Asparagus stem as a new lignocellulosic biomass feedstock for anaerobic digestion: increasing hydrolysis rate, methane production and biodegradability by alkaline pretreatment

Effects of hematite on two types of dissolved organic compounds in lignocellulosic anaerobic hydrolysate: Lignin-derived aromatic compounds and denitrifying carbon sources

The utilization of anaerobic hydrolysate from agroforestry wastes is limited by dissolved lignin and aromatics, which have received insufficient attention despite their potential as excellent carbon sources for denitrification. This study aims to investigate the influence of hematite on lignin-derived aromatic compounds and denitrifying carbon sources, as well as to identify iron-reducing bacteria that utilize lignin-derived aromatic compounds as electron donors. The findings revealed that hematite facilitated the anaerobic fermentation of plant biomass, resulting in the production of small molecular organic acids. Moreover, biodegradation of lignin-derived aromatic compounds led to the formation of phenolic acids, while an increased generation of denitrifying carbon sources enhanced nitrogen removal efficiency by 13.84 %. Additionally, due to adsorption by hematite and subsequent microbial degradation, there was a significant improvement (40.32%) in color removal rate within denitrification effluent. Notably, Azonexus strains were hypothesized to be involved in Fe(Ⅲ) reduction coupled with aromatic compounds oxidation.

Holistic biorefinery approach for biogas and hydrogen production: Integration of anaerobic digestion with hydrothermal carbonization and steam gasification

Recently, the integration of biochemical and thermochemical processes is recognized as a promising strategy for the valorization of lignocellulosic biomass into renewable energy production. In this study, different routes for the valorization of hemp hurd for biohydrogen and biomethane production were proposed, including anaerobic digestion (AD), hydrothermal carbonization, and steam gasification. AD results revealed that NaOH pre-treatment of hemp hurd improved biomethane production yield by 164%. Comparing hydrochars from raw hemp, digestate derived hydrochars had higher mass yield due to changes in composition during AD as well as high ash content of digestates. It was found that high ash content of digestates originated from inorganic compounds in inoculum that accumulated over hemp hurd during anaerobic digestion process. Among feedstocks (hydrochars and raw hemp hurd), hemp hurd derived hydrochar at 200 °C showed the best performance in terms of H2 yield (1278 mL/g) whereas carbon efficiency reached % 92 in case of digestate derived hydrochar at 200 °C. HTC improved the steam gasification performance of hemp hurd whereas hydrochars from NaOH pretreated digestate yielded lowest hydrogen production due to the high content of inorganics, particularly phosphorus (P) and aluminum (Al). According to BMP test, spent liquor obtained at the lowest HTC temperature (200 °C) exhibited the highest BMP, reaching 213 mL CH4/g COD. Considering the overall gas products of four different routes, it is concluded that HTC as a post-treatment exhibits slightly better performance than HTC as pre-treatment. Although alkali pretreatment enhanced the anaerobic digestion performance, the resulting hydrochars exhibited low gasification activity.

A simplified model for simulating anaerobic digesters: Application to valorisation of bagasse and distillery spent wash

Current anaerobic digestion (AD) design methods rely on crude empirical models or sophisticated anaerobic digestion models (like ADM1) requiring a large number of parameters which are difficult to obtain experimentally. A simplified model for simulating AD was developed in this work. The model requires knowledge of CH₄/CO₂ ratio in biogas or indigestible fraction in substrate and batch biomethane potential (BMP) data for estimating three kinetic parameters (maximum specific growth rate, half velocity constant and cell death rate). Reported lab scale BMP data of sugarcane bagasse and spent wash were used to first estimate the kinetics and then to simulate corresponding largescale AD. Simulated results of specific methane yield and digester performance were consistent with available largescale AD data. The potential of the model to simulate single and multi-stage AD were illustrated. The presented approach and model will be useful for effectively valorising a variety of complex biomass substrates to biogas.

Anaerobic Digestion of Wastewater Sludge and Alkaline-Pretreated Wheat Straw at Semi-Continuous Pilot Scale: Performances and Energy Assessment

During the last decade, the application of pretreatment has been investigated to enhance methane production from lignocellulosic biomass such as wheat straw (WS). Nonetheless, most of these studies were conducted in laboratory batch tests, potentially hiding instability problems or inhibition, which may fail in truly predicting full-scale reactor performance. For this purpose, the effect of an alkaline pretreatment on process performance and methane yields from WS (0.10 g NaOH g−1 WS at 90 °C for 1 h) co-digested with fresh wastewater sludge was evaluated in a pilot-scale reactor (20 L). Results showed that alkaline pretreatment resulted in better delignification (44%) and hemicellulose solubilization (62%) compared to untreated WS. Pilot-scale study showed that the alkaline pretreatment improved the methane production (261 ± 3 Nm3 CH4 t−1 VS) compared to untreated WS (201 ± 6 Nm3 CH4 t−1 VS). Stable process without any inhibition was observed and a high alkalinity was maintained in the reactor due to the NaOH used for pretreatment. The study thus confirms that alkaline pretreatment is a promising technology for full-scale application and could improve the overall economic benefits for biogas plant at 24 EUR t−1 VS treated, improve the energy recovery per unit organic matter, reduce the digestate volume and its disposal costs.

Socio-Economic and Environmental Impacts of Biomass Valorisation: A Strategic Drive for Sustainable Bioeconomy

In the late twentieth century, the only cost-effective opportunity for waste removal cost at least several thousand dollars, but nowadays, a lot of improvement has occurred. The biomass and waste generation problems attracted concerned authorities to identify and provide environmentally friendly sustainable solutions that possess environmental and economic benefits. The present study emphasises the valorisation of biomass and waste produced by domestic and industrial sectors. Therefore, substantial research is ongoing to replace the traditional treatment methods that potentially acquire less detrimental effects. Synthetic biology can be a unique platform that invites all the relevant characters for designing and assembling an efficient program that could be useful to handle the increasing threat for human beings. In the future, these engineered methods will not only revolutionise our lives but practically lead us to get cheaper biofuels, producing bioenergy, pharmaceutics, and various biochemicals. The bioaugmentation approach concomitant with microbial fuel cells (MFC) is an example that is used to produce electricity from municipal waste, which is directly associated with the loading of waste. Beyond the traditional opportunities, herein, we have spotlighted the new advances in pertinent technology closely related to production and reduction approaches. Various integrated modern techniques and aspects related to the industrial sector are also discussed with suitable examples, including green energy and other industrially relevant products. However, many problems persist in present-day technology that requires essential efforts to handle thoroughly because significant valorisation of biomass and waste involves integrated methods for timely detection, classification, and separation. We reviewed and proposed the anticipated dispensation methods to overcome the growing stream of biomass and waste at a distinct and organisational scale.

Enhanced lignocellulosic component removal and biomethane potential from chestnut shell by a combined hydrothermal-alkaline pretreatment

This study proposes new perspectives for the management and biorefinery of wastes deriving from the agri-food sector such as chestnut shell (CS), which was here used as an organic feedstock for biomethane production through anaerobic digestion (AD). 1-5% alkaline (i.e. NaOH and KOH), hydrothermal (i.e. at 100 °C) and combined hydrothermal-alkaline pretreatments were employed to enhance the CS biodegradability prior to biochemical methane potential (BMP) tests conducted under mesophilic conditions. The hydrothermally-pretreated CS with 3% NaOH achieved the highest biomethane yield of 253 (±9) mL CH₄·g VS^-1 coupled to a volatile solid reduction of 48%. The hydrothermal-alkaline pretreatment positively affected both delignification and hemicellulose polymerization, promoting an approximately 2.4-fold higher substrate biodegradability compared to the untreated CS, which only reached a CH₄ production of 104 (±5) mL CH₄·g VS^-1. AD proceeded via volatile fatty acid accumulation, subsequently followed by methane production that was effectively simulated via the modified Gompertz kinetic having a R² of 0.974-0.999. Among the physical-chemical parameters characterizing the CS, the soluble chemical oxygen demand (sCOD) was highly correlated with the BMP showing a Pearson coefficient of 0.952. The cumulative biomethane yield, the sCOD and the cellulose, hemicellulose and lignin amount of CS were also processed through the least square method, obtaining a useful regression equation to predict the BMP. The economic assessment indicated that the hydrothermal-alkaline pretreatment is a cost-effective method to improve the BMP of CS, also for future full-scale applications.

Microbial biotechnological approaches: renewable bioprocessing for the future energy systems

The accelerating energy demands of the increasing global population and industrialization has become a matter of great concern all over the globe. In the present scenario, the world is witnessing a considerably huge energy crisis owing to the limited availability of conventional energy resources and rapid depletion of non-renewable fossil fuels. Therefore, there is a dire need to explore the alternative renewable fuels that can fulfil the energy requirements of the growing population and overcome the intimidating environmental issues like greenhouse gas emissions, global warming, air pollution etc. The use of microorganisms such as bacteria has captured significant interest in the recent era for the conversion of the chemical energy reserved in organic compounds into electrical energy. The versatility of the microorganisms to generate renewable energy fuels from multifarious biological and biomass substrates can abate these ominous concerns to a great extent. For instance, most of the microorganisms can easily transform the carbohydrates into alcohol. Establishing the microbial fuel technology as an alternative source for the generation of renewable energy sources can be a state of art technology owing to its reliability, high efficiency, cleanliness and production of minimally toxic or inclusively non-toxic byproducts. This review paper aims to highlight the key points and techniques used for the employment of bacteria to generate, biofuels and bioenergy, and their foremost benefits.

Why can hydrothermally pretreating lignocellulose in low severities improve anaerobic digestion performances?

A lignocellulosic residue, rice straw, was hydrothermally pretreated for the whole slurry anaerobic digestion. In contrast to the unpretreated rice straw, 110-120 °C pretreatment promoted biogas yield by 35%-38%, while only 14% promotion happened on the pretreatment at 180 °C. To understand why this improvement happened at lower severities, the pretreated rice straw at 90 °C, 120 °C, and 180 °C were selected for the further investigation, in which the liquor and solid fraction were separated for digestion, and compared with the whole slurry digestion. Results indicated more methane was released from the derived liquor of 180 °C than that of 90 °C and 120 °C, however, solid fraction did not exhibit significantly different methane yields (187.77-193.91 mL/g TS). These results suggested that the released soluble fraction from pretreatment could facilitate the methanogenesis. Furthermore, the released inherent soluble fraction in rice straw was mainly responsible for higher biogas yield at lower temperatures. Pretreatment at higher temperatures disintegrated the rice straw recalcitrance more, and intensified the release of soluble fraction accordingly. Consequently, the methanogenesis of whole slurry could be promoted at the initial digestion; the hydrolysis/acidification of the solid fraction in whole slurry was weakened greatly, which resulted in a lower biogas yield. This can also be proved by the evolution of dominant bacteria and archaea in the anaerobic digestion of whole slurry, separated solid and liquor fraction.

A comprehensive review on the framework to valorise lignocellulosic biomass as biorefinery feedstocks

An effective pretreatment is the first step to enhance the digestibility of lignocellulosic biomass - a source of renewable, eco-friendly and energy-dense materials - for biofuel and biochemical productions. This review aims to provide a comprehensive assessment on the advantages and disadvantages of lignocellulosic pretreatment techniques, which have been studied at the lab-, pilot- and full-scale levels. Biological pretreatment is environmentally friendly but time consuming (i.e. 15-40 days). Chemical pretreatment is effective in breaking down lignocellulose and increasing sugar yield (e.g. 4 to 10-fold improvement) but entails chemical cost and expensive reactors. Whereas the combination of physical and chemical (i.e. physicochemical) pretreatment is energy intensive (e.g. energy production can only compensate 80% of the input energy) despite offering good process efficiency (i.e. > 100% increase in product yield). Demonstrations of pretreatment techniques (e.g. acid, alkaline, and hydrothermal) in pilot-scale have reported 50-80% hemicellulose solubilisation and enhanced sugar yields. The feasibility of these pilot and full-scale plants has been supported by government subsidies to encourage biofuel consumption (e.g. tax credits and mandates). Due to the variability in their mechanisms and characteristics, no superior pretreatment has been identified. The main challenge lies in the capability to achieve a positive energy balance and great economic viability with minimal environmental impacts i.e. the energy or product output significantly surpasses the energy and monetary input. Enhancement of the current pretreatment techno-economic efficiency (e.g. higher product yield, chemical recycling, and by-products conversion to increase environmental sustainability) and the integration of pretreatment methods to effectively treat a range of biomass will be the steppingstone for commercial lignocellulosic biorefineries.

Comparison of Calcium Oxide and Calcium Peroxide Pretreatments of Wheat Straw for Improving Biohydrogen Production

Wheat straw was pretreated with either CaO₂ or CaO to improve biohydrogen production. Both CaO and CaO₂ pretreatments improved the biodegradability of the wheat straw. CaO pretreatment raised the H₂ yield by between 48.8 and 163.9% at CaO contents ranging from 2 to 4%. The highest H₂ yield [144 mL/g total solid (TS)] was obtained at 121 °C and 6% CaO. In addition, the highest H₂ yield from wheat straw pretreated at the same temperature and dosage of CaO₂ was 71.8 mL/g TS, which was higher than that of the control group (43.2 mL/g TS), with hot water (121 °C) treatment. Considering pretreatment costs and H₂ production potential, CaO was a better pretreatment agent than CaO₂.

Optimisation and Modelling of Anaerobic Digestion of Whiskey Distillery/Brewery Wastes after Combined Chemical and Mechanical Pre-Treatment

Whiskey distillery waste streams consisting of pot ale (liquid residue) and spent grain (solid residue) are high strength organic wastes and suitable feedstock for anaerobic digestion (AD) from both economic and environmental stand points. Anaerobic digestion of pot ale and pot ale/spent grain mixtures (with mixing ratios of 1:1, 1:3, and 1:5 by wet weight) was performed after implementation of a novel hybrid pre-treatment (combined chemical and mechanical) in order to modify lignocellulosic structure and ultimately enhance digestion yield. Lignin, hemicellulose, and cellulose fractions were determined before and after chemical pre-treatment. Effects of different inoculum rates (10–30–50% on wet basis) and beating times (0–7.5–15 min) on anaerobic digestion of pot ale alone and of pot ale/spent grain mixtures were investigated in lab scale batch mode with a major focus of optimising biogas yield by using response surface methodology (RSM) in Design Expert Software. The highest biogas yields of 629 ± 8.5 mL/g vs. (51.3% CH4) and 360 ± 10 mL/g vs. (55.0 ± 0.4) with anaerobic digestion of pot ale alone and spent grain mix after 1M NaOH and 7.5 min beating pre-treatments with 50% inoculum ratio respectively. The optimum digestion conditions to maximise the biogas quality and quantity were predicted as 10 and 13 min beating times and 32 and 38 °C digestion temperatures for anaerobic digestion of pot ale alone and spent grain mix respectively.

Combination of steam explosion pretreatment and anaerobic alkalization treatment to improve enzymatic hydrolysis of Hippophae rhamnoides

The optimum condition of steam explosion pretreatment was screened for hippophae, and anaerobic calcium oxide (CaO) alkalization was further used to improve its enzymatic hydrolysis. Steam-exploded hippophae reached the lowest pH value (4.01) and the maximal hemicellulose removal (77.16%) at pressure 1.5 MPa and residence time 20 min. Lignocellulosic fractions of hippophae was remarkably reduced by CaO alkalization or steam explosion treatment, and enzymatic sugar yield was increased from 66 mg/g DM (untreated material) to 270 and 300 mg/g DM, respectively. The sequent pretreatment of steam explosion and CaO alkalization achieved a sugar yield of 330 mg/g DM, where 2% CaO loading rate was high enough. Besides, SEM, FTIR, and XRD analyses validated structural and physicochemical changes of hippophae. In conclusion, the sequent pretreatment of steam explosion at pressure 1.5 MPa for 20 min and anaerobic CaO alkalization at 2% loading rate could remarkably improve enzymatic hydrolysis of hippophae.

Effect of pre-treatment on sequential anaerobic co-digestion of chicken litter with agricultural and food wastes under semi-solid conditions and comparison with wet anaerobic digestion

Sequential co-digestion batch assays were conducted using feedstocks of chicken litter (CL), food waste (FW) and wheat straw (WS) mixed to a C/N ratio of 26.5 and 15% TS. Untreated mixture produced biogas of 321.6 ± 13.4 mL_N/g VS_added which improved up to 50% when either CL or WS pre-treated substrates were fed. However, when both pre-treated CL and WS were fed, 80% and 88% increase in total biogas were found with associated VS removal of 49% and 55%, respectively, for alkali and sequential acid pre-treatment. Also, reactors received pre-treated substrates showed reduction in ammonia and digestate cellulose fraction with an increase in water soluble contents. Biogas production using sequential AD at 15% was almost 38% less than BMP biogas at 4%, however this was negated with the pre-treatment indicating that co-digestion at high TS of 15% is achievable. Further testing is required to confirm these results under semi-continuous conditions.

Intermittent-vacuum assisted thermophilic co-digestion of corn stover and liquid swine manure: Salinity inhibition

In this study, the effects of Intermittent-Vacuum Stripping (IVS) on activities of methanogenesis in co-digestion of corn stover with liquid swine manure (LSM + CS) under thermophilic anaerobic digestion (TAD) conditions were evaluated. A 65% methanogenesis activity inhibition was observed in pretreated LSM plus corn stover (pLSM + CS), while 60 and 165 mL/L/day CH₄ productions were achieved in pLSM + CS and LSM + CS, respectively. The high salinity condition (5.28%) after IVS pretreatment was considered the primary inhibitor in pLSM + CS, while the ammonia (≤600 mg/L), C:N ratio (15.52) and volatile solid loading rate (3 g/kg^-1·day^-1) didn't show a negative effect on CH₄ production. When salinities were increased from 2% to 4% and 8%, 50% and 100% inhibition were observed respectively. The butyrate accumulation was a potential indicator of the non-salinity-inhibition status for methanogenesis in TAD.

Improving the co-digestion performance of waste activated sludge and wheat straw through ratio optimization and ferroferric oxide supplementation

Low anaerobic digestion efficiency of wheat straw (WS) has been an intractable problem owing to its high C/N ratio and complex structure. In this study, co-digestion of WS and waste activated sludge (WAS) at different ratios was performed to identify conditions that would elevate the acidic pH and increase methane production. The results showed that using a 1:1 ratio of WS and WAS, methane production in the co-digester was 26.9% higher than the sum of equal WAS and WS mono-digestion. When Fe₃O₄ was added to the co-digester, the acidic pH was further relieved and the anaerobic digestion efficiency was additionally enhanced. Microbial analysis showed that the ethanol-type fermentative bacterial genus Ethanoligenens was enriched in the WAS + WS-Fe₃O₄ reactor, in which the production of propionate was notably reduced, indicating that Fe₃O₄ could prevent the accumulation of volatile fatty acids by changing the types of fermentative bacteria present and promote anaerobic digestion efficiency.

Enhancement of methane production from Cotton Stalk using different pretreatment techniques

China produces large amount of cotton stalk (CS) residues as agricultural biomass, which are incinerated on-site, causing air pollution. The high organic content of CS could be utilized for biogas production, but the direct digestion without pretreatment always leads to a low methane yield and biodegradability, due to the complicated structure of lignocellulose. In order to search best fitting pretreatment methods in effective anaerobic digestion (AD) of CS, effects of various pretreatments including KOH, NaOH, Ca(OH)₂, alkali hydrogen peroxide (AHP), H₂SO₄, H₃PO₄ and steam explosion (SE) were studied. It was seen that all treatments resulted in varying methane yields. Among all the pretreatments, acid pretreatment is not suitable for AD of CS. The results showed that the highest cumulative methane yield (CMY) of 192.4 mL·gVS⁻¹ was obtained after 3% AHP pretreatment of CS, and the methane yield improved by 254.3% than the untreated CS. Therefore, AHP treatment was proven to be an efficient pretreatment technique. XRD and FTIR analyses had shown that pretreated CS had favorable structural changes. This research is beneficial in developing environment friendly and cost-effective pretreatment technologies to utilize CS for methane production in future application.

Can hydrothermal pretreatment improve anaerobic digestion for biogas from lignocellulosic biomass?

Hydrothermally-pretreated rice straw (HPRS) from various pretreatment temperatures was anaerobically-digested in whole slurry. Results indicated promoting pretreatment temperature significantly deconstructed rice straw, and facilitated the conversion of insoluble fractions to soluble fractions. Although 306.6 mL/g TS biogas was maximally yielded in HPRS-90 and HPRS-180, respectively, via digestion in whole slurry, it was only 3% promotion compared to the unpretreated rice straw. HPRS-210 yielded 208.5 mL/g TS biogas, which was 30% reduction with longer lag period of 19.8 d, suggesting serious inhibitions happened. Through slightly increasing organic loading, more serious acidification and reduction on biogas yield, especially at higher pretreatment temperatures, indicated the soluble fractions controlled digestion performances. Pearson correlation analysis suggested negative relationship existed between methane yield and the soluble fractions including soluble carbohydrates, formic acid and furfural. Hydrothermal pretreatment, especially at higher temperature, did not improve anaerobic digestion, thereby, was not recommended, however, lower temperature can be considered potentially.

Influence of pretreatment conditions on lignocellulosic fractions and methane production from grape pomace

The lignocellulosic structure of grape pomace requires the use of pretreatments facilitating microbial decomposition of the matter and enhancing methane production. In this study, the effects of various pretreatments (freezing, alkaline treatment using NaOH and NH₃, acid treatment using HCl, ultrasounds and pulsed electric fields) were examined in batch mode. The highest methane production (0.178Nm³kg^-1 of COD) was attained after alkaline treatment with 10% NaOH w/w dry basis, at 20°C and for 24h. This result is due to the degradation of more than 50% of lignin and about 22% of cellulose present in grape pomace. The coupling of this pretreatment with freezing at -20°C exhibited the highest methane production of 0.2194±0.0007Nm³kg^-1 of COD. When applied to a larger scale continuous digester, this coupled pretreatment increased methane production by about 27%, compared to the untreated samples, promoting the green valorization of the biomass.

Improvement of wheat straw anaerobic digestion through alkali pre-treatment: Carbohydrates bioavailability evaluation and economic feasibility

Lignocellulosic biomasses such as wheat straw are widely used as a feedstock for biogas production. However, these biomasses are mainly composed of a compact fibre structure and therefore, it is recommended to treat them prior to its usage for biogas production in order to improve their bioavailability. The aim of this work is to evaluate, in terms of performance stability, methane yield and economic feasibility, two different scenarios: a mesophilic codigestion of wheat straw and animal manure with or without a low-energy demand alkaline pre-treatment (0.08g_KOHg_TS^-1of wheat straw, for 24h and at 25°C). Besides this, said pre-treatment was also analysed based on the improvement of the bioavailable carbohydrate content in the untreated versus the pre-treated wheat straw. The results pointed out that pre-treated wheat straw prompted a more stable performance (in terms of pH and alkalinity) and an improved methane yield (128% increment) of the mesophilic codigestion process, in comparison to the "untreated" scenario. The pre-treatment increased the content of cellulose, hemicellulose and other compounds (waxes, pectin, oil, etc.) in the liquid fraction, from 5% to 60%, from 11.5% to 39.1% TS and from 57% to 79% of the TS in the liquid fraction for the untreated and pre-treated wheat straws, respectively. Finally, the pre-treated scenario gained an energy surplus of a factor 13.5 and achieved a positive net benefit of 90.4€t_VS-WS^-1d^-1, being a favourable case for an eventual scale-up of the combined process.

Process performance and methane production optimizing of anaerobic co-digestion of swine manure and corn straw

During anaerobic digestion (AD) process, process parameters e.g., VFA, pH, COD removal … and kinetic parameters e.g., hydrolysis rate, lag phase and methane production potential… are the important indicator for illustrating AD process performance, however, the AD process performance based on these parameters remains poorly understood. To estimate process performance focusing on initial pH and substrate composition, the effects of initial pH and swine manure to corn straw ratio on biogas production and these parameters and linkages of these parameters were analyzed. Also, the methane production was optimized. The results revealed that the maximum methane yield and methane production rate were obtained with initial pH 7.5 and SM/CS ratio of 70:30. Kinetic parameters are coupled with process parameters, especially for COD removal rate, VS degradation rate, VFA and pH. Hydrolysis constant positively correlated with pH, COD removal rate and VS degradation rate, then impacted methane production and lag phase. Meanwhile, lag phase and the maximum methane production rate were directly determined by VFA and COD removal rate. The optimum initial pH and SM/CS ratio were 7.15 and 0.62, respectively, with a predicted maximum methane content of 55.12%. Thinking these findings together, they provide a scientific theory for estimating AD performance.

Effect of alkaline pretreatment on mesophilic and thermophilic anaerobic digestion of a submerged macrophyte: Inhibition and recovery against dissolved lignin during semi-continuous operation

The long-term effect of alkaline pretreatment on semi-continuous anaerobic digestion (AD) of the lignin-rich submerged macrophyte Potamogeton maackianus was investigated using mesophilic and thermophilic conditions. In pretreated reactors, dissolved lignin accumulated to high levels. CH₄ production under the pretreated condition was higher than that of the untreated condition, but decreased from Days 22 (mesophilic) and 42 (thermophilic). However, CH₄ production subsequently recovered, although dissolved lignin accumulated. Further, the change in the microbial community was observed between conditions. These results suggest that dissolved lignin temporarily inhibited AD, although acclimatization to dissolved lignin occurred during long-term operation. During the steady state period, mesophilic conditions achieved a 42% increase in the CH₄ yield using pretreatment, while thermophilic conditions yielded an 8% increment. Because volatile fatty acids accumulated even after acclimatization during the thermophilic pretreated condition and was discharged with the effluent, improvement of the methanogenic step would enable enhanced CH₄ recovery.

Effect of hydrothermal pretreatment on Miscanthus anaerobic digestion

Miscanthus is a promising source of bioenergy with high lignocellulose content. This paper studied the effect of hydrothermal pretreatment on Miscanthus biogas production. Different pretreatment temperature from 125 to 200°C was tested. After pretreatment, hemicellulose was partially removed and this led to a change in cellulose accessibility. Enzymatic hydrolysis was used to examine the digestibility of different samples. There was no obvious enhancement in low temperature (125 and 150°C) conditions. According to the results, 200°C hydrothermal pretreatment was the optimal condition saving 50% on the digestion time and increasing glucose production 13.2 times compared with the raw material. Although the cellulose crystallinity increased after the pretreatment, its effect on biogas production and enzymatic hydrolysis was limited.

Enhancement of biohydrogen production from brewers' spent grain by calcined-red mud pretreatment

This paper investigated the utilization of calcined-red mud (CRM) pretreatment to enhance fermentative hydrogen yields from brewers' spent grain (BSG). The BSG samples were treated with different concentrations (0.0-20g/L) of CRM at 55°C for 48h, before the biohydrogen process with heat-treated anaerobic sludge inoculum. The highest specific hydrogen production of 198.62ml/g-VS was obtained from the BSG treated with 10g/L CRM, with the corresponding lag time of 10.60h. Hydrogen yield increments increased by 67.74%, compared to the control tests without CRM. The results demonstrated that the CRM could hydrolyze more cellulose and further provided adequate broth and suitable pH value for efficient fermentative hydrogen. The model-based analysis showed that the modified Gompertz model presented a better fit for the experimental data than the first-order model.

Effect of Ca(OH)2 pretreatment on extruded rice straw anaerobic digestion

It has been proven that extrusion can change the structure of rice straw and increase biogas production, but the effect of a single pretreatment is limited. Ca(OH)2 pretreatment was used to enhance the enzyme hydrolysis and biogas production of extruded rice straw. After Ca(OH)2 pretreatment, the glucose and xylose conversion rates in enzymatic hydrolysis increased from 36.0% and 22.4% to 66.8% and 50.2%, respectively. The highest biogas production observed in 8% and 10% Ca(OH)2 pretreated rice straw reached 564.7mL/g VS and 574.5mL/g VS, respectively, which are 34.3% and 36.7% higher than the non-Ca(OH)2-loaded sample. The Ca(OH)2 pretreatment can effectively remove the lignin and increase the fermentable sugar content. The structural changes in the extruded rice straw have also been analyzed by XRD, FTIR, and SEM. Considering all of the results, an 8% Ca(OH)2 loading rate is the best option for the pretreatment of extruded rice straw.

Semi-continuous anaerobic digestion of different silage crops: VFAs formation, methane yield from fiber and non-fiber components and digestate composition

This study presents the results of long-term semi-continuous experiments on anaerobic digestion at an HRT of 45d with ten silages: 2 annual and 4 perennial crops, and 4 mixtures of annual with perennial crops. The composition of substrates and digestates was determined with Van Soest's fractionation method. Removal of non-fiber materials ranged from 49.4% (Miscanthus sacchariflorus) to 89.3% (Zea mays alone and mixed with M. sacchariflorus), that of fiber materials like lignin ranged from 0.005% (Z. mays alone and mixed with grasses at VS ratio of 90:10%) to 46.5% (Sida hermaphrodita). The lowest stability of anaerobic digestion, as confirmed by normalized data concentrations of volatile fatty acids, was reported for both miscanthuses and sugar sorghum. The methane yield coefficients for non-fiber and fiber materials were 0.3666 and 0.2556L/g, respectively. All digestate residues had high fertilizing value, especially those from mixtures of crops.

Biomethane production and physicochemical characterization of anaerobically digested teff (Eragrostis tef) straw pretreated by sodium hydroxide

The biogas production potential and biomethane content of teff straw through pretreatment by NaOH was investigated. Different NaOH concentrations (1%, 2%, 4% and 6%) were used for each four solid loadings (50, 65, 80 and 95 g/L). The effects of NaOH as pretreatment factor on the biodegradability of teff straw, changes in main compositions and enhancement of anaerobic digestion were analyzed. The result showed that, using 4% NaOH for pretreatment in 80 g/L solid loading produced 40.0% higher total biogas production and 48.1% higher biomethane content than the untreated sample of teff straw. Investigation of changes in chemical compositions and physical microstructure indicated that there was 4.3-22.1% total lignocellulosic compositions removal after three days pretreatment with NaOH. The results further revealed that NaOH pretreatment changed the structural compositions and lignin network, and improved biogas production from teff straw.