# Special Issue on the Intensified Conversion of Organic Waste into Biogas
###### Abstract  
Anaerobic digestion is a sustainable technology used to produce renewable gas from organic wastes. There are thousands of biogas facilities in the world, but many fundamental aspects of anaerobic digestion processes are still being discovered. For example, the catalytic effect of conductive materials is undoubtedly one of the key breakthroughs in recent years, and it will improve the efficiency of every digester around the world [1]. Anaerobic digestion involves a series of syntrophic relationships where hydrogen (H)/Formate (F) electron carriers play an important role in the process of interspecies electron transfer (H/F IET) [1]. When syntrophy is compromised, conversion rates are reduced, which affects greatly the treatment efficiency. Several strategies have been implemented to improve anaerobic digestion of organic waste [2] such as empirical modifications of process design, operational conditions [3], application of substrate pretreatments [4], and waste co-digestion [5, 6]. Nevertheless, much work is needed to understand the fundamental phenomena involved in the anaerobic digestion process to accelerate the activity of anaerobic microorganisms and thus improve the self-sufficiency and sustainability of anaerobic digesters. This Special Issue brings together a broad range of recent advances in the field of anaerobic digestion. Notably, these range from co-gasification of municipal solid waste to a spatially explicit model and the definition of the best location for a co-digestion biogas production facility. Additionally, this Special Issue presents some advancements in pretreatments and co-digestion approaches and the application of conductive materials to improve methane production. A total of eight articles were selected for publication. 16 March 20220 March  
nanotubes on the methanogenic activity of anaerobic sludge and river sediment. Carbon nanotubes accelerated the initial specific methane production rate from acetate, hydrogen, ethanol, and butyrate, with a more pronounced effect on the assays with acetate and butyrate, i.e., 2.1 and 2.6 times, respectively. Experiments with river sediment showed that cumulative methane production was 10.2 times higher in the assays with carbon nanotubes impregnated with iron than in the assays without carbon nanotubes [9]. The conductive particle-mediated interspecies electron transfer mechanism facilitates the electron transfer between species and has been shown to be beneficial to accelerate methane production [10]. Other strategies include substrate pretreatment to overcome the recalcitrant complex structure of some softwood species, such as Norway spruce (_Picea abies_). [PERSON] et al. [11] evaluated the methane yield in thermophilic anaerobic digestion conditions from hot water extract of Norway spruce pretreated at two different pretreatment severities. A higher pretreatment severity (170 \({}^{\circ}\)C) yielded higher concentrations of AD inhibitors. Despite having lower sugar content, hydrolysate pretreated at the lower temperature of 140 \({}^{\circ}\)C had an 18% higher methane yield. This result show that these substrates can be utilized safely in continuous-flow industrial anaerobic digestion [11]. Furthermore, other recalcitrant wastes, such as cork boiling wastewater, were assessed through a biochemical methane potential [12]. Results showed that anaerobic treatment of this wastewater is possible and that co-digestion with food waste was not an advantage. Nevertheless, the co-digestion with cow manure increased the biomethane yields, demonstrating that this strategy increases biogas production and the quality of the final digestate [12]. Different thermal hydrolysis technologies were analyzed using data from the wastewater treatment plant of Burgos (Spain) as the base scenario when operating with conventional mesophilic anaerobic digestion [13]. [PERSON] et al.