title: The effect of discontinuous airlift mixing in outdoor flat panel photobioreactors on growth of Scenedesmus obliquus.
abstract: Discontinuous airlift mixing was realized by injecting pressured air at time intervals with a frequency between 0.033 and 0.25 Hz (at 80 kPa; i.e., every 4-30 s; valve opening time 800 ms) into outdoor flat panel photobioreactors ([Formula: see text]). This caused a flow velocity between 2 and 20 cm s(-1) of the culture medium within the photobioreactor and the mixing time was between 38 and 103.5 s, requiring 0.175-1.340 L(gas volume) L(photobioreactor volume)(-1) min(-1) pressured air. In order to detect the effect on growth of Scenedesmus obliquus during outdoor experiments and to be able to compare obtained results, a batch run with an airlift frequency of 0.25 Hz was simultaneously used as control. Growth at different airlift frequencies was measured by the increase of cell dry weight (CDW) during 3-5 days and biomass yield on light energy was calculated. With increasing airlift frequencies, growth increased from 52 to 91 % compared to the control. When CDW was at around 1.0-1.5 g L(-1), airlift frequency had no effect on growth, indicating that mass transfer gradients of nutrients and gas were not the limiting factors of growth. Above 1.5 g CDW L(-1), growth increased with increasing airlift frequency and light limitation for a single cell occurred. This effect was observed during low and high irradiance and it is concluded that a higher mean flow causes a better light distribution, resulting in an enhanced growth. Biomass productivity and demand of pressured air are correlated logarithmically, which enables to save mixing energy during cultivation.
authors.affiliation: Department of Cell Biology and Phycology, Biocentre Klein Flottbek, University of Hamburg, Ohnhorststraße 18, 22609, Hamburg, Germany, m.leupold@ssc-hamburg.de.
authors.forename: Marco
authors.initials: M
authors.lastname: Leupold
authors.forename: Stefan
authors.initials: S
authors.lastname: Hindersin
authors.forename: Martin
authors.initials: M
authors.lastname: Kerner
authors.forename: Dieter
authors.initials: D
authors.lastname: Hanelt
mesh_terms: D018533:Biomass; D019149:Bioreactors; D055668:Photochemical Processes; D044444:Scenedesmus / Q000254:growth & development* / Q000502:physiology; D018608:Ultrasonography, Doppler