Abstract: A process is disclosed for co-processing spent nitric acid wash water with organic or biological wastes under wet oxidation conditions whereby the nitrogen content of the waste and the chemical oxygen demand and the nitrogen content of the nitric acid wash water and of the organic or biological wastes are significantly reduced. The nitric acid wash water is the waste stream generated by descaling wet oxidation reactors used for the treatment of organic or biological wastes, especially municipal wastes. The process of this invention can be carried out in conventional above ground equipment or in a subterranean, down-hole reactor having a vertical configuration which utilizes gravitational forces to provide a high pressure reaction environment. The nitric acid wash water and organic wastes are reacted with an oxidant at a temperature greater than 150.degree. C. and a pressure suffient to keep water in the liquid state.