The understanding of the chemical nature of the oil is important for both the optimization of the process and the design of upgrading strategies for further use as an energy carrier or toward transportation fuels. Hydrothermal treatment (HTT) oil is a complex matrix, whose composition is strongly affected by the feedstock type and by the HTT experimental conditions. In the present work, HTT oil from Desmodesmus sp. was subjected to a detailed chemical analysis. Various characterization techniques (silica gel chromatography, methanolysis, size exclusion chromatography, analytical pyrolysis, elemental analysis, and thermogravimetric techniques) were coupled to gather clearer information on the chemical nature of HTT oil obtained at different reaction times, temperatures, and slurry concentrations. Special attention was paid to the fate of N in the HTT process and the nature of the N-containing species in the oil. By cross-checking results from the chemical characterization of the oil with process data, it was finally possible to identify some different competitive reactions involved in the formation of HTT oil at different conditions. Results show that main compounds obtained at low temperature are still classifiable as lipids, which are extractable without the HTT, together with some short chain algaenan and some hydrophobic protein fragments that end up in the organic solvent phase. At higher temperature (300–375 °C), proteins and cellulose started to break down, giving cyclic dipeptides and amino acids side chains (by pyrolysis-like reactions), carbohydrates derivatives (e.g., furans) and products from the cross reaction of proteins and carbohydrates (e.g., formation of alkyl-pyrrolidinones, pyrazines, pyrroles and melanoidin-like materials). This phenomenon is responsible for the observed increase in oil mass yield with increasing processing temperature, as well as the increase in nitrogen content of the oil. Optimization of the production of fuels and fuel precursors by HTT should be done in conjunction with evaluation of downstream processing options and/or the possibility to recycle unconverted material to the algae cultivation.

The hydrothermal treatment (HTT) technology is evaluated for its potential as a process to convert algae and algal debris into a liquid fuel, within a sustainable algae biorefinery concept in which, next to fuels (gaseous and liquid), high value products are coproduced, nutrients and water are recycled, and the use of fossil energy is minimized. In this work, the freshwater microalgae Desmodesmus sp. was used as feedstock. HTT was investigated over a very wide range of temperatures (175–450 °C) and reaction times (up to 60 min), using a batch reactor system. The different product phases were quantified and analyzed. The maximum oil yield (49 wt %) was obtained at 375 °C and 5 min reaction time, recovering 75% of the algal calorific value into the oil and an energy densification from 22 to 36 MJ/kg. At increasing temperature, both the oil yield and the nitrogen content in the oil increased, necessitating further investigation on the molecular composition of the oil. This was performed in the adjacent collaborative paper with special attention to the nitrogen-containing compounds and to gain insight in the liquefaction mechanism. A pioneering visual inspection of the cells after HTT showed that a large step increase in the HTT oil yield, when going from 225 to 250 °C at 5 min reaction time, coincided with a major cell wall rupture under these conditions. Additionally, it was found that the oil composition, by extractive recovery after HTT below 250 °C, did change with temperature, even though the algal cells were visually still unbroken. Finally, the possibilities of recycling growth nutrients became evident by analyzing the aqueous fractions obtained after HTT. From the results obtained, we concluded that HTT is most suited as post-treatment technology in an algae biorefinery system, after the wet extraction of high value products, such as protein-rich food/feed ingredients and lipids.

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The availability of green and cheap technologies to recover polyhydroxyalkanoates (PHAs) from microbial biomass is crucial for the development of a reliable and sustainable production chain. Here, two novel protocols are proposed to extract PHAs from Cupravidus necator. The first method is based on PHA-extraction with dimethyl carbonate (DMC), a green solvent that is completely biodegradable and less harmful to humans and the environment than most solvents. The procedure can be applied directly to concentrated microbial slurries or to dry biomass, affording very high polymer recovery (>85%) and excellent purity (>95%). No degradation/decomposition of the polymer is observed in both cases. The second protocol uses fatty acid carboxylates as surfactants, which disrupt cell membranes, providing excellent polymer recovery (>99%) and high purity (>90%). Ammonium laurate can be successfully used and easily recycled (98%) by lowering the pH through CO 2 addition. Therefore, both protocols reported here are effective and sustainable: the recovery and purity of the obtained PHAs are very high, the use of toxic chemicals is avoided, and the recycling of various solvents/surfactants used in the processes is optimal. Extraction of PHB with organic solventsFreeze-dried biomass extraction. C. necator freeze-dried samples (50 mg) were extracted with organic solvents (2 mL) for 1-4 h. The tested solvents and the corresponding temperatures of extraction were: DMC (90°C and 50°C), PC (90°C), DEC (90°C), ethyl acetate (80°C) and CH 2 Cl 2 (50°C). At the end of the extraction, the solutions were centrifuged at 4000 rpm for 1 min and then filtered with polypropylene membrane filters of 0.45 µm porosity. The polymer was recovered by solvent evaporation or by precipitation with EtOH, then dried at 60°C under vacuum overnight.Each extraction was performed in quadruplicate.