Fig. 2 shows the results of the sequential liquid–liquid extraction for acidified and non-acidified primary, secondary and blended sludges.The accumulated lipid yield was continually increasing in each extraction stage in all cases. As it was expected, primary sludge achieved the highest lipid yield followed by blended and secondary, irrespective of sludge acidification. Sludge acidification highly increased the lipid yield in each extraction stage. Nonetheless, this trend is more evident in the case of primary and blended sludges. In the last extraction
stage, the lipid yield obtained from primary sludge was 26.6% and 13.0% for acidified and non-acidified samples, respectively, whereas blended sludge gave 19.1% and 10.7 for acidifiedandnon-acidifieds amples, respectively