Carbon speciation and flocculation in Neochloris oleoabundans cultures using anaerobically digested stillage

The effects of bicarbonate loading rate (BLR) and pH on growth kinetics, inorganic carbon speciation, carbon fixation and lipid content in Neochloris oleoabundans cultures using anaerobically digested stillage (ADS) (2% v/v) were investigated. Four different cultures were established: culture A with BLR = 1 g l^-1 day^-1 and no pH adjustment, culture B with BLR = 0.5 g l^-1 day^-1 and no pH adjustment, culture C with BLR = 1 g l^-1 day^-1 and pH adjustment at 7.0, and culture D with BLR = 0.5 g l^-1 day^-1 and pH adjustment at 7.0. The experiments were carried out in flat plate reactors (4 l) at controlled conditions (light intensity of 134 µmol photon m^-1 s^-1 and photoperiod 16 light/8 darkness; temperature of 32 ± 1 °C). The effects of pH (7, 10.41, 10.65, and 12), time (15, 30, 60, and 90 min), and concentration of a cationic polyelectrolyte (CP) (10 and 20 mg l^-1) on the flocculation efficiency (FE) of N. oleoabundans were also investigated. The results showed that bicarbonate was the predominant carbon species in the media and the main carbon source for microalgae growth in all cultures. The highest productivity (87.70 ± 9.70 mg l^-1 day^-1) and CO_2(aq) fixation rate (0.15 g CO_2(aq) l^-1 day^-1) were found in culture B. The lipid content in N. oleoabundans was affected negatively by the pH adjustment at 7.0 during its growth; higher values were found in cultures with no pH adjustment (37.10% and 38.85% dw for culture A and B, respectively) as compared to those obtained in cultures with pH adjustment (27.35% and 22.20% dw for culture C and D, respectively) (p < 0.05). Regarding flocculation, the addition of 20 mg CP l^-1 was required to obtain a FE > 95% in cultures A and B, although a significant FE (40-59%) occurred without CP addition at a high pH (≥ 10.41) in all cultures.