*2.2.2. Industrial heterogeneous CWAO processes*

The development of stable and active heterogeneous catalysts for WO of PWWs is a difficult task, as the substrates to be oxidized are diverse, the wastes are multicomponent, and severe conditions are needed for the completion of the reactions. At high-temperature and highoxygen partial pressure even at basic pH of the reaction mixture the leaching of the active component(s) of the catalyst into the water solution frequently occurs. As mentioned in a review, there are two catalytic WAO technologies that have been developed in the late '80s in Japan. Both processes use heterogeneous catalysts, precious metals deposited on titaniazirconia carriers. They are able to oxidize two refractory compounds namely acetic acid and ammonia also [27].

The **NS-LC** process uses a Pt-Pd/TiO2-ZrO2 honeycomb catalyst. Typical operating conditions are 220°C and 4 MPa pressure with space velocity = 2 hour-1, which with these operating conditions the oxidation of compounds such as phenol, formaldehyde, acetic acid, glucose, etc., exceeds 99% conversion. In the absence of a catalyst the removal efficiency would go down to 5%-50% [38]. The specialty of the reactor is the segmented gas-liquid flow, which means that the liquid plugs are sandwiched between two gas plugs, and the flow has a mass transfer increasing and solid deposition preventing effect.

The other process, which is called **Osaka Gas,** is based on a mixture of precious and base metals on titania or titania-zirconia carriers in a form of honeycomb or sphere. This process has been applied in several industrial and urban wastes. A typical pilot plant at British Gas's London Research station works at 250°C and pressure of 9 MPa, with 200 L/h feed of waste [39].

Kurita Company developed a process to abate ammonia with the oxidation agent nitrite in the presence of a supported Pt catalyst. The reaction temperature (170°C) is lower than in usual WO.

One of the recent developments is the *CALIPHOX* process made by the National Institute of Chemistry of Slovenia and an engineering firm for treatment of industrial wastewaters with a metal oxide catalyst in the extruded form in a trickle-bed reactor. It is operating in relatively mild conditions (180°C, 4 MPa). The catalyst is based on the work of Pintar and Levec [40] who studied CuO- ZnO-Al2O3.
