The prevailing extractive economic model is unsustainable due to the finite nature of resources, thereby necessitating the development of alternative models and policies. The anaerobic digestion (AD) process is key to achieving this objective, as it facilitates the conversion of organic waste into biogas and nutrient-rich digestate. This approach is aligned with the principles of a circular economy and contributes to a reduction in carbon emissions. This study aims to conduct a comprehensive bibliometric analysis of the literature published over the past three decades (1993–2023). The analysis will be based on data drawn from the Scopus database and then analysed using the VOSviewer software, which allows for the interconnection of the revised bibliography through a series of selected keywords. The results demonstrated the existence of four clusters: (i) the beneficial valorisation of waste; (ii) volatile fatty acids and biohydrogen as added value by-products resulting from AD; (iii) lignocellulosic substrates and their by-products; and iv) the main products of AD, biogas and digestate. The bibliometric analysis demonstrates a growing interest in AD within the biorefinery concept in recent years, showcasing its potential for effective waste management and integration into the production chain through the principles of the circular economy.

This study shows, for the first time, how the natural biodegradation of the Phaeophyceae Rugulopteryx okamurae (R.o.) affects its methane yield, by biochemical methane potential assays, and the methane production kinetics. Additionally, a mechanical (zeolite-assisted milling) and a thermal (120 °C, 45 min) pretreatments were assessed. The highest methane yield was obtained from the mechanically pretreated fresh ashore biomass (219 (15) NL_CH4 kg_VS^-1), which presents the use of zeolite during milling as an economical alternative for heavy metal toxicity reduction. Moreover, no significant differences were observed between the other tests (with the exception of the lowest value obtained for the mechanically pretreated fresh R.o.). Low methane yields were linked to the heavy metal content. However, an increase of 28.5 % and 20.0 % in the k value was found for the untreated fresh R.o. biomass and fresh ashore biomass, respectively, when subjected to thermal pretreatment. Finally, an enhancement of 80.5 % in the maximum methane production rate was obtained for the fresh ashore biomass milled with zeolite compared to the untreated fresh ashore biomass.

This research evaluates the anaerobic digestion (AD) process of the residue generated in a new olive-oil manufacturing process for cold-pressed olive, a residue consisting of a mixture of the wastewater and solid waste obtained from this process. Additionally, in order to assess the possible influence of the level of ripening of the olives on the performance of anaerobic processing, olives of the Picual variety were collected at two stages, i.e., green olives and olives in veraison. The AD processes of the residues obtained from the cold-pressing process and the process without pressure (control) were comparatively assessed by means of biochemical methane potential (BMP) assays conducted at mesophilic temperature (35 ± 1 °C). Maximum values for methane yield (390 ± 1 NL CH₄/kg VS_added) and biodegradability (84.5%) were obtained from the cold-pressed green olive residues. For the rest of the wastes studied, biodegradability also reached high values, ranging from 79.1 to 79.6%. The logistic model adequately fit the experimental data and allowed for the assessment of the anaerobic biodegradation of these wastes and for obtaining the kinetic parameters for each case studied. The theoretical values for ultimate methane production predicted from this model showed less than a 1% deviation from the experimental values. A decrease was detected for both types of olives tested in the rate of maximum methane production, R_m, during the cold-pressing process, from 44.3 ± 0.1 to 30.1 ± 1.3 L CH₄/(kg VS·d) (green olives) and from 43.9 ± 1.5 to 38.7 ± 1.6 L CH₄/(kg VS·d) (olives in veraison). Finally, the highest energy output result was detected in the waste from cold-pressed green olives (15.7 kJ/g VS_removed), which coincided with its high methane yield.

The shedding of leaves by Posidonia oceanica (P. oceanica) in autumn results in the accumulation of shoreline debris, contributing to significant economic, social, and environmental problems. Due to the lack of alternative solutions, this waste biomass is disposed of in landfills, incurring an economic cost for the disposal process. In the context of the circular economy, anaerobic digestion (AD) can serve as a highly efficient biological alternative for treating and valorizing wastes with a high organic load. The aim of this research was to comparatively evaluate the performance and kinetics of the AD of ashore P. oceanica biomass and its anaerobic co-digestion (co-AD) with different nitrogen-rich co-substrates. To evaluate the effect of the nitrogen source in the co-AD system, peptone, casein, synthetic casein, urea, and the microalgae Raphidocelis subcapitata were used as co-substrates in biomethanization tests at a mesophilic temperature (35 ± 2 °C). The lowest methane yield was achieved for the sole AD of P. oceanica (79 ± 3 NL CH₄ kg⁻¹ VS), while the highest yields were found for the three co-ADs of P. oceanica with proteins (i.e., peptone, casein, and synthetic casein), showing no significant differences among them (380 ± 30 − 420 ± 30 NL CH₄ kg⁻¹ VS). Additionally, the first-order kinetics and the transference function model were proven and allowed for adequately fitting the experimental results of methane production with time.

The Mediterranean Sea has suffered recently from the unprecedent invasion of the alien macroalga Rugulopteryx okamurae due to global warming and climate change putting at risk the natural local ecosystem. Since 2015 this alga has colonized a great area on the western coast of the Mediterranean Sea and it has been also spotted in other areas such as the Azores Islands or the south of France. The arrival of alga tides into the coasts also provokes collateral environmental problems that need to be addressed. Seaweed-based biorefineries are considered a promising alternative within a circular economy model. This study aims to assess the potential of R. okamurae as raw material for the extraction of reducing sugars (RS) and total polyphenolic compounds (TPC) with antioxidant capacities, the subsequent production of methane from the extracted residue, and the final use of the anaerobic digestate as fertilizer. However, the presence of bioactive compounds greatly varies due to seasonality, location or even natural degradation. In order to provide some insights about these issues, two different batches were assessed: i.e. natural and ashore R. okamurae. As brown algae are characterised by a cell wall composed of crystalline cellulose and lignin, the biomasses studied were mechanically pretreated (dried at 100 °C and milled during 60 s) before the single and sequential extraction processes. Results showed that the extraction of the targeted compounds increased by 30-80 % when the biomasses were extracted sequentially. Similarly, the biochemical methane potential of the extracted solid residues increased as the RS and TPC content was reduced (120-150 NL_CH4 kg^-1_VS), with no significant impact regarding the biomass origin (natural or ashore) or the extraction process order. An increase in the kinetic constant k (first-order model) of 150% and 75% was observed when the fresh biomass was extracted with water and ethanol, respectively, compared to the value obtained for the unextracted biomass. Finally, the physicochemical characteristics of the different anaerobic digestates generated were assessed for their potential use as biofertilizer. In this study, most of the digestate' liquid fractions (7 out of 10) comply with European regulations for organic fertilizers and could be used directly.