Cost-effective production of biodiesel requires continuous operation plants. Therefore, design and scale-up of continuous processes must be done from batch data and operation. Lipid recovery data, starting from acidified primary sludge, obtained through batch liquid–liquid extraction experiments allow the determination of partition coefficients in a wide range of process conditions. Fig.4 presents an example of the equilibrium curve obtained in the experiment with these operative conditions: 200 mL of sludge, 400 mL of hexane, 20 min for each extraction,9 consecutive extractions with fresh hexane, and 330 rpm of agitation speed and ambient temperature. As the liquid–liquid equilibrium thermodynamic diagrams are then available, the application of design methods for typical extraction equipment gives optimised solvent to the feed flow rate ratio and number of stages in continuous operation