Wet oxidation procedures

Regulations on aqueous effluents, imposed by various regulations (ICPE, European laws on water and air), require compliance with increasingly drastic thresholds on the composition of discharges. Effluent decontamination processes must therefore be more efficient.

Aqueous industrial effluents are mainly made up of organic molecules synthesized by the chemical industry and sludge from wastewater treatment plants. In addition to the simplicity and low cost of treatment, which represent major economic criteria for industrialists, current environmental and societal challenges are emphasizing the need to develop "green" processes, i.e. those that respect the environment. One such process, using pressurized water under sub-critical conditions, is wet oxidation (WVO). The effluents targeted in the long term are specific effluents with low biodegradability rates, often posing treatment problems. Let's take a look at the different wastewater treatment processes: for concentrations below 1% by mass, the most commonly used process is biological (biodegradability permitting). For concentrations in excess of 20%, incineration or co-incineration may be considered. For effluents of intermediate content, particularly for waters with a high chemical oxygen demand (between 20 and 200 g.L- ¹ ), the OVH process can be considered. This process involves oxidation at a temperature of between 150 and 325°C over a wide range of total pressures (air or oxygen) from 2 to 30 MPa. In all cases, the process operates under sub-critical water temperature conditions. The low temperatures, compared with other thermochemical processes, limit the formation of toxic compounds in the gas phase (dioxins, NO _x ...). Residence time in the reactor ranges from a few minutes to several hours. Oxidation yields are in the order of 70-95%. The reaction may or may not be catalyzed.

The OVH process uses pressurized water, and its properties will be presented. The operating principle of the process will be explained, with particular emphasis on thermodynamic and kinetic aspects (gas/liquid transfer and chemistry). The different types of reactor and the constraints inherent in the process will be explained. Given the pressure and temperature conditions involved, aspects of energy recovery and optimization will also be discussed. Finally, a number of industrial realizations will be detailed.