Microalgae cultivation in thin stillage anaerobic digestate for nutrient recovery and bioproduct production

Anaerobically-digested thin-stillage, which is rich in nitrogen-ammonia (478 +/- 11 mg L-1) and phosphorus (508 +/- 5 mg L-1), offers great potential to be utilized as a source of nutrients for microalgae cultivation. However, the high concentration of ammonia is inhibitory to microalgal growth. In this study, ammonium present in the thin-stillage-digestate was partially recovered in the form of struvite to reduce the ammonia concentration to 267 +/- 13 mg L-1 and to improve the nitrogen to phosphorus ratio from 2.1 to 14.4 for microalgae cultivation. Chlorella sorokiniana in two times dilution of struvite-removed-digestate achieved a biomass concentration of 1.62 +/- 0.11 g L-1 and nutrient removal efficiencies of 95.3 +/- 1% (nitrogen) and 78.3 +/- 1.1% (phosphorous) at day 18. Protein, starch and lipid contents of C. sorokiniana biomass were 37.8 +/- 3.4%, 17.8 +/- 0.8% and 8.9 +/- 0.3% of dry weight, respectively at day 18. Moreover, a dramatic increase in genera of Alcaligenes and Acinetobacter (known as nitrifying bacteria) was observed in bacterial populations during algal cultivation.