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Anaerobic digestion has been widely applied for waste treatment, renewable energy generation and biofertilizer production. The biogas potential in Brazil is sizable, but the state of Rio de Janeiro is largely dependent on fossil fuels, and there is a lack of biogas potential assessments in the state. Thus, this study evaluated biomethane, electricity and biofertilizer potentials in the region. Three different scenarios of biomass supply were considered for four major biowaste streams: sewage sludge; cattle manure; sugarcane processing waste; and food waste. Biomethane generation from the assessed sources could reach 0.6–1.3 billion Nm3 year−1, corresponding to 1,768–3,961 GWh year−1 of electricity and 1.6–3.3 million Mg year−1 of biofertilizer. Cattle manure was responsible for 73–84% of the projected biomethane production, presenting an opportunity to reduce the significant emissions from livestock farming. The estimated biofertilizer production could meet the demands of the state, and the produced electricity could offset up to 10% of the demand. The gas grid could facilitate the distribution of upgraded biomethane, and 10–22% of the natural gas demand could be met. The findings of this work highlight the high potential for biogas generation in Rio de Janeiro, which is up to seven times larger than the current production. [ABSTRACT FROM AUTHOR]