Evaluation of dissolved oxygen (DO) profiles in microalgal suspension on outdoor thin-layer cultivation surface

Abstract

A new procedure for evaluation of mass transfer coefficient K L [m.h -1 ] for oxygen escape from microalgal Chlorella kessleri suspension and for net rate of oxygen evolution R [g O 2 per 1 m 2 of cultivation area per 1 h] by the alga has been developed and tested. The proposed approach is based on local and overall (on a whole cultivation area) mass balances of dissolved (DO) oxygen in suspension. The experimental longitudinal DO profiles in suspension flowing as 6-7 mm thick layer at 50 cm. s -1 on cultivation surface (with inclination 1.7%) of outdoor open unit, were evaluated by mathematical models based on O 2 mass balance. Overall DO mass balance for cultivation area gave most reliable estimation of K L , R. Mean K L = 0.24 m.h -1 for algal suspension was close to mean K L = 0.21 m.h -1 for water. Mean R values for cultivation area were linearly dependent on irradiance. R did not decrease (in some cases even increased) along the flow of suspension, despite increased DO accumulation (to about 22 mg O 2 . 1 -1 at 28 m at noon hours). DO level did not inhibit the rate of oxygen evolution by the alga and therefore a longer cultivation area would be recommended than it has been tested (length of suspension flow 28 m). Backmixing of suspension on cultivation surface was negligible and model derived for piston flow of suspension was sufficient for evaluation of DO profiles. For a reliable estimation of K L , R many DO profiles had to be evaluated, each profile composed of DO measurements made at several distances along the suspension flow.