Productivity, CO2/O2 exchange and hydraulics in outdoor open high density microalgal (Chlorella sp.) photobioreactors operated in a Middle and Southern European climate

Two variants of open photobioreactors were operated at surface-to-volume ratios up to 170 m(-1) stop. The mean values for July and September obtained for photobioreactor PB-1 of 224 m(2) culture area (length 28 m, inclination 1.7%, thickness of algal culture layer 6 mm), operated in Trebon (49 degrees N), Czech Republic, were: net areal productivity, P (net) = 23.5 and 11.1 g dry weight (DW) m(-2) stop d(-1) stop; net photosynthetic efficiency (based on PAR - Photosynthetic Active Radiation), eta = 6.48 and 5.98%. For photobioreactor PB-2 of 100 m(2) stop culture area (length 100 m, inclination 1.6%, thickness of algal culture layer 8 mm) operated in Southern Greece (Kalamata, 37 degrees N) the mean values for July and October were: P (net) = 32.2 and 18.1 g DW m(-2) d(-1) , eta = 5.42 and 6.07%. The growth rate of the alga was practically linear during the fed- batch cultivation regime up to high biomass densities of about 40 g DW L-1 stop, corresponding to an areal density of 240 g DW m(-2) stop in PB-1 and 320 g DW m(-2) stop in PB-2. Night biomass loss (% of the daylight productivity, P (L)) caused by respiration of algal cells were: 9-14% in PB-1; 6.6-10.8% in PB-2. About 70% of supplied CO2 was utilized by the algae for photosynthesis. The concentration of dissolved oxygen (DO) increased from about 12 mg L-1 at the beginning to about 35 mg L-1 at the end of the 100 m long path of suspension flow in PB-2 at noon on clear summer days. Dissipation of hydraulic energy and some parameters of turbulence in algal suspension on culture area were estimated quantitatively.