Propionate Degradation Research Articles

Thermal degradation and isothermal crystallization behavior of curdlan propionate and its melt-spinning fiber

The thermal properties and crystallization kinetics of Curdlan propionate, a type of bio-based plastic and β-1,3-glucan derivative, were investigated. Melt-spun fibers were also prepared, and the fiber formation mechanism and internal structure were analyzed. The decrease curve in molecular weight suggests that the thermal degradation of Curdlan propionate likely proceeds by random scission. Polarized light microscopy of the isothermal crystallization process showed Curdlan propionate forms a spherulite-like morphology, which is unusual among polysaccharides with a rigid backbone chain. By optimizing the melt spinning conditions, it was found that it is possible to spin with good spinnability using a no plasticizer. The crystallization rate of Curdlan propionate was significantly different between the isothermal crystallization process and the melt spinning process. There was a positive correlation between the take-up speed, crystallinity, and the tensile strength of the fiber. A highly ordered structural model of the fiber was suggested from wide-angle x-ray diffraction results and other measurements.

Enhanced syntrophic propionate degradation and methane production by granular activated carbon in AnSBR

Syntrophic degradation of propionate has been regarded as a limiting factor for methane formation in anaerobic digestion (AD) processes, due to its easier production but harder degradation than other volatile fatty acids. In the present study, 20 g/L granular activated carbon (GAC) was introduced into an anaerobic sequence batch reactor (AnSBR) and the enhancement of propionate degradation and methane production was evaluated with another AnSBR without GAC as a control. Inoculated with the same excess activated sludge, both AnSBRs were synchronously started up at 35 °C with the fed propionate increased gradually from the initial 340 mg/L to the final 1700 mg/L. The results showed that the introduction of GAC made the AnSBR get steady state 6 days earlier. With the fed propionate of about 1700 mg/L in the steady state, the average specific methane production and biomass was enhanced from 0.20 to 0.25 L/L·d, and 7.72 to7.96 g/L, respectively, by the introduced GAC. The results suggested that the GAC had functioned in stimulating microbial growth and enhancing direct interspecies electron transfer between hydrogen-producing acetogens and methanogens, which had resulted in the enhanced propionate degradation and methane production.

Methane Production from Propionate Enhanced by Met@Fe3O4 via Increasing Microbe–Material Attachment in a Direct Interspecies Electron-Transfer Process

Propionic acid is an important metabolic intermediate of anaerobic digestion, but its biotransformation to methane is difficult. Although Fe₃O₄ was reported to benefit the direct interspecies electron transfer (DIET) in propionate digestion, the poor attachment of Fe₃O₄ to microbes limited further improvement of methane yield. In this study, the use of methionine-modified Fe₃O₄ (Met@Fe₃O₄) aiming at increasing the attachment of Fe₃O₄ to microbes and enhancing the electron transfer to boost methane production from propionate was reported, and its role was explored. It was observed that in the presence of Met@Fe₃O₄, the methane yield and the maximal methane production rate were, respectively, 81.2 and 152.1% greater than those of the control and 37.6 and 82.7% higher than those of Fe₃O₄. The investigation of the mechanism indicated that the lower electrochemical impedance of Met@Fe₃O₄ made it more effective for promoting not only the intracellular electron transfer of acetoclastic methanogenesis but also extracellular DIET mainly rather than Fe₃O₄, resulting in the improvement of enzyme and microbial activities involved in anaerobic propionate degradation. Further studies demonstrated that the protein on the cell surface was more likely to bind to Met@Fe₃O₄ because of the enhanced hydrophilic nature, and the attachment of Met@Fe₃O₄ to microbes was increased, leading to a faster extracellular electron-transfer efficiency. This study provided an alternative method to improve propionate biodegradation for efficient energy recovery.

Structures, energetics, and kinetics of H-atom abstraction from methyl propionate by molecular oxygen: Ab initio and DFT investigations

Initial degradation of methyl propionate (MePr) through its reaction with triplet molecular (3Σg−) O2 was investigated using density functional theory (DFT) and ab initio CBS-QB3 calculations over temperature range 700–1600 K. Thermochemistry and kinetics of three hydrogen atom abstractions have been studied. These reactions encounter energy barriers of 44.17–48.10 kcal mol−1 at CBS-QB3. Rate coefficients of the studied channels were evaluated from transition state theory (TST) with tunneling correction. From kinetic and thermodynamic perspectives, the most favorable process is H-atom abstraction from the Cα position, followed by Cμ, and then Cβ position. This order is partially different for the reaction of MePr with singlet (1Δg) O2 where H-atom abstraction from the Cμ atom is the most preferable pathway, followed by Cα position, then Cβ position. The atom in molecules (AIM) theory in critical point and natural bond orbital (NBO) analysis was used to explain the existence of covalent interactions.

Enhancing anaerobic syntrophic propionate degradation using modified polyvinyl alcohol gel beads

Modified polyvinyl alcohol (PVA) beads serve as effective anaerobic microbe immobilization carriers. PVA beads were mixed with different conductive materials, activated carbon, magnetite, and green tuff stone powder. In this study, modified PVA beads were used to investigate the effect of using, promote methane production, and enhance direct interspecies electron transfer (DIET) on the anaerobic syntrophic degradation of propionate, which is an essential intermediate process for generating methane in anaerobic digesters. The batch experiment showed that PVA mixed with activated carbon had the highest methane conversion rate of 72%, whereas the rates for control (sludge) was 61%. Moreover, the lag time during the second and third feedings was shorter by 5-fold than for the first feeding when modified PVA beads were added. The syntrophic propionate degrading microorganisms in the modified PVA beads was Syntrophobacter and Methanobacterium, either Methanoculleus or Methanosaeta. The modified PVA beads hold at least 10 times larger syntrophs than normal PVA. Therefore, composite PVA with conductive materials can promote methane production, accelerate propionate consumption, and enhance electron transfer in related microbial species.

Effect of wood biochar dosage and re-use on high-solids anaerobic digestion of chicken litter

The use of biochar (pyrolysed biomass) in anaerobic digesters can improve the efficiency of methane production at a low cost. However, the biochar dosage and the ability to re-use biochar affects the viability of this process improvement. This study shows the effects of high dosages of both pristine and re-used wood-pellet biochar on the performance of high-solids anaerobic digesters processing chicken litter. Performance was assessed on a basis of methane yield per gram of volatile solids and the volumetric efficiency. The 90-day methane yield increased by 39% with a pristine biochar dosage of 1 gTS−char/gTS−feed but unchanged under pristine biochar at lower dosages and re-used biochar at all three dosages. The use of pristine biochar at the highest dosage increased propionate degradation compared with other treatments. In addition, the highest dosage of pristine biochar causes the largest reduction in lag time compared with the control, a reduction of 35%. Lowering the biochar dosage affects the concentration of methanogens on the biochar. Methanosaetaceae and hydrogenotrophic methanogens had greater concentrations on the biochar at dosages of 0.25 and 0.5 gTS−char/gTS−feed. The addition of both pristine and re-used biochar at all three dosages investigated allows for shorter retention times and a greater number of batches to be processed per annum compared with digesters without biochar.

Self-fluidized GAC-amended UASB reactor for enhanced methane production

Granular activated carbon (GAC) has been reported to benefit anaerobic digestion systems by stimulating direct interspecies electron transfer (DIET) as an alternative to interspecies hydrogen transfer (IHT). Conventional up-flow anaerobic sludge blanket (UASB) reactors typically have GAC settled at the bottom of the reactor, limiting the contact between biomass and GAC. This study demonstrated a modified UASB reactor that fluidizes GAC by encaging GAC in plastic carriers. Enhanced performance was achieved with respect to methane production and COD removal at an organic loading rate of 1500 g COD/m3/d under the temperature of 20 °C using the self-fluidized GAC configuration; with the methanation rate increased from 0.33 ± 0.08 g CH4-COD/g influent COD in the non-GAC reactor, to 0.66 ± 0.02 g CH4-COD/g influent COD in the GAC-only reactor, and further increased to 0.77 ± 0.02 g CH4-COD/g influent COD in the self-fluidized GAC reactor. The concentrations of medium-chain acyl-homoserine lactones increased in the GAC-amended reactors, potentially associated with the promotion of syntrophic interactions between bacteria and archaea. Batch tests were performed for syntrophic propionate degradation under hydrogen-inhibition conditions, and showed that microbial consortium enriched in both GAC amended reactors can overcome hydrogen inhibition. Further microbial community analysis unveiled a variation in spatial distribution in the GAC enriched microbial consortium (including DIET indicator microorganisms such as Geobacter and Methanosarcina) based on the location of GAC in the reactors. This study demonstrated an improved GAC amendment strategy in UASB reactors to facilitate biomethane recovery from wastewater anaerobic digestion.

Hydrothermal pretreatment of sewage sludge for enhanced anaerobic digestion: Resource transformation and energy balance

The objective of this study was to evaluate the effect of hydrothermal pretreatment (HTP) at 90, 125 and 155 °C on the extent of anaerobic digestion (AD) of sewage sludge, energy balance, and the distribution and availability of N and P. AD was evaluated with four bench-scale mesophilic, semi-continuous digesters with 20-d (Phase 1) and 10-d (Phase 2) solids retention time (SRT), fed with raw sludge (R1), 90 °C (R2), 125 °C (R3) and 155 °C (R4) HTP sludge. The two sludge mixtures used in this study had high ultimate biodegradability (58.3 for Phase 1 and 56.9% for Phase 2). With the exception of Phase 2 R4, HTP increased organic matter destruction and methane production. HTP of Phase 2 sludge mixture at 155 °C resulted in the formation of high propionate levels, which led to a lower methane production. HTP significantly changed the digesters’ bacterial communities, but had a minor effect on the archaeal communities. Abundance of well-known propionate degraders in all digesters was very low; however, Proteiniphilum, which is involved in the degradation of intermediates in the propionate degradation pathway, was found in relatively high abundance in Phase 2 R4. The highest net energy gain (ΔE) was obtained with the control (i.e., raw sludge AD without HTP) for both phases. HTP heat recovery greater than 85% is required to attain the same net energy as the control or higher. HTP at 155 °C followed by AD led to increased solids reduction, overall crude protein removal and release of ammonium N. HTP-AD decreased P availability. Thus, P and N recovery is recommended before and after HTP-AD, respectively.

Transition of microbial communities and degradation pathways in anaerobic digestion at decreasing retention time

Tuning of operational variables is a common practice to control the anaerobic digestion process and, in advanced applications, to promote the accumulation of fermentation products. However, process variables are interrelated. In this study, the hydraulic retention time (HRT) was decoupled from the organic loading rate (OLR) in order to isolate the effect of HRT as a selective pressure on: process performance, metabolic rates (hydrolytic, acetogenic, and methanogenic) and the microbial community. Four mesophilic anaerobic digesters were subjected to a sequential decrease in HRT from 15 to 8, 4 and 2 days while keeping the OLR constant at chemical oxygen demand of 1 gCOD L r−1 d−1. The results showed that HRT alone was insufficient to washout methanogens from the digesters, which in turn prevented the accumulation of volatile fatty acids (VFA). Methanosaeta was the dominant genus in the four digesters at all HRTs. Metabolic rates showed that process performance was controlled by hydrolysis, with a clear shift in acetogenic rates, from butyrate and propionate degradation to ethanol degradation at 4 and 2d HRT. The change in acetogenic pathways was attributed to a shift in the fermentation pathways co-current with changes in fermentative bacteria. At 2d HRT, biofilm was formed on the walls and paddles of the digesters, probably as a survival strategy. Most of the taxa in the biofilm were also present in the digester media. Overall, it is the combination of HRT with other operational parameters which promotes the washout of methanogens and the accumulation of VFA.

Graphene Addition to Digestion of Thin Stillage Can Alleviate Acidic Shock and Improve Biomethane Production

Production of biomethane from distillery byproducts (such as stillage) in a circular economy system may facilitate a climate neutral alcohol industry. Anaerobic digestion (AD) of easily degradable substrates can lead to rapid acidification and accumulation of intermediate volatile fatty acids, reducing microbial activity and biomethane production. Carbonaceous materials may function as an abiotic conductive conduit to stimulate microbial electron transfer and resist adverse impacts on AD. Herein, nanomaterial graphene and more cost-effective pyrochar were comparatively assessed in their ability to recover AD performance after acidic shock (pH 5.5). Results showed that graphene addition (1.0 g/L) could lead to a biomethane yield of 250 mL/g chemical oxygen demand; this is an 11.0% increase compared to that of the control. The recovered process was accompanied by faster propionate degradation (CH₃CH₂COO– + 2H₂O → CH₃COO– + CO₂ + 6H⁺ + 6e–). The enhanced performance was possibly ascribed to the high electrical conductivity of graphene. In comparison, pyrochar addition (1.0 and 10.0 g/L) did not enhance the biomethane yield, though it reduced the digestion lag-phase time by 18.1% and 12.2% compared to the control, respectively. Microbial taxonomy analysis suggested that Methanosarcina (81.5% in abundance) with diverse metabolic pathways and OTU in the order DTU014 (6.4% in abundance) might participate in direct interspecies electron transfer contributing to an effective recovery from acidic shock.

Zeolite favours propionate syntrophic degradation during anaerobic digestion of food waste under low ammonia stress

Zeolite addition has been widely suggested for its ability to overcome ammonia stress occurring during anaerobic digestion. However little is known regarding the underlying mechanisms of mitigation and especially how zeolite influences the microbial structuration. The aim of this study was to bring new contributions on the effect of zeolite on the microbial community arrangement under a low ammonia stress. Replicated batch experiments were conducted. The microbial population was characterised with 16S sequencing. Methanogenic pathways were identified with methane isotopic fractionation. In presence of ammonia, zeolite mitigated the decrease of biogas production rate. Zeolite induced the development of Izimaplasmatales order and preserved Peptococcaceae family members, known as propionate degraders. Moreover methane isotopic fractionation showed that hydrogenotrophic methanogenesis was maintained in presence of zeolite under ammonia low stress. Our results put forward the benefit of zeolite to improve the bacteria-archaea syntrophy needed for propionate degradation and methane production under a low ammonia stress.

Insights into the biological process performance and microbial diversity during thermophilic microalgae co-digestion in an anaerobic membrane bioreactor (AnMBR)

Harvested microalgae Chlorella spp. and primary sludge were co-digested in a laboratory-scale anaerobic membrane bioreactor (AnMBR) under thermophilic conditions (55 °C). The system was run for 700 days divided into four experimental phases to determine the influence of the organic loading rate on the process performance and the microbial community. The rise in organic loading rate from 0.17 to 0.5 gCOD·L−1·d−1 led to a 35% improvement in methane production. The system reached 69% biodegradability working at 0.5 gCOD·L−1·d−1 and a high solids retention time (70 d), indicating the efficient conversion of biomass into biogas through the AnMBR configuration while avoiding possible inhibition by free ammonia. The thermophilic AnMBR provided a stable microbial community dominated by hydrolytic and fermenter members such as Coprothermobacter, Fervidobacterium and Hydrothermae. Syntrophic oxidisers such as W5 were also key in propionate degradation and produced intermediates for later conversion into methane.

Substrate promiscuity of polyketide synthase enables production of tsetse fly attractants 3-ethylphenol and 3-propylphenol by engineering precursor supply in yeast

Tsetse flies are the transmitting vector of trypanosomes causing human sleeping sickness and animal trypanosomiasis in sub-saharan Africa. 3-alkylphenols are used as attractants in tsetse fly traps to reduce the spread of the disease. Here we present an inexpensive production method for 3-ethylphenol (3-EP) and 3-propylphenol (3-PP) by microbial fermentation of sugars. Heterologous expression in the yeast Saccharomyces cerevisiae of phosphopantetheinyltransferase-activated 6-methylsalicylic acid (6-MSA) synthase (MSAS) and 6-MSA decarboxylase converted acetyl-CoA as a priming unit via 6-MSA into 3-methylphenol (3-MP). We exploited the substrate promiscuity of MSAS to utilize propionyl-CoA and butyryl-CoA as alternative priming units and the substrate promiscuity of 6-MSA decarboxylase to produce 3-EP and 3-PP in yeast fermentations. Increasing the formation of propionyl-CoA by expression of a bacterial propionyl-CoA synthetase, feeding of propionate and blocking propionyl-CoA degradation led to the production of up to 12.5 mg/L 3-EP. Introduction of a heterologous ‘reverse ß-oxidation’ pathway provided enough butyryl-CoA for the production of 3-PP, reaching titers of up to 2.6 mg/L. As the concentrations of 3-alkylphenols are close to the range of the concentrations deployed in tsetse fly traps, the yeast broths might become promising and inexpensive sources for attractants, producible on site by rural communities in Africa.

Long-term enrichment of anaerobic propionate-oxidizing consortia: Syntrophic culture development and growth optimization

Propionate is a quantitatively important methanogenic intermediate in anaerobic digesters and only limited number of microbes can utilize it under syntrophic association with methanogens. The syntrophic propionate oxidizing bacterias are known to be slow growers due to the low energy yield. Thus, propionate get accumulated frequently in anaerobic digesters and it negatively affect digester performance. In this study, propionate degrading consortia from four different seeding sources were enriched in sequential bath mode in two phases; first adaption phase with 1 g/L of propionate concentration and later, high-strength phase with 3 g/L. From 16s rRNA gene based metagenomics analysis of the former phase, four syntrophic microbial groups, Syntrophaceae, Syntrophomonadaceae, Methanobacterium and Methanosaeta were found to be dominant with complete degradation of propionate. The substrate accelerated microbial shifts were observed at high-strength phase with significant decrease of Syntrophaceae up to 26.9 %. Using Response Surface Methodology, pH 6.8–6.9 and temperature 34.5–34.9 °C were found to be optimum growth conditions for the propionate degradation culture. Observed results could be useful to improve degradation efficiencies and obtained enriched culture can be used to recover propionate-accumulated digesters by bio-augmentation.

Kinetic and thermodynamic effects of temperature on methanogenic degradation of acetate, propionate, butyrate and valerate

This study elucidates volatile fatty acids (VFAs) accumulation reasons during organic solid wastes degradation via kinetics and thermodynamics assessments of degradation mechanisms of representative VFA (acetate, propionate, butyrate and valerate) under mesophilic (35 °C) and thermophilic (55 °C) conditions. Acetate exhibited the highest maximum microbial specific growth rates of 0.18 d−1 and 0.33 d−1 at mesophilic and thermophilic conditions, respectively, during the degradation process, followed by butyrate at 0.08 d−1 and 0.08 d−1. However, propionate showed a stronger inhibition to methanogenesis at thermophilic conditions, especially at the substrate concentration exceeding 5000 mg COD/L. The largest specific maximum methanogenic activity (SMA) decreased from 0.029 to 0.022 g CH4-COD/g-VSS/d. Furthermore, an inability of microorganisms to adapt to thermophilic propionate degradation conditions (at maximum lag time of 12 days) indicated substantial substrate inhibition. As an intermediate product of valerate degradation, the propionate degradation rate was considerably lower than that of valerate (0.036 g COD/d vs. 0.104 g COD/d). Hence, propionate caused a substantial rate limitation during the valerate degradation process under thermophilic conditions. In addition, under thermophilic conditions, valerate converted to methane via a long three-step syntrophic oxidation process. A large amount of hydrogen production accompanied this process. The inhibitory effect of propionate on methanogens and high hydrogen concentration (approximately 700 ppm) required time to subside. Thus, a 4-day plateau associated with a positive Gibbs free energy (1.45–2.11 kJ/mol) occurred during valerate degradation.

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.

Coupled effects of ferroferric oxide supplement and ethanol co-metabolism on the methanogenic oxidation of propionate

Direct interspecies electron transfer (DIET) is a new electron-transfer strategy for enhanced propionate degradation. Ethanol can enrich the DIET species of Geobacter and conductive ferroferric oxide (Fe3O4) can promote DIET. Therefore, coupled effects of ethanol and Fe3O4 on propionate degradation were investigated. The maximum CH4 production rate was increased by 81.4% by adding Fe3O4 when simultaneously fed with ethanol and propionate, while the improvement could not be observed without ethanol. The sludge conductivity and the electron transfer system activity by adding Fe3O4 were increased by 2.66 and 2.73 times, respectively. Besides, the relative abundance of functional microbes such as Geobacter, Syntrophobacter, Smithella, and Methanosaeta, and their functional genes were increased by the supplement of Fe3O4. The improvement of propionate degradation by adding Fe3O4 was largely attributed to the co-existence of ethanol degradation. The DIET between Geobacter and Methanosaeta might provide more energies or rapidly consume the oxidation products to promote the propionate degradation.

Propionate Converting Anaerobic Microbial Communities Enriched from Distinct Biogeochemical Zones of Aarhus Bay, Denmark under Sulfidogenic and Methanogenic Conditions.

The relationship between predominant physiological types of prokaryotes in marine sediments and propionate degradation through sulfate reduction, fermentation, and methanogenesis was studied in marine sediments. Propionate conversion was assessed in slurries containing sediment from three different biogeochemical zones of Aarhus Bay, Denmark. Sediment slurries were amended with 0, 3, or 20 mM sulfate and incubated at 25 °C and 10 °C for 514–571 days. Methanogenesis in the sulfate zone and sulfate reduction in the methane zone slurries was observed. Both processes occurred simultaneously in enrichments originating from samples along the whole sediment. Bacterial community analysis revealed the dominance of Desulfobacteraceae and Desulfobulbaceae members in sulfate-amended slurries incubated at 25°C and 10°C. Cryptanaerobacter belonging to the Peptococcaceae family dominated sulfate-free methanogenic slurries at 25°C, whereas bacteria related to Desulfobacteraceae were dominant at 10°C. Archaeal community analysis revealed the prevalence of different genera belonging to Methanomicrobiales in slurries incubated at different temperatures and amended with different sulfate concentrations. Methanosarcinaceae were only detected in the absence of sulfate. In summary, Aarhus Bay sediment zones contain sulfate reducers, syntrophs, and methanogens interacting with each other in the conversion of propionate. Our results indicate that in Aarhus Bay sediments, Cryptanaerobacter degraded propionate in syntrophic association with methanogens.

Metabolic remodeling in EscherichiacoliMG1655. A prophage e14-encoded small RNA,co293, post-transcriptionally regulates transcription factors HcaR and FadR.

In previous studies, we have shown the existence of metabolic remodeling in glucose-grown EscherichiacoliMG1655 cells expressing the esterase Orf306 from the opd island of Sphingobiumfuliginis. We now show that Orf306-dependent metabolic remodeling is due to regulation of a novel small RNA (sRNA). Endogenous propionate, produced due to the esterase/lipase activity of Orf306, repressed expression of a novel E.colisRNA,co293. This sRNA post-transcriptionally regulates expression of the transcription factors HcaR and FadR either by inhibiting translation or by destabilizing their transcripts. Hence, repression of co293 expression elevates the levels of HcaR and FadR with consequent activation of alternative carbon catabolic pathways. HcaR activates the hca and MHP operons leading to upregulation of the phenyl propionate and hydroxy phenyl propionate (HPP) degradation pathways. Similarly, FadR stimulates the expression of the transcription factor IclR which negatively regulates the glyoxylate bypass pathway genes, aceBAK.

Integrating microbial electrochemical technologies with anaerobic digestion to accelerate propionate degradation

The aim of this study is to evaluate the integration of microbial electrochemical technologies (MET) with anaerobic digestion (AD) to overcome AD limitations caused by propionate accumulation. The study focuses on understanding to what extent the inoculum impacts on the behaviour of the integrated systems (AD-MET) from the perspective of propionate degradation, methane production and microbial population dynamics. Three different inocula were used: two from environmental sources (anaerobic sludge and river sediment) and another one from a pre-enriched electroactive consortium adapted to propionate degradation. Contrary to expectations, the reactor inoculated with the pre-enriched consortium was not able to maintain its initial good performance in the long run, and the bioelectrochemical activity collapsed after three months of operation. On the other hand, the reactor inoculated with anaerobic sludge, although it required a relatively longer time to produce any observable current, was able to maintain an electrogenic activity operation (0.8 A m−2), whilst showcasing the positive contribution of AD-MET integration into tackling propionate accumulation and enhancing methane yield (338 mL gCOD−1). However, it must also be highlighted that from a purely energetic point of view the AD-MET was not favourable.