Recently, vigorous studies have been made on methods for producing organic substances such as ethanol by subjecting a synthesis gas containing carbon monoxide and hydrogen, such as waste derived gas, coal gas, natural gas, petroleum exhaust gas or the like, to microbial fermentation using gas-utilizing bacteria. Particularly drawing attention from all over the world as an important technology for realizing the future recycling society is a method that produces organic substances using, as a raw material, a synthesis gas obtained by partially oxidizing a carbon source generally collected as waste, because such a method enables a production of organic substances without consuming newly obtained petroleum resources or edible resources.
However, the synthesis gas contains many impurities derived from raw materials, as well as carbon monoxide and hydrogen. If a synthesis gas containing a large amount of impurities is supplied to microorganisms as it is, the impurities adversely affect the microorganisms contained in the synthesis gas, which may kill the microorganisms or decrease the utilization rate of the microorganisms. It is known that, for avoiding such disadvantage, it is necessary to reduce components that adversely affect microorganisms through an impurity concentration reducing step before supplying the synthesis gas to the microorganisms.
Specifically, a temperature swing adsorption (TSA: Temperature Swing Adsorption) method and a pressure swing adsorption (PSA: Pressure Swing Adsorption) method are known as means for removing impurities contained in the synthesis gas to reduce the concentration thereof (Patent Documents 1 and 2). In these adsorption methods, impurities are adsorbed on the adsorbent and removed.