Digestate management is essential to both environmental protection and sustainable agriculture. However, its safe and effective agricultural use is increasingly challenged by strong variability in composition and the presence of persistent contaminants, including heavy metals, pharmaceuticals, microplastics and per- and polyfluoroalkyl substances. While numerous treatment and recovery technologies have been developed, existing research remains fragmented, often addressing individual contaminants or processes without a unified framework linking environmental risk, technological performance and system-level decision-making. This review systematically synthesizes current knowledge on digestate characteristics, contaminant profiles and treatment technologies, with particular emphasis on nutrient recovery, contaminant control and circular valorisation pathways. Conventional, advanced and emerging technologies are critically compared across Technology Readiness Levels, highlighting trade-offs between recovery efficiency, contaminant fate, energy demand and the generation of secondary waste streams. The analysis reveals that many widely applied technologies primarily redistribute contaminants into concentrated side streams, underscoring the need for integrated treatment trains that combine separation, recovery and destructive steps. Building on this synthesis, the review proposes a structured decision framework that links digestate composition and management objectives with appropriate technology combinations, explicitly integrating techno-economic feasibility, environmental performance and technology maturity. This framework provides a practical tool for selecting and designing digestate valorisation strategies tailored to feedstock characteristics and regulatory constraints. Finally, critical research gaps and policy needs are identified to support the transition toward environmentally safe, economically viable and circular digestate management systems.

In this study, an analytical methodology for the determination of pesticides commonly found in sludge and wastewater was developed and validated based on the citrate-buffered QuEChERS (Quick, Easy, Cheap, Effective, Rugged, Safe) extraction procedure. Particularly, to succeed a limit of quantification of 0.001 mg/L in a matrix of synthetic wastewater or sludge, a 1:8 preconcentration was used with the utilization of a rotary evaporator. The method was tested for twelve pesticides identified and quantified via liquid chromatography tandem mass spectrometry (LC/MS-MS) and validated according to the quality control guidelines of EU’s SANTE 11312/2021. Obtained measurements provided acceptable linearity, selectivity, sensitivity, precision and accuracy at very low concentrations, indicating the potential of the method to study the fate of pesticides in wastewater and sludge samples, especially those related with fruit processing industry wastes. Recoveries at two fortification levels (0.001 and 0.125 mg/L) ranged 72-107 % for the vast majority of the target analytes, with relative standard deviations of less than 20% for all analytes except one. The effect of matrix was found to be different for the wastewater and sludge with a higher ion suppression response identified for the sludge samples.

Amid escalating environmental challenges and the global pursuit of carbon neutrality, sustainable pathways for platform chemical synthesis are a critical research priority. Acetic acid, a high-demand compound with extensive industrial applications, is still predominantly produced via petroleum-based processes, raising environmental and economic concerns. Considering these challenges, this study explores the biological conversion of biogenic carbon dioxide (CO2) into acetic acid, promoting a circular carbon economy. Specifically, it investigates the role of packing and conductive materials in influencing biological CO2-to-acetic acid conversion, demonstrating that their effects are material-specific, with some enhancing and others inhibiting process performance. To enhance acetogenic activity, 2-bromoethanesulfonic acid was used to inhibit methanogenesis, thereby favoring the Wood-Ljungdahl pathway. The experimental design incorporated a multilevel categorical design (MCD) to assess the individual and combined effects of support materials, namely packing and conductive types, on key process parameters: pH, yield (g acetic acid/g CO2), and efficiency (%). Zero-valent iron was identified as the most effective conductive material, achieving a yield of 0.68 g acetic acid/g CO2 and an efficiency up to 99.8 %. The statistical models developed through MCD exhibited strong predictive performance (R2 > 0.80; prediction errors < 3 %) and were validated experimentally. Furthermore, an artificial neural network (ANN) was trained and integrated with a genetic algorithm (GA) to optimize system performance. The ANN-GA model demonstrated slightly higher predictive accuracy compared to MCD. Overall, this study confirms the significant role of support material selection in enhancing biological CO2 conversion and presents an advanced modeling framework for process optimization.

This work studies and analyzes the transition from a linear to a circular economy through wastewater treatment and resource recovery. As wastewater volumes grow, sustainable management becomes critical. This study highlights the reuse of treated effluent, beneficial sludge utilization, and energy generation via anaerobic digestion. Wastewater treatment plants should be envisioned as hubs for recovering water, materials, and energy, rather than disposal facilities. Emphasizing resource efficiency, the circular economy approach offers viable solutions to challenges related to resource scarcity, climate change, and ecological impact.

Waste valorization involves reusing and recycling waste materials to create useful products such as materials, chemicals, fuels, or energy. The primary goal is the transition to a circular economy model while minimizing the impacts of hazardous waste. Adopting such policies appears to be a one-way path due to the continuous increase in the consumption of raw materials. According to recent projections, by 2050, 180 billion tonnes of materials will be consumed annually. Since natural resources cannot meet these requirements, new sources must be explored. Waste can serve as an alternative source and cover at least part of the needs that arise. In this work, good practices regarding waste valorization are presented. The case studies examined include the waste/by-products of ultrabasic rocks resulting in chromite and magnesite mining, as well as the tannery sludge produced after the corresponding wastewater treatment.

The biological methanation process has emerged as a promising alternative to thermo-catalytic methods due to its ability to operate under milder conditions. However, challenges such as low hydrogen solubility and the need for precise trace element supplementation (Fe(II), Ni(II), Co(II)) constrain methane production yield. This study investigates the combined effects of trace element concentrations and applied pressure on biological methanation, addressing their synergistic interactions. Using a face-centered composite design, batch mode experiments were conducted to optimize methane production. Response Surface Methodology (RSM) and Artificial Neural Network (ANN)—Genetic Algorithm (GA) approaches were employed to model and optimize the process. RSM identified optimal ranges for trace elements and pressure, while ANN-GA demonstrated superior predictive accuracy, capturing nonlinear relationships with a high R² (>0.99) and minimal prediction errors. ANN-GA optimization indicated 97.9% methane production efficiency with a reduced conversion time of 15.9 h under conditions of 1.5 bar pressure and trace metal concentrations of 25.0 mg/L Fe(II), 0.20 mg/L Ni(II), and 0.02 mg/L Co(II). Validation experiments confirmed these predictions with deviations below 5%, underscoring the robustness of the models. The results highlight the synergistic effects of pressure and trace metals in enhancing gas–liquid mass transfer and enzymatic pathways, demonstrating the potential of computational modeling and experimental validation to optimize biological methanation systems, contributing to sustainable methane production.

Fossil fuel combustion accounts for more than 40% of global greenhouse gas emissions, with carbon dioxide (CO2) being the predominant pollutant. Carbon capture and utilization (CCU) technologies involve the separation of CO2 from flue gases produced during industrial processes and repurposing it for a wide range of industrial applications, such as fuel production. Among available methods, membrane-based separation emerges as a particularly promising solution, offering key advantages including environmental friendliness, lower energy consumption compared to conventional techniques (e.g., chemical absorption using amine solutions), operational flexibility, and easy integration into existing industrial facilities. This study focuses on the design and operation of a pilot membrane unit as an innovative approach for CO2 separation from gas mixtures and its subsequent use in biofuel-based energy production, implemented at the Agios Dimitrios Power Plant in Kozani. The pilot unit comprises four hollow fiber (HF) membranes made of polyimide (PI), arranged in a two-stage configuration: three membranes in the first stage and one in the second, with recirculation of the $2^{\text {nd }}$ retentate stream. The system targets the separation of synthetic binary $\mathrm{CO}_{2} / \mathrm{N}_{2}$ mixtures, formulated to resemble the composition of flue gases produced by major industrial CO2 emitters, such as power plants and cement factories. Before entering each stage, the gas mixture is compressed; CO2 then permeates through the membrane fibers, leaving behind a nitrogen $\left(\mathrm{N}_{2}\right)$-enriched stream. The objective is to identify optimal operating conditions and feed gas characteristics (composition and volumetric flow rate) required to achieve a CO2 concentration above 90% in the 2 nd stage permeate stream- a critical threshold for using this stream in a biological methanogenesis unit aimed at hybrid energy production. In this process, CO2 is combined with hydrogen, generated from an electrolyser, to produce CH4. The results obtained serve as a starting point for the optimization of membrane-based systems and, more broadly, pilot-scale CCU systems for flue gas applications.

Hydrogenotrophic methanogenesis is a microbial process converting H2 and CO2 into CH4 under mild conditions, offering a sustainable route for renewable methane production. However, the low aqueous solubility of H2 and limited understanding of trace metal interactions constrain process efficiency. This study investigates the synergistic effects of pressure and trace metal concentrations- Fe (II), Ni (II), and $\mathbf{C o (I I) - o n ~ b i o m e t h a n a t i o n ~ p e r f o r m a n c e ~ u s i n g ~ R e s p o n s e ~ S u r f a c e ~}$ Methodology (RSM). A face-centered composite design (FCC) with 52 experimental runs was employed to model and optimize key operational variables: applied pressure ($0.5-2$ bar) and metal concentrations (Fe(II): $\mathbf{1}-\mathbf{5 0 ~ m g} / \mathbf{L}, \mathbf{N i} {(\mathbf{I I})} {:} \mathbf{0. 0 1}-\mathbf{0. 5 ~ m g} / \mathbf{L}, \mathbf{C o} {(\text { II): }} \mathbf{0. 0 1}-\mathbf{0. 1} \mathbf{~ m g} / \mathbf{L})$. Two response variables were assessed: $\mathbf{H}_{\mathbf{2}} / \mathbf{C O}_{\mathbf{2}}$ conversion efficiency and conversion time. Quadratic models demonstrated strong predictive accuracy $\left(R^{2}=0.9458\right.$ for conversion time; $\mathbf{R}^{\mathbf{2}} \boldsymbol{=} \mathbf{0. 9 2 0 4}$ for conversion rate). Results revealed that pressure and Fe (II)/ Ni (II) were the most influential factors, significantly enhancing hydrogen mass transfer and enzymatic activity. Optimal conditions- 1.25 bar, $30 \mathrm{mg} / \mathrm{L} \mathrm{Fe}$ (II), and $\mathbf{0. 1 ~ m g} / \mathbf{L ~ N i}$ (II)—achieved 97.4% methane purity within 16.9 hours, with $ \lt 5 \%$ prediction error. This study establishes a reproducible, multivariate optimization framework, highlighting the combined benefits of physicochemical and biochemical parameters in maximizing biomethanation efficiency and supporting scalable renewable energy applications.

Wastewater treatment (WWT) is among the main challenges in environmental engineering. However, conventional wastewater treatment methods are limited by several aspects, mostly related to efficiency, excessive energy requirements, and surplus sludge production. Thus, the alternative use of biofilms (instead of suspended biomass/activated sludge systems) has garnered particular interest, especially due to their ability to sustain high microbial activity and withstand extreme conditions. This review aims to provide an interdisciplinary and comprehensive approach to understanding the main interactions occurring in biofilms, emphasizing, specifically, the quorum sensing (QS) and the quorum quenching (QQ) mechanisms, as well as to address their relative applications in controlling biofouling problems, e.g., during the operation of membrane bioreactors (MBRs). The review summarizes and analyzes the latest developments, highlights the relevant research gaps in the literature, and links microbiological knowledge with related technological applications.

In this study, two different mining waste streams were combined in the context of circular and green economy. Acid mine drainage (AMD) is considered to be the main environmental issue of the corresponding industry due to its acidic nature and high concentration of heavy metals. On the other hand, a large volume of tailings is produced during chromite mining and beneficiation processes. These tailings are mainly ultrabasic rocks, namely, serpentine and olivine. Despite not posing a significant environmental concern, they need to be properly managed. This work aimed to evaluate the AMD treatment (neutralization and metal removal) by the chromite mining tailings, contributing to sustainable practices within the mining industry. As experimental material, field samples of ultrabasic rocks were obtained, sieved, homogenized, and thermally upgraded (700oC). Artificial AMD was synthesized to simulate real conditions, and the homogenized material was tested under batch and column setups to determine its neutralization efficiency. According to the results, the AMD was successfully neutralized by the upgraded ultrabasic rock. In detail, an equilibrium pH value of 7.9 was achieved by applying a liquid-to-solid ratio of 12.5 after 1 h of reaction time. Regarding the continuous flow column setup, neutralization was obtained by applying an empty bed contact time equal to 30 min. The heavy metal removal efficiency was mainly dependent on their solubility and experimental duration. Even if most of them presented high removal rates, magnesium concentration increased since it was leached from the upgraded ultrabasic rock to the aquatic phase.

The present study evaluates the application of heterogeneous catalytic ozonation for the removal of micropollutants from wastewater effluent in a pre-industrial-scale unit, consisting of a post-filtration, an ozone dilution, a catalytic ozonation, and a final biological stabilization step. The important step of ozone dilution is optimized by the use of a hollow fiber membrane that minimizes the loss of ozone gas due to the transfer of ozone to the liquid phase mainly by diffusion. It is observed that the efficiency of this sub-system is maximized for the dead-end operation of the membrane and the introduction of ozone gas to the shell side and liquid phase to the lumen side of the membrane module. Under these conditions, the concentration of dissolved ozone is directly dependent on the ratio of ozone gas feed to the wastewater flow subjected to post-treatment. Regarding the removal of MPs, part of their degradation already takes place at this stage (i.e., during ozone dilution), while after the post-treatment of wastewater effluent in the catalytic ozonation bed, the MP degradation yield ranges from 35% up to complete removal, depending on the type and properties of the specific MP. The addition of a final biological filtration bed to the overall treatment unit significantly increased its performance, regarding the removal of MPs, enhancing it by an additional removal rate that can reach up to 30%.

The major issue of raw materials’ depletion, and more specifically, of phosphorous (an important fertilizer) has currently become an emergent aspect due to expected depletion problems needing immediate handling. This was the reason for the implementation of the PhoReSe project that aimed to remove and recover phosphorus from the secondary (biologically treated) effluent of a municipal wastewater (biological) treatment plant (WWTP “AINEIA”, located near Thessaloniki, N. Greece), treating the wastewaters of the nearby touristic area. Regarding the phosphorous supplementary removal and recovery treatment options, two methods were examined, initially at the laboratory scale (batch experiments), i.e., (1) the adsorption of phosphorous, and (2) the chemical precipitation of phosphorus. Both methods were further applied at the pilot scale by initially performing the adsorption of phosphorous onto the AquAsZero commercial sorbent, which is a mixed manganese iron oxy-hydroxide, followed by the chemical precipitation of phosphorous implemented after the desorption process of the previously saturated adsorbent. The final precipitate of this procedure was examined as an alternative/supplementary fertilizer, this way returning phosphorus into the natural cycle. These experiments, as applied successfully in at the pilot scale, set the basis for larger-scale relevant applications for similar WWTP facilities.

The process of biological methanation (BM), which involves the reaction between carbon dioxide (CO2) and hydrogen (H2) producing methane (CH4), is a circular economy strategy that can alleviate the problem of energy storage and also reduce CO2 emissions. By harnessing a renewable energy source to facilitate H2 production through water electrolysis and by capturing CO2, e.g. from industrial flue gases or biogas, this Power-to-Gas (PtG) technology produces CH4, the predominant compound of synthetic natural gas (following appropriate purification/enhancement towards biomethane). During the last decade, BM process has been examined fundamentally by various research groups both in lab-scale, as well as in pilot-scale experiments/applications. Nevertheless, a significant portion of these studies lack presentation of technical details and implementation strategies, regarding the operation of existing BM units in real-world applications. The current work aims to offer a thorough review of biological methanation, focusing on its practical implementation, particularly examining the most notable full-scale (industrial) applications utilizing this technology. To the best of the authors’ knowledge, the specific review provides the first comprehensive insight on the application of biological methanation and potentially consists a ‘technology catalogue’ that includes both the latest lab-/pilot-scale research advancements and the most significant demonstration-/full-scale plants currently installed in Europe. After the presentation of BM process fundamentals and the respective basic reactor types, the study presents the current research, and also substantial information, regarding the operation of certain (largest) full-scale BM plants, installed in Germany, Switzerland and Denmark. Biomethanation integrates two solutions to pressing environmental challenges, likely related to waste and carbon emissions reduction and aligns with the principles of the circular economy, that aims to minimize waste and make the most of resources by reusing and recycling. Additionally fits into the emerging field of Carbon Capture and Utilization (CCU) aimed at mitigating Greenhouse Gas (GHG) emissions. However, although the scale-up of BM process has evolved, further operating information and economic data on demo- and full-scale plants should be appropriately investigated to ensure the economic feasibility and, therefore, the widespread application of this PtG technology.

This study investigated the impact of two low-temperature thermal pre-treatments on continuous anaerobic reactors’ performance, sequentially fed with sludge of different total solids content (∼3% and ∼6%) and subjected to progressively increasing Organic Loading Rates (OLR) from 1.0 to 2.5 g volatile solids/(LReactor⋅day). Assessing pre-treatments’ influence on influent sludge characteristics revealed enhanced organic matter hydrolysis, facilitating sludge solubilization and methanogenesis; volatile fatty acids concentration also increased, particularly in pre-treated sludge of ∼6% total solids, indicating improved heating efficiency under increased solids content. The reactor fed with sludge pre-treated at 45°C for 48 h and 55°C for an extra 48 h exhibited the highest methane yield under all applied OLRs, peaking at 240 ± 3.0 mL/g volatile solids at the OLR of 2.5 g volatile solids/(LReactor⋅day). 16S rRNA gene sequencing demonstrated differences in the reactors’ microbiomes as evidence of sludge thickening and the different pre-treatments applied, which promoted the release of organic matter in diverse concentrations and compositions. Finally, the microbial analysis revealed that specific foam-related genera increased in abundance in the foam layer of reactors’ effluent bottles, dictating their association with the sludge foaming incidents that occurred inside the reactors during their operation at 2.0 g volatile solids/(LReactor⋅day).

Ultrafiltration and reverse osmosis proved effective in treating the liquid fraction of anaerobically digested sewage sludge, both with and without an intermediate selective electrodialysis treatment step. The incorporation of selective electrodialysis significantly increased clean water recovery rates (from 38% to 88%) while concurrently reducing its dissolved solids load by almost two orders of magnitude, from 869.0 to 10.3mg/L. Selective electrodialysis efficiently separated ions into stable fractions that could be post-processed to produce tailored solid fertilizers, such as struvite and ammonium sulfate, on demand. Throughout the system, heavy metals and pharmaceuticals were monitored for potential toxicity and harm to ecosystems. Heavy metals in the effluents of the treatment process both with and without the selective electrodialysis inclusion were below regulatory limits. Traces of ibuprofen, ciprofloxacin, oxytetracycline, and tetracycline were found in ultrafiltration and reverse osmosis concentrates. Currently, there are no established regulations for monitoring pharmaceuticals in digestate, but their presence could potentially complicate the direct use of these concentrates in agricultural applications.

Developing an efficient recycling route for spent single-use medical devices is essential for recovering precious metals. The proposed complete hydrometallurgical route goes through the initial pyrolysis and acid digestion steps, expanding upon our previous relevant work in the field, followed by solvent extraction, stripping, and precipitation procedures. In this study, a complete hydrometallurgical process was developed for the recovery of gold, platinum, iridium, and tantalum, separating them from other metals, i.e., from iron, chromium, and nickel, also present in the examined medical devices, i.e., (i) diagnostic electrophysiology catheters, containing gold, (ii) diagnostic guide wires, containing platinum and iridium alloys, and (iii) self-expanding stents, containing tantalum. This study reports the experimental results of selecting an efficient extractant, stripping, and precipitation agent, along with the effects of key factors that influence each consecutive step of the process, i.e., agent concentration, aqueous to organic phase ratio, contact time, and pH, using simulated metal solutions and also applying the obtained optimal conditions to the treatment of real sample solutions. For the selective separation of gold, Aliquat 336 was used to extract it in the organic phase; it was then stripped using a thiourea solution and precipitated by utilizing an iron sulfate (II) solution and proper pH adjustment. The selective separation of platinum was achieved by using Aliquat 336 for the organic phase extraction and a perchlorate acid solution for stripping it back into the aqueous solution and adding a sodium bromate solution to precipitate it. Due to the similar chemical behavior, the selective recovery of iridium followed the same processes as that of platinum, and the separation between them was achieved through selective precipitation, as heating the solution and adjusting the pH value resulted in the selective precipitation of iridium. Lastly, the selective recovery of tantalum consists of extraction by using Alamine 336, then stripping it back to the aqueous phase by using sodium chloride, and precipitation by using potassium salt solution and proper pH adjustment. A total recovery of 88% for Au, 86% for Pt, 84% for Ir, and 80% for Ta was obtained, thus achieving a high uptake of precious metals from the examined real spent/waste samples.

Due to their unique properties, i.e., fluoroscopy response and inertness, noble metals and alloys are present in several widespread medical–technological products, such as catheters, guide-wires, and stents. Despite their value, these products serve as single-use consumables, following a fate of solid waste disposal and loss of their valuable metals. This work studies the development of a treatment methodology to recover noble metals such as Pt, Ir, Au, and Ta from certain commercial products commonly used for medical practices. In particular, a sequence of preliminary pyrolysis, aiming at polymer elimination, as well as an acid digestion step for selective metals dissolution, is suggested. Pyrolysis was capable of enriching samples with the targeted metals, though a small change in their oxidation states was observed. Still, acid digestion was fully able to successfully separate Au using a 50% v/v aqua regia solution for 30 min at room temperature and the Pt/Ir using concentrated aqua regia for 72 h under heating. Dissolution of Ta required a different leaching solution, i.e., a 50% v/v HF/H2SO4 mixture for 10 h under heating. According to the developed method, selective extraction of such noble metals in a concentrated slurry provides a high potential for the complete recovery and valorization of otherwise disposed medical wastes.

In the present work, the construction, and operation of a pilot-scale biogas upgrading system is presented, employing 2 commercial polyimide (PI) membranes. The Upgrading system treats biogas produced via anaerobic digestion of the sludge, produced from the treatment of municipal wastewater in the facilities of Thessaloniki's Wastewater Treatment Plant. The goal of the separation unit is the production of high purity biomethane (>95%) for potential reuse in terms of energy. The fabrication of the pilot scale system includes the scale up of a laboratory setup separating CO2 from binary CH4-CO2 gas mixture. After the stability tests of the process, for the operation of 5 months (February to June 2023) the purity and recovery of CH4 in the final gas product. The experimental results showed an average recovery of CH4 of 95.7% for an average 55% feed composition, whereas the average purity in the final product was equal to 82.4%. The purity results were lower because of the N2 presence in the product stream (average 17.5%). After normalization with the help of the lab-scale binary results, the expected results assuming N2 absence would be 99.8% CH4 purity and 67% CH4 recovery. Finally, 3 different membrane configurations are compared in terms of their energy production, concluding to the efficiency of 2-stage configuration with recycling stream for the optimal combination of theoretical stage cut fractions.

A series of technologies have been employed in pilot-scale to process digestate, i.e. the byproduct remaining after the anaerobic digestion of agricultural and other wastes, with the aim of recovering nutrients and reducing the load of solids and organics from it, hence improving the quality of digestate for potential subsequent reuse. In this case the digestate originated from a mixture of dairy and animal wastes and a small amount of agricultural wastes. It was processed by the application of several treatments, applied in series, i.e. microfiltration, ultrafiltration, reverse osmosis, selective electrodialysis and combined UV/ozonation. The initially applied membrane filtration methods (micro- and ultra-filtration) removed most of the suspended solids and macromolecules with a combined efficiency of more than 80%, while the reverse osmosis (at the end) removed almost all the remaining solutes (85-100%), producing sufficiently clarified water, appropriate for potential reuse. In the selective electrodialysis unit over 95% of ammonium and potassium were recovered from the feed, along with 55% of the phosphates. Of the latter, 75% was retrieved in the form of struvite.

Achieving carbon neutrality in Europe hinges on the exploitation of renewable energy resources. Although these resources seem plentiful, critical challenges emerge from the excess energy that cannot be effectively stored or from insufficient electricity production. A promising approach to sustaining a balanced electricity network that aligns production with demand involves integrating the transformation of surplus energy into biomethane through a two-stage process. The surplus energy is utilized to produce hydrogen through water electrolysis, followed by the biological methanogenesis of hydrogen and carbon dioxide to synthesize biomethane. Investigating energy undersupply scenarios is crucial to understanding the resilience of biological processes, requiring evaluation of intermittent hydrogen supply modes and their microbial impacts. The present study focused on simulating actual demand-driven operational conditions by intermittently halting the supply of input gas, thereby inducing disruptions within the biological processes. Various sequences of consecutive starvation and regular operation phases, spanning one to five weeks, were assessed. The experimental framework was executed in two thermophilic Trickle Bed Reactors under anaerobic conditions, each utilizing distinct packing materials; specifically, activated carbon pellets and polyethylene K1 Media Raschig rings. The objective was to scrutinize the influence of these materials on the composition of the output gas, process stability and resilience of the microbial community. Remarkably, in both reactors, the biomethanation process demonstrated high adaptability, with capabilities to cease and recommence almost instantaneously, even following a five-week starvation period, effectively returning the process performance to its optimal pre-starvation state.