Wet air oxidation: past, present and future

Wet air oxidation (WAO) is a liquid-phase reaction between organic material in water and oxygen. The WAO process is used around the world to treat industrial wastewaters and sludges, at moderate temperatures (180-315 degrees C), and at pressures from 2 to 15 MPa. Under these conditions, complex organic compounds are mostly oxidized into carbon dioxide and water along with simpler forms which are biodegradable. Unlike other thermal processes, WAO produces no NO(x), SO(2), HCl, dioxins, furans and fly ash. The efficiency of aqueous phase oxidation can be largely improved by the use of catalysts, either heterogeneous or homogeneous. However, key points to be solved are stability of heterogeneous catalysts and recycling of homogeneous catalysts. This paper presents an update on the development of commercial catalytic WAO (CWAO) processes, which started as early as the mid-fifties in the United States. Several Japanese companies developed CWAO technologies relying on heterogeneous catalysts based on precious metals deposited on titania or titania-zirconia. Conversely, the focus in Europe was more on homogeneous CWAO, where three processes are already commercial and one under development. Compared to conventional WAO, CWAO offers lower energy requirements and much higher oxidation efficiencies. Further developments of this technology should include high durability/low cost catalysts. Catalytic wet air oxidation would, thus, provide an environmentally attractive option to manage the growing organic sludge and toxic wastewater treatment problems. (C) 1999 Elsevier Science B.V. All rights reserved.