Fats, oil and grease (FOG) can be recovered efficiently from grease interceptors for biodiesel production. FOG is susceptible to hydrolysis because of its inherent high moisture content and the presence of lipases associated with food residuals in the grease interceptors. This study reveals that the FFA content of FOG derived from grease interceptors did not exceed 8% (w/w) due to constant influx of fresh FOG from waste-water. However, if the FOG is allowed to hydrolyze without dilution, the FFA content can reach 15% (w/w) in more than 20 days. Experiments were conducted to optimize reaction parameters for the esterification of FOG prior to the conventional alkali-catalyzed biodiesel production process. Sulphuric acid (H(2)SO(4)) was a more efficient catalyst than Fe(2)(SO(4))(3) in reducing the acid value to <= 1 mg KOH/g under identical reaction conditions. At reaction temperatures of <= 50 degrees C, only H(2)SO(4) was capable of reaching the recommended acid value within 24 h. The optimum methanol to FFA ratio for an H(2)SO(4)-catalyzed reaction was 20:1, whereas for Fe(2)(SO(4))(3) it was above 26:1. Esterification occurred under static, non-mixed conditions, although conversion rates were low. The rate of conversion increased with mixing speed, with a 200 rpm orbital shaking speed as optimum. (C) 2010 Elsevier Ltd. All rights reserved.