*2.2.3. Types of CWAO catalysts*

autothermal, no heat energy input is needed. A critical issue is the choice of structural materials of the reactor and of other hot parts of the system. Enameled or PTFE lined steel can be used up to 160°C, titanium and titanium-palladium alloys are applied up to 200°C because of the acidic pH. Several LOPROX plants are in operation at Bayer AG. They dispose PWWs from intermediate, dyestuff, pharmaceutical, paper and pulp production, and clarified sludge.

Veolia developed the **ATHOS®** process for the treatment of clarified sludge at WWTPs. It works with Cu ion catalysis at 250°C and 50-60 bar pressure with pure oxygen. The reactor is perfectly mixed, because it has a circulation loop, not the usual bubble column. The heat exchangers are working with extremely high-temperature water as heating source. Such plant is working at the Brussels WWTP, in a complex line for the abatement of clarified sludge [37].

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

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

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

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

*2.2.2. Industrial heterogeneous CWAO processes*

increasing and solid deposition preventing effect.

ammonia also [27].

162 Wastewater Treatment Engineering

WO.

studied CuO- ZnO-Al2O3.

The objective of catalyst application, beside reaction rate enhancement, is to operate among milder conditions. The catalyst is usually a metal salt, a metal oxide, or the metal itself [Table 2.]. As we know, heterogeneous catalysts based on precious metals deposited on stable supports are less sensitive to leaching [25, 31, 41-44]. Different catalysts were applied and their effects were investigated on different catalytic wet oxidation processes in the past years. Pt and Ru on ceria and zirkonia-ceria supports were tested in the oxidation of acetic acid that was accompanied by the loss of activity [32]. In a following paper, the same authors described the reason for deactivation of the Pt catalyst, the accumulation of carbonate species on the surface [45]. Recently, activity of Ru-oxide on different oxide supports in acetic acid oxidation was reported. The mixed Zr, Ce oxide supported catalyst proved to be the most active [46].


**Table 2.** CWAO of organic pollutants and industrial effluents. [40-41, 47-56]
