Thermophilic Reactor Research Articles

Determination of effects of thermophilic and hyperthermophilic temperatures on anaerobic hydrolysis and acidogenesis of pig manure through a one-year study

Improving hydrolysis and acidogenesis through thermophilic and hyperthermophilic temperatures is critical for enhancing the anaerobic decomposition of organic waste like pig manure. However, whether higher temperatures can provide more enhanced performance has not been elucidated experimentally. This study, therefore, conducted a 375-day continuous operation experiment at 55 and 70 °C with a 5-day hydraulic retention time. The two temperature reactors entered a stable state after about 200 days and long-term microbial acclimation markedly changed their performances. In the thermophilic and hyperthermophilic reactor, the hydrolysis efficiencies were obtained at 29.7 % and 27.3 % respectively, whereas the acidogenesis efficiency was relatively low at 1.0 % and 3.1 %. Due to the occurrence of methanogenesis, the volatile fatty acid concentration in the thermophilic reactor was only 45 % of that in the hyperthermophilic reactor. The thermophilic reactor exhibited higher bacterial diversity; however, this difference between the two reactors apparently did not correlate with hydrolysis and acidogenesis performance.

Distillery Plant Upflow Anaerobic Sludge Blanket Reactor comparision of Energy regime in Mesophilic and Thermophilic conditions

In this study, comparison of energy regime analysis of mesophilic and thermophilic condition upflow anaerobic sludge blanket (UASB) reactor treating distillery effluent is studied. Percentage COD reduction in thermophilic condition was more by 6.96% than mesophilic condition due to which biogas production increased by 0.1 m3/kg COD removed. In thermophilic temperature condition, methenogenesis was more than acidification than in mesophilic temperature condition; effluent VFA concentration in mesophilic condition was more in thermophilic condition. Stability of thermophilic reactor was more than mesophilic reactor from VFA/ Alkalinity ratio. Waste water treatment plant consumes 2.10 kWh/m3 total energy which comprises of electrical, mechanical and manual energy in mesophilic temperature condition. Input given in the form of electrical energy was 2.07 kWh/m3 and energy recovered in the form of biogas was 185 kWh/m3. Total energy input in thermophilic temperature condition was 1.82 kWh/ m3 and biogas recovery was 211 kWh/m3 in the form of energy. Net saving in energy input was 0.28 kWh/m3 (12.98%) and biogas production increased by 25.9 kWh/m3 (14%) in the form of energy in thermophic temperature condition. Treatment of distillery wastewater in thermophilic temperature condition can be a good option as compared to mesophilic temperature conditions considering the aspect of energy and it seems feasible due to the availability of elevated temperature of the feed for UASB.

Improving mesophilic anaerobic digestion of food waste by side-stream thermophilic reactor: Activation of methanogenic, key enzymes and metabolism.

Anaerobic digestion (AD) is a favorable way to convert organic pollutants, such as food waste (FW), into clean energy through microbial action. This work adopted a side-stream thermophilic anaerobic digestion (STA) strategy to improve a digestive system's efficiency and stability. Results showed that the STA strategy brought higher methane production as well as higher system stability. It quickly adapted to thermal stimulation and increased the specific methane production from 359mL CH4/g·VS to 439mL CH4/g·VS, which was also higher than 317mL CH4/g·VS from single-stage thermophilic anaerobic digestion. Further exploration of the mechanism of STA using metagenomic and metaproteomic analysis revealed enhanced activity of key enzymes. The main metabolic pathway was up-regulated, while the dominant bacteria were concentrated, and the multifunctional Methanosarcina was enriched. These results indicate that STA optimized organic metabolism patterns, comprehensively promoted methane production pathways, and formed various energy conservation mechanisms. Further, the system's limited heating avoided adverse effects from thermal stimulation, and activated enzyme activity and heat shock proteins through circulating slurries, which improved the metabolic process, showing great application potential.

Acclimatizing waste activated sludge in a thermophilic anaerobic fixed-bed biofilm reactor to maximize biogas production for food waste treatment at high organic loading rates

Thermophilic anaerobic digestion (TAD) provides a promising solution for sustainable high-strength waste treatment due to its enhanced methane-rich biogas recovery. However, high organic loading rates (OLR) exceeding 3.0 kgCOD/m3/day and short hydraulic retention times (HRT) below 10 days pose challenges in waste-to-energy conversion during TAD, stemming from volatile fatty acids (VFAs) accumulation and methanogenesis failure. In this study, we implemented a stepwise strategy for acclimatizing waste activated sludge (WAS) in a thermophilic anaerobic fixed-bed biofilm reactor (TA-FBBR) to optimize methanogen populations, thereby enhancing waste-to-energy efficiencies under elevated OLRs in food waste treatment. Results showed that following stepwise acclimatization, the TA-FBBR achieved stable methane production of approximately 5.8 L/L-reactor/day at an ultrahigh OLR of ∼20 kgCOD/m3/day and ∼15 kgVS/m3/day at 6-day HRT in food waste treatment. The average methane yield reached 0.45 m3/kgCODremoval, attaining the theoretical production in TAD. Moreover, VFA concentrations were stabilized below 1000 mg/L at the ultrahigh OLR under 6-day HRT, while maintaining an acetate/propionate ratio of > 1.8 and a VFA/TAK ratio of < 0.3 serving as effective indicators of system stability and methane yield potential. The microbial community analysis revealed that the WAS acclimatization strategy fostered the microbial diversity and abundance of Methanothermobacter and Methanosarcina. Methanosarcina in the biofilm were observed to be twice as abundant as Methanothermobacter, indicating a potential preference for biofilm existence among methanogens. The findings demonstrated an effective strategy, specifically the stepwise acclimatization of WAS in a thermophilic fixed-bed biofilm reactor, to enhance the food waste treatment performance at high OLRs, contributing valuable mechanistic and technical insights for future sustainable high-strength waste management.

Biogas Production and Energy Balance in a Two-Stage Anaerobic Digestion of Fruit and Vegetable Waste: Thermophilic versus Mesophilic

This study aimed to investigate biogas production and energy balance in a two-stage anaerobic digestion system of fruit/vegetable waste under thermophilic and mesophilic conditions. Firstly, the feedstock was hydrolyzed and acidified in an acidic reactor at 37 °C with a retention time of 5 d. Then, the liquid hydrolysate was collected and pumped into an up-flow methane reactor under a mesophilic temperature with a retention time of 5 d and a thermophilic condition with a retention time of 3 d. The experimental results showed that in the thermophilic methane reactor, the COD removal, biogas yield, and methane concentration were 96.3%, 492 mL/g-VS, and 70.4%, respectively. These values were 3%, 10%, and 3% higher, respectively, than those obtained in the mesophilic methane reactor. In terms of energy, the mesophilic and thermophilic methane reactors consumed the same thermal energy demand for temperature control. They were much lower than the heat values produced by the power engine. The two-stage anaerobic digestion system using a thermophilic methane reactor obtained a gross energy of 11.20 kJ/g-VS and a net energy of 9.83 kJ/g-VS. These values were 13.2% and 14.8% higher, respectively, than those obtained by the system with a mesophilic condition. Moreover, the use of a thermophilic reactor helped reduce the reactor volume by 40%, leading to significant investment cost savings.

Hyper-thermophilic anaerobic pretreatment enhances the removal of transferable oxazolidinone and phenicol cross-resistance gene optrA in enterococci

The extensive use of florfenicol in poultry industry results in the emergence of optrA gene, which also confers resistance to clinically important antibiotic linezolid. This study investigated the occurrence, genetic environments, and removal of optrA in enterococci in mesophilic (37 °C) and thermophilic (55 °C) anaerobic digestion systems, and a hyper-thermophilic (70 °C) anaerobic pretreatment system for chicken waste. A total of 331 enterococci were isolated and analyzed for antibiotic resistance against linezolid and florfenicol. The optrA gene was frequently detected in enterococci from chicken waste (42.7%) and effluents from mesophilic (72%) and thermophilic (56.8%) reactors, but rarely detected in the hyper-thermophilic (5.8%) effluent. Whole-genome sequencing revealed that optrA-carrying Enterococcus faecalis sequence type (ST) 368 and ST631 were the dominant clones in chicken waste, and they remained dominant in mesophilic and thermophilic effluents, respectively. The plasmid-borne IS1216E-fexA-optrA-erm(A)-IS1216E was the core genetic element for optrA in ST368, whereas chromosomal Tn554-fexA-optrA was the key one in ST631. IS1216E might play a key role in horizontal transfer of optrA due to its presence in different clones. Hyper-thermophilic pretreatment removed enterococci with plasmid-borne IS1216E-fexA-optrA-erm(A)-IS1216E. A hyper-thermophilic pretreatment is recommended for chicken waste to mitigate dissemination of optrA from animal waste to the environment.

Kinetic study on the effects of elevated temperature and micro aeration in volatile fatty acids production by anaerobic fermentation of alkaline-pretreated oil palm empty fruit bunch

This research studied the conversion of alkaline-pretreated oil palm empty fruit bunch (OPEFB) into volatile fatty acids (VFA) through anaerobic fermentation (AF). In AF, the methanogens that consume VFAs must be suppressed to increase the accumulated VFA as an intermediate product of AF. In-depth research has not been conducted on the effects of temperature and micro aeration to maximize the VFA accumulation. To investigate the temperature effect, the experiment was run at mesophilic (RM) and thermophilic conditions (RT). The impact of the micro aeration dose was studied in the thermophilic reactor set to the ORP −50 mV (RTM −50) and ORP −150 mV (RTM −150). The experimental data were fitted to the Anaerobic Digestion Model 1 (ADM 1) using AQUASIM®. The khyd.ch value of the RT reactor is higher than that of the RM, suggesting that high temperatures encourage hydrolysis and enhance hydrolase activity. Based on the simulation results, RT reactors without micro-aeration had a substantially higher specific growth rate of methanogens compared to RM. Therefore, micro aeration should be applied in RT to suppress the potentially faster growth of methanogen bacteria at elevated temperature. The result of this study indicated that both temperature and micro-aeration were important process variable to be further optimized to make VFAs production from OPEFB attractive.

Does carbon cloth really improve thermophilic anaerobic digestion performance on a larger scale? focusing on statistical analysis and microbial community dynamics

Currently, the phenomenon of direct interspecies electron transfer (DIET) is of great interest in the technology of anaerobic digestion (AD) due to potential performance benefits. However, the conditions for the occurrence of DIET and its limits on improving AD under conditions close to real have not been studied enough. This research is concentrated on the effect of conductive carbon cloth (R3), in comparison with a dielectric fiberglass cloth (R2) and control (R1), on the AD performance in large (90 L) thermophilic reactors, fed with a mixture of simulated organic fraction of municipal solid waste and sewage sludge. While organic loading rate (OLR) was gradually increased from 2.4 to 8.66 kg VS/(m3 day), a statistically significant (p < 0.05) difference in biogas production was observed between R1 and both R2 and R3. However, at a maximum OLR of 12.12 kg VS/(m3 day) in R3, an increase in biogas production (p < 0.05) was observed both compared to R1 (by 8.97%) and R2 (by 4.24%). The content of volatile fatty acids in R3 as a whole was the lowest, especially at the maximum OLR. Biofilm on carbon cloth was rich in syntrophic microorganisms of the genera Tepidanaerobacter, as well as Defluviitoga, capable of DIET in mixed cultures with Methanothrix, which was the most abundant methanogen in biofilm. Suspended Bifidobacterium, Fervidobacterium and Anaerobaculum were negatively affected, while Defluviitoga, Methanothermobacter and Methanosarcina, on the contrary, were positively affected by the increase in OLR and showed, respectively, a negative and positive correlation (p < 0.05) with the main AD performance parameters.

Biological Hydrogen Methanation with Carbon Dioxide Utilization: Methanation Acting as Mediator in the Hydrogen Economy

Hydrogen is one of the main energy carriers playing a prominent role in the future decarbonization of the economy. However, several aspects regarding the transport and storage of this gas are challenging. The intermediary conversion of hydrogen into high-density energy molecules may be a crucial step until technological conditions are ready to attain a significant reduction in fossil fuel use in transport and the industrial sector. The process of transforming hydrogen into methane by anaerobic digestion is reviewed, showing that this technology is a feasible option for facilitating hydrogen storage and transport. The manuscript focuses on the role of anaerobic digestion as a technology driver capable of fast adaptation to current energy needs. The use of thermophilic systems and reactors capable of increasing the contact between the H2-fuel and liquid phase demonstrated outstanding capabilities, attaining higher conversion rates and increasing methane productivity. Pressure is a relevant factor of the process, allowing for better hydrogen solubility and setting the basis for considering feasible underground hydrogen storage concomitant with biological methanation. This feature may allow the integration of sequestered carbon dioxide as a relevant substrate.

Recovering the sulfidogenic activity under harsh sulfide-rich fermentative conditions in a thermophilic structured-bed reactor: Performance and microbial aspects

Anaerobic treatment of sulfate-rich wastewaters is challenging due to the inevitable biological sulfate reduction to sulfide, which limits biogas use and can negatively impact the treatment performance due to the toxic character of sulfide. Establishing sulfidogenesis during fermentation has recently been highlighted as a novel application for acidogenic systems. Different strategies were applied to recover the efficiency of a thermophilic (55 °C) ethanol-fed anaerobic structured-bed reactor operated under acetogenic/sulfidogenic conditions after stress events: [i] the progressive re-insertion of liquid phase recirculation, [ii] the decrease in the sulfate loading rate (SLR) towards the minimization of residual sulfate levels, and [iii] the decrease in the organic loading rate (OLR). Long-term monitoring (631 d) divided into 13 operating phases (7 of which are detailed herein) showed a gradual improvement in the removal efficiency of sulfate, achieving 93.7% when applying suitable OLR (20.0 kg-COD m−3 d−1), SLR (3.2 kg-SO42- m−3 d−1), hydraulic retention time (15.0 h, theoretical value) and liquid upflow velocity (3.34 m h−1). A relatively restricted number of key microbial groups mediating both the sulfate reduction and acetate production from ethanol was identified, including the genera Coprothermobacter, Thermacetogenium, Fervidobacterium, Anaerobaculum and Thermotoga. This pattern most likely resulted from using ethanol as the only carbon source and explains the relatively long-term period (ca. 80 d) required to optimize the sulfidogenic activity. From a practical perspective, the operating strategies studied herein could be efficiently applied in industrial wastewater treatment plants, such as sulfate-rich sugarcane vinasses.

Effects of phenyl acids on different degradation phases during thermophilic anaerobic digestion.

Aromatic compounds like phenyl acids (PA) can accumulate during anaerobic digestion (AD) of organic wastes due to an increased entry of lignocellulose, secondary plant metabolites or proteins, and thermodynamic challenges in degrading the benzene ring. The effects of aromatic compounds can be various - from being highly toxic to be stimulating for methanogenesis - depending on many parameters like inoculum or molecular characteristics of the aromatic compound. To contribute to a better understanding of the consequences of PA exposure during AD, the aim was to evaluate the effects of 10 mM PA on microbial communities degrading different, degradation phase-specific substrates in thermophilic batch reactors within 28 days: Microcrystalline cellulose (MCC, promoting hydrolytic to methanogenic microorganisms), butyrate or propionate (promoting syntrophic volatile fatty acid (VFA) oxidisers to methanogens), or acetate (promoting syntrophic acetate oxidisers to methanogens). Methane production, VFA concentrations and pH were evaluated, and microbial communities and extracellular polymeric substances (EPS) were assessed. The toxicity of PA depended on the type of substrate which in turn determined the (i) microbial diversity and composition and (ii) EPS quantity and quality. Compared with the respective controls, methane production in MCC reactors was less impaired by PA than in butyrate, propionate and acetate reactors which showed reductions in methane production of up to 93%. In contrast to the controls, acetate concentrations were high in all PA reactors at the end of incubation thus acetate was a bottle-neck intermediate in those reactors. Considerable differences in EPS quantity and quality could be found among substrates but not among PA variants of each substrate. Methanosarcina spp. was the dominant methanogen in VFA reactors without PA exposure and was inhibited when PA were present. VFA oxidisers and Methanothermobacter spp. were abundant in VFA assays with PA exposure as well as in all MCC reactors. As MCC assays showed higher methane yields, a higher microbial diversity and a higher EPS quantity and quality than VFA reactors when exposed to PA, we conclude that EPS in MCC reactors might have been beneficial for absorbing/neutralising phenyl acids and keeping (more susceptible) microorganisms shielded in granules or biofilms.

Effects of biochar addition towards the treatment of blackwater in anaerobic dynamic membrane bioreactor (AnDMBR): Comparison among room temperature, mesophilic and thermophilic conditions

Effects of biochar addition on the performance of anaerobic dynamic membrane bioreactor (AnDMBR) under different temperatures for blackwater treatment was investigated. When the organic load ratios (OLR) was 1.0 g COD/L·d, the specific methane yield for the three biochar-amended reactors were 125.7, 148.0 and 182.3 mLCH4/g CODremoved, respectively. Compare to those digesters without biochar participation, the methane production in the thermophilic reactor with biochar increased by 12% while the other two digesters increased by 6–10%. An analysis of membrane filtration resistance showed a reduction in total resistance (Rt) of 6.2 × 1011–7.3 × 1011 m−1 when biochar was added to the three reactors. The thermophilic reactors with biochar increased the relative abundance of Methanothermobacter and promoted gene expression of metabolic pathways related to hydrolysis, acid production and methane production. Overall, biochar showed great potential as an inexpensive conductive material to increase methane production with reduced membrane fouling in AnDMBR systems.

Effect of temperature and solids retention time on the removal of antibiotic resistance genes during anaerobic digestion of sludge

This study investigated how anaerobic digestion (AD) of sludge under different operating temperatures (35 °C and 55 °C) may respond to changes in solids retention time (SRT = 20, 30, and 40 days) in terms of the removal of antibiotic resistance genes (ARGs) and biogas production. It was observed that at short SRTs (20 days), ARGs in thermophilic AD (TAD) were less abundant than in mesophilic AD (MAD) with a higher removal rate of 75.80 % vs. 45.78 %, respectively. At longer SRTs (30 and 40 days), the removal of ARGs in MAD increased to 51.79 % and 79.63 % while it decreased to 53.95 % and 24.65 % in TAD. These findings indicate that MAD needs longer SRTs to remove more ARGs, while thermophilic reactors are more effective at shorter SRTs. Moreover, 34–42 % more biogas was produced at thermophilic conditions relative to mesophilic conditions at different SRTs, suggesting that shortening the SRT and increasing the temperature are effective strategies for producing biogas.

Ex-situ biological hydrogen methanation in trickle bed reactors: Integration into biogas production facilities

Biological hydrogen methanation (BHM) is a biocatalytic process for biogas upgrading. Integrating ex-situ BHM processes into biogas facilities has the advantage of using inoculum, CO2, and nutrients directly from anaerobic digestion (AD) processes to enhance CH4 productivity. This study investigated the potential of biowaste digestate as an inoculum and digested biowaste reject water as the trickling liquid and nutrient source in thermophilic trickle bed reactors (TBRs). Use of reject water improved H2 conversion efficiency to up to 99 %, thus achieving a H2 loading rate of 10.8 LH2 L Rv-1 d-1 at a gas retention time (GRT) of 1.8 h and CH4 productivity of 2.6 L LRv-1 d-1 implying that reject water contains macronutrients beneficial to enriching hydrogenotrophic methanogens. However, at high H2 loading rates, a trace element supply was necessary to stabilize process performance. Hydrogenotrophic methanogens Methanothermobacter and Methanobacterium were selectively enriched, mainly due to the increased H2 loading rate.

Metabolic effects of Fe0 on simultaneously eliminating excessive acidification and upgrading biogas in mesophilic or thermophilic anaerobic reactor

Excessive acidification and low methane (CH4) percentage are technical challenges restricting anaerobic digestion (AD) of kitchen waste, especially under high organic loading rate (OLR) conditions. Although the contribution of Fe0 powder to excessive acidification elimination and CH4 yield improvement in mesophilic AD (MAD) has been extensively reported, its impact on the thermophilic AD (TAD) process with more frequent excessive acidification problem is rarely studied. Moreover, the difference in microbial interactions and metabolic pathways between MAD and TAD reactors under Fe0 regulation was still unclear. In this study, we compared the performance of Fe0 dosing on excessive acidification elimination and biogas upgrading quality in both MAD and TAD reactors at different OLRs. The results showed that compared with excessive acidification or low CH4 percentage (less than 70%) in control reactors without Fe0 addition, the CH4 yield of MAD and TAD reactors with addition of Fe0 at OLR of 35 gVS/L were 470.88 mLCH4/gVS and 270.40 mLCH4/gVS, with higher CH4 composition of 81.32%–82.23% and 83.68%–88.48% respectively. Microbial diversity analysis showed a sharp distinction between MAD and TAD reactors with addition of Fe0. Analyses of co-occurrence networks and metabolic pathway presented hydrogenotrophic methanogenic pathway, in addition to acetoclastic methanogenesis, was dominant in MAD reactors as results of enhanced interspecies hydrogen transfer between Methanoculleus and some hydrogen producers, especially under high OLR condition. In contrast, syntrophilic acetate oxidization - hydrogenotrophic methanogenesis, which dominated by Methanothermobacter and Alkaliphilus or Pelotomaculum, was the sole methanogenic pathway in TAD reactors. The results will shed lights on the regulation mechanisms of Fe0 in AD of kitchen waste under mesophilic and thermophilic conditions, offering new insight into development of more efficient waste to energy conversion technologies.

Biomethane recovery through co-digestion of cheese whey and glycerol in a two-stage anaerobic fluidized bed reactor: Effect of temperature and organic loading rate on methanogenesis

Anaerobic digestion for CH4 recovery in wastewater treatment has been carried out with different strategies to increase process efficiency, among which co-digestion and the two-stage process can be highlighted. In this context, this study aimed at evaluating the co-digestion of cheese whey and glycerol in a two-stage process using fluidized bed reactors, verifying the effect of increasing the organic loading rate (OLR) (2–20 g-COD.L−1.d−1) and temperature (thermophilic and mesophilic) in the second stage methanogenic reactor. The mesophilic methanogenic reactor (R-Meso) (mean temperature of 22 °C) was more tolerant to high OLR and its best performance was at 20 g-COD.L−1.d−1, resulting in methane yield (MY) and methane production (MPR) of 273 mL-CH4.g-COD−1 and 5.8 L-CH4.L−1.d−1 (with 67% of CH4), respectively. Through 16S rRNA gene massive sequencing analysis, a greater diversity of microorganisms was identified in R-Meso than in R-Thermo (second stage methanogenic reactor, 55 °C). Firmicutes was the phyla with higher relative abundance in R-Thermo, while in R-Meso the most abundant ones were Proteobacteria and Bacteroidetes. Regarding the Archaea domain, a predominance of hydrogenotrophic microorganisms could be observed, being the genera Methanothermobacter and Methanobacterium the most abundant in R-Thermo and R-Meso, respectively. The two-stage system composed with a thermophilic acidogenic reactor + R-Meso was more adequate for the co-digestion of cheese whey and glycerol than the single-stage process, promoting increases of up to 47% in the energetic yield (10.3 kJ.kg-COD−1) and 14% in organic matter removal (90.5%).

Two-stage thermophilic anaerobic digestion of cheese whey: Process optimization, comparison with single-stage, and full-scale estimation

The purpose of this study was to determine whether a single-stage or two-stage system of an anaerobic sequencing batch and fed-batch biofilm reactor (AnSBBR) would be more efficient for thermophilic whey treatment and bioenergy production. The best results were achieved at 55 °C and 15 kg-COD.m−3.d−1 with organic matter removal of 96%, methane productivity of 142.9 mol-CH4.m−3.d−1, and yield of 10.3 mol-CH4.kg-COD−1. In addition, temperature strongly impacted the microbial community diversity and had an inverse relation with the genus Methanosarcina. To treat all of the whey produced by a dairy industry, which generates 3.4 × 103 m3-whey.year−1, a single-stage anaerobic digestion system of 80.0 m³ would be required, generating a power of 75.1 kW and an energy yield of 9.89 MJ.kg-COD−1. A two-stage system designed for the same industry would have a total working volume of 134.9 m³, combining thermophilic acidogenic reactors and mesophilic methanogenic reactors, with a power of 39.1 kW and energy yield of 5.14 MJ.kg-COD−1. The single-stage system would need an investment cost of U$$ 271,681.00, which is 42% cheaper compared to the two-stage system and would be able to supply 23.5% of the energy demand required by the dairy industry.

Relating biomass composition and the distribution of metabolic functions in the co-fermentation of sugarcane vinasse and glycerol

This study aimed to determine the effect of increasing the organic loading rate (OLR) from 60 to 90 and 120 kg COD m−3 d−1 in the co-fermentation of glycerol and sugarcane vinasse (50%: 50% proportion on a COD basis) in a thermophilic anaerobic fluidized bed reactor (55 °C) at the fixed hydraulic retention time of 4 h. The highest values of hydrogen production rate (1851 mL H2 d−1 L−1bed) and yield (0.29 mmol H2 g−1 CODadded) were found at 120 kg COD m−3 d−1 and coincided with butanoate as a major liquid metabolite (2620 mg L−1). The reverse β-oxidation of lactate into butanoate contributed to its synthesis and was linked to synergism between Clostridium (relative abundance of 77.8%) and Lacticaseibacillus (7.2%) in the reactor. The identification of the butyryl-CoA/acetate-CoA transferase gene, which may have catalyzed the conversion of butyryl-CoA into butanoate using acetate as an acceptor, also supported this.

Methanogenic consortia from thermophilic molasses-fed structured-bed reactors: microbial characterization and responses to varying food-to-microorganism ratios

The heterogeneous character of fixed-film reactors may create highly specialized zones with a stratified distribution of microbial groups and varying capabilities to withstand high organic loads in anaerobic digestion (AD) systems. The microbial distribution and methane-producing potential of biomass from different regions (feeding zone and structured bed) of two second-stage thermophilic (55 ºC) fixed-film reactors were assessed. Three levels of food-to-microorganism (F/M) ratio (0.4, 1.0 and 3.0 g-COD g−1VS) using fermented (two-stage AD) and fresh (single-stage AD) sugarcane molasses were tested in batch reactors, simulating low to high organic loads. Specific methane production rates increased as the F/M increased when using fermented molasses, maintaining efficient methanogenesis at substrate availability levels threefold higher than single-stage schemes (3.0 vs. 1.0 g-COD g−1VS). Success in methane production derived from the homogenous establishment (similar in both feeding zone and bed) of syntrophic associations between acetogens (Pelotomaculum, Syntrophothermus, Syntrophomonas and Thermodesulfovibrio), acetate oxidizers (Thermoacetogenium, Mesotoga and Pseudothermotoga) and hydrogenotrophic methogens (Methanothermobacter and Methanoculleus) replacing acetoclastic methanogens (Methanosaeta). Phase separation under thermophilic conditions was demonstrated to boost methane production from sugar-rich substrates, because the process depends on microbial groups (hydrogenotrophs) that grow faster and are less susceptible to low pH values compared to acetotrophs.

Solving the bottlenecks of sugarcane vinasse biodigestion: Impacts of temperature and substrate exchange on sulfate removal during dark fermentation

Removing sulfate prior to methanogenesis and maintaining year-round operation of the plant are the two main bottlenecks to be solved prior to implementing highly efficient bioenergy recovery from sugarcane vinasse via anaerobic digestion. Sulfidogenesis establishment and optimization during vinasse fermentation at two temperature levels (30 °C and 55 °C) were assessed using continuous fixed-film reactors. Simulating the off season through replacing vinasse by molasses was also assessed. Performance results and the distribution of microbial communities were correlated in all operating conditions. Sulfidogenesis was not efficiently established at 30 °C, because sulfate-reducing bacteria (SRB) did not grow favorably during vinasse natural fermentation. Acetate accumulation (∼3 g L−1) resulting from homoacetogenic activity (most likely by Lachnospirales) was observed in this condition. Conversely, sulfate removal (driven by Desulfohalotomaculum and Desulfotomaculum at much lower abundances compared to fermenters, such as Clostridia) was successfully established (84–97 %) under varied hydraulic retention times (18, 12 and 6 h) at 55 °C. Replacing vinasse by molasses negatively impacted SRB due to a sharp drop in pH (<6.0) triggered by lactate accumulation (up to 16.8 g L−1) mainly by Bacillus. However, SRB were resilient to such severe acidic and thermophilic conditions, and sulfate removal was maximized once vinasse was re-introduced. These results strategically represent the best operating strategy for the compositional conditioning of sugarcane vinasse prior to methanogenesis, because the high temperature in which vinasse is released from distillation columns (85–90 °C) facilitates the implementation of thermophilic reactors by eliminating any energy demands related to system heating.