Production of volatile fatty acid from fruit waste by anaerobic digestion at high organic loading rates: Performance and microbial community characteristics

Combined semi-continuous feeding and stripping: Improving volatile fatty acid production and sulfate reduction in a two-phase anaerobic sulfate reduction system

This study developed a two-phase anaerobic sulfate reduction system with semi-continuous feeding, integrated with vapor stripping, to assess its treatment performance at varying chemical oxygen demand (COD)/SO42- ratios. At a COD/SO42- ratio of 2, the system achieved high removal efficiencies of 91.85 % for COD and 92.70 % for sulfate, respectively. Vapor stripping effectively maintained low free H2S concentrations, remaining below 50 mg/L in the first-phase reactor (Ra) and below 30 mg/L in the second-phase reactor (Rm). Additionally, the scouring effect of vapor stripping altered the functional group composition on the sludge surface, promoting the formation of sulfate-reducing granular sludge. Microbial analysis revealed a synchronized enrichment of Desulfovibrio and Enterococcus in Ra, as well as Methanosaeta and Enterococcus in Rm under a semi-continuous feeding regime combined with vapor stripping. Conductivity measurements and correlation analyses suggested that electroactive Methanosaeta and Enterococcus engaged in syntrophic metabolism via extracellular electron transfer, which was particularly beneficial for organic removal under low COD/SO42- ratio conditions. At a COD/SO42- ratio of 2, dissimilatory sulfate reduction (DSR) prevailed in Ra, whereas assimilatory sulfate reduction (ASR) dominated in Rm. This balance optimized sulfate removal efficiency while mitigating sulfide toxicity toward methanogens. Overall, this study presents a promising approach for the efficient treatment of high-sulfate organic wastewater with low COD/SO42- ratio.

Harnessing conductive materials for sustainable food waste treatment: Comparative evaluation of biochar and magnetite in volatile fatty acid production

Anaerobic fermentation offers a sustainable and eco-friendly approach to converting food waste (FW) into high-value volatile fatty acids (VFAs). Although both carbon- and metal-based conductive materials can enhance VFAs yields during FW fermentation, a comprehensive comparison of these materials and their underlying mechanisms remains unexplored, limiting their practical selection and application. This study systematically investigated the differences between biochar and magnetite, which are representative carbon- and metal-based conductive materials, in promoting VFAs production during FW fermentation. The results indicated that VFAs yields increased by 106 % and 81.2 % in the presence of biochar and magnetite, respectively, compared to the control, with notable enhancements during critical fermentation stages, including solubilization, hydrolysis, and acidification. Microbial analysis demonstrated that biochar more effectively enriched electroactive bacteria with acid-forming functions (e.g., Clostridium was enriched 1.3-fold more than in the control) compared to magnetite. Additionally, biochar more efficiently upregulated metabolic pathways associated with VFAs biosynthesis, including pyruvate metabolism (e.g., aceE and pckA), acetate production (e.g., pta and acyP), and butyrate production (e.g., ptb and bok). Biochar also enhanced microbial adaptability to external environmental conditions through quorum sensing (e.g., luxS and lsrA) and two-component systems (e.g., phoA and atoA). The enhanced TCA cycle, which provides electrons and energy for VFAs production and environmental adaptability, was more pronounced with biochar, ultimately leading to increased overall VFAs production. This work provides a deeper understanding about the impact of conductive materials on anaerobic fermentation and offers valuable direction for optimizing FW treatment.

Anaerobic Volatile Fatty Acid Production Performance and Microbial Community Characteristics from Solid Fraction of Alkali-Thermal Treated Waste-Activated Sludge: Focusing on the Effects of Different pH Conditions

The waste-activated sludge (WAS) is rich in organic matter and various nutrients. Alkali-thermal hydrolysis of WAS can be employed to produce a liquid fertilizer with high plant-promoting nutrient content. However, the solid fraction (abbreviated as SF) generated from this process requires further treatment. Although there have been studies on the recovery of plant nutrients from WAS via alkali-thermal hydrolysis, researches on the safe treatment of the SF are limited. This study aims to explore the potential and the microbiological mechanisms on anaerobic volatile fatty acid (VFA) production from the SF under different pH conditions (i.e., 6, 7, 8, 9, and 10). The results showed that the VFA yield was highest at pH 6, reaching 4095.84 mg COD/L (i.e., 0.16 g-COD/g-volatile solids), followed by pH 10, 8, 7, and 9, with acetate being the main component (> 56%). Microbial community analysis revealed that members in phyla Firmicutes and Bacteroidota constituted the main acid-producing microbial community during the anaerobic fermentation of SF. Furthermore, different pH conditions influenced the yield and composition of VFAs by altering the structure and functions of microbial community. This research provides a new direction for the fully resourceful utilization of sludge by producing both liquid fertilizer and VFAs from WAS.

Effect of Saccharomyces cerevisiae inoculation on the co-fermentation of Clostridium kluyveri and Clostridium tyrobutyricum: A strategy for controlling acidity and enhancing aroma in strong-flavor Baijiu

Microbial synergistic fermentation plays a vital role in the intelligent brewing and industrial upgrading of the Chinese traditional Baijiu fermentation industry. In this study, a chain-elongating microbial assemblages consisting of Clostridium and varying proportions of S. cerevisiae was applied to a solid-state simulated fermentation system to validate its functionality during strong-flavor Baijiu fermentation. The addition of S. cerevisiae promoted the hydrolysis of fermented grains and reduced the acidity compared with Clostridium biofortification (Group CFE; P < 0.05). The most significant enhancement in volatile flavor substances was achieved by the addition of S. cerevisiae at a high proportion (Group SFB), where the yields of ethyl hexanoate, phenylethyl alcohol, and ethanol increased by 191.2 %, 109.8 %, and 59.7 %, respectively. The OPLS-DA model (R2X = 0.976, Q2 = 0.992) identified seven volatile flavor substances that effectively distinguished the different co-fermented grains (VIP > 1, P < 0.05). S. cerevisiae accelerated the enrichment of Lentilactobacillus, Lactiplantibacillus, Loigolactobacillus, and Clostridium_sensu_stricto_12. Metabolic pathway and correlation analysis revealed that S. cerevisiae provides endogenous ethanol to chain-elongating microorganisms, and this fungal-bacterial synergistic fermentation enhances the reverse β-oxidation pathway, ultimately contributing to the production of volatile flavor substances. Overall, the microbial assembly pattern of chain-elongating microbial assemblages will help achieve quality enhancement and intelligent control by increasing the production of flavor ethyl esters and ethanol for Baijiu solid-state fermentation system.

Biochar@MIL-88A(Fe) accelerates direct interspecies electron transfer and hydrogen transfer in waste activated sludge anaerobic digestion: Exploring electron transfer and biomolecular mechanisms

Adding additives exogenously is an effective strategy to enhance methanogenic activity and improve AD stability. Corn straw-based biochar@MIL-88A(Fe) (BM) was synthesized herewith and used as an exogenous additive to boost methane (CH4) production. After adding BM at 250 mg/g WAS VS, the accumulative CH4 production and maximum CH4 yield increased by 1.2 and 1.9 times, respectively, with CH₄ comprising 88% of the biogas. BM accelerated electron transfer through its unsaturated sites and surface functional groups, while also enhancing metabolic functions for facilitating enzymatic activities and converting organic substrates. The abundance of syntrophic bacteria and methanogen were higher after BM addition. BM-mediated DIET and IHT pathways effectively oxidized propionate and butyrate, promoting methane generation. Higher expression of key genes involved in methane production correlated with shifts in microbial structure and increased CH4 yield after BM dosage. The invention of BM may provide more solutions for addressing low energy recovery during AD.

Microbial transitions and degradation pathways driven by butyrate concentration in mesophilic and thermophilic anaerobic digestion under low hydrogen partial pressure

Butyrate accumulation significantly affects the efficiency and stability of anaerobic digestion, while its specific impact on methane yield and butyrate degradation remains unclear. This study investigated how butyrate concentrations (2.0, 5.0, 10.0, and 20.0 g COD/L) affected methane production and butyrate degradation at 37 °C and 55 °C. Methane yield decreased when butyrate concentrations exceeded 10.0 g COD/L. Methanogenesis transitioned from the acetoclastic to the hydrogenotrophic pathway with butyrate concentration increasing at 37 °C, but this transition wasn't observed at 55 °C. Butyrate was fully degraded at butyrate concentrations of 2.0-20.0 g COD/L. Iso-butyrate production was observed at 37 °C, while it only occurred with 20 g COD/L butyrate at 55 °C. Metagenomic analysis identified distinct microbes responsible for butyrate degradation at each temperature, and revealed a novel iso-butyrate metabolic pathway. These insights significantly advance the comprehension of microbial and enzymatic mechanisms driving butyrate degradation and methane production.

Enhanced methane production from bloom algal biomass using hydrothermal and hydrothermal-alkaline pretreatment with anaerobic digestion

Coalbeds have the potential as geobioreactors for producing renewable natural gas from biomass derived from photosynthesis. This brings about a number of benefits, including support for sustainable energy and the sequestration of carbon dioxide in coal. In this study, freshwater bloom algae were employed as the substrate to examine the influence of hydrothermal and hydrothermal-alkaline pretreatment on methane production using an inoculum from an anaerobic digester. The morphology and chemical structures of the biomass, as well as the volatile fatty acids (VFAs) in the liquid fraction of the post-treatment and gas production, were analyzed to understand their relationship with the efficacy of methane yields and changes in microorganisms. The results revealed that both hydrothermal and hydrothermal-alkaline pretreatment, under the right conditions, can lead to an increase in methane production. Particularly, a pretreatment condition of 0.2 mol/L NaOH at 150 °C for 30 min resulted in a significant increase in methane yield by up to 303.9%. The addition of NaOH facilitated the hydrothermal-alkaline pretreatment, effectively destroying the cell structure of the bloom algae, promoting the dissolution of intracellular sugars and other substances, and reducing the loss of VFAs caused by heating. Moreover, hydrothermal-alkaline pretreatment was found to support the growth of acetoclastic methanogens and enhance methane production by mitigating pH drops. Overall, the results of this study suggest that hydrothermal-alkaline pretreatment offers significant advantages in methane production compared to hydrothermal pretreatment. These findings have important implications for harnessing bloom algae as a viable source for generating renewable natural gas.

In-depth exploration of microbial electrolysis cell coupled with anaerobic digestion (MEC-AD) for methanogenesis in treating protein wastewater at high organic loading rates

High concentrations of protein wastewater often reduce treatment efficiency due to ammonia inhibition and acid accumulation caused by its low carbon-to-nitrogen ratio (C/N) after digestion, as well as its complex structure. This study investigates the performance of a microbial electrolysis cell (MEC) driving a protein digestion system with gradually increasing organic loading rates (OLR) of bovine serum albumin, elucidating microbial changes and methanogenic metabolic pathways on bioelectrodes under high OLR "inhibited steady-state" (ISS) conditions. The results showed that the accumulation of ammonia nitrogen (AN) from protein hydrolysis under high OLR conditions disrupted microbial growth and caused cell death on the electrode surface, hindering the electron transfer rate. Toxic AN reduced protein hydrolysis, led to propionate accumulation, inhibiting the acetoclastic methanogenesis process and favoring the hydrogenotrophic pathway. As OLR increased from 6 to 11 gCOD/L, cumulative methane production increased significantly from 450.24 mL to 738.72 mL, while average methane yield and production rate decreased by 10.51% and 50.28%, from 375.20 mL/gCOD and 75.04 mL/(gCOD·d) to 335.78 mL/gCOD and 37.31 mL/(gCOD·d), respectively. Despite these declines, the system maintained an ISS. Moderate OLR increases can achieve an ISS, boosting protein waste treatment capacity, methane production, and net energy output (NEO), with an OLR of 6 gCOD/L being optimal for maximizing NEO per unit substrate. These findings provide theoretical insights into the methanogenesis pathway of high OLR proteins in MEC-AD systems and offer an effective method for treating high OLR protein wastewater in future practical applications.

Effect of acidification pretreatment on two-phase anaerobic digestion of acidified food waste

Acidified food waste significantly disrupts anaerobic digestion, highlighting the need for effective solutions to mitigate its impact. This study presents a method that utilizes acidified sludge to pretreat acidified food waste, thereby significantly improving the efficiency of hydrolysis and acidogenesis. After acidification pretreatment, hydrolysis efficiency improved from 64.54 % to 96.51 %, while acidogenesis efficiency increased from 34.82 % to 49.95 %. Additionally, the concentration of short-chain fatty acids and hydrogen production in the acidification pretreatment group increased by 45.89 % and 48.67 %, respectively. The pretreatment group exhibited a biochemical methane potential of 512.84 ± 13.73 mL/(g volatile suspended solids), which was 35.77 % higher than that of the control group. Mechanism analysis revealed that the higher abundance of genes associated with lactate dehydrogenase in the acidified sludge facilitated the rapid degradation of lactic acid. Moreover, the abundant Clostridium butyricum in the acidified sludge promoted the targeted conversion of lactic acid and other organic matter into butyric acid within the food waste system. This efficient butyric acid fermentation improved the fermentation environment and provided abundant substrates for methane production. This study introduces a promising bio-based strategy to improve the anaerobic digestion efficiency of acidified food waste.

Predicting metabolic pathways and microbial interactions in dark fermentation systems treating real cheese whey effluents

Dark fermentation of agro-industrial effluents is a promising way for waste valorization. However, understanding the complex microbial dynamics and metabolic interactions within the microbial communities remains challenging. This study investigates the microbial communities involved in continuous hydrogen production from cheese whey and fermented cheese whey using functional profiling with PICRUSt2. The analysis reveals the primary roles of key microbial genera. Lactobacillus dominates carbohydrate consumption and lactate production, while Clostridium sensu stricto 12 and Caproiciproducens are engaged in a competitive dynamic for lactate utilization. Clostridium sensu stricto 12 drives hydrogen production via electron bifurcation reactions, whereas Caproiciproducens may utilize alternative energy conservation mechanisms. The interaction between these genera is influenced by substrate availability and process conditions. This study highlights the utility of functional profiling in elucidating microbial interactions and metabolic pathways in dark fermentation. The findings emphasize the importance of understanding microbial interactions to optimize biohydrogen production processes.

High value-added chemical production through anaerobic codigestion of corn straw with a microbial consortium, cow manure and cow digestion solution

OBJECTIVES

This study investigated the codigestion of corn straw (CS) with cow manure (CM), cow digestion solution (CD), and a strain consortium (SC) for enhanced volatile fatty acid (VFA) production. The aims of this study were to develop a sustainable technique to increase VFA yields, examine how combining microbial reagents with CS affects VFA production by functional microorganisms, and assess the feasibility of improving microbial diversity through codigestion.

METHODS

Batch experiments evaluated VFA production dynamics and microbial community changes with different combinations of CS substrates with CM, CD, and SC. Analytical methods included measuring VFAs by GC, ammonia and chemical oxygen demand (COD) by standard methods and microbial community analysis by 16S rRNA gene sequencing.

RESULTS

Codigesting CS with the strain consortium yielded initial VFA concentrations ranging from 0.6 to 1.0 g/L, which were greater than those of the other combinations (0.05-0.3 g/L). Including CM, and CD further increased VFA production to 1.0-2.0 g/L, with the highest value of 2.0 g/L occurring when all four substrates were codigested. Significant ammonium reduction (194-241 mg/L to 29-37 mg/L) and COD reduction (3310-5250 mg/L to 730-1210 mg/L) were observed. Codigestion with CM and CD had greater Shannon diversity indices (3.19-3.24) than did codigestion with the other consortia (2.26). Bacillota dominated (96.5-99.6 %), with Clostridiales playing key roles in organic matter breakdown.

CONCLUSIONS

This study demonstrated the feasibility of improving VFA yields and harnessing microbial diversity through anaerobic codigestion of lignocellulosic and animal waste streams. Codigestion substantially enhanced VFA production, which was dominated by butyrate, reduced ammonium and COD, and enriched fiber-degrading and fermentative bacteria. These findings can help optimize codigestion for sustainable waste management and high-value chemical production.

Natural flocculant chitosan inhibits short-chain fatty acid production in anaerobic fermentation of waste activated sludge

Chitosan (CTS) serves as an excellent natural flocculant in wastewater purification and sludge conditioning, but its potential impact on anaerobic fermentation of waste-activated sludge is unclear. The current study investigated the role of CTS in short-chain fatty acids (SCFAs) generation via sludge alkaline anaerobic fermentation. The results showed a drastic reduction in SCFA production with CTS, showing a maximum inhibition of 33 % at 6 mg/g of total suspended solids. CTS hindered sludge solubilization through flocculation, and acted as a humus precursor, promoting humus formation, and consequently reduced the amount of available substrates. Further, CTS promoted free ammonia production, posing a challenge to enzymes and cell viability. Additionally, CTS increased the population of Rikenellaceae sp. and weakened the dominance of hydrolyzing and acidifying bacteria. This study deepens the understanding of the potential impact of CTS on anaerobic fermentation and provides a theoretical basis for reducing the risk of polymeric flocculants.

A review of volatile fatty acids production from organic wastes: Intensification techniques and separation methods

High value-added products from organic waste fermentation have garnered increasing concern in modern society. VFAs are short-chain fatty acids, produced as intermediate products during the anaerobic fermentation of organic matter. VFAs can serve as an essential organic carbon source to produce substitutable fuels, microbial fats and oils, and synthetic biodegradable plastics et al. Extracting VFAs from the fermentation broths is a challenging task as the composition of suspensions is rather complex. In this paper, a comprehensive review of methods for VFAs production, extraction and separation are provided. Firstly, the methods to enhance VFAs production and significant operating parameters are briefly reviewed. Secondly, the evaluation and detailed discussion of various VFAs extraction and separation technologies, including membrane separation, complex extraction, and adsorption methods, are presented, highlighting their specific advantages and limitations. Finally, the challenges encountered by different separation technologies and novel approaches to enhance process performance are highlighted, providing theoretical guidance for recycling VFAs from organic wastes efficiently.

Microbiome dynamics and products profiles of biowaste fermentation under different organic loads and additives

Biowaste fermentation is a promising technology for low-carbon print bioenergy and biochemical production. Although it is believed that the microbiome determines both the fermentation efficiency and the product profiles of biowastes, the explicit mechanisms of how microbial activity controls fermentation processes remained to be unexplored. The current study investigated the microbiome dynamics and fermentation product profiles of biowaste fermentation under different organic loads (5, 20, and 40 g-VS/L) and with additives that potentially modulate the fermentation process via methanogenesis inhibition (2-bromoethanesulfonate) or electron transfer promotion (i.e., reduced iron, magnetite iron, and activated carbon). The overall fermentation products yields were 440, 373 and 208 CH4-eq/g-VS for low-, medium- and high-load fermentation. For low- and medium-load fermentation, volatile fatty acids (VFAs) were first accumulated and were gradually converted to methane. For high-load fermentation, VFAs were the main fermentation products during the entire fermentation period, accounting for 62% of all products. 16S rRNA-based analyses showed that both 2-bromoethanesulfonate addition and increase of organic loads inhibited the activity of methanogens and promoted the activity of distinct VFA-producing bacterial microbiomes. Moreover, the addition of activated carbon promoted the activity of H2-producing Bacteroides, homoacetogenic Eubacteriaceae and methanogenic Methanosarcinaceae, whose activity dynamics during the fermentation led to changes in acetate and methane production. The current results unveiled mechanisms of microbiome activity dynamics shaping the biowaste fermentation product profiles and provided the fundamental basis for the development of microbiome-guided engineering approaches to modulate biowaste fermentation toward high-value product recovery.

Enhanced methane production with co-feeding spent coffee grounds using spare capacity of existing anaerobic food waste digesters

With increasing coffee consumption worldwide, the efficient and sustainable management of spent coffee grounds (SCG) has become increasingly challenging. This study investigated the anaerobic co-digestion of small amounts of SCG with food waste (FW) at increasing co-feeding ratios of 1:100-1:10 (volatile solids basis) to assess the possibility of SCG treatment using the spare capacity of existing anaerobic digesters. Co-feeding SCG increased methane production compared to FW mono-digestion in the tested range of co-feeding ratios without compromising process stability. Methane yield did not further increase when the SCG/FW ratio increased above 4%, and process failure occurred at a 1:10 co-feeding ratio without trace element supplementation. The enhanced methanogenic performance was attributed to increased protein removal efficiency, which was potentially related to the promotion of peptide hydrolysis. The overall results suggest that co-feeding appropriate small amounts of SCG to FW digesters can be a realistic sustainable option for SCG management.

Enhanced anaerobic digestion of human feces by ferrous hydroxyl complex (FHC): Stress factors alleviation and microbial resistance improvement

Anaerobic digestion (AD) offers a reliable strategy for resource recovery from source-separated human feces (HF), but is limited by a disproportionate carbon/nitrogen (C/N) ratio. Ferrous hydroxyl complex (FHC) was first introduced into the HF-AD system to mediate methanogenesis. Mono-digestion of undiluted HF was inhibited by high levels of volatile fatty acids (VFAs), ammonia, and hydrogen sulfide (H2S). FHC addition at optimum dosage (500-1000 mg/L) increased the cumulative methane (CH4) yield by 22.7%, enhanced the peak value of daily CH4 production by 60.5%, and shortened the lag phase by 24.7%. H2S concentration in biogas was also greatly decreased by FHC via precipitation. FHC mainly facilitated the hydrolysis, acidification, and methanogenesis processes. The production and transformation of VFAs were optimized in the presence of FHC, thus relieving acid stress. FHC elevated the activities of alkaline protease, cellulase, and acetate kinase by 32.3%, 18.2%, and 30.3%, respectively. Microbial analysis revealed that hydrogenotrophic methanogens prevailed in mono-digestion at high HF loading but were weakened after FHC addition. FHC also enriched Methanosarcina, thereby expanding the methanogenesis pathway and improving the resistance to ammonia stress. This work would contribute to improving the methanogenic performance and resource utilization for HF anaerobic digestion.

Granular activated carbon enhances volatile fatty acid production in the anaerobic fermentation of garden wastes

Garden waste, one type of lignocellulosic biomass, holds significant potential for the production of volatile fatty acids (VFAs) through anaerobic fermentation. However, the hydrolysis efficiency of garden waste is limited by the inherent recalcitrance, which further influences VFA production. Granular activated carbon (GAC) could promote hydrolysis and acidogenesis efficiency during anaerobic fermentation. This study developed a strategy to use GAC to enhance the anaerobic fermentation of garden waste without any complex pretreatments and extra enzymes. The results showed that GAC addition could improve VFA production, especially acetate, and reach the maximum total VFA yield of 191.55 mg/g VSadded, which increased by 27.35% compared to the control group. The highest VFA/sCOD value of 70.01% was attained in the GAC-amended group, whereas the control group only reached 49.35%, indicating a better hydrolysis and acidogenesis capacity attributed to the addition of GAC. Microbial community results revealed that GAC addition promoted the enrichment of Caproiciproducens and Clostridium, which are crucial for anaerobic VFA production. In addition, only the GAC-amended group showed the presence of Sphaerochaeta and Oscillibacter genera, which are associated with electron transfer processes. Metagenomics analysis indicated that GAC addition improved the abundance of glycoside hydrolases (GHs) and key functional enzymes related to hydrolysis and acidogenesis. Furthermore, the assessment of major genera influencing functional genes in both groups indicated that Sphaerochaeta, Clostridium, and Caproicibacter were the primary contributors to upregulated genes. These findings underscored the significance of employing GAC to enhance the anaerobic fermentation of garden waste, offering a promising approach for sustainable biomass conversion and VFA 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.

Microbial co-cultures for biochemicals production from lignocellulosic biomass: A review

Global reliance on fossil oil should shift to cleaner alternatives to get a decarbonized society. One option to achieve this ambitious goal is the use of biochemicals produced from lignocellulosic biomass (LCB). The inherent low biodegradability of LCB and the inhibitory compounds that might be released during pretreatment are two main challenges for LCB valorization. At microbiological level, constraints are mostly linked to the need for axenic cultures and the preference for certain carbon sources (i.e., glucose). To cope with these issues, this review focuses on efficient LCB conversion via the sugar platform as well as an innovative carboxylate platform taking advantage of the co-cultivation of microorganisms. This review discusses novel trends in the use of microbial communities and co-cultures aiming at different bioproducts co-generation in single reactors as well as in sequential bioprocess combination. The outlook and further perspectives of these alternatives have been outlined for future successful development.

Enhancement of volatile fatty acids production through anaerobic co-digestion of navel orange residue and waste activated sludge: Effect of pre-treatment and substrate proportions

In this study, the co-digestion system with Navel orange residues (NOR) and Waste activated sludge (WAS) was established, by pre-treating the NOR and setting different volatile solids (VS) ratios of NOR to WAS to motivate the production of volatile fatty acids (VFA). The pre-treatment method (pH 7 and temperature 70 °C) promoted the release of dissolved organic matter, and the concentration of soluble chemical oxygen demand (SCOD) increased by 45.56% compared with the untreated group (pH 3 and temperature 20 °C). In the co-digestion system, the highest VFA yield (5716.69 mg/L) was obtained at VS ratio of 2. When the VS ratio was increased to 4, the imbalance in proportions of carbon and nitrogen affected VFA production, and the high concentration of essential oils (EO) present in the NOR inhibited the methane production; the cumulative yield of methane gas decreased by 24.10% compared with the yield obtained when the VS ratio was 2. Analysis of microbial community revealed that an increase in the number of VFA-producing microbial populations and the abundance of Methanobacteria resulted in the accumulation of acetic acid. This study demonstrated that co-digestion of NOR with WAS improve VFA production, thus realizing the utilization of solid wastes and reducing environmental pollution.

Cellulase-lactic acid bacteria synergy action regulates silage fermentation of woody plant

BACKGROUND

Feed shortage is an important factor limiting livestock production in the world. To effectively utilize natural woody plant resources, we used wilting and microbial additives to prepare an anaerobic fermentation feed of mulberry, and used PacBio single-molecule real-time (SMRT) sequencing technology to analyse the "enzyme-bacteria synergy" and fermentation mechanism.

RESULTS

The fresh branches and leaves of mulberry have high levels of moisture and nutrients, and also contain a diverse range of epiphytic microorganisms. After ensiling, the microbial diversity decreased markedly, and the dominant bacteria rapidly shifted from Gram-negative Proteobacteria to Gram-positive Firmicutes. Lactic acid bacteria (LAB) emerged as the dominant microbial population, resulting in increased in the proportion of the carbohydrate metabolism and decreased in the proportion of the amino acid and "global and overview map" (GOM) metabolism categories. The combination of cellulase and LAB exhibited a synergistic effect, through which cellulases such as glycanase, pectinase, and carboxymethyl cellulase decomposed cellulose and hemicellulose into sugars. LAB converted these sugars into lactic acid through the glycolytic pathway, thereby improving the microbial community structure, metabolism and fermentation quality of mulberry silage. The GOM, carbohydrate metabolism, and amino acid metabolism were the main microbial metabolic categories during ensiling. The presence of LAB had an important effect on the microbial community and metabolic pathways during silage fermentation. A "co-occurrence microbial network" formed with LAB, effectively inhibiting the growth of harmful microorganisms, and dominating the anaerobic fermentation process.

CONCLUSIONS

In summary, PacBio SMRT was used to accurately analyse the microbial network information and regulatory mechanism of anaerobic fermentation, which provided a scientific basis for the study of woody silage fermentation theory. This study reveals for the first time the main principle of the enzyme-bacteria synergy in a woody silage fermentation system, which provides technical support for the development and utilization of woody feed resources, and achieves sustainable livestock production.

Enhanced aquaculture wastewater treatment in a biofilm reactor filled with sponge/ferrous oxalate/biochar composite (Sponge-C2FeO4@NBC) biocarriers: Performance and mechanism

Fabricating low-cost and efficient biofilm carriers for moving bed biofilm reactors in wastewater treatment is crucial for achieving environmental sustainability. Herein, a novel sponge biocarrier doped with NaOH-loaded biochar and nano ferrous oxalate (sponge-C₂FeO₄@NBC) was prepared and evaluated for nitrogenous compounds removal from recirculating aquaculture systems (RAS) wastewater by stepwise increasing ammonium nitrogen (NH₄⁺-N) loading rates. The prepared NBC, sponge-C₂FeO₄@NBC, and matured biofilms were characterized using SEM, FTIR, BET, and N₂ adsorption-desorption techniques. The results reveal that the highest removal rates of NH₄⁺-N reached 99.28 ± 1.3% was yielded by the bioreactor filled with sponge-C₂FeO₄@NBC, with no obvious nitrite (NO₂^--N) accumulation in the final phase. The reactor packed with sponge-C₂FeO₄@NBC biocarrier had the highest relative abundance of functional microorganisms responsible for nitrogen metabolism than in the control reactor, confirmed from 16S rRNA gene sequencing analysis. Our study provides new insights into the newly developed biocarriers for enhancing RAS biofilters treatment performance in keeping water quality within the acceptable level for the rearing of aquatic species.

Enhancing sulfate reduction and hydrogen sulfide removal through gas stripping in the acidogenesis phase of a two-phase anaerobic process

This study aims at evaluating two-phase and single-phase reactors for treating sulfate wastewater with low COD/SO₄^2- ratios. Additionally, a new process of gas stripping in an acidogenesis phase is proposed to reduce hydrogen sulfide (H₂S) inhibition and enhance biomethanation. The two-phase performed better than the single-phase in terms of COD removal, CH₄ production and H₂S resistance. After 30 days of stripping, the COD and sulfate degradation rates increased from 85.16% to 91.09% and from 49.39% to 63.07% in the two-phase, respectively. In contrast, without stripping, they were from 79.21% to 64.37% and from 50.26% to 53.15% in the single-phase, respectively. The microbial biodiversity was augmented via stripping, including norank_f__Spirochaetaceae, Petrimonas, Desulfurella and Blvii28_wastewater-sludge_group. Stripping operation enhanced the dissimilatory sulfate reduction, amino acid metabolism and possibly sulfate-dependent anaerobic ammonia oxidation (S-ANAMMOX). This study provides a promising strategy to improve sulfate reduction and reduce H₂S inhibition under a low COD/SO₄^2- ratio.

High-rate anaerobic digestion of sewage sludge using anaerobic dynamic membrane bioreactor under various sludge composition and organic loading rates

This study investigates the effects of sludge compositions and organic loading rates (OLRs) on stable biomethane production during sludge digestion. Batch digestion experiments evaluate the effects of alkaline-thermal pretreatment and waste activated sludge (WAS) fractions on the biochemical methane potential (BMP) of sludge. A lab-scale anaerobic dynamic membrane bioreactor (AnDMBR) is fed with a mixture of primary sludge and pretreated WAS. Monitoring of volatile fatty acid to total alkalinity (FOS/TAC) helps maintain operational stability. The highest average biomethane production rate of 0.7 L/L·d is achieved when the OLR, hydraulic retention time, WAS volume fraction, and FOS/TAC ratio are 5.0 g COD/L·d, 12 days, 0.75, and 0.32, respectively. This study finds functional redundancy in two pathways: hydrogenotrophic and acetolactic. An increase in OLR promotes bacterial and archaeal abundance and specific methanogenic activity. These results can be applied to the design and operation of sludge digestion for stable, high-rate biomethane recovery.

Unravelling roles of the intermediate settler in a microaerobic hydrolysis sludge in situ reduction process

The roles of the intermediate settler in the sludge process reduction activated sludge process (SPRAS), and the influences of its hydraulic retention time (HRT_ST) on pollutant removal and sludge reduction were investigated. Prolonging HRT_ST from 3.0 to 4.5 and 6.0 h resulted in sludge reduction efficiencies increased from 46.8% to 61.5% and 62.7%. The sludge accumulation in the intermediate settler formed an anaerobic zone but inhibited methane production, and the alternating microaerobic and anaerobic environment in the sludge process reduction (SPR) module increased the microbial diversity and enriched the hydrolytic and fermentative bacteria. Prolonging HRT_ST accelerated dissolved organic matter release and elevated the degradation of refractory fraction, and improved the sludge properties of the SPRAS. Metagenomic analysis showed that the SPR module enhanced the glycolysis pathway and decoupling metabolism for sludge reduction. The results revealed that the intermediate settler plays dual roles in solid-liquid separation and sludge reduction metabolism.

Culture adaptation for enhanced biogas production from birch wood applying stable carbon isotope analysis to monitor changes in the microbial community

Birch wood is a potential feedstock for biogas production in Northern Europe; however, the lignocellulosic matrix is recalcitrant preventing efficient conversion to methane. To improve digestibility, birch wood was thermally pre-treated using steam explosion at 220 °C for 10 min. The steam-exploded birch wood (SEBW) was co-digested with cow manure for a period of 120 days in continuously fed CSTRs where the microbial community adapted to the SEBW feedstock. Changes in the microbial community were tracked by stable carbon isotopes- and 16S r RNA analyses. The results showed that the adapted microbial culture could increase methane production up to 365 mL/g VS day, which is higher than previously reported methane production from pre-treated SEBW. This study also revealed that the microbial adaptation significantly increased the tolerance of the microbial community against the inhibitors furfural and HMF which were formed during pre-treatment of birch. The results of the microbial analysis indicated that the relative amount of cellulosic hydrolytic microorganisms (e.g. Actinobacteriota and Fibrobacterota) increased and replaced syntrophic acetate bacteria (e.g. Cloacimonadota, Dethiobacteraceae, and Syntrophomonadaceae) as a function of time. Moreover, the stable carbon isotope analysis indicated that the acetoclastic pathway became the main route for methane production after long-term adaptation. The shift in methane production pathway and change in microbial community shows that for anaerobic digestion of SEBW, the hydrolysis step is important. Although acetoclastic methanogens became dominant after 120 days, a potential route for methane production could also be a direct electron transfer among Sedimentibacter and methanogen archaea.

Insights into the impact of quaternary ammonium disinfectant on sewage sludge anaerobic digestion: Dose-response, performance variation, and potential mechanisms

Wide commercial applications of antimicrobial quaternary ammonium compounds (QACs) inevitably lead to the release into wastewater and enrichment in sewage sludge. This study evaluated the impacts of levels and structures of QACs on sewage sludge properties, microbial community, and methane production during anaerobic digestion. Methane production was stimulated or not affected at low QACs concentrations, but significantly inhibited at high QACs concentrations. Compared with benzyl and alkyltrimethyl QACs, dialkyl QACs showed least toxicity on digestion performance. Meanwhile, microbial community analysis indicated that shifts in bacterial communities mainly depended on QACs doses, but the archaeal communities were affected by both QACs doses and types. The dominant methanogenic pathway shifted from acetotrophic/methylotrophic methanogens to mixotrophic methanogens by low levels of benzyl and alkyltrimethyl QACs but not dialkyl QACs, and further to hydrogenotrophic methanogens at high QACs concentration. Mechanism exploration revealed that the presence of QACs promoted sludge solubilization by the integrated effects of cell lysis, electric neutralization, and hydrophobicity improvement, but inhibited methanogenesis due to the accumulation of volatile fatty acids and susceptibility of methanogens to QACs. These findings provided a reference for potential impacts of different QACs on sludge biological treatment, which had implications for the use and selection of QACs disinfectants.

Strategies for enhancing the efficacy of anaerobic digestion of food industry wastewater: An insight into bioreactor types, challenges, and future scope

Food waste have become a growing concern worldwide with raising population and economic growth. Wastewater discharged from food industries contains many valuable and toxic components that have a negative impact on the ecological system. Large amounts of wastewater are discharged from the food industry, which necessitates the creation of effective technologies. Wastewater from the food industry can be seen as a rich source of energy and a primary source for generating valuable products. Waste disposal and resource recovery are sustainably valued by anaerobic digestion of wastewater from the food sector. The characteristics, composition, and nature of wastewater produced from various food sectors are elaborated upon in this review. An overview of the anaerobic digestion process for wastewater treatment in the food industry is included. Enhancement strategies for the anaerobic digestion process have been discussed in detail. In addition, various types of reactors utilized for performing anaerobic digestion is illustrated. Though anaerobic digestion process possesses advantages, the challenges and future scope are examined for improving the outcome.

Effect of the initial pH on the anaerobic digestion process of dairy cattle manure

Anaerobic digestion (AD) has recently been studied to obtain products of greater interest than biogas, such as volatile fatty acids (VFAs) and phytoregulators. The effect of the initial pH of cow manure and the fermentation time of the AD on the microbial composition, VFAs, indole-3-acetic acid (IAA) and gibberellic acid (GA₃) production was evaluated. The cow manure (7% solids) was adjusted to initial pH values of 5.5, 6.5, 7.5, and 8.5, and the AD products were analyzed every four days until day 20. The initial pH and the fermentation time had an important effect on the production of metabolites. During AD, only the hydrolytic and acidogenic stages were identified, and the bacteria found were from the phyla Firmicutes, Bacteroidetes, Actinobacteria, and Spirochaetes. The most abundant genera produced in the four AD were Caproiciproducens, Clostridium sensu stricto 1, Romboutsia, Paeniclostridium, Turicibacter, Peptostreptococcaceae, Ruminococcaceae and Fonticella. The highest amount of VFAs was obtained at pH 8.5, and the production of the acids was butyric > acetic > propionic. The maximum production of GA₃ and IAA was at an initial pH of 6.5 on day 20 and a pH of 5.5 on day 4, respectively. There was a strong correlation (> 0.8) between the most abundant microorganisms and the production of VFAs and GA₃. The anaerobic digestion of cow manure is a good alternative for the production of VFAs, GA₃ and IAA.

Enhanced chloride-free snow-melting agent generation from organic wastewater by integrating bioconversion and synthesis

In this study, a new process for producing chloride-free snow-melting agents (CSAs) was proposed. Organic wastewater was converted to total volatile fatty acids (TVFA) by anaerobic acidogenic fermentation. The experiments for acid generation showed that the maximum TVFA concentration of 45.9 g/L was obtained at an organic loading rate of 5 g chemical oxygen demand /(L·d), and the proportion of acetic acid reached 78.8 %. Forward osmosis was used for concentrating the TVFA solution. The obtained CSAs, after evaporation and crystallization, had a better ice-melting capacity and less corrosion on metal and concrete than NaCl and CaCl₂. Additionally, the damage caused by CSAs to the germination of plant seeds was significantly lesser than that caused by chloride salts. This study proposed a feasible method for the high-value conversion of organic wastewater, providing a new direction for the reuse of organic wastewater.

Combined effects of liquid digestate recirculation and biochar on methane yield, enzyme activity, and microbial community during semi-continuous anaerobic digestion

The combined effects of liquid digestate recirculation (LDR) and biochar on methanogenesis and microbial communities were studied in semi-continuous anaerobic reactors fed with wheat straw and swine manure. The tolerated organic loading rate (OLR) was expanded from 5 g- volatile solids (VS)∙L-1∙d-1 in the control to higher than 6 g-VS∙L-1∙d-1 in the LDR. At the OLR of 5.0 g-VS∙L-1∙d-1, average special methane yield in LDR with biochar was 0.234 L∙g-VS-1, which was 5.4 % higher than that of the LDR alone. Moreover, enzyme activity and microbial community analysis indicated that LDR with biochar enhanced the processes of hydrolysis and methanogenesis, and balanced the pathway between hydrogenotrophic and acetoclastic methanogenesis. The co-application of LDR and biochar synergistically enhanced the degradation pathways of substrates and the loading shock resistance of anaerobic digestion system. This study could offer strategies for developing sustainable applications of full and continuous LDR in industrial biogas projects.

Anaerobic co-digestion of kitchen waste with hyperthermophilically pretreated grass for biohydrogen and biomethane production

Anaerobic digestion of kitchen waste with grass after hyperthermophilic pretreatment was performed in semi-continuously operated reactors. The greatest methane yield of 293 NmlCH4/gVS (volatile solids) was reported for the mixture of both substrates at 55 °C with a solids retention time of 30 d and the corresponding organic lading rate of 1.72 kgVS/m3/d. In contrast, pretreated grass subjected to thermophilic digestion produced only 131 NmlCH4/gVS. However, when mesophilic conditions were applied, the digestion process turned into dark fermentation, especially visible for the mixture. Metagenomic analysis revealed the dominance Ruminococcaceae, Atopobiaceae and Lactobacillaceae at a family level in mesophilic processes, whereas Petrotogaceae, Synergistaceae, Hungateiclostridiaceae, Planococcaceae and two methanogens Methanosarcinaceae and Methanothermobacteriaceae were the most frequent microbes of thermophilic digestion. Kitchen waste can successfully be co-digested with hyperthermophilically pretreated grass at high loading rates, however the digesters must be operated at thermophilic temperatures.

Medium-chain fatty acid production from Chinese liquor brewing yellow water by electro-fermentation: Division of fermentation process and segmented electrical stimulation

Electro-fermentation (EF) has been proposed as a method to improve the yield of medium-chain fatty acid (MCFA). In this study, MCFA production from Chinese liquor wastewater (yellow water) was investigated and corresponding composite electron donors (lactate and ethanol in yellow water) were investigated by different electrical stimulation modes. The caproate yield under whole period electrical stimulation increased by 250.9% compared with open circuit. The oxidation-dominated and reduction-dominated periods of the fermentation process were divided, and the segmented electrical stimulation experiment showed the caproate yield under reduction-dominated EF system further increased by 288.5% compared with open circuit. The microbial diversity analysis demonstrated that Clostridium 12 might be enriched better by keeping open circuit during EDs consumption, meanwhile the bacteria with potential negative effects on CE were inhibited. The electrical stimulation mode of EF process was optimized and provided a new way to recycle organic wastewater.

Upgrade the high-load anaerobic digestion and relieve acid stress through the strategy of side-stream micro-aeration: biochemical performances, microbial response and intrinsic mechanisms

In high-load anaerobic digestion such as in kitchen waste, side-stream micro-aeration (SMA) shows excellent operational performance to direct micro-aeration (DMA). It immediately restores the acidification to stability. Methanogenic performance remained stable when organic load ratios (OLR) was further increased to 5.5 g VS/L. Enhanced enzyme activity, microbial aggregation, and proliferation of bacteria and archaea were observed in SMA. The results indicates that SMA enriched Methanosaeta (relative abundance exceeded 93%) and induced the change of the main methanogenic pathway to acetoclastic methanogenesis. Mechanisms was further explored by using metagenomic analysis, and the results show SMA avoids mass formation of ROS (reactive oxygen species) by cycling the aerated slurry, and retains benefits of trace O₂ on material and energic metabolism, which poses great application potentials and deserves further investigation.

Technological and Energetic Aspects of Multi-Component Co-Digestion of the Beverage Industry Wastes and Municipal Sewage Sludge

In the present study, the co-digestion effectiveness of the selected beverage wastes and municipal sewage sludge in two- and three-component mixtures was evaluated. Orange peels and orange pulp, as well as brewery spent grain were applied as co-substrates to sewage sludge at the following doses: 1.5 and 3.0 g of orange peels, 2.5 and 5 g of orange pulp, and 1.5 g brewery spent grain. Mono-digestion of sewage sludge was used as a control. The experiments were performed under mesophilic conditions in batch reactors. As compared to the control, only in the presence of the highest dose of pulp, brewery spent grain and sewage sludge was the increased methane production of 395 mL CH4 g−1 VS accompanying an additional energy profit of 82% observed. Moreover, in this case, the enhanced volatile solids removal and lower accumulation of p-cymene were found. These results were despite the increased limonene and phenol content in the feedstock, confirming a synergistic effect at the highest dose of pulp, brewery spent grain and sewage sludge.

Effects of pretreatment methods on biomethane production kinetics and microbial community by solid state anaerobic digestion of sugarcane trash

Solid state anaerobic digestion (SS-AD) of lignocellulose is effective in improving biomethane productivity but is limited by low biomass digestibility and lack of substrate-specific working microorganisms. In this study, the effects of different pretreatment methods on biomethane production by SS-AD of sugarcane trash were studied. The biomethane production, fitted to a modified Gompertz's model, predicted a maximum methane yield of 214.2 L/kg volatile solids (VS) and productivity of 6.9 L/kg VS/day from KOH-pretreated trash, respectively. Microbial community analysis showed that bacterial community was significantly associated with volatile acids and pretreatment types while archaeal community was significantly associated with methane yield. Microbial community dynamics was revealed in SS-AD. Main genera related to pretreatment method were identified and discussed. This study generated important information on SS-AD of lignocellulosic biomass pretreated by different methods, which is useful for developing bioaugmentation strategies to improve biomethane production by SS-AD.