Algal biofuels have the potential to provide a scalable source of renewable fuels. The high nutrients use in algae cultivation may limit the scalability and sustainability of algal biofuels. The present study evaluates the use of aqueous phase obtained after hydrothermal liquefaction of Scendesmus sp./Desmodesmus cf. quadrica at 280 oC as a nutrients source for microalgae cultivation. Rapid hydrothermal liquefaction (R-HTL) (Flash Hydrolysis) nutrients recycle was compared with conventional hydrothermal liquefaction (C-HTL) nutrients recycle. Oocystis sp. and Desmodesmus cf. quadrica were cultivated using aqueous phase from both processes as a partial Phosphorous (P) and Nitrogen (N) source. Every day TSS was measured as an algae growth indicator (filtration with glass fiber discs 1.4 micron pore size Whatman 934-AH 47 mm). Temperature and pH (Extech PH220-C pH Meter) were also measured every day during hour 9 of 12 of the light period. Every two days a 10 mL sample (after filtration) was collected and preserved at 4°C until analyzed. The parameters that were measured in the liquid samples were: Soluble peptides (Lowry’s method), Nitrate (IC), Phosphate (IC), Sulfate (IC), Nitrite (IC), Ammonia (HACH DR 2800 spectrophotometer), and Total Nitrogen (Shimatzu TOC/TN model TOC V-csn). After finishing the experiment all the bottles used were harvested by centrifuging (Beckman Coulter Avanti Centrifuge J-26 XP). The collected biomass was freeze dried and stored at -4 °C until analyzed. All the samples were analyzed for elemental composition using a Thermo Finnigan Flash 1112 Elemental Analyzer with a nicotinamide standard for calibration. The method used for analysis was a furnace at 900°C, oven at 75°C, and carrier gas helium at 91 mL/min.
The study shows that 50% of phosphorus required in the culture media could be replaced with aqueous phase from R-HTL; also, 50% nitrogen was provided from the same source. Ammonia toxicity is one of the limitations for the higher percentages of N replacement in this study. R-HTL had a high initial soluble peptide composition that slowly degraded under culturing conditions to free ammonia, preventing toxicity in low percentages of N replacement. C-HTL had a high initial soluble ammonia concentration in the media with a slow removal rate. The effect was significant for the 50% replacement treatment where almost no algae growth was observed in the first 11 days of the experiment.