Microalgal community isolated from urban wastewater was cultivated in the same wastewater to produce microalgae. The mixed algal cells with the average biomass concentration of 1 g/l were harvested and dried for lipid extraction resulted in 20% lipid content. The analysis of the lipid based on “acidity” and “triglyceride value” was performed and the lipid from pure Chlorella vulgaris and vegetable oil were used as controls. The fist control showed the acidity of 13.5% (based on C16:0, NaOH) while vegetable oil showed 0.13% (on the same basis). Basically, acidity of higher than 5% prescribes acidic while that of lower than 2% prescribes base catalytic trans-esterification. Moreover, the triglyceride value of vegetable oil was measured as 3.35 gr/100ml oil, while for pure algal lipid it was measured as 0.1 gr/100 ml sample. The two parameters were measured for wastewater-derived lipid as 2.56% (based on C16:0, NaOH) and 1.95 gr/100 ml sample; respectively. These figures indicated that wastewater-derived algal lipid had a profile between two limits i.e. vegetable oil and pure algal lipid. As a result, a two-step reaction including an acid catalyzed esterification followed by a base catalyzed transesterification was presumed. The results indicated that due to the low content of triglyceride, and very slow reaction of acidic estrification, the conversion of free fatty acids to triglyceride was negligible and water wash at the end of the reaction resulted in high amount of soap. Therefore, acid catalytic trans-esterification was used to convert wastewater-derived algal lipid to biodiesel. Optimization results showed that this reaction reached 60% fatty acid methyl ester content in product mixture under operating condition of 70 C and 35 h reaction time. The fatty acid profile of the produced biodiesel was a combination of saturated, mono unsaturated, and poly unsaturated fatty acids, predominantly C16:0 and C18:1 and C18:3.

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