function of microbial communities. The addition of ZVI in AD can not only enhance bio-energy recovery and toxic contaminants removal from bio-wastes, but also have the potential to buffer adverse effect caused by inhibitors. Moreover, the electron transfer modes induced by ZVI include both interspecies hydrogen transfer and direct interspecies electron transfer pathways. How to comprehensively evaluate the effects of ZVI on AD and further improve the roles of ZVI in AD is urgently needed for practical application of ZVI in AD. This review aims to provide some references for the introduction of ZVI in AD for enhancing bio-energy recovery from bio-wastes.

Enhanced methane production in anaerobic digestion: A critical review on regulation based on electron transfer

Anaerobic digestion (AD) is a potential bioprocess for waste biomass utilization and energy conservation. Various iron/carbon-based CMs (e.g., magnetite, biochar, granular activated carbon (GAC), graphite and zero valent iron (ZVI)) have been supplemented in anaerobic digestors to improve AD performance. Generally, the supplementation of CMs has shown to improve methane production, shorten lag phase and alleviate environmental stress because they could serve as electron conduits and promote direct interspecies electron transfer (DIET). However, the CMs dosage varied greatly in previous studies and CMs wash out remains a challenge for its application in full-scale plants. Future work is recommended to standardize the CMs dosage and recover/reuse the CMs. Moreover, additional evidence is required to verify the electrotrophs involved in DIET.

Coupled mechanism of enhanced and inhibitory effects of nanoscale zero-valent iron on methane production and antibiotic resistance genes in anaerobic digestion of swine manure

In this study, the turning point for nanoscale zero-valent iron's (NZVI) promotion and inhibition effects of methane production coupled with the reduction of antibiotic resistance genes (ARGs) was investigated. Adding 150 mmol/L NZVI increased methane production by maximum of 23.8 %, which was due to the chemical reaction producing H₂ and enhancement of direct interspecies electron transfer (DIET) by NZVI. NZVI350 dramatically repressed methane generation by 48.0 %, which might be associated with the large quantity of reactive oxygen species (ROS) and excessive H₂ inhibiting the functioning of microorganisms. The fate of ARGs was significantly related to daily methane production, indicating that the more methane production finally generated, the less the abundance of ARGs at last left. The reduction of ARGs was enhanced by maximum of 61.0 %, which was attributed to the inhibition of vertical gene transfer (VGT) and horizontal gene transfer (HGT) caused by steric hindrance associated with NZVI corrosion.

Producing insect protein from food waste digestate via black soldier fly larvae cultivation: A promising choice for digestate disposal

The treatment of food waste digestate with high salinity is a big challenge. This paper evaluated the possibility of using black soldier fly larvae for food waste digestate disposal and insect protein production. Results showed that both digestates from hydrogen and methane fermentations were rich in protein and lipid contents, which benefited the BSFL cultivation. The BSFL reared on digestates from hydrogen and methane fermentations of food waste performed better in pre-pupal weight (19.12% and 41.13% higher, respectively), body length (3.62% and 18.21% higher, respectively) and crude protein contents (7.85% and 39.05% higher, respectively) than that reared on raw food waste. In addition, the maximum body weight growth rate (R_m) of BSFL cultivated on both digestates were 28.28% and 47.10% higher than that of BSFL cultivated on raw food waste, respectively. During BSFL cultivation, organic matter reduction between 40.97% and 46.07% were achieved. Digestates from hydrogen and methane fermentations represent favorable feeding substrates for BSFL cultivation. Using BSFL to treat AD digestate not only provides a digestate disposal approach, but also produces insect biomass and organic fertilizer as value-added byproducts, which shows tremendous potential in digestate disposal.

Co-Combustion of Food Solid Wastes and Pulverized Coal for Blast Furnace Injection: Characteristics, Kinetics, and Superiority

The combustion characteristics and kinetics of food solid wastes (FSW), pulverized coal (PC), and their mixtures were studied by a non-isothermal thermogravimetric method. In the co-combustion of FSW and PC, with the increase in FSW content in the mixture, the initial decomposition temperature, burnout temperature, and ignition temperature of the mixture decreased, and the flammability index and comprehensive combustion characteristic index gradually increased. The co-combustion of FSW and PC showed an inhibitory effect in the devolatilization stage but exhibited a combustion-promoting effect in the fixed carbon combustion stage. The interaction between FSW and PC while co-combusting them appeared to be dominated by thermal effects. On one hand, FSW combusted first and released heat that was partially absorbed by the PC, which hence suppressed the devolatilization stage of the co-combustion process. On the other hand, the PC absorbed the heat released by the combustion of the FSW, which increased the combustion rate of the PC in the fixed carbon combustion stage of the co-combustion process. The activation energy of the devolatilization stage and the fixed carbon combustion stage of the co-combustion process was calculated to be 34.16–74.52 kJ/mol and 15.04–36.15 kJ/mol, respectively. In general, the combustion performance of FSW is better than that of PC. The mixed injection of FSW and PC can improve the overall combustion efficiency and reduce CO2 emissions in the iron-making process.

Fe0-Supported Anaerobic Digestion for Organics and Nutrients Removal from Domestic Sewage

Results from different research suggest that metallic iron (Fe0) materials enhance anaerobic digestion (AD) systems to remove organics (chemical oxygen demand (COD)), phosphorus and nitrogen from polluted water. However, the available results are difficult to compare because they are derived from different experimental conditions. This research characterises the effects of Fe0 type and dosage in AD systems to simultaneously remove COD and nutrients (orthophosphate (PO43−), ammonium (NH4+), and nitrate (NO3− Lab-scale reactors containing domestic sewage (DS) were fed with various Fe0 dosages (0 to 30 g/L). Batch AD experiments were operated at 37 ± 0.5 °C for 76 days; the initial pH value was 7.5. Scrap iron (SI) and steel wool (SW) were used as Fe0 sources. Results show that: (i) SW performed better than SI on COD and PO43− removal (ii) optimum dosage for the organics and nutrients removal was 10 g/L SI (iii) (NO3− + NH4+) was the least removed pollutant (iv) maximum observed COD, PO43− and NO3− + NH4+ removal efficiencies were 88.0%, 98.0% and 40.0% for 10 g/L SI, 88.2%, 99.9%, 25.1% for 10 g/L SW, and 68.9%, 7.3% and 0.7% for the reference system. Fe0-supported AD significantly removed the organics and nutrients from DS.

Performance and metagenomics analysis of anaerobic digestion of food waste with adding biochar supported nano zero-valent iron under mesophilic and thermophilic condition

A large amount of food waste (FW) brings environmental pollution and sanitation problems. Anaerobic digestion (AD) is an effective technology to treat FW and generate biogas energy. This study investigated the effect of biochar supported nano zero-valent iron (BC-nZVI) on AD performance of FW. Results showed that the cumulative methane yield (CMY) increased by 21.52%-54.90% and the lag time decreased significantly with BC-nZVI. Under mesophilic and thermophilic condition, the peak of CMY was achieved at 178.82 ± 5.27 mL/g VS and 193.01 ± 6.81 mL/g VS with 5 g/L BC-nZVI, respectively. Besides, BC-nZVI stimulated hydrolysis process and reduced the inhibition of NH₄⁺-N and volatile fatty acids accumulation, and it could improve the system stability. Structural equation model analysis indicated that digestion time, BC-nZVI, NH₄⁺-N, temperature and total volatile fatty acid had significant effects on CMY, explaining 92.20% of its total variation. The metagenomic analysis of key microorganisms and related metabolism pathways involved in AD system was further investigated. The results suggested that BC-nZVI contributed to strengthen methanogenesis through enriching the various predominant methanogenic pathways and activating most enzymes related to methane metabolism. BC-nZVI could improve the AD system function and provided a better AD performance by shifting the microbial communities and altering functional genes. This study provided a theoretical basis for BC-nZVI applications and improvements in AD process of FW.

Plastic-containing food waste conversion to biomethane, syngas, and biochar via anaerobic digestion and gasification: Focusing on reactor performance, microbial community analysis, and energy balance assessment

To manage the mixture of food waste and plastic waste, a hybrid biological and thermal system was investigated for converting plastic-containing food waste (PCFW) into renewable energy, focusing on performance evaluation, microbial community analysis, and energy balance assessment. The results showed that anaerobic digestion (AD) of food waste, polyethylene (PE)-containing food waste, polystyrene (PS)-containing food waste, and polypropylene (PP)-containing food waste generated a methane yield of 520.8, 395.6, 504.2, and 479.8 mL CH₄/gVS, respectively. CO₂ gasification of all the plastic-containing digestate produced more syngas than pure digestate gasification. Syngas from PS-digestate reached the maximum yield of 20.78 mol/kg. During the digestate-derived-biochar-amended AD of PCFW, the methane yields in the biochars-amended digesters were 6-30% higher than those of the control digesters. Bioinformatic analysis of microbial communities confirmed the significant difference between control and biochar-amended digesters in terms of bacterial and methanogenic compositions. The enhanced methane yields in biochars-amended digesters could be partially ascribed to the selective enrichment of genus Methanosarcina, leading to an improved equilibrium between hydrogenotrophic and acetoclastic methanogenesis pathways. Moreover, energy balance assessment demonstrated that the hybrid biological and thermal conversion system can be a promising technical option for the treatment of PCFW and recovery of renewable biofuels (i.e., biogas and syngas) and bioresource (i.e., biochar) on an industrial scale.

The role of iron-based nanoparticles (Fe-NPs) on methanogenesis in anaerobic digestion (AD) performance

Several strategies have been proposed to improve the performance of the anaerobic digestion (AD) process. Among them, the use of various nanoparticles (NPs) (e.g. Fe, Ag, Cu, Mn, and metal oxides) is considered one of the most effective approaches to enhance the methanogenesis stage and biogas yield. Iron-based NPs (zero-valent iron with paramagnetic properties (Fe⁰) and iron oxides with ferromagnetic properties (Fe₃O₄/Fe₂O₃) enhance microbial activity and minimise the inhibition effect in methanogenesis. However, comprehensive and up-to-date knowledge on the function and impact of Fe-NPs on methanogens and methanogenesis stages in AD is frequently required. This review focuses on the applicative role of iron-based NPs (Fe-NPs) in the AD methanogenesis step to provide a comprehensive understanding application of Fe-NPs. In addition, insight into the interactions between methanogens and Fe-NPs (e.g. role of methanogens, microbe interaction and gene transfer with Fe-NPs) beneficial for CH₄ production rate is provided. Microbial activity, inhibition effects and direct interspecies electron transfer through Fe-NPs have been extensively discussed. Finally, further studies towards detecting effective and optimised NPs based methods in the methanogenesis stage are reported.

Combined effect of zero valent iron and magnetite on semi-dry anaerobic digestion of swine manure

Combined effect of zero valent iron (ZVI) and magnetite on semi-dry anaerobic digestion of swine manure was studied. Compared with control, the addition of 5 g/L ZVI, magnetite and their mixture (1:1 wt) increased the CH₄ yield by 17.6%, 22.7% and 21.9%, respectively. The three additives improved CH₄ production through altering the metabolism pathways, rather than improving the solid degradation efficiency. ZVI promoted interspecies hydrogen transfer (IHT) by enriching H₂-comsuming Methanolinea and acetate-oxidizing bacteria (Sedimentibacter and Clostridium). Magnetite enriched dissimilatory iron reduction bacteria (Acinetobacter) to accelerate organic hydrolysis and established direct interspecies electron transfer (DIET) by enriching Methanothrix and Methanospirillum. Key microorganisms relative to IHT (Clostridium) and DIET (Methanothrix and Methanospirillum) were simultaneously enriched with ZVI + magnetite, but they only showed an additive effect on methanogenesis, the lack of synergetic effect was attributable to the possible trade-off between IHT and DIET, or the little improvement effect of additives on substrate biodegradability.

Zero valent iron improved methane production and specifically reduced aminoglycoside and tetracycline resistance genes in anaerobic digestion

It is unadvisable to discuss the antibiotic resistance genes (ARGs) reduction in anaerobic digestion (AD) system neglecting its main purpose-methane production. The methane production improvement coupling with antibiotic resistance genes (ARGs) reduction in anaerobic digestion (AD) by zero valent iron (ZVI) were simultaneously investigated. Whether the role of ZVI on the ARGs fate was random or specific was clarified through the high-throughput qPCR (HT-qPCR). Results indicated that ZVI improved methane production and ARGs reduction by 23.9% and 25.0%, respectively. The improved methane production was associated with chemical reaction and variances of microbial community caused by ZVI, where DIET between Petrimonas, Clostridium and Syntrophomonas, Methanosarcina was established along with ACAS being enriched. ZVI specifically, not randomly, facilitated the reduction of aminoglycoside resistance genes of antibiotic inactivation and tetracycline resistance genes of ribosomal protection proteins. The specifical reduction could be attributed to enzyme activity inhibition and intracellular ionic disturbance caused by higher amounts of ZVI, although most of ARGs fate could be well explained by microbial community which contributed the most to ARGs dynamics as a whole. ZVI-based AD was a promising way for the improvement of methane production coupling ARGs reduction.

Long-term characterization and resource potential evaluation of the digestate from food waste anaerobic digestion plants

The management of digestate from food waste (DFW) has become a big challenge for anaerobic digestion (AD) plants. It is crucial to understand the characteristics of DFW for its beneficial utilization. This study investigated the long-term characteristics of DFW from an industrial-scale AD plant in China for 16 months. The result showed that the characteristics of the DFW were relatively stable. The DFW contained considerable amounts of organic matter (23-40% of lignin and 12-26% of protein) and abundant nutrients (N, P, and K), with high concentrations of metals (e.g., 55.17 mg g^-1 and 15.55 mg g^-1 of Ca and Fe) and sulfur (1.40%) on a dry basis. Based on the results, pyrolysis and composting were evaluated as optional conversion ways of DFW. The pyrolysis temperature range of 500 °C to 600 °C was recommended for producing biochar. In this temperature range, the Brunauer-Emmett-Teller surface area of the produced biochar is over 120 m² g^-1. The composting offered the best potential for recovering the nutrients from DFW, but the high ammonia gas content (6970 ppm) should be paid attention to during composting.

Mini art review for zero valent iron application in anaerobic digestion and technical bottlenecks

Zero valent iron (ZVI) has been used extensively to control environmental pollution owing to its strong reducibility and low cost. Herein, we evaluate the impact of ZVI (iron scrap and ZVI powder with different scales) on anaerobic digestion (AD) reactor performance improvement and syntrophic relationship stimulation among various microbial groups in the methanogenesis process. In recent studies, ZVI addition significantly enhanced methane and volatile fatty acid (VFA) yields and alleviated excessive acidification, ammonia accumulation, and odorous gas production. Further, we reviewed the changes in enzyme activity and microbial metabolism after the addition of ZVI throughout the reaction process. Certain innovative technologies, such as bioelectrochemical system assistance and combined usage of conductive materials, may improve AD performance compared to the use of ZVI alone, the mechanism of which has been discussed from various viewpoints. Furthermore, the primary technical bottlenecks, such as poor mass transfer efficiency in dry AD and high ZVI dosage, have been illustrated, and syntrophic methanogenesis regulated by ZVI addition can be further studied by conducting theoretical research.

Silica/Lignin Carrier as a Factor Increasing the Process Performance and Genetic Diversity of Microbial Communities in Laboratory-Scale Anaerobic Digesters

The article aims to present results of research on anaerobic digestion (AD) of waste wafers (WF-control) and co-substrate system–waste wafers and cheese (WFC-control), combined with digested sewage sludge, as inoculum. The purpose of this paper is to confirm the outcome of adding silica/lignin (S/L; 4:1) material, as a microbial carrier, on the process performance and genetic diversity of microbial communities. The experiment was conducted in a laboratory under mesophilic conditions, in a periodical operation mode of bioreactors. Selected physicochemical parameters of the tested carrier, along with the microstructure and thermal stability, were determined. Substrates, batches and fermenting slurries were subjected to standard parameter analysis. As part of the conducted analysis, samples of fermented food were also tested for total bacterial count, dehydrogenase activity. Additionally, DNA extraction and next-generation sequencing (NGS) were carried out. As a result of the conducted study, an increase in the volume of produced biogas was recorded for samples fermented with S/L carrier: in the case of WF + S/L by 18.18% to a cumulative biogas yield of 833.35 m3 Mg−1 VS, and in the case of WFC + S/L by 17.49% to a yield of 950.64 m3 Mg−1 VS. The largest total bacterial count, during the process of dehydrogenase activity, was maintained in the WFC + S/L system. The largest bacterial biodiversity was recorded in samples fermented with the addition of cheese, both in the case of the control variant and in the variant when the carrier was used. In contrast, three phyla of bacteria Firmicutes, Proteobacteria and Actinobacteria predominated in all experimental facilities.

Roles of modified biochar in the performance, sludge characteristics, and microbial community features of anaerobic reactor for treatment food waste

Anaerobic digestion (AD) is a green technology widely applied to food waste treatment. Although the AD has high efficiency, instability often occurs. The main purpose of the study is to understand the mechanism of modified biochar improving AD performance. The effects of different modified biochar on the efficiency and microecology of an anaerobic reactor treating food waste were investigated. Bagasse biochar was used as the substrate to explore the effects of iron-modified (A), chitosan-modified (B), iron-chitosan-modified (C) and iron‑magnesium-chitosan-modified (D) biochar on the anaerobic digestion process, sludge characteristics and microbial community. The results show that the average COD removal efficiency of the four reactors during the last five days of the experimentation period was 86.95%, 85.90%, 92.22% and 93.29%, respectively. Adding iron‑magnesium-chitosan-modified biochar could improve the efficiency of COD removal in the anaerobic reactor under ammonia nitrogen stress. On day 10 of operation, the content of coenzyme F₄₂₀ in the sludge of anaerobic reactors C and D reached to 0.44 and 0.57 mmol/g, respectively, indicating that the metal-chitosan complex biochar could promote the production of coenzyme F₄₂₀ in the early stage of the experiment. Within the four anaerobic reactors, Firmicutes, Bacteroidetes, Proteobacteria and Chloroflexi were the dominant bacteria, and the abundance of Chloroflexi reached a maximum of 26.24% in the reactor C. As for archaea, Methanobacterium and Methanothrix were the most dominant accounting for 44.03%, 49.88%, 31.29%, 52.01% and 38.34%, 34.52%, 50.9%, 35.72% respectively in the four reactors. KEGG functional analysis showed that the energy metabolism of bacteria and archaea in the reactor D was the largest among the four reactors. Meanwhile, the gene abundance associated with carbohydrate metabolism and membrane transport of microorganisms in the reactor D was greater than that of other groups.

Effect of biochar addition on microbial community and methane production during anaerobic digestion of food wastes: The role of minerals in biochar

The effect of biochar addition on the microbial community and methane (CH₄) production during anaerobic digestion was experimentally investigated, focusing on the role of minerals in biochar. The biochar was prepared from pine sawdust by pyrolysis at 650 °C and 900 °C, respectively, and a subsample was leached with citric acid. The cultures with the addition of biochar, leached biochar, Fe, and leached biochar combined with Fe, respectively, were placed in bench-scale bioreactors for anaerobic digestion. Daily biogas production was measured by volume displacement method and analysed for CH₄ concentration, which allowed the cumulative CH₄ yield (Y_M) and daily CH₄ production rate (R_M) to be determined. Culture samples were also taken daily for volatile fatty acids (VFAs) and microbial community analysis. Compared to the control without biochar addition, the addition of raw biochar significantly increased Y_M by 46.9% and R_M by 43.0%, while leached biochar only increased the Y_M by 33.2% and R_M by 18.2%, respectively. The Fe-containing minerals in biochar were found to enhance VFA degradation and increase population of Clostridia and Methanosaeta, improving the CH₄ production.