Biological determinants of photobioreactor design. 6. quarterly report, February 1--July 31, 1995

Interest has developed in using algae to fix CO{sub 2} to produce oxygen and biomass for life-support in space, and for reducing CO{sub 2} emissions from power plants. A novel photobioreactor (PBR) system using light-emitting diodes (LEDs) as a sole light source was constructed and operated with continuous medium perfusion. Direct internal illumination by 680 nm LEDs could deliver as high as 50 mW/cm{sup 2} of light into the culture medium. Gas transfer by internal sparging had the capacity to transfer 250 mmol O{sub 2}/L culture/h. Nutritional limitations could be overcome by continuous perfusion, supplying the medium components to the culture without increasing osmolarity, while removing potentially inhibitory cellular wastes. When the PBR operated in a continuous perfusion mode with a perfusion rate of 6 reactor volumes a day (6 VVD), it could support ultra high-density algal cultures up to cell concentrations of 4 {times} 10{sup 9} cells/mL and total cell volume fractions of 9.4% v/v (about 25 g dry weight/L). The oxygen production rate at its peak was 13 to 15 mmol/L culture/h. This performance represents the highest reported cell densities attained in photoautotrophic cultures. Continuous perfusion allowed for long-term stable oxygen production, while oxygen production in batch mode ceased when stationary phase was reached. The results presented suggest that PBR technology can still be significantly improved