SO2 is easily soluble in water, and after absorption by solution, it changes into SO42 -, SO32 - into liquid phase. SRB reduced SO42 -, SO32 - to sulfide using organic carbon as hydrogen donor. No gas is difficult to dissolve in water. In the trickling biofilter, no gas contacts with microorganism mainly through the adsorption of biofilm. Thiobacillus denitrifier takes S2O32 - as electronic donor and reduces no to N2