Mesophilic continuous fermentative hydrogen production from acid pretreated de-oiled jatropha waste hydrolysate using immobilized microorganisms

In-situ membrane fouling control and performance improvement by adding materials in anaerobic membrane bioreactor: A review

Anaerobic membrane bioreactor (AnMBR) is a promising treatment technique for various types of wastewaters, and is preferred over other conventional aerobic and anaerobic methods. However, membrane fouling is considered a bottleneck in AnMBR system, which technically blocks membrane pores by numerous inorganics, organics, and other microbial substances. Various materials can be added in AnMBR to control membrane fouling and improve anaerobic digestion, and studies reporting the materials addition for this purpose are hereby systematically reviewed. The mechanism of membrane fouling control including compositional changes in extracellular polymeric substances (EPSs) and soluble microbial products (SMPs), materials properties, stimulation of antifouling microbes and alteration in substrate properties by material addition are thoroughly discussed. Nonetheless, this study opens up new research prospects to control membrane fouling of AnMBR, engineered by material, including compositional changes of microbial products (EPS and SMP), replacement of quorum quenching (QQ) by materials, and overall improvement of reactor performance. Regardless of the great research progress achieved previously in membrane fouling control, there is still a long way to go for material-mediated AnMBR applications to be undertaken, particularly for materials coupling, real scale application and molecular based studies on EPSs and SMPs, which were proposed for future researches.

A comprehensive review on the use of conductive materials to improve anaerobic digestion: Focusing on landfill leachate treatment

Globally, around 70% of waste is disposed of in open dumps or landfill sites, with the leachate generated from these sites containing high concentrations of organic and inorganic compounds, which will adversely affect aquatic environments if discharged without proper treatment. Anaerobic digestion of landfill leachate is an environmentally-friendly method that efficiently converts organic compounds into methane-rich biogas. However, the widespread application of anaerobic digestion has been hindered by poor system stability, low methanogenic activity and a high level of volatile fatty acids (VFAs) accumulation, increasing the operational costs of treatment. Conductive materials can be added to the digester to improve the performance of anaerobic digestion in landfill leachate treatment systems and studies reporting the use of conductive materials for this purpose are hereby thoroughly reviewed. The mechanism of microbial growth and enrichment by conductive materials is discussed, as well as the subsequent effect on waste metabolism, methane production, syntrophic relationships and interspecies electron transfer. The porous structure, specific surface area and conductivity of conductive materials play vital roles in the facilitation of syntrophic relationships between fermentative bacteria and methanogenic archaea. In addition, the mediation of direct interspecies electron transfer (DIET) by conductive materials increases the methane content of biogas from 16% to 60% as compared to indirect interspecies electron transfer (IIET) in conventional anaerobic digestion systems. This review identifies research gaps in the field of material-amended anaerobic systems, suggesting future research directions including investigations into combined chemical-biological treatments for landfill leachate, microbial management using conductive materials for efficient pollutant removal and the capacity for material reuse. Moreover, findings of this review provide a reference for the efficient and large-scale treatment of landfill leachate by anaerobic digestion with conductive materials.

Dynamic membrane bioreactor for high rate continuous biohydrogen production from algal biomass

This study aimed to achieve continuous biohydrogen production from red algal biomass using a dynamic membrane bioreactor (DMBR). The DMBR was continuously fed with pretreated Echeuma spinosum containing 20 g/L hexose. The highest average hydrogen production rate (HPR) of 21.58 ± 1.59 L/L-d was observed at HRT 3 h, which was higher than previous reports for continuous H₂ production from biomass feedstock. Metabolic flux analysis revealed that butyric acid and propionic acid were the major by-products of the H₂-producing and H₂-consuming pathways, respectively, of the algal biomass fermentation. Hydrogen consumption by propionic acid pathway could not be prevented completely by heat treatment. PICRUSt2 analysis predicted that Clostridium sp., Anaerostipes sp., and Caproiciproducens sp. might significantly contribute to the expression of both ferredoxin hydrogenase and propionate CoA-transferase. This study would provide the design and operational information on high-rate bioreactor for continuous hydrogen production using biomass.

Food Waste Biorefinery: Pathway towards Circular Bioeconomy

Food waste biorefineries for the production of biofuels, platform chemicals and other bio-based materials can significantly reduce a huge environmental burden and provide sustainable resources for the production of chemicals and materials. This will significantly contribute to the transition of the linear based economy to a more circular economy. A variety of chemicals, biofuels and materials can be produced from food waste by the integrated biorefinery approach. This enhances the bioeconomy and helps toward the design of more green, ecofriendly, and sustainable methods of material productions that contribute to sustainable development goals. The waste biorefinery is a tool to achieve a value-added product that can provide a better utilization of materials and resources while minimizing and/or eliminating environmental impacts. Recently, food waste biorefineries have gained momentum for the production of biofuels, chemicals, and bio-based materials due to the shifting of regulations and policies towards sustainable development. This review attempts to explore the state of the art of food waste biorefinery and the products associated with it.

High-rate mesophilic hydrogen production from food waste using hybrid immobilized microbiome

This study examined the feasibility of dark fermentative biohydrogen production from food waste using hybrid immobilization in mesophilic condition. Among four different organic loading rates (OLRs), the highest average hydrogen production rate (HPR) of 9.82 ± 0.30 L/L-d was found at an OLR of 74.7 g hexose/L-d, which was higher than reported values from particulate feedstock in mesophilic condition. The average hydrogen yield (HY) at the condition was 1.25 ± 0.04 mol H₂/mol hexose_consumed. Whereas the average HPR and HY at an OLR 80 g hexose/L-d were 5.82 ± 0.12 L/L-d and 0.64 ± 0.02 mol H₂/mol hexose_consumed, respectively. Metabolic flux analysis showed the low HY was concurrent with the highest propionic acid and homoacetogenis. Bacterial population was shift from Clostridium sp. to non-hydrogen producers including Bifidobacterium, Bacteriodes, Olsenella, Dysgonomonas, and Dialister sp.

Methane production from wheat straw pretreated with CaO2/cellulase

There are various lignocellulosic biomass pretreatments that act as attractive strategies to improve anaerobic digestion for methane (CH₄) generation. This study proposes an effective technique to obtain more CH₄via the hydrothermal coupled calcium peroxide (CaO₂) co-cellulase pretreatment of lignocellulosic biomass. The total organic carbon in the hydrolysate of samples treated with 6% CaO₂ and 15 mg enzyme per g-cellulose was 7330 mg L⁻¹, which represented an increase of 92.39% over the total organic carbon value of samples hydrolyzed with the enzyme alone. The promotion of the anaerobic digestion of wheat straw followed this order: hydrothermal coupled CaO₂ co-cellulase pretreatment > hydrothermal coupled CaO₂ pretreatment > enzymatic pretreatment alone > control group. The sample treated with 6% CaO₂ and 15 mg enzyme per g-cellulose gave the highest CH₄ production with a CH₄ yield of 214 mL g⁻¹ total solids, which represented an increase of 64.81% compared to the control group. The CH₄ yield decreased slightly when the amount of added cellulase exceeded 15 mg enzyme per g-cellulose.

This work reports methane production from wheat straw pretreated with CaO₂/cellulase.

Sustainable energy from waste organic matters via efficient microbial processes

This review emphasizes utilization of waste organic matters from water bodies and soil sources for sustainable energy development. These organic waste matters (including microplastics) from a variety of environmental sources have created a big challenge to utilize them for energy development for human needs, maintaining a cleaner environment and thereby, producing useful bioproducts (sustainable bioenergy or other primary metabolites). Anaerobic digestions as well as other effective wastewater treatment approaches are discussed. From the water bodies, waste organic matter reduction can be achieved by a reduction of chemical oxygen demand and biological oxygen demand after the waste treatment. Other forms of organic waste matter are available in the form of agro wastes or residues (stalk of wheat or rice, maize, corn etc.) due to crop cultivation, which are generally burnt into ashes. Such wastes can be utilized for bioenergy energy production, which would help for the reduction of climate changes or other toxic gases. Hydrogen, bioelectricity, ethanol, butanol, methane and algal diesel or other types of fuel sources would help to provide sustainable source of bioenergy that can be produced from these wastes via degradation by the biological processes. This review will discuss in depths about the sustainable nature of organic matters to produce clean energy via application of efficient biological methods to maintain a clean environment, thereby providing alternative options to fossil energy fuels.

Comparison of Calcium Oxide and Calcium Peroxide Pretreatments of Wheat Straw for Improving Biohydrogen Production

Wheat straw was pretreated with either CaO₂ or CaO to improve biohydrogen production. Both CaO and CaO₂ pretreatments improved the biodegradability of the wheat straw. CaO pretreatment raised the H₂ yield by between 48.8 and 163.9% at CaO contents ranging from 2 to 4%. The highest H₂ yield [144 mL/g total solid (TS)] was obtained at 121 °C and 6% CaO. In addition, the highest H₂ yield from wheat straw pretreated at the same temperature and dosage of CaO₂ was 71.8 mL/g TS, which was higher than that of the control group (43.2 mL/g TS), with hot water (121 °C) treatment. Considering pretreatment costs and H₂ production potential, CaO was a better pretreatment agent than CaO₂.

State of the art and challenges of biohydrogen from microalgae

Biohydrogen from microalgae has attracted extensive attention owing to its promising features of abundance, renewable and self sustainability. Unlike other well-established biofuels like biodiesel and bioethanol, biohydrogen from microalgae is still in the preliminary stage of development. Criticisms in microalgal biohydrogen centered on its practicality and sustainability. Various laboratory- and pilot-scale microalgal systems have been developed, and some research initiatives have exhibited potential for commercial application. This work provides a review of the state of the art of biohydrogen from microalgae. Discussions include metabolic pathways of light-driven transformation and dark fermentation, reactor schemes and system designs encompassing reactor configurations and light manipulation. Challenges, knowledge gaps and the future directions in metabolic limitations, economic and energy assessments, and molecular engineering are also delineated. Current scientific and engineering challenges of microalgal biohydrogen need to be addressed for technology leapfrog or breakthrough.

Biohydrogen and polyhydroxyalkanoate production from original hydrolyzed polyacrylamide-containing wastewater

This work aimed to study biohydrogen (H₂) and polyhydroxyalkanoate (PHA) production from original hydrolyzed polyacrylamide (HPAM)-containing wastewater. NH₄⁺-N from HPAM hydrolysis was removed efficiently through short-cut nitrification and anoxic ammonia oxidation (anammox). Carbon/Nitrogen (C/N) ratios of effluent reached 51-97, and TOC decreased only 2%-4%, providing potential for subsequent H₂ and PHA production. The maximum yields of H₂ (0.833 mL·mg^-1_substrate) and Volatile Fatty Acid (VFA) (465 mg·L^-1) occurred at influent C/N ratio of 51. Substrate removal increased linearly with the activities of dehydrogenase and hydrogenase (R² ≥ 0.990), and H₂ yield rose exponentially with enzyme activities (R² ≥ 0.989). The maximum PHA yield (54.2% VSS) occurred at the 42nd hour and influent C/N ratio of 97. PHA yield was positively correlated with substrate uptake. The change of H₂-producing, PHA-accumulating and HPAM-degradating bacteria indicated that those functional microorganisms had synergistic effects on H₂ production and substrate uptake, as well as PHA accumulation and substrate uptake.

Continuous production of biohydrogen from brewery effluent using co-culture of mutated Rhodobacter M 19 and Enterobacter aerogenes

This study investigated the biohydrogen production from brewery effluents using free and immobilized co-culture of mutated Rhodobacter M 19 and Enterobacter aerogenes obtained from random mutagenesis with ultra violet (UV) and ethidium bromide (EtBr) treatment. The best mutant for biohydrogen production was screened based on the sugar utilization efficiency. Maximum hydrogen production of 87% was achieved with immobilized EtBr mutated co-culture. The mutant immobilized strains showed around 30% enhanced hydrogen production than wild strains at pH 6.9. Gompertz and Richard's model were used to fit the augmenting biohydrogen production and Logistics equation determines the fitness of biomass growth data. The maximal biomass concentration of co-cultures strains was 3.145 g/L with carrying capacity coefficient 0.137 h^-1. Gompertz model showed the best fit with minimal error in predicting the biohydrogen potential.

Recent achievements in enhancing anaerobic digestion with carbon- based functional materials

Carbon-based materials such as graphite, graphene, biochar, activated carbon, carbon cloth and nano-tube, and maghemite and magnetite carbons are capable for adsorbing chemicals onto their surfaces. Currently, this review is to highlight the relevance of carbons in enhancing hydrogen or methane production. Some key roles of carbons in improving cell growth, enrichment and activity, and accelerating their co-metabolisms were elaborated with regard to their effects on syntrophic communities, interspecies electron transfer, buffering capacity, biogas upgrading, and fertilizer nutrient retention and land application. Carbons can serve as a habitation for microbial immobilization, and a provision for bioelectrical connections among cells, and provide some essential elements for anaerobes. Besides, an outlook on the possible options towards the large scale and improvement solutions has been provided. Further studies in this area could be encouraged to intend and operate continuous mode by designing carbon-amended bioreactor with stability and reliability.

A review of research trends in the enhancement of biomass-to-hydrogen conversion

Different types of biomass are being examined for their optimum hydrogen production potentials and actual hydrogen yields in different experimental set-ups and through different chemical synthetic routes. In this review, the observations emanating from research findings on the assessment of hydrogen synthesis kinetics during fermentation and gasification of different types of biomass substrates have been concisely surveyed from selected publications. This review revisits the recent progress reported in biomass-based hydrogen synthesis in the associated disciplines of microbial cell immobilization, bioreactor design and analysis, ultrasound-assisted, microwave-assisted and ionic liquid-assisted biomass pretreatments, development of new microbial strains, integrated production schemes, applications of nanocatalysis, subcritical and supercritical water processing, use of algae-based substrates and lastly inhibitor detoxification. The main observations from this review are that cell immobilization assists in optimizing the biomass fermentation performance by enhancing bead size, providing for adequate cell loading and improving mass transfer; there are novel and more potent bacterial and fungal strains which improve the fermentation process and impact on hydrogen yields positively; application of microwave irradiation and sonication and the use of ionic liquids in biomass pretreatment bring about enhanced delignification, and that supercritical water biomass processing and dosing with metal-based nanoparticles also assist in enhancing the kinetics of hydrogen synthesis. The research areas discussed in this work and their respective impacts on hydrogen synthesis from biomass are arguably standalone. Thence, further work is still required to explore the possibilities and techno-economic implications of combining these areas for developing robust and integrated biomass-to-hydrogen synthetic schemes.

A novel approach of modeling continuous dark hydrogen fermentation

In this study a novel modeling approach for describing fermentative hydrogen production in a continuous stirred tank reactor (CSTR) was developed, using the Aquasim modeling platform. This model accounts for the key metabolic reactions taking place in a fermentative hydrogen producing reactor, using fixed stoichiometry but different reaction rates. Biomass yields are determined based on bioenergetics. The model is capable of describing very well the variation in the distribution of metabolic products for a wide range of hydraulic retention times (HRT). The modeling approach is demonstrated using the experimental data obtained from a CSTR, fed with food industry waste (FIW), operating at different HRTs. The kinetic parameters were estimated through fitting to the experimental results. Hydrogen and total biogas production rates were predicted very well by the model, validating the basic assumptions regarding the implicated stoichiometric biochemical reactions and their kinetic rates.

An integrated green process: Subcritical water, enzymatic hydrolysis, and fermentation, for biohydrogen production from coconut husk

The objective of this work is to develop an integrated green process of subcritical water (SCW), enzymatic hydrolysis and fermentation of coconut husk (CCH) to biohydrogen. The maximum sugar yield was obtained at mild severity factor. This was confirmed by the degradation of hemicellulose, cellulose and lignin. The tendency of the changing of sugar yield as a result of increasing severity factor was opposite to the tendency of pH change. It was found that CO₂ gave a different tendency of severity factor compared to N₂ as the pressurizing gas. The result of SEM analysis confirmed the structural changes during SCW pretreatment. This study integrated three steps all of which are green processes which ensured an environmentally friendly process to produce a clean biohydrogen.

Hydrogen production by dark fermentation from pre-fermented depackaging food wastes

In this study, a specific fraction of food waste, i.e. depackaging waste, was studied as substrate for hydrogen production by dark fermentation. During storage and transport of this liquid mixture, inhibitory compounds like acids or alcohol might be produced by endogenous flora. A factorial fractional design based on the composition of the substrate was used to determine the best condition to convert this substrate into hydrogen. First results indicated that the consortium used might convert high quantity of lactate into hydrogen. A batch culture confirmed that lactate was used as the main carbon source and a global yield of 0.4mol_H2·mol_lactate^-1 was obtained. This study demonstrated the ability of the consortium tested to convert different carbon sources (carbohydrates or lactate) with good efficiency. These data represented an important parameter in the prospect of using an industrial substrate whose composition is liable to vary according to the conditions of storage and transport.

Bio-immobilization of dark fermentative bacteria for enhancing continuous hydrogen production from cornstalk hydrolysate

Mycelia pellets were employed as biological carrier in a continuous stirred tank reactor to reduce biomass washout and enhance hydrogen production from cornstalk hydrolysate. Hydraulic retention time (HRT) and influent substrate concentration played critical roles on hydrogen production of the bioreactor. The maximum hydrogen production rate of 14.2mmol H₂L^-1h^-1 was obtained at optimized HRT of 6h and influent concentration of 20g/L, 2.6 times higher than the counterpart without mycelia pellets. With excellent immobilization ability, biomass accumulated in the reactor and reached 1.6g/L under the optimum conditions. Upon further energy conversion analysis, continuous hydrogen production with mycelia pellets gave the maximum energy conversion efficiency of 17.8%. These results indicate mycelia pellet is an ideal biological carrier to improve biomass retention capacity of the reactor and enhance hydrogen recovery efficiency from lignocellulosic biomass, and meanwhile provides a new direction for economic and efficient hydrogen production process.

From grass to gas: microbiome dynamics of grass biomass acidification under mesophilic and thermophilic temperatures

BACKGROUND

Separating acidification and methanogenic steps in anaerobic digestion processes can help to optimize the process and contribute to producing valuable sub-products such as methane, hydrogen and organic acids. However, the full potential of this technology has not been fully explored yet. To assess the underlying fermentation process in more detail, a combination of high-throughput sequencing and proteomics on the acidification step of plant material (grass) at both mesophilic and thermophilic temperatures (37 and 55 °C, respectively) was applied for the first time.

RESULTS

High-strength liquor from acidified grass biomass exhibited a low biodiversity, which differed greatly depending on temperature. It was dominated by Bacteroidetes and Firmicutes at 37 °C, and by Firmicutes and Proteobacteria at 55 °C. At the methane stage, Methanosaeta, Methanomicrobium and Methanosarcina proved to be highly sensitive to environmental changes as their abundance in the seed sludges dropped dramatically after transferring the seed sludges from the respective reactors into the experimental setup. Further, an increase in Actinobacteria coincided with reduced biogas production at the end of the experiment. Over 1700 proteins were quantified from the first cycle of acidification samples using label-free quantitative proteome analysis and searching protein databases. The most abundant proteins included an almost complete set of glycolytic enzymes indicating that the microbial population is basically engaged in the degradation and catabolism of sugars. Differences in protein abundances clearly separated samples into two clusters corresponding to culture temperature. More differentially expressed proteins were found under mesophilic (120) than thermophilic (5) conditions.

CONCLUSION

Our results are the first multi-omics characterisation of a two-stage biogas production system with separated acidification and suggest that screening approaches targeting specific taxa such as Methanosaeta, Methanomicrobium and Methanosarcina could be useful diagnostic tools as indicators of environmental changes such as temperature or oxidative stress or, as in the case of Actinobacteria, they could be used as a proxy of the gas production potential of anaerobic digesters. Metaproteome analyses only detected significant expression differences in mesophilic samples, whereas thermophilic samples showed more stable protein composition with an abundance of chaperones suggesting a role in protein stability under thermal stress.