A process for disposing of sanitary sewage sludge by producing a pumpable slurry of sewage sludge with or without solid carbonaceous fuel and burning said slurry as fuel in a partial oxidation gas generator, furnace, or boiler. The aqueous slurry of sewage sludge is concentrated by removing water by means of a conventional belt filter press, centrifuge, or hydroclone. The dewatered slurry of sewage sludge is heated by direct contact with steam while air and organic vapors are simultaneously removed, to reduce the pressure in the system; hydrothermally treated in two steps with indirect and direct heat exchange with steam; dewatered by flash evaporation and centrifuging; and burned with or without admixture with solid carbonaceous fuel in a partial oxidation gasifier, furnace, boiler, or incinerator to produce synthesis gas or a hot raw effluent gas stream. In a preferred embodiment, the effluent gas stream is cleaned and purified and non-contaminating ash and slag are separated. By this process, noxious sewage sludge may be disposed of without contaminating the environment. Synthesis gas, reducing gas, or fuel gas may be produced.