Abstract: The present specification discloses operation of a waste treatment system for treating a feed by bringing the feed into contact with a molten metal in a first vessel. A jet of air is ejected from a lance into the molten metal to react with the molten metal to form a layer of molten slag-oxide. The feed is selected from coal, coal-liquid slurry, biomass, waste-derived material, crude oil, tar sands, shale-derived material, or a combination thereof. The molten metal bath material comprises carbon, silicon, manganese, chromium, sulfur, phosphorus, aluminum and titanium. Exhaust gases evolving from the molten metal and molten slag-oxide layer are directed to a second vessel to treat the exhaust gases to a pre-determined proximate gas molar composition.