Conversion Of Volatile Fatty Acids Research Articles

Effects of microbial electrolysis cell on anaerobic digestion of food waste: Acidification and recovery

The effects of a microbial electrolysis cell (MEC) applying 1.2 V on the acidification and recovery in the anaerobic digestion (AD) of food waste were studied by semi-continuous experiments. The experimental results showed that the MEC reduced the accumulation of volatile fatty acids (VFAs), enhanced acidification, accelerated the conversion of VFAs during recovery, and accelerated recovery by decreasing the relative abundance of Aminobacterium and norank_f__W27 and increasing that of Methanobacterium. The MEC reduced the production of propionic and butyric acids by decreasing the abundance of functional enzymes (such as fumarate reductase, propionate CoA-transferase, phosphate butyryltransferase, and butyrate kinase) and bacteria (such as Aminobacterium and norank_f__W27). The MEC increased the rates of four AD steps with a maximal rate of 11.47–32.32 %. The MEC promoted electron transfer by enriching the electroactive bacterium Trichococcus (10.91–28.80 %) and Methanosaeta (8.17–24.43 %). The MEC increased the abundance of the most critical functional enzymes in the bacterial community during the entire test and those in the archaeal community during recovery.

Anhydrous volatile fatty acid extraction through omniphobic membranes by hydrophobic deep eutectic solvents: Mechanistic understanding and future perspective

Volatile fatty acids (VFAs) derived from arrested anaerobic digestion (AD) can be recovered as a valuable commodity for value-added synthesis. However, separating VFAs from digestate with complex constituents and a high-water content is an energy-prohibitive process. This study developed an innovative technology to overcome this barrier by integrating deep eutectic solvents (DESs) with an omniphobic membrane into a membrane contactor for efficient extraction of anhydrous VFAs with low energy consumption. A kinetic model was developed to elucidate the mechanistic differences between this novel omniphobic membrane-enabled DES extraction and the previous hydrophobic membrane-enabled NaOH extraction. Experimental results and mechanistic modeling suggested that VFA extraction by the DES is a reversible adsorption process facilitating subsequent VFA separation via anhydrous distillation. High vapor pressure of shorter-chain VFAs and low Nernst distribution coefficients of longer-chain VFAs contributed to DES-driven extraction, which could enable continuous and in-situ recovery and conversion of VFAs from AD streams.

Fulvic acid-mediated efficient anaerobic digestion for kitchen wastewater: Electrochemical and biochemical mechanisms

Fulvic acid, prevalent in humus derived from the anaerobic digestion of kitchen wastewater, is crucial in organic matter transformation. However, its effects and underlying mechanisms remain unclear. In this study, the fate of anaerobic digestion of artificial and kitchen wastewater with different fulvic acid contents was investigated. The results showed that 125 mg/L fulvic acid resulted in a 64.02 and 51.72 % increase in methane production in synthetic and kitchen wastewater, respectively. Fulvic acid acted as an electron mediator and increased substrate oxidation by boosting NAD and ATP levels, thereby increasing microbial metabolic rates and ensuring an adequate substrate for methane generation. Isotope analysis suggested that fulvic acid boosts the conversion of volatile fatty acids to methane via the interspecies electron transfer pathway. Gene expression analysis revealed that cytochrome c, FAD, and other electron transport coenzymes were upregulated by fulvic acid, thereby enhancing substrate utilisation and biogas quality. Fulvic acid presented a dual stimulatory and inhibitory effect on anaerobic digestion, with concentrations over 125 mg/L diminishing its positive impact. This dual effect may stem from the properties and concentrations of fulvic acid. This study revealed the effect mechanism of fulvic acid and provided insights into the humus performance in anaerobic digestion

Magnetite modified zeolite as an alternative additive to promote methane production from anaerobic digestion of waste activated sludge

Methane recovery from bio-wastes is one promising renewable energy to relieve the energy crisis faced around the world. Zeolite is one of attractive exogenous additives for promoting methane production from anaerobic digestion due to its potentials in regulating microorganism enrichment and anaerobic environment. However, the poor electron mediated ability limited its positive effects. Hence, this study hypothesized that the electron mediated ability of zeolite would be improved by loading magnetite, thereby generating more positive impacts on the performance of sludge anaerobic digestion. Compared to control group, magnetite modified zeolite (MZ) effectively improved methane yield by 37.4%. The reason was that the electron mediated ability and adsorption capacity of zeolite were improved by loading magnetite, facilitating the degradation of refractory organics and the conversion of volatile fatty acids. Moreover, the activities of cytochrome c and F420 increased by 33.3% and 29.8% with MZ additive, contributing to stimulating efficient electron transfer process. In addition, the electro-active genera were enriched, such as Bacteroidetes_vadinHA17, Methanobacterium, and Methanosaeta, and the direct interspecies electron transfer was possibly established for methanogenesis with the help of MZ. This study will provide valuable references for the application of zeolite in anaerobic digestion.

The Bioaugmentation of Electroactive Microorganisms Enhances Anaerobic Digestion

Direct interspecies electron transfer (DIET) between electroactive microorganisms (EAMs) offers significant potential to enhance methane production, necessitating research for its practical implementation. This study investigated enhanced methane production through DIET in an anaerobic digester bio-augmented with EAMs. A horizontal anaerobic digester (HAD) operated for 430 days as a testbed to validate the benefits of bioaugmentation with EAMs. Anaerobic digestate slurry, discharged from the HAD, was enriched with EAMs in a bioelectrochemical auxiliary reactor (BEAR) under an electric field. This slurry enriched with EAMs was then recirculated into the HAD. Results showed bio-augmentation with EAMs led to an increase in volatile solids removal from 56.2% to 77.5%, methane production rate from 0.59 to 1.00 L/L.d, methane yield from 0.26 to 0.34 L/g CODr, and biogas methane content from 59.9% to 71.6%. It suggests that bio-augmentation enhances DIET, promoting the conversion of volatile fatty acids to methane and enhancing resilience against kinetic imbalances. The enrichment of EAMs reached optimal efficacy under an electric field intensity of 2.07 V/cm with a mean exposure time of 2.53 days to the electric field in the BEAR. Bio-augmentation with externally enriched EAMs is a feasible and effective strategy to optimize anaerobic digestion processes.

Comprehensive study of the combined effects of biochar and iron-based conductive materials on alleviating long chain fatty acids inhibition in anaerobic digestion

This study investigated the feasibility of alleviating the negative influence of long-chain fatty acids (LCFAs) on anaerobic digestion by biochar, micron zero-valent iron, micron-magnetite (mFe3O4) and their combination. The results demonstrate that co-addition of biochar and 6 g/L mFe3O4 (BC+6 g/L mFe3O4) increased cumulative methane production by 50% as suffered from LCFAs inhibition exerted by 2 g/L glycerol trioleate. The BC+6 g/L mFe3O4 did best in accelerating total organic carbon degradation and volatile fatty acids conversion, through successively enriching Bacteroides, Corynebacterium, and DMER64 to dominant the bacterial community. The proportion of acetotrophic Methanothrix that could alternatively reduce CO2 to methane by accepting electrons via direct interspecies electron transfer (DIET) was 0.09% with BC+6 g/L mFe3O4, nine times more than the proportion in control. Prediction of functional genes revealed the enrichment of the bacterial secretion system, indicating that BC+6 g/L mFe3O4 promoted DIET by stimulating the secretion of extracellular polymeric substances. This study provided novel insights into combining biochar and iron-based conductive materials to enhance AD performance under LCFAs inhibition.

Performance enhancement of integrating microbial electrolysis cell on two-stage anaerobic digestion of food waste: Electro-methanogenic stage versus electro-two stages

The effects of microbial electrolysis cell (MEC) integration stage on two-stage anaerobic digestion (TSAD) of food waste (FW) were studied via semi-continuous experiments. The results showed that both MEC (with 1.2 V) integrations enhanced the performances of the TSADs, with the enhancement of electro-two stages being higher. The methane production of TSAD increased from 1.36 ± 0.04 L/L/d to 1.53 ± 0.05 L/L/d (electro-methanogenic stage) and 1.54 ± 0.04 L/L/d (electro-two stages) during the steady period. Electro-acidogenesis decreased propionic acid production and enhanced hydrogen production, while electro-methanogenesis promoted the conversion of volatile fatty acids to methane. The MEC integration improved energy recovery from the organic matter in FW by 11.65–16.15% and reduced the mass loss, with those of the electro-two stages being higher and the electro-methanogenic stage being dominant in the electro-two stages. The integration of MEC enhanced anaerobic fermentation by enriching Olsenella, norank_f__ST-12K33 and Proteiniphilum and improved methanogenesis by enriching Methanobacterium and Candidatus_Methanofastidiosum.

Recycling cinder in efficient methane production from wheat straw via solid-state anaerobic digestion (SS-AD)

Solid-state anaerobic digestion (SS-AD) has received widespread attention due to its high methane productivity and organic loading, as well as its almost nonexistent wastewater emission. However, local acidification of the reaction system resulted in low material transfer efficiency due to low free water content leading to low energy conversion efficiency. Cinder is a byproduct of the coal chemical industry that has the potential to enhance the SS-AD performance due to its porous structure, electrocatalytic activity, and high trace elements concentration. The effect of cinder addition on SS-AD of wheat straw at 35 °C was studied herein. The cumulative methane of 2.5%, 5.0%, and 10.0% cinder groups were 41.25, 33.34, 28.94, and 26.68 mL/g VS, respectively, which increased 54.61%, 24.96%, and 8.47% compared to control (CK), with 2.5% cinder obtained the highest methane production. Cinder considerably enhanced volatile fatty acids conversion, particularly the propionic acid. The modified Gompertz model was used to predict the methane production potential and found to be able to accurately fit the characteristics of gas production. The maximum methane production of three test groups (2.5%, 5.0%, and 10.0%) was predicted to increase by 53.28%, 26.60%, and 11.56%, respectively, compared to CK. Cinder greatly enhanced the abundance of Methanosarcina, Fibrobacteres, and Spirochaetoma, which contributed to the switch from hydrogenophilic to acetylated methanogenesis. The prediction of the abundance of enzymes associated with microbial metabolism verified that cinder contributed to the greatest abundance of enzymes working in the hydrolysis phase, including peptidases, pyruvate metabolism, and genes encoding methane metabolism. Based on the concept of circular economy, this study presents a significant reference for synergistic and efficient disposal of industrial and agricultural wastes with high-value utilization.

Biochar-promoted methane production and mitigation of acidification during thermophilic anaerobic co-digestion of food waste with crude glycerol: comparison with re-inoculation

Food waste and crude glycerol were anaerobically co-digested for 100 days at 52 ± 1 °C with an organic loading rate of 1.0 g L−1 d−1. This long-term thermophilic anaerobic digestion (AD) system encountered severe inhibition from volatile fatty acids (VFAs). The study investigated the impacts of re-inoculation (RI) and biochar addition (BA) on this AD process, and monitored the variation of pH, VFAs, total alkalinity and total ammonia nitrogen during treatment. RI treatment was effective in the short term by recovering reactivity after inhibited sludge was mixed 1:1 with active inoculant. In the long term, RI could not reverse process imbalance and finally failed on day 56. Superior performance in methane production and process stability was observed in BA reactors when compared with control and RI reactors. Overall, the biochar contributed to alkalinity and facilitated the activation of methanogenesis and stimulated the conversion of VFAs.

Current Status and Prospects of Valorizing Organic Waste via Arrested Anaerobic Digestion: Production and Separation of Volatile Fatty Acids

Volatile fatty acids (VFA) are intermediary degradation products during anaerobic digestion (AD) that are subsequently converted to methanogenic substrates, such as hydrogen (H2), carbon dioxide (CO2), and acetic acid (CH3COOH). The final step of AD is the conversion of these methanogenic substrates into biogas, a mixture of methane (CH4) and CO2. In arrested AD (AAD), the methanogenic step is suppressed to inhibit VFA conversion to biogas, making VFA the main product of AAD, with CO2 and H2. VFA recovered from the AAD fermentation can be further converted to sustainable biofuels and bioproducts. Although this concept is known, commercialization of the AAD concept has been hindered by low VFA titers and productivity and lack of cost-effective separation methods for recovering VFA. This article reviews the different techniques used to rewire AD to AAD and the current state of the art of VFA production with AAD, emphasizing recent developments made for increasing the production and separation of VFA from complex organic materials. Finally, this paper discusses VFA production by AAD could play a pivotal role in producing sustainable jet fuels from agricultural biomass and wet organic waste materials.

Revealing mechanism of micro-aeration for enhancing volatile fatty acids production from swine manure

Micro-aeration is considered a new strategy for improving volatile fatty acids (VFAs) production of agricultural waste. This study investigated the effect and mechanism of micro-aeration of air and oxygen (O2) on VFAs production from swine manure. The results showed that Air-micro-aeration had the most significant improvement effect, with the highest VFAs of 8.21 g/L, which was increased by 22.4%. Moreover, the mixing effects of different micro-aeration were limited, and the microbial communities significantly varied. Firmicutes and Bacteroidota were the dominant hydrolytic and acidogenic bacteria, and Air-micro-aeration preferentially promoted electron transfer activity and energy generation. Methanosarcina, Methanocorpusculum, and Methanobrevibacter can adapt to environmental changes according to their different oxygen tolerance, and the consumption and conversion of VFAs by methanogens were slow under Air-micro-aeration condition. This study revealed mechanism of micro-aeration for improving VFAs production from swine manure, providing a theoretical basis for micro-aeration regulation optimization.

Valorization of pretreated waste activated sludge to organic acids and biopolymer.

Polyhydroxybutyrate (PHB) is a natural polyester that may be made by utilizing volatile fatty acids (VFAs) as a substrate. VFA generated by continuous anaerobic fermentation of waste activated sludge (WAS) was fed into bioreactors for PHB synthesis in this work. Series of optimization tests were conducted to increase the biodegradability and hydrolysis of waste activated sludge. It was found out that 0.05g/g TS of SDBS (sodium dodecylbenzene sulfonate), 70°C (heat treatment) and 2hr (time) as pretreatment condition would give the highest solubilization. Impact of pH adjustment on the acidogenesis of pretreated WAS was evaluated in batch experiments at varying initial pH (4-10). The result indicated that when operational pH was between 7.5 and 8, the VFA yield was increased by 5.3-18.1%. Continuous acidogenic operation validated the SDBS pretreatment and pH adjustment warranted stable VFA conversion from WAS at a yield of 47% in COD basis. Firmicutes, Actinobacteria and Proteobacteria were affiliated as dominant bacterial phyla in the continuous acidogenesis. The effluent of the continuous acidogenesis was converted to biopolymer with the average yields of 0.23g PHB-COD/g VFAadded-COD in the feast mode and 0.34g PHB-COD/g VFAadded-COD in the famine mode. In feast and famine cycle, the average VFA utilization was 55% and 60% respectively. The sequential SDBS pretreatment, acidogenesis and PHB production would produce 162g of PHB from 1kg of WAS as COD basis.

Strategy to enhance semi-continuous anaerobic digestion of food waste by combined use of calcium peroxide and magnetite

Optimizing methane production from food waste (FW) efficiently is always a hot topic in the field of anaerobic digestion (AD). In this study we aimed to improve the conversion of organics to methane by using CaO2 and magnetite to enhance the semi-continuous AD of food waste. Under the organic load of 2.5 g VS/L·d−1, the specific methane yield was increased from 333.9 mL CH4/g·VS to 423.4 mL CH4/g·VS by adding 0.01 g/L CaO2 with 0.4 g/L magnetite, improving the production of methane from FW. We assessed reactor performance, ORP changes, mass balance, enzyme activities and characterized the metagenomic profile of microorganisms involved in digestion. These microorganisms showed rapid conversion of volatile fatty acids and increased expression of genes related to hydrolysis and acid production. Thus, the addition of CaO2 and magnetite optimized the relationship between fermentation bacteria and methanogenic archaea to enhance the overall production of methane. Microorganisms evolved unique adaptive mechanisms in the co-operative environment of CaO2 and magnetite, as their energy metabolism patterns combined those controlled by individual CaO2 and magnetite addition. This method of combining a micro-aeration environment with conductive materials provides a new perspective for optimizing the AD of FW.

Enhancement mechanisms of iron powder on co-digestion of kitchen waste and Pennisetum hybrid

To investigate iron enhanced anaerobic digestion performance, a survey was conducted to examine how iron powder (IP) affected the stability of a co-digestion reactor for kitchen waste and Pennisetum hybrid in a semi-continuous mode. In addition, the mechanisms underlying this process were analyzed according to various parameters and microbial communities. The results showed that the maximum stable operational organic loading rate (OLR) for the IP reactor increased by 50% compared to that of the control reactor, while the specific methane yields and volumetric methane yields increased by 4.8%− 106.8% and 6.3%− 105.9%, respectively, when OLRs were 1.0–3.0 g VS/(L·d). Meanwhile, selective enrichment of propionate-utilizing bacteria and hydrogenotrophic methanogens may have promoted the conversion of volatile fatty acids while increasing gene abundance via a methane-forming pathway. The addition of IP changed the dominant microorganisms related to the methanogenesis pathway. Additionally, the occurrence of interspecies formate transfer promoted anaerobic digestion stability in the IP reactor.

Nickel-loaded shrimp shell biochar enhances batch anaerobic digestion of food waste

This study evaluated the effectiveness of shrimp shell biochar (SBC) and nickel (Ni) loaded SBC in increasing methane yield during anaerobic digestion of food waste. The results indicated that the methane yields of control (without SBC), SBC, SBC loaded with the low concentration of Ni, and SBC loaded with the high concentration of Ni were 81.8, 116.1, 134.7, and 99.2 mL/(g·VS), respectively. SBC promoted the efficiency and stability of the whole anaerobic digestion process including hydrolysis, volatile fatty acid conversion and methanogenesis. While the invigorating effect of loaded Ni at the low concentration of 0.88 mg/g was mainly concentrated in methanogenesis, the inhibition effect of the high Ni concentration was comprehensive. SBC helped Methanosarcina proliferation, and low concentration Ni promoted the number and activity of Methanosarcina and Methanosaeta. The results show that biochar loaded with a low level of trace elements such as Ni can promote the anaerobic digestion process.

A strategy for food waste-to-biofuels: Co-production of gasoline alternatives from volatile fatty acids

Volatile fatty acids (VFA), byproducts of anaerobic digestion of organic waste, have emerged as a platform molecule for the production of liquid fuels and chemicals. This study reports a co-production process that allows the conversion of VFAs into promising gasoline alternatives: acetic acid to ethanol (EtOH) via methyl acetate and butyric acid to butanol (BuOH) via methyl butyrate. The effect of process integration combining acetic acid to EtOH with butyric acid to BuOH on a large-scale is investigated to improve energy efficiency and economics of the co-production process. The process model simulation results show an overall (EtOH + BuOH)/VFA mass yield of 46.2% and a high energy efficiency of 37.3%, in which most (79.6%) of heat requirements for the co-production process are satisfied internally by the heat exchanger network design using the burning of VFAs residues. Techno-economic feasibility of the co-production process was US$ 5.29 per gallon of gasoline equivalent as a blend fuel of E10 (i.e., 10% EtOH and 90% gasoline).

In-situ biogas upgrading assisted by bioaugmentation with hydrogenotrophic methanogens during mesophilic and thermophilic co-digestion

In this study, the effects of bioaugmentation of typically dominant hydrogenotrophic methanogens to CSTR co-digesting cheese whey and manure, under in-situ biomethanation operations were investigated. Reactors working at mesophilic (37 °C) and thermophilic (55 °C) conditions were independently treated and examined in terms of microbial composition and process dynamics. Addition of Methanoculleus bourgensis in the mesophilic reactor led to a stable biomethanation, and an improved microbial metabolism, resulting in 11% increase in CH4 production rate. 16S rRNA and biochemical analyses revealed an enrichment in syntrophic and acidogenic species abundance. Moreover, nearly total volatile fatty acids conversion was observed. Differently, Methanothermobacter thermautotrophicus addition in the thermophilic reactor did not promote biogas upgrading performance due to incomplete H2 conversion and inefficient community adaptation to H2 excess, ultimately favoring acetoclastic methanogenesis. Bioaugmentation constitutes a viable tool to strengthen in-situ upgrading processes and paves the way to the development of more sophisticated and robust microbial inoculants.

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.

Effect of microwave/hydrothermal combined ionic liquid pretreatment on straw: Rumen anaerobic fermentation and enzyme hydrolysis

To explore green technology for wheat straw pretreatment, this study combined the microwave or hydrothermal with ionic liquid ([Bmim][OAc]) on wheat straw followed by rumen fermentation. The optimal conditions of microwave assisted ionic liquids pretreatment (M-I) and hydrothermal assisted ionic liquids pretreatment (H–I) treatment were 360 W and 200 °C, and the corresponding lignin removal rates reached 35.3% and 25.4%, respectively. Rumen fermentation showed that the highest volatile fatty acid (VFA) yield was found in M-I group, followed by H–I group at 234 and 180 mg/g, respectively. As for enzymatic hydrolysis, the saccharification rates at 3 days of M-I (360 W) and H–I (200 °C) were determined to be 393 and 320 mg/g. The optimal ionic liquid dosage was determined to be 30% in consideration of cost and VFA conversion rate. M-I pretreatment plus the rumen fermentation enjoyed the benefit of no enzyme addition and high product recovery, which was worth further investigating.

Formation of ethane and propane via abiotic reductive conversion of acetic acid in hydrothermal sediments

A mechanistic understanding of formation pathways of low-molecular-weight hydrocarbons is relevant for disciplines such as atmospheric chemistry, geology, and astrobiology. The patterns of stable carbon isotopic compositions (δ13C) of hydrocarbons are commonly used to distinguish biological, thermogenic, and abiotic sources. Here, we report unusual isotope patterns of nonmethane hydrocarbons in hydrothermally heated sediments of the Guaymas Basin; these nonmethane hydrocarbons are notably 13C-enriched relative to sedimentary organic matter and display an isotope pattern that is reversed relative to thermogenic hydrocarbons (i.e., δ13C ethane > δ13C propane > δ13C n-butane > δ13C n-pentane). We hypothesized that this pattern results from abiotic reductive conversion of volatile fatty acids, which were isotopically enriched due to prior equilibration of their carboxyl carbon with dissolved inorganic carbon. This hypothesis was tested by hydrous pyrolysis experiments with isotopically labeled substrates at 350 °C and 400 bar that demonstrated 1) the exchange of carboxyl carbon of C2 to C5 volatile fatty acids with 13C-bicarbonate and 2) the incorporation of 13C from 13C-2-acetic acid into ethane and propane. Collectively, our results reveal an abiotic formation pathway for nonmethane hydrocarbons, which may be sufficiently active in organic-rich, geothermally heated sediments and petroleum systems to affect isotopic compositions of nonmethane hydrocarbons.