The bubble column consists of a glass column mounted on a steel base (Datar et al., 2004). The substrate gases are introduced into the reactor through a fritted glass disk with a pore size of 4-6 microns. These reactors are mainly designed for industrial applications with large reac­tor volumes. High mass transfer rate and relatively low operational and maintenance cost are among the merits of this type of reactor, while back mixing and coalescence are common drawbacks of the system (Datar et al., 2004).

4.3.1 Trickle-bed Reactors

Trickle-bed reactor is a slender column with a packing media. The microbial culture media continuously flows down and the gaseous substrate flows either upward (countercurrent) or downward (cocurrent) direction depending on the application (Munasinghe and Khanal, 2010). The trickle-bed reactors are operated under the atmospheric pressure and no agitation is necessary. Therefore, trickle-bed reactors consume less energy than the conventional CSTR (Bredwell et al., 1999).