NOVELTY - Producing zero-waste bioenergy with reduced carbon dioxide emissions, a minimized-waste or waste-free bioenergy production process capable of providing an optimal biomass treatment efficiency and bioenergy productivity, comprising a combination of biosaccharification and alcohol fermentation processes including a biomass pretreatment.    USE - The method is used for producing zero-waste bioenergy with reduced carbon dioxide emissions, a minimized-waste or waste-free bioenergy production process capable of providing an optimal biomass treatment efficiency and bioenergy productivity (claimed).    ADVANTAGE - Provided is a method for producing zero-waste bioenergy with reduced carbon dioxide emissions, a minimized-waste or waste-free bioenergy production process providing an optimal biomass treatment efficiency and bioenergy productivity (all claimed). The delays or prevents global warming. Also provided is a method to enhance the cost-effectiveness of biodiesel production via the development of a novel usage of glycerol waste by supplying a high glycerol content waste produced during the biodiesel production process into an algae cultivation liquid of the heterotrophic algae cultivation process, or recycling it into the biosaccharification/alcohol fermentation process to utilize it as a biomass raw material for producing bioalcohol, preferably producing a biofuel using a biodiesel waste, thus enabling an eco-friendly production of a biofuel.    DETAILED DESCRIPTION - Producing zero-waste bioenergy with reduced carbon dioxide emissions, a minimized-waste or waste-free bioenergy production process capable of providing an optimal biomass treatment efficiency and bioenergy productivity, comprising a combination of biosaccharification and alcohol fermentation processes including a biomass pretreatment, and steps described below, comprises (a) an anaerobic digestion process for (i) generating a biogas consisting of: methane, carbon dioxide, and hydrogen sulfide using a liquid byproduct including a solid biomass waste produced during the biosaccharification/alcohol fermentation process, and a liquid fermentation waste produced during bioalcohol purification, and (ii) recycling the generated anaerobic sludge to the biomass pretreatment process; (b) an autotrophic or mixotrophic algae cultivation process comprising supplying carbon dioxide liquid necessary for algal photosynthesis during the autotrophic or mixotrophic algae cultivation process to an algae cultivation liquid by introducing the biogas generated from the anaerobic digestion process into a lower part of the algae cultivation liquid; and purifying the biogas into a methane biogas with the absorption of hydrogen sulfide; and (c) recycling the biomass harvested during the algae cultivation process to the biosaccharification/alcohol fermentation process for producing the bioalcohol, with or without going through a biodiesel production process.