Abstract:  The air gasification characteristics of five different sewage sludges were carried out in an external heated downdraft fixed-bed. The results show that the higher gasification temperature can improve the quality of product gases, and the contents of CO, CH₄, H₂ and the lower heating value (LHV) of the gases decrease with the increase of air flux. Anaerobic digestion process of the sewage sludge reduces the quality of the gases which is also affected by wastewater treatment of the sludges. The contents of CO, CO₂ in the gases from the undigested sludge of continuous SBR process are the highest, and that of H₂, CH₄, C_mH_n are the least. There are the highest contents of H₂ and C_mH_n in the gas from the undigested sludge of activated sludge process. The gases from the undigested sludge of A²/O process contain the least contents of CO, CO₂ and the highest CH₄ content. The LHV of the gases from the three sludges increase in turn.