Studies on the scale-up of biomass production with Scenedesmus spp. in flat-plate gas-lift photobioreactors.

Microalgae are flagged as next-generation biomass feedstock for sustainable chemicals and fuels, because they actively metabolize the climate gas CO2, do not impact food production, and are not associated with land-use change. Scaling microalgae cultivation processes from lab to pilot scale is key to assessing their economic and ecologic viability. In this work, process performances of two different Scenedesmus species were studied using a 300 L flat-plate gas-lift photobioreactor system (14 m2 photosynthetically active area) equipped with a customized, broad-spectrum LED illumination system. Scaling up of batch processes from laboratory scale (1.8 L, 0.09 m2) to the geometrically equivalent pilot scale resulted in reduced volumetric biomass productivities of up to 11% and reduced areal biomass productivities of up to 7.5% at the pilot scale. Since biofilm formation was solely detected at pilot scale, biofilm most likely impaired scalability. Nevertheless, repeated addition of nutrients (BG-11) at pilot scale resulted in a 13.5 gCDW L-1 biomass concentration within a 15 day process time with S. obtusiusculus at constant incident-photon flux densities of 1400 µmol photons m-2 s-1 and more than 19.5 gCDW L1 after 30 days with Scenedesmus ovalternus SAG 52.80 at constant incident-photon flux densities of 750 µmol photons m-2 s-1. This resulted in areal biomass productivities of 14 gCDW m-2 day-1 (S. ovalternus) and 19 gCDW m-2 day-1 (S. obtusiusculus), respectively.

Reaction engineering analysis of Scenedesmus ovalternus in a flat-plate gas-lift photobioreactor.

Microalgal strains of the genus Scenedesmus are a promising resource for commercial biotechnological applications. The temperature-, pH- and light-dependent growth of Scenedesmus ovalternus has been investigated on a laboratory scale. Best batch process performance was obtained at 30°C, pH 8.0 and an incident photon flux density of 1300µmolphotonsm-2s-1 using a flat-plate gas-lift photobioreactor. Highest growth rate (0.11h-1) and space-time yield (1.7±0.1gCDWL-1d-1) were observed when applying these reaction conditions. Biomass concentrations of up to 7.5±0.1gCDWL-1 were achieved within six days (25.0±0.5gCDWm-2d-1). The light-dependent growth kinetics of S. ovalternus was identified using Schuster's light transfer model and Andrews' light inhibition model (KS=545µmolphotonsm-2s-1; KI=2744µmolphotonsm-2s-1; µmax=0.21h-1). The optimal mean integral photon flux density for growth of S. ovalternus was estimated to be 1223µmolphotonsm-2s-1.

Model-supported phototrophic growth studies with Scenedesmus obtusiusculus in a flat-plate photobioreactor.

Light-dependent growth of microalgae can vary remarkably depending on the cultivation system and microalgal strain. Cell size and the pigmentation of each strain, as well as reactor geometry have a great impact on absorption and scattering behavior within a photobioreactor. In this study, the light-dependent, cell-specific growth kinetics of a novel green algae isolate, Scenedesmus obtusiusculus, was studied in a LED-illuminated flat-plate photobioreactor on a lab-scale (1.8 L, 0.09 m2 ). First, pH-controlled batch processes were performed with S. obtusiusculus at different constant incident photon flux densities. The best performance was achieved by illuminating S. obtusiusculus with 1400 µmol photons m-2 s-1 at the surface of the flat-plate photobioreactor, resulting in the highest biomass concentration (4.95 ± 0.16 gCDW L-1 within 3.5 d) and the highest specific growth rate (0.22 h-1 ). The experimental data were used to identify the kinetic parameters of different growth models considering light inhibition for S. obtusiusculus. Light attenuation within the flat-plate photobioreactor was considered by varying light transfer models. Based on the identified kinetic growth model of S. obtusiusculus, an optimum growth rate of 0.22 h-1 was estimated at a mean integral photon flux density of 1072 µmol photons m-2 s-1 with the Beer-Lambert law and 1590 µmol photons m-2 s-1 with Schuster's light transfer model in the flat-plate photobioreactor. LED illumination was, thus, increased to keep the identified optimum mean integral photon flux density constant in the batch process assuming Schuster's light transfer model. Compared to the same constant incident photon flux density (1590 µmol photons m-2 s-1 ), biomass concentration was up to 24% higher using the lighting profile until a dry cell mass concentration of 14.4 ± 1.4 gCDW L-1 was reached. Afterward, the biomass concentration remained constant, whereas cell growth continued in the batch process with constant incident photon flux density. Finally, biomass concentration was 15.5 ± 1.5 gCDW L-1 and, thus, 7% higher compared to the corresponding batch process with lighting profile. Biotechnol. Bioeng. 2017;114: 308-320. © 2016 Wiley Periodicals, Inc.