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•Acidogenesis of citrus waste was better at controlled pH 6 at low substrate loading.•Addition of inhibitor for methanogens did not significantly increase VFA yield.•The presence of ...initial O2 did not negatively impact the VFA yield.•D-Limonene and peel oil also can also inhibit acidification of citrus waste.
Citrus waste from e.g., juice production is a potential substrate for anaerobic digestion (AD). However, due to the toxic citrus peel oil content, citrus waste has several challenges in biogas production. Hence, volatile fatty acids (VFAs) are very interesting intermediate products of AD. This paper was aimed to investigate VFA production from citrus wastes by boosting its production and inhibiting methane formation. Therefore, the effects of inoculum to substrate ratio (ISR), O2 presence, pH, and inhibitor for methanogens, in VFA production from citrus waste through acidification process were studied. The addition of 2 g/L methanogens inhibitor and the presence of O2 in the reactors were able to reduce methane production. The highest yield of VFA (0.793 g VFA/g VSadded) was achieved at controlled pH at 6 and low substrate loading (ISR 1:1). Acetic acid (32%), caproic acid (21%), and butyric acid (15%) dominate the VFA composition in this condition.
The popularization of large-scale biogas project makes the disposal of fermentative residue an urgent issue to be solved. Hydrothermal carbonization (HTC) technology is suitable for treating wet ...biomass to produce carbonaceous materials. In this study, the solid residue from the two-phase anaerobic digestion (AD) was hydrothermally converted in the range of 180–240 °C, and the hydrochar and aqueous components were characterized for subsequent utilization. The heating values of hydrochar were indicated to be increased by 14.2% and 16.6% at 210 °C and 240 °C as compared with feedstock, and also the specific surface areas were 34.8 m2/g and 27.1 m2/g with 17.4- and 13.3-fold enhancement, respectively. The migration of elements such as S, Cl, K to aqueous phase was beneficial for fuel application. The mesoporous pores were dominant in hydrochars with ample oxygenated functional groups. In addition, the wastewater involved organic acids, phenols, and nitrogen-containing compounds, etc. Evaluating the biodegradability by AD, it was found that when the initial concentration was ≤8 g COD/L, the maximum methane yields up to 275.9 mL CH4/g CODremoval and 277.6 mL CH4/g CODremoval were obtained. The enhanced toxicity/inhibition of representative pollutants on microorganisms was significant at higher organic loading, which could be indicated in the microbial structure and diversity. As a conclusion, the integrated production of hydrochar and methane will provide an extended route for further processing of lignocellulosic fermentative residue.
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•Moderate temperature was beneficial for the formation of high-performance hydrochar.•The exhibited characteristics of hydrochar made HTC more suitable to treat wet biomass.•The fermentative residue was conducive to the formation of porous structure.•The biodegradability and toxicity during AD of HTCWW was further demonstrated.•The production of hydrochar and methane was integrated to maximize the energy recovery.
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•Recent pretreatment techniques for improving anaerobic digestion of sewage sludge were reviewed.•Sludge pretreatment ameliorates anaerobic digestion and increases the biogas ...yield.•Pretreatment technique efficiency depends on sewage sludge composition.•Microbes play a crucial role in sewage sludge stabilization.•Very few pretreatment techniques are suitable for full-scale implementation.
Anaerobic digestion (AD) of sewage sludge is one of the most efficient, effective, and environmentally sustainable remediation techniques; however, the presence of complex floc structures, hard cell walls, and large amounts of molecular organic matter in the sludge hinder AD hydrolysis. Consequently, sewage sludge pretreatment is a prerequisite to accelerate hydrolysis and improve AD efficiency. This review focuses on pretreatment techniques for improving sewage sludge AD, which include mechanical, chemical, thermal, and biological processes. The various pretreatment process effects are discussed in terms of advantages and disadvantages, including their effectiveness, and recent achievements are reviewed for improved biogas production.
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•Thermophilic food waste digestion with varied particle size of biochar was examined.•All examined biochars (<50 μm to 3 cm) significantly improved methane production.•Bacteria ...Thermotogae and methanogens Methanothermobacter &Methanosarcina enriched.•The synergy of hydrogenotrophic and acetoclastic methanogenic pathways was achieved.•Both technical and economic feasibility of adding biochar strategy were validated.
Effectiveness of biochar addition to enhance thermophilic semi-continuous anaerobic digestion (AD) of food waste for methane production was investigated with a focus on dosage and particle size of biochar, pilot-scale application and elucidation of methanogenic pathways. Optimal dosage range of biochar was determined as 7.5 to 15 g per L working volume based on lab-scale batch AD. Effects of biochar with different particle sizes at a model dosage of 15 g/L were evaluated in a semi-continuous AD experiment, results of which showed that all the examined biochars with different particle sizes (<50 μm to 3 cm) substantially enhanced the average methane yields (0.465–0.543 L/gVS) compared to control digesters which failed due to overloading (≥3.04 gVS/L/d). No significant difference in methane yields, however, was observed among digesters with different particle sizes of biochars, except for 1–3 cm. The core reason for this phenomenon was that the biochars with different particle sizes had similar properties (e.g. density, surface area and pore size) and that the floating of large particle size (1–3 cm) of biochar with a density of 847 kg/m3 was not conducive to microbial growth. Metagenomic analysis was performed to determine the predominant microbial species and to explain the main methanogenic pathways in biochar-amended digesters using 16S rRNA sequencing. In the biochar-amended digester, bacterial phylum Thermotogae containing a major genus of Defluviitoga was selectively enriched with gradual increase of organic loadings, while simultaneously enriched methanogen genera Methanothermobacter and Methanosarcina, which showed a synergy of hydrogenotrophic and acetoclastic methanogenic pathways, jointly enhanced the methane productivity. Both technical feasibility and economic feasibility of adding biochar with simple pretreatment (e.g. smash) were validated in the pilot-scale thermophilic semi-continuous AD operations.
The dense structure of lignocellulosic biomass hinders efficient energy recovery from anaerobic digestion (AD). However, microorganisms or specific compounds can effectively break down ...lignocellulosic structure, increasing energy generation efficiency. Biogas slurry (BS) contains microorganisms and inorganic components that can act on lignocellulose. This study investigated the effects of BS on wheat straw (WS) pretreatment, focusing on key components such as ammonia nitrogen, acids, microorganisms, and metal ions in BS. To evaluate the mechanism, three-dimension excitation emission matrix (3D-EEM), fourier transform infrared spectrometer (FTIR), X-Ray diffraction (XRD), and lignocellulose composition analysis of were used. The effect of pretreatment on methane production was assessed using batch AD and the modified Gompertz model. Additionally, an economic analysis was conducted to compare different pretreatment methods. The results indicated that BS pretreatment significantly improved biodegradability (increased by 32.6 %), reduced crystallinity (by 24.6 %), and modified the composition of lignocellulose components. Furthermore, the abundance of hydrolysis and acidification bacteria, as well as acetoclastic methanogen, increased in the AD system after WS pretreatment with BS. Consequently, the methane production potential of WS increased by 23.8 %. Notably, ammonia nitrogen and microorganisms emerged as the primary active components in BS, exhibiting a synergistic reinforcing effect. This synergy significantly contributed to the breakdown of lignocellulose structure and improved the effectiveness of WS pretreatment. Moreover, the economic analysis demonstrated that BS pretreatment resulted in 33.2 % higher economic benefits compared to no pretreatment. These findings highlight the promising potential of BS pretreatment as a cost-effective and efficient strategy for improving biogas production from lignocellulosic biomass.
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•A green pretreatment of straw method for methane production was proposed.•BS changed the biodegradability (+32.6 %) and crystallinity (-24.6 %) of straw.•Methane yield and benefit increased by 23.8 % and 33.2 % after pretreatment.•Microorganisms and ammonia were the key components for pretreatment.
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•The disintegration rate constant affects the methane production.•The hydrogen concentration strongly affects the inhibition of acetogenesis.•Acetogenic microorganisms are sensitive ...to inhibitory concentration of ammonia.•The hydrogen inhibition constant is the most sensitivity parameter.
This article proposes a modification of the Anaerobic Digestion Model No. 1 (ADM1) to simulate biogas production in a full-scale agricultural biogas plant. The structure of the model was modified by dividing carbohydrates into three fractions: starch, cellulose, and hemicelluloses, and by dividing the protein fraction into rapidly and slowly degraded proteins. Lactic acid was also introduced to the model as a component of maize silage. The model was calibrated in several stages. In the first stage, model coefficients were selected based on a review of the literature to ensure that the results of the simulation were logically correct. In the following stage, the results of the sensitivity analysis were used to select parameters that significantly influenced the modeled results. The final values of the parameters were determined by analyzing the model’s accuracy within the 95% confidence interval. The variance of model coefficients, used in the analysis of confidence intervals, was determined based on the calculated optimal values. Even when biogas production suddenly decreased around day 225 of the experiment, due to a lack of fresh substrate, the model predictions were accurate. In real-world processes, a considerable decrease in biogas production is observed only after several days. The value of the Nash-Sutcliff efficiency coefficient (NSE = 0.5665) calculated for the calibration dataset demonstrated that the modified model fit the experimental data well. Among the inhibition constants, the inhibition constants for the uptake of propionic acid (KI_H2_pro = 4.6·10−8 kg COD·m−3), butyric acid, and valeric acid (KI_H2_c4 = 5.0·10−8 kg COD·m−3) indicate that the hydrogen concentration strongly affects the inhibition of acetogenesis. The hydrogen concentration should be monitored because an increase predicts process failure.
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•Rice straw was pretreated with liquid fraction of biogas digestion and hot water.•The straw was subjected to enzymatic hydrolysis and SSF processes.•The methane produced via solid ...state and liquid anaerobic digestion and compared.•Rice straw composition, crystallinity, and porosity were affected by pretreatment.
Effluent of biogas digestate, a waste stream with serious environmental problems, was used to pretreat rice straw in order to improve biofuel production. To investigate the effect of components presented in this waste stream and compare the results, water was also applied at the same conditions for the pretreatment. The straw was pretreated at 130, 160, and 190 °C for 30 and 60 min and subjected to enzymatic hydrolysis, simultaneous saccharification and fermentation (SSF), liquid anaerobic digestion (L-AD), and dry anaerobic digestion (D-AD). The highest improvements in hydrolysis and ethanol yield were 100 and 125%, achieved from the straw pretreated with effluent of biogas digestate at 190 °C for 60 min. The best methane yield was obtained through L-AD and D-AD of straw pretreated at 190 °C and 30 min with effluent of biogas digestate, resulting in 24 and 26% enhancements in produced methane. However, treatment with water had no significant effect on methane yield. Compositional and FTIR analyses indicated hemicellulose omission through the treatment under severe conditions. Furthermore, SEM images showed major enhancement in straw porosity by the pretreatment.
•Two-stage and one-stage AD of vinasse were investigated and compared.•Two-stage AD of vinasse from Chinese liquor industry was first conducted.•Lag-phase and T80 of two-stage AD were shorter than ...that of one-stage.•The energy recovery and VS removal efficiency of two-stage AD was higher.•Two-stage fermentation was more suitable for the AD of vinasse.
Vinasse is mainly produced during the brewing or liquor production, which is an potential substrate for biogas production. However, due to the rich content of easily degradation components in vinasse, the fermentation system during the initial AD stage of vinasse is easily acidifying, which leads to the fermentation system unstable, even failed. In this study, mesophilic hydrogen and methane production from vinasse through two-stage anaerobic digestion was investigated. In addition, one-stage anaerobic digestion of vinasse was also conducted to compare with two-stage anaerobic digestion. During two-stage anaerobic digestion, the hydrogen and methane yield were 14.8 and 274ml/g VSsubstrate, respectively. The methane yield from two-stage anaerobic digestion was 10.8% higher than that of one-stage. During the methane production process, the lag-phase of two-stage anaerobic digestion was 9.1days less than that of one-stage. In addition, the VS removal efficiency and energy recovery of two-stage anaerobic digestion were 10.4% and 12.9% higher than those of one-stage. Two-stage anaerobic digestion for hydrogen and methane production could efficiently improve the substrate utilization efficiency and energy recovery of vinasse.
This study evaluated the feasibility of H2 and CH4 production in two-stage thermophilic (55 °C) anaerobic digestion of sugarcane stillage (5,000 to 10,000 mg COD.L−1) using an acidogenic anaerobic ...fluidized bed reactor (AFBR-A) with a hydraulic retention time (HRT) of 4 h and a methanogenic AFBR (AFBR-S) with HRTs of 24 h–10 h. To compare two-stage digestion with single-stage digestion, a third methanogenic reactor (AFBR-M) with a HRT of 24 h was fed with increasing stillage concentrations (5,000 to 10,000 mg COD.L−1). The AFBR-M produced a methane content of 68.4 ± 7.2%, a maximum yield of 0.30 ± 0.04 L CH4.g COD−1, a production rate of 3.78 ± 0.40 L CH4.day−1.L−1 and a COD removal of 73.2 ± 5.0% at an organic loading rate (OLR) of 7.5 kg COD.m−3.day−1. In contrast, the two-stage AFBR-A system produced a hydrogen content of 23.9 ± 5.6%, a production rate of 1.30 ± 0.16 L H2.day−1.L−1 and a yield of 0.34 ± 0.08 mmol H2.g CODap−1. Additionally, the decrease in the HRT from 18 h to 10 h in the AFBR-S favored a higher methane production, improving the maximum methane content (74.5 ± 6.0%), production rate (5.57 ± 0.38 L CH4.day−1.L−1) and yield (0.26 ± 0.06 L CH4.g COD−1) at an OLR of 21.6 kg COD.m−3.day−1 (HRT of 10 h) with a total COD removal of 70.1 ± 7.1%. Under the applied COD of 10,000 mg L−1, the two-stage system showed a 52.8% higher energy yield than the single-stage anaerobic digestion system. These results show that, relative to a single-stage system, two-stage anaerobic digestion systems produce more hydrogen and methane while achieving similar treatment efficiencies.
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•Two-stage AFBR was more efficient for stillage digestion than the one stage AFBR.•Lower HRT (10 h) favors CH4 production (5.57 ± 0.38 L CH4.day−1.L−1) two-stage AFBR.•52.8% higher energy yield in H2–CH4 system under the applied COD of 10,000 mg. L−1.•Better stability of the two-stage system in the OLR of 17.1 kg COD.m−3.day−1.•Max HY (0.34 mmol H2.g COD-1) and MY (0.26 ± 0.06 L CH4.g COD-1) in H2–CH4 system.
Vinegar residue (VR), a major by‐product in the vinegar brewing process, is generated at least three million tons annually in China. Currently major part of the disposal of VR is fulfilled by ...landfill and incineration, however, serious environment pollution along with such processes is a challenging issue. Anaerobic digestion (AD) can provide benefits of environment and renewable methane generation, which seems to be a reasonable alternative to traditional treatment. Noticeably, total solids (TS) is an important influential parameter to AD performance. Therefore, VR was evaluated for methane production and associated kinetic characteristics during the liquid‐state AD (LS‐AD) and solid‐state AD (SS‐AD) in this study. The results indicated that the highest specific methane yield of 194.8 mL gVS−1 and volumetric methane yield of 9.4 Lmethane Lreactor volume−1 were achieved in LS‐AD (TS = 10%) and SS‐AD (TS = 20%), respectively. The kinetic analysis showed a higher specific methane production rate for LS‐AD than SS‐AD, while volumetric methane production rate presented a reverse trend. This study provides useful guidance for the sustainable utilization of VR while contributes to environmental protection and alleviates the dependence on fossil fuels.
