Microbubble sparged reactor is a combination of a CSTR reactor and a microbubble dis­perser. In this reactor configuration, the large specific interfacial area of the gas bubbles and the longer retention time stimulate the high gas-liquid mass transfer.

4.3.2 Membrane-based Reactors

CHFM have been proposed as a technologically and economically feasible method for syn­gas fermentation. Even though the technology is yet to be adopted for syngas fermentation, it has been widely studied in water and wastewater treatment. In CHFM reactors, the gas is introduced through the membrane fibers. The microbes grown as a biofilm on the surface of the membrane fibers utilize the CO and H2 and produce biofuels. This novel CHFM system offers several advantages such as higher microbial cell retention, higher yield, and higher tol­erance to toxic compounds (tar, acetylene, NOx, etc.).

4.4 pH

pH is a critical parameter to obtain optimal microbial activity in the culture media. In gen­eral, acidogenic reactions are more favorable at higher pH values (6.0-7.0), whereas solventogenesis (alcohol production) requires low pH values (4.0-4.5) (Klasson et al., 1993). Researchers employed pH shift approach in order to achieve high alcohol yields. Low pH, however, inhibits the cell growth (Klasson et al., 1992). The optimum pH for most of the syngas-fermenting microbes varies between 4.5 and 7.3 depending on the species. For example, C. Ijungdahlii has an optimum pH of 5.8-6.0.