The transition toward a sustainable bioeconomy relies on efficient biomass valorisation processes, which critically depends on the ability to predict and control the flow behaviour of solid biomaterials like lignocellulosic biomass (LCB). However, the rheological properties of LCB remain difficult to characterise due to its intrinsic complexity, including broad particle size distributions, irregular and non-spherical morphologies, presence of moisture and additional variability arising from biological and seasonal factors. Building on these challenges, this study investigates how particle-scale properties influence LCB macroscopic flow under quasi-static and dense regimes, using wood sawdust as a model material. Beech and fir sawdust were characterised then tested under above mentioned flow regimes using a shear cell, a powder rheometer and a rotating drum. Under quasi-static conditions, flow function coefficient (FFC) values ranged from 5.2 to 9.8 for both materials, indicating easy-flowing behaviour. In dense regimes, FT4 tests showed that beech required higher mechanical energy than fir to achieve steady flow, while rotating drum experiments revealed cohesion indices of 0.15–0.22 for beech and 0.20–0.28 for fir. Additionally, experimental videos were analysed using Particle Image Velocimetry (PIV) to assess internal flow dynamics. These results demonstrate that flow behaviour is strongly influenced by particle size and shape, with fibrous and irregular morphologies promoting interlocking and entanglement.

The accumulation of intermediate products, particularly volatile fatty acids (VFAs) like propionic acid (HPr) or its dissociated form, can inhibit biogas production during anaerobic digestion (AD) at low concentrations. Knowledge about the response of microorganisms to VFA inhibition can help control the digesters. In this study, we aimed to determine how sodium propionate (NaPr) inhibits the AD of municipal sewage sludge by identifying shifts in the microbial community. Four 5 L reactors were operated in semi-continuous mode using sewage sludge and then loaded with different levels of NaPr. The reactors operated at 37 degrees C with two hydraulic retention times. The results show that there was no apparent inhibition of biogas production at NaPr loading up to 20.3 mmol & centerdot;L-1. However, moderate inhibition was observed at 81 mmol & centerdot;L-1, corresponding to an approximate 10% decrease in methane production, while a approximate to 40% decrease in methane production was observed at 135.3 mmol & centerdot;L-1. Sequencing analysis revealed that the community composition was dominated by Bacillota, Bacteroidota, Proteobacteria, Chloroflexi, and Cloacimonadota, with Halobacterota and Euryarchaeota as the main archaeal groups. PERMANOVA revealed incubation time as the primary driver of community structure, followed by NaPr concentration. Elevated NaPr levels resulted in a decline in Methanothrix and Methanobrevibacter and promoted distinct syntrophic propionate-oxidizing bacteria (SPOB).

This study aimed to evaluate the feasibility of treating a unique mixture of substrates through two-phase anaerobic digestion at reduced retention times. Sewage sludge was co-digested with organic fraction of municipal solid waste and straw-rich equine manure in 20 L laboratory-scale anaerobic digesters operated under thermophilic and mesophilic conditions. Despite short hydraulic retention times (HRT) of 2 d in the thermophilic reactor followed by 12 or 15 d in the mesophilic reactor, no apparent process inhibition was observed. The system operated under high organic loading rates in the mesophilic phase (6.7 and 5.3 gVS center dot L- 1 center dot d- 1). Under these conditions, methane yields of approximately 235-245 NLCH4 center dot kgVS- 1 were achieved, corresponding to nearly 80% of the biochemical methane potential of the co-digested substrate (301 NLCH4 center dot kg VS- 1). The methanogenic reactor also showed good resilience, with gas production improving over the course of the experiment, with the volumetric methane production rate reaching 1.60 NLCH4 center dot L- 1 reactor d- 1 at an HRT of 12 days and 1.27 NLCH4 center dot L- 1 reactor center dot d- 1) at 15 days. The addition of strawed equine manure improved the C/N ratio and avoided potential inhibition by ammonia, reducing ammoniacal nitrogen concentrations from 4.7 g center dot L- 1 to 2.6 g center dot L- 1. The laboratory results indicated a strong potential for scale-up, because the system exhibited consistent performance and adaptability. These results were subsequently used in the design of a pilot plant processing 2-3 t center dot d- 1, which is currently operational.

Propionic acid (HPr) accumulation is a major indicator of anaerobic digestion (AD) dysfunction, yet the respective contributions of acidity, undissociated HPr and the propionate ion (Pr-) to inhibition and microbial community composition remain unclear. Using mesophilic batch microcosms fed with municipal sewage sludge, we compared HPr, sodium propionate (NaPr), NaCl and HCl across two experimental series. Inhibition by HPr was governed by the pH drop rather than by Pr-: 20 mM HPr reduced the maximal methane rate by 22%, while 81 mM HPr (pH 5.1) abolished methane production, an effect reproduced by HCl at the same pH. At near-neutral pH, the propionate salt had only a secondary, concentration-dependent effect (81 mM NaPr, -40% on the rate), largely attributable to ionic strength (81 mM NaCl, -23%). 16S rRNA gene amplicon sequencing revealed compound-specific community restructuring: methanogenic archaea fell from 2 to 3% to < 0.2% of relative abundance under 81 mM HPr, and the > 100 differentially abundant ASVs were largely shared with HCl reactors, confirming the dominant role of acidity. The number of differentially abundant ASVs was negatively correlated with the methane rate (R & sup2; = 0.97). These results clarify the respective roles of pH, undissociated HPr and Pr- in both methane inhibition and microbial community restructuring.

Two-stage anaerobic digestion (TSAD) is an alternative to single-stage anaerobic digestion, separating hydrolytic–acidogenic and methanogenic phases to improve stability, organic matter degradation, and methane production. This review examines TSAD for co-digestion of sewage sludge (SS) and the organic fraction of municipal solid waste (OFMSW), emphasizing performance, scale-up challenges, digestate intensification, and nutrient recovery. TSAD can increase methane production by 25–50% compared with single-stage systems, while volatile solids removal can reach 87–93% depending on substrate type, temperature regime, hydraulic retention time, and organic loading rate. However, improvement remains variable and depends on substrate biodegradability, reactor configuration, and process control. Beyond methane recovery, the review highlights valorizing digestate as a secondary resource. Digestate post-treatment technologies, including thermal hydrolysis and steam explosion, report methane improvements from 26% to more than 300%, although energy demand and economic feasibility remain constraints. Nitrogen recovery technologies, including ammonia stripping and membrane contactors, can achieve efficiencies above 80–95% under optimized conditions, while phosphorus may be recovered through struvite precipitation, calcium phosphate recovery, or biochar-based pathways. Future TSAD development should integrate biological conversion, digestate recirculation, nutrient recovery, techno-economic assessment, and life-cycle evaluation to support circular, resource-efficient organic waste treatment systems.

The accurate determination of volatile fatty acids (VFAs) and total alkalinity (TAC, mostly carried by bicarbonate ions) is critical for operating anaerobic digesters. The FOS/TAC titration method developed by Nordmann is widely used due to its simplicity and affordability. This method has known limitations in dosing VFAs and TAC, since the presence of one interferes with the determination of the other, especially at higher VFA or bicarbonate concentrations. This study builds upon our prior research in 2021 by integrating the influence of phosphate (H2PO4−/HPO42−) into numerical models correcting FOS/TAC titration results. A Scilab-based program was used to assess the impact of phosphate on titration results, revealing significant biases at lower concentrations. A revised multivariate regression formula was developed, incorporating phosphate effects, and demonstrating superior accuracy. The mean absolute percentage errors (MAPE) for TAC and VFA estimation were reduced to less than 0.3%. The model maintains compatibility with standard Nordmann’s titration protocols and equipment while significantly improving reliability. These findings highlight the necessity of considering phosphate interference in FOS/TAC titration, particularly in AD systems with variable buffering conditions. The proposed correction model enhances process monitoring and control, providing a more robust tool for both research and industrial practice in anaerobic digestion.

Solid-state anaerobic digestion (SS-AD) is a promising technology for treating organic waste and producing renewable energy. This study explores the feasibility of using 3D printing to rapidly design cost-effective laboratory-scale digesters for optimization experiments. Batch reactors were designed using fused deposition modeling (FDM) with polylactic acid (PLA) and stereolithography (SLA) with High Temp V2 resin. PLA had a negligible impact on methane yields, while raw SLA resin positively influenced methanogenic potential, likely due to residual isopropanol used in post-processing, causing a 19% increase in CH4 yield. The performance of the 3D-printed reactors was compared to that of a conventionally machined PMMA reactor using cattle manure as a substrate, showing comparable methane yields and process stability. Three-dimensional printing technologies have demonstrated remarkable efficiency in designing laboratory-scale digesters, with a 70% cost reduction for SLA technology and an 80% reduction in design time compared to conventional reactors designed by plastics processing, while maintaining comparable biogas production. FDM technologies with PLA have shown that they are not suitable for these uses. This study demonstrates the potential of additive manufacturing to accelerate SS-AD research and development. However, care must be taken in material selection and post-processing to avoid introducing experimental bias.

Inhibition by propionic acid is a major challenge for the smooth operation of anaerobic digestion (AD) of sewage sludge for biogas production. We conducted an original study to better understand the respective roles of propionic acid, its conjugate base propionate and pH in the AD inhibition, and to identify potential inhibition biomarkers for an early warning. We focused on both methane production and microbial community composition, during the digestion of wastewater treatment sludge. A partial decrease in methane production was detected with the addition of 13 mM of propionic acid (1,000 mg.L-1), while total inhibition occurred for 81 mM (6,000 mg.L-1). Comparisons with the effects of adding sodium propionate, sodium chloride, and lowering the pH indicated that this inhibition was largely attributed to the H+ counter-ion. In particular, only moderate inhibition was observed when adding sodium propionate at a concentration of 81 mM. The composition of the microbial communities was significantly modified in the presence of propionate, with this adaptation specifically depending on the nature and concentration of the compound added, more precisely of the counterion. Various functional groups were affected, including syntrophs and methanogenic archaea. The relative abundance of the latter decreased significantly in the presence of propionic acid, consistent with the negative effects observed on the methane production rate, and with previous studies. Several microbial groups highly sensitive to propionic acid were identified in both our tests, in particular the family ST-12K33 (order Sphingobacteriales), which emerged as a strong biomarker candidate. ### Competing Interest Statement The authors have declared no competing interest. French research program MOCOPEE

This study aimed to define the optimal composition of three heterogeneous substrates of the anaerobic digestion process to maximize methane production. The investigated substrates were sewage sludge (SS), the organic fraction of municipal solid waste (OFMSW), and horse waste (HW). The optimal composition of these substrates was defined using the mixture design and, more specifically, the simplex–centroid mixture design. Customized methods and materials were employed to study the complex mixture design of these substrates. The findings revealed that the optimal mixture involved all three substrates with the composition 0.17 HW, 0.66 SS, and 0.17 OFMSW, which demonstrated the highest methane yield at 269 NmL·gVS−1. In addition, a mathematical model was developed to predict methane production based on a specific composition of co-substrates. The results were validated at the small pilot scale.

Straw is a substantial agricultural by-product for biogas production. Hydrolysis of straw is found to be a rate-limiting step during its anaerobic digestion and could be enhanced by pretreatment. In this paper, the effect of various combinations of particle size reduction, autoclaving, and low-level Fenton reaction was studied on straw for biogas production. Grinding of straw contributed to the maximum increase in the biomethane potential. Only Fenton or only the autoclave process improves the kinetics slightly but does not considerably improve the biomethane potential. Combining autoclaving and low-concentration Fenton pretreatment considerably improves the BMP values. Lignin content, CHNSO elemental analysis, Scanning Electronic Microscopy (SEM), Simon’s staining, infrared spectroscopy (DRIFT and ATR), Nuclear magnetic resonance spectroscopy, and wide-angle X-ray diffraction analysis (WAXD) were used to characterize the physical and chemical changes of straw due to pretreatment. Results show a poor correlation between biogas production and the different physical and chemical biomass characteristics. It makes it difficult to explain the outcome of various pretreatment methods applied to biomass. Without further improvement and development of analytical techniques, the prediction of the biomethane potential of a feedstock with the aid of pretreatment can only be considered in case-by-case studies.

Solid residence time distribution (SRTD) analysis is important in the design, modeling, and operation of solid biomass in continuous dry anaerobic digestion (D-AD) reactors. SRTD analysis could allow the appropriately adjust of the applicable organic charge and the study of unusable volume due to solid accumulation and preferential pathways. The lack of sustainable, environmentally friendly, and cost-effective solid tracers limits the study of solid flow patterns in D-AD digesters. In this study, 3D printing enabled the fabrication of customized Poly Lactic Acid (PLA) tracers with a specific shape to perform SRTD analysis of the semi-continuous digestion of straw-cattle manure (SCM) in a horizontal reactor of 0.5 m3. The recovery of tracers at the reactor outlet allowed the determination of the average SRTD at 29.43 days. Solid convection was predominant over diffusion, indicating that the reactor behaves closer to a plug flow reactor (PFR) rather than a complete stirred tank reactor.

A simplified AM2 model was developed to characterize mussel solid-state anaerobic digestion. This model considers two different substrates for mussels' degradation: the mussel meat and the mussel juice obtained after sanitization. This model was implemented to characterize the anaerobic degradation of Mytilus edulis species. This model was verified, implemented, and validated in 60 L batch reactors in mesophilic conditions. Two different experiments were used to calibrate kinetics using reaction invariants and an interior point optimization method. A conditioning study and a sensitivity analysis were done and had shown a better sensitivity with delayed substrate injections throughout the experiment with a factor of 10. An 88.6 % accumulation of methane yield of the BMP measurement was observed, corresponding to 57.7 % volatile removal with a minimum mass balance of 96.1 %. Additionally, the model proposed in this study was able to successfully predict the two characteristic methane yield peaks observed during solid-state anaerobic digestion.

Anaerobic digestion (AD) is a promising way to produce renewable energy. The solid-state anaerobic digestion (SSAD) with a dry matter content more than 15% in the reactors is seeing its increasing potential in biogas plant deployment. The relevant processes involve multiple of evolving chemical and physical phenomena that are not crucial to conventional liquid-state anaerobic digestion processes (LSAD). A good simulation of SSAD is of great importance to better control and operate the reactors. The modeling of SSAD reactors could be realized either by theoretical or statistical approaches. Both have been studied to a certain extent but are still not sound. This paper introduces the existing mathematical tools for SSAD simulation using theoretical, empirical and advanced statistical approaches and gives a critical review on each type of model. The issues of parameter identifiability, preference of modeling approaches, multiscale simulations, sensibility analysis, particularity of SSAD operations and global lack of knowledge in SSAD media evolution were discussed. The authors call for a stronger collaboration of multidisciplinary research in order to further developing the numeric simulation tools for SSAD.

Cet article propose un état des lieux des méthodes pour la détermination des paramètres rhéologiques des biomasses grossières appliquées à la méthanisation en voie solide. La méthanisation en voie solide est un procédé biologique permettant la valorisation sous forme de biogaz des sous-produits à forte teneur en matières sèches, en particulier les sous-produits agricoles, les déchets ménagers et les déchets alimentaires. Afin d’assurer le développement de plusieurs technologies de méthanisation, des connaissances précises sur les propriétés rhéologiques des substrats et du milieu réactionnel sont indispensables pour la conception, l’opération et la simulation de procédés. Malgré l’importance des propriétés rhéologiques, très peu de données du comportement rhéologique de la biomasse grossière sont présentes dans la littérature. Cela s’explique par le fait que les rhéomètres rotatifs conventionnels ne sont pas bien adaptés à la biomasse hétérogène comportant des particules grossières et de longues fibres. Jusqu’à présent, les équipements rotatifs conçus permettent de déterminer les propriétés rhéologiques de biomasse concentrée contenant des particules d’une taille maximale de 3 cm alors que les biomasses peuvent contenir des fibres lignocellulosiques ayant jusqu’à 25 cm de longueur. Compte tenu de cette limitation instrumentale, différents appareils alternatifs issus des domaines de l’analyse du béton, des sols ou des aliments ont été récemment adaptés pour accéder à des propriétés rhéologiques des biomasses agricoles. Dans cet article, ces différents outils ont été répertoriés en précisant des informations sur les facteurs d’influence, les principes physiques théoriques, le montage expérimental et leur gamme d’application pour la biomasse. Hélas, pour certains de ces appareils, les relations trouvées sont à ce jour plutôt qualitatives que quantitatives et les raisonnements mathématiques permettant d’intégrer complètement les phénomènes physiques observés avec les facteurs influençant la rhéologie de la biomasse doivent encore être investigués afin de mettre en place des méthodes normalisées spécifiques à une biomasse grossière.

In the anaerobic digestion world, the dry matter (DM) and organic matter (OM) contents of the samples are major data for the design, handling, and maintenance of the processing of biogas plants. Volatile compounds, such as volatile fatty acids and ammonia, are likely to volatilize during drying at 105 °C and, thus, distort DM values; they have been described at length in the literature. On the other hand, the partial decomposition of bicarbonate, present in the digestion media, has been little described in the scientific literature. In this work, it appears that, in accordance with the stoichiometry of the partial decomposition of bicarbonate into CO2 and H2O, about 37% is volatilized as early as 70 °C and a fortiori at 105 °C, whether in aqueous solutions of pure bicarbonate or in different real digester media. With freeze-drying, no degradation of bicarbonate was observed, confirming the thermo-dependence of this reaction. This decomposition leads to an underestimation of DM measurements and some erroneous DM mass balances. It also led to an indirect overestimation of the OM, when expressed as a percentage of the DM.

Knowledge of rheological evolution of biomass during dry anaerobic digestion (D-AD) is important in the engineering design, modeling, and operation of D-AD reactors. In this work, two methods of rheological analysis, the slump test and the shear-box, were used to measure the evolution of the yield stress, cohesion and friction angle of the straw-cattle manure (SCM) during the D-AD. Firstly, four 60 L batch leach-bed reactors (LBR) were started in parallel and stopped at different stages of the D-AD process on days 0, 10, 21 and 31. Secondly, a 500 L and 2 m length plug flow reactor (PFR) was operated with 40 days of solid retention time and samples were recovered at different positions. The solid degradation during D-AD process was monitored by analysis of the degradation of volatile solids, the fiber content and the Flash BMP. Similar degradation patterns of SCM and rheological evolution were observed in both reactors type. VS content decreased of 10.7% and 10.2% in 30 days in PFR and LBR respectively. VS degradation in both cases was well explained by hemicellulose and cellulose consuming in D-AD process. Considering the rheological analysis, the results showed that D-AD induced a reduction of the yield stress of 28.1 and 24.2% in 30 days in PFR and LBR respectively. Moreover, a similar evolution of cohesion and friction angle value for samples from both reactors was observed. This study demonstrates the close relationship between the state of degradation of the solid biomass and its rheological properties.

Lignocellulosic biomass is a low-cost and environmentally-friendly resource that can be used to produce biofuels such as bioethanol and biogas, which are the leading candidates for the partial substitution of fossil fuels. However, the main challenge of using lignocellulosic materials for biofuel production is the low accessibility to cellulose for hydrolysis of enzymes and microorganisms, which can be overcome by pretreatment. Biological and chemical pretreatments have their own disadvantages, which could be reduced by combining the two methods. In this article, we review biological–chemical combined pretreatment strategies for biogas and bioethanol production. The synergy of fungal/enzyme–NaOH pretreatment is the only biological–chemical combination studied for biogas production and has proven to be effective. The use of enzyme, which is relatively expensive, has the advantage of hydrolysis efficiency compared to fungi. Nonetheless, there is vast scope for research and development of other chemical–biological combinations for biogas production. With respect to ethanol production, fungal–organosolv combination is widely studied and can achieve a maximum of 82% theoretical yield. Order of pretreatment is also important, as fungi may reduce the accessibility of cellulose made available by prior chemical strategies and suppress lignin degradation. The biofuel yield of similarly pretreated biomass can vary depending on the downstream process. Therefore, new strategies, such as bioaugmentation and genetically engineered strains, could help to further intensify biofuel yields.