Two-stage Anaerobic Digestion System Research Articles

Effects of hydrothermal pretreatment on sulfadiazine degradation during two-stage anaerobic digestion of pig manure

Antibiotics in animal manure pose significant risks to the environment and health. While anaerobic digestion (AD) is commonly used for pig manure treatment, its efficiency in antibiotic removal has been considerably limited. This study investigated the impact of hydrothermal pretreatment (HTP) on sulfadiazine (SDZ) removal in a two-stage AD system. Results indicated that the HTP process reduced SDZ concentration by 40.61%. Furthermore, the SDZ removal efficiency of the AD system coupling HTP increased from 50.90% to 65.04% compared to the untreated system. Biogas yield was also improved by 26.17% while maintaining system stability. Changes induced by HTP in the microbial communities revealed that Firmicutes, Bacteroidetes, Caldatribacteriota, and Proteobacteria emerged as the primary bacterial phyla. Following HTP, the relative abundance of Prevotella, which exhibited a strong negative correlation with SDZ concentration, increased significantly by 25-fold in the acidogenic stage. Proteiniphilum, Syntrophomonas and Sedimentibacter showed notable increases in the methanogenic stage after HTP. The N-heterocyclic metabolism carried out by Prevotella might have been the predominant SDZ degradation pathway in the acidogenic stage, while the benzene ring metabolism and hydroxylation by the Proteiniphilum emerged as the primary degradation pathways in the methanogenic stages. Furthermore, biodegradation intermediates were proven to be less toxic than SDZ itself, indicating that the HTP-enhanced two-stage AD process could be a viable way to lower the environmental risks associated with SDZ. The findings from this study provide valuable insights for removing SDZ from the environment via two-stage AD.

Quorum-sensing molecules regulate biochar-assisted anaerobic digestion system for methane production: Single-stage vs. two-stage digestion

Promoting methane production performance from waste activated sludge is of great significance for energy self-sufficiency operation of wastewater treatment plants. This study evaluated the methane production enhancement strategies of biochar and quorum-sensing molecules like N-acylated homoserine lactone (AHL) in single-stage and two-stage anaerobic digestion systems. The methane yield increased to 134.9 mL/g volatile suspended solids in single-stage digestion system, which was 41.7 % higher than that of control group, while it in two-stage digestion system was not only lower than that obtained in single-stage digestion system but also less than that observed in groups adding AHL either at the beginning or on the third day of experiment period. The mechanism studies indicated that the single-stage digestion strategy promoted solubilization, hydrolysis, acetogenesis, and methanogenesis stages and improved the conversion and utilization of organic matters with the enhanced syntrophic metabolisms between microorganisms. In addition, the electron transfer activity between microorganisms was promoted, with an increase ratio of 54.7 %, meanwhile, the syntrophic relationships between methanogens such as Methanosarcina and electroactive bacteria such as norank_f__Bacteroidetes_vadinHA17 were promoted. As a result, both acetotrophic and hydrogenotrophic methanogenesis pathways were improved. This study provided some novel approaches for promoting anaerobic digestion performance while using biochar as the additive.

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 (24–30 %) 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 337.7–385.6 mL/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.

Comprehensive insights into the effects of acidogenic off-gas utilization on successive biogas production, microbial community structure and metabolite distribution during two-stage anaerobic digestion

Although two-stage anaerobic digestion (TSAD) technology has been investigated, the mechanisms regarding the impact of acidogenic off-gas (AOG) on successive methane production have not been well addressed. In this study, a novel TSAD system was designed. Food waste, as the main substrate, was co-digested with chicken manure and corn straw. The acidogenic gas beyond atmospheric pressure was introduced into the bottom of the methanogenesis reactor through a stainless steel diffuser. Results showed the addition of AOG increased the methane yield from 435.2 to 597.1 mL/g VSin in successive methanogenesis stage, improved by 37.2 %, and increased the energy yield from 9.0 to 11.3 kJ/g VSsubstrate. However, the theoretical contribution of hydrogenotrophic methanogenesis using H2 contained in AOG was only 15.2 % of the increased methane yield. After the addition of AOG, the decreased levels of ammonia nitrogen and butyrate indicate that the stability of the AD system was improved. The electron transfer system and co-enzyme F420 activity were enhanced; however, the decrease in acetate kinase activity indicates aceticlastic methanogenesis may have been weakened. The microbial diversity and species richness were improved by the added AOG. Methanosarcina was more competitive than Methanothermobacter, enhancing the syntrophic effect. The relative abundance of protein degradation bacteria norank_f_Anaerolineaceae and lipid degradation bacteria Syntrophomonas was increased. Metabolite analysis confirmed that the addition of AOG promoted amino acid metabolism, the biosynthesis of other secondary metabolism and lipid metabolism. The improved degradation of recalcitrant organic components (lipids and proteins) in food waste was responsible for the increased methane yield. This study provides an in-depth understanding of the impact of AOG utilization on successive methane production and has practical implications for the treatment of food waste.

The Advantage of Citrus Residues as Feedstock for Biogas Production: A Two-Stage Anaerobic Digestion System

Anaerobic digestion (AD) is an important step in waste recovery. In Colombia, the production of citrus food significantly contributes to environmental impact via waste generation. In 2021, the waste produced, specifically citrus rind, amounted to 725,035 tons/year. During degradation, wastes generate leachate and greenhouse gases (GHGs), which negatively impact water sources (leachate), soil, and human and animal health. This article describes the design of a two-phase biodigestion system for the degradation of organic matter and biogas production. The system uses citrus waste to produce biogas with neutral emissions. The biodigestion process begins with the stabilization of the methanogenesis reactor (UASB), which takes approximately 19 days. During this period, the biogas produced contains approximately 60% methane by volume. Subsequently, the packed bed reactor operates for 7 days, where hydrolytic and acetogenic bacteria decompose the citrus waste, leading to the production and accumulation of volatile fatty acids. The final step involves combining the two phases for 5 days, resulting in a daily biogas production ranging from 700 to 1100 mL. Of this biogas, 54.90% is methane (CH4) with a yield of 0.51 LCH4gSV−1. This study assesses the methane production capacity of citrus waste, with the process benefiting from the pH value of the leachate, enhancing its degradability. Consequently, this approach leads to a notable 27.30% reduction in solids within the digestion system. The two-phase anaerobic biodigestion system described in this article demonstrates a promising method to mitigate the environmental impact of citrus waste while concurrently producing a renewable source of energy.

Effects of organic loading rate on hydrogen and methane production in a novel two-stage reactor system: performance, enzyme activity and microbial structure

Two-stage anaerobic digestion (AD) system is an efficient technology; however, organic loading rate (OLR), a critical design and process parameter, influences the performance and capacity of digesters. This study investigates the impact of various OLRs on biogas production during co-digestion of food waste, chicken manure and corn straw in an innovatively designed two-stage reactor system, including variations in the enzyme and electron transfer system (ETS) activities and microbial community. The production of hydrogen and methane increased gradually with increasing OLR, with maximum production at 8 g VS/L d (690 mLH2/L d and 690 mLCH4/L d), which is 30–97 % higher than OLR of 1.5 to 6 g VS/L d. However, the maximum methane yield of 535 mL/g VSadded was attained at OLR of 4 g VS/L d consistent with the highest coenzyme F420 activity. Although no substantial ETS activity was seen during the acidogenic stage, the ETS activity decreased with increasing OLR in the methanogenesis stage, indicating that high substrate loading results in low syntrophic methanogenesis. Microbial analysis revealed a more diverse enrichment of acidogens and acetogens at high OLR with a wider range of substrate consumption, resulting in balanced acetoclastic and hydrogenotrophic methanogens (Methanosarcina and Methanothermobacter) in the biomethane process. These findings suggest the strengthening of syntrophic methanogenesis in high substrate-loading AD systems.

Effects of sudden shock load on simultaneous biohythane production in two-stage anerobic digestion of high-strength organic wastewater

The two-stage anaerobic digestion (AD) for biohythane production is a sustainable solution, but it is sensitive to organic shock load that disrupts reactors and inhibits biohythane production. This study investigated biohythane production, reactor performance, and the possibility of post-failure restoration in a two-stage AD system designed for treating high-strength organic wastewater. Sudden shock load was applied by increasing the OLR threefold higher after reaching steady state phase. During shock load phase, hydrogen content, hydrogen yield and methane production rate (MPR) reached its peak values of 62.61 %, 1.641 mol H2/mol glucose, and 1.003 L CH4/L⋅d respectively before declining significantly. Interestingly, during the restorative phase, hydrogen production sharply declined to nearly zero, while methane production exhibited a resilience and reached its peak methane content of 52.2 %. The study successfully demonstrated the system’s resilience to sudden shock load, ensuring stable methane production, while hydrogen production did not exhibit the same capability.

Changes in the prokaryotic communities occurring along a two-stage anaerobic digestion system treating the organic fraction of solid waste

Two-stage anaerobic digestion is useful for recycling of the organic fraction of municipal solid waste and wastewater; the prokaryotic communities involved in this process were characterized using massive sequencing of 16S rRNA gene amplicons. During the 60-day study the prokaryotic communities were stable in time but were affected by their position in the system. The first stage of solid waste digestion was operated in an anaerobic-hydrolytic leach bead reactor with high organic solids content, the microbial communities were dominated by members of the orders Bacteroidales (62%), Selenomonadales (15%), Lactobacillales (13%), Clostridiales (4%) and Rhodospirillales (1%). At this stage, a vertical stratification acted as a transition zone from aerobic to anaerobic environments modifying significantly the composition and structure of bacterial communities.The second stage occurred in an up-flow anaerobic sludge bed reactor specialized in acidogenesis-methanogenesis, where the leachate of the first stage was co-fed with municipal wastewater. In this stage, the bacterial community was dominated by members of the orders Pseudomonadales, Clostridiales, Burkholderiales, Bacteroidales and Syntrophobacterales (accounting for 52%), while the most abundant orders of the archaeal community corresponded to Methanosarcinales (72%) and Methanobacteriales (13%), both of them with methanogenic capacity.

Enhancement of biogas production from industrial solid pineapple wastes by two-stage anaerobic digestion systems

Enhancement of biogas production from industrial solid pineapple wastes by two-stage anaerobic digestion systems

Application of an Integrated Granular and Suspended Sludge Methane Reactor for a Two-Stage Anaerobic Digestion System to Deal with Biodegradable Municipal Solid Waste

This study aims to investigate the performance of a two-stage anaerobic digestion system using a hybrid methane reactor to deal with biodegradable municipal solid waste. The reactor allowed both suspended sludge and granular sludge to work together. The feedstock was fermented in one continuous stirred tank at different pH conditions for 5 d. Furthermore, the liquid hydrolysate was diluted and pumped into a methane reactor with different organic loading rates. In the fermentative reactor, raising the pH condition from 4.5 to 6.5 caused a sharp increase in volatile fatty acids concentration, mainly due to the increase in acetate and propionate. The efficiency of the methane reactor was proven by the results of hydrodynamic analysis and biogas production. The relationship between biogas production and operating parameters in this reactor was modeled using a quadratic multivariate regression model. Overall, by maintaining the fermentative reactor at a pH of 6.0–6.5, the methane reactor was able to achieve an organic loading rate of 7.6 g-TS.L−1·d−1 with outstanding biogas quality and yield. In terms of microbiology, the most dominant phyla in the reactor included Firmicutes, Bacteroidetes, Proteobacteria, Euryarchaeota, Synergistetes, and Chloroflexi. Among them, the species with the highest relative abundance in granular sludge was Firmicutes, while that in suspended sludge was Bacteroidetes.

Digestate recirculation rate optimization for the enhancement of hydrogen production: The case of disposable nappies and fruit/vegetable waste valorization in a mesophilic two-stage anaerobic digestion system

After the anaerobic digestion involvement in waste management, several attempts have been made for the biogas yields enhancement. The implementation of methanogenic effluent (digestate) recirculation has been proposed as an alternative for the pH adjustment in two-stage reactors, leading to decreased alkali solution requirements, which could subsequently reduce operational costs. In the current study, a mixture of 40–60% (w/w) fruits/vegetable waste-disposable nappies hydrolysate was valorized through the anaerobic digestion process. The digestate recirculation rate (RR) optimization took place in a continuous acidogenic reactor, followed by the two-stage system operation under various experimental conditions. Particularly, the RRs of 0-0.25-0.5-0.75-1 were tested, indicating up to 208% H2 production increase (RR0.25). The single- and two-stage configurations comparison followed, under the hydraulic retention time (HRT) of 25 d, while the two-stage operation was evaluated for pH adjustment (5.5) not only with alkali solution but also with the applied RR0.5 (reduced alkali solution requirements compared to RR0.25). During the two-stage system operation under RR, additional scenarios were investigated regarding the HRTmethanogenic reduction and the pH adjustment, while using exclusively digestate. Although the RR implementation seems promising, the digestate alkalinity content can significantly determine the system's efficiency.

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.

A Novel Method Using Two-stage Anaerobic Digestion Integrated with Greenhouse Solar Dryer for Increased Biogas Production

Solar energy is infinite and environmental-friendly energy that is widely used in many fields such as solar cells, solar roofs, solar dryers and so on. However, solar energy has never been used in anaerobic digestion processes for biogas production. The use of solar energy may accelerate the biogas reaction to achieve higher gas production rates. Hence, the purpose of this research was to study a two-stage anaerobic digestion system in combination with a greenhouse solar dryer to increase the rate of biogas production. The study consisted of two parts: the first was to study the temperature variations in greenhouse solar dryers using computational fluid dynamic techniques, and the second was to conduct two-stage anaerobic digestion integrated with the greenhouse solar dryer system for biogas production. The results showed that the average temperature and biogas production rate in the integrated two-stage anaerobic digestion system compared to a conventional system increased by 16.2% and 19.69 %, respectively

Effects of adding Thermoanaerobacterium thermosaccharolyticum in the hydrogen production stage of a two-stage anaerobic digestion system on hydrogen-methane production and microbial communities

Microorganisms play a vital role in anaerobic digestion, however, bioaugmentation in a two-stage co-digestion system to directly regulate the microbial structure is limited. In this study, the different dosage of Thermoanaerobacterium thermosaccharolyticum was added in the hydrogen production stage of a two-stage anaerobic digestion system operating at a thermophilic temperature (55 °C). Results showed that Thermoanaerobacterium thermosaccharolyticum of 1.12 g had the best effect, with the cumulative hydrogen and methane yields reaching 81.54 mL/g VS and 550.98 mL/g VS respectively, which were 68.72 % and 84.45 % higher than the control group. According to the microbial analysis, the addition of Thermoanaerobacterium thermosaccharolyticum significantly changed the microbial community structure, and also its relative abundance increased in the hydrogen production stage, thereby increasing volatile fatty acids (VFAs) and hydrogen content. Although the methane production stage did not contain Thermoanaerobacterium thermosaccharolyticum, the microbial community structure changed, increasing the dominant species Anaerolineae and Limnochordia, thereby improving the efficiency of substrate decomposition and methane production. This study shows that Thermoanaerobacterium thermosaccharolyticum can promote not only hydrogen production but also the production of methane in two-stage anaerobic digestion.

Feasibility of successive hydrogen and methane production in a single-reactor configuration of batch anaerobic digestion through bioaugmentation and stimulation of hydrogenase activity and direct interspecies electron transfer

Two-stage anaerobic digestion is a promising way to increase the efficiency of decomposition of organic waste. However, spatial separation of stages makes this technology less economically attractive. In this work, we studied a model of two-stage anaerobic digestion with stages separated in time in one reactor. To implement this model, we used a combination of (1) bioaugmentation with a hydrogen-producing Thermoanaerobacterium thermosaccharolyticum SP-H2, resistant to low pH, (2) setting a low initial pH (5.5) to reduce the activity of hydrogenotrophic methanogens, (3) supplementing additives in the form of soluble iron (II) and granular activated carbon (GAC) to stimulate hydrogenase activity and direct interspecies electron transfer (DIET), respectively. For comparison, two methanogenic inoculums with different dominant microbial groups and initial iron concentrations were used. With the simultaneous addition of GAC and iron (II), sequential production of hydrogen and methane was observed for both methanogenic inoculums. Without the addition of GAC, methane formation was practically not observed, which, apparently, indicated the impossibility of syntrophic degradation of acidogenesis products due to the high partial pressure of hydrogen and the absence of conductive material for activation of DIET. Significant differences in hydrogenase activity and the kinetics of hydrogen and methane formation according to the modified Gompertz equation depending on the iron content in the system may indicate the importance of this additive. Further studies should be aimed at determining the optimal initial pH, concentrations of stimulating additives, bioaugmentation and methanogenic cultures, as well as confirming the stability of the proposed single-reactor two-stage anaerobic digestion system in semi-continuous mode.

Microbial communities and metabolic pathways involved in reductive decolorization of an azo dye in a two-stage AD system

Multiple stage anaerobic system was found to be an effective strategy for reductive decolorization of azo dyes in the presence of sulfate. Bulk color removal (56–90%) was achieved concomitant with acidogenic activity in the 1st-stage reactor (R1), while organic matter removal (≤100%) and sulfate reduction (≤100%) occurred predominantly in the 2nd-stage reactor (R2). However, azo dye reduction mechanism and metabolic routes involved remain unclear. The involved microbial communities and conditions affecting the azo dye removal in a two-stage anaerobic digestion (AD) system were elucidated using amplicon sequencing (16S rRNA, fhs, dsrB and mcrA) and correlation analysis. Reductive decolorization was found to be co-metabolic and mainly associated with hydrogen-producing pathways. We also found evidence of the involvement of an azoreductase from Lactococcus lactis. Bacterial community in R1 was sensitive and shifted in the presence of the azo dye, while microorganisms in R2 were more protected. Higher diversity of syntrophic-acetate oxidizers, sulfate reducers and methanogens in R2 highlights the role of the 2nd-stage in organic matter and sulfate removals, and these communities might be involved in further transformations of the azo dye reduction products. The results improve our understanding on the role of different microbial communities in anaerobic treatment of azo dyes and can help in the design of better solutions for the treatment of textile effluents.

Long term performance of pilot methanogenic digester filled with seashell wastes treating slaughterhouse wastes: Biogas production and environmental impact

High ammonia and organic acids contents of decomposed slaughterhouse waste (SHW) often result in the severe inhibition or even failure of the anaerobic digestion (AD). The two-stage AD system can alleviate the ammonia inhibition; however, addition of alkali is often needed to maintain the neutral condition. In this study, seashell wastes containing high percentage of calcium carbonate (CaCO3) were for the first time added in a pilot scale methanogenic digester treating chicken blood waste to retain the microorganisms and provide pH buffers for the substrate. A long-duration performance of this digester was conducted for 400 days under different organic loading rates (OLRs). The results show that methane yields of the digester were 244.8 and 224.9 mL g−1 CODadded at OLR of 0.4 and 0.6 g COD L−1 d−1, respectively, higher than the results of previous studies treating the similar feedstock. An AD system treating blood wastes of a typical poultry slaughterhouse was also proposed for which energy balance and carbon footprint analyses were conducted. The energy production of 0.35 MJ kg−1 blood and carbon emissions reduction of 0.05 kg CO2-eq kg−1 blood can be achieved from treating blood waste of the broiler chicken. This study reveals that using seashell wastes in an AD treating SHW can facilitate the reuse of aquaculture wastes and improve the sustainability of meat industry.

Anaerobic digestion of organic fraction of municipal solid waste using a novel two-stage solid-liquid system

Organic fraction of municipal solid waste (OFMSW) has high volatile solid and moisture content, which is mainly treated via anaerobic digestion (AD). Since the composition of OFMSW is complicated and inconstant, it is difficult to maintain the AD process stable and effective. In this study, a novel two-stage AD system, including a tank-type acidogenic phase and a fixed-bed methanogenic phase, was implemented for OFMSW treatment to demonstrate the feasibility of the two-stage configuration. Results showed that 84% of potential CH4 yield was achieved and the methane content reached 77.83% on average in methanogenic phase. The acetoclastic methanogens, Methanosaeta and Methanosarcina, which can participate in direct interspecies electron transfer, were the dominant genera in solid digestate and the biofilm. The hydrogenotrophic methanogens, Methanospirillum and Methanoculleus, accounted for high proportions in liquid digestate. Our study provides a novel approach to treat OFMSW with less water consumption, high stability and efficiency.

Nonlinear Optimization-Based Robust Control Approach for a Two-Stage Anaerobic Digestion Process

A two-stage anaerobic digestion (AD) process has been applied to improve the efficiency of methane production from various organic materials. However, the performance of traditional process controllers may be limited by differences in the rate of biochemical reactions, process uncertainties, and the consequences of interconnection between the two bioreactors. In this work, a nonlinear optimization-based control strategy that applies an analytical model predictive control (AMPC) scheme with an adaptive optimal set-point is proposed for the control of the two-stage AD system. The objectives of the proposed control system are to stabilize the system under uncertain operating conditions and maximize biomethane production. The optimal set-points for the controller are adapted in real-time operation, and then the control system is performed to manipulate the controlled output to the optimal trajectories. Compensators and nonlinear state observers are applied to handle the process/model mismatch and estimate unmeasured variables. The proposed control system is applied to the process with disturbances, fluctuations of inlet stream concentrations, and changes in the bacterial growth rate, and the control performance is investigated. Simulation results show that the developed control scheme automatically adjusts the optimal set-points and provides adequate control actions to maintain the maximum rate of methane production. The results of this investigation demonstrate that the control strategy promotes different biochemical reactions, avoids the inhibition effect, and handles the mutual effects between acidogenic and methanogenic bioreactors for methane production effectively.

Multi-advantageous sorghum as feedstock for biogas production: A comparison between single-stage and two-stage anaerobic digestion systems.

Anaerobic digestion (AD) is a keystone in the circular economy concept . The produced biogas can be demand-driven controlled and regionally adaptable, but sustainable feedstocks are required. Sorghum is a valuable alternative considering its bioenergy potential and wide adaptability to different environmental conditions. In this study, AD of sorghum silage was performed in a single-stage continuously stirred tank reactor (CSTR) and in a two-stage leach-bed reactor (LBR) system. The two-stage LBR system revealed higher methane yields (434 L N kg VS −1 ) compared to the single-stage CSTR (308 L N kg VS −1 ). Methane content of biogas ranged from 53 to 59% in both systems. Only the CSTR exhibited acids accumulation, led by propionic acid concentration (2.0 g L −1 ). Buffer capacity of the two-stage LBR system was superior to the CSTR. Technical disruptions and constant ammonium content declination were shown in both systems, associated to inherent characteristics of fibrous sorghum. AD of sorghum biomass, however, can be effectively performed under controlled conditions, generating high methane yields in both types of systems. • Mesophilic AD of fibrous sorghum in a single- and in a two-stage digestion system. • Accumulation of volatile fatty acids occurred during experiments in CSTR. • Two-stage system produced higher methane yields and showed better process stability. • A constant declination of the ammonium-nitrogen content was observed. • Fibrous sorghum caused technical disruption in both types of system.