Bubble columns generally consist of cylindrically shaped transparent vessels that are aerated by a gas distributor feeding gas bubbles with limited diameter and thus high gas/liquid exchange area into the system. Aeration does not only supply cultures
with carbon dioxide but also induces liquid movement and dispersion and thus contributes to a more equalized distribution of dissolved gasses and also prevents cells from settling. The superficial gas velocity (typical values between 0.01 and

0. 05 m/s (e. g., [53, 9]) affects the radial dispersion coefficient and together with the diameter of gas bubbles determines the interfacial area for mass transfer. In an attempt to improve mass transfer and radial dispersion a system with two different gas distributors was tested. A first gas sparger provides bubbles with a smaller diam­eter mainly for carbon dioxide supply. The second gas distributor provides bubbles with a larger diameter that induce turbulences for improved radial mixing and increased radial dispersion coefficients. At the same time this system should dimin­ish wall adhesion [16].

With regard to the geometrical structure of a bubble column photobioreactor there are several degrees of freedom. The ratio of length to diameter varies significantly. With regard to scale-up the diameter is limited by the light path length. The length of the column, in contrast, is limited by mass transfer and energetic con­siderations because a high hydrostatic pressure requires high power input for the aeration system [40]. A scale-up approach, in this case numbering-up, was demon­strated by the placement of several columns in specific distances and taking into consideration that column reactors mutually shade each other depending on the angle of incident light [9, 53].

A second cylinder can be installed in the center of the column to form an annular reactor [9]. Therewith, the dark liquid volume that does not contribute to the overall productivity is reduced. Simultaneously, the thickness of the column is adjusted to the short light path length and the scalability of the diameter is less restricted. Moreover, if the material chosen for the inner cylinder is transparent, this surface can additionally contribute to illumination of cultures. If energy input is not decisive for the economy of a process, additional light sources could be installed to illumi­nate cultures even from the inside of the annular reactor. However, this approach should not be taken into account for algae cultivation for the energy market.

The airlift principle can be applied to column reactors to improve axial transport. In this case, two interconnected compartments are separated in longitudinal direc­tion. Aeration in just one compartment, the riser, induces and upward flow, while the liquid volume with a lower gas hold up flows down in the downcomer section [41].

Depending on the specific design of the reactor, illumination and dimension of riser and downcomer one should keep in mind that light/dark cycles can be induced. Especially unfavorable slow cycles should be avoided [30] .