Partial oxidative gasification of municipal sludge in subcritical and supercritical water.

Subcritical and supercritical water gasification of dewatered sewage sludge obtained from a typical municipal wastewater treatment plant was investigated in a one-litre high-pressure autoclave at temperatures of 300-400 degrees C and pressures of 17.5-23.5 MPa. The sludge (without catalyst) was gasified at subcritical and supercritical water conditions, with different reaction times ranging from 30 to 60 min. The results showed that gaseous product yield increased with increasing temperature and reaction time. Gas products consisted primarily of hydrogen, carbon dioxide, methane, carbon monoxide and other light hydrocarbons. The liquid products contained high levels of organic matter, ammonia nitrogen and a few heavy metals. Compared with the landfilling of sewage sludge, the solid residues were in accordance with the Chinese standard for sludge quality in co-landfilling even without further treatment. In addition, the heavy metals in solid products exhibited more stable characteristics attributable to the reduced leaching toxicity after supercritical water gasification.