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International audience; Anaerobic digestion (AD) is a waste treatment technology based on organic matter degradation by microorganisms in the absence of oxygen. This process generates two valuables products, biogas and digestate. While biogas is recognized and exploited for its energy potential, digestate could be an interesting fertilizer thanks to its rich-nutrient composition. However, implementation of AD still raises concerns about energy transportation, consequences of digestate spreading and poor conversion to carbon during digestion. To overcome the issues linked to AD, to integrate anaerobic digestion with the pyrolysis of digestate is an innovate path. This study aims to analyze sustainability of a multifunctional process coupling anaerobic digestion and pyrolysis. The process is applied to the treatment of sewage sludge and quinoa residue in co-digestion. To evaluate environmental impacts, life cycle assessment (LCA) is realized from a cradle-to-gate perspective, using SimaPro software (V8.5.2) and Environmental Footprint EF 3.0 method. Results underline that the main contributor to environmental impacts is the treatment of bio-oil, a pyrolysis product, while other impacts could be counterbalance thanks to energy production

This document presents the minimal requirements for measurement and validation of biochemical methane potential (also called biomethane potential)(BMP) in batch tests, and represents the consensus of more than 40 biogas researchers. The list of requirements is based on Holliger et al.[2016], with some recent modifications of validation criteria as described in Hafner et al.[2020c] and additional details on calculation standardization.

The aim of this study was to explore the efficiency of a mechano-enzymatic deconstruction of two macroalgae species for sugars and bioethanol production, by using a new enzymatic cocktail (Haliatase) and two types of milling modes (vibro-ball: VBM and centrifugal milling: CM). By increasing the enzymatic concentration from 3.4 to 30 g/L, the total sugars released after 72 h of hydrolysis increased (from 6.7 to 13.1 g/100 g TS and from 7.95 to 10.8 g/100 g TS for the green algae U. lactuca and the red algae G. sesquipedale, respectively). Conversely, total sugars released from G. sesquipedale increased (up to 126% and 129% after VBM and CM, respectively). The best bioethanol yield (6 g(eth)/100 g TS) was reached after 72 h of fermentation of U. lactuca and no increase was obtained after centrifugal milling. The latter led to an enhancement of the ethanol yield of G. sesquipedale (from 2 to 4 g/100 g TS).

Over the past decade, increasing interest has been addressed to anaerobic fermentation. These bioprocesses have been investigated to not only produce hydrogen but also other high-value by-products, such as carboxylic acids (mainly acetate, butyrate, lactate), ethanol and other solvents. Indeed, hydrogen is characterised by a clean combustion process and is an efficient energy carrier with a considerable calorific value (122 MJ/kg), but represent only 20% of the total COD in fermentation, with more than 80% of valuable metabolites. Interestingly, anaerobic dark fermentative processes imply complex mixed microbial consortia able to operate in non-sterile conditions and to use various kinds of waste as feedstocks. Nevertheless, hydrogen yields from solid waste are directly correlated to the initial content in soluble carbohydrates (Guo et al., 2010; Monlau et al., 2012). In this context, lignocellulosic residues, rich in carbohydrates, constitute relevant feedstocks for dark fermentation but pretreatment is required to solubilise hemicelluloses and cellulose and make carbohydrates more available to microorganisms. High release of soluble carbohydrates can be obtained by thermal-acid pretreatment, enzymatic hydrolysis, combined or not with alkali upstream pretreatment. This paper will present a synthetic view of several studies combining thermo-chemical and/or enzymatic pretreatment of lignocellulosic biomass (wheat straw (WS) and sunflower stalks (SS)) with the purpose of H2 production by dark fermentation. As main findings, enzymatic hydrolysis of WS with an enzyme cocktail secreted by Trichoderma strain showed the necessity of working under sterile conditions to avoid a re-consumption of solubilized sugars by the endogenous bacteria naturally present in the substrate. A two-fold hydrogen production from 10 to 20 mL/g VS was obtained after addition of 5 mg enzymes/ g wheat straw. Interestingly, same results were obtained by adding enzymes directly into the fermenter, leading to a simpler and cheaper process (Quemeneur et al., 2012). The combination of thermal-alkali pretreatment (55°C, 4% NaOH (w/wTS) for 24 h) and enzymatic hydrolysis (cellulose 50 FPU/gTS; glucosidase 25 U/gTS and xylanase 50 U/gTS) on SS led to a 21 fold increase in hydrogen production from 2.4 to 49 mL/gVS (Monlau et al., 2013b). In comparison, thermal acid (170°C, 4% HCl for 1 hour) of SS led to a decrease of hydrogen production which was explained by the release of inhibitory by-products during pretreatments (furfural, 5-HMFand phenolic compounds). Interestingly, performing glucose fermentation with increasing amounts of acid pretreatment hydrolysate showed a metabolism shift from acetate/butyrate/hydrogen to lactate/ethanol and finally ethanol production with a yield of 2 mol ethanol/mol glucose consumed (Monlau et al., 2013a).

Keynote; Anaerobic digestion is a key process for urban solid waste management converting organic waste into biogas, mainly composed of methane and carbon dioxide, and a residue called digestate which is generally separated into solid and liquid fractions. Based on an overview of the abundant literature published on municipal solid waste and lignocellulosic biomasses, the potentialities of anaerobic digestion processes will be presented. The first part of the lecture will discuss the interest of using pretreatment techniques to improve the conversion of wastes into biogas. Some intermediary products of anaerobic digestion such as fatty acids, ethanol and hydrogen present a higher added value than methane. Different anaerobic process parameters and the selection of specific microbial consortia allow an optimal production of these products while preventing methane production in the so-called dark fermentation process. The impact of waste pretreatment on the production of hydrogen and metabolites will also be discussed. Dark fermentation effluents may be treated in anaerobic digestion to produce biohythane, consisting of a mixture of biohydrogen and methane, and leading a cleaner and more efficient combustion than that of methane alone. In addition, digestates are rich in nitrogen, phosphorous and more or less stabilized carbon and can be used as fertilizers or soil improvers. More original uses of digestates have been proposed such as the conversion of the digestate solid fraction into activated biochar, bio-oil and syngas through thermal processes, or the use of nutrients present in the liquid fraction in biological processes such as algae growth or bioethanol production.

In many European countries, biogas production through anaerobic digestion (AD) is a very fastgrowing market in the agricultural sector. AD converts biologically organic matrix into biogas anddigestate, the latter corresponding to the anaerobically not degraded fraction. Until yet, digestate ismostly used as soil improver or fertilizer at farm scale, but its increasing amount causes problemsrelative to its transport cost, greenhouse gases emissions during its storage and its used is regulate bythe European Nitrate Directive.In this study, we investigated two energetically valorization routes (i.e. biofuels, pyrolysis) tomaximize the energy recovery from the initial feedstock and reduce the digestate amount.In the first experiment, digestate that contains still a high amount of sugars especially cellulose wasinvestigated for soluble sugars production for further biofuels used (i.e. bioethanol, biohydrogen…).For this purpose, digestate was mechanically treated (i.e. centrifugal milling, 0.12 mm) and furthersubmitted to enzymatic hydrolysis. Energy requirement of mechanical pretreatment and enzymatichydrolysis yields were compared between the anaerobic digestate and the original feedstock. First,energy requirement for mechanical pretreatment was less important for digestate (1.3 kWh.kg-1) thanoriginal feedstocks (3.3 kWh.kg-1). AD has a positive effect on the physicochemical structures of theoriginal feedstocks and consequently leads to a reduction of the energy requirement. Then, theenzymatic hydrolysis yield of digestate (63%) was increased compared to raw feedstock (54%). Theseresults suggests that digestate from AD process could represent a suitable residues for sugar release inthe optic of biofuels production.In the second experiment, a new concept of coupling AD with digestate post-pyrolysis wasinvestigated. Pyrolysis of digestate at 500°C results to 8.8 wt. %, 58.4 wt. % and 32.8 wt. % of syngas,oil and pyrochar, respectively. Like methane produced from AD process, syngas and bio-oil can befurther converted into heat and electricity through a combined Heat and Power system. By couplinganaerobic digestion with pyrolysis process the electricity producted raised from 9896 KWhel day-1 to14066 KWhel day-1 corresponding to an increase of 42% compared to AD process alone. On the otherhand, pyrochar can be used as soil amendment in order to preserve the fragile soil quality, a practicewhich is in line with the principles of ecology and sustainable agriculture.

The high crude oil prices, the decline of fossil fuel reserves and the increase of greenhousegas emissions are motivating the development of biorefineries that produce energy fromrenewable resources (i.e. lignocellulosic biomass). However, the key driver for the successfulconversion of biomasses into biofuels is the selection of efficient pretreatments that permit tomaximize the sugars recovery and to minimize the consumption of water, chemicals andenergy. This study proposes the application of “dry” chemo-mechanical pretreatments thatpermit to enhance enzymatic hydrolysis and thereafter bioethanol production fromlignocellulosic biomasses.In a first experiment, wheat straw was pretreated with NH3, NaOH, NaOH-H2O2 and NH3-H2O2 at both high (5 kg L-1) and at low (0.2 kg L-1) biomass concentrations, named “dry” and“diluted” pretreatments. Then, untreated and pretreated samples were subjected to centrifugaland ball milling, followed by enzymatic hydrolysis. Compared to diluted mechano -NaOHpretreatments, dry mechano -NaOH and -NaOH-H2O2 were found to be more effective indecreasing the particle size (up to 55%), increasing the surface area (up to 88%) anddecreasing the total energy requirement (up to 50%). Enzymatic hydrolysis was enhanced bypretreatments, as suggested by higher glucose yields (332, 320, 140, 322, 141 g glucose kg-1biomass for diluted –NaOH, dry – NaOH, dry -NH3, dry -NaOH-H2O2 and dry -NH3-H2O2,respectively), compared to that of untreated wheat straw (118 g glucose kg-1). However, lowerenergy efficiency was obtained for dilute mechano -NaOH treatment, compared to drymechano -NaOH treatments.In a second experiment, sugarcane bagasse was pretreated with NaOH and H3PO4 at highbiomass concentration (5 kg L-1) and then milled by using different methods, such asvibratory milling (VBM), ball milling (BM) and centrifugal milling (CM). Results indicatethat NaOH-BM and NaOH-VBM was preferred to enhance glucose yields and bioethanolproduction (up to 76%), while CM consumed 75% and 58% lesser energy than BM and VBM,respectively. The highest energy efficiency was obtained with NaOH-CM. Therefore, thecombination of dry NaOH and CM appears the most suitable and interesting pretreatment forthe production of bioethanol from SB.

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International audience; Second generation biofuels such as biohythane, a mixture of biohydrogen (H2) and methane (CH4), produced through two-stage H2 / CH4 anaerobic processes represent a promising alternative to fossils fuels. However, lignocellulosic substrates contain recalcitrant structures that require a pretreatment step before anaerobic fermentative processes. In the present study, the impact of alkaline pretreatment (55°C, 24h, 4% NaOH) before a two-stage H2(batch)/CH4(continuous) process was investigated. Alkaline pretreatment did not enhance hydrogen yield probably due to the low solubilization of holocelluloses into soluble sugars. However, alkaline pretreatment enhanced the methane yield resulting in a 30 % increase of the total energy produced compared to the two-stage H2 (batch)/ CH4 (continuous) with no pretreatment.

International audience; This paper reviews and discusses the impact of byproducts released during pretreatment of lignocellulosic materials on anaerobic mixed cultures producing hydrogen and methane. 5-HMF, furfural, phenolic compounds and aldehydes, are strong inhibitors of biohydrogen production but can be further converted into methane. This finding can be explained by differences in both process parameters: anaerobic digestion is performed with more complex mixed cultures, lower substrate/inoculum or byproducts/inoculum ratios and longer batch incubation times than dark fermentation. Indeed, the presence of byproducts may require an adaptation phase of the microbial community leading to longer lag phase in dark fermentation. Finally, the presence of pretreatment by-products may lead to a metabolic shift from hydrogen production to no H2-producing ethanol and lactate pathways and whatever the route of dark fermentation, metabolites can be all further converted into methane, at different rates.

In the future, various forms of renewable energy, such as second generation biofuels from lignocellulosic residues, will be required to replace fossil fuels. Among these, biohydrogen and methane produced through fermentative processes appear as interesting candidates. However, biohydrogen and/or methane production of lignocellulosic residues is often limited by the recalcitrant structure and a pretreatment step prior to fermentative processes is often required. Up to date, informations on lignocellulosic characteristics limiting both hydrogen and methane production are limited. Therefore, this work aims to investigate the effect of compositional and structural features of lignocellulosic residues on biohydrogen and methane performances for further developping appropriate pretreatments strategies. Firstly, a panel of twenty lignocellulosic residues was used to correlate both hydrogen and methane potentials with the compositional and structural characteristics. The results showed that hydrogen potential positively correlated with soluble carbohydrates only. Secondly, methane potential correlated negatively with lignin content and, in a lesser extent, with crystalline cellulose, but positively with the soluble carbohydrates, amorphous holocelluloses and protein contents. Pretreatments strategies were further developed to enhance both hydrogen and methane production of sunflower stalks. Dilute-acid and combined alkalineenzymatic pretreatments, which were found efficient in solubilizing holocelluloses into soluble carbohydrates, were applied prior to biohydrogen potential tests. By combined alkaline-enzymatic pretreatment, hydrogen potential was fifteen times more than that of untreated samples. On the contrary, hydrogen production was inhibited after dilute-acid pretreatments due to the release of byproducts (furfural, 5-HMF and phenolic compounds) that led to microbial communities shift toward no hydrogen producing bacteria. Similarly, methane production, five thermo-chemical pretreatments (NaOH, H2O2, Ca(OH)2, HCl and FeCl3) found efficient in delignification or solubilization of holocelluloses, were considered. Among these pretreatments, the best conditions were 55°C with 4% NaOH for 24 h and led to an increase of 29-44 % in methane potential of sunflower stalks. This pretreatment condition was validated in one stage anaerobic mesophilic continuous digester for methane production and was found efficient to enhance from 26.5% the total energy produced compared to one stage-CH4 alone. Two-stage H2 (batch) / CH4 (continuous) process was also investigated. Nevertheless, in term of energy produced, no significant differences were observed between one-stage CH4 and two-stage H2 /CH4.; Dans le futur, différentes sources d'énergies renouvelables comme les energies de seconde génération produites à partir de déchets lignocellulosiques seront nécessaires pour palier à l'épuisement des énergies fossiles. Parmi ces énergies de seconde génération, le biohydrogène, le méthane et l'hythane produits à partir de procédés fermentaires anaérobies représentent des alternatives prometteuses. Cependant la production de biohydrogène et de méthane à partir de résidus lignocellulosiques est limitée par leurs structures récalcitrantes et une étape de prétraitement en amont des procédés fermentaires est souvent nécessaire. A ce jour, peu d'informations sur les paramètres limitant la conversion des matrices lignocellulosiques en hydrogène ou méthane sont disponibles. Ce travail a pour but d'étudier l'impact des facteurs biochimiques et structurels des résidus lignocellulosiques sur les performances de production d'hydrogène et de méthane, pour pouvoir par la suite développer des stratégies de prétaitements adaptées. Tout d'abord, sur un panel de vingt substrats lignocellulosiques, les potentiels hydrogène et méthane ont été corrélés aux paramètres biochimiques et structurels. Les résultats ont mis en évidence que le potentiel hydrogène est uniquement corrélé positivement à la teneur en sucres solubles. La production de méthane quant à elle est négativement corrélée à la teneur en lignine et, à un moindre degré, à la cristallinité de la cellulose, mais positivement à la teneur en sucres solubles, holocelluloses amorphes et protéines. Par la suite, des stratégies de prétraitements ont été établies pour améliorer la production d'hydrogène et de méthane. Le couplage prétaitements alcalins/enzymatique ainsi que les prétraitements à l'acide dilué, efficaces pour solubiliser les holocelluloses en sucres solubles ont été appliqués en amont de la production d'hydrogène. En combinant le pretraitement alcalin avec une hydrolyse enzymatique, le potentiel hydrogène des tiges de tournesol fut multiplié par quinze. En revanche, suite aux prétraitements acides, la production d'hydrogène fut inhibée à cause de la libération de sous-produits (furfural, 5-HMF et composés phénoliques) engendrant un changement d'espèces bactériennes vers des espèces non productrices d'hydrogène. Pour la production de méthane, cinq prétraitements thermochimiques (NaOH, H2O2, Ca(OH)2, HCl and FeCl3) efficaces pour délignifier ou solubiliser les holocelluloses ont été étudiés. Parmi ces prétraitements, la meilleure condition fut 55°C à une concentration de 4% NaOH pendant 24 h, résulant en une augmentation du potentiel méthane variant de 29 à 44 % en fonction des tiges de tournesol. Cette condition fut par la suite validée en réacteurs anaérobies continus avec une augmentation de 26.5% de la production de méthane. Un procédé à deux étages couplant la production d'hydrogène en batch suivi de la production de méthane en continu fut aussi étudié. Néanmoins, aucune différence significative en termes d'énergie produite ne fut observée entre les procédés à deux étages (H2/CH4) et à un étage (CH4).

International audience; Biomass deconstruction processes have been extensively studied as pretreatement of enzymatic hydrolysis of cellulose for second generation bioethanol production. This paper proposes to review these processes with a special attention on their impact on biomass structure and characteristics and to discuss the interest of using them as pretreatment to enhance anaerobic digestion of lignocellulosic compounds. Studies showing the performance of pretreatments on biogas production from lignocellulosic biomass are also rewieved.

The conversion of sunflower stalks into biomethane by batch mesophilic anaerobic digestion was investigated in this work. Methane potential of sunflower stalks (192 ± 2 mLCH4/g VS) was increased by several thermo-chemical pretreatments: NaOH, H2O2, Ca(OH)2, HCl and FeCl3. The highest production (259 ± 6mLCH4/ g VS) corresponding to a 31.4% increase was reached for the treatment at 55°C with 4% NaOH during 24h. More than 90 % of hemicelluloses were removed by applying acidic pretreatment (10% FeCl3 or 4% HCl) at 170 °C. Oxidative and alkaline pretreatment (4% NaOH, 4% H2O2, 4% Ca(OH)2) at 55°C were found to be more efficient to dissolve lignin than acidic pretreatment with a lignin removal around 22%.A negative linear correlation (R2 = 0.895) was found between the lignin content of raw and pretreated sunflower stalks and the methane potential.

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