Volatile Fatty Acids Degradation Research Articles

Thermodynamics of volatile fatty acid degradation during anaerobic digestion under organic overload stress: The potential to better identify process stability

Anaerobic digestion (AD) operating under organic overload stress usually increases the potential for process instability, leading to significant economic and ecological consequences. Volatile fatty acids (VFAs) accumulation is regularly considered a major factor during AD and their degradation is subject to thermodynamic constraints. To date, no study has systematically investigated the mechanisms of VFA degradation on process stability from the perspective of thermodynamics. Hence, increased substrate-to-inoculum ratio was applied in this study to simulate organic overload stress using batch tests with Hybrid Pennisetum. As a result, VFAs accumulation increased, accompanied by decreased methane yield, slower methane production kinetics and even severe process instability. Metagenomic analysis demonstrated that the accumulated propionate and butyrate were degraded by methyl-malonyl-CoA and the β-oxidation pathway while syntrophic acetate oxidation was preferred during acetate degradation. The deviation of stability parameters to varying degrees from the recommended threshold values was observed. However, a subsequent thermodynamic analysis revealed that moderate organic overload stress merely retarded the syntrophic oxidation of propionate, butyrate, and acetate. As a result, the methanogenic activity decreased, and the lag phase of AD was extended, but no adverse thermodynamic effects actually occurred. Changes in the Gibbs free energy for syntrophic propionate and acetate oxidation have the potential to better identify process stability. This study provided novel insights into the underlying thermodynamic mechanisms of VFA degradation and may have important implications for improving the current diagnostic mode for AD process stability.

Introducing electrolysis to enhance anaerobic digestion resistance to acidification.

The phenomenon that the anaerobic system is inhibited by acid has always been a bottleneck hindering the application of anaerobic digestion (AD) technology. We tried to introduce electrolysis into AD to improve the acidification resistance, and eventually the productivity of the energy. In a batch fermentation device, the ability of electrochemical anaerobic digestion (EAD) to resist acidification was evaluated in current intensity, electrode potential, AC impedance, microbial community, pH value, and volatile fatty acids (VFAs). The results showed that the average concentration of VFAs in EAD was 32.9% lower than that in AD, the energy efficiency of EAD is 53.25% higher than AD, indicating that EAD has stronger anti-acidification ability and energy conversion efficiency than AD. When the EAD reaches a steady state, the current intensity fluctuates in the range of 7-12mA, the electrode potential difference is maintained at 600 ± 5mV, and the internal resistance decreases from 3333.3 ± 16Ω at startup to 68.9 ± 1.4Ω at the steady state, indicating that the EAD has stronger resistance to acidification may be due to the degradation of some VFAs on the electrode surface. Furthermore, the 16S rRNA sequencing analysis showed that the dominant electricity-producing bacteria on EAD anode surface were Clostridium, Hydrogenophaga and Trichloromonas, with a relative abundance of 40.32%, while the relative abundance of electrogenic bacteria in AD bulk solution and EAD bulk solution were about 1/2 and 1/4 that of EAD anode film, suggesting that the electricity-producing bacteria on the electrode surface play an important role in the degradation of VFAs.

Study on the performance of HNO3-modified biochar for enhanced medium temperature anaerobic digestion of food waste.

Biochar can help promote direct interspecies electron transfer (DIET) and increase methane production; the surface redox groups play a constructive role in these processes. This study attempted to improve the anaerobic digestion (AD) performance by modifying biochar with HNO3 to increase its redox activity. A comparative experimental study, raw biochar (BC0) and biochar treated with HNO3 for 6h (BC6), were conducted to investigate the effect of HNO3 treatment on the medium temperature AD performance of food waste. Both BC0 and BC6 can enhance CH4 yield and facilitate the degradation of volatile fatty acids. The enhanced yield of CH4 was 36% for BC0 and 90% for BC6, respectively. Biochar can also enhance methanogenesis, presumably owing to direct interspecific electron transfer (DIET). Compared with BC0, BC6 had a higher redox activity and a smaller conductivity. It was supposed that BC0 mediated DIET through its conductivity, whereas BC6 accelerated DIET by surface redox groups.

Effects of pine sawdust and shrimp shell biochar on anaerobic digestion under different acidification conditions

Effects of different types of biochar (pine sawdust biochar (PSBC) and shrimp shell biochar (SSBC) on anaerobic digestion under different acidification conditions were studied and compared. Compared with PSBC, SSBC had more alkaline functional groups and higher conductivity. As a result, PSBC and SSBC posed different impacts on the anaerobic digestion system under different acid inhibition conditions. In the case of less acidification, PSBC and SSBC reduced the lag time in methane production from 10.2 d to 6.3 d and 3.5 d, but didn’t present significantly improvement in the final CH4 production. But regarding more serious acid stress, PSBC-added and SSBC-added digesters respectively obtained the specific methane yield of 266.8 and 280.0 mL CH4/g VSS, 7.6% and 12.9% higher than the control digester. Both PSBC and SSBC showed pH buffering potency and the ability to promote the degradation of volatile fatty acids, while the latter was superior to the former. On the one hand, this might be due to more basic functional groups and more areas for microbial attachment existing on the surface of SSBC, which was expected to buffer the acid shock and promote microbial growth; on the other hand, according to microbial diversity analysis, the SSBC was likely to stimulate direct interspecific electron transfer, thereby accelerating methane production.

Derivation of volatile fatty acids concentration-independent optimum trace elements configuration and elucidation of optimization kinetics of biomethanization processes

Trace elements (TEs) requirements for improved volatile fatty acids (VFA) degradation during biomethanization depend on VFA concentration of a reactor and the temperature of the process. While temperature remains relatively constant, VFA concentrations change in the course of biomethanization and this implies that for efficient VFA degradation, different trace elements configurations (TEC) should be supplemented. While this is the most efficient approach, it is impractical and constitutes a challenge for the effective use of TEs in the optimization of biomethanization processes. To alleviate this challenge, we modelled the biomethanization efficiency of various VFA concentration-dependent (VCD) TEs configuration as scenarios and derived a TEs configuration that produced optimum biomethanization across a wider range of VFA concentrations. The study was carried out at 37oC using different concentrations of fixed VFA composition and TEs configurations as scenarios. Response surface model and desirability function were used to determine and compare the biomethanization efficiency of the scenarios, and to derive a VFA concentration-independent (VCI) TEs configuration. Michaelis-Menten kinetics for two parameters was used to ascertain that the mechanism by which TEs supplementation enhanced mesophilic biomethanization was through an increase in maximum reaction rate (MRR). However, the enhancement was accompanied by an insignificant decline in inverse affinity (IA).

Promoting anaerobic co-digestion of sewage sludge and food waste with different types of conductive materials: Performance, stability, and underlying mechanism

In this research, we investigated and compared the effects of three widely used conductive materials, e.g., zero-valent iron (Fe0), magnetite (Fe3O4), and biochar on the performance, stability, and in-depth mechanism during the anaerobic co-digestion process of sewage sludge and food waste. Among the three conductive materials, Fe0 could achieve the highest cumulative methane production of 394.0 mL/g volatile solids (VS) added, which was 1.24-fold and 1.17-fold higher than that receiving Fe3O4 and biochar. The mechanistic studies indicated that compared to the Fe3O4 and biochar groups, Fe0 could significantly enhance the release of soluble protein, polysaccharide, and dissolved organic matters, the degradation of volatile fatty acids and VS, and the activities of key enzymes and direct interspecies electron transfer (DIET). Consequently, the methane yield and digestate dewaterability were notably improved. Collectively, these findings will offer suggestions of the preferable conductive materials in the anaerobic co-digestion process for decision makers.

Effects of Lighter Dose of Oxytetracycline on the Accumulation and Degradation of Volatile Fatty Acids in the Process of Thermophilic Anaerobic Digestion of Swine Manure

Oxytetracycline (OTC) is a commonly used antibiotic in livestock farming for controlling intestinal and respiratory infections in farm animals. However, the absorption of antibiotics by animals is limited, and most antibiotics are excreted in the original form with manure, which will have an impact on the environment. The removal of antibiotics from swine manure could generally be performed via anaerobic digestion (AD). In this study, the effect of oxytetracycline (OTC) at doses of 0.1, 0.5, and 1.0 mg/L on the thermophilic anaerobic digestion of swine manure (55 °C) in batch digesters was studied. The methane production, volatile fatty acid (VFA) levels, and dissolved organic matter (DOM) were determined and compared with the control (0 mg/L of OTC). The results indicate that (1) OTC at 0.1 mg/L had no inhibitory effect on methane production or on the accumulation of VFAs, while 0.5 mg/L and 1.0 mg/L inhibited methane production, with inhibition rates of 4.03% and 14.12% (p < 0.05), respectively; (2) the VFAs of each reactor peaked on the first day of the reaction, and as the OTC dose increased from 0 to 1.0 mg/L, the maximum VFA accumulation increased from 1346.94 mg/g to 2370 mg/g of volatile solids (VS); and (3) oxytetracycline (0.5 and 1.0 mg/L) could promote the temporary accumulation of propionic acid, which did, however, not result in significant VFA accumulation. Further, OTC at 1.0 mg/L can promote DOM production, and therefore, VFA accumulation.

Biochar addition supports high digestion performance and low membrane fouling rate in an anaerobic membrane bioreactor under low temperatures

The enhancement effects of biochar to an anaerobic membrane bioreactor (AnMBR) treating sewage at low temperatures was investigated in this study through analyzing organics removal, digestion performance, mixed liquor properties, membrane resistance, and foulant compositions. The chemical oxygen demand (COD) removal efficiency and the COD converted to methane rate increased by more than 12.5% at 10 °C, mainly because of the promotion of biochar to volatile fatty acids degradation. Although biochar caused higher dissolved organic matter (DOM) concentration in the AnMBR, it improved the filtration property of the bulk sludge and absorbed the hydrophobic DOM. The decreased filtration resistance assisted by biochar leads to a prolonged membrane operation duration over 200%. Surface foulants, especially cake foulants, were largely mitigated by the enhanced scouring intensity of mixed liquor at the membrane surface, and hence, decreasing the cake/gel foulants ratio.

Effect of biochar addition on microbial community and methane production during anaerobic digestion of food wastes: The role of minerals in biochar

The effect of biochar addition on the microbial community and methane (CH4) production during anaerobic digestion was experimentally investigated, focusing on the role of minerals in biochar. The biochar was prepared from pine sawdust by pyrolysis at 650 °C and 900 °C, respectively, and a subsample was leached with citric acid. The cultures with the addition of biochar, leached biochar, Fe, and leached biochar combined with Fe, respectively, were placed in bench-scale bioreactors for anaerobic digestion. Daily biogas production was measured by volume displacement method and analysed for CH4 concentration, which allowed the cumulative CH4 yield (YM) and daily CH4 production rate (RM) to be determined. Culture samples were also taken daily for volatile fatty acids (VFAs) and microbial community analysis. Compared to the control without biochar addition, the addition of raw biochar significantly increased YM by 46.9% and RM by 43.0%, while leached biochar only increased the YM by 33.2% and RM by 18.2%, respectively. The Fe-containing minerals in biochar were found to enhance VFA degradation and increase population of Clostridia and Methanosaeta, improving the CH4 production.

Responses of anaerobic digestion of food waste to coupling effects of inoculum origins, organic loads and pH control under high load: Process performance and microbial characteristics

This study investigated responses of anaerobic digestion (AD) of food waste (FW) with different inocula to varying organic loads and to pH control under high load in terms of process performance and microbial characteristics. Without pH control, digester inoculated by thickened sludge obtained high methane yield of 547.8 ± 27.8 mL/g VS under organic load of 7.5 g VS/L but was inhibited by volatile fatty acids (VFAs) under higher loads (15 and 30 g VS/L). However, digesters inoculated by anaerobic sludge obtained high methane yields of 575.9 ± 34.2, 569.3 ± 24.8 and 531.9 ± 26.2 mL/g VS under organic loads of 7.5, 15 and 30 g VS/L and VFAs inhibition only appeared under extremely high load of 45 g VS/L. Digesters under VFA inhibition with high load were significantly enhanced by controlling single ecological factor pH at 6.5, 7.0 and 7.5, as indicated by shorter lag phases, higher peak values of methane production rate, greater methane yields and fast VFAs degradation. Maximum methane recovery was obtained with pH control at 7.5 under high load. VFA inhibition was accompanied by the degeneration of ecological functions of Syntrophomonadaceae and unidentified Bacteroidales and the dominant growth of unidentified Clostridiales. Under high load and pH control, high stability was strongly associated with obvious growth of Methanosarcina, which enriched methanogenic pathways thus improved system robustness and tolerance to VFAs. Moreover, pH control stimulated the growth of syntrophic Bacteria Syntrophomonadaceae while maintaining the high activity of hydrogenotrophic methanogens therefore sustained efficient syntrophic communities of Bacteria and methanogens and avoided over accumulation of VFAs. pH control promoted adaptive selection of methanogens, leading to obvious decline of archaeal community diversity. This study provided practical guidance on digester configurations of high-load AD of FW and expanded the understanding of responses to coupling effects of inoculum origins, organic loads and pH control under high load concerning process performance and microbial community dynamics.

Variation of volatile fatty acid oxidation and methane production during the bioaugmentation of anaerobic digestion system: Microbial community analysis revealing the influence of microbial interactions on metabolic pathways

Anaerobic digestion (AD) is widely used on waste treatment for its great capability of organic degradation and energy recovery. Accumulation of volatile fatty acids (VFAs) caused by impact loadings often leads to the acidification and failure of AD systems. Bioaugmentation is a promising way to accelerate VFA degradation but the succession of microbial communities usually caused unpredictable consequences. In this study, we used the sludge previously acclimated with VFAs for the bioaugmentation of an acidified anaerobic digestion system and increased the methane yield by 8.03–9.59 times. To see how the succession of microbial communities affected bioaugmentation, dual-chamber devices separated by membrane filters were used to control the interactions between the acidified and acclimated sludges. The experimental group with separated sludges showed significant advantages of VFA consumption (5.5 times less final VFA residue than the control), while the group with mixed sludge produced more methane (4.0 times higher final methane yield than the control). Microbial community analysis further highlighted the great influences of microbial interaction on the differentiation of metabolic pathways. Acetoclastic methanogens from the acclimated sludge acted as the main contributors to pH neutralization and methane production during the early phase of bioaugmentation, and maintained active in the mixed sludge but degenerated in the separated sludges where interactions between sludge microbiotas were limited. Instead, syntrophic butyrate and acetate oxidation coupled with nitrate and sulfate reduction was enriched in the separated sludges, which lowered the methane conversion rate and would cause the failure of bioaugmentation. Our study revealed the importance of microbial interactions and the functionality of enriched microbes, as well as the potential strategies to optimize the durability and efficiency of bioaugmentation.

Inhibition mitigation of methanogenesis processes by conductive materials: A critical review

Methanogenesis can be promoted by the addition of conductive materials. Although stimulating effects of conductive materials on methane (CH4) production has been extensively reported, the crucial roles on recovering methanogenic activities under inhibitory conditions have not been systematically discussed. This critical review presents the current findings on the effects of conductive materials in methanogenic systems under volatile fatty acids (VFAs), ammonia, sulfate, and nano-cytotoxicity stressed conditions. Conductive materials induce fast VFAs degradation, avoiding VFAs accumulation during anaerobic digestion. Under high ammonia concentrations, conductive materials may ensure sufficient energy conservation for methanogens to maintain intracellular pH and proton balance. When encountering the competition of sulfate-reducing bacteria, conductive materials can benefit electron competitive capability of methanogens, recovering CH4 production activity. Conductive nanomaterials stimulate the excretion of extracellular polymeric substances, which can prevent cells from nano-cytotoxicity. Future perspectives about unraveling mitigation mechanisms induced by conductive materials in methanogenesis processes are further discussed.

Role of trace elements in anaerobic digestion of food waste: Process stability, recovery from volatile fatty acid inhibition and microbial community dynamics

Role of trace elements (TEs) in long-term anaerobic digestion of food waste (FW) under fixed and stepwise increasing loads and under early and medium volatile fatty acid (VFA) inhibition was investigated. Digesters under high load suffered VFA inhibition. Mismatch between scarce TEs in FW and essential TEs for sustainable methanogenesis suppressed Methanosaeta causing blocked aceticlastic methanogenesis and shift to CO2 reduction pathway, as indicated by decreased Methanosaeta from above 70.0% to below 42.0% and enriched hydrogenotrophic methanogens (Methanospirillum, Methanoculleus, Methanobacterium) from below 15.0% to above 53.6%. Dual stresses of VFA inhibition and TEs deficiency resulted in recession of syntrophic Bacteria Syntrophomonadaceae. Conversely, digesters with TEs supplementation maintained high activity of Syntrophomonadaceae and ensured predominant aceticlastic methanogenesis and powerful methanogenic community functions. Early and medium VFA inhibition were reversed by TEs supplementation or coupling with pH adjustment by stimulating VFAs degradation via syntrophic metabolism and unclogging acetate conversion via aceticlastic methanogenesis.

Methanogenic pathway and microbial succession during start-up and stabilization of thermophilic food waste anaerobic digestion with biochar

One of the major obstacles for thermophilic anaerobic digestion is the process instability during start-up. This study proposed the use of a cost-effective additive, biochar, to accelerate and stabilize the start-up of thermophilic semi-continuous food waste anaerobic digestion. The results showed that the reactors with biochar addition resulted in up to 18% higher methane yield as compared to the control reactors (without biochar). The key microbial networks were elucidated through thermochemical and microbial analysis. Particularly, the addition of biochar promoted the growth of electroactive Clostridia and other electroactive bacteria, while the absence of biochar promoted the growth of homoacetogenic Clostridia and syntrophic acetate oxidizing bacteria. It was revealed that biochar promoted direct interspecies electron transfer between the microbes and was responsible for the faster degradation of volatile fatty acids. Furthermore, reactors with biochar also enhanced the thermodynamically favourable acetoclastic methanogenic pathway due to the higher abundance of Methanosarcina.

Effects of Fe-Mn-modified biochar addition on anaerobic digestion of sewage sludge: Biomethane production, heavy metal speciation and performance stability

This study investigated the effects of MnFe2O4-biochar on sludge anaerobic digestion performance, methane production and heavy metal stabilization. The highest cumulative methane yield was achieved when the MnFe2O4-biochar dose was 1.50 g, which was 55.86% higher than control. A suitable dose of MnFe2O4-biochar stimulated methanogenic activities and improved methane yield, whereas excessive addition shows an inhibitory effect on anaerobic digestion. MnFe2O4-biochar addition significantly enhanced the biodegradation of organic matter, as the average volatile fatty acids degradation rate increased by 35.44%, compared to the control. Chemical speciation analyses demonstrated that MnFe2O4-biochar addition was more helpful for immobilization and risk reduction of heavy metals in sewage sludge. Simultaneously, microbial community analysis indicated that MnFe2O4-biochar obviously enriched acetoclastic methanogens Methanosarcina that capable of participating in direct interspecies electron transfer, which could accelerate substrate decomposition and methane production. These findings supply useful information for the resource utilization of sewage sludge.

Enhanced syntrophic metabolism of propionate and butyrate via nickel-containing activated carbon during anaerobic digestion

The accumulation of volatile fatty acids (VFAs) is commonly occur during anaerobic digestion, affecting methane production rate and system stability. Batch experiments were conducted to investigate the effects of nickel-containing granular activated carbon (GAC-Ni), an industrial waste, on the degradation of acetate (HAc), propionate (HPr) and butyrate (HBu), respectively. The results showed that the maximum methane production rate of the reactors fed with HPr and HBu were increased by 54.06% and 16.55%, respectively, with the supplementation of GAC-Ni. Correspondingly, the degradation rates of HPr and HBu were improved with GAC-Ni addition, which was 1.14–20.70 times and 1.01–2.16 times of the control, respectively. And the microbial analysis revealed that GAC-Ni facilitated the syntrophic relationship between Syntrophomonas and hydrogenotrophic methanogens (i.e. genera Methanolinea and Methanobacterium), leading to faster degradation of HPr and HBu. Additionally, the surface morphology analysis indicated that the presence of Ni in GAC-Ni could promote the formation of condense reticulate structures, which might be beneficial for linking the functional microbes together. This study suggested that GAC-Ni waste could mitigate the accumulation of VFAs in anaerobic digestion reactor by promoting the syntrophic metabolism between VFAs degradation bacteria and methanogens.

Eco-compatible biochar mitigates volatile fatty acids stress in high load thermophilic solid-state anaerobic reactors treating agricultural waste

A high concentration of accumulated volatile fatty acids (VFAs) is one of the most important factors resulting in reactor failure during solid-state anaerobic digestion. In this study, the feedstock-to-inoculum (F/I) ratio (0.5, 2, 3, 4 and 6) and the recovery method after failure (biochar addition or inoculum addition) were investigated in batch solid-state anaerobic digestion fed with rice straw and pig urine. An F/I ratio of 3 was the threshold for stable operation, while the reactors failed at F/I ratios of 4 and 6 because of high accumulated VFAs concentrations (above 30 g HAc/kg). Biochar addition (10% or 20% (wet weight) of the mixture) was as effective as inoculum addition (by adjusting the F/I ratio to 2 or 3) in promoting VFAs degradation in failed reactors within a short period (<1 day). The buffering capacity of biochar was important in promoting VFAs degradation.

Dynamic shifts within volatile fatty acid-degrading microbial communities indicate process imbalance in anaerobic digesters.

Buildup of volatile fatty acids (VFAs) in anaerobic digesters (ADs) often results in acidification and process failure. Understanding the dynamics of microbial communities involved in VFA degradation under stable and overload conditions may help optimize anaerobic digestion processes. In this study, five triplicate mesophilic completely mixed AD sets were operated at different organic loading rates (OLRs; 1-6g chemical oxygen demand [COD] LR-1day-1), and changes in the composition and abundance of VFA-degrading microbial communities were monitored using amplicon sequencing and taxon-specific quantitative PCRs, respectively. AD sets operated at OLRs of 1-4g COD LR-1day-1 were functionally stable throughout the operational period (120days) whereas process instability (characterized by VFA buildup, pH decline, and decreased methane production rate) occurred in digesters operated at ≥ 5g COD LR-1day-1. Though microbial taxa involved in propionate (Syntrophobacter and Pelotomaculum) and butyrate (Syntrophomonas) degradation were detected across all ADs, their abundance decreased with increasing OLR. The overload conditions also inhibited the proliferation of the acetoclastic methanogen, Methanosaeta, and caused a microbial community shift to acetate oxidizers (Tepidanaerobacter acetatoxydans) and hydrogenotrophic methanogens (Methanoculleus). This study's results highlight the importance of operating ADs with conditions that promote the maintenance of microbial communities involved in VFA degradation.

Enhanced volatile fatty acid degradation and methane production efficiency by biochar addition in food waste-sludge co-digestion: A step towards increased organic loading efficiency in co-digestion

This work investigated the effect of biochar addition to mitigate VFA accumulation and enhance methane production in mesophilic food waste/sludge co-digestion. Different types of biochar derived from agricultural and forestry residues at two pyrolysis temperatures were tested. Results showed that wheat straw biochar 550 °C supported the highest specific methane yield of 381.9 LCH4/kg VSadded and VS removal efficiency of 41.62% among all treatments. Degradation of propionic acid and long-chain fatty acids such as valeric, caproic and isovaleric acids was observed. This also corresponded to an increase in methanogenic favorable substrates including acetic acid (>40%) and butyric acid (~20%) over the control. Consequently, a 24% increase in overall methane production was obtained as compared to control. This demonstrated that biochar addition had positive effects on VFA degradation and methane production which could be a useful strategy to increase the organic loading in co-digestions without the fear of process failure.

Comparative facilitation of activated carbon and goethite on methanogenesis from volatile fatty acids

To provide insight into direct interspecies electron transfer (DIET) via carbon-based materials and ferric oxides, the effects of three conductive materials (i.e. activated carbon (AC), iron modified activated carbon (FEAC) and goethite (FEOOH)), on methanogenesis from volatile fatty acids (VFAs) were evaluated. Under the acid stress (~4 g/L VFAs), the maximum methane yield of 266 mL/g-chemical oxygen demand (COD) was found in the FEOOH supplemented reactor, which was 48% higher than that of AC reactor. The reasons for the enhanced activity of the electron transport chain and extracellular electron transfer ability by FEOOH include: 1) the activation on iron-containing enzymes that involved in methanogenesis and acidogenesis; 2) selective enrichment on functional microorganism. The higher electron donating capacities (EDC) value of FEOOH may be a triggering factor on the growth of Syntrophomonadaceae, which perform DIET with methanogens (Methanosaeta and Methanosarcina) for the syntrophic degradation of VFAs.