Microbial characteristics in anaerobic digestion process of food waste for methane production-A review

Compounding one problem with another? A look at biodegradable microplastics

Environmental concerns about microplastics (MPs) have motivated research of their sources, occurrence, and fate in aquatic and soil ecosystems. To mitigate the environmental impact of MPs, biodegradable plastics are designed to naturally decompose, thus reducing the amount of environmental plastic contamination. However, the environmental fate of biodegradable plastics and the products of their incomplete biodegradation, especially micro-biodegradable plastics (MBPs), remains largely unexplored. This comprehensive review aims to assess the risks of unintended consequences associated with the introduction of biodegradable plastics into the environment, namely, whether the incomplete mineralization of biodegradable plastics could enhance the risk of MBPs formation and thus, exacerbate the problem of their environmental dispersion, representing a potentially additional environmental hazard due to their presumed ecotoxicity. Initial evidence points towards the potential for incomplete mineralization of biodegradable plastics under both controlled and uncontrolled conditions. Rapid degradation of PLA in thermophilic industrial composting contrasts with the degradation below 50 % of other biodegradables, suggesting MBPs released into the environment through compost. Moreover, degradation rates of <60 % in anaerobic digestion for polymers other than PLA and PHAs suggest a heightened risk of MBPs in digestate, risking their spread into soil and water. This could increase MBPs and adsorbed pollutants' mobilization. The exact behavior and impacts of additive leachates from faster-degrading plastics remain largely unknown. Thus, assessing the environmental fate and impacts of MBPs-laden by-products like compost or digestate is crucial. Moreover, the ecotoxicological consequences of shifting from conventional plastics to biodegradable ones are highly uncertain, as there is insufficient evidence to claim that MBPs have a milder effect on ecosystem health. Indeed, literature shows that the impact may be worse depending on the exposed species, polymer type, and the ecosystem complexity.

Transitioning weathered oil fields towards new energy: A review on utilizing hydrogenotrophic methanogens for petroleum hydrocarbons remediation

The weathering process can cause the volatilization of light components in crude oil, leading to the accumulation of total petroleum hydrocarbons (TPH) in weathered oil field soils. These TPH compounds are relatively resistant to biodegradation, posing a significant environmental hazard by contributing to soil degradation. TPH represents a complex mixture of petroleum-based hydrocarbons classified as persistent organic pollutants in soil and groundwater. The release of TPH pollutants into the environment poses serious threats to ecosystems and human health. Currently, various methods are available for TPH-contaminated soil remediation, with bioremediation technology recognized as an environmentally friendly and cost-effective approach. While converting TPH to CO2 is a common remediation method, the complex structures and diverse types of petroleum hydrocarbons (PHs) involved can result in excessive CO2 generation, potentially exacerbating the greenhouse effect. Alternatively, transforming TPH into energy forms like methane through bioremediation, followed by collection and reuse, can reduce greenhouse gas emissions and energy consumption. This process relies on the synergistic interaction between Methanogens archaea and syntrophic bacteria, forming a consortium known as the oil-degrading bacterial consortium. Methanogens produce methane through anaerobic digestion (AD), with hydrogenotrophic methanogens (HTMs) utilizing H2 as an electron donor, playing a crucial role in biomethane production. Candidatus Methanoliparia (Ca. Methanoliparia) was found in the petroleum archaeal community of weathered Oil field in northeast China. Ca. Methanoliparia has demonstrated its independent ability to decompose and produce new energy (biomethane) without symbiosis, contribute to transitioning weathered oil fields towards new energy. Therefore, this review focuses on the principles, mechanisms, and developmental pathways of HTMs during new energy production in the degradation of PHs. It also discusses strategies to enhance TPH degradation and recovery methods.

A comprehensive review on the production and enhancement techniques of gaseous biofuels and their applications in IC engines with special reference to the associated performance and emission characteristics

The increasing global demand for energy, coupled with environmental concerns associated with fossil fuels, has led to the exploration of alternative fuel sources. Gaseous biofuels, derived from organic matter, have gained attention due to their renewable nature and clean combustion characteristics. The paper extensively explores production pathways for gaseous biofuels, including biogas, syngas, and hydrogen, providing insightful discussions on various sources and processes. The energy content, physical, and chemical properties of gaseous biofuels have been analysed, highlighting their potential as viable alternatives to conventional fuels. Distinctive properties of biogas, producer gas, and hydrogen that impact combustion characteristics and engine efficiency in IC engines are underscored. Furthermore, the review systematically reviews enhancement techniques for gaseous biofuels, encompassing strategies to augment quality, purity, and combustion efficiency. Various methods, ranging from substrate pretreatment for biogas to membrane separation for hydrogen, illustrate effective means of enhancing fuel performance. Rigorous examination of performance parameters such as brake thermal efficiency, specific fuel consumption and emissions characteristics such as NOx, CO, CO2, HC of gaseous biofuels in dual-fuel mode emphasizes efficiency and environmental impact, offering valuable insights into their feasibility as engine fuels. The findings of this review will serve as a valuable resource for researchers, engineers, and policymakers involved in alternative fuels and sustainable transportation, while also highlighting the need for further research and development to fully unlock the potential of gaseous biofuels in IC engines.

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.

Comparison of anaerobic co-digestion of vacuum toilet blackwater and kitchen waste under mesophilic and thermophilic conditions: Reactor performance, microbial response and metabolic pathway

Co-digestion of kitchen waste (KW) and black water (BW) can be considered as an attractive method to efficiently achieve the clean energy from waste. To find the optimal operation parameters for the co-digestion, the effects of different temperatures (35 and 55 °C) and BW:KW ratios on the reactor performances, microbial communities and metabolic pathways were studied. The results showed that the optimum BW:KW ratio was 1:3.6 and 1:4.5 for mesophilic and thermophilic optimal reactors, with methane production of 449.04 mL/g VS and 411.90 mL/g VS, respectively. Microbial communities showed significant differences between the reactors under different temperatures. For bacteria, increasing BW:KW ratio significantly promoted Defluviitoga enrichment (1.1%-9.5%) under thermophilic condition. For Archaea, the increase in BW:KW ratio promoted the enrichment of Methanosaeta (8.6%-56.4%) in the mesophilic reactor and Methanothermobacter (62.0%-89.2%) in the thermophilic reactor. The analysis of the key enzymes showed that, acetoclastic methanogenic pathway performed as the dominant under mesophilic condition, with high abundance of Acetate-CoA ligase (EC:6.2.1.1) and Pyruvate synthase (EC:1.2.7.1). Hydrogenotrophic methanogenic pathway was the main pathway in the thermophilic reactors, with high abundance of Formylmethanofuran dehydrogenase (EC:1.2.99.5).

Enhanced Sb(V) removal of sulfate-rich wastewater by anaerobic granular sludge assisted with Fe/C amendment

Antimony (Sb) and sulfate are two common pollutants in Sb mine drainage and Sb-containing textile wastewater. In this paper, it was found that iron‑carbon (Fe/C) enhanced Sb(V) removal from sulfate-rich wastewater by anaerobic granular sludge (AnGS). Sulfate inhibited Sb(V) removal (S + Sb, k = 0.101), while Fe/C alleviated the inhibition and increased Sb(V) removal rate by 2.3 times (Fe/C + S + Sb, k = 0.236). Fe/C could promote the removal of Sb(III), and Sb(III) content decreased significantly after 8 h. Meanwhile, Fe/C enhanced the removal of sulfate. The 3D-EEM spectrum of supernatant in Fe/C + S + Sb group (at 24 h) showed that Fe/C stimulated the production of soluble microbial products (SMP) in wastewater. SMP alleviated the inhibition of sulfate, promoting AnGS to reduce Sb(V). Sb(V) could be reduced to Sb(III) both by AnGS and sulfides produced from sulfate reduction. Further analysis of extracellular polymeric substances (EPS) and AnGS showed that Fe/C increased the adsorbed Sb(V) in EPS and the c-type cytochrome content in AnGS, which may be beneficial for Sb(V) removal. Sb(V) reduction in Fe/C + S + Sb group may be related to the genus Acinetobacter, while in Sb group, several bacteria may be involved in Sb(V) reduction, such as Acinetobacter, Pseudomonas and Corynebacterium. This study provided insights into Fe/C-enhanced Sb(V) removal from sulfate-rich wastewater.

Biohythane production from two-stage anaerobic digestion of food waste: A review

The biotransformation of food waste (FW) to bioenergy has attracted considerable research attention as a means to address the energy crisis and waste disposal problems. To this end, a promising technique is two-stage anaerobic digestion (TSAD), in which the FW is transformed to biohythane, a gaseous mixture of biomethane and biohydrogen. This review summarises the main characteristics of FW and describes the basic principle of TSAD. Moreover, the factors influencing the TSAD performance are identified, and an overview of the research status; economic aspects; and strategies such as pre-treatment, co-digestion, and regulation of microbial consortia to increase the biohythane yield from TSAD is provided. Additionally, the challenges and future considerations associated with the treatment of FW by TSAD are highlighted. This paper can provide valuable reference for the improvement and widespread implementation of TSAD-based FW treatment.

Thiosulfate/FeCl3 pre-treatment enhances short-chain fatty acid production and mitigates H2S generation during anaerobic fermentation of waste activated sludge: Performance, microbial community and ecological analyses

Anaerobic fermentation (AF) has been identified as a promising method of transforming waste activated sludge (WAS) into high-value products (e.g., short-chain fatty acids (SCFAs)). This study developed thiosulfate/FeCl3 pre-treatment and investigated the effects of different thiosulfate/FeCl3 ratios (S:Fe = 3:1, 3:2, 1:1, 3:4 and 3:5) on SCFA production and sulfur transformation during the AF of WAS. At a S:Fe ratio of 1:1, the maximal SCFA yield (933.3 mg COD/L) and efficient H2S removal (96.5 %) were obtained. S:Fe ratios ≤ 1:1 not only benefited hydrolysis and acidification but largely mitigated H2S generation. These results were supported by the enriched acidogens and reduced sulfur-reducing bacteria (SRB). Molecular ecological network analysis further revealed that the keystone taxon (g_Saccharimonadales) was found in S:Fe = 1:1, together with reductions in associations among methanogens, acidogens and SRB. This work provides a strategy for enhancing high-value product recovery from WAS and minimising H2S emissions.

Comparison of anaerobic digestion of starch- and petro-based bioplastic under hydrogen-rich conditions

To identify an economically viable waste management system for bioplastics, thermoplastic starch (TPS) and poly(butylene adipate-co-terephthalate) (PBAT) were anaerobically digested under hydrogen (H2)/carbon dioxide (CO2) and nitrogen (N2) gas-purged conditions to compare methane (CH4) production and biodegradation. Regardless of the type of bioplastics, CH4 production was consistently higher with H2/CO2 than with N2. The highest amount of CH4 was produced at 307.74 mL CH4/g volatile solids when TPS digested with H2/CO2. A stepwise increased in CH4 yield was observed, with a nominal initial increment followed by accelerated methanogenesis conversion as H2 was depleted. This may be attributed to a substantial shift in the microbial structure from hydrogenotrophic methanogen (Methanobacteriales and Methanomicrobiales) to heterotrophs (Spirochaetia). In contrast, no significant change was observed with PBAT, regardless of the type of purged gas. TPS was broken down into numerous derivatives, including volatile fatty acids. TPS produced more byproducts with H2/CO2 (i.e., 430) than with N2 (i.e., 320). In contrast, differential scanning calorimetry analysis on PBAT revealed an increase in crystallinity from 10.20 % to 12.31 % and 11.36 % in the H2/CO2- and N2-purged conditions, respectively, after 65 days of testing. PBAT surface modifications were characterized via Fourier transform infrared spectroscopy and scanning electron microscopy. The results suggest that the addition of H2/CO2 can enhance the CH4 yield and increase the breakdown rate of TPS more than that of PBAT. This study provides novel insights into the CH4 production potential of two bioplastics with different biodegradabilities in H2/CO2-mediated anaerobic digestion systems.

Microbial community dynamics and functional potentials in the conversion of oil palm wastes into biomethane

Oil palm processing generates substantial waste materials rich in organic content, posing various environmental challenges. Anaerobic digestion (AD), particularly for palm oil mill effluent (POME), offers a sustainable solution, by converting waste into valuable biomethane for thermal energy or electricity generation. The synergistic activities of the AD microbiota directly affect the biomethane production, and the microbial community involved in biomethane production in POME anaerobic digestion has been reported. The composition of bacterial and archaeal communities varies under different substrate and physicochemical conditions. This review discusses the characteristics of POME, explores the microbial members engaged in each stage of AD, and elucidates the impacts of substrate and physicochemical conditions on the microbial community dynamics, with a specific focus on POME. Finally, the review outlines current research needs and provides future perspectives on optimizing the microbial communities for enhanced biomethane production from oil palm wastes.

Food waste to resource recovery: a way of green advocacy

Due to the massive growth in population and urbanization, there has been a huge increase in the volume of food waste globally. The Food and Agriculture Organization (FAO) has estimated that around one-third of all food produced each year is wasted. Food waste leads to the emission of greenhouse gas and depletion of the soil fertility. Nevertheless, it has immense potential for the recovery of high-value energy, fuel, and other resources. This review summarizes the latest advances in resource recovery from food waste by using technologies that include food waste-mediated microbial fuel cell (MFC) for bioenergy production. In addition to this, utilization of food waste for the production of bioplastic, biogas, bioethanol, and fertilizer has been also discussed in detail. Competitive benefits and accompanying difficulties of these technologies have also been highlighted. Furthermore, future approaches for more efficient use of food waste for the recovery of valuable resources have been also offered from an interdisciplinary perspective.

Comparative study of high-performance mesophilic and thermophilic anaerobic membrane bioreactors in the co-digestion of sewage sludge and food waste: Methanogenic performance and energy recovery potential

To support smart cities in terms of waste management and bioenergy recovery, the co-digestion of sewage sludge (SeS) and food waste (FW) was conducted by the anaerobic membrane bioreactor (AnMBR) under mesophilic and thermophilic conditions in this study. The biogas production rate of the thermophilic AnMBR (ThAnMBR) at the SeS to FW ratio of 0:100, 75:25, 50:50 and 100:0 was 2.84 ± 0.21, 2.51 ± 0.26, 1.54 ± 0.26 and 1.31 ± 0.08 L-biogas/L/d, inconspicuous compared with that of the mesophilic AnMBR (MeAnMBR) at 3.00 ± 0.25, 2.46 ± 0.30, 1.63 ± 0.23 and 1.30 ± 0.17 L-biogas/L/d, respectively. The higher hydrolysis ratio and the poorer rejection efficiencies of the membrane under thermophilic conditions, resulting that the permeate COD, carbohydrate and protein of the ThAnMBR was higher than that of the MeAnMBR. The lost COD that might be converted into biogas was discharged with the permeate in the ThAnMBR, which was partly responsible for the inconspicuous methanogenic performance. Furthermore, the results of energy recovery potential assessment showed that the energy return on investment (EROI) of the MeAnMBR was 4.54, 3.81, 2.69 and 2.22 at the four SeS ratios, which was higher than that of the ThAnMBR at 3.29, 2.97, 2.02 and 1.80, respectively, indicating the advantage of the MeAnMBR over the ThAnMBR in energy recovery potential. The outcomes of this study will help to choose a more favorable temperature to co-digest SeS and FW to support the construction of smart cities.

Bacterial community composition in a two-stage anaerobic membrane bioreactor for co-digestion of food waste and food court wastewater

This study investigated the microbial community of a two-stage anaerobic membrane bioreactor (2S-AnMBR) co-digesting food waste and food court wastewater. The hydrolysis reactor (HR) was dominated by Bacteroidetes and Firmicutes phylum, with genus Lactobacillus enriched due to food waste fermentation. The up-flow anaerobic sludge blanket (UASB) was dominated by genus such as Methanobacterium and Methanosaeta. The presence of Methanobacterium (91 %) and Methanosaeta (7.5 %) suggested that methane production pathways inevitably undergo both hydrogenotrophic and acetoclastic methanogenesis. Hydrogen generated during hydrolysis fermentation in the HR contributed to methane production in the UASB via hydrogenotrophic pathways. However, the low abundance of Methanosaeta in the UASB can be attributed to the limited inffluent of volatile fatty acids (VFA) and the competitive presence of acetate-consuming bacteria Acinetobacter. The UASB exhibited more excellent dispersion and diversity of metabolic pathways compared to the HR, indicating efficient methane production.

Co-enhancing effects of zero valent iron and magnetite on anaerobic methanogenesis of food waste at transition temperature (45 °C) and various organic loading rates

Deoiling of food waste (FW) after hydrothermal pretreatment occurs at high temperatures, and more energy is required for substrate cooling before the anaerobic digestion (AD) process. AD at the transition temperature (for example 45 °C) is good for energy saving and carbon emission reducing when treating deoiling FW. However, the metabolic activity of methanogens must increase at the transition temperatures. This study proposes the use of zero-valent iron (Fe0) and magnetite (Fe3O4) to boost CH4 yield from deoiling FW. The results showed a co-enhancing effect on CH4 yield upgradation when using Fe0 and Fe3O4 simultaneously, and the highest CH4 yield reached 536.23 mLCH4/gVS, which was 67.5 % higher than that of Fe0 alone (320.14 mLCH4/gVS). In addition, a high organic loading was favorable for increasing the CH4 yield from deoiling FW. Microbial diversity analysis suggested that the dominant methanogenic pathway at 45 °C was hydrogenotrophic methanogenesis. Herein, a potential metabolic pathway analysis revealed that the co-enhancing effects of Fe0 and Fe3O4 enhanced syntrophic methanogenesis and possibly boosted electron transfer efficiency.

Biotechnology to reduce logistics burden and promote environmental stewardship for Air Force civil engineering requirements

This review provides discussion of advances in biotechnology with specific application to civil engineering requirements for airfield and airbase operations. The broad objectives are soil stabilization, waste management, and environmental protection. The biotechnology focal areas address (1) treatment of soil and sand by biomineralization and biopolymer addition, (2) reduction of solid organic waste by anaerobic digestion, (3) application of microbes and higher plants for biological processing of contaminated wastewater, and (4) use of indigenous materials for airbase construction and repair. The consideration of these methods in military operating scenarios, including austere environments, involves comparison with conventional techniques. All four focal areas potentially reduce logistics burden, increase environmental sustainability, and may provide energy source, or energy-neutral practices that benefit military operations.

Effects of ozone on activated sludge: performance of anaerobic digestion and structure of the microbial community

The treatment and disposal of activated sludge are currently challenging tasks in the world. As a common biological engineering technology, biological fermentation exists with disadvantages such as low efficiency and complex process. Ozone pretreatments are commonly applied to improve this problem due to their high efficiency and low cost. In this study, the significant function of ozone in anaerobic fermentation gas production was verified with excess sludge. Compared with other untreated sludge, ozone pretreatment can effectively degrade activated sludge. After ozone treatment and mixing with primary sludge, the methane production of excess sludge increased by 49.30 and 50.78%, and the methanogenic activity increased by 69.99 and 73.83%, respectively. The results indicated that the mixing of primary sludge with excess sludge possessed synergistic effects, which contributed to the anaerobic fermentation of excess sludge. The results of microbial community structure exhibited that methanogenic processes mainly involve hydrogenogens, acidogens and methanogens. The relative abundance of both bacteria and microorganisms changed significantly in the early stage of hydraulic retention time, which coincided exactly with the gas production stage. This study provided a feasible pretreatment strategy to improve sludge biodegradability and revealed the role of microorganisms during anaerobic digestion.

Fermentative Biohydrogen and Biomethane Production from High-Strength Industrial Food Waste Hydrolysate Using Suspended Cell Techniques

The food waste was very difficult to treat in a proper way since its high-organic matter. The novel biohythane (H2 + CH4) production from high-strength industry food waste hydrolysate in two steps anaerobic well mixed batch bioreactor was carried out in this study using cultivated microflora. The temperature was controlled at 37 °C and initial substrate concentration of industrial food waste hydrolysate varied from 60, 80, 100, and 120 g COD/L, respectively. The pH, TS, VS, and SCOD were analyzed from the influent and effluent samples. These analytical parameters showed the correlations between the biogas production rates and yields in the batch fermentation system. This study was the first time to use the industry food waste hydrolysate which was collected from the subcritical water hydrolysis process. In this study, the optimal biohydrogen and biomethane yield production by using suspended cells were 0.65 mL H2/g COD and 203.72 mL CH4/g COD where the initial substrate concentrations of total COD and SCOD were 60 g/L and 39.80 g/L, respectively. The optimal of the biohydrogen and biomethane yields production by using suspended cells were 0.65 mL H2/g COD and 203.72 mL CH4/g COD where the initial substrate concentrations of total COD and SCOD were 60 g/L and 39.80 g/L, respectively. The results of this study supported that the cultivation of inoculum in a suspended cell type can have a higher tolerance for the biohydrogen and biomethane production in a high-strength initial substrate concentration of 60 g COD/L.

Cultivation of Chlorella sp. HS2 using wastewater from soy sauce factory

Incorporation of wastewater from industrial sectors into the design of microalgal biorefineries has significant potential for advancing the practical application of this emerging industry. This study tested various food industrial wastewaters to assess their suitability for microalgal cultivation. Among these wastewaters, defective soy sauce (DSS) and soy sauce wastewater (SWW) were chosen but DSS exhibited the highest nutrient content with 13,500 ppm total nitrogen and 3051 ppm total phosphorus. After diluting DSS by a factor of 50, small-scale cultivation of microalgae was conducted to optimize culture conditions. SWW exhibited optimal growth at 25-30 °C and 300-500 μE m-2 s-1, while DSS showed optimal growth at 30-35 °C. Based on a 100-mL lab-scale and 3-L outdoor cultivation with an extended cultivation period, DSS outperformed SWW, exhibiting higher final biomass productivity. Additionally, nutrient-concentrated nature of DSS is advantageous for transportation at an industrial scale, leading us to select it as the most promising feedstock for microalgal cultivation. With further optimization, DSS has the potential to serve as an effective microalgal cultivation feedstock for large-scale biomass production.

Enhancement of anaerobic digestion of high salinity food waste by magnetite and potassium ions: Digestor performance, microbial and metabolomic analyses

The study investigated the effectiveness of magnetite and potassium ions (K+) in enhancing anaerobic digestion of high salinity food waste. Results indicated that both magnetite and K+ improved anaerobic digestion in high-salt environments, and their combination yielded even better results. The combination of magnetite and K+ promoted microorganism activity, and resulted in increased abundance of DMER64, Halobacteria and Methanosaeta. Metabolomic analysis revealed that magnetite mainly influenced quorum sensing, while K+ mainly stimulated the synthesis of compatible solutes, aiding in maintaining osmotic balance. The combined additives regulated pathways such as ATP binding cassette transport, methane metabolism, and inhibitory substance metabolism, enabling cells to resist environmental stress and maintain normal metabolic activity. Overall, this study demonstrated the potential of magnetite and K+ to enhance food waste anaerobic digestion in high salt conditions and provided valuable insights into the molecular mechanism.

Linking the shifts in the metabolically active microbiota in a UASB and hybrid anaerobic-aerobic bioreactor for swine wastewater treatment

Due to the high concentration of pollutants, swine wastewater needs to be treated prior to disposal. The combination of anaerobic and aerobic technologies in one hybrid system allows to obtain higher removal efficiencies compared to those achieved via conventional biological treatment, and the performance of a hybrid system depends on the microbial community in the bioreactor. Here, we evaluated the community assembly of an anaerobic-aerobic hybrid reactor for swine wastewater treatment. Sequencing of partial 16S rRNA coding genes was performed using Illumina from DNA and retrotranscribed RNA templates (cDNA) extracted from samples from both sections of the hybrid system and from a UASB bioreactor fed with the same swine wastewater influent. Proteobacteria and Firmicutes were the dominant phyla and play a key role in anaerobic fermentation, followed by Methanosaeta and Methanobacterium. Several differences were found in the relative abundances of some genera between the DNA and cDNA samples, indicating an increase in the diversity of the metabolically active community, highlighting Chlorobaculum, Cladimonas, Turicibacter and Clostridium senso stricto. Nitrifying bacteria were more abundant in the hybrid bioreactor. Beta diversity analysis revealed that the microbial community structure significantly differed among the samples (p < 0.05) and between both anaerobic treatments. The main predicted metabolic pathways were the biosynthesis of amino acids and the formation of antibiotics. Also, the metabolism of C5-branched dibasic acid, Vit B5 and CoA, exhibited an important relationship with the main nitrogen-removing microorganisms. The anaerobic-aerobic hybrid bioreactor showed a higher ammonia removal rate compared to the conventional UASB system. However, further research and adjustments are needed to completely remove nitrogen from wastewater.

Mechanistic insights into Fe3O4-modified biochar relieving inhibition from erythromycin on anaerobic digestion

Anaerobic digestion (AD) of antibiotic manufacturing wastewater to degrade residual antibiotics and produce mixture of combustible gases has been investigated actively in the past decades. However, detrimental effect of residual antibiotic to microbial activities is commonly faced in AD process, leading to the reduction of treatment efficiency and energy recovery. Herein, the present study systematically evaluated the detoxification effect and mechanism of Fe3O4-modified biochar in AD of erythromycin manufacturing wastewater. Results showed that Fe3O4-modified biochar had stimulatory effect on AD at 0.5 g/L erythromycin existence. A maximum methane yield of 327.7 ± 8.0 mL/g COD was achieved at 3.0 g/L Fe3O4-modified biochar, leading to the increase of 55.7% compared to control group. Mechanistic investigation demonstrated that different levels of Fe3O4-modified biochar could improve methane yield via different metabolic pathways involved in specific bacteria and archaea. Low levels of Fe3O4-modified biochar (i.e., 0.5-1.0 g/L) led to the enrichment of Methanothermobacter sp., strengthening the hydrogenotrophic pathway. On the contrary, high levels of Fe3O4-modified biochar (2.0-3.0 g/L) favored the proliferation of acetogens (e.g., Lentimicrobium sp.) and methanogen (Methanosarcina sp.) and their syntrophic relations played vital role on the simulated AD performance at erythromycin stress. Additionally, the addition of Fe3O4-modified biochar significantly decreased the abundance of representative antibiotic resistant genes (ARGs), benefiting the reduction of environmental risk. The results of this study verified that the application of Fe3O4-modified biochar could be an efficient approach to detoxify erythromycin on AD system, which brings high impacts and positive implications for biological antibiotic wastewater treatment.

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.

Enhanced Anaerobic Wastewater Treatment by a Binary Electroactive Material: Pseudocapacitance/Conductance-Mediated Microbial Interspecies Electron Transfer

Anaerobic digestion (AD) is a promising method to treat organic matter. However, AD performance was limited by the inefficient electron transfer and metabolism imbalance between acid-producing bacteria and methanogens. In this study, a novel binary electroactive material (Fe3O4@biochar) with pseudocapacitance (1.4 F/g) and conductance (10.2 μS/cm) was exploited to store-release electrons as well as enhance the direct electron transfer between acid-producing bacteria and methanogens during the AD process. The mechanism of pseudocapacitance/conductance on mediating interspecies electron transfer was deeply studied at each stage of AD. In the hydrolysis acidification stage, the pseudocapacitance of Fe3O4@biochar acting as electron acceptors proceeded NADH/NAD+ transformation of bacteria to promote ATP synthesis by 21% which supported energy for organics decomposition. In the methanogenesis stage, the conductance of Fe3O4@biochar helped the microbes establish direct interspecies electron transfer (DIET) to increase the coenzyme F420 content by 66% and then improve methane production by 13%. In the complete AD experiment, electrons generated from acid-producing bacteria were rapidly transported to methanogens via conductors. Excess electrons were buffered by the pseudocapacitor and then gradually released to methanogens which alleviated the drastic drop in pH. These findings provided a strategy to enhance the electron transfer in anaerobic treatment as well as guided the design of electroactive materials.

Acclimatizing waste activated sludge in a thermophilic anaerobic fixed-bed biofilm reactor to maximize biogas production for food waste treatment at high organic loading rates

Thermophilic anaerobic digestion (TAD) provides a promising solution for sustainable high-strength waste treatment due to its enhanced methane-rich biogas recovery. However, high organic loading rates (OLR) exceeding 3.0 kgCOD/m3/day and short hydraulic retention times (HRT) below 10 days pose challenges in waste-to-energy conversion during TAD, stemming from volatile fatty acids (VFAs) accumulation and methanogenesis failure. In this study, we implemented a stepwise strategy for acclimatizing waste activated sludge (WAS) in a thermophilic anaerobic fixed-bed biofilm reactor (TA-FBBR) to optimize methanogen populations, thereby enhancing waste-to-energy efficiencies under elevated OLRs in food waste treatment. Results showed that following stepwise acclimatization, the TA-FBBR achieved stable methane production of approximately 5.8 L/L-reactor/day at an ultrahigh OLR of ∼20 kgCOD/m3/day and ∼15 kgVS/m3/day at 6-day HRT in food waste treatment. The average methane yield reached 0.45 m3/kgCODremoval, attaining the theoretical production in TAD. Moreover, VFA concentrations were stabilized below 1000 mg/L at the ultrahigh OLR under 6-day HRT, while maintaining an acetate/propionate ratio of > 1.8 and a VFA/TAK ratio of < 0.3 serving as effective indicators of system stability and methane yield potential. The microbial community analysis revealed that the WAS acclimatization strategy fostered the microbial diversity and abundance of Methanothermobacter and Methanosarcina. Methanosarcina in the biofilm were observed to be twice as abundant as Methanothermobacter, indicating a potential preference for biofilm existence among methanogens. The findings demonstrated an effective strategy, specifically the stepwise acclimatization of WAS in a thermophilic fixed-bed biofilm reactor, to enhance the food waste treatment performance at high OLRs, contributing valuable mechanistic and technical insights for future sustainable high-strength waste management.

Microbial community composition of food waste before anaerobic digestion

Anaerobic digestion is widely used to process and recover value from food waste. Commercial food waste anaerobic digestion facilities seek improvements in process efficiency to enable higher throughput. There is limited information on the composition of microbial communities in food waste prior to digestion, limiting rational exploitation of the catalytic potential of microorganisms in pretreatment processes. To address this knowledge gap, bacterial and fungal communities in food waste samples from a commercial anaerobic digestion facility were characterised over 3 months. The abundance of 16S rRNA bacterial genes was approximately five orders of magnitude higher than the abundance of the fungal intergenic spacer (ITS) sequence, suggesting the numerical dominance of bacteria over fungi in food waste before anaerobic digestion. Evidence for the mass proliferation of bacteria in food waste during storage prior to anaerobic digestion is presented. The composition of the bacterial community shows variation over time, but lineages within the Lactobacillaceae family are consistently dominant. Nitrogen content and pH are correlated to community variation. These findings form a foundation for understanding the microbial ecology of food waste and provide opportunities to further improve the throughput of anaerobic digestion.

Different stages of microbial community during the anaerobic digestion of food waste

Large-scale food waste (FW) disposal has resulted in severe environmental degradation and financial losses around the world. Although FW has a high biomass energy contents and a growing large number of national projects to recover energy from FW by anaerobic digestion (AD) are being developed. AD is a promising solution for FW management and energy generation when compared to typical disposal options including landfill disposal, incineration, and composting. AD of FW can be combined with an existing AD operation or linked to the manufacture of value-added products to reduce costs and increase income. AD is a metabolic process that requires four different types of microbes: hydrolyzers, acidogens, acetogens, and methanogens. Microbes use a variety of strategies to avoid difficult situations in the AD, such as competition for the same substrate between sulfate-reducing bacteria and methane-forming bacteria. An improved comprehension of the microbiology involved in the anaerobic digestion of FW will provide new insight into the circumstances needed to maximize this procedure, including its possibilities for use in co-digestion mechanisms. This paper reviewed the present scientific knowledge of microbial community during the AD and the connection between microbial diversity during the AD of FW.

Trace elements' deficiency in energy production through methanogenesis process: Focus on the characteristics of organic solid wastes

Excessive or insufficient supplementation of trace elements (TEs) limits the progression of anaerobic digestion. The main reason for this is the lack of sufficient understanding of digestion substrate characteristics, which significantly affects the demand for TEs. In this review, the relationship between TEs requirements and substrate characteristics is discussed. We mainly focus on three aspects. 1) The basis for TE optimization and existing problems: The optimization of TEs often based on the total solids (TS) or volatile solids (VS) of substrates, does not fully consider substrate characteristics. 2) TE deficiency mechanisms for different types of substrates: nitrogen-rich, sulfur-rich, TE-poor, and easily hydrolyzed substrates are the four main types of substrates. The mechanisms underlying TEs deficiency in the different substrates are investigated. 3) Regulation of TE bioavailability: characteristics of substrates affect digestion parameters, which disturb the bioavailability TE. Therefore, methods for regulating bioavailability of TEs are discussed.

Conductive materials enhance microbial salt-tolerance in anaerobic digestion of food waste: Microbial response and metagenomics analysis

Previous studies have shown that high salinity environments can inhibit anaerobic digestion (AD) of food waste (FW). Finding ways to alleviate salt inhibition is important for the disposal of the growing amount of FW. We selected three common conductive materials (powdered activated carbon, magnetite, and graphite) to understand their performance and individual mechanisms that relieve salinity inhibition. Digester performances and related enzyme parameters were compared. Our data revealed that under normal and low salinity stress conditions, the anaerobic digester ran steady without significant inhibitions. Further, the presence of conductive materials promoted conversion rate of methanogenesis. This promotion effect was highest from magnetite > powdered activated carbon (PAC) > graphite. At 1.5% salinity, PAC and magnetite are beneficial in maintaining high methane production efficiency while control and the graphite added digester acidified and failed rapidly. Additionally, metagenomics and binning were used to analyze the metabolic capacity of the microorganisms. Some species enriched by PAC and magnetite possessed higher cation transport capacities and were to accumulate compatible solutes. PAC and magnetite promoted direct interspecies electron transference (DIET) and syntrophic oxidation of butyrate and propionate. Also, the microorganisms had more energy available to cope with salt inhibition in the PAC and magnetite added digesters. Our data imply that the promotion of Na+/H+ antiporter, K+ uptake, and osmoprotectant synthesis or transport by conductive materials may be crucial for their proliferation in highly stressful environments. These findings will help to understand the mechanisms of alleviate salt inhibition by conductive materials and help to recover methane from high-salinity FW.

Effects of different concentrations of butyrate on microbial community construction and metabolic pathways in anaerobic digestion

Investigating the effect of butyric acid concentration on anaerobic digestion systems in complex systems is important for the efficient degradation of butyric acid and improving the efficiency of anaerobic digestion. In this study, different loadings of butyric acid with 2.8, 3.2, and 3.6 g/(L·d) were added to the anaerobic reactor. At a high organic loading rate of 3.6 g/(L·d), methane was efficiently produced with VBP (Volumetric Biogas Production) of 1.50 L/(L·d) and biogas content between 65% and 75%. VFAs concentration remained below 2000 mg/L. Metagenome sequencing revealed changes in the functional flora within different stages. Methanosarcina, Syntrophomonas, and Lentimicrobium were the main and functional microorganisms. That the relative abundance of methanogens exceeded 35% and methanogenic metabolic pathways were increased indicated the methanogenic capacity of the system significantly improved. The presence of a large number of hydrolytic acid-producing bacteria also indicated the importance of the hydrolytic acid-producing stage in the system.

Synergetic effects of anaerobic co-digestion of food waste and algae on biogas production

Anaerobic co-digestion of food waste and algae was assessed to offset the drawbacks of anaerobic mono-digestion of each substrate. Batch test results indicated that a food waste and algae mixture ratio of 8:2 facilitated the highest CH₄ yield (334 mL CH₄/g COD_Input). This ratio was applied to the anaerobic co-digestion reactor, resulting in a CH₄ yield that was twice that of the anaerobic mono-digestion reactors, thereby facilitating high operational stability. In contrast to the anaerobic mono-digestion, anaerobic co-digestion resulted in stable CH₄ production by overcoming volatile fatty acid accumulation and a decreased pH, even under a high organic loading rate (3 kg COD/m³∙d). Furthermore, a comparative metagenomic analysis revealed that the abundance of volatile fatty acid-oxidizing bacteria and hydrogenotrophic and methylotrophic methanogens was significantly increased in the anaerobic co-digestion reactor. These findings indicate that the anaerobic co-digestion of food waste and algae significantly improves CH₄ production and process stability.

Enhanced methane production from the anaerobic co-digestion of food waste plus fruit and vegetable waste

Food waste (FW) and fruit, vegetable waste (FVW) are important components of municipal solid waste, yet the performance and related mechanisms of anaerobic co-digestion of FW and FVW for methane production have been rarely investigated. In order to get a deeper understanding of the mechanisms involved, the mesophilic FW and FVW anaerobic co-digestion in different proportions was investigated. The experimental results showed that when the ratio of FW and FVW was 1/1 (in terms of volatile suspended solid), the maximum biomethane yield of 269.9 mL/g TCOD from the codigested substrate is significantly higher than that in FW or FVW anaerobic digestion alone. FW and FVW co-digestion promoted the dissolution and biotransformation of organic matter. When the recommended mixing ratio was applied, the maximum concentration of dissolved chemical oxygen demand (COD) was high as 11971 mg/L. FW and FVW co-digestion reduced the accumulation of volatile fatty acids (VFA) in the digestive system, thus reducing its negative impact on the methanogenesis process. FW and FVW co-digestion process synergistically enhanced microbial activity. The analysis of microbial population structure showed that when FW and FVW were co-digested at the recommended ratio, the relative abundance of Proteiniphilum increased to 26.5%, and the relative abundances of Methanosaeta and Candidatus Methanofastidiosum were also significantly increased. The results of this work provide a certain amount of theoretical basis and technical support for the co-digestion of FW and FVW.

Emerging Strategies for Enhancing Propionate Conversion in Anaerobic Digestion: A Review

Anaerobic digestion (AD) is a triple-benefit biotechnology for organic waste treatment, renewable production, and carbon emission reduction. In the process of anaerobic digestion, pH, temperature, organic load, ammonia nitrogen, VFAs, and other factors affect fermentation efficiency and stability. The balance between the generation and consumption of volatile fatty acids (VFAs) in the anaerobic digestion process is the key to stable AD operation. However, the accumulation of VFAs frequently occurs, especially propionate, because its oxidation has the highest Gibbs free energy when compared to other VFAs. In order to solve this problem, some strategies, including buffering addition, suspension of feeding, decreased organic loading rate, and so on, have been proposed. Emerging methods, such as bioaugmentation, supplementary trace elements, the addition of electronic receptors, conductive materials, and the degasification of dissolved hydrogen, have been recently researched, presenting promising results. But the efficacy of these methods still requires further studies and tests regarding full-scale application. The main objective of this paper is to provide a comprehensive review of the mechanisms of propionate generation, the metabolic pathways and the influencing factors during the AD process, and the recent literature regarding the experimental research related to the efficacy of various strategies for enhancing propionate biodegradation. In addition, the issues that must be addressed in the future and the focus of future research are identified, and the potential directions for future development are predicted.

Hydrochar mediated anaerobic digestion of bio-wastes: Advances, mechanisms and perspectives

Bio-wastes treatment and disposal has become a challenge because of their increasing output. Given the abundant organic matter in bio-wastes, its related resource treatment methods have received more and more attention. As a promising strategy, anaerobic digestion (AD) has been widely used in the treatment of bio-wastes, during which not only methane as energy can be recovered but also their reduction can be achieved. However, AD process is generally disturbed by some internal factors (e.g., low hydrolysis efficiency and accumulated ammonia) and external factors (e.g., input pollutants), resulting in unstable AD operation performance. Recently, hydrochar was wildly found to improve AD performance when added to AD systems. This review comprehensively summarizes the research progress on the performance of hydrochar-mediated AD, such as increased methane yield, improved operation efficiency and digestate dewatering, and reduced heavy metals in digestate. Subsequently, the underlying mechanisms of hydrochar promoting AD were systematically elucidated and discussed, including regulation of electron transfer (ET) mode, microbial community structure, bio-processes involved in AD, and reaction conditions. Moreover, the effects of properties of hydrochar (e.g., feedstock, hydrothermal carbonization (HTC) temperature, HTC time, modification and dosage) on the improvement of AD performance are systematically concluded. Finally, the relevant knowledge gaps and opportunities to be studied are presented to improve the progress and application of the hydrochar-mediated AD technology. This review aims to offer some references and directions for the hydrochar-mediated AD technology in improving bio-wastes resource recovery.

Thiosulfate pretreatment enhancing short-chain fatty acids production from anaerobic fermentation of waste activated sludge: Performance, metabolic activity and microbial community

A novel strategy based on thiosulfate pretreatment for enhancing short-chain fatty acids (SCFAs) from anaerobic fermentation (AF) of waste activated sludge (WAS) was proposed in this study. The results showed that the maximal SCFA yield increased from 206.1 ± 4.7 to 1097.9 ± 17.2 mg COD/L with thiosulfate dosage increasing from 0 to 1000 mg S/L, and sulfur species contribution results revealed that thiosulfate was the leading contributor to improve SCFA yield. Mechanism exploration disclosed that thiosulfate addition largely improved WAS disintegration, due to thiosulfate serving as a cation binder for removing organic-binding cations, especially Ca²⁺ and Mg²⁺, dispersing the extracellular polymeric substance (EPS) structure and further entering into the intracellularly by stimulated carrier protein SoxYZ and subsequently caused cell lysis. Typical enzyme activities and related functional gene abundances indicated that both hydrolysis and acidogenesis were remarkably enhanced while methanogenesis was substantially suppressed, which were further strengthened by the enriched hydrolytic bacteria (e.g. C10-SB1A) and acidogenic bacteria (e.g. Aminicenantales) but severely reduced methanogens (e.g. Methanolates and Methanospirillum). Economic analysis confirmed that thiosulfate pretreatment was a cost-effective and efficient strategy. The findings obtained in this work provide a new thought for recovering resource through thiosulfate-assisted WAS AF for sustainable development.

Biogas production of food waste with in-situ sulfide control under high organic loading in two-stage anaerobic digestion process: Strategy and response of microbial community

A two-stage anaerobic digestion process utilizing food waste was investigated in this study, without any additive and co-digestion. Solid content, temperature and pH value were key controlling factors for hydrolysis, which results the optimized food waste hydrolysate with COD/VS_{food waste} of 2.67. Efficient biogas production was maintained in long-term operation (>150 d) without any additive, and methane production yields up to 699.7 mL·gVS^-1·d^-1 was achieved under organic loading rate (OLR) of 31.0 gVS·d^-1. Methane production can be recovered (70.4 %) after temperature shock within 30 days. This study confirmed the possibility to establish two-stage food waste anaerobic digestion system under high organic load. pH, OLR, and temperature are key factors to maintain stable biogas production, while pH control was performed as a in situ sulfide control technology (75.8 % sulfide reduction). This study provides practical strategies for food waste utilization and decreasing carbon footprint.

Microalgal Feedstock for Biofuel Production: Recent Advances, Challenges, and Future Perspective

Globally, nations are trying to address environmental issues such as global warming and climate change, along with the burden of declining fossil fuel reserves. Furthermore, countries aim to reach zero carbon emissions within the existing and rising global energy crisis. Therefore, bio-based alternative sustainable feedstocks are being explored for producing bioenergy. One such renewable energy resource is microalgae; these are photosynthetic microorganisms that grow on non-arable land, in extreme climatic conditions, and have the ability to thrive even in sea and wastewater. Microalgae have high photosynthetic efficiencies and biomass productivity compared to other terrestrial plants. Whole microalgae biomass or their extracted metabolites can be converted to various biofuels such as bioethanol, biodiesel, biocrude oil, pyrolytic bio-oil, biomethane, biohydrogen, and bio jet fuel. However, several challenges still exist before faster and broader commercial application of microalgae as a sustainable bioenergy feedstock for biofuel production. Selection of appropriate microalgal strains, development of biomass pre-concentrating techniques, and utilization of wet microalgal biomass for biofuel production, coupled with an integrated biorefinery approach for producing value-added products, could improve the environmental sustainability and economic viability of microalgal biofuel. This article will review the current status of research on microalgal biofuels and their future perspective.

Applying Dynamic Magnetic Field To Promote Anaerobic Digestion via Enhancing the Electron Transfer of a Microbial Respiration Chain

Electrochemical methods have been reported to strengthen anaerobic digestion, but the continuous electrical power supply and the complicated electrode installed inside the digester have restricted it from practical use. In this study, a dynamic magnetic field (DMF) was placed outside a digester to induce an electromotive force to electrically promote anaerobic digestion. With the applied DMF, an electromotive force of 0.14 mV was generated in the anaerobic sludge, and a 65.02% methane increment was obtained from the anaerobic digestion of waste-activated sludge. Experiments on each stage of anaerobic digestion showed that acidification and methanogenesis that involve electron transfer of respiration chains were promoted with the DMF, while solubilization and hydrolysis less related to respiration chains were not enhanced. Further analysis indicated that the induced electromotive force polarized the protein-like substances in the sludge to increase the conductivity and capacitance of the sludge. Electrotrophic methanogens (Methanothrix) and exoelectrogens (Exiguobacterium) were enriched with DMF. The kinetic isotope effect test confirmed that electron transfer was accelerated with DMF. Consistently, the concentration ratio of co-enzymes (NADH/NAD⁺ and F₄₂₀H₂/F₄₂₀₎ that reflects the electron exchange with respiration chains significantly increased. Applying the DMF seemed a more accessible strategy to electrically strengthen anaerobic digestion.

Advancing the bioconversion process of food waste into methane: A systematic review

With the continuous rise of food waste (FW) throughout the world, a research effort to reveal its potential for bioenergy production is surging. There is a lack of harmonized information and publications available that evaluate the state-of-advance for FW-derived methane production process, particularly from an engineering and sustainability point of view. Anaerobic digestion (AD) has shown remarkable efficiency in the bioconversion of FW to methane. This paper reviews the current research progress, gaps, and prospects in pre-AD, AD, and post-AD processes of FW-derived methane production. Briefly, the review highlights innovative FW collection and optimization routes such as AI that enable efficient FW valorization processes. As weather changes and the FW sources may affect the AD efficiency, it is important to assess the spatio-seasonal variations and microphysical properties of the FW to be valorized. In that case, developing weather-resistant bioreactors and cost-effective mechanisms to modify the raw substrate morphology is necessary. An AI-guided reactor could have high performance when the internal environment of the centralized operation is monitored in real-time and not susceptible to changes in FW variety. Monitoring solvent degradation and fugitive gases during biogas purification is a challenging task, especially for large-scale plants. Furthermore, this review links scientific evidence in the field with full-scale case studies from different countries. It also highlights the potential contribution of ADFW to carbon neutrality efforts. Regarding future research needs, in addition to the smart collection scheme, attention should be paid to the management and utilization of FW impurities, to ensure sustainable AD operations.

Evaluation of methane generation rate and energy recovery potential of municipal solid waste using anaerobic digestion and landfilling: A case study of Dhanbad, India

In this study, two most common biological waste to energy conversion techniques have been analysed and compared on the basis of methane generation and energy recovery potential. The biogas generation potential has been estimated using anaerobic co-digestion experiment. The main substrate used for this study was food waste, while cow dung and anaerobic digester sludge were used as co-digestion substrates. Food waste was used because of its maximum composition (36% of total wastes) in the study area (Dhanbad city, India) with high biogas generation potential. Cow dung acted as a natural buffer, while anaerobic digester sludge was a source of active inoculum. Based on the maximum biogas yield of 402 mL g^-1 VS and annual food waste generation rate in the study area, the energy recovery potential using anaerobic digestion was estimated to be 6.59 × 10⁶ kWh year^-1. Presently, the wastes in the mixed form are being dumped on a large abandoned land with an approximate area of 93 ha in the outskirts of Dhanbad city in an uncontrolled manner. The annual landfill gas generation from the existing landfill has been estimated using Landfill Gas Emissions Model (LandGEM) based on the waste characteristics, anticipated landfill life and other region-specific parameters of the present study area. The maximum electrical energy recovery potential of 44.62 × 10⁵ kWh from landfill gas has been estimated for the year 2041. Although, the results are based on the waste generation and characteristics of Dhanbad city, the comparison methodology can be applied to other cities.

A Review of Basic Bioinformatic Techniques for Microbial Community Analysis in an Anaerobic Digester

Biogas production involves various types of intricate microbial populations in an anaerobic digester (AD). To understand the anaerobic digestion system better, a broad-based study must be conducted on the microbial population. Deep understanding of the complete metagenomics including microbial structure, functional gene form, similarity/differences, and relationships between metabolic pathways and product formation, could aid in optimization and enhancement of AD processes. With advancements in technologies for metagenomic sequencing, for example, next generation sequencing and high-throughput sequencing, have revolutionized the study of microbial dynamics in anaerobic digestion. This review includes a brief introduction to the basic process of metagenomics research and includes a detailed summary of the various bioinformatics approaches, viz., total investigation of data obtained from microbial communities using bioinformatics methods to expose metagenomics characterization. This includes (1) methods of DNA isolation and sequencing, (2) investigation of anaerobic microbial communities using bioinformatics techniques, (3) application of the analysis of anaerobic microbial community and biogas production, and (4) restriction and prediction of bioinformatics analysis on microbial metagenomics. The review has been concluded, giving a summarized insight into bioinformatic tools and also promoting the future prospects of integrating humungous data with artificial intelligence and neural network software.

Multi-faceted analysis of thermophilic anaerobic biodegradation of poly(lactic acid)-based material

Currently, the production of bio-based polymeric materials, of which poly(lactic acid) (PLA) is the most popular, has been increasing, causing the growth of PLA waste in municipal waste. Thus, it is necessary to develop sustainable methods for treating it. Methane production, resulting from anaerobic digestion (AD), is a potential end-of-life scenario for PLA waste that needs to be investigated. To obtain high efficiency of AD, thermophilic fermentation was applied, and to overcome low rates of biodegradation, hydrothermal (HT) and alkaline (A) pretreatments were used. For a deep insight into the process, differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), and microscopic and microbial analyses (based on 16S rDNA) were applied. For both untreated (PLA) and pretreated (PLA_HT, PLA_A) samples a high maximal methane production (MP) of 453 L/kg volatile solids (VS) was obtained, almost 100 % of the theoretical methane yield from PLA. The use of pretreatment allowed shortening of the time for obtaining maximal MP, especially the hydrothermal pretreatment, which shortened the overall time of MP 1.3-fold, and methane was produced at an almost 10 % higher rate (8.35 vs 7.79 L/(kg VS·d)). However, DSC and microscopic analyses revealed that, in all cases, methane was intensively produced i) after the reduction of the molecular mass of the PLA material and ii) also when PLA pieces were not visible. This should be considered when designing the operational time for the AD process. Parallel to the gradual biodegradation of PLA, the abundances of Firmicutes, Thermotogae, and Euryarcheota increased. With PLA_HT, Syntrophobacteraceae, Thermoanaerobacteraceae, and methanogens were identified as potential key thermophilic PLA biodegraders.

Elucidating interactive effects of sulfidated nanoscale zero-valent iron and ammonia on anaerobic digestion of food waste

In our previous study, anaerobic digestion of food waste could be effectively enhanced by adding sulfidated nanoscale zero-valent iron (S-nZVI) under high-strength ammonia concentrations. In this study, in order to further elucidate the specific interactive effects of S-nZVI and ammonia on anaerobic digestion of nitrogen-rich food waste, the methanogenic performance of anaerobic digestion systems respectively added with nanoscale zero-valent iron (nZVI) and S-nZVI were compared and monitored under different ammonia stress conditions. Both nZVI and S-nZVI could effectively stimulate the methanogenesis process among ammonia concentrations ranging from 0 to 3500 mg/L. However, the enhancing effects of S-nZVI and nZVI on anaerobic digestion of food waste were different, in which anaerobic digestion systems added with S-nZVI and nZVI performed best under 2500 mg/L of ammonia and 1500 mg/L of ammonia, respectively. Furthermore, the analysis of microbial communities suggested that ammonia stress enriched acetoclastic methanogens, while adding nZVI and S-nZVI into anaerobic digestions stimulated the process of hydrogenotrophic methanogenesis. Moreover, S-nZVI performed better in promoting the evolution of DIET-related microorganisms than nZVI, resulting in enhanced methane production under high ammonia-stressed conditions. This work provided fundamental knowledge about the interactive effects of S-nZVI and ammonia on the anaerobic digestion of food waste.

Changes in the Microbial Structure of the Root Soil and the Yield of Chinese Baby Cabbage by Chemical Fertilizer Reduction with Bio-Organic Fertilizer Application

Using high-throughput sequencing, this study aimed to explore the response of soil microbial community and Chinese baby cabbage yield to the reduction of chemical fertilizers combined with bio-organic fertilizer. Our experiments consisted of conventional fertilizer (CK), 30% chemical fertilizer reduction + 6,000 kg/ha bio-organic fertilizer (T1), 30% chemical fertilizer reduction + 9,000 kg/ha bio-organic fertilizer (T2), 40% chemical fertilizer reduction + 6,000 kg/ha bio-organic fertilizer (T3), and 40% chemical fertilizer reduction + 9,000 kg/ha bio-organic fertilizer (T4). Compared with CK, soil microbial diversity and richness were higher for all treatments with added bio-organic fertilizer. Principle coordinate analysis (PCoA) showed that the bacterial and fungal communities in T2 and T4 were similar to each other. Redundancy and Spearman's correlation analyses of microbial communities and soil physicochemical properties revealed that reductions in chemical fertilizer rate combined with bio-organic fertilizer had a stronger impact on the fungal than the bacterial community. They also increased the relative abundance of the dominant bacterial and fungal phyla. Chinese baby cabbage yield was relatively higher under the combined bio-organic fertilizer plus reduced chemical fertilizer rate with T2 showing the highest yield. Therefore, this approach is feasible for sustainable agricultural, cost-effective and profitable crop production. IMPORTANCE Chemical fertilizers are commonly used for agriculture, though bio-organic fertilizers may be more efficient. We found that a mixture of bio-organic and moderately reduced chemical fertilizer was more effective than chemical fertilizer alone, as it raised the Chinese baby cabbage yield. Further, the presence of bio-organic fertilizer enhanced overall soil physicochemistry, as well as improved the beneficial bacterial and fungal abundance and diversity. Thus, we found that fertilizer combination sustainably & cost-effectively improves crop & soil quality.

Rigid bioplastics shape the microbial communities involved in the treatment of the organic fraction of municipal solid waste

While bioplastics are gaining wide interest in replacing conventional plastics, it is necessary to understand whether the treatment of the organic fraction of municipal solid waste (OFMSW) as an end-of-life option is compatible with their biodegradation and their possible role in shaping the microbial communities involved in the processes. In the present work, we assessed the microbiological impact of rigid polylactic acid (PLA) and starch-based bioplastics (SBB) spoons on the thermophilic anaerobic digestion and the aerobic composting of OFMSW under real plant conditions. In order to thoroughly evaluate the effect of PLA and SBB on the bacterial, archaeal, and fungal communities during the process, high-throughput sequencing (HTS) technology was carried out. The results suggest that bioplastics shape the communities' structure, especially in the aerobic phase. Distinctive bacterial and fungal sequences were found for SBB compared to the positive control, which showed a more limited diversity. Mucor racemosus was especially abundant in composts from bioplastics' treatment, whereas Penicillium roqueforti was found only in compost from PLA and Thermomyces lanuginosus in that from SBB. This work shed a light on the microbial communities involved in the OFMSW treatment with and without the presence of bioplastics, using a new approach to evaluate this end-of-life option.

Plant-scale biogas production prediction based on multiple hybrid machine learning technique

The parameters from full-scale biogas plants are highly nonlinear and imbalanced, resulting in low prediction accuracy when using traditional machine learning algorithms. In this study, a hybrid extreme learning machine (ELM) model was proposed to improve prediction accuracy by solving imbalanced data. The results showed that the best ELM model had a good prediction for validation data (R² = 0.972), and the model was developed into the software (prediction error of 2.15 %). Furthermore, two parameters within a certain range (feed volume (FV) = 23-45 m³ and total volatile fatty acids of anaerobic digestion (TVFA_AD) = 1750-3000 mg/L) were identified as the most important characteristics that positively affected biogas production. This study combines machine learning with data-balancing techniques and optimization algorithms to achieve accurate predictions of plant biogas production at various loads.

Microbial life strategy with high rRNA operon copy number facilitates the energy and nutrient flux in anaerobic digestion

Microbial life strategy, reflected by rRNA operon (rrn) copy number, determines microbial ecological roles. However, the relationship between microbial life strategy and the energy and nutrient flux in anaerobic digestion (AD) remains elusive. This study investigated microbial rrn copy number and expression ratio using amplicon sequencing of 16S rRNA gene and 16S rRNA, and monitored CH₄ daily production to approximate the status of energy and nutrient flux in semi-continuous AD. A significantly positive correlation between the mean rrn copy number of microbial communities in digestate and CH₄ daily production was detected in the control treatment fed swine manure. The reduced feedstock complexity, by replacing parts of swine manure with fructose or apple waste, weakened the correlation. When feedstock complexity was increased again, the correlation was strengthened again. Similar results were detected in mean rrn expression ratio of microbial communities. The responses of mean rrn copy number and expression ratio of communities to feedstock addition differed between the reduced feedstock complexity and the control treatment, as well as between in digestate and in straw. Our findings reveal a novel relationship between microbial community life strategy and the energy and nutrient flux, and the roles of feedstock characteristics therein in AD.

Intermittent voltage induced sludge polarization to enhance anaerobic digestion

Intermittent voltage supply has been reported to improve the performance of electro-assisted anaerobic digestion but has not been well understood. In this study, an intermittent voltage of 0.6 V (1 day on-1 day off) was applied in an electro-assisted anaerobic digester to explore its effects. Compared to those without the voltage, the methane yield increased nearly by 20.0%, and organic decomposition increased by 9.5% with the intermittent voltage, which was similar to those with the continuous voltage. The amide groups of the sludge protein after the electro-treatment were polarized to enhance electron transfer and electron storage of protein-like substances of the sludge. Although the voltage was supplied intermittently, the increased conductivity and capacitance of the sludge and EPS could effectively transport electrons between exoelectrogens and electrotrophs (such as Firmicutes and Methanothrix) to promote the anaerobic digestion. This study explained the essence of electrochemical enhancement of anaerobic digestion from the perspective of molecular structure, that is, the polarization of functional groups by voltage could improve the sludge electro-activity to maintain effective interspecies electron transfer in the periodic voltage supply.

An accelerated approach of biogas production through a two-stage BioH2/CH4 continuous anaerobic digestion system from Napier grass

Napier grass found to be greatest potential for gaseous bioenergy production. The biohydrogen and biomethane productions from untreated Napier grass in single and two-stage continuous bioreactors was evaluated using anaerobic digestion technology. The bioreactors were fed Napier grass with organic loading rate (OLR) of 0.5, 1.0 and 2.0 kg VS/m³-d, respectively. The hydrogen, methane, and energy yields were evaluated. The methane yield of single-stage system was 282.08 CH₄/g vS with OLR of 0.5 kg VS/m³-d. For two-stage system, the biohydrogen and biomethane yields were 90.06 mL H₂/g vS and 367.00 mL CH₄/g vS with OLRs of 1.0 and 0.5 kg VS/m³-d, respectively. The energy yields of single and two-stage systems were 13.14 and 10.10 kJ/g vS respectively. The peak OLR of Napier grass was 0.5 kg VS/m³-d for the two-stage system whereas the total energy recovery was 30 % higher than the single-stage system.

Determination of methanogenesis by nutrient availability via regulating the relative fitness of methanogens in anaerobic digestion

Response of microbial community to nutrient availability in anaerobic digestion (AD) remains elusive. Prokaryotic communities in AD batch cultures with 0, 1, 3, 5, 7, 11, 15, 20, and 25 g/L peptone were monitored using massive parallel sequencing and quantitative PCR over a 34-day experimental period. Methane production displayed a hump-shaped response to the nutrient gradient (peaking at 15 g/L peptone). Moreover, total and acetoclastic methanogens showed hump-shaped responses (both peaking at 11 g/L peptone). However, prokaryotic population increased with nutrient concentration (linear regression, R² = 0.86) while diversity decreased (R² = 0.94), and ordination analysis showed a gradual succession of community structure along the first axis. Network analysis revealed that extent of interspecific interactions (e.g., edge number and clustering coefficient) exhibited a hump-shaped response. The combined results indicate that abundant species became more dominated with increasing nutrient, which can result in a gain or loss of interspecific interaction within the community. Network module analysis showed that one module dominated the network at each nutrient level (comprising 41%-65% of the nodes), indicating that AD community formed a core microbial guild. The most abundant phylotypes, Macellibacteroides and Butyricicoccaceae, were consistently negative with acetoclastic methanogens in the dominant modules. Their predominance at ≥15 g/L peptone can explain the hump-shaped responses of methanogenesis and methanogens. Collectively, methanogenesis and microbial network exhibited hump-shaped responses, although microbial community exhibited monotonic responses. Therefore, nutrient availability can determine the methanogenesis through regulating the relative fitness of methanogens within the community.