Dark fermentation of complex waste biomass for biohydrogen production by pretreated thermophilic anaerobic digestate

Optimization of fermentation conditions for enhanced hydrogen production by newly isolated Clostridium butyricum ZYZCB strain

AIMS

The foundation of biohydrogen production lies in the utilization of efficient hydrogen-producing strains. This study aims to isolate highly efficient hydrogen-producing strains and enhance hydrogen production performance by optimizing the fermentation process.

METHODS AND RESULTS

A hydrogen-producing bacterial strain was isolated from air and identified by 16S rDNA sequence analysis as belonging to the species Clostridium butyricum, and was named C. butyricum ZYZCB. The results indicate that the ZYZCB strain exhibits notable advantages in terms of rapid growth and a shortened lag phase. ZYZCB can produce hydrogen within a pH range of 5-9 and a temperature range of 25°C-40°C. The utilization of starch, maltose, and sucrose as substrates, resulted in a hydrogen production exceeding 1750 ml l-1, with a maximum hydrogen yield of 2.28 mol H2 mol-1 glucose for ZYZCB, proving the suitability of strain ZYZCB for H2-producing dark fermentation.

CONCLUSIONS

The ZYZCB strain exhibits favorable characteristics during fermentation and hydrogen production, showing potential for application in the field of biohydrogen production.

Dark-fermentative biohydrogen production from vegetable residue using wine lees as novel inoculum

This work studied wine lees as a novel source of microorganisms for biohydrogen production from vegetable residue (VR). Green tomatoes (WLGT), bell peppers (WLBP), green beans (WLGB), zucchini (WLZ), peas (WLP), and (WLE) eggplants were used as a substrate for dark fermentation, which was conducted in batch assays at 37 °C for 60 d. The cumulative hydrogen yield was approximately 350, 344, 319, 314, 302, 170, and 149 mL H2/g VS in WLZ, WLE, WLRT, WLGT, WLP, WLGB, and WLBP, respectively. A total volatile fatty acid (VFA) accumulation of about 2059 - 4995 mg HAc/L accompanied the bio-H2 production. From day 61 to day 147, dark-fermentative digestate was subjected to an anaerobic digestion batch process under mesophilic conditions to allow the bioconversion of VFAs into renewable methane, as confirmed by a Pearson correlation value of 0.778, final VFA concentrations ≤ 131 mg HAc/L and key functional genes (e.g., K00925). Clostridium_sensu_stricto_12 and Caproiciproducens genera accounted for 44 - 78 % of relative abundance after the dark fermentation stage. The taxonomic classification also revealed a presence of Methanosaeta archaea comprised between 45 and 98 % after the two-stage anaerobic fermentation. Finally, a rough energy comparison was performed to evaluate the feasibility of the bioprocess by including practical implications and limitations.

Improving hydrogen and volatile fatty acids production through pretreatment of spent coffee grounds

Consumption of coffee produces large amounts of waste in the form of spent coffee grounds (SCG), a lignocellulosic material rich in carbohydrates, proteins, and polyphenols. This abundant feedstock is promising in terms of biofuels and value-added product generation. This study investigated the impact of pretreatments, such as alkaline (NaOH), ultrasound, and static magnetic field, on SCG bioconversion in terms of biomolecule release, H2 potential and volatile fatty acids production. Following treatment, the slurry (solid and liquid fraction mixture) was utilised in anaerobic fermentation tests at varying volatile solid (VS) concentrations (23.3 and 46.7 g VS/L). The highest H2 production range, 25 - 30 mL H2/g VS, was obtained using the alkaline-pretreated SCG slurry at 23.3 g VS/L. Nevertheless, inhibition of H2 production was observed when utilising the alkaline-pretreated slurry at 46.7 g VS/L owing to the excessive use of NaOH for pretreatment and chemicals to adjust the initial pH. In contrast, increasing the VS concentration had a positive impact on volatile fatty acids accumulation, with acetic (HAc) and caproic acid being dominant. Ultrasound-pretreated SCG achieved 3260.0 mg HAceq/L at a concentration of 46.7 g VS/L.

Metabolic engineering of Caldicellulosiruptor bescii for hydrogen production

Hydrogen is an alternative fuel for transportation vehicles because it is clean, sustainable, and highly flammable. However, the production of hydrogen from lignocellulosic biomass by microorganisms presents challenges. This microbial process involves multiple complex steps, including thermal, chemical, and mechanical treatment of biomass to remove hemicellulose and lignin, as well as enzymatic hydrolysis to solubilize the plant cell walls. These steps not only incur costs but also result in the production of toxic hydrolysates, which inhibit microbial growth. A hyper-thermophilic bacterium of Caldicellulosiruptor bescii can produce hydrogen by decomposing and fermenting plant biomass without the need for conventional pretreatment. It is considered as a consolidated bioprocessing (CBP) microorganism. This review summarizes the basic scientific knowledge and hydrogen-producing capacity of C. bescii. Its genetic system and metabolic engineering strategies to improve hydrogen production are also discussed. KEY POINTS: • Hydrogen is an alternative and eco-friendly fuel. • Caldicellulosiruptor bescii produces hydrogen with a high yield in nature. • Metabolic engineering can make C. bescii to improve hydrogen production.

Mixotrophic cultivation of microalgae-bacteria consortia enhances dark fermentation effluent treatment

Dark fermentation (DF) is a waste treatment bioprocess which produces biohydrogen and volatile fatty acids (VFAs) such as acetate or butyrate. DF can be coupled with microalgae cultivation, allowing VFA conversion into valuable biomass. Nevertheless, the process is hindered by slow butyrate consumption. In this study, novel artificial microalgae-bacteria consortia were used as a strategy to accelerate butyrate removal. Three microalgal strains with various trophic metabolisms, Chlorella sorokiniana, Euglena gracilis and Ochromonas danica, were cultivated on DF effluent that was either sterile or contained endogenous bacteria. Bacteria did not impact microalgal biomass production of C. sorokiniana or E. gracilis while accelerating butyrate removal rates 2 to 10-fold. O. danica greatly impacted microbial diversity, probably due to its phagotrophic metabolism. These results show that bacteria in organic rich effluents can greatly aid in substrate removal while allowing microalgal growth, inspiring bioprocesses coupling raw fermentation effluents with microalgae biomass production and valorization.

Valorisation of industrial hemp (Cannabis sativa L.) residues and cheese whey into volatile fatty acids for single cell protein production

The production of single cell protein (SCP) using lignocellulosic materials stands out as a promising route in the circular bioeconomy transition. However, multiple steps are necessary for lignocellulosics-to-SCP processes, involving chemical pretreatments and specific aerobic cultures. Whereas there are no studies that investigated the SCP production from lignocellulosics by using only biological processes and microbial biomass able to work both anaerobically and aerobically. In this view, the valorisation of industrial hemp (Cannabis sativa L.) biomass residues (HBRs), specifically hurds and a mix of leaves and inflorescences, combined with cheese whey (CW) was investigated through a semi-continuous acidogenic co-fermentation process (co-AF). The aim of this study was to maximise HBRs conversion into VFAs to be further used as carbon-rich substrates for SCP production. Different process conditions were tested by either removing CW or increasing the amount of HBRs in terms of VS (i.e., two and four times) to evaluate the performance of the co-AF process. Increasing HBRs resulted in a proportional increase in VFA production up to 3115 mg HAc L-1, with experimental production nearly 40% higher than theoretical predictions. The synergy between HBRs and CW was demonstrated, proving the latter as essential to improve the biodegradability of the former. The produced VFAs were subsequently tested as substrates for SCP synthesis in batch aerobic tests. A biomass concentration of 2.43 g TSS L-1 was achieved with a C/N ratio of 5.0 and a pH of 9.0 after two days of aerobic fermentation, reaching a protein content of 42% (g protein per g TSS). These results demonstrate the overall feasibility of the VFA-mediated HBR-to-SCP valorisation process.

Biomass-derived carbon dots as significant biological tools in the medicinal field: A review

Early disease detection is crucial since it raises the likelihood of treatment and considerably lowers the cost of therapy. Therefore, the improvement of human life and health depends on the development of quick, efficient, and credible biosensing methods. For improving the quality of biosensors, distinct nanostructures have been investigated; among these, carbon dots have gained much interest because of their great performance. Carbon dots, the essential component of fluorescence nanoparticles, having outstanding chemical characteristics, superb biocompatibility, chemical inertness, low toxicity and potential optical characteristics have attracted the researchers from every corner of the globe. Several carbon dots applications have been thoroughly investigated in recent decade, from optoelectronics to biomedical investigations. This review study primarily emphasizes the recent advancements in the field of biomass-derived carbon dots-based drug delivery, gene delivery and bioimaging, and highlights achievements in two major areas: in vivo applications that involve carbon dots absorption in zebrafish and mice, tumour therapeutics, and imaging-guided drug delivery. Additionally, the possible advantages, difficulties, and future possibilities of using carbon dots for biological applications are also explored.

Valorization of Purple Phototrophic Bacteria Biomass Resulting from Photo Fermentation Aimed at Biohydrogen Production

This study evaluated the feasibility of contextually producing hydrogen, microbial proteins, and polyhydroxybutyrate (PHB) using a mixed culture of purple phototrophic bacteria biomass under photo fermentative conditions. To this end, three consecutive batch tests were conducted to analyze the biomass growth curve and to explore the potential for optimizing the production process. Experimental findings indicated that inoculating reactors with microorganisms from the exponential growth phase reduced the duration of the process. Furthermore, the most effective approach for simultaneous hydrogen production and the valorization of microbial biomass was found when conducting the process during the exponential growth phase of the biomass. At this stage, achieved after 3 days of fermentation, the productivities of hydrogen, PHB, and microbial proteins were measured at 63.63 L/m3 d, 0.049 kg/m3 d, and 0.045 kg/m3 d, respectively. The biomass composition comprised a total intracellular compound percentage of 56%, with 27% representing PHB and 29% representing proteins. Under these conditions, the estimated daily revenue was maximized, amounting to 0.6 $/m3 d.

Carbon dots as potential candidate for photocatalytic treatment of dye wastewater

Water is the utmost important element for the existence of life. In recent decades, water resources have become highly contaminated by a variety of pollutants, especially toxic dyes that are harmful to both living beings and environment. Hence, there is an urgent need to develop more effective methods than traditional wastewater treatment approaches for treatment of hazardous dyes. Herein, we have addressed the various aspects related to the effective and economically feasible method for photocatalytic degradation of these dyes employing carbon dots. The photocatalysts based on carbon dots including those mediated from biomass have many superiorities over conventional methods such as utilization of economically affordable, non-toxic, rapid reactions, and simple post-processing steps. The current study will also facilitate better insight into the understanding of photocatalytic treatment of dye-polluted wastewater for future wastewater treatment studies. Additionally, the possible mechanistic pathways of photocatalytic dye decontamination, several challenges, and future perspectives have also been summarized.

Acidogenic fermentation of food waste to generate electron acceptors and donors towards medium-chain carboxylic acids production

This study investigated the optimum pH, temperature, and food-to-microorganisms (F/M) ratio for regulating the formation of electron acceptors and donors during acidogenic fermentation to facilitate medium-chain carboxylic acids (MCCAs) production from food waste. Mesophilic fermentation at pH 6 was optimal for producing mixed volatile fatty acids (719 ± 94 mg COD/g VS) as electron acceptors. Under mesophilic conditions, the F/M ratio (g VS/g VS) could be increased to 6 to generate 22 ± 2 g COD/L of electron acceptors alongside 2 ± 0 g COD/L of caproic acid. Thermophilic fermentation at pH 6 was the best condition for producing lactic acid as an electron donor. However, operating at F/M ratios above 3 g VS/g VS under thermophilic settings significantly reduced lactic acid yield. A preliminary techno-economic evaluation revealed that converting lactic acid and butyric acid generated during acidogenic fermentation to caproic acid was the most profitable food waste valorization scenario and could generate 442-468 €/t VS/y. The results presented in this study provide insights into how to tailor acidogenic fermentation reactions to desired intermediates and will help maximize MCCAs synthesis.

Sustainable production of biofuels and bioderivatives from aquaculture and marine waste

The annual global fish production reached a record 178 million tonnes in 2020, which continues to increase. Today, 49% of the total fish is harvested from aquaculture, which is forecasted to reach 60% of the total fish produced by 2030. Considering that the wastes of fishing industries represent up to 75% of the whole organisms, the fish industry is generating a large amount of waste which is being neglected in most parts of the world. This negligence can be traced to the ridicule of the value of this resource as well as the many difficulties related to its valorisation. In addition, the massive expansion of the aquaculture industry is generating significant environmental consequences, including chemical and biological pollution, disease outbreaks that increase the fish mortality rate, unsustainable feeds, competition for coastal space, and an increase in the macroalgal blooms due to anthropogenic stressors, leading to a negative socio-economic and environmental impact. The establishment of integrated multi-trophic aquaculture (IMTA) has received increasing attention due to the environmental benefits of using waste products and transforming them into valuable products. There is a need to integrate and implement new technologies able to valorise the waste generated from the fish and aquaculture industry making the aquaculture sector and the fish industry more sustainable through the development of a circular economy scheme. This review wants to provide an overview of several approaches to valorise marine waste (e.g., dead fish, algae waste from marine and aquaculture, fish waste), by their transformation into biofuels (biomethane, biohydrogen, biodiesel, green diesel, bioethanol, or biomethanol) and recovering biomolecules such as proteins (collagen, fish hydrolysate protein), polysaccharides (chitosan, chitin, carrageenan, ulvan, alginate, fucoidan, and laminarin) and biosurfactants.

Valorisation of industrial hemp (Cannabis sativa L.) biomass residues through acidogenic fermentation and co-fermentation for volatile fatty acids production

In line with the emerging circular bioeconomy paradigm, the present study investigated the valorisation of abundant hemp biomass residues (HBRs) such as hurds (HH) and a mix of leaves and inflorescences (Mix), and other organic wastes (i.e., cheese whey and grape pomace) through the volatile fatty acid (VFA) production in mono- and co-acidogenic fermentation. The highest VFA yields, measured as acetic acid (HAc) per unit of volatile solids (VS), were obtained with the untreated Mix in mono-fermentation (185 ± 57 mg HAc/g VS) and with the combination of Mix and CW in co-fermentation (651 ± 65 mg HAc/g VS), while the highest HAc percentage reached up to 94% of total VFAs. Finally, a preliminary techno-economic evaluation revealed that the mono-fermentation of alkali pretreated HH could lead to the highest revenues among HBRs, reaching up to 710-1810, 618-1577 and 766-3722 €/ha∙year for the production of HAc, single cell protein and polyhydroxybutyrates, respectively.

Novel strategies towards efficient molecular biohydrogen production by dark fermentative mechanism: present progress and future perspective

In the scenario of alarming increase in greenhouse and toxic gas emissions from the burning of conventional fuels, it is high time that the population drifts towards alternative fuel usage to obviate pollution. Hydrogen is an environment-friendly biofuel with high energy content. Several production methods exist to produce hydrogen, but the least energy intensive processes are the fermentative biohydrogen techniques. Dark fermentative biohydrogen production (DFBHP) is a value-added, less energy-consuming process to generate biohydrogen. In this process, biohydrogen can be produced from sugars as well as complex substrates that are generally considered as organic waste. Yet, the process is constrained by many factors such as low hydrogen yield, incomplete conversion of substrates, accumulation of volatile fatty acids which lead to the drop of the system pH resulting in hindered growth and hydrogen production by the bacteria. To circumvent these drawbacks, researchers have come up with several strategies that improve the yield of DFBHP process. These strategies can be classified as preliminary methodologies concerned with the process optimization and the latter that deals with pretreatment of substrate and seed sludge, bioaugmentation, co-culture of bacteria, supplementation of additives, bioreactor design considerations, metabolic engineering, nanotechnology, immobilization of bacteria, etc. This review sums up some of the improvement techniques that profoundly enhance the biohydrogen productivity in a DFBHP process.

Thermophilic Dark Fermentation of Olive Mill Wastewater in Batch Reactors: Effect of pH and Organic Loading

In recent decades, olive oil consumption has almost tripled worldwide. Olive oil production is linked with the production of enormous amounts of olive mill wastewater, the main by-product derived from three-phase olive mills. Due to the environmental risks of olive mill wastewater disposal, the management and valorization of the specific waste stream is of great importance. This work focuses on the thermophilic dark fermentation of olive mill wastewater in batch reactors, targeting pH optimization and the organic loading effect. A series of experiments were performed, during which the organic load of the substrate remained at 40 g/L after dilution with tap water, and the pH was tested in the range of 4.5 to 7.5. The maximum yield in terms of produced hydrogen was obtained at pH 6.0, and the yields were 0.7 mol H2/mol glucose or 0.5 L H2/Lreactor. At the same conditions, a reduction of 62% of the waste’s phenols was achieved. However, concerning the effect of organic loading at the optimized pH value (6.0), a further increase in the organic load minimized the hydrogen production, and the overall process was strongly inhibited.

Olive Mill Waste-Based Anaerobic Digestion as a Source of Local Renewable Energy and Nutrients

This study focused on what combination of anaerobic digestion (AD) temperature (ambient, mesophilic, and thermophilic) and olive mill waste (OMW) to dairy manure (DM) ratio mixture delivers the desired renewable energy and digestate qualities when using AD as olive mill waste treatment. OMW is widespread in the local environment in the North Sinai region, Egypt, which causes many environmental hazards if left without proper treatment. Three different mixtures consisting of OMW, dairy manure (DM), and inoculum (IN) were incubated under ambient, mesophilic, and thermophilic conditions for 45 days. The results showed that mixture B (2:1:2, OMW:DM:IN) at 55 °C produced more methane than at 35 °C and ambient temperature by 40% and 252%, respectively. Another aim of this study was to investigate the effects of the different concentrations of the digestate taken from each mixture on faba bean growth. The results showed that the maximum fresh weight values of the shoot system were observed at 10% and 15% for mixture B at ambient temperature. The best concentration value for the highest root elongation rate is a 5% addition of digestate mixture A at 55 °C, compared with other treatments.

Mixed-culture biocathodes for acetate production from CO2 reduction in the microbial electrosynthesis: Impact of temperature

The temperature effect on bioelectrochemical reduction of CO₂ to acetate with a mixed-culture biocathode in the microbial electrosynthesis was explored. The results showed that maximum acetate amount of 525.84 ± 1.55 mg L^-1 and fastest acetate formation of 49.21 ± 0.49 mg L^-1 d^-1 were obtained under mesophilic conditions. Electron recovery efficiency for CO₂ reduction to acetate ranged from 14.50 ± 2.20% to 64.86 ± 2.20%, due to propionate, butyrate and H₂ generation. Mesophilic conditions were demonstrated to be more favorable for biofilm formation on the cathode, resulting in a stable and dense biofilm. At phylum level, the relative abundance of Bacteroidetes phylum in the biofilm remarkably increased under mesophilic conditions, compared with that at psychrophilic and thermophilic conditions. At genus level, the Clostridium, Treponema, Acidithiobacillus, Acetobacterium and Acetoanaerobium were found to be dominated genera in the biofilm under mesophilic conditions, while genera diversity decreased under psychrophilic and thermophilic conditions.

Regulation of volatile fatty acid accumulation from waste: Effect of inoculum pretreatment

The study investigates the implications of waste feedstock, inoculum origin, and pretreatment on volatile fatty acids accumulation (VFA). The acidogenic fermentation of the feedstocks, rice mill effluent (RME), and brewery effluent (BE) was studied using untreated and pretreated (cyclic heat-acid shock) brewery anaerobic sludge as inoculum. The pretreatment was successful in refining and stabilizing VFA production from the feedstocks. The fermentation of RME with pretreated sludge had an enhanced acetate yield of 0.37 ± 0.02 mgCOD/mgCOD, even to odd ratio of 20.97 ± 0.08 mg/mg and the highest butyrate yield of 0.39 ± 0.01 mgCOD/mgCOD compared to untreated system. The pretreated system had stability in COD and pH profile, while VFA content depends on the origin of inoculum. Pretreatment inhibited the carbon sinks and augmented acetate-butyrate type metabolism with stable performance. The fermentation of RME by pretreated sludge produced a higher even-numbered VFAs and enhanced even to odd ratio in comparison with fermentation of BE, thereby affecting polymer composition and property. PRACTITIONER POINTS: The pretreated system had stable acidification, chemical oxygen demand, and pH profile. The pretreated system had higher acetate and butyrate yield compared to the untreated system. Rice mill effluent acidified with pretreated sludge had the highest even to odd ratio, 20.97 mg/mg. The even to odd ratio for acidification of brewery effluent was insignificant. Pretreatment, the origin of sludge, and the effluent had a regulatory effect on acidification.

Dark Fermentation of Sweet Sorghum Stalks, Cheese Whey and Cow Manure Mixture: Effect of pH, Pretreatment and Organic Load

The aim of this study was to determine the optimal conditions for dark fermentation using agro-industrial liquid wastewaters mixed with sweet sorghum stalks (i.e., 55% sorghum, 40% cheese whey, and 5% liquid cow manure). Batch experiments were performed to investigate the effect of controlled pH (5.0, 5.5, 6.0, 6.5) on the production of bio-hydrogen and volatile fatty acids. According to the obtained results, the maximum hydrogen yield of 0.52 mol H2/mol eq. glucose was measured at pH 5.5 accompanied by the highest volatile fatty acids production, whereas similar hydrogen productivity was also observed at pH 6.0 and 6.5. The use of heat-treated anaerobic sludge as inoculum had a positive impact on bio-hydrogen production, exhibiting an increased yield of 1.09 mol H2/mol eq. glucose. On the other hand, the pretreated (ensiled) sorghum, instead of a fresh one, led to a lower hydrogen production, while the organic load decrease did not affect the process performance. In all experiments, the main fermentation end-products were volatile fatty acids (i.e., acetic, propionic, butyric), ethanol and lactic acid.

Dynamics of bacterial communities and substrate conversion during olive-mill waste dark fermentation: Prediction of the metabolic routes for hydrogen production

The aim of this work was to study the biological catalysts and possible substrate conversion routes in mesophilic dark fermentation reactors aimed at producing H₂ from olive mill wastewater. Bacillus and Clostridium were the most abundant phylotypes during the rapid stage of H₂ production. Chemical analyses combined with predictive functional profiling of the bacterial communities indicated that the lactate fermentation was the main H₂-producing route. In fact, during the fermentation process, lactate and acetate were consumed, while H₂ and butyrate were being produced. The fermentation process was rich in genes that encode enzymes for lactate generation from pyruvate. Lactate conversion to butyrate through the generation of pyruvate produced H₂ through the recycling of electron carriers via the pyruvate ferredoxin oxydoreductase pathway. Overall, these findings showed the synergy among lactate-, acetate- and H₂-producing bacteria, which complex interactions determine the H₂ production routes in the bioreactors.

A Review of Carbon Dots Produced from Biomass Wastes

The fluorescent carbon dot is a novel type of carbon nanomaterial. In comparison with semiconductor quantum dots and fluorescence organic agents, it possesses significant advantages such as excellent photostability and biocompatibility, low cytotoxicity and easy surface functionalization, which endow it a wide application prospect in fields of bioimaging, chemical sensing, environmental monitoring, disease diagnosis and photocatalysis as well. Biomass waste is a good choice for the production of carbon dots owing to its abundance, wide availability, eco-friendly nature and a source of low cost renewable raw materials such as cellulose, hemicellulose, lignin, carbohydrates and proteins, etc. This paper reviews the main sources of biomass waste, the feasibility and superiority of adopting biomass waste as a carbon source for the synthesis of carbon dots, the synthetic approaches of carbon dots from biomass waste and their applications. The advantages and deficiencies of carbon dots from biomass waste and the major influencing factors on their photoluminescence characteristics are summarized and discussed. The challenges and perspectives in the synthesis of carbon dots from biomass wastes are also briefly outlined.

Fruit and vegetable waste management: Conventional and emerging approaches

Valorization of Fruit and Vegetable Wastes (FVW) is challenging owing to logistic-related problems, as well as to their perishable nature and heterogeneity, among other factors. In this work, the main existing routes for food waste valorization are critically reviewed. The study focuses on FVW because they constitute an important potential source for valuable natural products and chemicals. It can be concluded that FVW management can be carried out following different processing routes, though nowadays the best solution is to find an adequate balance between conventional waste management methods and some emerging valorization technologies. Presently, both conventional and emerging technologies must be considered in a coordinated manner to enable an integral management of FVW. By doing so, impacts on food safety and on the environment can be minimized whilst wasting of natural resources is avoided. Depending on the characteristics of FVW and on the existing market demand, the most relevant valorization options are extraction of bioactive compounds, production of enzymes and exopolysaccharides, synthesis of bioplastics and biopolymers and production of biofuels. The most efficient emergent processing technologies must be promoted in the long term, in detriment of the conventional ones used nowadays. In consequence, future integral valorization of FVW will probably comprise two stages: direct processing of FVW into value-added products, followed by processing of the residual streams, byproducts and leftover matter by means of conventional waste management technologies.

Development of novel strategies for higher fermentative biohydrogen recovery along with novel metabolites from organic wastes: The present state of the art

Depletion of fossil fuels and environmental concern has compelled us to search for alternative fuel. Hydrogen is considered as a dream fuel as it has high energy content (142 kJ g^-1 ) and is not chemically bound to carbon. At present, fossil fuel-based methods for producing hydrogen require high-energy input, which makes the processes expensive. The major processes for biohydrogen production are biophotolysis, microbial electrolysis, dark fermentation, and photofermentation. Fermentative hydrogen production has the additional advantages of potentially using various waste streams from different industries as feedstock. Novel strategies to enhance the productivity of fermentative hydrogen production include optimization in pretreatment methods, integrated fermentation systems (sequential and combined fermentation), use of nanoparticles as additives, metabolic engineering of microorganisms, improving the light utilization efficiency, developing more efficient photobioreactors, etc. More focus has been given to produce biohydrogen in a biorefinery approach in which, along with hydrogen gas, other metabolites (ethanol, butyric acid, 1,3-propanediol, etc.) are also produced, which have direct/indirect industrial applications. In present review, various emerging technologies that highlight biohydrogen production methods as effective and sustainable methods on a large scale have been critically reviewed. The possible future developments are also outlined.

Effect of Inoculum Pretreatment on the Composition of Microbial Communities in Anaerobic Digesters Producing Volatile Fatty Acids

Volatile fatty acids (VFAs) are intermediates in the methane formation pathway of anaerobic digestion and can be produced through the fermentation of organic wastes. VFAs have become an anticipated resource- and cost-effective way to replace fossil resources with higher added value and more versatile fuels and chemicals. However, there are still challenges in the production of targeted compounds from diverse and complex biomasses, such as urban biowastes. In this study, the aim was to modulate the microbial communities through inoculum treatment to enhance the production of green chemicals. Thermal and freeze-thaw treatments were applied to the anaerobic digester inoculum to inhibit the growth of methanogens and to enhance the performance of acidogenic and acetogenic bacteria. VFA fermentation after different inoculum treatments was studied in batch scale using urban biowaste as the substrate and the process performance was assessed with chemical and microbial analyses. Inoculum treatments, especially thermal treatment, were shown to increase VFA yields, which were also correlating with the dynamics of the microbial communities and retention times of the test. There was a strong correlation between VFA production and the relative abundances of the microbial orders Clostridiales (families Ruminococcaceae, Lachnospiraceae and Clostridiaceae), and Lactobacillales. A syntrophic relationship of these taxa with members of the Methanobacteriales order was also presumed.

Biohythane production from food processing wastes - Challenges and perspectives

The food industry generates enormous quantity of food waste (FW) either directly or indirectly including the processing sector, which turned into biofuels for waste remediation. Six types of food processing wastes (FPW) such as oil, fruit and vegetable, dairy, brewery, livestock and finally agriculture based materials that get treated via dark fermentation/anaerobic digestion has been discussed. Production of both hydrogen and methane is daunting for oil, fruit and vegetable processing wastes because of the presence of polyphenols and essential oils. Moreover, acidic pH and high protein are the reasons for increased concentration of ammonia and accumulation of volatile fatty acids in FPW, especially in dairy, brewery, and livestock waste streams. Moreover, the review brought to forefront the enhancing methods, (pretreatment and co-digestion) operational, and environmental parameters that can influence the production of biohythane. Finally, the nature of feedstock's role in achieving successful circular bio economy is also highlighted.

Implementation of artificial neural network model for continuous hydrogen production using confectionery wastewater

In the present study, an artificial neural network (ANN) was implemented to estimate the hydrogen production from confectionery wastewater. From the experimental investigation, it could be concluded that maximum COD removal efficiency of 99% and hydrogen production rate of 6570 mL/d was achieved at 7.00 kg COD/m³d and 24 h HRT. To validate this, a back propagation ANN configuration of 4-12-4-2 was opted. The modelling was performed using the input parameters like time, influent chemical oxygen demand (COD), effluent pH and volatile fatty acids (VFA). The correlation coefficient between the experimental and predicted hydrogen production rate was 0.996. The result of the tested data for hydrogen production rate was successful. The calculated average percentage error (APE) for hydrogen production rate was 0.0004. As the APE values were closer to zero, the trained ANN model fitted well with the experimental data.

Fermentative Conversion of Two-Step Pre-Treated Lignocellulosic Biomass to Hydrogen

Fermentative hydrogen production via dark fermentation with the application of lignocellulosic biomass requires a multistep pre-treatment procedure, due to the complexed structure of the raw material. Hence, the comparison of the hydrogen productivity potential of different lignocellulosic materials (LCMs) in relation to the lignocellulosic biomass composition is often considered as an interesting field of research. In this study, several types of biomass, representing woods, cereals and grass were processed by means of mechanical pre-treatment and alkaline and enzymatic hydrolysis. Hydrolysates were used in fermentative hydrogen production via dark fermentation process with Enterobacter aerogenes (model organism). The differences in the hydrogen productivity regarding different materials hydrolysates were analyzed using chemometric methods with respect to a wide dataset collected throughout this study. Hydrogen formation, as expected, was positively correlated with glucose concentration and total reducing sugars amount (YTRS) in enzymatic hydrolysates of LCMs, and negatively correlated with concentrations of enzymatic inhibitors i.e., HMF, furfural and total phenolic compounds in alkaline-hydrolysates LCMs, respectively. Interestingly, high hydrogen productivity was positively correlated with lignin content in raw LCMs and smaller mass loss of LCM after pre-treatment step. Besides results of chemometric analysis, the presented data analysis seems to confirm that the structure and chemical composition of lignin and hemicellulose present in the lignocellulosic material is more important to design the process of its bioconversion than the proportion between the cellulose, hemicellulose and lignin content in this material. For analyzed LCMs we found remarkable higher potential of hydrogen production via bioconversion process of woods i.e., beech (24.01 mL H2/g biomass), energetic poplar (23.41 mL H2/g biomass) or energetic willow (25.44 mL H2/g biomass) than for cereals i.e., triticale (17.82 mL H2/g biomass) and corn (14.37 mL H2/g biomass) or for meadow grass (7.22 mL H2/g biomass).

Recent Progresses in Application of Membrane Bioreactors in Production of Biohydrogen

Biohydrogen is a clean and viable energy carrier generated through various green and renewable energy sources such as biomass. This review focused on the application of membrane bioreactors (MBRs), emphasizing the combination of these devices with biological processes, for bio-derived hydrogen production. Direct biophotolysis, indirect biophotolysis, photo-fermentation, dark fermentation, and conventional techniques are discussed as the common methods of biohydrogen production. The anaerobic process membrane bioreactors (AnMBRs) technology is presented and discussed as a preferable choice for producing biohydrogen due to its low cost and the ability of overcoming problems posed by carbon emissions. General features of AnMBRs and operational parameters are comprehensively overviewed. Although MBRs are being used as a well-established and mature technology with many full-scale plants around the world, membrane fouling still remains a serious obstacle and a future challenge. Therefore, this review highlights the main benefits and drawbacks of MBRs application, also discussing the comparison between organic and inorganic membranes utilization to determine which may constitute the best solution for providing pure hydrogen. Nevertheless, research is still needed to overcome remaining barriers to practical applications such as low yields and production rates, and to identify biohydrogen as one of the most appealing renewable energies in the future.

Nutrients balance for hydrogen potential upgrading from fruit and vegetable peels via fermentation process

The sole, dual and multi-fermentations of fruit and vegetable peels (FVPs) were investigated in order to balance nutrition hierarchy for maximizing hydrogen potential via Batch experiments. The highest volumetric hydrogen production of 2.55 ± 0.07 L/L and hydrogen content of 64.7 ± 3.7% were registered for multi-fermentation of M-PTBO (25% pea +25% tomato + 25% banana +25% orange). These values outperformed sole and dual fermentation. The multi-fermentation of FVPs provided sufficient nutrients and trace elements for anaerobes, where C/N and C/P ratios were at levels of 24.7 ± 0.2 and 113.2 ± 9.4, respectively. In specific, harmonizing of macro and micro-nutrients remarkably maximized activities of amylase, protease and lipase to 4.23 ± 0.42, 0.035 ± 0.002 and 0.31 ± 0.02 U/mL, respectively, as well as, substantially incremented counts of Clostridium and Enterobacter sp. up to 5.81 ± 0.23 × 10⁵ and 2.17 ± 0.09 × 10⁶ cfu/mL, respectively. Furthermore, multi-fermentation of M-PTBO achieved the maximum net energy gain and profit of 1.82 kJ/g_feedstock and 4.11 $/kg_feedstock, respectively. Nutrients balance significantly develops bacterial activity in terms of hydrogen productivity, anaerobes reproduction, enzyme activities and soluble metabolites. As a result, overall fermentation bioprocess performance was improved.

Sustainable bio-hydrothermal sequencing treatment for asbestos-cement wastes

In this paper, the treatment of asbestos-cement waste (ACW) has been attempted by a dark fermentation (DF) pre-treatment followed by hydrothermal and anaerobic digestion (AD) treatments. During DF, glucose, employed as a biodegradable substrate, was mainly converted to H₂-rich biogas and organic acids (OAs). The latter caused the dissolution of the cement matrix and the partial structural collapse of chrysotile (white asbestos). To complete the chrysotile degradation, hydrothermal treatment of the DF effluents was performed under varying operating conditions (temperature, acid type, and load). After the addition of 5.0 g/L sulfuric acid, a temperature decrease, from 80 °C to 40 °C, slowed down the treatment. Similarly, at 100 °C, a decrease of sulfuric, lactic or malic acid load from 5.0 g/L to 1.0 g/L slowed down the process, regardless of acid type. The acid type did not affect the hydrothermal treatment but influenced the AD of the hydrothermal effluents. Indeed, when malic acid was used, the AD of the hydrothermally treated effluents resulted in the highest production of methane. At the end of the AD treatment, some magnesium ions derived from ACW dissolution participated in the crystallization of struvite, an ecofriendly phosphorous-based fertilizer.

Anaerobic Process for Bioenergy Recovery From Dairy Waste: Meta-Analysis and Enumeration of Microbial Community Related to Intermediates Production

Dairy wastes are widely studied for the hydrogen and methane production, otherwise the changes in microbial communities related to intermediate valuable products was not deeply investigated. Culture independent techniques are useful tools for exploring microbial communities in engineered system having new insights into their structure and function as well as potential industrial application. The deep knowledge of the microbiota involved in the anaerobic process of specific waste and by-products represents an essential step to better understand the entire process and the relation of each microbial population with biochemical intermediates and final products. Therefore, this study investigated the microbial communities involved in the laboratory-scale anaerobic digestion of a mixture of mozzarella cheese whey and buttermilk amended with 5% w/v of industrial animal manure pellets. Culture-independent methods by employing high-throughput sequencing and microbial enumerations highlighted that lactic acid bacteria, such as Lactobacillaceae and Streptococcaceae dominated the beginning of the process until about day 14 when a relevant increase in hydrogen production (more than 10 ml H₂ gVS^-1 from days 13 to 14) was observed. Furthermore, during incubation a gradual decrease of lactic acid bacteria was detected with a simultaneous increase of Clostridia, such as Clostridiaceae and Tissierellaceae families. Moreover, archaeal populations in the biosystem were strongly related to inoculum since the non-inoculated samples of the dairy waste mixture had a relative abundance of archaea less than 0.1%; whereas, in the inoculated samples of the same mixture several archaeal genera were identified. Among methanogenic archaea, Methanoculleus was the dominant genus during all the process especially when the methane production occurred, and its relative abundance increased up to 99% at the end of the incubation time highlighting that methane was formed from dairy wastes primarily by the hydrogenotrophic pathway in the reactors.

Dark fermentation metabolic models to study strategies for hydrogen consumers inhibition

A Flux Balance Analysis (FBA) metabolic model of dark fermentation was developed for anaerobic mixed cultures. In particular, the model was applied to evaluate the effect of a specific inoculum pre-treatment strategy, addition of waste frying oil (WFO) on H₂-producing and H₂-consuming metabolic pathways. Productions of volatile fatty acid (VFAs), CO₂, H₂ and CH₄ measured through triplicate batch experiments, were used as constraints for the FBA model, to compute fluxes trough different metabolic pathways. FBA model could estimate the effect of pre-treatment with WFO on major microbial populations present in the mixed community (H₂ producing bacteria, homoacetogen and methanogens). Results revealed that low concentrations of WFO did not completely inhibited hydrogenotrophic methanogens. FBA showed that acetoclastic methanogens were more sensitive to WFO, in comparison to hydrogenotrophic methanogens. The proposed model can be used to study H₂ production by any other mixed microbial culture with similar substrates.

Development of a production chain from vegetable biowaste to platform chemicals

BACKGROUND

A future bioeconomy relies on the development of technologies to convert waste into valuable compounds. We present here an attempt to design a biotechnological cascade for the conversion of vegetable waste into acetoin and electrical energy.

RESULTS

A vegetable waste dark fermentation effluent containing mainly acetate, butyrate and propionate was oxidized in a bioelectrochemical system. The achieved average current at a constant anode potential of 0 mV against standard hydrogen electrode was 177.5 ± 52.5 µA/cm². During this step, acetate and butyrate were removed from the effluent while propionate was the major remaining component of the total organic carbon content comprising on average 75.6%. The key players with regard to carbon oxidation and electrode reduction were revealed using amplicon sequencing and metatranscriptomic analysis. Using nanofiltration, it was possible to concentrate the propionate in the effluent. The effluent was revealed to be a suitable medium for biotechnological production strains. As a proof of principle, the propionate in the effluent of the bioelectrochemical system was converted into the platform chemical acetoin with a carbon recovery of 86%.

CONCLUSIONS

To the best of our knowledge this is the first report on a full biotechnological production chain leading from vegetable waste to the production of a single valuable platform chemical that integrates carbon elimination steps leading to the production of the valuable side product electrical energy.

Dark fermentation process as pretreatment for a sustainable denaturation of asbestos containing wastes

A cement asbestos compound (CAC) sample was detoxified by a treatment train based on a dark fermentation (DF) process followed by a hydrothermal phase, which led to the complete degradation of the chrysotile fibers. During the biological pretreatment, the glucose was converted in biogas rich in H₂ and volatile fatty acids (VFA). The latter caused the dissolution of all the Ca-based compounds and the solubilisation of 50% brucite-like layers of chrysotile fibers contained in the CAC suspended in the bioreactor (5 g/L). XRD analysis of the solids contained in the effluents of the DF process highlighted the disappearance of the chrysotile fiber peaks. However, a complete destruction of all the asbestos fibers is hard to prove and a hydrothermal treatment was carried out to dissolve the "brucite" layers still present in solution. Due to the presence of the VFA produced during the DF, a complete destruction of chrysotile fibers was achieved by a 24 h hydrothermal process performed with a [H₂SO₄]/[CAC] ratio 50% lower than that adopted in a previous finding. Consequently, the DF pre-treatment can contribute to lower the H₂SO₄ and the energy consumption of a CAC hydrothermal treatment, due to the production of VFA and H₂.

Pre-treating anaerobic mixed microflora with waste frying oil: A novel method to inhibit hydrogen consumption

An innovative method was introduced to inhibit methanogenic H₂ consumption during dark fermentative hydrogen production by anaerobic mixed cultures. Waste frying oil was used as an inhibitor for hydrogenotrophic methanogens. Simultaneous effect of waste frying oil concentrations (0-20 g/L) and initial pH (5.5, 6.5 and 7.5) on inhibition of methanogenic H₂ consumption and enhancement of H₂ accumulation were investigated using glucose as substrate. Enhanced hydrogen yields with decreased methane productions were observed with increasing the waste frying oil concentrations. On average, CH₄ productions from glucose in the cultures received 10 g/L WFO were reduced by 88%. Increased WFO concentration up to 20 g/L led to negligible CH₄ productions and in turn enhanced H₂ yields. Hydrogen yields of 209.26, 195.35 and 185.60 mL/g glucose_added were obtained for the cultures pre-treated with 20 g/L waste frying oil with initial pH of 5.5, 6.5 and 7.5 respectively. H₂ production by pre-treated cultures was also studied using a synthetic food waste. Anaerobic mixed cultures were pre-treated with 10 g/L WFO and varying durations (0, 24 and 48 h). A H₂ yield of 71.46 mL/g VS was obtained for cultures pre-treated with 10 g/L WFO for 48 h that was 475% higher than untreated control. This study suggests a novel and inexpensive approach for suppressing hydrogenotrophic methanogens during dark fermentative H₂ production.

Pre-treatment technologies for dark fermentative hydrogen production: Current advances and future directions

Hydrogen is regarded as a clean and non-carbon fuel and it has a higher energy content compared to carbon fuels. Dark fermentative hydrogen production from organic wastes is the most promising technology for commercialization among chemical and biological methods. Using mixed microflora is favored in terms of easier process control and substrate conversion efficiencies instead of pure cultures. However, mixed cultures should be first pre-treated in order to select sporulating hydrogen producing bacteria and suppress non-spore forming hydrogen consumers. Various inoculum pre-treatments have been used to enhance hydrogen production by dark fermentation including heat shock, acid or alkaline treatment, chemical inhibition, aeration, irradiation and inhibition by long chain fatty acids. Regarding substrate pre-treatment, that is performed with the aim of enhanced substrate biodegradability, thermal pre-treatment, pH adjustment using acid or base, microwave irradiation, sonication and biological treatment are the most commonly studied technologies. This article reviews the most investigated pre-treatment technologies applied for either inoculum or substrate prior to dark fermentation, the long-term effects of varying pre-treatment methods and the subsequently feasibility of each method for commercialization.

Modified Sample Preparation Approach for the Determination of the Phenolic and Humic-Like Substances in Natural Organic Materials By the Folin Ciocalteu Method

A novel modification of the sample preparation procedure for the Folin-Ciocalteu colorimetric assay for the determination of total phenolic compounds in natural solid and semisolid organic materials (e.g., foods, organic solid waste, soils, plant tissues, agricultural residues, manure) is proposed. In this method, the sample is prepared by adding sodium sulfate as a solid diluting agent before homogenization. The method allows for the determination of total phenols (TP) in samples with high solids contents, and it provides good accuracy and reproducibility. Additionally, this method permits analyses of significant amounts of sample, which reduces problems related to heterogeneity. We applied this method to phenols-rich lignocellulosic and humic-like solids and semisolid samples, including rice straw (RS), peat-rich soil (PS), and food waste (FW). The TP concentrations measured with the solid dilution (SD) preparation were substantially higher (increases of 41.4%, 15.5%, and 59.4% in RS, PS and FW, respectively) than those obtained with the traditional method (solids suspended in water). These results showed that the traditional method underestimates the phenolic contents in the studied solids.

Selective enrichment of electrogenic bacteria for fuel cell application: Enumerating microbial dynamics using MiSeq platform

This study is intended to examine the effect of pretreatment on selective enrichment of electrogenic bacteria from mixed culture. It has been observed that the iodopropane and heat-shock pretreatments suppress the growth of non-exoelectrons, while selecting only a limited number of strains belonging to genera Xanthomonas, Pseudomonas and Prevotella while untreated control inoculum showed more diverse community comprising of both exoelectrogens and non-exoelectrogens. High power output was observed in iodopropane (180mW/m(2)) pretreated microbial fuel cell (MFC) compared to heat-shock pretreated MFC (128mW/m(2)) and untreated control (92mW/m(2)). Coulombic efficiency of iodopropane and heat-shock pretreated MFC was higher compared to untreated control MFC, while drop in pH and volatile fatty acids (VFA) production was less in iodopropane pretreated MFC signifying the shifts in bacterial community structure toward electrogenesis instead of fermentation. These results signify the role of iodopropane and heat pretreatments on enrichment of electrogenic bacteria for fuel cell application.

Enhanced biohydrogen and subsequent biomethane production from sugarcane bagasse using nano-titanium dioxide pretreatment

Nano-titanium dioxide (nanoTiO2) under ultraviolet irradiation (UV) followed by dilute sulfuric acid hydrolysis of sugarcane bagasse was used to enhance the production of biohydrogen and biomethane in a consecutive dark fermentation and anaerobic digestion. Different concentrations of 0.001, 0.01, 0.1 and 1g nanoTiO2/L under different UV times of 30, 60, 90 and 120min were used. Sulfuric acid (2%v/v) at 121°C was used for 15, 30 and 60min to hydrolyze the pretreated bagasse. For acidic hydrolysis times of 15, 30 and 60min, the highest total free sugar values were enhanced by 260%, 107%, and 189%, respectively, compared to samples without nanoTiO2 pretreatment. The highest hydrogen production samples for the same acidic hydrolysis times showed 88%, 127%, and 25% enhancement. The maximum hydrogen production of 101.5ml/g VS (volatile solids) was obtained at 1g nanoTiO2/L and 120min UV irradiation followed by 30min acid hydrolysis.

Energy conversion of biomass crops and agroindustrial residues by combined biohydrogen/biomethane system and anaerobic digestion

Aim of this study was to evaluate the suitability of ensiled giant reed, ensiled maize, ensiled olive pomace, wheat bran for combined systems (CS: dark fermentation+anaerobic digestion (AD)) producing hydrogen-rich biogas (biohythane), tested in batch under basic operational conditions (mesophilic temperatures, no pH control). Substrates were also analyzed under a single stage AD batch test, in order to investigate the effects of DF on estimated energy recovery (ER) in combined systems. In CS, maize and wheat bran exhibited the highest hydrogen potential (13.8 and 18.9NLkgVS(-1)) and wheat bran the highest methane potential (243.5NLkgVS(-1)). In one-stage AD, giant reed, maize and wheat bran showed the highest methane production (239.5, 267.3 and 260.0NLkgVS(-1)). Butyrate/acetate ratio properly described the dark fermentation, correlating with hydrogen production (r=0.92). Wheat bran proved to be a promising residue for CS in terms of hydrogen/methane potential and ER.