A comprehensive review on two-stage integrative schemes for the valorization of dark fermentative effluents

Biohydrogen production by lactate-driven dark fermentation of real organic wastes derived from solid waste treatment plants

This study evaluated the hydrogen production potential through lactate-driven dark fermentation (LD-DF) of organic wastes from solid waste treatment plants, including the organic fraction of municipal solid waste (OFMSW), mixed sewage sludge, and two OFMSW leachates. In initial batch fermentations, only OFMSW supported a significant hydrogen yield (70.1 ± 7.7 NmL-H2/g-VS added) among the tested feedstocks. Lactate acted as an important hydrogen precursor, requiring the presence of carbohydrates for sequential two-step lactate-type fermentation. The impact of operational pH (5.5-6.5) and initial total solids (TS) concentration (5-12.5 % w/w) was also evaluated using OFMSW as substrate, obtaining hydrogen yields ranging from 6.6 to 55.9 NmL-H2/g-VSadded. The highest yield occurred at 6.5 pH and 7.5 % TS. The LD-DF pathway was indicated to be present under diverse pH and TS conditions, supported by employing a specialized microbial consortium capable of performing LD-DF, along with the observed changes in lactate levels during fermentation.

Bioelectrochemical systems in aid of sustainable biorefineries for the production of value-added products and resource recovery from wastewater: A critical review and future perspectives

Bioelectrochemical systems (BES) have the potential to be used in a variety of applications such as waste biorefinery, pollutants removal, CO₂ capture, and the electrosynthesis of clean and renewable biofuels or byproducts, among others. In contrast, many technical challenges need to be addressed before BES can be scaled up and put into real-world applications. Utilizing BES, this review article presents a state-of-the-art overall view of crucial concepts and the most recent innovative results and achievements acquired from the BES system. Special attention is placed on a hybrid approach for product recovery and wastewater treatment. There is also a comprehensive overview of waste biorefinery designs that are included. In conclusion, the significant obstacles and technical concerns found throughout the BES studies are discussed, and suggestions and future requirements for the virtual usage of the BES concept in actual waste treatment are outlined.

Hydrodynamic cavitation pre-treatment of urban waste: Integration with acidogenic fermentation, PHAs synthesis and anaerobic digestion processes

Urban waste can be valorized within a biorefinery approach, producing platform chemicals, biopolymers and energy. In this framework, hydrodynamic cavitation (HC) is a promising pre-treatment for improving biodegradability due to its high effectiveness and low cost. This paper deals with the effect of HC pre-treatment on the acidogenic co-fermentation process of thickened sewage sludge from a WWTP and seasonal vegetable waste from a wholesale market. Specifically, HC was assessed by testing two sets of parameters (i.e., treatment time of 30 and 50 min; vacuum pressure 1.4 and 2.0 bar; applied power 8 and 17 kW) to determine its effectiveness as a pre-treatment of the mixture. The highest increase in sCOD (+83%) and VFAs (from 1.93 to 17.29 gCOD_VFA L^-1) was gained after 50 min of cavitation. Fermentations were conducted with not cavitated and cavitated mixtures at 37 °C on 4 L reactors in batch mode, then switched to semi-continuous with OLR of 8 kg_TVS m-³ d^-1 and HRT of 5-6.6 d. Good VFAs concentrations (12.94-18.27 gCOD_VFA L^-1) and yields (0.44-0.53 gCOD_VFA g_VS(0)^-1) were obtained, which could be enhanced by pre-treatment optimization and pH control. The organic acid rich broth obtained was then assessed as a substrate for PHAs storage by C. necator. It yielded 0.37 g g^-1 of polyhydroxybutyrate, such biopolymer resulted to have analogous physicochemical characteristics of commercial equivalent. The only generated side-stream would be the solid-rich fraction of the fermented effluent, which valorization was assessed through BMP tests, showing a higher SGP of 0.42 Nm³ kgTVS^-1 for the cavitated.

Dark fermentative biohydrogen production from vinicultural biomass without exogenous inoculum in a semi-batch reactor: A kinetic study

This work employed a unique kind of vinicultural biomass (grape residues) to generate fermentative hydrogen. This form of biomass serves two purposes (contains substrate and inoculum). Four mathematical model methods were established; these models were used to represent the fluctuation of hydrogen generation and other fermentation products (organic acids, alcohols), the consumption of substrates included in biomass, and bacterial growth. One of these models was verified using experimental data and used to represent all of the metabolic pathways of bacteria contained in the medium and the interaction between products and substrates. The optimal biomass load, 60 g COD (Chemical Oxygen Demand)/L with a concentration of 0.22 mol of hexose and 0.0444 mol of tartrate offers the best hydrogen yield.

An Overview of Recent Advancements in Microbial Polyhydroxyalkanoates (PHA) Production from Dark Fermentation Acidogenic Effluents: A Path to an Integrated Bio-Refinery

Global energy consumption has been increasing in tandem with economic growth motivating researchers to focus on renewable energy sources. Dark fermentative hydrogen synthesis utilizing various biomass resources is a promising, less costly, and less energy-intensive bioprocess relative to other biohydrogen production routes. The generated acidogenic dark fermentative effluent [e.g., volatile fatty acids (VFAs)] has potential as a reliable and sustainable carbon substrate for polyhydroxyalkanoate (PHA) synthesis. PHA, an important alternative to petrochemical based polymers has attracted interest recently, owing to its biodegradability and biocompatibility. This review illustrates methods for the conversion of acidogenic effluents (VFAs), such as acetate, butyrate, propionate, lactate, valerate, and mixtures of VFAs, into the value-added compound PHA. In addition, the review provides a comprehensive update on research progress of VFAs to PHA conversion and related enhancement techniques including optimization of operational parameters, fermentation strategies, and genetic engineering approaches. Finally, potential bottlenecks and future directions for the conversion of VFAs to PHA are outlined. This review offers insights to researchers on an integrated biorefinery route for sustainable and cost-effective bioplastics production.

Dynamic membrane bioreactor for high rate continuous biohydrogen production from algal biomass

This study aimed to achieve continuous biohydrogen production from red algal biomass using a dynamic membrane bioreactor (DMBR). The DMBR was continuously fed with pretreated Echeuma spinosum containing 20 g/L hexose. The highest average hydrogen production rate (HPR) of 21.58 ± 1.59 L/L-d was observed at HRT 3 h, which was higher than previous reports for continuous H₂ production from biomass feedstock. Metabolic flux analysis revealed that butyric acid and propionic acid were the major by-products of the H₂-producing and H₂-consuming pathways, respectively, of the algal biomass fermentation. Hydrogen consumption by propionic acid pathway could not be prevented completely by heat treatment. PICRUSt2 analysis predicted that Clostridium sp., Anaerostipes sp., and Caproiciproducens sp. might significantly contribute to the expression of both ferredoxin hydrogenase and propionate CoA-transferase. This study would provide the design and operational information on high-rate bioreactor for continuous hydrogen production using biomass.

Use of Biochar-Based Cathodes and Increase in the Electron Flow by Pseudomonas aeruginosa to Improve Waste Treatment in Microbial Fuel Cells

In this paper, we tested the combined use of a biochar-based material at the cathode and of Pseudomonas aeruginosa strain in a single chamber, air cathode microbial fuel cells (MFCs) fed with a mix of shredded vegetable and phosphate buffer solution (PBS) in a 30% solid/liquid ratio. As a control system, we set up and tested MFCs provided with a composite cathode made up of a nickel mesh current collector, activated carbon and a single porous poly tetra fluoro ethylene (PTFE) diffusion layer. At the end of the experiments, we compared the performance of the two systems, in the presence and absence of P. aeruginosa, in terms of electric outputs. We also explored the potential reutilization of cathodes. Unlike composite material, biochar showed a life span of up to 3 cycles of 15 days each, with a pH of the feedstock kept in a range of neutrality. In order to relate the electric performance to the amount of solid substrates used as source of carbon and energy, besides of cathode surface, we referred power density (PD) and current density (CD) to kg of biomass used. The maximum outputs obtained when using the sole microflora were, on average, respectively 0.19 Wm−2kg−1 and 2.67 Wm−2kg−1, with peaks of 0.32 Wm−2kg−1 and 4.87 Wm−2kg−1 of cathode surface and mass of treated biomass in MFCs with biochar and PTFE cathodes respectively. As to current outputs, the maximum values were 7.5 Am−2 kg−1 and 35.6 Am−2kg−1 in MFCs with biochar-based material and a composite cathode. If compared to the utilization of the sole acidogenic/acetogenic microflora in vegetable residues, we observed an increment of the power outputs of about 16.5 folds in both systems when we added P. aeruginosa to the shredded vegetables. Even though the MFCs with PTFE-cathode achieved the highest performance in terms of PD and CD, they underwent a fouling episode after about 10 days of operation, with a dramatic decrease in pH and both PD and CD. Our results confirm the potentialities of the utilization of biochar-based materials in waste treatment and bioenergy production.

Advanced biomaterials for sustainable applications in the food industry: Updates and challenges

Maintaining the safety and quality of food are major concerns while developing biomaterial based food packaging. It offers a longer shelf-life as well as protection and quality control to the food based on international standards. Nano-biotechnology contributes to a far extent to make advanced packaging by developing multifunctional biomaterials for potential applications providing smarter materials to consumers. Applications of nano-biocomposites may thus help to deliver enhanced barrier, mechanical strength, antimicrobial and antioxidant properties to novel food packaging materials. Starch derived bioplastics, polylactic acid and polyhydroxybutyrate are examples of active bioplastics currently in the food packaging sector. This review discusses the various types of biomaterials that could be used to improve future smarter food packaging, as well as biomaterials' potential applications as food stabilizers, pathogen control agents, sensors, and edible packaging materials. The regulatory concerns related to the use of biomaterials in food packaging and commercially available biomaterials in different fields are also discussed. Development of novel biomaterials for different food packaging applications can therefore guarantee active food packaging in future.

A strategy for successive feedstock reuse to maximize photo-fermentative hydrogen production of Arundo donax L

This study proposed a strategy to maximize the hydrogen yield by reusing feedstock of Arundo donax L. For this purpose, a successive 4-batch photo-fermentative hydrogen production (PFHP) was carried out to test the strategy. About 50% of total hydrogen yield was additionally obtained by reusing the Arundo donax L for successive 4 times in comparison to single 1st batch (161.4 mL/U. cell dry weight). In addition to the highest hydrogen yield, the maximum hydrogen production rate (6.0 mL/U. cell dry weight /h), and the highest volatile fatty acids (VFAs) concentration (32 mM) were also obtained from the 1st batch, while the 2nd batch gave the maximum substrate conversion efficiency (96.5%). Moreover, a positive relationship between the sum of acetic and butyric acids with hydrogen yields was observed. This strategy would help in enhancing hydrogen yield that coupled with cost reduction for biohydrogen production.

Electro-fermentation for biofuels and biochemicals production: Current status and future directions

Electro-fermentation is an emerging bioporcess that could regulate the metabolism of electrochemically active microorganisms. The provision of electrodes for the fermentation process that functions as an electron acceptor and supports the formation and transportation of electrons and protons, consequently producing bioelectricity and value-added chemicals. The traditional method of fermentation has several limitations in usability and economic feasibility. Subsequently, a series of metabolic processes occurring in conventional fermentation processes are most often redox misaligned. In this regard, electro-fermentation emerged as a hybrid technology which can regulate a series of metabolic processes occurring in a bioreactor by regulating the redox instabilities and boosting the overall metabolic process towards high biomass yield and enhanced product formation. The present article deals with microorganisms-electrode interactions, various types of electro-fermentation systems, comparative evaluation of pure and mixed culture electro-fermentation application, and value-added fuels and chemical synthesis.

Digestate as Sustainable Nutrient Source for Microalgae—Challenges and Prospects

The interest in microalgae products has been increasing, and therefore the cultivation industry is growing steadily. To reduce the environmental impact and production costs arising from nutrients, research needs to find alternatives to the currently used artificial nutrients. Microalgae cultivation in anaerobic effluents (more specifically, digestate) represents a promising strategy for increasing sustainability and obtaining valuable products. However, digestate must be processed prior to its use as nutrient source. Depending on its composition, different methods are suitable for removing solids (e.g., centrifugation) and adjusting nutrient concentrations and ratios (e.g., dilution, ammonia stripping). Moreover, the resulting cultivation medium must be light-permeable. Various studies show that growth rates comparable to those in artificial media can be achieved when proper digestate treatment is used. The necessary steps for obtaining a suitable cultivation medium also depend on the microalgae species to be cultivated. Concerning the application of the biomass, legal aspects and impurities originating from digestate must be considered. Furthermore, microalgae species and their application fields are essential criteria when selecting downstream processing methods (harvest, disintegration, dehydration, product purification). Microalgae grown on digestate can be used to produce various products (e.g., bioenergy, animal feed, bioplastics, and biofertilizers). This review gives insight into the origin and composition of digestate, processing options to meet requirements for microalgae cultivation and challenges regarding downstream processing and products.

A Comprehensive Understanding of Electro-Fermentation

Electro-fermentation (EF) is an upcoming technology that can control the metabolism of exoelectrogenic bacteria (i.e., bacteria that transfer electrons using an extracellular mechanism). The fermenter consists of electrodes that act as sink and source for the production and movement of electrons and protons, thus generating electricity and producing valuable products. The conventional process of fermentation has several drawbacks that restrict their application and economic viability. Additionally, metabolic reactions taking place in traditional fermenters are often redox imbalanced. Almost all metabolic pathways and microbial strains have been studied, and EF can electrochemically control this. The process of EF can be used to optimize metabolic processes taking place in the fermenter by controlling the redox and pH imbalances and by stimulating carbon chain elongation or breakdown to improve the overall biomass yield and support the production of a specific product. This review briefly discusses microbe-electrode interactions, electro-fermenter designs, mixed-culture EF, and pure culture EF in industrial applications, electro methanogenesis, and the various products that could be hence generated using this process.

Enhancement of dark fermentative H2 production by gas separation membranes: A review

Biohydrogen production via dark fermentation is currently the most developed method considering its practical readiness for scale-up. However, technological issues to be resolved are still identifiable and should be of concern, particularly in terms of internal mass transfer. If sufficient liquid-to-gas H₂ mass transfer rates are not ensured, serious problems associated with the recovery of biohydrogen and consequent inhibition of the process can occur. Therefore, the continuous and effective removal of H₂ gas is required, which can be performed using gas separation membranes. In this review, we aim to analyze the literature experiences and knowledge regarding mass transfer enhancement approaches and show how membranes may contribute to this task by simultaneously processing the internal (headspace) gas, consisting mainly of H₂ and CO₂. Promising strategies related to biogas recirculation and integrated schemes using membranes will be presented and discussed to detect potential future research directions for improving biohydrogen technology.

A review on the conversion of volatile fatty acids to polyhydroxyalkanoates using dark fermentative effluents from hydrogen production

The production of bio/microbial-based polymers, polyhydroxyalkanoates (PHAs) from volatile fatty acids (VFAs) of dark fermentative effluents in the bio-H₂ reactor is being paid attention, owing to their commercial demand, applications and as carbon as well as energy storage source. Since, they are the cheap precursors for such valuable renewable biopolymers which all possess the properties; those are analogous to the petro-derived plastics. Several studies were stated, related to the consumption of both individual and mixed VFAs for the potential PHAs production. Their biodegradability nature makes them extremely desirable alternative to fossil-derived synthetic polymers. In this regard, this review summarizes the use of bio-based PHAs production via both microbial and biochemical pathways using dark fermentative bio-H₂ production from waste streams as feedstock. Furthermore, this review deals the characteristics, synthesis and production of the bio-based PHAs along with their co-polymers and applications to give an outlook on future research.

Whey and molasses as inexpensive raw materials for parallel production of biohydrogen and polyesters via a two-stage bioprocess: New routes towards a circular bioeconomy

Consecutive dark-fermentation and photo-fermentation stages were investigated for a profitable circular bio-economy. H₂ photo-production versus poly(3-hydroxybutyrate) (P3HB) accumulation is a modern biotechnological approach to use agro-food industrial byproducts as no-cost rich-nutrient medium in eco-sustainable biological processes. Whey and molasses are very important byproducts rich in nutrients that lactic acid bacteria can convert, by dark-fermentation, in dark fermented effluents of whey (DFE_W) and molasses (DFE_M). These effluents are proper media for culturing purple non-sulfur bacteria, which are profitable producers of P3HB and H₂. The results of the present study show that Lactobacillus sp. and Rhodopseudomonas sp. S16-VOGS3 are two representative genera for mitigation of environmental impact. The highest productivity of P3HB (4.445 mg/(L·h)) was achieved culturing Rhodopseudomonas sp. S16-VOGS3, when feeding the bacterium with 20% of DFE_M; the highest H₂ production rate of 4.46 mL/(L·h) was achieved when feeding the bacterium with 30% of DFE_M.

Hydrogen and electrical energy co-generation by a cooperative fermentation system comprising Clostridium and microbial fuel cell inoculated with port drainage sediment

This research work has succeeded in recovering energy from glucose by generating H₂ with the aid of a Clostridium beijerinckii strain and obtaining electrical energy from compounds present in the H₂ fermentation effluent in a microbial fuel cell (MFC) seeded with native port drainage sediment. In the fermentation step, 49.5% of the initial glucose concentration (56 mmol/L) was used to produce 104 mmol/L H₂; 5, 33, 3, and 1 mmol/L acetate, butyrate, lactate, and ethanol also emerged, respectively. MFC tests by feeding the anodic compartment with acetate, butyrate, lactate (individually or as a mixture), or the H₂ fermentation effluent provided power density values ranging between 0.6 and 1.2 W/m². Acetate furnished the highest power density with a nanowire-rich biofilm despite the lowest anode bacterial concentration (10¹² 16S gene copies/g of sediment). Non-conventional exoelectrogenic microbial communities were observed in the acetate-fed MFC; e.g., Pseudomonadaceae (Pseudomonas) and Clostridia (Acidaminobacter, Fusibacter).