Environmental analyses of energy systems usually lack a comprehensive perspective that takes into account their life cycle and a set of relevant impact categories. The present study tries to fulfil this need in the field of biofuel production from free fatty acid-rich wastes, therefore providing a life cycle assessment of four biodiesel production systems including esterification-transesterification of waste vegetable oils (used cooking oil) and animal fats (beef tallow, poultry fat), and in situ transesterification of sewage sludges. Reference inventory data for these systems were gathered from a literature review. Thereafter, environmental characterization values were computed for a selection of impact categories: global warming, acidification, eutrophication, ozone layer depletion, photochemical oxidant formation, and cumulative non-renewable energy demand. A comparison among the environmental profiles of these second generation biodiesel alternatives and those of first generation rapeseed and soybean biodiesel fuels and conventional low-sulphur diesel was also performed through a well-to-wheels analysis. Thus, biodiesel from waste vegetable oils potentially entailed the most favourable environmental performance. Nevertheless, actions aimed at minimizing thermal and electric energy demands are encouraged as they would lead to relevant environmental improvements. (C) 2011 Elsevier Ltd. All rights reserved.