Biogasification of solid wastes by two-phase anaerobic fermentation

Municipal, industrial and agricultural solid wastes, and biomass deposits cause large-scale pollution of land and water. Gaseous products of waste decomposition pollute the air and contribute to global warming. This paper describes the development of a two-phase fermentation system that alleviates methanogenic inhibition encountered with high-solids feeds, accelerates methane fermentation of solid bed, and captures methane (renewable energy) for captive use to reduce global warming. The innovative system consisted of a solid-bed reactor packed with simulated solid waste at a density of 160 kg/m³ and operated with recirculation of the percolated culture (bioleachate) through the bed. A rapid onset of solids hydrolysis, acidification, denitrification and hydrogen gas formation was observed under these operating conditions. However, these fermentative reactions stopped after about 2.5 months of solid-bed fermentation at which time total volatile fatty acids (VFA) concentration accumulated to 13,000 mg/l (as acetic) at pH 5, and the reactor head-gas consisted of 75% carbon dioxide, 20% nitrogen, 2% hydrogen and 3% methane. The VFA concentration and gas composition remained virtually constant for an additional 2.5 months of solid-bed fermentation indicating inhibition of the hydrolysis–acidification process. Inhibition of acidogenic fermentation was alleviated by moving the bioleachate to a separate methane-phase fermenter, and recycling methanogenic effluents at pH 7.5 to the solid bed. Coupled operation of the two reactors during the following 4.5 months of two-phase fermentation achieved methanogenic conversion of about 30% of the volatile solids (VS) content of the high-solids feed. Process operation was continuing.     

Direct hydrogen production from cellulosic waste materials with a single-step dark fermentation process

Biohydrogen production from cellulosic waste materials using dark fermentation is a promising technology for producing renewable energy. The purpose of this study was to evaluate residual cellulosic materials generated from local sources for their H₂ production potential without any pretreatment. Clostridium thermocellum ATCC 27405, a cellulolytic, thermophilic bacterium that has been shown to be capable of H₂ production on both cellobiose and α-cellulose substrates, was used in simultaneous batch fermentation experiments with dried distillers grain (DDGs), barley hulls (BH) and fusarium head blight contaminated barley hulls (CBH) as the carbon source. Overall, the dried distillers grain produced the highest concentration of hydrogen gas at 1.27 mmol H₂/glucose equivalent utilized. CBH and BH produced 1.18 and 1.24 mmol H₂/glucose equivalent utilized, respectively. Overall, this study indicates that hydrogen derived from a variety of cellulosic waste biomass sources is a possible candidate for the development of sustainable energy.     

Electro-stimulated biological production of hydrogen from municipal solid waste

Hydrogen production from municipal solid wastes was investigated by applying a weak current (0.06 A) to a slurry of municipal solid waste in an anaerobic reactor at 55 °C using 4 electrodes (carbon graphite for the cathode and platinum electroplated titanium for the anode). Current application to the organic waste stimulated the hydrogen producing bacteria especially bacteria related to the Thermotogales and Bacillus families. Measured hydrogen production rates were comprised between 16 and 41 mL/h. Comparison of bacterial and archaeal communities in methane-producing (control) and electro-stimulated reactors showed similar species but with different dynamics correlated to hydrogen or methane production. Energy efficiency of the overall bioelectrolysis process using municipal solid waste and an applied voltage of 3V was approximately 12.4%, which is relatively low compared to values reported in the literature for organic wastes and can be explained by the low organic carbon content and availability in the municipal solid waste. Results of this study highlight some important operational constraints with respect to electro-stimulated hydrogen production from organic wastes; related in particular to electrode lifetime expectancies. Results nevertheless illustrate the potential for hydrogen production from municipal solid waste as a possible route for energy recovery.     

Efficient hydrogen gas production from cassava and food waste by a two-step process of dark fermentation and photo-fermentation

A two-step process of sequential anaerobic (dark) and photo-heterotrophic fermentation was employed to produce hydrogen from cassava and food waste. In dark fermentation, the average yield of hydrogen was approximately 199 ml H₂ g⁻¹ cassava and 220 ml H₂ g⁻¹ food waste. In subsequent photo-fermentation, the average yield of hydrogen from the effluent of dark fermentation was approximately 611 ml H₂ g⁻¹ cassava and 451 ml H₂ g⁻¹ food waste. The total hydrogen yield in the two-step process was estimated as 810 ml H₂ g⁻¹ cassava and 671 ml H₂ g⁻¹ food waste. Meanwhile, the COD decreased greatly with a removal efficiency of 84.3% in cassava batch and 80.2% in food waste batch. These results demonstrate that cassava and food waste could be ideal substrates for bio-hydrogen production. And a two-step process combining dark fermentation and photo-fermentation was highly improving both bio-hydrogen production and removal of substrates and fatty acids.     

Parametric optimization of the dark fermentation process for enhanced biohydrogen production from the organic fraction of municipal solid waste using Taguchi method

In this work, the Taguchi method was used to optimize the dark fermentative H₂ production from the organic fraction of municipal solid waste (OFMSW). The experiments were planned using the L16 orthogonal array design with each trial conducted at different levels of substrate concentration, inoculum-to-substrate ratio (ISR), and temperature. Based on the results, the optimal setting of the process parameters was the substrate concentration of 6 g-VS/L, ISR 0.5, and temperature of 55 °C. Furthermore, substrate concentration was the most important parameter affecting bio-H₂ production among the three process parameters considered. Finally, a confirmation experiment under optimal conditions yielded 62.5 mL H₂/g-VS_added, which was higher than all the bio-H₂ yield values obtained in the other conditions tested in this study. The measured and predicted bio-H₂ yields in the verification test were also very close to each other, confirming the reliability of the Taguchi method in optimizing the bio-H₂ production process.     

Hydrogen production from agricultural waste by dark fermentation: A review

The degradation of the natural environment and the energy crisis are two vital issues for sustainable development worldwide. Hydrogen is considered as one of the most promising candidates as a substitute for fossil fuels. In this context, biological processes are considered as the most environmentally friendly alternatives for satisfying future hydrogen demands. In particular, biohydrogen production from agricultural waste is very advantageous since agri-wastes are abundant, cheap, renewable and highly biodegradable. Considering that such wastes are complex substrates and can be degraded biologically by complex microbial ecosystems, the present paper focuses on dark fermentation as a key technology for producing hydrogen from crop residues, livestock waste and food waste. In this review, recent findings on biohydrogen production from agricultural wastes by dark fermentation are reported. Key operational parameters such as pH, partial pressure, temperature and microbial actors are discussed to facilitate further research in this domain.     

Air-steam gasification of municipal solid wastes (MSWs) for hydrogen production

Gasification process can be considered as a partial thermal oxidation, which results in the production of a mixture of useful gases (CO, H₂, CH₄, and other gaseous hydrocarbons), little quantities of carbon black (char), ash, and several organic impurities (tar). In this study, we introduced an artificial neural network (ANN) model to simulate the influence of operating conditions on the concentration of products during the gasification process of municipal solid wastes (MSW). Results showed when increasing the residence time, more char is gasified, leading to an increase in the greenhouse gas emissions. It is also found that a further increase in the residence time results in a constant rate of products due to the heat and mass transfer limitations.     

Biohydrogen production from biomass and industrial wastes by dark fermentation

Hydrogen is a clean energy carrier which has a great potential to be an alternative fuel. Abundant biomass from various industries could be a source for biohydrogen production where combination of waste treatment and energy production would be an advantage. This article summarizes the dark fermentative biohydrogen production from biomass. Types of potential biomass that could be the source for biohydrogen generation such as food and starch-based wastes, cellulosic materials, dairy wastes, palm oil mill effluent and glycerol are discussed in this article. Moreover, the microorganisms, factors affecting biohydrogen production such as undissociated acid, hydrogen partial pressure and metal ions are also discussed.     

A multivariable evaluation of biohydrogen production by solid substrate fermentation of organic municipal wastes in semi-continuous and batch operation

This work focused on the hydrogen production from the organic fraction of municipal solid waste (OFMSW) in solid substrate fermentation (SSF) with a double purpose: (i) to evaluate the effect of the total solids content (20.9 and 35% TS), temperature (35 and 55 °C) and mass retention time (MRT, 21 and 14 d) on semi-continuous fermentation, and (ii) to test the supplementation of OFMSW with nutrient nitrogen in the form of waste activated sludge in batch mini-reactors.     

Hydrogen production characteristics of the organic fraction of municipal solid wastes by anaerobic mixed culture fermentation

The hydrogen production from the organic fraction of municipal solid waste (OFMSW) by anaerobic mixed culture fermentation was investigated using batch experiments at 37 °C. Seven varieties of typical individual components of OFMSW including rice, potato, lettuce, lean meat, oil, fat and banyan leaves were selected to estimate the hydrogen production potential. Experimental results showed that the boiling treated anaerobic sludge was effective mixed inoculum for fermentative hydrogen production from OFMSW. Mechanism of fermentative hydrogen production indicates that, among the OFMSW, carbohydrates is the most optimal substrate for fermentative hydrogen production compared with proteins, lipids and lignocelluloses. This conclusion was also substantiated by experimental results of this study. The hydrogen production potentials of rice, potato and lettuce were 134 mL/g-VS, 106 mL/g-VS, and 50 mL/g-VS respectively. The hydrogen percentages of the total gas produced from rice, potato and lettuce were 57–70%, 41–55% and 37–67%.     

Enhancement of hydrogen production by recycling of methanogenic effluent in two-phase fermentation of food waste

The feasibility of operational strategies was investigated for hydrogen and methane production from food waste. Food waste was heat-treated at 70 °C and fed to a two-phase anaerobic sequencing batch fermenting system. Maximum hydrogen productivity of 1.19 m³ H₂/m³ d was observed at a food waste concentration of 30 g carbohydrate/L, a hydraulic retention time of 2 d, and a solids retention time of 3.4 d. The effluent from hydrogenesis was efficiently converted to methane at an organic loading rate of up to 3.6 kg COD/m³.d. The methanogenic effluent was then recycled to the hydrogenesis reactor without any pretreatment. The recycled effluent not only successfully replaced external dilution water and decreased alkaline dosage by 75%, but also increased hydrogen production by 48%, resulting in hydrogen productivity of 1.76 m³/m³ d. The two-phase digestion with recycling would convert 91% of organic pollutants in food waste to hydrogen (8%) and methane (83%) without any external dilution water.     

Fungal solid-state fermentation of food waste for biohydrogen production by dark fermentation

The dark fermentation process was evaluated for biohydrogen production from food waste through fungal solid-state fermentation (SSF). Three fungal cultures (one strain of Aspergillus tubingensis and two strains of Meyerozyma caribbica) were compared, being A. tubingensis the best hydrolyser culture for releasing soluble carbohydrates. The biochemical hydrogen potential of food waste hydrolysate (FWH) at different substrate-inoculum ratios obtained a lower hydrogen yield than untreated food waste (RFW). The highest hydrogen yield value corresponded to treatments RFW-20 and RFW-30 with 77.0 ± 2.6 and 76.9 ± 1.4 mL H₂ normalized by per gram volatile solid added (NmL H₂/gVS_added), respectively. The microbial community of food waste was analysed, being detected lactic-acid bacteria genera as Latilactobacillus and Leuconostoc. The presence of actively growing bacteria during the SSF could explain the lowest hydrogen yield (20.1–36.0 NmL H₂/gVS_added) in the FWH treatment due to the substrate competition between lactic-acid bacteria and hydrogen-producing bacteria, where the lactic-acid bacteria were favoured by their faster growth rate.     

Thermophilic dark fermentation of acid hydrolyzed waste ground wheat for hydrogen gas production

Batch dark fermentation experiments were performed to investigate the effects of biomass and substrate concentration on bio-hydrogen production from acid hydrolyzed ground wheat at 55 °C. In the first set of experiments, the substrate concentration was constant at 20 g total sugar L⁻¹ and biomass concentration was varied between 0.52 and 2.58 g L⁻¹. Total sugar concentration was varied between 4.2 and 23.7 g L⁻¹ in the second set of experiments with a 1.5 g L⁻¹ constant biomass concentration. The highest cumulative hydrogen formation (582 mL, 30 °C, 1 atm), formation rate (5.43 mL h⁻¹) and final total volatile fatty acid (TVFA) concentration (6.54 g L⁻¹) were obtained with 1.32 g L⁻¹ biomass concentration. In variable substrate concentration experiments, the highest cumulative hydrogen (365 mL) and TVFA concentration (4.8 g L⁻¹) were obtained with 19.25 g L⁻¹ initial total sugar concentration while hydrogen gas formation rate (12.95 mL h⁻¹) and the yield (200 mL H₂ g⁻¹ total sugar) were the highest with 4.2 g L⁻¹ total sugar concentration.     

Effect of bioaugmentation on hydrogen production from organic fraction of municipal solid waste

In the present study, the effect of bioaugmentation with three bacterial species (i.e. E. coli, Bacillus subtilis and Enterobacter aerogenes) on the hydrogen production from organic fraction of municipal solid waste was evaluated at different bacteria/sludge ratios (0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35 and 0.40). Cumulative hydrogen production, lag phases, and maximum hydrogen production rates were analyzed using modified Gompertz model. The highest cumulative and volumetric hydrogen production of 564.4 ± 10.9 mL and 1.61L_H2/L_substrate respectively was achieved for bioaugmentation with Bacillus subtilis at bacteria/sludge ratio of 0.25. The corresponding highest hydrogen yield was 43.68 mLH₂/gCarbo. For bioaugmentation with E. coli and Enterobacter aerogenes, the maximum cumulative hydrogen production of 423.4 ± 10.6 mL and 486.3 ± 10.6 mL respectively was obtained from bacteria/sludge ratio of 0.20. Corresponding highest hydrogen yields were 32.9 mLH₂/gCarbo and 37.1 mLH₂/gCarbo respectively. Bioaugmentation shortened the lag phases and improved COD removal. Volatile fatty acid generation was also improved with the bioaugmentation.     

Bio-hythane production from food waste by dark fermentation coupled with anaerobic digestion process: A long-term pilot scale experience

In this paper are presented the results of the investigation on optimal process operational conditions of thermophilic dark fermentation and anaerobic digestion of food waste, testing a long-term run, applying an organic loading rate of 16.3 kgTVS/m³d in the first phase and 4.8 kgTVS/m³d in the second phase. The hydraulic retention times (HRTs) were maintained at 3.3 days and 12.6 days, respectively, for the first and second phase. Recirculation of anaerobic digested sludge, after a mild solid separation, was applied to the dark fermentation reactor in order to control the pH in the optimal hydrogen production range of 5–6. It was confirmed the possibility to obtain a stable hydrogen production, without using external chemicals for pH control, in a long-term test, with a specific hydrogen production of 66.7 l per kg of total volatile solid (TVS) fed and a specific biogas production in the second phase of 0.72 m³ per kgTVS fed; the produced biogas presented a typical composition with a stable presence of hydrogen and methane in the biogas mixture around 6 and 58%, respectively, carbon dioxide being the rest.     

Inhibition by the ionic strength of hydrogen production from the organic fraction of municipal solid waste

Composition of the Organic Fraction of Municipal Solid Waste (OFMSW) in organic compounds and inorganic ions is highly variable and might impact the microbial activity in dark fermentation processes. In this study, the effect of the total amount of inorganic ions on fermentative hydrogen production was investigated. Batch experiments were carried out at pH 6 and under a temperature of 37 °C. A freshly reconstituted organic fraction of municipal solid waste (OFMSW) was used as model substrate. At low concentrations in ammonium or chloride ions (2.9–5.1 g L⁻¹, respectively), the hydrogen yield reached a maximum of 40.8 ± 0.5. mLH₂.gVS⁻¹ and 25.1 ± 5.6 mLH₂.gVS⁻¹. In contrast, at high total ionic concentrations of ammonium and chloride (11.1–35.5 g L⁻¹ respectively), a strong inhibition of the fermentative microbial activity and more particularly hydrogen production, was observed. When considering the ionic strength of each ion, the effects of ammonia, chloride or a mixture of different ions (Na⁺, K⁺, H⁺, Li⁺, NH₄⁺, Mn²⁺, NH₄⁺, Mg²⁺, Cl⁻, PO₄³⁻, Br⁻, I⁻, SO₄²⁻) showed very similar inhibitory trends regardless the type of ion or the composition of the ionic mixture. A threshold inhibitory value of the ionic strength was estimated at 0.75 ± 0.13 M with a substantial impact on the fermentative activity from 0.81 ± 0.12 M, with hydrogen yields of 18.1 ± 3.3 and 6.2 ± 4.1 mLH₂.gVS⁻¹, respectively. Microbial community composition was also significantly impacted with a specific decrease in relative abundance of hydrogen-producing bacteria from the genus Clostridium sp. at high ionic strength.     

A bibliometric analysis of the hydrogen production from dark fermentation

Hydrogen energy plays an important role in solving the environmental problems caused by the fuel crisis and greenhouse gas emissions. However, hydrogen application on an industrial scale still requires technological advances, especially in choosing the best technological route for the recovery of renewable and cost-effective hydrogen. Therefore, this bibliometric review evaluated the research progress, trends, updates, and hotspots on hydrogen production from dark fermentation. The Web of Science© database was used to select the documents from 2000 to 2021, and the VOSviewer© and Bibliometrix softwares were used to carry out the bibliometric investigation. The results demonstrated that 3071 documents (2755 articles and 316 reviews) studied the hydrogen production from dark fermentation over the last 21 years. The number of publications exponentially increased in the last five years, which can be associated with the demand for new technologies to produce clean energy sources and decrease the environmental impacts caused by petroleum-based fuel. Keyword analysis revealed that the studies focused on the operational parameters, process optimization, pretreatment, and microbial community, aiming to increase the hydrogen yield during dark fermentation. Finally, this comprehensive review provides future directions for applying dark fermentation to produce hydrogen as a sustainable and renewable fuel in a biorefiney concept.     

Net energy gain from solid organic wastes by dark fermentation and its end products

This study evaluated the feasibility of improving net energy gain from solid organic wastes by dark fermentation (DF) and its aqueous end products. Batch experiments were conducted with dairy cattle manure as a typical solid waste blended with sucrose at different sucrose:manure ratios. This study differs from previous DF studies in two aspects; these experiments were conducted at ambient temperature without any external nutrient supplements or pH control measures while previous studies had resorted to mesophilic conditions, nutrient supplements, and external pH control; this study evaluated the feasibility of DF in terms of net energy yield rather than in terms of hydrogen yield as in previous studies. Hydrogen yields (2.9–5.3 M H₂/M sucrose) and net energy gains (2.0–3.7 kJ/g COD) demonstrated in this study are higher than in previous reports. Based on this study, sucrose:manure ratio of 4.5% is suggested as the optimal ratio.     

Techno-economic feasibility assessment of sorption enhanced gasification of municipal solid waste for hydrogen production

Waste-to-fuel coupled with carbon capture and storage is forecasted to be an effective way to mitigate the greenhouse gas emissions, reduce the waste sent to landfill and, simultaneously, reduce the dependence of fossil fuels. This study evaluated the techno-economic feasibility of sorption enhanced gasification, which involves in-situ CO₂ capture, and benchmarked it with the conventional steam gasification of municipal solid waste for H₂ production. The impact of a gate fee and tax levied on the fossil CO₂ emissions in economic feasibility was assessed. The results showed that the hydrogen production was enhanced in sorption enhanced gasification, that achieved an optimum H₂ production efficiency of 48.7% (T = 650 °C and SBR = 1.8). This was 1.0% points higher than that of the conventional steam gasification (T = 900 °C and SBR = 1.2). However, the total efficiency, which accounts for H₂ production and net power output, for sorption enhanced gasification was estimated to be 49.3% (T = 650 °C and SBR = 1.8). This was 4.4% points lower than the figure estimated for the conventional gasification (T = 900 °C and SBR = 1.2). The economic performance assessment showed that the sorption enhanced gasification will result in a significantly higher levelised cost of hydrogen (5.0 €/kg) compared to that estimated for conventional steam gasification (2.7 €/kg). The levelised cost of hydrogen can be reduced to 4.5 €/kg on an introduction of the gate fee of 40.0 €/t_MSW. The cost of CO₂ avoided was estimated to be 114.9 €/tCO₂ (no gate fee and tax levied). However, this value can be reduced to 90.1 €/tCO₂ with the introduction of an emission allowance price of 39.6 €/tCO₂. Despite better environmental performance, the capital cost of sorption enhanced gasification needs to be reduced for this technology to become competitive with mature gasification technologies.     

Homoacetogenesis during hydrogen production by mixed cultures dark fermentation: Unresolved challenge

This paper is a comprehensive review of H₂ consumption during anaerobic mixed culture H₂ dark fermentation with a focus on homoacetogenesis. Homoacetogenesis consumed from 11% to 43% of the H₂ yield in single and repeated batch fermentations, respectively. However, its quantification and extent during continuous fermentation are still not well understood. Models incorporating thermodynamic and kinetic controls are required to provide insight into the dynamic of homoacetogenesis during H₂ dark fermentation. Currently, no adequate method exists to eliminate homoacetogenesis because it does not depend on the culture's source, pre-treatment, substrate, type of reactor, or operation conditions. Controlling CO₂ concentrations during dark fermentation needs further investigation as a potential strategy towards controlling homoacetogenesis. Incorporating radioactive labeling technique in H₂ fermentation research could provide information on simultaneous production and consumption of H₂ during dark fermentation. Genetic studies investigating blocking H₂ consuming pathways and enhancing H₂ evolving hydrogenases are suggested towards controlling homoacetogenesis during dark fermentation.