Dark fermentation and microalgae cultivation coupled systems: Outlook and challenges

The implementation of a sustainable bio-based economy is considered a top priority today. There is no doubt about the necessity to produce renewable bioenergy and bio-sourced chemicals to replace fossil-derived compounds. Under this scenario, strong efforts have been devoted to efficiently use organic waste as feedstock for biohydrogen production via dark fermentation. However, the technoeconomic viability of this process needs to be enhanced by the valorization of the residual streams generated. The use of dark fermentation effluents as low-cost carbon source for microalgae cultivation arises as an innovative approach for bioproducts generation (e.g., biodiesel, bioactive compounds, pigments) that maximizes the carbon recovery. In a biorefinery context, after value-added product extraction, the spent microalgae biomass can be further valorised as feedstock for biohydrogen production. This integrated process would play a key role in the transition towards a circular economy. This review covers recent advances in microalgal cultivation on dark fermentation effluents (DFE). BioH₂ via dark fermentation processes and the involved metabolic pathways are detailed with a special focus on the main aspects affecting the effluent composition. Interesting traits of microalgae and current approaches to solve the challenges associated to the integration of dark fermentation and microalgae cultivation are also discussed.

Membrane-based technologies for biohydrogen production: A review

Overcoming the existing environmental issues and the gradual depletion of energy sources is a priority at global level, biohydrogen can provide a sustainable and reliable energy reserve. However, the process instability and low biohydrogen yields are still hindering the adoption of biohydrogen production plants at industrial scale. In this context, membrane-based biohydrogen production technologies, and in particular fermentative membrane bioreactors (MBRs) and microbial electrolysis cells (MECs), as well as downstream membrane-based technologies such as electrodialysis (ED), are suitable options to achieve high-rate biohydrogen production. We have shed the light on the research efforts towards the development of membrane-based technologies for biohydrogen production from organic waste, with special emphasis to the reactor design and materials. Besides, techno-economic analyses have been traced to ensure the suitability of such technologies in bio-H₂ production. Operation parameters such as pH, temperature and organic loading rate affect the performance of MBRs. MEC and ED technologies also are highly affected by the chemistry of the membrane used and anode material as well as the operation parameters. The limitations and future directions for application of membrane-based biohydrogen production technologies have been individuated. At the end, this review helps in the critical understanding of deploying membrane-based technologies for biohydrogen production, thereby encouraging future outcomes for a sustainable biohydrogen economy.

Intensification of Acidogenic Fermentation for the Production of Biohydrogen and Volatile Fatty Acids—A Perspective

Utilising ‘wastes’ as ‘resources’ is key to a circular economy. While there are multiple routes to waste valorisation, anaerobic digestion (AD)—a biochemical means to breakdown organic wastes in the absence of oxygen—is favoured due to its capacity to handle a variety of feedstocks. Traditional AD focuses on the production of biogas and fertiliser as products; however, such low-value products combined with longer residence times and slow kinetics have paved the way to explore alternative product platforms. The intermediate steps in conventional AD—acidogenesis and acetogenesis—have the capability to produce biohydrogen and volatile fatty acids (VFA) which are gaining increased attention due to the higher energy density (than biogas) and higher market value, respectively. This review hence focusses specifically on the production of biohydrogen and VFAs from organic wastes. With the revived interest in these products, a critical analysis of recent literature is needed to establish the current status. Therefore, intensification strategies in this area involving three main streams: substrate pre-treatment, digestion parameters and product recovery are discussed in detail based on literature reported in the last decade. The techno-economic aspects and future pointers are clearly highlighted to drive research forward in relevant areas.

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.

A review on biological recycling in agricultural waste-based biohydrogen production: Recent developments

Hydrogen has become a research highlight by virtue of its clean energy production technology and high energy content. The technology of biohydrogen production from biological waste via fermentation has lower costs, provides environment-friendly methods regarding energy balance, and creates a pathway for sustainable utilization of massive agricultural waste. However, biohydrogen production is generally limited by lower productivity. Many studies have been conducted aimed at improving biohydrogen production efficiency. Hence, this review is intended to describe improving routes for biohydrogen production from agricultural waste and highlights recent advances in these approaches. In addition, the critical factors affecting biohydrogen production, including the pretreatment method, substrate resource, fermentation conditions, and bioreactor design, were also comprehensively discussed along with challenges and future prospects.

New Insights into Controlling Homoacetogenesis in the Co-digestion of Coffee Waste: Effect of Operational Conditions and Characterization of Microbial Communities

In this research batch reactors were operated with coffee processing waste and autochthonous microbial consortium, and a taxonomic and functional analysis was performed for phase I of stabilization of maximum H₂ production and for phase II of maximum H₂ consumption. During phase I, the reactor's operating conditions were pH 4.84 to 8.18, headspace 33.18% to 66.82%, and pulp and husk from 6.95 to 17.05 g/L. These assays continued for phase II, with initial pH conditions of 5.8-8.1, headspace of 33.18-66.82%, and pulp and husk remaining from phase I. The highest homoacetogenesis was observed in assay 5 with pH 7.7, 40% headspace, and 15 g/L of pulp and husk (initial concentrations of phase I). A relative abundance of Clostridium 41%, Lactobacillus 20% and Acetobacter 14% was observed in phase I. In phase II, there was a change in relative abundance of 21%, 63%, and 1%, respectively, and functional genes involved with autotrophic (formyltetrahydrofolate synthase) and heterotrophic (enolase) homoacetogenesis, butanol (3-hydroxybutyryl-CoA dehydrogenase), and propionic acid (propionate CoA-transferase) were identified. This study provides a new and amplified insight into the physicochemical and microbiological factors, which can be used to propose adequate operational conditions to maximize the bioenergy production and reduce homoacetogenesis in biological reactors.

A preliminary study on a novel bioaugmentation technique enhancing lactic acid production by mixed cultures fermentation

The paper is a preliminary study on the selection of lactic acid producing microorganisms from a mixed microbial population via bioaugmentation. The bioaugmentation technique is based on pH sudden variations occurring in sequential batch steps of a dark fermentation process applied to simple substrates. Different conditions are tested and compared. The structure of microbial communities and concentrations of metabolic intermediates are analyzed to study the possible substrate conversion routes. Obtained results indicate that the initial mixed culture produced a lactic acid percentage of 5% in terms of COD_LA/COD_PRODUCTS. In the most favourable conditions, the selected culture produced a lactic acid percentage of 59%. The analysis of the composition of microbial communities before and after the bioaugmentation processes, indicates that lactic acid production mainly results from the population change to bacteria belonging to the genus Bacillus. Indeed, the relative abundance of Bacilli increased from 0.67%, to 8.40% during the bioaugmentation cycle.

Identification of Hydrogen Gas Producing Anaerobic Bacteria Isolated from Sago Industrial Effluent

In this study, the biohydrogen production ability of isolated strains with sago industrial effluent in anaerobic batch fermentation was investigated. The bacteria responsible for the biohydrogen were isolated and identified as Clostridium sartagoforme NASGE 01 and Enterobacter cloacae NASGE 02. The volume of biohydrogen gas generated from the effluent was determined by gas chromatography (GC) and the organic acids formed during the biohydrogen production were determined by GC equipped with a flame ionization detector (GC-FID). In batch fermentation, C. sartagoforme NASGE 01 produced high amount of biogas (232 ± 11.02 mL/L) and biohydrogen (41.5%) followed by E. cloacae NASGE 02 produced 212.8 ± 8 mL/L biogas containing 31.5% of biohydrogen. Moreover, the hydrogen production potential (P), production rate (Rm) and lag time (λ) were analyzed from Gompertz non-linear curve fit model. The peak hydrogen yield was obtained with C. sartagoforme NASGE 01 was 158.7 mL/g glucose (1.26 mol H₂/mol glucose) with the substrate degradation of 56.7%. Butyric acid was the major organic acid formed while hydrogen production with Clostridium sartagoforme NASGE 01 (176.4 mg/L) and Enterobacter cloacae NASGE 02 (285.1 mg/L). These experimental data demonstrated the feasibility of biohydrogen production using pure culture of anaerobic bacteria with sago industrial waste water as substrate.

Continuous thermophilic hydrogen production from an enzymatic hydrolysate of agave bagasse: Inoculum origin, homoacetogenesis and microbial community analysis

In this research, the performance of two thermophilic inocula of different origin on continuous hydrogen production from an enzymatic hydrolysate of agave bagasse were compared; one of them was obtained from a thermophilic reactor and the second one was taken from a mesophilic reactor and acclimated to thermophilic conditions. The acclimation process in one-step quickly established a high-performance hydrogen producing community, obtaining a volumetric hydrogen production rate of 3811 ± 19 mL H2/L-d with an hydrogen yield of 121 L H2/kg bagasse compared to 1473 ± 6 mL H2/L-d and 26.6 L H2/kg obtained with the thermophilic-origin inoculum. The differences in the performance of both inocula were closely linked to the profile of volatile fatty acids produced, the homoacetogenic pathway and the microbial community, the latter being the determining factor. The use of mesophilic-origin inoculum acclimated to thermophilic conditions can significantly improve the hydrogen production from lignocellulosic bagasse.

Waste-to-energy nexus for circular economy and environmental protection: Recent trends in hydrogen energy

The energy demand has increased exponentially worldwide owing to the continuously growing population and urbanization. The conventional fossil fuels are unable to satiate this requirement causing price inflation and significant environmental damage due to unrestrained emission of greenhouse gases. The focus now has shifted towards alternative, economical, renewable and green sources of energy such as hydrogen to deal with this bottle-neck. Hydrogen is a clean energy-source having high energy content (122 kJ/g). Recently, biological methods for the hydrogen production have attracted much attention because traditional methods are expensive, energy-exhaustive and not eco-friendly. The employment of biological methods promises utilization of waste or low-value materials for producing energy and building waste-to-energy nexus. Around 94% of the waste is discarded precariously in India and waste generation is growing at an alarming rate of 1.3% per year. The "waste-to-energy" techniques follow 'Reuse, Reduce, Recycle, Recovery and Reclamation' system solving three subjects at once; waste-management, energy-demand and environmental concern. Moreover, these methods have easy operability, cost-effectiveness and they help to shift from linear to circular model of economy for sustainable development. Biological processing of waste materials like agricultural discard (lignocellulosic biomass), food-waste and industrial discharge can be used for biohydrogen production. Dark and photo fermentation are the chief biological processes for the transformation of organic substrates to hydrogen. Dark fermentation is the acidogenic fermentation of carbohydrate-rich materials without light and oxygen. Clostridia, Enterobacter and Bacillus spp. are appropriate heterotrophic bacteria for dark fermentation. Various pretreatment methods like heat treatment, acid or base treatment, ultrasonication, aeration, electroporation, etc., can be applied on inoculums to increase H₂ producing bacteria eventually improving the hydrogen yield. However, only around 33% of COD in organic materials is transformed to H₂ by this method. Photofermentation by the photosynthetic non-sulfur bacteria (PNS) converts organic substrate to H₂ and CO₂ in the presence of nitrogenase enzyme in ammonium-limited and anoxygenic conditions. Rhodobacter or Rhodopseudomonas strains have been widely examined in this regard. But these methods are only able to produce H₂ with a poor yield. Combining dark and photofermentation is a noteworthy alternative for procuring enhanced hydrogen yields. Two-stage sequential method utilizes volatile fatty acids accumulated as byproducts after dark fermentation (in the first stage) for photofermentation by suitable bacteria (in the second stage). A proper investigation of the dark fermenter effluents is required before using them as a substrate for photo-fermentation. In a single-stage dark and photofermentation, co-culture of anaerobic and PNS bacteria in a single reactor is carried out for obtaining improved yield. The single stage system is comparatively inexpensive and less laborious; moreover, a limited requirement for an intermediate dilution stage is necessary. Economic analysis of hydrogen production showed that H₂ production by the present methods, save pyrolysis, is reasonably higher than the conventional approaches of fuel production. Probable routes to make H₂ production more cost-effective are reducing the cost of photobioreactor, installing proper storage system, etc. A constructive effort in the area of research and development of biological approaches of H₂ production technologies is vital. The commercial viability of biohydrogen production is imperative for accomplishment of circular economy system and sustainable development.

No difference in inhibition among free acids of acetate, propionate and butyrate on hydrogenotrophic methanogen of Methanobacterium formicicum

Free volatile fatty acids such as free acetic acid (FAA) and free butyrate acid (FBA) are true inhibitors of hydrogenotrophic methanogens (HM) in mixed culture. However, their inhibitory effects on pure culture of HM remain unclear. In this study, a typical HM of Methanobacterium formicicum demonstrated no difference in toxicity conferred by FAA, free propionate acid (FPA), or FBA in regard to the specific methanogenic activity (SMA) based on the C_50% (0.19, 0.17, and 0.23 g/L, respectively) and recoverable concentration values (0.97, 0.69, and 0.61 g/L, respectively). These results were within the same order of magnitude. The concentrations of FAA, FBA, and FPA all correlated well with the SMA values according to the inhibition model. Additionally, changes in the activity of the electron transport system also agreed well with the trend in the SMA variation. Together, the results of this study provide a benchmark to control methanogenesis during industrial applications.

Improvement of continuous hydrogen production using individual and binary enzymatic hydrolysates of agave bagasse in suspended-culture and biofilm reactors

Continuous hydrogen (H₂) production from individual (Stonezyme, IH) and binary (Celluclast-Viscozyme, BH) enzymatic hydrolysates of agave bagasse was evaluated in continuous stirred-tank reactors (CSTR) and trickling bed reactors (TBR). The volumetric H₂ production rates (VHPR) in CSTR were 13 and 2.25 L H₂/L-d with BH and IH, respectively. Meanwhile, VHPR of 5.76 and 2.0 L H₂/L-d were obtained in the TBR configuration using BH and IH, respectively. Differences on VHPR between reactors could be explained by substrate availability, which is intrinsic to the growth mode of each reactor configuration; while differences of VHPR between hydrolysates were possibly related to the composition of enzymatic hydrolysates. Furthermore, homoacetogenesis was strongly influenced by H₂ and substrate transfer conditions. Considering VHPR, H₂ yields, and costs of hydrolysis, hydrogen production from binary hydrolysates of agave bagasse was identified as the most promising alternative evaluated with scale-up potential for the production of energy biofuels.

Enhancement of hydrogen production using untreated inoculum in two-stage food waste digestion

This research investigated the possibility to enhance H₂ production using untreated inoculum in a two-stage hydrogen-methane process from food waste. Batch experiments were conducted to evaluate the H₂ production efficiency at different F/M ratios (ranging from 1:1 to 64:1). The results showed that when a proper F/M ratio was selected, significant H₂ production was feasible to be achieved even inoculated with untreated anaerobic sludge. Among the F/M ratios studied, maximum H₂ yield (217.98 mL H₂ g VS^-1 FW) was found in the digester at the F/M of 64:1, which was 93.75 times higher than that of the digester at the F/M of 1:1. Higher hydrogen yield was achieved at the greater F/M ratio, due to the enrichment of the H₂ producing bacteria and the reduction of the antagonistic bacteria. The two-stage process allowed more stable methane production and higher overall energy yield compared to the single-stage process.

Inhibitory effects of free propionic and butyric acids on the activities of hydrogenotrophic methanogens in mesophilic mixed culture fermentation

The aim of this work was to study the inhibitory of free propionic acid (FPA) and free butyric acid (FBA) on enriched hydrogenotrophic methanogens. It demonstrated that concentrations of FPA and FBA were correlated well with the specific methanogenic activity. Coenzyme M concentrations also agreed well with the trends of FPA and FBA. Two fators of C_50% (concentration at 50% inhibition) and C_RC (recoverable concentration from inhibition) were used to quantitively analyze the inhibitory order using the former result of free acetic acid (FAA) and the results of FBA and FPA. The order according to C_50% was FAA (5.2 mM) > FBA (8.3 mM) > FPA (8.5 mM), while for C_RC it was FPA (9.3 mM) > FAA = FBA (13.5 mM). After comparing with literatue, it suggests that the toxicities of these three organic acids are similar. Thus, accumulating free organic acid offers a cost-effective method to inhibit methanogenesis.

Effect of pH and temperature on microbial community structure and carboxylic acid yield during the acidogenic digestion of duckweed

BACKGROUND

Duckweeds (Lemnaceae) are efficient aquatic plants for wastewater treatment due to their high nutrient-uptake capabilities and resilience to severe environmental conditions. Combined with their rapid growth rates, high starch, and low lignin contents, duckweeds have also gained popularity as a biofuel feedstock for thermochemical conversion and alcohol fermentation. However, studies on the acidogenic anaerobic digestion of duckweed into carboxylic acids, another group of chemicals which are precursors of higher-value chemicals and biofuels, are lacking. In this study, a series of laboratory batch experiments were performed to determine the favorable operating conditions (i.e., temperature and pH) to maximize carboxylic acid production from wastewater-derived duckweed during acidogenic digestion. Batch reactors with 25 g/l solid loading were operated anaerobically for 21 days under mesophilic (35 °C) or thermophilic (55 °C) conditions at an acidic (5.3) or basic (9.2) pH. At the conclusion of the experiment, the dominant microbial communities under various operating conditions were assessed using high-throughput sequencing.

RESULTS

The highest duckweed-carboxylic acid conversion of 388 ± 28 mg acetic acid equivalent per gram volatile solids was observed under mesophilic and basic conditions, with an average production rate of 0.59 g/l/day. This result is comparable to those reported for acidogenic digestion of other organics such as food waste. The superior performance observed under these conditions was attributed to both chemical treatment and microbial bioconversion. Hydrogen recovery was only observed under acidic thermophilic conditions, as 23.5 ± 0.5 ml/g of duckweed volatile solids added. More than temperature, pH controlled the overall structure of the microbial communities. For instance, differentially abundant enrichments of Veillonellaceae acidaminococcus were observed in acidic samples, whereas enrichments of Clostridiaceae alkaliphilus were found in the basic samples. Acidic mesophilic conditions were found to enrich acetoclastic methanogenic populations over processing times longer than 10 days.

CONCLUSIONS

Operating conditions have a significant effect on the yield and composition of the end products resulting from acidogenic digestion of duckweed. Wastewater-derived duckweed is a technically feasible alternative feedstock for the production of advanced biofuel precursors; however, techno-economic analysis is needed to determine integrated full-scale system feasibility and economic viability.

Hydrogen production from a thermophilic alkaline waste activated sludge fermenter: Effects of solid retention time (SRT)

This study aims to investigate the effects of solid retention times (SRTs) on hydrogen production via thermophilic alkaline fermentation of waste activated sludge. The reactor was subjected to a SRT from 10 to 6 days during approximately 82 days of operation. The results revealed that SRT had minor influence on hydrolysis and hydrolysis efficiency in different phases were from 48.11% to 50.55%. Nevertheless, the efficiency of acidogenesis process was highly related to SRT and longer SRT could enhance the acidogenesis. On the other hand, acidogenesis efficiency was also related to H₂ partial pressure and high H₂ partial pressure negatively affected the acidogenesis. Thus, the maximum acidification was achieved in phase 1 (21.29%) resulting in the maximum H₂ yield in phase 1 (95.94 mL/g VSS; SRT = 10 days; H₂ partial pressure = 0-18%). Phyla Actinobacteria and Proteobacteria, who are highly related to hydrolytic microbial population, were abundant in all phases that resulted in high hydrolysis extent. H₂ production was attributed to the relative high abundance of Clostridia. Thus, this study suggested that longer SRT and lower H₂ partial pressure was necessary to improve the H₂ yield under alkaline pH condition.

Free acetic acid as the key factor for the inhibition of hydrogenotrophic methanogenesis in mesophilic mixed culture fermentation

The inhibition of acetate under acidic pH is an ideal way to reduce methanogenesis in mesophilic mixed culture fermentation (MCF). However, the effects of acetate concentration and acidic pH on methanogenesis remain unclear. Besides, although hydrogenotrophic methanogens can be suitable targets in MCF, they are generally ignored. Therefore, we intentionally enriched hydrogenotrophic methanogens and found that free acetic acid (FAA, x) concentration and specific methanogenic activity (SMA, y) were correlated according to the equation: y = 0.86 × 0.31/(0.31 + x) (R² = 0.909). The SMA was decreased by 50% and 90% at the FAA concentrations of 0.31 and 2.36 g/L, respectively. The coenzyme M concentration and relative electron transport activity agreed well with the FAA concentration. Moreover, the methanogenic activity could not be recovered when the FAA concentration exceeded 0.81 g/L. These findings indicated that neither acetate nor acidic pH, but FAA was the key factor to inhibit methanogenesis in MCF.

A novel process for volatile fatty acids production from syngas by integrating with mesophilic alkaline fermentation of waste activated sludge

The present study proposed and demonstrated a novel process for the bioconversion of syngas (mainly CO and H₂) to valuable volatile fatty acids (VFA) by integrating with mesophilic alkaline fermentation of waste activated sludge (WAS). The results showed that although pH 9 was suitable for VFA production from WAS, 62.5% of the consumed CO was converted to methane due to the presence of hydrogenogenic pathway for CO conversion. The increase of pH from 9 to 9.5 inhibited the methane production from CO because of the possible presence of only acetogenic pathway for CO conversion. However, methane was still produced from H₂ contained in syngas through hydrogenotrophic methanogenesis, and around 32-34% of the consumed syngas was converted to methane. At both pH 9 and 9.5, methane was produced by hydrogenotrophic methanogens Methanobacteriales. Further increase of pH to 10 effectively inhibited methane production from syngas, and efficient VFA (mainly acetate with the concentration of around 135 mM) production by simultaneous conversion of syngas and WAS was achieved. High acetate concentrations (>150 mM) were shown to have serious negative effects on the conversion of syngas. The addition of syngas to the mesophilic alkaline fermentation of WAS at pH 10 not only resulted in the enrichment of some known bacteria related with syngas conversion, but also changed the microbial community compositions for the fermentation of WAS.

Continuous hydrogen and methane production from Agave tequilana bagasse hydrolysate by sequential process to maximize energy recovery efficiency

Continuous H₂ and CH₄ production in a two-stage process to increase energy recovery from agave bagasse enzymatic-hydrolysate was studied. In the first stage, the effect of organic loading rate (OLR) and stirring speed on volumetric hydrogen production rate (VHPR) was evaluated in a continuous stirred tank reactor (CSTR); by controlling the homoacetogenesis with the agitation speed and maintaining an OLR of 44 g COD/L-d, it was possible to reach a VHPR of 6 L H₂/L-d, equivalent to 1.34 kJ/g bagasse. In the second stage, the effluent from CSTR was used as substrate to feed a UASB reactor for CH₄ production. Volumetric methane production rate (VMPR) of 6.4 L CH₄/L-d was achieved with a high OLR (20 g COD/L-d) and short hydraulic retention time (HRT, 14 h), producing 225 mL CH₄/g-bagasse equivalent to 7.88 kJ/g bagasse. The two-stage continuous process significantly increased energy conversion efficiency (56%) compared to one-stage hydrogen production (8.2%).

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.

Acidogenic fermentation of the organic fraction of municipal solid waste and cheese whey for bio-plastic precursors recovery - Effects of process conditions during batch tests

The problem of fossil fuels dependency is being addressed through sustainable bio-fuels and bio-products production worldwide. At the base of this bio-based economy there is the efficient use of biomass as non-virgin feedstock. Through acidogenic fermentation, organic waste can be valorised in order to obtain several precursors to be used for bio-plastic production. Some investigations have been done but there is still a lack of knowledge that must be filled before moving to effective full scale plants. Acidogenic fermentation batch tests were performed using food waste (FW) and cheese whey (CW) as substrates. Effects of nine different combinations of substrate to inoculum (S/I) ratio (2, 4, and 6) and initial pH (5, 7, and 9) were investigated for metabolites (acetate, butyrate, propionate, valerate, lactate, and ethanol) productions. Results showed that the most abundant metabolites deriving from FW fermentation were butyrate and acetate, mainly influenced by the S/I ratio (acetate and butyrate maximum productions of 21.4 and 34.5g/L, respectively, at S/I=6). Instead, when dealing with CW, lactate was the dominant metabolite significantly correlated with pH (lactate maximum production of 15.7g/L at pH = 9).

Pyrolysis gas as a carbon source for biogas production via anaerobic digestion

Different biomass was pyrolyzed to pyrolysis gas, which was converted to CH₄ by bio-fermentation. SPG was bioupgraded to high quality biogas by the addition of H₂.

Mesophilic and thermophilic alkaline fermentation of waste activated sludge for hydrogen production: Focusing on homoacetogenesis

The present study compared the mesophilic and thermophilic alkaline fermentation of waste activated sludge (WAS) for hydrogen production with focus on homoacetogenesis, which mediated the consumption of H2 and CO2 for acetate production. Batch experiments showed that hydrogen yield of WAS increased from 19.2 mL H2/gVSS at 37 °C and pH 10-80.1 mL H2/gVSS at 55 °C and pH 10. However, the production of volatile fatty acids (mainly acetate) was higher at 37 °C and pH 10 by comparison with 55 °C and pH 10. Hydrogen consumption due to homoacetogenesis was observed at 37 °C and pH 10 but not 55 °C and pH 10. Higher expression levels of genes relating with homoacetogenesis and lower expression levels of genes relating with hydrogen production were found at 37 °C and pH 10 compared to 55 °C and pH 10. The continuous experiment demonstrated the steady-state hydrogen yield of WAS was comparable to that obtained from batch experiments at 55 °C and pH 10, and homoacetogenesis was still inhibited. However, the steady-state hydrogen yield of WAS (6.5 mL H2/gVSS) was much lower than that (19.2 mL H2/gVSS) obtained from batch experiments at 37 °C and pH 10 due to the gradual enrichment of homoacetogens as demonstrated by qPCR analysis. The high-throughput sequencing analysis of 16S rRNA genes showed that the abundance of genus Clostridium, containing several homoacetogens, was 5 times higher at 37 °C and pH 10 than 55 °C and pH 10.

Volatile fatty acids platform from thermally hydrolysed secondary sewage sludge enhanced through recovered micronutrients from digested sludge

The extracellular polymeric substances and microbial cytoplasmic contents seem to hold inorganic ions and organic products, such as proteins and carbohydrates that are of critical importance for the metabolism of hydrolytic and acidogenic anaerobic microorganisms. The addition of soluble microbially recovered nutrients from thermally treated digestate sludge, for the fermentation of thermally hydrolysed waste activated sludge, resulted in higher volatile fatty acids yields (VFAs). The yield of VFAs obtained from the recovered microbial nutrients was 27% higher than the no micronutrients control, and comparable to the yield obtained using a micronutrients commercial recipe. In addition, the use of a low pH resulting from a high sucrose dose to select spore forming acidogenic bacteria was effective for VFA production, and yielded 20% higher VFAs than without the pH shock and this associated with the addition of recovered microbial nutrients would overcome the need to thermally pre-treat the inoculum.

Optimization performance of an AnSBBR applied to biohydrogen production treating whey

The present study investigated the influence of the influent concentration of substrate, feeding time and temperature on the production of biohydrogen from cheese whey in an AnSBBR with liquid phase recirculation. The highest hydrogen yield (0.80 molH2.molLactose(-1)) and productivity (660 mLH2 L(-1) d(-1)) were achieved for influent concentrations of 5400 mgDQO L(-1). No significant difference was noted in the biological hydrogen production for the feeding time conditions analyzed. The lowest temperature tested (15 °C) promoted the highest hydrogen yield and productivity (1.12 molH2 molLactose(-1) and 1080 mLH2 L(-1) d(-1)), and for the highest temperature (45 °C), hydrogen production did not occur. The indicator values for the hydrogen production obtained with this configuration were higher than those obtained in other studies using traditional configurations such as UASBr and CSTR. A phylogenetic analysis showed that the majority of the analyzed clones were similar to Clostridium. In addition, clones phylogenetically similar to the Lactobacilaceae family, notably Lactobacillus rhamnosus, and clones with similar sequences to Acetobacter indonesiensis were observed in small proportion in the reactor.

Selective conversion of carbon monoxide to hydrogen by anaerobic mixed culture

A new method for the conversion of CO to H2 was developed by anaerobic mixed culture in the current study. Higher CO consumption rate was obtained by anaerobic granular sludge (AGS) compared to waste activated sludge (WAS) at 55 °C and pH 7.5. However, H2 was the intermediate and CH4 was the final product. Fermentation at pH 5.5 by AGS inhibited CH4 production, while the lower CO consumption rate (50% of that at pH 7.5) and the production of acetate were found. Fermentation at pH 7.5 with the addition of chloroform achieved efficient and selective conversion of CO to H2. Stable and efficient H2 production was achieved in a continuous reactor inoculated with AGS, and gas recirculation was crucial to increase the CO conversion efficiency. Microbial community analysis showed that high abundance (44%) of unclassified sequences and low relative abundance (1%) of known CO-utilizing bacteria Desulfotomaculum were enriched in the reactor.

Photocatalytic hydrogen production of the CdS/TiO2-WO3 ternary hybrid under visible light irradiation

An attractive and effective method for converting solar energy into clean and renewable hydrogen energy is photocatalytic water splitting over semiconductors. The study aimed at utilizing organic sacrificial agents in water, modeled by formic acid, in combination with visible light driven photocatalysts to produce hydrogen with high efficiencies. The photocatalytic hydrogen production of cadmium sulfide (CdS)/titanate nanotubes (TNTs) binary hybrid with specific CdS content was investigated. After visible light irradiation for 3 h, the hydrogen production rate of 25 wt% CdS/TNT achieved 179.35 μmol·h(-1). Thanks to the two-step process, CdS/TNTs-WO3 ternary hybrid can better promote the efficiency of water splitting compared with CdS/TNTs binary hybrid. The hydrogen production of 25 wt% CdS/TNTs-WO3 achieved 212.68 μmol·h(-1), under the same condition. Coating of platinum metal onto the WO3 could further promote the reaction. Results showed that 0.2 g 0.1 wt% Pt/WO3 + 0.2 g 25 wt% CdS/TNTs had the best hydrogen production rate of 428.43 μmol·h(-1). The resultant materials were well characterized by high-resolution transmission electron microscope, X-ray diffraction, scanning electron microscopy, and UV-Vis spectra.

Inoculum pre-treatment affects the fermentative activity of hydrogen-producing communities in the presence of 5-hydroxymethylfurfural

To enhance the productivity of mixed microbial cultures for fermentative bio-hydrogen production, chemical-physical pre-treatments of the original seed are needed to suppress the activity of hydrogen (H2)-consuming microbes. This approach might influence negatively the composition and diversity of the hydrogen-producing community with consequences on the functional stability of the H2-producing systems in case of perturbations. In this study, we aimed at investigating the effect of different types of pre-treatment on the performance of hydrogen production systems in the presence of an inhibitor, such as 5-hydroxymethylfurfural (HMF). The efficiency and the microbial community structure of batch reactors amended with HMF and inoculated with non-pretreated and pretreated (acid, heat shock, and aeration) anaerobic sludge were evaluated and compared with control systems. The type of pre-treatments influenced the microbial community assembly and activity in inhibited systems, with significant effect on the performance. Cumulative H2 production tests showed that the pre-aerated systems (control and HMF inhibited) were the most efficient, while the difference of the lag phase of the pre-acidified control and HMF-added test was negligible. Analyses of the structure of the enriched microbial community in the systems through PCR-denaturing gradient gel electrophoresis (DGGE) followed by band sequencing revealed that the differences in performance were mostly related to shifts in the metabolic pathways rather than in the predominant species. In conclusion, the findings suggest that the use of specific inoculum pre-treatment could contribute to regulate the metabolic activity of the fermentative H2-producing bacteria in order to enhance the bio-energy production.

Influence of glucose fermentation on CO₂ assimilation to acetate in homoacetogen Blautia coccoides GA-1

Fermentation of glucose influences CO2 assimilation to acetate in homoacetogens. Blautia coccoides was investigated for a better understanding of the metabolic characteristics of homoacetogens in mixotrophic cultures. Batch cultures of the strain with H2/CO2 as a sole carbon source reached an acetate yield of 5.32 g/g dry cell weight after 240 h of incubation. Autotrophic metabolism was inhibited as glucose was added into the culture: the higher the glucose concentration the lower the autotrophic ability of the bacterium. Autotrophy was inhibited by high glucose concentration, probably due to the competition for coenzyme A between the Embden-Meyerhof-Parnas pathway and the Wood-Ljungdahl carbon fixation pathway, the energy (adenosine triphosphate) allocation for synthesis of cell carbon and reduction of CO2, in combination with the low pH caused by the accumulation of acetate.

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

The Biohydrogen Potential (BHP) of six different types of waste biomass typical for the Campania Region (Italy) was investigated. Anaerobic sludge pre-treated with the specific methanogenic inhibitor sodium 2-bromoethanesulfonic acid (BESA) was used as seed inoculum. The BESA pre-treatment yielded the highest BHP in BHP tests carried out with pre-treated anaerobic sludge using potato and pumpkin waste as the substrates, in comparison with aeration or heat shock pre-treatment. The BHP tests carried out with different complex waste biomass showed average BHP values in a decreasing order from potato and pumpkin wastes (171.1 ± 7.3 ml H2/g VS) to buffalo manure (135.6 ± 4.1 ml H2/g VS), dried blood (slaughter house waste, 87.6 ± 4.1 ml H2/g VS), fennel waste (58.1 ± 29.8 ml H2/g VS), olive pomace (54.9 ± 5.4 ml H2/g VS) and olive mill wastewater (46.0 ± 15.6 ml H2/g VS). The digestate was analyzed for major soluble metabolites to elucidate the different biochemical pathways in the BHP tests. These showed the H2 was produced via mixed type fermentation pathways.

Comparative study of the methane production based on the chemical compositions of Mangifera Indica and Manihot Utilissima leaves

Leaves of Mangifera Indica (MI, mango leaves) and Manihot Utilissima (MU, cassava leaves) are available in tropical regions and are the most accessible vegetal wastes of Kinshasa, capital of Democratic Republic of Congo. These wastes are not suitably managed and are not rationally valorized. They are abandoned in full air, on the soil and in the rivers. They thus pollute environment. By contrast, they can be recuperated and treated in order to produce methane (energy source), organic fertilizer and clean up the environment simultaneously. The main objective of this study was to investigate methane production from MI and MU leaves by BMP tests at 30°C. The yields achieved from the anaerobic digestion of up to 61.3 g raw matter in 1 l medium were 0.001 l/g and 0.100 l CH₄/g volatile solids of MI and MU leaves, respectively. The yield of MU leaves was in the range mentioned in the literature for other leaves because of a poor presence of bioactive substrates, and low C/N ratio. This methane yield corresponded to 7% of calorific power of wood. By contrast, the methane yield from MI leaves was almost nil suggesting some metabolism inhibition because of their rich composition in carbon and bioactive substrates. Whereas classical acidogenesis and acetogenesis were recorded.

Therefore, methane production from the sole MI leaves seems unfavorable by comparison to MU leaves at the ambient temperature in tropical regions. Their solid and liquid residues obtained after anaerobic digestion would be efficient fertilizers. However, the methane productivity of both leaves could be improved by anaerobic co-digestion.

Deciphering the electron transfer mechanisms for biogas upgrading to biomethane within a mixed culture biocathode

This study describes the electron transfer mechanism of a BES fed with the effluent from water scrubbing to improve biogas upgrading.

Dark fermentation from real solid waste. Evolution of microbial community

The purpose of this paper was to study the evolution of microbial community and its relation to the hydrogen production (HP) steps in thermophilic-dry dark fermentation from real organic fraction of municipal solid waste (OFMSW). Nine organic loading rates (OLRs) (from 9 to 220 g TVS/l/d) were investigated. Population dynamics study showed that increasing OLR (between 9 and 110 g TVS/l/d) resulted in an increase in the relations between Eubacteria:Archaea and hydrolytic-acidogenic bacteria (HABs):acetogens. This was strongly influenced by the microbial content of the OFMSW. The presence of acetogens and Archaea was due to contribution of these microorganisms in the substrate (the biogas produced was methane-free). The maximum value of hydrolysis (63±7%) was observed at 110 g TVS/l/d OLR according to maximum HP and HAB activity. The highest average values of acidification yields (57-60%) were achieved for OLR between 28 and 43 g TVS/l/d.

Upgrading dilute ethanol from syngas fermentation to n-caproate with reactor microbiomes

Fermentation of syngas from renewable biomass, which is part of the syngas platform, is gaining momentum. Here, the objective was to evaluate a proof-of-concept bioprocessing system with diluted ethanol and acetic acid in actual syngas fermentation effluent as the substrate for chain elongation into the product n-caproic acid, which can be separated with less energy input than ethanol. Chain elongation is performed with open cultures of microbial populations (reactor microbiomes) as part of the carboxylate platform. The highest concentration of n-caproic acid of ~1 g L(-1) was produced at a pH of 5.44 and a production rate of 1.7 g L(-1) day(-1). A higher n-butyrate production rate of 20 g L(-1) day(-1) indicated that product toxicity was limiting the chain elongation step from n-butyric acid to n-caproic acid. This result shows that the syngas and carboxylate platforms can be integrated within a biorefinery, but that product separation is necessary.

Fatty acids production from hydrogen and carbon dioxide by mixed culture in the membrane biofilm reactor

Gasification of waste to syngas (H2/CO2) is seen as a promising route to a circular economy. Biological conversion of the gaseous compounds into a liquid fuel or chemical, preferably medium chain fatty acids (caproate and caprylate) is an attractive concept. This study for the first time demonstrated in-situ production of medium chain fatty acids from H2 and CO2 in a hollow-fiber membrane biofilm reactor by mixed microbial culture. The hydrogen was for 100% utilized within the biofilms attached on the outer surface of the hollow-fiber membrane. The obtained concentrations of acetate, butyrate, caproate and caprylate were 7.4, 1.8, 0.98 and 0.42 g/L, respectively. The biomass specific production rate of caproate (31.4 mmol-C/(L day g-biomass)) was similar to literature reports for suspended cell cultures while for caprylate the rate (19.1 mmol-C/(L day g-biomass)) was more than 6 times higher. Microbial community analysis showed the biofilms were dominated by Clostridium spp., such as Clostridium ljungdahlii and Clostridium kluyveri. This study demonstrates a potential technology for syngas fermentation in the hollow-fiber membrane biofilm reactors.

Effect of temperature and temperature shock on the stability of continuous cellulosic-hydrogen fermentation

Three continuous stirred tank reactors (CSTR) were operated under mesophilic (37 ± 1°C), thermophilic (55 ± 1°C) and hyper-thermophilic (80 ± 1°C) temperatures for 164 days to investigate the effect of temperature and temperature shock on the cellulosic-dark hydrogen fermentation by mixed microflora. During steady state condition, the sudden decreases in the fermentation temperature occurred twice in each condition for 24h. The results show that the 55 ± 1 and 80 ± 1°C presented stable hydrogen yields of 12.28 and 9.72 mmol/g cellulose, respectively. However, the 37 ± 1°C presented low hydrogen yield of 3.56 mmol/g cellulose and methane yield of 5.4 mmol/g cellulose. The reactor performance under 55 ± 1 or 80 ± 1°C appeared to be more resilient to the sudden decreases in the fermentation temperature than 37 ± 1°C. The experimental analysis results indicated that the changing in soluble by-products could explain the effect of temperature and temperature shock, and the thermophilic temperature is expected having a better economic performance for cellulosic-hydrogen fermentation.

Microbial community structure and dynamics in two-stage vs single-stage thermophilic anaerobic digestion of mixed swine slurry and market bio-waste

The microbial community of a thermophilic two-stage process was monitored during two-months operation and compared to a conventional single-stage process. Qualitative and quantitative microbial dynamics were analysed by Denaturing Gradient Gel Electrophoresis (DGGE) and real-time PCR techniques, respectively. The bacterial community was dominated by heat-shock resistant, spore-forming clostridia in the two-stage process, whereas a more diverse and dynamic community (Firmicutes, Bacteroidetes, Synergistes) was observed in the single-stage process. A significant evolution of bacterial community occurred over time in the acidogenic phase of the two-phase process with the selection of few dominant species associated to stable hydrogen production. The archaeal community, dominated by the acetoclastic Methanosarcinales in both methanogen reactors, showed a significant diversity change in the single-stage process after a period of adaptation to the feeding conditions, compared to a constant stability in the methanogenic reactor of the two-stage process. The more diverse and dynamic bacterial and archaeal community of single-stage process compared to the two-stage process accounted for the best degradation activity, and consequently the best performance, in this reactor. The microbiological perspective proved a useful tool for a better understanding and comparison of anaerobic digestion processes.