Single-stage Anaerobic Digestion Research Articles

Unveiling antibiotic resistance dynamics in single and two-stage anaerobic digestion of dairy cow manure: Implications for environmental health

This study investigated antibiotic-resistant bacteria (ARB) survival during mesophilic anaerobic digestion (AD) of dairy cow manure with cefazolin (CEZ) antibiotic. Comparing single- and two-stage AD systems, we assessed ARB removal rates, biogas yield, volatile fatty acid concentration, and microbial populations. The results revealed that in the two-stage AD system, the methanogenic phase (MP) exhibited the highest removal rates of CEZ-resistant (CEZ-r) and oxytetracycline-resistant (OTC-r) bacteria at 9–16% and 69–72%, respectively. However, the MP had a higher proportion of ARB among the total culturable bacteria (24% to 30%) compared to the acidogenic phase (AP) and single-stage AD. CEZ negatively impacted ARB removal rates and biogas production in both systems, with a more pronounced effect in single-stage AD. Biogas yield in the two-stage AD ranged from 337.7–385.6mL/gVS, which was 10–20% higher than that of the single-stage AD, regardless of CEZ addition. The reduction in biogas production due to CEZ was primarily attributed to the suppression of VFA production. As for microbial populations, the changes in the percentages of Firmicutes, Bacteroidetes, and Proteobacteria were likely related to the variations in ARB numbers, and the abundance of Methanosaetaceae significantly influenced biogas production.

Critical Review on Two-Stage Anaerobic Digestion with H2 and CH4 Production from Various Wastes

Anaerobic digestion (AD) is a promising method for resource recovery from various wastes. Compared to the conventional single-stage AD process, a two-stage AD process with separate H2 and CH4 production provides higher energy recovery efficiency and enhanced operation stability. The stage separation makes it possible to apply optimal conditions for different functional microorganisms in their respective stages. This review elaborates the mechanisms of the two-stage AD process and evaluates recent research trends on this topic. A comprehensive comparison between single- and two-stage AD processes is made from the perspective of biogas production, organics degradation, energy recovery, and operation stability. The main influence factors on the two-stage AD process are discussed, including substrates, inoculum, and operation parameters, such as pH, temperature, etc. Upgrading technologies for the two-stage AD process are assessed. The microbial communities in the two-stage AD process for treating different substrates and the influence factors on microbial systems are also summarized. Furthermore, future research opportunities for enhancing the application of this technology are highlighted.

Enhancing Single- and Two-Stage Anaerobic Digestion of Thickened Waste-Activated Sludge through FNA-Heat Pretreatment

This study aimed to investigate the effect of combined Free Nitrous Acid (FNA)-Heat (i.e., FNH) pretreatment on single- and two-stage anaerobic digestion (AD) of thickened waste-activated sludge (TWAS). Single-stage AD was conducted in batches, while two-stage AD involved acidogenic fermentation under semi-continuous flow followed by batch methanogenesis. FNH pretreatment was applied before the acidogenic stage, using 1.4 mg HNO2-N/L FNA concentration at 25 °C, 37 °C, and 60 °C for 24 h. Among the scenarios, the most promising results were observed with two-stage AD fed with FNH-pretreated TWAS at 60 °C, showing higher COD solubilization and a reduction in volatile solids. Combined FNA-Heat pretreatment in two-stage AD yielded elevated methane production (363–415 mL CH4/g VS added) compared to single-stage digestion. Methane yields from FNA-Heat pretreated single-stage ranged from 332 to 347 mL CH4/g VS added, contrasting with 212 mL CH4/g VS added for untreated TWAS. Methane generation commenced early in both untreated and pretreated samples, attributed to soluble substrate abundance.

Effect of reactor operation modes on mitigating antibiotic resistance genes (ARGs) and methane production from hydrothermally-pretreated pig manure

Numerous efforts have been made to enhance the performance of anaerobic digestion (AD) for accelerating renewable energy generation, however, it remains unclear whether the intensified measures could enhance the proliferation and transmissions of antibiotic resistance genes (ARGs) in the system. This study assessed the impact of an innovative pig manure AD process, which includes hydrothermal pretreatment (HTP) and a two-stage configuration with separated acidogenic and methanogenic phases, on biomethane (CH4) production and ARGs dynamics. Results showed that HTP significantly increase CH4 production from 0.65 to 0.75 L/L/d in conventional single-stage AD to 0.82 and 0.91 L/L/d in two-stage AD. This improvement correlated with a rise in the relative abundance of Methanosarcina, a key methanogenesis microorganism. In the two-stage AD, the methanogenic stage offered an ideal environment for methanogens growth, resulting in substantially faster and higher CH4 production by about 10% compared to single-stage AD. Overall, the combined use of HTP and the two-stage AD configuration enhanced CH4 production by 40% compared to traditional single-stage AD. The abundance and diversity of ARGs were significantly reduced in the acidogenic reactors after HTP. However, the ARGs levels increased by about two times in the following methanogenesis stage and reached similar or higher levels than in single stage AD. The erm(F), erm(G), ant(6)-Ia, tet(W), mef(A) and erm(B) were the six main ARGs with significant differences in relative abundances in various treatments. The two-stage AD mode could better remove sul2, but it also had a rebound which elevated the risk of ARGs to the environment and human health. Network analysis identified pH and TVFAs as critical factors driving microbial communities and ARG proliferation in the new AD process. With the results, this study offers valuable insights into the trade-offs between AD performance enhancement and ARG-related risks, pinpointing essential areas for future research and practical improvements.

Comparison of suspended versus immobilized digested sludge for 2-stage anaerobic digestion of algae

A 2-stage anaerobic digestion (AD) approach to extract biogas from algae using microbes immobilized in calcium-alginate (CA) beads is demonstrated here for the first time. The biomethane released from the 2nd stage of AD is 61% and 72% of the yield from single-stage AD, digesting whole alga and Post-Extracted Algae Residue (PEAR) of S.obliquus, respectively. However, additional 56scc/gVS and 32scc/gVS of biohydrogen produced from the 1st stage of AD makes the 2-stage AD more energy efficient. Adopting glucose as a model compound, the mass transfer behavior of the substrates in CA was studied. Diffusion time and diffusivity of glucose increased with increase in sodium alginate concentration while higher diffusivities of glucose were obtained in digested-sludge-loaded beads due to exclusion effect. The AD reaction is rate-limiting. The data in this work can be used to predict the residence time of substrates in packed bed reactors filled with microbes encapsulated in CA.

Benchmarking composting, anaerobic digestion and dark fermentation for apple vinasse management as a strategy for sustainable energy production

Looking for renewable energy sources is one of the main targets in the transition to low-carbon economies. Bioenergy appears as an alternative with great potential, but firstly, it requires addressing all aspects that can limit its environmental viability. Stillage has been identified as an environmental concern of the bioethanol production. Thus, eco-friendly strategies for valorizing this resource should be pursued. In this work, three strategies such as composting (non-energy valorization), one-stage anaerobic digestion (with the aim of biogas production), and dark fermentation followed by anaerobic digestion (scheme under development to produce biohydrogen and biogas) were evaluated under a life-cycle perspective; and through process modeling based on literature data. The aim is to identify the advantages and disadvantages of dark fermentation for the management of apple stillage. To do this, a cradle-to-gate and attributional framework were followed, considering impact and damage perspectives. Results showed a very significant environmental load (around 99%) of composting compared to the selected energy recovery schemes, considering the added value of compost and the associated emissions versus those of biofuels. Otherwise, small differences were found between single-stage anaerobic digestion and dark fermentation (up to 16%), mainly, due to changes in chemical consumption and organic matter degradation efficiency.

Hydrothermal pre-treatment followed by anaerobic digestion for the removal of tylosin and antibiotic resistance agents from poultry litter.

Hydrothermal pretreatment (HPT) followed by anaerobic digestion (AD) is an alternative for harvesting energy and removing organic contaminants from sewage sludge and animal manure. This study investigated the use, in an energetically sustainable way, of HPT and AD, alone or combined, to produce methane and remove tylosin and antimicrobial resistance genes (ARG) from poultry litter (PL). The results showed that HPT at 80°C (HPT80), followed by single-stage AD (AD-1S), led to the production of 517.9 ± 4.7 NL CH4 kg VS-1, resulting in 0.11 kWh kg PL-1 of electrical energy and 0.75MJkg PL-1 of thermal energy, thus supplying 33.6% of the energy spent on burning firewood at a typical farm. In this best-case scenario, the use of HPT alone reduced tylosin concentration from PL by 23.6%, while the process involving HPT followed by AD-1S led to the removal of 91.6% of such antibiotic. The combined process (HPT80 + AD-1S), in addition to contributing to reduce the absolute and relative abundances of ARG ermB (2.13 logs), intI1 (0.39 logs), sul1 (0.63 logs), and tetA (0.74 logs), led to a significant removal in the relative abundance of tylosin-resistant bacteria present in the poultry litter.

Sequential production of hydrogen and methane using hemicellulose hydrolysate from diluted acid pretreatment of sugarcane straw

Sugarcane straw is not effectively used in the industry currently. However, sugarcane straw hemicellulose hydrolyzate (HH) application as raw material for H2 and CH4 production is a promising alternative. In this work, the two-stage anaerobic digestion (TS-AD) approach led to 37.86 mLH2/L.h and 40.12 mLCH4/L.d, while the single-stage anaerobic digestion (SS-AD) generated 46.11 mLCH4/L.d. Hence, the two-stage process was energetically favorable than the single-stage by approximately 33%. Additionally, the comparison with standard medium (composed of glucose, xylose, and arabinose) applied as raw material indicated that although hydroxymethylfurfural and furfural from HH were not responsible for the decrease in H2 production, they extended the adaptive phase of methanogenic archaea during the methanogenesis. Hemicellulose hydrolysate is an attractive raw material for two-stage anaerobic digestion.

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.

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.

Effect of nitrogen and iron supplementation on the process performance and microbial community structure of a hydrogen-producing reactor continuously fed with tequila vinasse

Dark fermentation is gaining importance as a bioprocess able to valorize different organic wastes/wastewaters through the co-production of organic acids and biogenic hydrogen (bioH2). In this study, the process performance and microbial community structure of a mesophilic lab-scale fermenter inoculated with an enriched hydrogenogenic consortium and fed with undiluted tequila vinasse (TV) were evaluated for 28 days equivalent to 112 hydraulic retention times (HRT). The influence of external nutrients supply on process performance was carefully evaluated. The fermenter was operated at 6 h HRT and fed with i) TV without additional nutrients, ii) TV with additional nitrogen (0.3 g/L), and iii) TV supplemented with nitrogen (0.3 g/L) and iron (0.01 g/L) chemical sources. Despite the inherent presence of macro- and micronutrients in TV, the use of raw TV with no additional nutrients resulted in a very low hydrogen production rate of 24 NmL-H2 L−1h−1. Contrarily, a nitrogen dosing of 1.2 g-NH4Cl L−1 led to a sudden increase in the bioH2 production, reaching productivity and yield values of 283 NmL-H2 L−1h−1 and 82 NmL-H2 g−1-volatile solids added, respectively. The further addition of FeSO4·7H2O (0.05 g/L) did not improve the TV-to-bioH2 conversion. Bridging hydrogen production and organic acids with molecular analysis, 16S rRNA gene sequencing, indicated that maintaining a suitable C/N ratio in the fermenter could ensure balance and syntrophy between lactic acid bacteria (e.g., Sporolactobacillus, Lactobacillus) and bioH2 producers (Clostridium). Further biochemical methane potential tests with the fermentation broth demonstrated that biohythane process (bioH2 + CH4) resulted in 5 % higher bioenergy recovery than that derived from the single-stage anaerobic digestion.

Hydrogen and Methane Production by Single- and Two-Stage Anaerobic Digestion of Second Cheese Whey: Economic Performances and GHG Emissions Evaluation

This study aimed at evaluating the economic performances of and carbon footprint associated with innovative systems for the energetic valorization of second cheese whey (SCW), a by-product of whey cheese manufacture, through anaerobic digestion processes. Three systems were modeled: a conventional single-stage anaerobic digester (FAD), located at about 50 km from the dairy factory; an on-site conventional single-stage anaerobic digester (CAD), located at the dairy industry; and an on-site two-stage anaerobic digester (TAD). The TAD technology enables the simultaneous production of hydrogen and methane on site. The biogases produced were combusted in combined heat and power plants (CHP), but only the onsite systems provided process heat to the dairy factory. In the specific conditions assumed, TAD configuration exhibited a higher energy output, which led to a GHG emission reduction of about 60% compared to FAD, mostly thanks to the additional hydrogen (H2) production and the improved engine performances. A detailed cost analysis confirmed the results of the environmental analysis, pointing to the TAD solution as the most economically viable, with a payback period of 9 years, while the CAD had a payback time of 12 years. The results here presented aim at providing the dairy industry with a robust economic analysis on the opportunity of building an innovative system for SCW valorization, as well as providing policymakers with environmental reliable data to support the promotion of this technology.

Yak rumen fluid inoculum increases biogas production from sheep manure substrate

Hydrolytic bacteria are essential for the degradation of lignocellulose to produce biogas and organic fertilizers. In this study, sheep manure was used as substrate, and sheep manure slurry, yak rumen fluid and slurry from a biogas reactor (SBR) were used as inocula in single-stage anaerobic digestion. The SBR and rumen fluid inocula increased biogas production by 23% and 43%, respectively, when compared to solely sheep manure in the single-stage anaerobic digestion. The two-stage anaerobic digestion, with yak rumen fluid as inoculum in the hydrolytic reactor, increased the biogas production by 59, 86, and 58% compared with the control. Microbial analysis of the effluent revealed that yak rumen fluid contained hydrolytic bacteria such as Proteiniphilum, Jeotgalibaca, Fermentimonas, and Atopostipes to enhance the degradation of sheep manure and increase biogas production. It was concluded that yak rumen fluid, rich in hydrolytic bacteria, increases the degradability of sheep manure and improves production of volatile fatt acids and biogas.

Modelling of anaerobic digester for the conversion of organic waste into hydrogen & methane

Multistage Anaerobic digestion (AD) nowadays plays a beneficial role compared to single-stage AD to achieve a high rate of waste conversion with higher energy recovery. Hence, obtaining a stable operation with optimal operating conditions is of paramount importance in the modeling study. In the current study, modeling of two stages continuous stirred anaerobic digesters was considered using mass balances over the substrates (organic waste) and biomass in the respective reactor. All the simulation work was performed using MATLAB, from which steady-state concentration values were taken for the calculation of Hydrogen flow rate (QH2) and Methane flow rate (QCH4). The operating volumes of both the reactors were computed by looking at the effect on the production rates (QH2 &QCH4) as a function of dilution rate of first (D1) and second (D2) reactors for constant initial substrate concentration (S0). At last, the effect of Initial substrate concentration on both QH2 &QCH4 was studied by considering different S0 values as 4 g/L, 8 g/L, and 12 g/L. From our modeling result, it was found out that for Maximum Hydrogen production, operating D1 (in day-1) was 0.8, 0.9 & 1 and the Methane production rate was Maximum at operating D2 (inday-1) of 0.25, for S0 values of 4 g/L, 8 g/L, and 12 g/L respectively.

Dual role of grass clippings as buffering agent and biomass during anaerobic co-digestion with food waste

There is a dire need to replace the chemical buffers that regulate the redox environment in single-stage anaerobic digestion of food waste. Hence, the applicability of grass clippings as an eco-friendly buffering agent and biomass during the anaerobic co-digestion of food waste was explored. A focus was primarily given on the effects of grass clippings on the redox environment and acidogenesis. Concomitantly the production of volatile fatty acids, hydrogen and methane in mesophilic conditions was monitored. Organic load and substrate-to-inoculum ratio were kept constant in all the experiments, and no chemical buffer was used. The results revealed that the redox environment was regulated with 10% grass clippings by inhibiting rapid pH drop in the digester. The addition of 2, 4, and 6% grass clippings promoted acidogenesis with increased production of acetic and butyric acids, whereas 8 and 10% grass clippings promoted solventogenesis with ethyl alcohol production. Hydrogen generation from the experiments with grass clippings was in the range of 27–30% of the total biogas, which was marginally higher than the control (25%). Methane concentration was negligible in the biogas generated from all experiments. The acidification rate, VFA production/consumption rate, specific hydrogen yield, hydrogen conversion efficiency, and volatile solids removal were maximum and minimum in the reactors with 6 and 10% grass clippings, respectively. From the above results, it can be concluded that adding grass clippings to food waste would regulate the sudden pH changes and enhance the production of value-added biochemicals, making the process cost-effective.Graphic abstract

Biodegradation efficiencies and economic feasibility of single-stage and two-stage anaerobic digestion of desulfated Skim Latex Serum (SLS) by using rubber wood ash

The efficiencies of single-stage anaerobic digestion (SSAD) and two-stage anaerobic digestion (TSAD) of desulfated skim latex serum (DSLS) using various rubber wood ash (RWA) loadings were investigated in this study. The experiments on batch processes showed that DSLS gave a higher yield (6–21%) than raw SLS in both SSAD and TSAD. The highest H2 and CH4 yields of 90.64 and 294.53 mL/g-CODadded were achieved with DSLS using RWA loading of 5 g/L (DSLS5) and 10 g/L (DSLS10), respectively in TSAD (thermophilic and mesophilic conditions, respectively). The maximum 305.09 mL/g-CODadded CH4 yield in SSAD was observed for DSLS10. Total energy recovery in TSAD was 5% higher than that in SSAD. However, the cost assessment on continuous AD using kinetics and yield from the batch experiments suggests longer payback time for TSAD (4.36 years) than for SSAD (2.52 years). TSAD is not economically attractive with DSLS10 due to the large total volume of digesters required. This study revealed that RWA can remove sulfate from SLS to enhance biogas production and reduces H2S in the biogas, while TSAD of DSLS was not attractive compared to the conventional SSAD like for some other substrates reported in the literature.

Scale-up and energy estimations of single- and two-stage vinasse anaerobic digestion systems for hydrogen and methane production

This study evaluated the full-scale vinasse anaerobic digestion for industrial application. Twelve scenarios were simulated, considering single (methanogenic reactor) and two-stage systems (acidogenic + methanogenic reactors), raw and concentrated vinasse, thermophilic and mesophilic conditions for methanogenesis and usage of acidogenic or methanogenic reactor effluent to dilute vinasse. A single-stage anaerobic digestion system of 17,433 m³ necessary to treat all the raw vinasse produced by a mill with a grinding capacity of 3.3 million tons of sugarcane per season can generate a power of 11.1 MW with an energy yield of 4.6 MJ/kg CODremoved. Two-stage system scenarios where vinasse was diluted with acidogenic reactor effluent had worse energetic recoveries than the single-stage system. Nonetheless, the usage of the methanogenic reactor effluent to dilute vinasse and its own influent in a two-stage system achieved an energetic recovery at least 34% higher than that of the single-stage system. In the best two-stage scenario, the total working volume was 19,666 m³, combining thermophilic acidogenic reactors and mesophilic methanogenic reactors, with an energy yield of 7.0 MJ/kg CODremoved. The different combinations of temperatures allow for a safer and more economic operation. The proposed novel industrial configuration can be divided into two parallel acidogenic reactors of 1,800 m³ each, and those reactors are in series with eight parallel methanogenic reactors of 2,828 m³ each. The electricity produced by the full-scale system can supply an energy demand of 24 thousand inhabitants per harvest season (9.7 kWh per ton of sugarcane). This study presents a novel discussion for sugarcane industry decision makers, such as plant managers, process and project engineers, providing a trade-off between highest energy production, optimal COD removal and lowest reactor volume (CAPEX investment) that better fits their needs.

Enhancing Methane Production in a Two-Stage Anaerobic Digestion of Spent Mushroom Substrate and Chicken Manure via Activation of Sludge, Optimization of Temperature, and C/N Ratio

Management of spent mushroom substrate (SMS) is causing a global environmental concern due to tremendous increase in mushroom production globally. Therefore, in this research, the performance of a two-stage anaerobic co-digestion (TS-AD) of spent mushroom substrate and chicken manure was evaluated in terms of methane and biogas production and process stability with respect to single stage anaerobic digestion (SS-AD). Activation of anaerobic sludge using aeration or heat treatment in the first stage at mesophilic temperature followed by thermophilic co-digestion with chicken manure in the second stage was investigated. TS-AD exhibited better performance and enhanced methane generation over SS-AD. The optimal temperatures were determined as 35°C and 50°C for the first and the second stage of TS-AD, respectively. C/N ratio of 10 was the most suitable for biogas and methane production. TS-AD with C/N ratio of 10 and mesophilic digestion of SMS and sludge for 3 days at 35°C followed by co-digestion of the first stage effluent with chicken manure at 50°C was the optimized state producing 1359 mL of biogas of which 614.42 mL was methane, showing an increment by 59.44% in methane production as compared to SS-AD. TS-AD might be promising approach for utilization of SMS as feed stocks for biogas and methane production.

Two-stage anaerobic digestion of ozonated sewage sludge predominantly took over by acetotrophic methanogens with increased biogas and methane production

Ozonation of sewage sludge prior to single-stage anaerobic digestion (SAD) has been reported to increase biogas and/or methane production. However, no studies have evaluated the impact of ozone pre-treatment on two-stage anaerobic digestion (TAD) of sewage sludge over the production of these renewable fuels. Present study, therefore, aimed at investigating the impact of 0.06 g O3/g TSS ozone-pretreated sewage sludge on the productivity of mesophilic semi-batch lab-scale TAD and on its methanogenic consortium. For this purpose, a TAD configuration was constructed by two reactors connected in series to perform acidogenic and methanogenic phases separately. Measurements of methane, biogas and quantification of microbial consortiums were done by gas chromatography, water displacement and fluorescence in situ hybridization techniques, respectively. The ozone pretreatment results highlighted average daily solid removals between 51 and 64%. Ozonation of sewage sludge prior to TAD also yielded 69 and 38% more methane and biogas, respectively. This increase accounted for the predominance shiftfrom strictly hydrogenotrophic to acetotrophic methanogens in the ozonated TAD.

Anaerobic digestion of vinasse in fluidized bed reactors: Process robustness between two-stage thermophilic-thermophilic and thermophilic-mesophilic systems

Two-stage anaerobic digestion (AD) is a solution for the biodegradation of complex wastes, but the best association of temperatures between acidogenesis and methanogenesis is unclear. Our study investigated whether it is best to associate a thermophilic acidogenic reactor with a thermophilic or a mesophilic methanogenic reactor and results were compared to single-stage AD. The effect of increasing the organic loading rate (OLR from 2.7 to 24.7 kg COD/m3/d) in thermophilic and mesophilic second-stage methanogenesis of two-stage AD was also assessed. A thermophilic acidogenic reactor (R2-H2-Thermo) treating sugarcane stillage fed thermophilic (R2-CH4-Thermo) and mesophilic (R2-CH4-Meso) methanogenic reactors. A single-stage methanogenic thermophilic reactor was used as control (R1-CH4-Thermo). Reactors R2-CH4-Thermo and R2-CH4-Meso showed similar performance at OLR of 2.7–13.3 kg COD/m3/d, but the latter achieved the highest methane production rate at 24.7 kg COD/m3/d (3.2 L CH4/d/L – 78% higher than R2-CH4-Thermo). The two-stage system (R2-H2-Thermo + R2-CH4-Meso) achieved the best energetic yield: 5.5 kJ/g COD, which was 41% higher than the single-stage system. Operation of a second-stage reactor at mesophilic temperature is the most suitable for vinasse AD due to its flexibility, robustness, and stability of CH4 production, even in situations of organic overload and accumulation of toxic components. Digestate at ambient temperature is also more adequate for fertirrigation.