A redox-based strategy to enhance propionic and butyric acid production during anaerobic fermentation

Enhancement of 3-hydroxyvalerate fraction in poly(3-hydroxybutyrate-co-3-hydroxyvalerate) produced by Haloferax mediterranei fed with food waste pretreated via arrested anaerobic digestion integrated with microbial electrolysis cells

Bioplastics made of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) with a 20 mol% HV fraction are highly desirable in the market for 3-Hydroxyvalerate (HV)-conferred superior thermal, biological, and mechanical properties. Although Haloferax mediterranei (HM) is capable of producing PHBV from food waste, its HV fraction is generally lower than 10 mol%. This study for the first time investigated the engineering approach to increasing HV fraction through elevating the propionic and valeric acid fractions in volatile fatty acids (VFAs) produced from food waste via arrested anaerobic digestion with and without microbial electrolysis cells (MECs) incorporation. Results showed that HV fraction in PHBV produced by HM is proportional to the fractions of propionic and valeric acids in VFAs. A 20 mol% HV fraction can be achieved by MECs incorporation, which might be attributable to pH regulation by the MECs. These findings lay a foundation for developing waste-processing technologies that enable the production of high-value, microbially-derived materials.

Effect of anthocyanins on the in vitro fermentation of high-amylose starch

Resistant starch provides beneficial metabolites through gut microbiota fermentation, and the gut microbiota is modulated by the gut redox status. However, the impact of redox status on high-amylose maize starch (HAMS) fermentation metabolism remains uncertain. Anthocyanins (AC) are dietary antioxidants that modulate the redox status. The effect of antioxidant AC on the fermentation metabolism of HAMS was examined using an in vitro fermentation system. AC lowers the system redox potential. The antioxidant AC and fermentation substrate HAMS (HAMS-AC) synergistically increased gas production and short-chain fatty acids and reduced the pH compared to the individual components. AC promotes HAMS fermentation metabolism to produce butyric acid. HAMS-AC also influences gut microbiota composition and metabolic functions. It facilitated beneficial bacteria proliferation, including Collinsella, Faecalibacterium, Agathobacter, Ruminococcus, and Megasphaera, and suppressed harmful bacteria proliferation, including Desulfovibrio, Barnesiella, Alistipes, Parabacteroides, Dorea, Colidextribacter, and Bilophila. HAMS-AC facilitates amino acid and protein synthesis and metabolism, sugar, nucleotide, energy metabolism, and transport. Overall, the antioxidant AC, by lowering the redox potential of the fermentation system, affects the gut microbiota structure and metabolic function and contributes to HAMS metabolism to produce butyric acid. This provides a novel approach for the modulation of intestinal homeostasis and organismal health.

Scalable propionic acid production using Cutibacterium acnes ZW-1: Insights into substrate and pH-driven carbon flux

An acid-resistant Cutibacterium acnes ZW-1 was isolated from human skin, and propionic acid (PA) production under different substrate and pH conditions was studied. When the molar ratio of lactic acid (LA) to acetic acid (AA) was 7:1 and the pH was 6.5, the PA concentration could reach 64.84 mM. Meanwhile, the index analysis and enzyme activity revealed that the PA carbon flux was 59 %, the PA electronic efficiency reached 79 %, and the propionyl-CoA carboxylase activity was 1.075 mmol·mg protein-1. Considering the competition between AA/PA production and biomass synthesis, although the slightly acidic pH (<6.5) would promote the flow of carbon to PA, its concentration was severely inhibited due to the limitation of biomass. Further scale-up verification in an automated bioreactor indicated that PA production improved, up to 83.31 mM, and the production rate reached 1.066 g·L-1·d-1. This work may provide support for the industrial application of PA bioproduction.

The enhancement of caproic acid synthesis from organic solid wastes: A review

Organic solid waste (OSW) significantly harms the environment and threatens human health. Producing caproic acid (CA) from OSW presents a cost-effective, sustainable, and resource-efficient solution. This study comprehensively examines the various methods for synthesizing CA from OSW, focusing on waste material selection, pretreatment processes to improve dissolution and hydrolysis of OSW, key substrates, and optimization strategies. Using OSW resources has been extensively studied and applied across numerous industries, presenting a promising solution for reducing environmental pollution. This study provides insights into CA synthesis pathways and substrate selection while emphasizing the optimization of CA production from OSW. It also highlights key areas for future research.

Perspectives on carboxylates generation from Ecuadorian agro-wastes

Carboxylates generation from banana (peel and pulp), coffee, and cacao fermentation agro-waste, upon uncontrolled and controlled pHs of 6.6 (heat-driven methanogens inactivation) and 5.2 (pH inactivation), was studied. Regarding volatile fatty acids (VFAs), acetic was the highest for cocoa (96.2 g kg-1TVS) at pH 4.5. However, butyric was relevant for banana pulp (90.7 g kg-1TVS), at controlled pH 6.6. The highest medium chain fatty acid (MCFAs) level was hexanoic (cocoa, 3.5 g kg-1TVS), while octanoic reached a maximum of 2.8 g kg-1TVS for coffee at pH 6.6. At pH 5.2 MCFAs yield was relatively low. Uncontrolled pH conditions, using banana resulted in superior VFAs production compared to controlled conditions. Thus, pH became a determining variable when deciding the time and kind of carboxylic acid to be recovered. The bacterial community at the end of the chain elongation process was dominated by phyla Firmicutes, and Clostridium as the most common genera.

Comprehensive study on pilot nitrification-sludge fermentation coupled denitrification system with extended sludge retention time

The sludge fermentation-coupled denitrification process, utilized for sludge reduction and nitrogen removal from wastewater, is frequently hindered by its hydrolysis step's efficacy. This study addresses this limitation by extending the sludge retention time (SRT) to 120 days. As a result, the nitrate removal efficiency (NRE) of the nitrification-sludge fermentation coupled denitrification (NSFD) pilot system increased from 67.1 ± 0.2 % to 96.7 ± 0.1 %, and the sludge reduction efficiency (SRE) rose from 40.2 ± 0.5 % to 62.2 ± 0.9 %. Longer SRT enhanced predation and energy dissipation, reducing intact cells from 99.2 % to 78.0 % and decreasing particle size from 135.2 ± 4.6 μm and 19.4 ± 2.1 μm to 64.5 ± 3.5 μm and 15.5 ± 1.6 μm, respectively. It also created different niches by altering the biofilm's adsorption capacity, with interactions between these niches driving improved performance. In conclusion, extending SRT optimized the microbial structure and enhanced the performance of the NSFD system.

Unraveling the effects and mechanisms of microplastics on anaerobic fermentation: Exploring microbial communities and metabolic pathways

To investigate the effects of microplastics (MPs) on hydrolysis, acidification and microbial characteristics during waste activated sludge (WAS) anaerobic fermentation process, five different kinds of MPs were added into the WAS fermentation system and results indicated that, compared to the control group, the addition of polyvinyl chloride (PVC)-MPs exhibited the least inhibition on volatile fatty acids (VFAs), reducing them by 13.49 %. Conversely, polyethylene (PE)-MPs resulted in the greatest inhibition, with a reduction of 29.57 %. MPs, while accelerated the dissolution of WAS that evidenced by an increase of lactate dehydrogenase (LDH) release, concurrently inhibited the activities of relevant hydrolytic enzymes (α-Glucosidase, protease). For microbial mechanisms, MPs addition affected the proliferation of key microorganisms (norank_f_Bacteroidetes_vadinHA17, Ottowia, and Propioniclava) and reduced the abundance of genes associated with hydrolysis and acidification (pfkb, gpmI, ilvE, and aces). Additionally, MPs decreased the levels of key hydrolytic and acidogenic enzymes to inhibit hydrolysis and acidification processes. This research provides a basis for understanding and unveils impact mechanisms of the impact of MPs on sludge anaerobic fermentation.

Electrogenerated singlet oxygen and reactive chlorine species enhancing volatile fatty acids production from co-fermentation of waste activated sludge and food waste: The key role of metal oxide coated electrodes

Electrochemical pretreatment (EPT) has shown to be superior in improving acidogenic co-fermentation (Co-AF) of waste activated sludge (WAS) and food waste (FW) for volatile fatty acids (VFAs). However, the influence of EPT electrode materials on the production of electrogenerated oxidants (such as singlet oxygen (1O2) and reactive chlorine species (RCS)), as well as their effects on properties of electrodes, the microbial community structure and functional enzymes remain unclear. Therefore, this study investigated the effects of various metal oxide coated electrodes (i.e., Ti/PbO2, Ti/Ta2O5-IrO2, Ti/SnO2-RuO2, and Ti/IrO2-RuO2) on EPT and subsequent Co-AF of WAS-FW. The results showed that EPT with Ti/PbO2, Ti/Ta2O5-IrO2, Ti/SnO2-RuO2 and Ti/IrO2-RuO2 electrodes generated 165.3-848.2 mg Cl2/L of RCS and 5.643 × 1011-3.311 × 1012 spins/mm3 of 1O2, which significantly enhanced the solubilization and biodegradability of WAS-FW by 106.4 %-233.6 % and 177.3 %-481.8 %, respectively. Especially with Ti/Ta2O5-IrO2 as the electrode material, an appropriate residual RCS (2.0-10.4 mg Cl2/L) remained in Co-AF step, resulted in hydrolytic and acidogenic bacteria (e.g., Prevotella_7, accounting for 78.9 %) gradually become dominant rather than methanogens (e.g., Methanolinea and Methanothrix) due to their different tolerance to residual RCS. Meanwhile, the functional gene abundances of hydrolytic and acidogenic enzymes increased, while the methanogenic enzymes deceased. Consequently, this reactor produced the highest VFAs up to 545.5 ± 36.0 mg COD/g VS, which was 101.8 % higher than that of the Control (without EPT). Finally, the economic analysis and confirmatory experiments further proved the benefits of WAS-FW Co-AF with EPT.

Inhibition of hyphal formation together with biochar addition promotes erythritol production by Yarrowia lipolytica

This study aimed to investigate the effect of hyphal formation in Yarrowia lipolytica and biochar addition on erythritol production by submerged fermentation. Hyphal formation significantly inhibited erythritol production by Y. lipolytica. Transcriptome analysis suggested that the impaired erythritol synthesis of hyphal cells was associated with the differential expression of genes involved in amino acid metabolism, lipid metabolism, and cell wall stability. Deletion of RAS2 responsible for yeast-to-hypha transition and EYD1 included in erythritol degradation blocked hyphal formation and improved erythritol production. Biochar prepared from corncob, sugarcane bagasse (SB), corn straw, peanut shell, coconut shell, and walnut shell (WS) had a positive effect on erythritol production, of which WS pyrolyzed at 500°C (WSc) performed the best in flask fermentation. In a 3.7 L bioreactor, 220.20 ± 10 g/L erythritol with a productivity of 2.30 ± 0.10 g/L/h was obtained in the presence of 1.4% (w/v) WSc and 0.7% SBc (SB pyrolyzed at 500°C) within 96 h. These results suggest that inhibition of hyphal formation together with biochar addition is an efficient way to promote erythritol production.

Promoting short-chain fatty acids production from sewage sludge via acidogenic fermentation: Optimized operation factors and iron-based persulfate activation system

Promoting short-chain fatty acids (SCFAs) production and ensuring the stability of SCFAs-producing process are becoming the two major issues for popularizing the acidogenic fermentation (AF). The key controlling operating and influencing factors during anaerobic fermentation process were thoroughly reviewed to facilitate better process performance prediction and to optimize the process control of SCFAs promotion. The wide utilization of iron salt flocculants during wastewater treatment could result in iron accumulating in sewage sludge which influenced AF performance. Additionally, appropriate ferric chloride (FC) could promote the SCFAs accumulation, while poly ferric sulfate (PFS) inhibited the bioprocess. Iron/persulfate (PS) system was proved to effectively enhance the SCFAs production while mechanism analysis revealed that the strong oxidizing radicals remarkably enhanced the solubilization and hydrolysis. Moreover, the changes of oxidation-reduction potential (ORP) and pH caused by iron/PS system exhibited more negative effects on the methanogens, comparing to the acidogenic bacteria. Furthermore, performance and mechanisms of different iron species-activating PS, organic chelating agents and iron-rich biochar derived from sewage sludge were also elucidated to extend and strengthen understanding of the iron/PS system for enhancing SCFAs production. Considering the large amount of generated Fe-sludge and the multiple benefits of iron activating PS system, carbon neutral wastewater treatment plants (WWTPs) were proposed with Fe-sludge as a promising recycling composite to improve AF performance. It is expected that this review can deepen the knowledge of optimizing AF process and improving the iron/PS system for enhancing SCFAs production and provide useful insights to researchers in this field.

Biochar applications in microbial fermentation processes for producing non-methane products: Current status and future prospects

The objective of this review is to encourage the technical development of biochar-assisted microbial fermentation. To this end, recent advances in biochar applications for microbial fermentation processes (i.e., non-methane products of hydrogen, acids, alcohols, and biofertilizer) have been critically reviewed, including process performance, enhanced mechanisms, and current research gaps. Key findings of enhanced mechanisms by biochar applications in biochemical conversion platforms are summarized, including supportive microbial habitats due to the immobilization effect, pH buffering due to alkalinity, nutrition supply due to being rich in nutrient elements, promoting electron transfer by acting as electron carriers, and detoxification of inhibitors due to high adsorption capacity. The current technical limitations and biochar's industrial applications in microbial fermentation processes are also discussed. Finally, suggestions like exploring functionalized biochar materials, biochar's automatic addition and pilot-scale demonstration are proposed. This review would further promote biochar applications in microbial fermentation processes for the production of non-methane products.

Biological methanation in trickle bed reactors - a critical review

Biological methanation of H₂ and CO₂ in trickle bed reactors is a promising energy conversion and storage approach that can support the energy transition towards a renewable-based system. Research in trickle bed reactor design and operation has significantly increased in recent years, but most studies were performed at laboratory scale and conditions. This review provides a comprehensive overview of the trickle bed reactor concept and current developments to support the decision-making process for future projects. In particular, the key design and operational parameters, such as trickling or nutrient provision, are presented, introducing the most recent advances. Furthermore, reactor operation, including the inoculation, long-term and dynamic operation, is described. To better assess the reactor upscaling, several parameters that enable reactor comparison are discussed. On the basis of this review, suitable operational strategies and further research needs were identified that will improve the overall trickle bed reactor performance.

Enhancing anaerobic fermentation of waste activated sludge by investigating multiple electrochemical pretreatment conditions: Performance, modeling and microbial dynamics

Electrochemical pretreatment (EPT) is an efficient technology to improve volatile fatty acids (VFAs) production during anaerobic fermentation of waste activated sludge (WAS). This study investigated the co-effects of different current intensities, electrolyte NaCl dosage and pretreatment time for promoting VFAs production. The results showed that it was considerably enhanced by 51.6 % when EPT was performed at 1.0 A, 1.0 g/L and 60 min, and response surface methodology strategy adjusted the optimal EPT conditions to 1.0 A, 1.2 g/L and 66 min. The potential mechanisms were proposed that EPT at optimal conditions greatly enhanced solubilization and hydrolysis of WAS and selectively inactivated methanogens, causing the enrichment of acidogenic bacteria (i.e., Lactobacillus, Saccharimonadales, Tetrasphaera and Prevotella) due to generated reactive chlorine species. Finally, the economic analysis indicated the promising application potential with the profit of EPT at optimal conditions increasing by 36.0 %.

Biohydrogen production from brown algae fermentation: Relationship between substrate reduction degree and hydrogen production

In this study, mannitol and mannitol-rich seaweed were fermented to investigate the relationship between substrate reduction degree and hydrogen production performance. The results showed that acetate was required in mannitol fermentation with an optimum acetate/mannitol mass ratio of 1:5. Hydrogen production and yield of mannitol fermentation reached 123.76 mL and 2.12 mol/mol-mannitol, respectively, 42.02 % and 26.95 % higher than that of glucose, respectively. The acetate was fully assimilated and the butyrate selectivity reached 100 % in the effluent. Redox potential and electron distribution showed that mannitol increased the overall electron input from mannitol and acetate, leading to the increase in hydrogen and butyrate generation. Hydrogen yield reached 2.33 mol/mol-mannitol with brown algae hydrolysate, which was the highest ever reported. This study demonstrated that substrate with a higher reduction degree could yield higher hydrogen and showed the great application potential of brown algae fermentation for the co-production of hydrogen and butyrate.