Fig. 3 shows the results of the optimisation of the lipid extraction. In all cases , the accumulated yields of lipids increased with consecutive extraction stages,reaching a constant value at the last stages of the extraction. The best value of the accumulated yield of lipids at the last stage of extraction was 29.6%(based on dry sludge),attained for the experiment with a sludge to hexane volume ratio 1:2.The 1:1 volume ratio was able to achieve 29.5% , the 2:1 gave 28.8% and the 4:1 only 28.1%.As expected,the lower the amount of solvent, the lower the extraction efficiency
achieved