Journal of Applied Phycology 2: 341-350, 1990. © 1990 Kluwer Academic Publishers. Printed in Belgium. 341 Carbon dioxide absorption characterization of a bioreactor for biomass production of Phormidium bohneri: comparative study of three types of diffuser P. Talbot', 2, R.W. Lencki 3 & J. de la Nouie 2 .* 'Groupe de recherche en recyclage biologique et aquaculture, Ddpartement de sciences et technologie des aliments; 2 Dipartement de biologie; 3 Ddpartement de gnie chimique, Universite Laval, Sainte-Foy GIK 7P4, Canada (* author for correspondence) Received 29 September 1990; accepted 4 October 1990 Key words: Carbon dioxide, absorption, Phormidium bohneri, bioreactor Abstract Under both laboratory-controlled and outdoor conditions, mass culture of Phormidium bohneri has shown low growth rates and filament aggregate or floc instability. In order to test for a possible carbonaceous supply limitation, a mass transfer characterization study of the reactor used was conducted. To examine different conditions of aeration, the overall volumetric mass transfer coefficient, KLa(CO2), and the energy and physical transfer efficiency for CO 2 were determined for three air-flow rates and three types of diffuser. For the different aeration conditions studied, the reactor showed adequate mixing properties with respect to CO 2 . However, under low air-flow rate conditions, even with the most efficient diffuser, the CO 2 transfer capacity appeared limiting. On the other hand, at high air-flow rates, floc breakage as a result of shear stress appeared to limit bioreactor efficiency. Introduction Under both laboratory-controlled and subtropi- cal outdoor conditions, the open-air culture of Phormidium bohneri, a filamentous cyanobacterium with a specific self-aggregation capacity (Talbot and de la Notie, 1988), has demonstrated low growth performance and floc or filament instability when grown on synthetic medium. To avoid algal biomass sedimentation problems in reactor dead zones, a new triangular aerated bioreactor was developed. The per- formance of the microalgal culture in this reactor with respect to growth and nutrient uptake for production and epuration systems, respectively, depends not only upon the intrinsic characteris- tics of the alga as well as environmental factors, but also upon physical processes such as the prevailing mass transfer and power input during aeration. According to the literature, the CO 2 supply to microalgae in algal mass culture systems is one of the principal difficulties and limitations that must be resolved (Oswald, 1988; Tapie & Bernard, 1989). In spite of the number and variety of experi- mental studies on microalgal cultivation (Shelef, 1980; Richmond, 1986; de la Nofle & De Pauw, 1988; Borowitzka & Borowitzka, 1988; Stadler et al., 1988), CO 2 mass transfer characterization studies of aerated culture systems are still frag-