Animal fat residues (AFR) from waste water were used as feedstock to produce biodiesel by a two-step acid-catalyzed process. Treatment of the AFRs with 5.4% (w/w) of 17 M H2SO4 at a methanol/AFR ratio of 13:1 (50% w/w) at 60 degrees C converted more than 95% of the triglycerides into fatty acid methyl esters (FAMEs) with an acid value (AV) of 1.3 mg(KOH)/g(biodiesel). Response surface methodology indicated that a lower AV cannot be reached using a one-step acid catalyzed process. Thus a two-step acid catalyzed process was employed using 3.6% catalyst and 30% methanol for 5 h for the first step and 1.8% catalyst and 10% methanol for I h in the second step, resulting in a yield higher than 98% and an AV of 0.3 mg(KOH)/g(biodiesel). The product thus conforms to the European norm EN14214 concerning biodiesel. (C) 2012 Elsevier Ltd. All rights reserved.