Enhanced mesophilic anaerobic digestion of waste sludge with the iron nanoparticles addition and kinetic analysis

Impact of Iron Oxide on Anaerobic Digestion of Frass in Biogas and Methanogenic Archaeal Communities' Analysis

With the increasing prominence of the global energy problem, socioeconomic activities have been seriously affected. Biofuels, as a renewable source of energy, are of great significance in promoting sustainable development. In this study, batch anaerobic digestion (AD) of frass (swine manure after bioconversion by black soldier fly larvae) and co-digestion with corn straw after the addition of iron oxide (Fe3O4) nanoparticles is investigated, as well as the start-up period without inoculation. The biochemical methane potential of pure frass was obtained using blank 1 group and after the addition of various sizes of Fe3O4 nanoparticles for 30 days period, and similarly, the digestion of frass with straw (blank 2) and after the addition of various sizes of Fe3O4 nanoparticles for 61 days period. The results showed that the average gas production was 209.43 mL/gVS, 197.68 mL/gVS, 151.85 mL/gVS, and 238.15 mL/gVS for the blank, ~176 nm, ~164 nm, and ~184 nm, respectively. The average gas production of frass with straw (blank 2) was 261.64 mL/gVS, 259.62 mL/gVS, 241.51 mL/gVS, and 285.98 mL/gVS for blank 2, ~176 nm, ~164 nm, and ~184 nm, respectively. Meanwhile, the accumulated methane production of the ~184 nm group was 2312.98 mL and 10,952.96 mL, respectively, which significantly increased the biogas production compared to the other groups. The methanogenic results of the frass (30 days) indicated that Methanocorpusculum, Methanosarcina, and Methanomassiliicoccus are the important methanogenic species in the AD reactor, while the microbial diversity of the ~184 nm group was optimal, which may be the reason for the high gas production of ~184 nm.

Functional biochar in enhanced anaerobic digestion: Synthesis, performances, and mechanisms

Anaerobic digestion technology is crucial in bioenergy recovery and organic waste management. At the same time, it often encounters challenges such as low organic digestibility and inhibition of toxic substances, resulting in low biomethane yields. Biochar has recently been used in anaerobic digestion to alleviate toxicity inhibition, improve the stability of anaerobic digestion processes, and increase methane yields. However, the practical application of biochar is limited, for the properties of pristine biochar significantly affect its application in anaerobic digestion. Although much research focuses on understanding original biochar's fundamental properties and functionalization, there are few reviews on the applications of functional biochar and the effects of critical properties of pristine biochar on anaerobic digestion. This review systematically reviewed functionalization strategies, key performances, and applications of functional biochar in anaerobic digestion. The properties determining the role of biochar were reviewed, the synthesis methods of functional biochar were summarized and compared, the mechanism of functional biochar was discussed, and the factors affecting the function of functional biochar were reviewed. This review provided a comprehensive understanding of functional biochar in anaerobic digestion processes, which would be helpful for the development and applications of engineered biochar.

Nanotechnology based technological development in biofuel production: Current status and future prospects

Depleting fossil fuels and net carbon emissions associated with their burning have driven the need to find alternative energy sources. Biofuels are near-perfect candidates for alternative energy sources as they are renewable and account for no net CO2 emissions. However, biofuel production must overcome various challenges to compete with conventional fuels. Conventional methods for bioconversion of biomass to biofuel include chemical, thermochemical, and biological processes. Substrate selection and processing, low yield, and total cost of production are some of the main issues associated with biofuel generation. Recently, the uses of nanotechnology and nanoparticles have been explored to improve the biofuel production processes because of their high adsorption, high reactivity, and catalytic properties. The role of these nanoscale particles and nanocatalysts in biomass conversion and their effect on biofuel production processes and yield are discussed in the present article. The applicability of nanotechnology in production processes of biobutanol, bioethanol, biodiesel, biohydrogen, and biogas under biorefinery approach are presented. Different types of nanoparticles, and their function in the bioprocess, such as electron transfer, pretreatment, hydrolysis, microalgae cultivation, lipid extraction, dark and photo fermentation, immobilization, and suppression of inhibitory compounds, are also highlighted. Finally, the current and potential applications of nanotechnology in biorefineries are also discussed.

Effects of zero-valent iron and magnetite on ethanol and lactic acid production in the anaerobic fermentation of food waste

With the increasing global concern about food waste management, finding efficient ways to convert it into valuable products is crucial. The addition of zero-valent iron and magnetite to enhance ethanol and lactic acid fermentation yields from food waste emerges as a potential solution. This study compared the effects of 50-nm and 500-nm particle sizes of zero-valent iron and magnetite on ethanol and lactic acid fermentation and analyzed the mechanism of action from the perspective of organic matter material transformation and microbiology. The experimental results showed that 500-nm particle size magnetite and zero-valent iron could promote the hydrolysis of polysaccharides and proteins. 500-nm particle size magnetite could increase ethanol production (1.4-fold of the control), while 500-nm particle size zero-valent iron could increase lactic acid production (2.8-fold of the control). Metagenomic analysis showed that 500-nm magnetite increased the abundance of genes for amino acid metabolic functions, while 500-nm zero-valent iron increased the abundance of glycoside hydrolase genes (1.3-fold of the control). It's worth noting that while these findings are promising, they are based on controlled experimental conditions, and real-world applications may vary. his research not only offers a novel approach to augmenting anaerobic fermentation yields but also contributes to sustainable food waste management practices, potentially reducing environmental impacts and creating valuable products.

Magnetic biochar promotes the risk of mobile genetic elements propagation in sludge anaerobic digestion

Mobile genetic elements (MGEs) mediated horizontal gene transfer is the primary reason for the propagation of antibiotic resistance genes in environment. The behavior of MGEs under magnetic biochar pressure in sludge anaerobic digestion (AD) is still unknown. This study evaluated the effects of different dosage magnetic biochar on the MGEs in AD reactors. The results showed that the biogas yield was highest (106.68 ± 1.16 mL g^-1 VS_added) with adding optimal dosage of magnetic biochar (25 mg g^-1 TS_added), due to it increased the microorganism's abundance involved in hydrolysis and methanogenesis. While, the total absolute abundance of MGEs in the reactors with magnetic biochar addition increased by 11.58%-77.37% compared with the blank reactor. When the dosage of magnetic biochar was 12.5 mg g^-1 TS_added, the relative abundance of most MGEs was the highest. The enrichment effect on ISCR1 was the most significant, and the enrichment rate reached 158.90-214.16%. Only the intI1 abundance was reduced and the removal rates yield 14.38-40.00%, which was inversely proportional to the dosage of magnetic biochar. Co-occurrence network explored that Proteobacteria (35.64%), Firmicutes (19.80%) and Actinobacteriota (15.84%) were the main potential host of MGEs. Magnetic biochar changed MGEs abundance by affecting the potential MGEs-host community structure and abundance. Redundancy analysis and variation partitioning analysis showed that the combined effect of polysaccharides, protein and sCOD exhibited the greatest contribution (accounted for 34.08%) on MGEs variation. These findings demonstrated that magnetic biochar increases the risk of MGEs proliferation in AD system.

Simultaneous addition of CO2-nanobubble water and iron nanoparticles to enhance methane production from anaerobic digestion of corn straw

In this research, CO₂-nanobubble water (CO₂-NBW) and iron nanoparticles (Fe⁰NPs) were added simultaneously to exploit individual advantages to enhance the methanogenesis process from both the stability of anaerobic digestion (AD) system and the activity of anaerobic microorganism aspects. Results showed that the AD performance was enhanced by supplementing with CO₂-NBW or Fe⁰NPs individually, and could be further improved by simultaneous addition of the two additives. The maximum methane yield was achieved in the CO₂-NBW + Fe⁰NPs reactor (141.99 mL/g-VS_added), which increased by 26.16% compared to the control group. Similarly, the activities of the electron transfer system (ETS) and enzyme were improved. The results of microbial community structure revealed that the addition of CO₂-NBW and Fe⁰NPs could improve the abundance of dominant bacteria (Anaerolineaceae, Bacteroidales, and Prolixibacteraceae) and archaea (Methanotrichaceae and Methanospirillaceae). Additionally, the functional metabolic prediction heatmap indicated that metabolic functional genes favorable for AD of corn straw were enhanced.

Prediction of biogas production of industrial scale anaerobic digestion plant by machine learning algorithms

In the anaerobic digestion (AD) process there are some difficulties in maintaining process stability due to the complexity of the system. The variability of the raw material coming to the facility, temperature fluctuations and pH changes as a result of microbial processes cause process instability and require continuous monitoring and control. Increasing continuous monitoring, and internet of things applications within the scope of Industry 4.0 in AD facilities can provide process stability control and early intervention. In this study, five different machine learning (ML) algorithms (RF, ANN, KNN, SVR, and XGBoost) were used to describe and predict the correlation between operational parameters and biogas production quantities collected from a real-scale anaerobic digestion plant. The KNN algorithm had the lowest accuracy in predicting total biogas production over time, while the RF model had the highest prediction accuracy of all prediction models. The RF method produced the best prediction, with an R² of 0.9242, and it was followed by XGBoost, ANN, SVR, and KNN (with R² values of 0.8960, 0.8703, 0.8655, 0.8326, respectively). Real-time process control will be provided and process stability will be maintained by preventing low-efficiency biogas production with the integration of ML applications into AD facilities.

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.

Effects of Magnetic Biochar Addition on Mesophilic Anaerobic Digestion of Sewage Sludge

As a low-cost additive to anaerobic digestion (AD), magnetic biochar (MBC) can act as an electron conductor to promote electron transfer to enhance biogas production performance in the AD process of sewage sludge and has thus attracted much attention in research and industrial applications. In the present work, Camellia oleifera shell (COS) was used to produce MBC as an additive for mesophilic AD of sewage sludge, in order to explore the effect of MBC on the mesophilic AD process and its enhancement mechanism. Analysis by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectrometry (FTIR), and X-ray diffraction (XRD) further confirmed that biochar was successfully magnetized. The yield of biogas from sewage sludge was enhanced by 14.68-39.24% with the addition of MBC, and the removal efficiency of total solid (TS), volatile solids (VS), and soluble chemical oxygen demand (sCOD) were 28.99-46.13%, 32.22-48.62%, and 84.18-86.71%, respectively. According to the Modified Gompertz Model and Cone Model, the optimum dosage of MBC was 20 mg/g TS. The maximum methane production rate (R_m) was 15.58% higher than that of the control reactor, while the lag-phase (λ) was 43.78% shorter than the control group. The concentration of soluble Fe²⁺ and Fe³⁺ were also detected in this study to analyze the function of MBC for improving biogas production performance from sewage sludge. The biogas production was increased when soluble Fe³⁺ was reduced to soluble Fe²⁺. Overall, the MBC was beneficial to the resource utilization of COS and showed a good prospect for improving mesophilic AD performance.

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.

Employing micro-aeration in anaerobic digestion of poultry litter and wheat straw: Batch kinetics and continuous performance

In this study, different micro-aeration (MA) strategies for anaerobic digestion (AD) of poultry litter (PL) and wheat straw (WS) were examined. MA at different stages (pretreatment, middle, pretreatment plus middle, and daily) in batch AD of WS showed that daily MA had the highest increase (16.5 %) of the cumulative methane yield (CMY) compared to the control. Batch co-digestion (Co-AD) of WS and PL with daily MA obtained a furtherly improved (15.1 %) CMY of 225.44 N mL CH₄/g vS _added. The modified Gompertz model and Cone model were good in fitting the methane yield kinetics of MA engaged AD process (R² greater than 0.99). Daily MA shortened the lag phase of Co-AD by 3.4 %. The sequencing batch reactor for the Co-AD of WS and PL showed an increased (21.5 %) daily methane yield when 0.5-h/d MA was employed. The results provided support for the application of micro-aeration in the AD of agricultural wastes.

The role of iron nanoparticles on anaerobic digestion: mechanisms, limitations, and perspectives

Anaerobic digestion (AD) is the most widely used technology for organic matter treatment. However, multiple types of research have reported on improving the process because different operation inhibition factors and limitations affect the performance of AD process. Owing to the increasing use of iron-nanoparticles (Fe-NP) on AD, this review addresses the knowledge gaps and summarizes the finding from academic articles based on (i) the AD upgrading operations: limitations and upgrade techniques, (ii) Fe-NPs mechanisms on AD, (iii) Fe-NP effect on microbial communities associated to AD systems, and (iv) perspectives. The selected topics give the Fe-NP positive effects on the AD methane-production process in terms of gas production, effluent quality, and process optimization. The main results of this work indicate that (i) Fe-NP addition can be adapted among different feedstocks and complement other pretreatments, (ii) Fe-NP physicochemical characteristics enhance biogas production via direct interspecies electron transfer (DIET) mechanisms, and Fe-ion release due to their structure and their conductivity capability, and (iii) syntrophic bacteria and acetoclastic methanogens have been reported as the communities that better uptake Fe-NPs on their metabolisms. Finally, our research perspectives and gaps will be discussed to contribute to our knowledge of using Fe-NPs on AD systems.

The advantages of co-digestion of vegetable oil industry by-products and sewage sludge: Biogas production potential, kinetic analysis and digestate valorisation

The production of edible vegetable oils generates considerable amounts of energy-rich waste, which is usually not utilised fully. Besides, inefficient management of such wastes can have a negative impact on the environment. On the other hand, this waste can also serve as a raw material for the production of high value-added products, such is biogas. The mono-digestion of seven different by-products and wastes from the vegetable oil industry was investigated in this study: Pumpkin seeds press cake (PSPC), grape seeds press cake (GSPC), olive mill pomace (OMP), coconut oil cake (CC), filtration additive (FA), spent bleaching earth (SBE) and sludge from a vegetable oil industry (SOI) wastewater treatment plant. In addition, co-digestion of these substrates was performed with municipal sewage sludge (SS). Besides inoculum, rumen fluid was added to the reactors to enhance biogas production. The biogas production potential of the tested substrates was monitored by measuring various parameters. A kinetic analysis was later carried out and a growth test was performed on the digestates to evaluate their potential for agricultural use. The highest biogas yields in the mono-digestion test were obtained with the substrates with the highest fat content: 1402, 1288, 830 and 750 mL of biogas/gVS for SOI, FA, PSPC and CC substrate, respectively. Co-digestion of SS with by-products of vegetable oil industry such as FA, SBE, CC, SOI and PSPC increased the biogas yields by 94.9%, 74.1%, 30.8%, 27.4% and 23.6% compared to SS mono-digestion. Furthermore, the data for mono-digestion of PSPC, GSPC, and FA, and co-digestion of SS with these substrates, CC and SBE, have not been found in the literature to date. The maximum methane content ranged from 61 to 74 vol%, while the chemical oxygen demand removal efficiency ranged from 42 to 78%. Relatively high fatty acids contents and ammonium concentrations were measured in the reactors. Kinetic analysis showed the best fit to the experimental data for the Cone kinetic model (R² > 0.98). The First order kinetic model, Monod, and the modified Gompertz model also exhibited high R² values. The digestates obtained from co-digestion proved to be excellent in the cress seeds growth test at digestate concentrations of 5-10 wt%, while higher concentrations had a toxic effect.

Nanomaterials for biogas augmentation towards renewable and sustainable energy production: A critical review

Nanotechnology is considered one of the most significant advancements in science and technology over the last few decades. However, the contemporary use of nanomaterials in bioenergy production is very deficient. This study evaluates the application of nanomaterials for biogas production from different kinds of waste. A state-of-the-art comprehensive review is carried out to elaborate on the deployment of different categories of nano-additives (metal oxides, zero-valent metals, various compounds, carbon-based nanomaterials, nano-composites, and nano-ash) in several kinds of biodegradable waste, including cattle manure, wastewater sludge, municipal solid waste, lake sediments, and sanitary landfills. This study discusses the pros and cons of nano-additives on biogas production from the anaerobic digestion process. Several all-inclusive tables are presented to appraise the literature on different nanomaterials used for biogas production from biomass. Future perspectives to increase biogas production via nano-additives are presented, and the conclusion is drawn on the productivity of biogas based on various nanomaterials. A qualitative review of relevant literature published in the last 50 years is conducted using the bibliometric technique for the first time in literature. About 14,000 research articles are included in this analysis, indexed on the Web of Science. The analysis revealed that the last decade (2010–20) was the golden era for biogas literature, as 84.4% of total publications were published in this timeline. Moreover, it was observed that nanomaterials had revolutionized the field of anaerobic digestion, methane production, and waste activated sludge; and are currently the central pivot of the research community. The toxicity of nanomaterials adversely affects anaerobic bacteria; therefore, using bioactive nanomaterials is emerging as the best alternative. Conducting optimization studies by varying substrate and nanomaterials’ size, concentration and shape is still a field. Furthermore, collecting and disposing nanomaterials at the end of the anaerobic process is a critical environmental challenge to technology implementation that needs to be addressed before the nanomaterials assisted anaerobic process could pave its path to the large-scale industrial sector.

Potential promotion of activated carbon supported nano zero-valent iron on anaerobic digestion of waste activated sludge

A large amount of waste activated sludge (WAS) harms the ecological environment, and anaerobic digestion (AD) is an effective method for WAS treatment. In this study, activated carbon (AC)/ nano zero-valent iron (NZVI) was synthesized by a liquid-phase reduction method, and was used to boost methane production. The associated mechanisms and effects of additives on AD during the addition and removal stage were investigated systematically. Compared to the blank group, the cumulative methane production was increased by 14.3%, 26.3% and 34.1% in the groups of AC, NZVI and AC/NZVI, respectively. The addition of AC/NZVI significantly increased the concentration of VFAs and promoted the hydrolysis and acidification of WAS. After the AD of the additives addition stage was finished, the additives were removed and the sludge was replenished in all groups, the methanogenesis performance of the experimental groups was significantly inhibited. The cumulative methane production in the AC and AC/NZVI groups was 21.7% and 13.5% lower than the blank group, respectively. The experimental results have a good correlation with curve fitting by the modified Gompertz model. The modified Gompertz model found that AC, NZVI and AC/NZVI increased the methanogenic potential and maximum methane production rate of WAS, but also prolonged the lag-phase time. AC/NZVI might play a role in coupling effects. It could not only maintain the original characteristics of NZVI and increase its stability, but also develop the advantages of AC promoting direct interspecies electron transfer. Microbial community analysis indicated that the abundance of hydrogenotrophic methanogens was enriched by AC/NZVI.

Impact of Iron oxide nanoparticles on sustainable production of biogas through anaerobic co-digestion of chicken waste and wastewater

As The effect of iron oxide nanoparticles (IONPs) on the anaerobic co-digestion (AD) of olive mill wastewater and chicken manure was investigated. In mesophilic conditions, biogas yield, methane (CH4) content, the removal efficiency of TS, VS., acidification and hydrolysis percentage, and contaminant removal efficiency were investigated. Supplementing AD with IONPs at a concentration of 20 mg/g VS. > IONPs and INOPs >30 mg/g VS. causes an inhibitor impact on biogas, methane generation, and hydrolysis. Furthermore, implantation with 20–30 mg of IONPs/kg VS. has induced an equivalent favorable impact, with hydrolysis percentages reaching roughly 7.2%–15.1% compared to the control test, in addition to a 1.3%–4.2% enhancement in methane generation yield. The maximum acidification concentration after five days of the incubation of 1,084, 9,463, and 760 g/L was attained with IONPs dosages of 25, 30, and 20 mg/g VS., respectively, compared to 713 g/L obtained with the control test. The results have illustrated that supplementing AD with a specific concentration of IONPs (20–30 mg/g VS.) has a significant effect and enhances the inhibitor removal efficiency, most possibly due to the small surface area of IONP particles. The resultant increase in the active surface area enhances the enzyme diffusion within the substrate. This study provides new data specifying the enhancement of iron oxide nanoparticles (IONPs) and identifies the impact of IONP doses at various concentrations on the AD of olive mill wastewater and chicken waste.

Preparation of phosphoric acid modified antibiotic mycelial residues biochar: Loading of nano zero-valent iron and promotion on biogas production

Antibiotic mycelial residues (AMRs), as recyclable hazardous waste, can realize efficient utilization by reasonable treatment. To solve the problems of undeveloped pore structure and low specific surface area existed in AMR biochar, this study first modified biochar by phosphoric acid (H₃PO₄) to prepare PBC (H₃PO₄-modified biochar). Then, PBC was used as carrier to load nano zero-valent iron (nZVI) for preparation of nZVI/PBC. Finally, the biochar materials were used to promote anaerobic digestion (AD) of corn straw. The results showed that H₃PO₄-modification can effectively improve the specific surface area, pore structure, and electron donating capacity of AMRs biochar. The using of PBC as carrier to load nZVI attenuated the agglomeration of nZVI particles. Both PBC and nZVI/PBC improved the AD process, with biogas yield enhanced by 29.63% and 29.26%, respectively. The nZVI/PBC exhibited higher ability in maintaining the stability of AD system and promotion of fiber degradation than PBC.

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.

Facile synthesis of recyclable 3D gelatin aerogel decorated with MIL-88B(Fe) for activation peroxydisulfate degradation of norfloxacin

The application of traditional powder catalysts is limited by particle agglomeration and difficult recovery. In this work, a three-dimensional porous aerogel catalyst for organic pollutants degradation in water by activating peroxydisulfate (PDS) was successfully synthesized, which was obtained via directly mixing of MIL-88B(Fe) with sol precursors followed by vacuum freeze-drying and low-temperature calcination. MIL-88B(Fe)/gelatin aerogel-150/PDS (MGA-150/PDS) system displayed satisfactory norfloxacin (NOR) degradation performance, which could remove 98.7% of NOR in 90 min. Its reaction rate constant was 23.2 times higher than the gelatin aerogel/PDS (GA/PDS) system. In addition, Electron paramagnetic resonance (EPR) results and radical trapping experiments revealed both radicals (SO₄^•-, •OH) and non-radical (¹O₂) pathways had participated in NOR degradation, of which •OH was dominant. Possible degradation pathways were proposed. Moreover, the high degradation efficiency of NOR by MGA-150 composites could still be reached more than 90.0% even after 10 cycles, and the morphology and chemical structure of MGA-150 composites exhibited no significant changes, indicating the arrestive stability of aerogel composites. This progress not only proposed an effective catalyst for PDS activation, but also expanded views for the design and development of 3D functional materials.

Volatile Fatty Acids (VFA) Production from Wastewaters with High Salinity—Influence of pH, Salinity and Reactor Configuration

The hydrocarbon-based economy is moving at a large pace to a decarbonized sustainable bioeconomy based on biorefining all types of secondary carbohydrate-based raw materials. In this work, 50 g L−1 in COD of a mixture of food waste, brine and wastewater derived from a biodiesel production facility were used to produce organic acids, important building-blocks for a biobased industry. High salinity (12–18 g L−1), different reactors configuration operated in batch mode, and different initial pH were tested. In experiment I, a batch stirred reactor (BSR) at atmospheric pressure and a granular sludge bed column (GSBC) were tested with an initial pH of 5. In the end of the experiment, the acidification yield (ηa) was similar in both reactors (22–24%, w/w); nevertheless, lactic acid was in lower concentrations in BSR (6.3 g L−1 in COD), when compared to GSBC (8.0 g L−1 in COD), and valeric was the dominant acid, reaching 17.3% (w/w) in the BSR. In experiment II, the BSR and a pressurized batch stirred reactor (PBSR, operated at 6 bar) were tested with initial pH 7. The ηa and the VFA concentration were higher in the BSR (46%, 22.8 g L−1 in COD) than in the PBSR (41%, 20.3 g/L in COD), and longer chain acids were more predominant in BSR (24.4% butyric, 6.7% valeric, and 6.2% caproic acids) than in PBSR (23.2%, 6.2%, and 4.2%, respectively). The results show that initial pH of 7 allows achieving higher ηa, and the BSR presents the most suitable reactor among tested configurations to produce VFA from wastes/wastewaters with high salinity.

Historical Perspective of the Addition of Magnetic Nanoparticles Into Anaerobic Digesters (2014-2021)

The addition of magnetic nanoparticles to batch anaerobic digestion was first reported in 2014. Afterwards, the number of works dealing with this subject has been increasing year by year. The discovery of the enhancement of anaerobic digestion by adding iron-based nanoparticles has created a multidisciplinary emerging research field. As a consequence, in the last years, great efforts have been made to understand the enhancement mechanisms by which magnetic nanoparticles (NPs) addition enhances the anaerobic digestion process of numerous organic wastes. Some hypotheses point to the dissolution of iron as essential iron for anaerobic digestion development, and the state of oxidation of iron NPs that can reduce organic matter to methane. The evolution and trends of this novel topic are discussed in this manuscript. Perspectives on the needed works on this topic are also presented.

Thermal treatment's enhancement on high solid anaerobic digestion: effects of temperature and reaction time

The methane production rate of high solid anaerobic digestion (HSAD) was poor although it was a promising technology with the advantages of small reactor, low energy consumption, and less digestate. In our research before, thermal treatment was proved to enhance HSAD's methane production rate via both batch experiments and continuous experiments of swine manure. However, the effect or investigation of thermal treatment's temperature-time combinations was not yet reported. In this study, swine manure was firstly thermally treated in 500-mL glass bottles with 400-mL work volume at 45-65 °C for 1-4 days. HSAD experiment of 10% solid content was then set up. The VS ratio of substrate to inoculum was 1:1. Thermal treatment at 45 °C (3 days), 55 °C (1 day), and 65 °C (3 days) could obtain the highest methane production rate, which was around 40% higher. Kinetics analysis suggested that the degradation of swine manure was quite different at different temperatures. Furthermore, energy assessment indicated that "thermal treatment + HSAD" had significant advantages in improving HSAD economic feasibility, because the improved methane production rate could compensate for the extra energy utilized for thermal treatment. Heat treatment at 45 °C (4 days) was preferred when the heating equipment was limited. Heat treatment at 55 °C (1 day) was preferred when the floor space and reactor volume were restricted. Heat treatment at 65°C (3 days) was preferred when the requirement of the digestate's sanitary condition is strict.

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.

Mitigation of acidogenic product inhibition and elevated mass transfer by biochar during anaerobic digestion of food waste

Anaerobic digestion (AD) of food waste is widely accepted as a promising technology for both waste disposal and resource recovery. With the advancing of AD technology, to exploit the capacity of organic waste for maximum energy/resource recovery becomes the new focus and hence, improve the viability of this technology for practical application. Product inhibition and mass transfer are the common limitations encountered during AD of putrescible organic waste. Biochar materials have been widely used to promote AD process in recent years. This review summarizes the mechanism and regulation strategies of biochar and its modified derivatives in promoting AD of solid waste (mainly food waste) from the three aspects of hydrolysis, syntrophic acetogenesis, and methane production. At the same time, the relationship between carbon materials and electron transfer among anaerobic microbes is summarized from the perspective of microbial community. In addition, the market application of this technology was evaluated.

Enhanced anaerobic digestion of waste-activated sludge via bioaugmentation strategy-Phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt2) analysis through hydrolytic enzymes and possible linkage to system performance

In this study, anaerobic digestion of waste-activated sludge was bioaugmented with hydrolytic bacteria, Bacteroidetes uniformis (Bacteroidetes, B) and Clostridium sp. (Firmicutes, F) at various dosages. Bioaugmentation resulted in enhanced methane conversion of waste-activated sludge. The highest methane yield of 298.1 mL CH₄/g-COD, 85.2% COD conversion efficiency was obtained when Bacteroidetes uniformis and Clostridium sp. were augmented at 100 and 900 CFU/mL, respectively. The microbial community analysis demonstrated that bioaugmentation increased the proportion of Bacteroidetes, Firmicutes, and Proteobacteria. Furthermore, at the highest methane yield, the principal methanogenic pathway was altered from acetoclastic to a mixture of hydrogenotrophic and acetoclastic; the major species shifted from Methanosaeta concilii to Methanobacterium subterraneum. Predicted gene analysis revealed that increased expression of hydrolases resulted in enhanced methane conversion through bioaugmentation.

The impact and fate of clarithromycin in anaerobic digestion of waste activated sludge for biogas production

Clarithromycin retained in waste activated sludge (WAS) inevitably enters the anaerobic digestion system. So far, the complex impacts and fate of clarithromycin in continuous operated WAS anaerobic digestion system are still unclear. In this study, two semi-continuous long-term reactors were set up to investigate the effect of clarithromycin on biogas production and antibiotic resistance genes (ARGs) during WAS anaerobic digestion, and a batch test was carried out to explore the potential metabolic mechanism. Experimental results showed that clarithromycin at lower concentrations (i.e., 0.1 and 1.0 mg/L) did not affect biogas production, whereas the decrease in biogas production was observed when the concentration of clarithromycin was further increased to 10 mg/L. Correspondingly, the relative abundance of functional bacteria in WAS anaerobic digestion (i.e., Anaerolineaceae and Microtrichales) was reduced with long-term clarithromycin exposure. The investigation of ARGs suggested that the effect of methylation belonging to the target site modification played a critical role for the anaerobic microorganisms in the expression of antibiotic resistance, and ermF, played dominated ARGs, presented the most remarkable proliferation. In comparison, the role of efflux pump was weakened with a significant decrease of two detected efflux genes. During WAS anaerobic digestion, clarithromycin could be partially degraded into metabolites with lower antimicrobial activity including oleandomycin and 5-O-desosaminyl-6-O-methylerythronolide and other metabolites without antimicrobial activity.

Addition of nanoparticles increases the abundance of mobile genetic elements and changes microbial community in the sludge anaerobic digestion system

This study explored the fate of mobile genetic elements (MGEs) in anaerobic digestion (AD) system with four nanoparticles (NPs) added, including carbon NPs, Al₂O₃ NPs, ZnO NPs, and CuO NPs. 16S rRNA amplicon sequencing and quantitative PCR to investigate the microbial community, MGEs abundance and the potential host in the AD process. The results of high-throughput sequencing showed that ZnO NPs and CuO NPs significantly reduced the microbial diversity and significantly changed the microbial community structure. Simultaneously, the absolute abundance of MGEs increased by 145.01%, 159.67%, 354.70%, and 132.80% on the carbon NPs, Al₂O₃ NPs, ZnO NPs, and CuO NPs. The enrichment rate of tnpA-03 in ZnO NPs group was the highest, which could reach up to 2854.80%. Co-occurrence analysis revealed that Proteobacteria harbored the vast majority of MGEs followed by Firmicutes. Redundancy analysis and variation partitioning analysis showed that metabolites were the main factors that shifted the succession of bacterial communities. Moreover, there were significant positive correlations between metabolites and part MGEs (such as tnpA-01, tnpA-02, tnpA-03, tnpA-04, tnpA-05, tnpA-07 and ISCR1). This study provides a new perspective that NPs increase the risk of antibiotic resistance through MGEs during AD process.

The effect of brewery spent grain application on biogas yields and kinetics in co-digestion with sewage sludge

The present study examines the effect of introducing dried brewery spent grain (BSG), known as the main solid by-product of the brewery industry on biogas yields and kinetics in co-digestion with sewage sludge (SS). The experiment was conducted in semi-continuous anaerobic reactors (supplied once a day) operating under mesophilic conditions (35°C) at different hydraulic retention times (HRT) of 18 and 20 d. In co-digestion runs, the BSG mass to the feed volume ratio was constant and maintained 1:10.The results indicated that the addition of BSG did not influence the biogas production, by comparison with SS mono-digestion (control run). At HRT of 18 d, in the co-digestion run, the average methane yield was 0.27 m³ kg/VS_added, while in the control run the higher value of 0.29 m³ kg/VS_addedwas observed. However, there was no difference in terms of statistical significance. At HRT of 20 d, the methane yield was 0.21 m³ kg/VS_added for both mono- and co-digestion runs. In the BSG presence, the decrease in kinetic constant values was observed. As compared to SS mono-digestion, reductions by 21 and 35% were found at HRT of 20 and 18 d, respectively. However, due to the supplementation of the feedstock with BSG rich in organic compounds, the significantly enhanced energy profits were achieved with the highest value of approx. 40% and related to the longer HRT of 20 d. Importantly, the mono- and co-digestion process proceeded in stable manner. Therefore, the anaerobic co-digestion of SS and BSG might be considered as a cost-effective solution that could contribute to the energy self-efficiency of wastewater treatment plants (WWTPs) and sustainable waste management for breweries.

Relieving ammonia inhibition by zero-valent iron (ZVI) dosing to enhance methanogenesis in the high solid anaerobic digestion of swine manure

Relieving from ammonia inhibition and enhancing the utilization of thermodynamically unfavorable propionate are crucial for methane harvest in the high solid anaerobic digestion (HSAD) of ammonia-rich swine manure. In this study, the potential of dosing zero-valent iron (ZVI, 150 um) for enhancing the methanogenesis to resist total ammonia (TAN) over 5.0 g-N·L^-1 was investigated via batch experiments under mesophilic condition. The cumulative methane production was enhanced by 22.2% at ≥160 mM ZVI dosage and the HSAD duration was further shortened by 50.6% at ≥320 mM ZVI dosage. The enhanced methanogenesis was mainly resulted from the full utilization of propionate and the accelerated collapse of posterior-biodegradable organics which might be driven by ZVI. Results of microbial community and qPCR (mcrA) showed that ZVI might trigger the blooming of Methanosarcina (from 27.9% to 78.3%) and Syntrophomonas (0.5% to 3.7%) and attribute to their possible direct interspecies electron transfer (DIET) to enhance propionate utilization. Besides, the main methanogenesis might remain in the effective aceticlastic pathway even under free ammonia (FAN) almost 1.0 g-N·L^-1 because syntrophic acetate oxidizing bacteria (SAOB) decreased to almost none at 320 mM ZVI dosage. Dosing ZVI could relieve HSAD from TAN inhibition and more dosage was required to resist FAN inhibition.

Evolutions of antibiotic resistance genes (ARGs), class 1 integron-integrase (intI1) and potential hosts of ARGs during sludge anaerobic digestion with the iron nanoparticles addition

In this work, we investigated the impact of iron nanoparticle, including magnetite nanoparticles (Fe₃O₄ NPs) and nanoscale zero-valent iron (nZVI), on the anaerobic digestion (AD) performance. Moreover, the evolutions of antibiotic resistance genes (ARGs), class 1 integrons-integrase (intI1) and potential hosts of ARGs were also investigated. The optimal addition of Fe₃O₄ NPs and nZVI to promote methane production was 0.5 g/L and 1 g/L, which led to 22.07% and 23.02% increase in methane yield, respectively. The degradation rate of organic matter was also enhanced with the addition of Fe₃O₄ NPs or nZVI. The results of high-throughput sequencing showed that the reactors with iron NPs exhibited significant differences in microbial community structure, compared to the reactors with the non‑iron NPs. Iron NPs have caused the relative abundance of the dominant bacteria (Proteobacteria, Firmicutes and Actinobacteria) generally decreased, while the dominant archaea (Euryarchaeota) increased in AD sludge. Quantitative PCR results revealed that iron NPs accelerated the reductions in total absolute abundance of ARGs, especially a beta-lactamase resistance encoded gene (blaOXA). Network analysis displayed that the attenuation of ARGs was mainly attributed to the decline of potential hosts (Proteobacteria, Firmicutes and Actinobacteria). Meanwhile, environmental factors (such as pH, soluble chemical oxygen demand and heavy metals) were also strongly correlated with ARGs.

Effects of different conductive nanomaterials on anaerobic digestion process and microbial community of sludge

The effects of four conductive nanomaterials (nano-carbon powder, nano-Al₂O₃, nano-ZnO, nano-CuO) on sludge anaerobic digestion (AD) performance and microbial community were investigated through a 36-day fermentation experiment. Results showed that biogas production enhanced by 16.9% and 23.4% with nano-carbon powder and nano-Al₂O₃ added but decreased by 90.2% and 17.3% with nano-ZnO and nano-CuO. Total solids (TS) removal efficiency was increased by 38.73% and 27.11% with nano-carbon powder and nano-Al₂O₃ added but decreased by 70.67% and 43.70% with nano-ZnO and nano-CuO. Kinetic analysis indicated four conductive nanomaterials could shorten the lag phase of AD sludge with an average rate of 51.75%. 16S rRNA amplicon sequencing results demonstrated microbes such as Syntrophomonas and Methanosaeta were enriched in nano-carbon powder and nano-Al₂O₃ reactors. However, microbial community diversity and richness were both inhibited by adding nano-ZnO and nano-CuO. Redundancy analysis (RDA) revealed that genera belong to Firmicutes and Chloroflexi could conduce to methanogenesis process.

Impact of Nanoscale Magnetite and Zero Valent Iron on the Batch-Wise Anaerobic Co-Digestion of Food Waste and Waste-Activated Sludge

As a potential approach for enhanced energy generation from anaerobic digestion, iron-based conductive nanoparticles have been proposed to enhance the methane production yield and rate. In this study, the impact of two different types of iron nanoparticles, namely the nano-zero-valent-iron particles (NZVIs) and magnetite (Fe3O4) nanoparticles (NPs) was investigated, using batch test under mesophilic conditions (35 °C). Magnetite NPs have been applied in doses of 25, 50 and 80 mg/L, corresponding to 13.1, 26.2 and 41.9 mg magnetite NPs/gTS of substrate, respectively. The results reveal that supplementing anaerobic batches with magnetite NPs at a dose of 25 mg/L induces an insignificant effect on hydrolysis and methane production. However, incubation with 50 and 80 mg/L magnetite NPs have instigated comparable positive impact with hydrolysis percentages reaching approximately 95% compared to 63% attained in control batches, in addition to a 50% enhancement in methane production yield. A biodegradability percentage of 94% was achieved with magnetite NP doses of 50 and 80 mg/L, compared to only 62.7% obtained with control incubation. NZVIs were applied in doses of 20, 40 and 60 mg/L, corresponding to 10.8, 21.5 and 32.2 mg NZVIs/gTS of substrate, respectively. The results have shown that supplementing anaerobic batches with NZVIs revealed insignificant impact, most probably due to the agglomeration of NZVI particles and consequently the reduction in available surface area, making the applied doses insufficient for measurable effect.

Kinetics of methane production during anaerobic fermentation of chicken manure with sawdust and fungi pre-treated wheat straw

The goal of this study was to determine the kinetic parameters of methane production during anaerobic fermentation of mixtures of chicken manure with sawdust and wheat straw overgrown with fungi. Pre-treatment of wheat straw was carried out with Pleurotus ostreatus and Trametes versicolor white-rot fungi. Mixtures of chicken manure with sawdust and wheat straw overgrown with fungi at different mass ratios (50:50, 60:40 and 80:20) were used as a substrate for anaerobic fermentation. For the control, ordinary wheat straw was used. Anaerobic fermentations were performed at (35, 40 and 45) °C. An individual process of anaerobic fermentation was maintained at constant temperature for 21 days. During the process, the volume and concentration of biogas produced were monitored. The most biogas produced was recorded for the straw overgrown with Pleurotus ostreatus fungi (ratio 50:50) at 45 °C, and the least in the case of straw overgrown with Trametes versicolor fungi (ratio 80:20) at 35 °C. At the beginning of anaerobic fermentation, the methane concentration increased faster at a higher temperature, while after 21 days, it was between 53 and 56% regardless of temperature.

Influence of nanoscale zero-valent iron and magnetite nanoparticles on anaerobic digestion performance and macrolide, aminoglycoside, β-lactam resistance genes reduction

The effect of nanoscale zero-valent iron (NZVI) and magnetite nanoparticles (Fe₃O₄ NPs) on anaerobic digestion (AD) performance was investigated through a series of 100-day semi-continuous mesophilic anaerobic digestions. The results indicated that biogas production had increased by 24.44% and 21.66% with the addition of 0.5 g/L Fe₃O₄ NPs and 1.0 g/L NZVI, respectively. Besides, the abundance of five widespread antibiotic resistance genes (ARGs) (ermF, ermA, ermT, aac(6')-IB, blaOXA-1) was also studied. The decrease in abundance of aac(6')-IB and blaOXA-1 was observed during the AD process with an average removal rate of 95.69% and 44.82%, respectively. Most of the ARGs, especially ermA and ermT, were less abundant in NZVI group compared with control group. The overall results suggested that the addition of NZVI and Fe₃O₄ NPs contributed to a better sludge anaerobic digestion performance, and NZVI was beneficial to the removal of some ARGs.

Achievements of biochar application for enhanced anaerobic digestion: A review

Anaerobic digestion (AD) and pyrolysis are two promising technologies used worldwide for waste biomass treatment. Interests on intensification techniques of AD has been increasing to obtain sufficient and sustainable methane production with stable digester performance. For instance, considerable attention has been devoted to the coupling of AD with biochar, which is produced by biomass thermochemical conversion. This manuscript presents a comprehensive review about recent achievements in enhancing AD efficiency with the utilization of biochar. The key roles of biochar include enhancing and equilibrating hydrolysis, acidogenesis-acetogenesis, and methanogenesis, as well as alleviating inhibitor stress were summarized. Biochar can promote biomethane process mainly by serving as a provision for bioelectrical connections between fermentative bacteria and methanogens, a support for microbial colonies, and a reinforcer for buffer capacity. Through an overview of the early applications, this paper aims to pinpoint the potential mechanism and future explorative directions of biochar enhancing AD performance.