###### Abstract  
Anaerobic digestion is one of the technologies that will play a key role in the decarbonization of the economy, due to its capacity to treat organic waste, recover nutrients and simultaneously produce biogas as a renewable biofuel. This feature also makes this technology a relevant partner for approaching a circular economic model. However, the low biogas yield of traditional substrates such as sewage sludge and livestock waste along with high installation costs limit its profitability. Further expansion of this technology encounters several barriers, making it necessary to seek improvements to attain a favorable financial balance. The use of co-substrates benefits the overall digestion performance thanks to the balancing of nutrients, the enhanced conversion of organic matter and stabilization, leading to an increase in biogas production and process economics. This article reviews the main co-substrates used in anaerobic digestion, highlighting their characteristics in terms of methane production, kinetic models commonly used and the synergistic effects described in the literature. The main process parameters and their influence on digestion performance are presented, as well as the current lines of research dedicated to improving biogas yields, focusing on the addition of hydrogen, bioaugmentation, supplementation with carbon compounds and nanoparticles, the introduction of bioelectrodes and adsorbents. These techniques allow a significant increase in waste degradation and reduce inhibitory conditions, thus favoring process outcomes. Future research should focus on global process efficiency, making particular emphasis on the extrapolation of laboratory achievements into large-scale applications, by analyzing logistical issues, global energy demand and economic feasibility. m 2022  
is rich in humic, fulvic substances and nutrients making it suitable as raw material to produce organic fertilizers [2]. Recently, digestate is also being considered as an organic soil impover, growing medium or organic non-microbial plant biostimulant [3]. Digestion plants can also become an excellent ally for mitigating greenhouse gases (GHG). The treatment of organic waste avoids uncontrolled degradation and thus the release of methane into the atmosphere. Biogas obtained from this process is easily oxidized for energy production (thermal or electrical) or upgraded to obtain a gaseous fuel with similar characteristics to that of natural gas. There is a rising concern regarding the effect of CO\({}_{2}\) concentration in the atmosphere and how the global climate responds to the continuous increase in CO\({}_{2}\) levels. Research efforts should focus on attenuating these changes, reducing the negative impact on the economy and searching for efficient ways of producing energy from renewable sources. The current energetic crisis needs urgent solutions provided by mature technologies capable of producing huge amounts of energy. Anaerobic digestion is capable of generating eco-friendly energy and, at the same time, addressing the waste management crisis [4]. However, several aspects are still pending a solution such as the profitability of the whole treatment system and the need to increase conversion efficiency to reduce installation costs. Anaerobic digestion has been traditionally linked to the treatment of sewage sludge in large-scale wastewater treatment plants and the treatment of livestock waste. Both applications are characterized by the use of substrates with high organic content but lacking suitable nutrient balances. The addition of a co-substrate to any of these systems aids in balancing the C/N ratio and improving the global process performance allowing a better economic balance by increasing profits [5]. In the case of sewage sludge digestion, its composition based on primary and secondary sludge results in a mixture that would demand an excessive amount of energy if stabilization is carried out by aerobic treatment.