Cultivation of a microalga Chlorella vulgaris using recycled aqueous phase nutrients from hydrothermal carbonization process

This study investigated the feasibility of using recovered nutrients from hydrothermal carbonization (HTC) for cultivation of microalga Chlorella vulgaris. Different dilution multiples of 50, 100 and 200 were applied to the recycled process water from HTC and algal growth was compared among these media and a standard growth medium BG-11. Algae achieved a biomass concentration of 0.79 g/L on 50× process water after 4 days. Algae removed total nitrogen, total phosphorus and chemical oxygen demand by 45.5–59.9%, 85.8–94.6% and 50.0–60.9%, respectively, on differently diluted process water. The fatty acid methyl ester yields for algae grown on the process water were 11.2% (50×), 11.2% (100×) and 9.7% (200×), which were significantly higher than 4.5% for BG-11. In addition, algae cultivated on process water had 18.9% higher carbon and 7.8% lower nitrogen contents than those on BG-11, indicating that they are very suitable as biofuel feedstocks.

► Process water from hydrothermal carbonization was recycled for algae cultivation.
► Algae grew faster on process water than on BG-11 medium.
► Algae removed TN, TP and COD by 45.5–59.9%, 85.8–94.6% and 50.0–60.9%.
► Algae grown on process water had higher carbon, hydrogen and lipids content.
► Recycling process water has the potential of reducing algae production cost.