Several industrial wastes that are either insoluble in water at room temperature, very concentrated or very dilute are difficult to treat by supercritical water oxidation (SCWO). This limitation leads to different operating problems related to pumping, preheating and thermal control during their treatment. A new feed system that is described here avoids the aforementioned problems and is suitable to treat water-insoluble organics and wastewaters with a wide range of concentrations. The new feed system has been tested in a pilot plant that includes three independent feed streams: (1) a preheated aqueous feed stream that contains water or a dilute wastewater, (2) a non-preheated feed stream containing the water-insoluble waste (i.e., oil waste) or the highly concentrated wastewater and (3) an air stream used as a source of oxygen. Once the aqueous feed stream in the supercritical state was mixed with the water-insoluble waste, a supercritical homogeneous phase was obtained and the oxidant was subsequently added so that the oxidation reactions took place at a high rate and released a significant amount of heat. Appropriate control of these feed streams with this technological solution would extend the versatility of the SCWO process in respect of the concentration of residues to be treated (normally limited to between 5 and 20% organic matter), since it allows the combined treatment of dilute residues (<5%) in the aqueous feed and very concentrated residues (close to 100%) in the water-insoluble feed. The optimum concentration in the reactor can thus be controlled, making it possible for the plant to remain self-sufficient in energy or to generate surplus energy, and to maintain the pilot plant operation under safe conditions by simply adjusting the flow rate of the water-insoluble stream. The work described here concerns the best procedure and operating conditions to mix the three aforementioned streams (water-oil-air) leading to safe and efficient performance of a highly exothermal process. (C) 2012 Elsevier B.V. All rights reserved.