The most important technologies and highlights for biogas production worldwide.

Bioenergy or green fuel has been considered the fuel of the future for being a type of renewable energy that contributes to the preservation of the environment as it helps to reduce greenhouse gas emissions. In this way, biogas offers a potential alternative to fossil fuels from anaerobic digestion (AD) bioprocess, which allows the action of several microorganisms in the transformation of substrates into biogas and secondary bioproducts. Over the years, researchers have discussed that low yields in AD are associated with different factors such as type of wastewater, reactor configuration, substrate concentration, temperature, organic loading rates, and biomass concentration inside of the reactor. In this way, to better conduct the AD, studies point to the reactor configuration as one of the factors in the determination of high biogas production for a long period. Understanding and knowing the type of reactor and how the parameters such as biomass accumulation and immobilization, pH, or temperature occur in the system would provide information and can help to improve the bioenergy production in different systems. Moreover, research opportunities about different technologies are essential for the anaerobic digestion of many substrates and the stability of interest production. Thus, this type of scientific study gives a broad overview of the principal systems used in the AD process and information about the circular economy in the production of biogas in the world. Important considerations are highlighted.Implication of Statement: The review paper provides information about the scenario of biogas in the world state-of-art and the biogas production from AD. Afterwards, an extensive analysis of different and principal types of reactors applied to the AD process, aimed at presenting an overview of the advantages and disadvantages of each configuration intending to gain new insights to improve traditional reactors or propose novel ones. This article enables us to have a perspective about the different technologies available and about new alternatives from an operational point of view for bioenergy from AD, not only in bench studies or pilot scale studies but also at an industrial level. Thus, this type of scientific study gives a broad overview of the principal systems used in the AD process and information about the circular economy in the production of biogas in the world.