This study proposes a circular economy strategy to recover phenolic compounds by valorizing shrimp shell waste into a chitosan biosorbent (CH-B). Its adsorption efficiency was evaluated compared to commercial activated carbon (AC) and synthetic XAD-4 resin. Kinetic analysis revealed that while both pseudo-first-order (PFO) and pseudo-second-order (PSO) models exhibited high correlations (R2 ≥ 0.96), both CH-B and XAD-4 resin were best described by the PFO model. This aligns with diffusion-controlled processes consistent with the porous and physical nature of these adsorbents. In contrast, AC followed the PSO model. Isotherm modeling indicated that CH-B and AC fit the Temkin model, reflecting heterogeneous surfaces, whereas XAD-4 followed the Langmuir model (monolayer adsorption). Notably, CH-B exhibited a maximum adsorption capacity (qm) of 229.2 mg/g, significantly outperforming XAD-4 (104.8 mg/g) and AC (90.2 mg/g). Thermodynamic and kinetic modeling confirmed that the adsorption mechanism was governed by a combination of electrostatic interactions, π–π stacking, and hydrogen bonding between the hydroxyl/amine groups of chitosan and phenolic compounds. Optimization using Box–Behnken design for CH-B showed optimal acidic pH and moderate temperature but non-significant effect of CH-B dose in the experimental domain. Optimisation results showed unexpected high removal efficiency at low CH-B dosages. A tentative explanation may be adsorbent aggre-gation, which needs to be confirmed by further experimental evidence.

This study proposes a scale-down approach to validate the consistency of laboratory and pilot-scale anaerobic digestion (AD) data through comparison with a semi-industrial AD system. The approach aims to evaluate the consistency and transferability of experimental findings across different scales, thereby contributing to more robust and predictive insights in AD project development. To this purpose four systems were operated for 200 days under mesophilic conditions in parallel at different volumes: 5 L, 12 L, 20 L, and 150 m³ (semi-industrial scale). Ensiled sweet corn waste and pig slurry (35% / 65 % respectively in terms of raw biomass) were codigested. Biological parameters (pH, VFAs, NH₄⁺), energetic performance (biogas and methane yields), digestate quality, and microbial community structure were assessed. Results showed stable operation across all scales, with methane yields ranging from 255 to 312 NL CH₄/kg VS. Digestate characteristics (nutrients, heavy metals, pathogens) complied with regulatory thresholds regardless of scale, and rheological behavior exhibited similar shear-thinning trends. In terms of microbial communities, 16S rRNA analysis revealed initially distinct communities from the same inoculum source that become similar by the third HRT. During the scale comparison, feedstock preparation emerged as a key factor influencing performance, emphasizing the need to apply similar practices for laboratory and pilot trials in order to improve the representativeness of full-scale AD units.

Anaerobic digestion (AD)-derived digestate can be used as an organic fertilizer or for soil amendment. However, its utilization for resource recovery raises valid biosafety concerns. Despite extensive research on the capacity of AD for pathogen reduction, the variability in results poses challenges for drawing definitive conclusions. To address this lack of unification, results from 121 scientific articles were compiled, and a comprehensive metaanalysis was conducted. Findings indicate that artificial pathogen spiking leads to performance overestimation. Current most common indicators represent accurately their respective microbial groups. Clostridiaceae are barely affected by AD and may be favored by some pre-treatment technologies. The impact of operational parameters and the coupling of pre- and post-treatments with AD on pathogen reduction was also investigated. While an optimal batch duration was identified, the hydraulic retention time in (semi)continuous systems did not affect the overall pathogen reduction. Heat-based post-treatments coupled with thermophilic AD resulted in the highest pathogen reductions, fulfilling legislations. Unprecedented statistical analyses allowed categorizing quantitatively key parameters. Results confirmed that temperature is the most relevant parameter. Thermophilic conditions resulted in the highest pathogen reductions, while psychrophilic and mesophilic temperatures showed similar performances. The impact of pH on pathogen removal was confirmed, with acidic and basic values enhancing pathogen reductions. More research considering all AD products within a multicriteria optimization approach (e.g., pathogen reduction, biogas production, and digestate quality) is needed to determine optimal conditions considering all aspects. This study provides novel and relevant conclusions for AD at research and industrial scale, drawing several R&D perspectives.

Olive mill wastewater (OMWW) has high energetic potential due to its organic load, but its complex composition and toxicity limit efficient energy recovery. This study proposes an innovative integrated process combining continuous resin adsorption with anaerobic digestion to detoxify OMWW and recover renewable energy simultaneously. It studies the recovery of polyphenols, methane production, and substrate degradation efficiency using resin column bed heights (C1: 5.7 cm, C2: 12.1 cm, C3: 18.5 cm), as well as kinetic modeling of organic matter degradation. Adsorption reduced chemical oxygen demand (COD) by up to 80% and polyphenols by up to 64%, which significantly improved substrate biodegradability from 34% to 82%, corresponding to a methane yield of 287 mL CH4/g COD. Organic matter was fractioned into rapid (S1), moderate (S2), and slow (S3) biodegradable fractions. The highest degradation kinetics was C3, with methane production rates of K1 = 23.86, K2 = 2.47, and K3 = 2.92 mL CH4/d. However, this condition produced the lowest volumetric methane production due to excessive COD removal, including readily biodegradable matter. These results highlight the importance of optimizing the adsorption step in order to find to a balance between detoxification and energy recovery from OMWW, thus supporting the principles of circular economy and promoting renewable energy production.

Les procédés de séparation de phases des digestats de méthanisation des boues d’épuration nécessitent encore d’être optimisés pour garantir une valorisation noble des phases liquide et solide obtenues. L’ajout de réactifs de coagulation-floculation, fréquemment de type polyacrylamide cationique, est un outil pertinent à cette optimisation. Il existe actuellement un intérêt grandissant pour les biopolymères naturels, produits à partir de ressources renouvelables et présentant une innocuité environnementale. Le chitosane fait partie des biopolymères les plus abondants dans le monde, et a fait l’objet de plusieurs recherches en coagulation-floculation d’effluents et de digestat. Toutefois, ses applications sont limitées à des conditions acides puisqu’il n’est pas soluble aux valeurs de pH supérieures à 6. Afin d’étendre sa solubilité sur toute la gamme de pH, il est possible de le fonctionnaliser, notamment avec du chlorure de glycidyltriméthylammonium pour y greffer une charge cationique (ammonium quaternaire). Au cours de cette étude, trois lots de chitosane fonctionnalisé de différents degrés de quaternisation (DQ) ont été synthétisés et testés sur un digestat de méthanisation de boue de station de traitement des eaux usées tout en étant comparés à un floculant de type polyacrylamide cationique usuel. Ont été évalués les effets du DQ sur les propriétés physico-chimiques du digestat et son comportement rhéologique, sa décantabilité et sa filtrabilité. Les tests ont révélé que l’ajout de chitosane quaternisé entraîne peu ou pas de variations du pH et de la conductivité électrique du digestat, tandis qu’il génère une baisse de la viscosité du digestat, au contraire du polymère commercial qui augmente son caractère visqueux. Concernant la séparation de phases, les floculants améliorent tous la décantabilité du digestat et diminuent sa résistance spécifique à la filtration. Quel que soit le paramètre étudié, l’efficacité du chitosane fonctionnalisé augmente avec l’augmentation du DQ et de la dose appliquée.

The electro-Fenton (EF) disintegration using iron electrodes was performed for the pretreatment of waste activated sludge (WAS). The effect of this electro-chemical pretreatment on anaerobic digestion (AD) performance and microbial population structure was studied. An improvement of biodegradability and bioaccessibility of organic matter was demonstrated. AD of pretreated WAS in an up-flow anaerobic sludge blanket reactor (UASB) resulted to an increase of biogas production by 60 % compared to control reactor without disintegration. PCR-DGGE and real-time qPCR analyses showed that the high abundance of bacteria and the coexistence of Coprothermobacter in the UASB digestate fed with disintegrated sample established a stable bacterial association which is in line with the AD performance. Besides, the increased number of methanogens along the process allowed the improvement of methane production in comparison to control reactor.

Although drawing inspiration from live animals to develop processes, technologies, or materials is not a new approach, there is rising interest toward biomimicry for implementing new ideas. This review presents the mechanisms and strategies of mechanical pretreatment developed by animals to enhance the digestion of their food. There is ample data in the literature describing animal digestive systems; however, very few compilations encompass all clades with a main focus on mechanical pretreatment. The objective is to improve, through a bioinspired approach, the design of anaerobic digestion processes that are comparable but less efficient than animal digestion. To assess the variety and diversity of strategies that animals have developed during their evolution to adapt and optimize this pretreatment step, the following items were identified, classified, and related: (i) types of mechanical pretreatments, (ii) organs used, (iii) body location of mechanical pretreatment organs, and (iv) substrates, according to their accessibility. Animals are found to have converged toward several solutions to mechanically process their food. The type of organ is associated with the phylogeny while the type of pretreatment is rather associated with the type of food protection. Grinding is the most commonly used pretreatment, covering the widest range of substrates and ensured by the largest number of organs. Finally, animal pretreatment strategies and industrial grinders are compared and discussed in order to identify potential technological improvements.

Biomass can be used as feedstock for the production of biomaterials, chemicals, platform molecules and biofuels. It is the most reliable alternative to reduce fossil fuel consumption and greenhouse gas emissions. Within the framework of the circular economy, resource recovery from organic waste, including sewage sludge, biowaste, manure and slaughterhouse waste, is particularly useful, as it helps saving resources while reducing environmental pollution. In contrast to energy crops, lignocellulosic biomass and algae do not compete for food production; therefore, they represent an important source of biomass for bioenergy and bioproducts. However, biomass may require a pretreatment step in order to enhance its conversion into valuable products in terms of process yield and/or productivity. Furthermore, a pretreatment step may be mandatory for waste management (i.e., animal by-products). Pretreatment technologies are applied upstream of various conversion processes of biomass into biofuels or biomaterials, including bioethanol, biohydrogen, biomethane, biomolecules or bioproducts. Pretreatments may include mechanical, thermal, chemical and biological techniques, which represent a crucial, cost-intensive step for the development of biorefineries. Thus, research is needed to help identify the most effective, economic, and environmentally friendly pretreatment options for each feedstock. This Special Issue aims to gather recent developments of biomass pretreatments for bioproduct and biofuel production.

Fast development of centralized agricultural biogas plants leads to high amounts of digestate production. The treatment and disposal of liquid fractions after on-site digestate solid–liquid separation remains problematic due to their high organic, nutrient and aromatic contents. This work aims to study the variability of the remaining compounds in the digestate liquid fractions in relation to substrate origin, process parameters and solid–liquid separation techniques. Twenty-nine digestates from full-scale codigestion biogas plants and one waste activated sludge (WAS) digestate were collected and characterized. This study highlighted the combined effect of the solid–liquid separation process and the anaerobic digestion feedstock on the characteristics of liquid fractions of digestates. Two major clusters were found: (1) liquid fractions from high efficiency separation process equipment (e.g., centrifuge and others with addition of coagulant, flocculent or polymer) and (2) liquid fractions from low efficiency separation processes (e.g., screw press, vibrating screen and rotary drum), in this latter case, the concentration of chemical oxygen demand (COD) was associated with the proportion of cow manure and energy crops at biogas plant input. Finally, SUVA254, an indicator for aromatic molecule content and the stabilization of organic matter, was associated with the hydraulic retention time (HRT).

Anaerobic digestion (AD), being the most effective treatment method of waste activated sludge (WAS), allows for safe disposal. The present study deals with the electro-Fenton (EF) pretreatment for enhancing the WAS biogas potential with low-cost iron electrodes. The effect of pretreatment on the physicochemical characteristics of sludge was assessed. Following EF pretreatment, the pH, conductivity, soluble chemical oxygen demand (SCOD), and volatile fatty acids (VFA) increased to 7.5, 13.72 mS/cm, 4.1 g/L, and 925 mg/L, respectively. Capillary suction time (CST) analysis highlighted the dewaterability effect of EF on WAS, as demonstrated by the decrease in CST from 429 to 180 s following 30 min of pretreatment. Batch digestion assays presented an increase in the biogas yield to 0.135 L/g volatile solids (VS) after 60 min of EF pretreatment in comparison to raw sludge (0.08 L/g VS). Production of biogas was also found to improve during semi-continuous fermentation of EF-pretreated sludge conducted in a lab-scale reactor. In comparison to raw sludge, EF-pretreated sludge produced the highest biogas yield (0.81 L biogas/g VS) with a high COD removal rate, reaching 96.6% at an organic loading rate (OLR) of 2.5 g VS/L. d. Results revealed that the EF process could be an effective WAS disintegration method with maximum recovery of bioenergy during AD.

Abstract Anaerobic digestion is a process where microorganisms degrade organic matter in the absence of oxygen to produce biogas, mainly composed of methane and carbon dioxide. Recent years have seen a strong development of anaerobic digestion units worldwide, especially in USA, Europe and China. Countries like China, Germany, USA, Italy, UK and France are seen leading in the biogas sector in the world due to long establishment, intensive research and government incentives for renewable energy as well as waste management solution option. Asian countries have also shown their interests in the biogas technology. With more proper research and studies on-going, Asia will see the growth of biogas sector in the next few coming years. Even though African countries are in their primary phase to develop this biogas technology and there are still more hurdles to overcome, there have been interests, development and implementation work to apply this technology for its potential renewable energy production as well as its waste management solution.

By-products of anaerobic digestion, digestate is commonly managed via several ways for its optimal transportion and application. The common practice for digestate management is through solid-liquid separation. The common use for solid fraction of digestates are either through land spreading; directly applied or after composting, as organic fertilizer. Several routes proposed for valorization of solid digestate include production of biochar, bio-fuel for domestic furnaces, bioethanol production after centrifugal milling as well as post treatments (enzymatic, thermal and alkaline) for the recovery of methane. Liquid fraction of digestates contain high concentration of nutrients; from 1.5 to 6.5 g/L total nitrogen and from 0.94 to 2.51 g/L total phosphorus (P2O5) as well as high ions concentrations from 0.5 to 3.1 g/L ammonium (NH4+), from 1.05 to 5.48 g/L potassium (K+) and from 0-2.13 g/L phosphate (PO43-). Besides it also contains other ions such as sodium, chloride, magnesium, calcium and sulfate. High nutrients concentration limits its application to land with maximum application of 60 kg/ha/y of phosphate and 100kg/ha/y of potassium. Therefore, the removal of these nutrients is important before land application or disposal. In addition, these nutrients could be marketed to regions with high demand of nutrient or to the non-agricultural sector. The opportunities for nutrients marketing from digestate are largely unexploited and the strategies for marketing is still immature. This paper reviews the current technology on the removal, recovery as well as reuse of nutrients from liquid fraction of digestate. The discussion on the removal of nutrients include ammonia stripping, anaerobic ammonium oxidation (ANNAMOX), direct contact membrane distillation, constructed wetland system and vapor pressure membrane contactor. Nutrients recovery technology discussed in this paper include vacuum evaporation, struvite recovery, vacuum thermal stripping with acid absorption, combined evaporation and reverse osmosis. Meanwhile, the current technology on nutrients reuse include cultivation with microalgal for biomass production, nutrients recycling back to digester, soil application and subsurface injection into soils.