LARGE SCALE OUTDOOR CULTIVATION OF MICROALGA SCENEDESMUS ARMATUS WITH AIRLIFT PHOTOBIOREACTOR

Abstract

The aim of this work was to investigate the effect of the airlift photobioreactor configurations and CO2 supplement on the cultivation of the green microalga Scenedesmus armatus. The scale-up from small scale indoor to large scale outdoor cultivations was also examined. The optimal condition for small scale indoor cultivation was 35oC, 10 kLux and superficial gas velocity (usg) 1 cm s‑1, this was equivalent to the specific biomass productivity of 106±6.6 mg L-1d-1. The associate biochemical compositions of lipid, protein and carbohydrate were 21.7±1.0, 32.9±2.6, 45.4±1.6 %w/w, respectively. The outdoor large scale operation was conducted in 100 L flat panel airlift photobioreactors (FPAPs) with usg 0.35 cm s-1 or aeration rate of 0.2 vvm in the batch mode under Thailand climate. The results indicated that cells could tolerate high light intensity and relatively high temperature where the highest actual growth rate occurred at around 9:00 AM to 3:00 PM with a wide pH range of 6.4 to 11. The average specific biomass productivity was around 39.2 to 55.5 mg L-1d-1. The supply of CO2 had a slight influence on growth characteristics but did exert some observable effects on biochemical accumulations. Adding CO2 from 2 to 15% by volume in the aeration (0.2 vvm) during daytime caused an increase in lipid and protein from 19.8 to 25.6 and 37.8 to 48.2% w/w, respectively, whereas carbohydrate decreased from 42.5 to 26.2% w/w. S. armatus cultivated with 2% CO2 enriched air provided the highest the average of the average biomass productivity of 91.3 mg L-1d-1 which corresponded to a CO2 fixation of 165 mg CO2 L-1d-1 with the average lipid, protein and carbohydrate productivities of 22.2, 38.3 and 30.8 mg L-1d-1. CO2 was also used to control pH in the range of 7-7.7 and similar results with the addition of 2%CO2 were obtained. A novel large scale airlift systems without the physical gas separating baffle, i.e Non-baffled cone airlift photobioreactors (NB-CAPs) and Non-baffled flat panel airlift photobioreactors (NB-FPAPs), could be successfully used to cultivate the microalgal culture. Growth was independent of the slope of the cone bottom NB-CAPs (30o, 45o and 53o) where the 30o cone bottom NB-CAP was suggested for S. armatus outdoor cultivation as it provided the highest volume using the same area. S. armatus was cultivated in NB-FPAPs with various the widths of the reactor from 20, 30, 40 and 50 cm and the unaerated medium heights from 40, 50 and 60 cm. The optimal medium height of NB-FPAPs was 40-50 cm whereas the width was 50 cm. The specific biomass productivities were around 44.6-45.5 mg L-1d-1.