Lactate- and acetate-based biohydrogen production through dark co-fermentation of tequila vinasse and nixtamalization wastewater: Metabolic and microbial community dynamics

Presence of lactic acid bacteria in hydrogen production by dark fermentation: competition or synergy

Dark fermentation in mixed cultures has been extensively studied due to its great potential for sustainable hydrogen production from organic wastes. However, microbial composition, substrate competition, and inhibition by fermentation products can affect hydrogen yield and production rates. Lactic acid bacteria have been identified as the key organisms in this process. On one hand, lactic acid bacteria can efficiently compete for carbohydrate rich substrates, producing lactic acid and secreting bacteriocins that inhibit the growth of hydrogen-producing bacteria, thereby decreasing hydrogen production. On the other hand, due to their metabolic capacity and synergistic interactions with certain hydrogen-producing bacteria, they contribute positively in several ways, for example by providing lactic acid as a substrate for hydrogen generation. Analyzing different perspectives about the role of lactic acid bacteria in hydrogen production by dark fermentation, a literature review was done on this topic. This review article shows a comprehensive view to understand better the role of these bacteria and their influence on the process efficiency, either as competitors or as contributors to hydrogen production by dark fermentation.

Mixed culture biotechnology and its versatility in dark fermentative hydrogen production

Over the years, extensive research has gone into fermentative hydrogen production using pure and mixed cultures from waste biomass with promising results. However, for up-scaling of hydrogen production mixed cultures are more appropriate to overcome the operational difficulties such as a metabolic shift in response to environmental stress, and the need for a sterile environment. Mixed culture biotechnology (MCB) is a robust and stable alternative with efficient waste and wastewater treatment capacity along with co-generation of biohydrogen and platform chemicals. Mixed culture being a diverse group of bacteria with complex metabolic functions would offer a better response to the environmental variations encountered during biohydrogen production. The development of defined mixed cultures with desired functions would help to understand the microbial community dynamics and the keystone species for improved hydrogen production. This review aims to offer an overview of the application of MCB for biohydrogen production.

A mini review on microwave and contemporary based biohydrogen production technologies: a comparison

Hydrogen gas, along with conventional fossil fuels, has been used as a green fuel with enormous potential. Due to the rapid depletion of fossil fuels, a new dimension of hydrogen production technology has arrived to reduce reliance on nonrenewable energy sources. Microwave-based hydrogen production is a more promising and cost-effective technology than other existing green hydrogen production methods such as fermentation and gasification. Microwave heating may be superior to traditional heating due to several advantages such as less power consumption compared to other methods, higher yield, and a higher rate of conversion. Compared to another process for hydrogen production, the microwave-driven process worked efficiently at lower temperatures by providing more than 70% yield. The process of production can be optimized by using properly sized biomass, types of biomass, water flow, temperature, pressure, and reactor size. This method is the most suitable, attractive, and efficient technique for hydrogen production in the presence of a suitable catalyst. Hot spots formed by microwave irradiation would have a substantial impact on the yield and properties of microwave-processed goods. The current techno-economic situation of various technologies for hydrogen production is discussed here, with cost, efficiency, and durability being the most important factors to consider. The present review shows that a cost-competitive hydrogen economy will necessitate continual efforts to increase performance, scale-up, technical prospects, and political backing.

A New Mechanism of Carbon Metabolism and Acetic Acid Balance Regulated by CcpA

Catabolite control protein A (CcpA) is a critical regulator in Gram-positive bacteria that orchestrates carbon metabolism by coordinating the utilization of different carbon sources. Although it has been widely proved that CcpA helps prioritize the utilization of glucose over other carbon sources, this global regulator's precise mechanism of action remains unclear. In this study, a mutant Bacillus licheniformis deleted for CcpA was constructed. Cell growth, carbon utilization, metabolites and the transcription of key enzymes of the mutant strain were compared with that of the wild-type one. It was found that CcpA is involved in the regulation of glucose concentration metabolism in Bacillus. At the same time, CcpA regulates glucose metabolism by inhibiting acetic acid synthesis and pentose phosphate pathway key gene zwF. The conversion rate of acetic acid is increased by about 3.5 times after ccpA is deleted. The present study provides a new mechanism of carbon metabolism and acetic acid balance regulated by CcpA. On the one hand, this work deepens the understanding of the regulatory function of CcpA and provides a new view on the regulation of glucose metabolism. On the other hand, it is helpful to the transformation of B. licheniformis chassis microorganisms.

Metatranscriptomic Analysis Reveals the Coexpression of Hydrogen-Producing and Homoacetogenesis Genes in Dark Fermentative Reactors Operated at High Substrate Loads

Microbial communities in dark fermentation continuous systems are affected by substrate type, concentration, and product accumulation (e.g., H2 and CO2). Metatranscriptomics and quantitative PCR (qPCR) were used to assess how high organic loading rates (OLR) from 60 to 160 g total carbohydrates (TC)/L-d modify the microbial community diversity and expression of key dark fermentative genes. Overall, the microbial communities were composed of H2-producing bacteria (Clostridium butyricum), homoacetogens (Clostridium luticellarii), and lactic acid bacteria (Enteroccocus gallinarum and Leuconostoc mesenteroides). Quantification through qPCR showed that the abundance of genes encoding the formyltetrahydrofolate synthetase (fthfs, homoacetogens) and hydrogenase (hydA, H2-producing bacteria) was strongly associated with the OLR and H2 production performance. Similarly, increasing the OLR influenced the abundance of the gene transcripts responsible for H2 production and homoacetogenesis. To evaluate the effect of decreasing the H2 partial pressure, silicone oil was added to the reactor at an OLR of 138 and 160 g TC/L-d, increasing the production of H2, the copies of genes codifying for hydA and fthfs, and the genes transcripts related to H2 production and homoacetogenesis. Moreover, the metatranscriptomic analysis also showed that lactate-type fermentation and dark fermentation simultaneously occurred without compromising the reactor performance for H2 production.

Influence of key operational parameters on biohydrogen production from fruit and vegetable waste via lactate-driven dark fermentation

This study aims at investigating the influence of operational parameters on biohydrogen production from fruit-vegetable waste (FVW) via lactate-driven dark fermentation. Mesophilic batch fermentations were conducted at different pH (5.5, 6.0, 6.5, 7.0, and non-controlled), total solids (TS) contents (5, 7, and 9%) and initial cell biomass concentrations (18, 180, and 1800 mg VSS/L). Higher hydrogen yields and rates were attained with more neutral pH values and low TS concentrations, whereas higher biomass densities enabled higher production rates and avoided wide variations in hydrogen production. A marked lactate accumulation (still at neutral pH) in the fermentation broth was closely associated with hydrogen inhibition. In contrast, enhanced hydrogen productions matched with much lower lactate accumulations (even it was negligible in some fermentations) along with the acetate and butyrate co-production but not with carbohydrates removal. At pH 7, 5% TS, and 1800 mg VSS/L, 49.5 NmL-H2/g VSfed and 976.4 NmL-H2/L-h were attained.

Biotreatment Potential and Microbial Communities in Aerobic Bioreactor Systems Treating Agro-Industrial Wastewaters

The thriving agro-industry sector accounts for an essential part of the global gross domestic product, as the need for food and feed production is rising. However, the industrial processing of agricultural products requires the use of water at all stages, which consequently leads to the production of vast amounts of effluents with diverse characteristics, which contain a significantly elevated organic content. This fact reinforces the need for action to control and minimize the environmental impact of the produced wastewater, and activated sludge systems constitute a highly reliable solution for its treatment. The current review offers novel insights on the efficiency of aerobic biosystems in the treatment of agro-industrial wastewaters and their ecology, with an additional focus on the biotechnological potential of the activated sludge of such wastewater treatment plants.

Effect of inoculum pretreatment on the microbial and metabolic dynamics of food waste dark fermentation

This study systematically evaluated and compared different inoculum pretreatment methods to quickly select dark fermentative bacteria from anaerobic sludge for the bioconversion of food waste. The hydrogen (H₂) production rate was found to be highest for 'heat + CO₂' treated inoculum at 140.75 ± 2.61 mL/L/h compared to control experiments (60.27 ± 2.61 mL/L/h). At the same time, H₂ yield was found to be highest for alkali-treated inoculum at 157.25 ± 7.62 mL/g of volatile solids (VS) added compared to control experiments (91.61 ± 1.93 mL/g VS). Analysis of organic acids suggests a Clostridial-type fermentation with acetate (0.52 to 1.60 g/L) and butyrate (1.69 to 2.42 g/L) being the major by-products. The microbial data analysis showed that Firmicutes (63.64-90.39%), Bacteroidota (1.16-21.88%), and Proteobacteria (2.09-9.93%) were dominant at the phylum level, whereas genus-level classification showed Clostridium sensu stricto 1 (6.37-42.63%), Streptococcus (1.87-28.96%), Prevotella (0.57-16.59%), and Enterococcus (0.56-14.51%) dominated under different experimental conditions.

Vacuum-enhanced anaerobic fermentation: Achieving process intensification, thickening and improved hydrolysis and VFA yields in a single treatment step

This study assessed the feasibility of a novel vacuum-enhanced anaerobic digestion technology, referred to as IntensiCarb^TM (IC), under mild vacuum pressure (110 mbar), compared to a control (conventional fermenter), and evaluated the impact of the vacuum on the activities of various microbial groups. Both fermenters (test and control) were operated with mixed (50% v/v) municipal sludge at solids concentrations of 2-2.5%, pH of 7.8-8.1, 40-45 °C, a theoretical solids retention time (SRT) of 3 days with different hydraulic retention times (HRT). The intensification factor (IF) of the IC, defined as SRT/HRT, was controlled at 1.3 and 2.0. Simultaneous thickening and fermentation intensification were achieved. Compared with the control, the IC, despite the shorter HRTs, achieved 29.5 to 90.2% increase in the VFA yield (79 to 116 mg ΔVFA/ g VSS vs 61 mg ΔVFA/ g VSS), and 16.2% to 56.4% increase (280 to 377 mg ΔsCOD/ g VSS vs 241 mg ΔsCOD/ g VSS), in the hydrolysis yield. Fermentate from the IC exhibited comparable specific denitrification rates to acetate. Further, the solids-free condensate contained low nutrient concentrations, and thus was far superior to a typical centrates from dewatering as a carbon source. No adverse effects of vacuum on the activity of fermentative bacteria and methanogens were observed. This study demonstrated that the IC can be deployed as an intensification technology for both fermentation and anaerobic digestion of biosolids with the additional significant advantage, i.e. elimination of sidestream ammonia treatment requirements.

Evaluation of inhibitory compounds produced by bacteria isolated from a hydrogen-producing bioreactor during the self-fermentation of wheat straw

AIMS

The objective of this study was to evaluate the inhibitory activity of compounds secreted by bacteria isolated from a hydrogen-producing bioreactor to understand how these microorganisms interact in this community.

METHODS AND RESULTS

In vitro inhibitory assays were performed using samples secreted by bacteria subject to different treatments to determine if their inhibitory effect was due to organic acids, non-proteinaceous compounds, or bacteriocin-like inhibitory substances (BLIS). Bacterial isolated were suppressed 43%, 30%, and 27% by neutralized, precipitated, and non-neutralized cell-free supernatants, respectively. Non-hydrogen producers (Non-H₂ P) LAB (Lactobacillus plantarum LB1, L. pentosus LB7, Pediococcus acidilactici LB4) and hydrogen producers (H₂ P) LAB (Enterococcus faecium F) were inhibited by the production of organic acids, non-proteinaceous compounds, and BLIS. Meanwhile, the obligate anaerobe H₂ P (Clostridium beijerinckii B) inhibited by the production of non-proteinaceous compounds and BLIS. The presence of BLIS was confirmed when proteolytic enzymes affected the inhibitory activity of secreted proteins in values ranging from 20 to 42%. The BLIS produced by L. plantarum LB1, P. acidilactici LB4, L. pentosus LB7, and E. faecium F showed molecular masses of ~ 11 kDa, 25 kDa, 20 kDa, and 11 kDa, respectively.

CONCLUSIONS

It was demonstrated antagonistic interactions between Lactobacillus- Enterococcus, and Pediococcus-Enterococcus species, generated by the secretion of organic acids, non-proteinaceous compounds, and BLIS.

SIGNIFICANCE AND IMPACT OF THE STUDY

We report the interactions between LAB isolated from hydrogen-producing bioreactors. These interactions might impact the dynamics of the microbial population during hydrogen generation. Our work lays a foundation for strategies that allow controlling bacteria that can affect hydrogen production.

Effect of external acetate on lactate-based carboxylate platform: Shifted lactate overloading limit and hydrogen co-production

Chain elongation is an anaerobic biotechnological process that converts short chain carboxylates and an electron donor (e.g. ethanol, lactate) into more valuable medium chain carboxylates. Caproate production in lactate-based chain elongation is gaining popularity, however, the relation between lactate (electron donor) and acetate (electron acceptor) has not yet been fully elucidated. Herein, for the first time, the effect of an external acetate on the lactate-based chain elongation in a continuously-fed bioreactor was tested to verify how the external acetate would affect the product spectrum, gas production, as well as stability and efficiency of carboxylates production. Periodic fluctuations in caproate production were observed in bioreactor continuously fed with lactate as a sole carbon source due to the lack of an electron acceptor (acetate) and low chain elongation performance. The recovery of stable caproate production (68.9 ± 2.2 mmol C/L/d), total lactate consumption, and high hydrogen co-production (748 ± 76 mL_H2/d) was observed as an effect of the addition of an external acetate. The lactate conversion with the external acetate in the second bioreactor ensured stable and dominant caproate production from the beginning of the process. Moreover, despite the continuous lactate overloading in the process with external acetate, stable caproate production was achieved (71.7 ± 2.4 mmol C/L/d) and previously unobserved hydrogen production occurred (213 ± 30 mL_H2/d). Thus, external electron acceptor addition (i.e. acetate) was proposed as an effective method for stable lactate-based caproate production. Microbiological analysis showed the dominance of microbes closely related to Ruminococcaceae bacterium CPB6 and Acinetobacter throughout the process. Co-occurrence networks based on taxon abundances and process parameters revealed microbial sub-networks responding to lactate concentrations.

Dark Fermentation Process Response to the Use of Undiluted Tequila Vinasse without Nutrient Supplementation

The technical feasibility of valorizing tequila vinasse (TV), a wastewater with high pollution potential, through the production of biogenic hydrogen via dark fermentation, has long been proven in diverse lab-scale reactors that were operated either in batch or continuous mode. However, such systems have mainly been tested with diluted streams and nutrient supplementation, hindering the techno-economic attractiveness of the TV-to-hydrogen concept at large scale. In this study, the feasibility of producing hydrogen from high-strength undiluted TV with no added extra nutrients was evaluated under batch mesophilic conditions. Additionally, the use of two different acidogenic inocula obtained either by heat or heat-aeration pretreatment was investigated to get a greater understanding of the effect of inoculum type on the process. The results obtained showed that the TV utilized herein contained macro- and micro-nutrients high enough to support the hydrogenogenic activity of both cultures, entailing average hydrogen yields of 2.4–2.6 NL H2/L vinasse and maximum hydrogen production rates of 1.4–1.9 NL H2/L-d. Interestingly, the consumption of lactate and acetate with the concomitant production of butyrate was observed as the main hydrogen-producing route regardless of the inoculum, pointing out the relevance of the lactate-driven dark fermentative process. Clostridium beijerinckii was ascertained as key bacteria, but only in association with microorganisms belonging to the genera Enterobacter and Klebsiella, as revealed by phylogenetic analyses.

Dynamics of dark fermentation microbial communities in the light of lactate and butyrate production

BACKGROUND

This study focuses on the processes occurring during the acidogenic step of anaerobic digestion, especially resulting from nutritional interactions between dark fermentation (DF) bacteria and lactic acid bacteria (LAB). Previously, we have confirmed that DF microbial communities (MCs) that fed on molasses are able to convert lactate and acetate to butyrate. The aims of the study were to recognize the biodiversity of DF-MCs able and unable to convert lactate and acetate to butyrate and to define the conditions for the transformation.

RESULTS

MCs sampled from a DF bioreactor were grown anaerobically in mesophilic conditions on different media containing molasses or sucrose and/or lactate and acetate in five independent static batch experiments. The taxonomic composition (based on 16S_rRNA profiling) of each experimental MC was analysed in reference to its metabolites and pH of the digestive liquids. In the samples where the fermented media contained carbohydrates, the two main tendencies were observed: (i) a low pH (pH ≤ 4), lactate and ethanol as the main fermentation products, MCs dominated with Lactobacillus, Bifidobacterium, Leuconostoc and Fructobacillus was characterized by low biodiversity; (ii) pH in the range 5.0-6.0, butyrate dominated among the fermentation products, the MCs composed mainly of Clostridium (especially Clostridium_sensu_stricto_12), Lactobacillus, Bifidobacterium and Prevotella. The biodiversity increased with the ability to convert acetate and lactate to butyrate. The MC processing exclusively lactate and acetate showed the highest biodiversity and was dominated by Clostridium (especially Clostridium_sensu_stricto_12). LAB were reduced; other genera such as Terrisporobacter, Lachnoclostridium, Paraclostridium or Sutterella were found. Butyrate was the main metabolite and pH was 7. Shotgun metagenomic analysis of the selected butyrate-producing MCs independently on the substrate revealed C.tyrobutyricum as the dominant Clostridium species. Functional analysis confirmed the presence of genes encoding key enzymes of the fermentation routes.

CONCLUSIONS

Batch tests revealed the dynamics of metabolic activity and composition of DF-MCs dependent on fermentation conditions. The balance between LAB and the butyrate producers and the pH values were shown to be the most relevant for the process of lactate and acetate conversion to butyrate. To close the knowledge gaps is to find signalling factors responsible for the metabolic shift of the DF-MCs towards lactate fermentation. Video Abstract.

A review on the factors influencing biohydrogen production from lactate: The key to unlocking enhanced dark fermentative processes

Dark fermentation (DF) is one of the most promising biological methods to produce bio-hydrogen and other value added bio-products from carbohydrate-rich wastes and wastewater. However, process instability and low hydrogen production yields and rates have been highlighted as the major bottlenecks preventing further development. Numerous studies have associated such concerns with the inhibitory activity of lactate-producing bacteria (LAB) against hydrogen producers. However, an increasing number of studies have also shown lactate-based metabolic pathways as the prevailing platform for hydrogen production. This opens a vast potential to develop new strategies to deal with the "Achilles heel" of DF - LAB overgrowth - while untapping high-performance DF. This review discusses the key factors influencing the lactate-driven hydrogen production, paying particular attention to substrate composition, the operating conditions, as well as the microbiota involved in the process and its potential functionality and related biochemical routes. The current limitations and future perspectives in the field are also presented.

Dynamics and Complexity of Dark Fermentation Microbial Communities Producing Hydrogen From Sugar Beet Molasses in Continuously Operating Packed Bed Reactors

This study describes the dynamics and complexity of microbial communities producing hydrogen-rich fermentation gas from sugar-beet molasses in five packed-bed reactors (PBRs). The bioreactors constitute a part of a system producing hydrogen from the by-products of the sugar-beet industry that has been operating continuously in one of the Polish sugar factories. PBRs with different working volumes, packing materials, construction and inocula were tested. This study focused on analysis (based on 16S rRNA profiling and shotgun metagenomics sequencing) of the microbial communities selected in the PBRs under the conditions of high (>100 cm3/g COD of molasses) and low (<50 cm3/g COD of molasses) efficiencies of hydrogen production. The stability and efficiency of the hydrogen production are determined by the composition of dark fermentation microbial communities. The most striking difference between the tested samples is the ratio of hydrogen producers to lactic acid bacteria. The highest efficiency of hydrogen production (130-160 cm3/g COD of molasses) was achieved at the ratios of HPB to LAB ≈ 4:2.5 or 2.5:1 as determined by 16S rRNA sequencing or shotgun metagenomics sequencing, respectively. The most abundant Clostridium species were C. pasteurianum and C. tyrobutyricum. A multiple predominance of LAB over HPB (3:1-4:1) or clostridia over LAB (5:1-60:1) results in decreased hydrogen production. Inhibition of hydrogen production was illustrated by overproduction of short chain fatty acids and ethanol. Furthermore, concentration of ethanol might be a relevant marker or factor promoting a metabolic shift in the DF bioreactors processing carbohydrates from hydrogen-yielding toward lactic acid fermentation or solventogenic pathways. The novelty of this study is identifying a community balance between hydrogen producers and lactic acid bacteria for stable hydrogen producing systems. The balance stems from long-term selection of hydrogen-producing microbial community, operating conditions such as bioreactor construction, packing material, hydraulic retention time and substrate concentration. This finding is confirmed by additional analysis of the proportions between HPB and LAB in dark fermentation bioreactors from other studies. The results contribute to the advance of knowledge in the area of relationships and nutritional interactions especially the cross-feeding of lactate between bacteria in dark fermentation microbial communities.

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.

Robust operation through effluent recycling for hydrogen production from the organic fraction of municipal solid waste

The stability of fermentative hydrogen production from the organic fraction of municipal solid waste (OFMSW) was evaluated in this work using a strategy of effluent recycling. Three pretreatment conditions were applied on the recycled effluent: a) no heat shock treatment, b) one initial heat shock treatment (90 °C, 30 min) and c) systematic heat shock treatment at the beginning of each fermentation. When a systematic heat shock was applied, a maximal hydrogen yield of 17.2 ± 3.8 mLH₂/gVS was attained. The hydrogen productivity was improved by 331% reaching a stable value of 1.51 ± 0.29 mLH₂/gVS/h, after 8 cycles of effluent recycling. This strategy caused a sharp decrease of diversity with stable co-dominance of hydrogen- and lactate-producing bacteria, ie. Clostridiales and Lactobacillales, respectively. For the other conditions, a sharp decrease of the hydrogen yields was observed showing the importance of applying a heat shock treatment for optimal hydrogen production with effluent recycling.

Emerging techniques assisting nixtamalization products and by-products processing: an overview

The production of worldwide nixtamalized products has increased in Latin American countries over the last years. For a better maize handling and exploitation of its nutritional elements, maize is subjected to a nixtamalization pretreatment protocol, which produces meaningful chemical, nutritional and quality changes in maize and its derived products, but large amounts of its primary by-product, well-known as 'nejayote', are also produced. Importantly, nejayote is usually discarded into the urbanized sewage with minimal treatment. Today, according to the recent research reports, new emerging techniques and protocols have been implemented to improve the nixtamalization products and by-products processing. New valorization approaches and biotechnological developments (including biotransformations) toward the reuse of nejayote have been developed according to its considerable content of biomolecules. Therefore, the goal of this paper is to provide a comprehensive review of the main development works at assisting nixtamalization products and by-products processing. Herein, particular attention is paid to experimental insights dealing with the valorization of nejayote.

A Review of Biohydrogen Productions from Lignocellulosic Precursor via Dark Fermentation: Perspective on Hydrolysate Composition and Electron-Equivalent Balance

This paper reviews the current technological development of bio-hydrogen (BioH2) generation, focusing on using lignocellulosic feedstock via dark fermentation (DF). Using the collected reference reports as the training data set, supervised machine learning via the constructed artificial neuron networks (ANNs) imbedded with feed backward propagation and one cross-out validation approach was deployed to establish correlations between the carbon sources (glucose and xylose) together with the inhibitors (acetate and other inhibitors, such as furfural and aromatic compounds), hydrogen yield (HY), and hydrogen evolution rate (HER) from reported works. Through the statistical analysis, the concentrations variations of glucose (F-value = 0.0027) and acetate (F-value = 0.0028) were found to be statistically significant among the investigated parameters to HY and HER. Manipulating the ratio of glucose to acetate at an optimal range (approximate in 14:1) will effectively improve the BioH2 generation (HY and HER) regardless of microbial strains inoculated. Comparative studies were also carried out on the evolutions of electron equivalent balances using lignocellulosic biomass as substrates for BioH2 production across different reported works. The larger electron sinks in the acetate is found to be appreciably related to the higher HY and HER. To maintain a relative higher level of the BioH2 production, the biosynthesis needs to be kept over 30% in batch cultivation, while the biosynthesis can be kept at a low level (2%) in the continuous operation among the investigated reports. Among available solutions for the enhancement of BioH2 production, the selection of microbial strains with higher capacity in hydrogen productions is still one of the most phenomenal approaches in enhancing BioH2 production. Other process intensifications using continuous operation compounded with synergistic chemical additions could deliver additional enhancement for BioH2 productions during dark fermentation.

Industrial wastewater to biohydrogen: Possibilities towards successful biorefinery route

The aim of this review is to summarize the modern developments and enhancement strategies reported for improving the biorefinery route of industrial wastewater to biohydrogen. Recent developments towards biohydrogen production chiefly involves culture enrichment, pretreatment of biocatalysts, co culture fermentation, metabolic and genetic engineering, ecobiotechnological approaches and the coupling process of biohydrogen. In addition, an overview of dark fermentation, pathways involved, microbes involved in biohydrogen production, industrial wastewater as substrate have been focused. The utilization of organic residuals of dark fermentation for subsequent value added products are highlighted. More apparently, the two stage coupling process and its possibilities towards biorefinery has been reviewed comprehensively. Moreover, comparative energy and economic aspects of biohydrogen production from industrial wastewater and its prospects towards pilot scale applications are also spotlighted. Though all the enhancement strategies have both benefits and disadvantages, coupling process is considered as the most successful biorefinery route for biohydrogen production.

Anaerobic co-digestion of cattle manure and liquid fraction of digestate (LFD) pretreated corn stover: Pretreatment process optimization and evolution of microbial community structure

Liquid fraction of digestate (LFD) was used to pretreat corn stover to enhance the biomethane production of anaerobic co-digestion (AcoD) with cattle manure. The effects of LFD concentration and water content (WC) for pretreatment on co-digestion performance and microbial community structure were investigated in a batch system. Results showed that the cumulative biomethane yield (CBY) for co-digestion was improved by 16.85%-41.78% compared with the control. The highest biomethane yield of 238.25 mL g VS^-1 was obtained at 85% WC for pretreatment and a 5 M LFD concentration, and this yield was 41.78% higher than that in the control. The LFD pretreatment enriched the dominant bacterial phyla (Firmicutes and Bacteroidetes), but had little influence on the prevalent archaeal genus (Euryarchaeota). This study demonstrated that LFD pretreatment can greatly enhance the biomethane yield of co-digestion of corn stover and cattle manure under optimal parameters.

Changes in performance and bacterial communities in a continuous biohydrogen-producing reactor subjected to substrate- and pH-induced perturbations

The performance and microbial communities of a continuous dark fermentation reactor exposed to perturbations induced by substrate change and acidic pH shock were investigated. A mesophilic well-mixed reactor separately fed with two types of tequila vinasses (TVs) and lactose was operated at a fixed pH of 5.5, except during short-term pH (3.8) stress, for ~61 days at decreasing hydraulic retention times (HRTs) from 24 to 4 h. During the first ~23 days of operation with TV, a decrease in HRT down to 4 h resulted in the highest reported biohydrogen productivity from TV of 12.4 NL/L-d. It was shown that abrupt change in TV type (even with temporal feeding of lactose) and transient over-acidification impaired the normal operation of the reactor. However, it rapidly recovered from such disturbances, sustaining similar high-rate productivity to that previously encountered. Recovery was attributed to resistant and resilient microbial community features, as supported by molecular characterisation.

State of the art and challenges of biohydrogen from microalgae

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

Novel insights on the versatility of biohydrogen production from sugarcane vinasse via thermophilic dark fermentation: Impacts of pH-driven operating strategies on acidogenesis metabolite profiles

An innovative application of the anaerobic structured-bed reactor (AnSTBR) in thermophilic dark fermentation of sugarcane vinasse targeting biohydrogen (bioH₂) production was assessed. A detailed metabolite monitoring program identified the major substrates and primary metabolic pathways within the system. Increasing the applied organic loading rate positively affected bioH₂ production, reaching 2074 N mL-H₂ L^-1 d^-1 and indicating an optimal load of approximately 70 kg-COD m^-3 d^-1. Controlling the fermentation pH (5.0-5.5) was the primary strategy to maintain bioH₂-producing conditions, offsetting negative impacts associated with the compositional variability of vinasse. Metabolic correlations pointed out lactate as the primary substrate for bioH₂ production, indicating its accumulation as evidence of impaired reactors. The versatility of the acidogenic system was confirmed by identifying three major metabolic pathways according to the pH, i.e., lactate-producing (pH <5.0), bioH₂-/butyrate-producing (pH = 5.0-5.5) and bioH₂-producing/sulfate-reducing (pH >6.0) systems, which enables managing the operation of the reactors for diversified purposes in practical aspects.