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 ind ustrial wastewaters and sludges, at moderate temperatures (180-315 degrees C), and at pressures from 2 to 15 MPa. Under these conditions, complex orga nic compounds are mostly oxidized into carbon dioxide and water along with simpler forms which are biodegradable. Unlike other thermal processes, WAO produces no NOx, SO2, HCl, dioxins, furans and fly ash. The efficiency of aqueous phase oxidation can be largely improved by the us e of catalysts, either heterogeneous or homogeneous. However, key points to be solved are stability of heterogeneous catalysts and recycling of homoge neous catalysts. This paper presents an update on the development of commer cial catalytic WAO (CWAO) processes, which started as early as the mid-fift ies in the United States. Several Japanese companies developed CWAO technol ogies relying on heterogeneous catalysts based on precious metals deposited on titania or titania-zirconia. Conversely, the focus in Europe was more o n homogeneous CWAO, where three processes are already commercial and one un der development. Compared to conventional WAO, CWAO offers lower energy requirements and muc h higher oxidation efficiencies. Further developments of this technology sh ould include high durability/low cost catalysts. Catalytic wet air oxidatio n would, thus, provide an environmentally attractive option to manage the g rowing organic sludge and toxic wastewater treatment problems. (C) 1999 Els evier Science B.V. All rights reserved.