Propionic acid accumulation and degradation during restart of a full-scale anaerobic biowaste digester

Brooklawnia propionicigenes sp. nov., a facultatively anaerobic, propionate-producing bacterium isolated from a methanogenic reactor treating waste from cattle farms

Facultatively anaerobic bacterial strains were isolated from samples of a methanogenic reactor and, based on 16S rRNA gene sequences, found to be affiliated with the family Propionibacteriaceae in the phylum Actinomycetota. Four strains with almost-identical 16S rRNA gene sequences were comprehensively characterized. The most closely related species to the strains was Brooklawnia cerclae BL-34T (96.4 % sequence similarity). Although most of the phenotypic characteristics of the four strains were identical, distinct differences in some cellular and physiological properties were also detected. Cells of the strains were Gram-stain-positive, non-spore-forming, pleomorphic rods. The strains utilized carbohydrates and organic acids. The strains produced acetate, propionate and lactate from glucose, but the molar ratios of the products were variable depending on the strains. The strains grew at 10-40 °C (optimum at 35 °C) and pH 5.3-8.8 (optimum at pH 6.8-7.5.) The major cellular fatty acids of the strains were anteiso-C15 : 0, C15 : 0 and C15 : 0 dimethylacetal (as a summed feature). The major respiratory quinone was menaquinone MK-9(H4) and the diagnostic diamino acid in the peptidoglycan was meso-diaminopimelic acid. The genome size of the type strain (SH051T) was 3.21 Mb and the genome DNA G+C content was 65.7 mol%. Genes responsible for propionate production through the Wood-Werkman pathway were detected in the genome of strain SH051T. Based on the results of phylogenetic, genomic and phenotypic analyses of the novel strains, the name Brooklawnia propionicigenes sp. nov. is proposed to accommodate the four strains. The type strain of the novel species is SH051T (=NBRC 116195T=DSM 116141T).

Modeling key intermediates during anaerobic digestion of lipid rich kitchen waste with an extended ADM1

Quantitative dynamics of the key intermediates, gases and carbohydrates during anaerobic digestion of different lipid rich kitchen waste and lipid rich model kitchen waste were modeled. Six batch reactors loaded with 25 gVS  l- 1 ( ∼ 39  g O 2  l- 1 ) kitchen waste and model kitchen waste during a batch experiment were considered in simulation. Observed dynamics of carbohydrates, volatile organic acids and gases were described by an extended benchmark simulation model no. 2 (BSM2). In this study the extended BSM2 included a more detailed β -oxidation for prediction of caproic acid. Furthermore, the extensions included carbohydrate digestion with an additional intermediate before propionic acid was released. In addition, a novel simplification approach for initial pH estimation was successfully applied. For parameter estimation a Markov Chain Monte Carlo method was used to obtain parameter distributions. With the presented model it was possible even with no calibrated data to predict point of times of intermediates maxima and propionic acid with relative stable concentration over several days for kitchen waste.

Syntrophic entanglements for propionate and acetate oxidation under thermophilic and high-ammonia conditions

Propionate is a key intermediate in anaerobic digestion processes and often accumulates in association with perturbations, such as elevated levels of ammonia. Under such conditions, syntrophic ammonia-tolerant microorganisms play a key role in propionate degradation. Despite their importance, little is known about these syntrophic microorganisms and their cross-species interactions. Here, we present metagenomes and metatranscriptomic data for novel thermophilic and ammonia-tolerant syntrophic bacteria and the partner methanogens enriched in propionate-fed reactors. A metagenome for a novel bacterium for which we propose the provisional name 'Candidatus Thermosyntrophopropionicum ammoniitolerans' was recovered, together with mapping of its highly expressed methylmalonyl-CoA pathway for syntrophic propionate degradation. Acetate was degraded by a novel thermophilic syntrophic acetate-oxidising candidate bacterium. Electron removal associated with syntrophic propionate and acetate oxidation was mediated by the hydrogen/formate-utilising methanogens Methanoculleus sp. and Methanothermobacter sp., with the latter observed to be critical for efficient propionate degradation. Similar dependence on Methanothermobacter was not seen for acetate degradation. Expression-based analyses indicated use of both H2 and formate for electron transfer, including cross-species reciprocation with sulphuric compounds and microbial nanotube-mediated interspecies interactions. Batch cultivation demonstrated degradation rates of up to 0.16 g propionate L-1 day-1 at hydrogen partial pressure 4-30 Pa and available energy was around -20 mol-1 propionate. These observations outline the multiple syntrophic interactions required for propionate oxidation and represent a first step in increasing knowledge of acid accumulation in high-ammonia biogas production systems.

Effects of initial volatile fatty acid concentrations on process characteristics, microbial communities, and metabolic pathways on solid-state anaerobic digestion

Solid-state anaerobic digestion (SSAD) is vulnerable to excess volatile fatty acids (VFA), mainly acetate and propionate. The co-effects of VFAs and microbial dynamics under VFA accumulation were investigated in SSAD of pig manure and corn straw. Adding 2 and 4 mg/g acetate or propionate caused initial increases in total VFAs, followed by decreases after day 6, resulting in 'mild' VFA accumulation, while adding 6 mg/g caused similarly increased VFAs, but with no subsequent decrease, causing 'severe' VFA accumulation and poor methanation performance. Mild propionate accumulation promoted acetate consumption, whereas acetate accumulation inhibited propionate degradation by affecting crucial redox reactions. Under severe VFA accumulation, hydrolysis and acidification mainly conducted by acid-tolerant Clostridium sp. exacerbated VFA inhibition, causing a competition between Methanosarcina and Methanosaeta, and impairments of acetoclastic and hydrogenotrophic methanogenesis and interspecies formate transfer. This study provides new insights into mechanisms of VFA accumulation in SSAD, and its effects on methanogenesis.

Molecular biology and modeling analysis reveal functional roles of propionate to acetate ratios on microbial syntrophy and competition in electro-assisted anaerobic digestion

This study examined the significance of propionate to acetate (HPr/HAc) ratios on microbial syntrophy and competition in microbial electrolysis cell-assisted anaerobic digestion (MEC-AD). In addition to molecular biology and phylogenetic analysis, a numerical MEC-AD model was developed by modifying Anaerobic Digestion Model No.1 to predict the effects of different HPr/HAc ratios (0.5, 1.5, 2.5, and 5). The HPr/HAc ratios of 0.5 and 1.5 maintained efficient syntrophy among electroactive bacteria, hydrogenotrophic methanogens, and homoacetogens, leading to higher methane yields. In contrast, higher HPr/HAc ratios of 2.5 and 5 were detrimental to methanogenesis. Both microbial community analysis and numerical modeling results suggested that higher propionate levels could promote the enrichment of H₂-utilizing acetogens, thereby triggering their competition with hydrogenotrophic methanogens. Moreover, protein fraction in extracellular polymeric substances and the relative expression of genes associated with extracellular electron transfer in both anode and cathode biofilms were markedly decreased with increasing HPr/HAc ratios, indicating partial inhibition of microbial electroactivity. Overall, these results illuminate deep insight into anaerobic syntrophy, contributing to the process kinetics and methane yields in MEC-AD systems. Furthermore, from a practical viewpoint, the results can also be helpful in effective control of MEC-AD operation without propionate accumulation.

Syntrophic propionate-oxidizing bacteria in methanogenic systems

The mutual nutritional cooperation underpinning syntrophic propionate degradation provides a scant amount of energy for the microorganisms involved, so propionate degradation often acts as a bottleneck in methanogenic systems. Understanding the ecology, physiology and metabolic capacities of syntrophic propionate-oxidizing bacteria (SPOB) is of interest in both engineered and natural ecosystems, as it offers prospects to guide further development of technologies for biogas production and biomass-derived chemicals, and is important in forecasting contributions by biogenic methane emissions to climate change. SPOB are distributed across different phyla. They can exhibit broad metabolic capabilities in addition to syntrophy (e.g. fermentative, sulfidogenic and acetogenic metabolism) and demonstrate variations in interplay with cooperating partners, indicating nuances in their syntrophic lifestyle. In this review, we discuss distinctions in gene repertoire and organization for the methylmalonyl-CoA pathway, hydrogenases and formate dehydrogenases, and emerging facets of (formate/hydrogen/direct) electron transfer mechanisms. We also use information from cultivations, thermodynamic calculations and omic analyses as the basis for identifying environmental conditions governing propionate oxidation in various ecosystems. Overall, this review improves basic and applied understanding of SPOB and highlights knowledge gaps, hopefully encouraging future research and engineering on propionate metabolism in biotechnological processes.

Patterns of syntrophic interactions in methanogenic conversion of propionate

Methanogenesis is central to anaerobic digestion processes. The conversion of propionate as a key intermediate for methanogenesis requires syntrophic interactions between bacterial and archaeal partners. In this study, a series of methanogenic enrichments with propionate as the sole substrate were developed to identify microbial populations specifically involved in syntrophic propionate conversion. These rigorously controlled propionate enrichments exhibited functional stability with consistent propionate conversion and methane production; yet, the methanogenic microbial communities experienced substantial temporal dynamics, which has important implications on the understanding of mechanisms involved in microbial community assembly in anaerobic digestion. Syntrophobacter was identified as the most abundant and consistent bacterial partner in syntrophic propionate conversion regardless of the origin of the source culture, the concentration of propionate, or the temporal dynamics of the culture. In contrast, the methanogen partners involved in syntrophic propionate conversion lacked consistency, as the dominant methanogens varied as a function of process condition and temporal dynamics. Methanoculleus populations were specifically enriched as the syntrophic partner at inhibitory levels of propionate, likely due to the ability to function under unfavorable environmental conditions. Syntrophic propionate conversion was carried out exclusively via transformation of propionate into acetate and hydrogen in enrichments established in this study. Microbial populations highly tolerant of elevated propionate, represented by Syntrophobacter and Methanoculleus, are of great significance in understanding methanogenic activities during process perturbations when propionate accumulation is frequently encountered. Key points • Syntrophobacter was the most consistent bacterial partner in propionate metabolism. • Diverse hydrogenotrophic methanogen populations could serve as syntrophic partners. • Methanoculleus emerged as a methanogen partner tolerant of elevated propionate.

Enhanced in-situ biomethanation of food waste by sequential inoculum acclimation: Energy efficiency and carbon savings analysis

The increasing rate of food waste (FW) generation globally, makes it an attractive resource for renewable energy through anaerobic digestion (AD). The biogas recovered from AD can be upgraded by the methanation of internally produced carbon dioxide, CO₂ with externally sourced hydrogen gas, H₂ (biomethanation). In this work, H₂ was added to AD reactors processing FW in three successive phases, with digestate from preceding phases recycled in succession with the addition of fresh inoculum to enhance acclimation. The concentration of H₂ was increased for succeeding phases: 5%, 10% and 15% of the reactor headspace in Phase 1 (EH1), Phase 2 (EH2) and Phase 3 (EH3), respectively. The H₂ utilisation rate and biomethane yields increased as acclimation progressed from EH1 through EH3. Biomethane yield from the controls: EH1_Control, EH2_Control and EH3_Control were 417.6, 435.4 and 453.3 NmL-CH₄/gVS_added accounting for 64.8, 73.9 and 77.8% of the biogas respectively. And the biomethane yield from the test reactors EH1_Test, EH2_Test and EH3_Test were 468.3, 483.6, and 499.0 NmL-CH₄/gVS_added, accounting for 77.2, 78.1 and 81.0% of the biogas respectively. A progressive in-situ biomethanation could lead to biomethane production that meets higher fuel standards for gas-to-grid (GtG) injections and vehicle fuel - i.e. >95% CH₄. This would increase the energy yield and carbon savings compared to conventional biogas upgrade methods. For example, biogas upgrade for GtG by in-situ biomethanation could yield 7.3 MWh/t_FW energy and 1343 kg-CO₂e carbon savings, which is better than physicochemical upgrade options (i.e., 4.6-4.8 MWh/t_FW energy yield and 846-883 kg-CO₂e carbon savings).

New insights into the effect of ethanol and volatile fatty acids proportions on methanogenic activities and pathways

During anaerobic digestion, methanogenic activities and pathways can be affected by intermediates. Here, the effects of intermediates acetate, propionate, and ethanol on methanogenesis were investigated. Four anaerobic sequencing batch reactors were acclimated with propionate (ASBR_P), ethanol/propionate (ASBR_EP), acetate/propionate (ASBR_AP), and ethanol/acetate/propionate (ASBR_EAP). Ethanol was the easiest one to be biodegraded, thereby enhancing the maximum methane production rate and shortening the lag phase, while the longest acclimation time and lowest methane production rate were observed in ASBR_P. Different microbial communities and syntrophic patterns existed in four reactors. Desulfovibrio and Geobacter were the dominant ethanol-oxidizing bacteria in ASBR_EP and ASBR_EAP, respectively. Both Desulfovibrio and Geobacter possessed the potential of extracellular electron transfer, which might be the advantage of ethanol dosage for enhancing methanogenesis through direct interspecies electron transfer. Methanosarcina was enriched in ASBR_P and ASBR_AP, while Methanosaeta in ASBR_EP and ASBR_EAP. Genes responsible for acetoclastic methanogenesis were significantly enriched in ASBR_EAP, possibly resulting in the highest methanogenic activity from acetate. Results from this study will advance the optimization of practical anaerobic systems, which can be achieved by regulating the intermediates with different fermenting pathways.

Towards the practical application of bioelectrochemical anaerobic digestion (BEAD): Insights into electrode materials, reactor configurations, and process designs

Anaerobic digestion (AD) is one of the most widely adopted bioenergy recovery technologies globally. Despite the wide adoption, AD has been challenged by the unstable performances caused by imbalanced substrate and/or electron availability among different reaction steps. Bioelectrochemical anaerobic digestion (BEAD) is a promising concept that has demonstrated potential for balancing the electron transfer rates and enhancing the methane yield in AD during shocks. While great progress has been made, a wide range of, and sometimes inconsistent engineering and technical strategies were attempted to improve BEAD. To consolidate past efforts and guide future development, a comprehensive review of the fundamental bioprocesses in BEAD is provided herein, followed by a critical evaluation of the engineering and technical optimizations attempted thus far. Further, a few novel directions and strategies that can enhance the performance and practicality of BEAD are proposed for future research to consider. This review and outlook aim to provide a fundamental understanding of BEAD and inspire new research ideas in AD and BEAD in a mechanism-informed fashion.

Synergetic promotion of direct interspecies electron transfer for syntrophic metabolism of propionate and butyrate with graphite felt in anaerobic digestion

Direct interspecies electron transfer (DIET) was considered to replace interspecies H₂ transfer to enhance syntrophic metabolism during the anaerobic digestion. The graphite felt with excellent electrical conductivity was added into a semi-continuous reactor as a conductive material, strengthening DIET in enhancing the degradation of propionate and butyrate. The final average CH₄ production rate increased by 19.1% and 16.7%, respectively, during the propionate and butyrate degradation. Microbial community analysis revealed that the relative abundance of the microflora that could participate in DIET, like Geobacter, Methanosaeta and Methanosarcina species, was greatly improved in the presence of graphite felt. Further experiments showed that the high H₂ partial pressure had no obvious effect on the degradation of propionate and butyrate in the graphite felt-supplement reactors.

Candidatus Syntrophosphaera thermopropionivorans: a novel player in syntrophic propionate oxidation during anaerobic digestion

Propionate is an important intermediate in the anaerobic mineralization of organic matter. In methanogenic environments, its degradation relies on syntrophic associations between syntrophic propionate-oxidizing bacteria (SPOB) and Archaea. However, only 10 isolated species have been identified as SPOB so far. We report syntrophic propionate oxidation in thermophilic enrichments of Candidatus Syntrophosphaera thermopropionivorans, a novel representative of the candidate phylum Cloacimonetes. In enrichment culture, methane was produced from propionate, while Ca. S. thermopropionivorans contributed 63% to total bacterial cells. The draft genome of Ca. S. thermopropionivorans encodes genes for propionate oxidation via methymalonyl-CoA. Phylogenetically, Ca. S. thermopropionivorans affiliates with the uncultured Cloacimonadaceae W5 and is more distantly related (86.4% 16S rRNA gene identity) to Ca. Cloacimonas acidaminovorans. Although Ca. S. thermopropionivorans was enriched from a thermophilic biogas reactor, Ca. Syntrophosphaera was in particular associated with mesophilic anaerobic digestion systems. 16S rRNA gene amplicon sequencng and a novel genus-specific quantitative PCR assay consistently identified Ca. Syntrophosphaera/Cloacimonadaceae W5 in 9 of 12 tested full-scale biogas reactors thereby outnumbering other SPOB such as Pelotomaculum, Smithella and Syntrophobacter. Taken together the ubiquity and abundance of Ca. Syntrophosphaera, those SPOB might be key players for syntrophic propionate metabolism that have been overlooked before.

Co-digestion of wastewater sludge: Choosing the optimal blend

Anaerobic co-digestion (AcoD) is a promising strategy to increase the methane production of anaerobic digestion plants treating wastewater sludge (WAS). In this work the degradability of six different mixtures of WAS with fish waste (FW) or garden-grass (GG) was evaluated and compared to the three mono-digestions. Degradation performances and methanogenic pathways, determined with the isotopic signatures of biogas, were compared across time. Fish and grass mono-digestion provided a higher final methane production than WAS mono-digestion. In co-digestion the addition of 25% of fish was enough to increase the final methane production from WAS while 50% of grass was necessary. To determine the optimal blend of WAS co-digestion two indicators were specifically designed, representing the maximum potential production (ODI) and the expected production in mono-digestion conditions (MDI). The comparison between these indicators and the experimental results showed that the most productive blend was composed of 75% of co-substrate, fish or grass, with WAS. Indeed, the final methane production was increased by 1.9 times with fish and by 1.7 times with grass associated to an increase of the methane production rate by 1.5 times. Even if the same succession of methanogenic pathways across time was observed for the different mixtures, their relative proportions were different. Sewage sludge degradation was mostly achieved through hydrogenotrophic pathway while acetoclastic pathway was dominant for fish and grass degradation. These results were confirmed by the identification of Archaea with 16S sequencing.

Response of treatment performance and microbial community structure to the temporary suspension of an industrial anaerobic bioreactor

In this study, a novel type of mesophilic anaerobic bioreactor-an expanded granular sludge bed (EGSB)-was utilized to explore the effect of suspending reactor operation on the treatment performance and the microbial community structure. The parameters of performance and bacterial community before and after a four-week suspension were compared for the starch processing wastewater treatment bioreactor. The results indicate that the removal rate of the organic matter remained higher than 90%, although the biomass significantly decreased after restarting the reactor. However, the relatively stable microbial community structure before the suspension was altered significantly during the restart and post-running stages. This change was primarily due to variability in satellite species and the substitution effect of different dominant bacteria. For example, some non-major carbohydrate-degrading bacteria that were sensitive to nutrition deficiency, such as Desulfovibrio and Geobacter, were dramatically reduced after the suspension. In contrast, the stress of starvation stimulated the reproduction of hydrolytic bacteria, such as Macellibacteroides. However, the high bacterial diversity index (6.12-6.65) and the longstanding core species, including Chloroflexi, Cloacimonetes, Ignavibacteriae, Thermotogae and Euryarchaeota, maintained the functional stability of the reactor. Consequently, although the total bacteria decreased significantly after reactor operation was suspended, sufficient functional bacteria supported by the high diversity, as well as the longstanding core species, guaranteed the effective degradation after suspension.

Biodiversity-function relationships in methanogenic communities

Methanogenic communities play a crucial role in carbon cycling and biotechnology (anaerobic digestion), but our understanding of how their diversity, or composition in general, determines the rate of methane production is very limited. Studies to date have been correlational because of the difficulty in cultivating their constituent species in pure culture. Here, we investigate the causal link between methanogenesis and diversity in laboratory anaerobic digesters by experimentally manipulating the diversity of cultures by dilution and subsequent equilibration of biomass. This process necessarily leads to the loss of the rarer species from communities. We find a positive relationship between methane production and the number of taxa, with little evidence of functional saturation, suggesting that rare species play an important role in methane‐producing communities. No correlations were found between the initial composition and methane production across natural communities, but a positive relationship between species richness and methane production emerged following ecological selection imposed by the laboratory conditions. Our data suggest methanogenic communities show little functional redundancy, and hence, any loss of diversity—both natural and resulting from changes in propagation conditions during anaerobic digestion—is likely to reduce methane production.

Use of artificial neuronal networks for prediction of the control parameters in the process of anaerobic digestion with thermal pretreatment

This article focuses on the analysis of the behavior patterns of the variables involved in the anaerobic digestion process. The objective is to predict the impact factor and the behavior pattern of the variables, i.e., temperature, pH, volatile solids (VS), total solids, volumetric load, and hydraulic residence time, considering that these are the control variables for the conservation of the different groups of anaerobic microorganisms. To conduct the research, samples of physicochemical sludge were taken from a water treatment plant in a poultry processing factory, and, then, the substrate was characterized, and a thermal pretreatment was used to accelerate the hydrolysis process. The anaerobic digestion process was analyzed in order to obtain experimental data of the control variables and observe their impact on the production of biogas. The results showed that the thermal pre-hydrolysis applied at 90°C for 90 min accelerated the hydrolysis phase, allowing a significant 52% increase in the volume of methane produced. An artificial neural network was developed, and it was trained with the database obtained by monitoring the anaerobic digestion process. The results obtained from the artificial neural network showed that there is an adjustment between the real values and the prediction of validation based on 60 samples with a 96.4% coefficient of determination, and it was observed that the variables with the major impact on the process were the loading rate and VS, with impact factors of 36% and 23%, respectively.

Bioaugmentation with Mixed Hydrogen-Producing Acetogen Cultures Enhances Methane Production in Molasses Wastewater Treatment

Hydrogen-producing acetogens (HPA) have a transitional role in anaerobic wastewater treatment. Thus, bioaugmentation with HPA cultures can enhance the chemical oxygen demand (COD) removal efficiency and CH4 yield of anaerobic wastewater treatment. Cultures with high degradation capacities for propionic acid and butyric acid were obtained through continuous subculture in enrichment medium and were designated as Z08 and Z12. Bioaugmentation with Z08 and Z12 increased CH4 production by glucose removal to 1.58. Bioaugmentation with Z08 and Z12 increased the COD removal rate in molasses wastewater from 71.60% to 85.84%. The specific H2 and CH4 yields from COD removal increased by factors of 1.54 and 1.63, respectively. Results show that bioaugmentation with HPA-dominated cultures can improve CH4 production from COD removal. Furthermore, hydrogen-producing acetogenesis was identified as the rate-limiting step in anaerobic wastewater treatment.

Control of accumulated volatile fatty acids by recycling nitrified effluent

BACKGROUND

Volatile fatty acids (VFA) often accumulate in anaerobic digestion systems, decreasing pH levels and causing unstable operational performance and poor biogas production. The aim of this study is to improve anaerobic digestion efficiency by controlling/reducing the accumulation of VFAs in a continuous anaerobic digestion system.

METHODS

NO₃ ^--N was added to the digester and its effects on VFAs were investigated. When the system reached an unstable condition with the accumulation of VFAs, the digester was fed at an organic loading rate of 6 kg COD (chemical oxygen demand)/m³∙d and 0.5Q of aeration tank effluent (1500 mg/L of NO₃ ^--N) was recirculated.

RESULTS

With the addition of NO₃ ^--N, VFAs were utilized during denitrification, after which methane production started. Furthermore, the accumulated VFAs could be used as a carbon substrate by denitrifying bacteria. After 56 d, a normal VFA concentration could be achieved. Methane production was 0.02-0.03 L CH₄/g VS higher with NO₃ ^--N recirculation and feeding than that without feeding.

CONCLUSIONS

The results show that the addition of NO₃ ^--N is a potentially feasible method to control VFAs. Combined with recirculation and feeding, the method can be used to effectively prevent the inhibition of methanogenic microbial activities caused by accumulated VFAs and enhance denitrification and methane production in anaerobic digesters.

Mediative mechanism of bicarbonate on anaerobic propionate degradation revealed by microbial community and thermodynamics

Syntrophic acetogenesis of volatile fatty acids (VFAs) such as propionate and butyrate is considered as the rate-limiting step of anaerobic digestion. Though being extensively researched, the mechanism is not well understood as the main constraint on developing effective solutions to the practical problem. In the present research work, the mediation of methanogenic propionate degradation by exogenous bicarbonate was evaluated, while the mechanism was revealed by microbial community and thermodynamics. It was found that the exogenous bicarbonate not more than 0.10 mol/L acted as a mediative role to enrich syntrophic acetogenic bacteria and decrease the actual Gibbs free energy change (ΔG) of syntrophic acetogenesis reaction, resulted in the increased degradation rate and methane production rate of propionate. The remarkably increased ΔG of methanogenic propionate degradation by the exogenous bicarbonate more than 0.15 mol/L decreased the degradation rate and methane production rate of propionate, though the ΔG of syntrophic acetogenesis reaction was also decreased by the exogenous bicarbonate. This research work provided a control strategy to enhance syntrophic acetogenesis, as well as the methanogenic VFAs degradation.

Wet biowaste digestion: ADM1 model improvement by implementation of known genera and activity of propionate oxidizing bacteria

Anaerobic digestion of biowaste not only reduces environmental burden but also plays an important role for sustainable energy supply. For process optimization simulation based on the Anaerobic Digestion Model No. 1 (ADM1) is commonly used. The ADM1 was extended to include the known three genera of propionate oxidizing bacteria (POB) and the two routes of propionate degradation (methyl-malonyl CoA and C6-dismutation pathway). Kinetic parameters for anaerobic propionate oxidation by single strains of the three propionate oxidizing genera were determined from defined tri-cultures of the POB with hydrogenotrophic and acetotrophic methanogens and implemented into ADM1. The such improved model ADM1xpro was evaluated with operational data from a full scale wet biowaste digestion plant. Predicted amounts of biogas and composition with ADM1xpro (2201 m³ d^-1, 68.1 % CH₄ and 31.9 % CO₂) correlated well with full-scale process data (2171 m³ d^-1, 67.5 % CH₄ and 31.9 % CO₂).

Acclimation of acid-tolerant methanogenic propionate-utilizing culture and microbial community dissecting

The acid-tolerant methanogenic propionate degradation culture was acclimated in a propionate-fed semi-continuous bioreactor by daily adjusting the digestate pH. The performance of propionate fermentation, the respond of microbial community structure to the acidic environment, and the microbial network for propionate degradation in the acid-tolerant culture was investigated. The results demonstrated that after long term of acclimation to low pH, the digester could produce methane from propionate at pH 4.8-5.5 with 0.3-0.4 L g^-1 propionic acid (HPr) d^-1 of the volatile solids (VS) methane production. The predominant methanogens shifted from acetoclastic methanogens (∼87%) to hydrogenotrophic methanogens (∼67%) in the bioreactor with the dropping pH, indicating that hydrogenotrophic methanogens were more acid-tolerant than acetoclastic methanogens. Smithella (∼11%), Syntrophobacter (∼7%) and Pelotomaculum (∼3%) were the main propionate oxidizers in the acid-tolerant propionate-utilizing culture. Methanothrix dominant acetoclastic methanogens, while Methanolinea and Methanospirillum were the major H₂ scavengers to support Syntrophobacter and Pelotomaculum syntrophic propionate degradation.

A review on the bioenergetics of anaerobic microbial metabolism close to the thermodynamic limits and its implications for digestion applications

The exploration of the energetics of anaerobic digestion systems can reveal how microorganisms cooperate efficiently for cell growth and methane production, especially under low-substrate conditions. The establishment of a thermodynamically interdependent partnership, called anaerobic syntrophy, allows unfavorable reactions to proceed. Interspecies electron transfer and the concentrations of electron carriers are crucial for maintaining this mutualistic activity. This critical review summarizes the functional microorganisms and syntroph partners, particularly in the metabolic pathways and energy conservation of syntrophs. The kinetics and thermodynamics of propionate degradation to methane, reversibility of the acetate oxidation process, and estimation of microbial growth are summarized. The various routes of interspecies electron transfer, reverse electron transfer, and Poly-β-hydroxyalkanoate formation in the syntrophic community are also reviewed. Finally, promising and critical directions of future research are proposed. Fundamental insight in the activities and interactions involved in AD systems could serve as a guidance for engineered systems optimization and upgrade.

Effects of carbon source on methanogenic activities and pathways incorporating metagenomic analysis of microbial community

In this study, the effects of four types of organic compounds (tryptone, acetate/propionate, glucose and ethanol) on methanogenesis, electron transfer processes and microbial community structure were examined. When tryptone and acetate/propionate were used, the dominant methanogenic pathway was aceticlastic methanogenesis and Methanosarcina was the most abundant methanogen. When glucose or ethanol were provided as the external carbon source, the aceticlastic and hydrogenotrophic pathways were utilised simultaneously, and Methanosarcina and Methanobacterium were enriched. However, the reactor fed with glucose was prone to acidification because volatile fatty acids accumulated in the medium, which inhibited methane synthesis. Geobacter was dominant in the reactor fed with ethanol and 45% of genes encoding pili synthesis were attributable to Geobacter, indicating that direct interspecies electron transfer may be a possible mechanism during syntrophic methanogenesis.

Temporal Microbial Community Dynamics in Microbial Electrolysis Cells - Influence of Acetate and Propionate Concentration

Microbial electrolysis cells (MECs) are widely considered as a next generation wastewater treatment system. However, fundamental insight on the temporal dynamics of microbial communities associated with MEC performance under different organic types with varied loading concentrations is still unknown, nevertheless this knowledge is essential for optimizing this technology for real-scale applications. Here, the temporal dynamics of anodic microbial communities associated with MEC performance was examined at low (0.5 g COD/L) and high (4 g COD/L) concentrations of acetate or propionate, which are important intermediates of fermentation of municipal wastewaters and sludge. The results showed that acetate-fed reactors exhibited higher performance in terms of maximum current density (I: 4.25 ± 0.23 A/m²), coulombic efficiency (CE: 95 ± 8%), and substrate degradation rate (98.8 ± 1.2%) than propionate-fed reactors (I: 2.7 ± 0.28 A/m²; CE: 68 ± 9.5%; substrate degradation rate: 84 ± 13%) irrespective of the concentrations tested. Despite of the repeated sampling of the anodic biofilm over time, the high-concentration reactors demonstrated lower and stable performance in terms of current density (I: 1.1 ± 0.14 to 4.2 ± 0.21 A/m²), coulombic efficiency (CE: 44 ± 4.1 to 103 ± 7.2%) and substrate degradation rate (64.9 ± 6.3 to 99.7 ± 0.5%), while the low-concentration reactors produced higher and dynamic performance (I: 1.1 ± 0.12 to 4.6 ± 0.1 A/m²; CE: 52 ± 2.5 to 105 ± 2.7%; substrate degradation rate: 87.2 ± 0.2 to 99.9 ± 0.06%) with the different substrates tested. Correlating reactor's performance with temporal dynamics of microbial communities showed that relatively similar anodic microbial community composition but with varying relative abundances was observed in all the reactors despite differences in the substrate and concentrations tested. Particularly, Geobacter was the predominant bacteria on the anode biofilm of all MECs over time suggesting its possible role in maintaining functional stability of MECs fed with low and high concentrations of acetate and propionate. Taken together, these results provide new insights on the microbial community dynamics and its correlation to performance in MECs fed with different concentrations of acetate and propionate, which are important volatile fatty acids in wastewater.

Optimization of micronutrient supplement for enhancing biogas production from food waste in two-phase thermophilic anaerobic digestion

The aim of this study was to enhance the biogas productivity of two-phase thermophilic anaerobic digestion (AD) using food waste (FW) as the primary substrate. The influence of adding four trace metals (Ca, Mg, Co, and Ni) as micronutrient supplement in the methanogenic phase of the thermophilic system was investigated. Initially, Response Surface Methodology (RSM) was applied to determine the optimal concentration of micronutrients in batch experiments. The results showed that optimal concentrations of 303, 777, 7 and 3mg/L of Ca, Mg, Co and Ni, respectively, increased the biogas productivity as much as 50% and significantly reduced the processing time. The formulated supplement was tested in continuous two-phase thermophilic AD system with regard to process stability and productivity. It was found that a destabilized thermophilic AD process encountering high VFA accumulation recovered in less than two weeks, while the biogas production was improved by 40% yielding 0.46L CH₄/gVS_added/day. There was also a major increase in soluble COD utilization upon the addition of micronutrient supplement. The results of this study indicate that a micronutrient supplement containing Ca, Mg, Co and Ni could probably remedy any type of thermophilic AD process.

Set anode potentials affect the electron fluxes and microbial community structure in propionate-fed microbial electrolysis cells

Anode potential has been shown to be a critical factor in the rate of acetate removal in microbial electrolysis cells (MECs), but studies with fermentable substrates and set potentials are lacking. Here, we examined the impact of three different set anode potentials (SAPs; −0.25, 0, and 0.25 V vs. standard hydrogen electrode) on the electrochemical performance, electron flux to various sinks, and anodic microbial community structure in two-chambered MECs fed with propionate. Electrical current (49–71%) and CH₄ (22.9–41%) were the largest electron sinks regardless of the potentials tested. Among the three SAPs tested, 0 V showed the highest electron flux to electrical current (71 ± 5%) and the lowest flux to CH₄ (22.9 ± 1.2%). In contrast, the SAP of −0.25 V had the lowest electron flux to current (49 ± 6%) and the highest flux to CH₄ (41.1 ± 2%). The most dominant genera detected on the anode of all three SAPs based on 16S rRNA gene sequencing were Geobacter, Smithella and Syntrophobacter, but their relative abundance varied among the tested SAPs. Microbial community analysis implies that complete degradation of propionate in all the tested SAPs was facilitated by syntrophic interactions between fermenters and Geobacter at the anode and ferementers and hydrogenotrophic methanogens in suspension.

Thermodynamically enhancing propionic acid degradation by using sulfate as an external electron acceptor in a thermophilic anaerobic membrane reactor

In this study, sulfate was employed as an external electron acceptor for enhancing the degradation of propionate in a thermophilic anaerobic membrane reactor (AnMBR). The organic loading rate (OLR) was increased gradually from the initial 3.9 kg-COD/m³d to the inhibiting OLR of 14.6 kg-COD/m³d. Feeding was stopped for 98 days but the process did not recover until 500 mg/L of sulfate was added into the AnMBR. After that, the enhanced propionate degradation was achieved up to an OLR of 15 kg-COD/m³d with a reduced sulfate addition of 300 mg/L. However, the thermodynamic calculation indicated that the syntrophic propionic acid degradation, coupled with methanogenesis, was unfavorable with a △G of +3 kJ/mol under the enhanced conditions. Conversely, the utilization of propionic acid by sulfate reduction bacterial (SRB) would be more favourable by having a much lower △G of -180 kJ/mol. The hydrogen conversion was presumed to go through the methanogenesis pathway according to the thermodynamic results. The mechanism of the propionic and hydrogen metabolism was supported as well by comparing the microbial communities with and without sulfate addition. As a result, the role of the sulfate enhancing propionic degradation can be concluded by combining the process performance, thermodynamic, and microbiology results.

Potential enhancement of direct interspecies electron transfer for syntrophic metabolism of propionate and butyrate with biochar in up-flow anaerobic sludge blanket reactors

Promoting direct interspecies electron transfer (DIET) to enhance syntrophic metabolism may be a strategy for accelerating the conversion of organic wastes to methane, but microorganisms capable of metabolizing propionate and butyrate via DIET under methanogenic conditions have yet to be identified. In an attempt to establish methanogenic communities metabolizing propionate or butyrate with DIET, enrichments were initiated with up-flow anaerobic sludge blanket (UASB), similar to those that were previously reported to support communities that metabolized ethanol with DIET that relied on direct biological electrical connections. In the absence of any amendments, microbial communities enriched were dominated by microorganisms closely related to pure cultures that are known to metabolize propionate or butyrate to acetate with production of H2. When biochar was added to the reactors there was a substantial enrichment on the biochar surface of 16S rRNA gene sequences closely related to Geobacter and Methanosaeta species known to participate in DIET.

Multiple paths of electron flow to current in microbial electrolysis cells fed with low and high concentrations of propionate

Microbial electrolysis cells (MECs) provide a viable approach for bioenergy generation from fermentable substrates such as propionate. However, the paths of electron flow during propionate oxidation in the anode of MECs are unknown. Here, the paths of electron flow involved in propionate oxidation in the anode of two-chambered MECs were examined at low (4.5 mM) and high (36 mM) propionate concentrations. Electron mass balances and microbial community analysis revealed that multiple paths of electron flow (via acetate/H2 or acetate/formate) to current could occur simultaneously during propionate oxidation regardless of the concentration tested. Current (57-96 %) was the largest electron sink and methane (0-2.3 %) production was relatively unimportant at both concentrations based on electron balances. At a low propionate concentration, reactors supplemented with 2-bromoethanesulfonate had slightly higher coulombic efficiencies than reactors lacking this methanogenesis inhibitor. However, an opposite trend was observed at high propionate concentration, where reactors supplemented with 2-bromoethanesulfonate had a lower coulombic efficiency and there was a greater percentage of electron loss (23.5 %) to undefined sinks compared to reactors without 2-bromoethanesulfonate (11.2 %). Propionate removal efficiencies were 98 % (low propionate concentration) and 78 % (high propionate concentration). Analysis of 16S rRNA gene pyrosequencing revealed the dominance of sequences most similar to Geobacter sulfurreducens PCA and G. sulfurreducens subsp. ethanolicus. Collectively, these results provide new insights on the paths of electron flow during propionate oxidation in the anode of MECs fed with low and high propionate concentrations.

Dynamics of propionic acid degradation in a two-phase anaerobic system

This paper reports on propionic acid (HPr) degradation in a laboratory scale two-phase anaerobic system, where HPr was accumulated in the acidogenic reactor and degraded in the methanogenic reactor. Batch tests using biomass from the two-phase anaerobic system showed HPr degradation was rarely detectable in the acidogenic reactor when HPr concentration ranged from 639 to 4531mgHPrL(-1) and at pH 4.50 to 6.50. Biomass from the methanogenic reactor could, however, successfully degrade HPr at its initial concentration of up to 4585mgHPrL(-1) at pH 6.40-7.30. ATP results showed that differences in the degradation ability of HPr by the acidogenic and methanogenic biomass may be related with their respective different biomass activities. Results from pyrosequencing showed that the predominant propionic acid oxidizing bacteria (POB) in the methanogenic reactor were Smithella (2.68%) and Syntrophobacter (0.35%); while poor degradation of HPr in the acidogenic reactor may be associated with the low abundance of POB (0.02% Desulfacinum and 0.08% Desulfobulbus). This might have been induced by the long-term unfavorable environment for POB growth in the acidogenic reactor.

Microbial diversity in support of anaerobic biomass valorization

Microbial diversity provides an immense reservoir of functions and supports key steps in maintaining ecosystem balance through matter decomposition processes and nutrient recycling. The use of microorganisms for biomolecule production is now common, but often involves single-strain cultures. In this review, we highlight the significance of using ecosystem-derived microbial diversity for biotechnological researches. In the context of organic matter mineralization, diversity of microorganisms is essential and enhances the degradation processes. We focus on anaerobic production of biomolecules of interest from discarded biomass, which is an important issue in the context of organic waste valorization and processing. Organic waste represents an important and renewable raw material but remains underused. It is commonly accepted that anaerobic mineralization of organic waste allows the production of diverse interesting molecules within several fields of application. We provide evidence that complex and diversified microbial communities isolated from ecosystems, i.e. microbial consortia, offer considerable advantages in degrading complex organic waste, to yield biomolecules of interest. We defend our opinion that this approach is more efficient and offers enhanced potential compared to the approaches that use single strain cultures.

Co-digestion of wheat and rye bread suspensions with source-sorted municipal biowaste

Acidification of wheat bread (WBS), rye bread (RBS) and fresh biowaste suspensions (FBS), leading to lactate+acetate, lactate+acetate+n-buyrate, and acetate+propionate+n-butyrate, respectively, and biogas production as well as population dynamics were investigated. Co-fermentation of FBS (14 kg m(-3) d(-1) organic loading rate (OLR)) with WBS or RBS was stable up to an OLR of 22 kg m(-3) d(-1) and resulted in up to 3 times as much biogas. During co-fermentation at more than 20 kg m(-3) d(-1) OLR the total population increased more than 2-fold, but the originally low share of propionate-oxidizing bacteria significantly decreased. The proportion of methanogens also decreased. Whereas the proportion of Methanosarcinales to Methanomicrobiales in biowaste and biowaste+WBS remained constant, Methanosarcinales and in particular Methanosaeta spec. in the biowaste+RBS assay almost completely disappeared. Methanomicrobiales increased instead, indicating propionate oxidation via acetate cleavage to CO2 and hydrogen.

Effect of ammoniacal nitrogen on one-stage and two-stage anaerobic digestion of food waste

This research compares the operation of one-stage and two-stage anaerobic continuously stirred tank reactor (CSTR) systems fed semi-continuously with food waste. The main purpose was to investigate the effects of ammoniacal nitrogen on the anaerobic digestion process. The two-stage system gave more reliable operation compared to one-stage due to: (i) a better pH self-adjusting capacity; (ii) a higher resistance to organic loading shocks; and (iii) a higher conversion rate of organic substrate to biomethane. Also a small amount of biohydrogen was detected from the first stage of the two-stage reactor making this system attractive for biohythane production. As the digestate contains ammoniacal nitrogen, re-circulating it provided the necessary alkalinity in the systems, thus preventing an eventual failure by volatile fatty acids (VFA) accumulation. However, re-circulation also resulted in an ammonium accumulation, yielding a lower biomethane production. Based on the batch experimental results the 50% inhibitory concentration of total ammoniacal nitrogen on the methanogenic activities was calculated as 3.8 g/L, corresponding to 146 mg/L free ammonia for the inoculum used for this research. The two-stage system was affected by the inhibition more than the one-stage system, as it requires less alkalinity and the physically separated methanogens are more sensitive to inhibitory factors, such as ammonium and propionic acid.

Microbial Community Shifts during Biogas Production from Biowaste and/or Propionate

Propionate is the most delicate intermediate during anaerobic digestion as its degradation is thermodynamically unfavorable. To determine its maximum possible degradation rates during anaerobic digestion, a reactor was fed Monday to Friday with an organic loading rate (OLR) of 12/14 kg COD_biowaste·m⁻³·d⁻¹ plus propionate up to a final OLR of 18 kg COD·m⁻³·d⁻¹. No feed was supplied on weekends as it was the case in full-scale. To maintain permanently high propionate oxidizing activity (POA), a basic OLR of 3 kg COD_propionate·m⁻³·d⁻¹ all week + 11 kg COD_biowaste·m⁻³·d⁻¹ from Monday to Friday was supplied. Finally a reactor was operated with an OLR of 12 kg COD_biowaste·m⁻³·d⁻¹ from Monday to Friday and 5 kg COD_propionate·m⁻³·d⁻¹ from Friday night to Monday morning to maintain a constant gas production for permanent operation of a gas engine. The propionate degradation rates (PDRs) were determined for biowaste + propionate feeding. Decreasing PDRs during starvation were analyzed. The POA was higher after propionate supply than after biowaste feeding and decreased faster during starvation of a propionate-fed rather than a biowaste-fed inoculum. Shifts of the propionate-oxidizing and methanogenic community were determined.

Thermophilic anaerobic co-digestion of coffee grounds and excess sludge: long term process stability and energy production

In this research, thermophilic anaerobic digestion of coffee grounds and sludge was carried out using a 12 liter continuously stirred tank reactor (CSTR) to identify the inhibitory factors and to evaluate the energy production.

Bioclogging in porous media: influence in reduction of hydraulic conductivity and organic contaminants during synthetic leachate permeation

In this study the concept of biofilm accumulation in the sand column was promoted to assess the changes in hydraulic conductivity and concentration of organic contaminants of the synthetic leachate. Four different combinations of column study were carried out using synthetic leachate as a substrate solution. Mixed and stratified mode of experiments with two different sizes (0.3 mm and 0.6 mm) of sand grains were used for column filling. Two columns were acting as a blank, the remaining two columns amended with mixed microbial cultures which were isolated from leachate. The column was operated with continuous synthetic leachate supply for 45 days. The results indicated that the highest hydraulic conductivity reduction occurred in the mixed sand microbial column with 98.8% when compared to stratified sand microbial column. The analysis of organic contaminants of the effluent leachate was also clearly shown that the mixed sand amended with microbes poses a suitable remedial measure when compared to natural and synthetic liners for controlling the leachate migration in the subsurface environment.

A metagenomic study of the microbial communities in four parallel biogas reactors

BACKGROUND

Biogas is a renewable energy carrier which is used for heat and power production or, in the form of purified methane, as a vehicle fuel. The formation of methane from organic materials is carried out by a mixed microbial community under anaerobic conditions. However, details about the microbes involved and their function are limited. In this study we compare the metagenomes of four parallel biogas reactors digesting a protein-rich substrate, relate microbiology to biogas performance, and observe differences in these reactors' microbial communities compared to the original inoculum culture.

RESULTS

The biogas process performance during the startup phase of four parallel continuous stirred tank reactors (designated R1, R2, R3, and R4) co-digesting fish waste and cow manure was studied. The microbial composition of the inoculum (day 0) and the four reactors at day 59 was studied and compared using 454 FLX Titanium pyrosequencing. In the inoculum and the reactor samples, the Bacteria Clostridium and Syntrophomonas were highly abundant, and the dominating methanogen was the hydrogenotrophic Methanoculleus. Syntrophic prokaryotes frequently found in biogas reactors with high concentrations of ammonium and volatile fatty acids were detected in all samples. The species Candidatus Cloacimonas acidaminovorans of the candidate phylum Cloacimonetes (WWE1) increased in all reactors and was the dominating bacterium at day 59. In particular, this bacterium showed a very high abundance in R1, which distinguished this reactor significantly from the other reactors in terms of microbial composition. Methane production and the reactor slurry characteristics were monitored in the digestion period. Generally all four reactors operated stably and showed rather similar characteristics. The average methane production in the reactors varied between 0.278 and 0.296 L gVS(-1), with the lowest production in R1.

CONCLUSIONS

This study showed that four parallel reactors co-digesting manure and fish waste silage operated stably during a startup phase. Several important Archaea and Bacteria degrading the protein-rich substrate were identified. In particular, microorganisms involved in syntrophic methane production seemed to be important. The detailed characterization of the microbial communities presented in this work may be useful for the operation of biogas plants degrading substrates with high concentrations of proteins.

Enhanced anaerobic fermentation with azo dye as electron acceptor: simultaneous acceleration of organics decomposition and azo decolorization

Accumulation of hydrogen during anaerobic processes usually results in low decomposition of volatile organic acids (VFAs). On the other hand, hydrogen is a good electron donor for dye reduction, which would help the acetogenic conversion in keeping low hydrogen concentration. The main objective of the study was to accelerate VFA composition through using azo dye as electron acceptor. The results indicated that the azo dye serving as an electron acceptor could avoid H2 accumulation and accelerate anaerobic digestion of VFAs. After adding the azo dye, propionate decreased from 2400.0 to 689.5mg/L and acetate production increased from 180.0 to 519.5mg/L. It meant that the conversion of propionate into acetate was enhanced. Fluorescence in situ hybridization analysis showed that the abundance of propionate-utilizing acetogens with the presence of azo dye was greater than that in a reference without azo dye. The experiments via using glucose as the substrate further demonstrated that the VFA decomposition and the chemical oxygen demand (COD) removal increased by 319.7mg/L and 23.3% respectively after adding the azo dye. Therefore, adding moderate azo dye might be a way to recover anaerobic system from deterioration due to the accumulation of H2 or VFAs.

Effect of moisture of municipal biowaste on start-up and efficiency of mesophilic and thermophilic dry anaerobic digestion

Methane production from biowaste with 20-30% dry matter (DM) by box-type dry anaerobic digestion and contributing bacteria were determined for incubation at 20, 37 and 55 °C. The same digestion efficiency as for wet anaerobic digestion of biowaste was obtained for dry anaerobic digestion with 20% DM content at 20, 37 and 55 °C and with 25% DM content at 37 and 55 °C. No or only little methane was produced in dry anaerobic reactors with 30% DM at 20, 37 or 55 °C. Population densities in the 20-30% DM-containing biowaste reactors were similar although in mesophilic and thermophilic biowaste reactors with 30% DM content significantly less but phylogenetically more diverse archaea existed. Biogas production in the 20% and 25% DM assays was catalyzed by Methanosarcinales and Methanomicrobiales. In all assays Pelotomaculum and Syntrophobacter species were dominant propionate degraders.

Effects of a gradually increased load of fish waste silage in co-digestion with cow manure on methane production

This study examined the effects of an increased load of nitrogen-rich organic material on anaerobic digestion and methane production. Co-digestion of fish waste silage (FWS) and cow manure (CM) was studied in two parallel laboratory-scale (8L effective volume) semi-continuous stirred tank reactors (designated R1 and R2). A reactor fed with CM only (R0) was used as control. The reactors were operated in the mesophilic range (37°C) with a hydraulic retention time of 30 days, and the entire experiment lasted for 450 days. The rate of organic loading was raised by increasing the content of FWS in the feed stock. During the experiment, the amount (volume%) of FWS was increased stepwise in the following order: 3% - 6% - 13% - 16%, and 19%. Measurements of methane production, and analysis of volatile fatty acids, ammonium and pH in the effluents were carried out. The highest methane production from co-digestion of FWS and CM was 0.400 L CH4 gVS(-1), obtained during the period with loading of 16% FWS in R2. Compared to anaerobic digestion of CM only, the methane production was increased by 100% at most, when FWS was added to the feed stock. The biogas processes failed in R1 and R2 during the periods, with loadings of 16% and 19% FWS, respectively. In both reactors, the biogas processes failed due to overloading and accumulation of ammonia and volatile fatty acids.

Fatty acid metabolism and population dynamics in a wet biowaste digester during re-start after revision

Volatile fatty acid (VFA) metabolism and population changes during re-start of a wet anaerobic digester for treatment of biowaste suspensions were investigated. Initially acetate and propionate accumulated. However, VFA degradation rates improved within 2 weeks, reaching a balanced metabolism with constantly low VFA concentrations. The total microbial population consisted of 60% acidogenic+acetogenic and 40% methanogenic bacteria. Maximally 5.1% of all bacteria in the digester were propionate-oxidizing bacteria. Pelotomaculum species were dominant, followed by much lower numbers of Smithella species and Syntrophobacter species. Pelotomaculum and Smithella spec. decreased during acetate and propionate accumulation and recovered later on, whereas Syntrophobacter spec. was steadily increasing during start-up. A nearly constant proportion of Methanosaeta spec. was present all time, while Methanosarcina spec. decreased and Methanomicrobiales increased during accumulation and degradation of acetate and propionate. During steady state proportions as in the inoculum were obtained.

Enhancement of biogas production by co-digestion of potato pulp with cow manure in a CSTR system

Anaerobic digestion (AD) process is a well-established method to generate energy from the organic wastes both from the environmental and economical perspectives. The purpose of present study is to evaluate energy production from potato wastes by incorporating cow manure into the process. Firstly, a laboratory pilot of one-stage biogas production was designed and built according to continuously stirred tank reactor (CSTR) system. The setup was able to automatically control the environmental conditions of the process including temperature, duration, and rate of stirring. AD experiment was exclusively performed on co-digestion of potato peel (PP) and cow manure (CM) in three levels of mixing ratio including 20:80, 50:50, 80:20 (PP:CM), and 0:100 as control treatment based on the volatile solid (VS) weight without adding initial inoculums. After hydraulic retention time (HRT) of 50 days on average 193, 256, 348, and 149 norm liter (LN) (kg VS)(-1), methane was produced for different mixing ratios, respectively. Statistical analysis shows that these gas productions are significantly different. The average energy was determined based on the produced methane which was about 2.8 kWh (kg VS)(-1), implying a significant energy production potential. The average chemical oxygen demand (COD) removal of treatments was about 61%, showing that it can be leached significantly with high organic matter by the employed pilot. The energy efficiency of 92% of the process also showed the optimum control of the process by the pilot.

Effect of a high strength chemical industry wastewater on microbial community dynamics and mesophilic methane generation

A high strength chemical industry wastewater was assessed for its impact on anaerobic microbial community dynamics and consequently mesophilic methane generation. Cumulative methane production was 251 mL/g total chemical oxygen demand removed at standard temperature and pressure at the end of 30 days experimental period with a highest recorded methane percentage of 80.6% of total biogas volume. Volatile fatty acids (VFAs) analysis revealed that acetic acid was the major intermediate VFAs produced with propionic acid accumulating over the experimental period. Quantitative analysis of microbial communities in the test and control groups with quantitative real time polymerase chain reaction highlighted that in the test group, Eubacteria (96.3%) was dominant in comparison with methanogens (3.7%). The latter were dominated by Methanomicrobiales and Methanobacteriales while Methanosarcinaceae in test groups increased over the experimental period, reaching a maximum on day 30. Denaturing gradient gel electrophoresis profile was performed, targeting the 16S rRNA gene of Eubacteria and Archaea, with the DNA samples extracted at 3 different time points from the test groups. A phylogenetic tree was constructed for the sequences using the neighborhood joining method. The analysis revealed that the presence of organisms resembling Syntrophomonadaceae could have contributed to increased production of acetic and propionic acid intermediates while decrease of organisms resembling Pelotomaculum sp. could have most likely contributed to accumulation of propionic acid. This study suggested that the degradation of organic components within the high strength industrial wastewater is closely linked with the activity of certain niche microbial communities within eubacteria and methanogens.