Impact of pretreatment on food waste for biohydrogen production: A review

Food waste (FW) can be utilized as a raw material to produce energy such as hydrogen via fermentation, which is a more attractive and environmentally friendly approach compared to incineration and land-filling. Food waste must be pretreated before being used in various biological processes. The choice of the pretreatment method usually depends on the composition of the food waste. Therefore, various pretreatment methods generally employed to treat FW, including physical, physiochemical, chemical and biological pretreatments, are summarized in this review. The different pretreatment methods are compared in terms of their efficiency and biohydrogen yield. Additionally, the energy efficiencies of the various pretreatment methods are compared, thereby leading to the selection of the most efficient pretreatment method.     

Cladophora sp. as a sustainable feedstock for dark fermentative biohydrogen production

Macroalgae are rich in carbohydrates which can be used as a promising substrate for fermentative biohydrogen production. In this study, Cladophora sp. biomass was fermented for biohydrogen production at various inoculum/substrate (I/S) ratios against a control of inoculum without substrate in laboratory-scale batch reactors. The biohydrogen production yield ranged from 40.8 to 54.7 ml H₂/g-VS, with the I/S ratio ranging from 0.0625 to 4. The results indicated that low I/S ratios caused the overloaded accumulation of metabolic products and a significant pH decrease, which negatively affected hydrogen production bacteria's metabolic activity, thus leading to the decrease of hydrogen fermentation efficiency. The overall results demonstrated that Cladophora sp. biomass is an efficient fermentation feedstock for biohydrogen production.     

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.     

Fermentative biohydrogen production by mixed anaerobic consortia: Impact of glucose to xylose ratio

Glucose and xylose are the dominant monomeric carbohydrates present in agricultural materials which can be used as potential building blocks for various biotechnological products including biofuels production. Hence, the imperative role of glucose to xylose ratio on fermentative biohydrogen production by mixed anaerobic consortia was investigated. Microbial catabolic H₂ and VFA production studies revealed that xylose is a preferred carbon source compared to glucose when used individually. A maximum of 1550 and 1650 ml of cumulative H₂ production was observed with supplementation of glucose and xylose at a concentration of 5.5 and 5.0 g L⁻¹, respectively. A triphasic pattern of H₂ production was observed only with studied xylose concentration range. pH impact data revealed effective H₂ production at pH 6.0 and 6.5 with xylose and glucose as carbon sources, respectively. Co-substrate related biohydrogen fermentation studies indicated that glucose to xylose ratio influence H₂ and as well as VFA production. An optimum cumulative H₂ production of 1900 ml for 5 g L⁻¹ substrate was noticed with fermentation medium supplemented with glucose to xylose ratio of 2:3 at pH 6. Overall, biohydrogen producing microbial consortia developed from buffalo dung could be more effective for H₂ production from lignocellulosic hydrolysates however; maintenance of glucose to xylose ratio, inoculum concentration and medium pH would be essential requirements.     

Whey waste as potential feedstock for biohydrogen production

In-house isolate Clostridium sp. IODB-O3 was exploited for biohydrogen production using cheese whey waste in batch fermentation. Analysis of cheese whey shows, it is enriched with lactose, lactic acid and protein components which were observed most favourable for biohydrogen production. Biohydrogen yield by IODB-O3 was compared with the cultures naturally occurring in waste solely or in combinations, and found that Clostridium sp. IODB-O3 was the best producer. The maximum biohydrogen yield obtained was 6.35 ± 0.2 mol-H₂/mol-lactose. The cumulative H₂ production (ml/L), 3330 ± 50, H₂ production rate (ml/L/h), 139 ± 5, and specific H₂ production (ml/g/h), 694 ± 10 were obtained. Clostridium sp. IODB-O3 exhibited better H₂ yield from cheese whey than the reported values in literature. Importantly, the enhancement of biohydrogen yield was observed possibly due to absence of inhibitory compounds, presence of essential nutrients, protein and lactic acid fractions which supported better cell growth than that of the lactose and glucose media. Carbon balance was carried out for the process which provided more insights in IODB-O3 metabolic pathway for biohydrogen production. This study may help for effective utilization of whey wastes for economic large scale biohydrogen production.     

Modeling dark fermentation for biohydrogen production: ADM1-based model vs. Gompertz model

Biohydrogen production by dark fermentation in batch reactors was modeled using the Gompertz equation and a model based on Anaerobic Digestion Model (ADM1). The ADM1 framework, which has been well accepted for modeling methane production by anaerobic digestion, was modified in this study for modeling hydrogen production. Experimental hydrogen production data from eight reactor configurations varying in pressure conditions, temperature, type and concentration of substrate, inocula source, and stirring conditions were used to evaluate the predictive abilities of the two modeling approaches. Although the quality of fit between the measured and fitted hydrogen evolution by the Gompertz equation was high in all the eight reactor configurations with r² ∼0.98, each configuration required a different set of model parameters, negating its utility as a general approach to predict hydrogen evolution. On the other hand, the ADM1-based model (ADM1BM) with predefined parameters was able to predict COD, cumulative hydrogen production, as well as volatile fatty acids production, albeit at a slightly lower quality of fit. Agreement between the experimental temporal hydrogen evolution data and the ADM1BM predictions was statistically significant with r² > 0.91 and p-value <1E-04. Sensitivity analysis of the validated model revealed that hydrogen production was sensitive to only six parameters in the ADM1BM.     

Techno-economic evaluation of biohydrogen production from wastewater and agricultural waste

The world is facing serious climate change caused in part by human consumption of fossil fuel. Therefore, developing a clean and environmentally friendly energy resource is necessary given the depletion of fossil fuels, the preservation of the earth's ecosystem and self-preservation of human life. Biological hydrogen production, using dark fermentation is being developed as a promising alternative and renewable energy source, using biomass feedstock. In this study, beverage wastewater and agricultural waste were examined as substrates for dark fermentation to produce clean biohydrogen energy.     

Converting waste molasses liquor into biohydrogen via dark fermentation using a continuous bioreactor

This study investigated the effects of substrate concentration, HRT (hydraulic retention time), and pre-treatment of the substrate molasses on biohydrogen production from waste molasses (condensed molasses fermentation solubles, CMS) with a CSTR (continuously-stirred tank reactor). First, the hydrogen production was performed with various CMS concentrations (40–90 g COD/L, total sugar 8.7–22.6 g/L) with 6 h HRT. The results show that the maximal hydrogen production rate (HPR) occurred at 80 g COD/L substrate (19.8 g ToSu/L, ToSu: Total Sugar), obtaining an HPR of 0.417 mol/L/d. However, maximum hydrogen yield (HY) of 1.44 mol H₂/mol hexose and overall hydrogen production efficiency (HPE) of 25.6% were achieved with a CMS concentration of 70 g COD/L (17.3 g ToSu/L). The substrate inhibition occurred when CMS concentration was increased to 90 g COD/L (22.6 g ToSu/L). Furthermore, it was observed that the optimal HPR, HY, and HPE all occurred at HRT 6 h. Operating at a lower HRT of 4 h decreased the hydrogen production performance because of lower substrate utilization efficiency. The employment of pre-heating treatment (60 °C for 1 h) of the substrate could markedly enhance the fermentation performance. With 6 h HRT and substrate pre-heating treatment, the HPE raised to 29.9%, which is 18% higher than that obtained without thermal pretreatment.     

Continuous biohydrogen production by a degenerated strain of Clostridium acetobutylicum ATCC 824

Degenerated strains of Clostridium acetobutylicum lack the ability to produce solvents and to sporulate, allowing the continuous production of hydrogen and organic acids. A degenerated strain of Clostridium acetobutylicum was obtained through successive batch cultures. Its kinetic characterization showed a similar specific growth rate than the wild type (0.25 h^?1), a higher butyric acid production of 6.8 g·L^?1 and no solvents production. A steady state was reached in a continuous culture at a dilution rate of 0.1 h^?1, with a constant hydrogen production of 507 mL·h^?1, corresponding to a volumetric rate of 6.10 L·L^?1 d^?1, and a yield of 2.39 mol of H₂ per mole of glucose which represents 60% of the theoretical maximum yield. These results suggest that the degeneration is an interesting alternative for hydrogen production with this strain, obtaining a high hydrogen production in a continuous culture with cells in a permanent acidogenic state.     

The CRISPR technology: A promising strategy for improving dark fermentative biohydrogen production using Clostridium spp.

Generating hydrogen gas (H₂) using the dark fermentation method has attracted much attention due to its lower energy requirement and environmental friendliness. However, producing a high yield of bio-H₂ is as challenging as ever due to low energy conversion by microorganisms. In this respect, the advancement of genome editing tools including the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-Cas technology could overcome the established maximum ceiling of product yield. To date, CRISPR-Cas systems, particularly those based on Type II CRISPR-Cas9 and Type V CRISPR-Cas12, are widely used in manipulating novel bacteria to improve the yield of specific biofuel. However, studies using the CRISPR-Cas technology for improving bio-H₂ production remain scarce. Understanding the metabolic pathways of Clostridium spp. is essential for using the CRISPR-Cas technology Thus, this review highlighted the state-of-the-art in CRISPR-Cas systems for bacterial genome editing while paying attention to bioprocess optimization strategies for modulating the biohydrogen production.     

Enhanced biohydrogen production from cornstalk through a two‐step fermentation: Dark fermentation and photofermentation

The pretreated cornstalk (CS) was employed for biohydrogen production through combining dark‐fermentative bacteria, cow dung, and photosynthetic bacteria, Rhodobacter capsulatus mutant in this work. In the first step, the cornstalk was pretreated with 0.75% NaOH, 12 IU/g‐CS cellulase, and 2400 IU/g‐CS hemicellulase under different hydrolysis time and temperature. The reducing sugar yields were very close under different conditions, and its maximum was 0.51 ± 0.01 g/g‐CS at hydrolysis 108°C for 2.0 hours. However, the H₂ yield of dark fermentation enhanced with the increasing hydrolysis time and temperature, and the maximum was 156.7 ± 8.6 mL/g‐CS at hydrolysis 126°C for 2.0 hours. The biogas was only H₂ and CO₂, and no methane was found. In the second step, R. capsulatus ZYHAB3 with the mutation on both hvrA and hupAB genes was obtained from wild type (R. capsulatus SB1003) and the inactivation of both hvrA and hupAB genes remarkably enhanced nitrogenase activity. When the dark‐fermentation effluent was employed as a substrate, the H₂ yield and maximum H₂ evolution rate of ZYHAB3 were 2827.5 ± 283.5 mL/L and 40.9 ± 2.3 mL/(Lh), which increased by 44.5 and 39.1% compared with those of wild type, respectively. The high H₂ yield of 439.4 mL/g‐CS was obtained from pretreated cornstalk through the 2‐step process, and the chemical oxygen demand removal rate achieved 90.6%. The results suggest that combining cow dung and R. capsulatus mutant (hvrA ^? hupAB ^?) could be a promising way to produce H₂ form agricultural wastes.     

Promoting dark fermentation for biohydrogen production: Potential roles of iron-based additives

Dark fermentation (DF) is a promising technology for biohydrogen production. Low efficiency of biohydrogen production is a bottleneck of the scale-up prospects for DF. Additives have been extensively studied to improve the biohydrogen production efficiency. Among of them, iron-based additives present a promising application potential due to their demonstrated significant enhancement of DF efficiency and among the low-cost bioactive agents. However, current reviews mainly examined the effects of nano-materials on DF and an in-depth analysis of enhancing mechanisms with addition of iron-based additives in DF is still lacking. To this end, this article comprehensively reviewed and evaluated the effects of iron-based additives on DF. Further, the potential mechanisms, including altering metabolic pathways, improving activities of microbes and enzymes, promoting electron delivery, and enriching hydrogen-producing bacteria, were discussed. Lastly, prospects and challenges of iron-based additives for subsequent research and large-scale application for DF were summarized.     

Extreme thermophilic condition: An alternative for long-term biohydrogen production from sugarcane vinasse

Boosted by the high temperatures in which vinasse is generated (90 °C–100 °C), this study evaluated the effect of an extreme thermophilic condition (70 °C) on sugarcane vinasse Dark Fermentation (DF) in an Anaerobic Structured Bed Reactor (ASTBR). Four hydraulic retention times (HRT) (19, 15, 12 and 8 h) were evaluated. Higher HRT resulted in a greater H₂ production rate (690 mLH₂.d⁻¹.L⁻¹), higher yields (1.8 molH₂.mol_Glucose⁻¹) and greater stability. The extreme temperature inhibits microorganisms' extracellular polymer production, thus leading to a disperse growth, preventing excess biomass accumulation, which was previously reported as the main drawback in H₂ production at lower temperatures. The ASTBR higher void index is also responsible for lower biomass/solids retention. The H₂ production main route was through the lactic/acetic acid pathway, which is highly reliant on the pH of fermentation broth. The main genus involved in H₂ production at 70 °C were Clostridium, Pectinatus, Megasphaera and Lactobacillus.     

Modeling microbial growth of dynamic membrane in a biohydrogen production bioreactor

Biohydrogen is renewable and has a huge potential to replace fossil fuels. Understanding mechanisms of controlling microbial processes of the dynamic membrane is critical for effective dark fermentative biohydrogen production in a dynamic membrane bioreactor (DMBR). This paper aims to develop a sophisticated model of biofilm growth, dynamic membrane formation, and dark fermentative hydrogen production within a platform of coupled lattice Boltzmann and cellular automata. The model was validated against the experimental data available and then was applied for the investigation of biohydrogen production in bioreactors under different membrane structures and inlet velocities. The results showed that porous twisted channels in the dynamic membrane could significantly affect biohydrogen extraction and biofilm patterns. In all cases, the dynamic membrane formation has three phases: the initial bacteria deposit, stable biofilm growth, and stable maximum biofilm biomass. The biohydrogen production could increase by 16.4% by optimizing the porous structure and increase 30%–40% of the hydrogen extraction. Inlet velocity also affects biohydrogen extraction in a range of ?28.3%–71.2%. Both porous structure and inlet velocity would be critical operational parameters for continuous biohydrogen production. The present model demonstrated its capability to investigate dark fermentative hydrogen production and its potential applications to porous bioreactors.     

Production of biohydrogen from waste wheat in continuously operated UPBR: The effect of influent substrate concentration

Utilization of waste materials is one of the most economical approaches to biohydrogen production. Continuous generation of biohydrogen in a bioreactor makes the process more economical with respect to the conventional physical and chemical method. The two main parameters that affect the biohydrogen production in a continuously operated bioreactor are hydraulic retention time (HRT) and influent substrate concentrations. The effect of influent substrate concentration on biohydrogen generation in an up-flow packed bed reactor (UPBR) at HRT = 3 h was investigated in this study. The substrate was waste wheat which was acid hydrolyzed in H₂SO₄ by adjusting the pH value to pH = 2, under high temperature as T = 90 °C in an autoclave to obtain fermentable sugar solution. A natural and porous support particle namely, aquarium biological sponge (ABS) was the microbial immobilization surface in the reactor. Total and hydrogen gas volumes, hydrogen percentage, influent and effluent substrate concentrations, VFA concentrations were monitored. The influent substrate concentration (TS_o) was varied between TS_o = 10 g/L and TS_o = 35 g/L. The process performance was evaluated as biohydrogen volume, percentages, rate and yield under varying operating conditions. The production volume (4275 ml/day) and the rate (3.05 L H₂/L day) were maximum at influent sugar concentration of TS_o = 25 g/L, but the yield reached to its maximum value as Y = 1.22 mol H₂/mol glucose at TS_o = 19 g/L. Substrate limitation and inhibitions were observed at influent concentrations of TS_o = 10 g/L and TS_o = 35 g/L, respectively. The results indicated that ABS could be suggested as a microbial support particle for hydrogen generation in immobilized systems.     

Biohydrogen production from household solid waste (HSW) at extreme-thermophilic temperature (70 °C) – Influence of pH and acetate concentration

Hydrogen production from household solid waste (HSW) was performed via dark fermentation by using an extreme-thermophilic mixed culture, and the effect of pH and acetate on the biohydrogen production was investigated. The highest hydrogen production yield was 257 ± 25 mL/gVS_added at the optimum pH of 7.0. Acetate was proved to be inhibiting the dark fermentation process at neutral pH, which indicates that the inhibition was caused by total acetate concentration not by undissociated acetate. Initial inhibition was detected at acetate concentration of 50 mM, while the hydrogen fermentation was seriously inhibited at acetate concentration of 200 mM. At 200 mM acetate concentration, the hydrogen yield was 36 ± 25 mL/gVS_added, which was almost 7 times lower than the yield of 254 ± 13 mL/gVS_added, which was achieved at lower acetate concentration (5–25 mM). Additional to the negative effect on the hydrogen yield, acetate was resulting in the longer lag phase during batch fermentations. The lag phase was more than 100 h at acetate concentration of more than 150 mM, while it was only 3–4 h at 5–25 mM acetate.     

A standardized biohydrogen potential protocol: An international round robin test approach

Hydrogen production by dark fermentation is an emerging technology of increasing interest due to its renewable feature. Recent scientific advances have well investigated the operational conditions to produce hydrogen through the valorization of several wastes or wastewaters. However, the development of standardized protocols to accurately assess the biohydrogen potential (BHP) is of crucial importance. This work is the first interlaboratory and international effort to validate a protocol estimating hydrogen potential using batch tests, using glucose as individual model substrate. The repeatability of the hydrogen potential (HP) increased with variations of the proposed protocol: reducing substrate concentration, increasing the buffer capacity, and using an automatic device. The interlaboratory variation of the HP was reduced from 32 to 12%, demonstrating the reproducibility and robustness of the proposed protocol. Recommendations to run BHP tests were formulated in terms of i) repeatability and reproducibility of results, ii) criteria for results validation and acceptance, iii) workload of the proposed protocols.     

Effects of pH value and substrate concentration on hydrogen production from the anaerobic fermentation of glucose

A series of batch experiments were conducted to investigate the effects of pH and glucose concentrations on biological hydrogen production by using the natural sludge obtained from the bed of a local river as inoculant. Batch experiments numbered series I and II were designed at an initial and constant pH of 5.0–7.0 with 1.0 increment and four different glucose concentrations (5.0, 7.5, 10 and 20 g glucose/L). The results showed that the optimal condition for anaerobic fermentative hydrogen production is 7.5 g glucose/L and constant pH 6.0 with a maximum H₂ production rate of 0.22 mol H₂ mol⁻¹ glucose h⁻¹, a cumulative H₂ yield of 1.83 mol H₂ mol⁻¹ glucose and a H₂ percentage of 63 in biogas.     

Evaluation of biohydrogen production from glucose and protein at neutral initial pH

Organic wastes are considered as potential substances for economical biohydrogen production, because the carbohydrate and protein are main components. Previous investigations indicate that an optimum hydrogen production appear in acidic conditions to carbohydrates, or in alkali condition to protein. However, in practice, the treatment of these organic wastes by anaerobic fermentation usually carries out at neutral pH condition, in which biohydrogen production is only a middle process. So, the purpose of this paper is to evaluate the biohydrogen production at neutral pH condition from carbohydrates or protein. Batch tests were conducted to investigate the differences in biohydrogen production by anaerobic fermentation at neutral initial pH using carbohydrate and protein (glucose and peptone) as the sole carbon source. The experimental results showed that the maximal hydrogen yields of two substrates were about 0.14 ml H₂/mg glucose and 0.077 ml H₂/mg protein, respectively, at neutral initial pH. Although the hydrogen yields of glucose is far greater than that of protein at neutral pH, they were lower than previous results of hydrogen production in acidic condition to carbohydrate or in alkali condition to protein. This result shows that the neutral pH is not an optimal condition for biohydrogen production. In this experiment of biohydrogen production, a phenomenon has been observed that the hydrogen production and hydrogen consumption occurred simultaneously in the fermentation of protein, whereas the hydrogen production occurred only in the fermentation of glucose. Furthermore, the different evaluation of the main components of the organic liquid by-products produced by fermentation of each substrate implied that the biohydrogen production pathways of these two substrates were different. Molecular analysis indicated that the dominant microorganisms during the anaerobic fermentation of these two substrates are greatly different.