TECHNOLOGY FOCUS - BIOLOGY - Preferred Method: The phototrophic algae are cultured in one or more open ponds and/or in one or more bioreactors, where each of the open ponds and each of the bioreactors have surfaces of 1-5000 m2 and depth of 0.05-2 m and are maintained under solar irradiation. The heterotrophic algae are cultivated in bioreactors and/or in closed fermenters, where each of the bioreactors and each of the fermenters have a volume of 0.010-1000 m3. Water required for culturing phototrophic algae and/or heterotrophic algae is fresh water or salt water from natural sources (e.g. seawater or river water) or artificial sources (e.g. co-produced waters or wastewaters from industrial and/or municipal discharge waters). The phototrophic algae are phototrophic microalgae (unicellular algae) chosen from Tetraselmis, Nannochloropsis, Nannochloris, Scenedesmus, Ankistrodesmus, Phaeodactylum, Chlorella, Amphipleura, Amphora, Chaetoceros, Cyclotella, Cymbella, Fragilaria, Navicula, Nitzschia, Dunaliella, Oscillatoria, Porphyridium, Spirulina and their mixtures. The heterotrophic algae are heterotrophic microalgae (unicellular algae) chosen from Chlorella, Nannochloropsis, Nitzschia, Thraustochytrium and Spirulina and their mixtures. The concentration of first aqueous suspension of algal biomass is performed by (a) gravitational separation by units of settlers and/or thickeners usually adopted in water treatment plants, (b) flotation, (c) gravimetric separation by units of cyclones or spirals, (d) centrifugation, (d) filtration by units of membranes for ultra- or micro-filtration, or vacuum filtration, or (e) treatment by units of filter presses or belt presses. The first aqueous suspension of algal biomass is subjected to flocculation. The flocculation is performed by addition of at least one flocculating agent chosen from metallic salts (e.g. iron chloride, aluminum sulfate, iron sulfate or their mixtures), cationic polyelectrolytes (e.g. polyethyleneimines, polyacrylamides or their mixture), chitosan and their mixtures. The first concentrated aqueous suspension of algal biomass has 0.1-10 wt.% (preferably 0.5-5 wt.%) of dry algal biomass (i.e. dry phototrophic algae) with respect to total weight of the first concentrated aqueous suspension of algal biomass. The concentrated aqueous suspension of algal biomass has 4-30 wt.% of dry algal biomass (i.e. dry phototrophic algae and dry heterotrophic algae) with respect to total weight of the second concentrated aqueous suspension of algal biomass. The second concentrated aqueous suspension of algal biomass has 5-25 wt.% of dry algal biomass (i.e. dry phototrophic algae and dry heterotrophic algae) with respect to total weight of the second concentrated aqueous suspension of algal biomass. The thermal treatment is performed at 80-350 degrees C (preferably 150-330 degrees C) and 0.1-25 MPa (preferably 0.5-18 MPa). The thermal treatment is performed for greater than or equal to 15 minutes or for 30 minutes to 3 hours. At the end of thermal treatment, the obtained mixture (oily phase, aqueous phase and gaseous phase) is cooled to 40-90 degrees C, the heat useful for thermal treatment is recovered, and the cooled mixture is subjected to separation.