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Microalgae have great potential to be used as feedstock for biofuel production. Municipal wastewater is an economically appealing culture medium to produce microalgae. Downstream processing of wastewater-adapted microalgae, i.e. transesterification of extracted lipid, needs to be more investigated due to the difference between lipid profile of pure culture microalgae and wastewater-adapted microalgae. Based on quantitative parameters of acidity, acid value, and triglyceride content it was indicated that free fatty acid and triglyceride contents of wastewater-adapted algal lipid were between the two limits i.e pure microalgal lipid and vegetable oil. Optimal transesterification conditions for wastewater-adapted algal lipid were investigated in terms of acid-catalyzed versus alkaline-catalyzed reactions, temperature and reaction time, and the type of catalyst. It was found that acid-catalyzed transesterification at 70 °C for 35 h using sulfuric acid as the catalyst is required to achieve the highest percentage of fatty acid methyl esters of 53.9 ± 9.6% in the end product. This is quite different from reaction conditions recommended for pure microalgal lipid and vegetable oil transesterification. These operating conditions resulted in a dominant fatty acid methyl ester profile of palmitic acid, stearic acid, oleic acid, linoleic acid, and linolenic acid in the product that satisfied international standards specified for biodiesel. The results of this study customizes the transesterification reaction conditions for wastewater-adapted algal lipid and is one step forward towards commercial production of clean fuel from microalgae feedstock obtained from wastewater. • Wastewater-adapted algal lipid contains more FFA than TAG. • Acid transesterification is more efficient than alkaline for wastewater algal lipid. • Longer reaction time is required for transesterification of wastewater algal lipid. • The best condition is 35 h and 70 °C that produces suitable FAME profile for biofuel. [ABSTRACT FROM AUTHOR]

Biodiesel, as a renewable and eco-friendly energy source that can be produced through algae oil esterification, has recently received much attention. Maximization of algal biomass and lipid content is crucial for commercial biodiesel production. In this study, Chlorella sp. PG96, a microalgal strain isolated from urban wastewater, was identified considering its morphological and molecular characteristics. Fractional factorial design (2 11 - 7) was employed to screen medium and environmental factors for achieving high lipid productivity. The effects of eleven factors including light intensity, light spectrum, aeration rate, temperature, salinity, NaHCO 3 , CO 2 , NaNO 3 , NH 4 Cl, MgSO 4.7H 2 O, and K 2 HPO 4 and their interactions on growth characteristics of Chlorella sp. PG96 (biomass and lipid production) were statistically assessed. Based on the experimental results, lipid productivity was at its maximum (54.19 ± 8.40 mg lipid L−1 day−1) under a combination of high levels of all factors. The analysis also showed that physical parameters of light intensity and temperature were more effective on algal growth compared to nutritional parameters. Furthermore, nitrogen source of ammonium and carbon source of bicarbonate played more significant roles in biomass and lipid production, compared with nitrate and CO 2 , respectively. Although the effect of sulfur limitation on cellular growth was similar to phosphorus deficiency, S-limitation had a greater impact on lipid accumulation. The interaction between NaHCO 3 and NH 4 Cl was the most prominent interaction affecting all responses. It is concluded that Chlorella sp. PG96 at a high level of light intensity and temperature (22500 Lux and 32 °C, respectively) can be a prospective candidate for biodiesel production. [Display omitted] • Chlorella sp. PG96 was found to be a suitable candidate for biodiesel production. • FFD (2 11 - 7) successfully determined combined effects of 11 factors on algal growth. • Maximum lipid productivity was obtained at high levels of all factors. • Light intensity and temperature had the most impact on biomass and lipid. • Interaction between NaHCO 3 and NH 4 Cl was the most effective term for all responses. [ABSTRACT FROM AUTHOR]

Highlights • A fully compartmentalized metabolic model was developed for algal metabolism. • Flux balance analysis was used to optimize growth and lipid of Chlorella vulgaris. • Growth behavior was predicted at conditions without and with nitrogen feeding. • Physiological pathways of microalgae for lipid biosynthesis were identified. • CO 2 was the most important limiting factor for lipid under nitrogen starvation. Abstract Microalgae, the world's largest group of photosynthetic organisms, convert atmospheric CO 2 to polar and neutral lipids using sunlight, which after esterification can be utilized for biodiesel production. In the present study, a fully compartmentalized metabolic network was developed to describe the metabolism of Chlorella vulgaris based on known enzymatic reactions and typical metabolic pathways of green algae. Flux balance analysis was employed to optimize the specific growth rate and the lipid production rate using measured exchange fluxes of the metabolites. The experimental data for batch and fed batch algal fermentation systems acquired from the literature were used to validate the accuracy of the pseudo- steady state model. The physiological pathways of the microalgae for lipid biosynthesis were identified. The simulation revealed that the microalgae would be able to produce higher levels of lipid content (43.6%) during N-starvation cultivation under 100 μmol m−2 s−1 light intensity, 0.25 vvm aeration with 2% (v/v) CO 2 , 2 mg L−1 PO 4 -P, and 5 mg L−1 NO 3 -N. Sensitivity analysis showed that CO 2 , light energy, O 2 , and nitrate were the most important factors affecting the lipid production at N-deficient conditions. The findings consequential for manipulation of the metabolism of the microalgae with optimal activity. [ABSTRACT FROM AUTHOR]