Benefit of sodium hydroxide pretreatment of ensiled sorghum forage on the anaerobic reactor stability and methane production

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.

Effect of alkaline pre-treatment on hydrolysis rate and methane production during anaerobic digestion of paunch solid waste

Paunch is comprised of the partially digested feed contained in cattle or sheep and contributes 20-50% of organic waste produced at red meat processing facilities. Anaerobic digestion has been identified as a promising technology for paunch treatment, however treatment times can be long and when combined with the moderate degradability of paunch this results in high treatment costs that need to be improved. Pre-treatment was investigated as a strategy to improve AD of paunch, alkaline treatment (NaOH or KOH) was selected due to the high lignin content. A range of alkaline loadings (1-20 g 100gTS-1) were tested with an equivalent hydroxide molar concentration of 9-250 mM [OH-]. Alkaline pre-treatment improved both the hydrolysis rate and the overall degradability of paunch solid by up to 4.4 times and 60%, respectively. The enhanced hydrolysis rate and methane yield was correlated to changes in material composition during pre-treatment. While alkaline concentration was an important factor, there were no significant improvements at alkaline concentrations above 12 g 100gTS-1 (150 mM [OH-]).

The current status and challenges of biomass biorefineries in Africa: A critical review and future perspectives for bioeconomy development

Africa benefits from diverse biomasses that are rich in high-added value materials and precursors for energy, food, agricultural, cosmetic and medicinal applications. Many African countries are interested in valorizing biomasses to develop efficient and integrated biorefinery processes and their use for local and regional economic development. Thus, this report critically reviews the current status of African biomass richness, its diversity, and potential applications. Moreover, particular attention is given to bioenergy production, mainly by biological and thermochemical conversion processes. This also includes biomass valorization in agriculture, particularly for the production of plant-based biostimulants, which are a potential emerging agri-input sector worldwide. This study points out that even though several processes for biofuel, biogas, biofertilizer and biostimulant production have already been established in Africa, their development on a larger scale remains limited. This study also reports the different socioeconomic and political aspects of biomass applications, along with their challenges, opportunities, and future research perspectives, to promote concrete technologies transferable into an industrial level.

Biochemical methane potential and active microbial communities during anaerobic digestion of biodegradable plastics at different inoculum-substrate ratios

The influence of the inoculum-substrate ratio (ISR) on the mesophilic and thermophilic biochemical methane potential test of two biodegradable plastics was evaluated. Poly(lactic acid) (PLA) and polyhydroxybutyrate (PHB) were selected for this study, the first for being recalcitrant to mesophilic anaerobic digestion (AD) and the second, by contrast, for being readily biodegradable. Several ISRs, calculated on the basis of volatile solids (VS), were tested: 1, 2, 2.85, 4, and 10 g(VS of inoculum).g(VS of substrate)^-1. A high ISR was associated with an enhanced methane production rate (i.e., biodegradation kinetics). However, the ultimate methane production did not change, except when inhibition was observed. Indeed, applying the lowest ISR to readily biodegradable plastics such as PHB resulted in inhibition of methane production. Based on these experiments, in order to have reproducible degradation kinetics and optimal methane production, an ISR between 2.85 and 4 is recommended for biodegradable plastics. The active microbial communities were analyzed, and the active bacteria differed depending on the plastic digested (PLA versus PHB) and the temperature of the process (mesophilic versus thermophilic). Previously identified PHB degraders (Ilyobacter delafieldii and Enterobacter) were detected in PHB-fed reactors. Thermogutta and Tepidanaerobacter were detected during the thermophilic AD of PLA, and they are probably involved in PLA hydrolysis and lactate conversion, respectively.

Hydrogen and Methane Production from Anaerobic Co-Digestion of Sorghum and Cow Manure: Effect of pH and Hydraulic Retention Time

The need for alternative energy sources is constantly growing worldwide, while the focus has shifted to the valorization of biomass. The aim of the present study was to determine the optimal pH and hydraulic retention time (HRT) values for treating a mixture of sorghum biomass solution with liquid cow manure (in a ratio 95:5 v/v) through anaerobic digestion, in a two-stage system. Batch tests were initially carried out for the investigation of the pH effect on bio-hydrogen and volatile fatty acids (VFA) production. The highest hydrogen yield of 0.92 mol H2/mol carbohydratesconsumed was obtained at pH 5.0, whereas the maximum degradation of carbohydrates and VFA productivity was observed at pH 6.0. Further investigation of the effect of HRT on hydrogen and methane production was carried out. The maximum yield of 1.68 mol H2/mol carbohydratesconsumed was observed at an HRT of 5 d, with H2 productivity of 0.13 L/LR·d. On the other hand, the highest CH4 production rate of 0.44 L/LR·d was achieved at an HRT of 25 d, with a methane yield of 295.3 mL/g VSadded, whereas at a reduced HRT of 20 d the process exhibited inhibition and/or overload, as indicated by an accumulation of VFAs and decline in CH4 productivity.

Impact of mechanical and thermo-chemical pretreatments to enhance anaerobic digestion of poly(lactic acid)

To date, the introduction of biodegradable plastics such as PLA in anaerobic digestion systems has been limited by a very low rate of biodegradation. To overcome these limitations, pretreatment technologies can be applied. In this study, the impact of pretreatments (mechanical, thermal, thermo-acid, and thermo-alkaline) was investigated. Mechanical pretreatment of PLA improved its biodegradation rate but did not affect the ultimate methane potential (430-461 NL CH₄ kg^-1 VS). In parallel, thermal and thermo-acid pretreatments exhibited a similar trend for PLA solubilization. Both of these pretreatments only achieved substantial solubilization (>60%) at higher temperatures (120 and 150 °C). At lower temperatures (70 and 90 °C), negligible solubilization (between 1 and 6%) occurred after 48 h. By contrast, coupling of thermal and alkaline pretreatment significantly increased solubilization at the lower temperatures (70 and 90 °C). In terms of biodegradation, thermo-alkaline pretreatment (with 5% w/v Ca(OH)₂) of PLA resulted in a similar methane potential (from 325 to 390 NL CH₄ kg^-1 VS) for 1 h at 150 °C, 6 h at 120 °C, 24 h at 90 °C, and 48 h at 70 °C. Reduction of the Ca(OH)₂ concentration (from 5% to 0.5% w/v) highlighted that a concentration of 2.5% w/v was sufficient to achieve a substantial level of biodegradation. Pretreatment at 70 and 90 °C using 2.5% w/v Ca(OH)₂ for 48 h resulted in biodegradation yields of 73% and 68%, respectively. Finally, a good correlation (R² = 0.90) was found between the PLA solubilization and its biodegradation.

Can anaerobic digestion be a suitable end-of-life scenario for biodegradable plastics? A critical review of the current situation, hurdles, and challenges

Growing concern regarding non-biodegradable plastics and the impact of these materials on the environment has promoted interest in biodegradable plastics. The intensification of separate biowastes collection in most European countries has also contributed to the development of biodegradable plastics, and the subject of their end-of-life is becoming a key issue. To date, there has been relatively little research to evaluate the biodegradability of biodegradable plastics by anaerobic digestion (AD) compared to industrial and home composting. However, anaerobic digestion is a particularly promising strategy for treating biodegradable organic wastes in the context of circular waste management. This critical review aims to provide an in-depth update of anaerobic digestion of biodegradable plastics by providing a summary of the literature regarding process performances, parameters affecting biodegradability, the microorganisms involved, and some of the strategies (e.g., pretreatment, additives, and inoculum acclimation) used to enhance the degradation rate of biodegradable plastics. In addition, a critical section is dedicated to suggestions and recommendations for the development of biodegradable plastics sector and their treatment in anaerobic digestion.

Industrial symbiosis of anaerobic digestion and pyrolysis: Performances and agricultural interest of coupling biochar and liquid digestate

The sustainability of the anaerobic digestion industry is closely related to proper digestate disposal. In this study, an innovative cascading biorefinery concept coupling anaerobic digestion and subsequent pyrolysis of the digestate was investigated with the aim of enhancing the energy recovery and improving the fertilizers from organic wastes. Continuous anaerobic co-digestion of quinoa residues with wastewater sludge (45/55% VS) exhibited good stability and a methane production of 219 NL CH₄/kg VS. Subsequent pyrolysis of the solid digestate was carried out (at 500 °C, 1 h, and 10 °C/min), resulting in a products distribution of 40 wt% biochar, 36 wt% bio-oil, and 24 wt% syngas. The organic phase (OP) of bio-oil and syngas exhibited higher and lower heating values of 34 MJ/kg and 11.8 MJ/Nm³, respectively. The potential synergy of coupling biochar with liquid digestate (LD) for agronomic purposes was investigated. Interestingly, coupling LD (at 170 kg N/ha) with biochar (at 25 tons/ha) improved the growth of tomato plants up to 25% compared to LD application alone. In parallel, co-application of biochar with LD significantly increased the ammonia volatilization (by 64%) compared to LD application alone, although their simultaneous use did not impact the C and N mineralization rates.

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.

Pretreatments for enhanced biomethane production from buckwheat hull: Effects on organic matter degradation and process sustainability

Buckwheat manufacturing produce a large amount of lignocellulosic residue (buckwheat hull), which could be used as substrate in anaerobic digestion, even if hard lignocellulosic structure represents the main obstacle for its degradation. This study presents the results of a laboratory experiment conducted to evaluate the effects of different pretreatments on buckwheat hull anaerobic digestion. To achieve the aim, five pretreatments (alkaline, thermo-alkaline, microwave, ultrasonication and low temperature thermal pretreatment) were studied and the results were compared to non treated buckwheat hull. Cumulative biomethane yields significantly increased after alkaline and thermo-alkaline pretreatments (+61% and +122% with respect to non treated hull, respectively). These results were mainly related to organic matter solubilisation (+772% and +859% of soluble reducing sugars, respectively) and lignin, hemicellulose and cellulose degradation. Overall, process parameters behaviour and digestate quality were not affected by the pretreatments. Alkaline and thermo-alkaline pretreatments were evaluated for their energetic and economic affordability, showing that combination of thermal and alkaline pretreatments ensures significant advantages.

Nutrient augmentation enhances biogas production from sorghum mono-digestion

This study investigated the effects of the addition of micro- (Fe, Co, Ni, and Mo) and macro-(Sulfur) nutrients on mono-digestion of sorghum under mesophilic conditions. A continuous stirred-tank reactor was operated for more than 420 days under seven different experimental conditions. The experimental results showed poor performance for methane production and process stability without nutrient supplementation. Serious deficiencies in Co and S were confirmed by nutrient analysis of dry sorghum and digestate. Nutrient augmentation efficiently enhanced methane production and volatile fatty acid (VFA) removal. Methane production reached 223 mL-CH₄/g-VS, almost matching the yield predicted by biochemical methane potential (BMP) test. S was demonstrated to have a critical effect on metal availability in the digester. Consequently, to maintain stable methane fermentation, suitable supplementations of S and Co are recommended for anaerobic sorghum mono-digestion.

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.

Effect of coupling alkaline pretreatment and sewage sludge co-digestion on methane production and fertilizer potential of digestate

This study aims at investigating how organic waste co-digestion coupled with alkaline pretreatment can impact the methane production and agronomic value of produced digestates. For this purpose, sludge alone and mixed with olive pomace or macroalgal residues were subjected to anaerobic digestion with and without alkaline pretreatment. In addition, co-digestion of pretreated sludge with raw substrates was also carried out and compared to the whole mixture pretreatment. KOH pretreatment enhanced methane production by 39%, 15% and 49% from sludge, sludge mixed with olive pomace and sludge mixed with macroalgal residues, respectively. The digestates were characterised according to their physico-chemical and agronomic properties. They were then applied as biofertilizers for tomato growth during the first vegetative stage (28 days of culture). Concentrations in chlorophyll a and carotenoids in tomato plants, following sludge digestate addition, rose by 46% and 41% respectively. Sludge digestate enhanced tomato plant dry weight by 87%, while its nitrogen content increased by 90%. The impact of nitrogen and phosphorus contents in the digestate was strongest on tomato plant dry weight, thus explaining the efficiency of sludge digestate relative to other types of digestate. However, when methane production is considered, the combination of pre-treatment with co-digestion of macroalgal residues and sludge appears most beneficial for maximizing energy recovery and for biofertilizer generation.

Evaluation of agronomic properties of digestate from macroalgal residues anaerobic digestion: Impact of pretreatment and co-digestion with waste activated sludge

The aim of this paper is to investigate the impact of pretreating macroalgal residue (MAR) from agar-agar extraction and its co-digestion with sewage sludge on methane production and the agronomic quality of the digestates produced. First, different pretreatments were assessed on BMP tests. Among milling technologies used, knife milling with a 4 mm-screen improved methane production by 25%. The MAR was then knife milled before alkaline, acid and thermal pretreatment. KOH pretreatment (5% TS basis, 25 °C for 2 days) led to the highest methane improvement. It was applied to semi-continuous anaerobic digestion and methane production achieved 237 Nml/gVS which was 20% higher than the control (198 Nml/gVS). In comparison to MAR mono-digestion, co-digestion with thickened activated sludge produced less methane (184 Nml/gVS) but reduced H₂S emission by 91%. None of the digestates was toxic for the germination or growth of wheat and tomato plants. Particularly, co-digestion had the highest impact on tomato plant dry weight (+94% compared to soil alone) mainly due to the phosphorous brought by sludge. However, the impact of alkaline pretreatment on plant growth was not significant.

A Review on Anaerobic Digestion of Lignocellulosic Wastes: Pretreatments and Operational Conditions

Anaerobic digestion (AD) has become extremely popular in the last years to treat and valorize organic wastes both at laboratory and industrial scales, for a wide range of highly produced organic wastes: municipal wastes, wastewater sludge, manure, agrowastes, food industry residuals, etc. Although the principles of AD are well known, it is very important to highlight that knowing the biochemical composition of waste is crucial in order to know its anaerobic biodegradability, which makes an AD process economically feasible. In this paper, we review the main principles of AD, moving to the specific features of lignocellulosic wastes, especially regarding the pretreatments that can enhance the biogas production of such wastes. The main point to consider is that lignocellulosic wastes are present in any organic wastes, and sometimes are the major fraction. Therefore, improving their AD could cause a boost in the development in this technology. The conclusions are that there is no unique strategy to improve the anaerobic biodegradability of lignocellulosic wastes, but pretreatments and codigestion both have an important role on this issue.

Evaluation on the Methane Production Potential of Wood Waste Pretreated with NaOH and Co-Digested with Pig Manure

Wood waste generated during the tree felling and processing is a rich, green, and renewable lignocellulosic biomass. However, an effective method to apply wood waste in anaerobic digestion is lacking. The high carbon to nitrogen (C/N) ratio and rich lignin content of wood waste are the major limiting factors for high biogas production. NaOH pre-treatment for lignocellulosic biomass is a promising approach to weaken the adverse effect of complex crystalline cellulosic structure on biogas production in anaerobic digestion, and the synergistic integration of lignocellulosic biomass with low C/N ratio biomass in anaerobic digestion is a logical option to balance the excessive C/N ratio. Here, we assessed the improvement of methane production of wood waste in anaerobic digestion by NaOH pretreatment, co-digestion technique, and their combination. The results showed that the methane yield of the single digestion of wood waste was increased by 38.5% after NaOH pretreatment compared with the untreated wood waste. The methane production of the co-digestion of wood waste and pig manure was higher than that of the single digestion of wood waste and had nonsignificant difference with the single-digestion of pig manure. The methane yield of the co-digestion of wood waste pretreated with NaOH and pig manure was increased by 75.8% than that of the untreated wood waste. The findings indicated that wood waste as a sustainable biomass source has considerable potential to achieve high biogas production in anaerobic digestion.

Microwave vs. alkaline-microwave pretreatment for enhancing Thickened Waste Activated Sludge and fat, oil, and grease solubilization, degradation and biogas production

The effects of microwave (MW) and combined alkaline-MW pretreatments on the co-digestion of TWAS:FOG mixtures with 20, 40 and 60% FOG were investigated. MW pretreatment at a high temperature of 175ᵒC was shown to be the most effective MW pretreatment option in solubilizing TWAS:FOG mixtures and boosting methane yield. MW pretreatment at 175ᵒC resulted in maximum solubilization (%) of 68.2% for the 20%FOG samples and a maximum methane yield that was 137% higher than the control for samples with 60%FOG. The combined alkaline-MW (NaOH-MW) pretreatment at pH 10 proved to be not an effective option for TWAS:FOG pretreatment before the anaerobic co-digestion. Despite the benefits of MW pretreatment on the TWAS:FOG samples, including a significant increase in solubilization, dewaterability improvement, high VS reductions, and high methane yield productions, the energy analysis resulted in negative net energy values for all MW-pretreated samples.

Data on energy and economic evaluation and microbial assessment of anaerobic co-digestion of fruit rind of Telfairia occidentalis (Fluted pumpkin) and poultry manure

The data described in this article was obtained in an experiment designed for the generation of biogas from the anaerobic co-digestion of Telfairia occidentalis (Fluted pumpkin) fruit rind and poultry manure both of which currently constitute an environmental nuisance in the localities where they are found. The data presented in this article is on the use of combined heat and power (CHP) system to assess the energy and economic feasibility of applying thermo-alkali pretreatment procedures to one of the substrates (Fluted pumpkin) prior to anaerobic digestion. Also, the microbial characterization and succession pattern of important microbes during the anaerobic digestion process was evaluated and the data reported in this paper.

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.

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.

Comparison of dry and wet milling pre-treatment methods for improving the anaerobic digestion performance of the Pennisetum hybrid

A planetary ball mill increased the specific methane yield of thePennisetumhybrid by 42%.

Innovative pretreatment strategies for biogas production

Biogas or biomethane is traditionally produced via anaerobic digestion, or recently by thermochemical or a combination of thermochemical and biological processes via syngas (CO and H₂) fermentation. However, many of the feedstocks have recalcitrant structure and are difficult to digest (e.g., lignocelluloses or keratins), or they have toxic compounds (such as fruit flavors or high ammonia content), or not digestible at all (e.g., plastics). To overcome these challenges, innovative strategies for enhanced and economically favorable biogas production were proposed in this review. The strategies considered are commonly known physical pretreatment, rapid decompression, autohydrolysis, acid- or alkali pretreatments, solvents (e.g. for lignin or cellulose) pretreatments or leaching, supercritical, oxidative or biological pretreatments, as well as combined gasification and fermentation, integrated biogas production and pretreatment, innovative biogas digester design, co-digestion, and bio-augmentation.

Improving methane production from digested manure biofibers by mechanical and thermal alkaline pretreatment

Animal manure digestion is associated with limited methane production, due to the high content in fibers, which are hardly degradable lignocellulosic compounds. In this study, different mechanical and thermal alkaline pretreatment methods were applied to partially degradable fibers, separated from the effluent stream of biogas reactors. Batch and continuous experiments were conducted to evaluate the efficiency of these pretreatments. In batch experiments, the mechanical pretreatment improved the degradability up to 45%. Even higher efficiency was shown by applying thermal alkaline pretreatments, enhancing fibers degradability by more than 4-fold. In continuous experiments, the thermal alkaline pretreatment, using 6% NaOH at 55°C was proven to be the most efficient pretreatment method as the methane production was increased by 26%. The findings demonstrated that the methane production of the biogas plants can be increased by further exploiting the fraction of the digested manure fibers which are discarded in the post-storage tank.

Review of feedstock pretreatment strategies for improved anaerobic digestion: From lab-scale research to full-scale application

When properly designed, pretreatments may enhance the methane potential and/or anaerobic digestion rate, improving digester performance. This paper aims at providing some guidelines on the most appropriate pretreatments for the main feedstocks of biogas plants. Waste activated sludge was firstly investigated and implemented at full-scale, its thermal pretreatment with steam explosion being most recommended as it increases the methane potential and digestion rate, ensures sludge sanitation and the heat needed is produced on-site. Regarding fatty residues, saponification is preferred for enhancing their solubilisation and bioavailability. In the case of animal by-products, this pretreatment can be optimised to ensure sterilisation, solubilisation and to reduce inhibition linked to long chain fatty acids. With regards to lignocellulosic biomass, the first goal should be delignification, followed by hemicellulose and cellulose hydrolysis, alkali or biological (fungi) pretreatments being most promising. As far as microalgae are concerned, thermal pretreatment seems the most promising technique so far.

Optimization of hydrolysis and volatile fatty acids production from sugarcane filter cake: Effects of urea supplementation and sodium hydroxide pretreatment

Different methods for optimization the anaerobic digestion (AD) of sugarcane filter cake (FC) with a special focus on volatile fatty acids (VFA) production were studied. Sodium hydroxide (NaOH) pretreatment at different concentrations was investigated in batch experiments and the cumulative methane yields fitted to a dual-pool two-step model to provide an initial assessment on AD. The effects of nitrogen supplementation in form of urea and NaOH pretreatment for improved VFA production were evaluated in a semi-continuously operated reactor as well. The results indicated that higher NaOH concentrations during pretreatment accelerated the AD process and increased methane production in batch experiments. Nitrogen supplementation resulted in a VFA loss due to methane formation by buffering the pH value at nearly neutral conditions (∼ 6.7). However, the alkaline pretreatment with 6g NaOH/100g FCFM improved both the COD solubilization and the VFA yield by 37%, mainly consisted by n-butyric and acetic acids.

Effects of alkalinity sources on the stability of anaerobic digestion from food waste

This study investigated the effects of some alkalinity sources on the stability of anaerobic digestion (AD) from food waste (FW). Four alkalinity sources, namely lime mud from papermaking (LMP), waste eggshell (WES), CaCO3 and NaHCO3, were applied as buffer materials and their stability effects were evaluated in batch AD. The results showed that LMP and CaCO3 had more remarkable effects than NaHCO3 and WES on FW stabilization. The methane yields were 120.2, 197.0, 156.2, 251.0 and 194.8 ml g(-1) VS for the control and synergistic digestions of CaCO3, NaHCO3, LMP and WES added into FW, respectively. The corresponding final alkalinity reached 5906, 7307, 9504, 7820 and 6782 mg l(-1), while the final acidities were determined to be 501, 200, 50, 350 and 250 mg l(-1), respectively. This indicated that the synergism between alkalinity and inorganic micronutrients from different alkalinity sources played an important role in the process stability of AD from FW.

Anaerobic mesophilic co-digestion of ensiled sorghum, cheese whey and liquid cow manure in a two-stage CSTR system: Effect of hydraulic retention time

The aim of this study was to investigate the effect of hydraulic retention time (HRT) on hydrogen and methane production using a two-stage anaerobic process. Two continuously stirred tank reactors (CSTRs) were used under mesophilic conditions (37°C) in order to enhance acidogenesis and methanogenesis. A mixture of pretreated ensiled sorghum, cheese whey and liquid cow manure (55:40:5, v/v/v) was used. The acidogenic reactor was operated at six different HRTs of 5, 3, 2, 1, 0.75 and 0.5d, under controlled pH5.5, whereas the methanogenic reactor was operated at three HRTs of 24, 16 and 12d. The maximum H2 productivity (2.14L/LRd) and maximum H2 yield (0.70mol H2/mol carbohydrates consumed) were observed at 0.5d HRT. On the other hand, the maximum CH4 production rate of 0.90L/LRd was achieved at HRT of 16d, whereas at lower HRT the process appeared to be inhibited and/or overloaded.

Enzymatic and metabolic activities of four anaerobic sludges and their impact on methane production from ensiled sorghum forage

Biochemical methane potential (BMP) tests were run on ensiled sorghum forage using four inocula (urban, agricultural, mixture of agricultural and urban, granular) and differences on their metabolic and enzymatic activities were also discussed. Results indicate that no significant differences were observed in terms of BMP values (258±14NmLCH4g(-1)VS) with a slightly higher value when agricultural sludge was used as inoculum. Significant differences can be observed among different inocula, in terms of methane production rate. In particular the fastest biomethanization occurred when using the urban sludge (hydrolytic kinetic constant kh=0.146d(-1)) while the slowest one was obtained from the agricultural sludge (kh=0.049d(-1)). Interestingly, positive correlations between the overall enzymatic activities and methane production rates were observed for all sludges, showing that a high enzymatic activity may favour the hydrolysis of complex substrate and accelerate the methanization process of sorghum.

High pressure assist-alkali pretreatment of cotton stalk and physiochemical characterization of biomass

Ground cotton stalks were pretreated with sodium hydroxide (NaOH) at concentrations of 1-4% (w/v), pressures of 30-130 kPa, durations of 15-75 min, and liquid/solid ratios of 10:1-30:1. Modeling of the high pressure assist-alkali pretreatment (HPAP) of cotton stalk was attempted. The levels of NaOH concentration, pressure, and duration were optimized using a Box-Behnken design to enhance the cellulose content of treated solid residue. The optimum pretreatment conditions were as follows: liquid/solid ratio, 20:1; pressure, 130 kPa; NaOH concentration, 3.0%; duration, 40 min. During the conditions, cellulose content of pretreated cotton stalk residue was 64.07%. The maximum cellulose conversion of 45.82% and reducing sugar yield of 0.293 g/g upon hydrolysis were obtained. Significant differences were observed in biomass composition and physiochemical characteristics between native and alkali-treated biomass. High NaOH concentration and pressure were conducive to lignin dissolution and resulted in increased cellulose content and conversion.