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Microalgae biofilms could be used as a production platform for microalgae biomass. In this study, a photobioreactor design based on a rotating biological contactor (RBC) was used as a production platform for microalgae biomass cultivated in biofilm. In the photobioreactor, referred to as Algadisk, microalgae grow in biofilm on vertical rotating disks partially submerged in a growth medium. The objective is to evaluate the potential of the Algadisk photobioreactor with respect to the effects of disk roughness, disk rotation speed and CO2 concentration. These objectives where evaluated in relationship to productivity, photosynthetic efficiency, and long-term cultivation stability in a lab-scale Algadisk system. Although the lab-scale Algadisk system is used, operation parameters evaluated are relevant for scale-up. Chlorella Sorokiniana was used as model microalgae. In the lab-scale Algadisk reactor, productivity of 20.1 ± 0.7 g per m(2) disk surface per day and a biomass yield on light of 0.9 ± 0.04 g dry weight biomass per mol photons were obtained. Different disk rotation speeds did demonstrate minimal effects on biofilm growth and on the diffusion of substrate into the biofilm. CO2 limitation, however, drastically reduced productivity to 2-4 g per m(2) disk surface per day. Productivity could be maintained over a period of 21 weeks without re-inoculation of the Algadisk. Productivity decreased under extreme conditions such as pH 9-10, temperature above 40°C, and with low CO2 concentrations. Maximal productivity, however, was promptly recovered when optimal cultivation conditions were reinstated. These results exhibit an apparent opportunity to employ the Algadisk photobioreactor at large scale for microalgae biomass production if diffusion does not limit the CO2 supply.

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The loss of carbon dioxide (CO ) to the environment during microalgae cultivation is undesirable for both environmental and economic reasons. In this study, a phototrophic biofilm growth model was developed and validated with the objective to maximize both CO utilization efficiency and production of microalgae in biofilms. The model was validated in growth experiments with CO as the limiting substrate. The CO utilization and biomass productivity were maximized by changing the gas flow rate, the number of biofilm reactors in series and gas composition. Based on simulations, the maximum CO utilization efficiency that was reached was 96% based on a process employing flue gas. The corresponding drop in productivity was only 2% in comparison to the non-CO limited reference situation. In order to achieve this, 25 biofilm reactors units, or more, must be operated in series. Based on these results, it was concluded that concentrated CO streams and plug flow behavior of the gaseous phase over the biofilm surface are essential for high productivity and CO utilization efficiency. Biotechnol. Bioeng. 2017;114: 769-776. © 2016 Wiley Periodicals, Inc.