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Biowaste is a prominent source for second generation biofuels and other biobased products since it constitutes an abundant feedstock, the current treatment of which poses challenges to the environment. In the current paper, the environmental sustainability of a biorefinery system producing ethanol from biowaste is assessed by means of Life Cycle Assessment. It is shown that the biorefinery system presents very good environmental performance, as the net emissions are quite low for all the impact categories examined, while in many of the cases the emissions are negative. The total net global warming impact is estimated to be -15 kg CO(2)eq/ton biowaste. Following, the environmental impacts from biowaste treatment with the biorefinery system are compared to those from biowaste treatment with the conventional municipal waste management methods. It is shown that the biorefinery system presents very good performance compared to the current waste management methods. Furthermore, a sensitivity analysis is conducted for investigating the impact of increased enzyme dosage to the overall environmental performance of the system. It is found that, the increase in ethanol production due to increased enzyme dosage has a smaller impact to the system performance compared to the effect of increased enzyme dosage. Furthermore, it was shown that the range of enzyme contribution in the global warming impact of ethanol biorefineries is wide (11-62%) which is attributed to a number of parameters, such as the high variation of the reported global warming impact of enzymes, the different enzyme loadings and ethanol yields.