Method of reducing nitrogen oxide concentration in sample, bioreactor, and plug flow reactor

A plug flow reactor for reducing a concentration of a nitrogen oxide in a nitrogen oxide-containing sample, the plug flow reactor comprising:

• two or more compartments separated by a porous plate, wherein the porous plate comprises a plurality of pores;
• a plurality of carriers to which a microorganism is adsorbed, wherein the plurality of carriers are disposed in each of the two or more compartments;
• an inlet through which a nitrogen oxide-containing sample is introduced into the plug flow reactor;
• and
• an outlet through which the reacted nitrogen oxide-containing sample flows out from the plug flow reactor,
• wherein the outlet is fluidly connected to the inlet so that a discharged sample is recirculated to the plug flow reactor through the inlet.